Mode control of broadband light sources based on photonic crystal fibers

Abstract

A mode control system and method for controlling an output mode of a broadband light source including a photonic crystal fiber (PCF) are disclosed. The mode control system comprises: at least one detection unit configured to measure one or more parameters of radiation emitted from the broadband light source to generate measurement data; and a processing unit configured to evaluate the mode purity of the radiation emitted from the broadband light source through the measurement data. Based on the evaluation, the mode control system is configured to generate a control signal for optimizing one or more pump coupling conditions of the broadband light source. The pump coupling condition is related to the coupling of a pump laser beam relative to a fiber core of the photonic crystal fiber.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP Application 19194974.2 filed on September 2, 2019, EP Application 19215183.5 filed on December 11, 2019, EP Application 20152635.7 filed on January 20, 2020, and EP Application 20165824.2 filed on March 26, 2020, all of which are incorporated herein by reference in their entirety. Technical Field

[0003] The present invention relates to mode control of broadband radiation generators based on photonic crystal fibers, and in particular to such broadband radiation generators relevant for metrology applications in integrated circuit manufacturing. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern to a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may project a pattern (also commonly referred to as a "design layout" or "design"), for example, at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate (e.g., a wafer).

[0005] In order to project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of the radiation determines the minimum size of a feature that can be formed on a substrate. Typical wavelengths currently used are 365nm (i-line), 248nm, 193nm, and 13.5nm. Compared to lithographic apparatus using radiation with a wavelength of, for example, 193nm, lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20nm (e.g., 6.7nm or 13.5nm) can be used to form smaller features on a substrate.

[0006] Low k1 lithography can be used to process features with dimensions smaller than the classical resolution limit of the lithographic apparatus. In such a process, the resolution formula can be expressed as CD=k1×λ / NA, where λ is the wavelength of radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (usually the smallest feature size printed, but in this case half pitch), and k1 is an empirical resolution factor. In general, the smaller the k1, the more difficult it is to reproduce a pattern on a substrate that is similar in shape and size to that planned by the circuit designer to achieve specific electrical functionality and performance. To overcome these difficulties, complex fine-tuning steps can be applied to the lithographic projection apparatus and / or the design layout. For example, these include, but are not limited to, optimization of NA, customized illumination schemes, use of phase-shifted patterning devices, various optimizations of the design layout such as optical proximity correction (OPC, sometimes also referred to as "optical and process correction") in the design layout, or other methods generally defined as "resolution enhancement techniques" (RET). Alternatively, a tight control loop for controlling the stability of the lithographic apparatus can be used to improve the reproduction of patterns at low k1.

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as alignment tools for correctly positioning the substrate before exposure, leveling tools for measuring the substrate surface topology, focus control and scatterometry-based tools for checking / measuring the products of exposure and / or etching in process control, for example. In each case, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly used for such metrology applications. It is desirable to improve existing devices for broadband radiation generation. Summary of the invention

[0008] In a first aspect of the present invention, a mode control system is provided, which is configured to control the output mode of a broadband light source including a photonic crystal fiber (PCF), and the mode control system includes: at least one detection unit, configured to measure one or more parameters of radiation emitted from the broadband light source to generate measurement data; and a processing unit, configured to evaluate the mode purity of the radiation emitted from the broadband light source through the measurement data; wherein based on the evaluation, the mode control system is configured to generate a control signal to optimize one or more pump coupling conditions of the broadband light source; the pump coupling condition is related to the coupling of the pump laser beam relative to the fiber core of the photonic crystal fiber.

[0009] In a second aspect of the present invention, a mode control method for a broadband light source including a photonic crystal fiber is provided, the method comprising: measuring one or more parameters of radiation emitted from the broadband light source to obtain measurement data; evaluating the mode purity of the radiation emitted from the broadband light source through the measurement data; and generating a control signal to optimize one or more pump coupling conditions of the broadband light source; the pump coupling condition is related to the coupling of the pump laser beam relative to the fiber core of the photonic crystal fiber.

[0010] Other aspects of the invention include a broadband light source and a measurement device including the mode control system of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0012] - Figure 1 depicts a schematic overview of a lithography apparatus;

[0013] - Figure 2 A schematic overview of a lithography unit is depicted;

[0014] - Figure 3 Depicted is a schematic representation of overall lithography showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0015] - Figure 4 depicts a schematic overview of a scatterometry apparatus used as a metrology device, which may include a radiation source according to an embodiment of the present invention;

[0016] - Figure 5 depicts a schematic overview of a level sensor device, which may include a radiation source according to an embodiment of the present invention;

[0017] - Figure 6 depicts a schematic overview of an alignment sensor device, which may include a radiation source according to an embodiment of the present invention;

[0018] - Figure 7 Schematic depiction of the cross-sections of two HC-PCF designs for white light generation, including (a) the Kagome design and (b) the single-ring design.

[0019] - Figure 8 An exemplary gas-filled HC-PCF-based broadband light source device is schematically depicted;

[0020] - Fig. 9 is a flowchart describing the operation procedure of the mode control system according to the embodiment of the present invention.

[0021] - Fig.10Schematically depicted is a broadband light source equipped with a mode control system according to a first embodiment of the invention for optimizing and stabilizing a fundamental transverse mode LP01 of the light source.

[0022] - Fig.11 A broadband light source equipped with a mode control system for optimizing and stabilizing a fundamental transverse mode LP01 of the light source according to a second embodiment of the invention is schematically depicted.

[0023] - Fig.12 A broadband light source equipped with a mode control system for optimizing and stabilizing a fundamental transverse mode LP01 of the light source according to a third embodiment of the invention is schematically depicted.

[0024] - Fig.13 A broadband light source equipped with a mode control system for optimizing and stabilizing a fundamental transverse mode LP01 of the light source according to a fourth embodiment of the invention is schematically depicted.

[0025] - Fig.14 schematically depicts a broadband light source equipped with a mode control system for optimizing and stabilizing a fundamental transverse mode LP01 of the light source according to different embodiments of the present invention;

[0026] - Fig.15 schematically depicts a coarse alignment arrangement according to a first embodiment for coarse alignment;

[0027] - Fig.16 schematically depicts a coarse alignment arrangement according to a second embodiment for coarse alignment;

[0028] - Fig.17 schematically depicts a coarse alignment arrangement according to a third embodiment for coarse alignment; and

[0029] - Fig.18 Schematically depicts a method for using at least Figures 15 to 17 A specific optical manipulation unit of the coarse alignment arrangement depicted in FIG.

[0030] - Fig.19 A broadband light source equipped with a specific timing control system for controlling and determining the timing of pulses of the broadband light source according to an embodiment is schematically depicted.

[0031] - Fig. 20 A broadband light source equipped with a specific polarization control system for optimizing and stabilizing the output polarization of the broadband light source according to an embodiment is schematically depicted. DETAILED DESCRIPTION

[0032] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 to 100 nm).

[0033] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incoming radiation beam, corresponding to the pattern to be created in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0034] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as an illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioner PM (configured to accurately position the patterning device MA according to certain parameters), a substrate support (e.g. a wafer stage) WT constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioner PW (configured to accurately position the substrate support according to certain parameters), and a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0035] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0036] The term "projection system" PS as used herein should be broadly interpreted as covering various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate for the exposure radiation used and / or other factors (such as the use of immersion liquid or the use of a vacuum). Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0037] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion technology is given in US6952253, which is incorporated herein by reference.

[0038] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also referred to as "dual stage"). In such a "multi-stage" machine, the substrate supports WT may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is used to expose a pattern on another substrate W.

[0039] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measuring table. The measuring table is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure properties of the projection system PS or properties of the radiation beam B. The measuring table may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, e.g. a part of the projection system PS or a part of a system for providing immersion liquid. The measuring table may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0040] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA), which is held on a mask support MT, and is patterned by a pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through a projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (not shown) are arranged in a plurality of positions. Figure 1 The patterning device MA (depicted explicitly in FIG. 1 ) can be used to accurately position the patterning device MA relative to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they can be located in the space between target portions. When the substrate alignment marks P1, P2 are located between target portions C, these are referred to as scribe lane alignment marks.

[0041] like Figure 2As shown, the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography cell) cluster, which typically also includes equipment for performing pre-exposure and post-exposure processes on a substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (e.g. for regulating the solvent in the resist layer). A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between the different process equipment, and delivers the substrates W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell (often also collectively referred to as tracks) are typically controlled by a track control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA, for example via a lithography control unit LACU.

[0042] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on substrate W, particularly if the inspection is done before other substrates W of the same batch or lot are still to be exposed or processed.

[0043] An inspection apparatus (which may also be referred to as a metrology apparatus) is used to determine properties of a substrate W, in particular how properties vary from one substrate W to another, or how properties associated with different layers of the same substrate W vary from layer to layer. The inspection apparatus may alternatively be configured to identify defects on the substrate W, and may for example be part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or on a developed resist image (where exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0044] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in dimensioning and placement of structures on the substrate W. To ensure this high accuracy, the three systems can be combined into a so-called "holistic" control environment, such as Figure 3Schematically depicted in . One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MET (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process will produce a defined result (e.g. a functional semiconductor device), and typically within which process parameters of a lithography process or patterning process are allowed to vary.

[0045] The computer system CL can use (a portion of) the design layout to be patterned to predict the resolution enhancement technique to be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used to detect where within the process window the lithographic apparatus LA is currently operating (e.g. using input from a metrology tool MT) to predict whether defects may be present (e.g. due to suboptimal processing). Figure 3 0 in the figure).

[0046] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithographic apparatus LA to identify possible drifts, such as in the calibration state of the lithographic apparatus LA (in Figure 3 (depicted by multiple arrows in the third scale SC3).

[0047] During the lithographic process, it is desirable to frequently measure the structures created, for example for process control and verification. The tool that performs such measurements is typically referred to as a measurement tool MT. Different types of measurement tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer measurement tools MT. Scatterometers are versatile instruments that allow measurement of parameters of the lithographic process by placing a sensor in the pupil of a scatterometer objective or in a plane conjugate to the pupil (these measurements are generally referred to as pupil-based measurements) or by placing a sensor in an image plane or in a plane conjugate to the image plane (in this case, these measurements are generally referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, which are incorporated herein by reference in their entirety. The scatterometer described above can measure the grating using light from soft x-rays and visible to the near IR wavelength range.

[0048] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods may be applied to the measured signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction may be performed by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measured results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from an actual target.

[0049] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e. measures the intensity as a function of wavelength). From this data, the structure or profile of the target which produced the detected spectrum can be reconstructed, for example by rigorous coupled wave analysis and nonlinear regression or by comparison with a library of simulated spectra.

[0050] In a third embodiment, the scatterometer MT is an ellipsometer. An ellipsometer allows the parameters of a lithography process to be determined by measuring the scattered radiation for each polarization state. Such a metrology device emits polarized light (such as linear, circular or elliptical) by using, for example, appropriate polarization filters in the illumination section of the metrology device. A source suitable for the metrology device may also provide polarized radiation. Various embodiments of existing ellipsometers are described in U.S. patent applications 11 / 451,599, 11 / 708,678, 12 / 256,780, 12 / 486,449, 12 / 920,968, 12 / 922,587, 13 / 000,229, 13 / 033,135, 13 / 533,110 and 13 / 891,410, which are incorporated herein by reference in their entirety.

[0051] Measurement equipment such as scatterometers are used in Figure 4 It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate W. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 6 of the specularly reflected radiation (i.e. the intensity as a function of wavelength). From this data, for example, by rigorous coupled wave analysis and nonlinear regression or by comparing Figure 3 The structure or profile 8 that produces the detected spectrum can be reconstructed by the processing unit PU by comparing it to the simulated spectral library shown at the bottom. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed by knowledge of the process by which the structure was manufactured, only a few parameters of the structure are to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0052] The overall measurement quality of the lithographic parameters measured via the measurement target is at least partially determined by the measurement scheme used to measure the lithographic parameters. The term "substrate measurement scheme" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement scheme is an optical measurement based on diffraction, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the radiation orientation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement scheme may be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. patent application US2016-0161863 and published U.S. patent application US 2016 / 0370717A1, which are incorporated herein by reference in their entirety.

[0053] Another type of metrology tool used in IC manufacturing is a topography measurement system, a level sensor or a height sensor. This tool can be integrated in a lithographic device to measure the topography of a substrate (or wafer). A map of the topography of a substrate (also referred to as a height map) can be generated from these measurements, which indicates the height of the substrate according to the position on the substrate. The height map can then be used to correct the position of the substrate during the pattern transfer to the substrate so as to provide an aerial image of the pattern forming device in a properly focused position on the substrate. It is to be understood that in this context, "height" refers to a dimension (also referred to as the Z axis) that is clearly outside the plane for the substrate. Typically, a level or height sensor performs measurement at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor causes height measurement to be performed at a position on the entire substrate.

[0054] Examples of level or height sensors LS known in the art are Figure 5 As schematically shown in Figure 5 Only the operating principle is illustrated. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO, which provides a radiation beam LSB applied by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrowband or broadband radiation source (such as a supercontinuum light source), polarized or unpolarized, pulsed or continuous, such as a polarized or unpolarized laser beam. The radiation source LSO may include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible radiation, but may additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0055] The projection grating PGR is a periodic grating including a periodic structure, which causes the radiation beam BE1 to have a periodically varying intensity. The radiation beam BE1 with periodically varying intensity is directed to a measurement position MLO on the substrate W with an incident angle ANG relative to an axis perpendicular to the incident substrate surface (Z axis), the incident angle ANG being between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement position MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed to the detection unit LSD.

[0056] In order to determine the height level at the measuring position MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal indicating the received light (e.g., such as a photodetector indicating the intensity of the received light) or representing the spatial distribution of the received intensity (such as a camera). The detector DET may comprise any combination of one or more detector types.

[0057] By means of triangulation techniques the height level at the measurement position MLO can be determined.The detected height level is typically related to the signal intensity measured by the detector DET, which has a periodicity that depends inter alia on the design of the projection grating PGR and the (oblique) angle of incidence ANG.

[0058] The projection unit LSP and / or the detection unit LSD may comprise further optical elements such as lenses and / or mirrors along the path of the patterned radiation beam between the projection grating PGR and the detection grating DGR (not shown).

[0059] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0060] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of reference beams BE1 onto the surface of the substrate W, thereby generating a measurement area MLO or an array of spots covering a larger measurement range.

[0061] For example, various height sensors of the general type are disclosed in US7265364 and US7646471, both of which are incorporated herein by reference. Height sensors using UV radiation rather than visible or infrared radiation are disclosed in US2010233600A1, which is incorporated herein by reference. In WO2016102127A1 (which is incorporated herein by reference), a compact height sensor is described that uses a multi-element detector to detect and identify the position of a grating image without the need for a detection grating.

[0062] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to correctly and accurately place an applied pattern with respect to features laid down in a previous layer (either by the same apparatus or a different lithographic apparatus). For this purpose, a substrate is provided with one or more sets of marks or targets. Each mark is a structure whose position can be measured at a later time using a position sensor (usually an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark".

[0063] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the positions of alignment marks provided on the substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena, such as diffraction and interference, to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor currently used in a lithographic apparatus is based on a self-referencing interferometer described in US6961116. For example, as disclosed in US2015261097A1, various enhancements and modifications of position sensors have been developed. The contents of all of these publications are incorporated herein by reference.

[0064] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS, such as described, for example, in US6961116 (which is incorporated herein by reference). A radiation source RSO provides a radiation beam RB of one or more wavelengths, which is steered by steering optics onto a mark, such as a mark AM located on a substrate W, as an illumination spot SP. In this example, the steering optics include a spot mirror SM and an objective lens OL. The diameter of the illumination spot SP, with which the mark AM is illuminated, may be slightly smaller than the width of the mark itself.

[0065] Radiation diffracted by the alignment mark AM is collimated (in this example, via the objective lens OL) into an information bearing beam IB. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI (for example of the type disclosed in US6961116 mentioned above) causes the beam IB to interfere with itself, after which the beam is received by a photodetector PD. In the event that more than one wavelength is created by the radiation source RSO, additional optics (not shown) may be included to provide separate beams. If desired, the photodetector may be a single element, or it may comprise several pixels. The photodetector may comprise an array of sensors.

[0066] The steering optics comprising in this example a spot mirror SM can also be used to block zero-order radiation reflected from the mark so that the information-bearing beam IB comprises only higher-order diffracted radiation from the mark AM (which is not essential for the measurement but improves the signal-to-noise ratio).

[0067] The intensity signal SI is supplied to the processing unit PU. By a combination of optical processing in block SRI and computational processing in unit PU, values ​​of the X and Y position on the substrate relative to the reference frame are output.

[0068] A single measurement of the type illustrated only fixes the position of the mark within a specific range corresponding to one pitch of the mark. A coarse measurement technique is used in conjunction with this measurement to identify which period of the sine wave is the period that contains the marked position. The same process can be repeated at different wavelengths with coarser and / or finer levels to increase accuracy and / or robustly detect the mark regardless of the material the mark is made of and the material on and / or below the location where the mark is placed. The wavelengths can be optically multiplexed and demultiplexed so as to be processed simultaneously, and / or they can be multiplexed by time or frequency division.

[0069] In this example, the alignment sensor and the spot SP remain stationary while the substrate W is moving. The alignment sensor can therefore be rigidly and accurately mounted to the reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by being mounted on a substrate support and a substrate positioning system (controlling the movement of the substrate support). A substrate support position sensor (e.g., an interferometer) measures the position of a substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measuring the position of the mark provided on the substrate support allows the position of the substrate support determined by the position sensor to be calibrated (e.g., relative to a frame to which the alignment system is connected). Measurement of the position of the alignment mark provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0070] For optical semiconductor measurement and inspection applications, such as in any of the above-mentioned measurement tools, a bright light source that outputs coherent radiation while covering a wide wavelength range (e.g., from UV to IR) is generally preferred. Such a broadband light source can help improve the flexibility and robustness of the application by allowing wafers with different material properties to be optically inspected in the same setup / system without any hardware changes (e.g., changing the light source to have a specific wavelength). Allowing the wavelength to be optimized for a specific application also means that measurement accuracy can be further improved.

[0071] Gas lasers that emit multiple wavelengths simultaneously based on the gas discharge effect can be used for these applications. However, inherent problems associated with gas lasers (such as high intensity instability and low spatial incoherence) may make them unsuitable. Alternatively, the outputs from multiple lasers with different wavelengths (e.g., solid-state lasers) can be spatially combined into the optical path of the measurement or inspection system to provide a multi-wavelength source. As the number of desired wavelengths increases, complexity and high implementation costs prevent the widespread use of this solution. In contrast, fiber-based broadband or white light lasers (also called supercontinuum lasers) are able to emit radiation with high spatial coherence and wide spectral coverage, such as from UV to IR, and are therefore a very attractive and practical option.

[0072] Hollow core photonic crystal fiber (HC-PCF) is a special type of optical fiber that includes a central hollow core region and an internal cladding structure surrounding the hollow core, both of which extend axially along the entire fiber. The light guiding mechanism is realized by the internal cladding waveguide structure, which can include, for example, thin-walled glass elements. Therefore, the radiation is mainly confined within the hollow core and propagates along the fiber in the form of transverse core modes.

[0073] Various types of HC-PCFs can be designed, each based on a different physical guiding mechanism. Two such HC-PCFs include: hollow core photonic bandgap fiber (HC-PBF) and hollow core antiresonant reflective fiber (HC-ARF).

[0074] HC-PCFs include hollow channels filled with fluids, so they have properties desirable for a variety of light-guiding applications; such as high-power beam delivery using HC-PBFs and gas-based white light generation (or supercontinuum generation) using HC-ARFs. Details on the design and manufacture of HC-PCFs can be found in U.S. Patent US2004175085 (for HC-PBFs) and European Patent Application EP3136143A1 (for HC-ARFs), which are incorporated herein by reference. The HC-PBF is configured to provide a low-loss but narrow-bandwidth light guide via the photonic bandgap effect established by the cladding structure of the central hollow core of the present invention. However, the HC-ARF is designed to significantly widen the transmission bandwidth via anti-resonant reflection of light from the cladding.

[0075] Figure 7 Two well-known types of HC-ARF are depicted in cross-section. Figure 7 (a) shows a Kagome optical fiber, comprising a Kagome lattice structure as its cladding CLA, defining a hollow optical fiber core FCO. This arrangement may be surrounded by one or more outer coatings OCO. Figure 7(b) shows a single ring or revolving optical fiber, in which a hollow core region FCO is formed and surrounded by a layer of non-contact ring CLA.

[0076] For gas-based white light generation, the HC-ARF can be included in a gas cell designed to operate at pressures of, for example, up to tens of bars (e.g., between 3 and 100 bars). When pumped by ultrashort pump laser pulses with sufficient peak power, the gas-filled HC-ARF can act as an optical frequency converter. The frequency conversion from ultrashort pump laser pulses to broadband laser pulses is achieved by a complex interplay of dispersion and nonlinear optical processes inside the gas-filled fiber. The converted laser pulses are mainly confined within the hollow core in the form of transverse core modes and are guided to the end of the fiber. The transverse core modes supported by the fiber can be described as linearly polarized (LP) modes. In LP notation, LP modes are referred to as LP mn , where the m and n subscripts are integers representing the azimuthal and radial order of a particular mode. The fundamental mode is LP 01 mode. Part of the radiation (e.g., high-order transverse core modes or specific wavelengths) may leak from the hollow core through the internal cladding waveguide structure and experience strong attenuation during its propagation along the fiber. The core region and cladding region of the HC-ARF can be configured so that the high-order core mode is phase-matched with the high-order cladding mode. In this way, the high-order core mode can resonantly couple with the high-order cladding mode, which is then attenuated or suppressed. In this way, low-loss and efficient single transverse mode transmission can be obtained over a wide spectral range.

[0077] The spatiotemporal transmission characteristics of the laser pulses (e.g., the spectral amplitude and phase thereof transmitted along a PCF (such as an HC-PCF)) can be changed and tuned by adjusting pump laser parameters, filling gas parameters, fiber parameters, and pump coupling conditions. The transmission characteristics may include one or more of the following: output power, output mode profile, output time profile, width of the output time profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The pump laser parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The pump parameters may include one or more of the following: fiber length, size and shape of the hollow core, size and shape of the cladding structure, thickness of the wall surrounding the hollow core. The filling gas parameters may include one or more of the following: gas type, gas pressure, and gas temperature. The pump coupling conditions that determine the extent to which the pump laser beam is coupled into the fiber core may include one or more of the following: angular offset of the pump laser beam relative to the fiber core, lateral offset of the pump laser beam relative to the fiber core, and mode matching between the pump laser beam and the fiber core. The mode matching between the pump laser beam and the optical fiber core may be determined by parameters such as the beam diameter of the pump laser beam, the divergence of the pump laser beam, the hollow core diameter, and the hollow core NA.

[0078] The filling gas of the HC-PCF can be an inert gas (such as helium, neon, argon, krypton and xenon), a Raman active gas (such as hydrogen, deuterium and nitrogen) or a gas mixture (such as an argon / hydrogen mixture, a xenon / deuterium mixture, a krypton / nitrogen mixture or a nitrogen / hydrogen mixture). Depending on the type of filling gas, nonlinear optical processes may include modulation instability (MI), soliton fission, Kerr effect, Raman effect and dispersion wave generation, the details of which are described in WO2018 / 127266A1 and US9160137B1 (both of which are incorporated herein by reference). Since the dispersion of the filling gas can be tuned by changing the gas chamber pressure, the generated broadband pulse dynamics and the associated spectral broadening characteristics can be adjusted to optimize frequency conversion. The generated broadband laser output can cover wavelengths from UV (e.g. <200nm) to mid-IR (e.g. >2000nm).

[0079] When applied to semiconductor metrology and alignment applications, such as in any of the above-mentioned metrology tools, it is desirable that the lateral mode of the output radiation of the HC-PCF based broadband light source is the fundamental lateral mode, i.e., the LP 01. In other words, a broadband laser beam with high or maximized mode purity is generally preferred, which is defined as the ratio between the power in the fundamental transverse mode and the total output power. This is due to the fact that the transmission losses of the fundamental transverse mode through a PCF (e.g. HC-PCF) are much lower than those of higher-order modes (HOMs). Therefore, the power efficiency is higher if all pump light is coupled into the fundamental transverse mode of the fiber. In addition, the presence of HOMs degrades the mode quality and intensity stability of the broadband output. In many applications where a Gaussian beam profile is desired, a broadband output with poor mode purity will experience significant power losses because the HOM content of the output will be removed during transmission, for example by spatial filtering. The reduction in intensity stability leads to high measurement noise and poor measurement consistency.

[0080] Figure 8 An exemplary HC-PCF based broadband light source 800 is schematically illustrated. A collimated pump laser beam 811 (comprising a pump pulse train having a specific repetition rate) is output from a pump laser 810 and is used as an input laser beam to generate broadband radiation in a HC-PCF 841. The propagation of the collimated pump laser beam is controlled by one or more beam steering components (e.g. forming part of a beam delivery system), depicted here as two steering mirrors 820 and 821, and is directed through a focusing lens 830. The focusing lens creates a suitable focus of the pump laser, which is matched to the core mode of the HC-PCF 841. The focused pump laser beam is transmitted through an input optical window 842 before being coupled into the fiber core of the HC-PCF 841. The HC-PCF 841 having a specific fiber length can be used with reference to FIG. Figure 7 Kagome design or single ring design. Alternatively, other fiber designs (not shown) may be used, such as solid core designs, suppressed coupling designs, hypocycloidal core Kagome, and nested tubular designs. In this example, the entire HC-PCF 841 is included in a single pressure-resistant gas chamber 840, which is filled with a working gas or gas mixture at a specific pressure or with a pressure distribution. After coupling to the inflated HC-PCF, the pump laser pulses propagate along the optical fiber, where they undergo significant spectral broadening. The resulting broadband laser pulses 880 are then released from the gas chamber 840 via the output optical window 843. The broadband laser beam 880 is then collimated to a suitable beam size by a collimating lens 831.

[0081] To fill the HC-PCF with working gas, the plenum may be in communication with a pressurized gas supply or reservoir (not shown). The walls of the plenum and the inner surface of the window enclose the cavity. The axis of the plenum is parallel to the axis of the HC-PCF.

[0082] The pump pulse duration may be selected to be greater than 10 fs, more specifically in the range of 10 fs to 100 ps, ​​10 fs to 30 ps or 10 fs to 1 ps. The pump wavelength may be selected from the visible region, the near IR region or the mid IR region. The pump laser pulses may have a repetition rate of hundreds of Hertz (Hz), kilohertz (kHz) or megahertz (MHz). Specifically, the repetition rate may be selected in the range of 100 kHz to 100 MHz, such as 100 kHz, 500 kHz, 1 MHz, 5 MHz, 10 MHz, 20 MHz, 30 MHz, 40 MHz.

[0083] The alignment of the pump laser beam relative to the HC-PCF can include two main steps, namely, rough alignment and fine alignment. Rough alignment is performed at a sufficiently low pump pulse energy or pump power to prevent damage to the HC-PCF. This step is to ensure that the pump beam is properly coupled into the hollow core of the HC-PCF and that the transverse core mode is excited at the front (or input) facet of the HC-PCF. Without rough alignment, damage may occur when the center of the high-power pump laser beam hits the cladding wall of the HC-PCF. Once the transverse core mode is excited and the transmission efficiency (defined as the ratio between the fiber output power and the fiber input power) is maximized at low power levels, fine alignment at high power levels is initiated. Again, the purpose of fine alignment is to further maximize the transmission efficiency. However, in this conventional optimization method, the maximized transmission efficiency (i.e., the maximized output power at a given input power) does not necessarily correspond to the highest mode purity. In other words, even after the transmission efficiency is optimized, HOM content can still exist in the output of the broadband light source. The main reason for this difference is that the transverse beam profile of the pump laser beam is usually not perfect, i.e., M 2 > 1. Therefore, no matter how well the overall transfer efficiency is optimized, the HOM will always be excited.

[0084] Both coarse alignment and fine alignment (e.g. as part of pump coupling optimization) can be achieved by one or more of the following: 1) movement of at least one beam steering component in the beam delivery system (in Figure 8and many subsequent figures as two steering mirrors, but this is only an exemplary beam delivery system); 2) movement of the gas chamber, and 3) movement of the focusing lens (if available). One or more beam steering components or steering mirrors can be held by a kinematic mirror mount that can be adjusted manually and / or electrically via an actuator such as a piezoelectric actuator. The mirror can be fixed by any suitable fixing arrangement, such as, for example, directly adhering a portion of its rear surface to the mirror mount or by using a fixing screw. The gas chamber can be mounted on a workbench module including one or more (e.g., piezoelectric) workbenches. The workbench module can provide movement with multiple degrees of freedom, for example, six degrees of freedom. Additionally or alternatively, alignment of the pump laser beam relative to the HC-PCF can also be achieved by inserting one or more additional optical components into the beam path of the pump laser beam (e.g., as part of a beam delivery system or otherwise). Such optical components can include, for example, two or more (rotatable) optical wedges or any other optical component that can generate the desired movement (angular movement and / or lateral movement) of the input pump laser beam. Any one or more of these alignment strategies and methods may be used for pump coupling optimization of the methods described herein.

[0085] In order to achieve good mode matching between the pump laser beam and the fiber core, the pump laser beam can be focused by an optical element (e.g., a lens) before entering the fiber, and the lens properties make the divergence and the diameter of the focused pump laser beam closely match the numerical aperture (NA) and mode field diameter of the fiber core. Since the characteristics of the focused pump laser beam are determined by the lens when the input pump laser beam is fixed, different HC-PCFs may have different fiber characteristics and thus may require different focusing lenses to achieve optimal mode matching. Similarly, when the characteristics of the HC-PCF are fixed, different pump laser beams (e.g., with different beam diameters or beam divergences) may require different focusing lenses to keep the focused beam diameter the same.

[0086] The presence of HOM degrades the mode quality as well as the intensity stability of the broadband output. Inconsistency in mode purity across the output spectrum results in wavelength-dependent output performance, which will negatively impact the reliability and repeatability of the measurement data if such a broadband source is used in any of the above-mentioned measurement tools. Typically, HOM is triggered when one or more pump coupling conditions are not optimized. In some scenarios where the mode purity of a broadband source is fully optimized across the entire output spectrum, HOM may still appear at the output after a certain amount of operating time. The appearance of HOM during operation is caused by degradation of pump coupling conditions caused by, for example, thermal and / or vibration drift.

[0087] Return to reference Figure 8In an exemplary setup of a broadband light source in, a small amount of the incident pump laser beam may leak through the reflective surface of a beam steering component, such as a steering mirror, and be incident on the component mounting mechanism. This pump leakage may heat the mounting mechanism and change its condition. For example, such heating may cause the adhesive used to bond the component to the mounting mechanism to soften, resulting in a minor misalignment of the component, which in turn results in a misalignment of the pump laser beam relative to the HC-PCF. Since the HC-PCF-based broadband light source is alignment sensitive, the misalignment of the pump laser beam relative to the HC-PCF may significantly degrade the coupling conditions of the pump laser beam into the fiber core, resulting in the generation of HOMs and / or a reduction in the mode purity of the broadband output. Misalignment and / or alignment changes of the pump laser beam may also be caused by other factors, such as ambient temperature oscillations or external vibrations. In a similar manner, thermal and / or vibration drifts of the HC-PCF also result in the appearance of HOMs and / or a reduction in the mode purity of the broadband output.

[0088] Therefore, it is highly desirable to optimize and / or stabilize the broadband output during operation of broadband light sources, especially LP 01 A mode control method and apparatus are therefore proposed to address the above-mentioned mode purity problem associated with PCF-based broadband light sources.

[0089] Fig. 9 The operating procedure of the proposed mode control system according to an embodiment is illustrated. In step 910, one or more parameters of a broadband output beam are measured. Such one or more beam parameters (i.e., parameters of the broadband output beam) indicate the output performance with respect to the purity of the fundamental mode. In step 920, the measured data is processed. In step 930, the processed data is evaluated by following a predefined set of criteria. The details of steps 910 to 930 depend largely on the monitored (multiple) beam parameters and / or hardware settings, and more detailed examples of these steps will be described below. Based on the results of such evaluation, a control signal will be generated in step 940. In step 950, the control signal is used to control one or more components of the broadband light source. The control of such components of the broadband light source optimizes the pump coupling conditions so that the fundamental transverse mode LP is outputted at the same level as the output of the broadband light source. 01 The pattern purity is maximized.

[0090] Optimization of pump coupling conditions can be achieved in a variety of ways. Any method that improves the coupling of the pump laser into the PCF may be used, and may be achieved by moving the pump beam relative to the PCF (e.g. via beam steering components or otherwise), moving the PCF relative to the pump beam, or a combination of both; or alternatively or additionally by changing the position or configuration of any intervening optical components (such as focusing components). This can be performed while monitoring the reference beam (as well as the output beam) to ensure that mode purity is optimized. Thus, the method can be implemented in closed-loop operation so that transverse mode changes of the broadband output can be continuously monitored and optimized. Depending on the type of beam parameter being monitored, different detection mechanisms may be used, each of which may require one or more different measurement devices or components.

[0091] Fig.10 Schematically illustrates a broadband light source equipped with a mode control system 1000 according to a first embodiment of the present invention for optimizing and stabilizing the fundamental transverse mode LP of the light source. 01 In this embodiment, the broadband light source is substantially similar to Figure 8 For the sake of brevity, equivalent components and features may be provided with similar reference numerals in the remaining drawings rather than being described separately (e.g. Figure 8 Tags in 811, Fig.10 1011 in Fig.11 1111 etc. describe the pump laser beam).

[0092] like Fig.10 As shown, a portion of the main broadband output beam 1080 is reflected by the front surface of the beam splitter 1051 (which may optionally be included in the detection unit 1050) and is used as a reference beam 1081. It is to be noted that the beam splitter 1051 should not cause any spatial and spectral distortion to the reference beam, so that the reference beam 1081 and the main broadband output beam 1080 are considered to share the same beam characteristics. The reference beam 1081 is used by the mode control system 1000 to optimize and stabilize the fundamental transverse mode LP of the broadband light source. 01. The mode control system 1000 comprises a detection unit 1050, a processing unit 1060 and a control unit 1070. The detection unit 1050 measures one or more parameters of the broadband output. The resulting measurement data is then sent to the processing unit 1060 for data processing and evaluation. Based on the results of the evaluation, a control signal is generated and used by the control unit 1070 to control one or more beam control system components accordingly. Such beam control system components may include, for example, one or more beam delivery or beam steering components (e.g., steering mirrors 1020, 1021 or actuators thereof), an actuator or a stage to move the gas cell 1040, an actuator to move the (optional) focusing lens 1030, and / or an actuator such as a rotating polarizer to change the absolute polarization angle (e.g., in the case where the HC-PCF maintains polarization and a certain absolute orientation of the polarization of the light is desired). The data measurement and evaluation may be performed in a continuous or periodic manner.

[0093] In a first embodiment, the detection unit comprises a bandpass filter 1052 and an illumination measuring device (e.g., a power meter) such as a power measuring device 1053, which is arranged so that the power measuring device 1053 measures the power of the broadband output in the passband spectral range of the filter. Optionally, the bandpass filter 1052 arrangement can have a variable passband arrangement. In this way, multiple power values ​​measured in multiple spectral ranges can be obtained, each power value corresponding to each spectral range of each bandpass filter. This can be achieved by mounting the bandpass filter 1052 on a filter wheel together with one or more different bandpass filters, so that when completing a power measurement in a specific spectral range (which corresponds to Fig. 9 In step 910 of the method), the filter wheel can be rotated to enable different bandpass filters. Other arrangements for obtaining variable bandpass characteristics can be envisioned, including, for example, providing a movable high-pass and a movable low-pass continuously variable filter in series.

[0094] The measured power values ​​from the power measurement device 1053 and the spectral information of the bandpass filter(s) used in the measurement are then sent to the processing unit 1060 to calculate one or more spectral parameter values, such as the power spectral density (PSD) or energy spectral density value (which corresponds to Fig. 9 The calculated PSD value is then evaluated in processing unit 1060 by following a certain set of criteria (which corresponds to Fig. 9 The processing unit 1060 may include a processor configured to process the measured data and then perform an evaluation on the processed data. Alternatively or additionally, the evaluation may be performed directly on the measured power value.

[0095] During data evaluation, the calculated PSD value can be compared with a reference PSD value within the corresponding spectral range, and based on the comparison, a set of deviation values ​​is generated; wherein the deviation value includes a measure of the degree of deviation of the calculated PSD value relative to the reference PSD value. The reference PSD value can correspond to a mode purity of a broadband output beam indicating optimal output. The set of deviation values ​​can be evaluated against a set of predefined deviation thresholds for the corresponding spectral range to determine whether the pump coupling conditions of the broadband light source are acceptable. The predefined deviation threshold can be set to a percentage in the range of, for example, 5% to 25% of the reference PSD value (e.g., 5%, 10%, 15% or 20% of the reference PSD value). If the deviation value indicates an unacceptable pump coupling condition such that the mode purity of the broadband output is suboptimal, a control signal is generated and / or changed accordingly (which corresponds to Fig. 9 Based on the control signal, the control unit 1070 will command one or more components to improve / optimize the pump coupling conditions and maximize the mode purity of the broadband output (which corresponds to Fig. 9 Step 950 of the method).

[0096] Several specific and purely exemplary methods for optimizing pump coupling conditions will now be described, in which the control unit 1070 (or processing unit 1060) can command two beam steering components (e.g., steering mirrors 1020, 1121) to perform incremental scans in the horizontal and vertical directions. The horizontal direction is defined as parallel to the optical table plane, and the vertical direction is defined as perpendicular to the table plane. This scanning implementation is applicable to all embodiments described herein, although the monitored (multiple) beam parameters may be different for later embodiments (as will be apparent).

[0097] In a first such specific scanning embodiment, the scan can be performed by scanning a first reflector (e.g., reflector 1020) of the reflectors in a zigzag or zigzag path: for example, it can be commanded to scan a repetition of the following sequence: a first incremental scan is performed within the desired range in the x direction, and a single move is performed in the y direction, repeated to cover the desired range in the y direction. While the first reflector 1020 scans, the second reflector 1021 is maintained in its original position. After each incremental movement, a measurement, such as a power measurement, is taken. In this way, a power map and / or a calculated PSD map in a predefined area is generated. Based on the (multiple) maps, the position of the first reflector is optimized, and the second reflector (e.g., 1021) will begin scanning in the same manner. At the end of the reflector scan, the position of the second reflector will also be optimized.

[0098] In the above scanning routine, the two mirrors are decoupled. When one mirror is scanning, the other mirror is assumed to be maintained in an optimal position. Therefore, this arrangement is not ideal when both mirrors drift from their respective optimal positions. Therefore, in a second scanning embodiment, a joint optimization of two or more beam steering components (e.g., mirrors 1020, 1021) can be performed. This specific embodiment can include a first mirror (e.g., mirror 1020) making incremental movements within a predefined range in the x direction, followed by a second mirror (e.g., mirror 1021) scanning the entire area determined by the desired range in the x direction and the desired range in the y direction. When the second mirror completes the area scan, the first mirror makes another incremental movement in the same direction, and the second mirror performs another area scan. This is repeated until the first mirror reaches the end of the desired range in the x direction, at which time it moves incrementally in the y direction and performs another incremental scan in the x direction. When the first mirror has scanned the entire area determined by the predefined ranges in the x and y directions, the complete mirror scanning process is completed. Hence, a power map and / or a calculated PSD map will be generated by the second mirror for each position of the first mirror, and therefore the optimal mirror position determined in this way should be more accurate.

[0099] Alternatively, or in addition to controlling (e.g., scanning) one or more beam steering components, further optimization of the output mode purity of the broadband light source can be achieved by controlling (e.g., incrementally scanning) the position of the gas cell 1040. The gas cell movement can be achieved by the stage module and can include lateral and / or angular movement in one or more directions. In addition, in an embodiment, the focusing lens 1030 can be mounted on a piezoelectric stage or stage module that allows the lens to move according to one or more degrees of freedom. Such lens movement can further optimize the mode purity of the broadband output 1080.

[0100] In another embodiment, a component is provided, comprising an optical element for receiving and modifying radiation, a receiving element for receiving the modified radiation, and a gas environment enclosing the receiving element, wherein the component further comprises a control element configured to stabilize a matching condition between the optical element and the receiving element by adjusting the modification of the received radiation or adjusting the distance between the optical element and the receiving element according to the properties of the gas environment.

[0101] In an example, the optical element may be a focusing lens 1030 and the receiving element is a nonlinear optical element, such as a nonlinear crystal or a hollow core optical fiber HC-PCF 1041. Radiation may be generated by a (monochromatic) pump laser, focused (modified) by the optical element 1030 and received by the receiving element 1041 enclosed in a gas chamber 1040. The gas chamber 1040 may be configured to allow adjustment of certain properties of the gas environment, such as pressure, temperature and / or gas composition. Typically, the adjustment of the gas environment aims to obtain a desired response of a nonlinear optical process generating broadband light, for example adjusting the wavelength spectrum of the broadband light. However, changes in said properties, typically the temperature and pressure of the gas environment, may have a direct impact on the matching conditions between the optical element 1030 and the receiving element 1041, such as the focus matching condition associated with the efficiency of coupling radiation into the HC-PCF 1041. If the properties of the gas environment are pressure and temperature variations, the underlying physical principles allow the corresponding changes in the focus matching conditions to be estimated. According to equation 1 (EQ[1]), the refractive index 'n' of the gas depends on the pressure P and the temperature T of the gas:

[0102]

[0103] P0 and T0 are reference pressure and temperature values, C1, C2, B1, B2 are the Sellmeier coefficients of the gas, and λ is the wavelength of the pump laser.

[0104] The change in the focus position of the pump laser relative to the entrance of the optical fiber 1041 due to the change in refractive index can be determined by basic optical analysis of the component (especially the power of the optical element and the distance between the optical element and the receiving element). For example, in the case of a 1um wavelength pump laser source, a change in the pressure of the xenon filling from 15 bar to 17 bar will result in an increase in the refractive index of approximately 0.0015. Using a simple optical model for a typical component design, it can be determined that this will result in a focus shift of 20 to 30um.

[0105] As mentioned, focus changes caused by pressure and / or temperature changes may be detrimental to the coupling efficiency of the pump laser into the optical fiber 1041. This may translate into a reduction in the power of the broadband light generated within the optical fiber 1041. Therefore, it is desirable to provide focus control means, for example by incorporating a control element, such as an actuator, to provide focus control based on available information about the properties of the gas environment.

[0106] The optical element 1030 and / or the gas cell 1040 may be configured to allow variable focusing of the pump laser relative to the entrance of the receiving element HC-PCF 1041. For example, the focusing lens 1030 and / or the gas cell 1040 may be moved within a certain range by a control element along the optical axis (longitudinal direction) of the receiving element HC-PCF 1041. Alternatively, the focusing lens may include an optical surface (lens) with variable optical power (e.g., a lens / optical surface that can be deformed by a control element) or lenses (elements) that can be moved relative to each other by a control element.

[0107] It is also proposed to periodically measure the pressure and / or temperature of the gas by means of a sensing component, determine the corresponding change in the refractive index of the gas, and subsequently determine the change in the focus position of the pump laser relative to the entrance of the receiving element 1041 due to said change in the refractive index.

[0108] In an embodiment, the assembly includes a focus control system that uses pressure and / or temperature measurements as input and outputs a value corresponding to a control signal for one or more control elements (actuators) coupled to the focusing lens 1030 and / or the gas cell 1040. The control signal can be configured to provide a focus position change that at least partially compensates for a determined focus position change of the pump laser relative to the entrance of the optical fiber 1041. One or more actuators can move the focusing lens 1030 and / or the gas cell 1040 along the optical axis. One or more actuators can position a lens (element) included in the focusing lens 1030, such as to cause a focus position change. One or more actuators can deform an optical surface or lens (element) included in the focusing lens 1030, thereby causing a focus position change. Alternatively, one or more actuators can control the position and / or optical power of an additional optical element (not shown) located in the optical path upstream of the gas cell to cause a focus position change.

[0109] The focus control system may include functionality to determine a change in the refractive index of the gas based on the pressure and / or temperature measurements, reference values ​​for pressure and / or temperature, the wavelength of the pump laser and the composition of the gas and / or a Sellmeier coefficient corresponding to the gas.

[0110] The focus control system may include functionality to determine changes in the focus position of the pump laser relative to the entrance of optical fiber 1041 based on determined changes in the gas refractive index and knowledge of the properties and positions of optical elements used to couple the pump laser into optical fiber 1041 .

[0111] The focus control system described herein allows for automatic focus adjustment in response to changing conditions of the gas (e.g., temperature, pressure, gas composition), ensuring that the pump laser is efficiently coupled into the optical fiber 1041. Thus, the focus control system increases the stabilization of the power of the broadband light delivered by the optical fiber 1041.

[0112] Alternatively, the power of the broadband light is periodically measured at the output of the optical fiber 1041 and used as an alternative or in addition to the pressure and / or temperature measurement readings to provide a control signal for one or more actuators. Alternatively, the ratio between the power at the output of the optical fiber and the power at the entrance of the optical fiber is determined. The determined ratio can be used to provide a control signal for one or more actuators.

[0113] Fig.11 A second embodiment is illustrated. In this embodiment, the detection unit includes a spatial filter 1152 (which may include (for example) a pinhole or a single-mode optical fiber) and a power measurement device 1153. Similar to the first embodiment, a reference beam 1181 is directed to the broadband output beam by the beam splitter 1151. The spatial filter 1152 is configured to remove the HOM content of the broadband output so that only the fundamental transverse mode is measured and monitored. Since the fundamental transverse mode and the HOM have different divergence angles and mode field diameters, only the fundamental mode of the output beam can be effectively coupled into the single-mode optical fiber, and the HOM is either not coupled into the single-mode optical fiber or is not guided to the power meter for power measurement. In a similar manner, a pinhole of carefully selected size only allows the fundamental transverse mode to be transmitted, thereby effectively removing the HOM of the output beam.

[0114] After being spatially filtered, the fundamental transverse mode LP 01 The power in is measured by a power measurement device 1153 placed after the spatial filter in the detection unit. One or more additional bandpass filters (not shown) or other filter arrangements can be used to select one or more desired spectral ranges for power measurement. When one or more pump coupling conditions are suboptimal, the output power of the fundamental transverse mode begins to decrease. Once the power in the fundamental mode drops below a predefined power threshold, a control signal is generated and / or changed and sent to the control unit 1170. The control unit 1170 will activate an optimization routine (e.g., as described above) to optimize the pump coupling conditions so that the output power of the fundamental transverse mode is sufficiently increased (above the threshold), indicating an increase in the purity of the output mode.

[0115] It should be noted that the power drop may be partially caused by the collimating lens 1131 and / or other downstream optical components (such as Fig.11 The position of the collimated output beam 1180 and the reference beam 1181 may be caused by thermal and / or vibration drift of the illustrated beam splitter 1151. Therefore, one or more beam alignment measurement devices (not shown) in the detection unit 1150 are used to continuously or intermittently monitor the position of the collimated output beam 1180 and the reference beam 1181. When the position of the collimated output beam 1180 and / or the reference beam 1181 is confirmed to have drifted, the position of the spatial filter 1152 can be optimized accordingly to compensate for the drift.

[0116] According to the third embodiment, Fig.12 As illustrated, the detection unit 1250 includes a beam shape measurement device 1253 (more generally, a beam shape and / or size measurement device) that measures various (e.g., far-field) shape / size parameters of the incident reference beam 1281, such as one or more of diameter / radius, ellipticity, centroid position, etc. For example, the beam shape measurement device 1253 can be a scanning slit beam analyzer or a CCD camera. One or more additional bandpass filters can be used to select one or more desired spectral ranges for beam profile measurement. Since the fundamental transverse mode of the HC-PCF 1241 has a Gaussian or near-Gaussian field distribution, and the HOM has a non-Gaussian field distribution, beam shape parameters such as ellipticity and beam diameter can be used (alone or in combination) to evaluate the fundamental mode purity. Once measured, the beam shape parameters are sent to the processing unit 1260 for data processing and evaluation. If the beam ellipticity measured is evaluated to be greater than a predefined ellipticity threshold, the mode purity of the broadband output beam 1280 is confirmed to be suboptimal. For example, the ellipticity threshold can be set to a value in the range between 1.04 and 1.20. Alternatively or in addition, the evaluation may include comparing the measured beam diameter to a reference value for a collimated Gaussian beam calculated using relevant parameters of the HC-PCF 1241 and the collimating lens 1231. If the difference between the measured beam diameter and the reference beam diameter is greater than a certain threshold, the mode purity of the broadband output beam 1280 is confirmed to be suboptimal. In an embodiment, both size and ellipticity are measured and evaluated against respective thresholds, as one of these parameters alone is not always fully indicative of mode purity. Alternatively or in addition, this approach may monitor the Laguerre-Gaussian mode shapes of the beam and fit these to Laguerre-Gaussian polynomials indicative of mode purity (or other). Alternatively or in addition, Zernike polynomial shapes may be monitored and fitted in a similar manner. Once suboptimal mode purity is confirmed, a control signal is generated and / or altered by the processing unit 1260 and sent to the control unit 1270 for a pump coupling optimization routine.

[0117] In an alternative embodiment, an optical lens 1252 may be included in the detection unit to image the end face of the HC-PCF 1241 onto the beam shape measurement device 1253. Compared to the above-mentioned third embodiment example in which the far-field distribution of the output mode is evaluated, this example uses the near-field distribution of the HC-PCF output for mode evaluation instead. Similarly, the ellipticity and diameter of the near-field distribution are evaluated based on theoretical and / or empirical values. In some cases, empirical values ​​derived experimentally may be more reliable.

[0118] According to the fourth embodiment, Fig.13As illustrated, the detection unit 1350 includes a spectral measurement arrangement; specifically: a multimode optical fiber 1352 and a spectral measurement device (e.g., a spectrometer or a spectrum analyzer) 1353. One end of the multimode optical fiber 1352 is placed in the beam path and is used to receive at least a portion of the reference beam 1381. The other end of the multimode optical fiber is optically connected to the spectral measurement device 1353, which is configured to analyze the spectral characteristics of the reference beam 1381. The intensity of the reference beam 1381 can be attenuated / controlled by a neutral density (ND) filter (not shown) to avoid damage to the fiber facets and / or saturation of the spectral measurement device. In different embodiments, the multimode optical fiber 1352 may not be required. The reference beam can be free-space coupled into the spectral measurement device.

[0119] Similar to the first embodiment in which the measured spectral parameter values ​​(in that case the PSD values) are compared with reference values ​​in the corresponding spectral range, in this embodiment, the spectral parameter values ​​(e.g. the measured spectrum) can be compared with a reference spectrum that can be acquired when the mode purity of the broadband output beam is known to be optimal. Depending on the degree of difference between the measured spectrum and the reference spectrum, a control signal will be generated and pump coupling optimization will be performed.

[0120] In another embodiment, if Fig.14 As shown, radiation 1481 that leaks through the fiber cladding is collected, for example, by a multimode fiber 1452. This radiation 1481 may be collected only from a section of the HC-PCF 1441, such as at or near an end section of the HC-PCF 1441 (e.g., at or near an output end) where the outer coating may be stripped. The collected leakage radiation 1481 is then directed to a spectrum measurement device 1453 for spectrum measurement. The higher order optical fiber core modes in the HC-PCF will experience more light than the fundamental LP as they propagate along the fiber. 01 Higher confinement losses of the core modes. Therefore, in case the mode purity is reduced or suboptimal, more power will leak through the cladding structure, resulting in an increase in the amplitude of the measured spectrum. Therefore, the amplitude of the measured spectrum can be used to evaluate whether the mode purity of the broadband output is optimal (e.g. by comparison with a threshold). If the mode purity proves to be suboptimal, a control signal will be generated by the processing unit and a pump coupling optimization routine will be activated by the control unit.

[0121] In addition to embodiments that improve coupling based on power or energy metrics or beam shape metrics, other parameters of the emitted radiation can be measured, such as polarization extinction and / or polarization angle. Note that these latter parameters cannot be measured directly from the beam or the light emitted from the fiber cladding, and can only be partially measured from the beam or the light from the second axis of the beam splitter.

[0122] The above pump coupling optimization method is mainly related to the fine alignment aspect of the pump laser beam relative to the HC-PCF alignment. Improvements mainly for coarse alignment will now be described. It is to be noted that the solution space of the above pump coupling optimization method and the improvements discussed below may overlap. For example, the pump coupling optimization method discussed above may also have the benefit of coarse alignment, and the improvements discussed below may also provide some fine alignment. The proposed method and apparatus use one or more suitable detectors (e.g., photodiodes and / or focal meters) to monitor the HC-PCF cladding, and more specifically, to monitor the light radially escaping from the HC-PCF. The concepts in this embodiment are similar to those described with respect to Fig.14 The concepts of fine alignment (mode purity assessment) described are similar. It will be appreciated that the method of any of the aforementioned embodiments may be used independently of any of the embodiments disclosed above or in combination therewith, in the latter case such that any of the aforementioned embodiments is used for an initial coarse alignment and any of the aforementioned embodiments is subsequently (i.e., while coarsely aligned) used for fine alignment. Note that the coarse alignment discussed may also be used in scenarios where the beam must be aligned relative to the solid core of a photonic crystal fiber having a cladding region around the fiber core of solid material.

[0123] Fig.15 (a) schematically depicts a rough alignment arrangement CA according to such an embodiment. The rough alignment arrangement CA comprises an optical element, in this example a positive lens POL, to focus the light beam LB onto the input surface INS of the HC-PCF. Figure 7 As in the example of the embodiment of the present invention, the HC-PCF has a (hollow) optical fiber core FCO and an inner cladding waveguide structure (optical fiber cladding CLA) around the optical fiber core FCO. An input surface INS defines one end of the HC-PCF and is configured to receive a light beam LB so as to couple at least a portion of the light beam LB into the optical fiber core FCO.

[0124] It is to be noted that the optical element may be any type of optical element and is not necessarily limited to a positive lens POL. For example, the optical element may be an off-axis parabolic reflector. The rough alignment arrangement CA further comprises a detector, such as a photosensor PHS, arranged near or on the optical fiber cladding CLA of the HC-PCF. The photosensor PHS is implemented here as a photodiode, but may be any other type of light or other electromagnetic energy sensor. The photosensor PHS is arranged so that it can receive light from a beam LB which is coupled into the optical fiber cladding CLA at the input surface INS. The photosensor PHS is also configured to output a signal SI representing the amount of light received by the photosensor, such that the output signal SI represents the amount of light coupled into the optical fiber cladding CLA. Thus, the photosensor measures light scattered from the optical fiber cladding CLA that occurs due to misalignment of the beam LB and the optical fiber core FCO. Optionally, there may be an optical filter before the photosensor PHS to filter unwanted wavelengths / polarizations and / or reduce the amount of light (e.g. within the dynamic range of the diode).

[0125] Fig.15 (b) A more detailed depiction of the HC-PCF by omitting the optical element POL and focusing on the free end of the HC-PCF. Fig.15 (a) shows a rough alignment arrangement CA, where the input surface INS receives the light beam LB. Fig.15 In (b), it can be clearly seen that the light beam LB is not fully aligned with the fiber core FCO, so a portion of the light beam LB is coupled into the fiber core FCO and another portion of the light beam LB is coupled into the fiber cladding CLA.

[0126] In prior art systems, the misalignment between the measurement beam LB and the fiber core FCO is usually measured by fiber tapping, where the amount of light coupled into the fiber core FCO is measured by modifying (e.g. damaging) the HC-PCF to tap off a portion of the light trapped inside the fiber core FCO and direct this portion to a detector or sensor. However, this results in transmission losses and may cause spectral changes and / or changes in the polarization extinction ratio.

[0127] In this embodiment, it is proposed to measure the light coupled into the fiber cladding CLA instead of the light coupled into the fiber core. Fig.15On the right side of (b), the input surface INS is shown as the beam LB is on the left side of the HC-PCF. Below the HC-PCF, a graph depicting the output signal In when the beam LB moves from the left side of the HC-PCF to the right side of the HC-PCF along the X direction is shown, but it is noted here that similar graphs can be obtained by moving in other degrees of freedom. The beam LB is shown as mostly incident on the outside of the optical fiber cladding CLA, corresponding to the alignment position AL1 (a specific illustration of the corresponding arrangement of the beam LB, the optical fiber core FCO and the optical fiber cladding CLA is shown directly below each value). At the alignment position Al2, the beam LB is mostly incident on the optical fiber cladding CLA, resulting in an increase in the value of the output signal In. Subsequently, the beam LB will be incident on the optical fiber core FCO, resulting in a decrease in the value of the output signal In; this corresponds to the optimal alignment position AL2. B Finally, the light beam LB will be incident on the fiber cladding CLA again, resulting in an increase in the value of the output signal In at the alignment position AL3. Therefore, when the light beam LB is coupled to the fiber cladding CLA to the maximum extent, when the output signal In is at the first maximum value In max1 and the second maximum value In max2 The minimum value between min , the optimal alignment between the light beam LB and the optical fiber core FCO is obtained.

[0128] Note that the minimum value In min Not necessarily zero. Minimum value In min A non-zero value of is very likely or even probable, especially for a coarse alignment phase, for example because in practice there may always be a certain level of light scattered from the core. min It may also be used to monitor the system and / or alignment and / or health of the optical fiber during operation of the broadband light source.

[0129] It should also be understood that any scan is in fact a two-dimensional scan on the input surface INS. Therefore, the model will be Fig.15 (c) The three-dimensional model shown in FIG. Fig.15 (b) A three-dimensional model of the two-dimensional curve diagram of FIG. 1 and its three different offset cross sections (here the maximum value is shown to be equal in all directions, although this may not be the case, as shown in FIG. 1 ). Fig.15 (b)). Therefore, the scanning algorithm will aim to find the position corresponding to the common minimum between the maxima in all directions on the X / Y plane.

[0130] The coarse alignment strategy may operate in a feedback loop based on the measured output signal and a control signal of a controller that controls the position of the beam relative to the input facet of the HC-PCF in order to find this minimum In minThis method may use a search algorithm that automatically searches whether the input beam is sufficiently aligned relative to the input of the hollow HC-PCF. This method may include a spiral scan to find a region bounded by a maximum value (e.g., forming an annular region), and find a minimum value within that region.

[0131] Fig.16 A further coarse alignment arrangement CA is schematically depicted. Fig.16 A side view and a front view of a HC-PCF and a photosensor PHS (or other detector) arranged around the HC-PCF are shown on the left and right sides, respectively. The HC-PCF comprises a hollow optical fiber core and an optical fiber cladding surrounding the optical fiber core, as depicted in other figures but not explicitly shown here. The HC-PCF further comprises an input surface INS, which is configured at one end of the HC-PCF to receive a beam so as to couple at least a portion of the beam into the optical fiber core. The photosensor PHS is arranged to receive light from the beam coupled into the optical fiber cladding at the input surface INS, wherein the photosensor is configured to output a signal representative of the amount of light received by the photosensor.

[0132] The photosensor PHS may have a plurality of individual areas capable of detecting the amount of light falling on the individual plurality of individual areas. If such a photosensor PHS having a plurality of individual areas is arranged around the HC-PCF, information about the direction in which a particular amount of light is output from the fiber cladding may be obtained. This direction information may be used to steer the alignment of the beam to a direction dependent on the detected direction information.

[0133] exist Fig.16 In an embodiment of the invention, the photosensor PHS is arranged on the fiber cladding close to the input surface INS. The photosensor PHS extends around the longitudinal axis LAF of the HC-PCF in the radial direction RAD of the HC-PCF by an angle α, which in this case is greater than 180 degrees, even greater than 270 degrees, and is almost 360 degrees. Such an embodiment can be advantageously used to increase the signal-to-noise ratio, since for an increased angle α more light is received by the photosensor PHS.

[0134] Fig.17 Another coarse alignment arrangement CA is schematically depicted. Fig.17 The rough alignment arrangement is similar to Fig.16 and only the front view is depicted here to explain the main differences between the two embodiments. The main difference is that Fig.16In an embodiment, the rough alignment arrangement CA comprises two photosensors, namely, a first photosensor PHS1 and a second photosensor PHS2, respectively having the same function. The first photosensor PHS1 and the second photosensor PHS2 are arranged to be substantially evenly distributed along the circumference of the HC-PCF in the radial direction RAD, wherein each photosensor extends around the longitudinal axis LAF of the HC-PCF in the radial direction RAD of the HC-PCF by angles α and β, respectively, which are less than 180 degrees, but preferably greater than 90 degrees.

[0135] The first photosensor PHS1 provides a first output signal In1, and the second photosensor PHS2 provides a second output signal In2. The combination of the signals In1 and In2 can be expressed as Fig.15 and 16 The output signal In of the photoelectric sensor PHS of the embodiment is used in a similar manner. However, the advantage of this arrangement is that the control unit receiving the signals In1 and In2 can also determine the linear difference or weighted difference of the signals In1 and In2, which can be used to determine in which direction the beam needs to be displaced or tilted to align the beam with the optical fiber core.

[0136] Although not shown, it is contemplated that in an embodiment, three or more photosensors may be provided and distributed substantially radially evenly along the circumference of the optical fiber, wherein each photosensor extends around the longitudinal axis of the optical fiber in the radial direction of the optical fiber over an angle of less than 360 / n degrees, where n is the number of photosensors, for example less than 120 degrees in the case of three photosensors.

[0137] exist Fig.17 In a variation of the depicted arrangement, the second photosensor PHS2 can be replaced by a reflector element used in combination with only a single photosensor PHS1. The reflector element reflects light coupled into the fiber cladding at the input surface towards the photosensor PHS1. As a result, the signal-to-noise ratio can be improved. The reflector element does not have to be arranged on the optical fiber, but can instead be arranged at a distance around the optical fiber. The reflector element can also be accompanied by one or more other reflector elements, which are all configured to reflect light coupled into the fiber cladding towards the photosensor PHS1.

[0138] The advantage of this coarse alignment arrangement is that light can be coupled into the core of the optical fiber after misalignment, for example due to replacement of components or drift, and this coupling can be done in-line without changing or disconnecting the system.

[0139] Fig.18An optical system OS with a specific beam steering arrangement or optical manipulation unit OMU is schematically depicted, which may be used for actual control of the beam on an input facet in a coarse alignment arrangement and / or in any fine alignment embodiments disclosed herein.

[0140] The optical system OS comprises a light source LIS and an optical manipulation unit OMU. The light source LIS provides a light beam LB to the optical manipulation unit OMU using an optical fiber HC-PCF and an output connector OC which may comprise a collimator to provide a collimated light beam LB to the optical manipulation unit OMU. The light source LIS may be a white light source or a supercontinuum source.

[0141] The optical manipulation unit OMU comprises an input device ID configured to receive an output connector OC. It is to be understood that the input device ID and the output connector OC have been depicted highly schematically, but that both components may comprise features allowing the output connector OC to be releasably but rigidly connected to the input device ID, thereby allowing the output connector OC to be replaced or disconnected and subsequently reconnected.

[0142] The optical manipulation unit OMU further comprises one or more optical elements configured to manipulate the light beam LB. Fig.18 Depicted as an example of such an optical element is a reflector MI which directs the light beam to a filter unit FU which is configured to filter the light beam passing through the filter unit FU. The filtering may include spectral filtering, polarization filtering and / or an overall attenuation of the light beam.

[0143] It is mentioned here that the presence of a filter unit does not mean that other optical elements may not also have a filtering function, for example in the form of a reflective or transmissive bandpass filter. Thus, the mirror MI may have such an alternative or additional filtering function.

[0144] In this embodiment, a beam tilt adjuster TA is provided downstream of the filter unit FU to adjust the propagation direction of the light beam LB. The beam tilt adjuster TA comprises a first wedge prism WP1 and a second wedge prism WP2 arranged in series, wherein each wedge prism WP1, WP2 comprises a corresponding tilt actuator A1, A2 to rotate the corresponding wedge prism WP1, WP2 around their respective optical axes. Fig.18 The axis A1 and A2 extend mainly along the X direction. The tilt actuators A1 and A2 are part of the tilt actuation system.

[0145] In this embodiment, arranged downstream of the beam tilt adjuster TA, a beam displacement device DD is provided to displace the light beam LB. The beam displacement device DD comprises a plane-parallel plate PP and a displacement actuation system which rotates the plane-parallel plate PP about a first axis using an actuator A3 and about a second axis using an actuator A4, the first and second axes being substantially perpendicular to each other and to the propagation direction of the beam. Since the propagation direction of the light beam LB is substantially in the X-direction, the first axis may, for example, be substantially parallel to the Y-direction, and the second axis may, for example, be substantially parallel to the Z-direction.

[0146] The optical manipulation unit OMU also comprises a control unit CU connected to a tilt actuation system (A1, A2) and a displacement actuation system (A3, A4) to adjust the propagation direction of the light beam and to displace the light beam to direct the light beam LB to the input connector INC. The input connector INC is received in the output device OD of the optical manipulation unit OMU and may comprise coupling means to couple the light into the optical fiber HC-PCF. Similar to the input device ID and the output connector OC, the output device ID and the input connector INC are depicted here highly schematically and may therefore comprise features allowing them to be rigidly and possibly releasably connected to each other, thereby allowing the input connector INC to be replaced with a new or different input connector INC or to be disconnected and connected again, for example for maintenance.

[0147] As in this example, the optical fiber HC-PCF may include a first optical fiber portion and a second optical fiber portion connected to each other using a connector CON. The connector CON or the optical fiber HC-PCF may be configured to direct a portion (preferably a small portion) of the light passing through the connector CON to a detector DE to determine the light intensity of the light beam in the optical fiber HC-PCF, which is a measure of the amount of light coupled into the optical fiber by the coupling means of the input connector INC.

[0148] Thus, the intensity of the light beam determined in the optical fiber HC-PCF can be used to operate the control unit CU to control the tilt actuation system until the light beam is received by the input connector INC. In an embodiment, the control unit CU can be configured to control the tilt actuation system so that the light beam moves along a spiral pattern, in this case in the ZY plane, in order to find a first estimate of the desired propagation direction of the light beam to be received by the input connector INC, and then the tilt actuation system and / or the displacement actuation system are controlled to move the manipulated light beam around the first estimate in order to find an improved first estimate of the desired propagation direction and / or displacement of the light beam. Moving the light beam in a spiral pattern can be achieved by rotating the two wedge prisms WP1, WP2 at different angular velocities. Both the first estimate and the second estimate can be rough alignment estimates, while the fine alignment strategy disclosed further herein is applied to the fine alignment. Alternatively, the first estimate can be related to the rough alignment and the second estimate can be related to the final alignment. In the latter example, the fine alignment can rely on one of the other measurement strategies described herein.

[0149] According to another embodiment, a timing control system is provided, which is configured to control the timing of pump laser pulses and / or broadband output pulses of a HC-PCF-based broadband light source. Timing control of laser pulses is generally desirable in applications that require precise temporal positions of laser pulses relative to a timing reference, and is generally implemented using, for example, microprocessor- or microcontroller-based techniques. A typical prior art timing control system may include one or more microprocessors, a central processing unit (CPU), and a memory unit. When such a microprocessor-based timing control system is used to control the timing of a HC-PCF-based broadband light source, the timing of pump laser pulses, broadband output pulses, and generated signals from one or more optical clients (e.g., optical sensors) may be determined and / or synchronized. However, using a prior art timing control system to control the timing of a HC-PCF-based broadband light source has many technical challenges or drawbacks. First, a HC-PCF-based light source includes multiple components that are typically located far apart from each other. For example, a pump laser (typically including a seed laser, a preamplifier, a pulse stretcher, a power booster, and a pulse compressor) is connected to a supercontinuum fiber that may be located 10 meters away. The communication between such components (e.g., between the seed laser and the supercontinuum fiber) may result in non-negligible time delays. Secondly, the timing of the pump laser is complex due to its complex optical architecture (e.g., an optical system including a seed laser, a preamplifier, a pulse stretcher, a power booster, and a pulse compressor). Therefore, each component of the pump laser has an impact on the timing of the pump laser pulse. The timing of the broadband output pulse is also complex due to different timings for different wavelengths caused by intra-pulse group delay dispersion. These complex timings place strict demands on the performance of the timing control system (e.g., timing accuracy), and are usually met with complex control architectures. In addition, the generated signal from each optical client (e.g., an optical sensor) needs to be processed before being used by other control units for other components. In order to meet this requirement, the timing control system of the prior art is usually equipped with signal processing functionality, resulting in a more complex control architecture.

[0150] The above shortcomings make it difficult to accurately predict and / or model the time delay between any two components, such as a seed laser and one of the optical clients used to monitor the supercontinuum output. Although the time delay determined by the timing control system can be calibrated based on the measurement data so that timing errors can be corrected, this approach is not practical for HC-PCF based light sources because their laser pulses are too short to be accurately measured. In order to obtain the best possible timing performance, a large amount of control firmware and / or software configured to operate within a predefined margin is required. The extensive control firmware and / or software and the complex microprocessor-based hardware make the entire timing control system overly complex and expensive. Since firmware or software based control systems are more prone to errors, a lot of time and effort is required to make them robust.

[0151] According to different aspects of the present disclosure, the embodiments described below provide better solutions to the above problems. Compared with the timing control system of the prior art, the significant advantage of the following embodiments is that it prevents the use of microprocessors or similar technologies.

[0152] Fig.19 A timing control system configured for timing control of a broadband light source based on HC-PCF according to an embodiment is schematically illustrated. Fig.19 , the timing control system may include a pressure sensor 1944 (e.g., a very high speed pressure sensor) configured to detect or monitor changes in pressure in the gas chamber 1940. In this embodiment, the pressure sensor 1944 may be held near the output of the HC-PCF 1941 by a support structure 1945 that is connected to the inside of the gas chamber wall 1946. In some different embodiments, the pressure sensor 1944 may be mounted directly on the inside of the gas chamber wall 1946. The gas sensor 1944 may communicate with external devices via one or more signal cables that pass through the gas chamber wall 1946 in a sealed manner, or are connected to external cables via a feed-through connector.

[0153] Continue to refer to Fig.19, a pump laser beam 1911 including a pump laser pulse train 1912 is focused by a focusing lens 1930. The focused pump laser beam 1911 passes through an input window 1942 of a gas cell 1940 before being coupled to the core of the HC-PCF 1941. While propagating along the optical fiber, each pump laser pulse 1912 is spectrally broadened into a broadband output pulse 1982 via the above-mentioned nonlinear optical process. After leaving the gas cell 1940, the broadband output beam 1980 including the broadband output pulse train 1982 is collimated by a collimating lens 1931. Starting from the nonlinear process, the spectral bandwidth of the pump laser pulse 1912 continues to increase (e.g., spectrally broadened) until the spectrally broadened pulse exits the optical fiber. Once exiting the optical fiber, the broadband output pulse 1982 may travel a short distance within the gas cell 1940 before exiting the output window 1943 of the gas cell 1940. Upon entering the gas cell 1940, the broadband output pulse 1982 generates a pressure wave within the gas cell 1940. The amplitude of such a pressure wave may be affected by many factors, such as the operating conditions of the gas cell 1940 (e.g., gas cell pressure, gas type) and the laser parameters of the broadband output pulse 1982 (e.g., pulse energy, pulse spectrum, pulse width). The pressure wave (temporarily) changes the internal pressure distribution of the gas cell 1940. The resulting pressure change at the location of the pressure sensor 1944 is detected by the pressure sensor and subsequently converted into an electrical signal by the pressure sensor 1944. In some embodiments, the electrical signal can be used as an output pulse trigger signal, which indicates the timing of the broadband output pulse 1982 being generated. Additionally or alternatively, such an electrical signal can be sent to the processing unit 1060, 1160, 1260, 1360, 1460 and / or the control unit 1070, 1170, 1270, 1370, 1470 of the broadband light source so that different functionalities can be achieved, such as outputting a train of broadband pulses.

[0154] Note that a pressure wave is also generated when the pump laser pulse enters the gas chamber 1940 before coupling to the HC-PCF 1941. Therefore, in some embodiments, the timing of the pump laser pulse 1911 entering the gas chamber 1940 can also be determined using the same pressure sensor 1944 and / or an additional pressure sensor (not shown), for example, located near the input window 1942.

[0155] In conjunction with the timing of the broadband output pulse 1982, the relative time delay between the pump laser pulse 1911 and the broadband output pulse 1982 can be determined. It should be noted that since the broadband output pulse 1982 is essentially synchronized with the pump laser pulse 1911 (in a fixed time relationship), the electrical signal of the broadband output pulse 1982 is also synchronized with the electrical signal of the pump laser pulse 1911. The two pulse trains (i.e., the pump laser pulse train and the broadband output pulse train) are offset in time by the amount of the above-mentioned time delay. In some other embodiments, the timing control system may also include an adjustable optical delay line, which is configured to adjust or minimize the time delay between the two pulse trains.

[0156] It is understandable that the above Fig.19 The described methods can be used independently of or in combination with any other embodiments disclosed herein. In combination, the mode purity based embodiments can be used for initial alignment (coarse and / or fine) and subsequently for any of the preceding embodiments for timing control (i.e., when the light source is properly aligned).

[0157] For some measurement or inspection tools, such as the above-mentioned scatterometry-based measurement tools, the performance of the measurement tool may be strongly affected by the polarization properties of the illumination radiation of the tool. Such polarization properties may include polarization extinction ratio (PER) or polarization quality, polarization stability and orientation of mainly linearly polarized light. PER is defined as the power ratio of two perpendicular polarizations, usually referred to as transverse electric (TE) and transverse magnetic (TM), and is usually used to characterize the quality of linear polarization. Polarization stability is used to characterize the stability of the polarization state that can be maintained over time. Due to component aging and / or movement, the polarization of the illumination radiation will change over time, resulting in polarization rotation and / or PER reduction. If an illumination beam with poor PER is used in a polarization-sensitive measurement tool (such as a scatterometer), the optical focal length in the unwanted polarization direction will not affect the measurement and may even cause background scattering, thereby reducing the detection signal-to-noise ratio (SNR). Moreover, since only the optical focal length in the desired polarization direction is used for measurement, the power efficiency of the measurement tool is low. Similarly, if an illumination beam with unstable polarization is used in, for example, a scatterometer, variations in the received polarization typically result in power fluctuations at the wafer level, compromising the fidelity of the metrology tool. It is therefore desirable to use an illumination source that provides good polarization stability.

[0158] When a broadband light source based on HC-PCF (such as Figures 10 to 14When a linearly polarized broadband output beam (such as those described in ) is used as an illumination source in, for example, a scatterometer, a linearly polarized broadband output beam with good PER is required. Since the broadband output beam mainly inherits the polarization properties of the pump laser beam, a linearly polarized pump laser beam with good PER can be used. For a perfectly and straight HC-PCF made and mounted without stress, the PER of the broadband output beam is expected to be independent of the input (pump) polarization direction. However, it was found that when low pump powers are coupled through the HC-PCF (i.e., no spectral broadening occurs in the HC-PCF), the PER of the transmitted pump laser beam varies periodically with the input (pump) polarization direction. This is explained by the fact that a slight asymmetry in the HC-PCF will result in a small optical birefringence, making the fiber effectively a long polarization retarder (or wave plate) with a fast axis and a slow axis. The slight asymmetry in the HC-PCF may be due to manufacturing tolerances and / or fiber stresses caused by mounting. Manufacturing tolerances may cause fluctuations in the structural core diameter of the HC-PCF, effectively making the core slightly elliptical. Therefore, orthogonal input polarizations may see different modal indices (i.e., form fast and slow axes) and experience different attenuation. When the polarization direction of the pump laser beam matches the fast or slow axis of the fiber, the polarization fidelity of the pump beam is maintained. However, when the polarization direction of the pump laser beam does not match the fast or slow axis of the fiber, the PER of the transmitted pump laser beam is reduced.

[0159] Therefore, in order to obtain a linearly polarized broadband output beam with high PER, the polarization direction of the pump laser beam needs to be accurately aligned with the preferred birefringence axis of the optical fiber. For industrial laser products, the polarization direction of the pump laser beam is fully optimized during factory construction. Control / optimization of the polarization direction of the pump laser beam is usually achieved at low power levels using a polarization control device, such as a half-wave plate (HWP), which can be placed in the beam path of the pump laser before coupling in the HC-PCF. When the polarization direction of the pump laser beam is rotated by the HWP, the PER of the pump laser beam after propagation through the optical fiber is measured using a polarimeter. However, whenever a critical component (such as a gas cell or HC-PCF) is replaced, for example for maintenance or repair purposes, the polarization direction of the pump laser beam relative to the preferred fiber axis needs to be re-optimized. This means that a polarimeter needs to be used to characterize and confirm the laser PER during and after re-optimization. In addition, custom-designed tools can also be used to access and / or control certain components (multiple), such as the HWP in packaged laser products. Although it is possible to characterize the axis orientation of the HC-PCF in advance and use this information to align the absolute rotation of the fiber, any change in the pump polarization may still result in a reduction in the output PER. Therefore, a polarimeter will still be used, for example, as a permanent in-product diagnostic to monitor the stability of the pump polarization, as manifested by the PER of the transmitted pump laser beam. A polarimeter is an expensive and bulky diagnostic device. Equipping each HC-PCF-based broadband laser source with a polarimeter would increase costs and limit the footprint of the laser product.

[0160] According to different aspects of the present disclosure, a method for solving the above-mentioned problems is proposed. The proposed method is based on the following discovery: when a linearly polarized radiation beam propagates through a waveguide structure with an asymmetric modal refractive index distribution, such as (for example, an optical fiber with a (slightly) elliptical cross-section), the PER of the output beam and the output power are closely related. Therefore, the power measured at the fiber output can be used to indirectly evaluate or infer the degree to which the polarization of the pump laser beam is aligned with the preferred axis of the optical fiber. If the relationship between the angular offset between the polarization direction of the pump beam and the preferred fiber axis and the power at the fiber output is known, then the power measured at the fiber output can also be used to determine the angular offset between the polarization direction of the pump beam and the preferred fiber axis of the fiber input. Since power measurement devices are the most common diagnostic devices and are much cheaper than polarimeters, this method is superior to prior art methods because it does not require a polarimeter.

[0161] The correlation between output power and PER can be explained by the structural characteristics of HC-PCF: the fundamental guided mode (LP 01 ) can be approximated by:

[0162]

[0163] where u 01 is the first zero of the Bessel function of the first kind J0, λ is the wavelength, and D is the inscribed core diameter. Thus, mode purity can also be monitored in this way.

[0164] Fig. 20 Schematically illustrates a polarization control system configured for polarization control of a HC-PCF based broadband light source according to various embodiments. Fig. 20 , the polarization control system may include a HWP 2029 for controlling the polarization of the pump laser beam 2011. The HWP 2029 may be placed anywhere between the pump laser 2010 and the input window 2042 of the gas cell 2040. In the case where a pump focusing lens 2030 is used to focus the pump laser beam 2011 into the core of the HC-PCF 2041, it is preferred to place the HWP 2029 before the focusing lens 2030, as this may prevent the HWP 2029 from being potentially damaged by the focused laser beam having a higher peak intensity.

[0165] In an embodiment, the polarization control system may further include a beam splitter 2051, which is placed in the beam path of the output beam 2080, preferably after the collimating lens 2031 to avoid potential damage. The beam splitter 2051 may split a portion of the output beam 2080 and may reflect it into the detection unit 2050 for diagnostic purposes. The portion of the output beam 2080 reflected from the beam splitter 2051 may be marked as a reference beam 2081. The detection unit 2050 may include a power measurement device 2053, such as a power meter that measures the power of the reference beam 2081. Before being received by the power measurement device 2053, the reference beam 2081 may pass through one or more optical filters 2052, which are used to select a desired wavelength range from the spectrum of the reference beam 2081. It is noted that the effect of the beam splitter 2051 on the optical properties of the reference beam 2081 may be negligible, rather than applying a power separation ratio to the output beam 2080.

[0166] In some embodiments, the polarization control system can be operated in a high power or energy optical region where the pump laser power / energy is high and the pump pulses experience significant spectral broadening after traversing the optical fiber. In some other embodiments, the polarization control system can be operated in a low power or energy optical region where the pump laser power / energy is low and the pump pulses do not experience spectral broadening when traversing the optical fiber. When operating in the low power optical region, the output beam 2080 has a spectrum substantially the same as the pump laser beam. Since the pump laser beam generally has a narrowband spectrum, spectral filtering may not be necessary, so the optical filter 2052 may not be used. In contrast, when operating in the high power optical region, the output beam 2080 has a broadband spectrum, much wider than the spectrum of the pump laser beam. In this case, spectral filtering may be required to measure only the power within the desired spectral range, so one or more optical filters 2052 may be used. In some embodiments, the beam splitter 2051 may be removed, and the detection unit 2050 may be placed in the beam path of the output beam 2080 so that the entire output beam 2080 is received by the power measurement device 2053.

[0167] Continue to refer to Fig. 20 , the polarization control system may further include a processing unit 2060 and a control unit 2070. The power measurement device 2053 measures the power of the reference beam 2081 and subsequently generates a power signal. The generated power signal may be sent to the processing unit 2060 that processes the received power signal (e.g., in a predefined manner). The processed power signal may then be sent to the control unit 2070, which generates a corresponding control signal based on the processed power signal. The control unit 2070 may include a memory unit for storing processed data. Finally, the control signal may be used to make appropriate adjustments to the HWP 2029. It should be noted that the processing unit 2060 and the control unit 2070 do not have to be separate units, but may be integrated into a processing and control unit (not shown) that performs all tasks performed by the processing unit 2060 and the control unit 2070.

[0168] As mentioned above, the polarization control system can be used to optimize the polarization direction of the pump laser beam relative to the preferred axis of the HC-PCF in at least two scenarios (e.g., component replacement and long-term polarization stability). The polarization control system can operate in two different routines. In a scenario where a key component (e.g., gas chamber 2040) is replaced, the polarization direction of the pump laser beam 2011 needs to be re-optimized to match the preferred axis of the new optical fiber 2041. In some embodiments, this can be achieved by rotating the HWP 2029 a full circle or 360°. In other embodiments, the HWP 2029 can be rotated more than 360°, such as 540°, 720°, 900°, or 1080°. In some embodiments, the HWP 2029 can be mounted on a motorized rotating base that rotates the HWP 2029 in small increments, for example. Depending on the resolution requirements, each incremental step can correspond to a small rotation angle, such as 1° per step, 2° per step, 5° per step, or 10° per step.

[0169] Reference Fig. 20 During the optimization process, the control unit 2070 sends a control signal to the HWP 2029 to command it to perform a rotation task, such as a 360° rotation with a step size increment of 5° (or 72 steps). After each rotation step, the power of the reference beam 2081 is measured by a power measurement device (e.g., a power meter). The power signal generated by the power measurement device 2053 is processed by the processing unit 2060, such as by averaging and / or filtering. The processed power signal can be sent to the control unit 2070, wherein the power value is stored in, for example, a memory unit relative to the current angular position of the HWP 2029. At the end of the rotation task, i.e., a 360° rotation with a step size of 5°, a data table with 72 pairs of power and angle values ​​will be generated and stored. Based on the data, the relationship between the power of the output beam 2080 (or the reference beam 2081) and the angular position of the HWP 2029 can be determined, for example, via interpolation of the data. Referring to the existing PER power correlation curve, indicating that each power minimum corresponds to a PER maximum, the power-angle relationship can be converted into a PER-angle relationship, from which the optimal HWP position that can give the maximum PER (or minimum power) can be determined.

[0170] In scenarios where long-term PER stability is to be monitored and / or maintained, the power of the output beam 2080 or the reference beam 2081 may be continuously or intermittently measured / sampled by the power measurement device 2053. It should be noted that it is assumed that the power-angle relationship of the broadband light source is already available before the long-term PER stability can be monitored and / or maintained. The power signal generated by the power measurement device 2053 may be processed by the processing unit 2060 before being sent to the control unit 2070, for example, averaged over a plurality of sampling points. Upon receiving the processed power signal, the control unit 2070 may compare the processed power signal with a predefined power threshold. If the power signal is above the power threshold, the control unit 2070 may not generate any control signal. However, if the power signal is below the power threshold, a control signal is generated. The control signal may command the HWP 2029 to perform a corresponding rotation task. In some embodiments, the rotation task may include a full 360° rotation of the HWP, such as the above example. Alternatively, in other embodiments, the rotation task may include a small angle rotation around the current angular position of the HWP 2029. When the PER (as represented by the measured power) of the output beam 2080 or the reference beam 2081 is optimized, the new angular position of the HWP 2029 can be determined. The control unit 2070 can then update the set angular position with the new angular position in the memory (if available). Thereafter, the polarization control system continues to monitor the long-term PER stability.

[0171] Since the power of the output beam 2080 or the reference beam 2081 can indicate the mode purity, the method of this section can be used in the mode control system described in this article.

[0172] It is to be noted that the number, location and type of power measurement devices 2053 are not limited to those already described. In some embodiments, a 'micro' power measurement device may be placed near the output end of the HC-PCF fiber so that the scattered light power at the fiber tip can be measured. Since the scattered light power at the fiber tip is linearly proportional to the power of the broadband output beam, the above PER optimization method is still valid. In some embodiments, the micro device may include a multimode fiber and a power meter. One or both of the gas cell 2040 or the HC-PCF 2041 may be mounted on a motorized rotating workbench, and the HWP 2029 may have a fixed angular position. In this way, instead of rotating the HWP 2029 for PER optimization, the gas cell 2040 or the HC-PCF 2041 or both may be rotated to minimize the angular difference between the pump polarization direction and one axis of the fiber, thereby maximizing the PER of the output beam 2080.

[0173] It will be appreciated that the methods of any of the foregoing embodiments may be used independently of or in combination with any of the other embodiments disclosed above, in the latter case as part of a mode control system and / or such that any of the foregoing embodiments are used for initial alignment (coarse and / or fine). Additionally or alternatively, other embodiments may subsequently (i.e., when the light source is properly aligned) be used for subsequent polarization optimization. When used independently, the above embodiments are not limited to application to HC-PCF based broadband light sources, which may be used to determine and / or optimize the relative angle between the polarization direction of the pump laser beam and the optical plane of a nominally cylindrical waveguide.

[0174] It should be noted that the configuration of the HC-PCF based broadband light source is not limited to the specific arrangement shown or described, and different configurations may be implemented. For example, the pump lasers 1010, 1110, 1210, 1310, 1410 may be configured to output a converging pump laser beam rather than a collimated pump laser beam 1011, 1111, 1211, 1311, 1411 whose waist has a good mode match with the fiber core. In this case, the focusing lens 1030, 1130, 1230, 1330, 1430 is not required. The beam delivery system may be different from the specific examples of the two or more steering mirrors 1020, 1120, 1220, 1320, 1420, 1021, 1121, 1221, 1321, 1421 shown. Alternatively or in addition, at least one of the mirrors may include a curved surface and / or another focusing beam delivery component provided having a carefully selected radius of curvature (ROC) to form a mode-matched pump spot without the use of focusing lenses 1030, 1130, 1230, 1330, 1430. According to different embodiments, the input optical window 1042, 1142, 1242, 1342, 1442 may be replaced by a focusing lens 1030, 1130, 1230, 1330, 1430 and / or the output optical window 1043, 1143, 1243, 1343, 1443 may be replaced by a collimating lens 1031, 1131, 1231, 1331, 1431. In this configuration, the distance between the two lenses and the two fiber ends is selected so that the mode matching condition is well maintained. A broadband light source configured in this way is more compact but less flexible. In another embodiment, the gas chamber 1040, 1140, 1240, 1340, 1440 may be composed of a plurality of sub-chambers; and the HC-PCF 1041, 1141, 1241, 1341, 1441 may be partially or fully included in a sub-chamber. The beam splitter 1051, 1151, 1251, 1351, 1451 may be located outside the corresponding detection unit 1050, 1150, 1250, 1350, 1450 instead of inside as shown. The processing unit 1060, 1160, 1260, 1360, 1460 and the control unit 1070, 1170, 1270, 1370, 1470 are not necessarily separate entities and may include a single unit with a single processor to perform processing and control functions.

[0175] Although any of the above embodiments is sufficient to independently perform mode optimization of a broadband light source, some embodiments may complement each other and may therefore be combined to improve the overall performance of the mode control system. For example, in an embodiment, the beam shape measurement device 1253 used in the third embodiment may be added to or combined with the detection unit 1050 (including the bandpass filter 1052 and the power measurement device 1053) of the first embodiment. In this way, the transverse mode profile of the broadband output beam can be directly monitored by the beam measurement device 1253, while the output PSD is measured by the power measurement device 1053. Since the power and / or spectral profile of the broadband light source may gradually degrade over time (for example, due to aging of the pump diode), the PSD value may decrease accordingly even if the mode purity remains unchanged. Therefore, the addition of the beam measurement device allows the mode control system to quickly verify whether any reduced PSD value is caused by mode degradation or power degradation caused by component aging, thereby preventing the mode control system from entering an optimization dead loop if it is the latter. In addition, it is desirable to regularly update the reference PSD value to reflect the power degradation caused by component aging. The use of the beam measurement device 1053 ensures that the reference PSD value can be regularly maintained and updated when the mode purity is optimal.

[0176] Thus, two or more detection units 1050, 1150, 1250, 1350, 1450 or any of their components may be used in combination to detect and evaluate the same reference beam or separate reference beams (eg generated by multiple beam splitters on the output beam).

[0177] Further embodiments are disclosed in the subsequent numbered clauses:

[0178] 1. A mode control system configured to control an output mode of a broadband light source comprising a photonic crystal fiber (PCF), the mode control system comprising:

[0179] at least one detection unit configured to measure one or more parameters of radiation emitted from the broadband light source to generate measurement data; and

[0180] a processing unit configured to evaluate, from the measurement data, a mode purity of the radiation emitted from the broadband light source;

[0181] Based on the evaluation, the mode control system is configured to generate a control signal for optimizing one or more pump coupling conditions of the broadband light source; the pump coupling conditions are related to coupling of the pump laser beam with respect to the fiber core of the photonic crystal fiber.

[0182] 2. A mode control system as defined in clause 1, wherein the one or more parameters of the output radiation include one or more parameters indicative of a mode purity of the broadband light source.

[0183] 3. A mode control system as defined in clause 1 or 2, wherein the radiation emitted from the broadband light source detected by the detection unit comprises output radiation emitted from an output end of a photonic crystal fiber.

[0184] 4. A mode control system as defined in clause 3, comprising a beam splitter positioned to separate a reference beam from a main output beam emitted by the photonic crystal fiber, the output radiation detected by the detection unit comprising the reference beam.

[0185] 5. A mode control system as defined in any preceding clause, wherein said at least one detection unit comprises a spectral measurement arrangement operable to measure one or more spectral parameter values ​​of the output radiation as said measurement data.

[0186] 6. A mode control system as defined in clause 5, wherein the spectral parameter values ​​comprise one or more parameters of a measured spectrum of the output radiation.

[0187] 7. A mode control system as defined in clause 5 or 6, wherein the spectral measurement arrangement comprises a spectral measurement device and a multimode optical fiber operable to guide at least part of the radiation emitted from the broadband light source to the spectral measurement device.

[0188] 8. A mode control system as defined in any of the preceding clauses, wherein the detection unit comprises one or more bandpass filters; each of the one or more bandpass filters is operable to select a corresponding spectral range of the output radiation, and the irradiation measurement device is operable to detect an irradiation parameter indicating the power of the filtered radiation, the measurement data including and / or derived from the irradiation parameter indicating the power.

[0189] 9. A mode control system as defined in clause 5 or 6, wherein the measurement data comprises power spectral density or energy spectral density values ​​within one or more spectral ranges derived from the illumination parameter indicative of power.

[0190] 10. A mode control system as defined in any of the preceding clauses, wherein the detection unit comprises: a spatial filter operable to filter out higher-order modes other than a fundamental mode from the output radiation; and an irradiation measurement device operable to detect an irradiation parameter indicating the power of the filtered radiation, the measurement data comprising and / or derived from the irradiation parameter indicating the power.

[0191] 11. A mode control system as defined in clause 10, wherein the spatial filter comprises a single mode optical fiber or a pinhole.

[0192] 12. A mode control system according to any one of the preceding clauses, further comprising:

[0193] a variable polarization arrangement operable to controllably configure a polarization property of the radiation emitted from the broadband light source;

[0194] wherein the detection unit comprises an illumination measurement device operable to detect an illumination parameter indicative of the power of the radiation emitted from the broadband light source as a function of the input polarization angle, the measurement data comprising and / or derived from the illumination parameter indicative of the power; and the mode control system is further operable to optimize the one or more pump coupling conditions by changing the polarization angle imposed by the variable polarizer relative to the fiber axis of the photonic crystal fiber, thereby optimizing the pump polarization condition of the radiation emitted from the broadband light source.

[0195] 13. A mode control system as defined in clause 12, wherein the variable polarization arrangement comprises one or both of the following:

[0196] a variable polarizer operable to change the polarization angle of the pump laser beam relative to the photonic crystal fiber, and / or

[0197] An actuator is operable to change the angle of the photonic crystal fiber about its optical axis.

[0198] 14. A mode control system as defined in any of the preceding clauses, wherein the detection unit includes a beam shape and / or size measuring device, capable of operating to measure one or more beam characteristics of the output radiation related to the shape and / or size of the beam, and the measurement data includes and / or is derived from the beam characteristics of the output radiation related to the shape and / or size of the beam.

[0199] 15. A mode control system as defined in clause 14, wherein the beam characteristics of the output radiation related to beam shape and / or size include one or more of: beam ellipticity, beam diameter, Laguerre-Gaussian mode shape or Zernike polynomial shape.

[0200] 16. A mode control system as defined in any preceding clause, wherein the one or more parameters of the radiation emitted from the broadband light source measured by the detection unit include leakage radiation emitted from a fiber cladding of the photonic crystal fiber.

[0201] 17. A mode control system as defined in any preceding clause, comprising one or more actuators for actuating movement of one or more components of the broadband light source; wherein the control signal is operable to control one or more of the actuators.

[0202] 18. A mode control system as defined in clause 17, further comprising a control unit configured to receive the control signal from the processing unit and to control the one or more actuators.

[0203] 19. A mode control system as defined in clause 17 or 18, wherein the one or more actuators are operable to optimize one or more pump coupling conditions by optimizing one or more of:

[0204] an angular offset of the pump laser beam relative to the fiber core of the photonic crystal fiber;

[0205] a lateral offset of the pump laser beam relative to the fiber core of the photonic crystal fiber;

[0206] beam diameter of the pump laser beam;

[0207] Absolute polarization angle; and

[0208] Divergence of the pump laser beam.

[0209] 20. A mode control system as defined in clause 17, 18 or 19, wherein the one or more actuators include one or more of the following:

[0210] at least one actuator for at least one beam steering component or a support thereof;

[0211] at least one actuator for the balloon of the photonic crystal fiber or its support;

[0212] At least one actuator is used for focusing the lens to focus the pump laser beam onto the fiber core of the photonic crystal fiber.

[0213] 21. A mode control system as defined in any of the preceding clauses, wherein the processing unit is operable to evaluate measurement data by comparing each of the one or more parameters of the radiation emitted from the broadband light source with an equivalent threshold parameter value indicative of optimal or acceptable mode purity.

[0214] 22. A mode control system as defined in any preceding clause, wherein the photonic crystal fiber comprises a hollow core photonic crystal fiber (HC-PCF).

[0215] 23. A mode control system as defined in any preceding clause, wherein said output radiation of said broadband light source comprises a wavelength range of 200 nm to 2000 nm or a sub-range within this range.

[0216] 24. A mode control system as defined in any preceding clause, wherein the mode purity describes the ratio between the power in the fundamental transverse mode and the total output power.

[0217] 25. A mode control system as defined in any preceding clause, wherein the mode control system is configured to generate a control signal to optimize one or more pump coupling conditions of the broadband light source comprising:

[0218] The method is configured to generate a control signal to optimize one or more pump coupling conditions to maximize mode purity.

[0219] 26. A mode control system as defined in any preceding clause, comprising at least one detector for detecting leakage radiation and operable to coarsely couple a pump laser beam relative to a fiber core of a photonic crystal fiber in an initial coarse pump coupling operation, the coarse pump coupling operation comprising measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam over an input facet of the photonic crystal fiber; and

[0220] Whether the pump laser beam is roughly aligned with the photonic crystal fiber is determined based on the measured leakage radiation.

[0221] 27. A mode control system as defined in clause 26, wherein determining whether the pump laser beam is coarsely aligned comprises positioning a minimum in the measured leakage radiation between at least two maxima in the measured leakage radiation.

[0222] 28. A mode control system as defined in clause 26 or 27, wherein determining whether the pump laser beam is coarsely aligned comprises positioning a minimum in the measured leakage radiation within an annular region around a maximum in the measured leakage radiation.

[0223] 29. A mode control system as defined in clause 26, 27 or 28, wherein the at least one detector comprises a plurality of detectors radially spaced about the optical fiber cladding.

[0224] 30. A mode control system as defined in clause 29, wherein each detector extends around the longitudinal axis of the photonic crystal fiber in a radial direction of the photonic crystal fiber by an angle less than 360 / n degrees, where n is the number of detectors

[0225] 31. A mode control system as defined in clauses 26 to 28, wherein the at least one detector comprises at least one pair of a detector and a reflector, each detector and reflector being positioned at radially opposite positions around the optical fiber cladding.

[0226] 32. A mode control system as defined in any one of clauses 26 to 31, wherein the scanning of the pump laser beam comprises scanning along a spiral path on the input facet.

[0227] 33. A mode control system as defined in any one of the preceding clauses, further comprising an optical manipulation unit for scanning the light beam, the optical manipulation unit comprising:

[0228] - one or more optical elements configured to manipulate the light beam;

[0229] - a beam tilt adjuster for adjusting the propagation direction of the light beam; and

[0230] - A control unit for controlling the beam tilt adjuster to set the propagation direction towards the photonic crystal fiber.

[0231] 34. A mode control system as defined in clause 35, wherein the beam tilt adjuster comprises at least two wedge prisms arranged in series and a tilt actuation system, and

[0232] The tilt actuation systems are controllable by a control unit to independently rotate the wedge prisms around their respective optical axes.

[0233] 35. A mode control system as defined in item 33 or 34, wherein the optical manipulation unit further comprises a beam displacement device for displacing the light beam, wherein the beam displacement device comprises a plane parallel plate and a displacement actuation system controllable by the control unit to rotate the plane parallel plate around a first axis substantially perpendicular to the propagation direction of the light beam, wherein preferably, the displacement actuation system is also controllable to rotate the plane parallel plate around a second axis substantially perpendicular to the propagation direction of the light beam and the first axis, and wherein preferably, the beam displacement device comprises a second plane parallel plate, and the displacement actuation system is controllable by the control unit to rotate the second plane parallel plate around a second axis substantially perpendicular to the propagation direction of the light beam and the first axis.

[0234] 36. A timing control system configured to determine the timing of a broadband light source comprising a photonic crystal fiber (PCF), the timing control system comprising:

[0235] At least one pressure sensor is configured to detect pressure changes in a gas environment surrounding the photonic crystal fiber and output at least one electrical signal based on the pressure changes, wherein the at least one electrical signal is used to determine the timing of at least one pulse of the broadband light source.

[0236] 37. A timing control system as defined in clause 36, wherein said at least one pulse of said broadband light source comprises a broadband pulse generated within said photonic crystal fiber and causing said pressure variation.

[0237] 38. A timing control system as defined in clause 36, wherein the at least one pulse of the broadband light source comprises a pump pulse configured to couple into the photonic crystal fiber and induce the pressure variation.

[0238] 39. A timing control system as defined in any one of clauses 36 to 38, wherein the photonic crystal fiber comprises a hollow core photonic crystal fiber (HC-PCF).

[0239] 40. A timing control system as defined in any one of clauses 36 to 39, wherein the at least one electrical signal is configured as a trigger signal for at least one of: the pump pulse or the broadband pulse.

[0240] 41. A broadband light source device comprising one or both of the following:

[0241] A mode control system as defined in any one of clauses 1 to 35; and / or

[0242] A timing control system as defined in any one of clauses 36 to 40.

[0243] 42. A measurement device comprising a broadband light source device as defined in item 41.

[0244] 43. A metrology apparatus as defined in clause 42, comprising a scatterometer metrology device, a level sensor or an alignment sensor.

[0245] 44. A method for mode control of a broadband light source comprising a photonic crystal fiber, the method comprising:

[0246] measuring one or more parameters of radiation emitted from the broadband light source to obtain measurement data;

[0247] evaluating, from the measurement data, a mode purity of the radiation emitted from the broadband light source; and

[0248] A control signal is generated to optimize one or more pump coupling conditions of the broadband light source; the pump coupling conditions are related to coupling of a pump laser beam with respect to a fiber core of a photonic crystal fiber.

[0249] 45. A method as defined in clause 44, wherein the one or more parameters of the output radiation include one or more parameters indicative of a mode purity of the broadband light source.

[0250] 46. ​​A method as defined in clause 44 or 45, wherein the radiation emitted from the broadband light source detected by the detection unit comprises output radiation emitted from an output end of a photonic crystal fiber.

[0251] 47. A method as defined in clause 46, comprising separating a reference beam from a main output beam emitted by a photonic crystal fiber, said measuring one or more parameters from said reference beam.

[0252] 48. A method as defined in any one of clauses 44 to 47, wherein the measuring step comprises measuring one or more spectral parameter values ​​of the output radiation to obtain the measurement data.

[0253] 49. A method as defined in clause 48, wherein the spectral parameter values ​​include power spectral density values ​​within one or more spectral ranges.

[0254] 50. A method as defined in clause 48 or 49, comprising bandpass filtering the radiation emitted from the broadband light source and measuring an illumination parameter indicative of the power of the filtered radiation.

[0255] 51. A method as defined in any one of clauses 44 to 50, wherein the step comprises spatially filtering out higher order modes other than a fundamental mode from the radiation emitted by the broadband light source and measuring an illumination parameter indicative of the power of the filtered radiation.

[0256] 52. A method as defined in any one of clauses 44 to 51, wherein the measuring step comprises measuring one or more beam characteristics of the output radiation related to the shape and / or size of the beam to obtain the measurement data.

[0257] 53. A method as defined in clause 52, wherein the beam characteristics of the output radiation related to beam shape and / or size include one or more of: beam ellipticity, beam diameter, Laguerre-Gaussian mode shape, or Zernike polynomial shape.

[0258] 54. A method as defined in any one of clauses 44 to 53, wherein the measuring step comprises measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber to obtain the measurement data.

[0259] 55. A method as defined in any one of clauses 44 to 54, comprising actuating movement of one or more components of a broadband light source based on a control signal to optimize one or more pump coupling conditions of the broadband light source.

[0260] 56. A method as defined in clause 55, wherein the one or more actuators are operable to optimize one or more pump coupling conditions by optimizing one or more of:

[0261] an angular offset of the pump laser beam relative to the fiber core of the photonic crystal fiber;

[0262] a lateral offset of the pump laser beam relative to the fiber core of the photonic crystal fiber;

[0263] Beam diameter of pump laser beam

[0264] Absolute polarization angle; and

[0265] Divergence of the pump laser beam.

[0266] 57. A method as defined in clause 55 or 56, wherein the one or more actuators include one or more of the following:

[0267] at least one actuator for at least one beam steering component or a support thereof;

[0268] at least one actuator for the balloon of the photonic crystal fiber or its support;

[0269] At least one actuator is used for focusing the lens to focus the pump laser beam onto the fiber core of the photonic crystal fiber.

[0270] 58. A method as defined in any one of clauses 44 to 57, wherein the evaluating step comprises comparing each of the one or more parameters of the radiation emitted from the broadband light source with an equivalent threshold parameter value indicative of optimal or acceptable mode purity.

[0271] 59. A method as defined in any one of clauses 44 to 58, wherein the photonic crystal fiber comprises a hollow core photonic crystal fiber (HC-PCF).

[0272] 60. A method as defined in any one of clauses 44 to 59, wherein the modal purity describes the ratio between the power in the fundamental transverse mode and the total output power.

[0273] 61. A method as defined in any one of clauses 44 to 60, wherein generating a control signal to optimize one or more pump coupling conditions optimizes one or more pump coupling conditions to maximize mode purity.

[0274] 62. A method according to any one of clauses 44 to 61, comprising an initial rough pump coupling step to roughly couple a pump laser beam relative to a fiber core of a photonic crystal fiber, the rough pump coupling step comprising measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam on an input facet of the photonic crystal fiber; and determining whether the pump laser beam is roughly aligned with the photonic crystal fiber based on the measured leakage radiation.

[0275] 63. A method as defined in clause 62, wherein determining whether the pump laser beam is coarsely aligned comprises positioning a minimum in the measured leakage radiation between at least two maxima in the measured leakage radiation.

[0276] 64. A method as defined in clause 62 or 63, wherein determining whether the pump laser beam is coarsely aligned comprises positioning a minimum in the measured leakage radiation within an annular region around a maximum in the measured leakage radiation.

[0277] 65. A method as defined in any one of clauses 62 to 64, wherein the scanning of the pump laser beam comprises scanning along a spiral path on the input facet.

[0278] 66. A method of performing a rough pump coupling step to roughly couple a pump laser beam relative to a fiber core of a photonic crystal fiber, the rough pump coupling step comprising measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam on an input facet of the photonic crystal fiber; and determining whether the pump laser beam is roughly aligned with the photonic crystal fiber based on the measured leakage radiation.

[0279] 67. A method as defined in clause 66, wherein determining whether the pump laser beam is coarsely aligned comprises positioning a minimum in the measured leakage radiation between at least two maxima in the measured leakage radiation.

[0280] 68. A method as defined in clause 66 or 67, wherein determining whether the pump laser beam is coarsely aligned comprises positioning a minimum in the measured leakage radiation within an annular region around a maximum in the measured leakage radiation.

[0281] 69. A method as defined in any one of clauses 66 to 68, wherein the scanning of the pump laser beam comprises scanning along a spiral path on the input facet.

[0282] 70. A method for determining an optimized value of a polarization parameter describing the polarization angle of radiation relative to an optical plane of a waveguide, the method comprising:

[0283] obtaining a relationship between the polarization angle and a power parameter indicative of radiated power traversing the waveguide;

[0284] obtaining a value of the power parameter, and

[0285] The optimized value of a polarization parameter is determined from the value of the power parameter and the relationship.

[0286] 71. A method as defined in clause 70, wherein the waveguide comprises a photonic crystal fiber or a hollow core photonic crystal fiber.

[0287] 72. A method as defined in any one of the preceding clauses 70 or 71, wherein the determining step comprises determining an optimized value corresponding to a minimum value of the power parameter.

[0288] 73. A method as defined in any one of clauses 70 to 72, comprising:

[0289] varying the polarization of the radiation between a plurality of angular positions;

[0290] obtaining a plurality of values ​​of the power parameter, each value corresponding to one of the angular positions; and

[0291] An optimized value of a polarization parameter corresponding to at least one minimum value is selected from the plurality of values ​​of the power parameter.

[0292] 74. A method as defined in clause 73, wherein said changing the polarization of said radiation comprises rotating any one or more of: a polarizer device, a light source emitting said radiation, or said nominally cylindrical waveguide.

[0293] 75. A polarization control system configured to control the output polarization of a broadband light source comprising a waveguide, the polarization control system comprising:

[0294] at least one detection unit configured to measure one or more parameters of radiation emitted from the broadband light source to generate measurement data; and

[0295] a processing unit configured to infer polarization properties of the radiation emitted from the broadband light source from the measurement data;

[0296] Wherein based on the evaluation, the polarization control system is configured to generate a control signal for optimizing a pump polarization condition of the broadband light source; the pump polarization condition is related to the coupling of the pump laser beam with respect to a fiber axis of the photonic crystal fiber.

[0297] 76. A polarization control system as defined in clause 75, wherein the waveguide comprises a photonic crystal fiber or a hollow core photonic crystal fiber.

[0298] 77. A polarization control system as defined in clause 75 or 76, comprising variable polarizer means for varying the pump polarization condition of said broadband light source relative to a fiber axis of the photonic crystal fiber.

[0299] 78. A polarisation control system as defined in any one of clauses 75 to 77, wherein the polariser arrangement comprises a rotatable half-wave plate.

[0300] 79. A polarisation control system as defined in any one of clauses 75 to 78, wherein the detection unit comprises a power measurement device and the one or more parameters of the radiation comprise power.

[0301] 80. A polarisation control system as defined in any of clauses 75 or 79, comprising an optical filter to select one or more desired wavelengths prior to the detection unit.

[0302] 81. A polarisation control system as defined in any one of clauses 75 to 80, comprising a measurement branch for collecting a portion of said radiation emitted from said broadband light source.

[0303] 82. An assembly comprising:

[0304] Optical components for receiving and modifying radiation;

[0305] a receiving element for receiving the modified radiation; and

[0306] Gas atmosphere, used to enclose the receiving element,

[0307] The component further includes a control element configured to stabilize a matching condition between the optical element and the receiving element by adjusting a modification of the received radiation or adjusting a distance between the optical element and the receiving element according to a property of the gas environment.

[0308] 83. A component according to item 82, wherein the radiation is substantially monochromatic, the optical element is a focusing element configured to provide focused monochromatic light to the element, and wherein the element is a nonlinear element configured to convert the focused monochromatic radiation into broadband radiation.

[0309] 84. An assembly according to clause 83, wherein the nonlinear element is a nonlinear optical fiber embedded in a gas environment.

[0310] 85. An assembly according to clause 84, wherein the property of the gas environment is the temperature, pressure or composition of the gas.

[0311] 86. An assembly according to item 85, wherein the control element includes an actuator configured to variably position the focusing element relative to the nonlinear optical fiber along the optical axis of the nonlinear optical fiber.

[0312] 87. An assembly according to item 85, wherein the control element includes an actuator configured to variably adjust the optical focal length of the focusing element.

[0313] 88. An assembly according to any of clauses 84 to 87, wherein the matching condition is associated with the efficiency of coupling focused monochromatic light into a nonlinear optical fiber.

[0314] 89. An assembly according to any of clauses 85 to 87, wherein the control element further comprises a processor configured to receive a measurement associated with the temperature, pressure or composition of the gas.

[0315] 90. An assembly according to clause 89, wherein the processor is configured to determine a change in a refractive index of the gas.

[0316] 91. The assembly of clause 90, wherein the processor is further configured to determine the focus change based on knowledge of an optical element positioned upstream in an optical path of the nonlinear optical fiber.

[0317] 92. An assembly according to any of clauses 89 to 91, further comprising a sensor configured to measure a property of the gaseous environment.

[0318] 93. An assembly comprising:

[0319] Optical components for receiving and modifying radiation; and

[0320] a receiving element for receiving the modified radiation,

[0321] The assembly further comprises a control element configured to stabilize a matching condition between the optical element and the receiving element by adjusting a modification of the received radiation or adjusting a distance between the optical element and the receiving element according to a property of a gas environment enclosing the receiving element.

[0322] 94. An assembly comprising:

[0323] Optical components for receiving and modifying radiation; and

[0324] a receiving element for receiving the modified radiation,

[0325] The assembly further comprises a control element configured to stabilize a matching condition between the optical element and the receiving element by adjusting a modification of the received radiation or adjusting a distance between the optical element and the receiving element based on an optical power determined at an exit of the receiving element.

[0326] 95. An assembly according to clause 94, wherein the adjustment is also based on an optical power determined at the entrance of the receiving element.

[0327] 96. An assembly according to clause 95, wherein the adjustment is based on a ratio between i) an optical power determined at an exit of the receiving element, and ii) an optical power determined at an entrance of the receiving element.

[0328] Although all of the above examples and embodiments of the present invention are related to a broadband light source based on HC-PCF, the present invention is equally applicable to the mode control of a broadband light source based on SC-PCF. In different embodiments, the detection unit of the mode control system measures one or more parameters of the broadband output beam of the broadband light source based on SC-PCF. Such parameters should be able to indicate the performance of the output mode of the broadband light source based on SC-PCF. The measured data is processed in the processing unit, and the processed data is then evaluated. Depending on the result of the evaluation, a feedback signal (or control signal) is generated and sent to the control unit of the mode control system. Finally, the control unit receives the control signal and controls the active components of the broadband light source based on SC-PCF, so that the pump coupling conditions of the light source are improved and the output mode purity of the light source based on SC-PCF is optimized.

[0329] Although the use of lithographic equipment in the manufacture of ICs may be specifically cited herein, it should be understood that the lithographic equipment described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0330] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes objects such as a wafer (or other substrate) or a mask (or other pattern forming device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0331] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0332] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner other than as described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A mode control system configured to control an output mode of a broadband light source comprising a photonic crystal fiber, the mode control system comprising: at least one detection unit configured to measure one or more parameters of radiation emitted from the broadband light source to generate measurement data; as well as a processing unit configured to evaluate, from the measurement data, a mode purity of the radiation emitted from the broadband light source; Based on the evaluation, the mode control system is configured to generate a control signal for optimizing one or more pump coupling conditions of the broadband light source; the pump coupling conditions are related to the coupling of the pump laser beam relative to the fiber core of the photonic crystal fiber. 2 . The mode control system of claim 1 , wherein the one or more parameters of radiation output from the broadband light source include one or more parameters indicative of a mode purity of the broadband light source.

3. The mode control system according to claim 1 or 2, wherein the radiation emitted from the broadband light source detected by the detection unit comprises output radiation emitted from an output end of the photonic crystal fiber.

4. A mode control system according to claim 3, comprising a beam splitter positioned to separate a reference beam from a main output beam emitted by the photonic crystal fiber, the radiation output from the broadband light source detected by the detection unit comprising the reference beam.

5. A mode control system according to claim 1 or 2, wherein the at least one detection unit comprises a spectral measurement arrangement operable to measure one or more spectral parameter values ​​of radiation output from the broadband light source as the measurement data.

6. A mode control system according to claim 5, wherein the spectral parameter values ​​include one or more parameters of a measured spectrum of radiation output from the broadband light source.

7. A mode control system according to claim 5 or 6, wherein the spectral measurement arrangement comprises a spectral measurement device and a multimode optical fibre operable to guide at least part of the radiation emitted from the broadband light source to the spectral measurement device.

8. A mode control system according to claim 1 or 2, wherein the detection unit includes one or more bandpass filters; each of the one or more bandpass filters is capable of operating to select a corresponding spectral range of the radiation output from the broadband light source, and the irradiation measurement device is capable of operating to detect an irradiation parameter indicating the power of the filtered radiation, and the measurement data includes and / or is derived from the irradiation parameter indicating the power.

9. A mode control system according to claim 8, wherein the measurement data comprises power spectral density or energy spectral density values ​​within one or more spectral ranges derived from the illumination parameter indicative of power.

10. The mode control system according to claim 1 or 2, wherein the detection unit comprises: a spatial filter operable to filter out higher order modes other than the fundamental mode from the output radiation; and an illumination measurement device operable to detect an illumination parameter indicative of a power of the filtered radiation, said measurement data comprising and / or being derived from said illumination parameter indicative of the power.

11. The mode control system of claim 10, wherein the spatial filter comprises a single mode optical fiber or a pinhole.

12. The mode control system according to claim 1 or 2, further comprising: a variable polarization arrangement operable to controllably configure a polarization property of the radiation emitted from the broadband light source; wherein the detection unit comprises an illumination measurement device operable to detect an illumination parameter indicative of the power of the radiation emitted from the broadband light source as a function of an input polarization angle, the measurement data comprising and / or derived from the illumination parameter indicative of the power; and the mode control system is further operable to optimize the one or more pump coupling conditions by changing the polarization angle imposed by the variable polarization arrangement relative to the fiber axis of the photonic crystal fiber, thereby optimizing the pump polarization condition of the radiation emitted from the broadband light source.

13. A mode control system according to claim 12, wherein the variable polarization arrangement comprises one or both of the following: a variable polarizer operable to change the polarization angle of the pump laser beam relative to the photonic crystal fiber, and / or An actuator is operable to change the angle of the photonic crystal fiber about its optical axis.

14. A mode control system according to claim 1 or 2, wherein the detection unit includes a beam shape and / or size measuring device, which is capable of operating to measure one or more beam characteristics of the radiation output from the broadband light source related to the shape and / or size of the beam, and the measurement data includes and / or is derived from the beam characteristics of the radiation output from the broadband light source related to the shape and / or size of the beam.

15. A mode control system according to claim 14, wherein the beam characteristics of the radiation output from the broadband light source related to the shape and / or size of the beam include one or more of the following: beam ellipticity, beam diameter, Laguerre-Gaussian mode shape or Zernike polynomial shape.

16. A mode control system according to claim 1 or 2, wherein the one or more parameters of the radiation emitted from the broadband light source measured by the detection unit are configured such that they include leakage radiation emitted from the fiber cladding of the photonic crystal fiber.

17. A mode control system according to claim 1 or 2, comprising one or more actuators for actuating movement of one or more components of the broadband light source; wherein the control signal is operable to control one or more of the actuators. 18 . The mode control system of claim 17 , further comprising a control unit configured to receive the control signal from the processing unit and control the one or more actuators.

19. A mode control system according to claim 17 or 18, wherein the one or more actuators are operable to optimize one or more pump coupling conditions by optimizing one or more of: an angular offset of the pump laser beam relative to the fiber core of the photonic crystal fiber; a lateral offset of the pump laser beam relative to the fiber core of the photonic crystal fiber; a beam diameter of the pump laser beam; Absolute polarization angle; and The divergence of the pump laser beam.

20. A mode control system according to claim 17, 18 or 19, wherein the one or more actuators include one or more of the following: at least one actuator for at least one beam steering component or a support thereof; at least one actuator for the balloon of the photonic crystal fiber or its support; At least one actuator is used for focusing the lens to focus the pump laser beam onto the fiber core of the photonic crystal fiber.

21. A mode control system according to claim 1 or 2, wherein the processing unit is operable to evaluate measurement data by comparing each of the one or more parameters of the radiation emitted from the broadband light source to an equivalent threshold parameter value indicative of optimal or acceptable mode purity.

22. A mode control system according to claim 1 or 2, wherein the photonic crystal fiber comprises a hollow core photonic crystal fiber.

23. A mode control system according to claim 1 or 2, wherein the radiation output from the broadband light source comprises a wavelength range of 200 nm to 2000 nm or a sub-range within this range.

24. A mode control system according to claim 1 or 2, wherein the mode purity describes the ratio between the power in the fundamental transverse mode and the total output power.

25. The mode control system of claim 1 or 2, wherein the mode control system is configured to generate a control signal to optimize one or more pump coupling conditions of the broadband light source comprising: The method is configured to generate a control signal to optimize one or more pump coupling conditions to maximize mode purity.

26. A mode control system according to claim 1 or 2, comprising at least one detector for detecting leakage radiation and operable to coarsely couple a pump laser beam with respect to a fiber core of a photonic crystal fiber in an initial coarse pump coupling operation, the coarse pump coupling operation comprising measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam on an input facet of the photonic crystal fiber; and Whether the pump laser beam is roughly aligned with the photonic crystal fiber is determined based on the measured leakage radiation.

27. The mode control system of claim 26, wherein determining whether the pump laser beam is coarsely aligned comprises: A minimum in the measured leakage radiation is positioned between at least two maxima in the measured leakage radiation.

28. The mode control system of claim 26 or 27, wherein determining whether the pump laser beam is coarsely aligned comprises: The minimum in the measured leakage radiation is positioned within an annular region around the maximum in the measured leakage radiation.

29. A mode control system according to claim 26, 27 or 28, wherein the at least one detector comprises a plurality of detectors radially spaced around the optical fibre cladding.

30. The mode control system of claim 29, wherein each detector extends around the longitudinal axis of the photonic crystal fiber by an angle less than 360 / n degrees in a radial direction of the photonic crystal fiber, where n is the number of detectors.

31. A mode control system according to any one of claims 26 to 28, wherein at least one detector comprises at least one pair of a detector and a reflector, each detector and reflector being positioned at radially opposite positions around the optical fibre cladding.

32. A mode control system according to any one of claims 26 to 31, wherein the scanning of the pump laser beam comprises scanning along a spiral path on the input facet.

33. The mode control system according to claim 1 or 2, further comprising an optical manipulation unit for scanning the light beam, the optical manipulation unit comprising: - one or more optical elements configured to manipulate the light beam; - Beam tilt adjuster, used to adjust the propagation direction of the light beam; as well as - A control unit for controlling the beam tilt adjuster to set the propagation direction towards the photonic crystal fiber.

34. The mode control system of claim 33, wherein the beam tilt adjuster comprises at least two wedge-shaped prisms and a tilt actuation system arranged in series, and The tilt actuation systems are controllable by a control unit to independently rotate the wedge prisms around their respective optical axes.

35. A mode control system according to claim 33 or 34, wherein the optical manipulation unit further comprises a beam displacement device for displacing the light beam, wherein the beam displacement device comprises a plane parallel plate and a displacement actuation system controllable by the control unit to rotate the plane parallel plate around a first axis substantially perpendicular to a propagation direction of the light beam.

36. The mode control system of claim 35, wherein the displacement actuation system is further controllable to rotate the plane parallel plate about a second axis substantially perpendicular to the propagation direction of the light beam and the first axis.

37. A mode control system according to claim 36, wherein the beam displacement device includes a second plane parallel plate, and the displacement actuation system can be controlled by the control unit to rotate the second plane parallel plate about a second axis substantially perpendicular to the propagation direction of the beam and the first axis.

38. A broadband light source device comprising one or both of the following: A mode control system as claimed in any one of claims 1 to 37.

39. A measuring device, comprising the broadband light source device according to claim 38.

40. The metrology apparatus of claim 39, comprising a scatterometer metrology device, a level sensor or an alignment sensor.

41. A method for mode control of a broadband light source comprising a photonic crystal fiber, the method comprising: measuring one or more parameters of radiation emitted from the broadband light source to obtain measurement data; evaluating, from the measurement data, a mode purity of the radiation emitted from the broadband light source; as well as generating a control signal to optimize one or more pump coupling conditions of the broadband light source; The pump coupling condition is related to the coupling of a pump laser beam with respect to a fiber core of the photonic crystal fiber.

42. The method of claim 41, wherein the one or more parameters of radiation output from the broadband light source include one or more parameters indicative of a mode purity of the broadband light source.

43. A method according to claim 41 or 42, wherein the radiation emitted from the broadband light source detected by the detection unit comprises output radiation emitted from an output end of a photonic crystal fiber.

44. The method of claim 43, comprising separating a reference beam from a main output beam emitted by a photonic crystal fiber, said measuring one or more parameters from said reference beam.

45. A method according to claim 41 or 42, wherein the measuring step comprises measuring one or more spectral parameter values ​​of the output radiation to obtain the measurement data.

46. ​​The method of claim 45, wherein the spectral parameter values ​​include power spectral density values ​​within one or more spectral ranges.

47. The method of claim 45, comprising bandpass filtering the radiation emitted from the broadband light source and measuring an illumination parameter indicative of the power of the filtered radiation.

48. The method of claim 41 or 42, wherein the step comprises spatially filtering out higher order modes other than a fundamental mode from the radiation emitted by the broadband light source and measuring an illumination parameter indicative of the power of the filtered radiation.

49. A method according to claim 41 or 42, wherein the measuring step comprises measuring one or more beam characteristics of radiation output from the broadband light source related to beam shape and / or size to obtain the measurement data.

50. The method of claim 49, wherein the beam characteristics of the radiation output from the broadband light source related to beam shape and / or size include one or more of: beam ellipticity, beam diameter, Laguerre-Gaussian mode shape, or Zernike polynomial shape.

51. The method of claim 41 or 42, wherein the measuring step comprises measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber to obtain the measurement data.

52. The method of claim 41 or 42, comprising actuating movement of one or more components of a broadband light source based on a control signal to optimize one or more pump coupling conditions of the broadband light source.

53. The method of claim 52, wherein the one or more actuators are operable to optimize one or more pump coupling conditions by optimizing one or more of: an angular offset of the pump laser beam relative to the fiber core of the photonic crystal fiber; a lateral offset of the pump laser beam relative to the fiber core of the photonic crystal fiber; beam diameter of the pump laser beam; Absolute polarization angle; and Divergence of the pump laser beam.

54. The method of claim 52, wherein the one or more actuators include one or more of: at least one actuator for at least one beam steering component or a support thereof; at least one actuator for the balloon of the photonic crystal fiber or its support; At least one actuator is used for focusing the lens to focus the pump laser beam onto the fiber core of the photonic crystal fiber.

55. A method according to claim 41 or 42, wherein the evaluating step includes comparing each of the one or more parameters of radiation emitted from the broadband light source to an equivalent threshold parameter value indicative of optimal or acceptable mode purity.

56. The method of claim 41 or 42, wherein the photonic crystal fiber comprises a hollow core photonic crystal fiber.

57. A method according to claim 41 or 42, wherein the mode purity describes the ratio between the power in the fundamental transverse mode and the total output power.

58. The method of claim 41 or 42, wherein said generating a control signal to optimize one or more pump coupling conditions optimizes one or more pump coupling conditions to maximize mode purity.

59. The method according to claim 41 or 42, comprising an initial rough pump coupling step to roughly couple the pump laser beam relative to the fiber core of the photonic crystal fiber, the rough pump coupling step comprising measuring leakage radiation emitted from the fiber cladding of the photonic crystal fiber during scanning of the pump laser beam on the input facet of the photonic crystal fiber; and determining whether the pump laser beam is roughly aligned with the photonic crystal fiber based on the measured leakage radiation.

60. The method of claim 59, wherein determining whether the pump laser beam is coarsely aligned comprises: A minimum in the measured leakage radiation is positioned between at least two maxima in the measured leakage radiation.

61. The method of claim 59, wherein determining whether the pump laser beam is coarsely aligned comprises: The minimum in the measured leakage radiation is positioned within an annular region around the maximum in the measured leakage radiation.

62. The method of claim 59, wherein the scanning of the pump laser beam comprises scanning along a spiral path on the input facet.

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