Mode control of broadband light source based on photonic crystal fiber

Abstract

The present disclosure provides a mode control system and method for controlling an output mode of a broadband light source including a photonic crystal fiber (PCF). The mode control system includes: at least one detection unit configured to measure one or more parameters of radiation emitted from a 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 from 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 relates to the coupling of a pump laser beam with respect to an optical fiber core of the photonic crystal fiber.

Description

[0001] This application is a divisional of the invention application with the application number 202080061680.1, which entered the Chinese national phase on March 1, 2022 from the international application PCT / EP2020 / 073422.

[0002] Cross-reference to related applications

[0003] This application claims the priority of 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

[0004] The present invention relates to mode control of broadband radiation generators based on photonic crystal fibers, and more particularly to such broadband radiation generators related to metrology applications in integrated circuit manufacturing. Background art

[0005] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can project a pattern (commonly also referred to as a "design layout" or "design") onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer) at a pattern-forming device (e.g., a mask), for example.

[0006] To project a pattern onto a substrate, a lithographic apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum feature size that can be formed on the substrate. Typical wavelengths currently in use are 365 nm (i-line), 248 nm, 193 nm, and 13.5 nm. Compared to a lithographic apparatus using radiation with a wavelength of, for example, 193 nm, a lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the range of 4 to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate.

[0007] Low-k1 lithography can be used to process features smaller than the classical resolution limit of a lithographic apparatus. In such a process, the resolution formula can be expressed as CD = k1×λ / NA, where λ is the wavelength of the radiation employed, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (the smallest feature size typically printed, but in this case the half-pitch), and k1 is an empirical resolution factor. Generally, the smaller k1 is, the more difficult it is to reproduce on a substrate a pattern similar in shape and size to that planned by the circuit designer to achieve a particular 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 the optimization of NA, customized illumination schemes, the use of phase-shifting 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.

[0008] 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, and focus control and scatterometry-based tools for inspecting / measuring products such as exposure and / or etching in process control. In each case, a radiation source is required. For various reasons, including measurement robustness and accuracy, broadband or white light radiation sources are increasingly being used for such metrology applications. There is a desire to improve existing devices for broadband radiation generation. Summary of the Invention

[0009] In a first aspect of the present invention, there is provided a mode control system configured to control the output mode of a broadband light source including a photonic crystal fiber (PCF), the mode control system comprising: at least one detection unit configured to measure one or more parameters of the 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 by means of 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 conditions being related to the coupling of a pump laser beam relative to the fiber core of the photonic crystal fiber.

[0010] In a second aspect of the present invention, there is provided a method for mode control of a broadband light source including a photonic crystal fiber, the method comprising: measuring one or more parameters of the radiation emitted from the broadband light source to obtain measurement data; evaluating the mode purity of the radiation emitted from the broadband light source by means of the measurement data; and generating a control signal to optimize one or more pump coupling conditions of the broadband light source; the pump coupling conditions being related to the coupling of a pump laser beam relative to the fiber core of the photonic crystal fiber.

[0011] Other aspects of the present invention include a broadband light source and a measuring device, which include the mode control system of the first aspect. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic diagrams, in which:

[0013] - Figure 1 A schematic overview of a lithographic apparatus is depicted;

[0014] - Figure 2 A schematic overview of a lithography cell is depicted;

[0015] - Figure 3 A schematic representation of overall lithography is depicted, showing the collaboration between three key technologies to optimize semiconductor manufacturing;

[0016] - Figure 4 A schematic overview of a scatterometry apparatus used as a measuring device is depicted, which may include a radiation source according to an embodiment of the present invention;

[0017] - Figure 5 A schematic overview of a horizontal sensor device is depicted, which may include a radiation source according to an embodiment of the present invention;

[0018] - Figure 6 A schematic overview of an alignment sensor device is depicted, which may include a radiation source according to an embodiment of the present invention;

[0019] - Figure 7 Cross-sections of two HC-PCF designs for white light generation are schematically depicted, including (a) the Kagome design and (b) the single-ring design;

[0020] - Figure 8 An exemplary inflated HC-PCF-based broadband light source device is schematically depicted;

[0021] - Figure 9 Is a flowchart describing the operating procedure of the mode control system according to an embodiment of the present invention.

[0022] - Figure 10Schematically depicts a broadband light source equipped with a mode control system according to a first embodiment of the present invention for optimizing and stabilizing the fundamental transverse mode LP01 of the light source.

[0023] - Figure 11 Schematically depicts a broadband light source equipped with a mode control system according to a second embodiment of the present invention for optimizing and stabilizing the fundamental transverse mode LP01 of the light source.

[0024] - Figure 12 Schematically depicts a broadband light source equipped with a mode control system according to a third embodiment of the present invention for optimizing and stabilizing the fundamental transverse mode LP01 of the light source.

[0025] - Figure 13 Schematically depicts a broadband light source equipped with a mode control system according to a fourth embodiment of the present invention for optimizing and stabilizing the fundamental transverse mode LP01 of the light source.

[0026] - Figure 14 Schematically depicts a broadband light source equipped with a mode control system according to different embodiments of the present invention for optimizing and stabilizing the fundamental transverse mode LP01 of the light source;

[0027] - Figure 15 Schematically depicts a rough alignment arrangement according to a first embodiment for rough alignment;

[0028] - Figure 16 Schematically depicts a rough alignment arrangement according to a second embodiment for rough alignment;

[0029] - Figure 17 Schematically depicts a rough alignment arrangement according to a third embodiment for rough alignment; and

[0030] - Figure 18 Schematically depicts a specific optical manipulation unit according to a rough alignment arrangement that can be used for at least Figures 15 to 17 depicted in the rough alignment arrangement.

[0031] - Figure 19 Schematically depicts a broadband light source equipped with a specific timing control system according to an embodiment for controlling and determining the pulse timing of the broadband light source.

[0032] - Figure 20 Schematically depicts a broadband light source equipped with a specific polarization control system according to an embodiment for optimizing and stabilizing the output polarization of the broadband light source. Detailed Description

[0033] In this document, the terms "radiation" and "beam" are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g., having wavelengths of 365, 248, 193, 157, or 126 nm) and EUV (extreme ultraviolet radiation, e.g., having wavelengths in the range of approximately 5 to 100 nm).

[0034] The terms "reticle", "mask", or "patterning device" as used herein can be broadly interpreted to refer to a general patterning device that can be used to endow an incoming radiation beam with a patterned cross-section corresponding to a pattern to be created in a target portion of a substrate. In this context, the term "light valve" can also be used. Examples of other such patterning devices include programmable mirror arrays and programmable LCD arrays in addition to classical masks (transmission or reflection, binary, phase-shifting, hybrid, etc.).

[0035] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA includes 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 table) 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 the pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0036] In operation, the illumination system IL receives the radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL can 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 can be used to condition the radiation beam B such that it has a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0037] The term "projection system" PS as used herein should be broadly interpreted to encompass various types of projection systems, including refractive, reflective, refraction-reflective, anamorphic, magnetic, electromagnetic, and / or electrostatic optical systems or any combination thereof, to be suitable for the exposure radiation used and / or other factors (such as the use of an immersion liquid or the use of a vacuum). Any use of the term "projection lens" herein can be considered synonymous with the more general term "projection system" PS.

[0038] The lithographic apparatus LA can be of the type in which at least a portion of the substrate can be covered by a liquid (e.g., water) having a relatively high refractive index to fill the space between the projection system PS and the substrate W, which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0039] The lithographic apparatus LA can also be of the type having two or more substrate supports WT (also referred to as a "dual stage"). In such a "multi-stage" machine, the substrate supports WT can be used in parallel, and / or the step of preparing a subsequent exposure of the substrate W can be performed on the substrate W located on one of the substrate supports WT while another substrate W on another substrate support WT is used for exposing a pattern on the other substrate W.

[0040] In addition to the substrate support WT, the lithographic apparatus LA can include a metrology stage. The metrology stage is arranged to hold sensors and / or cleaning devices. The sensors can be arranged to measure properties of the projection system PS or the radiation beam B. The metrology stage can hold multiple sensors. The cleaning device can be arranged to clean a part of the lithographic apparatus, such as a part of the projection system PS or a part of the system providing the immersion liquid. When the substrate support WT is away from the projection system PS, the metrology stage can move under the projection system PS.

[0041] In operation, the 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 the pattern (design layout) present on the patterning device MA. After traversing the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. By means of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, e.g., in order to position different target portions C in the path of the radiation beam B at a focus and alignment position. Similarly, a first positioner PM and possibly another position sensor (not explicitly depicted in Figure 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 mask alignment marks M1, M2 and 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 the target portions. When the substrate alignment marks P1, P2 are located between the target portions C, these are referred to as scribe alignment marks.

[0042] As Figure 2As shown, the lithography apparatus LA can form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography cell) cluster and 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 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 the substrate W from the input / output ports I / O1, I / O2, moves them between different process equipment, and delivers the substrate W to the feed table LB of the lithography apparatus LA. The devices in the lithography cell (which are usually also collectively referred to as a track) are typically controlled by a track control unit TCU, which itself can be controlled by a supervisory control system SCS, which can also control the lithography apparatus LA, e.g., via a lithography control unit LACU.

[0043] In order to expose the substrate W exposed by the lithography apparatus LA correctly and consistently, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimension (CD), etc. For this purpose, an inspection tool (not shown) may be included in the lithography cell LC. If an error is detected, then, for example, adjustments can be made to the exposure of subsequent substrates or other processing steps to be performed on the substrate W, especially if the inspection is completed before other substrates W in the same batch or lot still have to be exposed or processed.

[0044] Inspection equipment (which can also be referred to as metrology equipment) is used to determine the properties of the substrate W, in particular how the properties of different substrates W vary, or how the properties associated with different layers of the same substrate W vary between the layers. The inspection equipment can alternatively be configured to identify defects on the substrate W and can, for example, be part of the lithography cell LC, or can be integrated into the lithography apparatus LA, or can even be a stand-alone device. The inspection equipment can 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 a developed resist image (where the exposed or unexposed portions of the resist have been removed), or even an etched image (after a pattern transfer step such as etching).

[0045] Generally, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires highly accurate dimensional determination and placement of the structures on the substrate W. To ensure this high accuracy, three systems can be combined into a so-called "integrated" control environment, as Figure 3are schematically depicted. One of these systems is a lithographic apparatus LA, which is (virtually) connected to a metrology tool MET (second system) and a computer system CL (third system). The key to such an "integrated" environment is to optimize the collaboration 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 lithographic apparatus LA remains within the process window. The process window defines a range of process parameters (such as dose, focus, overlay) within which a specific manufacturing process yields a defined result (such as a functional semiconductor device), typically within which the process parameters in a lithography process or patterning process are allowed to vary.

[0046] The computer system CL can use (a part of) the design layout to be patterned to predict the resolution enhancement techniques to be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithographic apparatus settings achieve the maximum overall process window of the patterning process (depicted by the double arrows in the first scale SC1 in Figure 3 ). Typically, the resolution enhancement techniques are arranged to match the patterning capabilities of the lithographic apparatus LA. The computer system CL can also be used to detect where the lithographic apparatus LA is currently operating within the process window (e.g., using input from the metrology tool MT) to predict whether there might be defects due to, for example, sub-optimal processing (depicted by the arrow pointing to "0" in the second scale SC2 in Figure 3 ).

[0047] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithographic apparatus LA to identify possible drifts, e.g., in the calibration state of the lithographic apparatus LA (depicted by the multiple arrows in the third scale SC3 in Figure 3 ).

[0048] During the lithography process, frequent measurements of the created structures are desired, for example for process control and verification. Tools for performing such measurements are typically referred to as metrology tools MT. Different types of metrology tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer metrology tools MT. A scatterometer is a versatile instrument that allows the measurement of lithography process parameters by placing a sensor in the pupil of the scatterometer objective or in a plane conjugate to the pupil (these measurements are typically referred to as pupil-based measurements) or by placing the sensor in the image plane or a plane conjugate to the image plane (in which case these measurements are typically referred to as image- 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 hereby incorporated by reference in their entirety. The above scatterometers can measure gratings using light from soft x-rays and visible for the near-IR wavelength range.

[0049] In a first embodiment, the scatterometer MT is an angular-resolved scatterometer. In such a scatterometer, a reconstruction method can be applied to the measured signal to reconstruct or calculate the properties of the grating. For example, such reconstruction can 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 measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from the actual target.

[0050] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, the radiation emitted by a radiation source is directed onto a target, and the reflected or scattered radiation from the target is directed onto a spectrometer detector that 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 that produced the detected spectrum can be reconstructed, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library.

[0051] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows for the determination of parameters of a lithography process by measuring scattered radiation for each polarization state. Such metrology equipment emits polarized light (such as linear, circular, or elliptical) by using, for example, a suitable polarization filter in the illumination section of the metrology equipment. Sources suitable for the metrology equipment can also provide polarized radiation. Various embodiments of existing ellipsometric scatterometers 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 hereby incorporated by reference in their entirety.

[0052] Metrology equipment such as a scatterometer is depicted in Figure 4 It includes a broadband (white light) radiation projector 2 that projects radiation onto a substrate W. The reflected or scattered radiation is transmitted to a spectrometer detector 4 that measures the spectrum 6 of the specularly reflected radiation (i.e., measures the intensity as a function of wavelength). From this data, for example, by rigorous coupled-wave analysis and non-linear regression or by comparison with a simulated spectral library such as that shown at the bottom of Figure 3 , the structure or profile 8 of the detected spectrum can be reconstructed by a processing unit PU. Generally, for the reconstruction, the general form of the structure is known, and some parameters are assumed from knowledge of the process by which the structure is fabricated, and only a few parameters of the structure are to be determined from the scatter measurement data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0053] The overall measurement quality of lithography parameters via measurement of a measurement target is at least partially determined by the measurement scheme used to measure the lithography parameters. The term "substrate measurement scheme" can include one or more parameters of the measurement itself, one or more parameters of one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement scheme is a diffraction-based optical measurement, one or more parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One criterion in selecting a measurement scheme can 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 hereby incorporated by reference in their entirety.

[0054] Another type of metrology tool used in IC manufacturing is a topography measurement system, a level sensor or a height sensor. Such a tool can be integrated in a lithography device to measure the topography of a substrate (or wafer). A map of the topography of the 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 transfer of the pattern onto 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 out of plane for the substrate. Typically, a level or height sensor performs measurements 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 results in height measurements at positions across the substrate.

[0055] 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 can 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 can additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the substrate surface.

[0056] 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.

[0057] To determine the height level at the measurement location MLO, the level sensor further includes a detection system that includes a detection grating DGR, a detector DET, and a processing unit (not shown) for processing the output signal of the detector DET. The detection grating DGR can be the same as the projection grating PGR. The detector DET generates a detector output signal that indicates the received light (e.g., (such as a photodetector) indicates the intensity of the received light) or represents the spatial distribution of the received intensity (such as a camera). The detector DET can include any combination of one or more detector types.

[0058] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level typically correlates with the signal intensity measured by the detector DET, which has a periodicity that depends in particular on the design of the projection grating PGR and the (tilted) angle of incidence ANG.

[0059] The projection unit LSP and / or the detection unit LSD can include other 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).

[0060] In an embodiment, the detection grating DGR can be omitted, and the detector DET can be placed at the location where the detection grating DGR was located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0061] To effectively cover the surface of the substrate W, the level sensor LS can be configured to project an array of reference beams BE1 onto the surface of the substrate W, thereby generating an array of measurement regions MLO or spots covering a larger measurement range.

[0062] For example, various height sensors of a general type are disclosed in US7265364 and US7646471, both of which are incorporated herein by reference. A height sensor using UV radiation instead of visible or infrared radiation is 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 the grating image without the need for a detection grating.

[0063] Another type of metrology tool used in IC manufacturing is an alignment sensor. Thus, a key aspect of the performance of a lithographic apparatus is the ability to place the applied pattern correctly and accurately with respect to features laid down in a previous layer (by the same apparatus or a different lithographic apparatus). For this purpose, the 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, typically an optical position sensor. The position sensor may be referred to as an “alignment sensor”, and the marks may be referred to as “alignment marks”.

[0064] A lithographic apparatus may include one or more (e.g., multiple) alignment sensors by which the position of alignment marks provided on a 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. Examples of alignment sensors used in current lithographic apparatuses are based on the self - reference interferometer described in US6961116. For example, as disclosed in US2015261097A1, various enhancements and modifications of position sensors have been developed. The content of all these publications is incorporated herein by reference.

[0065] Figure 6 is a schematic block diagram of an embodiment of a known alignment sensor AS such as described 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 as an illumination spot SP onto a mark, such as a mark AM located on a substrate W, by steering optics. In this example, the steering optics includes a facular mirror SM and an objective lens OL. The diameter of the illumination spot SP by which the mark AM is illuminated may be slightly smaller than the width of the mark itself.

[0066] The 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 (which may be referred to as reflection) from the mark. A self - reference interferometer SRI (such as 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 case where 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 include a number of pixels. The photodetector may include a sensor array.

[0067] The steering optics, which includes the facular mirror SM in this example, may also be used to block the zero - order radiation reflected from the mark, such that the information - bearing beam IB includes only higher - order diffracted radiation from the mark AM (this is not essential for the measurement but improves the signal - to - noise ratio).

[0068] 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 positions on the substrate relative to the reference frame are output.

[0069] A single measurement of the type illustrated only fixes the position of the marker within a specific range corresponding to one spacing of the marker. A rough measurement technique is used in combination with such a measurement to identify which period of the sine wave is the period containing the marked position. The same process can be repeated at different wavelengths at a coarser and / or finer level in order to improve accuracy and / or robustly detect the marker, regardless of the material of which the marker is made and the material above and / or below the position where the marker is set. The wavelengths can be optically multiplexed and demultiplexed in order to be processed simultaneously, and / or they can be multiplexed by time division or frequency division.

[0070] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. The alignment sensor can thus be rigidly and accurately mounted to the reference frame while effectively scanning the marker 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 (which controls the movement of the substrate support). A substrate support position sensor (such as an interferometer) measures the position of the substrate support (not shown). In an embodiment, one or more (alignment) markers are provided on the substrate support. Measuring the position of the marker provided on the substrate support allows the position of the substrate support determined by the position sensor to be calibrated (e.g., relative to the frame to which the alignment system is connected). Measuring the position of the alignment marker provided on the substrate allows the position of the substrate relative to the substrate support to be determined.

[0071] For optical semiconductor metrology, inspection applications, such as in any of the above metrology tools, a bright light source that outputs coherent radiation and simultaneously covers a wide wavelength range (e.g., from UV to IR) is generally preferred. 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), such a broadband light source can help improve the flexibility and robustness of the application. Allowing the wavelength to be optimized for a specific application also means that the measurement accuracy can be further improved.

[0072] Gas lasers that simultaneously emit multiple wavelengths based on gas discharge effects can be used for these applications. However, the 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, such as solid-state lasers, can be spatially combined into the optical path of a metrology 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 known as supercontinuum lasers) are capable of emitting radiation with high spatial coherence and broad spectral coverage, such as from UV to IR, and are thus a very attractive and practical option.

[0073] A hollow-core photonic crystal fiber (HC-PCF) is a special type of fiber that includes a central hollow-core region and an internal cladding structure surrounding the hollow core, both extending along the entire fiber axis. The optical guiding mechanism is achieved by the internal cladding waveguide structure, which can include, for example, thin-walled glass elements. Thus, the radiation is mainly confined within the hollow core and propagates along the fiber in the form of transverse core modes.

[0074] Multiple types of HC-PCF can be designed, based on different physical guiding mechanisms. Two such HC-PCF include: hollow-core photonic bandgap fiber (HC-PBF) and hollow-core antiresonant reflecting fiber (HC-ARF).

[0075] HC-PCF include hollow channels filled with fluid, and thus they have properties desirable for a variety of optical guiding applications; for example, high-power beam delivery using HC-PBF and gas-based white light generation (or supercontinuum generation) using HC-ARF. Details regarding the design and manufacture of HC-PCF can be found in U.S. Patent US2004175085 (for HC-PBF) and European Patent Application EP3136143A1 (for HC-ARF), which are incorporated herein by reference. HC-PBF is configured to provide low-loss but narrow-bandwidth optical guiding via the photonic bandgap effect established by the cladding structure of the central hollow core herein. However, HC-ARF is designed to significantly broaden the transmission bandwidth via antiresonant reflection of light from the cladding.

[0076] Figure 7 Two well-known types of HC-ARF are depicted in cross-section. Figure 7 (a) shows a Kagome fiber, including a Kagome lattice structure as its cladding CLA, defining a hollow fiber core FCO. This arrangement can be surrounded by one or more outer coatings OCO. Figure 7(b) shows a single - loop or left - handed fiber, in which a hollow - core region FCO is formed and surrounded by a layer of non - contacting loop CLA.

[0077] For gas - based white - light generation, the HC - ARF can be included in a gas cell, which is designed to operate at pressures of, for example, up to several tens of bar (e.g., between 3 and 100 bar). When pumped by an ultrashort pump laser pulse with sufficient peak power, the gas - filled HC - ARF can act as an optical frequency converter. The frequency conversion from the ultrashort pump laser pulse to a broadband laser pulse is achieved through the complex interaction of dispersion and nonlinear optical processes inside the gas - filled fiber. The converted laser pulse is mainly confined in the hollow core in the form of a transverse core mode and is guided to the fiber end. The transverse core mode supported by the fiber can be described as a linearly polarized (LP) mode. In the LP notation, the LP mode is called LP mn , where the m and n subscripts are integers representing the azimuthal and radial orders of a particular mode. The fundamental mode is LP 01 mode. Some radiation (e.g., higher - 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 the cladding region of the HC - ARF can be configured such that the higher - order core modes are phase - matched with the higher - order cladding modes. In this way, the higher - order core modes can be resonantly coupled with the higher - order cladding modes, which are subsequently attenuated or suppressed. In this way, low - loss and efficient single - transverse - mode transmission can be obtained over a wide spectral range.

[0078] The spatio-temporal transmission characteristics of the laser pulse, such as its spectral amplitude and phase along the PCF (such as HC-PCF), can be changed and tuned by adjusting the pump laser parameters, the filling gas parameters, the fiber parameters, and the pump coupling conditions. The transmission characteristics may include one or more of the following: output power, output mode profile, output temporal profile, width of the output temporal 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 degree 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 with respect to the fiber core, lateral offset of the pump laser beam with respect to the fiber core, mode matching between the pump laser beam and the fiber core. The mode matching between the pump laser beam and the 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.

[0079] 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 the filling gas, the nonlinear optical processes may include modulation instability (MI), soliton fission, Kerr effect, Raman effect, and dispersive wave generation, the details of which are described in WO2018 / 127266A1 and US9160137B1 (both 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 associated spectral broadening characteristics can be adjusted to optimize the frequency conversion. The generated broadband laser output can cover wavelengths from UV (e.g., <200 nm) to mid-IR (e.g., >2000 nm).

[0080] When applied to semiconductor metrology and alignment applications, such as in any of the above metrology tools, it is desirable that the transverse mode of the output radiation of the broadband light source based on the HC-PCF is a fundamental transverse mode, i.e., LP 01In 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 because the transmission loss of the fundamental transverse mode through a PCF (such as an HC-PCF) is much lower than that of the higher-order modes (HOMs). Therefore, if all the pump light is coupled into the fundamental transverse mode of the fiber, the power efficiency is higher. In addition, the presence of HOMs reduces 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 loss because the HOM content of the output will be removed during transmission, for example, by spatial filtering. The reduction in intensity stability results in high measurement noise and poor measurement consistency.

[0081] Figure 8 Schematically illustrates an exemplary HC-PCF-based broadband light source 800. A collimated pump laser beam 811 (including a train of pump pulses with a specific repetition rate) is output from a pump laser 810 and is used as an input laser beam to generate broadband radiation in an HC-PCF 841. The propagation of the collimated pump laser beam is controlled by one or more beam steering components (such as 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 matches 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 with a specific fiber length can adopt the Kagome design or single-ring design with reference to Figure 7 . Alternatively, other fiber designs (not shown) can be used, such as a solid-core design, a suppressed-coupling design, an inner-cycloid core Kagome, and a nested tubular design. 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 having a pressure distribution. After being coupled into the gas-filled HC-PCF, the pump laser pulses propagate along the fiber, where they undergo significant spectral broadening. The resulting broadband laser pulses 880 are then released from the gas chamber 840 via an output optical window 843. The broadband laser beam 880 is then collimated by a collimating lens 831 to a suitable beam size.

[0082] To fill the HC-PCF with the working gas, the gas chamber can be in communication with a pressurized gas supply or reservoir (not shown). The walls of the gas chamber and the inner surfaces of the windows enclose the cavity. The axis of the gas chamber is parallel to the axis of the HC-PCF.

[0083] The pump pulse duration can be selected to be greater than 10 fs, more specifically in the following ranges: 10 fs to 100 ps, 10 fs to 30 ps, or 10 fs to 1 ps. The pump wavelength can be selected from the visible light region, the near-IR light region, or the mid-IR light region. The pump laser pulse can have a repetition rate of several hundreds of Hertz (Hz), kilohertz (kHz), or megahertz (MHz). Specifically, the repetition rate can 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.

[0084] The alignment of the pump laser beam with respect to the HC-PCF can include two main steps, namely, coarse alignment and fine alignment. The coarse 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 correctly coupled into the hollow core of the HC-PCF and the transverse core mode is excited at the front (or input) facet of the HC-PCF. Without coarse 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 a low power level, the fine alignment at a high power level is started. Again, the purpose of the 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 be present 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 transmission efficiency is optimized, HOMs will always be excited.

[0085] The coarse alignment and the fine alignment (e.g., as part of the pump coupling optimization) can both be achieved by one or more of the following: 1) the movement of at least one beam steering component in the beam delivery system (in Figure 8and are represented as two steering mirrors in many of the subsequent figures, but this is just an exemplary beam delivery system); 2) the movement of the gas cell, and 3) the movement of the focusing lens (if available). One or more beam steering components or steering mirrors can be held by a moving mirror mount, which can be adjusted manually and / or electrically via an actuator such as a piezoelectric actuator. The mirror can be fixed by any suitable fixed arrangement, such as for example directly adhering a part of its rear surface to the mirror mount or by using fixing screws. The gas cell can be mounted on a stage module that includes one or more (e.g., piezoelectric) stages. The stage module can provide movement with multiple degrees of freedom, for example six degrees of freedom. Additionally or alternatively, the 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 can be used for pump coupling optimization of the methods described herein.

[0086] 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, with the lens properties such that the divergence and the diameter of the focused pump laser beam match well with the numerical aperture (NA) and the 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 different focusing lenses may be required to achieve optimal mode matching. Similarly, when the characteristics of the HC-PCF are fixed, different pump laser beams (e.g., having different beam diameters or beam divergences) may require different focusing lenses to keep the focused beam diameter the same.

[0087] The presence of HOMs degrades the mode quality and intensity stability of the broadband output. Inconsistencies in mode purity across the entire output spectrum can lead to wavelength-dependent output performance, which will have a negative impact on the reliability and reproducibility of measurement data if such a broadband light source is used in any of the above-mentioned measurement tools. Typically, HOMs are triggered when one or more pump coupling conditions are not optimized. In some scenarios where the mode purity of the broadband light source is fully optimized across the entire output spectrum range, HOMs may still appear at the output after a certain amount of operating time. The appearance of HOMs during operation is caused by degradation of the pump coupling conditions due to, for example, thermal and / or vibration drifts.

[0088] Return to reference Figure 8In an exemplary setup of a broadband light source, a small amount of incident pump laser beam can leak through the reflective surface of a beam steering component such as a steering mirror and impinge on the component mounting mechanism. Such pump leakage can heat the mounting mechanism and change its conditions. 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 causes a misalignment of the pump laser beam relative to the HC-PCF. Since the HC-PCF-based broadband light source is sensitive to alignment, the misalignment of the pump laser beam relative to the HC-PCF may significantly degrade the coupling condition of the pump laser beam into the fiber core, resulting in a reduction in the mode purity of the generated HOM and / or broadband output. The misalignment and / or alignment change of the pump laser beam may also be caused by other factors such as environmental temperature oscillation or external vibration. In a similar manner, the thermal and / or vibration drift of the HC-PCF also results in the emergence of HOM and / or a reduction in the mode purity of the broadband output.

[0089] Therefore, it is highly desirable to optimize and / or stabilize the broadband output during the operation of the broadband light source, particularly the LP 01 mode. Accordingly, a mode control method and device are proposed to address the above-mentioned mode purity issues associated with PCF-based broadband light sources.

[0090] Figure 9 The operation procedure of the proposed mode control system according to an embodiment is illustrated. In step 910, one or more parameters of the broadband output beam are measured. Such one or more beam parameters (i.e., the parameters of the broadband output beam) indicate the output performance regarding the fundamental mode purity. In step 920, the measurement 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 largely depend on the monitored beam parameter(s) and / or the hardware setup, and more detailed examples of these steps will be described below. Based on the result of this 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 condition such that the mode purity with respect to the fundamental transverse mode LP 01 is maximized.

[0091] The optimization of the pump coupling conditions can be achieved in a variety of ways. Any method of improving the coupling of the pump laser into the PCF can be used and can be achieved by moving the pump beam relative to the PCF (e.g., via a beam steering component 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 component (such as a focusing component). This can be performed while monitoring the reference beam (and the output beam) to ensure that the mode purity is optimized. Thus, the method can be implemented in closed-loop operation such that the transverse mode variations of the broadband output can be continuously monitored and optimized. Depending on the type of beam parameters being monitored, different detection mechanisms can be used, each of which may require one or more different measurement devices or components.

[0092] Figure 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 the exemplary light source 800 illustrated in Figure 8 wherein the labels 811 in Figure 10 1011 in Figure 11 1111 in

[0093] etc. all describe the pump laser beam). For the sake of brevity, equivalent components and features may be provided with similar reference numerals in the remaining figures rather than being described individually (e.g., Figure 10 As illustrated, a portion of the main broadband output beam 1080 is reflected from the front surface of a beam splitter 1051 (which may optionally be included within the detection unit 1050) and is used as the reference beam 1081. It should be noted that the beam splitter 1051 should not cause any spatial and spectral distortion to the reference beam such 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 includes 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 result of the evaluation, a control signal is generated and used by the control unit 1070 to correspondingly control one or more beam control system components. Such beam control system components can include, for example, one or more beam delivery or beam steering components (such as steering mirrors 1020, 1021 or their actuators), the actuator of the movable gas cell 1040 or the workbench, the actuator for moving the (optional) focusing lens 1030, and / or an actuator, such as a rotating polarizer that changes the absolute polarization angle (for example, in the case of maintaining polarization in an HC-PCF and a certain absolute orientation of the polarization of the desired light). The data measurement and evaluation can be performed in a continuous or periodic manner.

[0094] In a first embodiment, the detection unit includes a bandpass filter 1052 and an irradiation measurement device (such as a power measurement device 1053, e.g., a power meter), and the power measurement device 1053 is arranged such that the power measurement 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, with 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, such that when the power measurement in a specific spectral range (which corresponds to Figure 9 step 910 of the method) is completed, the filter wheel can be rotated to enable a different bandpass filter. Other arrangements for obtaining variable bandpass characteristics can be envisioned, including, for example, providing a series of movable high-pass and movable low-pass continuously variable filters.

[0095] The measured power value from the power measurement device 1053 and the spectral information of the (multiple) bandpass filters 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 the energy spectral density value (which corresponds to Figure 9 step 920 of the method). The calculated PSD value is subsequently evaluated in the processing unit 1060 by following a certain set of criteria (which corresponds to Figure 9 step 930 of the method). The processing unit 1060 can include a processor configured to process the measured data and then perform an evaluation on the processed data. Alternatively or additionally, the evaluation can be directly performed on the measured power values.

[0096] During data evaluation, the calculated PSD values can be compared with reference PSD values within the corresponding spectral range, and based on the comparison, a set of deviation values is generated; where the deviation values include a measure of the degree of deviation of the calculated PSD values relative to the reference PSD values. The reference PSD values can correspond to the mode purity of the broadband output beam indicating the optimal output. The set of deviation values can be evaluated against a predefined set of deviation thresholds for the corresponding spectral range to determine whether the pump coupling conditions of the broadband light source are acceptable. The predefined deviation thresholds can be set as percentages within, for example, the range of 5% to 25% of the reference PSD value (e.g., 5%, 10%, 15%, or 20% of the reference PSD value). If the deviation values indicate unacceptable pump coupling conditions such that the mode purity of the broadband output is suboptimal, a control signal is generated and / or changed accordingly (which corresponds to Figure 9 step 940 of the method of Figure 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

[0097] step 950 of the method of

[0098] Several specific and purely exemplary methods for optimizing the pump coupling conditions will now be described, where the control unit 1070 (or the 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 platform plane, and the vertical direction is defined as perpendicular to the platform plane. This scanning implementation applies to all embodiments described herein, although the (multiple) beam parameters being monitored may be different for later embodiments (as will be apparent).

[0099] In the above scanning routine, the two mirrors are decoupled. When one mirror is scanning, the other mirror is assumed to be maintained at the optimal position. Therefore, when both mirrors drift from their respective optimal positions, this arrangement is not ideal. Thus, in a second scanning embodiment, co-optimization of two or more beam steering components (e.g., mirrors 1020, 1021) can be performed. This particular embodiment may include an incremental movement of a first mirror (e.g., mirror 1020) 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 point it makes an incremental movement 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 scan process is finished. Thus, a power map and / or a calculated PSD map will be generated by the second mirror for each position of the first mirror, and thus, the optimal mirror positions determined in this way should be more accurate.

[0100] 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 implemented by a stage module and can include lateral and / or angular movement in one or more directions. Additionally, 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.

[0101] In another embodiment, a component is provided that includes 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 includes a control element configured to stabilize the matching condition between the optical element and the receiving element by adjusting the modification of the received radiation or the distance between the optical element and the receiving element according to the nature of the gas environment.

[0102] In an example, the optical element can be the focusing lens 1030, and the receiving element is a non-linear optical element, such as a non-linear crystal or a hollow-core fiber HC-PCF 1041. The radiation can be generated by a (monochromatic) pump laser, focused (modified) by the optical element 1030, and received by the receiving element 1041 enclosed within the gas chamber 1040. The gas chamber 1040 can be configured to allow adjustment of certain properties of the gas environment, such as pressure, temperature, and / or gas composition. Generally, the adjustment of the gas environment is aimed at obtaining the desired response of the non-linear optical process that generates broadband light, for example, adjusting the wavelength spectrum of the broadband light. However, changes in the properties (usually the temperature and pressure of the gas environment) can have a direct impact on the matching conditions between the optical element 1030 and the receiving element 1041, such as the focusing matching conditions associated with the efficiency of coupling the radiation into the HC-PCF 1041. If the properties of the gas environment are pressure and temperature changes, the underlying physical principles allow the estimation of the corresponding changes in the focusing matching conditions. According to Equation 1 (EQ[1]), the refractive index 'n' of the gas depends on the pressure P and temperature T of the gas: P0 and T0 are reference pressure and temperature values, C1, C2, B1, B2 are the Sellmeier coefficients of the gas, and is the wavelength of the pump laser.

[0103] The change in the focusing position of the pump laser relative to the entrance of the fiber 1041 due to the change in the refractive index can be determined by a basic optical analysis of the components (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 xenon filling pressure 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 focusing offset of 20 to 30um.

[0104] As described, the focusing change caused by pressure and / or temperature changes can be detrimental to the coupling efficiency of the pump laser into the fiber 1041. This can translate into a reduction in the power of the broadband light generated within the fiber 1041. Therefore, it is desirable to provide a focusing control component, for example, by incorporating a control element, such as an actuator, to provide focusing control based on the available information about the properties of the gas environment.

[0105] The optical element 1030 and / or the gas chamber 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 chamber 1040 may be moved by a control element along the optical axis (longitudinal direction) of the receiving element HC-PCF 1041 within a certain range. 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.

[0106] It is also proposed to periodically measure the pressure and / or temperature of the gas by a sensing component, determine the corresponding change in the gas refractive index, and subsequently determine the change in the focusing position of the pump laser relative to the entrance of the receiving element 1041 due to the said change in refractive index.

[0107] In an embodiment, the assembly includes a focusing control system that uses the pressure and / or temperature measurement values as inputs and outputs values corresponding to control signals for one or more control elements (actuators) coupled to the focusing lens 1030 and / or the gas chamber 1040. The control signal may be configured to provide a change in the focusing position that at least partially compensates for the determined change in the focusing position of the pump laser relative to the entrance of the optical fiber 1041. One or more actuators may move the focusing lens 1030 and / or the gas chamber 1040 along the optical axis. One or more actuators may position the lenses (elements) included within the focusing lens 1030, such as to cause a change in the focusing position. One or more actuators may deform the optical surface or lenses (elements) included within the focusing lens 1030, thereby causing a change in the focusing position. Alternatively, one or more actuators may control the position and / or optical power of additional optical elements (not shown) in the optical path upstream of the gas chamber to cause a change in the focusing position.

[0108] The focusing control system may include a function for determining the change in the refractive index of the gas based on the said pressure and / or temperature measurement values, reference values of pressure and / or temperature, the wavelength of the pump laser, and the composition of the gas and / or the Sellmeier coefficients corresponding to the gas.

[0109] The focusing control system may include a functionality for determining the change in the focusing position of the pump laser relative to the entrance of the optical fiber 1041 based on the determined change in the refractive index of the gas and the knowledge of the properties and positions of the optical elements used to couple the pump laser into the optical fiber 1041.

[0110] The focusing control system described herein allows for automatic focusing adjustment in response to changing conditions of the gas (e.g., temperature, pressure, gas composition), ensuring efficient coupling of the pump laser into the optical fiber 1041. Thus, the focusing control system increases the stability of the power of the broadband light delivered by the optical fiber 1041.

[0111] Alternatively, the power of the broadband light is periodically measured at the output of the optical fiber 1041 and used as an alternative or additional 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 fiber output and the power at the fiber input is determined. The determined ratio can be used to provide a control signal for one or more actuators.

[0112] Figure 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, the reference beam 1181 is directed by the beam splitter 1151 to the broadband output beam. The spatial filter 1152 is configured to remove the HOM content of the broadband output such 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 not directed to the power meter for power measurement. In a similar manner, a pinhole with a carefully selected size only allows the fundamental transverse mode to be transmitted, thereby effectively removing the HOM of the output beam.

[0113] After being spatially filtered, the power in the fundamental transverse mode LP 01 is measured by the 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 (such as described above) to optimize the pump coupling conditions such that the output power of the fundamental transverse mode increases sufficiently (above the threshold), indicating an improvement in the output mode purity.

[0114] It should be noted that the power decrease may be caused in part by thermal and / or vibration drift of the collimating lens 1131 and / or other downstream optical components (such as Figure 11 the beam splitter 1151 shown). Therefore, one or more beam alignment measurement devices (not shown) in the detection unit 1150 continuously or intermittently monitor the positions of the collimated output beam 1180 and the reference beam 1181. When the positions of the collimated output beam 1180 and / or the reference beam 1181 are confirmed to have drifted, the position of the spatial filter 1152 can be optimized accordingly to compensate for the drift.

[0115] According to the third embodiment, as Figure 12 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 (individually 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 evaluated measured beam ellipticity is greater than a pre-defined ellipticity threshold, the mode purity confirmation of the broadband output beam 1280 is sub-optimal. For example, the ellipticity threshold can be set to a value within the range between 1.04 and 1.20. Alternatively or additionally, the evaluation can include comparing the measured beam diameter with a reference value of a collimated Gaussian beam calculated using the 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 as sub-optimal. In an embodiment, both the size and ellipticity are measured and evaluated against the respective thresholds, because one of these parameters does not always fully indicate the mode purity alone. Alternatively or additionally, this method can monitor the Laguerre-Gaussian mode shape of the beam and fit these to Laguerre-Gaussian polynomials indicating mode purity (or others). Alternatively or additionally, the Zernike polynomial shape can be monitored and fitted in a similar manner. Once sub-optimal mode purity is confirmed, a control signal is generated and / or changed by the processing unit 1260 and sent to the control unit 1270 for the pump coupling optimization routine.

[0116] In an alternative embodiment, an optical lens 1252 can be included within the detection unit to image the end face of the HC-PCF 1241 onto the beam shape measurement device 1253. Compared with the above example of the third embodiment where the far-field distribution of the output mode is evaluated, this example instead uses the near-field distribution of the HC-PCF output for mode evaluation. Similarly, the ellipticity and diameter of the near-field distribution are evaluated based on theoretical and / or empirical values. In some cases, the empirical values obtained through experiments may be more reliable.

[0117] According to the fourth embodiment, as Figure 13As illustrated, the detection unit 1350 includes a spectral measurement arrangement; specifically: a multimode optical fiber 1352 and a spectral measurement device (such as a spectrometer or a spectral analyzer) 1353. One end of the multimode optical fiber 1352 is placed in the beam path and is used to receive at least a part 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 facet 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.

[0118] Similar to the first embodiment where the measured spectral parameter value (in that case the PSD value) is compared with a reference value within the corresponding spectral range, in this embodiment, the spectral parameter value (such as the measured spectrum) can be compared with a reference spectrum, which can be obtained 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.

[0119] In another embodiment, as Figure 14 illustrated, the radiation 1481 leaking through the fiber cladding is collected, for example, by the multimode optical fiber 1452. This radiation 1481 can be collected only from a section of the HC-PCF 1441, such as at or near the end section of the HC-PCF 1441 (such as at or near the output end), where the outer coating can be stripped. Then the collected leaked radiation 1481 is guided to the spectral measurement device 1453 for spectral measurement. The higher-order optical fiber core modes in the HC-PCF will experience higher confinement losses when propagating along the fiber than the fundamental LP 01 core mode. Therefore, in the case where the mode purity is reduced or sub-optimal, 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 (for example, by comparing with a threshold). If the mode purity is proven to be sub-optimal, a control signal will be generated by the processing unit and a pump-coupling optimization routine will be activated by the control unit.

[0120] In addition to the 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 directly measured 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.

[0121] The above pump coupling optimization method mainly relates to the fine alignment aspect of the pump laser beam relative to the alignment of the HC-PCF. The improvement mainly for rough alignment will now be described. It should be noted that the solution space of the above pump coupling optimization method may overlap with the improvement discussed below. For example, the pump coupling optimization method discussed above may also have the benefit of rough alignment, and the improvement discussed below can also provide some fine alignment. The proposed method and apparatus use one or more suitable detectors (such as photodiodes and / or lensometers) to monitor the HC-PCF cladding, and more specifically, to monitor the light radially escaping from the HC-PCF. The concept in this embodiment is similar to the concept of Figure 14 the fine alignment (mode purity evaluation) described. It can be understood that the method of any of the foregoing embodiments can be used independently of or in combination with any of the embodiments disclosed above. In the latter case, any of the foregoing embodiments is used for the initial rough alignment, and any of the foregoing embodiments is subsequently (i.e., when the rough alignment is achieved) used for the fine alignment. Note that the rough alignment discussed can also be used in a scenario where the beam must be aligned relative to the solid core of the photonic crystal fiber, which has a cladding region around the fiber core of the solid material.

[0122] Figure 15 (a) schematically depicts a rough alignment arrangement CA according to such an embodiment. The rough alignment arrangement CA includes an optical element, which is a positive lens POL in this example, to focus the beam LB onto the input surface INS of the HC-PCF. Similar to the Figure 7 example of, the HC-PCF has a (hollow) fiber core FCO and an internal cladding waveguide structure (fiber cladding CLA) around the fiber core FCO. The input surface INS defines one end of the HC-PCF and is configured to receive the beam LB so as to couple at least a portion of the beam LB into the fiber core FCO.

[0123] Note that the optical element can be any type of optical element and need not be limited to a positive lens POL. For example, the optical element can be an off-axis paraboloidal mirror. The coarse alignment arrangement CA also includes a detector, such as a photoelectric sensor PHS, disposed near or on the fiber cladding CLA of the HC-PCF. The photoelectric sensor PHS is implemented here as a photodiode, but can be any other type of light or other electromagnetic energy sensor. The photoelectric sensor PHS is arranged such that it can receive light from the light beam LB, which is coupled into the fiber cladding CLA at the input surface INS. The photoelectric sensor PHS is also configured to output a signal SI representative of the amount of light received by the photoelectric sensor, such that the output signal SI represents the amount of light coupled into the fiber cladding CLA. Thus, the photoelectric sensor measures the light scattered from the fiber cladding CLA due to the misalignment of the light beam LB and the fiber core FCO. Optionally, there can be an optical filter before the photoelectric sensor PHS to filter out unwanted wavelengths / polarizations and / or reduce the amount of light (e.g., within the dynamic range of the diode).

[0124] Figure 15 (b) The coarse alignment arrangement CA of (a) is depicted in more detail by omitting the optical element POL and focusing on the free end of the HC-PCF Figure 15 (a), where the input surface INS receives the light beam LB. In Figure 15 (b), it can be clearly seen that the light beam LB is not perfectly 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.

[0125] In prior art systems, the misalignment between the light beam LB and the fiber core FCO is typically measured by fiber tapping, where the amount of light coupled into the fiber core FCO is measured by altering (e.g., damaging) the HC-PCF to tap a portion of the light trapped inside the fiber core FCO and directing 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.

[0126] In this embodiment, it is proposed to measure the light coupled into the fiber cladding CLA rather than the light coupled into the fiber core. In Figure 15To the right of (b), the input surface INS is shown with the light beam LB on the left side of the HC-PCF. Below the HC-PCF, a graph of the output signal In as the light beam LB moves from the left side to the right side of the HC-PCF along the X direction is shown. Note here that similar graphs can be obtained by moving in other degrees of freedom. The light beam LB is shown as mostly starting to impinge outside the fiber cladding CLA, corresponding to the alignment position AL1 (a detailed illustration of the respective arrangement of the light beam LB, the fiber core FCO, and the fiber cladding CLA is directly shown below each value). At the alignment position Al2, the light beam LB mostly impinges on the fiber cladding CLA, resulting in an increase in the value of the output signal In. Subsequently, the light beam LB will impinge on the fiber core FCO, resulting in a decrease in the value of the output signal In; this corresponds to the optimal alignment position AL B Finally, the light beam LB will be impinged on the fiber cladding CLA again, resulting in an increase in the value of the output signal In at the alignment position AL3. Thus, when the light beam LB is maximally coupled into the fiber cladding CLA, when the output signal In is at the minimum value In max1 between the first maximum value In max2 and the second maximum value In min the optimal alignment between the light beam LB and the fiber core FCO is obtained.

[0127] Note here that the minimum value In min is not necessarily a zero value. A non-zero value for the minimum value In min is very likely or even highly likely, especially for the rough alignment stage, for example because there may always be a certain level of light scattered from the core in practice. The signal In min can also be used to monitor the system and / or alignment and / or the health of the fiber during the operation of a broadband light source.

[0128] It should also be understood that any scanning is in fact a two-dimensional scan on the input surface INS. Therefore, the model will be Figure 15 the three-dimensional model illustrated in (c), which shows a three-dimensional model similar to Figure 15 the two-dimensional graph in (b) and its three different offset cross-sections (here the maximum values are shown as equal in all directions, although this may not be the case, as Figure 15 shown in (b)). Thus, the scanning algorithm will aim to find the position corresponding to the common minimum between the maximum values in all directions on the X / Y plane.

[0129] The rough alignment strategy can operate in a feedback loop based on the measured output signal and the control signal of the controller, which controls the position of the beam relative to the input facet of the HC-PCF in order to find this minimum value In minThis method can 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 can include a spiral scan to find a region bounded by a maximum value (e.g., forming an annular region), and find the minimum value within that region.

[0130] Figure 16 Schematically depicts a further rough alignment arrangement CA. Figure 16 Side views and front views of the HC-PCF and the optoelectronic sensors PHS (or other detectors) arranged around the HC-PCF are shown on the left and right, respectively. The HC-PCF includes a hollow fiber core and a fiber cladding around the fiber core, as depicted in other figures but not explicitly shown here. The HC-PCF also includes 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 fiber core. The optoelectronic sensor PHS is arranged to receive light from the beam coupled into the fiber cladding at the input surface INS, where the optoelectronic sensor is configured to output a signal representative of the amount of light received by the optoelectronic sensor.

[0131] The optoelectronic sensor PHS can have a plurality of individual regions that are capable of detecting the amount of light falling on the plurality of individual regions separately. If such an optoelectronic sensor PHS having a plurality of individual regions is arranged around the HC-PCF, information about the direction in which a specific amount of light is output from the fiber cladding can be obtained. This direction information can be used to steer the alignment of the beam to a direction depending on the detected direction information.

[0132] In Figure 16 the embodiment, the optoelectronic sensor PHS is arranged on the fiber cladding close to the input surface INS. The optoelectronic sensor PHS extends an angle α in the radial direction RAD of the HC-PCF around the longitudinal axis LAF of the HC-PCF, which in this case is greater than 180 degrees, even greater than 270 degrees, and almost 360 degrees. Such an embodiment can be advantageously used to increase the signal-to-noise ratio because for the increased angle α, more light is received by the optoelectronic sensor PHS.

[0133] Figure 17 Schematically depicts another rough alignment arrangement CA. Figure 17 The rough alignment arrangement of Figure 16 is similar to the arrangement of Figure 16In an embodiment, the coarse alignment arrangement CA includes two optoelectronic sensors, namely, a first optoelectronic sensor PHS1 and a second optoelectronic sensor PHS2, which have the same functions respectively. The first optoelectronic sensor PHS1 and the second optoelectronic sensor PHS2 are arranged to be substantially evenly distributed in the radial direction RAD along the circumference of the HC-PCF, where each optoelectronic sensor extends an angle α and β respectively in the radial direction RAD of the HC-PCF around the longitudinal axis LAF of the HC-PCF, and these angles are less than 180 degrees, but preferably greater than 90 degrees.

[0134] The first optoelectronic sensor PHS1 provides a first output signal In1, and the second optoelectronic sensor PHS2 provides a second output signal In2. The combination of the signals In1 and In2 can be used in a similar manner to the output signal In of the optoelectronic sensor PHS in the embodiment of Figure 15 and 16 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 between 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 fiber core.

[0135] Although not shown, it is conceivable that in an embodiment, three or more optoelectronic sensors can be provided and are substantially evenly distributed radially along the circumference of the fiber, where each optoelectronic sensor extends an angle less than 360 / n degrees in the radial direction of the fiber around the longitudinal axis of the fiber, where n is the number of optoelectronic sensors, for example, less than 120 degrees in the case of three optoelectronic sensors.

[0136] In Figure 17 In a variant of the depicted arrangement, the second optoelectronic sensor PHS2 can be replaced by a mirror element that is only used in combination with a single optoelectronic sensor PHS1. The mirror element reflects the light coupled into the fiber cladding at the input surface towards the optoelectronic sensor PHS1. As a result, the signal-to-noise ratio can be improved. The mirror element does not have to be arranged on the fiber, but can instead be arranged at a certain distance around the fiber. The mirror element can also be accompanied by one or more other mirror elements, all of which are configured to reflect the light coupled into the fiber cladding towards the optoelectronic sensor PHS1.

[0137] The advantage of this coarse alignment arrangement is that light can be coupled into the core of the fiber after misalignment, for example, due to component replacement or drift, and this coupling can be done in-line without changing or disconnecting the system.

[0138] Figure 18Schematically depicts an optical system OS having a particular beam steering arrangement or optical manipulation unit OMU, which can be used to effect actual control of a beam on an input facet in a coarse alignment arrangement and / or in any of the fine alignment embodiments disclosed herein.

[0139] The optical system OS includes a light source LIS and an optical manipulation unit OMU. The light source LIS provides a beam LB to the optical manipulation unit OMU using an optical fiber HC-PCF and an output connector OC, which may include a collimator to provide a collimated beam LB to the optical manipulation unit OMU. The light source LIS can be a white light source or a supercontinuum source.

[0140] The optical manipulation unit OMU includes an input device ID configured to receive the output connector OC. It is to be understood that the input device ID and the output connector OC have been depicted highly schematically, but these two components can include features that allow the output connector OC to be releasably but rigidly connected to the input device ID, thus allowing replacement of the output connector OC or disconnection of the output connector OC and subsequent reconnection of the output connector OC.

[0141] The optical manipulation unit OMU also includes one or more optical elements configured to manipulate the beam LB. Depicted as an example of such an optical element in Figure 18 is a mirror MI that directs the beam to a filter unit FU configured to filter the beam passing through the filter unit FU. Filtering can include spectral filtering, polarization filtering, and / or overall attenuation of the beam.

[0142] It is noted here that the presence of the filter unit does not mean that other optical elements may not also have a filtering function, such as in the form of a reflective or transmissive bandpass filter. Thus, the mirror MI can have such an alternative or additional filtering function.

[0143] In this embodiment, a beam tilt adjuster TA is provided downstream of the filter unit FU to adjust the propagation direction of the beam LB. The beam tilt adjuster TA includes a first wedge prism WP1 and a second wedge prism WP2 arranged in series, where each wedge prism WP1, WP2 includes a respective tilt actuator A1, A2 to rotate the corresponding wedge prism WP1, WP2 about their respective optical axes, which extend mainly along the X direction in Figure 18 . The tilt actuators A1, A2 are part of a tilt actuating system.

[0144] In this embodiment, disposed downstream of the beam tilt adjuster TA, a beam displacement device DD is provided to displace the beam LB. The beam displacement device DD includes a parallel plane plate PP and a displacement actuation system that rotates the parallel plane plate PP about a first axis using an actuator A3 and about a second axis using an actuator A4, the first axis and the second axis being substantially perpendicular to each other and to the propagation direction of the beam. Since the propagation direction of the beam LB is substantially in the X direction, the first axis can, for example, be substantially parallel to the Y direction, and the second axis can, for example, be substantially parallel to the Z direction.

[0145] The optical manipulation unit OMU further includes a control unit CU connected to the tilt actuation system (A1, A2) and the displacement actuation system (A3, A4) to adjust the propagation direction of the beam and displace the beam to direct the 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 can include coupling means to couple 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 can thus include features that allow them to be rigidly and possibly releasably interconnected, thereby allowing the input connector INC to be replaced or disconnected and reconnected with a new or different input connector INC, for example for maintenance.

[0146] As in this example, the optical fiber HC-PCF can 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 can 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 beam in the optical fiber HC-PCF, which is a measure of the amount of light coupled into the optical fiber through the coupling means of the input connector INC.

[0147] Accordingly, the light intensity of the 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 beam is received by the input connector INC. In an embodiment, the control unit CU can be configured to control the tilt actuation system such that the beam moves along a spiral pattern, in this case in the Z-Y plane, in order to find a first estimate of the desired propagation direction of the beam to be received by the input connector INC. Subsequently, the tilt actuation system and / or the displacement actuation system are controlled to move the manipulated beam around the first estimate in order to find an improved first estimate of the desired propagation direction and / or displacement of the beam. Moving the beam along a spiral pattern can be achieved by rotating 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 herein is applied for fine alignment. Alternatively, the first estimate can be related to rough alignment and the second estimate can be related to final alignment. In the latter example, the fine alignment can rely on one of the other measurement strategies described herein.

[0148] According to another embodiment, a timing control system is provided that is configured to control the timing of pump laser pulses and / or broadband output pulses of an HC-PCF-based broadband light source. Timing control of laser pulses is generally desirable in applications that require precise temporal positioning of the laser pulses relative to a timing reference and is typically achieved using techniques based on, for example, a microprocessor or a microcontroller. 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 an HC-PCF-based broadband light source, the timing of the pump laser pulses, the broadband output pulses, and the generated signals from one or more optical clients (such as optical sensors) can be determined and / or synchronized. However, there are many technical challenges or drawbacks in using a prior art timing control system to control the timing of an HC-PCF-based broadband light source. First, an HC-PCF-based light source includes multiple components that are typically located far apart from each other. For example, a pump laser (which typically includes 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 more than 10 meters away. Communication between such components (such as communication between the seed laser and the supercontinuum fiber) can result in non-negligible time delays. Second, due to its complex optical architecture (such as an optical system including a seed laser, a preamplifier, a pulse stretcher, a power booster, and a pulse compressor), the timing of the pump laser is complex. Thus, each component of the pump laser can affect the timing of the pump laser pulses. The timing of the broadband output pulses is also complex due to different wavelengths having different timings due to intrapulse group delay dispersion. These complex timings impose stringent requirements on the performance of the timing control system (such as timing accuracy) and are typically met with complex control architectures. In addition, the generated signals from each optical client in an optical client (such as an optical sensor) need to be processed before being used by other control units for other components. To meet this requirement, prior art timing control systems are typically equipped with signal processing functionality, resulting in a more complex control architecture.

[0149] The above drawbacks make it difficult to accurately predict and / or model the time delay between any two components, such as between a seed laser and one of the optical clients for monitoring the supercontinuum output. Although the time delay determined by the timing control system can be calibrated based on measurement data so that the timing error can be corrected, this method is not practical for HC-PCF-based light sources because their laser pulses are too short to be accurately measured. To achieve 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, along with complex microprocessor-based hardware, make the entire timing control system too complex and expensive. Since firmware- or software-based control systems are more error-prone, it takes a significant amount of time and effort to make them robust.

[0150] According to various aspects of the present disclosure, the embodiments described below provide a better solution to the above problems. Compared with the prior art timing control systems, a significant advantage of the following embodiments is the prevention of the use of microprocessors or similar technologies.

[0151] Figure 19 Schematically illustrates a timing control system configured for the timing control of an HC-PCF-based broadband light source according to an embodiment. Referring to Figure 19 , the timing control system may include a pressure sensor 1944 (such as a very high-speed pressure sensor) configured to detect or monitor pressure changes in the gas chamber 1940. In this embodiment, the pressure sensor 1944 may be held near the output end of the HC-PCF 1941 by a support structure 1945 that is connected to the inner side of the gas chamber wall 1946. In some alternative embodiments, the pressure sensor 1944 may be directly mounted on the inner side of the gas chamber wall 1946. The gas sensor 1944 may communicate with an external device via one or more signal cables that pass through the gas chamber wall 1946 in a sealed manner or are connected to an external cable via a feedthrough connector.

[0152] Continuing to refer to Figure 19, the pump laser beam 1911 including the pump laser pulse train 1912 is focused by a focusing lens 1930. The focused pump laser beam 1911 passes through the input window 1942 of the gas cell 1940 before being coupled into the core of the HC-PCF 1941. When propagating along the optical fiber, each pump laser pulse 1912 is spectrally broadened into a broadband output pulse 1982 via the above-described 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., spectral broadening) 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 leaving the output window 1943 of the gas cell 1940. When entering the gas cell 1940, the broadband output pulse 1982 generates a pressure wave within the gas cell 1940. The amplitude of this 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 may 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 may be sent to the processing units 1060, 1160, 1260, 1360, 1460 and / or the control units 1070, 1170, 1270, 1370, 1470 of the broadband light source, such that different functionalities may be achieved, such as outputting a train of broadband pulses.

[0153] It should be noted that when the pump laser pulse enters the gas cell 1940 before being coupled into the HC-PCF 1941, a pressure wave is also generated. Thus, in some embodiments, the timing of the pump laser pulse 1911 entering the gas cell 1940 may also be determined using the same pressure sensor 1944 and / or an additional pressure sensor (not shown) located, for example, near the input window 1942.

[0154] 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.

[0155] It is understandable that the above Figure 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).

[0156] For some metrology or inspection tools, such as the scatterometry-based metrology tools described above, the performance of the metrology tool may be strongly affected by the polarization properties of the illumination radiation of the tool. Such polarization properties may include, among others, the polarization extinction ratio (PER) or polarization quality, polarization stability, and orientation of predominantly linearly polarized light. PER is defined as the ratio of the powers of two perpendicular polarizations, commonly referred to as transverse electric (TE) and transverse magnetic (TM), and is often used to characterize the quality of linear polarization. Polarization stability is used to characterize how stable the polarization state can be maintained over time. Due to component aging and / or movement, the polarization of the illumination radiation may change over time, resulting in polarization rotation and / or PER degradation. If an illumination beam with a poor PER is used in a polarization-sensitive metrology tool (e.g., a scatterometer), optical power in the unwanted polarization direction will not contribute to the measurement and may even cause background scattering, thereby reducing the detection signal-to-noise ratio (SNR). Moreover, since only the optical power in the desired polarization direction is used for measurement, the power efficiency of the metrology 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.

[0157] When a broadband light source based on HC-PCF (such as Figures 10 to 14When those described in are 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 fabricating a perfect and straight HC-PCF without stress during installation, the PER of the broadband output beam is expected not to depend on the input (pump) polarization direction. However, it has been found that when coupling low pump power through an 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, effectively making the fiber 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 stress caused by installation. 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 a fast axis and a slow axis) and experience different attenuations. When the polarization direction of the pump laser beam matches the fast axis or the 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 axis or the slow axis of the fiber, the PER of the transmitted pump laser beam is reduced.

[0158] Therefore, in order to obtain a linearly polarized broadband output beam with a high PER, it is necessary to accurately align the polarization direction of the pump laser beam 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. The control / optimization of the polarization direction of the pump laser beam is typically achieved using a polarization control device, such as a half-wave plate (HWP), at a low power level, which can be placed in the beam path of the pump laser before coupling into 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 propagating 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. Additionally, custom-designed tools can also be used to access and / or control certain components, such as the HWP in a packaged laser product. Although the axis orientation of the HC-PCF can be pre-characterized and this information can be used to align the absolute rotation of the optical fiber, any change in the pump polarization may still result in a decrease 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, which is 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 increases the cost and limits the footprint of the laser product.

[0159] According to various aspects of the present disclosure, a method for solving the above problems is proposed. The proposed method is based on the following discovery: when a linearly polarized radiation beam propagates through a waveguide structure having an asymmetric modal refractive index distribution, such as an optical fiber having a (slightly) elliptical cross-section, the PER and the output power of the output beam are closely related. Therefore, the power measured at the output of the optical fiber 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 output of the optical fiber is known, then the power measured at the output of the optical fiber can also be used to determine the angular offset between the polarization direction of the pump beam and the preferred fiber axis at the input of the optical fiber. Since a power measurement device is the most common diagnostic device and is much cheaper than a polarimeter, this method is superior to the prior art methods because it does not require a polarimeter.

[0160] The correlation between the output power and the PER can be explained by the structural characteristics of the HC-PCF: the modal index and attenuation of the fundamental guided mode (LP 01 ) can be approximated as follows:

[0161]

[0162] where u 01 is the first zero of the Bessel function of the first kind J0, λ is the wavelength, and D is the inner core diameter. Thus, the mode purity can also be monitored in this way.

[0163] Figure 20 Schematically illustrated is a polarization control system configured for polarization control of an HC-PCF-based broadband light source according to different embodiments. Referring Figure 20 , the polarization control system may include an HWP 2029 for controlling the polarization of the pump laser beam 2011. The HWP 2029 may be placed at any position between the pump laser 2010 and the input window 2042 of the gas cell 2040. In the case where the pump focusing lens 2030 is used to focus the pump laser beam 2011 into the core of the HC-PCF 2041, the HWP 2029 is preferably placed before the focusing lens 2030 because this can avoid the HWP 2029 being potentially damaged by the focused laser beam with a higher peak intensity.

[0164] In an embodiment, the polarization control system may further include a beam splitter 2051, which is placed on the beam path of the output beam 2080, preferably after the collimating lens 2031 to avoid potential damage. The beam splitter 2051 may separate 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 labeled as the reference beam 2081. The detection unit 2050 may include a power measurement device 2053, such as a power meter for measuring 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 should be noted that the influence of the beam splitter 2051 on the optical properties of the reference beam 2081 can be ignored rather than imposing a power splitting ratio on the output beam 2080.

[0165] In some embodiments, the polarization control system may operate in a high-power or high-energy optical regime where the pump laser power / energy is high and the pump pulse experiences significant spectral broadening after traversing the optical fiber. In some other embodiments, the polarization control system may operate in a low-power or low-energy optical regime where the pump laser power / energy is low and the pump pulse does not experience spectral broadening when traversing the optical fiber. When operating in the low-power optical regime, the output beam 2080 has substantially the same spectrum as the pump laser beam. Since the pump laser beam typically has a narrowband spectrum, spectral filtering may not be necessary, and thus the optical filter 2052 may not be used. In contrast, when operating in the high-power optical regime, the output beam 2080 has a broadband spectrum that is 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, and thus 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 such that the entire output beam 2080 is received by the power measurement device 2053.

[0166] Continuing to refer to Figure 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 then 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). Then, the processed power signal may 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 the 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 single processing and control unit (not shown) that performs all the tasks performed by the processing unit 2060 and the control unit 2070.

[0167] 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, such as component replacement and long-term polarization stability. The polarization control system can operate in two different routines. In the scenario where a critical component (such as the gas cell 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 fiber 2041. In some embodiments, this can be achieved by rotating the HWP 2029 through a full circle or 360º. 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. Depending on the resolution requirements, each incremental step can correspond to a small rotation angle, such as 1º per step, 2º per step, 5º per step or 10º per step.

[0168] Referring to Figure 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 rotating 360º in 5º (or 72 steps) step increments. After each rotation step, the power of the reference beam 2081 is measured by a power measurement device (such as 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, where 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 360º rotation with a step size of 5º, 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 via, for example, 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 to a PER-angle relationship, from which the optimal HWP position that can give the maximum PER (or minimum power) can be determined.

[0169] 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 can be continuously or intermittently measured / sampled by the power measurement device 2053. It should be noted that the power-angle relationship of the broadband light source is assumed to be available before long-term PER stability can be monitored and / or maintained. The power signal generated by the power measurement device 2053 can be processed by the processing unit 2060 before being sent to the control unit 2070, for example, averaged over multiple sampling points. Upon receiving the processed power signal, the control unit 2070 can compare the processed power signal with a predefined power threshold. If the power signal is higher than the power threshold, the control unit 2070 may not generate any control signal. However, if the power signal is lower than the power threshold, a control signal is generated. The control signal can command the HWP 2029 to perform a corresponding rotation task. In some embodiments, the rotation task can include a full 360° rotation of the HWP, such as the example described above. Alternatively, in other embodiments, the rotation task can include a small-angle rotation about the current angular position of the HWP 2029. When the PER of the output beam 2080 or the reference beam 2081 (as manifested by the measured power) is optimized, a new angular position of the HWP 2029 can be determined. Then the control unit 2070 can 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.

[0170] Since the power of the output beam 2080 or the reference beam 2081 can indicate the mode purity, the methods of this section can be used in the mode control systems described herein.

[0171] It should be noted that the number, location, and type of the power measurement device 2053 are not limited to those already described. In some embodiments, a'miniature' power measurement device can be placed near the output end of the HC-PCF fiber such 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 remains valid. In some embodiments, the miniature device can include a multimode fiber and a power meter. One or both of the gas cell 2040 or the HC-PCF 2041 can be mounted on a motorized rotary stage, and the HWP 2029 can 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 can 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.

[0172] It can be understood that the method of any of the foregoing embodiments can be used independently of or in combination with any other embodiment disclosed above. In the latter case, it is part of a mode control system and / or enables any of the foregoing embodiments to be used for initial alignment (coarse and / or fine). Additionally or alternatively, other embodiments can subsequently (i.e., when the light source is correctly aligned) be used for subsequent polarization optimization. When used independently, the foregoing embodiments are not limited to being applied to HC-PCF-based broadband light sources, which can be used to determine and / or optimize the relative angle between the polarization direction of the pump laser beam and the optical plane of the nominal cylindrical waveguide.

[0173] It should be noted that the configuration of the HC-PCF-based broadband light source is not limited to the specific arrangements illustrated or described, and different configurations can be implemented. For example, the pump lasers 1010, 1110, 1210, 1310, 1410 can be configured to output converging pump laser beams instead of the collimated pump laser beams 1011, 1111, 1211, 1311, 1411 whose waists have good mode matching with the fiber core. In this case, the focusing lenses 1030, 1130, 1230, 1330, 1430 are not required. The beam delivery system can be different from the specific examples of the two or more steering mirrors 1020, 1120, 1220, 1320, 1420, 1021, 1121, 1221, 1321, 1421 illustrated. Alternatively or additionally, at least one of these mirrors can include a curved surface and / or another focusing beam delivery component provided with a carefully selected radius of curvature (ROC) to form a mode-matched pump spot without using the focusing lenses 1030, 1130, 1230, 1330, 1430. According to different embodiments, the input optical windows 1042, 1142, 1242, 1342, 1442 can be replaced by the focusing lenses 1030, 1130, 1230, 1330, 1430 and / or the output optical windows 1043, 1143, 1243, 1343, 1443 can be replaced by the collimating lenses 1031, 1131, 1231, 1331, 1431. In this configuration, the distances between the two lenses and the two fiber ends are selected such that the mode matching condition is well maintained. The broadband light source configured in this way is more compact but less flexible. In another embodiment, the gas chambers 1040, 1140, 1240, 1340, 1440 can be composed of multiple sub-chambers; and the HC-PCFs 1041, 1141, 1241, 1341, 1441 can be partially or fully included in the sub-chambers. The beam splitters 1051, 1151, 1251, 1351, 1451 can be located outside the corresponding detection units 1050, 1150, 1250, 1350, 1450 instead of inside as illustrated. The processing units 1060, 1160, 1260, 1360, 1460 and the control units 1070, 1170, 1270, 1370, 1470 are not necessarily separate entities and can include a single unit with a single processor to perform the processing and control functions.

[0174] Although any one of the above embodiments is sufficient to independently perform mode optimization of a broadband light source, some embodiments can complement each other and thus can be combined to improve the overall performance of the mode control system. For example, in an embodiment, the beam shape measuring device 1253 used in the third embodiment can be added to or combined with the detection unit 1050 (including the bandpass filter 1052 and the power measuring 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 measuring device 1253, while the output PSD is measured by the power measuring device 1053. Since the power and / or spectral profile of the broadband light source may gradually degrade over time (e.g., due to the aging of the pump diode), even if the mode purity remains unchanged, the PSD value may correspondingly decrease. Therefore, adding the beam measuring device allows the mode control system to quickly verify whether any decreased PSD value is caused by mode degradation or power degradation caused by component aging, thus preventing the mode control system from entering an optimization dead loop if it is the latter. In addition, it is desirable to periodically update the reference PSD value to reflect the power degradation caused by component aging. The use of the beam measuring device 1053 ensures that the reference PSD value can be periodically maintained and updated when the mode purity is optimal.

[0175] Therefore, any one of two or more detection units 1050, 1150, 1250, 1350, 1450 or components therein can be used in combination to detect and evaluate the same reference beam or separate reference beams (e.g., generated on the output beam by a plurality of beam splitters).

[0176] Other embodiments are disclosed in the subsequent numbered clauses:

[0177] 1. A mode control system configured to control the output mode of a broadband light source including a photonic crystal fiber (PCF), the mode control system comprising:

[0178] At least one detection unit configured to measure one or more parameters of the radiation emitted from the broadband light source to generate measurement data; and

[0179] A processing unit configured to evaluate the mode purity of the radiation emitted from the broadband light source based on the measurement data;

[0180] Wherein 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 with respect to the fiber core of the photonic crystal fiber.

[0181] 2. The mode control system as defined in clause 1, wherein the one or more parameters of the output radiation include one or more parameters indicating the mode purity of the broadband light source.

[0182] 3. The mode control system as defined in item 1 or 2, wherein the radiation emitted from the broadband light source detected by the detection unit includes output radiation emitted from the output end of the photonic crystal fiber.

[0183] 4. The mode control system as defined in item 3, including a beam splitter, the beam splitter being positioned to separate a reference beam from the main output beam emitted by the photonic crystal fiber, and the output radiation detected by the detection unit includes the reference beam.

[0184] 5. The mode control system as defined in any one of the foregoing items, wherein the at least one detection unit includes a spectral measurement arrangement capable of operating to measure one or more spectral parameter values of the output radiation as the measurement data.

[0185] 6. The mode control system as defined in item 5, wherein the spectral parameter values include one or more parameters of the measured spectrum of the output radiation.

[0186] 7. The mode control system as defined in item 5 or 6, wherein the spectral measurement arrangement includes a spectral measurement device and a multimode optical fiber, the multimode optical fiber being capable of operating to guide at least part of the radiation emitted from the broadband light source to the spectral measurement device.

[0187] 8. The mode control system as defined in any one of the foregoing items, wherein the detection unit includes one or more band-pass filters; each of the one or more band-pass filters is capable of operating to select a corresponding spectral range of the output radiation, and an illumination measurement device is capable of operating to detect an illumination parameter indicating the power of the filtered radiation, and the measurement data includes and / or is derived from the illumination parameter indicating the power.

[0188] 9. The mode control system as defined in item 5 or 6, wherein the measurement data includes a power spectral density or an energy spectral density value within one or more spectral ranges derived from the illumination parameter indicating the power.

[0189] 10. The mode control system as defined in any one of the foregoing items, wherein the detection unit includes: a spatial filter capable of operating to filter out higher-order modes other than the fundamental mode from the output radiation; and an illumination measurement device capable of operating to detect an illumination parameter indicating the power of the filtered radiation, and the measurement data includes and / or is derived from the illumination parameter indicating the power.

[0190] 11. The mode control system as defined in item 10, wherein the spatial filter includes a single-mode optical fiber or a pinhole.

[0191] 12. The mode control system defined by any of the foregoing clauses further comprises:

[0192] A variable polarization arrangement that can be operated to controllably configure the polarization properties of the radiation emitted from the broadband light source;

[0193] Wherein the detection unit includes an irradiation measurement device that can be operated to detect an irradiation parameter indicating the power of the radiation emitted from the broadband light source according to the input polarization angle, the measurement data including and / or originating from the irradiation parameter indicating the power; and the mode control system can also be operated to optimize the one or more pump coupling conditions by changing the polarization angle applied by the variable polarizer relative to the fiber axis of the photonic crystal fiber, thereby optimizing the pump polarization conditions of the radiation emitted from the broadband light source.

[0194] 13. The mode control system defined by clause 12, wherein the variable polarization arrangement includes one or both of the following:

[0195] A variable polarizer that can be operated to change the polarization angle of the pump laser beam relative to the photonic crystal fiber, and / or

[0196] An actuator that can be operated to change the angle of the photonic crystal fiber about its optical axis.

[0197] 14. The mode control system defined by any of the foregoing clauses, wherein the detection unit includes beam shape and / or size measurement means that can be operated to measure one or more beam characteristics of the output radiation related to the shape and / or size of the beam, the measurement data including and / or originating from the beam characteristics of the output radiation related to the shape and / or size of the beam.

[0198] 15. The mode control system defined by clause 14, wherein the beam characteristics of the output radiation 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.

[0199] 16. The mode control system defined by any of the foregoing clauses, wherein the one or more parameters of the radiation emitted from the broadband light source configured to be measured by the detection unit include leakage radiation emitted from the fiber cladding of the photonic crystal fiber.

[0200] 17. The mode control system defined by any of the foregoing clauses includes one or more actuators for actuating the movement of one or more components of the broadband light source; wherein the control signal can be operated to control one or more of the actuators.

[0201] 18. The mode control system as defined in item 17 further includes a control unit configured to receive the control signal from the processing unit and control the one or more actuators.

[0202] 19. The mode control system as defined in item 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 the following:

[0203] The angular offset of the pump laser beam relative to the fiber core of the photonic crystal fiber;

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

[0205] The beam diameter of the pump laser beam;

[0206] The absolute polarization angle; and

[0207] The divergence of the pump laser beam.

[0208] 20. The mode control system as defined in item 17, 18 or 19, wherein the one or more actuators include one or more of the following:

[0209] At least one actuator for at least one beam steering component or its support;

[0210] At least one actuator for the air bladder of the photonic crystal fiber or its support;

[0211] At least one actuator for a focusing lens to focus the pump laser beam onto the fiber core of the photonic crystal fiber.

[0212] 21. The mode control system as defined in any of the preceding items, wherein the processing unit is operable to evaluate the 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 indicating an optimal or acceptable mode purity.

[0213] 22. The mode control system as defined in any of the preceding items, wherein the photonic crystal fiber includes a hollow core photonic crystal fiber (HC-PCF).

[0214] 23. The mode control system as defined in any of the preceding items, wherein the output radiation of the broadband light source includes a wavelength range of 200 nm to 2000 nm or a sub-range within this range.

[0215] 24. The mode control system as defined in any of the preceding items, wherein the mode purity describes the ratio of the power in the fundamental transverse mode to the total output power.

[0216] 25. A mode control system as defined in any of the preceding clauses, 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 includes:

[0217] Configured to generate a control signal to optimize one or more pump coupling conditions to maximize mode purity.

[0218] 26. A mode control system as defined in any of the preceding clauses, including at least one detector for detecting leakage radiation and operable during an initial rough pump coupling operation to rough couple a pump laser beam to a fiber core of a photonic crystal fiber, the rough pump coupling operation including measuring leakage radiation emitted from a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam across an input facet of the photonic crystal fiber; and

[0219] Determining whether the pump laser beam is roughly aligned with the photonic crystal fiber based on the measured leakage radiation.

[0220] 27. The mode control system as defined in clause 26, wherein determining whether the pump laser beam is roughly aligned includes: locating a minimum value in the measured leakage radiation between at least two maximum values in the measured leakage radiation.

[0221] 28. The mode control system as defined in clause 26 or 27, wherein determining whether the pump laser beam is roughly aligned includes: locating a minimum value in the measured leakage radiation within an annular region surrounding a maximum value in the measured leakage radiation.

[0222] 29. The mode control system as defined in clause 26, 27 or 28, wherein the at least one detector includes a plurality of detectors radially spaced around the fiber cladding.

[0223] 30. The mode control system as defined in clause 29, wherein each detector extends at an angle less than 360 / n degrees in a radial direction of the photonic crystal fiber about a longitudinal axis of the photonic crystal fiber, where n is the number of detectors

[0224] 31. The mode control system as defined in clause 26 to 28, wherein the at least one detector includes at least a pair of detectors and mirrors, each detector and mirror being positioned at radially opposite positions around the fiber cladding.

[0225] 32. The mode control system as defined in any of clauses 26 to 31, wherein the scanning of the pump laser beam includes scanning along a helical path across the input facet.

[0226] 33. The mode control system defined by any of the foregoing clauses further includes an optical manipulation unit for scanning the beam, and the optical manipulation unit includes:

[0227] - One or more optical elements configured to manipulate the beam;

[0228] - A beam tilt adjuster for adjusting the propagation direction of the beam; and

[0229] - A control unit for controlling the beam tilt adjuster to set the propagation direction towards the photonic crystal fiber.

[0230] 34. The mode control system defined by clause 35, wherein the beam tilt adjuster includes at least two wedge prisms arranged in series and a tilt actuation system, and

[0231] wherein the tilt actuation system can be controlled by the control unit to independently rotate the wedge prism about its corresponding optical axis.

[0232] 35. The mode control system defined by clause 33 or 34, wherein the optical manipulation unit further includes a beam displacement device for displacing the beam, and the beam displacement device includes a plane parallel plate and a displacement actuation system that can be controlled by the control unit to rotate the plane parallel plate about a first axis substantially perpendicular to the propagation direction of the beam, and preferably, the displacement actuation system can also be controlled to rotate the plane parallel plate about a second axis substantially perpendicular to the propagation direction of the beam and the first axis, and preferably, 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.

[0233] 36. A timing control system configured to determine the timing of a broadband light source including a photonic crystal fiber (PCF), the timing control system including:

[0234] At least one pressure sensor configured to detect a pressure change in the gas environment around the photonic crystal fiber and output at least one electrical signal according to the pressure change, wherein the at least one electrical signal is used to determine the timing of at least one pulse of the broadband light source.

[0235] 37. The timing control system defined by clause 36, wherein the at least one pulse of the broadband light source includes a broadband pulse generated within the photonic crystal fiber and causing the pressure change.

[0236] 38. The timing control system defined by clause 36, wherein the at least one pulse of the broadband light source includes a pump pulse configured to be coupled into the photonic crystal fiber and causing the pressure change.

[0237] 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).

[0238] 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.

[0239] 41. A broadband light source device comprising one or both of the following:

[0240] A mode control system as defined in any one of clauses 1 to 35; and / or

[0241] A timing control system as defined in any one of clauses 36 to 40.

[0242] 42. A measuring device comprising the broadband light source device as defined in clause 41.

[0243] 43. The measuring device as defined in clause 42, comprising a scatterometer measuring device, a level sensor or an alignment sensor.

[0244] 44. A method for mode control of a broadband light source comprising a photonic crystal fiber, the method comprising:

[0245] Measuring one or more parameters of the radiation emitted from the broadband light source to obtain measurement data;

[0246] Evaluating the mode purity of the radiation emitted from the broadband light source by means of the measurement data; and

[0247] Generating a control signal to optimize one or more pump coupling conditions of the broadband light source; the pump coupling conditions being related to the coupling of a pump laser beam with respect to the fiber core of the photonic crystal fiber.

[0248] 45. The method as defined in clause 44, wherein the one or more parameters of the output radiation comprise one or more parameters indicating the mode purity of the broadband light source.

[0249] 46. The method as defined in clause 44 or 45, wherein the radiation emitted from the broadband light source detected by a detection unit comprises output radiation emitted from an output end of the photonic crystal fiber.

[0250] 47. The method as defined in clause 46, comprising separating a reference beam from a main output beam emitted from the photonic crystal fiber and measuring one or more parameters from the reference beam.

[0251] 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.

[0252] 49. A method as defined in clause 48, wherein the spectral parameter values comprise power spectral density values within one or more spectral ranges.

[0253] 50. A method as defined in clause 48 or 49, comprising band-pass filtering the radiation emitted from the broadband light source and measuring an irradiation parameter indicative of the power of the filtered radiation.

[0254] 51. A method as defined in any one of clauses 44 to 50, wherein the method comprises spatially filtering out higher-order modes other than the fundamental mode from the radiation emitted from the broadband light source and measuring an irradiation parameter indicative of the power of the filtered radiation.

[0255] 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.

[0256] 53. A method as defined in clause 52, wherein the beam characteristics of the output radiation related to the shape and / or size of the beam comprise one or more of the following: beam ellipticity, beam diameter, Laguerre-Gaussian mode shape, or Zernike polynomial shape.

[0257] 54. A method as defined in any one of clauses 44 to 53, wherein the measuring step comprises measuring leaky radiation emitted from the fiber cladding of the photonic crystal fiber to obtain the measurement data.

[0258] 55. A method as defined in any one of clauses 44 to 54, comprising actuating the movement of one or more components of the broadband light source based on a control signal to optimize one or more pump coupling conditions of the broadband light source.

[0259] 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 the following:

[0260] The angular offset of the pump laser beam with respect to the fiber core of the photonic crystal fiber;

[0261] The lateral offset of the pump laser beam with respect to the fiber core of the photonic crystal fiber;

[0262] The beam diameter of the pump laser beam

[0263] The absolute polarization angle; and

[0264] Divergence of the pump laser beam.

[0265] 57. The method according to item 55 or 56, wherein the one or more actuators comprise one or more of the following:

[0266] At least one actuator for at least one beam steering component or its support;

[0267] At least one actuator for the airbag of the photonic crystal fiber or its support;

[0268] At least one actuator for the focusing lens to focus the pump laser beam onto the fiber core of the photonic crystal fiber.

[0269] 58. The method according to any one of items 44 to 57, wherein the evaluation 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 indicating optimal or acceptable mode purity.

[0270] 59. The method according to any one of items 44 to 58, wherein the photonic crystal fiber comprises a hollow-core photonic crystal fiber (HC-PCF).

[0271] 60. The method according to any one of items 44 to 59, wherein mode purity describes the ratio of the power in the fundamental transverse mode to the total output power.

[0272] 61. The method according to any one of items 44 to 60, wherein generating the control signal to optimize one or more pump coupling conditions optimizes one or more pump coupling conditions to maximize mode purity.

[0273] 62. The method according to any one of items 44 to 61, comprising an initial rough pump coupling step of roughly coupling the pump laser beam with respect to the fiber core of the photonic crystal fiber, the rough pump coupling step comprising measuring the 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.

[0274] 63. The method according to item 62, wherein determining whether the pump laser beam is roughly aligned comprises: positioning the minimum value in the measured leakage radiation between at least two maximum values in the measured leakage radiation.

[0275] 64. The method as defined in clause 62 or 63, wherein determining whether the pump laser beam is roughly aligned includes: locating a minimum value in the measured leakage radiation within an annular region around a maximum value in the measured leakage radiation.

[0276] 65. The method as defined in any one of clauses 62 to 64, wherein the scanning of the pump laser beam includes scanning along a helical path on the input facet.

[0277] 66. A method of performing a rough pump coupling step to roughly couple a pump laser beam to a fiber core of a photonic crystal fiber, the rough pump coupling step including 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.

[0278] 67. The method as defined in clause 66, wherein determining whether the pump laser beam is roughly aligned includes: locating a minimum value in the measured leakage radiation between at least two maximum values in the measured leakage radiation.

[0279] 68. The method as defined in clause 66 or 67, wherein determining whether the pump laser beam is roughly aligned includes: locating a minimum value in the measured leakage radiation within an annular region around a maximum value in the measured leakage radiation.

[0280] 69. The method as defined in any one of clauses 66 to 68, wherein the scanning of the pump laser beam includes scanning along a helical path on the input facet.

[0281] 70. A method for determining an optimized value of a polarization parameter that describes a polarization angle of radiation with respect to an optical plane of a waveguide, the method including:

[0282] obtaining a relationship between the polarization angle and a power parameter indicative of the radiation power traversing the waveguide;

[0283] obtaining a value of the power parameter, and

[0284] determining the optimized value of the polarization parameter from the value of the power parameter and the relationship.

[0285] 71. The method as defined in clause 70, wherein the waveguide includes a photonic crystal fiber or a hollow-core photonic crystal fiber.

[0286] 72. The method as defined in any one of the preceding clauses 70 or 71, wherein the determining step includes determining an optimized value corresponding to a minimum value of the power parameter.

[0287] 73. The method defined in any one of the preceding clauses 70 to 72, comprising:

[0288] changing the polarization of the radiation between a plurality of angular positions;

[0289] obtaining a plurality of values of the power parameter, each value corresponding to one of the angular positions; and

[0290] selecting an optimized value of the polarization parameter corresponding to at least one minimum value from the plurality of values of the power parameter.

[0291] 74. The method defined in clause 73, wherein changing the polarization of the radiation comprises rotating any one or more of: a polarizer device, a light source emitting the radiation, or the nominal cylindrical waveguide.

[0292] 75. A polarization control system configured to control the output polarization of a broadband light source including a waveguide, the polarization control system comprising:

[0293] at least one detection unit configured to measure one or more parameters of the radiation emitted from the broadband light source to generate measurement data; and

[0294] a processing unit configured to infer the polarization properties of the radiation emitted from the broadband light source from the measurement data;

[0295] wherein based on the evaluation, the polarization control system is configured to generate a control signal for optimizing the 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 the fiber axis of the photonic crystal fiber.

[0296] 76. The polarization control system defined in clause 75, wherein the waveguide comprises a photonic crystal fiber or a hollow-core photonic crystal fiber.

[0297] 77. The polarization control system defined in clause 75 or 76, comprising a variable polarizer device for changing the pump polarization condition of the broadband light source with respect to the fiber axis of the photonic crystal fiber.

[0298] 78. The polarization control system defined in any one of clauses 75 to 77, wherein the polarizer device comprises a rotatable half-wave plate.

[0299] 79. The polarization control system 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.

[0300] 80. A polarization control system as defined in any one of clauses 75 or 79, comprising an optical filter to select one or more desired wavelengths before said detection unit.

[0301] 81. A polarization 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.

[0302] 82. A component, comprising:

[0303] an optical element for receiving and modifying radiation;

[0304] a receiving element for receiving the modified radiation; and

[0305] a gas environment for enclosing the receiving element,

[0306] wherein the component further comprises a control element configured to stabilize the matching condition between the optical element and the receiving element by adjusting the modification of the received radiation or by adjusting the distance between the optical element and the receiving element according to the nature of the gas environment.

[0307] 83. The component according to clause 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 non-linear element configured to convert the focused monochromatic radiation into broadband radiation.

[0308] 84. The component according to clause 83, wherein the non-linear element is a non-linear optical fiber embedded in the gas environment.

[0309] 85. The component according to clause 84, wherein the nature of the gas environment is the temperature, pressure or composition of the gas.

[0310] 86. The component according to clause 85, wherein the control element comprises an actuator configured to variably position the focusing element relative to the non-linear optical fiber along the optical axis of the non-linear optical fiber.

[0311] 87. The component according to clause 85, wherein the control element comprises an actuator configured to variably adjust the optical power of the focusing element.

[0312] 88. The component according to any one of clauses 84 to 87, wherein the matching condition is associated with the efficiency of coupling the focused monochromatic light into the non-linear optical fiber.

[0313] 89. The component according to any one of clauses 85 to 87, wherein the control element further comprises a processor configured to receive measurement values associated with the temperature, pressure or composition of the gas.

[0314] 90. The component according to item 89, wherein the processor is configured to determine a change in the refractive index of the gas.

[0315] 91. The component according to item 90, wherein the processor is further configured to determine a change in the focusing position based on knowledge of an optical element positioned upstream of the optical path of the non-linear optical fiber.

[0316] 92. The component according to any one of items 89 to 91, further comprising a sensor configured to measure a property of the gas environment.

[0317] 93. A component comprising:

[0318] an optical element for receiving and modifying radiation; and

[0319] a receiving element for receiving the modified radiation,

[0320] 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 by adjusting a distance between the optical element and the receiving element according to a property of the gas environment enclosing the receiving element.

[0321] 94. A component comprising:

[0322] an optical element for receiving and modifying radiation; and

[0323] a receiving element for receiving the modified radiation,

[0324] 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 by adjusting a distance between the optical element and the receiving element based on a focal power determined at an exit of the receiving element.

[0325] 95. The component according to item 94, wherein the adjustment is further based on a focal power determined at an entrance of the receiving element.

[0326] 96. The component according to item 95, wherein the adjustment is based on a ratio between i) a focal power determined at an exit of the receiving element, and ii) a focal power determined at an entrance of the receiving element.

[0327] Although all of the above examples and embodiments of the present invention are related to HC-PCF-based broadband light sources, the present invention is equally applicable to mode control of SC-PCF-based broadband light sources. In different embodiments, the detection unit of the mode control system measures one or more parameters of the broadband output beam of the SC-PCF-based broadband light source. Such parameters should be able to indicate the performance of the output mode of the SC-PCF-based broadband light source. The measurement data is processed in the processing unit, and subsequently the processed data is 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 SC-PCF-based broadband light source such that the pump coupling condition of the light source is improved and the output mode purity of the SC-PCF-based light source is optimized.

[0328] Although the use of a lithographic apparatus in the manufacture of ICs may be specifically recited herein, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, the guiding and detecting patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, and the like.

[0329] Although embodiments of the present invention may be specifically recited herein in the context of a lithographic apparatus, embodiments of the present invention may be used in other apparatuses. Embodiments of the present invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing an object such as a wafer (or other substrate) or a mask (or other patterning device). Such apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0330] Although the use of embodiments of the present invention may have been specifically recited above in the context of optical lithography, it is to be understood that, where the context permits, the present invention is not limited to optical lithography and may be used in other applications such as imprint lithography.

[0331] Although specific embodiments of the present invention have been described above, it is to be understood that the present invention may be practiced in a manner different from that described. The above description is intended to be illustrative, not restrictive. Thus, it will be apparent to those skilled in the art that modifications may be made to the present invention described without departing from the scope of the claims set forth below.

Claims

1. A method of performing a pump coupling step to couple a pump laser beam relative to a fiber core of a photonic crystal fiber, the method comprising: Measuring leakage radiation emitted from a site on an outer surface of a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam on an input facet of the photonic crystal fiber, the measuring using a detector having a radiation receiving detection surface facing the site on the outer surface of the fiber cladding and spaced apart from the fiber cladding; And Determining whether the pump laser beam is roughly aligned with the photonic crystal fiber based on the measured leakage radiation.

2. A system for performing a pump coupling step to couple a pump laser beam relative to a fiber core of a photonic crystal fiber, the system comprising: A detector configured to measure leakage radiation emitted from a site on an outer surface of a fiber cladding of the photonic crystal fiber during scanning of the pump laser beam on an input facet of the photonic crystal fiber, wherein the detector has a radiation receiving detection surface facing the site on the outer surface of the fiber cladding and spaced apart from the fiber cladding; And Instructions that, when executed by a processor or controller, determine whether the pump laser beam is roughly aligned with the photonic crystal fiber based on the measured leakage radiation.

3. A broadband radiation source device, comprising: A radiation source; And The system according to claim 2.

4. A measuring device, comprising: The broadband radiation source device according to claim 3; And Another detector.

5. A mode control system, comprising: A detector configured to measure leakage radiation emitted from a site on an outer surface of 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 Instructions that, when executed by a processor or controller, estimate a mode of radiation emitted from a broadband radiation source including the photonic crystal fiber from the measured leakage radiation, Wherein based on the estimation, the mode control system is configured to generate a control signal for optimizing one or more pump coupling conditions of the broadband radiation source, the one or more pump coupling conditions being related to the coupling of the pump laser beam relative to the fiber core of the photonic crystal fiber.

6. A broadband radiation source device, comprising: A radiation source; And The system according to claim 5.

7. A measuring device, comprising: The broadband radiation source device according to claim 6; And Another detector.

8. A timing control system configured to determine the timing of a broadband light source including a photonic crystal fiber (PCF), the timing control system comprising: At least one pressure sensor configured to detect a pressure change in a gas environment around the photonic crystal fiber and output at least one electrical signal according to the pressure change, wherein the at least one electrical signal is used to determine the timing of at least one pulse of the broadband light source.

9. A method for determining an optimized value of a polarization parameter that describes the polarization angle of radiation relative to an optical plane of a waveguide, the method comprising: Obtaining a relationship between the polarization angle and a power parameter indicating the radiation power traversing the waveguide; Obtain the value of the power parameter, and Determine the optimized value of the polarization parameter from the value of the power parameter and the relationship.

10. A polarization control system configured to control the output polarization of a broadband light source including a waveguide, the polarization control system comprising: At least one detection unit configured to measure one or more parameters of the radiation emitted from the broadband light source to generate measurement data; And A processing unit configured to infer the polarization properties of the radiation emitted from the broadband light source from the measurement data; Wherein, based on an evaluation, the polarization control system is configured to generate a control signal for optimizing the 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 the fiber axis of the photonic crystal fiber.

11. A component, comprising: An optical element for receiving and modifying radiation; A receiving element for receiving the modified radiation; And A gas environment for enclosing the receiving element, Wherein the component further comprises a control element configured to stabilize the matching condition between the optical element and the receiving element by adjusting the modification of the received radiation or by adjusting the distance between the optical element and the receiving element according to the properties of the gas environment.

12. A component, comprising: An optical element for receiving and modifying radiation; And A receiving element for receiving the modified radiation, Wherein the component further comprises a control element configured to stabilize the matching condition between the optical element and the receiving element by adjusting the modification of the received radiation or by adjusting the distance between the optical element and the receiving element according to the properties of the gas environment enclosing the receiving element.

13. A component, comprising: An optical element for receiving and modifying radiation; And A receiving element for receiving the modified radiation, Wherein the component further comprises a control element configured to stabilize the matching condition between the optical element and the receiving element by adjusting the modification of the received radiation or by adjusting the distance between the optical element and the receiving element based on the optical power determined at the exit of the receiving element.

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