Method for producing photonic crystal fiber

Abstract

A method of producing a photonic crystal fiber (PCF) preform, comprising: obtaining a plurality of preformed capillaries; measuring a wall thickness of each of the plurality of preformed capillaries at one or more axial positions along at least a portion of a circumference of the preformed capillaries; selecting a number of preformed capillaries from the plurality of preformed capillaries based at least on a thickness criterion; and placing the selected preformed capillary in a glass tube (or sleeve) to form a PCF preform having at least one annular arrangement of the selected preformed capillary; wherein the selected preformed capillaries are oriented such that a portion of each of the selected preformed capillaries in which a variation in measured wall thickness relative to nominal wall thickness does not exceed the maximum thickness variation faces a longitudinal axis of the glass tube, the glass tube is substantially symmetric about the longitudinal axis.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to EP application 22177524.0, filed on June 7, 2022, and incorporated herein by reference in its entirety. Technical Field

[0003] The present invention relates to a method for producing photonic crystal optical fibers, in particular hollow photonic crystal optical fibers, and its application in integrated circuit manufacturing. Background Art

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

[0005] To project a pattern onto a substrate, a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of a feature 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. Lithographic apparatus using extreme ultraviolet (EUV) radiation with a wavelength in the 4 nm to 20 nm heating range (e.g., 6.7 nm or 13.5 nm) can be used to form smaller features on a substrate than lithographic apparatus using, for example, radiation with a wavelength of 193 nm.

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

[0007] Metrology tools are used in many aspects of the IC manufacturing process, such as alignment tools for correctly positioning the substrate before exposure, leveling tools for measuring the substrate surface topology, focus control and scatterometry-based tools for checking / measuring the products of exposure and / or etching in process control, for example. In each case, a radiation source is required. Broadband or white light radiation sources are increasingly used for such metrology applications for various reasons, including measurement robustness and accuracy. It is desirable to improve existing devices for broadband radiation generation. Among other technologies for white light generation, photonic crystal fibers (PCFs) can be used to convert narrowband input radiation into broadband output radiation via one or more nonlinear optical processes. The output performance of PCF-based radiation sources has a strong dependence on the quality and characteristics of the PCF, which are mainly determined by the fiber manufacturing process. Currently, PCFs are drawn in conventional fiber drawing towers that draw optical fibers in a continuous manner. This conventional fiber manufacturing method typically results in large variations in one or more dimensions of the internal structure of the PCF. Radiation sources that use such PCFs to generate broadband radiation will output light with undesirable characteristics. It is therefore an object of the present invention to provide an improved method for producing PCF. Summary of the invention

[0008] According to a first aspect of the present invention, there is provided a method for producing a photonic crystal fiber (PCF) preform, comprising: obtaining a plurality of preformed capillaries; measuring the wall thickness of each of the plurality of preformed capillaries at one or more axial positions along at least a portion of the circumference of the preformed capillaries; selecting some preformed capillaries from the plurality of preformed capillaries based at least on a thickness criterion, the thickness criterion being that the variation of the wall thickness along at least a circumferential portion of the preformed capillary relative to the nominal wall thickness does not exceed a maximum thickness variation; and placing the selected preformed capillaries in a glass tube (or sleeve) to form a PCF preform having at least one annular arrangement of the selected preformed capillaries; wherein the selected preformed capillaries are oriented so that the portion of each of the selected preformed capillaries in which the measured wall thickness does not vary relative to the nominal wall thickness by more than the maximum thickness variation faces the longitudinal axis of the glass tube, and the glass tube is substantially symmetrical about the longitudinal axis.

[0009] According to a second aspect of the present invention, there is provided a method for producing a photonic crystal fiber (PCF), comprising: producing a PCF preform using the method of the first aspect of the present invention; drawing a PCF blank from the PCF preform; and drawing a final PCF from the PCF blank.

[0010] According to a third aspect of the present invention, there is provided a PCF product produced using the method of the second aspect of the present invention.

[0011] Other aspects of the present invention include a measurement device including a PCF-based broadband light source device including a PCF produced by the method of the first aspect or the second aspect. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

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

[0015] Figure 3 Depicted is a schematic representation of overall lithography showing the collaboration between three key technologies to optimize semiconductor manufacturing;

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

[0017] Figure 5 depicts a schematic overview of a level sensor device which may include a radiation source according to an embodiment of the invention;

[0018] Figure 6 depicts a schematic overview of an alignment sensor device that may include a radiation source according to an embodiment of the present invention;

[0019] Figure 7 is a schematic cross-sectional view in a transverse plane (ie, perpendicular to the axis of the optical fiber) of a hollow core optical fiber which may form part of a radiation source according to an embodiment;

[0020] Figure 8 depicts a schematic representation of a radiation source for providing broadband output radiation according to an embodiment;

[0021] Fig. 9 (a) and (b) schematically depict cross-sections of examples of hollow core photonic crystal fiber (HC-PCF) designs for supercontinuum generation;

[0022] Fig.10 (a), (b) and (c) show three cross-sectional images taken at three fiber drawing stages and corresponding to a preform, a billet and a HC-PCF, respectively;

[0023] Fig.11 is the HC-PCF as a function of wavelength (e.g. Figure 7 Example simulation of confinement loss for the optical fiber shown;

[0024] Fig.12 A flow chart showing a method of producing a photonic crystal optical fiber (PCF) preform according to an embodiment;

[0025] Fig.13 The use according to an embodiment is shown by, for example, Fig.12 A flow chart of a method for producing a PCF from a preform produced by the method shown; and

[0026] Fig.14 A block diagram of a computer system for controlling a broadband radiation source is depicted. DETAILED DESCRIPTION

[0027] In this document, the terms "radiation" and "beam" are used to cover all types of electromagnetic radiation, including ultraviolet radiation (e.g., wavelengths of 365 nm, 248 nm, 193 nm, 157 nm or 126 nm) and EUV (extreme ultraviolet radiation, e.g., wavelengths in the range of about 5 nm to 100 nm).

[0028] The terms "reticle", "mask" or "patterning device" as used herein may be broadly interpreted as referring to a general patterning device that can be used to impart a patterned cross-section to an incident radiation beam that corresponds to the pattern to be created in a target portion of a substrate. In this context, the term "light valve" may also be used. In addition to classical masks (transmissive or reflective, binary, phase-shifting, hybrid, etc.), other examples of such patterning devices include programmable mirror arrays and programmable LCD arrays.

[0029] Figure 1 A lithographic apparatus LA is schematically depicted. The lithographic apparatus LA comprises an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g. UV radiation, DUV radiation or EUV radiation), a mask support (e.g. mask table) MT configured to support a patterning device (e.g. 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. wafer stage) WT configured to hold a substrate (e.g. 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. refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by the patterning device MA onto a target portion C (e.g. comprising one or more dies) of the substrate W.

[0030] In operation, the illumination system IL receives a radiation beam from a radiation source SO (e.g., via a beam delivery system BD). The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic and / or other types of optical components, or any combination thereof, for directing, shaping and / or controlling the radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross-section at the plane of the patterning device MA.

[0031] The term "projection system" PS as used herein should be broadly understood to cover various types of projection systems, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems or any combination thereof, as appropriate with respect to the exposure radiation used and / or other factors such as the use of immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered synonymous with the more general term "projection system" PS.

[0032] The lithographic apparatus LA may be of a type in which at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, such as water, to fill the space between the projection system PS and the substrate W, which is also known as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.

[0033] The lithographic apparatus LA may also be of a 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 may be used in parallel, and / or steps for preparing a substrate W for subsequent exposure may be performed on a substrate W located on one of the substrate supports WT, while another substrate W on another substrate support WT is being used to expose a pattern on another substrate W.

[0034] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement platform. The measurement platform is arranged to hold sensors and / or cleaning means. The sensors may be arranged to measure characteristics of the projection system PS or characteristics of the radiation beam B. The measurement platform may hold a plurality of sensors. The cleaning means may be arranged to clean a part of the lithographic apparatus, e.g. a part of the projection system PS or a part of a system providing an immersion liquid. The measurement platform may be moved under the projection system PS when the substrate support WT is away from the projection system PS.

[0035] In operation, a radiation beam B is incident on a patterning device (e.g. a mask MA) and is patterned by a pattern (design layout) present on the patterning device MA, which is held on a mask support MT. After traversing the mask MA, the radiation beam B passes through a projection system PS which focuses the beam onto a target portion C of a substrate W. With the aid of a second positioner PW and a position measurement system IF, the substrate support WT can be accurately moved, for example in order to position different target portions C in the path of the radiation beam B at focused and aligned positions. Similarly, the first positioner PM and possibly another position sensor (not shown) are also provided. Figure 1 The patterning device MA may be accurately positioned relative to the path of the radiation beam B using the mask alignment marks M1, M2 and the substrate alignment marks P1, P2. Although the illustrated substrate alignment marks P1, P2 occupy dedicated target portions, they may be located in spaces between target portions. When the substrate alignment marks P1, P2 are located between target portions C, they are referred to as scribe line alignment marks.

[0036] like Figure 2 As shown, the lithography apparatus LA may form part of a lithography cell LC, which is sometimes also referred to as a lithography cell or (lithography) cluster, which typically also includes equipment for performing pre- and post-exposure processes on a substrate W. Conventionally, these include a spin coater SC for depositing a resist layer, a developer DE for developing the exposed resist, a chill plate CH and a bake plate BK, for example, for regulating the temperature of the substrate W (e.g. for regulating the solvent in the resist layer). A substrate handler or robot RO picks up substrates W from input / output ports I / O1, I / O2, moves them between the different process equipment, and delivers the substrates W to a feed station LB of the lithography apparatus LA. The devices in the lithography cell (often also collectively referred to as a coater-developer) are typically controlled by a coater-developer control unit TCU, which itself may be controlled by a supervisory control system SCS, which may also control the lithography apparatus LA (e.g. via a lithography control unit LACU).

[0037] In order for substrates W exposed by lithographic apparatus LA to be correctly and consistently exposed, it is desirable to inspect the substrates to measure characteristics of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) may be included in lithography cell LC. If errors are detected, then, for example, adjustments may be made to the exposure of subsequent substrates or other processing steps to be performed on substrate W, particularly if the inspection is completed before other substrates W of the same batch or lot are still to be exposed or processed.

[0038] The inspection apparatus (which may also be referred to as a metrology apparatus) is used to determine properties of the substrate W, in particular how properties vary from one substrate to another W, or how properties associated with different layers of the same substrate W vary from layer to layer. Alternatively, the inspection apparatus may be configured to identify defects on the substrate W, and may be, for example, part of the lithography cell LC, or may be integrated into the lithography apparatus LA, or may even be a stand-alone device. The inspection apparatus may measure properties on a latent image (the image in the resist layer after exposure) or a semi-latent image (the image in the resist layer after a post-exposure bake step PEB), or a developed resist image (where exposed or unexposed parts of the resist have been removed), or even on an etched image (after a pattern transfer step such as etching).

[0039] Typically, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires high accuracy in dimensioning and placement of structures on the substrate W. To ensure this high accuracy, the three systems can be combined into a so-called "holistic" control environment, such as Figure 3 One of these systems is the lithography apparatus LA, which is (in practice) connected to a metrology tool MT (a second system) and a computer system CL (a third system). The key to this "holistic" environment is to optimize the cooperation between these three systems to enhance the overall process window, and to provide a tight control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines the range of process parameters (e.g. dose, focus, overlay) within which a specific manufacturing process will produce a defined result (e.g. a functional semiconductor device), and within which process parameters of a lithography process or patterning process are typically allowed to vary.

[0040] The computer system CL may use (a portion of) the design layout to be patterned to predict the resolution enhancement technique to be used, and perform computational lithography simulations and calculations to determine which mask layouts and lithography equipment settings achieve the maximum overall process window for the patterning process (in Figure 3 Typically, the resolution enhancement technique is arranged to match the patterning possibilities of the lithographic apparatus LA. The computer system CL may also be used (e.g. using input from a metrology tool MT) to detect where within the process window the lithographic apparatus LA is currently operating, to predict whether defects may be present (e.g. due to suboptimal processing). Figure 3 ) is depicted by an arrow pointing to “0” in the second scale SC2.

[0041] The metrology tool MT may provide input to the computer system CL to enable accurate simulations and predictions, and may provide feedback to the lithography apparatus LA to identify possible drifts, e.g. in the calibration state of the lithography apparatus LA (in Figure 3 ) as depicted by multiple arrows in a third scale SC3.

[0042] During the lithographic process, it is desirable to frequently measure the created structures, for example for process control and verification. The tool for performing such measurements is typically referred to as a measurement tool MT. Different types of measurement tools MT for performing such measurements are known, including scanning electron microscopes or various forms of scatterometer measurement tools MT. A scatterometer is a multifunctional instrument that allows measurement of parameters of the lithographic process by placing a sensor in the pupil of a scatterometer objective or in a plane conjugated to the pupil (these measurements are generally referred to as pupil-based measurements) or by placing a sensor in an image plane or in a plane conjugated to the image plane (in this case, these measurements are generally referred to as image-based or field-based measurements). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP1,628,164A, the entire contents of which are incorporated herein by reference. The scatterometer described above can measure the grating using light from soft x-rays and visible to the near IR wavelength range.

[0043] In a first embodiment, the scatterometer MT is an angle-resolved scatterometer. In such a scatterometer, reconstruction methods may be applied to the measurement signals to reconstruct or calculate the properties of the grating. For example, such a reconstruction may be performed by simulating the interaction of the scattered radiation with a mathematical model of the target structure and comparing the simulation results with the measurement results. The parameters of the mathematical model are adjusted until the simulated interaction produces a diffraction pattern similar to that observed from a real target.

[0044] In a second embodiment, the scatterometer MT is a spectroscopic scatterometer MT. In such a spectroscopic scatterometer MT, radiation emitted by a radiation source is directed onto a target and reflected or scattered radiation from the target is directed to a spectrometer detector which measures the spectrum of the specularly reflected radiation (i.e., an intensity-side measurement 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 nonlinear regression or by comparison with a library of simulated spectra.

[0045] In a third embodiment, the scatterometer MT is an ellipsometric scatterometer. An ellipsometric scatterometer allows the parameters of a lithographic process to be determined by measuring the scattered radiation of each polarization state. Such a measurement device emits polarized light (such as linear, circular or elliptical) by using, for example, an appropriate polarization filter in the illumination section of the measurement device. A source suitable for the measurement device may 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, the entire contents of which are incorporated herein by reference.

[0046] In one embodiment of the scatterometer MT, the scatterometer MT is adapted to measure the overlap of two misaligned gratings or periodic structures by measuring the reflection spectrum and / or an asymmetry in the detection construction, the asymmetry being related to the degree of the overlap. The two (usually overlapping) grating structures may be applied in two different layers (not necessarily consecutive layers) and may be formed at substantially the same position on the wafer. The scatterometer may have a symmetrical detection construction, such as described in the co-owned patent application EP1,628,164A, so that any asymmetry is clearly distinguishable. This provides a direct way to measure misalignment in the grating. Other examples for measuring the overlap error between two layers containing a periodic structure as a target by measuring the asymmetry of the periodic structure may be found in PCT patent application publication number WO2011 / 012624 or in US patent application US 20160161863, the entire contents of which are incorporated herein by reference.

[0047] Other parameters of interest may be focus and dose. Focus and dose may be determined simultaneously by scatterometry (or alternatively by scanning electron microscopy), as described in U.S. Patent Application US2011-0249244, the entire contents of which are incorporated herein by reference. A single structure may be used that has a unique combination of critical dimension and sidewall angle measurements for each point in a focus energy matrix (FEM, also called a focus exposure matrix). If these unique combinations of critical dimension and sidewall angle are available, focus and dose values ​​may be uniquely determined from these measurements.

[0048] The metrology target may be the entirety of a composite grating, which is formed by a lithography process, primarily in resist, but may also be formed after etching. Typically, the pitch and line width of the structures in the grating are largely determined by the measurement optics (particularly the NA of the optics) in order to be able to capture the diffraction orders from the metrology target. As indicated earlier, the diffraction signal may be used to determine the shift between two layers (also called 'overlap'), or may be used to reconstruct at least part of the original grating produced by the lithography process. This reconstruction may be used to provide guidance on the quality of the lithography process, and may be used to control at least part of the lithography process. The target may have smaller sub-divisions configured to mimic the size of a functional portion of the design layout in the target. Due to the sub-divisions, the behavior of the target will be more similar to the functional portion of the design layout, so that the overall process parameter measurement better resembles the functional portion of the design layout. The target may be measured in an underfill mode or in an overfill mode. In the underfill mode, the measurement beam generates a smaller spot than the overall target. In the overfill mode, the measurement beam generates a larger spot than the overall target. In this overfill mode, it is also possible to measure different targets simultaneously and thus determine different processing parameters simultaneously.

[0049] The overall measurement quality of the lithography parameters using a specific target is at least partially determined by the measurement scheme used to measure the lithography parameters. The term "substrate measurement scheme" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement scheme is an optical measurement based on diffraction, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the radiation orientation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement scheme may be, for example, the sensitivity of one of the measurement parameters to process variations. More examples are described in U.S. Patent Application US2016-0161863 and published U.S. Patent Application US 2016 / 0370717A1, the entire contents of which are incorporated herein by reference.

[0050] exist Figure 4 An example of a metrology device such as a scatterometer is depicted in FIG. It comprises a broadband (white light) radiation projector 2 which projects radiation onto a substrate 6. The reflected or scattered radiation is passed to a spectrometer detector 4 which measures the spectrum 10 of the specularly reflected radiation (i.e. a measurement of the intensity as a function of wavelength). From this data, the spectrometer detector 4 can be used to generate a spectrum 10 of the specularly reflected radiation, for example by rigorous coupled wave analysis and nonlinear regression or by comparing the specularly reflected radiation to the spectrometer detector 4. Figure 3The structure or profile that produces the detected spectrum can be reconstructed by the processing unit PU for comparison with the simulated spectral library shown at the bottom. Typically, for the reconstruction, the general form of the structure is known and some parameters are assumed by knowledge of the process by which the structure was manufactured, with only a few parameters of the structure to be determined from the scatterometry data. Such a scatterometer can be configured as a normal incidence scatterometer or an oblique incidence scatterometer.

[0051] The overall measurement quality of the lithography parameters via the measurement of the measurement target is at least partially determined by the measurement scheme used to measure the lithography parameters. The term "substrate measurement scheme" may include one or more parameters of the measurement itself, one or more parameters of the one or more patterns being measured, or both. For example, if the measurement used in the substrate measurement scheme is an optical measurement based on diffraction, the one or more parameters measured may include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the radiation orientation relative to the pattern on the substrate, etc. One of the criteria for selecting the measurement scheme may be, for example, the sensitivity of one of the measurement parameters to process changes. More examples are described in U.S. Patent Application US2016-0161863 and published U.S. Patent Application US 2016 / 0370717A1, the entire contents of which are incorporated herein by reference.

[0052] Another type of measurement tool for 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 graph of the topography of a substrate, also referred to as a height map, can be generated by indicating these measurements of the height of the substrate that varies with the position on the substrate. This 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 a spatial image of the pattern forming device in the proper focus position on the substrate. It should be understood that "height" in this context generally refers to the dimension from the plane to the substrate (also referred to as the Z axis). Typically, a level or height sensor performs measurement at a fixed position (relative to its own optical system), and the relative movement between the substrate and the optical system of the level or height sensor facilitates height measurement at positions throughout the substrate.

[0053] exist Figure 5An example of a level or height sensor LS known in the art is schematically shown in , which only illustrates the operating principle. In this example, the level sensor comprises an optical system, which comprises a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB, which is given a pattern by a projection grating PGR of the projection unit LSP. The radiation source LSO can be, for example, a narrow-band (such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous) or a broadband light source (such as a polarized or non-polarized laser beam). The radiation source LSO may include a plurality of radiation sources with different colors or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not limited to visible light radiation, but may additionally or alternatively cover UV and / or IR radiation and any wavelength range suitable for reflection from the surface of the substrate.

[0054] The projection grating PGR is a periodic grating comprising a periodic structure, which generates a radiation beam BE1 with a periodically varying intensity. The radiation beam BE1 with a periodically varying intensity is directed to a measurement location 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 being between 0 and 90 degrees, typically between 70 and 80 degrees. At the measurement location MLO, the patterned radiation beam BE1 is reflected by the substrate W (indicated by arrow BE2) and directed towards the detection unit LSD.

[0055] In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET generates a detector output signal which is indicative of the received light, for example of the intensity of the received light, such as a photodetector, or which represents the spatial distribution of the received intensity, such as a camera. The detector DET may comprise any combination of one or more detector types.

[0056] By means of triangulation techniques the height level at the measurement location MLO can be determined.The detected height level is typically related to the signal intensity as measured by the detector DET, which has a periodicity that depends on the design of the projection grating PGR and the (tilted) angle of incidence ANG, among other things.

[0057] The projection unit LSP and / or the detection unit LSD may comprise further optical elements like lenses and / or mirrors along the path (not shown) of the patterned radiation beam between the projection grating PGR and the detection grating DGR.

[0058] In an embodiment, the detection grating DGR may be omitted and the detector DET may be placed where the detection grating DGR is located. This configuration provides a more direct detection of the image of the projection grating PGR.

[0059] In order to effectively cover the surface of the substrate W, the level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.

[0060] Various height sensors of the general type are disclosed, for example, in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation rather than visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In WO2016102127A1, incorporated by reference, a compact height sensor is described that uses a multi-element detector to detect and discern the position of a grating image without detecting the grating.

[0061] Another type of metrology tool used in IC manufacturing is an alignment sensor. Therefore, a key aspect of the performance of a lithographic apparatus is the ability to properly and accurately place an applied pattern relative to features placed in a previous layer (either 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 later using a position sensor (usually an optical position sensor). The position sensor may be referred to as an "alignment sensor" and the mark may be referred to as an "alignment mark."

[0062] The lithographic apparatus may include one or more (e.g., multiple) alignment sensors, by which the position of alignment marks provided on the substrate can be accurately measured. The alignment (or position) sensor may use optical phenomena such as diffraction and interference to obtain position information from alignment marks formed on the substrate. An example of an alignment sensor used in a current lithographic apparatus is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of position sensors have been developed, such as disclosed in US2015261097A1. All of these disclosed contents are incorporated herein by reference.

[0063] Figure 6is a schematic block diagram of an embodiment of a known alignment sensor AS such as described in, for example, US6961116 and incorporated by reference. A radiation source RSO provides a radiation beam RB having one or more wavelengths, which is steered as an illumination spot SP by a steering optic onto a mark (e.g., a mark AM located on a substrate W). In this example, the steering optics comprises a point mirror SM and an objective lens OL. The diameter of the illumination spot SP illuminating the mark AM may be slightly smaller than the width of the mark itself.

[0064] Radiation diffracted by the alignment mark AM (in this example, via the objective lens OL) is collimated into a beam IB carrying the information. The term "diffraction" is intended to include zero-order diffraction from the mark (which may be referred to as reflection). A self-referencing interferometer SRI, for example of the type disclosed in US6961116 mentioned above, causes the beam IB to interfere with itself, after which the beam is received by the photodetector PD. Additional optics (not shown) may be included to provide separate beams in the event that more than one wavelength is generated by the radiation source RSO. The photodetector may be a single element, or it may include a number of pixels as required. The photodetector may include an array of sensors.

[0065] The steering optics comprising a spot mirror SM in this example may also be used to block zeroth order radiation reflected from the mark so that the information carrying beam IB comprises only higher order diffracted radiation from the mark AM (this is not necessary for the measurement but improves the signal to noise ratio).

[0066] The intensity signal SI is supplied to the processing unit PU. By combining the optical processing carried out in block SRI with the computational processing carried out in unit PU, the values ​​of the X-position and Y-position of the substrate relative to the reference system are output.

[0067] A single measurement of the type illustrated only fixes the position of the mark within a certain range corresponding to one pitch of the mark. A coarser measurement technique is used in conjunction with this measurement to identify which period of the sine wave is the period that includes the marked position. The same process at coarser and / or finer levels can be repeated at different wavelengths for increased accuracy and / or for robust detection of the mark, regardless of the material the mark is made of and the material above and / or below which the mark is disposed. The wavelengths can be optically multiplexed and demultiplexed so as to process the wavelengths simultaneously, and / or the wavelengths can be multiplexed by time division or frequency division.

[0068] In this example, the alignment sensor and the spot SP remain stationary while the substrate W moves. Thus, the alignment sensor can be mounted securely and accurately to the reference frame while effectively scanning the mark AM in a direction opposite to the direction of movement of the substrate W. The substrate W is controlled in this movement by mounting the substrate W on a substrate support and by a substrate positioning system that controls the movement of the substrate support. A substrate support position sensor (e.g., an interferometer) measures the position of a substrate support (not shown). In an embodiment, one or more (alignment) marks are provided on the substrate support. Measurement of the position of the mark provided on the substrate support allows calibration of the position of the substrate support as determined by the position sensor (e.g., relative to a frame to which the alignment system is connected). Measurement of the position of the alignment mark provided on the substrate allows determination of the position of the substrate relative to the substrate support.

[0069] The measurement tools MT mentioned above (such as scatterometers, topography measurement systems or position measurement systems) can use radiation originating from a radiation source to perform measurements. The nature of the radiation used by the measurement tool may affect the type and quality of the measurements that can be performed. For some applications, it may be advantageous to use multiple radiation frequencies to measure the substrate, for example, broadband radiation may be used. Multiple different frequencies may be able to propagate, irradiate the measurement target and scatter away from the measurement target without interfering with other frequencies or with minimal interference with other frequencies. Therefore, different frequencies may be used, for example, to obtain more measurement data at the same time. Different radiation frequencies may also be able to query and discover different properties of the measurement target. Broadband radiation may be suitable for use in measurement systems MT such as level sensors, alignment mark measurement systems, scattering measurement tools or inspection tools. The broadband radiation source may be a supercontinuum source.

[0070] High quality broadband radiation, such as supercontinuum radiation, may be difficult to produce. One method for producing broadband radiation may be, for example, to broaden high power narrowband or single frequency input radiation or pump radiation using nonlinear high order effects. The input radiation (which may be produced using a laser) may be referred to as pump radiation. Alternatively, the input radiation may be referred to as seed radiation. To obtain high power radiation for the broadening effect, the radiation may be confined to a smaller region so that a largely localized high intensity radiation is achieved. In those regions, the radiation may interact with the broadening structure and / or the material forming the nonlinear medium to produce broadband output radiation. In the high intensity radiation region, different materials and / or structures may be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0071] In some implementations, broadband output radiation is generated in a photonic crystal fiber (PCF). In several embodiments, such a photonic crystal fiber has a microstructure around its fiber core that helps confine the radiation traveling through the fiber in the fiber core. The fiber core can be composed of a solid material that has nonlinear properties and is capable of generating broadband radiation when high-intensity pump radiation is transmitted through the fiber core. Although it is feasible to generate broadband radiation in a solid core photonic crystal fiber, there may be several disadvantages to using solid materials. For example, if UV radiation is generated in a solid core, such radiation may not be present in the output spectrum of the fiber because the radiation is absorbed by most solid materials.

[0072] In some embodiments, as described below with reference to Figure 8 As further discussed, methods and apparatus for broadening input radiation may use an optical fiber for limiting the input radiation and for broadening the input radiation to output broadband radiation. The optical fiber may be a hollow core optical fiber and may include an internal structure for achieving effective guidance and confinement of radiation in the optical fiber. The optical fiber may be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly inside the hollow core of the optical fiber, thereby achieving high radiation intensity. The hollow core of the optical fiber may be filled with a gas, which serves as a broadening medium for broadening the input radiation. Such an optical fiber and gas arrangement may be used to generate a supercontinuum radiation source. The radiation input to the optical fiber may be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation may consist of or include broadband radiation, which may be referred to herein as white light.

[0073] Some embodiments relate to new designs of such broadband radiation sources comprising optical fibers. The optical fiber is a hollow core photonic crystal fiber (HC-PCF). In particular, the optical fiber can be a hollow core photonic crystal fiber of the type comprising an anti-resonance structure for limiting radiation. Such optical fibers comprising anti-resonance structures are known in the art as anti-resonance fibers, tubular fibers, single-ring fibers, negative curvature fibers, or suppressed coupling fibers. Various different designs of such optical fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, such as a Kagome fiber).

[0074] Multiple types of HC-PCFs can be designed, each based on a different physical guiding mechanism. Two such HC-PCFs include: hollow core photonic bandgap fiber (HC-PBF) and hollow core antiresonant reflective fiber (HC-ARF). Details on the design and manufacture of HC-PCFs can be found in U.S. Patent US2004 / 015085A1 (for HC-PBF) and International PCT Patent Application WO2017 / 032454A1 (for hollow core antiresonant reflective fiber), which are incorporated herein by reference. Fig. 9 (a) shows a Kagome optical fiber including a Kagome lattice structure.

[0075] Now refer to Figure 7 Describe examples of optical fibers used in radiation sources, Figure 7 is a schematic cross-sectional view of an optical fiber OF in a transverse plane. Figure 7 Further embodiments of practical examples of optical fibers are disclosed in WO 2017 / 032454 A1.

[0076] The optical fiber OF comprises an elongated body, the optical fiber OF being longer in one dimension than the other two dimensions of the optical fiber OF. This longer dimension may be referred to as the axial direction and may define the axis of the optical fiber OF. The two other dimensions define a plane which may be referred to as a transverse plane. Figure 7 A cross section of the optical fiber OF is shown in such a transverse plane (ie perpendicular to the axis) marked as the xy plane.The transverse cross section of the optical fiber OF may be substantially constant along the fiber axis.

[0077] It will be understood that the optical fiber OF has a certain degree of flexibility and therefore, generally, the direction of the axis will not be uniform along the length of the optical fiber OF. Terms such as optical axis, transverse cross section, etc. should be understood to mean a local optical axis, a local transverse cross section, etc. Furthermore, where components are described as cylindrical or tubular, these terms will be understood to encompass such shapes that may have deformed when the optical fiber OF is bent.

[0078] The optical fiber OF may have any length and it will be appreciated that the length of the optical fiber OF may depend on the application.The length of the optical fiber OF may be between 1 centimeter (cm) and 10 meters (m), for example the length of the optical fiber OF may be between 10 cm and 100 cm.

[0079] The optical fiber OF includes: a middle hollow core portion HC; a cladding portion surrounding the middle hollow core portion HC; and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be regarded as including a body (including a cladding portion and a support portion SP) having a middle hollow core portion HC. The cladding portion includes a plurality of anti-resonance elements for guiding radiation through the middle hollow core portion HC. In particular, the plurality of anti-resonance elements are arranged to confine the radiation propagating through the optical fiber OF mainly inside the middle hollow core portion HC and guide the radiation along the optical fiber OF. The middle hollow core portion HC of the optical fiber OF can be substantially disposed in a central region of the optical fiber OF so that the axis of the optical fiber OF can also define the axis of the middle hollow core portion HC of the optical fiber OF.

[0080] The cladding portion comprises a plurality of anti-resonance elements for guiding radiation propagating through the optical fiber OF. In particular, in such an embodiment, the cladding portion comprises a single ring with six tubular capillaries CAP. Each tubular capillary in the tubular capillary CAP acts as an anti-resonance element.

[0081] The capillary CAP may also be referred to as a tube. The cross-section of the capillary CAP may be circular, or may have another shape. Each capillary CAP includes a generally cylindrical wall portion WP, which at least partially defines the middle hollow core portion HC of the optical fiber OF and separates the middle hollow core portion HC from the capillary cavity CC. It should be understood that the wall portion WP can be used as an anti-reflection Fabry-Perot resonator for radiation, which propagates through the middle hollow core portion HC (and the radiation may be incident on the wall portion WP at a grazing angle of incidence). The thickness of the wall portion WP may be suitable so as to ensure that the reflection of the returning middle hollow core portion HC is substantially enhanced while substantially suppressing the transmission into the capillary cavity CC. In some embodiments, the capillary wall portion WP may have a thickness between 0.01 μm and 10.0 μm.

[0082] It will be appreciated that, as used herein, the term "cladding portion" is intended to mean the portion of the optical fiber OF that is used to guide radiation propagating through the optical fiber OF (i.e., confine the radiation to the capillary CAP within the hollow core HC). The radiation may be confined in the form of transverse modes propagating along the optical fiber axis.

[0083] The support portion is generally tubular and supports the six capillaries CAP of the covering portion. The six capillaries CAP are evenly distributed around the inner surface of the inner support portion SP. The six capillaries CAP can be described as being arranged in a generally hexagonal form.

[0084] The capillary CAP is arranged so that each capillary does not contact any of the other capillary CAPs. Each capillary in the capillary CAP contacts the inner support portion SP and is spaced apart from the adjacent capillary CAP in the annular structure. This arrangement can be beneficial because it can increase the transmission bandwidth of the optical fiber OF (e.g., relative to an arrangement in which the capillaries contact each other). Alternatively, in some embodiments, each capillary in the capillary CAP can contact the adjacent capillary CAP in the annular structure.

[0085] The six capillaries CAP of the cladding portion are arranged in an annular structure around the middle hollow core HC. The inner surface of the annular structure of the capillaries CAP at least partially defines the middle hollow core HC of the optical fiber OF. The diameter d of the middle hollow core HC (which can be defined as the minimum dimension between the opposing capillaries, indicated by the arrow d) can be between 10 μm and 1000 μm. The diameter d of the middle hollow core HC may affect the mode field diameter of the middle hollow core HC optical fiber OF, affecting loss, dispersion, modal multiplicity and nonlinear properties.

[0086] In such an embodiment, the cladding portion comprises a single ring arrangement of capillaries CAP (which serve as anti-resonance elements).Thus, a line in any radial direction from the center of the hollow core HC to the outside of the optical fiber OF does not pass through more than one capillary CAP.

[0087] It will be appreciated that other embodiments may be provided with different arrangements of anti-resonance elements. These arrangements may include arrangements with multiple rings of anti-resonance elements and arrangements with nested anti-resonance elements. Fig. 9 (a) shows an embodiment of a HC-PCF having three rings of capillary CAPs, which are stacked on top of each other in the radial direction. In this embodiment, each capillary CAP is in contact with other capillaries both in the same ring and in different rings. In addition, although Figure 7 The embodiment shown in comprises a ring of six capillaries, but in other embodiments one or more rings comprising any number of anti-resonant elements (eg 4, 5, 6, 7, 8, 9, 10, 11 or 12 capillaries) may be provided in the cladding portion.

[0088] Fig. 9 (b) shows a modified embodiment of the HC-PCF discussed above with a single ring of tubular capillaries. Fig. 9 In the example of (b), there are two coaxial rings of tubular capillary tube 21. In order to hold the inner and outer rings of tubular capillary tube 21, a support tube ST may be included in the HC-PCF. The support tube may be made of silica.

[0089] Figure 7 as well as Fig. 9The tubular capillary of the examples of (a) and (b) may have a circular cross-sectional shape. For the tubular capillary, other shapes are also possible, such as an elliptical or polygonal cross-section. In addition, Figure 7 as well as Fig. 9 The solid material of the tubular capillary of the examples of (a) and (b) may include a plastic material such as PMA, glass such as silica, or soft glass.

[0090] Figure 8 A radiation source RDS for providing broadband output radiation is depicted. The radiation source RDS comprises: a pulsed pump radiation source PRS or any other type of source capable of generating short pulses of the desired length and energy level; an optical fiber OF with a hollow core HC (e.g., Figure 7 and a working medium WM (eg, gas) disposed within the hollow core HC. Figure 8 Medium radiation sources RDS include Figure 7 The optical fiber OF is shown in FIG. 1 , but in alternative embodiments other types of hollow core HC optical fibers OF may be used.

[0091] The pulsed pump radiation source PRS is configured to provide input radiation IRD. The hollow core HC of the optical fiber OF is arranged to receive input radiation IRD from the pulsed pump radiation source PRS and to broaden the input radiation to provide output radiation ORD. The working medium WM enables broadening of the frequency range of the received input radiation IRD in order to provide broadband output radiation ORD.

[0092] The radiation source RDS also includes a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV may also be referred to as a housing, a container, or a gas chamber. The reservoir RSV is configured to accommodate a working medium WM. The reservoir RSV may include one or more features known in the art for controlling, adjusting, and / or monitoring the composition of the working medium WM (which may be a gas) inside the reservoir RSV. The reservoir RSV may include a first transparent window TW1. When in use, the optical fiber OF is disposed inside the reservoir RSV so that the first transparent window TW1 is positioned adjacent to the input end IE of the optical fiber OF. The first transparent window TW1 may form a portion of the wall of the reservoir RSV. The first transparent window TW1 may be transparent at least to the received input radiation frequency so that the received input radiation IRD (or at least a larger portion thereof) may be coupled to the optical fiber OF located inside the reservoir RSV. It will be appreciated that an optical device (not shown) for coupling the input radiation IRD to the optical fiber OF may be provided.

[0093] The reservoir RSV comprises a second transparent window TW2 forming part of a wall of the reservoir RSV. In use, when the optical fiber OF is arranged inside the reservoir RSV, the second transparent window TW2 is positioned adjacent to the output end OE of the optical fiber OF. The second transparent window TW2 may be transparent at least for the frequencies of the broadband output radiation ORD of the device 120.

[0094] Alternatively, in another embodiment, the two opposite ends of the optical fiber OF may be placed inside different reservoirs. The optical fiber OF may include a first end section configured to receive input radiation IRD and a second end section for outputting broadband output radiation ORD. The first end section may be placed inside a first reservoir including a working medium WM. The second end section may be placed inside a second reservoir, wherein the second reservoir may also include the working medium WM. The operation of the reservoir may be as described above with respect to Figure 8 As described. The first reservoir may include a first transparent window, which is configured to be transparent to the input radiation IRD. The second reservoir may include a second transparent window, which is configured to be transparent to the broadband output broadband radiation ORD. The first reservoir and the second reservoir may also include a sealable opening to allow the optical fiber OF to be partially placed inside the reservoir and partially placed outside the reservoir so that the gas can be sealed inside the reservoir. The optical fiber OF may also include an intermediate section that is not contained inside the reservoir. This arrangement using two separate gas reservoirs may be particularly convenient for embodiments in which the optical fiber OF is relatively long (for example, when the length exceeds 1 m). It will be understood that for such an arrangement using two separate gas reservoirs, the two reservoirs (which may include one or more features known in the art for controlling, regulating and / or monitoring the composition of the gas inside the two reservoirs) can be regarded as providing a device for providing a working medium WM within the hollow core portion HC of the optical fiber OF.

[0095] In this context, a window may be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95% or 99% of incident radiation at that frequency is transmitted through the window.

[0096] Both the first transparent window TW1 and the second transparent window TW2 may form a gas-tight seal within the wall of the reservoir RSV so that a working medium WM (which may be a gas) may be contained within the reservoir RSV. It will be appreciated that the gas WM may be contained within the reservoir RSV at a pressure different from the ambient pressure of the reservoir RSV.

[0097] The working medium WM may include: rare gases such as argon, krypton and xenon; Raman active gases such as hydrogen, deuterium and nitrogen; or gas mixtures such as argon / hydrogen mixtures, xenon / deuterium mixtures, krypton / nitrogen mixtures or nitrogen / hydrogen mixtures. Depending on the type of filling gas, nonlinear optical processes may include modulation instability (MI), soliton self-compression, soliton splitting, Kerr effect, Raman effect and dispersed wave generation (DWG), details of which are described in WO2018 / 127266A1 and US9160137B1 (both of which are hereby incorporated by reference). Since the dispersion of the filling gas can be tuned by changing the pressure of the working medium WM in the reservoir RSR (i.e., the gas chamber pressure), the generated broadband pulse dynamics and the associated spectral broadening characteristics can be adjusted to optimize the frequency conversion.

[0098] In one embodiment, the working medium WM may be disposed within the hollow core portion HC at least during reception of input radiation IRD for generating broadband output radiation ORD. It should be understood that the gas WM may be completely or partially absent from the hollow core portion HC when the optical fiber OF is not receiving input radiation IRD for generating broadband output radiation.

[0099] In order to achieve frequency broadening, high intensity radiation may be desired. An advantage of having a HC fiber OF is that it can achieve high intensity radiation due to a strong spatial confinement of the radiation propagating through the fiber OF, thereby achieving a high localized radiation intensity. The radiation intensity inside the fiber OF may be higher, for example, due to the high input radiation intensity received and / or due to the strong spatial confinement of the radiation inside the fiber OF. An advantage of a HC fiber is that the HC fiber can guide radiation having a wider wavelength range than a solid core fiber, and in particular, the HC fiber can guide radiation in both the ultraviolet and infrared ranges.

[0100] An advantage of using a hollow core HC optical fiber OF may be that most of the radiation guided inside the optical fiber OF is confined to the hollow core HC. Therefore, most of the interaction of the radiation inside the optical fiber OF is with the working medium WM arranged inside the hollow core HC of the optical fiber OF. Therefore, the broadening effect of the working medium WM on the radiation may be increased.

[0101] The received input radiation IRD may be electromagnetic radiation. The input radiation IRD may be received as pulsed radiation. For example, the input radiation IRD may comprise ultrafast pulses, for example generated by a laser.

[0102] The input radiation IRD may be coherent radiation. The input radiation IRD may be collimated radiation, which may have the advantage of facilitating and improving the efficiency of coupling the input radiation IRD into the optical fiber OF. The input radiation IRD may include a single frequency or a narrow frequency range. The input radiation IRD may be generated by a laser. Similarly, the output radiation ORD may be collimated and / or coherent.

[0103] The broadband range of the output radiation ORD may be a continuous range, including a continuous range of radiation frequencies. The output radiation ORD may include supercontinuum radiation. Continuous radiation may be useful for use in a number of applications, such as in measurement applications. For example, a continuous frequency range may be used to obtain a large number of properties. The continuous frequency range may, for example, be used to determine and / or eliminate the frequency dependence of the measured properties. The supercontinuum output radiation ORD may include, for example, electromagnetic radiation in a wavelength range of 100 nm to 4000 nm. The broadband output radiation ORD frequency range may be, for example, 400 nm to 900 nm, 500 nm to 900 nm, or 200 nm to 2000 nm. The supercontinuum output radiation ORD may include white light.

[0104] The input radiation IRD provided by the pulsed pump radiation source PRS may be pulsed. The input radiation IRD may include electromagnetic radiation of one or more frequencies between 200 nm and 2 μm. The input radiation IRD may, for example, include electromagnetic radiation with a wavelength of 1.03 μm. The repetition rate of the pulsed radiation IRD may be of the order of 1 kHz to 100 MHz. The pulse energy may be of the order of 0.1 μJ to 100 μJ, for example 1 μJ to 10 μJ. The pulse duration of the input radiation IRD may be between 10 fs and 10 ps, ​​for example 300 fs. The average power of the input radiation IRD may be between 100 mW and several hundred W. The average power of the input radiation IRD may be, for example, 20 W to 50 W.

[0105] The pulsed pump radiation source PRS may be a laser. The spatiotemporal transmission characteristics (e.g., its spectral amplitude and phase) of such laser pulses transmitted along the optical fiber OF may be changed and tuned by adjusting (pump) laser parameters, working component WM changes, and optical fiber OF parameters. The spatiotemporal transmission characteristics may include one or more of the following: output power, output mode profile, output time profile, width of the output time profile (or output pulse width), output spectral profile, and bandwidth of the output spectral profile (or output spectral bandwidth). The pulsed pump radiation source PRS parameters may include one or more of the following: pump wavelength, pump pulse energy, pump pulse width, pump pulse repetition rate. The optical fiber OF parameters may include one or more of the following: optical fiber length, size, and shape of the hollow core HC; size and shape of the capillary; thickness of the wall of the capillary surrounding the hollow core HC. The working component WM (e.g., filling gas) parameters may include one or more of the following: gas type, gas pressure, and gas temperature.

[0106] The broadband output radiation ORD provided by the radiation source RDS may have an average output power of at least 1 W. The average output power may be at least 5 W. The average output power may be at least 10 W. The broadband output radiation ORD may be a pulsed broadband output radiation ORD. The broadband output radiation ORD may have a power spectral density of at least 0.01 mW / nm in the entire wavelength band of the output radiation. The power spectral density of the broadband output radiation in the entire wavelength band may be at least 3 mW / nm.

[0107] Similar to other types of specialty optical fibers such as solid core PCF (SC-PCF), hollow core PCF (HC-PCF) can also be manufactured through a conventional manufacturing process that includes the following three main stages.

[0108] In stage one, a PCF preform is produced. According to an exemplary method (the so-called stack and pull technique), (pure silica) preformed capillaries and spacers are first stacked together to form a stack assembly. The stack assembly is inserted into a large sleeve (e.g., with a diameter of several centimeters) to form a preform. The gap between the stack assembly and the sleeve can be filled with, for example, silica rods with various outer diameters. Fig.10 (a) shows a cross-sectional image of an example preform. As shown, the example preform is formed by inserting a stacking assembly including seven (or other number) preformed capillaries CAP-1 and other elements (such as spacers (not shown)) into a sleeve JT. In some cases, the spacers can be in the form of rods or columns. In some cases, the spacers can be made of the same material as the preformed capillaries.

[0109] According to another exemplary method, instead of inserting the stack of preformed capillaries into the sleeve JT, the preformed capillary CAP-1 is inserted into the sleeve JT individually and sequentially. In this exemplary method, the sleeve JT remains horizontal. Each preformed capillary CAP-1 in the preformed capillary CAP-1 is inserted into the sleeve JT via one end of the sleeve JT. Once inserted, the preformed capillary CAP-1 is located at the bottom of the sleeve JT. Then, the preformed capillary CAP-1 is fixed to the inner wall of the sleeve JT, for example, by melting or fusing. Then, the sleeve JT is rotated by a predefined angle (e.g., 60°) to allow the next preformed capillary CAP-1 to be inserted into the bottom of the sleeve JT. Repeat this insertion, fixing and rotation process until all preformed capillaries are inserted and fixed to the inner wall of the sleeve JT.

[0110] Note that the capillary used in this stage is usually drawn from a commercially available glass tube in a separate drawing process, which can also be considered a manufacturing stage. The outer diameter of the preform can be, for example, between 10 mm and 100 mm, and the length of the preform is, for example, between 0.3 m and 2 m, for example about 1 m.

[0111] In stage two, the preform is held at the top of a conventional optical fiber drawing tower, where it is fed downward into a furnace (e.g., a graphite resistance furnace) to heat the lower section of the preform. The heated area of ​​the preform softens and elongates, with teardrop-shaped drops (also called drops) that pull the optical fiber downward. Additional pulling force and tension control are used to ensure that the preform is drawn into a billet. Typically, a one-meter long preform can be drawn into a billet of 10 m to 1000 m in length. This long billet is then cut into shorter segments, each of which is, for example, between 0.5 m and 2 m in length, such as about 1 m. Fig.10 (b) shows the Fig.10 A cross-sectional image of an example billet drawn from an example preform shown in (a). The example billet includes seven capillaries CAP-2, which surround a hollow core HC-2 and are supported by a support portion SP-2. Although the example billet looks more like a final HC-PCF (or a target HC-PCF), it is rigid and its physical dimensions (e.g., capillary dimensions, support portion diameter) are larger than those of the final HC-PCF. At the end of stage two, the outer diameter of the billet may be, for example, between 0.5 mm and 10 mm, and the outer diameters of the billet capillaries CAP-2 surrounding the hollow core HC of the billet may be, for example, between 100 µm and 1000 µm, respectively, and the wall thickness may be, for example, between 1 µm and 100 µm, respectively.

[0112] In stage three, each preform obtained in stage two is then taken to another conventional drawing tower suitably equipped to draw HC-PCF, and the preform is drawn into a final HC-PCF having target dimensions. Typically, a one-meter-long preform can be drawn into 100 meters of optical fiber in a continuous manner. In its final form, the outer diameter of the HC-PCF can be reduced to, for example, between 100 µm and 300 µm. Compared to stage two, the optical fiber drawing process performed in stage three additionally requires connecting the preform to a pressure source, which is used to pressurize the internal structure of the preform so that each of the capillaries expands in a desired and controlled manner. By carefully adjusting the relative pressure between different parts of the internal structure of the preform, such as the capillary cavity CC and the hollow core HC, the preform capillaries CAP-2 expand, so that their outer diameter increases relative to the inner diameter of the support portion SP-2. By comparing the pressure in Fig.10 (b) and Fig.10 This effect is evident in the two cross-sectional images shown in (c). Fig.10 The ratio of the outer diameter of each capillary CAP-3 of the drawn optical fiber shown in (c) to the inner diameter of the support portion SP-3 is greater. At the end of stage three, the outer diameter of the capillary CAP-3 can be increased to, for example, between 10 µm and 50 µm, and correspondingly, the thickness of the capillary wall WP can be reduced to, for example, 0.05 µm to 1 µm. The positive pressure difference between each capillary cavity CC and the hollow core HC can be in the range between 1 millibar (mbar) and 1000 mbar.

[0113] As described above, the multiple anti-resonance elements (e.g. Figure 7 A single ring arrangement of a glass capillary CAP as shown) is used to guide the propagation of radiation through the optical fiber. The light guiding effect is established by antiresonant interference at the glass-air boundary (inverse Fabry-Perot effect) and is strongest at the core-facing region of the capillary wall WP. In hollow core negative curvature fibers such as HC-PCF, the total fiber losses are affected by both mode confinement losses and material losses. Confinement losses are losses caused by the leaky nature of the mode and the imperfect structure of the HC-PCF, while material losses are caused by the absorption of light in the fiber material. Importantly, the confinement losses of the HC-PCF are strongly affected by the thickness of the capillary wall WP. Specifically, the wall thickness forms a resonance at which light leaks out of the capillary CAP. The resonance position λq can be estimated by the following formula:

[0114] ,[1]

[0115] where q is a positive integer (the resonance order), t is the nominal wall thickness, and n g is the refractive index of the glass (e.g. n for quartz glassg is about 1.45).

[0116] Fig.11 An example simulation of the confinement loss of HC-PCF as a function of wavelength is shown. As can be seen in the figure, there is a sharp resonant feature with a peak at λq. This sharp resonant feature causes the confinement loss to be significantly increased by more than three orders of magnitude relative to the wavelength of, for example, the adjacent resonance.

[0117] Due to geometric variations in the raw materials and process errors during drawing, the dimensions of the capillaries are not perfect, including aspects such as capillary outer diameter, capillary wall thickness, capillary concentricity, and capillary bow. In state-of-the-art fiber drawing facilities, the outer diameter is monitored as a function of the capillary length. And in a post-processing step, the best matching capillary is picked for preform assembly. However, the measurement of wall thickness is more challenging because standard recording devices (e.g. based on optical shadowmetry) cannot evaluate wall thickness. Without accurate measurement of wall thickness, existing preform manufacturing processes lack control over the wall thickness of the capillaries used to manufacture the preforms. Therefore, there may be large differences in the wall thickness of the capillaries of the preforms and, therefore, the resulting HC-PCF.

[0118] HC-PCFs with capillary wall WP of varying or non-uniform thickness will result in broadened resonance peaks instead of sharp peaks (e.g. Fig.11 ), resulting in an undesirable overlap with the generated broadband spectrum. If the broadband spectrum is deliberately chosen to extend into the UV (e.g. 200 nm to 900 nm), the broad resonance can negatively impact the conversion efficiency from the input radiation IRD to the wavelength region below the resonance. In this case, more powerful pump lasers are required to maintain the desired or useful power levels in the corresponding wavelength region, resulting in higher equipment costs and, typically, a larger system footprint. Additionally, the wall thickness of the individual capillaries should not be significantly less than required, as this can significantly increase confinement losses, especially at longer wavelengths (e.g. wavelengths above 1000 nm).

[0119] As described above, during the fabrication of the final HC-PCF, the blank capillary CAP-2 is effectively expanded by overpressure relative to the hollow core region to reach its target dimensions (e.g., a diameter of about 10 to 25 um at the fiber level). It is critical that the blank capillary CAP-2 is expanded in a desired manner because: 1) the capillary dimensions (e.g., the outer diameter of each capillary) set the size of the inscribed core, which affects the generated broadband output radiation ORD; 2) the thickness of the capillary wall WP determines the resonant wavelength and should ideally correspond to a wavelength that falls outside the spectrum of the output radiation ORD; 3) the ratio between the capillary outer diameter and the hollow core diameter determines the optical fiber's light-guiding properties.

[0120] The inventors have found that in stage three of the above-mentioned manufacturing process, the expansion rate of the blank capillary CAP-2 is strongly determined by the wall thickness of the capillary CAP-2: the thinner the capillary wall, the higher the expansion rate and the higher the sensitivity to process variations. Therefore, when the blank capillary CAP-2 has different wall thicknesses, it is difficult to obtain a final HC-PCF with equal or similar capillary diameters. Using HC-PCFs with different capillary diameters can lead to deterioration of output performance. By way of example, a capillary diameter variation of ±5% will result in an output PSD variation of ±5%. Moreover, drawing a final HC-PCF from a blank having a capillary CAP-2 with different wall thicknesses and therefore different expansion rates may have a negative impact on yield. For example, if the expansion rate of one capillary is greater than that of the other capillaries, the capillary may contact its nearest neighboring capillary, thereby compromising the fiber structure and making the drawing unusable.

[0121] It is therefore an object of the present invention to alleviate at least some of the above-mentioned problems (e.g. output spectrum affected by broad fiber resonances, output performance or yield deteriorated by variations in capillary diameter or expansion ratio fiber manufacturing). The inventors have found that these problems can be alleviated if the variation in capillary wall thickness of a PCF preform is better controlled or reduced. The inventors have also found that the requirement for better consistency or less variation in the wall thickness of the preformed capillaries does not have to be applied to the entire circumference of each constituent capillary, but can be applied only to the portion facing the fiber core region. In order to be able to evaluate the wall thickness variation, it is necessary to accurately measure the wall thickness of each capillary along its circumference and the length of the fiber. Note that the wall thickness distribution along the circumference of the capillary is also referred to as the azimuthal distribution of the wall thickness of the capillary.

[0122] Based on these findings, the inventors proposed a method for producing a photonic crystal fiber (PCF) preform. Fig.12 , the proposed method 1200 may include, for example, the following five main steps.

[0123] Step 1210: Obtain a plurality of preformed capillaries (e.g., Fig.10 Capillary CAP-1 shown in (a). A plurality of preformed capillaries can be obtained by drawing a plurality of larger and thicker glass tubes using a conventional hot drawing method. The diameter and wall thickness of the preformed capillaries are much larger than the final (or target) PCF capillary. The diameter of each of the preformed capillaries can be, for example, between 0.5 mm and 20 mm, and the wall thickness can be, for example, between 50 µm and 1000 µm.

[0124] Step 1220: Measure the wall thickness of each of the plurality of preformed capillaries along at least a portion of the entire circumference of the preformed capillary. At least a portion of the circumference of the capillary may be, for example, at least 25%, at least 50%, at least 70%, at least 90%, or most or all of the circumference. The wall thickness of the capillary may be measured by an optical measuring device. In an embodiment, the measurement may be performed sequentially or in parallel at a plurality of (azimuth) positions along the circumference of the capillary. The plurality of (azimuth) locations may be, for example, at least 3, at least 5, at least 10, at least 20, or at least 50 different positions along the circumference of the capillary. At each measurement position, a wall thickness value may be obtained. The minimum angular difference (or azimuth resolution) between two immediately adjacent measurement positions may be limited by a measurement setup (e.g., the type and resolution of the measurement device, the physical arrangement of the measurement setup). The azimuth resolution may be, for example, less than 40°, less than 30°, less than 20°, less than 10°, less than 5°, or less than 1°.

[0125] In an embodiment, the wall thickness of each preformed capillary may be measured with the aid of an interferometric optical sensor (e.g., such as a tube measurement system sold by Nirox). In an embodiment, step 1220 may include measuring the wall thickness at multiple axial positions along at least a portion of the full length of each preformed capillary. In contrast to conventional wide-field microscope-based measurement methods that only inspect and measure the end faces of the capillaries, interferometry-based measurement methods allow the capillaries to be inspected and measured over any predetermined length or over the full length. For example, known tube measurement systems allow the preformed capillaries to be optically scanned over their entire length.

[0126] In an embodiment, step 1220 may further include generating a set of azimuthally resolved wall thickness data for each of a plurality of axial positions. In an embodiment, the azimuthally resolved wall thickness data may include a plurality of angular thickness pairs, each angular thickness pair corresponding to a data point obtained at a measurement position along the circumference of the capillary. In an embodiment, each set of azimuthally resolved wall thickness data may be represented by a polar plot showing the distribution of wall thickness along the circumference at an axial position.

[0127] Step 1230: Selecting some preformed capillaries from the plurality of preformed capillaries based at least on a thickness criterion, wherein the thickness criterion is that a wall thickness along at least a circumferential portion of the preformed capillary does not vary by more than a maximum thickness variation relative to a nominal wall thickness. The circumferential portion of the preformed capillary is a wall portion having a certain length along a circumferential direction.

[0128] At least the circumference portion may cover, for example, at least 10%, at least 25%, at least 40% or at least 50% of the circumference. The nominal wall thickness may be, for example, less than 200 μm, less than 180 μm, less than 160 μm, less than 140 μm, less than 120 μm or less than 100 μm. The maximum thickness variation is the absolute value of the maximum percentage difference between the measured wall thickness and the nominal wall thickness. The maximum thickness variation may be, for example, less than 15%, less than 10%, less than 5% or less than 1%. The combination of a given nominal wall thickness and a given maximum thickness variation defines a wall thickness range having a maximum thickness value and a minimum thickness value. By way of example, a maximum thickness variation of 10% and a nominal wall thickness of 200 μm define a wall thickness range, the maximum thickness value of the wall thickness range being calculated as 180 μm by (100%-10%)×200 μm, and the minimum thickness value of the wall thickness range being calculated as 220 μm by (100%+10%)×200 μm.

[0129] In an embodiment, the nominal wall thickness may be determined prior to performing the method.

[0130] In an embodiment, nominal wall thickness can be determined based on the measured wall thickness of a plurality of preformed capillaries. In an embodiment, the determination of nominal wall thickness can include obtaining a total average wall thickness by averaging the measured wall thickness on a plurality of preformed capillaries. The determined total average wall thickness can be used as nominal wall thickness subsequently. Determining total average wall thickness can include, for example, determining the individual capillary average wall thickness of each preformed capillary, and averaging these individual capillary average wall thicknesses. Determining individual capillary average wall thickness can include determining one or more circumferential average wall thickness values ​​of each preformed capillary, and each circumferential average wall thickness value is obtained by averaging the wall thickness measured along at least a portion of the circumference of the preformed capillary at the corresponding axial position. Then, the circumferential average wall thickness value can be averaged according to each capillary, to obtain the individual capillary average wall thickness according to each capillary, or the circumferential average wall thickness value is averaged on all axial positions of all capillaries, thereby directly determining total average wall thickness.

[0131] In an embodiment, step 1230 may include identifying, for each preformed capillary, a wall portion in which the measured thickness does not vary relative to the nominal wall thickness by more than a maximum thickness variation. In an embodiment, step 1230 may also include determining, for each preformed capillary, whether the identified wall portion covers a desired percentage of the circumference of the corresponding preformed capillary. This identification step may be performed using measured thickness data, such as azimuthally resolved wall thickness data. For example, for each capillary, one or more azimuthally resolved sets of wall thickness data (each set being obtained at an axial position) may be used for identification of the wall portion. When more than one azimuthally resolved set of wall thickness data is used, the wall thicknesses measured at the same azimuth position but at different axial positions may be averaged to generate an average thickness value for the corresponding azimuth position.

[0132] Step 1240, placing the selected preformed capillaries in a glass tube (or sleeve) to form a PCF preform; wherein the selected preformed capillaries are oriented so that a portion of each of the selected preformed capillaries in which the measured wall thickness varies from the nominal wall thickness by no more than a maximum thickness variation faces a longitudinal axis of the glass tube, the glass tube being substantially symmetrical about the longitudinal axis.

[0133] In an embodiment, step 1240 may include: assembling the selected preformed capillaries into a stacked assembly, wherein the selected preformed capillaries are oriented so that the portion of each of the selected preformed capillaries wherein the measured wall thickness varies by no more than a maximum thickness variation relative to the nominal wall thickness faces a longitudinal axis of the stacked assembly, the stacked assembly being substantially symmetrical about the longitudinal axis; and inserting the stacked assembly into a glass tube to form a PCF preform.

[0134] In an embodiment, step 1240 may include: inserting one of the selected preformed capillaries into the glass tube; orienting the inserted preformed capillary so that the portion of the inserted preformed capillary in which the measured wall thickness relative to the nominal wall thickness does not vary by more than the maximum thickness variation faces the longitudinal axis of the glass tube; fixing the inserted preformed capillary to the inner surface of the glass tube; rotating the glass tube by a predetermined angle to allow another selected preformed capillary to be inserted; and repeating the inserting, orienting, fixing, and rotating steps until all of the selected preformed capillaries have been inserted and fixed to the glass tube.

[0135] In an embodiment, step 1240 may further include marking a central region of the portion of each of the selected preformed capillaries where the measured wall thickness does not vary from the nominal wall thickness by more than a maximum thickness variation. In an embodiment, one or more lines may be engraved on one or both end faces of each selected preformed capillary (e.g., with the aid of a laser beam, a knife edge, or any other suitable tool) to indicate the midpoint or boundary of the wall portion having a satisfactory thickness and facing the longitudinal axis of the glass tube. In an embodiment, one or more lines may be engraved along the outer surface of the wall portion having a satisfactory thickness and facing the longitudinal axis of the glass tube.

[0136] The PCF preform produced by the above method 1200 can then be used to draw PCF. Therefore, the present invention also provides a method for producing PCF. Fig.13 , method 1300 may include, for example, the following three main steps.

[0137] Step 1310 : Produce a PCF preform according to the method 1200 for producing a PCF preform.

[0138] Step 1320: Drawing a PCF blank from the PCF preform. In an embodiment, step 1320 may be substantially the same as the second stage of a conventional optical fiber drawing process as described above.

[0139] Step 1330: Drawing the final PCF from the PCF preform. In an embodiment, step 1330 may be substantially the same as the third stage of the conventional optical fiber drawing process as described above.

[0140] In an embodiment, the final PCF produced using method 1300 may be a HC-PCF, which includes a hollow core extending axially along the HC-PCF and a cladding portion having a single ring arrangement of capillaries around the hollow core along the HC-PCF (e.g., as shown in FIG. Figure 7 ), wherein the variation in capillary wall thickness of the optical fiber can be, for example, less than 20% of the total average wall thickness, less than 15% of the total average wall thickness, less than 10% of the total average wall thickness, less than 5% of the total average wall thickness, or less than 1% of the total average wall thickness.

[0141] In embodiments, the total average wall thickness of the capillaries of the HC-PCF produced by method 1300 may be, for example, less than 200 nm, less than 180 nm, less than 160 nm, less than 140 nm, or less than 100 nm.

[0142] Compared to PCFs produced by conventional fiber drawing processes (e.g., as described above), the dimensions of the PCFs produced by the aforementioned embodiments are more accurately controlled and therefore have less deviation from the design targets. For example, in the case of HC-PCFs, the application of the aforementioned embodiments ensures that at least the portion of the capillary wall facing the hollow core of the fiber (where the light guiding effect is the strongest) has a more uniform (or less variable) thickness, which enables better control over the bandwidth and position of the fiber resonance caused by the interaction between the guided light and the capillary wall.

[0143] Fig.14 1400 is a block diagram illustrating a computer system 1400 that can assist in implementing the methods and processes disclosed herein. The computer system 1400 includes a bus 1402 or other communication mechanism for transferring information, and a processor 1404 (or multiple processors 1404 and 1405) coupled to the bus 1402 for processing information. The computer system 1400 also includes a main memory 1406, such as a random access memory (RAM) or other dynamic storage device, which is coupled to the bus 1402 for storing information and instructions to be executed by the processor 1404. The main memory 1406 can also be used to store temporary variables or other intermediate information during the execution of instructions to be executed by the processor 1404. The computer system 1400 also includes a read-only memory (ROM) 1408 or other static storage device, which is coupled to the bus 1402 for storing static information and instructions for the processor 1404. A storage device 1410, such as a magnetic disk or optical disk, is provided and coupled to the bus 1402 for storing information and instructions.

[0144] The computer system 1400 can be coupled to a display 1412, such as a cathode ray tube (CRT) or a flat panel or touch panel display, via the bus 1402 for displaying information to a computer user. An input device 1414 including alphanumeric keys and other keys is coupled to the bus 1402 for communicating information and command selections to the processor 1404. Another type of user input device is a cursor control 1416, such as a mouse, a trackball, or cursor direction keys, for communicating direction information and command selections to the processor 1404 and controlling cursor movement on the display 1412. The input device typically has two degrees of freedom on two axes, a first axis (e.g., x) and a second axis (e.g., y), which allows the device to specify a position in a plane. A touch panel (screen) display can also be used as an input device.

[0145] In response to processor 1404 executing one or more sequences of one or more instructions contained in main memory 1406, one or more methods described herein may be performed by computer system 1400. Such instructions may be read into main memory 1406 from another computer-readable medium such as storage device 1410. Execution of the sequences of instructions contained in main memory 1404 causes processor 1404 to perform the process steps described herein. One or more processors in a multi-processing arrangement may also be employed to execute the sequences of instructions contained in main memory 1406. In alternative embodiments, hard-wired circuitry may be used instead of software instructions or in combination with software instructions. Therefore, the description herein is not limited to any specific combination of hardware circuitry and software.

[0146] The term "computer-readable medium" as used herein refers to any medium that participates in providing instructions to the processor 1404 for execution. Such media can take many forms, including but not limited to non-volatile media, volatile media, and transmission media. Non-volatile media include, for example, optical or magnetic disks, such as storage device 1410. Volatile media include dynamic memory, such as main memory 1406. Transmission media include coaxial cables, copper wires, and optical fibers, including the wires that make up bus 1402. Transmission media can also take the form of sound waves or light waves, such as those generated during radio frequency (RF) and infrared (IR) data communications. Common forms of computer-readable media include, for example, floppy disks, flexible disks, hard disks, magnetic tapes, any other magnetic media, CD-ROMs, DVDs, any other optical media, punch cards, paper tapes, any other physical media with patterns of holes, RAM, PROMs and EPROMs, flash EPROMs, any other memory chips or memory cartridges, carrier waves described below, or any other media from which a computer can read.

[0147] Various forms of computer-readable media may be involved when carrying one or more sequences of one or more instructions to the processor 1404 for execution. For example, the instructions may initially be carried on a disk of a remote computer. The remote computer may load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to the computer system 1400 may receive data over the telephone line and convert the data into an infrared signal using an infrared transmitter. An infrared detector coupled to the bus 1402 may receive the data carried in the infrared signal and place the data on the bus 1402. The bus 1402 carries the data to the main memory 1406, from which the processor 1404 retrieves and executes the instructions. The instructions received by the main memory 1406 may optionally be stored on the storage device 1410 before or after execution by the processor 1404.

[0148] Preferably, the computer system 1400 also includes a communication interface 1418 coupled to the bus 1402. The communication interface 1418 provides a bidirectional data communication coupling to a network link 1420, which is connected to a local network 1422. For example, the communication interface 1418 can be an integrated services digital network (ISDN) card or a modem to provide a data communication connection with a corresponding type of telephone line. As another example, the communication interface 1418 can be a local area network (LAN) card to provide a data communication connection with a compatible LAN. A wireless link can also be implemented. In any such implementation, the communication interface 1418 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

[0149] The network link 1420 typically provides data communication to other data devices through one or more networks. For example, the network link 1420 can provide a connection to a host computer 1424 or data equipment operated by an Internet service provider (ISP) 1426 through a local network 1422. The ISP 1426, in turn, provides data communication services through a global packet data communication network (now generally referred to as the "Internet" 1428). Both the local network 1422 and the Internet 1428 use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on the network link 1420 and through the communication interface 1418 (carrying the digital data to and from the computer system 1400) are exemplary forms of carrier waves that transport the information.

[0150] The computer system 1400 can send messages and receive data, including program code, through (multiple) networks, network links 1420, and communication interfaces 1418. In the Internet example, a server 1430 can send a request code for an application program through the Internet 1428, ISP 1426, local network 1422, and communication interface 1418. For example, one such download application can provide one or more of the techniques described herein. The received code can be executed by the processor 1404 when it is received, and / or stored in the storage device 1410 or other non-volatile storage device for later execution. In this way, the computer system 1400 can obtain the application code in the form of a carrier wave.

[0151] Further embodiments are disclosed in the subsequent numbered list of items:

[0152] 1. A method for producing a photonic crystal fiber (PCF) preform, comprising:

[0153] obtaining a plurality of preformed capillaries;

[0154] measuring a wall thickness of each preformed capillary in the plurality of preformed capillaries at one or more axial locations along at least a portion of a circumference of the preformed capillary;

[0155] selecting some of the preformed capillaries from the plurality of preformed capillaries based at least on a thickness criterion that a wall thickness along at least a circumferential portion of the preformed capillary does not vary from a nominal wall thickness by more than a maximum thickness variation; and

[0156] The selected preformed capillaries are placed in a glass tube (or sleeve) to form a PCF preform having at least one annular arrangement of the selected preformed capillaries; wherein the selected preformed capillaries are oriented so that a portion of each of the selected preformed capillaries in which the measured wall thickness varies from the nominal wall thickness by no more than the maximum thickness variation faces the longitudinal axis of the glass tube, the glass tube being substantially symmetrical about the longitudinal axis.

[0157] 2. A method according to item 1, wherein the placing step includes: assembling selected preformed capillaries to obtain a stacked assembly, wherein the selected preformed capillaries are oriented so that the circumferential portion of each preformed capillary in the selected preformed capillaries wherein the measured wall thickness varies relative to the nominal wall thickness by no more than the maximum thickness variation faces the longitudinal axis of the stacked assembly, and the stacked assembly is substantially symmetrical about the longitudinal axis; and inserting the stacked assembly into the glass tube to form a PCF preform.

[0158] 3. The method of clause 1, wherein the placing step comprises:

[0159] inserting one of the selected preformed capillaries into the glass tube;

[0160] orienting the inserted preformed capillary tube so that the portion of the inserted preformed capillary tube in which the measured wall thickness does not vary from the nominal wall thickness by more than the maximum thickness variation faces the longitudinal axis of the glass tube;

[0161] fixing the inserted preformed capillary to the inner surface of the glass tube;

[0162] rotating the glass tube by a predetermined angle to allow another selected preformed capillary tube to be inserted; and

[0163] The inserting, orienting, securing and rotating steps are repeated until all of the selected preformed capillaries have been inserted and secured to the glass tube.

[0164] 4. The method according to any preceding clause, wherein the step of obtaining a plurality of preformed capillaries comprises drawing the plurality of preformed capillaries from a plurality of glass tubes.

[0165] 5. A method according to any preceding clause, wherein the wall thickness of each preformed capillary is measured by means of an interferometric optical sensor.

[0166] 6. A method according to any preceding clause, wherein the measuring step comprises measuring the wall thickness at a plurality of axial positions along at least a portion of the full length of each preformed capillary tube.

[0167] 7. A method according to any preceding clause, wherein the measuring step further comprises generating a set of azimuthally resolved wall thickness data for each axial position of the plurality of axial positions.

[0168] 8. The method of any preceding clause, wherein the measuring step comprises measuring the wall thickness of each preformed capillary of the plurality of preformed capillaries along at least 25% of the circumference of the preformed capillary.

[0169] 9. The method of any preceding clause, wherein the measuring step comprises measuring the wall thickness of each preformed capillary of the plurality of preformed capillaries along a majority or all of a circumference of the preformed capillary.

[0170] 10. A method according to any preceding clause, wherein the circumferential portion comprises at least 25% of the circumference.

[0171] 11. A method according to any preceding clause, wherein the circumferential portion comprises at least 50% of the circumference.

[0172] 12. A method according to any preceding clause, wherein the nominal wall thickness is determined prior to performing the method.

[0173] 13. The method of any one of clauses 1 to 11, wherein the nominal wall thickness is determined based on measured wall thicknesses of the plurality of preformed capillaries.

[0174] 14. The method of clause 13, wherein the determining of the nominal wall thickness further comprises determining an overall average wall thickness by averaging the measured wall thicknesses across the plurality of preformed capillaries.

[0175] 15. The method of clause 14, wherein determining the total wall thickness comprises determining an individual capillary average wall thickness for each preformed capillary and averaging the individual capillary average wall thicknesses.

[0176] 16. The method of clause 15, wherein determining the average wall thickness of the individual capillaries may comprise:

[0177] determining one or more circumferential average wall thickness values ​​per each preformed capillary, each circumferential average wall thickness value being obtained by averaging the wall thicknesses measured at corresponding axial locations along at least a portion of the circumference of the preformed capillary; and

[0178] The circumferentially averaged wall thickness values ​​averaged per capillary are averaged.

[0179] 17. The method of clause 14, wherein determining the total wall thickness comprises:

[0180] determining one or more circumferential average wall thickness values ​​per each preformed capillary, each circumferential average wall thickness value being obtained by averaging the wall thicknesses measured at corresponding axial locations along at least a portion of the circumference of the preformed capillary; and

[0181] The circumferential average wall thickness values ​​are averaged.

[0182] 18. A method according to any preceding clause, wherein the nominal wall thickness is less than 200 µm.

[0183] 19. A method according to any preceding clause, wherein the nominal wall thickness is less than 180 µm.

[0184] 20. A method according to any preceding clause, wherein the nominal wall thickness is less than 160 µm.

[0185] 21. A method according to any preceding clause, wherein the nominal wall thickness is less than 140 µm.

[0186] 22. A method according to any preceding clause, wherein the maximum thickness variation is less than 15%.

[0187] 23. A method according to any preceding clause, wherein the maximum thickness variation is less than 10%.

[0188] 24. A method according to any preceding clause, wherein the maximum thickness variation is less than 5%.

[0189] 25. The method of any preceding clause, further comprising: identifying, for each preformed capillary, a wall portion where the measured thickness falls within a thickness range.

[0190] 26. The method of any preceding clause, wherein the placing step further comprises: marking a central region of a circumferential portion of each of the selected preformed capillaries where the measured thickness falls within the thickness range.

[0191] 27. A method according to any preceding clause, wherein the number of preformed capillaries selected is at least six.

[0192] 28. A method according to any preceding clause, wherein the assembling step assembles the preformed capillaries into a single ring around the hollow core.

[0193] 29. A method for producing a photonic crystal fiber (PCF), comprising:

[0194] Producing a PCF preform according to any of the preceding clauses;

[0195] Drawing a PCF billet from the PCF preform; and

[0196] The final PCF is drawn from the PCF billet.

[0197] 30. A PCF product produced using the method described in item 29.

[0198] 31. The PCF product according to item 30 is a hollow core PCF (HC-PCF), which includes a hollow core and a cladding portion, wherein the hollow core extends axially along the HC-PCF, and the cladding portion has a single ring arrangement of multiple preformed capillaries around the hollow core along the HC-PCF, wherein the variation in wall thickness of the multiple preformed capillaries of the HC-PCF is less than 20% of the average wall thickness of the multiple preformed capillaries.

[0199] 32. A PCF product comprising a hollow core and a cladding portion, wherein the hollow core extends axially along a HC-PCF, and the cladding portion has a single ring arrangement of multiple glass capillaries around the hollow core along the HC-PCF, wherein the variation in wall thickness of the multiple glass capillaries of the HC-PCF is less than 20% of the average wall thickness of the multiple glass capillaries.

[0200] 33. A PCF according to clause 31 or 32, wherein the variation in wall thickness of the plurality of capillaries of the HC-PCF is less than 15% of the average wall thickness of the plurality of capillaries.

[0201] 34. A PCF according to any one of clauses 31 to 33, wherein the variation in wall thickness of the plurality of capillaries of the HC-PCF is less than 10% of the average wall thickness of the plurality of capillaries.

[0202] 35. A PCF according to any one of clauses 31 to 34, wherein the variation in wall thickness of the plurality of capillaries of the HC-PCF is less than 5% of the average wall thickness of the plurality of capillaries.

[0203] 36. A PCF according to any one of clauses 31 to 35, wherein the average wall thickness of the plurality of capillaries of the HC-PCF is less than 200 nm.

[0204] 37. A PCF product according to any one of clauses 31 to 36, wherein the average wall thickness of the plurality of preformed capillaries of the HC-PCF is less than 180 nm.

[0205] 38. A PCF product according to any one of clauses 31 to 37, wherein the average wall thickness of the plurality of preformed capillaries of the HC-PCF is less than 160 nm.

[0206] 39. A PCF product according to any one of clauses 31 to 38, wherein the average wall thickness of the plurality of preformed capillaries of the HC-PCF is less than 140 nm.

[0207] Although specific reference may be made herein to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, guidance and detection patterns for magnetic domain memories, and the like.

[0208] Although embodiments of the present invention may be specifically referenced herein in the context of lithographic equipment, embodiments of the present invention may be used in other equipment. Embodiments of the present invention may form part of a mask inspection equipment, a metrology equipment, or any equipment that measures or processes objects such as a wafer (or other substrate) or a mask (or other pattern forming device). These equipment may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0209] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention is not limited to optical lithography and may be used in other applications, such as imprint lithography, where the context permits.

[0210] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in a manner other than as described. The above description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that modifications may be made to the described invention without departing from the scope of the claims set forth below.

Claims

1. A method for producing a photonic crystal fiber (PCF) preform, comprising: obtaining a plurality of preformed capillaries; measuring a wall thickness of each preformed capillary in the plurality of preformed capillaries at one or more axial locations along at least a portion of a circumference of the preformed capillary; selecting some of the preformed capillaries from the plurality of preformed capillaries based at least on a thickness criterion that a variation of the wall thickness along at least a circumferential portion of the preformed capillary relative to a nominal wall thickness does not exceed a maximum thickness variation; as well as The selected preformed capillaries are placed in a glass tube or sleeve to form a PCF preform having at least one annular arrangement of the selected preformed capillaries; wherein the selected preformed capillaries are oriented so that a portion of each of the selected preformed capillaries in which the measured wall thickness varies from the nominal wall thickness by no more than the maximum thickness variation faces a longitudinal axis of the glass tube, the glass tube being substantially symmetrical about the longitudinal axis.

2. The method according to claim 1, wherein: The placement steps include: assembling the selected preformed capillaries to obtain a stacked assembly, wherein the selected preformed capillaries are oriented such that the circumferential portion of each of the selected preformed capillaries faces the longitudinal axis of the stacked assembly, in which circumferential portion the measured wall thickness varies from the nominal wall thickness by no more than the maximum thickness variation, the stacked assembly being substantially symmetrical about the longitudinal axis; and The stack assembly is inserted into the glass tube to form the PCF preform.

3. The method according to claim 1, wherein: The placement steps include: inserting one of the selected preformed capillaries into the glass tube; orienting the inserted preformed capillary tube so that the portion of the inserted preformed capillary tube in which the measured wall thickness does not vary from the nominal wall thickness by more than the maximum thickness variation faces the longitudinal axis of the glass tube; fixing the inserted preformed capillary to the inner surface of the glass tube; rotating the glass tube by a predetermined angle to allow another selected preformed capillary tube to be inserted; and The inserting, orienting, securing and rotating steps are repeated until all of the selected preformed capillaries have been inserted and secured to the glass tube.

4. A method according to any preceding claim, wherein: The step of obtaining the plurality of preformed capillaries includes drawing the plurality of preformed capillaries from a plurality of glass tubes.

5. A method according to any preceding claim, wherein: The wall thickness of each preformed capillary is measured by means of an interferometric optical sensor.

6. A method according to any preceding claim, wherein: The measuring step includes measuring the wall thickness at a plurality of axial locations along at least a portion of the overall length of each preformed capillary tube.

7. A method according to any preceding claim, wherein: The measuring step further includes generating a set of azimuthally resolved wall thickness data for each axial position of the plurality of axial positions.

8. A method according to any preceding claim, wherein: The measuring step includes measuring the wall thickness of each preformed capillary of the plurality of preformed capillaries along at least 25% of the circumference of the preformed capillary.

9. A method according to any preceding claim, wherein: The circumferential portion comprises at least 25% of the circumference.

10. A method according to any preceding claim, wherein: The nominal wall thickness stated is less than 200 µm.

11. A method according to any preceding claim, wherein: The maximum thickness variation is less than 15%.

12. The method according to any preceding claim, further comprising: Wall portions where the measured thickness falls within the thickness range are identified for each preformed capillary.

13. A method according to any preceding claim, wherein: The placing step further includes marking a central region of the circumferential portion of each of the selected preformed capillaries where the measured wall thickness falls within the thickness range.

14. A PCF product comprising a hollow core and a cladding portion, wherein the hollow core extends axially along a HC-PCF, and the cladding portion has a single ring arrangement of multiple glass capillaries around the hollow core along the HC-PCF, wherein the variation in wall thickness of the multiple glass capillaries of the HC-PCF is less than 20% of the average wall thickness of the multiple glass capillaries.

15. The PCF according to claim 14, wherein: The average wall thickness of the plurality of capillaries of the HC-PCF is less than 200 nm.

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