Hollow-core fiber light source and method for manufacturing a hollow-core fiber

Abstract

A method of treating an output coupling end of a hollow core optical fiber comprising a plurality of anti-resonant elements surrounding a hollow core, and a hollow core optical fiber that has been so treated. The method comprises: performing a tapering step to form a taper in the anti-resonant elements; performing a cleaving step at the taper to form at least one tapered output coupling end of the hollow core optical fiber; and performing an end treatment step comprising further heating the output coupling end in a controlled manner to smooth the output coupling end.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to EP application 20189800.4, filed on 6 August 2020, and EP application 20207371.4, filed on 13 November 2020, the entire contents of which are incorporated herein by reference. Technical Field

[0003] The present invention relates to a light source and a method for operating the light source, specifically, the light source being a broadband light source for use in photolithography equipment or measuring tools. Background Technology

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

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

[0006] Low-ki lithography can be used to process features with dimensions smaller than the classical resolution limit of a lithographic apparatus. In such process, the resolution formula can be expressed as CD = kixX / NA, where λ is the wavelength of radiation employed by the lithographic apparatus, NA is the numerical aperture of the projection optics in the lithographic apparatus, CD is the "critical dimension" (generally the smallest feature size that is typically used to characterize the resolution of the lithographic apparatus, but in this case half-pitch, i.e. half the distance between two adjacent features), and ki is an empirical resolution factor. In general, the smaller ki the more difficult it becomes to reproduce the shape and dimensions planned by a circuit designer in the lithographic apparatus. To overcome these difficulties, sophisticated fine tuning steps can be applied to the lithographic projection apparatus and / or the design layout. These steps can include, for example and without limitation: optimization of NA, customized illumination schemes, use of phase- shift 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 incorporated under the term "resolution enhancement techniques" (RET). Alternatively, tight control loops for controlling the stability of the lithographic apparatus can be used to improve the reproduction of the pattern at low ki.

[0007] A metrology apparatus can be used to measure a parameter of interest of a structure on the substrate. For example, a metrology apparatus can be used to measure a parameter such as a critical dimension, an overlay between layers on the substrate, and an asymmetry of a pattern on the substrate. A beam of measurement radiation is used to illuminate the substrate. The radiation is diffracted by the structure on the substrate. The diffracted radiation is collected by an objective and captured by a sensor.

[0008] The beam of measurement radiation is provided by light emitted by a light source. This light is directed onto the substrate via a beamsplitter and an objective that collects diffracted radiation from the substrate.

[0009] The light source that provides the measurement radiation can be a broadband light source. The broadband light source can be generated using a gas-filled optical fiber. A laser source can be coupled to an input of the optical fiber of the light source and spectrally broadened in the optical fiber. SUMMARY

[0010] The very high spectral power density of such light sources and the plasma generated as a byproduct of the light generation both have the risk of damaging the hollow core photonic crystal fiber (HC-PCF) material. Thus, for example, it is desirable to provide a broadband light source with an increased operational lifetime. In particular, it is desirable to provide a broadband light source that includes a gas-filled hollow core photonic crystal fiber that generates less contaminant material during operation.

[0011] According to a first aspect, there is provided a method of processing an output coupling end of a hollow core optical fiber, the hollow core optical fiber comprising a plurality of anti-resonant elements surrounding a hollow core, the method comprising: performing a tapering step to form a taper in the anti-resonant elements; performing a cleaving step at the taper to form at least one tapered output coupling end of the hollow core optical fiber; and performing an end processing step, the end processing step comprising further heating the output coupling end in a controlled manner to smooth the output coupling end.

[0012] According to a second aspect of the application, there is provided a hollow core optical fiber, the hollow core optical fiber comprising: a plurality of anti-resonant elements surrounding a hollow core; an outer cladding surrounding the anti-resonant elements; an input coupling end having a first tapered region; and an output coupling end having a second tapered region; wherein anti-resonant elements are substantially collapsed at the output coupling end; and the output coupling end is substantially smooth such that an inner edge and / or an outer edge of the cladding comprises a circular profile at the output coupling end. BRIEF DESCRIPTION OF DRAWINGS

[0013] Embodiments of the application will now be described, by way of example only, with reference to the accompanying schematic drawings in which:

[0014] - Figure 1A A schematic overview of a lithographic apparatus is depicted;

[0015] - Figure 1B A schematic overview of a lithographic cell is depicted;

[0016] - Figure 2 A schematic representation of overall lithography is depicted, representing the cooperation between three key technologies to optimize semiconductor manufacturing;

[0017] - Figure 3A A schematic block diagram of an alignment sensor is described;

[0018] - Figure 3B A schematic block diagram of a level sensor is described;

[0019] - Figure 4A A schematic cross-sectional view of a hollow core fiber, which can form part of a radiation source according to embodiments, in a lateral plane, i.e. perpendicular to the axis of the fiber;

[0020] - Figure 4B A schematic representation of a radiation source for providing broadband output radiation according to embodiments is depicted; and

[0021] - Figure 4C(a) and (b) depict schematically a transverse cross-sectional view of an example of a hollow core photonic crystal fiber (HC-PCF) design for supercontinuum generation, each hollow core photonic crystal fiber (HC-PCF) can form part of a radiation source according to embodiments; Figure 5 depicts a schematic of a HC-PCF based light source with a gas cell;

[0022] - Figure 6 depicts a schematic example of an end of a HC-PCF with contaminant growth;

[0023] - Figure 7 depicts a hollow core fiber tapering and cleaving process according to known methods; and Figure 8 depicts a hollow core fiber tapering and cleaving process according to embodiments of the present application. DETAILED DESCRIPTION

[0024] In this document, the terms“radiation” and“beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0025] The term“reticle”,“mask” or“patterning device” as employed herein can be broadly interpreted to refer to a generic patterning device that can be used to impart a pattern to a beam of radiation, which pattern corresponds to a pattern that is to be created in a target portion of the substrate. The term“optical valve” can also be used in this context. Examples of other such patterning devices include a programmable mirror array and a programmable LCD array.

[0026] Figure 1AAn lithographic apparatus LA is schematically depicted. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV, DUV or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

[0027] The term "projection system" PS used herein should be interpreted broadly to encompass any type of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein can be considered as synonymous with the more general term "projection system" PS.

[0028] In addition to the substrate support WT, the lithographic apparatus LA can include a measurement platform. The measurement platform can be arranged to hold a sensor and / or a cleaning device. The sensor can be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement platform can hold a plurality of sensors. The cleaning device can be arranged to clean a portion of the lithographic apparatus, for example a portion of the projection system PS or a portion of the system that provides the immersion liquid. The measurement platform can be movable under the projection system PS when the substrate support WT is distanced from the projection system PS.

[0029] In operation, the radiation beam B is incident on the patterning device (e.g., mask MA) held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. After having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and the position measurement system IF, the substrate support WT can be accurately moved, for example, so as to position different target portions C in the path of the radiation beam B at a focussed and aligned position. Similarly, the first positioning device PM and possibly another position sensor (not explicitly depicted in Figure 1) can be used to accurately position the patterning device MA in relation to the path of the radiation beam B. The patterning device MA and the substrate W can be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they can be located in spaces between target portions. When the substrate alignment marks Pl, P2 are located between the target portions C, they are referred to as scribe-lane alignment marks.

[0030] As Figure 1B indicated, the lithography apparatus LA can form part of a litho cell LC (sometimes also referred to as a litho cell or (litho) cluster), which typically also includes devices to perform pre- and post-exposure processes on the substrate W. Conventionally, these devices include a spin coater SC to deposit a coating of photoresist on the substrate W, a developing apparatus DE to develop the exposed resist, an chill plate CH and a bake plate BK (e.g., to adjust the temperature of the resist). A substrate transport device, or robot RO, picks up substrates W from input / output stations I / O1, I / O2, moves them between the different process devices and delivers them to the lithography apparatus LA at a loading bay LB. The devices in the litho cell, which are often collectively referred to as a track or track system, are typically under the control of a track control unit TCU, which itself can be controlled by a supervisory control system SCS, which can also control the lithography apparatus LA, e.g., via a lithography control unit LACU.

[0031] In order to properly and consistently expose the substrate W by the lithography apparatus LA, it is desirable to inspect the substrate to measure properties of the patterned structures, such as overlay errors between subsequent layers, line thickness, critical dimensions (CD), etc. For this purpose, an inspection tool (not shown) can be included in the lithography complex LC. If errors or mistakes are detected, adjustments can be made, for example, to the exposure of subsequent substrates or to other processing steps to be performed on the substrate W, especially if the inspection is performed before other substrates W of the same batch or lot are still to be exposed or processed.

[0032] An inspection apparatus, which can also be referred to as a metrology apparatus, is used to determine properties of the substrate W, and in particular how properties of different substrates W vary, or how properties associated with different layers of the same substrate W vary between the different layers. The inspection apparatus can alternatively be configured to identify defects on the substrate W, and can for example be part of the lithography complex LC, or can be integrated into the lithography apparatus LA, or can even be a separate device. The inspection apparatus can measure properties on a latent image (an image in a resist layer after the exposure), or a semi-latent image (an image in a resist layer after an exposure followed by 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).

[0033] Generally, the patterning process in the lithography apparatus LA is one of the most critical steps in the process, which requires a high accuracy for the determined size and placement of structures on the substrate W. To ensure this high accuracy, as Figure 2 As schematically depicted in Fig. 1, the three systems can be combined in a so-called “holistic” control environment. One of these systems is the lithography apparatus LA, which is (virtually) connected to a metrology tool MT (second system) and to a computer system CL (third system). The key of this “holistic” environment is to optimize the co-operation between these three systems to enhance the overall process window and to provide a tight, i.e. strict, control loop to ensure that the patterning performed by the lithography apparatus LA stays within the process window. The process window defines a range of process parameters (e.g. dose, focus, overlay) for which a certain manufacturing process yields a defined result (e.g. functional semiconductor devices), typically allowing the process parameters in the lithography or patterning process to vary within the range.

[0034] The computer system CL can use the (part of the) design layout to be patterned to predict which resolution enhancement techniques will be used, and to perform computation lithography simulations and calculations to determine which mask layouts and lithography apparatus settings achieve the largest overall process window for the patterning process (depicted in the first scale SCI by the double arrow). Figure 2 Typically, the resolution enhancement techniques are arranged to match the patterning possibilities of the lithography apparatus LA. The computer system CL can also be used to detect where the lithography apparatus LA is currently operating within the process window (e.g. using input from the metrology tool MT) to predict whether there are defects due to, for example, sub-optimal processing (depicted in the second scale SC2 by the arrow pointing to "0"). Figure 2

[0035] The metrology tool MT can provide input to the computer system CL to enable accurate simulations and predictions, and can provide feedback to the lithography apparatus LA to identify possible drifts, for example in the calibration state of the lithography apparatus LA (depicted in the third scale SC3 by the multiple arrows). Figure 2

[0036] In lithographic processes, it is desirable to frequently measure the resulting structure, for example for process control and verification. Different types of metrology apparatus MT for making these measurements are known, including a scanning electron microscope or various forms of scatterometer metrology apparatus MT.

[0037] A scatterometer is a versatile instrument which allows measuring parameters of a lithographic process either by disposing a sensor in the pupil of the objective lens of the scatterometer or in a conjugate plane to the pupil (the measurement is then generally referred to as pupil-based measurement), or by disposing a sensor in an image plane or in a plane conjugate to the image plane (in this case, the measurement is then generally referred to as image- or field-based measurement). Such scatterometers and associated measurement techniques are further described in patent applications US20100328655, US2011102753A1, US20120044470A, US20110249244, US20110026032 or EP 1,628,164 A, which are incorporated herein in their entirety by reference. The aforementioned scatterometers can measure gratings using light from embodiments of the light sources discussed in this document.

[0038] ​​The overall measurement quality for a lithography parameter using a particular target is determined at least in part by the measurement recipe used to measure the lithography parameter. The term "substrate measurement recipe" can include one or more parameters of the measurement itself, one or more parameters of the one or more patterns measured, or both. For example, if the measurement used in the substrate measurement recipe is a diffraction-based optical measurement, the one or more parameters of the measurement can include the wavelength of the radiation, the polarization of the radiation, the angle of incidence of the radiation relative to the substrate, the orientation of the radiation relative to the pattern on the substrate, etc. One of the criteria used to select a measurement recipe can be, for example, the sensitivity of one of the measurement parameters to process variations. Further examples are described in U.S. Patent Application US2016-0161863 and published U.S. Patent Application US2016 / 0370717A1, which are incorporated by reference herein in their entirety. The light source in this document can be configured to be controllable in terms of the light source requirements of these substrate measurement recipes.

[0039] The lithographic apparatus can include one or more (e.g., multiple) alignment sensors by which the position of an alignment mark disposed on a substrate can be accurately measured. The alignment (or position) sensors can use optical phenomena such as diffraction and interference to obtain position information from an alignment mark formed on the substrate. An example of an alignment sensor used in current lithographic apparatuses is based on a self-referencing interferometer as described in US6961116. Various enhancements and modifications of the position sensor have been developed, for example as disclosed in US2015261097A1. All of these disclosures are incorporated herein by reference.

[0040] The mark, or alignment mark, can include a series of bars formed on or in a layer disposed on the substrate or (directly) in the substrate. The bars can be regularly spaced apart and act as grating lines, such that the mark can be considered a diffractive grating with a known spatial period (pitch). Depending on the orientation of the grating lines, the mark can be designed to allow measurement of position along the X-axis or along the Y-axis (the Y-axis being oriented substantially perpendicular to the X-axis). A mark including bars arranged at +45 degrees and / or -45 degrees relative to both the X-axis and the Y-axis allows X and Y measurements combined using the techniques described in US2009 / 195768A, which is incorporated herein by reference.

[0041] The alignment sensor optically scans each mark with a radiation spot to obtain a periodically varying signal, such as a sinusoidal wave. The phase of such a signal is analyzed to determine the position of the mark, and hence the position of the substrate relative to the alignment sensor, which in turn is fixed relative to a reference frame of the lithography apparatus. So-called coarse marks and fine marks can be provided in relation to different (coarse and fine) mark sizes, so that the alignment sensor can distinguish between different periods of the periodic signal and the exact position (phase) within the period. Marks of different pitches can also be used for this purpose.

[0042] Measuring the position of the marks can also provide information about the deformation of the substrate on which the marks are provided (e.g. in the form of a wafer grid). Deformation of the substrate can occur, for example, by electrostatic clamping of the substrate to a substrate table, and / or heating of the substrate when it is exposed to radiation.

[0043] Figure 3A is a schematic block diagram of an embodiment of a known alignment sensor AS, such as for example described in US 6961116, which is incorporated by reference. A radiation source RSO provides a beam RB of one or more wavelengths, which is turned by turning optics onto a mark, such as a mark AM on a substrate W, as an illumination spot SP. In this example, the turning optics comprise a spot mirror SM and an objective lens OL. The radiation source RSO can be provided by an embodiment of the light source of the disclosure of this document. The diameter of the illumination spot SP used to illuminate the mark AM can be slightly smaller than the width of the mark itself.

[0044] Radiation diffracted by the mark AM (in this example via the objective lens OL) is collimated into an information-bearing beam IB. The term "diffracted" is intended to include zeroth order diffraction from the mark (which can be referred to as reflection). A self-referencing interferometer SRI, such as of the type disclosed in US 6961116 mentioned above, interferes the beam IB with itself, after which the beam is received by a photodetector PD. In case more than one wavelength is generated by the radiation source RSO, additional optics (not shown) can be included to provide multiple separate beams. The photodetector can be a single element, or it can comprise multiple pixels, if desired. The photodetector can comprise a sensor array.

[0045] A topography measurement system, level sensor or height sensor that can be integrated in a lithographic apparatus is arranged to measure the topography of the top surface of a substrate (or wafer). A topography map (also known as a height map) of the substrate can be generated from these measurements indicative of the height of the substrate as a function of position of the substrate. This height map can subsequently be used to correct the position of the substrate during the transfer of the pattern on the substrate in order to provide a spatial image of the patterning device at the appropriate focus position on the substrate. It will be understood that "height" in this context refers to the dimension that is significantly out of plane (also referred to as the Z-axis) relative to the substrate. Typically, a level sensor or height sensor performs measurements at a fixed location (relative to the optical system of itself) and the relative motion between the substrate and the optical system of the level sensor or height sensor results in height measurements at multiple locations across the substrate.

[0046] Figure 3B An example of a level sensor or height sensor LS known in the art is schematically illustrated, which only illustrates the principle of operation. In this example, the level sensor comprises an optical system comprising a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a radiation beam LSB which is imparted with a projection grating PGR of the projection unit LSP. The radiation source LSO can comprise embodiments of the disclosure of the present document.

[0047] The present disclosure aims to improve the operational lifetime of light sources, in particular the operational lifetime of broadband light sources comprising hollow core photonic crystal fibers (HC-PCF). The broadband light sources of the present disclosure can be used in metrology tools, such as the scatterometer alignment sensor, height or level sensor described above.

[0048] A metrology tool MT, such as the scatterometer, topography measurement system, or position measurement system described above, can use radiation originating from a radiation source to perform measurements. The properties of the radiation used by the metrology tool can influence the type and quality of measurements that can be performed. For some applications, it can be advantageous to use multiple radiation frequencies to measure a substrate, for example broadband radiation can be used. Multiple different frequencies can be able to propagate, illuminate a metrology target and scatter off a metrology target without or with minimal interference from other frequencies. Thus, for example, different frequencies can be used to obtain more metrology data simultaneously. Different radiation frequencies can also be able to interrogate and discover different properties of a metrology target. Broadband radiation can be used in a metrology system MT, such as for example a level sensor, an alignment mark measurement system, a scatterometry tool, or an inspection tool. A broadband radiation source can be a supercontinuum source.

[0049] It can be difficult to produce high quality broadband radiation, such as supercontinuum radiation. One approach to producing broadband radiation can be to broaden high power narrowband or single frequency input radiation, for example, using nonlinear, high order effects. The input radiation, which can be produced using a laser, can be referred to as pump radiation. Alternatively, the input radiation can be referred to as seed radiation. To obtain high power radiation for the broadening effect, the radiation can be confined, i.e., limited, to a small area, so that a locally intensified high intensity radiation is achieved. In these areas, the radiation can interact with a broadening structure and / or material forming a nonlinear medium, in order to produce broadband output radiation. In the high intensity radiation areas, different materials and / or structures can be used to achieve and / or improve radiation broadening by providing a suitable nonlinear medium.

[0050] In some embodiments, broadband output radiation is produced in a photonic crystal fiber (PCF). In several embodiments, such a photonic crystal fiber has a microstructure around its fiber core, thereby facilitating confinement of radiation traveling through the fiber in the fiber core. The fiber core can be made of a solid material having nonlinear properties and is capable of producing broadband radiation when high intensity pump radiation is transmitted through the fiber core. Although it is feasible to produce broadband radiation in a photonic crystal fiber with a solid core, there can be some disadvantages to using a solid material. For example, if UV radiation is produced in a solid core, this radiation can not appear in the output spectrum of the fiber, because the radiation is absorbed by most solid materials.

[0051] In some embodiments, as discussed further below with reference to Figure 4B Methods and apparatus for broadening input radiation can use an optical fiber for confining input radiation and for broadening the input radiation to output broadband radiation, as discussed further below. The optical fiber can be a hollow core fiber and can include an internal structure for enabling efficient guiding and confinement of radiation in the fiber. The optical fiber can be a hollow core photonic crystal fiber (HC-PCF), which is particularly suitable for strong radiation confinement mainly within the hollow core of the fiber, thereby enabling high radiation intensity. The hollow core of the optical fiber can be filled with a gas or a gas mixture that acts as a broadening medium for broadening the input radiation. Such a fiber and gas mixture arrangement can be used to produce a supercontinuum radiation source. The radiation input to the optical fiber can be electromagnetic radiation, such as radiation in one or more of the infrared, visible, UV, and extreme UV spectra. The output radiation can consist of or include broadband radiation, which can be referred to herein as white light. The output radiation can cover the UV, visible, and near infrared ranges. The exact spectrum and power density of the output radiation will be determined by a number of parameters, such as the fiber structure, the gas mixture composition, the gas pressure, the energy of the input radiation, the pulse duration and shape of the input radiation.

[0052] Some embodiments relate to new designs of 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 anti-resonant structures for confining the radiation. Such optical fibers comprising anti-resonant structures are known in the art as anti-resonant fibers, tubular fibers, single-ring fibers, negative curvature fibers or suppressed coupling fibers. Various different designs of such fibers are known in the art. Alternatively, the optical fiber can be a photonic bandgap fiber (HC-PBF, e.g. a Kagome fiber).

[0053] Various types of HC-PCF can be designed, each based on a different physical guiding mechanism. Two such HC-PCFs include: a hollow core photonic bandgap fiber (HC-PBF) and a hollow core anti-resonant reflecting fiber (HC-ARF). Details on the design and fabrication of HC-PCFs can be found in US patent US2004 / 015085 Al (for HC-PBF) and international PCT patent application WO2017 / 032454 Al (for hollow core anti-resonant reflecting fiber), which are incorporated herein by reference. Figure 4C (a) shows a Kagome fiber comprising a Kagome lattice structure.

[0054] Reference is now made to Figure 4A Examples of optical fibers for use in the radiation source are described, Figure 4A is a schematic cross-sectional view of the optical fiber OF in a transverse plane. Similar to Figure 4A Other embodiments similar to the actual example of the optical fiber of

[0055] The optical fiber OF comprises an elongated body that is longer in one dimension than the other two dimensions. This longer dimension can be referred to as an axial direction and can define an axis of the optical fiber OF. The other two dimensions define a plane that can be referred to as a transverse plane. Figure 4A A cross-sectional view of the optical fiber OF in this transverse plane, labeled as the x-y plane, is shown (i.e. perpendicular to the axis). The transverse cross-section of the optical fiber OF can be substantially constant along the optical fiber axis.

[0056] It will be appreciated that the optical fiber OF has a degree of flexibility or pliability and thus the direction of the axis will not generally be uniform along the length of the optical fiber OF. Terms such as optical axis, transverse cross-section, etc. will be understood to refer to local optical axis, local transverse cross-section, etc. Furthermore, when components are described as cylindrical or tubular, these terms will be understood to encompass these shapes as they can deform when the optical fiber OF is bent.

[0057] The optical fiber OF can have any length, and it will be appreciated that the length of the optical fiber OF can depend on the application. The length of the optical fiber OF can be between 1 cm and 10 m or between 0.1 cm and 10 m, for example the length of the optical fiber OF can be between 10 cm and 100 cm.

[0058] The optical fiber OF comprises a hollow core COR, a cladding portion surrounding the hollow core COR, and a support portion SP surrounding and supporting the cladding portion. The optical fiber OF can be considered to comprise a body (comprising the cladding portion and the support portion SP) having the hollow core COR. The cladding portion comprises a plurality of anti-resonant elements for guiding radiation through the hollow core COR. In particular, the plurality of anti-resonant elements are arranged to confine radiation propagating through the optical fiber OF mainly within the hollow core HC and to guide said radiation along the optical fiber OF. The hollow core HC of the optical fiber OF can be substantially disposed in a central region of said optical fiber OF, such that an axis of the optical fiber OF can also define an axis of the hollow core HC of the optical fiber OF.

[0059] The cladding portion comprises a plurality of anti-resonant elements for guiding radiation to propagate through the optical fiber OF. In particular, in this embodiment, the cladding portion comprises a single ring having six tubular capillaries CAP. Each tubular capillary CAP acts as an anti-resonant element.

[0060] The capillaries CAP can also be referred to as tubes. The cross-section of the capillaries CAP can be circular, or can have other shapes. Each capillary CAP comprises a substantially cylindrical wall portion WP which at least partially defines the hollow core HC of the optical fiber OF and separates the hollow core HC from a capillary cavity CC. It will be appreciated that said wall portion WP can act as an anti-reflection Fabry-Perot resonator for radiation propagating through said hollow core HC (and which can be incident on said wall portion WP at a grazing incidence angle). The thickness of said wall portion WP can be adapted to ensure that substantially enhanced reflection back into the hollow core HC is ensured, while substantially transmission into the capillary cavity CC is suppressed. In some embodiments, the capillary wall portion WP can have a thickness between 0.01 pm - 10.0 pm.

[0061] It will be appreciated that, as used herein, the term cladding portion is intended to mean the portion of the optical fiber OF for guiding radiation to propagate through the optical fiber OF (i.e. the capillaries CAP which confine said radiation within the hollow core COR). The radiation can be confined in the form of transverse modes propagating along the optical fiber axis.

[0062] The support portion is substantially tubular and supports six capillaries CAP of the cladding portion. If an inner support portion SP, the six capillaries CAP are evenly distributed around the inner surface. The six capillaries CAP can be described as being arranged in a substantially hexagonal formation.

[0063] The capillaries CAP are arranged such that each capillary is not in contact with any other capillary CAP. Each capillary CAP is in contact with the inner support portion SP and is spaced apart from adjacent capillaries CAP in the annular formation. This arrangement can be beneficial as it can increase the transmission bandwidth of the optical fibre OF (for example relative to an arrangement in which the capillaries are in contact with each other). Alternatively, in some embodiments, each capillary CAP can be in contact with adjacent capillaries CAP in the annular formation.

[0064] The six capillaries CAP of the cladding portion are arranged in an annular formation around the hollow core COR. The inner surface of the annular formation of capillaries CAP at least partially defines the hollow core HC of the optical fibre OF. The diameter d (which can be defined as the smallest dimension between opposing capillaries, indicated by arrow d) of the hollow core HC can be between 10 pm and 1000 pm. The diameter d of the hollow core HC can affect the mode field diameter, the impact loss, the dispersion, the modal complexity, and the non-linear properties of the hollow core fibre OF.

[0065] In this embodiment, the cladding portion comprises a single ring arrangement of capillaries CAP (which act as anti-resonant elements). Thus, a line in any radial direction from the centre of the hollow core HC to the outside of the optical fibre OF does not pass through more than one capillary CAP.

[0066] It will be appreciated that other embodiments can have different anti-resonant element arrangements. These can include arrangements with multiple rings of anti-resonant elements, and arrangements with nested anti-resonant elements. Furthermore, although the embodiments described above have been described in relation to a hollow core fibre, it will be appreciated that the principles described above can be applied to other optical fibres, such as a photonic crystal fibre. Figure 4A The embodiments shown include a ring of six capillaries, but in other embodiments one or more rings can be provided in the cladding portion, the one or more rings comprising any number of anti-resonant elements (for example 4, 5, 6, 7, 8, 9, 10, 11 or 12 capillaries).

[0067] Figure 4C (b) shows a modified embodiment of the HC-PCF with a single ring of multiple tubular capillaries discussed above. In this embodiment, the inner ring of tubular capillaries 21 is replaced by a ring of solid rods 22. The rods 22 can be made of silica. Figure 4C In the example of (b), there are two coaxial rings of tubular capillaries 21. To hold the inner and outer rings of tubular capillaries 21, a support tube ST can be included in the HC-PCF. The support tube can be made of silica.

[0068] Figure 4A and Figure 4C The tubular capillary in examples of (a) and (b) can have a circular cross-sectional shape. The tubular capillary can also be other shapes, such as an elliptical or polygonal cross-section. Alternatively, Figure 4A and Figure 4C The solid material of the tubular capillary in examples of (a) and (b) can comprise a plastic material, such as PMA, glass, such as silica, or a soft glass.

[0069] Figure 4B A radiation source RDS is described for providing broadband output radiation. The radiation source RDS comprises a pulsed pump radiation source PRS, or any other type of source capable of producing short pulses with a desired length and energy level; an optical fiber OF having a hollow core COR (e.g. Figure 4A The radiation source RDS comprises the optical fiber OF shown in Figure 4B but in alternative embodiments, other types of hollow core optical fibers can be used. Figure 4A

[0070] 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 the input radiation IRD from the pulsed pump radiation source PRS and to broaden the input radiation IRD to provide output radiation ORD. The working medium WM is capable of broadening the frequency range of the received input radiation IRD in order to provide broadband output radiation ORD.

[0071] The radiation source RDS further comprises a reservoir RSV. The optical fiber OF is disposed inside the reservoir RSV. The reservoir RSV can also be referred to as a housing, a container or a gas cell. The reservoir RSV is configured to contain the working medium WM. The reservoir RSV can comprise one or more features known in the art for controlling, regulating and / or monitoring the composition of the working medium WM (which can be a gas) inside the reservoir RSV. The reservoir RSV can comprise a first transparent window TW1. In use, the optical fiber OF is disposed within the reservoir RSV such that the first transparent window TW1 is located proximate to the input end IE of the optical fiber OF. The first transparent window TW1 can form part of a wall of the reservoir RSV. The first transparent window TW1 can be transparent at least to the received input radiation frequencies, such that the received input radiation IRD (or at least a substantial portion thereof) can be coupled into the optical fiber OF located inside the reservoir RSV. It will be appreciated that optics (not shown) can be provided for coupling the input radiation IRD into the optical fiber OF.

[0072] ​The reservoir RSV includes a second transparent window TW2 that forms part of a wall of the reservoir RSV. In use, when the optical fiber OF is disposed inside the reservoir RSV, the second transparent window TW2 is positioned proximate to the output end OE of the optical fiber OF. The second transparent window TW2 can be transparent at least to the frequencies of the broadband output radiation ORD of the device.

[0073] Alternatively, in another embodiment, two opposing ends of the optical fiber OF can be placed inside different reservoirs. The optical fiber OF can include a first end section configured to receive the input radiation IRD, and a second end section for outputting the broadband output radiation ORD. The first end section can be placed inside a first reservoir that includes the working medium WM. The second end section can be placed inside a second reservoir, where the second reservoir can also include the working medium WM. The operation of the reservoirs can be as described above with respect to Figure 4B the first reservoir can include a first transparent window configured to be transparent to the input radiation IRD. The second reservoir can include a second transparent window configured to be transparent to the broadband output broadband radiation ORD. The first and second reservoirs can also include sealable openings to allow the optical fiber OF to be placed partially inside the reservoirs and partially outside the reservoirs, such that a gas can be sealed inside the reservoirs. The optical fiber OF can also include an intermediate section that is not contained inside the reservoirs. This arrangement using two separate gas reservoirs can be particularly convenient for embodiments in which the optical fiber OF is relatively long (e.g., when the length is greater than 1 m). It will be appreciated that for this arrangement using two separate gas reservoirs, the two reservoirs (which can 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 considered to provide a device for providing the working medium WM into the hollow core HC of the optical fiber OF.

[0074] In this context, a window can be transparent to a frequency if at least 50%, 75%, 85%, 90%, 95%, or 99% of the incident radiation of that frequency that is incident on the window is transmitted through the window.

[0075] Both the first and second transparent windows TW1, TW2 can form a gas-tight seal within the wall of the reservoir RSV, such that the working medium WM (which can be a gas) can be contained within the reservoir RSV. It will be appreciated that the gas WM can be contained within the reservoir RSV at a pressure that is different from the ambient pressure of the reservoir RSV.

[0076] The working medium WM can comprise an inert gas (such as argon, krypton and xenon), a Raman active gas (such as hydrogen, deuterium and nitrogen), or a gas mixture (such as an argon / hydrogen mixture, a xenon / deuterium mixture, a krypton / nitrogen mixture, a krypton / helium or a nitrogen / hydrogen mixture). Depending on the type of the filling gas, the nonlinear optical processes can include modulation instability (MI), soliton self-compression, soliton fission, Kerr effect, Raman effect and dispersive wave generation, the details of which are described in WO2018 / 127266A1 and US9160137B1, both of which are incorporated herein by reference. Since the dispersion of the filling gas can be adjusted by varying the working medium WM pressure (i.e. gas cell pressure) in the reservoir RSR, the generated broadband pulse dynamics and associated spectral broadening characteristics can be tuned in order to optimize the frequency conversion.

[0077] In an embodiment, at least during the process of receiving input radiation IRD for generating broadband output radiation ORD, the working medium WM can be arranged within the hollow core HC. It will be understood that the gas WM can be entirely or partially absent from the hollow core COR when the optical fiber OF does not receive input radiation IRD for generating broadband output radiation.

[0078] For achieving frequency broadening, high intensity radiation can be required. Hollow core fibers OF have the advantage that by strongly spatially confining the radiation propagating through the optical fiber OF, high intensity radiation can be achieved, resulting in high local radiation intensities. For example, the radiation intensity inside the optical fiber OF can be high due to the high intensity of the received input radiation and / or due to the strong spatial confinement of the radiation inside the optical fiber OF. Hollow core fibers have the advantage that they can guide radiation having a wider wavelength range than solid core fibers, and in particular, hollow core fibers can guide radiation in both the ultraviolet and infrared ranges.

[0079] An advantage of using a hollow core fiber OF can be that a large portion of the radiation guided inside the optical fiber OF is confined to the hollow core COR. Thus, a large portion of the interactions of the radiation inside the optical fiber OF are interactions with the working medium WM, which is arranged inside the hollow core HC of the optical fiber OF. Thus, the broadening effect of the working medium WM on the radiation can be increased.

[0080] The received input radiation IRD can be electromagnetic radiation. The input radiation IRD can be received as pulsed radiation. For example, the input radiation IRD can comprise ultrafast pulses, for example generated by a laser.

[0081] The input radiation IRD can be coherent radiation. The input radiation IRD can be collimated radiation, which is advantageous in that it can facilitate and improve the efficiency of coupling the input radiation IRD to the optical fiber OF. The input radiation IRD can comprise a single frequency, or a narrow range of frequencies. The input radiation IRD can be generated by a laser. Similarly, the output radiation ORD can be collimated radiation and / or can be coherent radiation.

[0082] The broadband range of the output radiation ORD can be a continuous range, comprising a continuous range of frequencies of the radiation. The output radiation ORD can comprise supercontinuum radiation. Continuous radiation can be beneficial to use in many applications, such as metrology applications. For example, a continuous range of frequencies can be used to interrogate a large number of properties. For example, a continuous range of frequencies can be used to determine and / or cancel out frequency dependencies, i.e. frequency correlations, of the measured properties. For example, supercontinuum output radiation ORD can comprise electromagnetic radiation with a wavelength range of 100 nm - 4000 nm, or even up to 10 pm. For example, the frequency range of the broadband output radiation ORD can be 400 nm - 900 nm, 500 nm - 900 mm, or 200 nm - 2000 nm. Supercontinuum output radiation ORD can comprise white light.

[0083] The input radiation IRD provided by the pulsed pump radiation source PRS can be pulsed. The input radiation IRD can comprise electromagnetic radiation at one or more frequencies (between 200 nm and 2 pm). For example, the input radiation IRD can comprise electromagnetic radiation at a wavelength of 1.03 pm. The repetition rate of the pulsed radiation IRD can be in the order of 1 kHz to 100 MHz. The pulse energy can be in the order of 0.1 pJ to 100 pJ, for example 1 pJ - 10 pJ. The pulse duration of the input radiation IRD can be between 10 fs and 10 ps, for example 300 fs. The average power of the input radiation IRD can be between 100 mW and several 100 W. For example, the average power of the input radiation IRD can be 20 W - 50 W.

[0084] The pulsed pump radiation source PRS can be a laser. By adjusting the (pump) laser parameters, the working medium WM variations, and the optical fiber OF parameters, the spatio-temporal transmission characteristics of such laser pulses transmitted along the optical fiber OF (e.g. its spectral amplitude and phase) can be varied and adjusted. The spatio-temporal transmission characteristics can include one or more of: the output power, the output modal profile, the output temporal profile, the width of the output temporal profile (or output pulse width), the output spectral profile, and the bandwidth of the output spectral profile (or output spectral bandwidth). The pulsed pump radiation source PRS parameters can include one or more of: the pump wavelength, the pump pulse energy, the pump pulse width, the pump pulse repetition rate. The optical fiber OF parameters can include one or more of: the optical fiber length, the size and shape of the hollow core 101, the size and shape of the capillary, the thickness of the wall of the capillary surrounding the hollow core. The working medium WM (e.g. the fill gas) parameters can include one or more of: the gas type, the gas pressure, and the gas temperature.

[0085] The broadband output radiation ORD provided by the radiation source RDS can have an average output power of at least 1 W. The average output power can be at least 5 W. The average output power can be at least 10 W. The broadband output radiation ORD can be a pulsed broadband output radiation ORD. The power spectral density of the entire wavelength band of the output radiation of the broadband output radiation ORD can be at least 0.01 mW / nm. The power spectral density of the entire wavelength band of the broadband output radiation can be at least 3 mW / nm.

[0086] Figure 5 An example of a broadband light source apparatus comprising a HC-PCF is shown. The light source 500 can comprise a gas cell 502 in which a HC-PCF 501 is embedded. Each of the two ends of the HC-PCF 501 can be optically coupled to a window 503 / 504 through which the input and output beams travel.

[0087] In use, the gas cell 502 can be filled with a gas (which can be a gas mixture) and the HC-PCF 501 can be filled with the same or substantially similar gas. A gas flow can be established by providing a pressure difference between the input and output ends of the HC-PCF 501. Such pressure in the gas cell or / and HC-PCF 501 can be in the range of 0.1-100 bar. The gas cell can be a composite material.

[0088] An input source (such as a pump laser) enters the light source through the input window 503. The input beam 506 is focused and enters the HC-PCF 501. As the beam travels through the HC-PCF 501, the spectrum of the beam is broadened, resulting in a broadband output beam 505.

[0089] A gap 509 is provided between the end facet of the HC-PCF and the window, to prevent dielectric breakdown of the window, in particular the output window. As shown in Figure 5 the wideband beam segment 505 diverges as it exits the HC-PCF output facet, so that the intensity of the beam is lower once it reaches the output window 503. Thus, the intensity of the beam impinging on the output window 503 is lower than the intensity threshold for dielectric breakdown of the window.

[0090] When operating an HC-PCF based light source, such as the light source 500, contaminants grow over time on the end facets of the HC-PCF. In particular, the inventors have observed that the growth of contaminants develops, i.e. progresses, mainly on the output facet of the HC-PCF. The growth of contaminants appears to occur where the light intensity is the highest, in particular, the growth of contaminants is higher at the output facet of the HC-PCF than at the output window. The growth of contaminants also appears to occur where the spectrum of the light is broadened, in particular, the growth of contaminants on the input facet is less intense. Furthermore, contaminants are mainly observed on the output facet of the HC-PCF, rather than inside the HC-PCF itself.

[0091] Contaminants can be produced due to silica particles being ablated from the windows of the gas cell, or from the HC-PCF. The contaminants can undergo photo-induced processes with the broadened output light from the PCF, and change their chemical structure and / or crystallize on the output facet. After a number of hours of operation (e.g. after a certain dose (J) of laser energy has been transmitted), such contaminants can cause degradation of the performance of the optical fiber; this degradation can be referred to as the glassy growth phenomenon (GGP): for example, the growth of SiO x structures at the output end of the optical fiber.

[0092] The accumulation of these contaminants can cause the lifetime of the light source to be shortened. The GGP and the growth of contaminants produced at the output facet can protrude into the optical path of the diverging beam. This can cause scattering of the output light, and thus attenuation of the output power of the light source. The GGP can shorten the lifetime of the source: the GGP causes light scattering, and thus the optical fiber loses its performance. For example, this can cause the photon budget required by the sensor to not be met after about 200 hours. Furthermore, the GGP can cause drifts in the light source power / spectral density and modal profile, which, if not addressed, require frequent recalibration. As such, the short lifetime of the optical fiber means that the optical fiber is frequently replaced in the field, and can cause very serious downtime of the machine (of the order of a few days) per year. This is not acceptable for industrialized products.

[0093] Figure 6A schematic example of contaminant growth at the output facet of an HC-PCF after an extended period of operation is shown. To reduce these contaminant growths and extend the operational lifetime of the HC-PCF light source, it can be necessary to prevent the formation of contaminants, reduce their concentration in the light source apparatus or inhibit the contaminant-induced GGP growth. Possible techniques for reducing the contaminant concentration and preventing their formation include, rigorous cleaning of the parts of the light-emitting unit / bulb and the gaseous environment prior to use in the light source. However, achieving sufficient cleaning with known cleaning methods is challenging and it can not be possible to avoid all contaminants.

[0094] To describe the cause of the GGP, a description will be provided on how anti-resonant optical fibers can currently be manufactured. The HC-PCF is typically composed of pure fused silica (Si02) HC-PCF. In terms of geometry, the central hollow core is surrounded by a large number of thin-walled (e.g. ~ 170 nm wall thickness) anti-resonant optical fibers or tubes; e.g. arranged in a azimuthally uniform manner around the core. The core and tubes can be encapsulated in a fused silica cladding.

[0095] Figure 7 A process is illustrated that is currently available to form the ends of a hollow core fiber or HC-PCF. The optical fiber can undergo a tapering and cleaving stage. First, as shown in step 700, the optical fiber outer jacket OJ can be subjected to heat from a plasma discharge PL, causing the tubes ARE to partially collapse under surface tension. The result of which can be seen in step 710, where the length L tp over a relatively short tapering transition region of the Rayleigh length of the laser beam (non-adiabatic tapering), the diameter of the tubes ARE is reduced from a first diameter d to a second, smaller diameter d tp . As such, the diameter D of the hollow core is increased to a diameter D tp By applying a sharp blade BL (e.g. a diamond blade) to the optical fiber cladding, the optical fiber is cleaved at or near the waist of the tapered region, resulting in two parts, as shown in step 720, each part comprising a clean, new Si02 surface. During the cleaving process, small cracks created by the blade BL can be forced to propagate along the surface perpendicular to the longitudinal axis of the optical fiber by applying mechanical stress / tension. As illustrated in step 730, the tapering and cleaving process can be repeated on the second end of the optical fiber to produce a double-sided tapered optical fiber having an input coupling end IC and an output coupling end OC (of course, in comparison, the section of the optical fiber between the ends will be significantly longer). The ends can typically have substantially the same form and be interchangeable.

[0096] This process was initially developed for the input coupling side of the fiber to increase the damage threshold of the fiber by protecting it from the high intensity of the focus of the input laser. In this case, the capillary is pushed away from the laser focus so that the fiber is more robust or more robust to beam pointing instability, i.e. more resistant to beam pointing instability.

[0097] During the lifetime of the fiber, the growth of glassy structures at the OC end tip of the fiber is observed. The growth is more pronounced at the sharp edges of the tube and the sheath at the end tip. Several factors contribute to this, some of which are related to structural flaws or structural defects of the fiber end. One such problem is surface reduction in the presence of atomic hydrogen. It is known that silica glass can be reduced in the presence of atomic hydrogen at relatively low temperatures (~ 100 degrees Celsius) according to the following reaction: SiO2(s) + 4H → Si(s) + 2H2O(g). Molecular H2 requires temperatures in excess of 1000 °C to etch a silica surface. H2 sources include: H2 added deliberately to the gas mixture (e.g. 2% H2), which can be added to reduce tube variants, outgassing of the gas system surfaces and the fiber surface itself, hydrocarbons (C x H y ) and impurities.

[0098] Defect centers and structural flaws or structural defects (e.g. cuts and sharp edges) enhance surface reduction, for example by enhancing the production of atomic hydrogen. For example, defect centers act as "chemical centers" for the production of atomic hydrogen: it is a well-known phenomenon that molecular H2 at the defect site is broken with an activation energy of 0.3-0.4 eV. An example is the reaction at a silicon dangling bond: H2 + · Si (~ O3) → H + HSi (~ O3). In a controlled environment, this can be used to passivate the dangling bond. In an uncontrolled environment, this reaction can be a source of atomic hydrogen. In addition, defect centers form local hot spots (high temperature points) at the glass-gas interface. This provides or reduces the activation energy required for the photodissociation of H2 into atomic hydrogen. Defect centers enhance the local electric field at the glass-gas interface. This provides or reduces the activation energy required for the photodissociation of H2 into atomic hydrogen.

[0099] In addition, the structural cracks or structural defects enhance light absorption and multiphoton ionization in the glass. This enhances surface reduction at the local defects via increasing the temperature and atomic hydrogen concentration. It is known that the behavior of glass at structural cracks or structural defects is similar to that of a metal (absorbing light with wavelengths below 1 pm) [see, for example, "Metallic-like photoluminescence and absorption in fused silica surface flaws", Ted A. Laurence et al., Applied Physics Letters 94, 151114 (2009); which is incorporated herein by reference]. This is due to an increase in defect centers at the local of the structural cracks or structural defects. The defect centers provide local electrons. At these sites, the electronic structure of the glass matrix (which is essentially a dielectric) is disturbed. In the presence of short wavelength and high intensity light (i.e. supercontinuum pulses), multiphoton absorption and avalanche ionization in the glass contribute to the electrons being lifted from the valence band to the conduction band, forming an electron plasma at the surface of the glass. This enhances surface reduction by increasing the production of atomic hydrogen. The formation of a plasma in a transparent medium under high intensity light is a known effect and is used to produce a fusion of metal to glass, or a fusion of glass to glass. The electrons lifted to the conduction band during the duration of the supercontinuum pulse cause heating of the glass and extreme heat at the defects and cracks or structural defects.

[0100] The knowledge that cleaving produces atomically smooth surfaces is based on cleaving of crystals and the inventors have observed that this cannot be directly extrapolated to amorphous materials, such as fused silica used for optical fiber manufacturing. For such amorphous materials, the cleaving procedure results in a sudden termination of the fiber end and thus there are cases of (a) structural cracks and defects (e.g. sharp edges and defects) at the location of the blade impact on the cladding and (b) surface defects, such as dangling bonds (NBOHC and E'-centers) at the surface. As already explained, these become suitable centers for surface reduction and seeding of glass growth. As such, the current post-processing step leaves the end tip of the fiber with a rough surface and sharp edges and with a high concentration, i.e. a high number of glass defects (e.g. dangling bonds). It is proposed to employ an improved end processing step to address these issues and to control the glassy growth process at the end tip.

[0101] Therefore, an improved method for forming a tapered end of a hollow core fiber, such as an HC-PCF, is proposed. The main aim is to terminate the end tip of the fiber as smoothly as possible, thus making the glass properties at the tip as similar as possible to the bulk glass.

[0102] Figure 8 This is a schematic diagram of the modified processing steps. The method may include preparing the optical fiber in the same manner as existing methods, i.e., performing... Figure 7 The tapering and cutting steps are illustrated in the figure. Thus, steps 700, 710, 720, and 730 are essentially the same as steps 800, 810, 820, and 830. In this example, the first tapering step 810 partially opens the tube. Therefore, as shown in the cross-section CS... 830 As shown, the anti-resonant element ARE at the tapered end tp diameter d tp Smaller than the untapered fiber CS 800 The diameter d of the anti-resonant element ARE. Then, step 840 can be performed to heat the tip in a controlled manner; for example, by flame polishing or thermal polishing and smoothing at least the output coupling terminal OC. This thermal polishing and smoothing step 840 can be performed after the cutting step 820 by applying, for example, an additional electric arc (e.g., similar to the arc used in the tapering step), laser radiation (e.g., a CO2 laser), or a flame. The resulting heat will cause further collapse of the anti-resonant element, for example, causing the anti-resonant element ARE to substantially or completely collapse. cl And the inner edge or inner rim of the outer sheath OJ ir and / or outer edge or outer margin OJ or It is smoothed, as shown in the cross-sectional image CS. 840 As shown in the attached figures. Alternatively, the tapering step 810 before cutting can be modified to cause the anti-resonant element ARE to collapse substantially or completely, and then perform a thermal polishing and smoothing step 830.

[0103] Smooth inner edge OJ ir and / or outer edge OJ or These edges can be rounded, so that one or two of these edges have a substantially (partially) circular or elliptical profile. For example, the edge or edge radius of these edges (e.g., the radius of the white dashed circle in the figure) can be greater than 0.5 μm, greater than 2 μm, or greater than 5 μm.

[0104] Collapsed pipe ARE cl It may have an opening of no more than 10 μm, no more than 5 μm, no more than 2 μm or no more than 1 μm in at least one dimension of the opening plane (perpendicular to the fiber axis).

[0105] In embodiments, the length of the tapered section of the output coupling side OC can be longer or shorter than the length of the input coupling side IC. Alternatively, the lengths of the tapered sections of the input coupling and output coupling can be the same, but the taper profile at the output coupling side can be sharper (e.g., have a steeper profile gradient). As such, the process can result in an asymmetry between the input coupling end IC and the output coupling end OC of the optical fiber. This causes the anti-resonant elements ARE at the input coupling IC end to remain slightly open to counterbalance, i.e., balance, the positive gas pressure in the core.

[0106] The entire process can be performed in a protective atmosphere.

[0107] The method can include a final annealing step. As such, the process parameters (e.g., temperature, environmental components, i.e., atmosphere components, and / or pressure, etc.) can be selected to also provide annealing of the glass such that the entire optical fiber is annealed for removal of at least some remaining defects (i.e., dangling bonds) and residual stresses. As an example, heat can be provided by a laser (e.g., a CO2laser) or a flame.

[0108] Life tests performed on the end-processed optical fibers disclosed herein show a direct improvement in terms of lifetime. In particular, no decay of the power spectral density in the 500-900 nm spectral range was observed during the duration of the tests. This is the longest lifetime observed for optical fibers. A comparison between the glassy growth pattern of the end-processed optical fibers and a normal tapered optical fiber shows that for the end-processed optical fibers the dominant growth on the tube tip is completely eliminated and only a symmetric growth on the inner edge of the cladding is observable. The growth pattern is also much smoother. Thus, the total output power of a light source using such an optical fiber shows a much lower rate of gradual decrease compared to a source based on an unend-processed optical fiber.

[0109] Other embodiments are disclosed in the following numbered list of embodiments:

[0110] 1. A method of processing an output coupling end of a hollow core optical fiber, the hollow core optical fiber comprising a plurality of anti-resonant elements surrounding a hollow core, the method comprising:

[0111] performing a tapering step to form a taper in the anti-resonant elements;

[0112] performing a cleaving step at the taper to form at least one tapered output coupling end of the hollow core optical fiber; and

[0113] performing an end processing step, the end processing step comprising further heating the output coupling end in a controlled manner to smooth the output coupling end.

[0114] 2. A method as defined in aspect 1, comprising collapsing the anti-resonant elements substantially at the output coupling end.

[0115] 3. A method as defined in aspect 2, wherein the collapsing of the anti-resonant elements is performed as part of the end treatment step.

[0116] 4. A method as defined in aspect 2, wherein the collapsing of the anti-resonant elements is performed as part of the tapering step.

[0117] 5. A method as defined in aspect 2, 3 or 4, wherein the collapsing of the anti-resonant elements is such that each anti-resonant element has an opening at the output coupling end that is no larger than 1 pm in at least one dimension of the output plane.

[0118] 6. A method as defined in aspect 2, 3 or 4, wherein the collapsing of the anti-resonant elements is such that each anti-resonant element has an opening at the output coupling end that is no larger than 5 pm in at least one dimension of the output plane.

[0119] 7. A method as defined in any of aspects 2 to 6, wherein the method does not comprise an equivalent collapsing step of the anti-resonant elements at the input coupling end of the hollow core fiber.

[0120] 8. A method as defined in any of aspects 2 to 7, wherein the collapsing step is such that the length of the taper at the output coupling end is longer than the taper at the input coupling end.

[0121] 9. A method as defined in any of aspects 2 to 8, wherein the collapsing step is such that the profile of the taper at the output coupling end has a steeper gradient than the taper at the input coupling end.

[0122] 10. A method as defined in any preceding aspect, wherein the end treatment step comprises thermal polishing and smoothing the output coupling end.

[0123] 11. A method as defined in aspect 10, wherein the thermal polishing and smoothing step comprises applying one or more of an electric arc, laser radiation or a flame to the output coupling end.

[0124] 12. A method as defined in any preceding aspect, wherein the end treatment step is such that the resulting smoothed inner and / or outer rim at the output coupling end comprises a circular profile.

[0125] 13. A method as defined in aspect 12, wherein the circular profile of the inner and / or outer rim comprises a rim radius that is greater than 0.5 pm.

[0126] 14. The method as defined in aspect 12, wherein the rounded profile of the inner and / or outer edge comprises an edge radius greater than 5 pm.

[0127] 15. The method as defined in any preceding aspect, comprising performing an annealing step to anneal the output coupling end.

[0128] 16. The method as defined in any preceding aspect, wherein the hollow core fiber comprises a hollow core photonic crystal fiber.

[0129] 17. A hollow core fiber obtained by performing the method according to any preceding aspect.

[0130] 18. A hollow core fiber, comprising:

[0131] a plurality of anti-resonant elements surrounding a hollow core;

[0132] an outer cladding surrounding the anti-resonant elements;

[0133] an input coupling end having a first tapering region; and

[0134] an output coupling end having a second tapering region;

[0135] wherein anti-resonant elements are substantially collapsed at the output coupling end; and the output coupling end is substantially smooth such that an inner and / or outer edge of the cladding comprises a rounded profile at the output coupling end.

[0136] 19. The hollow core fiber as defined in aspect 18, wherein the rounded profile of the inner and / or outer edge comprises an edge radius greater than 0.5 pm.

[0137] 20. The hollow core fiber as defined in aspect 18, wherein the rounded profile of the inner and / or outer edge comprises an edge radius greater than 5 pm.

[0138] 21. The hollow core fiber as defined in any of aspects 18 to 20, wherein each of the anti-resonant elements has an opening at the output coupling end that is no greater than 1 pm in at least one dimension in an output plane.

[0139] 22. The hollow core fiber as defined in any of aspects 18 to 20, wherein each of the anti-resonant elements has an opening at the output coupling end that is no greater than 5 pm in at least one dimension in an output plane.

[0140] 23. The hollow-core fiber as defined in any of aspects 18 to 22, wherein each of the anti-resonant elements has an opening at the input-coupling end that is larger than an opening at the output-coupling end.

[0141] 24. The hollow-core fiber as defined in any of aspects 18 to 23, wherein the second tapering region has a length that is longer than a length of the first tapering region.

[0142] 25. The hollow-core fiber as defined in any of aspects 18 to 24, wherein the taper of the second tapering region has a steeper gradient than a taper of the first tapering region.

[0143] 26. The hollow-core fiber as defined in any of aspects 18 to 25, wherein the hollow- core fiber comprises a hollow-core photonic crystal fiber.

[0144] 27. A broadband light source device configured to produce a broadband output upon receipt of pump radiation, the broadband light source device comprising:

[0145] an optical component comprising:

[0146] a hollow-core fiber according to any of aspects 18 to 26; and

[0147] a gas mixture filling the hollow-core fiber.

[0148] 28. The broadband light source as defined in aspect 27, wherein the hollow-core fiber is coupled to an optical light source, the optical light source comprising a pump laser.

[0149] 29. A metrology device comprising a broadband light source according to the aspect, wherein the light source is configured to produce light for projection onto a substrate.

[0150] 30. The metrology device according to aspect 29, wherein the metrology device is one of: a scatterometer, an alignment sensor, or a leveling sensor.

[0151] Although specific reference can be made in this text to the use of the lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein can have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat-panel displays such as liquid-crystal displays (LCDs), thin-film magnetic heads, etc. It will be appreciated that, in the context of such alternative applications, any use of the terms wafer or die herein can be considered as being synonymous with the more general terms substrate or target portion, respectively. The substrate referred to herein can be processed, before or after exposure, in for example, a track or coating and development system (a tool that typically applies a resist layer to a substrate and develops the exposed resist), a metrology tool, and / or an inspection tool. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, e.g., in order to form a multi-layer IC, so that the term substrate as used herein can also refer to a substrate that already contains one or more processed layers.

[0152] Although specific embodiments of the application have been described above, it will be appreciated that the application can be practiced otherwise than as described.

[0153] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications can be made to the application as described without departing from the scope of the claims set out below.

Claims

1. A method for processing the output coupling end of a hollow core optical fiber, the hollow core optical fiber comprising a plurality of anti-resonant elements surrounding the hollow core, the method comprising: A tapering step is performed to form a tapered portion in the anti-resonant element; A cutting step is performed at the tapered portion to form at least one tapered output coupling end of the hollow core optical fiber; as well as An end-processing step is performed, which includes further heating the output coupling terminal in a controlled manner to smooth the output coupling terminal.

2. The method of claim 1, further comprising causing the anti-resonant element to collapse at the output coupling terminal.

3. The method according to claim 2, wherein, The collapse of the anti-resonant element is performed as part of the end processing step.

4. The method according to claim 2, wherein, The collapse of the anti-resonant element is performed as part of the tapering step.

5. The method according to claim 2, 3 or 4, wherein, The collapse of the anti-resonant element results in each anti-resonant element having an opening at the output coupling end, the opening being no greater than 1µm in at least one dimension of the opening plane perpendicular to the fiber axis.

6. The method according to claim 2, 3 or 4, wherein, The collapse of the anti-resonant element results in each anti-resonant element having an opening at the output coupling end, the opening being no greater than 5µm in at least one dimension of the opening plane perpendicular to the fiber axis.

7. The method according to claim 2, wherein, The method does not include the equivalent collapse step of the anti-resonant element at the input coupling end of the hollow core optical fiber.

8. The method according to claim 7, wherein, The collapse step causes the tapered portion at the output coupling end to be longer than the tapered portion at the input coupling end.

9. The method according to claim 7, wherein, The collapse step results in the profile of the tapered portion at the output coupling end having a steeper gradient than that at the input coupling end.

10. The method according to claim 1, wherein, The end processing steps include thermal polishing and smoothing the output coupling end.

11. The method according to claim 10, wherein, The thermal polishing and smoothing steps include applying one or more of an electric arc, laser radiation, or flame to the output coupling terminal.

12. The method according to claim 1, wherein, The end processing step results in a smoothed inner and / or outer edge at the output coupling end comprising a circular profile.

13. The method according to claim 12, wherein, The circular profile of the inner edge and / or the outer edge includes an edge radius greater than 0.5µm.

14. The method according to claim 12, wherein, The circular profile of the inner edge and / or the outer edge includes an edge radius greater than 5µm.

15. The method of claim 1, further comprising performing an annealing step to anneal the output coupling terminal.

16. The method according to claim 1, wherein, The hollow core optical fiber includes hollow core photonic crystal fiber.

17. A hollow core optical fiber, said hollow core optical fiber being obtained by performing the method according to any one of claims 1 to 16.

18. A hollow core optical fiber, comprising: Multiple anti-resonant elements surrounding the hollow core; The outer sheath surrounding the anti-resonant element; An input coupling end having a first tapered region; as well as The output coupling terminal has a second tapered region; The anti-resonant element is collapsed at the output coupling end; and the output coupling end is smooth, such that the inner and / or outer edges of the sheath include a circular profile at the output coupling end.

19. The hollow core optical fiber according to claim 18, wherein, The circular profile of the inner edge and / or the outer edge includes an edge radius greater than 0.5µm.

20. The hollow core optical fiber according to claim 18, wherein, The circular profile of the inner edge and / or the outer edge includes an edge radius greater than 5µm.

21. The hollow core optical fiber according to any one of claims 18 to 20, wherein, Each of the anti-resonant elements has an opening at the output coupling end, the opening being no greater than 1µm in at least one dimension of the opening plane perpendicular to the fiber axis.

22. The hollow core optical fiber according to any one of claims 18 to 20, wherein, Each of the anti-resonant elements has an opening at the output coupling end, the opening being no greater than 5µm in at least one dimension of the opening plane perpendicular to the fiber axis.

23. The hollow core optical fiber according to claim 18, wherein, Each of the anti-resonant elements has an opening at the input coupling terminal that is larger than the opening at the output coupling terminal.

24. The hollow core optical fiber according to claim 18, wherein, The length of the second tapered region is longer than the length of the first tapered region.

25. The hollow core optical fiber according to claim 18, wherein, The tapered portion of the second tapered region has a steeper gradient than the tapered portion of the first tapered region.

26. The hollow core optical fiber according to claim 18, wherein, The hollow core optical fiber includes hollow core photonic crystal fiber.

27. A broadband light source device configured to generate a broadband output upon receiving pump radiation, the broadband light source device comprising: Optical components, including: The hollow core optical fiber according to any one of claims 18 to 26; and A gas mixture that fills the hollow core of the optical fiber.

28. The broadband light source device according to claim 27, wherein, The hollow core fiber is coupled to an optical source, which includes a pump laser.

29. A measurement apparatus comprising the broadband light source apparatus of claim 27, wherein the light source is configured to generate light for projection onto a substrate.

30. The measuring device according to claim 29, wherein, The measuring device is one of a scattering instrument, an alignment sensor, or a leveling sensor.

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