Optical time-domain reflectometry for hollow-core optical fibers.

Abstract

A method of evaluating an optical system including a hollow core optical fiber includes the steps of providing an optical time domain reflectometry system including a light source configured to generate light pulses having a wavelength λ, a photodetector configured to detect light of wavelength λ, and an input / output fiber including a solid core optical fiber optically coupled at a proximal end to receive the light pulses from the light source and deliver light to the photodetector, the input / output fiber having a treated end facet at a distal end thereof, the end facet configured to suppress back reflections of light of wavelength λ caused by an interface between the glass forming the core of the solid core optical fiber and the air at the end facet. the steps of: aligning a distal end of the input / output fiber with a proximal end of the hollow-core optical fiber for optical transmission between the input / output fiber and the hollow-core optical fiber having gas present in the hollow core; operating a light source to generate light pulses for propagation along the input / output fiber and into the hollow-core fiber; receiving backscattered light generated by Rayleigh scattering of the light pulses from the gas in the hollow core of the hollow-core fiber and detecting the backscattered light with a photodetector to generate a detection signal; and processing the detection signal to produce an optical time-domain reflectometry profile including a distribution of the backscattered light power along the length of the hollow-core optical fiber.

Description

[Technical field]

[0001] The present invention relates to a method and system for performing optical time domain reflectometry using a hollow core optical fiber. [Background technology]

[0002] Optical time domain reflectometry (OTDR) is an established technique for spatially resolving and measuring continuous and local parameters of optical fibers. OTDRs are routinely used for measurements on fabricated fibers, fiber cables, and installed optical fiber systems such as communication links and networks.

[0003] OTDR is performed by shining a pulse of light into the end of a length of optical fiber. As the pulse propagates along the fiber, it undergoes scattering from the material forming the fiber, some of which is backscattered towards the illuminated end where it can be detected. The use of discrete pulses of known duration combined with a known propagation speed of light along the fiber allows the spatial origin of the detected light with respect to distance along the fiber to be determined, resulting in a profile of the reflected light level or power over the fiber length. Specific properties and conditions within or outside the fiber will modify the amount of backscattering, and the location of the scattering occurrence can be precisely localized with a spatial resolution that depends on the pulse length. This allows not only for fiber properties such as local defects and changes in structure to be identified, but also for the detection of external parameters that modify the fiber properties, such as temperature and pressure. Thus, fibers can be tested or characterized, or used as distributed sensors. In installed optical fiber systems containing multiple fiber segments, joints such as splices and connectors between different segments can be identified and characterized, and faults such as fiber breaks can be located.

[0004] To use backscatter in this way, it is crucial that the optical fiber is capable of generating backscatter for detection. In particular, OTDR detects Rayleigh scattering, which is the elastic scattering of an illuminated light pulse from the fiber material. Rayleigh scattering is caused by microscopic density variations in the fiber due to the non-crystalline structure of the glass from which the optical fiber is formed. Thus, OTDR is a proven and useful technique for evaluating solid core optical fibres (SCFs). SCFs comprise a longitudinal core of a first refractive index surrounded by a cladding of a lower refractive index, both the core and the cladding comprising glass. Propagating light is guided along the fibre by total internal reflection at the refractive index limit.

[0005] However, problems arise for other types of optical fibers. A class of optical fibers with hollow cores, i.e., cores defined by a longitudinal air gap surrounded by a cladding formed by a number of longitudinal capillaries including glass boundaries arranged in a defined structure, is becoming well developed. Light is guided via a mechanism different from total internal reflection in SCFs. These fibers, which may be referred to as hollow core fibres (HCFs), can demonstrate advantageously low optical propagation losses thanks to the lack of glass from the core, which causes absorption and attenuation in SCFs. HCFs are therefore of great interest for many applications, including communications and remote sensing, and have been shown to be capable of transmitting data over thousands of kilometres. However, the lack of a glass core, which gives HCFs their own attractive low-loss qualities, also removes most of the density variations that create Rayleigh scattering, making dense OTDRs with HCFs apparently unfeasible. Large reflections can be detected from the glass boundaries at the proximal and distal ends of the fiber, allowing simple measurements to be obtained, including fiber length measurements, but only information about the largest defects can be extracted from the reflected light (Reference [1]).

[0006] Alternative methods have been proposed for dispersion measurement and characterization of HCFs. One example is the optical side-scatter radiation measurement reported on an 11 km long hollow-core fiber of a type known as a photonic bandgap hollow-core fiber (Ref. [2]), where the authors noted that "the characterization requirements far exceed the capabilities of standard optical reflectance instruments."

[0007] A further example is given in US 10,845,268 (Reference [3]), which proposes alternating lengths of HCF and SCF and obtaining backscattering via OTDR from the SCF sections. This allows only local characterization of the fiber at the SCF location, but the introduction of the SCF negates the benefits of the HCF, such as low loss, low delay, high power handling, and low nonlinearity.

[0008] Therefore, methods to enable true OTDR in HCF are of interest. Summary of the Invention

[0009] Aspects and embodiments are set out in the accompanying claims.

[0010] According to a first aspect of certain embodiments described herein, there is provided a method of evaluating an optical system comprising a hollow-core optical fiber, the method comprising the steps of providing an optical time domain reflectometry system including a light source configured to generate light pulses having a wavelength λ, a photodetector configured to detect light of wavelength λ, and a solid-core optical fiber optically coupled at a proximal end to receive the light pulses from the light source and deliver light to the photodetector, the input / output fiber having a treated end facet at its distal end, the end facet configured to suppress back reflections of light of wavelength λ caused by an interface between the glass forming the core of the solid-core optical fiber and the air at the end facet. , and an input / output fiber; aligning a distal end of the input / output fiber and a proximal end of the hollow-core optical fiber for optical transmission between the input / output fiber and the hollow-core optical fiber having gas present in the hollow core; operating a light source to generate light pulses for propagation along the input / output fiber and into the hollow-core fiber; receiving backscattered light generated by Rayleigh scattering of the light pulses from the gas in the hollow core of the hollow-core fiber and detecting the backscattered light with a photodetector to generate a detection signal; and processing the detection signal to produce an optical time-domain reflectometry profile including a distribution of backscattered light power along a length of the hollow-core optical fiber.

[0011] According to a second aspect of certain embodiments described herein, there is provided an optical time domain reflectometry system, the optical time domain reflectometry system including: a light source configured to generate light pulses having a wavelength λ; a photodetector configured to detect light of wavelength λ; and a solid-core optical fiber optically coupled at a proximal end to receive the light pulses from the light source and deliver light to the photodetector; and an input / output fiber having a treated end facet at its distal end to suppress back reflection of light of wavelength λ caused by an interface between the glass forming the core of the solid-core optical fiber and the air at the end facet. an input / output fiber configured to receive a backscattered optical signal from a light source and a proximal end of the hollow-core optical fiber, the backscattered optical signal being generated by Rayleigh scattering of a light pulse from a light source in the gas within the hollow-core of the hollow-core optical fiber; and a processor configured to receive a detection signal generated by the light detector from a received backscattered light generated by Rayleigh scattering of a light pulse from a light source in the gas within the hollow-core of the hollow-core optical fiber, and to process the detection signal to produce an optical time-domain reflectometry profile comprising a distribution of the backscattered optical power along the length of the hollow-core optical fiber.

[0012] These and further aspects of certain embodiments are set out in the accompanying independent and dependent claims. It will be understood that the features of the dependent claims may be combined with each other and with the features of the independent claims in other combinations than those explicitly set out in the claims. Furthermore, the techniques described herein are not limited to the specific embodiments as set out below, but include and contemplate any appropriate combination of the features presented herein. For example, methods and systems according to the techniques described herein may be provided that include any one or more of the various features described below, as appropriate.

[0013] For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which: [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 shows a cross-sectional view of an example of a photonic-bandgap hollow-core optical fiber to which aspects of the present invention are applicable. [Diagram 2] FIG. 1 shows a cross-sectional view of a first example anti-resonant hollow-core optical fiber to which aspects of the present invention are applicable. [Diagram 3] FIG. 2 shows a cross-sectional view of a second example anti-resonant hollow-core optical fiber to which aspects of the present invention are applicable. [Figure 4] FIG. 13 shows a graph of simulation results of the component of backscattering in an anti-resonant hollow-core fiber as a function of wavelength. [Diagram 5] FIG. 13 shows a graph of simulation results of the backscattering component in an anti-resonant hollow-core fiber as a function of fiber core diameter. [Figure 6] FIG. 13 shows an experimentally obtained optical time-domain reflectometry profile of the backscattered power distributed over the fiber distance from a first sample of antiresonant hollow-core fiber. [Figure 7] FIG. 13 shows experimentally obtained optical time-domain reflectometry profiles of backscattered power distributed over fiber distance from a first sample of antiresonant hollow-core fiber at different optical pulse widths or durations. [Figure 8] FIG. 7 shows the optical time domain reflectometry profile of FIG. 6 normalized by fiber length. [Figure 9] FIG. 13 shows an experimentally obtained optical time-domain reflectometry profile of the backscattered power distributed over the fiber distance from a second sample of antiresonant hollow-core fiber. [Figure 10] FIG. 13 shows experimentally obtained optical time-domain reflectometry profiles of backscattered power distributed over fiber distance from a second sample of antiresonant hollow-core fiber at different optical pulse widths or durations. [Figure 11] FIG. 1 shows a schematic diagram of an optical time domain reflectometry system according to an example of the present invention. [Figure 12] FIG. 2 shows a simplified longitudinal cross-section of an input / output fiber of an optical time domain reflectometry system with distal end treatment for reduced back reflection according to a first example. [Figure 13] FIG. 13 shows a simplified longitudinal cross-section of an input / output fiber of an optical time domain reflectometry system with distal end treatment for reduced back reflection according to a second example. [Figure 14] FIG. 1 illustrates a simplified longitudinal cross-section of an input / output fiber of an optical time domain reflectometry system with an exemplary distal end treatment for low coupling loss. [Figure 15] FIG. 2 illustrates a schematic diagram of an exemplary alignment apparatus for an optical time domain reflectometry system, in accordance with an embodiment. [Figure 16] FIG. 2 illustrates a flowchart of an example method according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Aspects and features of certain examples and embodiments are discussed / described herein. Some aspects and features of certain examples and embodiments may be implemented in a conventional manner, and they are not discussed / described in detail for the purpose of brevity. Thus, it will be understood that aspects and features of the systems and methods discussed herein that are not described in detail may be implemented by conventional techniques for implementing such aspects and features.

[0016] A hollow-core optical fiber (HCF) has a structure that includes an array or configuration of holes, capillaries, or lumens in the fiber material that extend along the length of the fiber parallel to the longitudinal axis and are defined within a material such as glass. The configuration of holes may be referred to as a microstructure, and typically the microstructure surrounds a central hollow void or region that forms at least a portion of the cladding of the fiber and realizes a light-guiding core that may be filled with air or another gas. The capillaries of the microstructure are typically supported within a larger outer cladding tube made of glass. The propagation of light in air, made possible by the lack of a solid glass core, reduces the proportion of guided optical waves propagating in glass compared to solid-core fibers, resulting in benefits such as increased propagation speed, reduced losses from both absorption and scattering, and reduced nonlinear interactions. Hollow-core fibers are therefore highly attractive for applications including communications, enabling data transmission at nearly the speed of light in a vacuum, at higher optical powers, over a wider optical bandwidth, and relatively free from problems such as nonlinear and thermo-optical effects that can affect light traveling in solid fibers.

[0017] Hollow-core fibers can be classified into two main classes or types according to their optical guidance mechanism: hollow-core photonic bandgap fiber (HCPBF), alternatively called hollow-core photonic crystal fiber (HCPCF) (Reference [4]), and antiresonant hollow core fiber (AR-HCF or ARF) (Reference [5]), with various subcategories characterized by the geometry of the cladding capillary. The present disclosure is applicable to all types of hollow-core fibers, including these two main classes, and their associated subtypes as well as other hollow-core designs. It is noted that there is some overlapping terminology use in the art for the various classes of fibers. In this disclosure, the terms "hollow-core fiber" and "hollow-core microstructure fiber" are intended to cover all types of these fibers having the hollow cores described above. The terms "HCPBF" and "HCPCF" are used to refer to hollow-core fibers having a structure that achieves waveguiding through the photonic bandgap effect (described in more detail below). The terms "ARF" and "anti-resonant hollow-core fiber" are used to refer to hollow-core fibers having a structure that achieves waveguiding through the anti-resonant effect (also described in more detail below).

[0018] FIG. 1 shows a cross-sectional view of an example HCPBF 10. In this fiber type, the structured inner cladding 1 includes a regular, closely packed array of many small glass capillaries from which a central group is eliminated to define a substantially circular hollow core 2. The cladding capillaries are arranged in multiple rings around the core 2. The periodicity of the cladding structure 1 results in a substantially periodically structured refractive index and therefore a photonic band gap effect that confines optical waves propagating towards the core 2. These fibers may be described in terms of the number of cladding capillaries or "cells" that are eliminated to create the core 2. In the example of FIG. 1, the central 19 cells from the array are absent in the core region, resulting in a 19-cell core HCPBF. The structured cladding 1 is formed from six rings of cells surrounding the core 2, plus some cells in a seventh ring to improve the roundness of the outer surface of the structured cladding 1. In other examples, a different number of rings may be used to define the cladding 1. The outer cladding 3 surrounds the structured cladding 1 and is a tube that supports the capillaries of the structured cladding 1 .

[0019] In contrast to HCPBFs, anti-resonant hollow-core fibers guide light by anti-resonant optical guiding effects. The structured cladding of ARFs has a simpler configuration, including a much lower number of larger glass capillaries or tubes than HCPBFs, to give a structure without a high degree of periodicity so that photonic band gap effects are not important, but has some rotational periodicity on a larger scale, since the tubes are evenly arranged (with or without spaces). The cladding capillaries include only a single ring of capillaries around the core, and additional smaller capillaries may be included inside the primary single ring capillary. The structure means that anti-resonance is realized for propagation wavelengths that are not resonant with the cladding capillary wall thickness, in other words wavelengths within the anti-resonant window defined by the cladding capillary wall thickness. The cladding capillaries surround a central void or cavity that provides the hollow core of the fiber, which can accommodate anti-resonant guiding optical modes. The structured cladding can further accommodate cladding modes that may propagate primarily inside the capillary, in the glass of the capillary wall, or in the space or gap between the cladding capillary and the outer cladding of the fiber. The loss of these additional non-core guided modes is generally much higher than the loss of the core guided modes. The fundamental core guided mode typically has by far the lowest loss of the core guided modes. The anti-resonance achieved by the capillary wall thickness in anti-resonance with the wavelength of the propagating light acts to prevent coupling between the fundamental core mode and any of the cladding modes so that the light is confined to the core and can propagate with extremely low loss.

[0020] FIG. 2 shows a cross-sectional view of an exemplary simple anti-resonant hollow-core fiber. The fiber 10 has an outer tubular cladding 3. The structured inner cladding 1 includes a plurality of tubular cladding capillaries 4, in this example seven capillaries of the same cross-sectional size and shape, disposed inside the single-ring outer cladding 3, such that the long axis of each cladding capillary 4 is substantially parallel to the long axis of the outer cladding 3. The cladding capillaries 4 are evenly spaced around the inner circumference of the outer cladding 3, and are also spaced apart from one another such that adjacent capillaries do not touch, with each cladding capillary 4 in contact (bonded to) the inner surface of the outer cladding 3 at location 5. In some designs of ARFs, the cladding tubes 4 may be positioned in contact with one another (in other words, rather than spaced apart as in FIG. 2), but spacing them apart to eliminate this contact can improve the optical performance of the fiber. The spacing eliminates nodes that occur at the contact points between adjacent tubes and tend to cause undesirable resonances that result in high losses. Thus, a fiber containing spaced apart cladding capillaries is sometimes referred to as a "nodeless anti-resonant hollow-core fiber."

[0021] The formation of the cladding capillary 4 in an annulus around the inside of the tubular outer cladding 3 creates a central space, cavity, or void in the fiber 10 that is the hollow core 2 of the fiber, further having a longitudinal axis parallel to the longitudinal axes of the outer cladding 3 and the capillary 4. The core 2 is bounded by the inward-facing portion of the outer surface of the cladding capillary 4. This is the core boundary, and the capillary wall material (e.g., glass or polymer) that makes up this boundary provides the required anti-resonant optical guiding effect or mechanism. The capillary 4 has a thickness at the core boundary that defines the wavelength at which anti-resonant optical guiding occurs in the ARF.

[0022] FIG. 3 shows a cross-sectional view of a second exemplary ARF. The ARF 10 has a structured inner cladding 1 including six cladding capillaries 4 evenly spaced around the inner surface of the tubular outer cladding 3, forming a single primary annulus surrounding the hollow core 2. Each cladding capillary 4 has a secondary, smaller capillary 6 nested within it, bonded to the inner surface of the cladding capillary 4, in this example at the same azimuthal location 5 as the point of bonding between the primary capillary 4 and the outer cladding 3. These additional smaller capillaries 6 can reduce optical losses. Also, additional smaller tertiary capillaries may be nested within the secondary capillary 6. This type of ARF design, including secondary capillaries and optionally additional smaller capillaries, may be referred to as "nested antiresonant nodeless fibres" or NANFs (reference [6]). In other NANF designs, one or more additional secondary capillaries may be nested within the primary capillary. Many other capillary configurations for the structured cladding of the ARF are feasible, and the disclosure is not limited to the above examples. For example, the capillaries need not be of circular cross section and / or may or may not all be the same size and / or shape. The number of capillaries surrounding the core in the primary ring may be, for example, 4, 5, 6, 7, 8, 9, 10, or more.

[0023] Hollow core optical fibers may be made from any of the glass-based materials known for making solid-core fibers, especially silica. Types of glass include "silicate glasses" or "silica-based glasses" based on chemically synthesized silica (silicon dioxide, or quartz), of which there are many examples. Other glasses suitable for optical fibers include, but are not limited to, doped silica glasses. The materials may contain one or more dopants to tailor the optical properties of the fiber, such as modifying absorption or transmission, or to tailor the properties of the material, such as to facilitate fiber manufacture, improve reliability, or enable or enhance a particular end use. Fibers may also be made from polymeric materials.

[0024] As used herein, terms including hollow core optical fiber, hollow core fiber, hollow core waveguide, hollow core optical waveguide, hollow core microstructured fiber, hollow core microstructured waveguide, and similar terms are intended to cover optical waveguiding structures constructed according to any of the above examples and similar structures, where light is guided by any of several guiding mechanisms (photonic bandgap guiding, antiresonant guiding, and / or blocked coupling guiding) within a hollow, elongated air gap or core surrounded by a structured (microstructured) cladding that includes a plurality of longitudinal capillaries. These various terms may be used interchangeably in this disclosure.

[0025] For all hollow-core fibers, the common attribute is the lack of a glass core and therefore the lack of microscopic glass density variations that produce Rayleigh scattering, traditionally detected in optical time-domain reflectometry. HCFs, although highly developed and practical, can be manufactured in multi-kilometer lengths with extremely low loss or attenuation levels (reportedly 0.22 dB / km), can transmit data over thousands of kilometers (demonstrated in recirculating loop experiments), and are beginning to be cabled and installed in ducts to carry live data traffic. To further drive manufacturing improvements and to enable testing of HCF cables in the lab and in the field, it would be useful to be able to utilize established distributed characterization techniques to identify fiber faults, manufacturing imperfections, or irregularities along the fiber length. While these functions are accomplished by using OTDRs for solid-core fiber systems, conventional understanding suggests that this cannot be usefully applied to HCFs due to the extremely low levels of backscatter.

[0026] Rayleigh backscattering, as detected in an OTDR, is a weak optical effect. Current conventional solid-core optical fibers made from silica glass (such as standard single-mode fiber (SMF)) can typically produce a backscattering signal from a 1-meter length of fiber that is about 72 dB below the incident power (a backscattering coefficient of -72 dB / m). With commercially available OTDR instruments, this level of backscattering is consistent with normal characteristics for fibers with lengths up to about 160-400 km (e.g., using the FTB7600 model from EXFO, Canada, or the LOR-200 model from Luciol, Switzerland). In contrast, the lowest loss data transmission reported to date, HCF, is a NANF design (Ref. [7]), and has a much lower level of backscattering from glass surfaces within the fiber structure, reportedly as low as -118 dB / m, more than 45 dB lower than the backscattering signal in a typical SMF. A custom-built OTDR reflectometer capable of measuring backscattered signals as weak as -140 dB / m with a spatial resolution better than 1 m was required to perform this measurement (Ref. [8]). However, such equipment is not commercially available, so extensive testing and evaluation of HCFs, especially in the field, cannot currently be performed.

[0027] However, this disclosure proposes an approach to adapt optical time domain reflectometry to hollow core optical fiber applications.

[0028] Backscattering from antiresonant hollow-core fibers (ARFs) has been studied (ref. [9]). Three backscattering contributors have been analyzed. To compare their (relative) magnitudes, calculations given at 1550 nm for a fiber with a core diameter of 35 μm are considered here. The Rayleigh backscattering from the microstructured glass mass (i.e., forming the cladding capillaries) was found to be very low (-150 dB / m), which is understandable considering that only a very small proportion of the guided light propagates through the glass (less than 0.01% of the light energy). Two other contributors are identified as the backscattering caused by the surface roughness of the glass microstructured surface (-115 dB / m) and the Rayleigh backscattering from gases that may fill the hollow regions of the HCF (-100 dB / m for air at atmospheric pressure). These low levels of backscattering are beyond the detection capabilities of standard OTDR equipment. A review of other known optical reflectance measurement techniques may suggest the use of phase-sensitive measurement techniques such as optical frequency domain reflectometry (OFDR) or phase OTDR to measure such low levels of backscattering. However, when using these techniques, the thermal motion of gas molecules at room temperature must be taken into account. This causes a Doppler broadening of the received optical signal, thereby "blurring" the useful signal. As a result, the backscattered signal from the gas cannot be detected and only the weaker signal scattered from stationary scatterers on the glass surface is measured. The information carried by the scattering from the gas is lost.

[0029] This disclosure is based on the realization that, contrary to common understanding, low levels of backscattering from gas within the HCF's air gap can in fact be effectively detected by typical non-custom OTDR equipment. To measure backscattering from inside the HCF, the use of a highly sensitive OTDR is proposed, whose standard implementation is not phase sensitive and therefore insensitive to Doppler broadening in moving gases. As mentioned above, Rayleigh backscattering from gas (typically air) inside an anti-resonant HCF is predicted to be about 15 dB stronger than surface scattering, and the inventors have estimated that the sensitivity of current top quality OTDRs will be sufficient for its detection. From a fiber characterization perspective, this represents the use of a less sensitive method (OTDR rather than OFDR) to measure a stronger signal (air scattering rather than surface scattering), resulting in a lower or no overall loss in signal-to-noise ratio. It also allows the use of commercially available instruments. A further advantage is that, compared to phase-sensitive methods, most OTDRs are polarization insensitive, thereby providing a direct measurement of the total scattered power, information of interest in routine fiber characterization and testing.

[0030] Considering the amount of backscatter expected to be available from a gas-filled HCF, the inventors propose that successful OTDR measurement results of an HCF can be achieved by careful handling of the coupling losses of the light pulses coupled into the HCF (if necessary to maximize the amount of available light illuminated into the HCF) and the strong backreflection that occurs at the glass-air interface between the portion of the SMF by which light is coupled from and to the OTDR instrument or device, and the input facet of the HCF under test. Commercially available OTDR instruments are designed for use with solid-core fibers (SCFs), so the output / input fiber that forms part of the instrument for connection to the fiber under test is a portion of solid-core fiber, typically an SMF. As is well known, 4% reflection of light occurs at the glass-air interface, and such an interface is created when an HCF is coupled to the SCF input / output of an OTDR instrument, since the hollow core of the HCF is air-filled or gas-filled. If this 4% reflection is not effectively suppressed, it will be directed back into the OTDR instrument and the OTDR measurement will be limited not by the sensitivity of the instrument but by saturation due to the strong early reflection peak from the interface, and the backscattering will likely go undetected. It is therefore proposed to eliminate or reduce this backreflection in order to utilize the highest sensitivity of the OTDR instrument so that low levels of backscattering from the gas in the HCF can be measured.

[0031] Simulations (computer modeling) of the backscattering contributions identified as occurring within the HCF have been performed to support the concept of OTDR based on Rayleigh scattering from air or gas within the HCF. A more detailed simulation of backscattering in an anti-resonant HCF has been presented elsewhere (ref. [9]), showing that the total backscattering coefficient depends primarily on the radius R of the fiber core and the wavelength λ of the propagating pulse, and is relatively insensitive to the anti-resonant HCF microstructure. The backscattering contribution due to surface scattering is λ 3 / R 6 It was found that the contribution from the gas inside the hollow core is proportional to 1 / (λR) 2 Follow.

[0032] Figure 4 shows a graph of the variation of backscattering coefficient versus wavelength, determined by simulation, for both air within the fiber and the glass surface. The fiber used in the simulation was a NANF type hollow core fiber (similar to the fiber used in the experiments described below), assumed to be filled with air at atmospheric pressure. Note that while actual manufactured fibers may have different pressures or gas compositions, the invention is not limited in this manner and is generally applicable to HCFs with gas-filled cores, where the gas may be air or a specific gas or gas composition, either of which may be present from the fiber fabrication process or may be added later. The diameter of the fiber core was fixed at 36 μm. The backscattering from air is shown as a solid line, and the backscattering from the glass surface is shown as a dashed line. As can be seen, the Rayleigh backscattering from atmospheric air inside the fiber is well above the predicted surface scattering for all modeled wavelengths, between 1300 nm and 2000 nm. The expected decrease in both is observed as the wavelength increases.

[0033] Figure 5 shows a graph of the variation of backscatter coefficient versus core diameter, determined by simulation, for both air inside the fiber (solid line) and the glass surface (dashed line). The fibers had the same NANF design as in the simulation of Figure 4, and were also filled with atmospheric air, but the core diameter was varied from 24 μm to 40 μm, and the wavelength of the OTDR pulse was fixed at 1550 nm. Again, the Rayleigh backscatter from the atmospheric air inside the fiber is well above the predicted surface scattering for all core diameters. The expected decrease of both is observed as the core diameter increases.

[0034] This identification of levels of Rayleigh backscatter from air or gas in hollow core fibers that are within the detection capabilities of known OTDR technology is a surprising result, unexpected from previously reported observations, and when combined with certain equipment adaptations described below, enables the proposed OTDR method disclosed herein. The proposed method may be utilized for evaluation (testing, analyzing, investigating) of optical systems and optical fiber systems that comprise or include hollow core optical fibers, alone or in combination with other optical fiber types, and waveguiding components.

[0035] From these simulations, it can be concluded that scattering from air is substantially more important over a wide spectral range and for all practical ranges of core diameters. The simulations suggest air-dominated Rayleigh backscattering at a level of -100 dB / m at 1550 nm and at a core diameter of 36 μm. With respect to the change in backscattering level with core diameter, FIG. 5 shows that the backscattering coefficient is expected to change by ±0.35 dB at a core diameter of about 36 μm and with a dimensional error of ±1 μm. Therefore, an OTDR instrument configured to increase or maximize the detection of air-origin Rayleigh backscattering from the HCF is proposed. To increase the signal-to-noise ratio and dynamic range, steps are taken by reducing or suppressing the 4% reflection at the glass-air interface where the HCF is coupled to the SCG for optical communication with the OTDR instrument, allowing low-loss coupling between the SCF and the HCF.

[0036] Experiments were performed to demonstrate the OTDR results that can be achieved according to this approach. A commercial OTDR (LOR-200 from Luciol, Switzerland) based on high sensitivity photon counting detection was used. It offers state-of-the-art dynamic range for optical pulses of 1 μs duration (corresponding to a spatial resolution of the HCF of about 300 m). Pulses of about 10 mW peak power are generated, but with the proposed OTDR method, higher pulse powers can be used, leading to more satisfactory results in addition to being tolerated by the HCF, which has a much lower optical nonlinearity than the SCF. HCFs of NANF design were tested. In particular, long fibers with lengths of more than 4 km were investigated. This allows the use of longer pulses, which provide a lower spatial resolution for the OTDR dispersion profile, but a better signal-to-noise ratio due to the higher total power per pulse. It is noted that commercial OTDR equipment typically allows the pulse duration to be changed, but the peak pulse power remains the same, so the longer the pulse, the higher the total power. A low back reflection and low loss coupling technique was used to splice the SCF and HCF, which is described further below. To match the simulation, an HCF with a core diameter of approximately 36 μm was used with a pulse wavelength of 1550 nm. As can be seen, the experiment yielded an estimated backscattering coefficient of -102 dB / m from the measured data, which is very close to the predicted value of -100 dB / m mentioned above. The ability to identify scattered scattering events along the fiber, such as those that may be caused by scattered fiber imperfections, was demonstrated down to a spatial resolution of 10 m.

[0037] The OTDR instrument used in the experiments is a field-deployable instrument that uses a highly sensitive photon-counting photodetector that provides sufficient sensitivity that backscattering from air-filled NANF·HCF at atmospheric pressure is expected to be measurable. The OTDR is designed to have a backscattering dynamic range of 57 dB for SMF at 1550 nm (60 dB at 1.3 μm) when the longest (1 μs) pulse is used. As a result, with a 1 μs pulse (corresponding to a spatial resolution of 300 m at HCF), it is expected to measure a backscattering signal from air-filled NANF at atmospheric pressure that is 27 dB above the noise floor (about 30 dB lower for SMF). The shorter pulses required for higher resolution contain less power and therefore a lower signal-to-noise ratio, so measurements at 10 times the resolution (30 m) should produce a signal up to 17 dB above the noise floor.

[0038] A further limiting factor in OTDR measurements is the maximum difference between the strongest and weakest signals that can be measured, which is typically 30-50 dB, depending on the OTDR instrument and measurement setup. Therefore, when measuring HCF, it is appropriate to carefully manage the 4% reflection that typically occurs at the air-glass boundary between the OTDR input / output SMF and the HCF, since this corresponds to -14 dB, a large percentage of the available measurement range. For a 1 μs pulse, the back reflection peak at -14 dB is 35 dB higher than the SMF backscatter level (-49 dB) and 65 dB higher than the expected backscatter level of the NANF·HCF (-79 dB), making measurements of the NANF backscatter level impossible. To suppress this backreflection, an angle-polished solid-core fiber, i.e. a fiber with end facets cleaved or polished at a non-orthogonal angle to the optical axis of the fiber, was used and coupled to the HCF. This suppressed the backreflection at the input to the OTDR to below -60 dB. Additionally, a simple four-layer antireflection (AR) coating was applied to the angled SCF to reduce insertion loss (by 0.15 dB). An angled cleaved fiber with -49 dB back reflection without any AR coating was also used and found to be sufficient for collecting backscattering data. The angled facets and / or AR coatings are intended to provide a low reflection interface with the HCF and minimize the amount of optical pulse power reflected back to the OTDR instrument to avoid saturating the detection signal with light that has not been backscattered in the optical system. The angled cleavage was placed on the distal end of a short piece of graded index multimode fiber that was spliced ​​to the input / output SMF·SCF. It served as a mode-field adapter to reduce the coupling loss between the SMF and the HCF, and to expand the mode field diameter (MFD) from the 10 μm MFD of the SMF to the 24 μm MFD of the NANF·HCF in order to maximize the amount of optical pulse power launched into the HCF, thereby maximizing the signal-to-noise ratio and increasing the detection capability of the backscattered light.Thus, the mode field adapter helps to achieve low-loss optical coupling. If the MFDs are better matched (equal or nearly equal), the mode field adaptation may be omitted and angled facets and / or AR coatings are provided directly on the OTDR input-output fiber (or on some portion of the intermediate SCF). Angled facets, AR coatings, and mode field adapters are described in more detail below.

[0039] To further ensure low-loss coupling to the HCF, the NANF·HCF and the input / output SMF·SCF (with attached angle-polished and AR-coated mode-field adapters) were butt-coupled using a pair of alignment stages with a total of five axes of alignment, with a fiber mounted on each. Five-axis alignment allows adjustment in three orthogonal linear directions plus pitch and yaw, allowing for better alignment of the fiber cores than three-axis (or xyz) alignment, especially when angled facets are involved.

[0040] Two geometrically similar NANF·HCF samples were tested: NANF1 had a length of 4.3 km and a core diameter of 35.4 / 37.0 μm at its beginning / end (except the beginning or proximal end was coupled to the OTDR instrument), and NANF2 had a length of 3.4 km and a core diameter of 35.3 / 35.8 μm. In addition, 1 km of SMF was inserted in front of the NANF sample to allow visual comparison of the backscattering from the SMF and the NANF.

[0041] Figure 6 shows a graph of the measured detected backscattering power from the sample NANF1, plus the aforementioned SMF, resolved over the distance along the fiber for dispersion measurements with 1 μs pulses in a conventional OTDR manner. The distances shown on the horizontal axis are adjusted to take into account the group refractive indices of typical SMFs (1.46) and NANFs (1.003) (this also applies to the following figures). The relative sensitivity (i.e., the noise level visible beyond the end of the NANF at 5.5-6.0 km) was -106 dB. This could have been lowered by up to another 4 dB by increasing the time between the pulses sent through the fiber during the test (i.e., reducing the pulse repetition rate), but this would increase the measurement time accordingly. Instead, the smallest feasible time between pulses was used throughout the measurements presented here (limited by the pulse round trip time through the fiber during the test). The much higher backscattering detected from the 1 km inserted SMF can be seen at the beginning of the profile. However, the backscattering levels from the NANFs are much lower, but still clearly higher than the noise level, and so are usefully detectable, demonstrating what OTDR on HCFs can achieve. It can be seen that the backscattering signal from the distal end of the NANF1 sample (remote from the OTDR instrument) was 4.6 dB lower than at its starting or proximal ends. From this, the attenuation of the NANFs can be determined, but as noted above, the backscattering varies with core radius, so the slightly larger core diameter (by 1.6 μm) at the distal end of NANF1 must be taken into account. Considering the rate of -0.35 dB / μm mentioned previously in the discussion of the data in Figure 5, a reduction in the backscattering coefficient of about 0.55 dB is expected in this section of fiber. This gives a fiber attenuation of (4.6-0.55) / 2=2.0 dB, which is consistent with the transmission loss measurement of 2.0 dB for NANF1. From both of these measurements, the average NANF1 attenuation is 0.47 dB / km.

[0042] FIG. 7 shows further graphs of detected backscatter power measured from sample NANF1, plus the aforementioned SMF, resolved over distance along the fiber for dispersion measurements in a conventional OTDR manner, for optical pulses of different widths launched into the fiber from an OTDR instrument. Pulse widths of 100, 300, and 1000 ns (1 μs) were used as shown. It is evident by comparing the profiles for 100 ns and 1000 ns that a pulse 10 times shorter (and therefore a 10 times reduction in the launched optical power) would be expected to reduce the measured power by 10 dB, while the noise level is unchanged for the different pulse lengths. Backscatter from the leading and proximal ends of the NANF are approximately 20, 15, and 10 dB above the noise floor, respectively, for the decreasing pulse lengths, which gives estimated measurement ranges of up to 20, 15, and 10 km of fiber with 0.5 dB / km attenuation for these three pulse durations. For 1 μs pulses, a maximum dynamic range of 27 dB was predicted as mentioned above, which is about 7 dB higher than experimentally achieved. However, the predicted value included 4 dB that could be gained by reducing the pulse repetition rate as discussed above, but does not take into account the 0.6 dB return loss at the SMF-NANF interface, nor the estimated 1-2 dB return loss at the connectorized interface between the SMF·SCF input / output fiber and the OTDR instrument. Thus, the actually obtained value of 20 dB is in good agreement with the expectations from the advertised performance of the instrument. Considering a state-of-the-art NANF with a loss of 0.22 dB / km, this should allow the measurement of 45 km of fiber with a resolution of 300 m. Measuring from both ends, this would allow the characterization of up to 90 km of fiber. Longer fiber lengths can be expected with OTDR instruments at higher pulse powers.

[0043] Figure 8 shows a graph where the data from Figure 6 has been converted to length-normalized backscattering values ​​that take into account factors including pulse length, insertion loss, and instrument calibration. It can be seen that the backscattering from the SMF is at -72 dB / m, matching the expected value for such a solid-core fiber, while for the NANF, the backscattering is at -102 dB / m, roughly matching the -100 dB / m expected from simulation.

[0044] All measurement data acquired on the NANF1 sample show a small bump between 2.75 km and 2.95 km, with an amplitude of about 0.7 dB. Based on the simulation shown in Figure 5, the rise in backscatter at that point can be explained by a core size that is about 2 μm smaller, considering the discussed rate of 0.35 dB / μm. Considering the dependency of backscatter level on HCF core radius, as discussed above, it is proposed that OTDR measurements can be used to evaluate, measure, or monitor the core size. This may be performed during fiber fabrication, quality control monitoring is performed during fiber draw to ensure consistent core size, and corrections to the draw are performed to maintain a constant backscatter level and therefore a constant core size, or to achieve the required core size variation. Otherwise, the finished fiber may be tested so that the core size can be measured and characterized.

[0045] FIG. 9 shows a graph of the backscattered power over the distance measured from the NANF2 sample, plus the preceding 1 km of SMF as before, and normalized as in the FIG. 8 graph. The NANF2 sample exhibited small manufacturing imperfections along its length. Therefore, to increase the spatial resolution and to be able to resolve these imperfections as individual features in the measurement results, data was acquired using shorter pulses of 300 ns duration. The first 400 m of the NANF2 sample showed similar behavior to NANF1, with a backscattering coefficient of −102 dB / m, identical to that measured for NANF1. The total round trip loss was 9 dB (determined from the difference between the values ​​at both ends of the fiber), but indicates a one-way loss of 4.5 dB. This corresponds to an average NANF2 loss of 1.3 dB / km, which is in good agreement with the attenuation measured via direct transmission loss. Along the length, several peaks can be identified, but all of them produce backscattering levels well below the backscattering levels of the SMF, and therefore represent very small backscattering events.

[0046] Figure 10 shows a graph of the measured backscattering power over distance, taken from NANF2, for different pulse lengths. These measurements were taken to further investigate localized scattering events by decreasing the pulse duration to increase spatial resolution. 100 ns and 30 ns pulses were used, and they are shown together with the 300 ns results from Figure 9, and also measurements using μs pulses. The measurements are limited to a range of distances along the fiber of 1.5 to 2.1 km, which has a high concentration of defects as determined from the data in Figure 9. It can be seen that for the 30 ns pulse, scattering features as close together as -10 m separation can be resolved. Note that for the 30 ns pulse, the signal is close to the noise floor measurement, but the scattering peaks rise high enough above the noise floor to allow determination of their longitudinal location with an accuracy of -10 m. Furthermore, scattering events are visible down to the much lower resolution of 1 μs pulses, enabling identification of defects and other scattering sources from even the fastest and highest dynamic range measurements.

[0047] The numerous backscattering events exhibited by the NANF2 sample all exhibit backscattering levels below that of the SMF. Judging from the backscattering levels on each side of each of these events, none of them appear to add significant attenuation. This suggests that any scattering points, such as manufacturing defects or damage caused during or after installation that add significant attenuation to the fiber, would be visible in the measured OTDR trace, confirming that OTDR measurement results can be useful for fiber characterization and loss point identification.

[0048] Overall, it is proposed that OTDR can be used to characterize HCFs in the same way that it is used to characterize SMFs and other SCFs, and continuous (dispersion) data can be generated along the fiber. Although the experimental results have focused on anti-resonant HCFs in the form of NANFs, the principles are applicable to other anti-resonant HCF designs, and indeed to all kinds of HCF fibers, such as photonic bandgap hollow core fibers. All hollow core fibers can have a gas filling within the hollow core, either present from manufacture or introduced by design, and it has been shown that with proper treatment of the OTDR instrumentation, it is possible to detect detectable levels of Rayleigh backscattering from the gas, which can be used in place of backscattering from the glass, so that hollow core fibers can be evaluated equivalently to SCFs using OTDR. Such evaluation is not limited to fiber characterization as further discussed above. The same technique may alternatively be used for dispersion sensing along the fiber length to detect external parameters that act on the fiber and change the backscattering properties of the gas filling, such as temperature or pressure. Similarly, OTDR may be used to sense variations in gas pressure or concentration along the HCF, among others. This measurement can be used to resolve the internal gas pressure along the fiber length and is a useful diagnostic technique for many sensing applications.

[0049] Thus, the feasibility of using commercially available OTDR instruments for characterization and measurement of attenuation in HCFs has been proposed and demonstrated by utilizing the backscattered signal generated by the air / gas inside the hollow core of the fiber. Given the commercial availability of the field-deployable OTDR instruments used, the proposed measurement approach may find widespread use as more and more HCFs start to be commercially installed. The technique provides a means to exploit the highly useful length resolution information of conventional OTDR testing on SCFs for characterization of HCFs. This provides a post-fabrication tool that is useful for testing cabled and installed fibers and may enable new and interesting applications of the technique, for example in distributed gas sensing.

[0050] FIG. 11 shows a simplified schematic diagram of an apparatus for performing OTDR on an HCF according to an embodiment of the present invention. The apparatus includes an OTDR system or apparatus 20. The OTDR system includes an OTDR instrument 21 having an input / output optical fiber 22 extending from a housing of the instrument. The input / output fiber 22 may be permanently optically coupled to the housing or may be connectable and disconnectable by a fiber coupler 23 provided on the housing. The optical coupling allows pulses of light generated by a light source 24 in the housing to be coupled into the input / output fiber 22 and allows light returning along the input / output fiber 22 via back propagation, in particular backscatter of interest from the optical fiber under investigation that forms the received optical signal, to be delivered from the input / output fiber 22 to a photodetector 29 in the housing. The light source 24 generates optical pulses at a wavelength λ (which may be adjustable in some cases), the pulse length of which is typically adjustable to select the spatial resolution of the OTDR measurement results, as discussed above. The wavelength λ may be 1550 nm, which is a standard wavelength for communications, and therefore optical fibers of interest for OTDR are often configured to propagate this wavelength. However, other wavelengths may also be deliverable. The photodetector 29 has high sensitivity to detect Rayleigh backscattering, a weak optical effect that produces only low power levels from optical fibers, as described. Importantly, the detector sensitivity may be designed to be sufficient to capture the level of Rayleigh backscattering produced by solid-core optical fibers, but need not be of a special or significantly more sensitive design targeted at the lower levels of Rayleigh backscattering obtained from the air or gas-filled core of hollow-core fibers. In other words, the OTDR instrument may include standard commercial off-the-shelf products designed to be used with solid-core fibers such as SMF. However, custom-made instruments may also be used. By way of example, the photodetector 29 may include a photon-counting photon detector capable of detecting or counting individual incident photons.The photodetector 29 generates an electrical output signal indicative of the received optical signal in the usual manner which is provided to a processor or controller 25 which is configured to process and convert the electrical signal in the usual manner into an OTDR output trace or profile 26 indicating the detected optical power level P as a function of distance d along the interrogated fiber. The profile 26 may be provided as a data output or may be visually shown, for example, on a display on the OTDR instrument housing. The optional display, and further processor 25, may be included as part of the OTDR instrument or may be separate elements on the system.

[0051] In general, however, the present invention is not concerned with the specifics of the implementation of the OTDR instrument, which may be thought of as a "black box" configured to obtain OTDR measurements from a solid core optical fiber in any known manner.

[0052] The input / output fiber 22 has a distal end 22a remote from the proximal end coupled to the OTDR instrument 21. This distal end 22a is treated or configured according to an embodiment of the present invention to provide low reflection coupling of optical pulses from the input / output fiber 22 that require interrogation or analysis via OTDR to the proximal end 30a of the hollow-core fiber 30. The treatment of the distal end 22a may include various configurations, which are further discussed below. Thus, in FIG. 11, the treatment applied to the distal end 22a is simply indicated by box 26. Furthermore, if necessary, the treatment may be further configured to enable low optical loss coupling between the input / output fiber 22 and the hollow-core fiber 30. To perform OTDR measurements, the input / output fiber 22 and the hollow-core fiber 30 are non-permanently aligned with each other to enable maximum or high optical power transmission from one core to the other. This may be made possible by an alignment device 28 configured to allow relative spatial adjustment of the fiber ends 22a, 30a so that they may be properly positioned relative to one another. This is discussed further below. Alternatively, the input / output fiber 22 and the hollow-core fiber 30 may be permanently aligned to one another to allow maximum or high optical power transmission from one core to the other, and the hollow-core fiber 30 may be permanently or non-permanently optically connected to an additional length of hollow-core fiber desired to test. After the OTDR measurement is performed, the hollow-core fiber 30 may be decoupled from the input / output fiber 22, and the OTDR system may be removed and utilized elsewhere.

[0053] The input / output fiber 22 is a length or portion of a solid-core optical fiber, typically, but not necessarily, a single-mode optical fiber. Additionally, one or more additional lengths of the same or different type of solid-core optical fiber may be spliced ​​or coupled to the distal end of the input / output fiber 22, as the input / output fiber 22 may be provided by a manufacturer along with the OTDR instrument. This may be done, for example, to increase the length of the input / output fiber or to provide some optical effect. According to this disclosure, any such additions are considered part of the input / output optical fiber 22, and the treatment of the distal end 22a is the treatment of the end of any additional portion of fiber added to the input / output fiber. In other words, the treatment pertains to a solid-core fiber end facet coupled to a hollow-core fiber, the solid-core fiber end facet providing the glass portion of the air / glass interface between the two fiber types.

[0054] FIG. 12 shows a simplified diagram of a solid core input / output fiber including a distal end treatment according to a first example, shown as a longitudinal cross section. The input / output fiber 22 includes a solid glass core 32 surrounded by a solid glass cladding 34 in a known manner. The distal end 22 is treated by having an end facet or end surface 33 cleaved at an angle, so that the plane of the facet 33 is not perpendicular to the long axis of the fiber 22. The cleaved facet 33 makes an angle θ with respect to the normal. The angled cleaved facet 33 serves to reduce or eliminate the 4% back reflection at the glass / air boundary experienced by light leaving the distal end 22a by directing the reflected light away from the long axis of the fiber 22 so that it cannot propagate back to the OTDR detector. The angle θ required to achieve this will depend on the wavelength of the light pulse and the refractive index difference between the core 32 and the cladding 34, which supports light guiding by allowing total internal reflection at the core-cladding boundary. The reflected light from the facet 33 needs to be directed back towards the cladding 34 at an angle that does not accommodate total internal reflection. Typically, a small angle will suffice, for example in the range of 1°-10°, or 2°-8° or 2°-6°, which is believed to be potentially useful in the present situation to suppress or eliminate back reflections from the input / output fiber distal end facets. However, smaller or larger angles may be used as appropriate.

[0055] FIG. 13 shows a simplified diagram of a solid-core input / output fiber including a distal end treatment according to a second example, shown as a longitudinal cross section. As previously mentioned, the input / output fiber 22 includes a solid glass core 32 surrounded by a solid glass cladding 34 in a known manner. In this example, back reflections from the glass-to-air boundary at the distal end facet 33 of the fiber 22 are reduced by an anti-reflection coating 35 applied on the surface of the end facet 33. Anti-reflection coatings typically include multiple optically thin layers arranged in a stack to provide multiple reflective surfaces that create destructive interference and thus remove power from the reflected light. The optically thin layers are configured for operation at a specific wavelength or range of wavelengths, for example at 1550 nm or 1625 nm, which may be used for optical pulses in OTDR instruments intended for testing solid-core fibers for telecommunications. The latter wavelength may be a feature of OTDR instruments that allows testing to be performed simultaneously with live data transmission in the fiber at the conventional telecommunications wavelength of 1550 nm. In the experiments reported above, a four-layer anti-reflection coating was used (reference

[10] ), but more or fewer layers may be used.

[0056] A third example of a distal end treatment for solid-core input / output fibers that can be applied to achieve low reflection behavior is an anti-reflective microstructured / nanostructured surface (Ref.

[11] ). The end facet of the fiber, with or without angled cleavage, is treated to create a textured structure that includes randomly distributed surface features or pillars of varying depth and profile, with depth greater than width and subwavelength dimensions. The features act to suppress reflection of the incident light. This type of anti-reflective microstructured surface can be referred to as a "moth-eye" surface.

[0057] To achieve a low or reduced reflective interface when coupled to a hollow core fiber, either angled cleavage or anti-reflective coatings or microstructured surfaces may be used to achieve an input / output fiber with low or reduced back reflection capabilities. In other examples, a combination of these treatments may be used. The angled end facets may have an anti-reflective coating applied thereto or an anti-reflective microstructured surface formed thereon (not shown).

[0058] The distal end of the input / output fiber may be further processed to improve optical coupling from the solid-core fiber to the hollow-core fiber, thereby achieving low coupling loss. Typically, a solid-core fiber, such as a single mode fiber, provided or used as an input / output fiber for an OTDR instrument will be configured such that the optical mode field size or diameter of the corresponding propagating optical mode matches that of the solid-core fiber type likely to be tested with the OTDR instrument, such as a telecommunications optical fiber. On the other hand, a hollow-core fiber typically has a larger mode field diameter, requiring a larger core size to propagate the same wavelength, such as 1550 nm. Thus, there may be a mismatch in the mode field diameter between the input / output fiber and the hollow-core fiber, which may lead to a loss of transmitted optical power when the two are coupled together. To address this, it is proposed to achieve low-loss coupling through the use of a mode field adapter.

[0059] FIG. 14 shows a simplified diagram of a solid core input / output fiber including a distal end treatment according to a third example, shown as a longitudinal cross section. As previously described, the input / output fiber 22 includes a solid glass core 32 surrounded by a solid glass cladding 34 in a known manner. In this example, the treatment of the distal end 22a of the fiber 22 includes the addition of a mode field adapter 36 that is spliced ​​or otherwise optically joined to the end facet of the fiber 22. The mode field adapter 36 serves to expand the mode field diameter of the light received from the fiber 22 to the mode field diameter of the expected light propagation mode of the hollow core fiber to be tested using the OTDR instrument. The mode field adapter 36 has a proximal end that is joined to the distal end of the input / output fiber 22 and a distal end that delivers the expanded mode field. An end facet 37 at the distal end is coupled to the hollow core fiber to perform OTDR and defines a glass-to-air interface between the solid core and the hollow core. The mode field adapter 36 may thus be considered a section of fiber that is added to the input / output fiber 32, and as described above, a low-reflection treatment is applied to the end facet 37 of the mode field adapter since it forms part of it. In other examples, the mode field adapter 36 may be formed directly from the end of the input / output fiber 22, rather than comprising a separate section of fiber. As discussed above, the low-reflection treatment may include one or both of angled facets and an anti-reflection coating (neither of which are shown in FIG. 14). The mode field adapter 36 may take any convenient form. Although FIG. 14 illustrates the mode field adapter 36 as a short section of solid-core fiber having a tapered core that expands in diameter from the proximal end to the distal end, other configurations may be used to achieve mode field expansion, such as graded index optical fiber, graded index optical fiber plus large mode area solid-core fiber, and solid-core fiber that includes a core expanded by thermal expansion.Further details of a mode field adapter suitable for achieving low-loss optical coupling between a solid-core fiber and a hollow-core fiber, and therefore suitable for use in the present context, can be found in WO2020 / 070487 (reference

[12] ).

[0060] The purpose of the mode field adapter is to perform low-loss optical coupling from the solid-core input / output fiber to the hollow-core fiber to launch as much pulse power as possible into the hollow-core fiber and back from the hollow-core fiber to the solid-core fiber, preserving as much backscattered power as possible for detection by the OTDR instrument. Low-loss coupling may be understood as any configuration that operates to reduce coupling losses between the fibers compared to the amount of optical power loss that would occur in the absence of low-loss coupling. Configurations other than mode field adapters may be used if preferred. While achieving the smallest amount of loss achievable is desirable and useful, some loss at this interface may be tolerable or unavoidable. Thus, the purpose of a low-loss coupling is to transmit or preserve at least 90% of the power of the optical pulse output from the OTDR instrument for propagation into the hollow-core fiber, or to lose at most about 0.5 dB of that power.

[0061] In some cases, however, the mode field adapter or other low-loss coupling means may be omitted. For example, the mismatch in mode field diameter may be insufficient to cause unacceptable power loss levels across the solid-core to hollow-core interface. The solid-core input / output fiber may be configured as a large mode area fiber specifically intended to match or nearly match the mode field diameter of the hollow-core fiber.

[0062] FIG. 15 shows a simplified schematic plan view depiction of an example of an alignment apparatus for enabling optical alignment between the core of a solid core and the core of a hollow core fiber. In this example, the alignment apparatus 28 includes a pair of alignment stages, both having five axes of adjustment. A first stage 36a has a distal end of the input / output fiber 22 mounted thereon, and a second stage 36b has a proximal end of the hollow core fiber 30 mounted thereon. The two stages 36a, 36b may be physically separate items, or alternatively may be configured as a single unit, such as held on a common support. In this manner, the input / output fiber 22 may be mounted and held on its stage 36a, and the second stage 36b is available as an integral part of an OTDR system, ready to accept the hollow core fiber 30 for investigation or analysis. An alignment stage or stages with five adjustment axes provide five degrees of movement. Three of these are orthogonal, one parallel to the optical axis of the fiber corresponding to the long axis of the end of the fiber, and two directions in a plane perpendicular to the optical axis. The fourth degree of movement is pitch, which allows the optical axis to be tilted about a horizontal axis in a plane perpendicular to the optical axis. The fifth direction is yaw, which allows the fiber to be rotated about a vertical axis in a plane perpendicular to the optical axis. This range and amount of adjustability allows the fiber ends to be aligned for high optical transmission between the cores and helps achieve low loss coupling between fibers. This is particularly important when the input / output fibers 22 have angled end facets, since refraction at the glass-air interface causes light pulses to emit from the fiber 22 at an angle relative to the optical axis. The fiber ends may be directly abutted, placed in contact with one another, or placed with some space between them, again this is important for angled end facets where true abutment is more tricky.

[0063] Other structures for achieving alignment of the fiber ends to maximize optical coupling between the fibers may alternatively be used.

[0064] As mentioned above, OTDR instruments typically have an adjustable pulse duration, so that the spatial resolution of the OTDR profile can be selected. A shorter pulse gives a higher resolution, while a longer pulse corresponds to a reduced resolution. In the current situation, a longer pulse length may be preferred, or indeed required, since the power per pulse will be higher, and the amount of backscattering to be detected will be correspondingly higher, resulting in a higher signal-to-noise ratio. As shown by experimental results, a longer pulse can still give a useful resolution, and a shorter pulse can still provide a detectable amount of backscattering, although it will be less distinguishable from the noise floor. Therefore, a range of pulse durations that is particularly useful for OTDR on hollow-core fibers is proposed to be 30ns to 1000ns. However, shorter or longer pulses are not excluded.

[0065] The experimental results discussed above indicate that it is expected that a hollow-core fiber with a length of 45 km and an attenuation of 0.22 dB / km can be usefully analyzed using the described OTDR method. By irradiating a pulse into both ends of the hollow-core fiber, a full fiber length of 90 km can be observed. However, with a lower-loss hollow-core fiber, with increased pulse power, optimized low reflection, and low coupling management, a larger backscattered power may be achieved, which can propagate further before being attenuated to the noise floor of the system. Thus, the method is believed to be applicable to hollow-core fiber lengths up to at least 200 km or 400 km when investigated from both ends.

[0066] 16 shows a flow chart of steps in an exemplary method for evaluating, analyzing, testing, or measuring a hollow core optical fiber according to an embodiment of the present invention. A first step S1 includes providing an optical time domain reflectometry system including a treatment at an end facet of an input / output fiber for reduced back reflection and, optionally, reduced coupling loss. This may include, for example, providing an optical time domain reflectometry system including a light source configured to generate light pulses having a wavelength λ, a photodetector configured to detect light of wavelength λ, and a solid core optical fiber optically coupled at a proximal end to receive light pulses from the light source and deliver light to the photodetector, the input / output fiber having a treated end facet at its distal end, the end facet configured to suppress back reflection of light of wavelength λ caused at an interface between the glass forming the core of the solid core optical fiber and the air at the end facet.

[0067] In a second step S2, the method proceeds with aligning the input / output fiber with the hollow core optical fiber having a gas-filled core, such as by aligning a distal end of the input / output fiber with a proximal end of the hollow core optical fiber, for optical transmission between the input / output fiber and the hollow core optical fiber having gas present in the hollow core.

[0068] A third step S3 of the method includes irradiating a light pulse into the hollow-core fiber and detecting backscattering from the gas in the core, which may be accomplished, for example, by operating a light source to generate a light pulse for propagation into the hollow-core fiber along the input / output fiber, receiving backscattered light generated by Rayleigh scattering of the light pulse from the gas in the hollow core of the hollow-core fiber, and detecting the backscattered light with a photodetector to generate a detection signal.

[0069] Finally, the method ends with step S4, which includes processing the detected backscattering to provide an optical time domain reflectometry profile. This may include processing the detected signal to create an optical time domain reflectometry profile including a distribution of backscattered optical power along the length of the hollow core optical fiber. In other examples, the method may further include using the profile to determine further information about the fiber, such as determining its attenuation, identifying peaks in the profile resulting from defects or damage, or determining information about the core size. The method may be applied to any hollow core optical fiber type, including fibers in communication networks, and the method may evaluate fibers during installation or for post-installation maintenance, or for fibers during fabrication, for monitoring fiber characterization and for defect identification for quality control. The hollow core fiber may be a gas-filled cell for optical sensing, and the method includes analyzing a parameter of interest that affects the backscattered optical power and therefore can be monitored, measured, or determined from the amount of backscattering.

[0070] Any gas may be used within the voids, and particularly within the core, of the hollow-core fiber, as long as it is capable of producing the required Rayleigh backscattering. The gas may include air, including atmospheric air, which may enter the core during or after fabrication of the fiber. Alternatively, a specific gas or gas mixture or composition may be introduced into the fiber, such as to provide a hollow-core fiber gas cell for sensing purposes. Argon is typically used as the fill for the gas cell, although other gases are not excluded. In summary, a hollow-core fiber simply has a non-vacuum core.

[0071] The various embodiments described herein are presented only to aid in the understanding and teaching of the claimed features. These embodiments are provided as merely representative samples of embodiments and are not exhaustive and / or exclusive. The advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein should not be considered limitations on the scope of the invention as defined by the claims, or limitations on the equivalents of the claims, and it is understood that other embodiments may be utilized and changes may be made without departing from the scope of the claimed invention. Various embodiments of the present invention may suitably comprise, consist of, or consist essentially of any suitable combination of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, the present disclosure may include other inventions that are not currently claimed but may be claimed in the future.

[0072] References [1] GT Jasion et al, “Hollow core NANF with 0.28 dB / km attenuation in the C and L bands, paper Th4B.4, 2020 Optical Fiber Communications Conference, March 2020 [2] SR Sandoghchi et al, “Optical side scattering radiometry for high resolution, “wide dynamic range longitudinal assessment of optical fibers”, Opt. Express, vol. 23, 27960-27974, 2015 [3] U.S. Patent No. 10,845,268 [4] U.S. Patent No. 9,904,008 [5] WO2015 / 185761 [6] F. Poletti, "Nested antiresonant nodeless hollow core fiber," Opt. Express, vol. 22, 23807-23828, 2014 [7] H. Sakr et al, “Hollow core NANFs with five nested tubes and record low loss at 850, 1060, 1300 and 1625 nm,” in 2021 Optical Fiber Communications Conference (OFC2021), paper F3A.4, 2021 [8] V. Michaud-Belleau et al, “Backscattering in antiresonant hollow-core fibers: over 40 dB lower than in standard optical fibers,” Optica, vol. 8, no 2, pp. 216-219, Feb 2021 [9] E. Numkam Fokoua, “Theoretical analysis of backscattering in hollow-core antiresonant fibers,” APL Photonics, vol. 6, 096106, Sept. 2021

[10] D. Suslov et al, “Angled interconnection between standard single-mode fiber and nested nodeless antiresonant fibers”, paper Stu1Q.5, 2021 Conference on Lasers and Electro-Optics, May 2021

[11] U.S. Patent No. 8,187,481

[12] WO2020 / 070487

Claims

1. 1. A method for evaluating an optical system comprising a hollow-core optical fiber, the method comprising: Providing an optical time domain reflectometry system, the optical time domain reflectometry system, a light source configured to generate light pulses having a wavelength λ; a photodetector configured to detect light of wavelength λ; an input / output fiber including a solid-core optical fiber optically coupled at a proximal end to receive light pulses from the light source and deliver light to the photodetector, the input / output fiber having a treated end facet at a distal end thereof, the end facet configured to suppress back reflection of light of wavelength λ caused by an interface between glass forming a core of the solid-core optical fiber and air at the end facet; aligning the distal end of the input / output fiber with a proximal end of the hollow-core optical fiber for optical transmission between the input / output fiber and the hollow-core optical fiber having gas present in the hollow core; operating the light source to generate light pulses for propagation along the input / output fiber and into the hollow-core fiber; receiving backscattered light generated by Rayleigh scattering of the light pulse from the gas within the hollow core of the hollow-core fiber and detecting the backscattered light with the photodetector to generate a detection signal; processing the detected signals to produce an optical time domain reflectometry profile comprising a distribution of backscattered optical power along the length of the hollow core optical fiber; A method comprising:

2. 2. The method of claim 1 , wherein the applied treatment at the end facet comprises an angled cleavage at the end facet at an angle relative to a longitudinal axis of the input / output fiber that directs backscattered light of wavelength λ away from a propagating optical mode of the input / output fiber.

3. 3. The method of claim 1 or claim 2, wherein the applied treatment at the end facet comprises an anti-reflection coating on the end facet configured to reduce reflection of light of the wavelength λ incident on the end facet.

4. 3. The method of claim 1 or claim 2, wherein the applied treatment at the end facet comprises an anti-reflective microstructured surface formed on the end facet configured to reduce reflection of light of the wavelength λ incident on the end facet.

5. 3. The method of claim 1 or claim 2, wherein the applied treatment at the end facet is further configured to reduce optical transmission loss between the input / output fiber and the hollow-core fiber from an amount of optical transmission loss that would occur in the absence of the additional configuration of the applied treatment.

6. 6. The method of claim 5, wherein the applied treatment at the end facet is configured to reduce optical transmission loss by providing a mode field adapter at the distal end of the input / output fiber providing the end facet of the input / output fiber, the mode field adapter configured to match a mode field diameter of propagating light between a mode field diameter of the input / output fiber and a mode field diameter of the hollow-core fiber.

7. The method of claim 1 , wherein the light pulse has a duration in the range of 30 ns to 1000 ns.

8. The method of claim 1 , wherein the photodetector comprises a photon-counting detector.

9. 2. The method of claim 1, wherein aligning the distal end of the input / output fiber and the proximal end of the hollow-core optical fiber comprises supporting each of the distal end and the proximal end on an alignment stage, the alignment stage having a total of five alignment axes; and using the alignment stage to adjust the relative positions of the distal end and the proximal end to achieve optical transmission between the input / output fiber and the hollow-core optical fiber.

10. The method of claim 1 , wherein the hollow-core optical fiber comprises an anti-resonant hollow-core optical fiber configured to guide light of the wavelength λ by an anti-resonant effect.

11. The method of claim 1 , wherein the hollow-core optical fiber comprises a photonic-bandgap hollow-core optical fiber configured to guide light of the wavelength λ by the photonic-bandgap effect.

12. The method of claim 1 , further comprising determining the attenuation of the hollow-core optical fiber from values ​​of backscattered optical power at different length values ​​in the optical time-domain reflectometry profile.

13. 10. The method of claim 1, further comprising the steps of finding any peaks in backscattered optical power in the optical time domain reflectometry profile and identifying the length value at which the peak is located as the location of a defect or damage in the hollow-core optical fiber.

14. 14. The method of claim 13, wherein the hollow-core optical fiber is or has been installed in a communications network for the transmission of optical data.

15. The method of claim 1 , further comprising extracting information about a core size of the hollow-core optical fiber from values ​​of backscattered optical power in the optical time-domain reflectometry profile.

16. 2. The method of claim 1, wherein the method is performed during fabrication of the hollow-core optical fiber, and the fabrication is adjusted in response to the optical time-domain reflectometry profile, and / or a quality assessment of the hollow-core optical fiber is performed in response to the optical time-domain reflectometry profile.

17. 10. The method of claim 1, wherein the hollow-core optical fiber is configured as a gas cell optical sensor, and the method further comprises determining or monitoring a parameter of interest from the optical time-domain reflectometry profile.

18. a light source configured to generate light pulses having a wavelength λ; a photodetector configured to detect light of wavelength λ; an input / output fiber including a solid-core optical fiber optically coupled at a proximal end to receive light pulses from the light source and deliver light to the photodetector, the input / output fiber having a treated end facet at a distal end thereof, the end facet configured to suppress back reflection of light of wavelength λ caused by an interface between glass forming a core of the solid-core optical fiber and air at the end facet; an alignment device for aligning the distal end of the input / output fiber with a proximal end of the hollow-core optical fiber for optical transmission between the input / output fiber and the hollow-core optical fiber having gas present in the hollow core; a processor that receives detection signals generated by the photodetector from received backscattered light produced by Rayleigh scattering of light pulses from the light source in the gas within the hollow core of the hollow core optical fiber, and processes the detection signals to produce an optical time domain reflectometry profile comprising a distribution of backscattered light power along a length of the hollow core optical fiber; 1. An optical time domain reflectometry system comprising:

19. 20. The optical time domain reflectometry system of claim 18, wherein the applied treatment at the end facet comprises an angled cleavage at the end facet at an angle relative to a longitudinal axis of the input / output fiber that directs backscattered light of wavelength λ away from a propagating optical mode of the input / output fiber.

20. 20. The optical time domain reflectometry system of claim 18 or claim 19, wherein the applied treatment at the end facet comprises an anti-reflection coating on the end facet configured to reduce reflection of light of the wavelength λ incident on the end facet.

21. 20. The optical time domain reflectometry system of claim 18 or claim 19, wherein the applied treatment on the end facet comprises an anti-reflective microstructured surface formed on the end facet configured to reduce reflection of light of the wavelength λ incident on the end facet.

22. 20. The optical time domain reflectometry system of claim 18 or claim 19, wherein the applied treatment at the end facet is further configured to reduce optical transmission loss between the input / output fiber and the hollow-core fiber from an amount of optical transmission loss that would occur in the absence of the additional configuration of the applied treatment.

23. 23. The optical time domain reflectometry system of claim 22, wherein the applied treatment at the end facet is configured to reduce optical transmission loss by providing a mode field adapter at the proximal end of the input / output fiber providing the end facet of the input / output fiber, the mode field adapter configured to match a mode field diameter of propagating light between a mode field diameter of the input / output fiber and a mode field diameter of the hollow-core fiber.

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