Distributed sensing via optical time domain reflectometry in hollow core optical fibres

Abstract

A method of optical time domain reflectometry comprises: launching a probe pulse of laser light into a proximal end of a hollow core optical fibre having a gas content in its core, wherein the hollow core optical fibre is deployed in a environment having a physical parameter that affects movement of gas molecules in the core; detecting, at the proximal end of the hollow core optical fibre, Rayleigh backscattered light produced by scattering of the probe pulse from the gas molecules in the core, wherein the light is detected with spectral resolution and temporal resolution; determining a spectral linewidth or a spectral shift of the detected light arising from Doppler shifting of the Rayleigh backscattered light caused by movement of the gas molecules; and determining from the spectral linewidth or the spectral shift a value of the physical parameter at one or more locations along a length of the hollow core optical fibre.

Description

[0001] TITLE OF THE INVENTION

[0002] DISTRIBUTED SENSING VIA OPTICAL TIME DOMAIN REFLECTOMETRY IN HOLLOW CORE OPTICAL FIBRES

[0003] BACKGROUND OF THE INVENTION

[0004] The present invention relates to methods and apparatus for distributed sensing via optical time domain reflectometry in hollow core optical fibres.

[0005] Optical time domain reflectometry (OTDR) in optical fibre is a well-established technique for measuring various physical parameters within and external to a deployed optical fibre. It enables distributed sensing to be carried out, yielding measurements which are spatially resolved along all or part of the length of the optical fibre.

[0006] Conventionally, distributed sensing via OTDR is performed by launching pulses of light (probe pulses) into an end of a solid-core optical fibre deployed in a region of interest. As a pulse propagates along the fibre it undergoes scattering from the material forming the fibre. Some of the scattered light is backscattered (counter-propagates) towards the launch end of the fibre, where it can be detected. The use of discrete pulses of known duration coupled with the known propagation speed of light along the fibre allows the spatial origin of the detected light with respect to distance along the fibre to be determined, yielding a profile of reflected light level or power distributed over the fibre length. Particular characteristics and circumstances within the fibre or externally thereto will modify the characteristics of the backscattered light, and the originating location of the scattering can be pinpointed with a spatial resolution that depends on the pulse length. This allows the detection of external parameters that modify fibre properties, such as temperature, strain and pressure, as well as the as identification of fibre characteristics such as localised defects and changes in structure. Hence a fibre can be used as a distributed sensor, or itself be tested or characterised.

[0007] In order for backscattering to be used in this way, it is crucial that the optical fibre can produce backscattered light for detection. A widely used example is distributed temperature sensing (DTS) based on the detection of Raman scattering, which is inelastic scattering of photons of the launched light pulses caused by interaction with optical phonons in the fibre. The ratio of the amplitude of Raman anti-Stokes scattering and Raman Stokes scattering (respectively shifted in wavelength to be shorter and longer than the wavelength of the probe pulses) is temperature-dependent, so detection of the Raman scattering allows the temperature along the length of the fibre to be mapped.Although well-established, Raman-based DTS suffers from a number of drawbacks. Raman scattering is a weak optical phenomenon, so that only low levels of backscattered light are available for detection. Coupled with the optical attenuation inherent in solid core optical fibres, this limits the length or range of the detectable distributed signal. A common application of DTS is temperature monitoring of high voltage direct current (HVDC) subsea (or generally under water) electrical power cables. These are now routinely installed with lengths in excess of 200 km, which is beyond the sensing range of available DTS systems, which have a relatively short range of only around 40 km, and have a long signal acquisition time of over ten minutes to achieve measurements of ±1°C temperature accuracy and 1 m spatial resolution. Hence, many existing cables lack any means to effectively assess their condition. Reliable detection of temperature hotspots along HVDC cables is critical for safety and routine maintenance, so this lack of monitoring is a major difficulty, particularly in view of an increasing need to interconnect power grids between nations. Another field in which temperature measurement is key is in nuclear and fusion reactors. In conventional nuclear reactors, monitoring the temperature distribution of the reactor core and of the pipes carrying heavy water is important. In the emerging area of fusion reactors, solutions are required for mapping the temperature of the entire reactor structure, including the superconductors and the main reactor chamber. However, radioactivity present in these environments causes photodarkening of the glass of solid core optical fibres. This increases the absorption of propagating pulses, leading to increased attenuation, which is detrimental to the already weak Raman scattering. Raman-based DTS systems are therefore unsuitable for nuclear industry applications. A third area of application is the gas and oil industry, where down-hole temperature measurements inside oil wells and gas wells are required. This is a harsh environment, with high temperatures and high pressures, and such conditions cause the ingress of hydrogen into deployed optical fibres. This increases attenuation caused by the glass of the fibre, again problematic for weak Raman scattering, so the accuracy of temperature measurements is reduced. Currently, this is addressed with costly specialised coatings on sensing fibres to reduce hydrogen diffusion into the glass, and the use of complex detection apparatus able to compensate for changes in attenuation.

[0008] Accordingly, distributed sensing systems that can operate successfully in these and other scenarios are of interest.

[0009] SUMMARY OF THE INVENTION

[0010] Aspects and embodiments are set out in the appended claims.According to a first aspect of certain embodiments described herein, there is provided a method of optical time domain reflectometry comprising: launching a probe pulse of laser light into a proximal end of a hollow core optical fibre having a gas content in its core, wherein the hollow core optical fibre is deployed in a environment having a physical parameter that affects movement of gas molecules in the core; detecting, at the proximal end of the hollow core optical fibre, Rayleigh backscattered light produced by scattering of the probe pulse from the gas molecules in the core, wherein the light is detected with spectral resolution and temporal resolution; determining a spectral linewidth or a spectral shift of the detected light arising from Doppler shifting of the Rayleigh backscattered light caused by movement of the gas molecules; and determining from the spectral linewidth or the spectral shift a value of the physical parameter at one or more locations along a length of the hollow core optical fibre.

[0011] According to a second aspect of certain embodiments described herein, there is provided optical time domain reflectometry apparatus configured to perform a method according to the first aspect.

[0012] According to a third aspect of certain embodiments described herein, there is provided optical time domain reflectometry apparatus comprising: a hollow core optical fibre having a gas content in its core, for deployment in an environment having a physical parameter that affects movement of gas molecules in the core; an optical source configured to generate probe pulses of laser light and launch the probe pulses into a proximal end of the hollow core fibre; an optical detection arrangement configured to detect, at the proximal end of the hollow core optical fibre, and with spectral resolution and temporal resolution, Rayleigh backscattered light produced by scattering of the probe pulses from the gas molecules in the core; and a processor configured to: determine a spectral linewidth or a spectral shift of the detected light arising from Doppler shifting of the Rayleigh backscattered light caused by movement of the gas molecules; and determine, from the spectral linewidth or the spectral shift, a value of the physical parameter at one or more locations along a length of the hollow core optical fibre.

[0013] These and further aspects of certain embodiments are set out in the appended independent and dependent claims. It will be appreciated that features of the dependent claims may be combined with each other and features of the independent claims in combinations other than those explicitly set out in the claims. Furthermore, the approach described herein is not restricted to specific embodiments such as set out below, but includes and contemplates any appropriate combinations of features presented herein. For example, methods and apparatus may be provided in accordance with approachesdescribed herein which includes any one or more of the various features described below as appropriate.

[0014] BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which:

[0016] Figure 1A shows a graph of the theoretically calculated velocity of air molecules as a function of temperature, together with the corresponding frequency shift of Rayleigh scattered light caused by Doppler broadening;

[0017] Figure 1B shows a graph of two theoretically calculated spectra of Rayleigh backscattered light, corresponding to two temperatures of gas from which the light has been scattered;

[0018] Figure 2 shows a transverse cross-sectional view of an example of a photonic bandgap hollow core optical fibre, to which aspects of the invention are applicable;

[0019] Figure 3 shows a transverse cross-sectional view of a first example antiresonant hollow core optical fibre, to which aspects of the invention are applicable;

[0020] Figure 4 shows a transverse cross-sectional view of a second example antiresonant hollow core optical fibre, to which aspects of the invention are applicable;

[0021] Figure 5 shows a flow chart of steps in an example method according to an aspect of the invention;

[0022] Figure 6 shows a simplified schematic representation of an example apparatus according to an aspect of the invention;

[0023] Figure 7 shows a graph of experimentally measured spectra of Rayleigh backscattered light detected from different optical fibres at different temperatures; and Figure 8 shows a graph of experimental data comprising the variation of the Rayleigh backscatter spectral linewidth with fibre temperature for a hollow core optical fibre.

[0024] DETAILED DESCRIPTION

[0025] Aspects and features of certain examples and embodiments are discussed I described herein. Some aspects and features of certain examples and embodiments may be implemented conventionally and these are not discussed I described in detail in the interests of brevity. It will thus be appreciated that aspects and features of apparatus and methods discussed herein which are not described in detail may be implemented in accordance with any conventional techniques for implementing such aspects and features.In order to address the problems and difficulties of OTDR distributed sensing based on Raman scattering in solid core optical fibres discussed above, the present disclosure proposes an alternative arrangement. Firstly, it is proposed to use hollow core optical fibre in place of the conventional solid core optical fibre, and secondly it is proposed to detect Rayleigh backscattered light instead of Raman scattering. Rayleigh scattering is an elastic optical process in which light scattered from stationary scattering centres has the same frequency and wavelength as the incident light. It is known as an alternative to Raman scattering in DTS, where, instead of interaction with optical phonons in the fibre, probe pulse light is backscattered from inhomogeneities in the glass core of the solid core optical fibre. Rayleigh scattering is stronger than Raman scattering so appears beneficial, but temperature measurements are problematic and can only be obtained indirectly via measuring changes in strain arising from thermal expansion. This approach is limited to detection of relative changes in temperature, and suffers from cross-talk with the mechanical strain measurements. Hence, Rayleighbased DTS is unsuitable for absolute measurement of temperature only, and overall, Raman-based systems tend to be preferred.

[0026] Solid core optical fibre (SCF) comprises a longitudinal core of a glass of a first refractive index surrounded by a glass cladding of a lower refractive index, such that propagating light is guided along the fibre by total internal reflection at the refractive index boundary. This solid glass structure makes such fibres obvious candidates for applications based on scattering of light, since scattering centres in the form of inhomogeneities (density fluctuations) in the glass core abound.

[0027] A newer, alternative, class of optical fibres is hollow core optical fibres (HCF), comprising a longitudinal central void defining a hollow core, surrounded by a cladding formed by a plurality of longitudinal capillaries with glass boundaries arranged in a defined structure. Light is guided via different mechanisms from the total internal reflection in a SCF. These fibres can demonstrate an advantageously low optical propagation loss, owing to the absence of glass from the core, which causes absorption and attenuation in a SCF. The lack of glass also enables the use of higher peak optical powers, which can undergo unwanted nonlinear effects in a SCF. Hence, HCFs are of great interest for many applications, including telecommunications, and have been shown to be capable of transmitting data over thousands of kilometres. However, the absence of a glass core, which gives HCFs their attractive low loss characteristics, also removes most of the material density fluctuations that produce scattering. This would seem to make OTDR distributed sensing unfeasible with HCFs.However, the voids in a HCF can contain gas. This can be residual from the fabrication process, during which gas at different pressures is applied to the various voids during drawing of the fibre to ensure required relative void sizes in the finished fibre, or can deliberately introduced into the fibre after drawing. Hence, there is typically a gas content within the core of a HCF, and the gas molecules can act as scattering centres, producing backscattered light available for detection in an OTDR system.

[0028] In a SCF, the glass forming the core has a fixed lattice structure, so the inhomogeneities that cause backscatter are fixed in place. If Rayleigh backscattering is detected from a SCF, the detected light has a fixed spectral linewidth substantially matching the linewidth of the launched probe pulses, which is typically very narrow since laser light is used in OTDR systems. In contrast, in a HCF, the gas molecules providing backscattering in the hollow core are mobile. This means that the Rayleigh backscattered light undergoes Doppler shifting as it is scattered from the individual moving gas molecules. Since the gas molecules will move randomly in all possible directions (Brownian motion), different photons experience different amounts of Doppler shift along the backscatter direction, and the cumulative effect is a broadening of the linewidth of the Rayleigh backscatter compared with the linewidth of the probe pulses. It is recognised that this occurs in HCF, and it is generally acknowledged as a problem, since the Doppler shift essentially causes a “blurring” of what is perceived as the useful detected signal (namely the Rayleigh scatter at the probe pulse wavelength), thereby reducing accuracy or making measurements unfeasible [1],

[0029] However, the present disclosure recognises that the Doppler shifting of Rayleigh backscatter in a HCF with a gas content in the core can be unexpectedly useful. The amount of Doppler shifting, and hence the amount of broadening of the linewidth of the detected Rayleigh backscattered light, depends on the speed of movement of the gas molecules in the hollow core with which the light of the probe pulse interacts. The speed of movement is affected by the environmental conditions to which the optical fibre is subjected. A particular example is temperature. A higher temperature in the environment around a HCF increases the temperature of the gas content in the core, causing the velocity of the gas molecules to also increase, with a corresponding broadening of the linewidth of the Rayleigh backscatter. Conversely a lower temperature slows the gas molecules and produces a narrower linewidth. Hence, monitoring of Rayleigh backscatter from launched probe pulses in a spectrally resolved manner that allows the linewidth to be determined enables the temperature to be spatially resolved along the length of a HCF. Other external parameters that affect movement of the gas molecules can be similarly detected. Hence, the present disclosure proposes to implement OTDRin a gas-containing hollow core optical fibre by detection of the spectral linewidth or spectral shift of Doppler-broadened or Doppler-shifted Rayleigh backscattered light in order to obtain distributed measurements of external physical parameters in an environment in which the HCF is deployed.

[0030] Considering temperature as an example, it is known that the average speed of gas molecules in free space, vrms, is a function of the temperature T of the gas:

[0031] >

[0032]

[0033] where KB is the Boltzmann constant and m is the mass of the gas molecules. Hence, the temperature can be determined if the average velocity is known (noting that the relationship may be slightly different for gas molecules bounded by the capillary walls in a hollow core fibre). Since the amount of Doppler broadening of the Rayleigh backscatter depends on the gas molecule velocity as noted above, it is possible to determine the gas temperature from a measurement of the linewidth of the Rayleigh backscattered light, since the linewidth depends on the Doppler broadening. If the linewidth (bandwidth) of the Rayleigh backscatter is detected in a distributed manner along the length of a hollow core optical fibre, in line with conventional time I distance resolved OTDR detection techniques, temperature can be mapped along the fibre. Hence, a temperature profile over all or part of the length of a hollow core optical fibre deployed in a region of interest can be obtained.

[0034] Figure 1A shows a graph of the velocity of air molecules as a function of temperature (left axis), together with the corresponding frequency shift (spectral shift) of Rayleigh scattered light caused by the Doppler shift (right axis). The data are theoretical, obtained using equation [1], It can be seen that the relationship between temperature and frequency shift is a simple linear relationship, and can be correspondingly determined for other gas types. One can therefore determine the temperature of a gas inside a HCF (which will roughly correspond to the temperature of the external environment around the fibre) from a measurement of the Doppler frequency shift, which is embodied in a measurement of the linewidth (bandwidth) of the Rayleigh backscattered light, the spectrum of which is broadened compared with the original light before scattering. When implemented in an OTDR arrangement, in which the use of pulsed input light enables time-resolved detection and hence distance-resolved measurement along the length of the optical fibre, a new arrangement for distributed temperature sensing is possible. Other environmental conditions can also affect the Rayleigh linewidth via Doppler shifting, so that distributed sensing can be implementedmore generally. Examples of other parameters that can be measured in this way are given later.

[0035] Figure 1B shows a graph of two spectra of Rayleigh backscattered light, corresponding to two temperatures of gas from which the light has been scattered (theoretical data). It can be seen that the spectra have different linewidths. A higher temperature of 373 K (and hence higher gas molecule velocity) produces more Doppler broadening and hence a larger linewidth than a lower temperature of 237 K (and hence lower gas molecule velocity). Measurement of the linewidth can thereby reveal the gas temperature. The nature of gas molecule movement affected by temperature is random as regards direction, so that the Doppler shifting of the Rayleigh backscattered light varies between molecules, and the overall cumulative effect of the Doppler shifting is to broaden the spectrum of the Rayleigh backscattered light (some light will be shifted up in frequency and some light will be shifted down in frequency, depending on the direction of movement of the gas molecules). Hence, to determine the temperature, it is appropriate to measure the linewidth, since this captures the maximum frequency shift corresponding to molecules moving parallel to the direction of light propagation in the optical fibre, and therefore reveals the actual molecule velocity and hence the temperature.

[0036] Other physical parameters may have a different effect on gas molecule movement, however. For example, molecules caused to move by the action of a magnetic field will generally all have movement along the same direction, depending on the orientation of the magnetic field. Hence, the light scattered by the gas molecules will all experience the same Doppler shift, in contrast with the random spread of Doppler shifts arising from molecules moving in random directions. In this case, the Rayleigh backscattered light will show a general overall shift in frequency, rather than the spectral broadening caused by randomly moving molecules. In such a case, it is appropriate to measure the spectral shift of the Rayleigh backscattered light (peak-to-peak frequency change, for example), rather than the linewidth. The size of the shift corresponds to the molecule velocity, caused by and dependent on the size of the applied magnetic field.

[0037] Hollow core optical fibres (HCFs) have a structure comprising an array or arrangement of holes, capillaries or lumen within the fibre material, extending along the length of the fibre parallel to the longitudinal axis and defined within a material such as glass. The arrangement of the holes and their defining boundary walls can be termed a microstructure. Typically the microstructure forms at least part of the cladding of the fibre, and surrounds a central hollow void or region that provides a light-guiding core, and which may be filled with air or another gas. The capillaries of the microstructure aretypically supported within a larger outer cladding tube made from glass. The propagation of light in air enabled by the absence of a solid glass core reduces the proportion of a guided optical wave which propagates in glass compared to a solid core fibre, offering benefits such as increased propagation speed, reduced loss from both absorption and scattering, and reduced nonlinear interactions. Hence hollow core fibres are very attractive for applications including telecommunications; they enable data transmission at nearly the speed of light in vacuum, at higher optical powers, and over broader optical bandwidths, with relative freedom from issues such as nonlinear and thermo-optic effects that can affect light travelling in solid fibres. These properties are also attractive for the use of HCFs in OTDR systems as proposed herein.

[0038] Hollow core fibres can be categorised according to their mechanism of optical guidance into two principal classes or types: hollow core photonic bandgap fibre (HCPBF, alternatively referred to as hollow core photonic crystal fibre, HCPCF) [2], and antiresonant hollow core fibre (AR-HCF or ARF) [3], of which there are various subcategories characterised by the geometric structure of the cladding capillaries. The present disclosure is applicable to all types of hollow core fibre, including these two main classes and their associated sub-types plus other hollow core designs. Note that in the art, there is some overlapping use of terminologies for the various classes of fibre. For the purposes of the present disclosure, the terms “hollow core fibre” and “hollow core microstructured fibre” are intended to cover all types of these fibres having a hollow core as described above. The terms “HCPBF” and “HCPCF” are used to refer to hollow core fibres which have a structure that provides waveguiding by photonic bandgap effects (described in more detail below). The terms “ARF” and “antiresonant hollow core fibre” are used to refer to hollow core fibres which have a structure that provides waveguiding by antiresonant effects (also described in more detail below).

[0039] Figure 2 shows a cross-sectional view of an example HCPBF 10. In this fibre type, a structured, inner, cladding 1 comprises a regular closely packed array of many small glass capillaries, from which a central group is excluded 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 provides a substantially periodically structured refractive index and hence a photonic bandgap effect that confines the propagating optical wave towards the core 2. These fibres can be described in terms of the number of cladding capillaries or “cells” which are excluded to make the core 2. In the Figure 2 example, the central nineteen cells from the array are absent in the core region, making this 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 thecircularity of the outer surface of the structured cladding 1. In other examples, different numbers of rings may be used to define the cladding 1. An outer cladding 3 surrounds the structured cladding 1; this is a tube that supports the capillaries of the structured cladding 1.

[0040] In contrast to HCPBF, antiresonant hollow core fibres guide light by an antiresonant optical guidance effect. The structured cladding of ARFs has a simpler configuration, comprising a much lower number of larger glass capillaries or tubes than a HCPBF to give a structure lacking a high degree of periodicity so that photonic bandgap effects are not significant, but with some rotational periodicity on a larger scale since the tubes are evenly disposed (with or without spaces). The cladding capillaries comprise only a single ring of capillaries around the core; additional smaller capillaries may be included inside the capillaries of the primary single ring. The structure means that antiresonance is provided for propagating wavelengths which are not resonant with a wall thickness of the cladding capillaries, in other words, for wavelengths in an antiresonance window which is defined by the cladding capillary wall thickness. The cladding capillaries surround a central void or cavity which provides the hollow core of the fibre, and which is able to support antiresonantly-guided optical modes. The structured cladding can also support cladding modes able to propagate primarily inside the capillaries, in the glass of the capillary walls or in the spaces or interstices between the cladding capillaries and the fibre’s outer cladding. The loss of these additional noncore guided modes is generally very much higher than that of the core guided modes. The fundamental core guided mode typically has by far the lowest loss amongst the core guided modes. The antiresonance provided by a capillary wall thickness which is in antiresonance with the wavelength of the propagating light acts to inhibit coupling between the fundamental core mode and any cladding modes, so that light is confined to the core and can propagate at very low loss.

[0041] Figure 3 shows a transverse cross-sectional view of an example simple antiresonant hollow core fibre. The fibre 10 has an outer tubular cladding 3. The structured, inner, cladding 1 comprises a plurality of tubular cladding capillaries 4, in this example seven capillaries of the same cross-sectional size and shape, which are arranged inside the outer cladding 3 in a single ring, so that the longitudinal axes of each cladding capillary 4 and of the outer cladding 3 are substantially parallel. Each cladding capillary 4 is in contact with (bonded to) the inner surface of the outer cladding 3 at a location 5, such that the cladding capillaries 4 are evenly spaced around the inner circumference of the outer cladding 3, and are also spaced apart from each other so there is no contact between neighbouring capillaries. In some designs of ARF, thecladding tubes 4 may be positioned in contact with each other (in other words, not spaced apart as in Figure 3), but spacing to eliminate this contact can improve the fibre’s optical performance. The spacing removes nodes that arise at the contact points between adjacent tubes and which tend to cause undesirable resonances that result in high losses. Accordingly, fibres with spaced-apart cladding capillaries may be referred to as “nodeless antiresonant hollow core fibres”.

[0042] The arrangement of the cladding capillaries 4 in a ring around the inside of the tubular outer cladding 3 creates a central space, cavity or void within the fibre 10, also with its longitudinal axis parallel to those of the outer cladding 3 and the capillaries 4, which is the fibre’s hollow core 2. The core 2 is bounded by the inwardly facing parts of the outer surfaces of the cladding capillaries 4. This is the core boundary, and the material (glass or polymer, for example) of the capillary walls that make up this boundary provides the required antiresonance optical guidance effect or mechanism. The capillaries 4 have a thickness at the core boundary which defines the wavelength for which antiresonant optical guiding occurs in the ARF.

[0043] Figure 4 shows a transverse cross-sectional view of a second example ARF. The ARF 10 has a structured inner cladding 1 comprising six cladding capillaries 4 evenly spaced apart around the inner surface of a tubular outer cladding 3 and forming a single, primary ring surrounding a hollow core 2. Each cladding capillary 4 has a secondary, smaller capillary 6 nested inside 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 the optical loss. Additional still smaller tertiary capillaries (not shown) may be nested inside the secondary capillaries 6. ARF designs of this type, with secondary and optionally smaller further capillaries, may be referred to as “nested antiresonant nodeless fibres”, or NANFs [4], In other NANF designs, one or more additional secondary capillaries may be nested within a primary capillary. Many other capillary configurations for the structured cladding of an ARF are possible, and the disclosure is not limited to the examples described above. For example, the capillaries need not be of circular cross-section, and / or may or may not be all of the same size and / or shape. The number of capillaries surrounding the core in the primary ring may be for example, four, five, six, seven, eight, nine, ten or more.

[0044] Hollow core optical fibres may be made from any of the glass-based materials known for the fabrication of solid core fibres, in particular silica. Types of glass include “silicate glasses” or “silica-based glasses”, based on the chemical compound silica (silicon dioxide, or quartz), of which there are many examples. Other glasses suitable foroptical fibres include, but are not limited to, doped silica glasses. The materials may include one or more dopants for the purpose of tailoring the optical properties of a fibre, such as modifying absorption or transmission, or tailoring properties of the materials for purposes such as facilitating fibre manufacture, improving reliability, or enabling or enhancing a particular end use. HCFs may also be made from polymer materials.

[0045] Herein, terms including hollow core optical fibre, hollow core fibre, hollow core waveguide, hollow core optical waveguide, hollow core microstructured fibre, hollow core microstructured waveguide, and similar terms are intended to cover optical waveguiding structures configured according to any of the above examples and similar structures, where light is guided by any of several guidance mechanisms (photonic bandgap guiding, antiresonance guiding, and / or inhibited coupling guiding) in a hollow elongate void or core surrounded by a structured (microstructured) cladding comprising a plurality of longitudinal capillaries. These various terms may be used interchangeably in the present disclosure.

[0046] As noted above, it is contemplated that any design or configuration of hollow core optical fibre may be used in an OTDR system as described herein. A required feature is the presence of a gas content within the hollow core; otherwise the structure of the fibre is immaterial. However, in some examples, a ARF design may be preferred. As can be appreciated from Figures 2-4, the core size in a ARF is typically proportionally larger than in a HCPBF, and the microstructured cladding comprises fewer glass walls. This allows a larger volume of gas to be contained within a given length of fibre. A larger number of gas molecules is therefore present, providing a larger number of scattering centres, and thereby increasing the amount of Rayleigh backscattering that can occur. The detectable signal strength is therefore higher, giving a larger signal-to-noise ratio and hence improved measurement accuracy. The usable length of fibre, corresponding to the detection range of the OTDR system, is also increased for a higher signal strength, since attenuation effects are proportionately reduced. The smaller overlap between the propagating light field and the glass in a ARF compared to a HCPBF may also be advantageous. Scattering from the glass will occur, but is not useful for measuring temperature using the Doppler shift. In a HCPBF the amount of scattering from the glass may be higher than the amount of backscattered light from the gas content, so that the desired Doppler-shifted signal may be swamped. Among ARFs, it is anticipated that a NANF design of ARF may be most beneficial, owing to the advantages of a NANF structure compared to a basic ARF design as described above. However, other HCF designs are not excluded, and an OTDR system as disclosed herein can be implemented with any HCF which is conveniently available.In contrast with conventional OTDR using a solid core fibre, the proposed approach using hollow core fibre is enabled by the detection of the linewidth of Rayleigh backscattered light, which is broadened by the Doppler effect in moving gas molecules in the fibre core. This is not possible in solid core fibre, since the scattering centres provided by inhomogeneities in the glass of the fibre core are fixed and produce no spectral broadening. In turn, the use of hollow core fibre addresses many problems which make conventional solid core fibre OTDR systems unsuitable. The measurement range can be greatly increased compared to a solid core fibre system. This is because the Rayleigh backscatter is a stronger effect than Raman backscatter, yielding a stronger signal for detection, and optical attenuation is much lower in a hollow core fibre, so that a higher proportion of the backscattered light can propagate back to the end of the fibre for detection. This will allow distributed temperature sensing over much greater distances than is currently achievable, enabling temperature monitoring of subsea I underwater HVDC cables in excess of 200 km in length. Additionally, probe pulses with much higher peak power can be used to interrogate the fibre, compared with solid core fibre systems which are subject to nonlinear optical effects at higher optical powers.

[0047] Also, the absence of a solid glass core makes hollow core fibres resistant to problems caused by photodarkening, which increases the attenuation of solid core fibres. Hence, the OTDR systems proposed herein are suitable for use in highly radioactive environments such as nuclear and fusion reactors and particle accelerators, where the radioactivity causes photodarkening of glass and hence makes solid core fibre systems unsuitable. Similarly, the optical attenuation of solid core fibre is increased by ingress of hydrogen into the glass structure, which occurs in the high pressure and high temperature environments within oil wells and gas wells. Hollow core fibres are also resistant to this deterioration, owing to the lack of glass, so the proposed OTDR systems are also usable in the oil and gas industry. This will avoid current requirements for costly specialised protective fibre coatings and complex signal processing that compensates for the increasing attenuation.

[0048] Figure 5 shows a flow chart of steps in an example method according to an aspect of the present disclosure. In a first step S1, a hollow core optical fibre having a gas content in its hollow core is deployed in an environment which has an associated physical parameter which is able to have an effect on the movement of gas molecules in the gas content. The optical fibre can be arranged along a particular distance over which it is desired to obtain a distributed measurement of the physical parameter. A distributed measurement is a measurement which is spatially resolved over a physical distance, yielding a profile of the value of the parameter with respect to distance. Themeasurement distance may correspond to the full length of the optical fibre, or to a part or parts of the length, or to one or more discrete locations along the length of the optical fibre. As is known with OTDR measurements, the distance at which a measured value of the parameter is obtained is resolved by the time of flight of an optical pulse to travel along the optical fibre to that distance and back, from an accessed end of the optical fibre. The time of detection of returned light corresponds to the location from which the light is returned. Step S1 may be optional depending on circumstances. It will be necessary to arrange the optical fibre in the measurement environment for a first time of taking measurements at the deployment location, but the optical fibre may be retained in its deployed position over time to enable repeated measurements to be made, for example to enable long-term monitoring of the environment. Example environments include the vicinity of an underwater electric cable such as a HVDC cable, in which case the optical fibre can be arranged alongside the cable, or within a nuclear or fusion reactor, or within an oil or gas well, but other environments are not excluded.

[0049] After deployment of the optical fibre, or on return to a previously deployed optical fibre, the remaining steps of the example method can be performed. In a second step S2, a probe pulse of laser light of a known wavelength is generated, and launched into an accessible end of the optical fibre for propagation along the fibre. The accessible end may be referred to as a proximal end of the fibre, with the opposite, distal end of the fibre in a remote location which may or may not be accessible. The duration of the probe pulse determines the spatial resolution of the measurement, as is known for OTDR, so can be selected accordingly. The wavelength of the laser light should preferably correspond to a wavelength for which the optical fibre is configured for low loss propagation, in order to minimise optical attenuation and maximise the returned light available for detection. For reasons discussed further below, a shorter wavelength may be preferred in order to also maximise the amount of returned light. The pulse peak power can be selected to also provide a high level of returned light, where maximum peak powers can be much higher than for solid core optical fibres owing to the lack of nonlinear optical effects in hollow core optical fibres. It is also possible to generate and launch additional probe pulses at different wavelengths, simultaneously, again to improve the level of returned light for detection. All these options are available for improvement of the signal-to-noise ratio to enhance measurement accuracy.

[0050] The launched probe pulse propagates along the optical fibre, interacts with the gas molecules of the gas content along the hollow core and undergoes Rayleigh scattering from the molecules. Some of the scattered light will be along the incident propagation direction of the probe pulse light in the reverse direction, thereby formingRayleigh backscatter. The Rayleigh backscattered light can propagate back along the optical fibre to the proximal end, arriving at a time dependent on the position along the fibre length at which the backscatter occurred. Also, movement of the gas molecules causes a Doppler shift in the wavelength of the light, causing a broadening or shift of the spectrum of the Rayleigh backscattered light compared to the original wavelength of the probe pulse.

[0051] In a third step S3, the Rayleigh backscattered light returning to the proximal end of the optical fibre is detected as it exits the optical fibre. Detection is carried out with temporal resolution, in order that the point of origin of the backscatter along the fibre length can be determined, and also with spectral resolution so that the Doppler broadening or shifting can be determined.

[0052] A fourth step S4 is a processing step in which the spectrally resolved detected Rayleigh backscattered light is assessed in order to determine its spectral linewidth and I or the spectral shift, arising from the Doppler shift caused by movement of the gas molecules from which the light was scattered.

[0053] In a further processing step, fifth step S5, the determined spectral linewidth or spectral shift is used to determine a value of the physical parameter, using a known relationship between the physical parameter and the size of the Doppler shift caused by resulting motion of the gas molecules (as discussed above with regard to Figures 1A and 1B. In conjunction with the time-resolved aspect of the measurement, the value of the physical parameter at one or more locations along the length of the hollow core optical fibre is thereby obtained. A full distributed measurement or profile along all or part of the fibre’s length may be obtained, or one or more discrete values at selected locations along the length. Step S5 may be performed in conjunction with step S4 in order to obtain an immediate measurement result, or the spectral linewidth or spectral shift data may be stored for later determination of the parameter value, or for transmission of the data for determination of the parameter value at a remote location away from the optical fibre deployment site. Similarly, data representing the spectrally and temporally resolved detected Rayleigh backscattered light may be stored and I or transmitted after step S3 for subsequent processing to extract the linewidth or spectral shift data and calculation of the measured parameter value(s).

[0054] Figure 6 shows a schematic representation of an example apparatus for performing OTDR measurements in accordance with the present disclosure. The apparatus 20 overall comprises four parts. A hollow core optical fibre 24 has a gas content in its core, and is configured for deployment in (or has been deployed in) an environment 18 having an associated physical parameter that affects the movement ofgas molecules in the hollow core of the optical fibre 24. The optical fibre 24 has a proximal end 24a located in conjunction with the remainder of the apparatus 20, and extends along its length to a remotely located distal end 24b, which is typically not accessible. The optical fibre 24 is shown with a short length for convenience, but Figure 6A is not to scale and the optical fibre 24 may extend over any length, including tens or hundreds of kilometres to enable remote measurement and highly extended distributed measurement.

[0055] An optical source 22 is configured to generate pulses of laser light, these being the probe pulses 30 of the OTDR system. The output of the optical source 22 is coupled via an optical circulator 32 to the proximal end 24a of the optical fibre 24 in order that the probe pulses can be launched into the proximal end 24a of the optical fibre 24 for propagation along the optical fibre 24. Within the optical fibre 24, a probe pulse undergoes Rayleigh scattering from the gas molecules in the core, and the light which is backscattered acquires a Doppler shift owing to the movement of the gas molecules. The Rayleigh backscatter 29 returns to the proximal end 24a of the optical fibre 24, and is passed by the optical circulator 32 to an optical detection arrangement 26. While an optical circulator 32 is convenient in an optical fibre system for passing a returned signal away from its source, other means may be used including free space optical elements such as a beam splitter.

[0056] The optical detection arrangement 26 is configured to detect the Rayleigh backscattered light 29 (comprising the detectable optical signal of the OTDR system) with both spectral resolution and temporal resolution. The detected signal is passed from the optical detection arrangement 26 to a processor 28. The processor 28 may be physically proximate to the optical detection arrangement 26 in order to provide an immediate “on the spot” measurement. Alternatively, the detected signal may be transmitted, either immediately or stored for later transmission, to a remotely located processor 28.

[0057] The processor 28 is configured to determine the spectral linewidth (bandwidth) and I or the spectral shift of the detected Doppler shifted Rayleigh backscatter. Additionally, the processor 28 is configured to determine or calculate, from the spectral linewidth or the spectral shift, a value of the physical parameter of the environment 18, at one or more locations along the length of the hollow core optical fibre, where the temporal resolution of the detected signal is used to ascertain the location information in the known manner for OTDR. The processor 28 can be configured for these operations by appropriately coded software, for example, and previously determined or obtained data relating values of the physical parameter with the size of the spectral linewidth orspectral shift (via the movement of the gas molecules produced by the physical parameter and causing the Doppler shifting and I or broadening of the probe pulse light spectrum as explained above). The processor 28 may be configured as a single entity, or may comprise a first part located in conjunction with the optical detector arrangement 26 which ascertains the spectral linewidth or spectral shift, and then transmits this data (immediately or after storage) to a second part proximally or remotely located that calculates the parameter value(s). The first part might also be comprised within the optical detection arrangement 28. Alternatively, the first part may store the spectral linewidth or spectral shift data for collection for subsequent processing to determine the parameter value(s) rather than transmitting the data.

[0058] Examples implementations of the parts of the apparatus 20 will now be described in more detail.

[0059] In the depicted example, the optical source 22 comprises a laser 34, such as a diode laser, configured to generate continuous wave laser light at an appropriate wavelength for transmission along the hollow core optical fibre 24. In order to generate probe pulses of the laser light, the output of the laser 34 is passed through an amplifier 36 such as an erbium-doped fibre amplifier (EDFA), then through an optical filter 38 to remove amplified spontaneous emission from the amplifier and retain only the wavelength of interest after amplification, and then through a modulator 40 such as an acousto-optic modulator or an electro-optic modulator, which outputs the probe pulses 30. . The pulse generation can be selected to give a desired spatial resolution of the distributed measurement, in the conventional manner for OTDR measurements as will be understood by the skilled person. A continuous wave laser is used in this particular example, rather than a laser that directly generates pulses of light, because the continuous wave output is used in the detection process.

[0060] The optical detection arrangement 26 is configured to achieve spectrally resolved detection of the returning Rayleigh backscattered light, which is necessary in order for the spectral linewidth or the spectral shift to be ascertained. Any technique for spectrally resolved optical detection can be employed. The example apparatus of Figure 6 utilises microwave detection. The continuous wave output of the laser 34 is split, with part being used to generate the probe pulses 30 as described above, and part diverted for use as an optical local oscillator (OLO) 42 which is provided to the optical detection arrangement 26. The optical detection arrangement 26 includes an optical coupler 44 which receives both the Rayleigh backscattered light 29 collected from the optical fibre 24 by the optical circulator 32, and the OLO signal 42, and mixes the two light signals together to generate a beat signal. The OLO signal 42 can be passed through apolarisation scrambler 46 prior to mixing with the Rayleigh backscattered light 29 to ensure effective mixing and generation of the beat signal, since the Rayleigh scattering within the optical fibre 24 randomises the original polarisation of the probe pulse light. The optical signal output by the optical coupler 44 is detected by a photodetector 48 in the optical detection arrangement 26, and the photodetector output signal is delivered to a microwave detector 50 for detection of the beat signal frequency which is in the microwave frequency range. The microwave detector 50 also receives the drive signal of the modulator 40 in the optical source 22, for synchronisation with the probe pulse repetition rate. The microwave signal is passed to the processor 28 which extracts the signal linewidth or the spectral shift. More details of this microwave detection technique for spectrally resolved detection can be found in [5] which describes this technique for distributed sensing in the context of Brillouin scattering in an optical fibre. Microwave detection for this purpose is a relatively slow process since the frequencies in the detected signal are scanned one at a time, but the frequency resolution is clean and fine.

[0061] An alternative technique for spectrally resolved detection of the Rayleigh backscattered light is described in [6], again in the context of Brillouin scattering. This approach involves dividing the detected Rayleigh backscattered signal into temporal windows and calculating the short-time Fourier transform (STFT) in each window to obtain the frequency components. This method is faster than the microwave detection method since it is effectively a single-shot measurement because there is no need for consecutive scanning of the frequencies, and is advantageous in not requiring a microwave detector. However, the size of the temporal window determines the spatial resolution, resulting in a trade-off between frequency resolution and spatial resolution in the final distributed measurement of the physical parameter of interest.

[0062] Other techniques for spectrally resolved measurements may also be used; the invention is not limited in this regard.

[0063] Figure 7 shows a graph of experimental data in the form of measured Rayleigh backscattered light spectra obtained at different temperatures from different optical fibres, using apparatus similar to that of Figure 6. The graph plots optical power (vertical axis) against frequency (horizontal axis). The spectra A, B and C were all measured from a NANF ARF hollow core optical fibre of 300 m in length. Spectrum A was measured at a fibre temperature of 23°C, spectrum B was measured at a fibre temperature of 28°C, and spectrum C was measured as a fibre temperature of 48°C. From these it can be seen than the linewidth increases with temperature, corresponding to the higher gas molecule velocity and associated larger Doppler shift at higher temperatures. Hence, it is readily apparent that temperature can be determined from such measurements. Forcomparison, spectrum D was measured using a standard single mode solid core optical fibre, and has a much narrower linewidth owing to the Rayleigh backscattered light originating from fixed scattering centres in the glass of the core and hence experiencing no Doppler shift, rather than from moving gas molecules in the core of the NANF.

[0064] Figure 8 shows a further graph of experimental data obtained from the NANF, and is a plot of measured spectral linewidth (vertical axis) against fibre temperature (horizontal axis). A linear relationship between these values is apparent, as predicted from the theoretical data shown in Figure 1A. Hence, it is straightforward to determine temperature from a measurement of the spectral linewidth of the Rayleigh backscattered light, for a calibrated system for which this relationship is known. Similarly, other physical parameters that cause movement of gas molecules in the hollow core can be measured in the same way, via measurement of the spectral linewidth or the spectral shift, as described elsewhere herein.

[0065] While distributed temperature sensing is a primary application of the methods and apparatus proposed herein, it will be apparent that other physical parameters can be detected using the described approach. Since the technique relies on the detection of Doppler-broadened Rayleigh scattered light, any environmental condition that affects the velocity of the gas molecules in the hollow core fibre and therefore causes Doppler broadening can in principle be measured, once the relationship between the physical parameter and the gas molecule movement is known. As an example, the strength of a magnetic field can be measured, monitored or assessed. Gas molecules that have a dipole moment will respond to magnetism, with the magnetic field causing an amount of movement of the gas molecules, and hence an amount of Doppler spectral broadening, that depends on the magnetic field strength. Hence, if a hollow core optical fibre having a gas content comprising molecules with a dipole moment is deployed in an environment for which the magnetic field strength is desired to be known, the magnetic field can be determined from a measurement of the Rayleigh backscatter linewidth. Carbon monoxide is an example of a gas suitable for magnetic measurements, but other dipole moment gases might be used instead.

[0066] Regardless of the detected parameter, there are a number of factors that can affect the amount of the Rayleigh backscattered light available for detection (signal strength of the system), and hence determine the signal-to-noise ratio and the accuracy of the measurements. Implementing a system in a manner that produces a strong detectable signal is hence important. Some factors, parameters, features and characteristics that can usefully be taken into account are now discussed.A first feature of significance is the choice of gas used to provide the gas content within the hollow core fibre. A simple approach is to utilise the gas content inherent in a hollow core fibre as a by-product of its fabrication process. Hollow core optical fibre is produced by drawing the fibre from a heated and softened glass preform, using a fibre drawing tower in the same manner as is established for solid core fibre production. The glass preform is assembled from a plurality of hollow glass tubes corresponding to the various voids and capillaries of the fibre design. It is common to apply gases to the various voids in the preform during the draw to achieve pressure differentials between the voids, in order to both prevent collapse of the voids, and to achieve the required relative dimensions of the voids in the finished fibre structure. Inert gases are typically used for this pressurisation, and some of this gas remains within the fibre structure at the end of the fabrication process. If a specific gas content is desired within a hollow core fibre, for example for a particular OTDR sensing application, it may be possible to use a desired gas species for pressurisation during the draw in order that the finished hollow core fibre contains required gas molecules once it is fabricated. Alternatively, the gas content inherent from the fabrication process may be supplemented or partially or wholly replaced by one or more different gas species after the hollow core fibre is fabricated. This can be achieved by deliberately forcing a gas into the hollow core fibre via an open end of the fibre, perhaps in combination with drawing the existing gas content out via the opposite end of the fibre so that the new gas replaces the old gas. Diffusion of gas along a hollow core fibre is a relatively slow process, however, owing to the very narrow width of the voids and capillaries, so this may not be suitable for very long fibre lengths. An alternative approach is to drill holes in the side of the hollow core fibre, introduce a desired gas species into the fibre through the holes, and then seal the holes to keep the gas content inside the fibre.

[0067] As an example, the hollow core fibre may be configured to have a gas content that comprises gas molecules having a particular scattering cross-section. This is a parameter that determines what proportion of light incident on a gas molecule undergoes Rayleigh scattering. A larger scattering cross-section produces a higher intensity of backscattered light, so gives a larger detectable signal and improves the performance of the OTDR system. As an example, the scattering cross-section of sulphur hexafluoride (SFe) is around 460 times larger than that of helium (He), so a hollow core fibre containing SFe would provide a much higher backscattered signal for detection than a fibre containing helium. Hence, it may be advantageous to select a gas species with a higher scattering cross-section for the gas content of the hollow core fibre. If the gas species of interest is not suitable for void pressurisation during drawing, it would need tobe added into the fibre post-fabrication, to replace or supplement the existing gas content. For example, SFe is toxic so is less suitable for use during fibre fabrication and would preferably be introduced into the fibre after drawing. The fibre should then be sealed to retain the gas inside. While a gas content with a larger scattering cross-section is useful for providing a stronger signal, this comes at the cost of a reduced measurement length (since the backscattered light has to traverse the core without further scattering in order to be detected at the fibre end), so use of a higher scattering cross-section may not be suitable for long-distance sensing applications.

[0068] Overall, however, any gas species might be selected for the gas content of the hollow core fibre, which can deliver an amount of Rayleigh backscattered light which is adequate for the required level of measurement accuracy (in conjunction with other parameters discussed below). Examples of gases suitable for temperature measurements include carbon dioxide (CO2), methane (CH4) and sulphur hexafluoride (SFe), although the disclosure is not limited in this regard.

[0069] A second feature of interest for the hollow core fibre is the pressure of the gas content. A higher gas pressure within the hollow core provides more gas molecules with which the probe pulse can interact and undergo Rayleigh scattering, thereby increasing the backscattered signal strength available for detection. Measurement accuracy can thereby be improved. While any internal pressure that can be supported by the fibre structure can in principle be used here, it is proposed that a pressure of one atmosphere (101.325 kPa) or near one atmosphere (around 100 kPa) is useful. This helps to maintain the gas content if the fibre is broken or damaged. An internal pressure above one atmosphere causes the gas content to leak out if the core is breached, while an internal pressure below one atmosphere can cause external gas to be drawn into the fibre if the core is breached. In either circumstance, the gas content is altered (in terms of pressure or species or both) which will modify the amount of backscattered light and affect the measurement accuracy. However, if the fibre is able to be protected, and I or deployed in circumstances where the risk of damage is low, or where the enhanced signal provided by higher pressure is required for a particular application, a gas content above one atmosphere may be used. Similarly, if a lower pressure is considered to provide an adequate signal, a gas content below one atmosphere may be used. Internal pressure of hollow core fibres can be below one atmosphere after drawing, owing to cooling of the fibre and the gas after the heat of the drawing tower, so the use of off-the-shelf hollow core fibres may fall into this category. Otherwise, the internal pressure of the hollow core fibre might be increased by modifying the pressure(s) of the applied gas during the drawing process, or by adding more gas into the fibre after fabrication.In addition to the gas content (gas species and gas pressure) within the fibre, the actual hollow core fibre itself can be chosen to enhance the performance of the OTDR system. As discussed above, any hollow core fibre design is potentially suitable, since any design can include the required gas content in its hollow core. Also as discussed above, an ARF may be preferred to a HCPBF, owing to the smaller amount of glass in the fibre cross-section, giving a lower overlap between the propagating light and the glass of the fibre. The less overlap there is, the better the potential performance of the OTDR system owing to more interaction between the light and the gas content. A single mode NANF ARF has been successfully employed in experimental work. However, it is also possible to use a few-mode NANF (supporting two, three or four optical modes, for example) if the detection arrangement is configured to detect and characterise the Doppler-shifted Rayleigh backscattered light independently for each mode.

[0070] Turning to the optical apparatus used with the hollow core optical fibre to implement the OTDR, the optical characteristics of the probe pulses may be selected to optimise or otherwise modify the performance of the system, in particular to enhance the signal strength. A first pulse feature of interest is the wavelength of light used for the probe pulses. The intensity (power PR) of Rayleigh scattering is dependent on the wavelength A of the incident light, being inversely proportional to the fourth power of the wavelength (sct i / ^4). Hence, the use of a shorter wavelength of probe light will increase the level of Rayleigh backscatter which is available for detection. For example, a probe pulse at 1064 nm wavelength can boost the intensity of the Rayleigh backscatter by a factor of five, compared to a probe pulse at 1550 nm wavelength (being a commonly used wavelength for solid core optical fibres). To achieve this improvement, a hollow core fibre configured for optical transmission at the selected probe pulse wavelength should be selected for use in conjunction with a suitable optical source to generate the probe pulses at the selected wavelength.

[0071] A second probe pulse feature of interest is the peak pulse power. In common with known OTDR systems, the intensity of the backscattered Rayleigh light is linearly proportional to the peak power of the probe pulses. In a solid core fibre, usable peak power is limited by the threshold at which nonlinear optical effects commence, caused by the interaction of the probe light with the glass through which it propagates. Hollow core fibres are advantageous in avoiding nonlinear effects, since the light largely propagates in the air (or other gas or vacuum) filling the core. Hence, hollow core fibres can support much higher peak pulse powers without nonlinear frequency distortion than solid core fibres, so a hollow core fibre OTDR system can be utilised with higher pulse powers than a solid core system, thereby obtaining higher levels of Rayleigh backscattered light andimproved measurement accuracy. This higher signal level, in conjunction with the lower attenuation of hollow core fibres, can also increase the detection range compared with a solid core fibre OTDR system.

[0072] Although less relevant to the strength of the backscattered signal, another probe pulse feature that requires selection is the pulse duration. As is well understood for OTDR systems, the spatial resolution of the distributed measurement is determined by the pulse duration, with shorter pulses giving a higher spatial resolution. Hence, the probe pulse duration may be chosen with reference to the desired level of spatial resolution required for the distributed measurement.

[0073] Hollow core optical fibres also show low frequency dispersion, again owing to the absence of a solid glass core. Low dispersion means that propagating light retains its original wavelength as it propagates along the fibre. It is therefore possible to simultaneously propagate multiple probe pulses of different wavelengths without risk of frequency shifts causing interference between the pulses. Accordingly, a hollow core fibre OTDR system can be operated with two or more separate probe pulses of different wavelengths at the same time, the separate backscatter from which can be independently detected. In other words, a wavelength division multiplexing regime can be implemented, which can be used to boost the detectable backscatter signal compared with use of single probe wavelength only.

[0074] In principle, any physical parameter that can cause sufficient gas molecule movement to produce a detectable Doppler shift or linewidth broadening of the Rayleigh backscattered light can be detected or measured using a hollow core OTDR system as described herein. For example, pressure might be detected in circumstances where a sufficiently high pressure change (mechanical or sound) occurs that squeezes or compresses the hollow core fibre, or alternatively allows expansion from a squeezed state. The partial pressure of the gas at the compressed I expanded location will increase I decrease and cause gas molecules to move away from or into the location. The movement, which is a transient effect until the gas pressure in the fibre reaches a steady state, will produce the desired Doppler shift of the backscattered light, and can be detected, allowing the location and magnitude of the pressure change to be measured.

[0075] The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and / or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and / or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to theclaims, and that other embodiments may be utilised and modifications may be made without departing from the scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in the future.

[0076] REFERENCES

[0077] [1] WO 2023 / 152468

[0078] [2] US 9,904,008

[0079] [3] WO 2015 / 185761

[0080] [4] F. Poletti, "Nested antiresonant nodeless hollow core fiber," Opt. Express, vol.

[0081] 22, 23807-23828, 2014

[0082] [5] M. N. Alahbabi et al, “High spatial resolution microwave detection system for Brillouin-based distributed temperature and strain sensors”, Meas. Sci. Technol., vol. 15, 1539-1543, 2004

[0083] [6] B. Li et al, “Dynamic strain measurement using small gain stimulated Brillouin scattering in STFT-BOTDR”, IEEE Sensors Journal, vol. 17, 2718-2724, 2017

Claims

CLAIMS1. A method of optical time domain reflectometry comprising:launching a probe pulse of laser light into a proximal end of a hollow core optical fibre having a gas content in its core, wherein the hollow core optical fibre is deployed in a environment having a physical parameter that affects movement of gas molecules in the core;detecting, at the proximal end of the hollow core optical fibre, Rayleigh backscattered light produced by scattering of the probe pulse from the gas molecules in the core, wherein the light is detected with spectral resolution and temporal resolution; determining a spectral linewidth or a spectral shift of the detected light arising from Doppler shifting of the Rayleigh backscattered light caused by movement of the gas molecules; anddetermining from the spectral linewidth or the spectral shift a value of the physical parameter at one or more locations along a length of the hollow core optical fibre.

2. A method according to claim 1, wherein the physical parameter is temperature.

3. A method according to claim 1 or claim 2, wherein the gas content comprises one or more of carbon dioxide, methane, and sulphur hexafluoride.

4. A method according to any one of claims 1 to 3, wherein the environment is adjacent to an underwater high voltage direct current cable.

5. A method according to any one of claims 1 to 3, wherein the environment is within a nuclear reactor or a fusion reactor.

6. A method according to any one of claims 1 to 3, wherein the environment is within an oil well or a gas well.

7. A method according to claim 1, wherein the gas content includes gas molecules with a dipole moment, and the physical parameter is magnetic field strength.

8. A method according to any one of claims 1 to 7, wherein the hollow core optical fibre is an antiresonant hollow core optical fibre.

9. A method according to any one of claims 1 to 8, wherein the gas content of the hollow core optical fibre is at a pressure of substantially 100 kPa or a pressure of 101.325 kPa.

10. A method according to any one of claims 1 to 9, wherein the detecting with spectral resolution comprises generating a microwave frequency beat signal between the Rayleigh backscattered light and light used to generate the probe pulse, and determining the spectral linewidth or the spectral shift from the beat signal.

11. A method according to any one of claims 1 to 9, wherein the detecting with spectral resolution comprises temporal windowing of the detected Rayleigh backscattered light and calculation of a short-time Fourier transform for each temporal window to obtain spectral data.

12. A method according to any one of claims 1 to 11, further comprising launching one or more additional probe pulses simultaneously with the said probe pulse of laser light, wherein each of the probe pulse and the one or more additional probe pulses has a different wavelength;and detecting, determining the spectral linewidth or spectral shift, and determining the value of the physical parameter for each probe pulse.

13. Optical time domain reflectometry apparatus configured to perform a method according to any one of claims 1 to 12.

14. Optical time domain reflectometry apparatus comprising:a hollow core optical fibre having a gas content in its core, for deployment in an environment having a physical parameter that affects movement of gas molecules in the core;an optical source configured to generate probe pulses of laser light and launch the probe pulses into a proximal end of the hollow core fibre;an optical detection arrangement configured to detect, at the proximal end of the hollow core optical fibre, and with spectral resolution and temporal resolution, Rayleigh backscattered light produced by scattering of the probe pulses from the gas molecules in the core; anda processor configured to:determine a spectral linewidth or a spectral shift of the detected light arising from Doppler shifting of the Rayleigh backscattered light caused by movement of the gas molecules; anddetermine, from the spectral linewidth or the spectral shift, a value of the physical parameter at one or more locations along a length of the hollow core optical fibre.

15. Optical time domain reflectometry apparatus according to claim 14, wherein the physical parameter is temperature.

16. Optical time domain reflectometry apparatus according to claim 14 or claim 15, wherein the gas content comprises one or more of carbon dioxide, methane and sulphur hexafluoride.

17. Optical time domain reflectometry apparatus according to claim 14, wherein the gas content includes gas molecules with a dipole moment, and the physical parameter is magnetic field strength.

18. Optical time domain reflectometry apparatus according to any one of claims 14 to 17, wherein the hollow core optical fibre is an antiresonant hollow core optical fibre.

19. Optical time domain reflectometry apparatus according to claim 18, wherein the antiresonant hollow core optical fibre is a nested antiresonant nodeless fibre.

20. Optical time domain reflectometry apparatus according to any one of claims 14 to 19, wherein the gas content of the hollow core optical fibre is at a pressure of substantially 100 kPa or a pressure of 101.325 kPa.

21. Optical time domain reflectometry apparatus according to one of claims 14 to 20, wherein the optical detection arrangement is configured to detect with spectral resolution by generating a microwave frequency beat signal between the Rayleigh backscattered light and light from the optical source, and the processor is configured to determine the spectral linewidth or the spectral shift from the beat signal.

22. Optical time domain reflectometry apparatus according to any one of claims 14 to 20, wherein the processor is configured to preform temporal windowing of the detectedRayleigh backscattered light and calculate of a short-time Fourier transform for each temporal window to obtain spectral resolution of the detected Rayleigh backscattered light.

23. Optical time domain reflectometry apparatus, wherein the optical source is further configured to generate additional probe pulses of laser light at one or more wavelengths different from a wavelength of the said probe pulses, and launch pulses at one or more different wavelengths simultaneously with the said probe pulses; andthe optical detection arrangement and the processor are configured to detect, determine the spectral linewidth or spectral shift, and determine the value of the physical parameter for each wavelength.

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