Optical Sensor

Abstract

An optical sensor for detecting one or more measurands, such as temperature or pressure, is disclosed, comprising a probe light source arranged to generate a probe light, and a sensor head arranged to receive the probe light from the probe light source and apply an interference signal to the probe light responsive to the one or more measurands. The sensor then also comprises an interrogator arranged to receive the probe light from the sensor head, measure the applied interference signal, and determine the one or more measurands from the measured interference signal, and an optical fiber arranged to carry the received probe light at least partly on a path from the sensor head to the interrogator, the optical fiber being disposed in a protective conduit. A granular material may then be filled in the conduit to limit or prevent lateral movement of the optical fiber within the conduit. The optical fiber may also or instead be disposed in one or more flexible sleeves within the conduit.

Description

[Technical field]

[0001] The present invention relates to optical sensors, for example optical sensors in which a sensor head adds an interference signal responsive to one or more measurands to a probe light, and an optical fiber carries the interference signal from the sensor head for reception by an interrogator. [Background technology]

[0002] Introduction Sensor systems using optical fibers are promising candidates for replacing or complementing traditional instruments in harsh environments. For example, sensor systems based on remotely interrogated Fabry-Perot cavities can be used to monitor the combustion process in internal combustion engines such as gas turbines (Pechstedt and Hemsley, "Fiber optical sensors for monitoring industrial gas turbines", Handbook of Optoelectronics, Vol. 3, Chapter 18, CRC Press, Taylor & Francis Group, 2018) and reciprocating engines (FCP Leach et al., "An optical method for measuring exhaust gas pressure from an internal combustion engine at high speed", Review of Scientific Instruments 88, 125004, 2017).

[0003] In these applications, the passive sensor head is typically mounted in or near the combustion zone where it may be exposed to extreme vibration levels and very high temperatures. More commonly, the sensor head may be mounted in the core of the engine, including the compressor, burner, and turbine, or in the exhaust system. As a guide, the sensor head is typically exposed to temperatures of 400°C to 600°C or more, and vibration levels may reach tens of g's, where g is the standard acceleration of gravity, g=9.81 m / s 2 It represents.

[0004] Optical pressure sensors such as those described in WO 2009 / 077727 may use a transducer element comprising a flexible diaphragm that provides the boundary of an optical cavity and deforms in response to applied pressure. The sensor head comprising the transducer element is typically connected to an interrogator (or signal conditioner) via an optical cable comprising an optical fiber. The optical fiber acts as a medium for transmitting probe light from the interrogator to the sensor head and back to the interrogator.

[0005] Optical interference produced by the probe light in the optical cavity produces an optical intensity of the returned probe light that varies with applied pressure. In the interrogator, the intensity of the returned probe light is received by a photodetector, such as a photodiode, and the signal therefrom is processed to determine and output a signal representative of the pressure at the diaphragm, which may typically be output as a voltage or current. Different sensor heads and transducer arrangements can be used to determine other measurands at the sensor head, such as temperature.

[0006] The inventors have observed that the harsh environments in which such sensors are typically deployed often introduce undesirable artifacts or biases into the interference signal that do not arise from the measurand itself. Such undesirable artifacts or biases are sometimes referred to in the prior art as being due to the cross-sensitivity of the sensor. To improve the accuracy of the determination of the measurand determined by such sensors, it would be desirable to reduce or eliminate such artifacts, biases, and cross-sensitivities.

[0007] The present invention seeks to address these and other limitations of the prior art. Summary of the Invention [Means for solving the problem]

[0008] Summary of the Invention Optical fibers may be used to carry probe light between a sensor head and an interrogator, and the inventors have identified that movement of such optical fibers contributes significantly to artifacts and biases in the measured interference signal and thus to errors in subsequently determined measurands such as pressure and / or temperature. Accordingly, embodiments of the present invention aim to eliminate or even suppress such motion-induced artifacts and biases in fiber optic sensors, enabling such sensors to operate more effectively and accurately in harsh environments, such as extreme vibration levels and / or high operating temperatures found, for example, in typical gas turbine or reciprocating engine applications.

[0009] The present invention thus provides an optical sensor for detecting one or more measurands such as pressure and / or temperature comprising a probe light source arranged to generate a probe light; a sensor head arranged to receive the probe light from the probe light source and to add an interference signal to the probe light responsive to the one or more measurands; an interrogator arranged to receive the probe light from the sensor head, measure the added interference signal and determine the one or more measurands from the measured interference signal; an optical fiber arranged to carry the received probe light on at least a part of the path from the sensor head to the interrogator, at least a part of the length of the optical fiber being disposed within a protective conduit; and granular material filled or packed within the conduit so as to limit or prevent lateral movement of the optical fiber within the conduit.

[0010] At the same time, however, the granular material is preferably filled or packed within the conduit in a manner that allows longitudinal movement of the optical fiber within the conduit, thereby allowing for differential thermal expansion without causing undue strain on the optical fiber. To this end, the granular material may be filled or packed fairly loosely without much or any compression, but preferably avoids significant voids or gaps that could result in instability and movement of the optical fiber within the conduit when the sensor is deployed.

[0011] Typically, the probe light source and interrogator (which may comprise the probe light source) may be located a significant distance from the sensor head, for example several meters or tens of meters, and at least part of the optical coupling between the two is provided by optical fibers.

[0012] The conduit may comprise an elongated tube or a corrugated hose, or a portion of the conduit may be provided by one or more elongated tubes and one or more appropriately joined corrugated or flexible hoses, either or both typically made of metal. In this manner, more rigid sections may be provided by metal tube configurations and more flexible sections by corrugated metal hose configurations, as desired depending on the application area and particular installation constraints.

[0013] The conduit may typically have an inner diameter of 2 mm to 10 mm, or 1 mm to 20 mm. The length of the conduit housing the optical fiber may typically extend from 100 mm to 3000 mm from the sensor head, or near or proximal to the sensor head. Different elongated sections of the conduit may have different inner diameters, for example a rigid section may have a different inner diameter than a flexible section.

[0014] The conduit may extend from the sensor head to a junction, the junction comprising an optical connector between a first portion of the optical fiber contained within the conduit and a second portion of the optical fiber extending further from the junction towards the interrogator. The junction may then comprise a slack section of the first portion of the optical fiber arranged to accommodate movement of the first portion of the optical fiber along the conduit. In this manner, some thermal mismatch between the optical fiber and the conduit can be accommodated without adding additional strain to the optical fiber within the conduit, the sensor head, or the junction.

[0015] The granular material may comprise ceramic granules or ceramic powders, in particular engineering ceramic granules or powders. To provide suitable properties for injection or packing into conduits and for stable positioning of optical fibers, the granular material may have an average or median particle size in the range of 10 μm to 200 μm, or 30 μm to 80 μm.

[0016] Whether or not a granular material is provided in the conduit as described above, the optical fiber may be disposed in a flexible sleeve disposed in the conduit, or in multiple coaxial flexible sleeves disposed in the conduit, each flexible sleeve may include a braided, knitted, or woven material, or some other textile construction. When multiple coaxial flexible sleeves are used, each sleeve of the coaxial combination may be formed using a different such textile construction. It is noted that when coaxial sleeves are referenced, they do not have to be concentric in the sense of having exactly the same center point, but may be offset to some degree while still being nested within one another. When a granular material is also provided, it may be filled between the conduit, the optical fiber, and one or more of the sleeve layers.

[0017] If two coaxial flexible sleeves are used, the inner one of the coaxial flexible sleeves may be formed from a woven textile material and the outer one of the coaxial flexible sleeves may be formed from a braided or braided textile material, so that the inner woven sleeve better protects the optical fiber from potential deformation by the outer layer, which acts to fill the volume outside the woven layer to better restrict movement of the optical fiber. Alternatively, the inner one of the coaxial flexible sleeves may be formed from a braided textile material and the outer one of the coaxial flexible sleeves may be formed from a woven textile material, so that the braided material may better allow the optical fiber to move longitudinally in the direction of the fiber axis through a smoother surface.

[0018] Where the conduit comprises multiple elongated sections through which the optical fiber passes, for each such elongated section, the optical fiber may be disposed within a different combination of two or more coaxial flexible sleeves disposed within that section of the conduit, where each sleeve in each combination may be of a particular textile construction type, and each different combination may comprise a different sequence of two or more such textile construction types. For example, a smaller diameter section of the conduit may use coaxial woven and braided flexible sleeves, and a larger diameter section may use coaxial woven and braided flexible sleeves.

[0019] A silica material may be used in the or each flexible sleeve, typically comprising, for example, at least 95% or at least 99% silica. If a particulate material is used, it may be filled or packed within the particular flexible sleeve, around the outside of the particular flexible sleeve, or both.

[0020] If the granular material is packed in layers both within the flexible sleeve and around the outside of the flexible sleeve, two types of granular material may be used such that the granular material within the flexible sleeve is a different type or has different properties than the granular material around the outside of the flexible sleeve. In such a case, the granular material within the flexible sleeve may have a lower coefficient of thermal expansion than the granular material around the outside of the flexible sleeve to help accommodate the optical fiber, which has a lower coefficient of thermal expansion than the conduit.

[0021] A typical optical fiber used in the prior art for a similar sensor typically has a diameter or cladding outer diameter of 125 μm. Whether or not the above-mentioned granular material is used and whether or not the above-mentioned flexible sleeve is used, the optical fiber, or the cladding layer of the optical fiber, may have an enlarged diameter or enlarged outer diameter of at least 150 μm, or at least 200 μm, or at least 250 μm. Increasing the fiber diameter or cladding outer diameter in this manner increases the stiffness of the optical fiber, thereby helping to further reduce bending and movement within the conduit. The enlarged outer diameter optical fiber may in particular be a single mode optical fiber.

[0022] Whether or not the granular material is used, whether or not the one or more flexible sleeves are used, and whether or not the enlarged diameter cladding layer of the optical fiber is used, the optical fiber may be a single mode fiber having a reduced mode field diameter such that bending of the optical fiber in the conduit has a reduced effect on the interference signal conveyed from the sensor head to the interrogator. In particular, the optical fiber may be arranged to have a mode field diameter of 10.0 μm or less, or 8.0 μm or less, or in the range of 6.0 μm to 8.0 μm. Since the mode field diameter varies with the wavelength of the probe light, these values ​​of the mode field diameter may be defined as being at the central wavelength, e.g., peak wavelength or mean wavelength, of the probe light.

[0023] For typical infrared probe light, e.g., in the region of about 1300-1800 nm or 1400-1700 nm, the optical fiber may have a core diameter of 5 μm-7 μm and a numerical aperture of 0.16-0.20 to achieve an appropriate range of mode field diameters.

[0024] Certain optical fiber structures may be used in place of, or as well as, reduced mode field diameter to reduce bending losses or similar effects, for example holey fibers such as photonic bandgap fibers or average index guiding fibers, or optical fibers with multi-layer core regions including annular trenches of depressed index surrounding a central core with an elevated step-index profile.

[0025] The probe light source may comprise one or more lasers, or one or more superluminescent diodes, arranged to generate the probe light. The wavelength characteristics of the probe light may depend on how the sensor is arranged to measure and use the applied interference signal. For example, the interrogator may be arranged to separately detect the intensity of two different wavelengths or wavelength bands of the probe light received from the sensor head and determine one or more of the one or more measured quantities in response to a relationship between the detected intensities of the two wavelengths or wavelength bands. In this case, probe light of two separate wavelengths or wavelength bands is required, which may be provided, for example, by two lasers, a single tunable or swept laser, a single broadband light source combined with two optical filters with high transmission characteristics at the two wavelengths, or two superluminescent diodes with respective central wavelengths (such as peak or average) selected to coincide with the two required wavelengths or wavelength bands.

[0026] Alternatively, the interrogator may comprise a spectral engine or spectrometer arranged to measure an interference spectrum comprising the applied interference signal and then to determine one or more of the one or more measurands from the measured interference spectrum, in which case a broadband probe light provided, for example, by a superluminescent diode or a swept laser source is required.

[0027] The sensor head may comprise one or more optical cavities arranged to add an interference signal to the probe light in response to one or more measurands. One or more or all of these optical cavities may be Fabry-Perot cavities. The one or more measurands may include one or more of temperature, pressure (e.g., static pressure, or pressure changes at acoustic frequencies), and acceleration at the sensor head, and the optical cavities may then be arranged to respond appropriately to these measurands such that changes in these optical cavities are detectable in the interference signal.

[0028] The sensor may be used to determine one or more measured quantities on, in or within various types of engines, such as internal combustion or gas turbine engines. To this end, the invention also provides a gas turbine engine or an internal combustion engine comprising one or more of the optical sensors of any preceding claim. In that case, the optical sensor may be particularly positioned to detect combustion instabilities in a gas turbine or other type of engine.

[0029] The present invention also provides methods corresponding to the above-described apparatus, including methods of operating such apparatus and methods of constructing or manufacturing such apparatus.

[0030] The present invention therefore provides a method for detecting one or more measurands, the method comprising generating a probe light, receiving the probe light from a probe light source and directing the probe light to a sensor head arranged to add an interference signal to the probe light responsive to the one or more measurands, receiving the probe light together with the added interference signal from the sensor head along an optical fibre and measuring the added interference signal, the optical fibre being disposed within a protective conduit.

[0031] The conduit may then contain the above-mentioned granular material to restrict lateral movement of the optical fiber within the conduit, and / or the optical fiber may be contained within the above-mentioned flexible sleeve or multiple coaxial flexible sleeves within the conduit, and / or the optical fiber may be provided with an enlarged outer diameter cladding, for example as described above, and / or the optical fiber may have a reduced mode field diameter, for example as described above, it being noted that the optical fiber may be a single mode optical fiber.

[0032] The method may then further include determining one or more measurements from the measured interference signal.

[0033] The invention also includes methods of providing or making or manufacturing the optical sensors described herein, the methods including providing an optical fiber to couple probe light from a sensor head for reception by an interrogator, the interrogator being arranged to measure an interference signal imposed on the probe light by the sensor head in response to one or more measurands; placing at least a portion of the optical fiber within a protective conduit; and one or more of providing a granular material within the protective conduit, providing one or more flexible sleeves coaxially around the optical fiber within the conduit, providing an increased cladding diameter on the optical fiber, and providing a reduced mode field diameter on the optical fiber.

[0034] A brief overview of the drawing Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: [Brief description of the drawings]

[0035] [Figure 1] FIG. 1 is a schematic diagram of a sensor according to the present invention using a granular material within a conduit carrying an optical fiber. [Diagram 2] FIG. 2 illustrates in more detail how the sensor head and optical fiber of FIG. 1 may be implemented. [Diagram 3]3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Figure 4] 3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Diagram 5] 3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Figure 6] 3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Figure 7] 3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Figure 8a] 3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Figure 8b] 3 is a cross-sectional view showing how the conduit and threaded optical fiber of FIGS. 1 and 2 may be implemented. FIG. [Figure 9] 1 is a graph of the sensitivity of an illustrated sensor arranged to measure pressure to incidental vibration when the sensor is mounted with (lower curve) and without (upper curve) granular material in a conduit. [Figure 10] FIG. 1 illustrates how increasing the cladding diameter of an optical fiber increases the stiffness and reduces the sensitivity to vibration or movement of the optical fiber within the conduit. [Figure 11] FIG. 13 is a cross-sectional view showing how a flexible sleeve can be used to mount the conduit and the optical fiber threaded therethrough without the use of particulate material within the conduit. [Figure 12] FIG. 2 shows how the mode field diameter of an optical fiber, like the other figures, varies with core diameter and numerical aperture (NA). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Detailed Description of the Embodiments Referring now to Figure 1, there is shown a schematic of an optical sensor 5 that may embody various aspects of the present invention. A probe light source 10 generates probe light that is coupled via an optical coupler 12 into an optical fiber 14 (typically a single mode optical fiber), which directs the probe light to a sensor head 16. The sensor head 16 may be mounted in a harsh environment, such as a wall 18 of a gas turbine or other engine, and is often flush with the inside of the wall rather than protruding as shown in Figure 1. The harsh environment may be characterized by high temperatures, perhaps hundreds of degrees Celsius, for example, and may be subject to high intensity vibrations, etc.

[0037] The sensor head 16 is positioned to impart an interference signal to the probe light that is responsive to one or more measurands at the sensor head, e.g., one or more of temperature T, static or dynamic pressure P, acceleration A, etc. As shown in Figure 1, the sensor head may be positioned to respond to such measurands within a space, such as within a wall 18 of a chamber of a gas turbine or other engine. The probe light carrying the interference signal is then directed from the sensor head 16 back along the optical fiber 14 to the optical coupler 12, from where it is directed to a photodetector 20 where the interference signal is measured.

[0038] The measured interference signal is then passed to an analyzer 22 which uses the measured interference signal to determine values ​​of, or signals representative of, one or more measurands which are then output or used in a variety of ways. Such signals may be in the form of a voltage or current representative of the measurand, in the form of a corresponding digital data signal, or in other forms.

[0039] In FIG. 1, the light source 10, optical coupler 12, photodetector 20, and analyzer 22 are shown as being housed in or forming part of an interrogator unit 24, although these or related functions or elements may be housed or distributed in different ways. In FIG. 1, a single optical fiber 14 is used to carry the probe light from the light source to the sensor head 16 and from the sensor head to the detector 20, but two different optical fibers could be used for these purposes. Various other configurations of one or more probe light sources, one or more sensor heads 26, and one or more photodetectors 20 could also be used. For example, one or more sensor heads could be arranged to operate in a transmission mode rather than a reflection mode, for example using Mach-Zehnder or Bragg grating interferometry techniques. Multiple such transmission or reflection geometry sensor heads could be daisy-chained together for coupling to a single interrogator unit, for example with the probe light entering and exiting along the daisy chain and the sensor heads coupled using a single optical fiber for both directions, or using a different optical fiber for each direction, or in a ring configuration.

[0040] Some examples of how the sensor head 16 and interrogator 24 or related elements may be implemented are described in WO 2009 / 077727, WO 2012 / 140411, WO 2013 / 136071 and WO 2013 / 136072. Some other specific examples of how the sensor head itself may be implemented are provided in WO 2013 / 024262. The contents of each of these documents are incorporated herein by reference for these and all other purposes.

[0041] The probe light may be narrowband in nature, e.g., generated using one or more laser sources contained in or forming the probe light source 10, or it may be broadband in nature, e.g., generated using one or more swept laser sources, or generated using one or more superluminescent diodes, typically having a bandwidth of tens of nanometers, contained in or forming the probe light source. In some embodiments, as described in more detail below, the probe light may include two or more distinct and distinct frequencies, wavelengths, or wavelength bands, e.g., using a probe light source that includes two superluminescent diodes with central wavelengths sufficiently spaced apart so that the wavelength bands do not overlap, or a single superluminescent diode in combination with two optical filters, each of which may have a bandwidth of about 10-20 nm.

[0042] The interference signal may be added to the probe light by one or more structures in the sensor head, for example one or more optical cavities 26. Such optical cavities may be, for example, Fabry-Perot cavities. Each such optical cavity is typically defined by two substantially parallel refractive index boundaries in the sensor head, for example a boundary between a solid material and a gas or vacuum, and thus each such optical cavity may include a solid material, a gas or a vacuum, or both. In other embodiments, the interference signal may be added to the probe light using one or more Michelson-type interferometer structures, see, for example, FIG. 8b and the relevant text of GB 2495518, the contents of which are incorporated herein by reference for these and all other purposes.

[0043] Such optical cavities 26 and other interferometric structures may respond in a variety of ways, for example, to temperature of the sensor head due to expansion and / or refractive index changes of the material of the sensor head, to pressure due to movement of a diaphragm whose boundaries form the boundaries of such optical cavities, to acceleration due to movement of a proof mass, or the like.

[0044] The interference signal may be measured by the interrogator and used to determine one or more of the one or more measurands in a variety of ways. According to the "dual wavelength" technique referred to elsewhere herein, the probe light source 10 is arranged to provide probe light of two different wavelengths or wavelength bands, for example using a suitably positioned laser or superluminescent diode light source. The sensor head then adds effectively separate interference signals to the probe light of each wavelength or wavelength band. When the probe light is received back at the interrogator from the sensor head, the two wavelengths or wavelength bands are detected separately, for example by two different photodetector components of the photodetector 20, providing separate detection signals. The analyzer 22 then receives these detection signals, which may represent, for example, the intensities of the two different wavelengths or wavelength bands at the photodetector, and determines one or more of the one or more measurands in response to a relationship, for example by comparison of the detection signals of the two wavelengths or wavelength bands.

[0045] Such techniques are discussed in the prior art, such as GB 2202936 and WO 2013 / 136072, the contents of which are incorporated herein by reference for these and all other purposes. This "dual wavelength" type of technique provides compensation for intensity or power losses that may be present in the optical system, which may be interpreted as a measurement signal, for example due to bending of the optical fiber 14. However, the inventors have found that such compensation is generally not sufficient to eliminate artifacts and biases due to the harsh environments to which the optical fiber 14 may be exposed.

[0046] The interference signal may also, or instead, be measured by the interrogator and used to determine one or more of the one or more measurands using spectral techniques. For example, the photodetector 20 may comprise a spectral engine arranged to measure an interference spectrum including the applied interference signal, and the analyzer 22 may be arranged to determine one or more of the one or more measurands from the measured interference spectrum. Such techniques are also discussed in prior art such as WO 2013 / 136072, the contents of which are incorporated herein by reference for these and all other purposes. However, with regard to the dual wavelength techniques described above, it may still be difficult to eliminate artifacts and biases due to the harsh environments to which the optical fiber 14 may be exposed.

[0047] The optical fiber 14 that carries the probe light from the sensor head 16 to the photodetector 20 for detection of an interference signal may be formed of a single length of optical fiber or two or more lengths coupled together, as desired. In some examples, multiple optical fibers may be used, for example, with different optical fibers carrying the probe light to and away from the sensor head.

[0048] 1, at least a portion of the optical fiber 14 is disposed within a protective conduit 30 that protects the optical fiber from damage and adverse environmental conditions, particularly those that may be experienced by the optical fiber 14 proximate the sensor head 16. Such adverse conditions may include, for example, high temperatures, excessive vibration, and the like. Typically, the conduit may extend from or near the sensor head along the optical fiber for a length of about 0.1 to 3.0 meters, or more preferably about 0.2 to 2.0 meters, although longer extensions may be used if desired.

[0049] If multiple optical fibers are used to connect the sensor head 16 to the interrogator 24 (e.g., if a different fiber is used to carry light in each direction for other purposes), they may be run together in the same conduit, or multiple optical fibers may be run in a single conduit 30 for other purposes.

[0050] The conduit is typically provided by a tube, pipe, or similar elongated structure having an internal diameter of, for example, 2-10 mm, or 1-20 mm, and may be designed to be flexible along at least a portion of its length, particularly to aid in the installation of the sensor. Such flexibility may be provided by one or more portions or the entirety of the conduit comprising a flexible metal hose, for example a corrugated metal hose. Where some or all sections of the conduit are rigid, these may comprise more rigid metal tube configurations. Suitable metals for the conduit may include austenitic stainless steels and nickel-chromium alloys, although non-metals such as suitable ceramic materials may also or instead be used. Further description of how the conduit may be implemented is provided below.

[0051] The inventors have found that lateral movement of the optical fiber 14 within the protective conduit 30 (i.e., towards and away from the conduit wall) can introduce undesirable artifacts, biases, or cross-sensitivities in the measured interference signal and thus errors in the determined measurand. More generally, the properties of the probe light propagating through the optical fiber can be affected by external stimuli acting on the conduit 30. The resulting variations in the properties of the probe light received at the photodetector 20 and / or the resulting variations in the interference signal can be erroneously interpreted as being due to changes in the measurand. For example, changes in the bend radius of the optical fiber, either in the sense of a static or slow moving or as a vibratory motion, can induce variations in the propagation loss of the optical fiber, which may itself also be wavelength dependent. Such propagation loss changes can then result in changes in the light intensity received at the interrogator at a particular wavelength or wavelengths, and a corresponding perceived change in the measurand. As described elsewhere herein, when dual wavelength interrogation techniques are used, different propagation losses between the two wavelengths or wavelength bands introduce errors into the determined measurement quantities.

[0052] Several strategies to mitigate such effects are already known in the prior art, for example the "dual wavelength" strategy mentioned above, and are described in GB 2202936, the contents of which are incorporated herein by reference for these and all other purposes. This document proposes sending a probe light containing at least two wavelength components to a sensor head, measuring separately the interference signal resulting from the pressure change at the sensor head at each of the two wavelengths, and taking the ratio of the two signals to arrive at a corrected pressure response. The rationale behind this is that the two different wavelength components propagating along an optical fiber are attenuated by similar amounts when the fiber is bent, which means that the ratio of the responses is largely independent of bending, but is simply a function of the applied pressure.

[0053] Such an interrogation scheme, using two wavelengths or wavelength band components or probe light, is sometimes referred to as "dual wavelength interrogation" and is described in more detail in A. Winterburn et al., "Extension of an optical dynamic pressure sensor to measure temperature and absolute pressure in combustion applications", The Future of Gas Turbine Technology, 6th International Conference, 17-18 October 2012, Brussels, Belgium, Paper No: 15.

[0054] However, the common mode rejection thus provided does not guarantee complete removal of artifacts and biases in the interference signal or, depending on the application, sufficient suppression and undesirable effects remain. Effects on the probe light propagating in the fiber, such as bending-induced losses, are generally a function of wavelength and result in a small differential loss between the two wavelengths or wavelength bands of the probe light used. As a result, the intensity ratio between the probe light at two different wavelengths or wavelength bands is not only a function of the measurand in the sensor head, but can also be affected by external stimuli such as vibration-induced bending or movement of the optical fiber 14.

[0055] As shown in FIG. 1, to reduce or eliminate such artifacts and biases, a granular material 32 may be filled into the conduit 30 to limit or prevent movement of the optical fiber within the conduit, particularly lateral movement across the axis of the conduit. Preferably, the granular material is loosely provided within the conduit to continue to allow essentially unrestricted longitudinal movement of the optical fiber, i.e., along the central axis of the conduit and / or along the direction of the optical fiber itself, while avoiding unfilled pockets that may allow lateral movement of the optical fiber or other less stable configurations within the conduit. In this way, relative longitudinal movement of the conduit and the optical fiber itself, particularly due to thermal expansion and contraction of the conduit and / or optical fiber, is still permitted, thereby avoiding excessive changes in strain within the optical fiber due to such effects.

[0056] The granular material may be filled into the conduit, for example by injection techniques, optionally including some agitation of the conduit to help ensure that no significant unintended voids are left unfilled by the granular material. It may be desirable to use other techniques to ensure proper packing density, for example by using a gas flow to carry the granular material along the conduit during the filling process.

[0057] The particulate material may be, for example, ceramic particulates, or, for example, alumina (Al 2 O 3 The material may be or may include an engineering ceramic particulate, including magnesium oxide (MgO). The particle size of the material should be suitable for filling around the optical fiber, whether or not enclosed within a sleeve as described below, such that particulate materials having an average or median particle size in the range of 10 μm to 200 μm, or 30 μm to 80 μm may be used. Further discussion of suitable particulate materials is provided below.

[0058] Other structures may also or instead be used within the conduit to protect and / or limit the movement of the optical fiber, for example, the optical fiber may be disposed within a sleeve or within multiple coaxial sleeves disposed coaxially within the conduit. Each such coaxial sleeve, typically provided as a flexible sleeve and / or woven sleeve, may be constructed using one of several different types of textile constructions, for example, providing a braided, woven, or knitted sleeve. Such a sleeve may, for example, be disposed between the optical fiber-filled granular material, and / or between the filled granular material and the conduit, and / or between two bodies of filled granular material, such as a first body of filled granular material surrounding the optical fiber and a second body of filled granular material surrounding the sleeve. Such a sleeve may, for example, comprise a braided, woven, or knitted silica material, or a braided, woven, or knitted form of other ceramic. In some embodiments, one such coaxial sleeve surrounds the optical fiber, but no filled granular material is used.

[0059] By providing one or more such sleeves, each including layers formed using any of several different textile construction types, different desirable properties of the corresponding sleeve or sleeves alone or in combination can be obtained by providing the corresponding material type best suited for the application.

[0060] For example, a braided sleeve may typically be formed by intertwining several strands at non-right angles, with the resulting acute angles defining several preferred directions that potentially provide a smoother surface in the direction along the axis of the conduit and sleeve, thereby reducing friction between the sleeve and other surfaces, such as the optical fiber.

[0061] A woven sleeve may typically consist of two sets of straight strands intertwined at substantially right angles, the strands themselves typically consisting of layers of individual parallel-laid fibers. Thus, a woven sleeve is typically somewhat stiffer than a braided sleeve, providing better mechanical protection for the optical fiber while still facilitating movement of the optical fiber along the axis of the conduit and sleeve and limiting lateral movement of the optical fiber within the conduit.

[0062] Braided sleeves may typically be formed by strands formed into loops that interlock with further loops into a structure, generally resulting in a more flexible material. Braided sleeves allow for larger diameter strands to be utilized without excessive loss of flexibility, allowing a larger volume between the optical fiber and the conduit to be filled by the sleeve.

[0063] Regardless of the type of fabric construction or sleeve material, the strands themselves may typically consist of, for example, silica (e.g., quartz) fibers twisted or laid in parallel to form a yarn or thread, respectively. Different fabric constructions or sleeve material types may also exhibit different mechanical damping characteristics, with looser, more flexible layers typically allowing stronger damping. Stronger damping is beneficial to further buffer the optical fiber within the conduit, thus helping to reduce lateral movement of the optical fiber without affecting movement along the sleeve axis.

[0064] Additional damping can be achieved by embedding one or more flexible sleeves within the mineral wool. Mineral wool is typically produced by spinning or drawing molten mineral material and is therefore usually resistant to very high temperatures.

[0065] It should be noted that in some embodiments, the granular material may be omitted entirely, but one or more flexible sleeves are still used to surround the optical fiber within the conduit to provide some degree of restriction of lateral movement of the optical fiber, as well as some degree of protection beyond that provided by the conduit itself.

[0066] Further discussion of how such sleeves, for example braided silica, can be used is provided below.

[0067] To further limit the movement of the optical fiber within the conduit, particularly lateral movement, the stiffness of the optical fiber itself may be increased. This can be achieved in a variety of ways, but in some embodiments, an optical fiber 14 with an increased outer diameter or a cladding with an increased outer diameter may be used. The most commonly used optical fibers have cladding with an outer diameter of about 125 μm. To further increase the stiffness and thereby reduce artifacts and biases in the interference signal and therefore errors in the determined measurement quantity or quantities, an optical fiber with a diameter or outer cladding diameter of at least 150 μm, or at least 200 μm, or at least 250 μm may be used, it being noted that the optical fiber may be a single mode optical fiber.

[0068] To reduce the magnitude of artifacts such as bending losses and biases in the probe light and interference signal resulting from a particular degree of movement of the optical fiber within the conduit, particularly lateral movement within the conduit, the mode field diameter of the optical fiber may be reduced. The mode field diameter of a particular optical fiber depends on the wavelength of the light in the optical fiber, in this case the wavelength of the probe light, but for a suitable infrared wavelength of about 1550 nm of the probe light, a typical step-index single mode optical fiber may have a core diameter of 9 μm, a mode field diameter of about 10.6 μm, and a numerical aperture of about 0.12. Thus, to reduce artifacts such as bending losses and biases, the optical fiber 14 may be provided to have a mode field diameter of 10 μm or less, more preferably 8 μm or less, at the wavelength or center wavelength (which may be the peak wavelength or average wavelength) of the probe light.

[0069] When a broadband or multi-wavelength or wavelength band probe light source is used, such a central wavelength may be defined, for example, as the wavelength of the principle or main peak of the probe light, or the average wavelength for the optical power across the spectrum of the probe light. Broadband light sources may typically have a spectral width of tens of nm.

[0070] Figure 2 shows in more detail how the sensor head 16 and at least a portion of the optical fiber 14 that interfaces with the interrogator 24 as shown in Figure 1 may be implemented. In Figure 2, a conduit 30 containing the optical fiber 14 extends from the sensor head 16 to a junction 36. The sensor head and the portion of the conduit closest to the sensor head will typically be exposed to the most extreme temperatures and / or vibrations.

[0071] The sensor head 16 shown in FIG. 2 comprises a sensor housing 40, an optional rigid section 42, a transducer element 44, and an optical coupling arrangement 46 for optically coupling between the optical fiber 14 and the transducer element 44. The sensor housing 40 protects the interior of the sensor head 16 and allows the sensor head to be mounted, for example, through an opening in a liner of a gas turbine. A preferred material for the housing 40 may be a high performance nickel-chromium alloy. For example, Inconel 625 may be a preferred choice due to its excellent mechanical properties, such as high tensile, rupture and creep strength, which are maintained over a wide range of operating temperatures.

[0072] The transducer element 44 provides a transducer mechanism by which one or more measurands are encoded onto the probe light. For example, in an interferometry-based pressure sensor, the transducer element 44 may comprise a flexible diaphragm that is part of an optical cavity and deflects in response to applied pressure. The flexible diaphragm provides one of the two parallel reflective surfaces of the optical cavity, whereby the second reflective surface is provided by a rigid member of the transducer element 44 on the opposite side of the diaphragm. The probe light entering the optical cavity is split into two return beams at the two reflective surfaces of the optical cavity, which when recombined generate an interference pattern. The application of pressure causes a change in the distance between the two surfaces, which changes the interference pattern and, as a result, the intensity of the reflected probe light.

[0073] The reflected probe light thus carries the pressure information as light intensity.For intended operation at high temperatures, the transducer elements may preferably be formed entirely of sapphire, for example as taught in WO 2009 / 077727, the contents of which are incorporated herein by reference for these and all other purposes.

[0074] The coupling arrangement 46 provides optical coupling between the probe light in the optical fiber 14 and the transducer element 44. For example, the optical fiber may be attached to a lens (not shown), and the probe light propagating in the optical fiber towards the transducer element 44 is then collimated by the lens and directed towards the transducer element. The probe light is then reflected back from the transducer element, recaptured by the lens and relaunched into the optical fiber 14 where it propagates back to the interrogator 24. For intended operation at extreme temperatures in the sensor head 16 of 1000° C. or more, the adjacent or attached ends of the lens and optical fiber may preferably be located at a certain minimum distance from the transducer element 44. Such a distance may be provided by a spacer, for example as described in WO 2009 / 077727. In this way, the sapphire-based transducer element 44 may be exposed to extreme temperatures of 1000° C. or more, while the lens and optical fiber, which may conveniently be formed of silica, may be kept at a lower temperature. Typically, silica fibers should not be exposed to temperatures above about 700°C for extended periods of time to avoid degradation of performance, such as damage to the fiber coating or out-diffusion of dopants from the core region into the cladding. However, in some arrangements, the optical fiber may be attached directly to the sensor element without a spacer and without a lens, or with only a lens.

[0075] An optional rigid section 42 may be provided as part of the sensor head 16 surrounding the optical fiber away from the housing 40, typically for additional mechanical protection during handling and / or installation. A typical length of the rigid section may be from 10 mm to 200 mm. The rigid section may be straight or curved.

[0076] Beyond or coupled to the rigid section, a conduit 30 is provided to protect the optical fiber and to act as an external sheath, typically extending along the optical fiber for a sufficient length to extend into a more benign environment, for example 100 mm to 3000 mm from the sensor head, and to accommodate the optical fiber. Typically, the conduit 30 needs to exhibit some large degree of mechanical flexibility, for example to allow routing of the conduit as required during installation. To this end, all or part of the conduit may be designed to be flexible, for example including or provided by a corrugated metal hose. However, one or more parts, or all, of the conduit may be rigid if required, for example including or provided by a relatively rigid section of a metal tube construction.

[0077] Austenitic stainless steels may be a suitable material choice for conduits or corrugated metal hoses due to their good mechanical properties and high corrosion resistance. Alternatively, nickel-chromium alloy based conduits or corrugated metal hoses such as Inconel may be used.

[0078] In some applications, the entire length of the conduit or one or more sections of the conduit may be rigid. The one or more rigid sections of the conduit may be straight or may be pre-bent prior to installation to allow for a predetermined routing. Typical lengths of the conduit may be from about 100 mm to 3000 mm. One or more portions, or all of the conduit, particularly any flexible sections of the conduit, may be further protected by a metal braid on the outside of the conduit.

[0079] A junction 36 at the end of the conduit 30 opposite the sensor head 16 provides an optical interface that allows a first portion of the optical fiber 14 to be coupled, typically via an optical connector forming part of the junction, to a second portion of the optical fiber provided in an optical extension cable 48. The optical extension cable 48 then provides an extension of the optical fiber 14 to the interrogator 24. The junction 36 may be of different geometric shapes, for example in the shape of a box or a tube. Thus, a complete sensor system may include the interrogator 24, the sensor head 16, the conduit 30, the junction 36, and one or more optical sensor assemblies each comprising one or more extension cables 48 connecting each of the sensor assemblies to the interrogator.

[0080] The junction 36 may accommodate an additional amount or slack section of optical fiber arranged to allow some movement of the optical fiber within and along the axis of the conduit to accommodate thermal expansion mismatch between the optical fiber and the conduit. ss316 ~15×10 -6 Consider a 1000 mm long flexible conduit made of stainless steel 316, which has a coefficient of thermal expansion (CTE) of α / °C. Optical fibers, on the other hand, typically have a CTE of α silica ~0.5×10 -6 / °C. Thus, the temperature rise ΔT of 100°C across a conduit of length L = 1 m is given by ΔL = L(α ss316 -α silica )ΔT=1m×(15-0.5)×10 -6 / °C × 100°C = 1.45 mm. A similar thermal mismatch with an optical fiber, for example, would result in a differential length increase of about 13×10 -6 / °C between a rigid conduit section made of Inconel 625, which has a similarly large CTE.

[0081] If no slack section is provided and the movement of the optical fiber within the conduit along its axis is restricted, the mismatch can result in a build-up of stresses in the optical fiber, resulting in damage or breakage of the fiber. Damage that is not initially apparent can lead to complete failure of the fiber over time or during repeated temperature cycling, as is the case for example with gas turbines operating under changing load conditions dictated by the needs of the power grid.

[0082] The requirement to allow movement of the optical fiber 14 along its own axis and the axis of the containing conduit, particularly for conduits that are bent or flexed for routing purposes during installation, typically requires that the inner diameter of the conduit be substantially larger than the optical fiber diameter, e.g., having an inner diameter or maximum internal dimension of 2 mm to 10 mm or 1 mm to 20 mm. As a result, when the sensor head and conduit are subjected to vibration, the optical fiber undergoes lateral movement within the conduit perpendicular to its axis, which creates vibration-induced bending losses that can appear as spurious measurand signals.

[0083] Thus, some embodiments of the present invention use a powder filler or granular material that is intended to eliminate lateral mechanical movement of the optical fiber within the conduit caused by vibration, while allowing axial movement of the fiber to allow compensation for thermal expansion mismatch between the fiber and the conduit for a conduit that is flexible enough for routing purposes during sensor installation.

[0084] For this purpose, as shown in FIG. 2 and the above-mentioned FIG. 1, a powder or other granular material 32 of suitable material and consistency may be introduced to fill the space between the optical fiber 14 and the conduit 30. The use of a suitable powder filler or other granular material 32 allows to completely fill the entire space between the optical fiber and the wall of the conduit without leaving any voids. Thus, the lateral movement of the fiber in the conduit perpendicular to its axis is effectively eliminated, and vibration-induced artifacts or biases in the probe light signal due to bending effects of the fiber in the conduit are significantly reduced. On the other hand, the optical fiber, while embedded in a powder or granular material 32 of suitable consistency, can still move along its axis and along the axis of the conduit, allowing compensation of thermal expansion mismatch between the optical fiber and the conduit.

[0085] Figure 3 shows a cross-sectional view of the flexible conduit 30 of Figure 2. The optical fiber 14 is embedded within, and in this case in contact with, a granular filler material 32 packed within the conduit 30. Advantageously, the addition of such granular material 32 is packed within the conduit such that the packing of the granular material is sufficiently loose, e.g., packed within the conduit to avoid voids and gaps, as may be required, e.g., during installation, but it should be noted that it should not unduly limit the ability of the flexible conduit 30 to bend unless significant compression or force is used.

[0086] In Figure 3, the optical fiber is positioned centrally within the conduit so that the conduit, the region of granular material, and the optical fiber are substantially concentric, although other arrangements are possible. For example, the optical fiber can be off-center within the conduit but within, for example, 10% of the conduit diameter from the central axis of the conduit. Ensuring that the optical fiber is reasonably close to the center of the conduit helps to reduce stresses applied to the fiber when the conduit is bent, for example, during installation, and to reduce vibration and other environmental effects on the optical fiber.

[0087] In FIG. 3, the conduit is further protected by an optional external metal braid 34 that surrounds the conduit 30 .

[0088] The granular material 32 may be selected to satisfy one or more of several conditions. First, the granular material should not change its structure or consistency across the operating and storage environmental conditions to which the sensor may be exposed. For example, the granular material should not melt when exposed to the temperature range of the sensor.

[0089] Second, the particulate material should not react or form bonds with any of the materials it is in contact with during exposure to the entire operating and storage temperature ranges of the sensor, including the optical fiber 14, and in particular the coating of that optical fiber. For example, it is generally accepted that a silica optical fiber requires a non-silica coating to ensure its long-term integrity. Conventional coating materials are typically acrylates, which have a maximum continuous operating temperature of about 85°C. Alternative coating materials may be used for higher temperature applications. Multiple layers of different coatings may also be used. For example, a thin layer of carbon may be deposited first to provide an airtight seal, followed by a polyimide coating.

[0090] Table 1 lists some of the known coating materials for optical fibers along with their typical maximum continuous operating temperatures. For temperature applications above 300°C, a metallic coating is typically required, with gold coating being the preferred choice for operation above 450°C. Thus, for applications in high temperature sensors such as those of FIG. 1 or FIG. 2, a granular material should be selected that does not react with the metallic coating of the optical fiber 14. Similarly, the granular material should not react with the material used to form the conduit, particularly the inner surface of the conduit, such as an Inconel or stainless steel material.

[0091] [Table 1]

[0092] Third, the coefficient of thermal expansion (CTE) of the particulate material should preferably be between the CTE of the optical fiber and the CTE of the conduit material, in this way the introduction of the particulate material does not exacerbate the thermal mismatch between the optical fiber and the conduit.

[0093] Fourth, the filler material should have non-binding properties in situations where moisture may be present, such as exposure to moisture during assembly or in situations where the conduit does not need to be sealed during operation. In such situations, the granular material should dry without solidifying when exposed to heat.

[0094] The powder or other particulate material filler meeting the above requirements may consist of a suitable high temperature ceramic particulate. For example, a preferred ceramic particulate is alumina Al, which has a melting point of about 2072°C. 2 O 3 It may comprise or be made from MgO, HfO 2 or SiO 2 Other suitable granular materials are also listed in Table 2.

[0095] [Table 2]

[0096] The grain size of the granular filler material may be selected to be within a certain range. For example, using a material with a grain size comparable to or larger than the size of the optical fiber outer diameter may cause undesirable mechanical deformation and potential damage to the fiber when bending the conduit. A typical single mode optical fiber has a cladding diameter of 125 μm, and therefore a preferred upper limit of the grain size may be on the order of 50 μm. Some embodiments described below may use fibers with diameters larger than 125 μm. In that case, the upper grain size scales accordingly at 40% of the fiber diameter with values ​​for 125 μm diameter fibers. Thus, for example, a 200 μm or 250 μm diameter fiber may provide a suitable grain size of up to about 80 μm or 100 μm, respectively, optionally up to about 200 μm. On the other hand, using a material with a very fine or small grain size creates practical difficulties in filling long sections during assembly. The grain size should also facilitate free axial movement of the fiber and free formation of the conduit. Thus, a suitable lower limit for the grain size of the material may be 10 μm or 30 μm. The lower limit is largely independent of the fiber diameter.

[0097] When considering the particle size of a powder, one must deal with the fact that commercially available powders usually contain a range of particle sizes. Thus, particle size may be characterized by statistical measures such as percentile Dx, which refers to the maximum particle size or diameter D to which x% of the population belongs. For example, D50 is commonly known as the median particle size and means that 50% of the particles in the population have a size or diameter smaller than D50. As a result, the remaining 50% of the particles in the population will have a size or diameter greater than (or equal to) D50. Thus, 90% of the particles in the population have a maximum size or diameter less than the D90 value, and 90% of the particles in the population have a size or diameter that is at least the D10 value.

[0098] Thus, for example, a suitable grain size of material for a 125 μm (or 200 μm or 250 μm) diameter fiber may be conveniently characterized by D10=30 μm (or at least 10 μm), and D90=40% of the fiber diameter (or no more than 50% of the fiber diameter), so that D90=50 μm or 80 μm or 100 μm for a 125 μm or 200 μm or 250 μm diameter fiber, respectively. For tighter size control, this can be set to D5=30 μm and D95=50 μm (80 μm or 100 μm).

[0099] Based on the above, it may be said that a suitable range of particle size for the material is, for example in terms of D50 or median particle size, between about 30 μm and 80 μm, or more broadly, between about 10 μm and 200 μm.

[0100] To demonstrate the effectiveness of using a granular material filled in a conduit, Figure 9 shows the effect of vibration on the output of a pressure sensor using an optical fiber 14 in a flexible metal conduit 30, both without a granular material filler (upper curve 102) and with granular material filled in the conduit (lower curve 104), as shown in cross section in Figure 3. A dual wavelength interrogation technique as described above was used.

[0101] For these measurements, the sensor head 16 arranged to measure the local pressure was mounted on a vibration table resulting in preset acceleration levels over a selected frequency band. Recording the AC output from the interrogator 24, which should represent the true pressure change at the sensor head, the sensitivity is calculated as the ratio of the apparent measured pressure to the applied acceleration. The arrangement without the granular material filler shows a significant residual sensitivity to vibrations seen in the curve 102 above, especially noticeable in the low frequency range. This is due to the fact that for a given acceleration level, the corresponding displacement is inversely proportional to the square of the vibration frequency, resulting in increased fiber bending and associated light attenuation effects at lower frequencies.

[0102] The use of a granular material filler in the conduit dramatically reduces the sensitivity to vibrations by limiting the lateral movement of the fiber within the conduit, resulting in a nearly flat response across the entire frequency range, substantially below the 0.5 mbar / g mark, as shown in curve 104 below.

[0103] FIG. 3 shows in cross section a conduit 30 in which the optical fiber 14 is embedded directly in a granular filler material that would otherwise fill the cross section of the conduit. However, this arrangement is subject to several possible variations. For example, in the arrangement of FIG. 4, the optical fiber is first disposed in a sleeve 40, particularly a woven or flexible sleeve that can withstand high temperatures, which is then surrounded by a granular material that fills the conduit between the sleeve and the wall of the conduit. This aspect of the invention may be practiced alone, or even without the use of granular material, or with one or more of the other aspects described herein, such as the use of granular material, the use of an expanded diameter cladding, and the use of reduced mode field diameters or other fiber types to reduce bending losses.

[0104] In Figure 4, the conduit, granular material, sleeve and optical fiber are substantially concentric, with the advantages already described with respect to Figure 3, although some deviation from this arrangement is naturally possible, for example if the optical fiber is within, for example, 10% of the conduit diameter of the central axis of the conduit, or as close to the central axis as is reasonably practical subject to the technology used to construct the illustrated configuration.

[0105] A variety of suitable flexible high temperature type sleeves for this purpose are available in different grades of purity, for example based on quartz, fused silica or alumina silica, which have high use temperatures on the order of about 1000° C. However, using a flexible sleeve 40 with a thermal expansion coefficient close to that of fused silica can help minimize the thermal expansion mismatch between the optical fiber 14 and the sleeve 40.

[0106] Suitable high temperature sleeves have a very high SiO 2The flexible sleeve 40 may include a braided or woven sleeve having a pore content. Such sleeve configurations are available, for example, from Hitex Composites (Ningbo, China) or Textile Technologies Europe Ltd. (Cheshire, UK). A powder or other granular material filler is then used to completely fill the entire space between the flexible sleeve 40 surrounding the optical fiber 14 and the inner wall of the conduit 30 to eliminate lateral fiber movement induced by vibrations in the conduit. It may therefore be desirable to tightly fit the flexible sleeve 40 around the optical fiber to avoid optical fiber movement within the sleeve. The granular material filler then limits lateral fiber movement in the same way as without the flexible sleeve 40, but additional cushioning of the optical fiber is provided by the flexible sleeve compared to the situation in FIG. 3 where the optical fiber is in direct contact with the granular material.

[0107] It should be noted that when flexible sleeves comprising braided or woven materials are used, braided materials are available that can provide a smoother surface in the direction along the axis of the sleeve and optical fiber, thereby reducing friction between the optical fiber and the inner surface of the sleeve. This promotes smoother axial movement of the fiber when exposed to temperature fluctuations, further minimizing the risk of stress variations in the optical fiber over time.

[0108] However, as an alternative to the arrangements of Figures 3 and 4, the granular material may instead be filled directly around the optical fiber, but within a flexible sleeve 40, which is adjacent to and / or in direct contact with the conduit. Such an arrangement is shown in Figure 5. In this arrangement, contact between the conduit and the granular material is avoided. However, the flexible sleeve in this arrangement may facilitate increased flexibility of the conduit, for example if bending is required during installation of the sensor.

[0109] As a further alternative to the arrangements of Figures 3, 4 and 5, the granular material may be filled both within the flexible sleeve and outside the flexible sleeve 40 as shown in Figure 6. The inner granular material within the flexible sleeve, and therefore typically in contact with the optical fiber 14, is shown in Figure 6 as inner material 32'. The outer granular material that is outside the flexible sleeve, and therefore typically in contact with the inner wall of the conduit 30, is shown as outer material 32".

[0110] As with Figures 3 and 4, in Figures 5 and 6 the optical fiber, sleeve, and one or more regions of granular material are substantially concentric within the conduit, although some variations from this shape are of course possible.

[0111] The same granular material having the same properties may be used for both the inner and outer granular materials, which may alternatively differ in one or more respects, such as chemical composition, granular size distribution, thermal properties, etc. Thus, the use of at least two distinct, typically concentric regions of granular material allows for the selection of properties such as grain size and coefficient of thermal expansion (CTE) that are better suited to the respective encapsulated or encapsulating materials and structures.

[0112] For example, the inner granular material 32' may be selected to have a CTE that more closely resembles the CTE of an optical fiber (typically made of silica), while the outer granular material 32" may be selected to have a CTE that more closely resembles the CTE of a conduit (usually formed of a metal such as Inconel or austenitic stainless steel). To this end, the inner material typically has a lower CTE than the outer material.

[0113] For example, the inner granular material 32' may have a diameter of about 0.5×10 -6 The outer granular material may include or be a silica granule having a CTE essentially the same as or very similar to the CTE of the optical fiber at 15×10 / °C. -6 / °C Stainless steel 316 conduit material CTE is close to approximately 8.4 x 10 -6 / °C.

[0114] Thus, using multiple concentric or at least multiple layers of different granular materials packed within the conduit, typically separated by one or more optionally concentric flexible sleeves, a more gradual change in the thermal expansion coefficient in the radial direction of the conduit can be created, mitigating the effects of CTE mismatch between the central optical fiber 14 and the surrounding conduit material.

[0115] Alternatively, or in addition to the above multiple granular materials of different CTE, the inner material 32' can be comprised of finer granules than the outer material 32", e.g., having a smaller median grain size. Use of a finer granular inner material can minimize indentation damage to coatings, such as metal coatings, of the optical fiber 14, while allowing a coarser granular outer material to be used, e.g., to help provide improved flexibility of the conduit during installation.

[0116] Although Figures 3-6 have been described as showing a filling of one or more granular materials with an optional additional sleeve around the optical fiber within the conduit, essentially the same or similar configurations may be used or extend within other structures of the sensor, such as the rigid section 42 of the sensor head shown in Figure 2, and junction 36.

[0117] Similarly, the above-described configurations using granular material may be limited to certain parts of the section of the conduit, for example being used only in the flexible section, or only in the rigid section.

[0118] Although one layer of sleeve 40 within the conduit is shown in Figures 4, 5 and 6, two or more such sleeve layers may be used, for example, two such layers in contact with each other which can reduce friction against longitudinal movement of the optical fiber within the conduit, or two or more such layers, each set of layers spaced apart by different regions of granular material.

[0119] When two or more such coaxial layers of sleeves are used, each of the two or more layers may be formed using a different textile construction type, such as using different ones of woven, braided, and knitted textile materials, each of which may have different properties and advantages as already described above. For example, FIG. 7 shows a variation of the construction of FIG. 6 in which two coaxial sleeves, one inside (and usually generally concentric with) the other, are used, all filled with granular material within inner flexible sleeve 42 as material 32', on the outside of outer flexible sleeve 44 as material 32", and between the inner and outer flexible sleeves as material 32"'.

[0120] Alternatively, any one, more than one, or all of the layers of granular material shown in Figure 7 may be omitted as desired. For example, Figures 8a and 8b show other constructions in which two coaxial (i.e., one inside the other), typically approximately concentric, layers of sleeve configurations are used without any filled granular material. In such an arrangement, the woven construction or other material type of each sleeve may be selected differently to meet specific design functions.

[0121] 8a, the outer sleeve 44 may be a braided or woven sleeve formed from strands having a relatively large diameter arranged to fill most of the diameter between the optical fiber 14 and the conduit 30. However, the use of larger diameter strands in the layers of the sleeve may cause excessive deformation of the optical fiber if used to directly surround the optical fiber, especially if tightly packed to avoid lateral movement of the optical fiber. Such excessive deformation may weaken the fiber or cause undesirable changes in its optical properties, such as additional attenuation of light propagating within the fiber.

[0122] This deformation problem can be avoided by providing an illustrated inner sleeve 42 between the optical fiber 14 and the outer sleeve, the inner sleeve having a different textile construction than that of the outer sleeve, e.g., a sleeve of woven material and formed from strands having a relatively small diameter compared to those of the outer sleeve. Such an inner woven sleeve typically has somewhat greater stiffness than the outer braided or knitted sleeve, thereby providing sufficient protection to the optical fiber 14 from deformations caused by the larger strands of the outer sleeve, while the outer sleeve 44 provides the volume required to fill most of the space between the optical fiber and the conduit 30 with a density sufficient to provide the mechanical damping required to limit or prevent lateral movement of the optical fiber within the conduit.

[0123] On the other hand, if the braided sleeve layer is made of sufficiently soft strands, the risk of deformation of the optical fiber when in contact with such a layer may be negligible, and it may be beneficial to first encapsulate the optical fiber in an inner braided sleeve layer 46, followed by a woven outer sleeve layer 48, as shown in FIG. 8b. Such an arrangement may facilitate smoother axial movement of the optical fiber when exposed to temperature fluctuations, since a braided material is available that provides a smoother surface in the direction along the axis of the sleeve and optical fiber, thereby reducing friction between the optical fiber and the inner surface of the sleeve. The outer woven sleeve layer 48 would then provide some additional stiffness, aiding in the desired reduction in lateral movement.

[0124] Although only two coaxial flexible sleeve layers are shown in Figures 8a and 8b, these embodiments may be adapted to include three or more such layers, with the layers being formed from any suitable combination of the same or different textile construction types and other properties.

[0125] In some embodiments, the optical fibers along each of two or more different elongated sections of the conduit may be surrounded by a different combination of one, two or more coaxial sleeves, each such coaxial sleeve combination may use the same or a different number of coaxial sleeves as other sleeve combinations, and the sequence of woven construction types of the coaxial sleeves may differ between sleeve combinations.

[0126] In some embodiments, all of the different coaxial sleeve combinations used in different elongated sections of the conduit may include at least two coaxial sleeve layers. For example, an inner woven sleeve and an outer braided sleeve may be used together in a portion of the conduit with a larger inner diameter, and an inner woven sleeve and an outer braided sleeve may be used in a portion of the conduit with a smaller inner diameter. For example, referring to FIG. 2, the rigid section 42 of the conduit may have a different inner diameter than the flexible section of the conduit. Using different coaxial sleeve combinations in different elongated sections of the conduit provides for different fill rates to be implemented in different sections to ensure that the space within the conduit is properly filled (even in the absence of granular fill material) and ensure that lateral movement of the optical fiber within the different sections of the conduit is properly restricted or prevented.

[0127] The inventors have also observed that increasing the stiffness of the optical fiber 14 tends to reduce vibration-induced movement of the optical fiber, resulting in suppression of vibration and other environmentally induced artifacts and biases in the interference signal, thus providing a more accurate determination of one or more measurands, and that such an increase in stiffness can be achieved by increasing the outer diameter of the optical fiber cladding. For example, the optical fiber 14 in the conduit 30 may have an increased outer cladding diameter of at least 150 μm, or at least 200 μm, or at least 250 μm. This outer cladding diameter typically does not include an additional protective layer, such as a non-silica coating. This aspect of the invention may be implemented alone or with one or more of other aspects, such as the use of granular materials, the use of one or more protective sleeves, and the use of reduced mode field diameters or other fiber types to reduce bending losses.

[0128] Single-mode optical fibers commonly used in the prior art typically consist of a 9 μm diameter germanium oxide (GeO) fiber in the center of a 125 μm diameter pure silica cladding. 2 ) doped silica core. Germanium oxide doping is used to slightly raise the refractive index of the optical core, creating a step-index profile. Light guided by the fiber is contained primarily within the core region of the fiber and decays exponentially in the cladding region. Increasing the cladding diameter of the fiber therefore does not tend to affect the light guiding properties of the fiber.

[0129] Mechanically, the fiber can be thought of as a homogeneous solid rod made of silica with a diameter D equal to the outer cladding diameter. The stiffness or resistance to bending deformation is proportional to the area moment of inertia I given by:

[0130]

number

[0131] To illustrate the stiffness improvement achievable compared to a standard single-mode optical fiber with a diameter of 125 μm, the moment of inertia is plotted in FIG. 10 as a function of the fiber outer cladding diameter D, normalized to that of a 125 μm diameter fiber.

[0132] To arrive at a suitable outer cladding diameter, a trade-off can be considered between the desired increased stiffness and the routing requirements of the conduit 30 during installation for a given application. For example, the maximum reinforcement stiffness must be such that it still guarantees the minimum bend radius of the conduit 30 required for routing purposes. For many applications, an increase in stiffness of about 15 times the optical fiber compared to a conventional 125 μm optical fiber is considered to be a suitable upper limit. On the other hand, a doubling of the stiffness compared to a standard 125 μm fiber can be considered as a minimum requirement to obtain a significant reduction in vibration-induced artifacts and bias in the interference signal.

[0133] Thus, an outer cladding diameter of at least 150 μm, or at least 200 μm, may be desirable according to the curves shown in Figure 10. However, to retain adequate flexibility, an outer cladding diameter of 250 μm or less may also be desirable.

[0134] Single mode optical fibers with outer cladding diameters larger than the standard 125 μm size can be produced by conventional drawing processes, but using tailor-made optical preforms that replicate the cross-sectional fiber shape and doping profile of the fiber being drawn. During the fiber drawing process, the corresponding core-cladding diameter ratio and doping profile remain unchanged. Thus, for example, if a single mode fiber with a core diameter of 9 μm and a cladding diameter of 200 μm is required, a preform with the same equivalent core-to-cladding ratio of 9 μm / 200 μm=0.045 is required.

[0135] For deployment of sensors in high temperature environments, a suitable coating capable of withstanding those temperatures should be applied to the expanded diameter cladding of the optical fiber 14. According to Table 1 above, for temperatures above about 300° C., the preferred choice is a metal coating. For applications above about 450° C., the preferred choice is a gold coating. Advantageously, methods such as “liquid freezing”, as described for example in US Pat. No. 6,600,863 and in VA Bogatyrev, S. Semjonov “Metal-Coated Fibers”, Chapter 15, Specialty Optical Fibers Handbook, A. Mendez, TF Marche (eds.), Elsevier, 2007, are equally applicable to fibers with cladding diameters of 150 μm, or in the range of 200 μm to 250 μm or more, to provide a metal coating during the drawing process.

[0136] While the optical fiber 14 having an increased outer diameter cladding may be used in the context of the various arrangements described above and illustrated in Figures 1-7 in which the granular material filled in the conduit limits or prevents lateral movement of the optical fiber within the conduit, the increased cladding diameter may also be used in arrangements in which the granular material is omitted, for example in the configurations of Figures 8a, 8b and 11.

[0137] FIG. 11 illustrates such an arrangement in which the conduit 30 is still optionally surrounded by a protective braided metal sheath 34 and the optical fiber 14 is housed within the conduit as already described above. However, in this arrangement, the optical fiber 14 is optionally housed within a flexible sleeve 40, but without the particulate material. The sleeve 40 may have the various properties and characteristics described above, for example a flexible sleeve such as a braided, knitted, or woven silica sleeve. The sleeve 40 then provides support for the optical fiber 14 within the conduit and helps to limit or prevent lateral movement of the optical fiber within the conduit, thereby reducing artifacts and biases in the interference signal that would otherwise cause errors in one or more measured quantities.

[0138] It should be noted that in arrangements where there is no particulate material filled or packed into the conduit, a conduit with a reduced inner diameter (and optionally outer diameter) may be used, e.g., having a diameter in the range of 1 mm to 5 mm. The diameter selected may depend, for example, on the characteristics of any sleeve or sleeves 40 used to support the optical fiber 14 within the conduit 30.

[0139] The inventors have also observed that using an optical fiber 14 with reduced effects of fiber bending on light propagation tends to reduce the effects of vibrations and other artifacts and biases in the interference signal, thus providing a more accurate determination of one or more measurands. The inventors have also noted that such reduction may be achieved by using an optical fiber with a reduced mode field diameter. This is particularly true when the optical fiber 14 is a single mode optical fiber or acts as a single mode optical fiber with respect to the probe light. This aspect of the invention may be implemented alone or with one or more of the other aspects, such as the use of granular materials, the use of one or more protective sleeves, and the use of an enlarged cladding diameter.

[0140] While a typical single mode fiber carrying light at a wavelength of about 1550 nm exhibits a mode field diameter of about 10.6 μm, the use of an optical fiber in conduit 30 having a mode field diameter of 10.0 μm or less, or 8.0 μm or less, or in the range of 6.0 μm to 8.0 μm, for example at the central (e.g., peak or average) wavelength of the probe light, may thus be advantageous in improving the accurate determination of one or more measurands. The reduction in bending losses, and therefore the reduction in artifacts and bias, that can be achieved in this manner may be understood to result from stronger confinement of one or more optical modes of the probe light within the optical fiber.

[0141] For a single-mode, step-index optical fiber, the mode field diameter (MFD) is approximately given by D. Marcuse, "Loss analysis of single-mode fiber splices," Bell Sys. Tec. J. 56(5):703-18 (1977), as follows:

[0142]

number

[0143] where α and V are the radius of the fiber core and the normalized frequency, respectively. The normalized frequency, V, is defined as follows:

[0144]

number

[0145] If V<2.405, the fiber is single mode. In the above formula, NA stands for the numerical aperture:

[0146]

number

[0147] n c and n cl are the refractive indices of the fiber core and fiber cladding, respectively, and Δn=n c -n cl where λ is the vacuum wavelength of the light guided in the fiber.

[0148] From the above equation, it follows that at a given operating wavelength, the MFD depends on the core radius, α, and the refractive index difference, Δn, between the core and the cladding, which is characterized by the numerical aperture, NA.

[0149] In Figure 12, the MFD of the fundamental mode is plotted as a function of core diameter 2α for different values ​​of NA at a typically suitable probe light wavelength of 1550 nm. The dotted lines indicate the region where the optical fiber 14 carries multiple optical modes, i.e., is no longer single mode, according to V>2.405. A typical step-index single mode fiber might have a core diameter of 9 μm, an MFD of 10.6 μm, and an NA of 0.12, as shown by the dots.

[0150] Reducing the core diameter of a fiber with NA=0.12 initially results in a modest reduction in MFD before the mode is "squeezed" out of the core and rapidly becomes less guided, as characterized by an increase in MFD. A larger reduction in MFD can be achieved by increasing the difference between the core and cladding refractive indices, illustrated by the lower curve in Figure 10, which represents a larger NA. Interferometer-based optical sensors typically require a single-mode fiber to carry the probe light. Thus, as can be seen from Figure 12, an increase in NA also requires a reduction in the core diameter to stay within the single-mode regime of the fiber.

[0151] Therefore, to reduce errors in one or more measured quantities, the optical fiber 14 of the present sensor may be provided with a core diameter of 5 μm to 7 μm and an NA in the range of 0.16 to 0.20, resulting in an MFD of approximately 6 μm to 8 μm for the approximately 1550 nm probe light.

[0152] More generally, however, keeping in mind that the MFD depends to some extent on the probe light wavelength, MFDs of less than 10 μm, or less than 8 μm, or between 6 μm and 8 μm may be considered advantageous, and these values ​​and ranges may be interpreted as specified in terms of the central (e.g., peak or average) wavelength of the probe light. Such central wavelength may be interpreted, for example, as the principal or dominant peak of the intensity or power of the probe light, or an average of two or more principal or dominant peaks, or as the average wavelength with respect to intensity or power over wavelength, or in any other suitable manner.

[0153] A value of NA=0.16 may also, or instead, be considered as a minimum value for providing a significant reduction in bending-induced vibration effects.

[0154] It should be noted that although a probe light of about 1550 nm is used in the example of FIG. 10, the proposed values ​​and ranges of mode field diameter may be equally applicable if the probe light has a wavelength in the near infrared range of about 1300-1800 nm or about 1400-1700 nm.

[0155] Optical fibers 14 having a core diameter of 5 μm to 7 μm and a NA value of 0.16 to 0.20, or other ranges of properties to provide a reduced mode field diameter, can be produced by preparing a preform with the same desired reduced core-to-cladding diameter ratio as the fiber, since this ratio is maintained during the process of drawing the fiber from the preform. For example, for a standard single-mode optical fiber with a core diameter and cladding diameter of 9 μm and 125 μm, respectively, the required preform ratio is equal to 9 μm / 125 μm=0.072. To draw a fiber with a core diameter and cladding diameter of 6 μm and 125 μm, respectively, a preform with a ratio of 6 μm / 125 μm=0.048 is required.

[0156] Increasing the NA of the optical fiber 14 to achieve a smaller mode field diameter can be accomplished by increasing the doping concentration and / or by modifying the doping profile of the preform. In standard single mode fibers, the silica core is typically doped with germanium oxide (GeO 2 ), which slightly raises the refractive index of the core, creating a step-index profile. The cladding material is typically cl For example, for a fiber with NA=0.12, the refractive index difference is Δn=(NA) 2 / 2n cl≒ 0.005.

[0157] A higher refractive index difference between the core and the cladding, and therefore a larger NA, can be achieved by increasing the concentration of germanium dopants in the core. For example, to achieve an NA of 0.16, the refractive index difference must be increased to about Δn≈0.009. Alternatively, or additionally, the cladding can be doped with a suitable doping material that reduces the refractive index of silica and results in a larger refractive index difference between the refractive index of the core and the cladding. An example of such a suitable dopant could be fluorine.

[0158] As an alternative method to reduce vibration-induced bending losses and associated undesirable artifacts and biases, more complex doping profiles can be used. For example, U.S. Patent No. 6,901,196, incorporated herein by reference for these and all other purposes, describes a suitable optical fiber having a multi-layer core region including an annular trench of reduced refractive index surrounding a first core with an elevated step-index profile. Other suitable optical fiber types for achieving a similar effect include photonic bandgap fibers and average index guiding fibers, each of which uses a different light guiding mechanism and can be used to achieve lower bending losses without necessarily reducing the mode field diameter.

[0159] Although reduced mode field diameter optical fiber 14 in which granular material and / or one or more flexible sleeves filled within the conduit limit or prevent lateral movement of the optical fiber within the conduit and / or the cladding diameter is increased to increase the stiffness of the optical fiber may be used in the context of the various configurations described above and illustrated in Figures 1-11, the reduced mode field diameter embodiment may also be used in arrangements in which the granular material is omitted and the cladding diameter is not increased. When an arrangement is used without granular material, whether or not increased fiber stiffness is used, the conduit 30 may still be optionally surrounded by a protective braided metal sheath 34, and the optical fiber 14 may be contained within the conduit as already described above, and may also be contained within one or more sleeves 40 within the conduit as described above.

[0160] It should be noted that in arrangements where there is no particulate material filled or packed into the conduit, a conduit with a reduced inner (and outer) diameter may be used, e.g., having a diameter in the range of 1 mm to 10 mm. The diameter selected may depend, for example, on the characteristics of any sleeve 40 used to support the optical fiber 14 within the conduit 30.

[0161] The above-described sensors can be used to perform time-varying, or "dynamic," pressure sensing, which is often used to detect or monitor combustion instabilities in gas turbines. The onset of combustion instability typically manifests as self-amplified pressure oscillations in the frequency range up to about 10 kHz, with frequency content up to about 20 kHz often being significant.

[0162] Typically, the analyzer 22 can apply a Fast Fourier Transform to the time series of the dynamic pressure measurement signal, and combustion instabilities are detectable as pressure peaks in the frequency domain. According to sampling theory, a signal frequency of 20 kHz requires a minimum update rate of 40 kHz. Therefore, preferably, to achieve a sufficiently high update rate, a fast interrogation technique may be used by the interrogator 24, such as, for example, the dual wavelength interrogation scheme described in A. Winterburn et al., "Extension of an optical dynamic pressure sensor to measure temperature and absolute pressure in combustion applications", The Future of Gas Turbine Technology, 6th International Conference, October 17-18, 2012, Brussels, Belgium, Paper ID No: 15, the contents of which are incorporated herein by reference for these and all other purposes.

[0163] Artifacts due to vibrations of the optical fiber 14 in the conduit 30 may then appear as additional peaks in the frequency domain and, if not sufficiently suppressed, may be mistaken for pressure oscillations associated with combustion instabilities. The described techniques can be used to adequately suppress such vibration-induced artifacts, allowing reliable detection of combustion instabilities. Preferably, such vibration-induced artifacts are suppressed to a level of less than 0.5 mbar / g in the dynamic pressure measurement signal.

[0164] More generally, the described sensors may be mounted in the core of a gas turbine engine, including the compressor, burner and turbine, or at the exhaust of such an engine, and may be arranged to measure temperature and / or dynamic pressure and / or static or quasi-static pressure.

[0165] It will be appreciated that although a dual wavelength interrogation scheme along the lines described above may be used by the interrogator 24, the described sensor may also be deployed using other schemes in which the measurand is encoded onto the probe light at the sensor head 16, for example by altering the phase or polarization of the probe light.

[0166] It will be further appreciated that the above dual wavelength interrogation schemes may be considered as special cases of more general multi-wavelength interrogation techniques that may be used by the interrogator 24. For example, a spectral interrogation technique may use a broadband probe light source 10 that emits light over a range of wavelengths, such as a superluminescent light emitting diode (SLED). The interference signal is then encoded into the spectrum of the probe light reflected back from the sensor head 16. One or more measurands may be extracted from the returned spectrum using methods such as those described in WO 2013 / 136071. A spectrum analyzer, typically including a dispersive element, may be used to identify the individual wavelength components. Alternatively, the probe light source 10 may be implemented using a tunable laser that sweeps the individual wavelength components in time, along with the photodiodes of the photodetector 20.

[0167] For spectral interrogation techniques, a suitable wavelength range of the probe light may be approximately 40 nm to 80 nm centered around a near infrared central wavelength such as 1550 nm. A suitable spectrum analyzer may then use 512 or 1024 pixels to provide the wavelength range for detection if a broadband probe light source is used, or if a tunable laser is used, hundreds or thousands of wavelength points can be defined by appropriate timing of detection relative to tuning of the laser.

[0168] Although certain detailed embodiments of the present invention have been described, those skilled in the art will appreciate that modifications and variations can be made thereto without departing from the scope of the invention as defined by the appended claims.

[0169] For example, although some embodiments have been described with respect to sensing pressure using an optical cavity in the sensor head, the embodiments are not limited to pressure sensors or to transducer elements 44 including only a single optical cavity. Rather, the transducer elements may include multiple, spatially separated, or spatially overlapping optical cavities that respond to different measurands or to the same measurand. For example, techniques for measuring temperature and acceleration using dual cavity transducer elements such as those described in WO 2013 / 136071, or for measuring temperature at two spatially separated locations within a transducer may be used.

[0170] The described embodiment implements a single conduit 30 that routes a single optical fiber 14 from the sensor head 16 to the junction 36, and the same optical fiber 14 may carry the probe light from the interrogator to the sensor head and back again in a reflective configuration, although other configurations may be used. For example, some embodiments of the invention may provide a sensor that operates in a transmission mode, whereby the probe light that reaches the sensor head appears to carry an interference signal through the sensor head into a different optical fiber 14 that is routed through the same or a different conduit 30. In this manner, multiple conduits as described above may be implemented to connect multiple sensor heads in series, with a selection of the probe light from all of the serial sensor heads being routed back to the same interrogator for analysis.

[0171] Although the described embodiments primarily refer to only one optical fiber being threaded through the conduit, two or more optical fibers can be threaded through a single conduit, for example, different optical fibers can be used to carry probe light to the sensor head and back to the interrogator, or two optical fibers can each carry light to or from a single sensor head or for other purposes.

Claims

1. 1. An optical sensor for detecting one or more measurands, comprising: a probe light source arranged to generate probe light; a sensor head positioned to receive the probe light from the probe light source and to apply an interference signal to the probe light responsive to the one or more measurands; an interrogator positioned to receive the probe light from the sensor head, measure the applied interference signal, and determine the one or more measurands from the measured interference signal; one or more flexible sleeves disposed within the conduit; an optical fiber arranged to carry the received probe light at least a portion of the way from the sensor head to the interrogator, the optical fiber being disposed within the one or more flexible sleeves; An optical sensor comprising:

2. The optical sensor of claim 1 , wherein each of the one or more flexible sleeves comprises one of a braided, woven, and knitted material.

3. The optical sensor of claim 1 or 2, wherein each flexible sleeve comprises a silica material.

4. The optical sensor of claim 1 , wherein the one or more flexible sleeves comprise at least two coaxial flexible sleeves.

5. 5. The optical sensor of claim 4, wherein at least two of the two coaxial flexible sleeves are formed using different textile construction types, optionally selected from woven, braided, or knitted textile construction types.

6. 6. The optical sensor of claim 5, wherein an inner one of the coaxial flexible sleeves is formed from a woven textile material and an outer one of the coaxial flexible sleeves is formed from a braided or knitted textile material, or wherein an inner one of the coaxial flexible sleeves is formed from a braided textile material and an outer one of the coaxial flexible sleeves is formed from a woven textile material.

7. 10. The optical sensor of claim 1, wherein the conduit comprises a plurality of elongated sections through which the optical fiber passes, and for each elongated section, the optical fiber is disposed within a different combination of two or more coaxial flexible sleeves disposed within the conduit, each sleeve of each combination being of a particular woven construction type, and each different combination comprising a different sequence of two or more such woven construction types.

8. The optical sensor of claim 1 , wherein the optical fiber comprises a cladding having an outer diameter of at least 150 μm, or at least 200 μm, or at least 250 μm.

9. the optical fiber has a mode field diameter of 10.0 μm or less, or 8.0 μm or less, or in the range of 6.0 μm to 8.0 μm at the center wavelength of the probe light; and / or 2. The optical sensor of claim 1, wherein the optical fiber has a core diameter of 5 μm to 7 μm and a numerical aperture of 0.16 to 0.

20.

10. 1. An optical sensor for detecting one or more measurands, comprising: a probe light source arranged to generate probe light; a sensor head positioned to receive the probe light from the probe light source and to apply an interference signal to the probe light responsive to the one or more measurands; an interrogator positioned to receive the probe light from the sensor head, measure the applied interference signal, and determine the one or more measurands from the measured interference signal; an optical fiber arranged to carry the received probe light at least part of the way from the sensor head to the interrogator, the optical fiber having an outer diameter of at least 150 μm, or at least 200 μm, or at least 250 μm; an optical fiber having a cladding having a An optical sensor comprising:

11. The optical sensor of claim 10 , wherein the optical fiber is disposed within a protective conduit.

12. the optical fiber has a mode field diameter of 10.0 μm or less, or 8.0 μm or less, or in the range of 6.0 μm to 8.0 μm at the center wavelength of the probe light; and / or 12. The optical sensor according to claim 10, wherein the optical fiber has a core diameter of 5 μm to 7 μm and a numerical aperture of 0.16 to 0.

20.

13. 1. An optical sensor for detecting one or more measurands, comprising: a probe light source arranged to generate probe light; a sensor head positioned to receive the probe light from the probe light source and to apply an interference signal to the probe light responsive to the one or more measurands; an interrogator positioned to receive the probe light from the sensor head, measure the applied interference signal, and determine the one or more measurands from the measured interference signal; an optical fiber arranged to carry the received probe light along at least a portion of a path from the sensor head to the interrogator, the optical fiber having a mode field diameter at a center wavelength of the probe light of 10.0 μm or less, or 8.0 μm or less, or in the range of 6.0 μm to 8.0 μm; An optical sensor comprising:

14. 14. The optical sensor of claim 13, wherein the optical fiber has a core diameter of 5 μm to 7 μm and a numerical aperture of 0.16 to 0.

20.

15. 14. The optical sensor of claim 1, 10, or 13, wherein the probe light source comprises one or more lasers or one or more superluminescent diodes arranged to generate the probe light.

16. 14. The optical sensor of claim 1, 10, or 13, wherein the sensor head comprises one or more optical cavities arranged to add the interference signal to the probe light in response to the one or more measurands.

17. 17. The optical sensor of claim 16, wherein the one or more optical cavities comprise one or more Fabry-Perot cavities.

18. 14. The optical sensor of claim 1, 10, or 13, wherein the optical fiber is a single-mode optical fiber.

19. The optical sensor of claim 1 , 10 , or 13 , wherein the one or more measurands include one or more of temperature, pressure, and acceleration at the sensor head.

20. 14. The optical sensor of claim 1, wherein the interrogator is arranged to separately detect intensities of two different wavelengths of the probe light received from the sensor head and to determine one or more of the one or more measurands in response to a relationship between the detected intensities of the two wavelengths.

21. 14. The optical sensor of claim 1, 10, or 13, wherein the interrogator comprises a spectral engine arranged to measure an interference spectrum including the applied interference signal, and arranged to determine one or more of the one or more measurands from the measured interference spectrum.

22. A gas turbine engine comprising an optical sensor as described in any one of claims 1, 10, or 13, wherein the optical sensor is positioned to detect combustion instability in the gas turbine engine.

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