Optical Measurement System

Abstract

An optical coupler (104) optically coupled to the optical interrogator (103) applies a portion of the stimulus light (129) to the coupling ends (109, 111) of one of the reflective optical arms (105) and another portion of the stimulus light (129) to the coupling ends (109, 111) of the other reflective optical arm (106). The optical coupler (104) provides response light (132, 133) that is an interferometric combination of return light (130) from one of the reflective optical arms (105) and return light (131) from the other reflective optical arm (106). The optical coupler (104) is located at a distance (107) from the optical interrogator (103) that is at least one order of magnitude greater than the distance range (108) over which the pair of reflective optical arms (105, 106) are deployed.

Description

[Technical Field]

[0001] One aspect of the present invention relates to an optical measurement system capable of measuring a physical quantity. The optical measurement system may be used, for example, to perform non-destructive testing of mechanical structures and / or to monitor mechanical structures that may be subject to wear, the latter commonly referred to as structural health monitoring. Another aspect of the present invention relates to the use of the optical measurement system to measure the physical quantity. [Background technology]

[0002] Optical sensors are used to measure physical quantities. Typically, an optical sensor has an optical path with optical properties that change depending on the physical quantity being measured. Light passing through this path is affected by the optical properties and therefore carries information about the measured physical quantity. Optical interferometry is sometimes used to obtain this information. In this case, light passing through one path interferes with light passing through another path, or light passing through a different path. This produces an interfering light that also carries information about the physical quantity, but in a form that is easier to detect.

[0003] In addition to the optical sensor, optical measurement systems generally comprise a so-called optical interrogator, which may be part of a group of optical sensors in the system. The optical interrogator essentially performs two functions: First, it provides light that passes through the optical path of the optical sensor. Second, it obtains information about the physical quantity being measured. This information is conveyed by the light that has passed through the optical path of the optical sensor, or, if optical interferometry is used, by the aforementioned interfering light.

[0004] Optical sensors are particularly suited to measuring physical quantities in harsh and / or hazardous environments. The optical path of an optical sensor, e.g., comprising an optical fiber, can withstand environmental harshness. This harshness can be caused by, for example, extreme temperatures, high pressure, chemically aggressive substances, strong electric and electromagnetic fields, strong radiation, or a combination of these. Furthermore, the light passing through the optical path is unlikely to cause a fire or explosion, which could make the environment hazardous.

[0005] The measured physical quantity may result, for example, from acoustic emissions by a mechanical structure subjected to forces. This is commonly referred to as acoustic emission testing. Acoustic emissions provide information about the mechanical structure, especially its health and wear state. Furthermore, defects within the mechanical structure can be identified and characterized. Specifically, defects such as cracks, delaminations, corrosion, and disbonds release energy as relatively high-frequency sound waves. These sound waves, traveling along the components of the mechanical structure, convey information about the defect in question.

[0006] Acoustic emission testing is therefore used, for example, in non-destructive testing and structural health monitoring, as mentioned above. In these applications, piezoelectric transducers are commonly used to detect and measure acoustic emissions. However, in harsh and / or hazardous environments, piezoelectric transducers may be unsuitable or unusable. Optical sensors may be able to withstand these environments. However, to date, there have been few commercially successful optical measurement systems for acoustic emission testing.

[0007] WO 2019 / 170194 describes a measurement device for determining a physical measurement variable. The device includes an optical coupler having at least three coupler inputs and an equal number of coupler outputs. A first lightwave signal is applied to the first coupler input. A second lightwave signal is applied to the second coupler input. The first and second lightwave signals are interfered with and have a measurement-variable-dependent phase shift corresponding to the physical measurement variable. A first optical interference signal is present at the first coupler output, representing a first interference of the first and second lightwave signals. A second optical interference signal is present at the second coupler output, representing a second interference of the first and second lightwave signals. The second optical interference signal has a coupler-dependent phase shift relative to the first optical interference signal, which is not equal to m·π (m ∈ Z). A first optical detector receives the first optical interference signal at a first detector input and provides a first electrical output signal at a first detector output. A second optical detector receives the second optical interference signal at a second detector input and provides a second electrical output signal. An evaluation unit determines the physical measurement variable from the first and second electrical output signals. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2019 / 170194 Summary of the Invention [Problem to be solved by the invention]

[0009] There is a need for optical measurement techniques that can measure physical quantities with high sensitivity and accuracy using optical probes installed at a relatively large distance from the location where the physical quantities are to be measured. [Means for solving the problem]

[0010] According to an aspect of the present invention as defined in claim 1, there is provided an optical measurement system comprising the following components: - an optical sensor assembly configured to receive a stimulus light and, in response thereto, provide a response light that conveys information about the physical quantity being measured; and - an optical interrogator configured to provide a stimulus light and to extract information about the physical quantity from the response light. Here, the optical sensor assembly includes: - a pair of reflective optical arms, each of which receives input light at a coupling end and responsively provides return light at the coupling end according to a return response, wherein the return response of one reflective optical arm of the pair varies differently depending on the physical quantity being measured. an optical coupler optically coupled to the optical interrogator, the optical coupler configured to apply a portion of the stimulus light to one coupled end of a pair of reflecting optical arms and another portion of the stimulus light to the other coupled end of the pair of reflecting optical arms, and configured to provide the response light as at least one interferometric combination of return light from one of the pair of reflecting optical arms and return light from the other of the pair of reflecting optical arms. Here, the optical coupler is located at a distance from the optical interrogator that is at least an order of magnitude greater than the range of distances from the optical coupler over which the pair of reflective optical arms are deployed.

[0011] A further aspect of the invention as defined in claim 19 relates to measuring a physical quantity using the optical measurement system described above.

[0012] In each of these aspects, Rayleigh scattering is prevented from significantly impairing measurement sensitivity and accuracy. Measurement sensitivity and accuracy are particularly impaired by Rayleigh scattering in the reflective optical arms of the sensor assembly. Rayleigh scattering in these reflective optical arms adds noise and interference to the information about physical quantities provided by the interferometric combination. Rayleigh scattering can also weaken this information. The longer the reflective optical arms of the sensor assembly, the greater the extent to which Rayleigh scattering adds noise and weakens the aforementioned information of interest.

[0013] In the optical measurement technique according to the present invention, the optical coupler can be located relatively close to the location where the physical quantity is measured, allowing the aforementioned reflective optical arm to be relatively short. Therefore, the extent to which Rayleigh scattering adds noise to and weakens information about the physical quantity is relatively low. This is particularly noticeable when compared to measurement techniques in which the optical coupler is located within the optical interrogator or relatively close to the optical interrogator. Therefore, the optical measurement technique according to the present invention allows for sensitive and precise measurements of the physical quantity, especially when the optical interrogator is located at a relatively large distance from the location where the physical quantity is measured.

[0014] For purposes of illustration, some embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which additional features defined in some dependent claims will be set forth and their advantages will become apparent. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic block diagram of an optical measurement system according to a first embodiment. [Figure 2] FIG. 10 is a schematic block diagram of an optical measurement system according to a second embodiment. [Figure 3] FIG. 10 is a schematic block diagram of an optical measurement system according to a third embodiment. [Figure 4] FIG. 10 is a schematic block diagram of an optical measurement system according to a fourth embodiment. [Figure 5] FIG. 10 is a schematic block diagram of an optical measurement system according to a fifth embodiment. [Figure 6] FIG. 10 is a schematic block diagram of an optical measurement system according to a sixth embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0016] 1-6 schematically illustrate various exemplary optical measurement systems 100-600. Each of FIGS. 1-6 provides a schematic block diagram of an exemplary optical measurement system. Each of the exemplary optical measurement systems 100-600, and variations thereof, can be used to measure physical quantities in harsh and / or hazardous environments 101. The physical quantities measured relate, for example, to acoustic emissions from mechanical structures subjected to forces. Thus, each exemplary optical measurement system 100-600 can be used, for example, to perform nondestructive testing of mechanical structures, or to monitor mechanical structures that may be subject to wear, or both. The latter is commonly referred to as structural health monitoring.

[0017] The exemplary optical measurement systems 100-600 share certain common features, which will be described first. Each of the exemplary optical measurement systems 100-600 includes an optical sensor assembly 102 and an optical interrogator 103. The optical sensor assembly 102 includes an optical coupler 104 and a pair of reflective optical arms 105, 106. The optical coupler 104 is, in turn, optically coupled to the optical interrogator 103. The optical coupler 104 is, in turn, coupled to the pair of reflective optical arms 105, 106.

[0018] The optical coupler 104 is located at a distance 107 from the optical interrogator 103 that is at least an order of magnitude greater than the distance range 108 over which the reflective optical arm pair is deployed. For example, assume that the distance range 108 over which the pair of reflective optical arms is deployed is less than 10 m. In this case, the distance 107 over which the optical coupler 104 is located from the optical interrogator 103 may be at least 100 m. This particular feature and its advantages will be discussed below.

[0019] In the exemplary optical measurement systems 100-600, one of a pair of reflective optical arms 105 at least partially traverses an environment 101 in which a physical quantity to be measured resides. This reflective optical arm 105 is hereinafter referred to as the sensing arm 105. The other reflective optical arm 106 of the pair does not traverse, or at least does not significantly traverse, the environment 101. For convenience, this reflective optical arm 106 is hereinafter referred to as the reference arm 106.

[0020] Specifically, the sensing arm 105 has two ends 109, 110. One end 109 is connected to the optical coupler 104 and is therefore hereinafter referred to as the coupled end 109. In this embodiment, the other end 110 is optically reflective and is therefore hereinafter referred to as the reflective end 110. Similarly, the reference arm 106 has two ends 111, 112, with one end 111 being coupled to the optical coupler 104 and therefore also hereinafter referred to as the coupled end 111. The other end 112 is optically reflective and is therefore hereinafter referred to as the reflective end 112.

[0021] The sensing arm 105 and the reference arm 106 each have an optical path length that extends from the coupling end 109, 111 to the reflecting end 110, 112, respectively. If these arms 105, 106 are exposed to the same environmental conditions, the optical path length of the sensing arm 105 may match the optical path length of the reference arm 106. That is, the optical path lengths may nominally correspond to each other.

[0022] In the exemplary optical measurement systems 100-600, the sensing arm 105 and the reference arm 106 may include optical fibers 113, 114, respectively. The optical fiber 113 of the sensing arm 105 is hereinafter referred to as the sensing fiber 113. The optical fiber 114 of the reference arm 106 is hereinafter referred to as the reference fiber 114. Furthermore, in the exemplary optical measurement systems 100-600, the reflective end 110 of the sensing arm 105 may have a Faraday rotator mirror 115 optically coupled to the sensing fiber 113. Similarly, the reflective end 112 of the reference arm 106 has a Faraday rotator mirror 116, which is optically coupled to the reference fiber 114.

[0023] In the exemplary optical measurement systems 100 to 600, the optical path length of the sensing arm 105 depends on the length of the sensing fiber 113 and the optical properties, particularly the effective refractive index, of the sensing fiber 113. Similarly, the optical path length of the reference arm 106 depends on the length of the reference fiber 114 and the optical properties, particularly the effective refractive index, of the reference fiber 114. The sensing fiber 113 and the reference fiber 114 may have similar optical properties. For example, the sensing fiber 113 and the reference fiber 114 may be sections cut from the same bulk optical fiber. In this case, if the length of the sensing fiber 113 matches the length of the reference fiber 114, the optical path length of the sensing arm 105 will match the optical path length of the reference arm 106.

[0024] Each of the exemplary optical measurement systems 100 to 600 comprises a coupling unit 117 that houses the optical coupler 104. The coupling unit 117 further houses the reflective end 110 of the sensing arm 105 and the reflective end 112 of the reference arm 106. This means that the coupling unit 117 can house a Faraday rotator mirror 115 that is coupled to the sensing fiber 113 and a Faraday rotator mirror 116 that is coupled to the reference fiber 114.

[0025] In the exemplary optical measurement systems 100-600, optical fiber groups 118-120 optically couple the optical coupler 104 and the optical interrogator 103. The optical fiber groups 118-120 include an optical fiber 118, which will be referred to as a forward fiber 118 for reasons that will be described below. The optical fiber group further includes two optical fibers 119, 120, which will be referred to as a first return fiber 119 and a second return fiber 120, respectively, for reasons that will be described below.

[0026] In the exemplary optical measurement systems 100-600, the optical coupler 104 is, for example, an N×N type, where N is an integer having a value of at least 3. Accordingly, the optical coupler 104 has two sets of optical ports, each set having N optical ports. One of the two sets has optical ports connected to optical fiber groups 118-120 that optically couple the optical coupler 104 to the optical interrogator 103. These optical ports are conveniently referred to as the front ports. The other set of the two sets has optical ports connected to the pair of reflective optical arms 105, 106. These optical ports are conveniently referred to hereinafter as the back ports. In the exemplary optical measurement systems 100-600 shown in FIGS. 1-6, the optical coupler 104 is, for example, a 3×3 coupler.

[0027] Specifically, a first front port of the optical coupler 104 may be connected to a forward fiber 118, a second front port may be connected to a first return fiber 119, and a third front port may be connected to a second return fiber 120. A first rear port of the optical coupler 104 may be coupled to an end of a sensing fiber 113 that constitutes a coupled end 109 of the sensing arm 105. A second rear port may be coupled to an end of a reference fiber 114 that constitutes a coupled end 111 of the reference arm 106. Yet another rear port may be coupled to a non-reflective optical termination.

[0028] In the exemplary optical measurement systems 100-600, the optical interrogator 103 includes a light source 121, a set of optical detectors 122, 123, and a measurement processor 124. The light source 121 may be in the form of, for example, a laser, more specifically, a coherent laser having a narrow linewidth, for example, less than 10 kHz. The set of optical detectors 122, 123 includes a first optical detector 122 and a second optical detector 123, each of which may include a photodiode or other type of optical-to-electrical conversion element. The measurement processor 124 may include one or more electrical circuits, including an analog-to-digital converter and a data processing circuit capable of executing instructions stored in a program memory.

[0029] The forward optical path extends from the light source 121 in the optical interrogator 103 to the optical coupler 104 in the optical sensor assembly 102. In addition to the forward fiber 118, the forward optical path includes an optical isolator 125, which will be referred to hereinafter as the forward isolator 125 for convenience. Meanwhile, the return optical path extends from the optical coupler 104 in the optical sensor assembly 102 to a pair of optical detectors 122, 123 in the optical interrogator 103. In the exemplary embodiment, the return optical path is comprised of two branches. The first branch of the return optical path includes a first return fiber 119 and a first optical isolator 126. The second branch of the return optical path includes a second return fiber 120 and a second optical isolator 127. For convenience, the first optical isolator 126 and the second optical isolator 127 described above will be referred to as the first return isolator 126 and the second return isolator 127, respectively, hereinafter.

[0030] The optical interrogator 103 may have an optical splitter 128 whose input is connected to the light source 121, as shown in Figures 1-6. The output of the optical splitter 128 is connected to the optical sensor assembly 102 via the forward fiber 118. One or more additional outputs of the optical splitter 128 may be connected to one or more additional optical sensor assemblies, which may be similar to any of the embodiments of the optical sensor assembly 102 shown in Figures 1-6. Indeed, the optical interrogator 103 may be configured to interrogate multiple optical sensor assemblies in a manner similar to the way the optical sensor assembly 102 shown in Figures 1-6 is interrogated, as described below.

[0031] The exemplary optical measurement systems 100-600 shown in Figures 1-6 basically operate as follows: A light source 121 in an optical interrogator 103 generates light, at least a portion of which constitutes stimulus light 129, which reaches an optical coupler 104 in an optical sensor assembly 102. In the exemplary optical measurement systems 100-600, the stimulus light 129 reaches the optical coupler 104 via a forward optical path that includes a forward fiber 118 and a forward isolator 125. The optical coupler 104 receives the stimulus light 129 at its first front port. The forward isolator 125 prevents unwanted reflections between the optical coupler 104 in the optical sensor assembly 102 and the light source 121 in the optical interrogator 103.

[0032] The optical coupler 104 splits the stimulus light 129 received from the optical interrogator 103 into several portions. One portion of the stimulus light 129 starts at a first rear port of the optical coupler 104. The optical coupler 104 then applies this portion to the coupling end 109 of the sensing arm 105. For convenience, this portion of the stimulus light 129 will be referred to hereinafter as the sensing input light. Another portion of the stimulus light 129 starts at a second rear port of the optical coupler 104. The optical coupler 104 then applies this portion to the coupling end 111 of the reference arm 106. For convenience, this portion of the stimulus light 129 will be referred to hereinafter as the reference input light.

[0033] In response to the sensing input light, the sensing arm 105 provides return light 130 at its coupling end 109 according to a return response. For convenience, the return light 130 and the return response are hereinafter referred to as sensing return light 130 and sensing response, respectively. The sensing response depends on the optical path length of the sensing arm 105. In turn, the optical path length of the sensing arm 105 depends on the length of the sensing fiber 113. This length may vary depending on the physical quantity to be measured.

[0034] Similarly, in response to the reference input light, the reference arm 106 provides return light 131 at its coupling end 111 according to a return response. For convenience, this return light 131 and the return response are hereinafter referred to as reference return light 131 and reference response, respectively. The reference response depends on the optical path length of the reference arm 106. In turn, the optical path length of the reference arm 106 depends on the length of the reference fiber 114. This length does not vary with the physical quantity being measured, or at least varies significantly less with the physical quantity than the length of the sensing fiber 113 varies with the physical quantity.

[0035] The optical coupler 104 receives sensing return light 130 from the sensing arm 105 at its first rear port. The optical coupler 104 also receives reference return light 131 from the reference arm 106 at its second rear port. The optical coupler 104 interferometrically combines the sensing return light 130 and the reference return light 131. In this embodiment, the optical coupler 104 performs two types of interferometric combinations of the sensing return light 130 and the reference return light 131. The optical coupler 104 provides a first interferometric combination 132 at its second front port. The optical coupler 104 also provides a second interferometric combination 133 at its third front port.

[0036] The two different interferometric combinations 132, 133 each have an intensity that depends on the phase difference between the sensing return light 130 and the reference return light 131. This phase difference depends on the optical path length difference between the sensing arm 105 and the reference arm 106. This optical path length difference can be directly related to the length difference between the sensing fiber 113 and the reference fiber 114. The aforementioned length difference depends on the physical quantity to be measured.

[0037] The two different interferometric combinations 132, 133 differ from each other in that different predefined phase offsets are applied to the phase difference between the sensing return light 130 and the reference return light 131. This allows the sign and magnitude of the phase difference between the sensing return light 130 and the reference return light 131 to be determined based on the first interferometric combination 132 and the second interferometric combination 133. In the exemplary optical measurement systems 100-600 shown in Figures 1-6, the optical combiner 104 is a 3x3 combiner, providing a relative phase offset of 120°.

[0038] In practice, the stimulus light 129 may have phase noise, which may affect measurement accuracy. The phase noise of the stimulus light 129 is transmitted to the sensing return light 130 and the reference return light 131. A phase noise cancellation effect is obtained when the sensing return light 130 and the reference return light 131 are interferometrically combined. As mentioned above, the phase noise cancellation effect is maximized when the optical path lengths of the sensing arm 105 and the reference arm 106 nominally correspond to each other. In this case, the interferometric combination has minimal phase noise. This applies to the first interferometric combination 132 and the second interferometric combination 133 formed in the optical coupler 104 mentioned above.

[0039] In response to the stimulus light 129, the optical sensor assembly 102 provides a response light having two components. One of the two components is a first interferometric combination 132 formed in the optical coupler 104. The other component of the response light is a second interferometric combination 133 formed in the optical coupler 104. The response light conveys information about the physical quantity being measured, as can be understood from the above description of the two different interferometric combinations 132, 133 that make up the two components of the response light.

[0040] The optical interrogator 103 receives the response light through the return optical path. Specifically, a first optical detector 122 within the optical interrogator 103 receives the first interferometric combination 132 via a first return fiber 119 and a first return isolator 126. A second optical detector 123 receives the second interferometric combination 133 via a second return fiber 120 and a second return isolator 127. The first return isolator 126 prevents unwanted reflections between the optical sensor assembly 102 and the first optical detector 122 within the optical interrogator 103. Similarly, the second return isolator 127 prevents unwanted reflections between the optical sensor assembly 102 and the second optical detector 123.

[0041] The first optical detector 122 provides a first detected signal whose magnitude varies depending on the intensity of the first interferometric combination 132. Similarly, the second optical detector 123 provides a second detected signal whose magnitude varies depending on the intensity of the second interferometric combination 133. Thus, the two detected signals convey information about the physical quantity being measured. Specifically, fluctuations in the physical quantity cause fluctuations in the magnitude of the first detected signal and the second detected signal. These fluctuations can be due to, for example, acoustic emissions picked up by the sensing arm 105 of the optical sensor assembly 102.

[0042] The measurement processor 124 generates output data related to the physical quantity based on the two detection signals from the optical detectors. To this end, the measurement processor 124 can apply a combination of various known relationships, such as the following: On the one hand, there is a known relationship between the intensity of the first interferometric combination 132 and the intensity of the second interferometric combination 133, and on the other hand, the phase difference between the sensing return light 130 and the reference return light 131. There is also a known relationship between this phase difference and the difference in optical path length between the sensing arm 105 and the reference arm 106. There is also another known relationship between the aforementioned difference in optical path length and the physical quantity. Any of these known relationships can be determined empirically, by calculation, or by a combination thereof.

[0043] The output data generated by the measurement processor 124 can take a variety of forms. For example, the output data may include a signal representative of the acoustic emissions. As another example, the output data may include parameters characterizing the acoustic emissions. These parameters may relate to at least one of amplitude, frequency, and duration. These examples relate to the use of the exemplary optical measurement systems 100-600 for acoustic emission testing. In other applications, the output data may include other elements. For example, the output data may include a value corresponding to the value of a physical quantity being measured, such as temperature, pressure, or strain.

[0044] The exemplary optical measurement systems 100-600 shown in Figures 1-6, and variations thereof, are particularly suited for performing sensitive and precise measurements in harsh and / or hazardous environments. First, the sensing fiber 113 can withstand harsh environments and be safe in hazardous environments. The optical interrogator 103, which is generally not designed for these conditions, can be installed at a relatively large distance from the environment, in a safe location. This distance may be, for example, hundreds of meters, kilometers, or even longer.

[0045] This configuration prevents Rayleigh scattering and external interference from significantly affecting measurement accuracy. Rayleigh scattering in the sensing arm 105 and reference arm 106 has a significantly greater negative impact on measurement sensitivity and accuracy than Rayleigh scattering in the forward and return paths. The same is true for external interference. The sensing arm 105 and reference arm 106, which are more susceptible to Rayleigh scattering and external interference, are relatively short. On the other hand, the forward and return paths, which are less sensitive to Rayleigh scattering and external interference, are significantly longer than the aforementioned arms 105, 106. The forward and return paths substantially cover the distance separating the environment 101 requiring measurement from the optical interrogator 103.

[0046] The exemplary optical measurement systems 100-600 shown in Figures 1-6 operate essentially as described above. However, the exemplary optical measurement systems 100-600 differ from one another in other details. That is, each of the exemplary optical measurement systems 100-600 has particular characteristics, which are described below.

[0047] 1 shows a first embodiment of an optical measurement system 100 having the following specific features: The coupling unit 117 integrally houses the reference arm 106, i.e., the reference arm 106 is integrally provided within the coupling unit 117. This isolates the reference arm 106 from the environment 101 in which the physical quantity is to be measured. As a result, the Faraday rotator mirror 116 forming the reflective end 112 of the reference arm 106 is housed within the coupling unit 117. This Faraday rotator mirror 116 is protected from the environment 101 in which the physical quantity is to be measured. Similarly, the Faraday rotator mirror 115 forming the reflective end 110 of the sensing arm 105 is housed within the coupling unit 117 for the same reasons. The rest of the sensing arm 105 is located outside the coupling unit 117 and traverses the environment 101 in which the physical quantity is to be measured.

[0048] The sensing fiber 113, which constitutes a part of the sensing arm 105, includes a section wound around the sensing transducer 134. The cross-sectional size of the sensing transducer 134 changes depending on the physical quantity of the object to be measured. As a result, the length of the section of the sensing fiber 113 wound around the sensing transducer 134 can change depending on the physical quantity. This sensing transducer 134 can make this change in length relatively large in response to a given change in the physical quantity. This causes the optical path length of the sensing arm 105 to change relatively greatly depending on the physical quantity. Therefore, relatively high measurement sensitivity can be obtained.

[0049] The sensing transducer 134 may be made of a solid material, such as metal, plastic, or ceramic, or a combination thereof. The sensing transducer 134 may have dimensions that match the frequency range of fluctuations in the physical quantity to be measured. These fluctuations may be due to sound waves from acoustic emissions. Therefore, the cross-sectional size of the sensing transducer 134 may change depending on the sound waves received by the sensing transducer 134. The sensing transducer 134 may be a resonant structure that exhibits a relatively large change in cross-sectional size in response to sound waves. By having the sensing transducer 134 have an appropriate shape and appropriate material properties, efficient coupling and extraction of the physical quantity may be achieved. For example, acoustic impedance matching may be achieved in this manner.

[0050] The reference fiber 114, which constitutes part of the reference arm 106, includes a section wound around a reference support 135. The cross-sectional area of ​​the reference support 135 varies depending on the physical quantity to be measured, but this variation is significantly smaller than the cross-sectional size of the sensing transducer 134. In extreme cases, the cross-sectional size of the reference support 135 may be completely independent of the physical quantity to be measured. Therefore, the optical path length of the reference arm 106 may be independent of the physical quantity. In other words, in contrast to the sensing arm 105, the optical path length of the reference arm 106 is less affected by the physical quantity.

[0051] The reference support 135 comprises a solid material with a high attenuation coefficient, such as rubber or cork, or a combination thereof, which makes the reference support 135 insensitive to acoustic waves, while in contrast the sensing transducer 134 is sensitive to acoustic waves.

[0052] The forward isolator 125 in the forward optical path is located in the optical interrogator 103. Similarly, the first return isolator 126 in the first leg of the return optical path is located in the optical interrogator 103. Similarly, the second return isolator 127 in the second leg of the return optical path is also located in the optical interrogator 103.

[0053] 2 illustrates a second exemplary optical measurement system 200 that differs from the first exemplary optical measurement system 100 in the following respects: In the second exemplary optical measurement system 200, the reference arm 106 is only partially mounted in the coupling unit 117. Specifically, a relatively large portion of the reference fiber 114 is located outside the coupling unit 117. The portion wrapped around the reference support 135, as well as the reference support 135 itself, are located outside the coupling unit 117. However, the coupling unit 117 houses the Faraday rotator mirror 116 that forms the reflective end 112 of the reference arm 106. As previously mentioned, this Faraday rotator mirror 116 is protected from the environment 101 in which the physical quantity is being measured.

[0054] FIG. 3 illustrates a third exemplary optical measurement system 300. This system differs from the first exemplary optical measurement system 100 in the following ways: Instead of being wound around the sensing support 134, a portion of the reference fiber 114 is distributed above or within the support. This support may constitute a distributed transducer. For example, a section of the reference fiber 114 may be embedded in a composite material. As another example, the section of the reference fiber 114 may be glued or otherwise fixed to the surface of a suitable material. In general, the section of the reference fiber 114 may constitute part of a structure whose length varies relatively significantly depending on a physical quantity. This structure may function as a microphone to pick up acoustic emissions. Furthermore, this structure may protect the reference fiber 114 from damage.

[0055] 4 shows a fourth exemplary optical measurement system 400. This system differs from the first exemplary optical measurement system 100 in the following respects: The sensing arm 105 has multiple optical sensing units arranged at different positions. In this embodiment, the sensing arm 105 has two optical sensing units 136, 137. For example, each of these has a portion of the sensing fiber 113 wound around a sensing transducer as described above. This makes it possible to measure two physical quantities at two different positions. The two physical quantities to be measured may be of the same nature or different natures.

[0056] The first interferometric combination 132 has two components. One component is generated by one of the two optical sensing portions 136, and the other component is generated by the other optical sensing portion 137. The same is true for the second interferometric combination 133. These two components can be generated by the existence of at least one optical characteristic that differs between the two optical sensing portions 136, 137. The optical interrogator 103 can be configured to distinguish between these two components.

[0057] 5 illustrates a fifth exemplary optical measurement system 500. This system differs from the first exemplary optical measurement system 100 in the following ways: Instead of being housed within the optical interrogator 103, the first return isolator 126 and the second return isolator 127 are housed within the coupling unit 117. This potentially prevents a significant amount of backscattered response light from entering the optical coupler 104 and then reaching the sensing arm 105, the reference arm 106, or both. If the distance 107 is relatively long, e.g., on the order of kilometers or longer, a significant amount of backscattered response light may occur in the return optical path.

[0058] FIG. 6 illustrates a sixth exemplary optical measurement system 600, which differs from the first exemplary optical measurement system 100 in the following respects. The optical interrogator 103 includes an optical modulator 138 connected between the light source 121 and the optical splitter 128. The optical modulator 138 may be, for example, an optical polarization modulator or a phase modulator. The polarization modulator can switch the stimulus light 129 between at least three polarization states. This polarization switching occurs at a rate at least three times faster than the rate at which the physical quantity being measured is sampled. Thus, the stimulus light 129 has at least three polarization states that alternate with each other at a relatively high rate. The optical interrogator 103 can deinterlace the response light in the opposite direction. This polarization modulation technique ensures that the components of the response light have a relatively high intensity in at least one polarization state. This improves measurement accuracy and reliability.

[0059] The optical phase modulator can modulate the phase or optical frequency of the stimulus light 129 using a relatively high-frequency modulation signal. The modulation signal can be, for example, a sinusoidal wave or band-limited noise. The relatively high frequency of the modulation signal can exceed the rate at which the physical quantity being measured is sampled by at least one order of magnitude. This allows noise that may be induced by Rayleigh backscattered light to be shifted to a frequency band outside the detectable range of the optical detectors 122, 123. This can prevent this noise from degrading measurement accuracy, especially when the distance 107 is on the order of kilometers or longer.

[0060] Note

[0061] The embodiments described above with reference to the drawings have been presented by way of example. The invention can be embodied in many different ways. To illustrate this, some alternatives will be briefly presented.

[0062] The present invention may be applied to various types of products or methods related to optical measurements. As previously mentioned, the present invention may be applied to optical measurements of acoustic emissions, which may be performed for structural health monitoring and / or non-destructive testing. Another potential application is partial discharge detection. Further potential applications include optical measurements of temperature, pressure, strain, and other physical quantities. In all of these applications, the present invention allows for relatively sensitive and precise measurements in harsh and / or hazardous environments.

[0063] There are many ways to implement an optical measurement system according to the present invention. For example, in the above-described embodiment, a Faraday rotator mirror is used to form the reflective ends of the sensing arm and the reference arm. In other embodiments, other optically reflective structures or elements can be used, such as a plane mirror or a fiber Bragg grating. Such other embodiments can include so-called polarization-diverse receivers to prevent polarization attenuation from affecting measurement sensitivity and accuracy. Other techniques for mitigating polarization attenuation can also be used.

[0064] As an example showing that many more alternatives exist, embodiments different from those described above may include additional optical isolators. For example, referring to the fifth exemplary optical measurement system 500 shown in FIG. 5 , in a variation thereof, the forward optical path may include an additional optical isolator with a forward directivity similar to that of the forward isolator 125. This additional forward isolator may be located within the combining unit 117. A variation of the fifth exemplary optical measurement system 500 may also include two additional optical isolators in the return optical path. These two additional optical isolators have return directivities similar to those of the first return isolator 126 and the second return isolator 127 and may be located within the optical interrogator 103. One of these two additional return isolators may be located between the first return fiber 119 and the first optical detector 122. The other of the two additional return isolators may be located between the second return fiber 120 and the second optical detector 123.

[0065] The foregoing description indicates that the embodiments described with reference to the drawings illustrate the invention, rather than limit it. The present invention can be embodied in numerous alternative ways within the scope of the appended claims. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope. Any reference signs in the claims should not be construed as limiting the claims. The verb "comprise" or "have" in the claims does not exclude the presence of elements or steps other than those stated in the claim. The same applies to the similar verbs "include" and "contain." The singular recitation of an element in a product claim does not exclude the possibility that the product contains a plurality of such elements. Similarly, the singular recitation of a step in a method claim does not exclude the possibility that the method contains a plurality of such steps. The mere fact that each dependent claim defines additional features does not exclude combinations of additional features other than those reflected in the claims.

[0066] In the optical measurement system 100, the optical sensor assembly 102 receives a stimulus light 129 and, in response, provides response lights 132, 133 that convey information about the physical quantity being measured. The optical interrogator 103 provides the stimulus light 129 and extracts information about the physical quantity from the response lights 132, 133. The optical sensor assembly 102 includes a pair of reflective optical arms 105, 106, each configured to receive input light at a coupling end 109, 111 and, in response, provide return light at the coupling end 109, 111 according to a return response. The return response of one reflective optical arm 105 of the pair and the return response of the other reflective optical arm 106 of the pair vary differently depending on the physical quantity being measured.

[0067] An optical coupler 104 optically coupled to the optical interrogator 103 applies a portion of the stimulus light 129 to coupling ends 109, 111 of one reflecting optical arm 105 and another portion of the stimulus light 129 to coupling ends 109, 111 of the other reflecting optical arm 106. The optical coupler 104 provides response lights 132, 133 that are interferometrically combined with return light 130 from one reflecting optical arm 105 and return light 131 from the other reflecting optical arm 106. The optical coupler 104 is located at a distance 107 from the optical interrogator 103 that is at least one order of magnitude greater than a distance range 108 over which the pair of reflecting optical arms 105, 106 are deployed.

Claims

1. an optical sensor assembly (102) configured to receive a stimulus light (129) and to provide in response a response light (132, 133) conveying information about the physical quantity to be measured; and an optical interrogator (103) configured to provide said stimulus light and to extract said information on said physical quantity from said response light, the optical sensor assembly includes a pair of reflective optical arms (105, 106) and an optical coupler (104) optically coupled to the optical interrogator; each of a pair of reflective optical arms receives input light at a coupling end (109, 111) and in response provides return light (130, 131) at said coupling end in response to a return response, said return response of one reflective optical arm of said pair and said return response of the other reflective optical arm of said pair varying differently as a function of said physical quantity to be measured; the optical coupler is configured to apply a portion of the stimulus light to the coupled end of one of the reflecting optical arms of the pair and another portion of the stimulus light to the coupled end of the other of the reflecting optical arms of the pair, and to provide the response light as at least one interferometric combination (132, 133) of return light from one of the reflecting optical arms of the pair and return light from the other of the reflecting optical arms of the pair; an optical measurement system (100 to 600) wherein the optical coupler is located at a distance (107) from the optical interrogator that is at least one order of magnitude greater than a distance range (108) from the optical coupler at which the pair of reflective optical arms are deployed.

2. 2. The optical measurement system of claim 1, wherein the optical coupler (104) is located at least 100 m away from the optical interrogator (103).

3. 3. The optical measurement system according to claim 1, further comprising a coupling unit (117) that houses the optical coupler (104) and at least one reflective end (112) of the pair of reflective optical arms (106).

4. 4. The optical measurement system of claim 3, wherein the coupling unit (117) accommodates the reflective end (110) of one of the pair of reflective optical arms (105) and the reflective end (112) of the other of the pair of reflective optical arms (106).

5. 5. The optical measurement system of claim 3, wherein the reflective end (110, 112) of at least one of the pair of reflective optical arms (105, 106) comprises a Faraday rotator mirror (115, 116).

6. 6. The optical measurement system of claim 3, wherein the coupling unit (117) further integrally houses one of the pair of reflective optical arms (106), and the other of the pair of reflective optical arms (105) traverses an environment (101) in which the physical quantity is measured.

7. 7. The optical measurement system of claim 1, wherein one of the pair of reflective optical arms (105) has an optical path length that matches the other of the pair of reflective optical arms (106).

8. 8. The optical measurement system according to claim 1, wherein at least one of the pair of reflective optical arms (105) has a plurality of optical sensing portions (136, 137) arranged at different positions.

9. 9. An optical measurement system according to any one of claims 1 to 8, wherein at least one of the pair of reflective optical arms (105, 106) comprises an optical fiber (113, 114).

10. 10. The optical measurement system of claim 9, wherein the optical fiber (113) of one of the pair of reflective optical arms (105) includes a section coupled to a sensing transducer (134) configured to change an optical characteristic within the section in response to a variation in the physical quantity.

11. 11. The optical measurement system of claim 10, wherein the optical fiber (114) of the other reflective optical arm (106) of the pair of reflective optical arms (105) includes a section coupled to a non-responsive medium (135) configured to prevent variations in optical properties within the section in response to variations in the physical quantity.

12. the optical coupler (104) is configured to provide several interferometric combinations (132, 133) of return light (130) from one of the pair of reflective optical arms (105) and return light (131) from the other of the pair of reflective optical arms (106); the several interferometric combinations differ from one another in that different predefined phase offsets are applied to a phase difference between the return light from one of the pair of reflective optical arms and the return light from the other of the pair of reflective optical arms; 12. The optical measurement system of claim 1, wherein upon interferometric combination, the several interference combinations form several components of the response light.

13. 13. The optical measurement system of claim 1, further comprising at least one optical isolator (125) in a forward optical path through which the stimulation light (129) passes.

14. 14. An optical measurement system according to any one of claims 1 to 13, comprising at least one optical isolator (126, 127) in a return optical path through which the response light (132, 133) passes.

15. 15. The optical measurement system of claim 1, wherein the optical interrogator (103) comprises an optical modulator (138) configured to modulate the stimulation light (129).

16. 16. The optical measurement system of claim 15, wherein the optical modulator is configured to switch the stimulus light between at least three polarization states at a rate at least three times faster than the rate at which the physical quantity to be measured is sampled.

17. 16. The optical measurement system of claim 15, wherein the optical modulator (138) is configured to phase-modulate the stimulation light (129) with a modulation signal having a frequency that is at least an order of magnitude greater than the rate at which the physical quantity to be measured is sampled.

18. - said optical sensor assembly (102) forms part of a plurality of optical sensor assemblies; - said optical interrogator (103) comprises an optical splitter (128) configured to provide stimulation light (129) to said plurality of optical sensor assemblies; An optical measurement system according to any one of claims 1 to 17, wherein the optical interrogator (103) is arranged to individually detect response light from each optical sensor assembly from among the plurality of optical sensor assemblies.

19. Use of an optical measurement system (100 to 600) according to any of claims 1 to 18 for measuring a physical quantity.

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