System and method for optical strain measurement

Abstract

A position-dependent strain measurement system uses an interferometer to detect strain that varies with position in an optical sensing fiber, the interferometer having an input coupled to a first end of the optical sensing fiber. An electronic phase measurement subsystem is coupled to an output of the interferometer. The electronic phase measurement subsystem defines a range of available light intensity of input light to the interferometer in which the electronic phase measurement subsystem is able to measure a phase of the input light. An optically pumped fiber amplifier is coupled in series with an electrically pumped semiconductor optical amplifier between the first end of the optical sensing fiber and the input of the interferometer. The electrically pumped semiconductor optical amplifier has a nonlinear intensity amplification range that overlaps the range of available light intensity. The optically pumped fiber amplifier is configured to amplify intensity of backscattered light from the optical sensing fiber to be within the nonlinear intensity amplification range.

Description

[0001] This application claims priority to European Patent Application No. EP19219909.9, filed December 27, 2019. TECHNICAL FIELD

[0002] The present invention relates to a system and method for position-dependent measurement of strain in an optical sensing fiber from backscattering of light from a position within the optical sensing fiber. BACKGROUND

[0003] It is known to perform position-dependent strain change measurements using Rayleigh backscattered light from within an optical sensing fiber. Long sensing fibers (up to tens of kilometers) can be used for practical applications of strain change measurements, such as monitoring structural deformations of structures such as bridges, buildings, etc., or detecting intruders by a sensing fiber buried in the ground.

[0004] Rayleigh backscattering is due to randomly distributed scattering centers in the optical sensing fiber. When a light pulse is transmitted to one end of the sensing fiber, the distributed scattering of the pulse produces backscattered light at that end, whose phase and intensity fluctuate over time. The frequency of occurrence of different backscattered intensities is characterized by an intensity distribution that varies with the backscattered intensity. For the backscattered light at any given time, the time required for the pulse to travel a selected distance from the end of the sensing fiber is half of the time for the pulse to travel that distance.

[0005] In a sensing system, the local strain of the sensing fiber at that distance is determined by detecting changes in the optical phase difference between scattered light at different positions along the fiber. As long as the distance between the positions remains unchanged, the phase difference remains unchanged. However, if the distance changes due to strain of the fiber between the positions, the phase difference changes.

[0006] The optical phase difference between scattered light from pairs of different positions can be determined by feeding backscattered light from the optical sensing fiber along different lengths of optical path and detecting interference between light from these different optical paths. The single intensity resulting from such interference depends on the phase difference and the (fluctuating) scattering intensity. However, the optical phase difference can be computed by digitizing optoelectronic signals obtained from backscattered light along the optical paths, which include at least one interference signal between backscattered light along different optical paths. This digitization defines a range of available light intensity combinations from which the phase can be determined.

[0007] It is preferable to use a high sampling frequency to achieve a high spatial resolution (e.g. for a spatial resolution of about 1 m, a sampling frequency of about 100 MHz). However, the Rayleigh intensity is low. It is desirable to use commercially available optical communication fibres. However, the scattering in such fibres is small because it is intentionally minimised as scattering is an unwanted effect for communication purposes. In contrast, specially designed fibres are often used in which the scattering along the fibre is not very low.

[0008] In order to obtain intensities in the range of usable optical intensities, it is known to amplify the backscattered light and to make a phase measurement on the amplified backscattered light. A considerable amplification is required for this. The amplification can be provided by adding a pump fibre amplifier (e.g. an erbium-doped fibre amplifier (EDFA) or other optical pump light amplifier) between the optical sensing fibre and the interferometer. In general, EDFA is a standard technique for amplification in optical fibre applications because of the high gain and high output.

[0009] However, in practice this still does not enable a phase measurement to be made at all locations along the optical sensing fibre. At certain locations, the intensity can be outside the range of usable optical intensity combinations. Thus, as an example for a very long optical sensing fibre, the backscattered intensity from locations near the end of the optical sensing fibre can often drop below the range of usable optical intensity combinations. For measurements at a large number of locations along the optical sensing fibre, a control circuit can be used to increase the gain of the fibre amplifier for certain time intervals (where the intensity at the input of the interferometer rises above the range of usable optical intensities) and to decrease the gain of the fibre amplifier for certain time intervals (where the intensity at the input of the interferometer will drop below the range of usable optical intensities). However, it has been found that this merely improves the envelope of the Rayleigh scattering intensity along the fibre and still does not enable a phase measurement to be made at all locations along the optical sensing fibre. SUMMARY

[0010] It is an object of the present invention to provide a position-dependent strain measurement system in which a phase measurement is possible at a large number of locations along the optical sensing fibre.

[0011] There is provided a position-dependent strain measurement system according to claim 1. The system uses Rayleigh backscattering from within the optical sensing fibre to measure strain. In this context, the series arrangement of an optical pump fibre amplifier and an electrical pump semiconductor optical amplifier and / or a non-linear optical absorber before the separation of the interferometer optical paths is used to ensure that the coherent combination of intensities obtainable from the average intensity of the Rayleigh backscattered intensity at at least one detector will be within the usable range of the detector of the electronic phase measurement subsystem.

[0012] This can be done alternatively by electrical-pumped semiconductor optical amplifiers and / or nonlinear optical absorbers for the respective optical paths. When electrical-pumped semiconductor optical amplifiers and / or nonlinear optical absorbers for the respective optical paths are used, the optical pump fiber amplifiers can also be replaced by optical pump fiber amplifiers for the respective optical paths. The amplification factor of the optical pump fiber amplifiers can be set to ensure that the coherent combination is within the available range of the detector of the electronic phase measurement subsystem, for example.

[0013] At the available coherent combination required for a sufficiently reliable phase determination and its highest quantifiable intensity level, the detector defines an available range in terms of a predetermined minimum of quantization steps (e.g. at least fifty or one hundred quantization steps). It should be noted that this available range is only used to indicate the condition of the intensity that is obtainable by the nominal coherent combination of the amplified average Rayleigh backscattering intensity, but not other combinations of the actual intensity.

[0014] One or more electrical-pumped semiconductor optical amplifiers and / or one or more nonlinear optical absorbers are coupled between the one or more optical pump fiber amplifiers and the electronic phase measurement subsystem. Because this provides an optical intensity dependent derivative transfer factor that causes the higher intensity parts of the Rayleigh backscattering light intensity distribution to be compressed to the lower intensity parts of the Rayleigh backscattering light intensity distribution (as used herein, the optical intensity dependent derivative transfer factor is the derivative of the transmitted light intensity with respect to the input light intensity of the electrical-pumped semiconductor optical amplifier or the nonlinear optical absorber). As a result, a wider intensity fluctuation range of the Rayleigh backscattering light intensity can be used for detecting phase variations, thereby reducing the number of locations where a reliable phase measurement is not possible.

[0015] In embodiments, the amplification of the one or more optical pump fiber amplifiers is set such that the light intensity obtainable from the average intensity of the Rayleigh backscattering intensity at the input of the one or more electrical-pumped semiconductor optical amplifiers and / or the one or more nonlinear optical absorbers reaches an intensity at which the optical intensity dependent derivative transfer factor is less than half of the small-signal transfer factor of the one or more electrical-pumped semiconductor optical amplifiers and / or the one or more nonlinear optical absorbers. The derivative transfer factor can even be less than a quarter of the small-signal transfer factor, or less than a quarter of the small-signal transfer factor (as used herein, the small-signal transfer factor is the asymptotic value of the derivative of the optical intensity dependent derivative transfer factor when the intensity goes to zero). By amplifying the average to such a level, a significant extension of the lower intensity range can be achieved (in which a reliable phase measurement is possible).

[0016] The one or more electrically pumped semiconductor optical amplifiers and / or one or more nonlinear optical absorbers can be characterized by a saturated optical output intensity (i.e., the asymptotic value of the output intensity in the limit of infinite input intensity, or the highest value of the output intensity reached for any admissible input intensity). The saturated optical output intensity can be configured to result in an intensity from each optical path at the at least one detector that is equal to or less than one quarter of the highest intensity of the usable range. Thus, the intensity of the coherent sum of the light having such intensities does not exceed the saturation value. This ensures that high backscattered intensities do not impede accurate phase measurements.

[0017] In an embodiment, the electronic phase measurement subsystem comprises an N-way coupler, where N is at least 3, configured to form N combinations of light from the first and second optical paths having N different relative phase shifts, N detectors configured to measure the intensity of a respective one of the N combinations, and a computing system configured to compute the phase difference between the light from the first and second optical paths from the measured intensities. Assuming that the maximum possible intensity of such a combination is within the usable range, such a subsystem can eliminate the effect of the intensity of the light in the optical paths on the phase determination.

[0018] In long optical fibers, the intensity can decrease significantly as the distance from the input end of the optical fiber and the scattering point increases. In an embodiment, the average is an average of the backscattered intensity from a distal portion of the optical fiber. By setting the amplification to ensure an average backscattered intensity of this distal portion (e.g., in the last hundred meters of the optical fiber), the number of points where a reliable phase measurement is not possible can be reduced.

[0019] In an embodiment, the measurement system comprises a gain control circuit configured to vary the amplification factor of the one or more optically pumped fiber amplifiers from the optical pulses over time such that the time-dependent average of the Rayleigh backscattered intensity at the electronic phase measurement subsystem remains within the usable intensity range. The gain control circuit can provide, for example, an amplification factor having a predetermined time dependence, or provide feedback control based on detection of the time-dependent average.

[0020] In an embodiment, the system comprises means for amplifying the two polarization components of the output light of the optically pumped fiber amplifier with a semiconductor optical amplifier or a combination of a semiconductor optical amplifier and another semiconductor optical amplifier, respectively, before entering the interferometer. This increases the signal-to-noise ratio.

[0021] In an embodiment, the apparatus includes a polarization splitter having an input coupled to an output of the optically pumped fiber amplifier, a first polarization output coupled to an input of the semiconductor optical amplifier; a polarization rotator having an input coupled to a second polarization output of the polarization splitter; another electrically pumped semiconductor optical amplifier having an input coupled to an output of the polarization rotator; and an optical combiner having inputs coupled to outputs of the electrically pumped semiconductor optical amplifier and the other electrically pumped semiconductor optical amplifier, the optical combiner having an output coupled to the interferometer. In another embodiment, the system includes a polarization splitter having an input coupled to an output of the optically pumped fiber amplifier; a polarization rotator having an input coupled to a first polarization output of the polarization splitter; and an optical combiner having inputs coupled to a second polarization output of the polarization splitter and an output of the polarization rotator, the optical combiner having an output coupled to an input of the semiconductor optical amplifier.

[0022] In an embodiment, the optically pumped fiber amplifier is a transient- suppressed fiber amplifier. This prevents problems due to sudden increases in backscattered intensity from the input end of the fiber. BRIEF DESCRIPTION OF DRAWINGS

[0023] These and other objects and advantageous aspects of the present application will become apparent upon reading the following description of exemplary embodiments, with reference to the accompanying drawings.

[0024] Figure 1 Figure la shows a strain change measurement system;

[0025] Figure 2 Figure shows Rayleigh backscattered intensity and response of a nonlinear optical amplifier;

[0026] Figure 3 Figure shows a dual semiconductor optical amplifier configuration;

[0027] Figure 3a Figure shows a semiconductor optical amplifier prior to a polarization equalizer. DETAILED DESCRIPTION

[0028] Figure 1 and Figure 1aA strain change measurement system is shown, which comprises a pulsed light source 10, an optical sensing fiber 12, a fiber amplifier unit 14, a semiconductor optical amplifier 16 and an interferometric measurement subsystem 18. The output of the pulsed light source 10 is coupled to a first end of the optical sensing fiber 12. The first end of the optical sensing fiber 12 is further coupled to an input of the fiber amplifier unit 14 to amplify light returning from within the optical sensing fiber 12 through the first end of the optical sensing fiber 12. As the pulsed light source 10 and the returning light propagate in opposite directions along the same optical path, an optical coupling device in this optical path is used to direct part or all of the returning light to the fiber amplifier unit 14. As an example, for this purpose an optical circulator 11 is shown between the pulsed light source 10 and the optical sensing fiber 12, but alternatively another type of coupler, such as a splitter, can be used.

[0029] The fiber amplifier unit 14 can comprise one or more fiber amplifiers in series, such as a low noise preamplifier and a further fiber amplifier. The output of the fiber amplifier unit 14 is coupled to an input of the semiconductor optical amplifier 16. The output of the semiconductor optical amplifier 16 is coupled to the interferometric measurement subsystem 18.

[0030] Preferably, the fiber amplifier unit 14 is an optically pumped amplifier, such as an EDFA or a YDFA, which comprises a pump light source configured to provide pump light to a doped region in a fiber section through which backscattered light from the optical sensing fiber 12 propagates to the semiconductor optical amplifier 16.

[0031] While the semiconductor optical amplifier 16 is an electrically pumped amplifier, it is preferably implemented on a planar substrate, in which case the pump signal is provided by an electronic circuit. When the semiconductor optical amplifier 16 is implemented on a planar substrate, the measurement system comprises a fiber-to-substrate interface between the semiconductor optical amplifier 16 and the fiber amplifier unit 14, and preferably a fiber-to-substrate interface between the semiconductor optical amplifier 16 and the interferometric measurement subsystem 18 or parts thereof.

[0032] In an electrically pumped semiconductor optical amplifier, the amplification depends on the population of states that provide stimulated emission. This population decreases with increasing intensity of the amplified light due to a change in the balance between pumping and emission. Typically, an electrically pumped semiconductor optical amplifier cannot output more than a predetermined maximum light intensity level, and its nonlinearity can further be characterized by a half of the predetermined maximum light intensity level at its input, which results in a half of the predetermined maximum light intensity level at its output.

[0033] Instead of the semiconductor optical amplifier 16, a nonlinear absorber or a series connection of a semiconductor optical amplifier and a nonlinear absorber can be used. Such embodiments can be used in a similar way as the semiconductor optical amplifier 16, but with the nonlinear absorber providing the nonlinearity instead of the semiconductor optical amplifier 16. Figure 1which is illustrated in the figure by replacing item 16 in the optical path by a nonlinear absorber or a semiconductor optical amplifier and a nonlinear absorber in series in the optical path.

[0034] One example of an effect that contributes linearly to absorption is two-photon absorption, which involves the absorption of two photons for a single absorption transition of the absorber. This requires the availability of two photons, and thus increases with the availability of photons. It depends on the square of the intensity of the incident light at the absorber. See (see https: / / en.wikipedia.org / wiki / Two-photon_absorption) and J Almeida et al. in Third-order nonlinear spectra and optical limiting of lead oxifluoroborate glasses, Optics Express 17221 (Vol. 19, No. 18).

[0035] In other examples, the nonlinearity of the absorption by the nonlinear absorber can depend on the number of certain states that are occupied. When the increased light intensity changes the number of occupied states, making more absorption processes possible, this can lead to an increase in the absorption that is nonlinear in the light intensity.

[0036] In operation, the pulsed light source 10 repeatedly transmits light pulses into the optical sensing fiber 12 and detects changes in strain by comparing measurements of the returned light that is received with the same time delay due to backscattering of light from the pulses transmitted at different points in time.

[0037] Such measurements are known per se. As a reference, such a method will be briefly described. Due to the distributed Rayleigh scattering within the optical sensing fiber 12, the return time required for each returned light pulse is twice the time required for the light to propagate between the ends of the optical sensing fiber 12.

[0038] The interferometric measurement subsystem 18 measures the interference between light returned from a pair of spaced-apart locations within the optical sensing fiber 12, preferably the phase difference between light returned from a pair of spaced-apart locations within the optical sensing fiber 12, and repeats this process for successive pairs of locations. Each of the successive pairs of locations includes a first location and a second location a distance from the first location. This results in a measurement of the interference signal for successive time delays from the transmission of a light pulse into the optical sensing fiber 12 to the return of backscattered light from the first location of the successive pairs of locations. When a strain occurs in the optical sensing fiber 12 between a pair of locations, this will result in a change in the fiber length and thus a change in the interferometric measurement for that pair. The interferometric measurement subsystem 18 compares the measurements obtained using light pulses transmitted at different points in time to detect such changes in strain.

[0039] Most commercially available optical fibers are developed to minimize transmission loss. Rayleigh backscattering is therefore very low. The Rayleigh backscattering coefficient for a standard commercial telecom optical fiber (e.g. Corning SMF-28 Ultra) is about -80 dB. The optical intensity of the backscattered signal is very low, typically on the order of -40 dBm for a Rayleigh scattering based distributed dynamic strain measurement system. In order to obtain a detectable interference signal, an optical fiber amplifier unit 14, such as a light pumped EDFA or YDFA, can be used to significantly amplify the optical intensity of the backscattered signal, e.g. by 30 to 40 dB. The pulsed light source 10 can be configured to produce light at a wavelength of 1550 nm or 1064 nm to best utilize such optical fiber amplifiers. In practice, when used for such large amplification, such optical fiber amplifiers add noise, significantly reducing the signal to noise ratio. By placing a low noise optical fiber amplifier with a lower amplification factor in front, a higher signal to noise ratio can be maintained.

[0040] Any type of interferometric measurement subsystem 18 can be used. Figure 1 An embodiment that measures a simple two-signal interference is shown. This embodiment of the interferometric measurement subsystem 18 includes a first optical path 180a and a second optical path 180b, an optical combiner 181, a photodetector 185, processing circuitry 188, and a memory 189. The photodetector 185 includes an analog to digital converter (as used herein, this encompasses the possibility that the analog to digital converter can be referred to as part of the processing circuitry 188 and convert the signal from the photodetector). The interferometric measurement subsystem 18 is configured to distribute the amplified light from the semiconductor optical amplifier 16 onto the first optical path 180a and the second optical path 180b, e.g. by a splitter in the interferometric phase measurement subsystem 18. The outputs of the first optical path 180a and the second optical path 180b are coupled to the optical combiner 181.

[0041] The optical path lengths from the semiconductor optical amplifier 16 to the first and second inputs of the three-way coupler 184 via the first optical path 180a and the second optical path 180b differ by, for example, one or more meters. As an example, a delay element 182, for example a fiber coil, is shown in the second optical path 180b to provide the difference in optical path lengths.

[0042] In operation, the optical combiner 181 combines light from the first optical path 180a and the second optical path 180b, between which light along the different optical paths interferes. The photodetector 185 produces a measurement of the intensity, and the processing circuitry 188 is configured to read out the intensity and calculate the phase difference. The processing circuitry 188 has inputs coupled to the outputs of the photodetectors 186a-c. For accurate measurement of the strain position, a high measurement bandwidth of, for example, at least 100 MHz is desirable. The photodetector 185 performs an analog-to-digital conversion. The processing circuitry 188 can store the intensity values from different delays of the optical pulse transmission in a memory 189.

[0043] Figure 1a An embodiment is shown in which the phase difference is calculated rather than the direct interference intensity. This embodiment includes a three-way coupler 184 rather than an optical combiner 181, and a first photodetector 186a, a second photodetector 186b, and a third photodetector 186c. The outputs of the first optical path 180a and the second optical path 180b are coupled to the first and second inputs of the three-way coupler 184. The first, second, and third outputs of the three-way coupler 184 are coupled to the first, second, and third photodetectors 186a-c. The photodetectors 186a-c include analog-to-digital converters. The processing circuitry 188 has inputs coupled to the outputs of the photodetectors 186a-c. The processing circuitry 188 is further coupled to a memory 189.

[0044] In operation, the three-way coupler 184 collects light from the first optical path 180a and the second optical path 180b and outputs it at different outputs of the three-way coupler 184 with different relative phase shifts. At each of the outputs of the three-way coupler 184, the light from the different optical paths interferes, and this results in an optical intensity at the output that is in accordance with the absolute square of the sum of the wave vectors at the output, which depends on the individual intensities of the light from the first optical path 180a and the second optical path 180b, their mutual polarization states, and their phase difference. Since the relative phase shifts imparted by the three-way coupler 184 are known, it is possible to mathematically cancel the individual intensities and visibility and solve for the phase difference from the intensities at the three outputs.

[0045] Although a preferred embodiment is shown in which the same fiber-optic amplifier unit 14 and the same semiconductor optical amplifier 16 are used to amplify the intensities of the first 180a and second 180b optical paths, it should be noted that each optical path 180a,b can have its own semiconductor optical amplifier 16 before or after the optical path. Such semiconductor optical amplifiers 16 can have the same amplification characteristics, but this is not necessary. Instead of or in addition to the semiconductor optical amplifiers 16, each optical path 180a,b can have its own linear absorber. When each optical path 180a,b has its own semiconductor optical amplifier 16 and / or nonlinear absorber, it can also have its own (partial) fiber-optic amplifier unit 14 before the semiconductor optical amplifier 16 and / or nonlinear absorber. Such own (partial) fiber-optic amplifier unit 14 can be located before or after the optical path.

[0046] The photodetectors 186a-c produce digitized measurements of these intensities, and the processing circuitry 188 is configured to compute the phase difference. For an accurate measurement of the strain location, a high measurement bandwidth of at least 100 MHz is desirable. The processing circuitry 188 can be configured to compute the phase difference in real time as the measurements arrive, but this is not necessary: instead, the measurements from the different delays of the optical pulse transmission, or intermediate results, from the photodetectors 186a-c can be stored in a memory 189, and the phase difference can be computed later. The processing circuitry 188 is configured to compare the phase difference computed from the backscatter of the first optical pulse with the phase difference computed from the backscatter of a different second optical pulse of the same delay of the first and second optical pulses. The result of this comparison can be used to detect the location of a change in strain along the optical sensing fiber 12. This comparison and / or the location detection can be performed in the processing circuitry 188 or another computer or computers (not shown) programmed to do so.

[0047] Although embodiments have been described by way of example with a three-way coupler 184, it should be noted that other methods can be used, for example using different types of interferometers. Also, other embodiments using a three-way coupler 184 are possible. For example, embodiments can be used with photodetectors on different sides of the three-way coupler 184 and / or optical paths on opposite sides of the semiconductor optical amplifiers.

[0048] It is advantageous to use phase measurements rather than just interference intensities, because the phase change is only linearly related to the length change, whereas the interference intensity is also related to amplitude changes. The amplitude can depend in an unpredictable way on the scattering location. The use of computed phase values makes quantitative strain change measurements possible.

[0049] In each case, a digital measurement of the intensity of at least one of the mixtures of light from the first optical path 180a and the second optical path 180b and other digital measurements of other intensities is required with sufficient precision to calculate the phase even if the amplitudes in 180a,b can unpredictably change. Thus, the analog-to-digital conversion defines the usable range of intensities that can be obtained by the coherent combination of light on the detector. If the intensity obtained by the coherent combination would have an intensity that exceeds the highest intensity value that the analog-to-digital converter can represent as a digital result, or if the coherent combination is so small that it is below the threshold number of quantization steps (e.g., too few significant bits) of the analog-to-digital converter for the accurate representation of the size, then the light from the optical path is outside the usable range. For example, if the signal is at least fifty (preferably one hundred) quantization steps above the lowest quantization level, then the signal is within the usable range (e.g., so that at least six or seven bits of the quantization result are significant for the representation of the size).

[0050] The usable range defines the usable light intensity range of the nominal coherent combination at the input of the photodetector 185 or 186a-c that would result in a signal that is within the usable analog-to-digital conversion range. Only when the sum of the squares of the absolute sizes of the wave vectors of the light from the first optical path 180a and the second optical path 180b is within the usable intensity range of the analog-to-digital converter 186a-c can a reliable calculation be made. It should be noted that an intensity within the usable range does not guarantee a reliable phase measurement. For example, if the scattering intensity at one location is zero or too small, this is not possible.

[0051] Figure 2 The problem of Rayleigh backscatter measurements is illustrated. The backscatter intensity fluctuates rapidly with position along the optical sensing fiber 12. This depends on the density of scattering centers as a function of position and the way they add up their scattering phases. This produces a probability density 20 of the backscatter intensity. The backscatter intensity can be amplified so that the range 22 of its intensity fluctuations overlaps with the usable intensity range 24 at the analog-to-digital converter 186a-c. But the range of fluctuations is too large for the amplification to fit the range 22 of fluctuations within the usable intensity range 24. When the intensity is amplified so that the highest value of the range of fluctuations does not exceed the usable intensity range, the minimum backscatter intensity of the amplification remains below the usable intensity range. Conversely, when the intensity is amplified so that the bottom of the range of fluctuations is within the usable intensity range, the highest value of the range of fluctuations of the amplification is above the highest value of the usable intensity range. In both cases, an accurate phase change measurement is not possible.

[0052] This problem is solved by including a semiconductor optical amplifier 16 or a nonlinear absorber in the nonlinear operating range between the fiber amplifier unit 14 and the interferometric phase measurement subsystem 18. The semiconductor optical amplifier 16 provides a nonlinear response relationship 26 between the light intensity at its input and the light intensity at its output. The same applies to the nonlinear absorber. The intensity variation of the amplified light due to the intensity variation of the incident light decreases with increasing incident light intensity. This has the effect of compressing the fluctuation range 22 to a compressed output range and thus reduces the minimum portion of the backscattered intensity that remains outside the available intensity range when the amplification of the fiber amplifier unit 14 is chosen to fit as much of the fluctuation range as possible into the available intensity range. The nonlinear response relationship 26 of the semiconductor optical amplifier 16 has the effect that the derivative of the transmitted light intensity with respect to the input light intensity of the electrically pumped semiconductor optical amplifier decreases with increasing input intensity. Therefore, the higher intensity range will be compressed to a smaller extent relative to the lower intensity range. The same applies to the nonlinear absorber. Furthermore, the semiconductor optical amplifier 16 has the effect that the input intensity is not amplified to an output intensity that is higher than a predetermined saturation output intensity. Preferably, the saturation intensity of the semiconductor optical amplifier 16 caused by the light from each individual optical path at the detector is equal to or less than one quarter of the highest intensity level that is measurable by the detector. This ensures that the interference of the two signals does not exceed this highest intensity level.

[0053] In principle, the fiber amplifier unit 14 can be used alone or extended to compress the fluctuation range. Like the semiconductor optical amplifier 16, it has a nonlinear response characteristic, but this depends on the consumption of the energy provided by the optical pumping, which occurs more slowly than the compression effect of the semiconductor optical amplifier 16. As a result, fast backscattering fluctuations cannot be kept within the available intensity range. The same applies to the feedback control of the gain fiber amplifier unit 14 using the pump energy of the fiber amplifier unit 14.

[0054] In an embodiment, the system comprises a control circuit configured to change the optical amplification factor of the fiber amplifier unit 14 as a function of time starting from the time point of the light pulse. A predetermined time dependence of the amplification factor can be used to compensate for the overall decrease in the received Rayleigh scattering intensity with time due to the increasing distance along the fiber from which the received scattering originates.

[0055] The functionality of the control circuit can be performed by, for example, the processing circuit 188 or any other suitable circuit. The overall decrease of the Rayleigh scattered intensity over time, also referred to as the envelope, received from the optical pulse can be determined, for example, by low-pass filtering the Rayleigh scattered intensity over time. The variable amplification factor can be chosen to be inversely proportional to the envelope without the variable amplification factor. Alternatively, a feedback circuit can be used to control the amplification factor, using the difference between the average intensity measured by the measurement circuit and the reference to control the amplification factor.

[0056] The combination of the non-linear amplification or absorption and the fiber amplifier unit 14, which then acts as an optical amplification factor, can be used to ensure that the relative position of the intensity profile at the output of the fiber amplifier unit 14 and the compression range of the non-linear semiconductor optical amplifier 16 or non-linear absorber remains the same over time from the point in time of the optical pulse.

[0057] This increases the signal-to-noise ratio and reduces the risk of damage or blindness. Likewise, it is preferred that the amplification factor of the fiber amplifier unit 14 is set such that the peak of the probability density 20 of the backscattered intensity and / or its average probability density 20 at the output of the fiber amplifier unit lies within the compression range.

[0058] For long optical fibers, the scattering position-dependent probability density 20 of the Rayleigh scattering can be defined, and preferably the peak and / or the average of the Rayleigh scattering probability density 20 at the end of the fiber.

[0059] Similar considerations apply to the non-linear absorber. Here, the compression range can be defined in terms of the intensity, wherein the input / output ratio of the non-linear absorber is less than half the input / output ratio for small intensities (within the limit of the intensity going to zero).

[0060] In some cases, using a semiconductor optical amplifier 16 in series with a fiber amplifier unit can pose a risk of damage. When a fiber amplifier unit 14 is pumped without significant input intensity being provided, for example before a light pulse from the pulsed light source 10, the fiber amplifier unit 14 can produce a temporary high gain once more input intensity is suddenly provided to its input, for example due to backscattering shortly after the light pulse. This high gain results in high intensity amplified light, which can damage the semiconductor optical amplifier 16. This can be prevented by, for example, using a transiently inhibited fiber amplifier, for example as disclosed in US20100221012 or as proposed by Shiraiwa (Shiraiwa M. et al., “Performance evaluation of a burst-mode EDFA in an optical packet and circuit integrated network”, Optics Express, Vol. 21, No. 26, DOB 10.1364 / OE:21.032589, 2013).

[0061] Another solution is to include a shutter between the fiber amplifier unit 14 and the semiconductor optical amplifier 16, and a control circuit that closes the shutter during the interval in which high intensity amplified light occurs after a light pulse. Another solution is to configure the optical pumping of the fiber amplifier unit 14 so that it starts pumping later relative to the light pulse, thereby preventing or reducing the temporary high gain. However, such solutions can make strain detection at the front of the optical sensing fiber 12 more difficult. The use of a transiently inhibited fiber amplifier makes it possible to measure more strain in the optical sensing fiber 12.

[0062] It is preferred that the semiconductor optical amplifier 16 is implemented in a planar semiconductor device, while the fiber amplifier unit 14 and the delay element 182 are implemented in optical fiber. However, planar semiconductor optical amplifiers 16 typically only amplify one polarization component, which can reduce the part of the location where the optical sensing fiber 12 is located, because Rayleigh backscattering produces randomly fluctuating polarization. To increase the part of the location, a double planar semiconductor optical amplifier or other configuration (for example a semiconductor optical amplifier with suppressed polarization sensitivity) can be used to address the polarization dependence of the semiconductor optical amplifier SOA.

[0063] Figure 3A dual-plane semiconductor optical amplifier 16 is shown, including a polarization splitter 30, a polarization rotator 32a, a polarization rotator 32b, a first semiconductor optical amplifier 34a and a second semiconductor optical amplifier 34b, and an optical combiner 36. An optical path from an output of a fiber amplifier unit (not shown) to an input of the polarization splitter 30 provides amplified backscattered light to the polarization splitter 30. Additional optical paths from an output of the polarization splitter 30 to the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b provide different polarization components of the light to the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b, respectively, through polarization rotators 32a, 32b in the additional optical paths.

[0064] The first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b can be implemented together on a single planar substrate, so that they amplify light primarily having a single polarization direction. The polarization rotators 32a, 32b rotate the polarization direction of the polarization components from the polarization splitter 30 to the polarization direction amplified by the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b, respectively. The polarization rotators in one of the additional optical paths can be sufficient, or can be completely unnecessary when the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b are oriented so that they amplify different polarization directions. The output optical path from the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b feeds the amplified polarization components from the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b to the optical combiner 36. The optical combiner 36 is configured to combine the amplified light from the first semiconductor optical amplifier 34a and the second semiconductor optical amplifier 34b and provide the combination as an output of the dual-plane semiconductor optical amplifier.

[0065] Figure 3a Another possible configuration is shown, in which the optical combiner 36 is moved after the combination of the polarization splitter 30 and the polarization rotators 32a, b to combine the optical power in both polarization directions into a single linearly polarized output and feed it to the semiconductor optical amplifier 34. In this embodiment, the single linearly polarized output has at least a portion in the direction in which the semiconductor optical amplifier 34 provides maximum gain, e.g., with a deviation from the direction of no more than forty-five degrees or thirty degrees. Preferably, the polarization direction of the single linearly polarized output is the direction in which the semiconductor optical amplifier 34 provides maximum gain.

[0066] With such embodiments, the interferometric phase measurement subsystem 18 is provided with amplified light having a compressed intensity range of both polarization components of the backscattered light from the optical sensing fiber.

[0067] In another embodiment, the following apparatus can be used to measure the interference of the polarization components using two interferometers: a polarization splitter having an input coupled to an output of an optically pumped fiber amplifier, a first polarization output coupled to an input of a semiconductor optical amplifier; a polarization rotator having an input coupled to a second polarization output of the polarization splitter; another electrically pumped semiconductor optical amplifier having an input coupled to an output of the polarization rotator; another interferometer having an input coupled to a first end of an optical sensing fiber; and another electronic phase measurement subsystem coupled to an output of the other interferometer.

Claims

1. A position-dependent strain measurement system for measuring strain using Rayleigh backscatter from within an optical sensing fiber, wherein, The frequency of occurrence of different backscattering intensities is characterized by an intensity distribution that varies as a function of backscattering intensity, the measuring system comprising: a pulsed light source; an optical sensing fiber, a first end of which is coupled to an output of the pulsed light source; an interferometer comprising a beamsplitter, an input of which is coupled to the first end of the optical sensing fiber, and a first optical path and a second optical path of different optical lengths, inputs of which are coupled to respective outputs of the beamsplitter; an electronic phase measurement sub-system coupled to outputs of the first and second optical paths, and comprising at least one light intensity detector configured to detect an interference intensity of a combination of light from the first and second optical paths, each of the at least one light intensity detector defining an available intensity range for a nominally coherent combination of intensities of light from the two optical paths; a first optical pump fiber amplifier coupled between the first end of the optical sensing fiber and the electronic phase measurement sub-system, the first optical pump fiber amplifier being connected in series with the first and second optical paths, respectively; or a first optical pump fiber amplifier and a second optical pump fiber amplifier coupled between the first end of the optical sensing fiber and the electronic phase measurement sub-system, the first and second optical pump fiber amplifiers being connected in series and connected in series with the first and second optical paths, respectively; The first electrically pumped semiconductor optical amplifier is connected in series between the first optical pump fiber amplifier and the electronic phase measurement subsystem and in series with the first optical path and the second optical path, respectively; or the first nonlinear optical absorber is connected in series between the first optical pump fiber amplifier and the electronic phase measurement subsystem and in series with the first optical path and the second optical path, respectively; or the first electrically pumped semiconductor optical amplifier and the first nonlinear optical absorber are connected in series and are connected as a whole between the first optical pump fiber amplifier and the electronic phase measurement subsystem and in series with the first optical path and the second optical path, respectively; or the first electrically pumped semiconductor optical amplifier and the second electrically pumped semiconductor optical amplifier are connected in series and are connected as a whole between the first optical pump fiber amplifier and the electronic phase measurement subsystem and in series with the first optical path and the second optical path, respectively; or the first nonlinear optical absorber and the second nonlinear optical absorber are connected in series and are connected as a whole between the first optical pump fiber amplifier and the electronic phase measurement subsystem and in series with the first optical path and the second optical path, respectively; or the first electrically pumped semiconductor optical amplifier is connected in series with the first nonlinear optical absorber, and the second electrically pumped semiconductor optical amplifier is connected in series with the second nonlinear optical absorber, and the two are connected in series and are connected as a whole between the first optical pump fiber amplifier and the electronic phase measurement subsystem and in series with the first optical path and the second optical path, respectively; or the first electrically pumped semiconductor optical amplifier and the second electrically pumped semiconductor optical amplifier are connected in series and are connected as a whole between the electronic phase measurement subsystem and the first optical pump fiber amplifier and the second optical pump fiber amplifier and in series with the first optical path and the second optical path, respectively; or the first nonlinear optical absorber and the second nonlinear optical absorber are connected in series and are connected as a whole between the electronic phase measurement subsystem and the first optical pump fiber amplifier and the second optical pump fiber amplifier and in series with the first optical path and the second optical path, respectively; or the first electrically pumped semiconductor optical amplifier is connected in series with the first nonlinear optical absorber, and the second electrically pumped semiconductor optical amplifier is connected in series with the second nonlinear optical absorber, and the two are connected in series and are connected as a whole between the electronic phase measurement subsystem and the first optical pump fiber amplifier and the second optical pump fiber amplifier and in series with the first optical path and the second optical path, respectively; wherein one or both optical pump fiber amplifiers and one or both electrically pumped semiconductor optical amplifiers and / or one or both nonlinear optical absorbers provide a plurality of amplification factors that ensure that the coherent combination of intensities available at the at least one optical intensity detector from the average intensity of Rayleigh backscattered intensity will be within the available intensity range; and wherein one or more optical intensity dependent derivative transmission factors of the first electrically pumped semiconductor optical amplifier and / or the first nonlinear optical absorber cause higher intensity portions of the intensity profile to be compressed to the intensity profile relative to lower intensity portions, or wherein multiple optical intensity dependent derivative transmission factors of the first electrically pumped semiconductor optical amplifier and the second electrically pumped semiconductor optical amplifier and / or the first nonlinear optical absorber and the second nonlinear optical absorber cause higher intensity portions of the intensity profile to be compressed to the intensity profile relative to lower intensity portions.

2. The measurement system of claim 1, wherein, The amplification of one or both optical fiber amplifiers is set such that the optical intensity obtainable from the average intensity of the Rayleigh backscattered intensity at the input of one or both electrically pumped semiconductor optical amplifiers and / or one or both nonlinear optical absorbers reaches an intensity at which the optical intensity dependent derivative transmission factor is less than half of the small signal transmission factor of one or both electrically pumped semiconductor optical amplifiers and / or one or both nonlinear optical absorbers.

3. The measurement system of claim 1 or 2, wherein, The electrically pumped semiconductor optical amplifier and / or the nonlinear optical absorber is characterized by a saturation optical output intensity, and wherein the saturation optical output intensity is configured to cause the intensity from each optical path at the at least one optical intensity detector to be equal to or less than one quarter of the highest intensity of the available intensity range.

4. The measurement system of claim 1 or 2, wherein, The electronic phase measurement subsystem comprises an N-way coupler, where N is at least 3, configured to form N combinations of light from the first optical path and the second optical path with N different relative phase shifts, N detectors configured to measure the intensity of a respective one of the N combinations, and a computing system configured to compute a phase difference between light from the first optical path and the second optical path from the measured intensities.

5. The measurement system of claim 1 or 2, wherein, The average intensity is an average of backscattered intensity from a distal portion of the optical fiber.

6. The measurement system of claim 1 or 2, comprising a gain control circuit configured to vary one or both optical fiber amplifier amplification factors over time from an optical pulse such that a time-dependent average of the Rayleigh backscattered intensity at the electronic phase measurement subsystem remains within the available intensity range.

7. The measurement system of claim 1 or 2, wherein, The electrically pumped semiconductor optical amplifier provides amplification of only a single polarization component, the measurement system comprising: a polarization splitter having an input coupled to an output of an optically pumped fiber amplifier, a first polarization output of the polarization splitter coupled to an input of an electrically pumped semiconductor optical amplifier; a polarization rotator having an input coupled to a second polarization output of the polarization splitter; another electrically pumped semiconductor optical amplifier having an input coupled to an output of the polarization rotator; and an optical combiner having inputs coupled to outputs of the electrically pumped semiconductor optical amplifier and the other electrically pumped semiconductor optical amplifier, an output of the optical combiner coupled to the interferometer.

8. The measurement system of claim 1 or 2, wherein, The electrically pumped semiconductor optical amplifier provides amplification of only a single polarization component, the measurement system comprising: a polarization splitter having an input coupled to an output of an optically pumped fiber amplifier; a polarization rotator having an input coupled to a first polarization output of the polarization splitter; and an optical combiner having inputs coupled to a second polarization output of the polarization splitter and an output of the polarization rotator, an output of the optical combiner coupled to an input of an electrically pumped semiconductor optical amplifier.

9. The measurement system of claim 1 or 2, wherein, The electrically pumped semiconductor optical amplifier provides amplification of only a single polarization component, the measurement system comprising: a polarization splitter having an input coupled to an output of an optically pumped fiber amplifier, a first polarization output of the polarization splitter coupled to an input of an electrically pumped semiconductor optical amplifier; a polarization rotator having an input coupled to a second polarization output of the polarization splitter; another electrically pumped semiconductor optical amplifier having an input coupled to an output of the polarization rotator; another interferometer having an input coupled to the first end of the optical sensing fiber; and another electronic phase measurement subsystem coupled to an output of the other interferometer.

10. The measurement system of claim 1 or 2, wherein, The optically pumped fiber amplifier is a transiently suppressed fiber amplifier.

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