Patent Application
20250093184
The invention relates to an optical measurement system, for performing measurements on an optical path such as an optical fibre.
There exist various techniques for measuring properties of an optical fibre using pulsed laser light transmitted along the optical fibre. These techniques make use of different scattering mechanisms within the optical fibre.
Distributed Acoustic Sensing (DAS) is an established technology with several commercial systems available. In these systems, a pulse or pulses of laser light are launched into a length of optical fibre and the light that is scattered within the fibre is analysed in order to derive the nature of the acoustic environment, i.e. any physical vibrations, of the fibre transducer. In particular, these systems typically make a measurement of the acoustic strain environment of an optical fibre transducer using an optical time domain reflectometer (OTDR) approach. This gives a differential strain measurement as a function of position along the optical fibre.
As an optical fibre is manufactured it is cooled or quenched from a high temperature as it is drawn. This process leads to the presence of small variations in the density of the optical fibre. These tiny variations in density equate to variations in the effective refractive index of the fibre. These discontinuities lead to scattering of laser light passing through the optical fibre, particularly by Rayleigh scattering. The amplitude of the scattering follows a Rayleigh distribution, but the phase angle of the scattering is uniformly distributed around a unit circle, i.e. −π≤Φ≤π where Φ is the phase angle.
Rayleigh scattering is an elastic scattering mechanism, such that the frequency of the scattered light is the same as the pulse of laser light used to probe the optical fibre. However, since the probe pulse is highly coherent, the scattered light interferes with itself as it scatters along the optical fibre. The bandwidth required to detect and fully utilise the Rayleigh scatter for DAS sensing is inversely proportional to the pulse duration. For typical applications, this bandwidth is of the order of 30 MHz to 200 MHZ.
Distributed Temperature Sensing (DTS) is a technique for measuring temperature along an optical fibre using a laser pulse which is transmitted along the optical fibre. DTS typically involves detecting Raman scattering of the laser pulse in the optical fibre. Unlike Rayleigh scattering, Raman scattering is an inelastic mechanism. When a pulse propagates along the fibre, a small portion of the incident photons interact with molecular bonds of the material constituting the fibre. These photons can excite vibrations of the molecular bonds of the material. These vibrating molecules will then scatter the photon, typically to a lower energy with the emission of an optical phonon. This scattering mechanism is termed Stokes Scattering. However, if a photon interacts and scatters from an already excited molecule, then it may scatter by absorbing a phonon and thus scatter to a higher energy. This is termed Anti-Stokes Scattering.
The frequency shift associated with Raman scattering is dependent on the material from which the fibre is constructed. For example, for Silica fibres, the frequency shift is typically of the order of 13 THz, which for a probe pulse at 1550 nm results in a wavelength shift of the order of 100 nm. The bandwidth of the scattering mechanism is also very wide, typically greater than 10 THz.
Raman scattering can be utilised for temperature sensing, due to the fact that the probability of Anti-Stokes scattering (i.e. scattering to a higher energy) is highly temperature dependent. In particular, the ratio of Stokes/Anti-Stokes scattering is related to the temperature of the fibre, such that the temperature of the fibre can be determined by measuring the Stokes/Anti-Stokes ratio.
Distributed Strain and Temperature Sensing (DSTS) is a technique that enables simultaneous measurement of temperature and strain in an optical fibre, using a laser pulse which is transmitted along the optical fibre. The scattering mechanism of interest for DSTS is Brillouin scattering. Like Raman scattering, Brillouin is an inelastic scattering process. However, instead of involving scattering from optical band phonon from vibrations of the chemical bonds, this Brillouin is due to photons interacting with and scattering from acoustic band phonons created by the lattice vibrations of the fibre material. The phonon energy is therefore much lower than in Raman scattering and hence the Stokes shift is also much lower, typically at around 10 to 11 GHZ.
Unlike Raman scattering, the amplitude of the Stokes and Anti-Stokes components for Brillouin scattering are approximately equal, at least in the spontaneous domain. At higher launch powers, Brillouin scattering can be stimulated whereby the Stokes component is amplified relative to the Anti-Stokes. Since this scattering is related to lattice vibrations, both the Stokes shift and the spontaneous amplitude of the Stokes and Anti-Stokes emission are related to the strain experienced by the fibre and the absolute temperature of the fibre. Signals related to temperature and strain of the optical fibre can be separated by measuring the frequency and amplitude of the Anti-Stokes emission.
At its most general, the present invention involves implementing DAS, together with DTS and/or DSTS using a single measurement system. In particular, the inventors have realised that it is possible to perform DAS, DTS and DSTS measurements on a same optical fibre and using a single light source. In contrast, conventionally, if it is desired to perform simultaneous DAS, DTS and/or DSTS measurements, then separate optical systems are used for each measurement, resulting in a complex and expensive optical setup. Indeed, in view of the different scattering mechanisms involved in each measurement technique, as well as the different detection bandwidths and frequencies required for these techniques, it is common practice to use a different measurement system for each technique. Additionally, this may require the use of multiple optical fibres which, in many installations would not be practical.
Using a single measurement system for implementing these different measurement techniques may reduce an amount and cost of hardware which is required for performing the measurements. In particular, sharing the same optical fibre and many optical components between the measurement techniques may avoid redundancy in the equipment needed for the different techniques. In particular, the inventors have found that DAS and DSTS measurements can share a common detector and acquisition stage. The measurement system of the invention may have a greater flexibility compared to conventional system, as it enables multiple measurement techniques to be implemented, depending on the properties of the optical fibre that are to be measured. Thus, it is possible to use different measurement techniques even in cases where it would not be practical to install multiple optical fibres.
DAS measurements can be used to obtain a spatial differential of the strain and/or rate of temperature change at positions along the optical fibre. However, DAS measurements do not typically yield absolute values of the strain or temperature. On the other hand, DTS measurements can yield absolute values of the temperature, whilst DSTS measurements can yield absolute values of strain and temperature. Therefore, by implementing DAS with DTS and/or DSTS the results obtained from the different measurement techniques may be used to complement one another, and may improve the accuracy and reliability with which strain and/or temperature can be determined for the optical fibre.
A first aspect of the invention provides an optical measurement system that enables combination of DAS and DSTS measurements. According to the first aspect of the invention, there is provided an optical measurement system comprising: a coherent light source configured to generate a light signal; a launch stage configured to receive the light signal from the light source and generate a pulsed test signal and launch the test signal along an optical path; a local oscillator stage configured to generate a local oscillator signal, wherein local oscillator signal comprises a first local oscillator component and a second local oscillator component which is frequency-shifted relative to the first local oscillator component; and a first detector stage configured to: receive the local oscillator signal from the local oscillator stage and a scattered signal from the optical path, wherein the scattered signal comprises a Rayleigh scattering component and a Brillouin scattering component; and interfere the local oscillator signal with the scattered signal to produce an output signal having a first component at a first frequency corresponding to a frequency difference between the first local oscillator component and the Rayleigh scattering component, and a second component at a second frequency corresponding to a frequency difference between the second local oscillator component and the Brillouin scattering component.
The first aspect of the invention is based on the realisation that, although Brillouin scattering is at a much higher frequency compared to Rayleigh scattering, the bandwidth for Brillouin scattering is relatively small and of a same order of magnitude as the bandwidth for Rayleigh scattering. Therefore, by generating a second local oscillator component which is frequency-shifted (e.g. to a frequency that is closer to that of the Brillouin scattering), it is possible to bring the second component of the output signal down to a frequency which is closer to that of the original test signal (which is the frequency at which Rayleigh scattering is observed). As a result, the same detector stage (and acquisition system) can be used for detecting both Rayleigh scattering and Brillouin scattering.
The coherent light source may be a laser or other suitable narrow linewidth source. The light signal corresponds to light that is emitted by the light source. The light signal may also be referred to as a carrier signal. The light source may be configured to emit a continuous wave of coherent light.
Light emitted by the light source may be split (e.g. by a beam-splitter), such that a first portion of the light is conveyed to the launch stage and a second portion of the light is conveyed to the local oscillator stage. In other words, the light source may be coupled to the launch stage and the local oscillator stage via a beam-splitter, such that the light signal is received at both the launch stage and the local oscillator stage.
The launch stage is configured to generate the test signal in response to receiving the light signal from the light source. The launch stage may include any suitable component(s) for generating the pulsed test signal based on the light signal. For example, the launch stage may include a pulse generator, which is configured to generate the pulsed test signal from the received light signal. By way of example, the pulse generator may comprise an acousto-optic modulator (AOM), an electro-optic modulator (EOM), an IQ modulator, a solid state optical amplifier (SOA), and/or any other suitable type of pulse generator. In some cases, the pulse generator may comprise a single sideband modulator (SSB), e.g. implemented by an IQ modulator driven as a single sideband modulator. A duration of the pulsed test signal may be set based on various considerations, such as a length of the optical path and a desired measurement resolution. As an example, the pulsed test signal may have a duration between 5 ns and 100 ns, but need not be limited to this range.
The launch stage is further configured to launch the pulsed test signal along the optical path. Thus, an output of the launch stage may be connectable (or connected) to the optical path, so that the test signal can be transmitted to the optical path. For example, the launch stage may comprise a connector for connecting the output of the launch stage to the optical path.
The optical path may comprise an optical fibre. As discussed above, the optical fibre may act as a transducer for DAS, DSTS and DTS measurements. In some cases, the optical path (e.g. optical fibre) may form part of the optical measurement system.
The local oscillator stage is configured to generate the local oscillator signal. The local oscillator signal includes a first component and a second, frequency-shifted component. In other words, at least one of the first and second local oscillator components is frequency-shifted relative to the light source. Thus, the second local oscillator signal may correspond to a frequency-shifted copy of the light signal. For example, the local oscillator stage may be configured to generate two copies of the light signal, and to apply a frequency shift to one of the two copies. The local oscillator stage may include any suitable arrangement of optical and electronic elements to generate the two components of the local oscillator signal.
Preferably, the first local oscillator component is not frequency-shifted relative to the light source and the second local oscillator component is frequency-shifted relative to the light source. In other words, there may be a 0 Hz frequency shift between the first local oscillator component and the light signal, and a non-zero frequency shift between the second local oscillator signal and the light signal. However, as discussed below, a frequency shift may be in some cases be applied to the first local oscillator component.
The local oscillator stage may be configured to receive the light signal from the light source. In this manner the local oscillator stage may generate the local oscillator signal based on the received light signal, e.g. by generating two copies of the light signal as mentioned above. Alternatively, a separate light source may be used by the local oscillator stage for generating the local oscillator signal.
The optical path (e.g. optical fibre) may include a plurality of scattering sites distributed along the length of the optical path, e.g. caused by variations in refractive index along the length of the optical path. When a pulse of the pulsed test signal travels along the optical path (e.g. optical fibre), light in the pulse may be scattered at various locations (i.e. at the scattering sites) along the length of the optical path. In particular, light in the pulse may be backscattered via Rayleigh scattering and by Brillouin scattering of the pulse at various locations along the length of the optical path. Scattering of light in the pulse at a location along the optical path may result in a scattered signal associated with that location. Thus, a pulse of the test signal travelling along the optical path may result in a plurality of scattered signals, each scattered signal having been scattered at a respective location (i.e. scattering site) along the optical path. The scattered signals may be guided back along the optical path, such that they are received by the detector stage. Moreover, each scattered signal will have a Rayleigh scattering component (i.e. which results from Rayleigh scattering of the test signal), and a Brillouin scattering component (i.e. which results from Brillouin scattering of the test signal).
The optical path (e.g. optical fibre) may be subjected to different conditions at locations along the optical path, such as acoustic modulations and/or temperature changes. Such variations in conditions along the optical path may cause changes in the scattering of the pulsed test signal, e.g. due to local variations in refractive index and length of the optical path.
The location on the optical path at which the scattered signal was scattered may be determined based on a time of receipt of the scattered signal at the detector stage. For example, the location on the optical path may be determined by comparing a time at which a pulse of the test signal was launched along the optical path, and the time of receipt of the scattered signal at the detector stage, taking into account the speed of light along the optical path.
The first detector stage is configured to receive the scattered signal from the optical path. Thus, the first detector stage may be connectable (or connected) to the optical path. For example, the first detector stage may include a connector for connecting an input of the detector stage to the optical path. Note that the term ‘first’ here is used to identify (or name) the detector stage, and does not imply the presence of multiple detector stages.
As noted above, the scattered signal will include a Rayleigh scattering component and a Brillouin scattering component, which result from respective scattering mechanisms in the optical path. The Rayleigh scattering component will be at a same frequency as the test signal, as Rayleigh scattering is an elastic scattering process. The Brillouin scattering component will be frequency shifted relative to the test signal, due to the inelastic nature of Brillouin scattering. Therefore, the Rayleigh scattering component and the Brillouin scattering component of the scattered signal can be distinguished based on their frequencies. In particular, the Rayleigh scattering component may be at a frequency corresponding to that of the test signal, whilst the Brillouin scattering component may have a frequency that is offset relative to that of the test signal.
The magnitude of the frequency offset for the Brillouin scattering component will depend on measurement details, such as the frequency of the test signal and the material of the optical fibre. Typically, the frequency shift observed for Brillouin scattering is in the range of 8 GHZ to 12 GHz, such that the Brillouin scattering component may have a frequency in the range of 8 GHZ to 12 GHZ, depending on temperature and strain of the fibre.
For example, the inventors have found that the Brillouin frequency shift is typically around 10-11 GHZ for silica fibres at room temperature under zero strain. Temperature sensitivity of the Brillouin frequency shift is typically around 1 MHz/K, meaning that for a 200° C. operating range, a change with temperature in the range of 0-200 MHz in the Brillouin frequency shift is expected. Strain sensitivity of the Brillouin frequency shift is typically around 0.05 MHz/μStrain. So, for a maximum 3% breaking strain of the fibre, a maximum Brillouin frequency shift of around 1.5 GHZ is expected with strain of the fibre.
In more detail, the Brillouin component may comprise a Stokes component, whose frequency is down-shifted relative to the test signal, and an Anti-Stokes component whose frequency is up-shifted relative to the test signal.
The first detector stage is further configured to interfere the local oscillator signal with received scattered signal, and produce an output signal based on an interference of the local oscillator signal and the scattered signal. The first detector stage may comprise an optical detector which is arranged to generate an output signal that is representative of an intensity of incident light on the detector. For example, the first detector stage may include a square law detector. A square law detector outputs a signal having an intensity that is proportional to the square of the field amplitude of incident light on the detector. Both the local oscillator signal and the scattered signal may be incident on the detector of the first detector stage, such that they interfere at the detector. As a result, the detector (e.g. square law detector) may produce an output signal that is related to an intensity produced by the interference of the local oscillator signal and the scattered signal.
As the local oscillator signal and the scattered signal each respectively comprise two frequency components, the output signal will include a series of different frequency components resulting from the interference between the two signals. Specifically, the output signal includes a first component which is at a first frequency (equal to the frequency difference between the first local oscillator component and the Rayleigh scattering component), and a second component which is at a second frequency (equal to the frequency difference between the second local oscillator component and the Brillouin scattering component). The first component of the output signal relates to Rayleigh scattering, and corresponds to a DAS signal, whilst the second component of the output signal relates to Brillouin scattering, and corresponds to a DSTS signal. The DAS signal and the DSTS signal can then be processed separately, using known techniques. For example, the DAS signal can be processed to estimate a strain and/or temperature differential at a location in the optical path, whilst the DSTS signal can be processed to estimate an absolute value of strain and/or temperature at the location in the optical path.
The frequency shift of the second local oscillator component acts to bring the frequency of the DSTS signal (i.e. the second component of the output signal) closer to the frequency of the DAS signal (i.e. the first component of the output signal). This may facilitate detecting both Rayleigh and Brillouin scattering at the first detector stage. In particular, if the frequency shift of the second local oscillator component is set to be close to the frequency shift cause by Brillouin scattering, then this may result in the second frequency being close to the first frequency. In practice, the frequency shift of the second local oscillator component may be set based on the measurement conditions (e.g. fibre material, temperature of the fibre), to ensure that the second local oscillator component is close in frequency to the Brillouin scattering. For example, the Brillouin scattering frequency along an optical fibre may be measured, and/or a look-up table may be used, to enable the frequency shift of the second local oscillator component to be set accordingly. In some cases, the frequency shift of the second local oscillator may be changed over time, e.g. if a given location or area of the fibre were at a significantly different temperature to the rest of the fibre.
The first detector stage may comprise an acquisition system configured to acquire the output signal, e.g. so that the output signal can be stored and analysed. For example, the first detector stage may comprise an analog-to-digital converter configured to receive the output signal from the optical detector (e.g. square law detector). The analog-to-digital converter may be configured to convert the output signal into a digital signal, which can subsequently be stored and processed by a computing system.
The local oscillator stage may comprise a single sideband modulator which is configured to receive the light signal from the light source and generate the second local oscillator component. Passing the light signal through a single sideband modulator may enable a frequency-shifted copy of the light signal to be created.
The single sideband modulator may further be configured to not fully suppress the light signal received from the light source, such that the first local oscillator component corresponds to a non-fully suppressed version of the light signal from the light source. In other words, the single sideband modulator may create a frequency-shifted copy of the light signal (i.e. the second local oscillator component), whilst not fully suppressing the original light signal received from the light source (i.e. to provide the local oscillator component). Thus, the single sideband modulator may output both the first component and the second component of the local oscillator signal, which may simplify a configuration of the local oscillator stage. This may be achieved by non-perfectly biasing a single sideband suppressed carrier (SSC-SC) modulator, so that it does not fully suppress a carrier signal (i.e. the light signal) which is fed to it. In this manner, it is possible to shift the relative power in the shifted and non-shifted components of the local oscillator signal.
Other techniques may also be used for generating the two local oscillator components based on the received light signal. For example, as mentioned above, the local oscillator stage may create two copies of the light signal, and apply a frequency shift to one of the two copies.
The first detector stage may have a detection bandwidth that is configured to cover the first frequency, and the frequency shift of the second component of the local oscillator signal may be set such that the second frequency is within the detection bandwidth. In this manner, it is possible to ensure that both the first frequency and the second frequency are within the detection bandwidth of the detector stage, such that the DAS signal and the DSTS signal can reliably be extracted from the detector stage's output signal. In particular, the second local oscillator signal acts to shift the DSTS signal closer in frequency to the DAS signal, such that both the DAS signal and the DSTS signal can be represented within the same detection bandwidth. Further, as mentioned above, the inventors have realised that Rayleigh scattering and Brillouin scattering have bandwidths of a similar order, such that by shifting the DSTS signal towards the DAS signal, both signals can be represented within the same detection bandwidth.
The frequency shift of the second local oscillator component may be selected such that a frequency of the second local oscillator component is within a predetermined range of a frequency of the Brillouin scattering component (i.e. so that the second frequency is in the predetermined range). This may help ensure that the second frequency is within the detection bandwidth. For example, the frequency shift of the second local oscillator signal may be set such that the second frequency is 2 GHz or less (e.g. 2 GHZ, 1.5 GHz, 1 GHZ or 0.5 GHZ). This may enable detection of the first and second components of the output signal using a detector having a detection bandwidth of 2 GHz or less.
As noted above, the frequency shift caused by Brillouin scattering is dependent on measurement conditions, such as the material and temperature of the optical path. Therefore, the frequency shift of the second local oscillator component may be set based on the measurement conditions, e.g. based on an observed frequency of the Brillouin scattering component.
Likewise, the detection bandwidth may be selected based on measurement conditions, such as the material of the optical path and the temperature of the optical path, as these will affect the offset of the Brillouin scattering which in turn determines the bandwidth requirements for detection. The measurement conditions may also affect the linewidths and amplitudes of the Rayleigh and Brillouin scattering.
The detection bandwidth may be between 100 MHz and 2 GHZ, as the bandwidths for Rayleigh scattering and Brillouin scattering are typically observable in this range.
The second local oscillator component may be frequency-shifted relative to the light source by a frequency in the range of 8 GHZ to 12 GHz. In typical measurement conditions (e.g. where the optical path includes a silica fibre), the frequency shift caused by Brillouin scattering is in the range of 8 GHZ to 12 GHZ, as discussed above. Thus, setting the frequency shift for the second local oscillator component in this range may bring the frequency of the second local oscillator component close to the frequency of the Brillouin scattering component. As a results, the second frequency will be less than the frequency of the Brillouin scattering component, and will be closer to the first frequency, thus facilitating detection of both DAS and DSTS signals. As an example, where the frequency shift from Brillouin scattering is 11 GHZ, the frequency shift of the second local oscillator component may be 10 GHz. Then, the second frequency will be 1 GHZ, meaning that the DSTS signal will be observed at 1 GHZ.
The launch stage may be configured to frequency-shift the test signal relative to the light source. In this manner, the DAS measurement may be an optical heterodyne measurement. The frequency shift may be applied by the pulse generator mentioned above, i.e. the pulse generator may generate the pulsed test signal as well as frequency-shift the test signal. Then, the first frequency may correspond to the frequency shift applied to the test signal, i.e. the DAS signal may be at a frequency corresponding to the frequency shift applied to the test signal.
The frequency shift of the test signal relative to the light source may be in the range of 50 MHz to 500 MHZ. In a preferred example, the frequency shift may be in the range of 100 MHz to 200 MHZ.
Alternatively, no frequency shift may be applied to the test signal, i.e. the DAS measurement may be an optical homodyne measurement. In such a case, the test signal and the first component of the local oscillator signal are at a same frequency. Where a homodyne DAS measurement is performed, the first detector stage may include an optical hybrid detection system, e.g. which is configured to mix the scattered signal with the local oscillator signal and a phase-shifted (e.g. 90 degree phase-shifted) copy of the local oscillator signal.
In some embodiments, the system may be configured to alternate over time between a first step for detecting the first scattering component and a second step for detecting the second scattering component; and the launch stage may be configured to generate a first test signal in the first step and a second test signal in the second step, the first test signal and the second test signal having a different property. In this manner, different test signals may be used for probing Rayleigh scattering and Brillouin scattering. Thus, rather than using the same test signal for probing both Rayleigh and Brillouin scattering, the first test signal and the second test signal can be specifically tailored for the detection of Rayleigh scattering and Brillouin scattering, respectively. This may improve an accuracy with which Rayleigh and Brillouin scattering can be measured, e.g. by enabling improvements in signal-to-noise ratio (SNR) for the first and second components of the output signal. Furthermore, as the DAS signal and DSTS signal are obtained from different test signals, it is possible to perform a calibration process for the DSTS signal (examples of which are discussed below), without interrupting the DAS measurement.
The first test signal and the second test signal may have one or more properties which are different. For example, the first test signal and the second test signal may have a different pulse power, a different pulse duration, a different amplitude, and/or a different frequency shift. For example, the pulse powers of the first test signal and the second test signal may be adjusted in order to obtain a desired SNR for the DAS and DSTS measurements. The properties of the first and second test signals may be adjusted and optimised during a setup phase, in order to provide a desired SNR for the DAS and DSTS measurements.
The first component of the output signal may be measured when the system is in the first step, whilst the second component of the output signal may be measured when the system is in the second step. In other words, the first component of the output signal may be obtained as a result of the first test signal, whilst the second component of the output signal may be obtained as a result of the second test signal. Thus, the DAS signal and the DSTS signal may be obtained using different pulsed test signals.
The launch stage may be configured to alternate between generating a pulse of the first test signal and a pulse of the second test signal, such that subsequent pulses of the test signal probe different scattering mechanisms. For example, the pulse generator in the launch stage may be configured to sequentially generate a pulse of the first test signal and a pulse of the second test signal.
The system may be configured to control the launch stage to vary a frequency shift applied to the second test signal upon successive iterations of the second step. Varying the frequency shift applied to the second test signal and the local oscillator signal may be equivalent to varying a frequency of the light source for the purposes of measurements on the optical path. Advantageously however, by controlling the launch stage and/or the local oscillator stage, the frequency of the light source can be kept constant. Whereas sweeping the frequency of the light source may take place on a timescale of minutes (as this usually involves varying a temperature of the light source), the frequency shift applied to the second test signal can be varied near-instantly. In particular, by controlling the pulse generator in the launch stage and/or the modulator in the local oscillator stage, signal frequency may be directly changed within a single pulse repetition.
Whilst the frequency shift applied to the second test signal is being varied upon successive iterations of the second step, the frequency of the first test signal and of the components of the local oscillator signal may be kept constant across successive iterations of the first step. In this manner, the DAS measurement performed during the first step may be unaffected by frequency changes taking place for the DSTS measurement.
Furthermore, the system may be configured to control the local oscillator stage to apply the same frequency shift to the first local oscillator component as for the second test signal. In this manner a frequency difference between the second test signal and the first local oscillator component may be kept constant, whilst the frequency of the second test signal is varied across successive iterations of the second step. As a result, the frequency of the first component of the output signal may be kept constant across successive iterations of the second step, thus facilitating obtaining the DAS and signal in the second step. In some cases, the same frequency shift may also be applied to the second local oscillator component, to keep constant the frequency of the second component of the output signal, which may facilitate obtaining the DSTS signal.
Varying the frequency of the second test signal upon successive iterations of the second step may serve to determine a loss profile of the optical path, which may be used for normalising the DSTS signal. In particular, varying the frequency shift applied to the second test signal may yield DAS signals that effectively correspond to different light source frequencies. In other words, the Rayleigh component of the scattering signal obtained across successive iterations of the second step may be equivalent to signals obtained for different light source frequencies. This enables a wideband ensemble average of the Rayleigh component (or DAS signals) obtained across successive iterations of the second step to be averaged, in order to determine a loss profile of the optical path.
Thus, in some cases, the system may be configured to perform a sweep (e.g. across a predetermined range) of the frequency shift applied to the second test signal over successive iterations of the second step. The system may then determine an average of the Rayleigh component of the scattering signal across a range of effective light source wavelengths, based on which a loss profile of the optical path can be determined.
Typical modulators which may be used in the launch stage and the local oscillator stage may have bandwidths of the order of 10 to 20 GHz, thus enabling the frequency shift applied to the second test signal to be swept over a range of 10 to 20 GHZ across multiple iterations of the second step. In this manner, a wideband average of the Rayleigh component (and/or of the DAS signal) can be obtained. The inventors have found that using a wideband average of the Rayleigh component (and/or of the DAS signal) for normalising the DSTS signal may minimise coherent Rayleigh noise (CRN) which results from the coherent nature of the light source.
In some cases, the system may be configured to vary a frequency difference between the second test signal and the second local oscillator component upon successive iterations of the second step. This may be achieved, for example, by varying the frequency shift applied to the second local oscillator component in the local oscillator stage upon successive iterations of the second step. Additionally or alternatively, this could be achieved by varying a frequency shift applied to the second test signal upon successive iterations of the second step. Varying the frequency difference between the second test signal and the second local oscillator component may enable various parameters of the Brillouin scattering to be determined. In particular, if the frequency difference between the second test signal and the second local oscillator component is varied upon successive iterations of the second step (e.g. upon successive pulses of the second test signal) by an amount which is less than a linewidth of the Brillouin scattering component, it is possible to map a width, line-shape and peak of the Brillouin scattering component.
Alternatively to the above discussion, instead of measuring Rayleigh scattering and Brillouin scattering on alternate pulses of the test signal, Rayleigh scattering and Brillouin scattering can be detected simultaneously, i.e. using the same pulsed test signal. In such a case, properties of the test signal may be set to enable detection of both scattering mechanisms. In such a case, the test signal may not be specifically tailored for one type of scattering mechanism, and may represent a compromise for detecting both types of scattering mechanism. However, obtaining both the DAS signal and the DSTS signal based on the same test signal may increase an acquisition speed for the DAS and DSTS signals, which may result in an increased SNR and bandwidth of the DAS and DSTS signals.
The launch stage may comprise a regenerative loop configured to provide a portion of the pulsed test signal as positive feedback for the launch stage. This may increase a bandwidth across which the frequency of the test signal can be shifted. For example, in the case discussed above where the frequency difference between the second test signal and the local oscillator signal is varied, use of a regenerative loop in the launch stage may increase a range across which the frequency difference can be varied.
The regenerative loop may be connected between an output and an input of the pulse generator in the launch stage, such that a portion of the test signal output from the pulse generator is provided to the input of the pulse generator. In this manner, a pulse of a desired amplitude may be built up using the regenerative loop.
The first detector stage may be a polarisation diverse detector stage. For example, the local oscillator signal or the scattered signal may be split into components having different polarisations, e.g. vertical and horizontal polarisation. The first detector stage may then include a respective detector for each polarisation state, such that a respective output signal may be generated for each polarisation state. This may serve to ensure that interference of the scattered signal and local oscillator signal can be detected for at least one of the polarisation states, e.g. in cases where polarisation of the scattered signal varies relative to polarisation of the local oscillator signal.
The optical measurement system of the first aspect may further be adapted to perform DTS measurements, in addition to the DAS and DSTS measurements discussed above. Thus, the scattered signal may further comprise a Raman scattering component, and the system further may further comprise: a second detector stage; and a wavelength splitter configured to split the scattered signal such that the first scattering component and the second scattering component are transmitted to the first detector stage, and such that the Raman scattering component is transmitted to the second detector stage; wherein the second detector stage is configured to split the Raman scattering component into a Stokes component and an Anti-Stokes component, and to detect an amplitude of each of the Stokes component and the Anti-Stokes component.
In addition to the Rayleigh and Brillouin scattering components discussed above, the scattering signal may also include a Raman scattering component, which results from Raman scattering of the test signal in the optical path. The Raman scattering component may typically be at a much higher frequency than the Brillouin scattering component. Indeed, the frequency shift caused by Raman scattering in an optical fibre (e.g. a silica fibre) is usually between 10 to 15 THZ.
The wavelength splitter acts to separate the Rayleigh and Brillouin components from the Raman component of the scattering signal, on the basis of the differences in frequency (and therefore in wavelength) between the different components. For instance, the wavelength splitter may be configured to transmit components of the scattered signal having a frequency below a predetermined threshold to the first detector stage, and to transmit components of the scattered signal having a frequency above the predetermined threshold to the second detector stage. In one example, the wavelength splitter may be configured to transmit components of the scattered signal having a frequency below 20 GHZ to the first detector stage (such that both the Rayleigh and Brillouin components reach the first detector stage), and to transmit components of the scattered signal having a frequency above 10 THz to the second detector stage (such that the Raman scattering component reaches the second detector stage).
The wavelength splitter may include any suitable optical components for separating components of a light signal on the basis of their frequency. In one example, the wavelength splitter may comprise a wavelength division demultiplexer.
The wavelength splitter may comprise an input which is connectable (or connected) to the optical path, to receive the scattered signal from the optical path. The wavelength splitter may further comprise a first output which is connected to the first detector stage, and a second output which is connected to the second detector stage, the wavelength splitter being configured to output the Rayleigh and Brillouin scattering components via its first output, and to output the Raman scattering component via its second output.
The second detector stage is configured to perform a DTS measurement. The inventors have realised that, unlike Brillouin and Rayleigh scattering, Raman scattering results in a broadband emission, such that using separate detector stages for the DTS measurement compared to the DAS and DSTS measurements is beneficial, and may improve a quality of measurement results. In particular, the first and second detector stages can be tailored to the specific scattering mechanisms of interest.
The second detector stage is configured to split the Raman scattering component into the Stokes component (which is down-shifted relative to the test signal) and the Anti-Stokes component (which is up-shifted relative to the test signal). This may be achieved using a second wavelength splitter, which is configured to split the Raman scattering component into the Stokes and Anti-Stokes components based on the different frequencies (or wavelengths) of the Stokes and Anti-Stokes components. The second wavelength splitter may include any suitable optical components for separating components of a light signal on the basis of their frequency. In one example, the second wavelength splitter may comprise a wavelength division demultiplexer.
The second wavelength splitter may have an input that is connected to the second output of the first wavelength splitter mentioned above, to receive the Raman scattering component. The second wavelength splitter may further have a first output configured to output the Stokes component, and a second output configured to output the Anti-Stokes component.
The second detector stage may further comprise a first detector configured to detect an amplitude of the Stokes component, and a second detector configured to detect an amplitude of the Anti-Stokes component. In this manner, a ratio of the Anti-Stokes and Stokes components may be determined, in order to determine a temperature of the optical path.
The first detector and the second detector may include any suitable type of detector. Preferably, the first and second detectors may comprise high gain detectors. For example, the first and second detectors may comprise avalanche photodiode (APD) detectors, or photomultiplier detectors.
In some cases, the second detector stage may comprise a first bandpass filter connected to an input of the first detector, and a second bandpass filter connected to an input of the second detector. In this manner, the Stokes and Anti-Stokes components may be passed through respective bandpass filters before reaching the first and second detectors, respectively. This may serve to isolate the Stokes and Anti-Stokes components, in order to improve an accuracy of the DTS measurement.
In an alternative implementation of the invention, instead of using a local oscillator signal which has two components, one of which is frequency-shifted, the test signal may include a first component and a second component which is frequency shifted relative to the first. In other words, instead of providing the frequency-shifted component in the local oscillator, the frequency-shifted component may be provided in the test signal. This may yield an equivalent result to the first aspect of the invention, with the frequency-shifted component of the test signal serving to shift the DSTS signal closer in frequency to the DAS signal.
Thus, according to a second aspect of the invention, there is provided an optical measurement system comprising: a coherent light source configured to generate a light signal; a launch stage configured to receive the light signal from the light source and generate a pulsed test signal and launch the test signal along an optical path, wherein the test signal comprises a first test signal component and a second test signal component which is frequency-shifted relative to the first test signal component; a local oscillator stage configured to generate a local oscillator signal; and a first detector stage configured to: receive the local oscillator signal from the local oscillator stage and a scattered signal from the optical path, wherein the scattered signal comprises a Rayleigh scattering component resulting from scattering of the first test signal component and a Brillouin scattering component resulting from scattering of the second test signal component; and interfere the local oscillator signal with the scattered signal to produce an output signal having a first component at a first frequency corresponding to a frequency difference between the local oscillator signal and the Rayleigh scattering component, and a second component at a second frequency corresponding to a frequency difference between the local oscillator signal and the Brillouin scattering component.
The second aspect of the invention corresponds to an alternative (or equivalent) implementation of the system of the first aspect of the invention, and may yield equivalent results. In particular, the first component and the second component of the output signal may correspond to a DAS signal and DSTS signal, respectively, and may be at the same frequencies as the first and second components of the output signal in the first aspect. Accordingly, any features discussed above in relation to the first aspect of the invention may be shared with the second aspect of the invention. In particular, equivalents of features relating to the frequency shift of the second local oscillator component in first aspect of the invention may be applied to the frequency shift of the second test signal component.
The second test signal component may be frequency-shifted based on similar considerations and in a manner analogous to the second local oscillator component in the first aspect. In particular, the frequency shift applied of the second test signal component serves to bring the frequency of the DSTS signal closer to that of the DAS signal.
The first test signal component may not be frequency-shifted relative to the light source (e.g. there is a 0 Hz shift between the first test signal component and the light source), whilst the second test signal is frequency-shifted relative to the light source.
The local oscillator signal may be frequency-shifted relative to the light source, e.g. to enable heterodyne DAS detection in a manner analogous to that discussed in relation to the first aspect of the invention. For example, the frequency shift of the local oscillator signal relative to the light source may be in the range of 50 MHz to 500 MHz. In a preferred example, the frequency shift may be in the range of 100 MHz to 200 MHZ.
Alternatively, no frequency shift may be applied to the local oscillator signal, i.e. the DAS measurement may be an optical homodyne measurement.
In a further alternative, the first test signal component may be frequency-shifted relative to the light source (e.g. by an amount between 100 MHZ-200 MHZ), and no frequency shift may be applied to the local oscillator signal, to allow for heterodyne DAS detection.
The Rayleigh scattering component of the scattered signal results from scattering of the first test signal component along the optical path, whilst the Brillouin scattering component results from scattering of the second test signal component along the optical path. For example, the Stokes component of the Brillouin scattering component may be used, as this will be down-shifted in frequency relative to the second test signal component, so that it may be closer in frequency to the Rayleigh scattering component. Thus, in a manner analogous to that discussed for the first aspect of the invention, the first and second components of the output signal may be observable within the detection bandwidth of the detector stage.
The first test signal component may be in the form of a first pulse, and the second test signal component may be in the form of a second pulse. Thus, the test signal may comprise a pair of pulses which are at different frequencies.
In view of the first aspect and the second aspect of the invention disclosed above, the local oscillator signal and/or the test signal may be generated so as to have first and second components which are shifted in energy relative to one another.
Thus, in a general aspect of the invention, the test signal and/or the local oscillator signal may comprise a first component and a second component which is frequency-shifted relative to the first component.
A third aspect of the invention provides an optical measurement system that enables combination of DTS measurements with DAS or DSTS measurements. According to the third aspect of the invention, there is provided an optical measurement system comprising: a coherent light source configured to generate a light signal; a launch stage configured to receive the light signal from the light source and generate a pulsed test signal and launch the test signal along an optical path; a local oscillator stage configured to generate a local oscillator signal; a first detector stage and a second detector stage; and a wavelength splitter configured to receive a scattered signal from the optical path, and split the scattered signal into a first scattering component which is transmitted to the first detector stage and a Raman scattering component which is transmitted to the second detector stage; wherein the first detector stage is configured to interfere the local oscillator signal with the scattered signal to produce an output signal having a frequency corresponding to a frequency difference between the local oscillator signal and the first scattering component; wherein the second detector stage is configured to split the Raman scattering component into a Stokes component and an Anti-Stokes component, and to detect an amplitude of each of the Stokes component and the Anti-Stokes component.
The optical measurement system of the third aspect of the invention is based on similar considerations as the first aspect of the invention, in particular embodiments of the first aspect which include DTS measurements. Any features discussed above in relation to the first aspect of the invention may be shared with the third aspect of the invention. For instance, the light source, launch stage, local oscillator stage, first detector stage, second detector stage, and wavelength splitter may be configured as discussed above in relation to the first aspect of the invention.
The first scattering component may be a Rayleigh scattering component or a Brillouin scattering component. Thus, depending on whether the Rayleigh scattering component or the Brillouin scattering component is used, the first detector stage may yield a DAS signal or a DSTS signal, in line with the discussion above relating to the first aspect.
According to a fourth aspect of the invention, there is provided a method for operating an optical measurement system, the method comprising: generating, using a coherent light source, a light signal, and conveying the light signal to a launch stage; generating, at the launch stage, a pulsed test signal and launching the test signal along an optical path; generating, at a local oscillator stage, a local oscillator signal, wherein local oscillator signal comprises a first local oscillator component and a second local oscillator component which is frequency-shifted relative to the first local oscillator component; receiving, at a first detector stage, the local oscillator signal from the local oscillator stage and a scattered signal from the optical path, wherein the scattered signal comprises a Rayleigh scattering component and a Brillouin scattering component; interfering, by the first detector stage, the local oscillator signal with the scattered signal to produce an output signal having a first component at a first frequency corresponding to a frequency difference between the first local oscillator component and the Rayleigh scattering component, and a second component at a second frequency corresponding to a frequency difference between the second local oscillator component and the Brillouin scattering component.
The method of the fourth aspect of the invention may be used to operate the system of the first aspect of the invention. Therefore, any features discussed above in relation to the first aspect of the invention may be shared with the fourth aspect of the invention.
The light source may be configured to convey the light signal to both the launch stage and the local oscillator stage (e.g. via a beam splitter), such that the local oscillator stage may use the light signal for generating the local oscillator signal. Alternatively, a separate light source may be employed by the local oscillator stage for generating the local oscillator signal.
The first detector stage may have a detection bandwidth that covers the first frequency, and the method may further comprise: setting the frequency shift of the second component of the local oscillator signal relative to the light source such that the second frequency is within the detection bandwidth.
The method may further comprise: alternating over time between a first step in which the launch stage generates a first test signal and a second step in which the launch stage generates a second test signal, the first test signal and the second test signal having a different property.
The method may further comprise varying a frequency shift applied to the second test signal upon successive iterations of the second step.
The method may further comprise determining a loss profile of the optical path based on an average of the Rayleigh scattering components obtained across multiple iterations of the second step. The loss profile of the optical path may relate an optical loss to a position (i.e. a scattering location) along the optical path.
For a location along the optical path, the average may, for example, correspond to an average over time of the Rayleigh scattering component at that location across multiple iterations of the second step (i.e. as the frequency shift applied to the second test signal is varied). The optical loss at that location on the optical path may be determined by comparing (e.g. dividing) the average of the Rayleigh scattering component for that location with a value of the Rayleigh scattering component obtained for a location at a proximal end of the optical path (i.e. at an end of the optical path closest to the launch stage). This process may be performed for multiple locations along the optical path, to build up the loss profile of the optical path. As noted above, averaging the Rayleigh scattering component over multiple iterations of the second step may serve to minimise CRN.
The method may further comprise using the loss profile to normalise the second term of the output signal. In other words, the loss profile may serve to normalise the DSTS signal, i.e. to take into account optical losses along the optical path.
The scattered signal may further comprise a Raman scattering component, and the method may further comprise: splitting, with a wavelength splitter, the scattered signal such that the first scattering component and the second scattering component are transmitted to the first detector stage, and such that the Raman scattering component is transmitted to a second detector stage; splitting, at the second detector stage, the Raman scattering component into a Stokes component and an Anti-Stokes component, and detecting an amplitude of each of the Stokes component and the Anti-Stokes component.
According to a fifth aspect of the invention, there is provided a method for operating an optical measurement system, the method comprising: generating, using a coherent light source, a light signal, and conveying the light signal to a launch stage; generating, at the launch stage, a pulsed test signal and launching the test signal along an optical path, wherein the test signal comprises a first test signal component and a second test signal component which is frequency-shifted relative to first test signal component; generating, at a local oscillator stage, a local oscillator signal; receiving, at a first detector stage, the local oscillator signal from the local oscillator stage and a scattered signal from the optical path, wherein the scattered signal comprises a Rayleigh scattering component resulting from scattering of the first test signal component and a Brillouin scattering component resulting from scattering of the second test signal component; and interfering, by the first detector stage, the local oscillator signal with the scattered signal to produce an output signal having a first component at a first frequency corresponding to a frequency difference between the local oscillator signal and the Rayleigh scattering component, and a second component at a second frequency corresponding to a frequency difference between the local oscillator signal and the Brillouin scattering component.
The method of the fifth aspect of the invention may be used to operate the system of the second aspect of the invention. Therefore, any features discussed above in relation to the first and second aspects of the invention may be shared with the fifth aspect of the invention.
In view of the fourth and fifth aspects of the invention, in a further general aspect of the invention, the test signal and/or the local oscillator signal may comprise a first component and a second component which is frequency-shifted relative to the first component.
According to a sixth aspect of the invention, there is provided a method for operating an optical measurement system, the method comprising: generating, using a coherent light source, a light signal, and conveying the light signal to a launch stage; generating, at the launch stage, a pulsed test signal and launching the test signal along an optical path; generating, at the local oscillator stage, a local oscillator signal; receiving, at a wavelength splitter, a scattered signal from the optical path, and splitting the scattered signal into a first scattering component which is transmitted to a first detector stage and a Raman scattering component which is transmitted to a second detector stage; interfering, at the first detector stage, the local oscillator signal with the scattered signal to produce an output signal having a frequency corresponding to a frequency difference between the local oscillator signal and the first scattering component; and splitting, at the second detector stage, the Raman scattering component into a Stokes component and an Anti-Stokes component, and detecting an amplitude of each of the Stokes component and the Anti-Stokes component.
The method of the sixth aspect of the invention may be used to operate the system of the third aspect of the invention. Therefore, any features discussed above in relation to the first and third aspects of the invention may be shared with the sixth aspect of the invention.
The first scattering component may be a Rayleigh scattering component or a Brillouin scattering component.
An embodiment of the invention is discussed below in more detail with reference to the accompanying drawings, in which:
The system 10 comprises a coherent light source 12 which produces a coherent light signal. The light source 12 may be a laser, which is used in continuous wave (CW) operation. The light source 12 is coupled to a launch stage 14 and a local oscillator stage 16, by an optical coupler 18 or beam splitter. In this manner, a first portion of light emitted by the light source is transmitted to the launch stage 14, and a second portion of the light emitted by the light source is transmitted to the local oscillator stage 16. The light may be split between the two stages by the optical coupler 18 such that 90% of the incoming light is directed into the launch path, and 10% of the incoming light is directed into the local oscillator path. Of course, the ratio of incoming light directed into each stage may be chosen by the operator or manufacturer depending on the nature of the operation for which the system 10 is used. Optionally, an optical isolator (not shown) may be coupled between the light source 12 and the optical coupler 18, to ensure that light is not passed back to the light source 12.
The launch stage 14 includes a pulse generator 20, into which the light signal received at the launch stage 14 from the light source 12 is directed. The pulse generator 20 is a device which is configured to generate a pulsed test signal from the received light signal. In addition, a drive signal can be applied to the pulse generator 20 to upshift or downshift the frequency of test signal by an amount equal to the frequency of the drive signal. This is shown in
The launch stage 14 further includes an optical amplifier 22 that is configured to amplify the pulsed test signal. The amplifier 22 may be in a master oscillator power amplifier (MOPA) configuration. The amplified test pulse may then be passed through an attenuator 24 for controlling a power of the test signal. The launch stage 14 is connected to the optical fibre 1000 via an optical circulator 26, which has three ports. The pulsed test signal enters the circulator 26 through a first port, where it is passed to a second port in order to enter the optical fibre 1000. As the test signal passes through the fibre 1000, a fraction of the light is backscattered from the fibre 1000 by a variety of scattering mechanisms, including Rayleigh scattering, Brillouin scattering and Raman scattering. A portion of the backscattered light is captured and guided back along the optical fibre 1000 towards the circulator 26. The scattered light, which may be referred to herein as a scattered signal, enters the circulator 26 at the second port, and leaves the circulator 26 to enter a detection stage 28 via a third port.
The local oscillator stage 16 includes a modulator 30 which is configured to receive the light signal from the light source 12 and generate a local oscillator signal having a first local oscillator component which is not frequency-shifted, and a second local oscillator component which is frequency-shifted relative to the light source 12. In the example shown, the modulator 30 is an IQ modulator which is driven as a partial single side band suppressed carrier modulator (SSB-SC), which is non-perfectly biased so as not to fully suppress the original light signal. Thus, the modulator 30 outputs a frequency-shifted copy of the light signal, as well as a copy of the original light signal which may be partially suppressed. The frequency shift applied to the second local oscillator component may be controlled by a drive signal that is applied to the modulator 30, and is illustrated as F2 in
The detector stage 28 has two inputs. The first input of the detector stage 28 is connected to the third output of the circulator 26, to receive the scattered signal from the optical fibre 1000. The second input of the detector stage is connected to the local oscillator stage 16, to receive the local oscillator signal. In a first part of the detector stage 28, the scattered signal is divided into two paths, for example using a polarising beam splitter (PBS) 34. The PBS 34 splits the scattered light into a horizontally polarised state and a vertically polarised state. The PBS 34 is used as the polarisation of the pulsed test signal and also of the scattered signal will evolve as a function of distance as they pass through the optical fibre 1000. The PBS 34 therefore enables polarisation diverse detection, to ensure that a signal can always be detected, regardless of a polarisation state of the scattered signal. The local oscillator (LO) signal, which is highly polarised, is split equally between two paths using a polarisation maintaining optical coupler 36. In other embodiments, the LO signal may be split into two polarisation states in preference to the scattered signal as described.
The detector stage 28 includes first and second optical couplers 38a, 38b, configured to mix the LO signal with a respective one of the horizontal and vertical states of the scattered signal. The detector stage further includes a first square law detector 40a and a second square law detector 40b, on which the light output from each optical coupler 28a, 38b is respectively interfered. The first and second square law detectors each provide a respective output signal, which is respectively taken and measured at an analog-to-digital converter 42a, 42b. Thus, a first output signal is produced for the vertical polarisation stage, and a second output signal is produced for the horizontal polarisation stage.
Due to the presence of the two components in the LO signal, and the multiple scattering components in the scattered signal, the output signal from each detector 42a, 42b will include a series of components at different frequencies, corresponding to frequency differences between components of the LO signal and the scattered signal. In particular, each output signal will include a first component at a first frequency corresponding to a frequency difference between the first local oscillator component and the Rayleigh scattering component, and a second component at a second frequency corresponding to a frequency difference between the second local oscillator component and the Brillouin scattering component. The first component of the output signal relates to Rayleigh scattering, and thus corresponds to a DAS signal. In the example shown in
The DAS signal can be processed in accordance with known techniques, in order to extract differential strain and/or temperature information about the optical fibre 1000. For example, the applicant's earlier application published as GB 2588177 A (the entirety of which is incorporated herein by reference) provides processing techniques that can be applied to a DAS signal. The second component of the output signal relates to Brillouin scattering, and thus corresponds to a DSTS signal. The DSTS signal can be processed in accordance with known techniques, in order to extract values of the strain and/or temperature in the optical fibre 1000. For example, strain and temperature measurements using Brillouin scattering are disclosed in “Distributed fibre sensing using microwave heterodyne detection of spontaneous Brillouin backscatter” by S. M. Maughan (PhD thesis, University of Southampton).
The system 10 described above makes use of a heterodyne sensing approach, wherein the frequency of the test signal is frequency-shifted relative to the light source 12 (and thus relative to the first component of the LO signal). Preferably, the difference in frequency difference between the test signal and the first component of the LO signal should be larger than the bandwidth required to represent Rayleigh scattering, without allowing crosstalk between the carrier and the DC terms which are also generated (see below), allowing the phase and amplitude information of the scattering to be recovered using a real carrier. Another method employs a complex carrier detection stage, replicating the polarisation diverse detection stage for two copies of the local oscillator shifted by 90 degrees relative to each other. This allows detection via a complex carrier, allowing either the positive sidelobe or the negative sidelobe of the resulting interference signal to be recovered independently. This allows homodyne operation whereby the first component of the local oscillator signal and test signal operate at the same optical frequency.
A treatment of the interference of the LO signal and the scattered signal for the system 10 shown in
Equation (1) shows the E-field of the first local oscillator component (LOA), with phase ØLOA and frequency ωLOA (which corresponds to the frequency of the light source 12). Equation (2) shows the E-filed of the second local oscillator component (LOB), with phase ØLOB and frequency ωLOB (shifted relative to the light source 12). Equation (3) shows the E-filed of the Rayleigh scattering component, with phase ØR and frequency ωR, which is offset from the light source frequency by F1 (e.g. 100-200 MHZ). Equation (4) shows the E-filed of the Anti-Stokes Brillouin scattering component, with phase ØAS and frequency was, which is shifted relative to the source frequency by the Stokes shift. Equation (5) shows the E-Field of the Stokes Brillouin scattering component, with phase ØS and frequency ωS, which is shifted relative to the source frequency by the negative-Stokes shift. Each of equations (1)-(5) relates to the corresponding E-field at a given scattering location along the optical fibre. As can be seen in equations (4) and (5), the Anti-Stokes and Stokes Brillouin scattering components have an amplitude and frequency which depend on temperature T and strain ϵ at the scattering location in the optical fibre.
When these waves interfere, and are observed by a square law detector, the resulting intensity measured at the detector corresponds to the sum of the E-fields multiplied by their complex conjugate, as shown by equation (6), below:
This results in several terms from each LO component beating with the Rayleigh, Stokes and Anti-Stokes scattering components, such that the interference intensity will include components at various sum and difference frequencies. In particular, the interference intensity includes (among others) the following three components:
The first component is an AC carrier corresponding to LOA mixing with the Rayleigh scattering component. The second component is an AC carrier corresponding to LOB mixing with the Anti-Stokes Brillouin scattering component. The third component corresponds to a DC component. Accordingly it can be seen that, amongst others, the interference at the detector stage generates a first component which is centred at the difference frequency between the first local oscillator component and the Rayleigh scattering component, and a second component which is centred at the difference frequency between the second local oscillator component and the Brillouin scattering component. The first component may be taken as the DAS signal, whilst the second component may be taken as the DSTS signal.
As an example, the Stokes shift maybe of the order of 10-11 GHz, the Rayleigh scattering component may be frequency shifted by 100-200 MHz relative to the LOA, and the frequency difference between LOA and LOB may be around 9-10 GHz. If the bandwidth of the detector stage is limited to 5 GHz then, of the many of the terms in IDet may average to zero over this bandwidth, however the three components noted above will be detectable within this bandwidth. Accordingly both the DAS signal and DSTS signal can be recovered using a single detector stage and acquisition system.
The system 10 of
A point worth noting is that the measured result output bandwidth of DAS and DSTS may be significantly different. The DAS measurement would be expected to yield an output phase result at the same rate as the pulse repetition of the test signal. However, the DSTS measurement would be expected to yield an output temperature and DC strain result over a longer time scale, e.g. once every few minutes as it generally requires a long-term average of several thousand pulses to calculate the output.
As noted above, the DAS signal can be used to obtain a differential value of the strain and/or rate of change of temperature in the optical fibre. Thus, once a reference temperature or strain value is obtained from the DSTS measurement, data from the DAS measurement can be used to update the strain and temperature measurements before the next data point is obtained from the DSTS measurement. In other words, data from the DAS measurement may be used to interpolate between data points obtained from the DSTS measurement. This may serve to increase the update rate and accuracy of the system.
When deployed on a standard fibre the DAS measurement may show a cross-sensitivity to strain and temperature. On the other hand, if the DAS measurement is deployed in either a temperature insensitive fibre or a strain decoupled loose clad buffered cable, the system can separate the effects of strain and temperature and this cross-sensitivity can be removed or reduced. Therefore, the DAS measurement could be employed to remove the cross-sensitivity inherent in the DSTS measurement, which may reduce the need to suppress CRN, and accurately measure the offset frequency of the Brillouin emission.
The system 300 is designed to use alternate pulses of the test signal for performing the DAS measurement and the DSTS measurement. Specifically, the system 300 is configured to alternate back and forth over time between a first step for performing the DAS measurement and a second step for performing the DSTS measurement. In the first step the pulse generator 20 generates a pulse of a first test signal, whilst in the second step the pulse generator 20 generates a pulse of a second test signal. Thus, the pulse generator 20 generates a sequence of pulses which alternate between the first test signal and the second test signal. The alternation between the first test signal and the second test signal may be controlled via the drive signal which is fed to the pulse generator 20. The first test signal and the second test signal may have different properties, such that each one may be specifically tailored for the detection of Rayleigh scattering and Brillouin scattering, respectively. In particular, properties of the first and second test signals such as pulse duration, pulse amplitude, and/or frequency shift may be adjusted to enhance detection of the corresponding scattering mechanism.
A benefit of performing the DAS measurement and the DSTS measurement on alternate pulses is that the calibration phase for the DSTS measurement may be performed without interrupting the DAS measurement. This may be achieved by applying a frequency shift to the second test signal, and varying the frequency shift applied to the second test signal across successive pulses of the second test signal. The inventors have found that applying a frequency shift to the test signal is equivalent to shifting the frequency of the light source for the purposes of the DAS and DSTS measurements. Thus, as the frequency shift applied to the second test signal can be controlled directly by controlling the pulse generator 20 (i.e. by controlling the drive signal for this component), the frequency of the light source 12 need not be varied for the calibration process. In addition to applying a frequency shift applied to second test signal in the second step, the same frequency shift may also be applied to the local oscillator signal (e.g. by controlling the drive signal for the modulator 30). In this manner, the frequency differences between the second test signal and the first and second local oscillator components may be kept constant, so that the DAS and DSTS signals remain at the same frequency across successive pulses of the second test signal.
Moreover, whilst the frequency shift applied to the second test signal is varied across successive pulses of the second test signal, the frequency of the first test signal and the LO signal may be kept constant across successive pulses of the of the first test signal. In this manner, the DAS measurement which is obtained from pulses of the first test signal is not affected by frequency variations occurring for the second test pulse.
As an example, in a calibration phase, the frequency shift applied by the pulse generator 20 to the second test signal may be swept across a bandwidth that is between 10 GHz to 20 GHZ. This may be equivalent to sweeping the frequency of the light source 12 across a similar range. This may be done by incrementing the frequency shift applied by the pulse generator 20 to the second test signal between successive pulses of the second test signal. Any suitable increment of the frequency shift may be used. For example, the frequency shift may be incremented by steps of 1 GHz or less. Preferably, the increment of the frequency shift may be of the order of half the bandwidth required to represent Rayleigh scattering. This may enable up a loss profile for averaging out CRN to be efficiently built up, whilst ensuring that the increment is not overly large, which could result in potential data of interest being missed. Thus, as the bandwidth for representing Rayleigh scattering is typically of the order of 30 MHz to 200 MHz, the increment of the frequency shift may be of the order of 15 MHZ. Additionally, the modulator 30 may be controlled so that the same frequency shift as for the second test signal is applied to the LO signal (including the first and second LO components), to keep the DAS and DSTS signals at the same frequency, as mentioned above.
Incrementally sweeping the frequency shift applied the second test signal over a wide bandwidth (e.g. 10 GHz to 20 GHZ) over successive pulses of the second test signal results in a set of DAS signals obtained at different frequencies. This is equivalent to measuring DAS signals at a corresponding set of light source frequencies. In this manner, an ensemble average of the DAS signals (or Rayleigh scattering components) obtained from the pulses of the second test signal during the calibration phase can be performed, in order to determine a loss profile of the optical fibre 1000. This can then be used to normalise the DSTS signal. Such a calibration phase performed with pulses of the second test signal may be performed periodically, in order to ensure that an accurate loss profile of the optical fibre 1000 is maintained.
A bandwidth of the order of 10 GHz to 20 GHz can be achieved using known optical modulators and pulse generators. However, this is typically not as large a bandwidth as can be achieved by varying the frequency of the light source 12 itself, which can be of the order of 60 GHz. In order to increase a bandwidth that can be achieved via control of the pulse generator 20, a regenerative loop 302 can be applied to the pulse generator 20 in the launch stage 14, as shown in
Performing DAS and DSTS measurements on alternate pulses may further enable various parameters of the Brillouin scattering to be determined. In particular, if the frequency difference between the second LO component and the second test signal is varied upon successive pulses of the second test signal, by an amount which is less than a linewidth of the Brillouin scattering component, then a width, line-shape and peak of the Brillouin scattering component. Typically, the linewidth of Brillouin scattering is of the order of 20-100 MHz. Therefore, the system 300 may be configured to vary the frequency difference between the second LO component and the second test signal by an increment between 20 and 100 MHZ, between successive pulses of the second test signal. This can be achieved by varying the frequency shift F2 applied to the second LO component by the modulator 30, and/or by varying a frequency shift applied to the second test signal by the pulse generator 20.
The launch stage 14 of system 400 is configured similarly to that of system 10, such that it generates a pulsed test signal which is transmitted to the first port of a circulator 26, to be launched along an optical fibre 1000. As with system 10, the pulse generator 20 may apply a frequency shift F1 to the pulsed test signal. However, the local oscillator stage 16 of system 400 is different compared to that of system 10. In particular, no modulator is provided in the local oscillator stage 16 of system 400. In the example shown, the local oscillator stage 16 of system 400 only includes an attenuator 32 for controlling a power of the LO signal. Thus, the LO signal generated in system 400 simply corresponds to the original light signal received from the light source 12, whose power may be adjusted by the attenuator 32. In other words, the LO signal in system 400 includes only one component, corresponding to the original light signal from the light source 12.
The second port of the circulator 26 in system 400 is coupled to the optical fibre 1000 via a wavelength splitter 52. The wavelength splitter 52 is configured to transmit the pulsed test signal to the optical fibre 1000. As discussed above, propagation of the test signal along the optical fibre 1000 results in a scattered signal having multiple scattering components, as illustrated in
The third port of the circulator 26 is connected to a first detector stage 28, such that the Rayleigh and Brillouin scattering components are received at the first detector stage 28. The wavelength splitter 52 is further coupled to a second detector stage 404, and configured to transmit the Raman scattering signal to the second detector stage 404.
The first detector stage 28 of system 400 operates in a corresponding manner to the detector stage 28 of system 10. In particular, the first detector stage 28 interferes different polarisation states of the received scattering components with the LO signal, to produce an output signal for each polarisation state. In this case however, as the LO signal only includes a single, non-frequency-shifted component, the output signal does not include a component corresponding to the DSTS signal discussed above. The output signal will include a component which is at a frequency F1, corresponding to the frequency shift between the test signal and the LO signal (which is at the same frequency as the light source 12). Thus, the first detector stage 28 of system 400 provides a DAS signal, in a similar manner to the detector stage 28 of system 10.
The second detector stage 404 is configured to enable a DTS measurement. Specifically, the second detector stage 404 comprises a wavelength splitter 54 (e.g. a WDM), which splits the Raman Stokes and Anti-Stokes components into two separate paths for detecting each component. A bandpass filter 56a, 56b is applied to each of the Stokes and Anti-Stokes components, in order to isolate each component. The second detector stage 404 includes a first detector 58a for detecting an amplitude of the Stokes component of the Raman scattering, and a second detector 58b for detecting an amplitude of the Anti-Stokes component of the Raman scattering. The detectors 58a, 58b may be high gain detectors, such as avalanche photodiode (APD) detectors, or photomultiplier detectors. The output from each detector 58a, 58b is measured by a respective analog-to-digital converter 60a, 60b. The ratio of the Stokes and Anti-Stokes components may then be calculated, to determine the temperature at a scattering location on the optical fibre 1000.
The system 600 is based on the architecture of the system 300, and further includes the second detection stage 404 of system 400. A wavelength splitter 52 is connected to the optical fibre 1000, to split the components of the scattered signal as discussed above in relation to the system 400. Thus, the first detector stage 28 receives the LO signal from the local oscillator stage 16 and the Rayleigh and Brillouin components of the scattered signal. The LO signal includes a first non-frequency shifted component and a second frequency-shifted component (e.g. as shown in
In embodiments discussed above, combined DAS and DSTS measurements are performed by generating a local oscillator signal with two components that are shifted in frequency relative to one another. An alternative approach for enabling combined DAS and DSTS measurements is to instead use a test signal which includes two components that are shifted in frequency relative to one another, which may yield analogous results to the approaches described above. Such an alternative approach for combined DAS and DSTS measurements will now be described, with reference to the optical measurement system 10 of
In this approach, the launch stage 14 is configured to generate a test signal having two components at different frequencies.
The test signal 700 may be generated by controlling the pulse generator 20, which may be in the form of an IQ modulator. In particular, the sequence of pulses 702 and 704 may be produced by modulating the amplitude and frequency of the signal over time with the pulse generator 20. The test signal 700 may then amplified by the optical amplifier 22 and passed through the attenuator 24, before being launched into the optical fibre 1000. The first pulse 702 and the second pulse 704 may be generated such that there is a suitable delay between the two pulses. For instance, the delay between the two pulses may be set so that cross phase modulation (XPM) between the two pulses is minimised (avoided), and so that the two pulses do not stimulate Brillouin emission for one another. As an example, the delay between the two pulses may be set such that there is a separation of approximately 100 m between the two pulses in the optical fibre 1000.
In this approach, the local oscillator stage 16 is configured to generate an LO signal with a single, non-frequency-shifted component. Thus, the LO signal is at the same frequency as the light source 12. Accordingly, the modulator 30 may be controlled so that no frequency shift is applied to the received light signal from the light source 12. Alternatively, the modulator 30 may be dispensed with in this approach, as no frequency modulation of the LO signal may be required.
When the scattered signal 706 received from the optical fibre 1000 is interfered with the LO signal on the detectors 40a, 40b, a series of interference components at different frequencies will be generated.
Typically, only the interference components 718 and 720 may be within the detection bandwidth of the detectors 40a, 40b, depicted by the dashed line 730 in
In the example illustrated in
A benefit of the approach depicted in
In some cases, the first approach described above in relation to
The below tables show example parameters of Brillouin scattering and Raman scattering in an optical fibre. In particular, Table 2 below shows example dependencies of the Brillouin scattering component on temperature and strain of the optical fibre. These dependencies may be used to obtain estimates of strain and/or temperature of the fibre from the DSTS signals obtained from embodiments discussed above. Due to the dependency of the Brillouin scattering component on both strain and temperature, in order to separate the strain and temperature effects, it may be necessary to determine both the frequency and the amplitude of the Brillouin scattering component. Table 2 also shows an example dependency of the Raman Anti-Stokes/Stokes ratio, which may be used to calculate the temperature of the fibre from the DTS signals obtained from embodiments discussed above.
Although a few preferred embodiments have been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except where at least some of such features and/or steps are mutually exclusive. In particular, various combinations of the systems 10, 300, 400 and 600 discussed above may be combined, in order to achieve various combinations of DAS, DTS and DSTS measurements on a same optical path.
Each feature disclosed in this specification (including any accompanying claims, abstract and drawings) may be replaced by alternative features serving the same, equivalent or similar purposes, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclose is one example only of a generic series of equivalent or similar features.
The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2200142.4 | Jan 2022 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/050121 | 1/4/2023 | WO |
