DE102023112656A1 - Apparatus, method and system for quantifying a change in environmental conditions along the length of an optical fiber - Google Patents

Abstract

An apparatus (100) for quantifying a change in environmental conditions along the length of an optical fiber (130) comprises a frequency-scanning optical pulse generator (110) for generating pulse trains of optical pulses; an optical component (120) coupled to the pulse generator (110) for injecting the optical pulses into an optical fiber (130) and redirecting the backscattered optical signal from the optical fiber (130); and a receiver module (150) coupled to the optical component (120). The receiver module (150) includes an interferometer (170) for determining the phase information of the backscattered optical signal as a function of time and optical frequency.

Description

background

A distributed fiber optic sensor system (DOFS) uses an optical fiber as a sensor element to detect environmental changes. Such systems enable seamless and spatially resolved measurements along the fiber. A phase-sensitive optical time domain reflectometry (φOTDR) is such a sensor system and is based on Rayleigh scattering within the optical fiber. When a series of optical interrogation pulses are coupled into the optical fiber, the light is backscattered at randomly distributed inhomogeneities in the fiber material. The backscattered optical signal is a fingerprint of the optical fiber that changes when the fiber is exposed to environmental variations such as temperature changes or mechanical stress. In recent years, this sensor system has been used in many fields, e.g. in structural health monitoring, intrusion monitoring, and seismic event detection.

A φOTDR system feeds coherent optical pulses into a sensor fiber and records the Rayleigh light backscattered by the pulses as a function of time. The obtained time domain signal can be converted to a distance based on the time of flight of the light. The environmental variations can change the properties of the local fiber, e.g. the refractive index and the size of the fiber section, so that the amplitude/intensity and the phase of the light backscattered from the corresponding location changes. The environmental variations can be quantified by the change in amplitude/intensity or the optical phase.

DE 10 2018 105 905 B4 and Liehr et al. “Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing” Optics Express Vol. 26, Issue 8, pp. 10573-10588 (2018 ) describe a method based on changing the amplitude/intensity of the backscattered light. The optical frequency of the interrogating optical pulses is changed periodically to obtain a reflection spectrum. A change in the fiber environment leads to a shift in the reflection spectrum and this shift can be detected. This method allows the measurement of environmental changes at a very low frequency (quasi-static) since the maximum measurable frequency is strongly limited by the scanning speed. A similar method is described in the US 2010 / 0 014 071 A1 revealed.

On the other hand, there are generally two methods to obtain the phase of the backscattered light in a φOTDR system. One method, as in US 9 170 149 B2 described is the coherent detection: the backscattered light is mixed with a local oscillator, its beat signal is detected and processed to obtain the phase information. The other method is based on various types of interferometric structures that convert the phase information into optical intensity. The output of the interferometer is recorded by direct detection and the obtained signal is processed to obtain the optical phase information. See eg A. Masoudi et al: “A distributed optical fiber dynamic strain sensor based on phase-OTDR”, Measurement Science and Technology, Volume 24 (2013), 085204 ; US 2021 / 0 033 430 A1 , and X. Lu et al: "Evaluating phase errors in phase-sensitive optical time-domain reflectometry based on I/Q demodulation", Journal of Lightwave Technology, Vol. 38, No. 15, (2020), 4133-4141 . The phase-based φOTDR systems described above can achieve a very high frequency which is limited by the length of the sensor fiber, but they suffer from different noise to measure the environmental changes at low frequency. Therefore, advanced signal processing for noise reduction is required to realize measurement at low frequencies, such as in US 2017 / 0 342 814 A1 explained.

With current φOTDR systems, only one property of the backscattered light, e.g. either the intensity/amplitude or the optical phase, can be determined and used for distributed measurements. However, the determined property is sensitive to many environmental parameters, such as strain and temperature, and it is very difficult to distinguish two environmental parameters based on a single property of the backscattered light. In practice, for example, cross-sensitivity has a negative influence on the strain measurement at low frequencies, since the temperature variations over time can also change the obtained properties of the backscattered light, resulting in an error. Additional sensor fibers or sensor systems are needed to compensate for the influence of the temperature change. As in Z. Ding et. al: “Distributed Strain and Temperature Discrimination Using Two Types of Fiber in OFDR,” IEEE Photonics Journal, Vol. 8, No. 5, (2016 ), Art No. 6804608, two different types of optical fibers can be used to distinguish temperature and strain changes based on the shift in the reflection spectrum. Or two distributed fiber measurement systems, such as a φOTDR system and a distributed temperature acquisition system system, can be used to simultaneously measure temperature and strain changes, as in R. Amer et al. described: “Field Applications of Distributed Fiber Optic Strain and Temperature Sensing for Caprock - Well Integrity and CO2 Leakage Monitoring”, Proceedings of the 16th Greenhouse Gas Control Technologies Conference (GHGT-16) 23-24 , Oct 2022. Consequently, the entire sensor system is complex and expensive.

Attempts have been made to measure both the intensity and the phase of the backscattered light, as in the above-mentioned US 2017 / 0 342 814 A1 mentioned. The US 2010 / 0 014 071 A1 However, the configuration described requires coherent detection to obtain the phase information, which requires a very high coherence of the light source for measurement over long distances.

Therefore, it is necessary to extend the frequency measurement range and to distinguish between strain and temperature changes in order to perform both high and low frequency measurements of environmental parameters of interest.

Summary

In view of the above, a device for quantifying a change in environmental conditions along the length of an optical fiber is proposed. The device enables the determination of two properties of the reflected optical signal by using a new configuration for the interrogating device such that at least two environmental parameters that affect the physical properties of the optical fiber can be measured. In particular, the phase and the intensity/amplitude of the backscattered optical signal can be determined, thereby reliably measuring both high frequency environmental changes such as vibrations and strains and low frequency environmental changes such as temperature changes. The device, which is coupled to an optical fiber, represents a distributed optical fiber sensing (DOFS) system.

The system and methods described here are based on both the phase and the amplitude/intensity of the backscattered Rayleigh light. The system sends frequency-sampling optical pulses into an optical fiber and detects backscattered light (backscattered optical signals) from the fiber. The amplitude/intensity correlation of the backscattered light obtained during scanning along the optical fiber provides a magnitude of the spectral shift that can be used for low frequency measurement. From the outputs of an interferometer in the measurement system, a phase difference is calculated that can be used for high frequency measurement. The phase difference obtained from a section of fiber isolated from environmental noise can also be used to determine the relative frequency change during scanning. A combination of the obtained spectral shift and the phase difference allows the changes in two parameters of interest to be determined.

An apparatus for quantifying a change in environmental conditions along the length of an optical fiber according to claim 1, a system including an optical fiber and an apparatus for quantifying a change in environmental conditions along the length of the optical fiber according to claim 15, and any of the methods according to claims 17-21 are provided to solve the above problem.

According to one embodiment, an apparatus for quantifying a change in environmental conditions along the length of an optical fiber comprises a frequency-sampling optical pulse generator, an optical component, and a receiver module comprising an interferometer, photodetectors, and a data processing device. The frequency-sampling optical pulse generator is configured to generate pulse trains of optical pulses having variable optical frequencies. The optical component is coupled to the frequency-sampling optical pulse generator to inject the optical pulses into an optical fiber and redirect the optical signal backscattered from the optical fiber. The receiver module is coupled to the optical component. The interferometer of the receiver module has an input for receiving the backscattered optical signal and a coupler for providing phase-shifted optical signal outputs, each optical signal output of the coupler being coupled to one of the photodetectors for providing electrical signals. The data processing device is coupled to the photodetectors for obtaining the electrical signals. The data processing device is configured to derive the phase information of the backscattered optical signal from the electrical signals as a function of time and optical frequency.

The use of an interferometer makes it possible to determine the phase of the backscattered optical signal with high reliability. The interferometer provides optical signal outputs with phases shifted relative to each other, so that when using suitable demodulation processing methods such as IQ demodulation, differentiation and cross-multiplication algorithm, the phase of the backscattered signal or the phase difference of the backscattered light along the optical fiber can be determined.

In a simple configuration, the interferometer has two optical outputs with a phase shift of, for example, 120°. The photodetectors can be arranged to form a balanced detector. If this configuration is extended, the interferometer has three outputs with a phase shift of, for example, 120°, with each optical output of the interferometer coupled to a corresponding photodetector.

The frequency-scanning optical pulse generator delivers coherent pulses so that the backscattered optical signals can interfere in the interferometer.

According to an embodiment that can be combined with any other embodiment described here, the data processing device is configured to derive, in addition to the phase information, amplitude and/or intensity information of the backscattered optical signal from the electrical signals as a function of time and the optical frequency. The amplitude and/or intensity information can be obtained from the evaluation of the electrical signals provided by the photodetectors.

Since only one optical source is required for determining the phase and amplitude/intensity, the device is cost-effective and space-saving.

According to an embodiment that can be combined with any other embodiment described herein, the frequency-scanning optical pulse generator is configured to periodically or repeatedly scan the optical frequency of the optical pulses through a predetermined optical frequency range. When the optical frequency is repeatedly scanned to generate a plurality of sequences of optical pulses, each sequence can be used to obtain a reflection spectrum. Changes in the reflection spectrum, such as a shift in the reflection spectrum, can be used as a parameter to quantify an environmental change (parameter of interest). One example is temperature variations.

According to an embodiment that can be combined with any other embodiment described herein, the frequency-sampling optical pulse generator comprises a coherent light source configured to receive an electrical drive signal that causes the light source to perform optical frequency scanning, and an optical gate configured to receive light from the light source and generate optical pulses. The drive signal can be provided by a driver to repeatedly or periodically scan the optical frequency. A linear change in the optical frequency within a sequence of pulses, with the optical frequency remaining constant for a single optical pulse, is advantageous for determining the spectral shift based on the amplitude/intensity information. The optical frequency of a single optical pulse can be considered constant because the sampling rate of the optical frequency is very low, in the kHz or sub-kHz range, and the optical sampling range is comparatively small, e.g., a few GHz. For a typical pulse duration (several tens of ns), the optical frequency can be approximately assumed to be constant within the duration of a pulse.

According to an embodiment that can be combined with any other embodiment described herein, the frequency-swept optical pulse generator comprises a coherent light source configured to operate at a fixed optical frequency, a modulator configured to be driven by an RF signal at a scanned microwave frequency, an optional narrow filter to select a sideband after modulation for optical frequency scanning, and an optical gate for generating optical pulses. The modulator generates sidebands of the light generated by the frequency-swept optical pulse generator, for example by amplitude modulation at microwave frequency. The narrow filter selects the desired sideband. However, the narrow filter is optional if the modulator only generates one sideband or if the modulator already suppresses unwanted sidebands.

According to an embodiment that can be combined with any other embodiment described herein, the frequency-sweeping optical pulse generator comprises a coherent light source configured to operate at a fixed optical frequency, a modulator configured to be driven by an RF pulse signal, and an optional narrow filter to select a post-modulation sideband for optical frequency scanning.

According to an embodiment that is compatible with any other embodiment described herein can be combined, the receiver module is configured to determine the relative optical frequency v' of the injected optical pulses relative to an optical reference frequency during scanning on the basis of the optical signal backscattered from the reference section of the optical fiber. For determining the phase and amplitude, in particular for obtaining reflection spectra, a linear change in the optical frequency of the pulses is desired. Determining the optical frequency v' of the injected optical pulses relative to an optical reference frequency during scanning offers a feedback option for adjusting the drive signal supplied to the generator for frequency-sampling optical pulses, in particular the light source. Alternatively or additionally, the determined optical frequency v' of the injected optical pulses can be used to interpolate the obtained reflection spectrum, for example if the optical frequency has not been changed linearly. Overall, the reflection spectra can be determined more reliably since information about the actually emitted optical frequency is obtained.

To determine the optical frequency v' of the injected optical pulses, optical signals can be used that are scattered back from a reference section of the optical fibers. The reference section is kept under defined and constant environmental conditions.

According to an embodiment that can be combined with any other embodiment described herein, the receiver module is configured to obtain reflection spectra for a particular position in the optical fiber based on sequences of pulses, wherein the receiver module is further configured to detect a frequency shift of the reflection spectra to acquire the local environmental information.

According to an embodiment, which can be combined with any other embodiment described here, the interferometer comprises two arms with different path lengths, the difference between the two arms being equal to or greater than the length of an optical pulse.

According to an embodiment that can be combined with any other embodiment described here, the receiver module comprises a splitter for splitting the backscattered optical signal into two optical sub-signals, a sub-module for determining amplitude and/or intensity information of the backscattered optical signal and a sub-module for determining phase information of the backscattered optical signal. The sub-module for determining amplitude and/or intensity information is coupled to the splitter to receive one optical sub-signal of the backscattered optical signal and the sub-module for determining phase information is coupled to the splitter to receive the other optical sub-signal of the backscattered optical signal.

The receiver module can therefore contain two optical branches, one for determining the amplitude/intensity and another for determining the phase. This opens up more possibilities in the operation of the device. For example, if only information on the amplitude/intensity is required for a certain time, e.g. to obtain the reflection spectra that allow the determination of low-frequency environmental changes, then only one branch, i.e. only one sub-module, can be in operation. This reduces the amount of data generated and thus facilitates data processing.

According to an embodiment that can be combined with any other embodiment described here, the submodule for determining amplitude and/or intensity information comprises a coupler for mixing the optical sub-signal with a local oscillator and a symmetrical detector for detecting the output of the coupler. The symmetrical detector can comprise two photodetectors: a beat signal of the mixed optical sub-signal and the signal of the local oscillator.

According to an embodiment that can be combined with any other embodiment described herein, the submodule for determining the phase information comprises the interferometer and the photodetectors for detecting the output of the interferometer.

The interferometer may comprise two arms, with one arm providing a delay of Δl. Note that a longer optical path corresponds to a delay, since path and time are coupled by the speed of light. The signal provided by the interferometer can thus be described as proportional to E(v _i ,z,t _m ) * E(v _i ,z + Δl,t _m ), where z represents a position in the optical fiber and v _i is the optical frequency of the pulse. The time t _m here represents the time at which the measurement was made. To obtain a single reflection spectrum for optical frequencies from v ₁ to v _p , it is assumed that the parameter of interest changes at a low frequency and can be considered constant over the time period needed to acquire a single reflection spectrum. A change in the parameter of interest, such as eg the temperature, can be determined by comparing reflection spectra recorded eg at t _m and t _m+q .

According to an embodiment that can be combined with any other embodiment described herein, the interferometer comprises a first arm and a second arm, the first arm having a longer optical path than the second arm. The coupler is a [3×3] coupler configured to output three optical signals with a phase shift of 120°, the first arm and the second arm being coupled to the respective inputs of the [3×3] coupler. Each optical signal output of the [3×3] coupler is coupled to one of the photodetectors to provide the electrical signals. The first arm introduces an optical delay as explained above.

und $$\begin{array}{c}{A}_{de}\left(z\right)\propto \sqrt{I^2\left(z\right)+Q^2\left(z\right)}\\ \mathrm{\Delta }\phi \left(z\right)={\text{tan}}^{-1}\left[I\left(z\right)\text{/}Q\left(z\right)\right]+2k\pi \end{array}$$

bestimmt werden, wobei P₁(z), P₂(z) und P₃(z) die von den Fotodetektoren erhaltenen bzw. erzeugten Signale bezeichnen, und wobei k eine ganze Zahl ist und der Term 2kπ einen Phasenentfaltungsprozess (Engl.: phase unwrapping process) darstellt, um den Wert der tan^-1 Funktion zu erweitern.According to an embodiment that can be combined with any other embodiment described here, the phase difference Δφ along the reference section and the detection section and the amplitude information A _de (z) are determined according to the following formula $$\{\begin{array}{c}I\left(z\right)=-P_1\left(z\right)\text{/}2+P_2\left(z\right)-P_3\left(z\right)\text{/}2\\ Q\left(z\right)=-\sqrt{3}{P}_1\left(z\right)\text{/}2+\sqrt{3}{P}_3\left(z\right)\text{/}2\end{array}$$

and $$\begin{array}{c}{A}_{de}\left(z\right)\propto \sqrt{I^2\left(z\right)+Q^2\left(z\right)}\\ \mathrm{\Delta }\phi \left(z\right)={\text{tan}}^{-1}\left[I\left(z\right)\text{/}Q\left(z\right)\right]+2k\pi \end{array}$$

where P ₁ (z), P ₂ (z) and P ₃ (z) denote the signals received and generated by the photodetectors, respectively, and where k is an integer and the term 2kπ represents a phase unwrapping process to expand the value of the tan ^-1 function.

According to an embodiment that can be combined with any other embodiment described herein, a system for quantifying a change in environmental conditions along the length of an optical fiber comprises a device according to any of the embodiments described herein and an optical fiber coupled to the optical component of the device. The device coupled to an optical fiber forms an operational system for distributed optical fiber monitoring (DOFS) based on phase-sensitive optical time domain reflectometry (φOTDR).

According to an embodiment that can be combined with any other embodiment described herein, the optical fiber comprises a reference section isolated from environmental disturbances and a detection section. The reference section can be used to determine the optical frequency v' of the injected optical pulses, as described above. The detection section can be used to determine the parameter(s) of interest in the environment.

According to an embodiment that can be combined with any other embodiment described herein, a method for determining changes in parameters of interest along the optical fiber for low and high frequency measurements comprises the following processes: providing optical pulses of different optical frequencies; injecting the optical pulses into an optical fiber; receiving an optical signal that is Rayleigh backscattered from the optical fiber; splitting the optical signal into two optical sub-signals; detecting one of the two optical sub-signals as an intensity of the backscattered optical signal as a function of time to obtain reflection spectra along the optical fiber; introducing the other of the two optical signals into an interferometer and detecting the output of the interferometer as a phase of the optical signal backscattered from the optical fiber as a function of time; and calculating a spectral frequency based on the correlation of the reflection spectrum along the optical fiber for a low frequency measurement and a phase difference based on the output of the interferometer for a high frequency measurement.

In this embodiment, the backscattered light is split, the intensity is determined directly, and the phase or phase difference is determined using the interferometer. Low frequency changes in the ambient conditions are determined using a correlation between the reflection spectra, while high frequency changes are determined using the phase difference.

According to an embodiment that can be combined with any other embodiment described herein, a method for determining changes of parameters of interest along the optical fiber for the measurement of low and high frequencies comprises the following processes: providing optical pulses of different optical frequencies; feeding the optical pulses into an optical fiber; receiving an optical signal Rayleigh backscattered from the optical fiber, which is fed into an interferometer; detecting the output of the interferometer as a function of time; calculating the amplitude information A _de and the phase difference of the backscattered optical signal based on the output of the interferometer and obtaining reflection spectra based on the amplitude information A _de ; and calculating a spectral frequency based on the correlation of the reflection spectrum and the amplitude information A _de for a low frequency measurement and the obtained phase difference for a high frequency measurement.

In this embodiment, the backscattered light is not split and the interferometer is used to determine both the amplitude (or intensity) and the phase difference. A direct determination of the intensity by a separate submodule is not required. The reflection spectra are determined from the amplitude. Low frequency changes in the environmental conditions are determined from a correlation between the reflection spectra, while high frequency changes are determined from the phase difference.

According to an embodiment that can be combined with any other embodiment described herein, a method for determining changes in parameters of interest along the optical fiber based on spectral shift and phase difference comprises the following processes: providing optical pulses of different optical frequencies; injecting the optical pulses into an optical fiber; receiving an optical signal that is Rayleigh backscattered from the optical fiber; splitting the optical signal into two optical sub-signals; detecting one of the two optical sub-signals as an intensity of the backscattered optical signal as a function of time to obtain reflection spectra along the optical fiber; introducing the other of the two optical signals into an interferometer and detecting the output of the interferometer as a phase of the optical signal backscattered from the optical fiber as a function of time; calculating a spectral frequency based on the correlation of the reflection spectrum along the optical fiber and a phase difference based on the output of the interferometer; and calculating the change in the two parameters of interest based on the spectral frequency and the phase difference.

In this embodiment, the backscattered light is split, the intensity is determined directly and the phase or phase difference is determined using the interferometer. The reflection spectra are determined based on the intensity. The changes in two parameters of the environment are determined based on the spectral frequency, which results from the correlation of the reflection spectra, and the phase difference.

According to an embodiment that can be combined with any other embodiment described herein, a method for determining changes of parameters of interest along the optical fiber based on the spectral shift and the phase difference comprises the following processes: providing optical pulses of different optical frequencies; feeding the optical pulses into an optical fiber; detecting the output of the interferometer as a function of time; calculating the amplitude information A _de and the phase difference of the backscattered optical signal based on the output of the interferometer and obtaining reflection spectra based on the amplitude information A _de ; calculating a spectral frequency based on the correlation of the reflection spectrum and the amplitude information A _de ; and calculating the change of the two parameters of interest according to a system of equations based on the spectral frequency and the phase difference.

In this embodiment, the backscattered light is not split and the interferometer is used to determine both the amplitude and the phase difference. A direct determination of the intensity by a separate submodule is not required. The changes in two parameters of the environment are determined using the spectral frequency, which is obtained from the correlation of the reflection spectra, and the phase difference.

According to an embodiment that can be combined with any other embodiment described herein, a method for determining changes in parameters of interest along the optical fiber based on the spectral shift and the phase difference comprises the following processes: providing optical pulses at preselected optical frequencies, one of the optical pulses being a reference pulse at a given optical frequency; injecting the optical pulses into an optical fiber comprising a reference section and a detection section, the reference section being maintained under defined and constant environmental conditions; receiving optical signals Rayleigh backscattered from the reference section of the optical fiber for each pulse, the optical signals being introduced into an interferometer; detecting the output of the interferometer as a function of time; calculating a phase difference at the reference section of the optical fiber based on the output of the interferometer; and calculating the relative ven change of the optical frequency relative to the given optical frequency of the reference pulse based on the obtained phase difference.

According to this embodiment, the change in the optical frequency of the pulses can be determined relative to the optical frequency of a reference pulse. This can be used either as feedback for driving the generator for frequency-scanning optical pulses including the coherent light source or for interpolating the optical frequency when determining the reflection spectra. The determination of the change in the optical frequency of the pulses can be used in combination with one of the above methods.

Furthermore, any of the above methods can be performed with the apparatus and system disclosed herein.

Short description of the drawings

• zeigt eine schematische Darstellung einer grundlegenden Ausführungsform für die verteilte Faserabtastung mit optischen Pulsen mit abgetasteter optischer Frequenz.
• zeigt eine schematische Darstellung einer beispielhaften Ausführungsform, die auf zwei Submodulen für die optische Phasenerfassung bzw. die Amplituden-/Intensitätserfassung basiert.
• zeigt eine schematische Darstellung einer anderen beispielhaften Ausführungsform, die auf einem einzigen Submodul zur gleichzeitigen Erfassung von optischer Phase und Amplitude/Intensität basiert.
• 4 zeigt mögliche Implementierungen des Submoduls für die Amplituden-/Intensitätserkennung, wobei 4(a) eine schematische Darstellung auf Basis der direkten Erfassung und 4(b) eine schematische Darstellung auf Basis der kohärenten Erfassung zeigt.
• 5 zeigt eine schematische Darstellung einer beispielhaften Ausführungsform eines Submoduls für die optische Phasenerfassung, das auch als Submodul für die Phasen- und Amplituden-/Intensitätserfassung verwendet werden kann.
• 6 zeigt einen beispielhaften Versuchsaufbau zur Demonstration einer Ausführungsform der vorliegenden Erfindung, basierend auf der in 3 gezeigten Ausführungsform.
• zeigt eine optische Phasendifferenz, die an einer Position des Referenzabschnitts der optischen Faser erhalten wurde, und die berechnete relative optische Frequenzänderung der Lichtquelle.
• 8 zeigt eine optische Phasendifferenz, die an einem Erfassungsabschnitt der optischen Faser erhalten wurde, der einer Vibration ausgesetzt war, wobei
• 8(a) und 8(b) die ungefilterte und die gefilterte Phasendifferenz über die Zeit zeigen.
• zeigt das Ergebnis der Kreuzkorrelation der Reflexionsspektren, die an einem Erfassungsabschnitt der optischen Faser, die Vibrationen ausgesetzt war, erhalten wurden.
• zeigt die spektralen Verschiebungen, die an einer ungestörten Position (1391,30 m) und einer Position (1407,65 m) des Erfassungsabschnitts der optischen Faser, die Vibrationen ausgesetzt war, erhalten wurden.

In the following, embodiments are described with reference to the drawings, without being limited thereto.

• shows a schematic representation of a basic embodiment for distributed fiber sensing with optical pulses of sampled optical frequency.
• shows a schematic representation of an exemplary embodiment based on two submodules for optical phase detection and amplitude/intensity detection, respectively.
• shows a schematic representation of another exemplary embodiment based on a single submodule for simultaneous detection of optical phase and amplitude/intensity.
• 4 shows possible implementations of the submodule for amplitude/intensity detection, where 4(a) a schematic representation based on direct recording and 4(b) shows a schematic representation based on coherent detection.
• 5 shows a schematic representation of an exemplary embodiment of a submodule for optical phase detection, which can also be used as a submodule for phase and amplitude/intensity detection.
• 6 shows an exemplary experimental setup for demonstrating an embodiment of the present invention, based on the 3 embodiment shown.
• shows an optical phase difference obtained at a position of the reference section of the optical fiber and the calculated relative optical frequency change of the light source.
• 8 shows an optical phase difference obtained at a detection portion of the optical fiber subjected to vibration, wherein
• 8(a) and 8(b) show the unfiltered and filtered phase difference over time.
• shows the result of cross-correlation of the reflection spectra obtained at a detection section of the optical fiber subjected to vibrations.
• shows the spectral shifts obtained at an undisturbed position (1391.30 m) and a position (1407.65 m) of the optical fiber sensing section subjected to vibrations.

Detailed description

The present disclosure provides a novel apparatus, method and system for measuring or determining the change of one or more parameters of interest, e.g. temperature and strain, along a sensor fiber. The method is based on the reflection spectrum and optical phase of the backscattered light, which can be obtained with a single device such as a φOTDR device.

When the present disclosure refers to "light" or "light source", this does not only mean visible light, but electromagnetic radiation or a "light source" that is capable of generating electromagnetic radiation in the visible range and also in the non-visible range such as ultraviolet, near infrared and mid-infrared. A typical operating range is the near infrared between about 780 nm and 3 µm. For the sake of simplicity, the term "light" or "light source" is used below without being limited to visible light.

shows a schematic representation of a device 100, eg a φOTDR device, for quantifying a change in environmental conditions along an optical fiber according to an embodiment.

The device 100 comprises an optical frequency control or sampling of optical pulses, which is embodied here by a generator for frequency-sampling optical pulses 110, hereinafter referred to as pulse generator 110. The Pulse generator 110 is coupled to an optical component 120 that can inject optical pulses generated by pulse generator 110 into an optical fiber 130. Optical fiber 130 is often referred to as a “fiber under test” (FUT).

The pulse generator 110 is configured to generate optical pulse trains at different optical frequencies. The generated pulses are coherent optical pulses. For example, a pulse train may comprise a plurality of pulses with varying optical frequency, e.g. with increasing or decreasing optical frequency. In a typical application, all pulses of a pulse train have the same pulse width, i.e., duration. The spacing between successive pulses may be chosen such that the next pulse is generated after the backscattered optical signal of the previous pulse has been received, so that successive pulses do not interfere with each other.

The optical element 120 serves two main purposes, namely coupling the optical pulses generated by the pulse generator 110 into the optical fiber 130 and coupling the backscattered optical signal of an optical pulse into a receiver module 150 that is coupled to the optical element 120. The optical element 120 can be, for example, a circulator, an optical coupler or a fast optical switch.

The optical fiber 130, or test fiber, is a fiber that can be coupled to the optical element 120 of the device and can be replaced by another optical fiber. Therefore, the device 100 can be used with different fibers and coupled to a fiber 130 that is, for example, integrated into a building or an infrastructure object such as a bridge or a geological site such as a cavern for oil or gas storage. When coupled to an optical fiber 130, the device 100 forms together with the optical fiber 130 a system for quantifying a change in environmental conditions along the length of an optical fiber, i.e. a distributed optical fiber sensor system (DOFS), which can be implemented as a φOTDR system.

According to one embodiment, the optical fiber 130 of the φOTDR system comprises a reference section 132 and a detection section 131. The reference section 132 of the optical fiber 130 is maintained under constant environmental conditions to isolate the reference section 132 from environmental changes. Since the reference section 132 is not subject to environmental disturbances, the optical signals backscattered by the reference section 132 can be used to determine the relative frequency change v' of the optical pulses introduced into the optical fiber 130. The detection section 131 serves to detect environmental changes that affect the physical properties of the optical fiber 130 to realize a distributed detection system.

As mentioned above, a φOTDR device detects the Rayleigh backscattered signal along the optical fiber 130. The Rayleigh scattering is caused by the inhomogeneities in the optical fiber 130. The inhomogeneity scatters the light in all directions, a small part of the scattered light is recaptured by the fiber 130 and propagates backwards to the input of the incident light.

In a φOTDR system, coherent optical pulses are generated by the frequency-scanning optical pulse generator 110, fed into the optical fiber 130 and propagated along it. Each pulse is continuously scattered at the inhomogeneity during propagation. The light that is backscattered within half the length of the pulse arrives at the receiver module 150 at the same time. The detected light at a certain position z is therefore the sum of the light that is backscattered at the inhomogeneities within half the pulse length.

The optical signal obtained in a φOTDR system depends on the optical frequency of the pulse and the scattering conditions. The properties of the inhomogeneity, such as location, size, density and refractive index, vary randomly along the fiber. Therefore, the scattering conditions are different at each fiber position. As a result, the detected backscattered light, i.e. the backscattered optical signal, e.g. the Rayleigh intensity trace, exhibits a stochastic profile along the fiber. This noise-like profile is static if the working conditions remain constant, e.g. no temperature and strain variations and no optical frequency change of the incident pulses.

When the ambient conditions and/or the optical frequency of the pulses vary, the scattering conditions change. As a result, the amplitude A(z), the intensity |A(z)| ² and the phase φ(z) of the backscattered light also change. The φOTDR device 100 detects the change in amplitude, intensity or phase to quantify the change in the environment.

According to an embodiment that is compatible with any other embodiment described herein bind, the φOTDR device 100 uses the frequency shift of the reflection spectrum at a particular position in the optical fiber 130 to capture the local environment information. In this technique, the optical frequency of the pulse is scanned, producing a sequence of pulses with, e.g., increasing optical frequency, and a group of Rayleigh intensity traces are recorded for each frequency scan ("sequence"), which can be viewed as a matrix in the distance and optical frequency domain. At each fiber position, the signal obtained changes with optical frequency, and this signal change is considered to be the local reflection spectrum at the corresponding position. When the local environment changes, this spectrum shifts in the frequency domain compared to the previously obtained spectrum at the same position. The spectral shift Δv can be determined, e.g., by cross-correlating the two spectra. Once the spectral shift is determined, the environment change can be quantified.

The method for determining the spectral shift typically uses linear optical frequency scanning so that the Rayleigh intensity trace is obtained in a uniform frequency step. In practice, linear frequency scanning can be realized by RF modulation of the light, but this requires expensive and sophisticated equipment. Direct modulation of the drive current of the light source, as used here in an embodiment that can be combined with any other embodiment described here, is an economical solution for optical frequency scanning, but usually in a non-linear manner. In this case, interpolation can be used if the optical frequency v can be determined during scanning so that the detected signal of the backscattered light can be reconstructed with a uniform frequency step. In this method, the change in optical frequency is monitored, e.g. by means of the light backscattered from the reference section 132 of the optical fiber 130. Based on the optical frequency change, the reflection spectrum obtained at a certain position after a frequency scan can be interpolated so that the frequency step of the interpolated spectrum is uniform. A more detailed description of the interpolation of the detection spectrum is given in S. Liehr et al. included: “Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing”, Optics Express, Vol. 26, Issue 8, (2018), 10573-10588 , which are incorporated herein by reference in their entirety.

$$\mathrm{\Delta }{v}_{\xi }\text{/}v=-\left(1-p_{\epsilon }\right)\mathrm{\Delta }\epsilon \approx -0.78\mathrm{\Delta }\epsilon$$

Wobei Δv_T und ΔV_ε die durch die Temperaturänderung ΔT bzw. die Dehnungsänderung Δε induzierte spektrale Verschiebung sind, ξ den thermooptischen Koeffizienten der Sensorfaser darstellt, α der thermische Ausdehnungskoeffizient der Sensorfaser ist und p_ε den effektiven optischen Ausdehnungskoeffizienten der Sensorfaser 130 darstellt.Temperature and strain variations can cause the shift of the reflection spectrum, and the induced spectral shift can be expressed as $$\mathrm{\Delta }{v}_T\text{/}v=-\left(\xi +\alpha \right)\mathrm{\Delta }T\approx -6.92\times {10}^{-6}\mathrm{\Delta }T$$

$$\mathrm{\Delta }{v}_{\xi }\text{/}v=-\left(1-p_{\epsilon }\right)\mathrm{\Delta }\epsilon \approx -0.78\mathrm{\Delta }\epsilon$$

Where Δv _T and ΔV _ε are the spectral shift induced by the temperature change ΔT and the strain change Δε, respectively, ξ represents the thermo-optical coefficient of the sensor fiber, α is the thermal expansion coefficient of the sensor fiber and p _ε represents the effective optical expansion coefficient of the sensor fiber 130.

The optical phase φ(z) of the backscattered light (backscattered optical signal) depends on the environmental conditions and can also be used for quantitative measurements. According to an embodiment that can be combined with any other embodiment described here, the φOTDR device 100 obtains the phase by coherent detection, in which a local oscillator is used to mix with the backscattered light. Various signal processing methods, such as IQ demodulation and analog circuits, can be used to extract the phase information from the beat signal between the local oscillator and the backscattered light. Based on the coherent detection, the optical phase profile along an optical fiber can be determined for each pulse.

The phase information is lost when the backscattered light directly hits a square detector. However, the φOTDR device 100 described here is configured to obtain the phase information using an interferometric structure, for example a Mach-Zehnder interferometer and a Michelson interferometer. The output of the interferometer can be directly detected. Various signal processing methods can be used to obtain the phase difference Δφ.

wobei t für die Zeit, n für den Brechungsindex der Faser und c für die Lichtgeschwindigkeit im Vakuum steht. Während des Frequenzscans ändert sich die optische Frequenz mit der Zeit.The phase actually varies randomly with an environmental change, but the phase difference Δφ between two fiber positions has a quasi-linear relationship with the external variation. The position interval Δl used to determine the phase difference is usually set uniformly along the optical fiber 130. The phase difference determined at position z and at measurement time t can be expressed as follows: $$\mathrm{\Delta }\phi \left(z,t\right)=4\pi n\mathrm{\Delta }lv\left(t\right)\text{/}c$$

where t is time, n is the refractive index of the fiber, and c is the speed of light in vacuum. During the frequency scan, the optical frequency changes with time.

wobei γ der thermo-optische Koeffizient der optischen Faser 130 ist, η der thermische Ausdehnungskoeffizient der optischen Faser 130 ist, l_T gleich dem kleineren Wert zwischen der Faserlänge unter der Temperaturänderung und dem Wert Δl ist.Temperature variations in the environment can change the refractive index and the length of the optical fiber 130, resulting in a change in the phase difference. The temperature-related change in the phase difference can be expressed as follows $$\mathrm{\Delta }{\phi }_T=4\pi \gamma \mathrm{\Delta }T{l}_{T}v\text{/}c+4\pi n\eta l_{T}v\text{/}c$$

where γ is the thermo-optical coefficient of the optical fiber 130, η is the thermal expansion coefficient of the optical fiber 130, l _T is equal to the smaller value between the fiber length under the temperature change and the value Δl.

wobei l_Δε gleich dem kleineren Wert zwischen der Faserlänge unter der Dehnungsänderung und dem Wert Δl ist.The strain-induced phase difference change can be expressed as follows $$\mathrm{\Delta }{\phi }_{\epsilon }\approx 4\pi n{l}_{\mathrm{\Delta }\epsilon }v\text{/}c\cdot \left(1-0.1n^2\right)\mathrm{\Delta }\epsilon$$

where l _Δε is equal to the smaller value between the fiber length under the strain change and the value Δl.

wobei t_ref die Referenzzeit ist. Die erhaltene relative Frequenz v' kann über die gesamte Referenzfaser gemittelt werden, um den Einfluss von Rauschen zu reduzieren und ein genaueres Ergebnis zu erzielen.As can be seen from Eq. (3), the phase difference also depends on the optical frequency v of the pulse. This means that the phase difference Δφ changes with the optical frequency even when there is no environmental variation. Thus, the change of the optical frequency during scanning can be monitored from the phase difference obtained from the reference section 132 of the optical fiber 130, which is well isolated from environmental disturbances. It is advantageous to determine the optical frequency v(t) to interpolate the obtained signal when the optical frequency scanning is not strictly linear. The optical frequency determined during scanning can also be used in a feedback loop to change the drive signal applied to the light source to realize linear scanning. Compared with the absolute optical frequency v, the frequency change v' relative to a reference time during scanning, e.g., at the beginning of each scan, is more important for the φOTDR detection method based on the frequency shift of the reflection spectrum. The relative frequency v' at any time t can be determined by the optical phase difference as $$v{\text{'}}^{\left(t\right)}=\left[\mathrm{\Delta }\phi \left(t\right)-\mathrm{\Delta }\phi \left(t_{ref}\right)\right]c\text{/}4\pi n\mathrm{\Delta }l$$

where t _ref is the reference time. The obtained relative frequency v' can be averaged over the entire reference fiber to reduce the influence of noise and obtain a more accurate result.

According to an embodiment that can be combined with any other embodiment described herein, the relative optical frequency v' of the injected optical pulses relative to an optical reference frequency is determined during scanning based on the optical signal backscattered from the reference section 132 of the optical fiber 130. Optical pulses are injected at preselected optical frequencies, one of the optical pulses being a reference pulse with a particular optical frequency. The relative optical frequency v' of the injected optical pulses is determined relative to an optical reference frequency, i.e. the predetermined optical frequency of the reference pulse.

Conventional φOTDR devices can only determine either the amplitude/intensity or the optical phase of the backscattered light to perform quantitative measurements. In contrast, the present disclosure provides a new φOTDR device 100 and method that can determine both amplitude/intensity and the optical phase, i.e., two properties of the backscattered optical signal, and that can therefore reliably evaluate at least two environmental parameters.

Another example of a φOTDR sensor device 100 according to an embodiment, which can be combined with any other embodiment described here, is shown in 2 The device 100 includes a coherent light source 111, which may be, for example, a distributed feedback semiconductor laser. A driver 115 provides a drive signal to the light source 111 to change the operating temperature or current of the light source 111 to scan the optical frequency of the output of the light source 111. Another way to realize the frequency scanning is to use a frequency shifter, such as an electro-optic modulator and an acousto-optic modulator, controlled by an RF signal with a tunable frequency. In this case, the drive signal for the light source 111 is optional since the light source 111 is operated at a fixed optical frequency.

The coherent light source 111 can operate at any wavelength, but typically in the near infrared range, e.g. between about 780 nm and 3 µm. The linewidth of the light source is typically narrow, e.g. on the order of kHz.

The device 100 also includes an optical gate 112 to convert the continuous wave of the light source 111 into optical pulses. The optical gate 112 may be, for example, an electro-optic modulator, an acousto-optic modulator, and a semiconductor optical amplifier.

Alternatively, the optical frequency scanning and pulse generation may be performed by only one opto-electrical component, e.g. an electro-optical modulator. In this case, the optical frequency of the light source 111 remains fixed over time. An RF pulse with tunable microwave frequency is applied to the modulator (optical gate 112) so that the modulation generates sidebands in the optical frequency range. The optical sidebands are modulated into pulses because the applied RF signal is pulsed. If more than one sideband is generated, an optical filter may be used to select one of the generated sidebands that is the optical pulse with the scanned frequency.

After pulse conversion, an optical amplifier 113 may be used in the device 100 to amplify the pulse to a desired level. For example, an erbium-doped fiber amplifier may be used as the optical amplifier 113.

The coherent light source 111, the optical gate 112, the optical amplifier 113 and the driver 115 may be part of the frequency-scanning optical pulse generator 110. According to an embodiment that may be combined with any other embodiment described herein, the frequency-scanning optical pulse generator 110 is configured to scan the frequency of the optical pulses through a predetermined optical frequency range. According to an embodiment that may be combined with any other embodiment described herein, the frequency-scanning optical pulse generator 110 may further comprise a light source 111 driven by an electrical signal for optical frequency scanning and an optical gate 112 for generating optical pulses.

The output of the frequency-scanning optical pulse generator 110, in the present embodiment the output of the optical amplifier 113, is coupled to the optical element 120. The optical element 120 has three ports (inputs and outputs, respectively) in the present embodiment. One port, designated as port A, is coupled to the frequency-scanning optical pulse generator 110, another port, designated as port B, is coupled to the optical fiber 130, and another port, designated as port C, is coupled to the receiver module. The light received at port A is only passed to port B, while the light received at port B is only passed to port C. No light enters port C.

Each pulse of the pulse trains is fed via the optical component 120 into the optical fiber 130, which includes the reference section 132 and the detection section 131. The optical component 120 also redirects the backscattered light, i.e. the backscattered optical signal, from the optical fiber 130 into the receiver module 150. In the receiver module 150, the backscattered light may be split by a splitter 156, and the partial optical signals generated by the splitter 156 are passed to a submodule 151 for determining amplitude and/or intensity information of the backscattered optical signal and a submodule 152 for determining phase information of the backscattered optical signal.

Amplitude and/or intensity information refers to one or more values representing the amplitude and/or intensity of the backscattered light representative of a particular location (position) in the fiber 130 when coherent detection is used to obtain the backscattered light. When an interferometer is used and the output of the interferometer is directly detected, the amplitude information refers to a value obtained as amplitude in the IQ demodulation process and which is proportional to the product of the amplitude of the light backscattered at two different positions. The intensity information in this case is the square of the amplitude. On the other hand, the phase information refers to a value representing the phase or a phase difference of the backscattered light at a particular location (position) in the fiber 130.

Another example of a φOTDR sensor device 100 according to an embodiment, which can be combined with any other embodiment described here, is shown in 3 which uses a single submodule within the receiver module 150 to determine both amplitude and/or intensity information of the backscattered optical signal and phase information of the backscattered optical signal.

The receiver module 150 can therefore comprise two submodules, a submodule 151 for determining amplitude and/or intensity information of the backscattered optical signal and a further submodule 152 for determining phase information of the backscattered optical signal, or a single submodule 152, which determines both amplitude and/or intensity information of the backscattered optical signal and phase information of the backscattered optical signal.

shows a schematic representation of the submodule 151 for determining amplitude and/or intensity information based on direct detection. The partial optical signal 154, which is generated by the splitter 156 from the backscattered optical signal, passes directly into a photodetector 161. The output of the photodetector 161 provides the information about the amplitude and/or intensity of the backscattered light.

Another possible scheme for the submodule 151 to determine amplitude and/or intensity information is based on coherent detection, as in The partial optical signal 154 generated by the splitter 156 and the light 153 from a local oscillator operating at a fixed frequency are combined by a coupler 155. The output of the coupler 155 is directed to a detector consisting of two photodetectors 162, 163 in a symmetrical configuration. The output of the symmetrical detector provides the beat signal between the backscattered light 154 and the light 153 from the local oscillator.

The output of the amplitude/intensity detection sub-module 151 is processed by a data processing device 158 using cross-correlation or other methods to obtain the spectral shift Δv along the reference section 132 and the detection section 131. The environmental change can then be quantified based on the obtained shift Δv for the low frequency and even quasi-static measurement.

5 is a schematic representation of a possible submodule 152 for determining the phase information. The backscattered light 154 is split by a splitter 172 and passes through a first arm 173 and a second arm 174 of a Mach-Zehnder interferometer 170. The first arm 173 and the second arm 174 are directed to two inputs of a [3×3] coupler 171, so that the light passing through the two arms 173, 174 is coupled and interferes at the [3×3] coupler 171. The third input of the [3×3] coupler 171 is unused and can be blocked. The [3×3] coupler 171 not only splits the incident light equally between three outputs, but also introduces a phase shift of 120° between the outputs. The length of the first arm 173 is different from the length of the second arm 174. For example, the first arm 173 is in longer, so that the light traveling along this path is delayed compared to the light in the second arm 174. Therefore, the interference result of the light traveling along the two arms 173, 174 of the interferometer 170 depends on the phase difference of the reflected light from two locations (positions) within the fiber that are spaced apart equal to the difference in length of the two arms 173, 174. The difference in length between the first arm 173 and the second arm 174, expressed as Δl in Eq. (3), is approximately equal to or greater than the length of the optical pulse. Note that the 5 The Mach-Zehnder interferometer 170 shown is only an example. Other interferometer structures, such as the Michelson interferometer, can also realize the same functionality.

The outputs of the phase interrogation submodule 152 can be processed to obtain the phase difference Δφ. For example, a differential and cross demodulation method can be used to obtain the phase difference Δφ along the reference and detection sections 131. The phase difference can be used to quantify environmental changes. In coherent detection, the optical phase along the fiber is first determined and then the difference is calculated. In direct detection using interferometers, the phase difference can be calculated directly.

zu verarbeiten, wobei P₁(z), P₂(z), P₃(z) die Signale bezeichnen, die von den Fotodetektoren 164, 165 bzw. 166 erhalten bzw. erzeugt werden.Another method is to use the output of the interferometer P ₁ (z), P ₂ (z), P ₃ (z) as $$\{\begin{array}{c}I\left(z\right)=-P_1\left(z\right)\text{/}2+P_2\left(z\right)-P_3\left(z\right)\text{/}2\\ Q\left(z\right)=-\sqrt{3}{P}_1\left(z\right)\text{/}2+\sqrt{3}{P}_3\left(z\right)\text{/}2\end{array}$$

where P ₁ (z), P ₂ (z), P ₃ (z) denote the signals received and generated by the photodetectors 164, 165 and 166, respectively.

und $E_B=\frac{1}{\sqrt{2}}E\left(z+\mathrm{\Delta }l\right)e^{\frac{j\mathrm{\pi }}{2}},$

wobei E(z) das optische Feld des rückgestreuten optischen Signals an der Position z darstellt, das Amplituden- und Phaseninformationen enthält, λ die Wellenlänge darstellt und der Koefffizient √1/2 und die π/2-Phasenverschiebung in E_B durch den Splitter 172 induziert werden.The first arm 173 of the interferometer 170 is longer than the second arm 174 by a length of Δl, which can be considered as the measuring length of the sensor. The light passing through the first arm 173 is delayed so that the optical signals appearing simultaneously at the inputs of the [3×3] coupler can be written as ${\text{E}}_{\text{A}}=\frac{1}{\sqrt{2}}\text{E}\left(\text{z}\right){\text{<figure-callout id="2" label="e j" filenames="DE102023112656A1\_0022.png" state="{{state}}">e</figure-callout>}}^{\frac{\text{j}}{}}$

and $E_B=\frac{1}{\sqrt{2}}E\left(z+\mathrm{\Delta }l\right)e^{\frac{j\mathrm{\pi }}{2}},$

where E(z) represents the optical field of the backscattered optical signal at position z, which contains amplitude and phase information, λ represents the wavelength, and the coefficient √1/2 and the π/2 phase shift in E _B are induced by the splitter 172.

The three optical outputs of the interferometer 170 are given by $$\{\begin{array}{c}{\text{<figure-callout id="1" label="E" filenames="DE102023112656A1\_0022.png" state="{{state}}">E</figure-callout>}}_1=\frac{1}{\sqrt{3}}\cdot \left({\text{E}}_{\text{A}}+{\text{E}}_{\text{<figure-callout id="2" label="B e j" filenames="DE102023112656A1\_0022.png" state="{{state}}">B</figure-callout>}}{\text{e}}^{\text{j}}{2\mathrm{\pi }\text{/}3}^{}\right)\\ {\text{<figure-callout id="2" label="E" filenames="DE102023112656A1\_0022.png" state="{{state}}">E</figure-callout>}}_2=\frac{1}{\sqrt{3}}\cdot \left({\text{E}}_{\text{<figure-callout id="2" label="A e j" filenames="DE102023112656A1\_0022.png" state="{{state}}">A</figure-callout>}}{\text{e}}^{\text{j}}{2\mathrm{\pi }/3}^{}+{\text{E}}_{\text{<figure-callout id="2" label="B e j" filenames="DE102023112656A1\_0022.png" state="{{state}}">B</figure-callout>}}{\text{e}}^{\text{j}}{2\mathrm{\pi }\text{/}3}^{}\right)\\ {\text{<figure-callout id="1" label="E" filenames="DE102023112656A1\_0022.png" state="{{state}}">E</figure-callout>}}_1=\frac{1}{\sqrt{3}}\cdot \left({\text{E}}_{\text{<figure-callout id="2" label="A e j" filenames="DE102023112656A1\_0022.png" state="{{state}}">A</figure-callout>}}{\text{e}}^{\text{j}}{2\mathrm{\pi }\text{/}3}^{}+{\text{E}}_{\text{B}}\right)\end{array}$$

ist, kann die obige Formel umgeschrieben werden in: $$\{\begin{array}{c}{\text{P}}_1\left(\text{z}\right)={\text{P}}_{\text{DC}}\left(\text{z}\right)-{\text{P}}_{\text{AC}}\left(\text{z}\right)\text{sin}\left[\mathrm{\Delta \phi }\left(\text{z}\right)+2\mathrm{\pi }\text{/}3\right]\\ {\text{P}}_2\left(\text{z}\right)={\text{P}}_{\text{DC}}\left(\text{z}\right)-{\text{P}}_{\text{AC}}\left(\text{z}\right)\text{sin}\mathrm{\Delta \phi }\left(\text{z}\right)\\ {\text{P}}_3\left(\text{z}\right)={\text{P}}_{\text{DC}}\left(\text{z}\right)-{\text{P}}_{\text{AC}}\left(\text{z}\right)\text{sin}\left[\mathrm{\Delta \phi }\left(\text{z}\right)-2\mathrm{\pi }\text{/}3\right]\end{array}$$

wobei Δφ = φ(z + Δl) - φ(z) -2π·Δl/λ die Phasendifferenz zwischen der Faserposition z und z + Δl ist, P_DC = [P(z) + P[z + Δl)]/6, und P_AC = /E(z)//E(z + Δl)//3, wobei /E(z)/ die Feldamplitude des rückgestreuten optischen Signals an der Position z darstellt. Der Term 2π·Δl/λ, der in der Verzögerung des ersten Arms 173 enthalten ist, kann als Konstante betrachtet werden und wird daher bei der Berechnung vernachlässigt.Da $P_i=E_i{E}_i^{*}$

the above formula can be rewritten as: $$\{\begin{array}{c}{\text{P}}_1\left(\text{z}\right)={\text{P}}_{\text{DC}}\left(\text{z}\right)-{\text{P}}_{\text{AC}}\left(\text{z}\right)\text{sin}\left[\mathrm{\Delta \phi }\left(\text{z}\right)+2\mathrm{\pi }\text{/}3\right]\\ {\text{P}}_2\left(\text{z}\right)={\text{P}}_{\text{DC}}\left(\text{z}\right)-{\text{P}}_{\text{AC}}\left(\text{z}\right)\text{sin}\mathrm{\Delta \phi }\left(\text{z}\right)\\ {\text{P}}_3\left(\text{z}\right)={\text{P}}_{\text{DC}}\left(\text{z}\right)-{\text{P}}_{\text{AC}}\left(\text{z}\right)\text{sin}\left[\mathrm{\Delta \phi }\left(\text{z}\right)-2\mathrm{\pi }\text{/}3\right]\end{array}$$

where Δφ = φ(z + Δl) - φ(z) -2π·Δl/λ is the phase difference between fiber position z and z + Δl, P _DC = [P(z) + P[z + Δl)]/6, and P _AC = /E(z)//E(z + Δl)//3, where /E(z)/ is the field amplitude of the backscattered optical signal at position z. The term 2π·Δl/λ contained in the delay of the first arm 173 can be considered as a constant and is therefore neglected in the calculation.

wobei k eine ganze Zahl ist und der Term 2kπ einen Phasenentfaltungsprozess darstellt, um den Wert der tan^-1 Funktion zu erweitern. Der Phasenentfaltungsprozess ist in der Regel notwendig, weil das Ergebnis der tan^-1 Funktion auf den Bereich [-π/2, π/2] begrenzt ist, der vom tatsächlichen Wert der Phasendifferenz abweichen kann. Ein Phasenentfaltungsprozess ist in der Lage, das Ergebnis der tan^-1 Funktion so zu erweitern, dass die richtige Phasendifferenz erhalten wird.Then the amplitude information A _de (z) and the phase difference can be calculated as follows $$\begin{array}{c}{A}_{de}\left(z\right)\propto \sqrt{I^2\left(z\right)+Q^2\left(z\right)}\\ \mathrm{\Delta }\phi \left(z\right)={\text{tan}}^{-1}\left[I\left(z\right)\text{/}Q\left(z\right)\right]+2k\pi \end{array}$$

where k is an integer and the term 2kπ represents a phase unfolding process to expand the value of the tan ^-1 function. The phase unfolding process is usually necessary because the result of the tan ^-1 function is limited to the range [-π/2, π/2], which may deviate from the actual value of the phase difference. A phase unfolding process is able to expand the result of the tan ^-1 function so that the correct phase difference is obtained.

The obtained phase difference changes during frequency sampling as shown in Eq. (3). The influence of frequency sampling on the phase difference can be easily removed by frequency filtering. For example, a notch filter can be used to suppress the phase difference at the sampling frequency. Based on the obtained phase difference, a high frequency measurement can be performed. The environmental fluctuations can be quantified from the filtered phase difference.

The in The configuration of the submodule 152 shown can also be used as a single submodule for both the phase and the amplitude/intensity detection 150 in the shown configuration. The phase difference can be obtained as described above. The spectral shift is determined based on the amplitude information A _de (z) obtained by equation (10). The obtained signal A _de (z) is basically proportional to the product of A(z) and A(z+Δl) of the backscattered light, therefore A _de (z) corresponds to the product of the reflection spectra at two different positions z and z+Δl. The two spectra can be considered independent if the two positions are separated by a distance greater than the pulse length. Consequently, the spectral shift can also be obtained from the amplitude information A _de (z) for the low frequency measurement.

The receiver module 150 can 5 Submodule 152 shown in combination with one of the 4(a) or 4(b) shown submodules 151, or alternatively it may only include the one shown in 5 shown submodule 152, since submodule 152 allows both the amplitude/intensity and the phase to be determined based on the evaluation of the signals from photodetectors 164, 165, 166 as described above. The use of one of the submodules 151 in addition to submodule 152 provides a more direct detection of the amplitude/intensity and can relieve the data processing device 158. It also allows the selection of only one submodule if only one environmental parameter is of interest or is to be detected.

The data processing device 158 receives the electrical signals from the photodetectors, which may also be preprocessed, and performs the calculations described herein. The data processing device 158 may include A/D converters to generate digital values that are used by a computer as input for the calculation.

With the 1 until 6 In the detection device shown, both the spectral shift Δv and the phase difference Δφ can be determined simultaneously. The spectral shift can be used to measure environmental changes in the low frequency range, while the phase difference can be used for high frequency measurement. In addition, the phase difference determined at the reference section of the fiber can also be used to determine the relative frequency change during scanning, as shown in Eq. (6).

wobei K_v,T und K_v,ε jeweils die Temperatur- und Dehnungsempfindlichkeiten für das Reflexionsspektrum und K_φ,T und K_φ,ε jeweils die Temperatur- und Dehnungsempfindlichkeiten für die optische Phase darstellen. Folglich können die Temperatur- und Dehnungsänderungen entlang der Sensorfaser gleichzeitig durch die spektrale Verschiebung Δv und die Phasendifferenz Δφ ermittelt werden: $$\{\begin{array}{c}\mathrm{\Delta }T\left(z\right)=\frac{K_{\phi ,\epsilon }\mathrm{\Delta }v\left(z\right)-K_{v,\epsilon }\mathrm{\Delta }\phi \left(z\right)}{K_{v,\epsilon }{K}_{\phi ,T}-K_{\phi ,\epsilon }{K}_{v,T}}\\ \mathrm{\Delta }\epsilon \left(z\right)=\frac{K_{\phi ,T}\mathrm{\Delta }v\left(z\right)-K_{v,T}\mathrm{\Delta }\phi \left(z\right)}{K_{v,\epsilon }{K}_{\phi ,T}-K_{\phi ,\epsilon }{K}_{v,T}}\end{array}$$

Both the spectral shift Δv and the phase difference Δφ are sensitive to temperature and strain changes, so that a system of equations can be set up as follows $$\{\begin{array}{c}\mathrm{\Delta }v\left(z\right)=K_{v,T}\mathrm{\Delta }T+K_{v,\epsilon }\mathrm{\Delta }\epsilon \\ \mathrm{\Delta \phi }\left(z\right)=K_{\mathrm{\phi },T}\mathrm{\Delta }T+K_{\mathrm{\phi },\epsilon }\mathrm{\Delta }\epsilon \end{array}$$

where K _v,T and K _v,ε are the temperature and strain sensitivities for the reflection spectrum, respectively, and K _φ,T and K _φ,ε are the temperature and strain sensitivities for the optical phase, respectively. Consequently, the temperature and strain changes along the sensor fiber can be determined simultaneously by the spectral shift Δv and the phase difference Δφ: $$\{\begin{array}{c}\mathrm{\Delta }T\left(z\right)=\frac{K_{\phi ,\epsilon }\mathrm{\Delta }v\left(z\right)-K_{v,\epsilon }\mathrm{\Delta }\phi \left(z\right)}{K_{v,\epsilon }{K}_{\phi ,T}-K_{\phi ,\epsilon }{K}_{v,T}}\\ \mathrm{\Delta }\epsilon \left(z\right)=\frac{K_{\phi ,T}\mathrm{\Delta }v\left(z\right)-K_{v,T}\mathrm{\Delta }\phi \left(z\right)}{K_{v,\epsilon }{K}_{\phi ,T}-K_{\phi ,\epsilon }{K}_{v,T}}\end{array}$$

As in As shown, three photodetectors 164, 165, 166 are used, which increases the amount of data acquired since a standard φOTDR device normally uses only one photodetector. However, the use of the three photodetectors 164, 165, 166 also improves the reliability of the phase and amplitude/intensity determination.

According to an embodiment that can be combined with any other embodiment described here, the number of 5 used photodetectors. As can be seen from equation (7), the quadrature component Q is proportional to the signal difference of the two outputs. Therefore, only two photodetectors can be used to obtain the signal from any two outputs of the [3×3] coupler 171, and the signal difference between the two photodetectors used is considered as the quadrature component Q. Or any two outputs of the [3×3] coupler 171 can be directly connected to two inputs of a balanced photodetector, which directly provides the signal difference. Once the quadrature component is determined, the in-phase component can be calculated as the Hilbert transform of Q, so that the amplitude information A _de (z) and the phase difference can be calculated using equation (10). Note that the obtained phase difference may have an offset of ±2π/3, depending on the choice of the [3×3] coupler outputs. This phase offset is a constant and can be easily removed for dynamic measurements. When a single symmetric photodetector or two photodetectors are used, the amount of data acquired is reduced compared to the case of three photodetectors, which facilitates data storage, data processing and data transmission.

To demonstrate an embodiment of the invention, a test setup of the type described in 6 The light source 111 used is a semiconductor laser operating at 1550 nm with a narrow line width of 3 kHz. The optical frequency of the laser is modulated by a ramp signal (drive signal) of 50 Hz provided by the driver 115 to realize frequency scanning. A semiconductor optical amplifier 112 receiving a drive signal from a driver 117 is used to convert the light of the laser 111 into optical pulses. Note that the optical gate can also be realized by the semiconductor optical amplifier 112. The pulse width is set to 10 ns and the pulse repetition rate to 20 kHz.

The generated pulses are amplified by an erbium-doped fiber amplifier (EDFA) 113. A narrowband filter 114 with a bandwidth of ±12.5 GHz is used to suppress the amplified spontaneous emission of the EDFA, and an optional attenuator 116 can be used to adjust the intensity of the pulses. Then the optical pulses are introduced into a fiber under test 130 via a circulator 120, which is the optical element here. The circulator 120 also redirects the weak backscattered light into another EDFA (optical amplifier 141) for pre-amplification, and another filter 142 is used to suppress the amplified spontaneous emission (ASE). The amplified light then enters the submodule 152 to determine both amplitude and/or intensity information of the backscattered optical signal as well as phase information. Submodule 152 contains a Mach-Zehnder interferometer as shown in The difference in length of the two arms 173, 174 in the interferometer 170 is set to 2 m. Three photodetectors 164, 165, 166 with very similar performance are used to detect the outputs of the interferometer 170.

The fiber 130 is about 1450 m long. The first 400 m of the fiber 130 form a reference section 132, which is well insulated in a housing and can be used to measure the relative frequency change of the light source 111. The rest of the fiber 130 is located outdoors and serves as the detection section 131. A short part of the detection section 131, about 10 m long, is wound around a piezoelectric tube (PZT) for vibration measurement. A sinusoidal voltage is applied to the PZT so that the wrapped fiber is stretched accordingly. The oscillation frequency of the PZT is set to 2 Hz and 1 kHz for low frequency and high frequency measurements, respectively.

The output of the photodetectors 164, 165, 166 is digitized at a rate of 250 MS/s and the digitized signal is processed by a computer (data processing device 158). The amplitude obtained by Eqs. (7) and (10) is used to create the reflection spectrum at each position along the entire fiber 130. The spectral shift can then be determined by cross-correlating the reflection spectra and used to quantify the environmental change at a low frequency equal to half the ramp frequency (in this case 25 Hz).

The phase difference can also be calculated by Eqs. (7) and (10) based on the digitized signal. The phase difference obtained in the reference section 132 of the fiber 130 is used to calculate the relative frequency shift of the light source 111, the phase difference obtained in the detection section 131 of the fiber 130 is used to quantify the environmental change at a high frequency which is half the pulse repetition rate (in this case 10 kHz).

shows the phase difference detected at a specific position in the reference section 132. The phase difference changes with a period of 20 ms due to the ramp signal applied to the laser 111. The relative optical frequency change can be calculated according to equation (6) based on this phase difference, which is represented by the y-axis on the right side of Therefore, the relative optical frequency change can be used to monitor each frequency scan and determine the spectral shift.

shows the phase difference at 1407.65 m, which is generated by the 1 kHz vibration of the PZT. The The temporal shape shown is a combination of a quasi-ramp wave at 50 Hz and a sine wave at 1 kHz, caused by the frequency sampling and the applied vibration, respectively. The two oscillating signals can be separated in the frequency domain. Since the sampling frequency of the laser is known, the signals at this frequency can easily be removed by notch filtering. The result of the filtering shows a sinusoidal waveform at 1 kHz, as shown in This shows that this invention is capable of performing dynamic measurements at high frequencies based on the phase difference.

The vibration frequency of the PZT is set to 2 Hz for measuring the low frequencies. Based on the obtained relative optical frequency change, the reflection spectrum is interpolated at each position along the fiber 130. shows the result of the cross-correlation between the reflection spectrum obtained at the first scan and later at the fiber position 1407.65 m. The position of the correlation peak in the frequency domain indicates the corresponding spectral shift.

shows the temporal variation of the spectral shifts obtained at two different fiber positions. The spectral shift at 1407.65 m fiber shows a clear oscillation at 2 Hz, since the fiber at this position is subject to the 2 Hz oscillation. The spectral shift at 1391.30 m, an undisturbed position, fluctuates slightly around 0 MHz, indicating that there are no changes in the environment.

reference sign

100

device

110

generator for frequency-sensing optical pulses

111

coherent light source / laser

112

optical gate

113

optical amplifier

114

optical filter

115

driver for coherent light source

116

attenuator

117

optical gate driver

120

optical component / circulator

130

optical fiber

131

registration section

132

reference section

141

optical amplifier

142

optical filter

150

receiver module

151

Submodule (151) for determining amplitude and/or intensity information

152

Submodule (152) for determining the phase information

153

light from the local oscillator

154

backscattered light / backscattered optical signal

155

coupler

156

splitter (divider)

158

data processing device

161, 162, 163

photodetector

170

interferometer

171

[3×3] coupler

172

splitter (divider)

173

first arm of the interferometer

174

second arm of the interferometer

QUOTES INCLUDED IN THE DESCRIPTION

This list of documents listed by the applicant was generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions.

Cited patent literature

• DE 10 2018 105 905 B4 [0003]
• US 2010 / 0 014 071 A1 [0003, 0006]
• US 9 170 149 B2 [0004]
• US 2021 / 0 033 430 A1 [0004]
• US 2017 / 0 342 814 A1 [0004, 0006]

Cited non-patent literature

• Liehr et al. “Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing” Optics Express Vol. 26, Issue 8, pp. 10573-10588 (2018 [0003]
• A. Masoudi et al: “A distributed optical fiber dynamic strain sensor based on phase-OTDR”, Measurement Science and Technology, Volume 24 (2013), 085204 [0004]
• X. Lu et al: “Evaluating phase errors in phase-sensitive optical time-domain reflectometry based on I/Q demodulation”, Journal of Lightwave Technology, Vol. 38, No. 15, (2020), 4133-4141 [0004]
• Z. Ding et. al: “Distributed Strain and Temperature Discrimination Using Two Types of Fiber in OFDR”, IEEE Photonics Journal, Vol. 8, No. 5, (2016 [0005]
• R. Amer et al. described: “Field Applications of Distributed Fiber Optic Strain and Temperature Sensing for Caprock - Well Integrity and CO2 Leakage Monitoring”, Proceedings of the 16th Greenhouse Gas Control Technologies Conference (GHGT-16) 23-24 [0005]
• S. Liehr et al. included: “Wavelength-scanning coherent OTDR for dynamic high strain resolution sensing”, Optics Express, Vol. 26, Issue 8, (2018), 10573-10588 [0059]

Claims (21)

An apparatus (100) for quantifying a change in environmental conditions along the length of an optical fiber (130), comprising: a frequency-sweeping optical pulse generator (110) configured to generate pulse trains of optical pulses having variable optical frequencies; an optical component (120) coupled to the frequency-sweeping optical pulse generator (110) for injecting the optical pulses into an optical fiber (130) and for redirecting the optical signal backscattered from the optical fiber (130); and a receiver module (150) coupled to the optical component (120), the receiver module (150) comprising an interferometer (170) and photodetectors (164, 165, 166), the interferometer (170) having an input for receiving the backscattered optical signal and a coupler (171) for providing optical signal outputs with shifted phases, each optical signal output of the coupler (171) being coupled to one of the photodetectors (164, 165, 166) for providing electrical signals; and a data processing device (158) coupled to the photodetectors (164, 165, 166) to receive the electrical signals, the data processing device (158) configured to derive phase information of the backscattered optical signal from the electrical signals as a function of time and optical frequency. Device (100) according to claim 1 , wherein the data processing device (158) is configured to derive, in addition to the phase information, amplitude and/or intensity information of the backscattered optical signal from the electrical signals as a function of time and the optical frequency. Device (100) according to claim 1 or 2 wherein the frequency-sampling optical pulse generator (110) is configured to periodically scan the optical frequency of the optical pulses through a predetermined optical frequency range. The apparatus (100) of any preceding claim, wherein the frequency-sweeping optical pulse generator (110) comprises a coherent light source (111) configured to receive an electrical drive signal that causes the light source (111) to perform optical frequency scanning, and an optical gate (112) configured to receive light from the light source (111) and generate optical pulses. Device (100) according to one of the Claims 1 until 3 wherein the frequency-swept optical pulse generator (110) comprises a coherent light source (111) configured to operate at a fixed optical frequency, a modulator configured to be driven by an RF signal having a sampled microwave frequency and an optional narrow filter to select a post-modulation sideband for optical frequency scanning, and an optical gate (112) for generating optical pulses. Device (100) according to one of the Claims 1 until 3 wherein the frequency-swept optical pulse generator (110) comprises a coherent light source (111) configured to operate at a fixed optical frequency, a modulator configured to be driven by an RF pulse signal, and an optional narrow filter to select a post-modulation sideband for optical frequency scanning. The apparatus (100) of any preceding claim, wherein the receiver module (150) is configured to determine the relative optical frequency v' of the injected optical pulses relative to an optical reference frequency during scanning based on the optical signal backscattered from a reference portion (132) of the optical fiber (130). The apparatus (100) of any preceding claim, wherein the receiver module (150) is configured to obtain reflection spectra for a given position in the optical fiber (130) based on pulse sequences, the receiver module (150) further configured to detect a frequency shift of the reflection spectra to obtain local environmental information. Device (100) according to one of the preceding claims, wherein the interferometer (170) has two arms (173, 174) with different path lengths, the difference between the two arms (173, 174) being equal to or greater than the length of an optical pulse. Device (100) according to one of the preceding claims, wherein the receiver module (150) comprises a splitter (156) for splitting the backscattered optical signal into two optical sub-signals, a submodule (151) for determining amplitude and/or intensity information of the backscattered optical signal and a submodule (152) for determining phase information of the backscattered optical signal, wherein the submodule (151) for determining amplitude and/or intensity information is coupled to the splitter (151) to receive one optical partial signal of the backscattered optical signal, and the submodule (152) for determining phase information is coupled to the splitter (151) to receive the other optical partial signal of the backscattered optical signal. Device (100) according to claim 10 , wherein the submodule (151) for determining amplitude and/or intensity information has a coupler (155) for mixing the optical partial signal with a local oscillator and a symmetrical detector (162, 163) for detecting the output of the coupler (155). Device (100) according to claim 10 or 11 , wherein the submodule (152) for determining the phase information comprises the interferometer (170) and the photodetectors (164, 165, 166) for detecting the output of the interferometer (170). The apparatus (100) of any preceding claim, wherein the interferometer (170) comprises a first arm (173) and a second arm (174), the first arm (173) having a longer optical path than the second arm (174), and the coupler is a [3×3] coupler (171) configured to output three optical signals with 120° phase shifts, the first arm (173) and the second arm (174) being coupled to respective inputs of the [3×3] coupler (171), each optical signal output of the [3×3] coupler (171) being coupled to one of the photodetectors (161, 162, 163) to provide the electrical signals. Device (100) according to claim 13 , wherein the phase difference Δφ along the reference section (132) and a detection section (131) of the optical fiber (130) and the amplitude information A _de (z) is determined according to $$\{\begin{array}{c}I\left(z\right)=-P_1\left(z\right)\text{/}2+P_2\left(z\right)-P_3\left(z\right)\text{/}2\\ Q\left(z\right)=-\sqrt{3}{P}_1\left(z\right)\text{/}2+\sqrt{3}{P}_3\left(z\right)\text{/}2\end{array}$$

and $$\begin{array}{c}{A}_{de}\left(z\right)\propto \sqrt{I^2\left(z\right)+Q^2\left(z\right)}\\ \mathrm{\Delta }\phi \left(z\right)={\text{tan}}^{-1}\left[I\left(z\right)\text{/}Q\left(z\right)\right]+2k\pi \end{array}$$

where P ₁ (z), P ₂ (z) and P ₃ (z) denote the signals received and generated by the photodetectors (161, 162 and 163), respectively, and where k is an integer and the term 2kπ represents a phase unfolding process to expand the value of the tan ^-1 function. A system for quantifying a change in environmental conditions along the length of an optical fiber (130), comprising: a device (100) according to any one of the preceding claims, and an optical fiber (130) coupled to the optical component (120) of the device (100). system according to claim 15 wherein the optical fiber (130) has a reference section (132) isolated from environmental noise and a detection section (131). A method for determining changes in parameters of interest along an optical fiber for low and high frequency measurements, the method comprising the steps of: providing optical pulses at different optical frequencies; injecting the optical pulses into an optical fiber (130); receiving a Rayleigh backscattered optical signal from the optical fiber (130); splitting the optical signal into two optical sub-signals; detecting one of the two optical sub-signals as an intensity of the backscattered optical signal as a function of time to obtain reflection spectra along the optical fiber; injecting the other of the two optical signals into an interferometer (170) and detecting the output of the interferometer (170) as a phase of the optical signal backscattered from the optical fiber (130) as a function of time; and calculating a spectral frequency based on the correlation of the reflection spectrum along the optical fiber for a low frequency measurement and a phase difference based on the output of the interferometer for a high frequency measurement. A method for determining changes in parameters of interest along an optical fiber for low and high frequency measurements, the method comprising the steps of: providing optical pulses at different optical frequencies; injecting the optical pulses into an optical fiber (130); receiving a Rayleigh backscattered optical signal from the optical fiber (130) which is introduced into an interferometer (170); detecting the output of the interferometer as Function of time; calculating the amplitude information A _de and the phase difference of the backscattered optical signal based on the output of the interferometer (170) and obtaining reflection spectra based on the amplitude information A _de ; and calculating a spectral frequency based on the correlation of the reflection spectrum and the amplitude information A _de for the measurement of low frequencies and the obtained phase difference for the measurement of high frequencies. A method for determining changes in parameters of interest along an optical fiber based on spectral shift and phase difference, the method comprising the steps of providing optical pulses having different optical frequencies; injecting the optical pulses into an optical fiber (130); receiving a Rayleigh backscattered optical signal from the optical fiber (130); splitting the optical signal into two optical sub-signals; detecting one of the two optical sub-signals as the intensity of the backscattered optical signal as a function of time to obtain reflection spectra along the optical fiber; injecting the other of the two optical signals into an interferometer (170) and detecting the output of the interferometer (170) as the phase of the optical signal backscattered from the optical fiber (130) as a function of time; calculating a spectral frequency based on the correlation of the reflection spectrum along the optical fiber and a phase difference based on the output of the interferometer (170); and calculating the change in the two parameters of interest based on the spectral frequency and the phase difference. A method for determining changes in two parameters of interest along an optical fiber based on spectral shift and phase difference, the method comprising the steps of: providing optical pulses with different optical frequencies; feeding the optical pulses into an optical fiber (130); receiving a Rayleigh backscattered optical signal from the optical fiber (130) which is introduced into an interferometer (170); detecting the output of the interferometer as a function of time; calculating the amplitude information A _de and the phase difference of the backscattered optical signal based on the output of the interferometer (170) and obtaining reflection spectra based on the amplitude information A _de ; calculating a spectral frequency based on the correlation of the reflection spectrum and the amplitude information A _de ; and calculating the change in the two parameters of interest according to a system of equations based on the spectral frequency and the phase difference. A method for determining a relative change in the optical frequency of the pulses, the method comprising the steps of: providing optical pulses at preselected optical frequencies, one of the optical pulses being a reference pulse at a given optical frequency; injecting the optical pulses into an optical fiber (130) having a reference section (132) and a detection section (131), the reference section (132) being maintained under defined and constant environmental conditions; receiving Rayleigh backscattered optical signals from the reference section (132) of the optical fiber (130) for the pulses, the optical signals being introduced into an interferometer (170); detecting the output of the interferometer (170) as a function of time; calculating a phase difference at the reference section (132) of the optical fiber (130) based on the output of the interferometer (170); and calculating the relative change in optical frequency relative to the given optical frequency of the optical pulse based on the obtained phase difference.

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