Apparatus and method for digitizing optical signals and for spatially resolving and measuring temperature and strain by Brillouin scattering.

Abstract

To provide an apparatus for spatially resolved measurement of temperature and strain via Brillouin scattering, and a method for spatially resolved measurement of temperature and strain.SOLUTION: An apparatus 10 for digitizing an optical signal comprises: an optical detector 1 configured to detect optical signals and to generate electric signals corresponding to the optical signals; an envelope curve detector 4 configured to determine the amplitudes of the electric signals generated by the optical detector 1 and to output electric signals corresponding to the amplitudes; an analog to digital converter 7 configured to digitize the electric signals output by the envelope curve detector 4 and to output corresponding data; and a variable voltage source 9 having an output unit connected or connectable to an input unit of the envelope curve detector 4, where the apparatus for digitizing the optical signal is configured to calibrate the envelope curve detector 4 with the variable voltage source 9.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an apparatus and method for digitizing optical signals, and an apparatus for spatially resolving and measuring temperature and strain by Brillouin scattering according to the generic concept of claim 12, and a method for spatially resolving and measuring temperature and strain by Brillouin scattering according to the generic concept of claim 13.

Background Art

[0002] Apparatuses and methods for spatially resolving and measuring temperature and strain by Brillouin scattering of the aforementioned type are known from EP3139133A1. In the apparatus described therein, laser radiation emitted from a laser light source is coupled into an optical fiber used for measurement. The Brillouin signal generated by Brillouin scattering from the laser radiation in the optical fiber is coupled. The coupled Brillouin signal is separated by an optical polarization beam splitter into two components with different polarizations from each other, and laser radiation is mixed into each component of the Brillouin signal by an optical coupler. These mixed signals are detected separately, for example, by sensor means configured as optical detectors. Further, evaluation means are also provided for determining the temperature and strain of the optical fiber cross-section with spatial resolution from the detected Brillouin signal. The frequency of the Brillouin signal is detected by the evaluation means.

[0003] Brillouin scattering in an optical fiber is available for distributed measurement and measurement with spatial resolution of temperature and strain along the optical fiber, because the frequency and amplitude of Brillouin scattering are functions of the measured values of the temperature and strain to be measured.

[0004] In many cases, only the Brillouin frequency is measured, which is very sensitive to the measured quantity, for example, about 1 MHz / °C or 0.05 MHz / με in fused silica, and can be determined very accurately. However, there is a problem of separating the effects of the two measured values of temperature and strain.

[0005] In some cases, the two measurement values ​​can be separated by comparative measurements using optical fibers installed in different configurations, such as loose fiber / loose tube or solid fiber / solid tube (see: Inaudi & Glisic, 2006, Reliability and Field testing of distributed strain and temperature sensors, 6167, 61671D-61671D-8). Alternatively, Brillouin frequency measurements can be performed using fibers with multiple Brillouin peaks (see Liu & Bao, 2012, Brillouin Spectrum in LEAF and Simultaneous Temperature and Strain Measurement. J. Lightwave Technol., 30(8), 1053-1059), or oligomode fibers with a few different spatial modes (see Weng, Ip, Pan, & Wang, 2015, Single-end simultaneous temperature and strain sensing techniques based on Brillouin optical time domain reflectometry in few-mode fibers, Opt. Express, 23(7), 9024-9039), where the measured quantities can be separated by using fibers with different frequency dependencies on temperature and strain.

[0006] However, none of these methods are generally applicable, as the appropriate optical fiber may not always be available for the specific application. Furthermore, installing and measuring multiple optical fibers, including specialized ones, is time-consuming.

[0007] Another method for separating two measurements is to measure the frequency and amplitude of one or more Brillouin peaks. Examples of measuring the frequency and amplitude of one or more Brillouin peaks are described in Parker, Farhadiroushan, Handerek, & Rogers, 1997, Temperature and strain dependence of the power level and frequency of spontaneous Brillouin scattering in optical fibers, Opt. Lett., 22(11), 787-789, and Maughan, Kee, & Newson, 2001, Simultaneous distributed fiber temperature and strain sensor using microwave coherent detection of spontaneous Brillouin backscatter, Measurement Science and Technology, 12(7), 834.

[0008] By measuring the frequency and amplitude of one or more Brillouin peaks, two independent parameters can be obtained, from which, in principle, two required physical parameters, temperature and strain, can be determined. However, the dependence of amplitude on temperature and strain is weak, for example, around 0.3% / °C. Therefore, to obtain a practical temperature resolution and an accuracy of about 1°C, the amplitude needs to be measured very precisely.

[0009] In particular, when separating the two measurements based on the frequencies of multiple Brillouin peaks, it was found that the frequency coefficients for temperature and strain dependence are very similar for different peaks in a single fiber, which poses a problem. Therefore, separating and calculating the measurements significantly increases noise. Furthermore, using Brillouin scattering for temperature and strain measurements in Brillouin DTS (Distributed Temperature Sensing) requires linear amplitude measurement over a large dynamic range of 20 dB or 30 dB. This is because, in part, attenuation in the optical fiber and losses in connectors, splices, and other fiber coupling components reduce the signal amplitude as the fiber length increases. For a typical fiber length of 100 km and attenuation of 0.2 dB / km, the total loss in the unconnected direction is 20 dB. In addition, further fluctuations in the signal level occur due to effects such as polarization fading, which also need to be covered by the detector's dynamic range. [Overview of the project]

[0010] The fundamental problem of the present invention is to fabricate an apparatus of the type described above for digitizing an optical signal, and to provide a method of the type described above for digitizing an optical signal that is capable of detecting the amplitude of the optical signal with great precision and / or with a large dynamic range. Furthermore, the fundamental problem of the present invention is to fabricate an apparatus of the type described above for spatially resolving and measuring temperature and strain via Brillouin scattering, and to provide a method of the type described above for spatially resolving and measuring temperature and strain via Brillouin scattering that is capable of easily and / or precisely determining temperature and strain.

[0011] This is achieved, according to the invention, by an apparatus of the type described above for digitizing an optical signal having the features of claim 1, and by a method of the type described above for digitizing an optical signal having the features of claim 7, and by an apparatus for spatially resolving and measuring temperature and strain by Brillouin scattering having the features of claim 12, and by a method for spatially resolving and measuring temperature and strain by Brillouin scattering according to the superordinate concept of claim 13. The lower claims relate to preferred embodiments of the invention.

[0012] According to claim 1, an apparatus for digitizing an optical signal is: A photodetector is provided to detect an optical signal and generate an electrical signal corresponding to the optical signal, An envelope detector provided to determine the amplitude of an electrical signal generated by a photodetector, or the amplitude of an electrical signal generated from this electrical signal, and to output an electrical signal corresponding to these amplitudes, An analog-to-digital converter, which is configured to digitize an electrical signal output by an envelope detector and output corresponding data, A variable voltage source having an output section connected to or connectable to the input section of an envelope detector, wherein the device for digitizing optical signals is provided to calibrate the envelope detector with the variable voltage source, include.

[0013] Since the influence of temperature and distortion effects on the amplitude of the Brillouin signal is small, extremely accurate amplitude measurement is necessary to obtain sufficient temperature resolution and accuracy using amplitude and frequency-based Brillouin DTS. Such accuracy cannot be provided by an uncalibrated envelope detector.

[0014] In linear measurements, a certain degree of change in the optical Brillouin signal translates into a certain degree of change in the measured voltage, independent of the absolute signal strength. Linear measurements over a large dynamic range are essential for accurate temperature and strain measurements at any point in the fiber. An uncalibrated envelope detector cannot provide this kind of linearity.

[0015] For example, BOTDR-DTS (Brillouin optical time domain reflectometry distributed temperature sensing) typically converts the optical Brillouin signal into a high-frequency electrical signal, which is then filtered and amplified. The amplitude of such a signal can be determined using an envelope detector. An envelope detector removes the carrier frequency from the signal, allowing the amplitude to be sampled and / or digitized at a low signal modulation frequency. Essentially, it rectifies the high-frequency signal, then filters and smooths it. Envelope detectors can be configured in various ways, including full-wave rectification, half-wave rectification, various filters, and active precision rectifiers using operational amplifiers. All of these configurations have strict constraints in terms of the nonlinearity and temperature dependence of the output signal.

[0016] Nonlinearity at small signal amplitudes is primarily due to the threshold voltage of the rectifier diode used in the detector. This threshold prevents the detection of weak signals and makes the diode characteristics nonlinear at low voltages. This limitation is partially overcome by active rectifiers, but not completely eliminated. Active rectifiers use amplification circuits that may have other problems, such as temperature drift, signal offset, and voltage-dependent gain.

[0017] In the case of high signals, the gain may decrease and nonlinearity may occur as the output voltage approaches the power supply voltage level. Furthermore, temperature dependence of the amplifier output may occur due to offsets and gain fluctuations caused by temperature-dependent active and passive components such as transistors and resistors. In addition, variations in component characteristics can cause subtle differences in the gain profiles of detectors and amplifiers within and between batches, which is also a problem.

[0018] The horizontal axis represents the input signal strength in dB, and the vertical axis represents the output signal in arbitrary units. Figure 4 illustrates the temperature dependence of a typical envelope detector. The solid line corresponds to the temperature of the envelope detector at 5°C, the dotted line to the temperature of the envelope detector at 25°C, and the dashed line to the temperature of the surrounding detector at 55°C. In particular, strong nonlinearity and a large temperature dependence of the output are observed at very small signals.

[0019] The variable voltage source configured according to the invention enables relatively precise measurements over a large dynamic range, despite the typical nonlinearity and temperature dependence of the intensity of envelope detectors.

[0020] The voltage source can be configured to generate a variable voltage signal having the same or similar frequency as the electrical signal generated from the optical signal. In particular, the voltage signal generated by the variable voltage source may have a frequency range equal to, smaller than, or larger than the frequency range of the electrical signal generated from the optical signal. Specifically, the frequency range of the voltage signal generated by the variable voltage source can cover the frequency range of the electrical signal generated from the optical signal. Alternatively, the voltage signal generated by the variable voltage source may have only a single frequency, or a narrow frequency range that lies within the frequency range of the electrical signal generated from the optical signal.

[0021] For example, the frequency range of the electrical signal generated from the optical signal may be between 823.5 MHz and 935 MHz. In this case, the voltage signal generated from the variable voltage source only needs to be in the frequency range of at least 823.5 MHz to 935 MHz. The frequency range of the voltage signal generated by the variable voltage source can be much wider, for example, 800 MHz to 960 MHz. Alternatively, in this example, the frequency range of the voltage signal generated by the variable voltage source may be small, within the frequency range of the electrical signal generated from the optical signal. For example, the voltage signal generated by the variable voltage source may be a constant frequency of 890 MHz + / - 3 ppm.

[0022] In addition to the Brillouin DTS, devices for digitizing optical signals can be used for many other applications requiring precise measurement of the amplitude of high-frequency optical or electrical signals.

[0023] The apparatus may include a bandpass filter positioned between the photodetector and the envelope detector, the bandpass filter being configured to filter out at least the DC component and / or frequency ranges that are not required when determining the amplitude via the envelope detector and / or are considered noise from the electrical signal generated by the photodetector. Furthermore, the apparatus may include an amplifier positioned between the photodetector and the envelope detector, or between the photodetector and the bandpass filter, the amplifier being configured to amplify the electrical signal generated by the photodetector, and the amplifier being, in particular, a transimpedance amplifier.

[0024] The apparatus may include a switch, which may be positioned on the one hand between the photodetector or amplifier or bandpass filter and the envelope detector, and on the other hand between the variable voltage source and the envelope detector, and may be configured to directly or indirectly supply the electrical signal generated by the photodetector to the input of the envelope detector, or to connect the output of the variable voltage source to the input of the envelope detector.

[0025] The device may include an amplifier disposed between the envelope detector and the analog / digital converter, and the amplifier is provided to amplify the electrical signal generated by the envelope detector.

[0026] The device may include a digital processing device, and the digital processing device is provided to store the generated calibration data and normalize the data output by the analog / digital converter using these calibration data when calibrating the envelope detector.

[0027] According to claim 7, a method for digitizing an optical signal is detecting an optical signal and generating an electrical signal corresponding to the optical signal; determining, by an envelope detector, the amplitude of the generated electrical signal or the amplitude of an electrical signal resulting from this electrical signal, and outputting an electrical signal corresponding to the amplitude; digitizing the electrical signal output by the envelope detector and outputting data corresponding to the digitization; calibrating the envelope detector by a variable voltage source; and including.

[0028] When calibrating the envelope detector, a plurality of different voltages can be generated by the variable voltage source and configured to be applied to the input of the envelope detector. Preferably, the voltage source can be used for calibrating the envelope detector and any amplifier behind the switch position over the entire measured voltage range. The voltage source preferably supplies a variable voltage signal having the same or a similar frequency as the detected optical voltage. Further, it is desirable that the output of the voltage source is adjustable over the entire range of the expected optical voltage.

[0029] A voltage source can generate a variable voltage signal having the same or similar frequency as the electrical signal generated from the optical signal. In particular, the voltage signal generated by the variable voltage source may have a frequency range equal to, smaller than, or larger than the frequency range of the electrical signal generated from the optical signal. Specifically, the frequency range of the voltage signal generated by the variable voltage source can cover the frequency range of the electrical signal generated from the optical signal. Alternatively, the voltage signal generated by the variable voltage source may have only a single frequency, or a narrow frequency range that lies within the frequency range of the electrical signal generated from the optical signal.

[0030] For example, the frequency range of the electrical signal generated from the optical signal may be between 823.5 MHz and 935 MHz. In this case, the voltage signal generated from the variable voltage source only needs to be in the frequency range of at least 823.5 MHz to 935 MHz. The frequency range of the voltage signal generated by the variable voltage source can be much wider, for example, between 800 MHz and 960 MHz. Alternatively, in this example, the frequency range of the voltage signal generated by the variable voltage source may be small, within the frequency range of the electrical signal generated from the optical signal. For example, the voltage signal generated by the variable voltage source may have a constant frequency of 890 MHz + / - 3 ppm.

[0031] Furthermore, the envelope detector can be configured to be calibrated at multiple different temperatures. Calibration can be performed at any temperature within the receiver's expected operating temperature range. This approach almost completely cancels out the nonlinearity and temperature effects of receiving components downstream of the switch position.

[0032] To further improve measurement accuracy, a configuration is also possible in which the variable voltage source is calibrated before use.

[0033] During the calibration of the envelope detector, it is also possible to generate calibration data, save it, and use it to normalize the output data.

[0034] In particular, calibration may be performed during the manufacture or maintenance of equipment for digitizing optical signals, or between different, consecutive digitizations of optical signals. The voltage source may be permanently connected to the receiver or temporarily connected. A temporarily connected voltage source can be used for calibration during the manufacture or maintenance of the equipment. Complete calibration data is stored in the digital processing unit of the equipment and used for numerical calibration or correction of the signal. A built-in fixed voltage source allows calibration data to be recorded at a convenient time between optical measurements. Calibration by such an integrated source may be more accurate because it is related to the current state of the system.

[0035] According to claim 12, the apparatus for digitizing an optical signal is an apparatus for digitizing an optical signal according to the invention.

[0036] According to claim 13, the apparatus for digitizing an optical signal is an apparatus for digitizing an optical signal according to the invention.

[0037] The frequency and amplitude of the Brillouin signal may be determined. In this case, the amplitude of the Brillouin signal can be measured continuously at different frequencies, and the peak frequency can be determined from the amplitude-frequency peak fit.

[0038] Other features and advantages of the present invention will become apparent from the following description of preferred embodiments with reference to the accompanying figures. [Brief explanation of the drawing]

[0039] [Figure 1] This is a schematic diagram of a device according to an invention for digitizing optical signals. [Figure 2] This is a schematic diagram of a first embodiment of an apparatus according to an invention for spatially resolving and measuring temperature and strain by Brillouin scattering. [Figure 3]This is a schematic diagram of a second embodiment of an apparatus according to an invention for spatially resolving and measuring temperature and strain by Brillouin scattering. [Figure 4] This graph shows the temperature dependence of an envelope detector, with the input signal intensity shown in decibels (dB) on the X-axis and the output signal shown in arbitrary units on the Y-axis. [Modes for carrying out the invention]

[0040] In the diagram, identical or functionally identical parts are denoted by the same reference numeral. Dashed connection lines represent optical signals, which are preferably transmitted via optical fibers. Solid connection lines represent electrical signal lines.

[0041] The embodiment of the optical signal digitization device 10 shown in Figure 1 consists of, for example, a photodetector 1 that receives a high-frequency amplitude-modulated optical signal and generates a photocurrent.

[0042] The device 10 further includes one or more amplifiers 2, which are transimpedance amplifiers. Note that the amplifiers 2 are optional and can be omitted. At least one amplifier 2 amplifies the electrical signal generated by the photodetector 1, thereby converting the current into a voltage.

[0043] It is quite possible that an amplifier designed as a transimpedance amplifier is built into the photodetector 1. In this case, the photocurrent within the photodetector 1 is converted into a voltage signal and applied to the output of the photodetector 1.

[0044] The device further includes a bandpass filter 3 for filtering out DC components from the amplified signal, and a filter for filtering out frequency bands that are not needed for further processing or that cause interference.

[0045] The device 10 further includes an envelope detector 4, the input of which is connected to the output of the bandpass filter 3 via a switch 5. Note that switch 5 is optional and can be omitted.

[0046] The envelope detector 4 determines the amplitude of the electrical signal filtered by the bandpass filter 3 and outputs an electrical signal corresponding to this amplitude. In doing so, the envelope detector 4 removes the carrier frequency from the signal, allowing the amplitude to be sampled and / or digitized at a lower signal modulation frequency. In principle, the high-frequency signal is rectified and then filtered for smoothing.

[0047] The device 10 further includes an amplifier 6 that amplifies the electrical signal output from the envelope detector 4 to a level suitable for subsequent digitization. The amplifier 6 is optional and can be omitted. The device 10 further includes an analog-to-digital converter 7 that digitizes the signal output from the amplifier 6.

[0048] The device 10 further includes a digital processing unit 8 that stores calibration data and uses this calibration data to normalize or linearize the data output from the analog-to-digital converter 7 over a wide dynamic range, as described below.

[0049] The device 10 further includes a variable voltage source 9. An optional switch 5 switches the input of the envelope detector 4 between the amplified optical voltage applied to the output of the bandpass filter 3 and the output of the variable voltage source 9.

[0050] In particular, the voltage source 9 can supply a variable voltage signal at the same or similar frequency as the optical voltage to be detected.

[0051] A voltage source can generate a variable voltage signal having the same or similar frequency as the electrical signal generated from the optical signal. In particular, the voltage signal generated by the variable voltage source may have the same, lower, or higher frequency range as the electrical signal generated from the optical signal. Specifically, the frequency range of the voltage signal generated by the variable voltage source may overlap the frequency range of the electrical signal generated from the optical signal. Alternatively, the voltage signal generated by the variable voltage source may have only a single frequency, or it may have a narrow frequency range that lies within the frequency range of the electrical signal generated from the optical signal.

[0052] For example, the frequency range of the electrical signal generated from the optical signal may be between 823.5 MHz and 935 MHz. In this case, the voltage signal generated by the variable voltage source may have a frequency range of at least 823.5 MHz to 935 MHz. In this case, the frequency range of the voltage signal generated by the variable voltage source can be larger, for example, between 800 MHz and 960 MHz. Alternatively, in this example, the frequency range of the voltage signal generated by the variable voltage source may be smaller, within the frequency range of the electrical signal generated from the optical signal. For example, the voltage signal generated by the variable voltage source may have a constant frequency of 890 MHz + / - 3 ppm.

[0053] The amplitude of the voltage signal generated by a variable voltage source changes by a significant amount depending on the operating temperature and time, and in particular, the precise value of this change can be stored within the device.

[0054] The output of the voltage source 9 is preferably adjustable over the entire range of the expected optical voltage. Alternatively, signal switching can be achieved by switching the variable voltage source 9 on or off, or by switching the photodetector 1 or amplifier 2, which functions as a signal source.

[0055] The voltage source 9 may be used to calibrate the envelope detector 4 and any amplifier 6 downstream of the switch position over the entire voltage range to be measured. Calibration can or should be performed at a number of temperatures preferably distributed over the operating temperature range of the envelope detector 4. Such a configuration can ideally almost completely cancel out the nonlinearity and temperature effects of the receiving components downstream of the switch position.

[0056] The voltage source 9 may be permanently or temporarily connected to the envelope detector 4 and the switch 5, respectively. A temporarily connected voltage source 9 may be used for calibration of the envelope detector 4 and optionally the amplifier 6 during manufacturing or maintenance. The complete calibration data is then stored in the digital processing unit 8 and used for numerical calibration or correction of the signals generated by the analog-to-digital converter 7.

[0057] The fixed integrating voltage source 9 can be used to record calibration data at any convenient time during optical measurements. Such calibration using the integrating voltage source 9 can be more accurate because it is related to the current state of the system.

[0058] Figure 2 shows a first embodiment of a device 20 that spatially resolves and measures temperature and strain using Brillouin scattering. The device 20 shown in Figure 2 utilizes optical superposition with laser light used to excite Brillouin scattering.

[0059] The apparatus 20 shown in Figure 2 consists of a laser light source 11 that emits, for example, a narrowband laser radiation with a linewidth of 1 MHz. Furthermore, the laser radiation from the laser light source 1 has a constant power of, for example, about 10 mW. Preferably, as the laser light source 11, a frequency-stabilized diode laser such as a DFB laser, or a narrowband laser with an emission wavelength in the near-infrared region, for example, 1550 nm, is used.

[0060] The apparatus 20 shown in Figure 2 further comprises a beam splitter 12 configured as an optical fiber splitter, which is capable of splitting the laser radiation from the laser light source 11 into two parts 13a and 13b. The first part 13a is coupled to an optical fiber 14 used for measurement, in which temperature and strain are determined with spatial resolution via Brillouin scattering excitation. The second part 13b is coupled from the optical fiber 14, as will be described in detail below, and is used to superimpose the Brillouin signal generated by Brillouin scattering.

[0061] The apparatus further comprises an optical modulator 15 capable of modulating a first portion 13a of the laser radiation according to a method used for local allocation of scattered signals. For example, when using the optical time-domain reflectivity measurement (OTDR) method, pulses are formed from the first portion 13a, and when using the optical frequency-domain reflectivity measurement (OFDR) method, amplitude-modulated signals are formed. An optical amplifier (not shown) can amplify the first portion 13a of the laser radiation used for measurement before it is guided into the optical fiber 14 used for measurement via light, particularly an optical fiber, circulator 16, also included in the apparatus.

[0062] A Brillouin scattering signal is generated in the optical fiber 14 used for measurement. This signal returns to the optical circulator 16 with a propagation delay of approximately 10 μs / km corresponding to the distance, and is guided by the circulator to the device's receiving path 17. By using optional optical filters (not shown), such as fiber Bragg gratings (FBGs), Rayleigh scattering can be suppressed to prevent interference with the measurement of the weaker Brillouin signal. Furthermore, an optional optical amplifier 18 can be used to perform optical amplification in the receiving path 17.

[0063] The Brillouin signal and the second component 13b of the laser radiation are coupled by an optical coupler, particularly an optical fiber coupler 19. The apparatus 20 constitutes the apparatus 10 shown in Figure 1 as an apparatus for digitizing optical signals. In this case, the Brillouin signal superimposed with the second component 13b of the laser radiation is detected by a photodetector 1.

[0064] In particular, this generates a beat signal in which the difference frequency between the Brillouin signal and the laser emission component is in the range of approximately 10 GHz. The frequency and amplitude of this beat signal depend on the material, temperature, strain, etc., of the optical fiber 14 used for measurement.

[0065] The amplitude of the beat signal is proportional to the square root of the product of the power of the Brillouin signal and the power of the laser emission component. Therefore, using a high-power laser can yield a much stronger measurement signal than directly measuring the Brillouin scattered light, significantly improving the detection strength of the device.

[0066] In contrast to the apparatus 20 shown in Figure 2, the apparatus 30 shown in Figure 3 includes a second narrowband laser source 21 in addition to the first laser source 11, whose laser radiation is used for superposition with the Brillouin signal. In this case, the frequency of the second laser source 21 is adjusted to be just slightly different from the frequency of the first laser source 11 so that the difference frequency between the Brillouin scattered light and the second laser source 21 is 1 GHz or less. Typical Brillouin frequencies are around 10-13 GHz, and especially around 10.8 GHz in standard single-mode fibers.

[0067] It should be noted that the temperature dependence of the Brillouin peak also depends on the fiber; for example, it is approximately 1.1 MHz / Kelvin for a typical single-mode fiber.

[0068] For example, when using a fused silica light guide, in order to keep the difference frequency between the Brillouin scattered light and the second laser light source 21 below 1 GHz, the relative frequency shift of the two laser light sources 11 and 21 needs to be slightly more than 10 GHz.

[0069] If the difference frequency is 1 GHz or less, a photodetector 1 with a cutoff frequency of 1 GHz or less can be used, resulting in a lower detection limit. Furthermore, amplification and filtering of signals in this frequency band are simpler and more efficient.

[0070] To stabilize the second laser light source 21 to a desired frequency distance relative to the first laser light source 11, a phase-locked loop using an optical input signal called an O-PLL (Optical Phase-Locked Loop) 22 is used, although it is only schematically shown below. A portion of the laser light from both laser light sources 11 and 21 is separated by beam splitters 12 and 23 designed as optical fiber splitters, and combined with an optical fiber coupler having the correct polarization, and superimposed on a photodetector. The measured signal contains a difference frequency component of both laser light sources 11 and 21, and its frequency should be in the range of around 10 GHz. The frequency of this signal is compared in the O-PLL 22 with the frequency of an electronic local oscillator set to the desired difference frequency. Using this comparison signal, the frequency of one of the two laser light sources 11 and 21 is readjusted so that the difference frequency of the laser light sources 11 and 21 matches the frequency of the local oscillator. When using a diode laser, it is preferable that the laser frequency is adjusted via the operating current.

[0071] The Brillouin signal is superimposed on a portion of the laser light emitted from the second laser light source 21 at the coupler 19. The apparatus 30 also includes the apparatus 10 shown in Figure 1, which digitizes the optical signal. The photodetector 1 detects the Brillouin signal superimposed on the component of the laser radiation emitted from the second laser light source 21. In this case, both the frequency and amplitude of the Brillouin signal can be determined.

[0072] In this case, the device 10 measures the amplitude at a specific frequency separated from the bandpass filter 3, which is given by the frequency difference between the Brillouin signal frequency and the frequency interval between the two laser light sources 11, 21. The frequency measurement consists of continuously measuring the amplitude at different frequencies and determining the peak frequency from the amplitude-frequency peak fit.

Claims

1. An apparatus (10) for spatially resolving and measuring temperature and strain using Brillouin scattering, A photodetector (1) is provided to detect an optical signal and generate an electrical signal corresponding to the optical signal, An envelope detector (4) is provided to determine the amplitude of an electrical signal generated by a photodetector (1), or the amplitude of an electrical signal generated from this electrical signal, and to output an electrical signal corresponding to these amplitudes. An analog-to-digital converter (7) is provided to digitize the electrical signal output by the envelope detector (4) and output the corresponding data, A variable voltage source (9) having an output section connected to or connectable to the input section of an envelope detector (4), wherein the device for digitizing optical signals is provided to calibrate the envelope detector (4) with the variable voltage source (9), The apparatus (10) is characterized in that the variable voltage source (9) is an integral voltage source that generates a variable voltage signal having the same or similar frequency as the electrical signal generated by the photodetector (1).

2. It includes a bandpass filter (3) disposed between the photodetector (1) and the envelope detector (4), The apparatus according to claim 1, characterized in that the bandpass filter (3) is provided to filter out a DC component from the electrical signal generated by the photodetector (1), and / or a frequency range that is not required when determining the amplitude via the envelope detector (4) and / or is considered noise.

3. The amplifier (2) is disposed between the photodetector (1) and the envelope detector (4), or between the photodetector (1) and the bandpass filter (3), The amplifier (2) is provided to amplify the electrical signal generated by the photodetector (1), The apparatus (10) according to claim 2, characterized in that the amplifier (2) is a transimpedance amplifier.

4. Includes switch (5), The apparatus (10) according to either 2 or 3, characterized in that the switch (5) is disposed on the one hand between the photodetector (1) or the amplifier (2) or the bandpass filter (3) and the envelope detector (4), and on the other hand between the variable voltage source (9) and the envelope detector (4), and is provided to directly or indirectly supply the electrical signal generated by the photodetector (1) to the input of the envelope detector (4), or to connect the output of the variable voltage source (1) to the input of the envelope detector (4).

5. It includes an amplifier (6) disposed between the envelope detector (4) and the analog / digital converter (7), The apparatus (10) according to any one of claims 1 to 4, characterized in that the amplifier (6) is provided to amplify the electrical signal generated by the envelope detector (4).

6. Includes a digital processing unit (8), The apparatus (10) according to any one of claims 1 to 5, characterized in that the digital processing device (8) is provided to store the calibration data generated when calibrating the envelope detector (4), and to normalize the data output by the analog / digital converter (7) using this calibration data.

7. A method for spatially resolving and measuring temperature and strain using Brillouin scattering, A process of detecting an optical signal and generating an electrical signal corresponding to the optical signal, The process involves determining the amplitude of the generated electrical signal or the amplitude of the electrical signal derived from this electrical signal using an envelope detector (4), and outputting an electrical signal corresponding to that amplitude. The process involves digitizing the electrical signal output by the envelope detector (4) and outputting data corresponding to the digitization, The process involves calibrating the envelope detector (4) with a variable voltage source (9) to generate a variable voltage signal with the same or similar frequency as the electrical signal corresponding to the optical signal, A method characterized by including the following.

8. The method according to the present invention, characterized in that, when calibrating the envelope detector (4) with the variable voltage source (9), a plurality of different voltages are generated and applied to the input of the envelope detector (4).

9. The method according to 7 or 8, characterized in that the calibration of the envelope detector (4) is performed at a plurality of different temperatures of the envelope detector (4).

10. The method according to any one of claims 7 to 9, characterized in that, when calibrating the envelope detector (4), calibration data is generated, stored, and used for normalizing the output data.

11. The method according to any one of claims 7 to 10, characterized in that calibration is performed during the manufacture or maintenance of an apparatus (10) for spatially resolving and measuring temperature and strain using Brillouin scattering, or between different consecutive digitizations of optical signals.

12. A device (20, 30) for spatially resolving and measuring temperature and strain by Brillouin scattering, A laser light source (11, 21) provided to produce laser radiation, The apparatus (20, 30) is configured such that laser radiation generated by at least one laser light source (11) is coupled into the optical fiber (14), and the Brillouin signal generated from the laser radiation by Brillouin scattering is separated from the optical fiber (14), and the optical fiber (14) is configured such that, Apparatus (20, 30) includes an apparatus (10) for spatially resolving and measuring temperature and strain using Brillouin scattering, which is provided to digitize the Brillouin signal separated from the optical fiber (14), The apparatus (20, 30) is characterized in that the apparatus (10) for spatially resolving and measuring temperature and strain using the Brillouin scattering is the apparatus (10) described in any one of claims 1 to 6.

13. A method for spatially resolving and measuring temperature and strain using Brillouin scattering, The process of generating laser radiation, For temperature and strain measurement, the laser radiation is coupled into the optical fiber, The Brillouin signal generated by laser radiation within the optical fiber (14) is separated from the optical fiber in the following steps: A method comprising the step of digitizing a Brillouin signal separated from an optical fiber by an apparatus (10) for spatially resolving and measuring temperature and strain using Brillouin scattering, A method characterized in that the apparatus (10) for spatially resolving and measuring temperature and strain using the Brillouin scattering is the apparatus (10) described in any one of claims 1 to 6.

14. The method according to 13, characterized in that the frequency and amplitude of the Brillouin signal are determined.

15. The method according to 14, characterized in that the amplitude of the Brillouin signal at different frequencies is measured continuously, and the peak frequency is determined from the amplitude-to-frequency peak fit.

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