Optical Fiber Sensing System

Abstract

This facilitates detection of the presence or absence of light and level adjustment when compensating for loss of time-varying light using an optical amplifier, and also prevents measurement errors by suppressing gain fluctuations in the optical amplifier. [Solution] The optical fiber sensing system uses the optical fiber itself as a sensor, inputs probe pulse light from an interrogator at a terminal station into the optical fiber, and the interrogator receives and analyzes the returning light that is affected by environmental information added to the optical fiber. An optical amplifier repeater that compensates for light attenuation is provided within the optical fiber path, and the terminal station wavelength-multiplexes the probe pulse light with continuous light that does not contain low-frequency intensity modulation components and has a wavelength within the amplification band of the optical amplifier repeater, suppressing gain fluctuations of the optical amplifier repeater, at an intensity greater than that of the probe pulse light, and inputs this into the optical fiber.

Description

[Technical Field]

[0001] The present invention relates to optical fiber sensing, and more particularly to the technical field of optical amplifier repeaters. [Background technology]

[0002] <Optical fiber sensing> In addition to being a communication medium, optical fibers are also used as sensors. For example, when sound waves or vibrations are applied to an optical fiber, the light passing through the optical fiber is modulated, and by detecting the reflected or transmitted light, environmental information such as sound and vibrations in remote locations can be detected and measured. One representative example of such a measuring device is called DAS (Distributed Acoustic Sensing).

[0003] DAS detects environmental information from the backscattered light that propagates through optical fiber. DAS is a type of OTDR (Optical Time Domain Reflectometry). The measuring instrument part of a DAS is called an interrogator. An interrogator means one that inquires, and its role is to shine light at each point on the optical fiber, which acts as a sensor, to obtain environmental information sensed by each point. By analyzing Rayleigh backscattered light, DAS can measure vibrations and distortions at each point through which the probe pulse light passes. Other well-known types include BOTDR, which analyzes Brillouin backscattered light.

[0004] <Combination of optical fiber sensing and optical amplifier repeater> The sensing signal light is attenuated due to transmission loss in the optical fiber, which limits the measurable distance. Therefore, a technique for compensating for the loss using an optical amplifier is disclosed in, for example, Patent Document 1. When using optical amplifier repeaters, it is necessary to pay attention to the power limit imposed by nonlinear optical effects in optical fibers. In other words, it is important to adjust the repeater output level to an approximately optimal level, neither too high nor too low. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] U.S. Patent No. 7,595,865 [Patent Document 2] Japanese Patent Application Publication No. 4-275530 Summary of the Invention [Problem to be solved by the invention]

[0006] In measuring instruments that use the OTDR principle, the probe pulse light is a pulsed light with a high duty ratio. In other words, it is light that has high power momentarily, but averages out to low power over time. When amplifying such light in an optical amplifier repeater, the following problems arise:

[0007] Issue (1) It is difficult to confirm the presence and level of probe pulse light and its return light. There are two reasons for this. First, the intensity of both the probe pulse light and its return light fluctuates from moment to moment, so the level cannot be determined unless measurements are taken over a sufficient averaging time. Second, the ASE (Amplified Spontaneous Emission) light emitted by the optical amplifier repeater has a higher power, so the level becomes obscured and cannot be seen. The same is true when using an optical spectrum analyzer, which resolves the power by wavelength. This makes it difficult to confirm whether the measurement light is reaching the target and to determine its level.

[0008] Issue (2) The intensity-modulated component of the probe pulse light modulates the gain of the optical amplifier repeater. This causes fluctuations in the optical output power, including the ASE light, which in turn fluctuates the gain of downstream optical amplifiers, potentially resulting in instability of the optical level of the entire system and optical surges that can damage optical components. Furthermore, this phenomenon can also self-modulate the intensity waveform and phase of the probe pulse light, potentially resulting in adverse effects such as fluctuations and errors in measurements. This type of gain modulation is particularly noticeable when the light passing through the amplifier is significantly intensity-modulated in the frequency range below 1 kHz. In OTDR-type measuring instruments, the pulse repetition frequency is limited to 1 kHz or less due to the round-trip time of light when the measurement range is 100 km or longer, making it more likely that low-frequency intensity-modulated components that cause gain modulation will appear.

[0009] The present invention has been made in view of the above circumstances, and aims to solve these problems. [Means for solving the problem]

[0010] The optical fiber sensing system of the present invention is an optical fiber sensing system in which an optical fiber itself is used as a sensor, an interrogator at a terminal station inputs probe pulse light into the optical fiber, and the interrogator receives and analyzes return light that has been affected by environmental information added to the optical fiber. The optical fiber is provided with an optical amplifier repeater in the path of the optical fiber that compensates for light attenuation, and the terminal station wavelength-multiplexes the probe pulse light with continuous light that has a wavelength within the amplification band of the optical amplifier repeater and does not contain low-frequency intensity-modulated components, thereby suppressing gain fluctuations of the optical amplifier repeater, at an intensity greater than that of the probe pulse light, and inputs the multiplexed light into the optical fiber. This allows the arrival of the probe pulse light to be confirmed by the presence or absence of continuous light, and the amplification gain can be adjusted by checking the level of the clamped light.In addition, since clamped light of a constant power is always input to the optical amplifier repeater, gain fluctuations can be suppressed. [Effects of the Invention]

[0011] According to the present invention, on-site adjustment of loss compensation by an optical amplifier repeater in OTDR-based optical fiber sensing measurement is facilitated. In addition, gain fluctuations in the optical amplifier repeater are suppressed, eliminating the risk of level instability and measurement failure due to pulsed light amplification. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a configuration diagram of a first embodiment. [Figure 2] FIG. 10 is a comparative explanatory diagram of OTDR traces when an optical amplifier repeater is used and when it is not used. [Figure 3] FIG. 10 is a diagram illustrating gain fluctuations when clamped light is not simultaneously transmitted. [Figure 4] 10A and 10B are diagrams illustrating gain fluctuations when clamped light is simultaneously transmitted. [Figure 5] FIG. 10 is a configuration diagram of a second embodiment. [Figure 6] 5A and 5B are explanatory diagrams illustrating changes in optical spectrum caused by an optical filter provided at the output of the optical amplifier according to the first embodiment. [Figure 7] 10A and 10B are diagrams illustrating changes in the optical spectrum caused by an optical filter provided at the output of a downstream optical amplifier according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] First Embodiment A first embodiment of an optical fiber sensing system will be described with reference to FIG. FIG. 1 shows a terminal station 10, a plurality of optical amplifier repeaters 11, and an optical cable 21.

[0014] The terminal station 10 includes a DAS interrogator 30 , a clamped optical transmitter 31 , and a multiplexer 32 . The terminal station 10 wavelength-multiplexes continuous light (clamped light) having a wavelength within the amplification band of the optical amplifier repeater 11 for suppressing gain fluctuations of the optical amplifier, and probe pulse light, and outputs the multiplexed light.

[0015] The optical fiber sensing system is configured in such a way that the optical fiber itself in the optical cable 21 is used as a sensor, probe pulse light is input from the DAS interrogator 30 into the optical fiber, and the DAS interrogator 30 receives and analyzes the return light that is affected by the environmental information added to the optical fiber. One core wire 41 of the optical fibers bundled in the optical cable 21 is a sensing optical fiber that senses information about the environment surrounding the optical cable 21. Figure 1 shows the optical cable 21 sensing vibrations as an example of environmental information it senses. In such an optical fiber sensing system, optical amplifier repeaters 11 that compensate for optical attenuation are provided in the optical fiber path. These optical amplifier repeaters 11 are inserted and arranged at appropriate intervals in the path of the optical cable 21. In this embodiment, they are arranged every 80 km.

[0016] In this embodiment, the wavelength of the probe pulse light transmitted and received by the DAS interrogator 30 is 1550.1 nm. The clamped light transmitter 31 transmits two clamped light wavelengths, approximately 1549.3 nm and 1550.9 nm. These three wavelengths of light are multiplexed and output by the multiplexer 32. The optical power after multiplexing is +10 dBm per wavelength for the clamped light, or +13 dBm for the two wavelengths in total, while the probe pulse light is approximately -30 dBm. The clamped light is divided into multiple wavelengths to avoid the occurrence of Stimulated Brillouin scattering (SBS). These wavelength arrangements and powers are merely examples, and it is sufficient if the power of the clamped light is sufficiently greater than the power of the probe pulse light. The clamped light may be intensity modulated or frequency dithered at a rate of change fast enough not to cause gain fluctuations in the optical amplifier.

[0017] The effect of optical amplifier repeaters on optical fiber transmission line loss compensation in optical fiber sensing is explained using Figure 2. Figure 2 shows an OTDR trace obtained using the OTDR function of the DAS, with the horizontal axis representing distance and the vertical axis representing the attenuation of reflected return light. Without using optical amplifier repeaters, the detection limit of this measuring instrument for returned light is just over 20 dB, and approximately 100 km is the upper limit of the measurement range when using this optical fiber.

[0018] When the optical amplifier repeater 11 of this embodiment is installed at points 80 km and 160 km, it can be seen that the loss of 80 km is compensated for by the optical amplifier before transmission to the next transmission section. This means that a range of 240 km can be measured. This figure simply shows that the measurement range of a typical OTDR, which measures the loss of optical fiber transmission lines, has been extended, and does not directly mean that the measurement range of a DAS has been expanded. However, in reality, if the DAS has sufficient processing performance, there is a strong correlation between the distance at which the returned light becomes so weak that it is difficult to detect in OTDR measurements and the measurement data becomes dominated by noise, and the similar distance in DAS measurements. In fact, in this case, vibrations applied to the optical fiber cable could be detected up to 240 km away.

[0019] <Configuration of optical amplifier repeater and explanation of optical filters used therein> The optical amplifier repeater 11 is a bidirectional optical amplifier configured to amplify light separately according to its traveling direction. Optical circulators 56 and 57 demultiplex and combine the light according to its traveling direction. In this embodiment, erbium-doped fiber amplifiers (EDFAs) 50 and 51 are used as the optical amplifiers. Dielectric multilayer filters are used as bandpass filters (BPFs) 54 and 55 to form optical bandpass filters. In this diagram, the upper path is the amplification path for the probe pulse light, and the lower path is the amplification path for the return light. For ease of explanation, the probe pulse light side will sometimes be referred to as the upstream path and the return light side as the downstream path. In the upstream path, light is amplified by the EDFA 50, and in the downstream path, light is amplified by the EDFA 51.

[0020] The BPFs 54 and 55 are of a filter type that passes the probe pulse light, clamp light, and return light, and attenuates unnecessary light such as ASE light.

[0021] The amplification band of the EDFAs 50 and 51 is usually about 1530 to 1562 nm, and the wavelength of the probe pulse light as well as the clamp light are set within the amplification band of the EDFAs 50 and 51 .

[0022] Figure 6 shows a schematic diagram of the optical spectrum before and after passing through the filter. This example shows the application of a filter shape that passes both the wavelength of the probe pulse light and the wavelength of the clamp light. Because the probe pulse light is weak on a time average, it is either at the same level as or buried in the ASE light emitted by EDFAs 50 and 51, making it difficult to confirm its presence or level. On the other hand, the clamp light has a power that is sufficiently high compared to the ASE light level, making it easy to confirm not only its presence but also its amplification level.

[0023] If an optical filter with a passband as shown by the dotted line in Figure 6A is applied, the result after passing through will be as shown in Figure 6B. In other words, the unnecessary ASE light has been removed. Although the power density per unit frequency of ASE light is low, it is distributed throughout the entire amplification band, so its power becomes too large to ignore. After multiple stages of optical amplification, the majority of the optical power will be dominated by ASE light. For this reason, it is desirable to attenuate the ASE light in unnecessary bands with an optical filter each time amplification is performed, as shown in Figures 6A and 6B. The BPFs 54 and 55 function as such optical filters.

[0024] FIG. 1 shows monitor port 39 in terminal station 10, which branches off a portion of the multiplexed light in multiplexing section 32 and outputs it, monitor port 90 in optical amplifier repeater 11, which branches off a portion of the passing light in coupler 92 and outputs it, and monitor port 91, which branches off a portion of the passing light in coupler 93 and outputs it. By observing the light from these monitor ports 39, 90, and 91 with an optical spectrum analyzer or the like, the arrival of the measurement light can be confirmed by the presence or absence of clamped light. Also, by checking the level of the clamped light, the amplification gain of the EDFAs 50 and 51 can be adjusted. For example, by monitoring the level of the clamped light, the gain of the optical amplifier repeater 11, i.e., the gain of the EDFAs 50 and 51, can be adjusted to a predetermined value. Therefore, on-site adjustments can be easily made when compensating for losses using the optical amplifier repeater 11 in OTDR-based optical fiber sensing measurements.

[0025] Furthermore, gain fluctuations are suppressed because clamped light of a constant power is always input to the optical amplifier repeater 11. This eliminates the risk of level instability and measurement failure due to pulsed light amplification.

[0026] The effect of suppressing gain fluctuations by simultaneously transmitting clamped light is explained using Figures 3 and 4. Figure 3 shows the case without clamped light, while Figure 4 shows the case when clamped light with sufficient power is added. The probe pulse light output from the measuring instrument is pulsed light with a very small duty ratio, so although the average value is small, the power is high when the pulse is present. When this pulse portion is input to an optical amplifier, such as an EDFA, stimulated emission occurs all at once, reducing the gain, which then gradually returns. In this way, nonlinear behavior occurs that deviates from the linear behavior of an ideal amplifier. If clamped light, which is larger than the pulse light, is wavelength-multiplexed and simultaneously input to the optical amplifier, the input power of the optical amplifier will be approximately constant regardless of whether or not there is a pulse portion, thereby suppressing gain fluctuations (Figure 4).

[0027] As described above, the clamped light of this embodiment not only serves the purpose of clamping the gain of the optical amplifier, but also serves as an indicator for signal arrival confirmation and level adjustment.

[0028] <Second embodiment> The second embodiment will be described with reference to Fig. 5. Since the configuration is the same as that of the first embodiment, some parts are omitted from the illustration. The second embodiment is characterized in that the filter shapes of the BPFs 54 and 55A in the optical amplifier repeater 11 are different between the upstream and downstream sides.

[0029] 6 and 7 are schematic diagrams illustrating the optical spectrum before and after passing through the filter. 6A and 6B show the optical spectra before and after passing through the upstream BPF 54, with the dotted line in FIG. 6A representing a schematic representation of the filter shape. The BPF 54 used is the same as in the first embodiment. 7A and 7B show the optical spectra before and after passing through the downstream BPF 55A, with the dotted line in FIG. 7A representing a schematic representation of the filter shape. The BPF 55A has a filter shape that passes the wavelength of the probe pulse light but attenuates the wavelength of the clamped light.

[0030] The probe pulse light and clamp light here are returned light that has been reflected by Rayleigh scattering and other phenomena in the optical fiber, and since the reflectivity is generally very low at -30 dB or less, the downstream signal light is even weaker than the upstream signal light, and even after being amplified and passed through the ASE elimination filter, most of the light remains as unwanted ASE light (unwanted light including ASE light). Therefore, in order to narrow the passband width of the ASE elimination filter BPF55A as much as possible, the clamp light is also attenuated.

[0031] The Rayleigh scattering return light of the probe pulse light is sawtooth rather than pulse-shaped, so it is relatively uniform in time, and most of the return light is occupied by ASE light that does not fluctuate over time, so there is almost no gain modulation of the optical amplifier. Therefore, there is no need to run clamped light in parallel, and the idea is to narrow the filter band as much as possible to reduce the ASE light as much as possible.

[0032] The returning light of the clamped light may slightly exceed the ASE light level, in which case it can be used to confirm that the signal is returning.

[0033] <Modifications not shown> In the above-described embodiment, the clamp light has two wavelengths, but multiple wavelengths may be used. Also, although the clamp light is used on both the short wavelength and long wavelength sides of the probe pulse light, it may be used on only one side.

[0034] When using clamped light for level control of an optical amplifier, the clamped light may be intensity-modulated at a frequency high enough that the optical amplifier is not subject to gain modulation, and the gain of the optical amplifier may be adjusted at each repeater amplifier so that the intensity-modulated component remains constant. Such a technique for using clamped light is disclosed, for example, in Patent Document 2. In this case, even if the power of the clamped light weakens and is drowned out by the ASE light, it can still serve as a more accurate level control index. [Explanation of symbols]

[0035] 10 Terminal 11 Optical amplifier repeater 21 Optical Cable 30 DAS Interrogator 31 Clamp optical transmitter 50,51 EDFA 54, 55, 55A BPF 56,57 Circulator

Claims

1. an optical fiber sensing system in which an optical fiber itself is used as a sensor, a probe pulse light is input from an interrogator at a terminal station to the optical fiber, and the interrogator receives and analyzes return light that is affected by environmental information added to the optical fiber; an optical amplifier repeater for compensating for optical attenuation is provided within the optical fiber path; The terminal wavelength-multiplexes the probe pulse light with continuous light having a wavelength within the amplification band of the optical amplifier repeater and not containing a low-frequency intensity modulation component, which suppresses gain fluctuations of the optical amplifier repeater, at an intensity stronger than that of the probe pulse light, and inputs the multiplexed light into the optical fiber. Fiber optic sensing system.

2. The continuous light is intensity-modulated or frequency-dithered at a rate of change fast enough not to cause gain fluctuations in the optical amplifier repeater. The optical fiber sensing system of claim 1 .

3. the optical amplifier repeater is configured to separate the probe pulse light and the return light in the traveling direction, amplify each light by each optical amplifier, and then combine and output the combined light, and is configured to include an optical bandpass filter on the output side of the optical amplifier, an optical bandpass filter on the probe pulse light side passes the probe pulse light and the continuous light, and separates unnecessary light emitted by the optical amplifier by wavelength band and attenuates the unnecessary light; The optical bandpass filter on the return light side passes the return light of the probe pulse light among the return light, and attenuates unnecessary light emitted by the optical amplifier and the return light of the continuous light. The optical fiber sensing system of claim 1 .

4. In the optical amplifier repeater, the level of the continuous light is monitored and the gain of the optical amplifier repeater is adjusted so that the level remains constant.

4. The optical fiber sensing system according to claim 1.

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