Optical fiber characteristic measurement device and optical fiber characteristic measurement method

Abstract

An optical fiber characteristic measurement device (1) is provided with: a light source (11) that emits laser light modulated at a predetermined modulation frequency; incident units (12, 13, 15, 16) that cause laser light to enter from one end and the other end of the optical fiber (14), respectively, as probe light (L1) and pump light (L2); a light detection unit (17) that has a photoelectric conversion element, a current source, and an amplifier circuit, and that detects light emitted from the optical fiber (14); a measurement unit (18) that measures the characteristics of the optical fiber (14) using the detection signal (D1) output from the light detection unit (17); and a current source control unit (19) that controls the current source on the basis of a detection result of the light detection unit (17) during a non-contained period, which is a period during which light including the probe light (L1) but not including the Brillouin scattered light is emitted from the optical fiber (14).

Description

Technical Field

[0001] The invention relates to an optical fiber characteristic measuring device and an optical fiber characteristic measuring method. Background Art

[0002] The spectrum (frequency) of Brillouin scattered light generated by incident light on an optical fiber changes with changes in the temperature and strain of the optical fiber. An optical fiber characteristic measurement device that utilizes this property measures the temperature distribution and strain distribution in the longitudinal direction of the optical fiber by detecting changes in the frequency of Brillouin scattered light (Brillouin frequency shift (BFS)) along the longitudinal direction of the optical fiber.

[0003] One of such optical fiber characteristic measurement devices is a BOCDA (Brillouin Optical Correlation Domain Analysis) method device disclosed in the following patent documents 1 and 2. This optical fiber characteristic measurement device allows frequency modulated light (pump light and probe light) to be injected from both ends of the optical fiber. Then, at the position where the modulation phases of the pump light and the probe light coincide (the position where the "correlation peak" appears), the optical fiber characteristics are measured by utilizing the property that the probe light is amplified due to the stimulated Brillouin scattering phenomenon.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: (Japan) Patent No. 3667132

[0007] Patent Document 2: (Japan) Patent No. 5654891 Summary of the invention

[0008] Problems to be solved by the invention

[0009] However, the light emitted from the optical fiber is a light in which weak Brillouin scattered light is superimposed on the probe light, so the detection signal obtained by detecting this light contains a Brillouin scattered light component whose signal intensity is significantly lower than that of the probe light component. If such a detection signal is amplified by an amplifier, the amplifier is saturated by the probe light component and cannot amplify the Brillouin scattered light component to a sufficient level. Therefore, it is impossible to improve the ratio of the Brillouin scattered light component to noise, that is, the SN ratio (signal-to-noise ratio).

[0010] Here, if the probe light component is removed from the detection signal and only the Brillouin scattered light component is amplified, the Brillouin scattered light component can be amplified to a sufficient level. However, according to the method of removing the probe light component from the detection signal, for example, when the loss changes suddenly due to external disturbance such as bending of the optical fiber, it is considered that there will be an influence on the tracking property, such as the measurement result not following up immediately.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical fiber characteristic measurement device and an optical fiber characteristic measurement method that can suppress the influence of followability due to external disturbance and improve the SN ratio.

[0012] Means for solving problems

[0013] In order to solve the above-mentioned problems, a first aspect of the present invention relates to an optical fiber characteristic measurement device (1-3) comprising: a light source (11) for emitting laser light modulated at a predetermined modulation frequency; an incident section (12, 13, 15, 16) for causing the laser light to be incident from one end and the other end of an optical fiber (14) as continuous light (L1) and pulsed light (L2), respectively; a light detection section (17, 17A) comprising a photoelectric conversion element (21), a current source (22) connected to the photoelectric conversion element, and an amplifier circuit (23) connected to a connection point (CP) between the photoelectric conversion element and the current source, for detecting light emitted from the optical fiber; a measurement section (18) for measuring the characteristics of the optical fiber using a detection signal (D1) output from the light detection section; and a current source control section (19) for controlling the current source based on a detection result (D1, D2, D11) of the light detection section during a period (T2) in which light including the continuous light but not including Brillouin scattered light is emitted from the optical fiber.

[0014] The optical fiber characteristic measurement device involved in the second embodiment of the present invention is the optical fiber characteristic measurement device as described in the first embodiment of the present invention, wherein the period (T) of the pulse light incident on the other end of the optical fiber is set to at least twice the reciprocation time required for the pulse light to reciprocate between one end and the other end of the optical fiber, and the non-containing period is the period from the time when the pulse light is incident on the other end of the optical fiber and the reciprocation time passes to the time when the next pulse light is incident on the other end of the optical fiber.

[0015] The optical fiber characteristic measurement device involved in the third aspect of the present invention is the optical fiber characteristic measurement device as described in the second aspect of the present invention, wherein the current source control unit uses a synchronization signal (SY) having a period identical to a period of the pulse light incident on the other end of the optical fiber to obtain the detection result of the light detection unit during the non-containing period.

[0016] The optical fiber characteristic measurement device according to a fourth aspect of the present invention is the optical fiber characteristic measurement device according to the third aspect of the present invention, wherein the current source control unit obtains the detection signal (D1) output from the light detection unit as the detection result of the light detection unit.

[0017] The optical fiber characteristic measurement device involved in the fifth aspect of the present invention is the optical fiber characteristic measurement device as described in the third aspect of the present invention, wherein the measuring unit is equipped with a synchronous detection device (20), which cuts out a detection signal obtained by detecting light set near a measurement point of the optical fiber from the detection signal output by the optical detection unit, and performs synchronous detection on the detection signal cut out using the synchronous signal, and the current source control unit obtains the detection signal (D2) cut out by the synchronous detection device as the detection result of the optical detection unit.

[0018] The optical fiber characteristic measuring device involved in the 6th mode of the present invention is the optical fiber characteristic measuring device as described in the 3rd mode of the present invention, wherein the optical detection unit has a current detector (24), which detects the current flowing through the photoelectric conversion element, and the current source control unit obtains the detection result (D11) of the current detector as the detection result of the optical detection unit.

[0019] The optical fiber characteristic measuring device involved in the 7th embodiment of the present invention is the optical fiber characteristic measuring device described in the 1st embodiment of the present invention, wherein the current source control unit controls the current source based on the detection result of the light detection unit in the non-containing period to remove the continuous light component contained in the light detected by the light detection unit.

[0020] A method for measuring optical fiber characteristics according to one embodiment of the present invention comprises: emitting laser light modulated at a predetermined modulation frequency through a light source, causing the laser light to be incident on one end and the other end of an optical fiber (14) as continuous light (L1) and pulse light (L2) respectively through an incident section, detecting the light emitted from the optical fiber through a light detection section (17), the light detection section (17) having a photoelectric conversion element (21), a current source (22) connected to the photoelectric conversion element, and an amplifier circuit (23) connected to a connection point (CP) between the photoelectric conversion element and the current source, measuring the characteristics of the optical fiber through a measurement section using a detection signal (D1) output from the light detection section, and controlling the current source through a current source control section based on a detection result of the light detection section during a period (T2) during which light including the continuous light but not including Brillouin scattered light is emitted from the optical fiber.

[0021] Effects of the Invention

[0022] According to the present invention, there is an effect of being able to suppress the influence of the follow-up performance due to external disturbance and improve the SN ratio. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a block diagram showing the main part configuration of the optical fiber characteristics measurement device according to the first embodiment of the present invention.

[0024] Figure 2 This is a circuit diagram showing a main part configuration of a light detection unit provided in the optical fiber characteristics measurement apparatus according to the first embodiment of the present invention.

[0025] Figure 3 This is a diagram for explaining processing performed by the synchronous detection device in the first embodiment of the present invention.

[0026] Figure 4 This is a diagram for explaining processing performed by the current source control unit in the first embodiment of the present invention.

[0027] Figure 5 It is a block diagram showing the main part configuration of an optical fiber characteristics measurement device according to a second embodiment of the present invention.

[0028] Figure 6 This is a diagram for explaining processing performed by the current source control unit in the second embodiment of the present invention.

[0029] Figure 7 It is a block diagram showing the main part structure of the optical fiber characteristics measurement device according to the third embodiment of the present invention.

[0030] Figure 8 It is a circuit diagram showing a main part configuration of a light detection unit provided in an optical fiber characteristics measurement apparatus according to a third embodiment of the present invention. DETAILED DESCRIPTION

[0031] Hereinafter, an optical fiber characteristic measurement device and an optical fiber characteristic measurement method according to embodiments of the present invention will be described in detail with reference to the accompanying drawings. Hereinafter, an overview of embodiments of the present invention will be described first, and then details of each embodiment of the present invention will be described.

[0032] 〔summary〕

[0033] The embodiment of the present invention suppresses the influence of tracking caused by external disturbance in the optical fiber characteristic measurement device and improves the SN ratio. Specifically, in the optical fiber characteristic measurement device of the BOCDA method, the component of the probe light is removed from the detection signal of the light emitted from the optical fiber, and only the component of the Brillouin scattered light is amplified to improve the SN ratio. At this time, even if the temperature and strain of the optical fiber change suddenly, its measurement result will immediately follow.

[0034] If light is incident on an optical fiber, tiny scattered light is generated at various locations within the optical fiber. Scattered light is roughly divided into three types (Rayleigh scattered light, Brillouin scattered light, and Raman scattered light) according to the cause of its generation. The frequency of Brillouin scattered light changes linearly with respect to the temperature and strain applied to the optical fiber. Therefore, if the generation position and frequency of Brillouin scattered light are obtained, the optical fiber itself can be used as a strain sensor, a temperature sensor, or both. Using this property, research and development of measurement technology for applications such as laying quality management of communication optical fibers themselves, diagnosis of the structural integrity of social infrastructure structures such as bridges and dams, or aircraft bodies are prevalent.

[0035] Brillouin scattering includes natural Brillouin scattering (SpBS: Spontaneous Brillouin Scattering) and stimulated Brillouin scattering (SBS: Stimulated Brillouin Scattering). Natural Brillouin scattering is the scattering caused by the sound waves that exist naturally. Stimulated Brillouin scattering is the scattering caused by the interaction between strong sound waves and light generated when two lights propagating in opposite directions meet specific conditions.

[0036] The optical fiber characteristic measuring apparatus of the BOCDA method disclosed in the above-mentioned patent documents 1 and 2 allows light obtained by frequency modulation (FM modulation) by a sine wave to be incident from both ends of the optical fiber. In addition, there is only one position in the optical fiber where the modulation phases of the two lights are consistent (the position where the correlation peak appears). At this time, high-intensity stimulated Brillouin scattered light is generated at the position where the correlation peak appears, and low-intensity stimulated Brillouin scattered light is generated at other positions.

[0037] When observing the spectrum of all stimulated Brillouin scattered light generated in the optical fiber (BGS: Brillouin Gain Spectrum), the shape of the stimulated Brillouin scattered light generated at the position where the correlation peak appears becomes dominant. In other words, by observing the spectrum of stimulated Brillouin scattered light generated at the position where the correlation peak appears, it is possible to obtain information on the temperature or strain at the position where the correlation peak appears based on the difference between the frequency of its maximum value and the frequency of the incident light (BFS: Brillouin Frequency Shift).

[0038] The position where the correlation peak appears can be moved by changing the frequency of the frequency modulation. Therefore, by moving the position where the correlation peak appears along the length direction of the optical fiber, it is possible to obtain information on the temperature or strain at any position along the length direction of the optical fiber.

[0039] On the other hand, when the light incident on the optical fiber is frequency modulated by a sine wave, correlation peaks appear at certain intervals along the length direction of the optical fiber. In order to make the correlation peak appear at only one position in the optical fiber, the length of the optical fiber must be shorter than the interval between the correlation peaks. As a method for avoiding this limitation, there is a method called the time gate method disclosed in the above-mentioned patent document 2. Simply put, the time gate method is a method of cutting out only the detection signal obtained by detecting the light generated at the measurement point set in the optical fiber and its vicinity from the light emitted sequentially from the optical fiber. As a result, even if there are multiple positions where correlation peaks appear in the optical fiber, it is possible to obtain only the detection signal obtained by detecting the light generated at the measurement point set in the optical fiber and its vicinity.

[0040] In such a BOCDA-type optical fiber characteristic measurement device, light obtained by superimposing weak Brillouin scattered light on the probe light is detected. The detection signal obtained by detecting this light contains a Brillouin scattered light component whose signal intensity is significantly lower than that of the probe light component. If such a detection signal is amplified by an amplifier, the amplifier is saturated by the probe light component and cannot amplify the Brillouin scattered light component to a sufficient level. Therefore, it is impossible to improve the ratio of the Brillouin scattered light component to noise, that is, the SN ratio.

[0041] Here, if the probe light component is removed from the detection signal and only the Brillouin scattered light component is amplified, the Brillouin scattered light component can be amplified to a sufficient level. However, according to the method of removing the probe light component from the detection signal, when the temperature or strain of the optical fiber changes suddenly, it is considered that there will be an influence on the tracking performance, such as the measurement result not following immediately.

[0042] In this embodiment, first, a laser modulated at a predetermined modulation frequency is made to enter from one end and the other end of the optical fiber as continuous light and pulse light, respectively. Then, the light emitted from the optical fiber is detected by a light detection unit, which has a photoelectric conversion element, a current source connected to the photoelectric conversion element, and an amplifier circuit connected to the connection point of the photoelectric conversion element and the current source. Then, based on the detection result of the light detection unit during the period when light including continuous light but not including Brillouin scattered light is emitted from the optical fiber, that is, the non-containing period, the current source provided in the light detection unit is controlled. Then, the detection signal output from the light detection unit is used to measure the characteristics of the optical fiber. Thus, the influence of tracking caused by external interference can be suppressed and the SN ratio can be improved.

[0043] [First embodiment]

[0044] 〈Optical Fiber Characteristics Measurement Device〉

[0045] Figure 11 is a block diagram showing the main structure of the optical fiber characteristic measurement device according to the first embodiment of the present invention. Figure 1 As shown, the optical fiber characteristic measurement device 1 of this embodiment includes a light source 11, a spectrometer 12 (incident part), a light modulator 13 (incident part), an optical fiber 14, a pulse modulator 15 (incident part), a directional coupler 16 (incident part), a light detection part 17, a measurement part 18, and a current source control part 19. Such an optical fiber characteristic measurement device 1 measures the characteristics of the optical fiber 14 in the longitudinal direction (for example, temperature distribution, strain distribution, etc.).

[0046] The light source 11 includes a semiconductor laser 11a and a signal generator 11b, and emits light at a predetermined modulation frequency f. m Modulated laser light. The semiconductor laser 11a emits laser light of a wavelength (eg, 1.55 μm) with little absorption in the optical fiber 14. The signal generator 11b outputs a signal at a modulation frequency f to the semiconductor laser 11a. m A sine wave signal (modulation signal) that frequency modulates the laser light emitted from the semiconductor laser 11 a. The beam splitter 12 splits the laser light emitted from the light source 11 into two branches at a one-to-one intensity ratio, for example.

[0047] The optical modulator 13 includes a microwave generator 13a and an SSB (Single Side Band) optical modulator 13b. The optical modulator 13 modulates (shifts the optical frequency) one of the laser beams branched by the optical splitter 12, and generates a sideband (single sideband) relative to the center frequency of the laser beam. In addition, in the present embodiment, a single sideband wave on the low frequency side is output from the optical modulator 13.

[0048] The microwave generator 13a outputs microwaves having a frequency corresponding to the frequency shift to be imparted to one of the laser beams branched by the optical splitter 12. The SSB optical modulator 13b generates a single sideband wave having a frequency difference equal to the frequency of the microwave output from the microwave generator 13a with respect to the center frequency of the input light. In addition, the frequency of the microwave output from the microwave generator 13a is variable. The light modulated by the optical modulator 13 is incident into the optical fiber 14 from one end of the optical fiber 14 as the probe light L1 (continuous light).

[0049] The pulse modulator 15 includes a signal generator 15a and an optical intensity modulator 15b, and generates pulsed light by pulsing the other laser light branched by the spectrometer 12. Here, the pulse modulator 15 generates pulsed light with a period T, and the period T is set to at least twice the time required for the pulsed light to reciprocate between one end and the other end of the optical fiber 14. The signal generator 15a outputs a timing signal that specifies the timing for pulsing the laser light. The optical intensity modulator 15b is, for example, an EO (Electro-Optic) switch, and pulses the laser light from the spectrometer 12 at the timing specified by the timing signal output from the signal generator 15a.

[0050] The directional coupler 16 allows the pulsed laser light output from the pulse modulator 15 to be incident as the pump light L2 (pulse light) into the optical fiber 14 from the other end of the optical fiber 14. In addition, the directional coupler 16 emits the light (detection light L11) included in the probe light L1 propagated in the optical fiber 14 and emitted from the other end of the optical fiber 14 toward the light detection unit 17. In addition, the intensity of the detection light L11 is affected by the stimulated Brillouin scattering phenomenon generated in the optical fiber 14. The light detection unit 17 detects (receives) the detection light L11 and outputs a detection signal D1.

[0051] Figure 2 FIG. 1 is a circuit diagram showing a main part structure of a light detection unit provided in an optical fiber characteristic measurement device according to a first embodiment of the present invention. Figure 2 As shown, the light detection unit 17 includes a photodiode 21 (photoelectric conversion element), a current source 22 , and an amplifier circuit 23 .

[0052] The photodiode 21 performs photoelectric conversion on the detection light L11 and outputs a current corresponding to the detection light L11. As the photodiode 21, for example, a highly sensitive light receiving element such as an avalanche photodiode can be used. The cathode of the photodiode 21 is connected to the bias terminal T11, and the anode of the photodiode 21 is connected to one end of the current source 22. In addition, the bias terminal T11 is a terminal for inputting a bias voltage applied to the photodiode 21.

[0053] The current source 22 outputs a current corresponding to the control signal C1 (described in detail later) input from the control signal input terminal T13. The current source 22 is provided to remove the component of the detection light L1 included in the detection light L11 from the current output from the photodiode 21. One end of the current source 22 is connected to the anode of the photodiode 21, and the other end of the current source 22 is connected to the negative power supply terminal T12.

[0054] The amplifier circuit 23 amplifies the current output from the photodiode 21 (more precisely, the current obtained by subtracting the current flowing through the current source 22 from the current output from the photodiode 21), converts the amplified current into a voltage, and outputs it. The amplifier circuit 23 includes an amplifier 23a and a feedback resistor 23b. The input end of the amplifier 23a is connected to the connection point CP of the photodiode 21 and the current source 22, and the output end of the amplifier 23a is connected to the output terminal T14. The feedback resistor 23b is connected between the input end and the output end of the amplifier 23a, and together with the amplifier 23a, constitutes a current-voltage conversion circuit. Therefore, the detection signal D1 output from the output terminal T14 has a voltage corresponding to the current obtained by subtracting the current flowing through the current source 22 from the current output from the photodiode 21.

[0055] The measuring unit 18 measures the characteristics of the optical fiber 14 in the longitudinal direction using the detection signal D1 output from the light detecting unit 17. The measuring unit 18 includes a synchronous detection device 20 for synchronously detecting the detection signal D1 output from the light detecting unit 17. The synchronous detection device 20 cuts out the detection signal D1 output from the light detecting unit 17, which is obtained by detecting the light including stimulated Brillouin scattered light generated at the measurement point (the point where the characteristics are to be measured) set in the optical fiber 14 and in the vicinity thereof. Then, the synchronous detection signal SY having a predetermined period (refer to Figure 3 ) performs synchronous detection on the detection signal D2 obtained by cutting out. The synchronous detection device 20 includes a timing adjuster 20a and a lock-in amplifier 20b (synchronous detector).

[0056] The timing adjuster 20a is realized by, for example, an electric switch (high-speed analog switch) that can switch between an on state (a state in which the detection signal D1 is passed) and an off state (a state in which the detection signal D1 is cut off) at high speed. The timing adjuster 20a allows the detection signal D1 output from the light detection unit 17 to pass, which is a detection signal obtained by detecting the light including the stimulated Brillouin scattered light generated at the measurement point set in the optical fiber 14 and its vicinity, to cut out the detection signal. In addition, the operation cycle of the timing adjuster 20a is set to a cycle that is half the cycle of the synchronization signal SY.

[0057] The lock-in amplifier 20b uses the above-mentioned synchronization signal SY to perform synchronous detection on the detection signal D2 (the detection signal cut out by the timing adjuster 20a) that has passed through the timing adjuster 20a. Here, the period of the above-mentioned synchronization signal SY is set to the same period as the period of the pump light L2 incident on the other end of the optical fiber 14 (at least twice the time required for the pulse light to reciprocate between one end and the other end of the optical fiber 14). In addition, the structure of the lock-in amplifier 20b is the same as the structure disclosed in the above-mentioned patent document 2 except that it is configured to output the detection signal D2 and the synchronization signal SY to the outside, so the detailed description is omitted.

[0058] The current source control unit 19 generates a control signal C1 for controlling the current source 22 of the light detection unit 17 using the detection signal D2 (the detection result of the light detection unit) output from the lock-in amplifier 20 b and the synchronization signal SY. Specifically, the current source control unit 19 extracts the period ( ) during which the light including the probe light L1 but not including the Brillouin scattered light is emitted from the optical fiber 14 using the detection signal D2 and the synchronization signal SY. Figure 3 , Figure 4 The detection signal D1 is output from the light detection unit 17 during T2 (non-containing period). Then, the current source control unit 19 generates a control signal C1 using the extracted detection signal D1, and outputs the generated control signal C1 to the light detection unit 17.

[0059] Here, the detection signal D1 extracted using the detection signal D2 and the synchronization signal SY represents the intensity of the probe light L1. The current source control unit 19 generates a control signal C1 based on the extracted detection signal D1. The control signal C1 can be obtained from Figure 2 The component of the detection light L1 contained in the detection light L11 is removed from the current output by the photodiode 21 shown in FIG. Figure 2 The amplifier circuit 23 shown is saturated by the component of the detection light L1 and improves the SN ratio (ratio of the Brillouin scattered light component to noise) by amplifying the Brillouin scattered light component to a sufficient level.

[0060] <Optical Fiber Characteristics Measurement Method>

[0061] When the measurement starts, the light source 11 emits a signal with a modulation frequency f m Frequency modulated laser light (first step). The laser light emitted from the light source 11 is branched by the spectrometer 12. One of the laser lights branched by the spectrometer 12 is incident on the optical modulator 13 and modulated by the SSB optical modulator 13b, thereby generating a single sideband with respect to the center frequency of the laser light. The laser light (continuous light) with a single sideband emitted from the optical modulator 13 is incident on the optical fiber 14 from one end of the optical fiber 14 as the probe light L1 (second step).

[0062] On the other hand, the other laser beam branched by the optical splitter 12 is incident on the pulse modulator 15 and intensity-modulated by the optical intensity modulator 15b, thereby being pulsed. Specifically, pulse light is generated with the aforementioned period T (a period set to at least twice the time required for the pulse light to reciprocate between one end and the other end of the optical fiber 14). The pulse light is incident on the optical fiber 14 from the other end of the optical fiber 14 as the pump light L2 via the directional coupler 16 (second step).

[0063] If the modulation frequency f m When the frequency-modulated probe light L1 as continuous light and the pump light L2 as pulsed light are incident on the optical fiber 14, correlation peaks are sequentially generated at different positions in the optical fiber 14 as the pump light L2 propagates in the optical fiber 14. At the position of each correlation peak, the probe light L1 obtains a gain (gain) based on stimulated Brillouin amplification by the pump light L2.

[0064] If the frequency difference between the pump light L2 and the probe light L1 is changed at the position of the correlation peak with the pump light L2 as the reference, a Brillouin frequency shift ν is obtained. B The spectrum of the shape of the Lorentzian function with a center frequency is called the Brillouin gain spectrum (BGS). The Brillouin frequency shift ν is known as B It changes depending on the material, temperature, strain, etc. of the optical fiber 14, and in particular changes linearly with respect to the strain. Therefore, the strain amount of the optical fiber 14 can be obtained by detecting the peak frequency of the Brillouin gain spectrum.

[0065] The probe light L1 passing through the optical fiber 14 and the stimulated Brillouin scattered light generated in the optical fiber 14 are emitted from the other end of the optical fiber 14 and then enter the light detection unit 17 as the detection light L11 via the directional coupler 16. Then, the detection light L11 is detected in the light detection unit 17, and a detection signal D1 indicating the detection result is output from the light detection unit 17 (third step). The detection signal D1 output from the light detection unit 17 is input to the measurement unit 18, and synchronous detection is performed by the synchronous detection device 20.

[0066] Figure 3 This is a diagram for explaining the processing performed by the synchronous detection device in the first embodiment of the present invention. In addition, in order to simplify the description below, it is assumed that the period T (the period of the synchronization signal SY) of the pump light L2 incident on the other end of the optical fiber 14 is set to twice the time required for the pump light L2 to reciprocate between one end and the other end of the optical fiber 14.

[0067] In the first half T1 of one cycle T of the synchronization signal SY, the detection light L11 including the probe light L1 passing through the optical fiber 14 and the stimulated Brillouin scattered light generated in the optical fiber 14 enters the light detection unit 17. Figure 3 As shown in FIG. 1 , a detection signal D1 affected by stimulated Brillouin scattered light is output from the light detection unit 17. Figure 3 In FIG. 1 , portions affected by stimulated Brillouin scattered light (eg, portions indicated by symbols P1 to P4 ) are represented by black bands.

[0068] On the other hand, in the second half T2 (non-inclusion period) of one cycle T of the synchronization signal SY, the detection light L11 including the probe light L1 passing through the optical fiber 14 but not including the stimulated Brillouin scattered light enters the light detection unit 17. Therefore, Figure 3 As shown, a detection signal D1 that is not affected by stimulated Brillouin scattered light (without a black band) is output from the light detection unit 17. In addition, since the pump light L2 is repeatedly incident with a period T, the detection signal D1 that is affected by stimulated Brillouin scattered light and the detection signal D2 that is not affected by stimulated Brillouin scattered light are alternately output every T / 2.

[0069] Here, if Figure 3 As shown, the operation cycle of the timing adjuster 20a is set to T / 2, and in each cycle, an operation is performed to pass a detection signal obtained by detecting light including stimulated Brillouin scattered light generated at a measurement point set in the optical fiber 14 and in the vicinity thereof. Figure 3 In the example shown, the timing adjuster 20a switches the on / off state so as to pass the portion indicated by the symbol P1 in the detection signal D1 output from the light detection unit 17. Thus, the lock-in amplifier 20b receives the signal cut out by the timing adjuster 20a. Figure 3 The detection signal D2 is shown.

[0070] When the detection signal D2 is input to the lock-in amplifier 20b, the polarity of the detection signal D2 is firstly inverted alternately using the synchronization signal SY. Specifically, the polarity is not inverted in the first half T1 of one cycle T of the synchronization signal SY, but inverted in the second half T2 of one cycle T of the synchronization signal SY. By performing such a process, a signal S1 in which the polarity of the portion of the detection signal D2 that is not affected by the stimulated Brillouin scattered light is inverted is obtained.

[0071] Next, a low-pass filter is applied to the signal S1. If this low-pass filter is applied, the signal d11, which is equivalent to the detection signal obtained by detecting only the detection light L1 in the first half T1, and the signal d12, which is equivalent to the detection signal obtained by detecting only the detection light L1 in the second half T2, cancel each other out. Figure 3 As shown, the measurement value V1 output from the lock-in amplifier 20b becomes a level representing stimulated Brillouin scattered light. The above-described operation is repeated while changing the position of the measurement point set in the optical fiber 14 by changing the cut-out timing of the timing adjuster 20a. Thus, the characteristics of the optical fiber 14 in the longitudinal direction are measured (step 4).

[0072] Figure 4 1 is a diagram for explaining the processing performed by the current source control unit in the first embodiment of the present invention. First, the current source control unit 19 generates an inverted synchronization signal SY￣ obtained by inverting the synchronization signal SY output from the lock-in amplifier 20b, and extracts the signal S2 from the detection signal D2 using the generated inverted synchronization signal SY￣. In addition, in this specification, for the sake of convenience, the symbol "SY￣" is used to represent a mark in which the symbol "￣" is added to the upper part of the symbol "SY".

[0073] Next, the current source control unit 19 smoothes the extracted signal S2 over one cycle T of the synchronization signal SY, and appropriately amplifies it to generate a control signal C1. Then, the current source control unit 19 outputs the generated control signal C1 to the light detection unit 17 to control the current output from the current source 22 (step 5). In addition, the current source control unit 19 generates the control signal C1 for each cycle T of the synchronization signal SY.

[0074] Here, the signal S2 extracted by the current source control unit 19 is a signal d13 that corresponds to the detection signal obtained by detecting only the detection light L1 in the second half T2. Therefore, by generating a control signal C1 corresponding to the signal d13 to control the current source 22, the current derived from the detection light L1 included in the detection light L11, among the current output from the photodiode 21, can flow through the current source 22 (absorb the current). As a result, the amplifier circuit 23 is not saturated by the component of the detection light L1, and the component of the Brillouin scattered light is amplified to a sufficient level.

[0075] As described above, in this embodiment, the current source 22 is controlled based on the detection result of the light detection unit 17 during the period (the second half T2 of one cycle T of the synchronization signal SY) when the light including the probe light L1 but not including the Brillouin scattered light is emitted from the optical fiber 14. As a result, the amplifier circuit 23 is not saturated by the component of the probe light L1, and the component of the Brillouin scattered light is amplified to a sufficient level, so that the SN ratio (ratio of the component of the Brillouin scattered light to the noise) can be improved.

[0076] In this embodiment, the time width required to generate the control signal C1 is at most one cycle of the synchronization signal SY. Therefore, even if external disturbances such as bending are applied to the optical fiber 14 and the loss changes suddenly, for example, the influence of the tracking property such as the measurement result not following up immediately can be suppressed.

[0077] [Second embodiment]

[0078] Figure 5 2 is a block diagram showing the main structure of the optical fiber characteristic measurement device according to the second embodiment of the present invention. Figure 5 In, with Figure 1 The same reference numerals are used for the same structures as those shown in the figure. Figure 1 The optical fiber characteristic measurement apparatus 1 shown is the same in basic configuration, but is different in that a control signal C1 is generated using a detection signal D1 (a detection result of the optical detection unit) output from the optical detection unit 17 .

[0079] like Figure 5 As shown, the detection signal D1 output from the light detection unit 17 and the synchronization signal SY output from the lock-in amplifier 20b are input to the current source control unit 19. The current source control unit 19 generates a control signal C1 using the detection signal D1 and the synchronization signal SY.

[0080] Figure 6 1 is a diagram for explaining the processing performed by the current source control unit in the second embodiment of the present invention. As in the first embodiment, the current source control unit 19 generates an inverted synchronization signal SY￣ that inverts the synchronization signal SY output from the lock-in amplifier 20b, and uses the generated inverted synchronization signal SY￣ to extract the signal S2 from the detection signal D1. Then, the current source control unit 19 smoothes the extracted signal S2 over one cycle T of the synchronization signal SY, and appropriately amplifies it to generate the control signal C1.

[0081] Here, the signal S2 extracted by the current source control unit 19 is a signal d14 that corresponds to the detection signal obtained by detecting only the detection light L1 in the entire second half T2 (T / 2). Therefore, by generating a control signal C1 corresponding to the signal d14 to control the current source 22, the current derived from the detection light L1 included in the detection light L11, among the current output from the photodiode 21, can flow through the current source 22 (absorb the current). As a result, the amplifier circuit 23 is not saturated by the component of the detection light L1, and the component of the Brillouin scattered light is amplified to a sufficient level.

[0082] In addition, the optical fiber characteristic measurement device 2 of this embodiment is Figure 1The optical fiber characteristic measurement device 1 shown is different only in the method of generating the control signal C1, and the basic operation of the optical fiber characteristic measurement device 2 is the same as that of the optical fiber characteristic measurement device 1. Therefore, the detailed operation of the optical fiber characteristic measurement device 2 is omitted.

[0083] As described above, in the present embodiment, the current source 22 is also controlled based on the detection result of the light detection unit 17 during the period (the second half T2 of one cycle T of the synchronization signal SY) when the light including the probe light L1 but not including the Brillouin scattered light is emitted from the optical fiber 14. As a result, the amplifier circuit 23 is not saturated by the component of the probe light L1, and the component of the Brillouin scattered light is amplified to a sufficient level, so that the SN ratio (ratio of the component of the Brillouin scattered light to the noise) can be improved.

[0084] In this embodiment, the time width required to generate the control signal C1 is at most one cycle of the synchronization signal SY. Therefore, even if external disturbances such as bending are applied to the optical fiber 14 and the loss changes suddenly, for example, the influence of the tracking property such as the measurement result not following up immediately can be suppressed.

[0085] [Third embodiment]

[0086] Figure 7 is a block diagram showing the main part structure of the optical fiber characteristic measurement device involved in the third embodiment of the present invention. Figure 7 In, with Figure 1 , Figure 5 The same reference numerals are used for the same structures shown in the figure. Figure 1 , Figure 5 The light detection unit 17 of the optical fiber characteristic measurement devices 1 and 2 shown is replaced with a light detection unit 17A, and the control signal C1 is generated using the current detection signal D11 (detection result of the light detection unit) output from the light detection unit 17A.

[0087] Figure 8 3 is a circuit diagram showing the main part structure of the light detection unit provided in the optical fiber characteristic measurement device involved in the third embodiment of the present invention. Figure 8 In, with Figure 2 The same structures as shown are given the same reference numerals. Figure 8 As shown, the light detection unit 17A includes a current detector 24 in addition to the photodiode 21 , the current source 22 , and the amplifier circuit 23 .

[0088] The current detector 24 is provided between the photodiode 21 and the bias terminal T11, and detects the current output from the photodiode 21. Alternatively, the current detector 24 may be provided between the photodiode 21 and the connection point CP. The detection result of the current detector 24 is output as a current detection signal D11 from the current detection signal output terminal T15.

[0089] like Figure 7 As shown in FIG. 1 , the current detection signal D11 output from the light detection unit 17A and the synchronization signal SY output from the lock-in amplifier 20b are input to the current source control unit 19. The current source control unit 19 generates the control signal C1 using the current detection signal D11 and the synchronization signal SY. The processing performed by the current source control unit 19 to generate the control signal C1 is the same as the processing performed by the current source control unit 19 in the first and second embodiments.

[0090] That is, in the current source control unit 19, first, an inverted synchronization signal SY￣ is generated by inverting the synchronization signal SY output from the lock-in amplifier 20b, and the generated inverted synchronization signal SY￣ is used to extract a signal corresponding to the signal S2 from the current detection signal D11. Next, the following processing is performed: the extracted signal is smoothed within one cycle T of the synchronization signal SY, and appropriately amplified to generate the control signal C1.

[0091] In addition, the optical fiber characteristic measurement device 3 of this embodiment is Figure 1 , Figure 5 The optical fiber characteristic measurement apparatuses 1 and 2 shown differ only in the method of generating the control signal C1, and the basic operation of the optical fiber characteristic measurement apparatus 3 is the same as that of the optical fiber characteristic measurement apparatuses 1 and 2. Therefore, the detailed operation of the optical fiber characteristic measurement apparatus 3 is omitted.

[0092] As described above, in the present embodiment, the current source 22 is also controlled based on the detection result of the light detection unit 17 during the period (the second half T2 of one cycle T of the synchronization signal SY) when the light including the probe light L1 but not including the Brillouin scattered light is emitted from the optical fiber 14. As a result, the amplifier circuit 23 is not saturated by the component of the probe light L1, and the component of the Brillouin scattered light is amplified to a sufficient level, so that the SN ratio (ratio of the component of the Brillouin scattered light to the noise) can be improved.

[0093] In this embodiment, the time width required to generate the control signal C1 is at most one cycle of the synchronization signal SY. Therefore, even if external disturbances such as bending are applied to the optical fiber 14 and the loss changes suddenly, the influence of the tracking property such as the measurement result not following up immediately can be suppressed.

[0094] The optical fiber characteristic measuring device and the optical fiber characteristic measuring method according to the embodiments of the present invention are described above, but the present invention is not limited to the above embodiments and can be freely changed within the scope of the present invention. For example, in the above-mentioned first to third embodiments, the current source control unit 19 smoothes the extracted signal S2 within one cycle T of the synchronization signal SY, and appropriately amplifies it to generate the control signal C1. However, the current source control unit 19 may also sample the extracted signal S2 (signals d13, d14) or the signal level equivalent to the signal S2 and hold it within one cycle T of the synchronization signal SY, and control the control signal C1 based on the level. In addition, the time width required to generate the control signal C1 is not limited to the range of one cycle of the synchronization signal SY, and may also be used by weighting the value of the previous control signal C1 as needed.

[0095] Description of Reference Numerals

[0096] 1~3 Optical fiber characteristic measurement device; 11 Light source; 12 Spectrometer; 13 Optical modulator; 14 Optical fiber; 15 Pulse modulator; 16 Directional coupler; 17, 17A Light detection unit; 18 Measurement unit; 19 Current source control unit; 20 Synchronous detection device; 21 Photodiode; 22 Current source; 23 Amplifier circuit; 24 Current detector; CP connection point; D1, D2 detection signals; D11 Current detection signal; L1 Probe light; L2 Pump light; SY Synchronous signal; T period; T2 The second half.

Claims

1. An optical fiber characteristic measuring device, comprising: A light source, emitting laser light modulated at a prescribed modulation frequency; An incident part that allows the laser light to be incident from one end and the other end of the optical fiber as continuous light and pulse light respectively; a light detection unit having a photoelectric conversion element, a current source connected to the photoelectric conversion element, and an amplifier circuit connected to a connection point between the photoelectric conversion element and the current source, and detecting light emitted from the optical fiber; a measuring unit that measures a characteristic of the optical fiber using a detection signal output from the light detecting unit; as well as The current source control unit controls the current source based on the detection result of the light detection unit during a period in which light including the continuous light but not including the Brillouin scattered light is emitted from the optical fiber, that is, a non-containing period.

2. The optical fiber characteristic measuring device according to claim 1, wherein: The period of the pulse light incident on the other end of the optical fiber is set to at least twice the time required for the pulse light to reciprocate between one end and the other end of the optical fiber. The non-containing period is a period from when the pulse light is incident on the other end of the optical fiber and the round trip time has elapsed until when the next pulse light is incident on the other end of the optical fiber.

3. The optical fiber characteristic measuring device according to claim 2, wherein: The current source control unit obtains the detection result of the light detection unit in the non-containing period using a synchronization signal having the same period as a period of the pulse light incident on the other end of the optical fiber.

4. The optical fiber characteristic measuring device according to claim 3, wherein: The current source control section obtains the detection signal output from the light detection section as a detection result of the light detection section.

5. The optical fiber characteristic measuring device according to claim 3, wherein: The measuring unit includes a synchronous detection device that cuts out a detection signal obtained by detecting light set near a measurement point of the optical fiber from the detection signal output by the light detection unit, and performs synchronous detection on the detection signal cut out using the synchronous signal. The current source control section obtains the detection signal cut out by the synchronous detection device as the detection result of the light detection section.

6. The optical fiber characteristic measuring device according to claim 3, wherein: The light detection unit includes a current detector that detects a current flowing through the photoelectric conversion element. The current source control unit obtains a detection result of the current detector as a detection result of the light detection unit.

7. The optical fiber characteristic measuring device according to claim 1, wherein: The current source control section controls the current source based on the detection result of the light detection section in the non-inclusion period so as to remove the continuous light component included in the light detected by the light detection section.

8. A method for measuring optical fiber characteristics, comprising the following steps: The laser light modulated at a predetermined modulation frequency is emitted by the light source; The laser beam is injected into one end and the other end of the optical fiber as continuous light and pulse light respectively through the injection portion; detecting the light emitted from the optical fiber by a light detection unit, the light detection unit having a photoelectric conversion element, a current source connected to the photoelectric conversion element, and an amplifier circuit connected to a connection point between the photoelectric conversion element and the current source; measuring, by a measuring unit, a characteristic of the optical fiber using a detection signal output from the light detecting unit; The current source is controlled by the current source control unit based on the detection result of the light detection unit during a period in which light including the continuous light but not including the Brillouin scattered light is emitted from the optical fiber, that is, a non-containing period.

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