JP2009047455A - Device for measuring optical fiber characteristics - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an device for measuring optical fiber characteristics, capable of acquiring high space resolution, even when measuring light is allowed to enter only from one end of an optical fiber, and of measuring the optical fiber characteristics, in a short time. <P>SOLUTION: This device for measuring optical fiber characteristics 1 is equipped with an optical pulse generating circuit 13, an optical phase modulator 14, a balance light-receiving circuit 19, and a signal processing part 25 or the like. The optical pulse generating circuit 13 generates a pulse train L3 comprising a plurality of pulsed lights L31, L32 having a pulse interval adjusted to be shorter than the lifetime of the acoustic wave in an optical fiber 18 to be measured. The optical phase modulator 14 generates measuring pulsed lights L41, L42 by performing code modulation of a Barker code, or the like, to each pulsed light L31, L32 forming the pulse train L3. The balance light-receiving circuit 19 receives a returning light L5 from the optical fiber 18 to be measured and acquires a light-receiving signal S1. The signal processing part 25 acquires the characteristics of the optical fiber 18 to be measured, by performing a prescribed correlation processing to the light-receiving signal S1. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an optical fiber characteristic measuring apparatus that measures characteristics in the length direction of an optical fiber.

  In recent years, active development of optical fiber characteristic measuring devices that measure strain distribution and temperature distribution in an environment in which the optical fiber is installed by measuring Brillouin scattered light obtained by making pulsed light incident on the optical fiber has been actively conducted. ing. Since this optical fiber characteristic measuring device uses the installed optical fiber itself as a medium for detecting strain or temperature, it has a simple structure compared to a measuring device in which a large number of point sensors are arranged and measured. Temperature distribution can be measured. Typical examples of the optical fiber characteristic measuring apparatus include BOTDR (Brillouin Optical Time Domain Reflectometry) and BOTDA (Brillouin Optical Time Domain Analysis).

  In the above BOTDR, pulsed light is incident from one end of an optical fiber to generate natural Brillouin scattered light (backward Brillouin scattered light) in the optical fiber, and time variation of backward Brillouin scattered light emitted from the same end of the optical fiber. This is an optical fiber characteristic measuring apparatus that obtains the Brillouin spectrum distribution along the optical fiber by measuring the above, and thereby measures the strain distribution and temperature distribution in the length direction of the optical fiber. For details of BOTDR, see, for example, Patent Documents 1 and 2 below.

  Further, the BOTDA described above causes a pulsed light (pump light) having a variable frequency to enter from one end of the optical fiber and a probe light which is a continuous light from the other end of the optical fiber to cause a probe light due to stimulated Brillouin scattering phenomenon In this optical fiber characteristic measuring apparatus, the Brillouin (gain) spectrum distribution along the optical fiber is obtained by sequentially measuring the change components of the optical fiber, thereby measuring the strain distribution and temperature distribution in the length direction of the optical fiber. For details of BOTDA, refer to Patent Document 3 below.

  By the way, in the above BOTDR and BOTDA, it is known that the spatial resolution is improved by reducing the pulse width of the pulsed light incident on the optical fiber. However, if the pulse width is set to a predetermined value or less, the spectral width of the pulsed light itself becomes wider than the spectral width of the Brillouin scattered light, and the center frequency of the Brillouin scattered light cannot be measured with high accuracy. For this reason, the spatial resolution of the conventional BOTDR and BOTDA is about 2 to 3 m. Patent Document 4 below discloses a distributed optical fiber sensor system capable of increasing spatial resolution by taking account of acoustic phonon transients.

  The distributed optical fiber sensor system disclosed in Patent Document 4 pays attention to the fact that the acoustic phonon causing Brillouin scattering is mechanical vibration, and therefore there is a transient phenomenon in which vibration cannot be started instantaneously. Specifically, the pump light is a first pump light and a second pump light having different frequencies, and after propagating the first pump light to the optical fiber, the second pump light that generates Brillouin scattered light for measurement is used. By propagating, a transient phenomenon is prevented from occurring in the Brillouin scattered light for measurement, and a high spatial resolution of about 10 cm can be realized.

In Patent Document 5 below, an apparatus capable of realizing a high spatial resolution of about cm is proposed based on a principle completely different from time domain measurement. This device uses a frequency converter to change the center frequency of the probe light so that the frequency difference between the center frequencies of the pump light and the probe light is close to the Brillouin frequency shift, and then modulates the oscillation frequency of the light source In this way, attention is paid to the fact that the power is selectively transferred from the pump light to the probe light at the position where the phases of both lights are synchronized. And the Brillouin spectrum in the position where the phase of both lights synchronizes is measured by detecting the power of the probe light inject | emitted from the optical fiber with a photodetector. According to such an apparatus, a high spatial resolution of about 1 cm can be realized. This apparatus is called BOCDA (Brillouin Optical Correlation Domain Analysis).
Japanese Patent No. 2575794 Japanese Patent No. 3481494 Japanese Patent No. 2589345 International Publication No. 04/040241 Pamphlet Japanese Patent No. 3667132

  By the way, although the distributed optical fiber sensor system disclosed in Patent Document 4 and the BOCDA disclosed in Patent Document 5 can achieve a high resolution of about several centimeters, measurement light (pumps) can be obtained from both ends of the optical fiber. Light and probe light) are required to be incident, which complicates the apparatus configuration and increases the apparatus cost. For this reason, it is desired to realize high spatial resolution with a device that allows measurement light to enter only from one end of an optical fiber such as BOTDR.

  Further, the Brillouin scattered light generated in the optical fiber is extremely weak. For this reason, an optical fiber characteristic measuring device that measures the characteristics of an optical fiber using Brillouin scattered light is obtained each time pulse light is incident on the optical fiber a plurality of times and each pulse light is incident. The S / N ratio (signal to noise ratio) is improved by averaging the measurement results obtained. Here, when a plurality of pulse lights are simultaneously present in the optical fiber, it cannot be determined which pulse light caused the Brillouin scattered light generated in the optical fiber. For this reason, usually, the time interval at which the pulsed light is incident on the optical fiber is divided into the time required for the pulsed light incident from one end of the optical fiber to reach the other end and the backward Brillouin scattering generated near the other end of the optical fiber. There is a problem that the time required for the light to reach one end of the optical fiber needs to be equal to or longer than the sum of the time required for the measurement, and it takes time for the measurement.

  The present invention has been made in view of the above circumstances, and is capable of obtaining high spatial resolution and measuring characteristics of an optical fiber in a short time even when measurement light is incident only from one end of the optical fiber. An object of the present invention is to provide a fiber characteristic measuring apparatus.

In order to solve the above-mentioned problems, the optical fiber characteristic measuring apparatus of the present invention receives measurement pulsed light from one end of an optical fiber (18), and receives backward Brillouin scattered light emitted from one end of the optical fiber. In the optical fiber characteristic measuring device (1-3) for measuring the characteristics of the optical fiber based on the received light signal obtained in this manner, the first pulsed light (L31) whose pulse interval is less than or equal to the life of the acoustic wave in the optical fiber ) And a second pulsed light (L32), a pulsed light generating unit (13) that generates a pulsed line (L3) from the coherent light (L0), and each pulsed light that forms the pulsed string generated by the pulsed light generating part A code modulation section (14) for generating measurement pulse light (L41, L42) to be incident on the optical fiber by performing a predetermined code modulation on the optical fiber, and emitting from one end of the optical fiber. A light receiving part (19) for receiving a combined light (L6) of the backward Brillouin scattered light (L5) and the coherent light to obtain a light receiving signal (S1), a light receiving signal from the light receiving part, and the pulsed light Correlation processing between the pulse interval of the pulsed light forming the pulse train generated by the generation unit and the code sequence (L4) according to the type of code modulation performed by the code modulation unit is performed, and based on the processing result of the correlation processing And a signal processing unit for obtaining characteristics of the optical fiber.
According to the present invention, when a pulse train composed of the first pulse light and the second pulse light whose pulse interval is less than or equal to the lifetime of the acoustic wave in the optical fiber is generated from the coherent light, each of the code modulators forms the pulse train. Predetermined code modulation is performed on the pulsed light, and then the code-modulated pulsed light is incident on the optical fiber. When this pulsed light enters from one end of the optical fiber, backward Brillouin scattered light is generated in the optical fiber and is emitted from one end of the optical fiber. The backward Brillouin scattered light and the coherent light are combined and subjected to photoelectric conversion, whereby a light receiving signal is output from the light receiving unit. When the light reception signal is obtained, in the signal processing unit, the light reception signal, a code sequence corresponding to the pulse interval of the pulse light generated by the pulse light generation unit and the type of code modulation performed by the code modulation unit, The correlation processing is performed, and the characteristics of the optical fiber are obtained based on the processing result of the correlation processing.
In the optical fiber characteristic measuring apparatus according to the present invention, the code modulation unit code-modulates each pulsed light forming the pulse train generated by the pulsed light generation unit using a Barker code.
Further, in the optical fiber characteristic measuring apparatus of the present invention, the code modulation unit divides each pulsed light forming the pulse train generated by the pulsed light generation unit into a plurality of parts, and codes each of the divided parts. The predetermined code modulation is performed by performing the corresponding phase modulation.
Further, in the optical fiber characteristic measuring apparatus according to the present invention, the signal processing unit sequentially delays the light reception signals and can extract the light reception signals having different delay times, and the tap delay. Correlation processing between the signal extracted from the line, the pulse interval of the pulsed light forming the pulse train generated by the pulsed light generation unit, and the code sequence (L4) according to the type of code modulation performed by the code modulation unit And a correlation processing unit (33) for performing the processing.
The optical fiber characteristic measuring apparatus according to the present invention further includes a correction processing unit (37) that performs a correction process for reducing the influence of the code side lobe on the processing result of the correlation processing performed by the correlation processing unit. It is characterized by that.
The optical fiber characteristic measuring apparatus according to the present invention includes a frequency converter (21, 22) that converts the frequency band of the received light signal into a predetermined frequency band before the signal processor performs correlation processing on the received light signal. ).
In addition, the optical fiber characteristic measuring apparatus of the present invention includes a polarization plane changing unit (41) capable of changing at least one of the polarization plane of the coherent light and the polarization plane of the rear Brillouin scattered light.
In addition, the optical fiber characteristic measuring apparatus of the present invention includes an unnecessary component removing unit (42) that removes an unnecessary component included in the pulse train incident on the optical fiber.
Furthermore, the optical fiber characteristic measuring apparatus according to the present invention is characterized in that the lifetime of the acoustic wave is a time from when the energy of the acoustic wave reaches the peak power to 5% or less of the peak power. Yes.

According to the present invention, a pulse train composed of two pulse lights whose pulse interval is made equal to or shorter than the lifetime of the acoustic wave in the optical fiber and predetermined code modulation is generated and made incident on the optical fiber. Then, a light reception signal is obtained from the return light obtained by making this pulse train incident on the optical fiber, and depending on the light reception signal, the pulse interval of the pulse light forming the pulse train and the type of code modulation performed in the code modulation section Correlation processing with the code string is performed.
Since the Brillouin spectrum of the back Brillouin scattered light obtained by performing such correlation processing shows a periodic change on the frequency axis due to interference of back scattered light from the same point and the same acoustic wave in the optical fiber, This is sharper than the Brillouin spectrum obtained when the pulsed light forming the pulse train is incident on the optical fiber alone. Therefore, by obtaining the characteristics of the optical fiber based on the signal obtained by the correlation processing, it is very easy to detect the Brillouin frequency shift, and the spatial resolution can be effectively improved.
Further, in the present invention, predetermined code modulation is performed on the pulsed light forming the pulse train, and the correlation processing is performed to demodulate the signal to indicate the intensity of the back Brillouin scattered light. Here, for example, when code modulation using an N-bit Barker code is performed, the S / N ratio of the signal indicating the intensity of the backward Brillouin scattered light obtained by demodulation is N times the power of the pulsed light. The S / N ratio is the same as that obtained.
From the above, the present invention has an effect that the spatial resolution can be effectively improved in a short time.

  Hereinafter, an optical fiber characteristic measuring apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings.

[First Embodiment]
FIG. 1 is a block diagram showing a configuration of an optical fiber characteristic measuring apparatus according to a first embodiment of the present invention. As shown in FIG. 1, an optical fiber characteristic measuring apparatus 1 according to this embodiment includes a light source 11, a branch coupler 12, an optical pulse generation circuit 13 (pulse light generation unit), an optical phase modulator 14 (code modulation unit), and an optical amplifier. 15, optical directional coupler 16, optical connector 17, optical fiber 18 to be measured (optical fiber), balance light receiving circuit 19 (light receiving unit), first amplifier 20, signal generating unit 21 (frequency converting unit), mixer 22 ( Frequency converter), a low-pass filter 23, a second amplifier 24, and a signal processor 25.

  The light source 11 is a light source that emits coherent light L0 having a narrow line width, and an MQW-DFB (multiple quantum well-distributed feedback type) semiconductor laser that emits laser light in a 1.55 μm band can be used. In the following description, it is assumed that the frequency of the coherent light L0 emitted from the light source 11 is f0. The branch coupler 12 is a 1 × 2 optical branching coupler having one incident port and two exit ports. The branch coupler 12 splits the coherent light L0 incident on the incident port into two, and outputs the coherent light L1 from the two exit ports. , L2 respectively.

  The optical pulse generation circuit 13 is a high-speed optical switch or the like, and generates pulsed light having a pulse width of about several seconds from the coherent light L1 by turning on / off the switch. This optical pulse generation circuit 13 generates not a single pulse light but a pulse train L3 composed of two pulse lights L31 and L32 having a pulse width of several n seconds. Of the two pulse lights forming the pulse train L3, the time between the pulse light (hereinafter referred to as the first pulse light) L31 generated first and the pulse light (hereinafter referred to as the second pulse light) L32 generated later. The interval is set to a time interval equal to or shorter than the lifetime of the acoustic wave in the optical fiber 18 to be measured, and is preferably 10 nsec or less. As will be described later, the pulsed lights L31 and L32 are divided into N pieces (N is an integer equal to or greater than 2) and phase-modulated (encoded) by the optical phase modulator 14. The pulsed lights L31 and L32 may be N pulse trains each having a pulse width less than a divided time instead of a single pulse.

In addition, the above-mentioned acoustic wave lifetime means the lifetime of an acoustic wave generated inside the optical fiber 18 to be measured in a broad sense, and means the time from when a predetermined acoustic wave is generated until it disappears. However, in this specification, from the viewpoint of more reliably generating an interference signal to be described later, it means the time from when the acoustic wave energy reaches the peak power to 5% or less of the peak power. To do. For example, when the energy of the acoustic wave is attenuated based on the following equation (1), the time until the peak power becomes 5% or less is the time until (t> 3Τa). be able to. However, the variable Τa in the following equation (1) is an acoustic wave decay time.

Further, the spatial resolution δz of the optical fiber characteristic measuring apparatus 1 is a time width (one code length (1 bit)) obtained by dividing the pulse width of each of the first pulsed light L31 and the second pulsed light L32 forming the pulse train L3 by N. Alternatively, when a pulse having a time width smaller than that is used, it is determined by the pulse width W. When the velocity of the pulse train L3 in the optical fiber 18 to be measured is v, the spatial resolution δz of the optical fiber characteristic measuring device 1 is expressed by the following equation (2).

Here, referring to the above equation (2), the N-divided time width (one code length (1 bit)) of each pulse width of the first pulsed light L31 and the second pulsed light L32 forming the pulse train L3 or its It can be seen that when a pulse having a time width less than that is used, the spatial resolution δz of the optical fiber characteristic measuring apparatus 1 is improved as the pulse width W is reduced. However, when the pulse width W becomes narrower than the predetermined value, the spectrum width of the pulse width W becomes wider than the spectrum width of the Brillouin scattered light, and the center frequency of the Brillouin scattered light cannot be measured with high accuracy. For this reason, in the conventional BOTDR using a single pulse, the pulse width W needs to be set to be equal to or larger than the pulse width W0 expressed by the following equation (3). However, the variable Δν _B in the following equation (3) is the spectrum width of the Brillouin scattered light.

  Further, the generation period of the pulse train L3 depends on the length (that is, the distance range) of the optical fiber 18 to be measured. That is, when measuring the characteristics of the optical fiber 18 to be measured, a plurality of pulse trains L3 are not allowed to be present in the optical fiber 18 to be measured at the same time. It needs to be longer than the time required to reciprocate the fiber 18. Strictly speaking, the generation period of the pulse train L3 includes the time required for the pulse train L3 to reach from one end of the optical fiber 18 to be measured to the other end, and the backward Brillouin scattered light generated at the other end of the optical fiber 18 to be measured. This time is equal to or longer than the time required to reach one end of the measurement optical fiber 18. For this reason, for example, when the distance range is 10 km, the generation cycle is about 200 μsec, and when the distance range is 1 km, the generation cycle is about 20 μsec.

  The optical phase modulator 14 performs predetermined N-bit code modulation on each of the first pulsed light L31 and the second pulsed light L32 forming the pulse train L3 generated by the optical pulse generating circuit 13, and measures the optical fiber 18 to be measured. A code string L4 composed of the measurement pulse lights L41 and L42 to be incident on and a no-signal section is generated. As a code modulation method performed by the optical phase modulator 14, for example, a binary code modulation method having codes “+1” and “−1” using a Barker code, a Golay code, or the like can be used. In particular, from the viewpoint of setting the pulse interval between the first pulsed light L31 and the second pulsed light L32 to be less than or equal to the lifetime of the acoustic wave, code modulation using a Barker code having a small code bit length N and a large S / N improvement is performed. It is desirable to do it. In the following description, it is assumed that the optical phase modulator 14 performs code modulation of the first pulsed light L31 and the second pulsed light L32 by a binary modulation method using a Barker code.

  Specifically, the optical phase modulator 14 divides each of the first pulsed light L31 and the second pulsed light L32 into N (bit) parts, and each of the divided parts (1 bit) has a phase corresponding to the code. By performing modulation, code modulation is performed on each of the first pulsed light L31 and the second pulsed light L32. For example, when the code value is “1”, phase modulation is not performed, but when the code value is “−1”, phase modulation is performed to invert the phase of the portion by 180 °.

  FIG. 2 is a diagram for explaining a specific example of code modulation performed by the optical phase modulator 14, and (a) is a diagram showing code modulation using a 3-bit (N = 3) Barker code. And (b) is a diagram showing code modulation using a 5-bit (N = 5) Barker code. As shown in FIG. 2A, when the Barker code is 3 bits, the optical phase modulator 14 divides the first pulsed light L31 forming the pulse train L3 into three parts d11 to d13, and the second The pulsed light L32 is divided into three parts d21 to d23.

  Then, since the 3-bit Barker code is “1”, “1”, “−1”, the optical phase modulator 14 performs the portion d13 of the first pulse light L31 and the portion d23 of the second pulse light L32. Thus, phase modulation is performed to invert the phase by 180 °, and the remaining portions (the portions d11 and d12 of the first pulse light L31 and the portions d21 and d22 of the second pulse light L32) are not subjected to phase modulation. By the above processing, as shown in FIG. 2A, code-modulated measurement pulse lights L41 and L42 are generated. In FIG. 2, the portions of the measurement pulse lights L41 and L42 whose phases are inverted by 180 ° are represented by negative phases, and the portions whose phases are not inverted are represented by positive phases. Further, a non-signal section between the first pulsed light L31 and the second pulsed light L32 is regarded as “0” in terms of code.

  As shown in FIG. 2B, when the Barker code is 5 bits, the optical phase modulator 14 divides the first pulsed light L31 forming the pulse train L3 into five parts d31 to d35, The second pulsed light L32 is divided into five parts d41 to d45. Then, since the 5-bit Barker code is “1”, “1”, “1”, “−1”, “1”, the optical phase modulator 14 has the portion d34 and the second portion of the first pulsed light L31. Phase modulation for inverting the phase by 180 ° is performed on the portion d44 of the pulse light L32, and the remaining portions (the portions d31 to d33 and d35 of the first pulse light L31 and the portions d41 to d43 and d45 of the second pulse light L32). ) Phase modulation is not performed. By the above processing, as shown in FIG. 2B, code-modulated measurement pulse lights L41 and L42 are generated.

  When the optical phase modulator 14 divides the first pulsed light L31 and the second pulsed light L32 with a time interval of 1 ns, for example, in the example shown in FIG. The pulse widths of the light L31 and the second pulse light L32 are set to 3 n seconds, and the interval between the first pulse light L31 and the second pulse light L32 is set to 6 n seconds. When the optical phase modulator 14 divides the first pulsed light L31 and the second pulsed light L32 with a time interval of 0.5 ns, for example, in the example shown in FIG. The pulse widths of the first pulse light L31 and the second pulse light L32 are set to 2.5 nsec, and the interval between the first pulse light L31 and the second pulse light L32 is set to 5 nsec.

  The optical amplifier 15 amplifies and emits the pulse train L3 emitted from the optical phase modulator 14 to a predetermined level. For example, an optical fiber amplifier using an Er (erbium) doped fiber can be used as the optical amplifier 15. The optical directional coupler 16 emits the code string L4 incident on the incident port P1 from the incident output port P2, and returns light from the measured optical fiber 18 incident on the incident output port P2 via the optical connector 17. L5 is injected from the injection port P3. An optical circulator or the like can be used as the optical directional coupler 16. The optical connector 17 connects the entrance / exit port P <b> 2 of the optical directional coupler 16 and one end (one end) of the optical fiber 18 to be measured.

  Here, the return light L5 from the measured optical fiber 18 includes natural Brillouin scattered light (rear Brillouin scattered light), Rayleigh scattered light, and Fresnel reflected light. The frequency of the natural Brillouin scattered light is shifted by about 9 to 12 GHz with respect to the frequency of the pulse train L3 incident on the measured optical fiber 18 (that is, the frequency f0 of the coherent light L0). If this frequency shift is now fs, the frequency of the natural Brillouin scattered light can be expressed as “f0 ± fs”. On the other hand, since the frequency shift amount of Rayleigh scattered light and Fresnel reflected light is 0, the frequency is “f0”. From the above, the frequency fb of the return light L5 from the measured optical fiber 18 includes “f0 ± fs” and “f0”.

  In the optical fiber characteristic measuring apparatus 1 of the present embodiment, the code string L4 incident on the measured optical fiber 18 includes the first pulsed light L31 and the first pulse light L31 whose time interval is equal to or shorter than the life of the acoustic wave. Measurement pulse lights L41 and L42 obtained by N-bit code modulation of the two-pulse light L32 are included. For this reason, in the return light L5 from the measured optical fiber 18, the return light related to the measurement pulse light L41 and the return light related to the measurement pulse light L42 overlap each other. Here, in the present embodiment, as described with reference to FIG. 2, the measurement pulse lights L41 and L42 are divided into a plurality of portions and code-modulated. For this reason, it can be considered that each of the return lights related to the measurement pulse lights L41 and L42 is overlapped with the return lights obtained when the divided portions are regarded as individual pulse lights.

  The balance light receiving circuit 19 includes a coupling / branching coupler 19a and a photoelectric conversion circuit 19b. The balance light receiving circuit 19 receives the combined light L6 obtained by combining the coherent light L2 and the return light L5 (photoelectric conversion), and receives a light reception signal ( Electrical signal) is converted to S1. The coupling / branching coupler 19a has the coherent light L2 having the frequency f0 emitted from the branching coupler 12 and the frequencies fb (= “f0 ± fs”, “f0”) emitted through the optical directional coupler 16. By combining the return light L5, the combined light L6 is obtained. The frequency component of the multiplexed light L6 is a DC component and three frequency components “± fs”. The photoelectric conversion circuit 19b converts the combined light L6 into a light reception signal S1.

  Here, as described above, the return light L5 from the measured optical fiber 18 includes the return light related to each of the measurement pulse lights L41 and L42. For this reason, also with respect to the multiplexed light L6 generated by the multiplexing at the multiplexing / branching coupler 19a and the received light signal S1 converted from the multiplexed light L6, the component (hereinafter referred to as the first component) related to the measurement pulse light L41 and the measurement A component related to the pulsed light L42 (hereinafter referred to as a second component) is included. Since the measurement pulse lights L41 and L42 are respectively generated from the first pulse light L31 and the second pulse light L32 having a time interval equal to or shorter than the lifetime of the acoustic wave, the light reception signals output from the balance light reception circuit 19 In S1, the first component and the second component are included with a delay difference corresponding to the same time interval as the time interval between the first pulsed light L31 and the second pulsed light L32.

  The first amplifier 20 amplifies the received light signal S1 to a level suitable for processing by a mixer 22 (described later). The DC component included in the light reception signal S1 is removed by AC coupling of an electric circuit, for example. The signal generation unit 21 operates under the control of the control circuit 21b, and controls the signal generation circuit 21a that outputs a sine wave RF (radio frequency) signal S2 having a predetermined frequency fr, and controls the signal generation circuit 21a. And a control circuit 21b for setting the frequency of the RF signal S2. In the present embodiment, the frequency fr of the RF signal S2 is about 8 to 12 GHz which is near the frequency shift of the return light (frequency shift of the natural Brillouin scattered light) fs in order to detect the Brillouin scattered light. Set to range. By performing signal processing while changing the frequency fr in the signal generation circuit 21a, the spectrum of the received light signal S1 can be measured.

  The mixer 22 mixes the light reception signal S1 amplified by the first amplifier 20 and the RF signal S2 output from the signal generator 21, and reduces the frequency of the light reception signal S1 by the frequency fr of the RF signal S2. The signal S3 (baseband signal) is output. Here, since the frequency fr of the RF signal S2 is set in the vicinity of the frequency shift fs of the return light, the frequency component obtained by reducing the value of the frequency shift fs by the frequency fr out of the two frequency components described above is a DC component. (Baseband) will be approached. Therefore, this frequency component becomes a frequency region that can be easily processed by an electric circuit (low-pass filter 23, second amplifier 24, and signal processing unit 25) located at the subsequent stage of the mixer 22.

  Here, in the optical fiber characteristic measuring apparatus 1 of the present embodiment, the first component and the second component from the balance light receiving circuit 19 are the same as the time interval between the first pulsed light L31 and the second pulsed light L32. An electrical signal S1 included with a delay difference corresponding to the time interval is output. For this reason, also in the electrical signal S3 output from the mixer 22, the first component and the second component have a delay difference corresponding to the same time interval as the time interval between the first pulse light L31 and the second pulse light L32. Is included. Further, by correlating the electric signal S3 and the code string L4, an interference signal can be obtained, and at the same time, N-bit codes are superimposed to obtain N times signal power.

  Here, the signal generator 21 and the mixer circuit 22 are used in order to obtain the electric signal S3. However, the coherent light L2 used for multiplexing is optically frequency-converted to generate coherent light having an optical frequency that substantially matches the Brillouin scattered light, and the coherent light and the return from the optical fiber 18 to be measured. An electrical signal similar to the electrical signal S3 may be obtained by multiplexing the light L5. Alternatively, the same effect can be obtained by using an optical frequency converter that shifts the code string L4 by a frequency that substantially matches the frequency shift amount of the backscattered light.

  The low-pass filter 23 is a circuit for improving the S / N ratio by removing high-frequency components such as noise included in the electric signal S3 output from the mixer 22. The second amplifier 24 amplifies the electric signal S3 output from the low-pass filter 23 to a level suitable for the signal processing unit 25.

  The signal processing unit 25 performs A / D (analog / digital) conversion processing on the electrical signal output from the second amplifier 24 to convert it into a digital signal, and then performs various processing such as correlation processing to be measured. The characteristic in the length direction of the optical fiber 18 is obtained. FIG. 3 is a block diagram illustrating an example of the internal configuration of the signal processing unit 25. As shown in FIG. 3, the signal processing unit 25 includes an A / D conversion unit 31, a tapped delay line 32, a correlation processing unit 33, a square detection processing unit 35, an averaging processing unit 36, and a correction processing unit 37.

The A / D converter 31 performs A / D conversion processing on the electrical signal output from the second amplifier 24 and outputs the digital signal. The tapped delay line 32 can sequentially delay the digital signals output from the A / D converter 31 and can extract digital signals having different delay times. For example, the tapped delay line 32 can sequentially delay the digital signal for 9 bits sequentially output from the A / D conversion unit 31 by 1 n seconds. Individual digital signals can be extracted. That is, if the digital signal obtained by A / D converting the electrical signal output from the second amplifier 24 at this time is D _k , the digital signal D _{k−8 that} is 8 bits before is sent from the tapped delay line 32 to the current digital signal. A signal for 9 bits of _Dk can be extracted.

  The correlation processing unit 33 performs correlation processing between the signal extracted from the tapped delay line 32 and the code string L4. Thereby, an interference signal between the first component and the second component is generated. The square detection processing unit 35 demodulates the signal output from the correlation processing unit 33 into a signal indicating the intensity of normal backscattered light by squaring the result of the correlation processing performed by the correlation processing unit 33. The averaging processing unit 36 performs an averaging process on the signal output from the square detection processing unit 35, and the correction processing unit 37 performs a correction for reducing code side lobes generated by performing code modulation using a Barker code. Process.

但し、上記（４）式及び（５）式において、変数Ｚは被測定光ファイバ１８の長さ方向における位置を示しており、Ｐ_Ｂは補正前の信号パワー、Ｐ_Ａは補正後の信号パワーをそれぞれ示している。また、変数ΔＺは、測定用パルス光Ｌ４１，Ｌ４２のパルス幅に相当する距離（１符号（ビット）長に相当する距離）を示している。尚、パルス幅が１ｎ秒である場合には変数ΔＺは１０ｃｍ程度である。 Specifically, when the 3-bit Barker code is used in the optical phase modulator 14, the correction processing unit 37 performs the correction process shown in the following equation (4), and the 5-bit Barker code is used. The correction processing shown in the following equation (5) is performed.

However, equation (4) and (5) In the equation, the variable Z indicates a position in the length direction of the measured optical fiber 18, P _B is the uncorrected signal power, P _A is the corrected signal power Respectively. A variable ΔZ indicates a distance corresponding to the pulse width of the measurement pulse lights L41 and L42 (a distance corresponding to one code (bit) length). When the pulse width is 1 ns, the variable ΔZ is about 10 cm.

  The signal processing unit 25 measures the characteristics (distortion and loss) of the measured optical fiber 18 using the signal output from the correction processing unit 37. Here, the Brillouin spectrum is obtained by repeating the above processing while varying the frequency fr of the RF signal S2 in the vicinity of the frequency shift fs in order to detect the Brillouin scattered light. Further, the return light L5 is detected on the time axis, and the distribution of strain characteristics and the distribution of optical loss characteristics in the length direction of the optical fiber 18 to be measured are obtained. Note that the function of each block of the signal processing unit 25 shown in FIG. 3 can also be realized by software.

  The Brillouin scattered light is generated when the Brillouin scattered light generated by the reflection of the pulsed light on the same acoustic wave in the optical fiber 18 to be measured interferes with each other and the pulsed light is reflected by a different acoustic wave. In addition, the Brillouin scattered light does not interfere with each other. Although the acoustic wave has a velocity, it is extremely slow with respect to the velocity of the pulsed light. Therefore, the Brillouin scattered light generated by reflecting the pulsed light to the same acoustic wave is not reflected in the optical fiber 18 to be measured. Can be considered to have been generated at the same location. That is, if the Brillouin scattered lights are generated at the same location in the optical fiber 18 to be measured, they interfere with each other, but do not interfere if they are generated at different locations in the optical fiber 18 to be measured. .

  Here, in the optical fiber characteristic measuring apparatus 1 of the present embodiment, the time interval between the measurement pulsed light L41 and the measurement pulsed light L42 is set to be equal to or shorter than the lifetime of the acoustic wave in the measured optical fiber 18. For this reason, when the backward Brillouin scattered light is generated by reflecting the measurement pulse light L41 with a predetermined acoustic wave in the optical fiber 18 to be measured, the measurement pulse light L42 is also reflected by the same acoustic wave, Back Brillouin scattered light capable of interfering with the return light L5 is included. Therefore, the signal processing unit 25 can generate an interference signal by summing the two together in time.

  In the present embodiment, the measurement pulse light L41 and the measurement pulse light L42 are divided into a plurality of pieces to perform code modulation, and the return lights related to the measurement pulse lights L41 and L42 are respectively divided. It can be considered that the return lights obtained when these parts are viewed as individual pulse lights overlap. Therefore, the correlation process between the signal delayed by the tapped delay line 32 and the code string L4 is considered to be equivalent to the process of adding the time positions of the return lights with respect to the divided individual pulse lights together. be able to. That is, by performing code modulation using an N-bit Barker code, it can be considered that N pieces of pulsed light are incident on the optical fiber 18 to be measured, and thereby the power of the measurement pulse lights L41 and L42 is set to N. An S / N ratio similar to that obtained is obtained.

  Next, the operation of the optical fiber characteristic measuring apparatus 1 according to the first embodiment of the present invention will be described. In the following description, it is assumed that the measurement pulse lights L41 and L42 reach the same acoustic wave in the optical fiber 18 to be measured. In the following description, it is assumed that the optical phase modulator 14 performs code modulation using a 3-bit Barker code. For this reason, the code string L4 has the 9 bits of “1”, “1”, “−1”, “0”, “0”, “0”, “1”, “1”, “−1” described above. It is assumed that However, the number of “0” is not uniquely determined, and is set in a range in which the time interval between the measurement pulse light L41 and the measurement pulse light L42 is equal to or less than the life of the acoustic wave in the optical fiber 18 to be measured. The

  When the coherent light L0 having the frequency f0 is emitted from the light source 11, the coherent light L0 enters the branch coupler 12 and is emitted to the optical pulse generation circuit 13, and the coherent light emitted to the balance light receiving circuit 19. Branches to the light L2. When the coherent light L1 emitted from the branching coupler 12 enters the optical pulse generation circuit 13, the first pulsed light L31 and the second pulsed light L32 that have a time interval equal to or shorter than the lifetime of the acoustic wave in the measured optical fiber 18 Is generated.

  The pulse train L3 generated by the optical pulse generation circuit 13 enters the optical phase modulator 14, and the first pulse light L31 forming the pulse train L3 is divided into three portions d11 to d13 shown in FIG. In addition to code modulation using the Barker code, the second pulsed light L32 forming the pulse train L3 is divided into three portions d21 to d23 shown in FIG. 2A and code-modulated using a 3-bit Barker code. . As a result, a code string L4 composed of the measurement pulse lights L41 and L42 incident on the optical fiber 18 to be measured is generated.

  The code string L4 generated by the optical phase modulator 14 is amplified by the optical amplifier 15 and then enters the incident port P1 of the optical directional coupler 16, and is emitted from the incident output port P2 of the optical directional coupler 16. Thereafter, the light enters from one end of the optical fiber 18 to be measured via the optical connector 17. When the code string L4 is incident from one end of the optical fiber 18 to be measured, the measurement pulse lights L41 and L42 reach the same acoustic wave in the optical fiber 18 to be measured, so that the first component Brillouin capable of interference is obtained. Scattered light and second component Brillouin scattered light are generated.

  Therefore, in the return light L5 from the optical fiber 18 to be measured, the rear Brillouin scattered light component of the measurement pulse light L41 and the rear Brillouin scattered light component of the measurement pulse light L42 are the first pulse light L31 and the second pulse light L31. The light is overlapped and emitted while having a time interval delay with the pulsed light L32. Here, as described above, since the return light L5 receives the frequency shift fs peculiar to the Brillouin scattering phenomenon, the frequency fb of the return light L5 includes the frequency “f0 ± fs”. Further, since the return light L5 also includes Rayleigh scattered light and Fresnel reflected light, the frequency fb also includes the frequency “f0”.

  Such return light L <b> 5 enters through the optical connector 17 from the entrance / exit port P <b> 2 of the optical directional coupler 16, exits from the exit port P <b> 3, and enters the balance light receiving circuit 19. The return light L5 incident on the balance light receiving circuit 19 is combined with the coherent light L2 by the combining / branching coupler 19a. As a result, a combined light L6 is generated, and the combined light L6 is converted into a light reception signal S1 by the photoelectric conversion circuit 19b. As described above, the light reception signal S1 includes a DC component and two frequency components “fs”.

  The light reception signal S1 is amplified to a predetermined level by the first amplifier 20 and then input to the mixer 22. When the light reception signal S1 is amplified by the first amplifier 20, the direct current component included in the light reception signal S1 is removed. On the other hand, in the signal generation unit 21, the control circuit 21b controls the signal generation circuit 21a to generate an RF signal S2 having a frequency fr set in the vicinity of the frequency shift fs, and this RF signal S2 is supplied to the mixer 22. Entered. When the mixer 22 mixes the light reception signal S1 from the first amplifier 20 and the RF signal S2 from the signal generation circuit 21a, an electric signal S3 in which the frequency of the light reception signal S1 is reduced by the frequency fr is obtained. That is, only the signal component having the frequency “fs” is detected from the two frequency components included in the light reception signal S1, and only the electric signal corresponding to the backward Brillouin scattered light can be processed. The electric signal S3 is removed of a high frequency component by the low-pass filter 23, amplified by the second amplifier 24, and then input to the signal processing unit 25.

The electric signal S3 input to the signal processing unit 25 is converted into a digital signal by the A / D conversion unit 31. The converted digital signal is input to the tapped delay line 32, and is sequentially delayed in units of the division time (for example, 1 nsec) of the measurement pulse lights L41 and L42 in the optical phase modulator 14. As a result, 9-bit digital signals D _{k−8 to} D _k sequentially delayed by 1 n seconds are output from the tapped delay line 32 to the correlation processing unit 33. The correlation processing unit 33 performs correlation processing between the digital signal output from the tapped delay line 32 and the code string L4. Specifically, 9-bit digital signals D _{k-8 to} D _k and “1”, “1”, “−1”, “0”, “0”, “0”, “1”, “1” As a result of performing correlation processing with a 9-bit code string of “−1”, an interference signal between the first component and the second component is generated.

  The processing result of the correlation processing unit 33 is output to the square detection processing unit 35, and demodulated into a signal indicating the intensity of normal backscattered light by being squared by the square detection processing unit 35. The signal indicating the intensity of the backscattered light is subjected to averaging processing in the averaging processing unit 36, and thereafter, correction processing shown in the above-described equation (4) is performed in the correction processing unit 37. Thereby, the influence by the code | symbol side lobe produced by performing code modulation using a Barker code | cord | chord is reduced. When the above processing is completed, the signal processing unit 25 measures the characteristics (distortion and loss) of the optical fiber 18 to be measured using the signal output from the correction processing unit 37. The above processing is repeatedly performed in the signal processing unit 25 while changing the frequency fr of the RF signal S2, and the distribution of distortion characteristics and the distribution of optical loss characteristics in the length direction of the measurement optical fiber 18 are obtained.

  Here, the electric signal S3 input to the signal processing unit 25 includes the first component and the second component. Further, the first component can be considered to be obtained by overlapping the return light obtained when each portion of the divided measurement pulse light L41 is viewed as individual pulse light. Similarly, the second component can be considered to be obtained by overlapping the return lights obtained when the respective portions of the divided measurement pulse light L42 are viewed as individual pulse lights.

  In the processing in the tapped delay line 32 and the correlation processing unit 33, the sum of the electrical signal S3 and the electrical signal obtained by delaying the electrical signal S3 by the time interval (6 n seconds) of the measurement pulse lights L41 and L42 is obtained. Processing to obtain is performed, and an interference signal between the first component and the second component is generated. The Brillouin spectrum of the interference signal is narrowed because the Brillouin spectrum based on the first component and the Brillouin spectrum based on the second component have a phase difference corresponding to the time interval between the measurement pulse lights L41 and L42. And become steep. Since the signal processing unit 25 measures the characteristics of the optical fiber 18 to be measured by using the interference signal having the narrowed and sharp Brillouin spectrum, the Brillouin frequency shift can be detected with high accuracy. , The spatial resolution is improved.

  Further, in the processing in the above-described tapped delay line 32 and the correlation processing unit 33, the time of the return light obtained when each of the divided portions is regarded as individual pulse light with respect to each of the measurement pulse lights L41 and L42. A process similar to the process of adding after aligning the positions is performed. For this reason, it can be considered that three pulse lights are made incident on the optical fiber 18 to be measured by performing code modulation using a 3-bit Barker code, whereby the power of the measurement pulse lights L41 and L42 is 3 An S / N ratio similar to that obtained is obtained. As a result, in the optical fiber characteristic measuring apparatus 1 of the present embodiment, the number of times that the code string L4 is incident on the measured optical fiber 18 can be reduced, so that the measurement time can be shortened.

  FIG. 4 is a diagram illustrating a simulation result of a signal output from the square detection processing unit 35 when measurement pulse light that has been code-modulated by a 3-bit Barker code is used. FIG. 4A is a diagram showing the power distribution of backscattered light generated at one point in the optical fiber 18 to be measured, and FIG. 4B shows the maximum power of backscattered light in FIG. It is a figure which shows the relationship between the power and the time in a certain frequency. Note that the measurement pulse lights L41 and L42 used here have a pulse width of 3 nsec and a pulse interval of 6 nsec. Also,

  4A and 4B, it can be seen that the power of the Brillouin scattered light takes a large peak value (maximum value) when the time is 0 ns and ± 6 ns. The peak at −6 nsec is the Brillouin spectrum contained in the first component, and the peak at +6 nsec is the Brillouin spectrum contained in the second component. The peak at 0 ns is the Brillouin spectrum included in the interference signal obtained by summing the time axes of the first component and the second component. Further, other peaks at ± 2n seconds and other peaks are due to side lobes generated along with the Barker code. In addition, referring to FIG. 4A, it can be seen that the peak at 0 ns periodically changes on the frequency axis. This is due to the interference between the first component and the second component. Paying attention to such periodic fluctuations in the Brillouin spectrum, and performing filter processing according to the periodic fluctuations, it becomes possible to measure the strain distribution with higher accuracy.

  FIG. 5 is a diagram showing a simulation result obtained when measuring pulsed light in which distortion is intentionally applied to the optical fiber 18 to be measured and code-modulated by a 3-bit Barker code is used. 5A is a diagram showing the power distribution of the return light L5, and FIG. 5B is a diagram obtained by converting the power distribution shown in FIG. 5A into a distortion distribution. Here, it is assumed that the length of the optical fiber 18 to be measured is 220 cm, and a distortion corresponding to 50 MHz in terms of Brillouin frequency shift is applied to a 20 cm portion at the center of the optical fiber 18 to be measured. The simulation result shown in FIG. 5 is obtained when the measurement pulse lights L41 and L42, which have a pulse interval of 6 ns and are code-modulated by a 3-bit Barker code, are incident on the optical fiber 18 to be measured. Is.

  Referring to FIG. 5 (b), it can be seen that the frequency is greatly changed to about 20 to 45 MHz when the distance is between 100 and 120 cm. This is due to the frequency shift of Brillouin scattered light caused by strain applied to the optical fiber 18 to be measured. However, the set distortion value (50 MHz in terms of frequency shift) has not been reached (section indicated by reference sign Q3), and in addition, a section having a distance of about 80 cm to 100 cm (section indicated by reference sign Q1) In addition, it can be seen that the frequency slightly changes in the section where the distance is around 120 cm to 140 cm (section indicated by reference sign Q2). This is considered to be an influence of a code side lobe generated by performing code modulation using a Barker code. In the present embodiment, the correction processing unit 37 of the signal processing unit 15 performs the correction processing shown in the above-described equation (4), thereby reducing the influence of such side lobes.

  FIG. 6 is a diagram illustrating a result of performing the correction process performed by the correction processing unit 37 of the signal processing unit 15 on the simulation result illustrated in FIG. It seems that there is no significant difference between FIG. 6 (a) and FIG. 5 (b), but when FIG. 6 (b) is compared with FIG. 5 (b), the sections indicated by reference numerals Q1 and Q2. It can be seen that the change in frequency at is reduced. Further, it can be seen that the frequency approaches the true value in the section indicated by the symbol Q3. Thereby, the influence of the code side lobe generated by performing the code modulation using the Barker code is reduced, and a more accurate distortion distribution measurement can be performed.

  FIG. 7 is a diagram illustrating a simulation result of a signal output from the square detection processing unit 35 in the case where measurement pulse light that has been code-modulated by a 5-bit Barker code is used. Here, the pulse widths of the measurement pulse lights L41 and L42 are 2.5 ns, and the optical phase modulator 14 divides the first pulse light L31 and the second pulse light L32 with a time interval of 0.5 ns. Thus, the measurement pulse lights L41 and L42 are generated. FIG. 7A is a diagram corresponding to FIG. 4A, and FIG. 7B is a diagram corresponding to FIG. 4B.

  7A and 7B, it can be seen that the power of the Brillouin scattered light takes a large peak value when the time is 0 ns and ± 5 ns. The peak at -5 ns is the Brillouin spectrum contained in the first component, the peak at +5 ns is the Brillouin spectrum contained in the second component, and the peak at 0 ns is the first and second components. It is a Brillouin spectrum included in an interference signal obtained by summing the time axes with two components. In addition to these peaks, fine peaks appear, but these are due to side lobes that occur with the Barker code. Referring to FIG. 7 (a), it can be seen that the peak at 0 ns periodically changes on the frequency axis, as in FIG. 4 (a). Therefore, in the same manner as when the measurement pulse lights L41 and L42 that have been code-modulated by the 5-bit Barker code are made incident on the optical fiber 18 to be measured, if the filtering process according to this periodic variation is performed, Highly accurate strain distribution measurement is possible.

  FIG. 8 is a diagram showing a simulation result obtained when measuring pulsed light in which distortion is intentionally applied to the optical fiber 18 to be measured and code modulation is performed using a 5-bit Barker code. FIG. 9 is a diagram illustrating a result of performing the correction process performed by the correction processing unit 37 of the signal processing unit 15 on the simulation result illustrated in FIG. 8. FIGS. 8A and 8B are the same as FIGS. 5A and 5B, respectively, and FIGS. 9A and 9B are FIGS. 6A and 6B, respectively. It is the same figure. 8 and 9, as in the case of FIGS. 5 and 6, the length of the optical fiber 18 to be measured is 220 cm, and the Brillouin frequency is 20 cm in the central portion of the optical fiber 18 to be measured. It is assumed that distortion equivalent to 50 MHz is added in terms of shift.

  Referring to FIG. 8B, similarly to FIG. 5B, when the distance is between 100 and 120 cm, the frequency is caused by the frequency shift of the Brillouin scattered light caused by the strain applied to the optical fiber 18 to be measured. It can be seen that there is a significant change from about 20 to 45 MHz. However, the set distortion value (50 MHz in terms of frequency shift) has not been reached (the section indicated by the reference sign Q3), the distance indicated by the reference sign Q1 is about 80 to 100 cm, and the distance indicated by the reference sign Q2 It can be seen that in the section of 120 cm to 140 cm, the frequency slightly changes due to the influence of the code side lobe generated by the code modulation using the Barker code.

  The correction processing unit 37 of the signal processing unit 15 performs the correction processing shown in the above-described equation (5), thereby reducing the influence of the code side lobe as shown in FIG. 9B. That is, referring to FIG. 9B, it can be seen that the change in frequency in the section indicated by the symbols Q1 and Q2 is reduced as compared with FIG. 8B. Further, it can be seen that the frequency approaches the true value in the section indicated by the symbol Q3. Thereby, the influence of the code side lobe generated by performing the code modulation using the Barker code is reduced, and a more accurate distortion distribution measurement can be performed. As described above, in both cases where code modulation is performed using a 3-bit Barker code and code modulation is performed using a 5-bit Barker code, the influence of code side lobes is reduced. Highly accurate strain distribution measurement is possible. Further, even when code modulation using an N-bit Barker code is performed, the same highly accurate distortion distribution can be measured.

  As described above, according to the optical fiber characteristic measuring apparatus 1 of the present embodiment, the measurement pulse lights L41 and L42 having a predetermined pulse width and subjected to code modulation using an N-bit Barker code are included. The code string L4 is incident from one end of the optical fiber 18 to be measured, receives the returned Brillouin scattered light and obtains a light reception signal, and then generates an interference signal and simultaneously decodes the encoded signal. ing. This interference signal exhibits a steep Brillouin spectrum. Further, the obtained signal has the same S / N improvement effect as the pulse power multiplied by N. This makes it extremely easy to detect the Brillouin frequency shift, and can effectively improve the spatial resolution in a short time. Further, by applying a noise removal filter to the Brillouin spectrum or approximating by curve fitting, it is possible to expect a more accurate distortion distribution measurement. Further, in the correlation processing using the Barker code string, side lobes are generated in principle to cause signal deterioration, but improvement can be made by performing correction processing.

[Second Embodiment]
Next, an optical fiber characteristic measuring apparatus according to a second embodiment of the present invention will be described. FIG. 10 is a block diagram showing a configuration of an optical fiber characteristic measuring apparatus according to the second embodiment of the present invention. In FIG. 10, the same blocks as those shown in FIG. As shown in FIG. 10, the optical fiber characteristic measuring apparatus 2 according to the present embodiment includes a polarization controller 41 (polarization plane changing unit) between the branch coupler 12 and the balance light receiving circuit 19. This polarization control device 41 changes the polarization state of the coherent light L2 at random by changing the polarization plane of the coherent light L2 at high speed.

  In the first embodiment described above, it is assumed that the coherent light L2 and the return light L5 input to the coupling coupler 19a of the balanced light receiving circuit 19 have a constant polarization state relationship. However, such a condition is satisfied when a special optical fiber such as a polarization maintaining optical fiber or a multimode optical fiber whose polarization plane is randomized is used as the optical fiber 18 to be measured. It is. That is, when a general optical fiber is used as the optical fiber 18 to be measured, the above condition is not satisfied.

  On the other hand, the detection sensitivity in the balance light receiving circuit 19 has a polarization dependence that becomes a maximum value when the polarization direction of the coherent light L2 and the polarization direction of the return light L5 coincide with each other, and becomes zero when they are orthogonal. Have. For this reason, in this embodiment, the polarization control device 41 randomizes the polarization state of the coherent light L2, thereby averaging the detection sensitivity in the balance light receiving circuit 19. As a result, the polarization dependence of the balance light receiving circuit 19 can be canceled.

  The polarization control device 41 also changes the polarization plane of the coherent light L2 by 90 ° every predetermined unit time, and takes the average of the square sum of the measurement results in a plurality of unit times. Dependencies can be negated. Further, in the optical fiber characteristic measuring device 2 of the present embodiment, a configuration in which the polarization control device 41 is installed between the branch coupler 12 and the balance light receiving circuit 19 is adopted. However, the configuration is not limited to this, and a polarization controller is provided between the branch coupler 12 and the optical directional coupler 16 or between the optical directional coupler 16 and the measured optical fiber 18. The same effect can be obtained by changing the polarization state of the pulse train L3 or the return light L5.

[Third Embodiment]
Next, an optical fiber characteristic measuring device according to a third embodiment of the present invention will be described. FIG. 11 is a block diagram showing a configuration of an optical fiber characteristic measuring apparatus according to the third embodiment of the present invention. In FIG. 11, the same blocks as those shown in FIG. As shown in FIG. 11, the optical fiber characteristic measuring apparatus 3 of the present embodiment includes an ASE light removal optical switch 42 (unnecessary component removal unit) between the optical amplifier 15 and the directional coupler 16. The ASE light removal optical switch 42 amplifies the pulse train L3 by the optical amplifier 15 to remove noise components (ASE light: Amplified Spontaneous Emission light) superimposed on the pulse train L3.

  In the first embodiment described above, it is assumed that noise components (unnecessary components) generated in the optical amplifier 15 can be ignored. However. Actually, since there is a possibility of causing S / N deterioration of the pulse train L3 and the return light L5, it is preferable to remove such noise components. Therefore, by providing the ASE light removal optical switch 42 as in the optical fiber characteristic measuring apparatus 3 of the present embodiment, it is possible to suppress the S / N deterioration of the pulse train L3 and the return light L5. Further, from the viewpoint of removing a noise component superimposed on the pulse train L3, a removal unit for removing leakage light when the optical pulse generation circuit 13 is turned off may be provided at the subsequent stage of the optical pulse generation circuit 13.

  As mentioned above, although the optical fiber characteristic measuring apparatus by embodiment of this invention was demonstrated, this invention is not restrict | limited to the said embodiment, It can change freely within the scope of the present invention. For example, in the embodiment described above, a case where binary code modulation is performed using a Barker code, a Golay code, or the like has been described as an example, but the present invention can also be applied to a case where a multilevel code modulation method is used. It is. In the above embodiment, the code string L4 composed of the two measurement pulse lights L41 and L42 is made incident on the optical fiber 18 to be measured. However, in the present invention, the code string L4 composed of three or more measurement pulse lights is used. The present invention can also be applied to the case where the light enters the optical fiber 18 to be measured.

It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus by 1st Embodiment of this invention. 6 is a diagram for explaining a specific example of code modulation performed by the optical phase modulator 14. FIG. 3 is a block diagram illustrating an example of an internal configuration of a signal processing unit 25. FIG. It is a figure which shows the simulation result of the signal output from the square detection process part 35 at the time of using the pulsed light for a measurement by which the code modulation by 3 bit Barker code | cord | chord was made. It is a figure which shows the simulation result obtained when distortion is intentionally applied to the optical fiber to be measured 18 and the measurement pulse light that has been code-modulated by a 3-bit Barker code is used. It is a figure which shows the result of having performed the correction process performed in the correction process part 37 of the signal processing part 15 with respect to the simulation result shown in FIG. It is a figure which shows the simulation result of the signal output from the square detection process part 35 when the pulse light for a measurement by which the code modulation by 5 bit Barker code | symbol was made was used. It is a figure which shows the simulation result obtained when the distortion is intentionally applied to the optical fiber to be measured 18 and the measurement pulse light subjected to the code modulation by the 5-bit Barker code is used. It is a figure which shows the result of having performed the correction process performed in the correction process part 37 of the signal processing part 15 with respect to the simulation result shown in FIG. It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus by 2nd Embodiment of this invention. It is a block diagram which shows the structure of the optical fiber characteristic measuring apparatus by 3rd Embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1-3 Optical fiber characteristic measuring apparatus 13 Optical pulse generation circuit 14 Optical phase modulator 18 Optical fiber to be measured 19 Balance light reception circuit 21 Signal generation circuit 22 Mixer 25 Signal processing part 32 Tap delay line 33 Correlation processing part 37 Correction processing part 41 Polarization Control Device 42 ASE Light Removal Optical Switch L0 Coherent Light L3 Pulse Train L4 Code Train L5 Return Light L6 Combined Light L31, L32 Pulse Light L41, L42 Measurement Pulse Light S1 Light Received Signal

Claims (9)

An optical fiber characteristic measuring apparatus for measuring the characteristics of the optical fiber based on a received light signal obtained by receiving measurement pulsed light from one end of the optical fiber and receiving backward Brillouin scattered light emitted from the one end of the optical fiber In
A pulsed light generating unit that generates a pulse train composed of a first pulsed light and a second pulsed light whose pulse interval is less than or equal to the lifetime of the acoustic wave in the optical fiber, from coherent light;
A code modulation unit that performs predetermined code modulation on each pulsed light that forms a pulse train generated by the pulsed light generation unit to generate measurement pulsed light that is incident on the optical fiber; and
A light receiving unit that receives the combined light of the back Brillouin scattered light and the coherent light emitted from one end of the optical fiber to obtain a light reception signal;
Correlation processing is performed between the light reception signal from the light receiving unit, the pulse interval of the pulsed light forming the pulse sequence generated by the pulsed light generating unit, and the code sequence according to the type of code modulation performed by the code modulating unit. And a signal processing unit for obtaining the characteristics of the optical fiber based on a processing result of the correlation processing.   2. The optical fiber characteristic measuring apparatus according to claim 1, wherein the code modulation unit code-modulates each pulse light forming the pulse train generated by the pulse light generation unit using a Barker code.   The code modulation unit divides the individual pulsed light forming the pulse train generated by the pulsed light generation unit into a plurality of parts, and performs phase modulation corresponding to the code on each of the divided parts, thereby performing the predetermined code 3. The optical fiber characteristic measuring apparatus according to claim 1, wherein modulation is performed. The signal processing unit sequentially delays the light reception signal, and a tapped delay line capable of taking out light reception signals having different delay times;
Correlation processing between a signal taken out from the tapped delay line and a code string according to a pulse interval of a pulse light generated by the pulse light generation unit and a type of code modulation performed by the code modulation unit The optical fiber characteristic measuring device according to any one of claims 1 to 3, further comprising: a correlation processing unit that performs the following.   5. The optical fiber characteristic measuring apparatus according to claim 4, further comprising: a correction processing unit that performs a correction process for reducing the influence of code side lobes on a processing result of the correlation processing performed by the correlation processing unit. .   6. The frequency converter according to claim 1, further comprising: a frequency converter that converts a frequency band of the received light signal into a predetermined frequency band before the signal processor performs correlation processing on the received light signal. An optical fiber characteristic measuring apparatus according to claim 1.   The optical fiber according to any one of claims 1 to 6, further comprising a polarization plane changing unit capable of changing at least one of a polarization plane of the coherent light and a polarization plane of the backward Brillouin scattered light. Characteristic measuring device.   The optical fiber characteristic measuring device according to any one of claims 1 to 7, further comprising an unnecessary component removing unit that removes an unnecessary component included in the pulse train incident on the optical fiber.   The lifetime of the acoustic wave is a time from when the energy of the acoustic wave becomes peak power until it becomes 5% or less of the peak power, according to any one of claims 1 to 8. The optical fiber characteristic measuring device according to item.

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