JP2024033286A - Optical fiber sensor and method of measuring filter property - Google Patents

Abstract

To detect property of an FBG filter.SOLUTION: A light source unit includes a variable wavelength light source and generates probe light having a predetermined frequency. An optical switch unit selects backward Brillouin scattering light caused in an optical fiber being a measurement target by the probe light or property measurement light having a predetermined frequency and sends the selected backward Brillouin scattering light as measurement light to a filter unit. The filter unit transmits a specific frequency component of the measurement light. An interference unit subjects the measurement light with the specific frequency component extracted therefrom, to self-delay interference to generate interference light. A signal processing unit acquires transmission property of the filter unit on the basis of signal intensity of the interference light and the frequency of the property measurement light.SELECTED DRAWING: Figure 1

Description

The present invention relates to an optical fiber sensor using Brillouin scattered light, and a method for measuring filter characteristics that can be performed using this optical fiber sensor.

With the development of optical fiber communications, distributed optical fiber sensing, in which optical fiber itself is used as a sensing medium, has been actively researched. In particular, optical fiber sensing that uses scattered light can measure a long-distance distribution, unlike an electric sensor that measures point by point, so it can measure physical quantities of the entire object to be measured.

Time-domain reflectometry (OTDR) is a distributed optical fiber sensing method that can measure long-distance distributions, in which a light pulse is input from one end of an optical fiber and the backscattered light in the optical fiber is measured over time. Optical Time Domain Reflectometry) is a typical example (see, for example, Non-Patent Document 1). Backscattering in optical fibers includes Rayleigh scattering, Brillouin scattering, and Raman scattering. Among these, the one that measures natural Brillouin scattering is called BOTDR (Brillouin OTDR) (for example, see Non-Patent Document 2).

Brillouin scattering is observed at positions shifted in frequency by approximately GHz toward the Stokes side and the anti-Stokes side with respect to the center frequency of the optical pulse incident on the optical fiber, and its spectrum is called the Brillouin Gain Spectrum (BGS). Called. The frequency shift and spectral linewidth of BGS are called Brillouin Frequency Shift (BFS) and Brillouin linewidth, respectively. BFS and Brillouin linewidth vary depending on the material of the optical fiber and the wavelength of the incident light. For example, in the case of a silica-based single mode optical fiber, it has been reported that the BFS size and Brillouin linewidth at a wavelength of 1.55 μm are approximately 11 GHz and approximately 30 MHz, respectively. Furthermore, from Non-Patent Document 1, the magnitude of BFS due to changes in strain and temperature in a single mode fiber is 0.049 MHz/με and 1.0 MHz/° C., respectively, at a wavelength of 1.55 μm.

Thus, BFS has strain and temperature dependence. For this reason, BOTDR is attracting attention because it can be used for purposes such as deterioration diagnosis of large structures such as bridges and tunnels, temperature monitoring of plants, and monitoring of areas where landslides may occur. .

In BOTDR, in order to measure the spectral waveform of naturally Brillouin scattered light generated in an optical fiber, it is common to perform heterodyne detection with a separately prepared reference light. The intensity of natural Brillouin scattered light is two to three orders of magnitude smaller than that of Rayleigh scattered light. For this reason, heterodyne detection is also useful in improving the minimum light receiving sensitivity.

Here, since the natural Brillouin scattered light is very weak, a sufficient signal-to-noise ratio (S/N) cannot be ensured even if heterodyne detection is applied. As a result, averaging processing is required to improve the S/N. Conventional optical fiber strain measurement devices that perform BOTDR acquire three-dimensional information of time, amplitude, and frequency, but it is difficult to shorten the measurement time due to averaging processing and acquisition of this three-dimensional information.

In contrast, self-delaying heterodyne BOTDR (SDH-BOTDR)
An optical fiber strain measuring device and an optical fiber strain measuring method using a delayed heterodyne BOTDR (Delayed Heterodyne BOTDR) have been proposed (for example, see Patent Document 1). In SDH-BOTDR, a change in BFS is observed as a phase change in the beat signal by comparing the phases of the received beat signal and the local signal. In this way, SDH-BOTDR can directly calculate BFS without requiring frequency sweep, and therefore can realize high-speed and inexpensive measurement.

Japanese Patent Application Publication No. 2016-191659

Yokogawa Technical Report, Vol. 49, No. 2, pp. 56-58 T. Kurashima et al. , “Brillouin Optical-fiber time domain reflectometry”, IEICE Trans. Commun. , vol. E76-B, no. 4, pp. 382-390 (1993)

In SDH-BOTDR, it is necessary to extract the Brillouin scattered light component from the received light that includes the Rayleigh scattered light component and the Brillouin scattered light component. Since the difference in frequency between the Rayleigh scattered light component and the Brillouin scattered light component is small, a narrow band FBG (Fiber Bragg Grating) filter is usually used as a filter for the received light.

When using this narrow band FBG filter, there is a possibility that the desired Brillouin scattered light cannot be extracted due to changes in environmental temperature or changes in filter characteristics due to deterioration over time.

This invention was made in view of the above-mentioned situation. An object of the present invention is to provide an optical fiber sensor and a filter characteristic measuring method that can detect the characteristics of an FBG filter.

In order to achieve the above object, the optical fiber sensor of the present invention includes a light source section, an optical switch section, a filter section, an interference section, and a signal processing section. The light source section includes a variable wavelength light source and generates probe light of a predetermined frequency. The optical switch section selects the backward Brillouin scattered light generated in the optical fiber to be measured by the probe light and the characteristic measurement light of a predetermined frequency, and sends the selected light to the filter section as measurement light. The filter section transmits a specific frequency component of the measurement light. The interference unit generates interference light by self-delay interference of the measurement light from which a specific frequency component has been extracted. The signal processing section obtains the transmission characteristics of the filter section based on the signal strength of the interference light and the frequency of the characteristic measurement light.

According to a preferred embodiment of the optical fiber sensor of the present invention, the frequency of the characteristic measuring light can be changed over a range including the frequency band transmitted by the filter section. The configuration may further include a characteristic measurement light source that generates characteristic measurement light. Further, it is also possible to have a configuration in which the continuous light generated by the variable wavelength light source included in the light source section is split into two, and one is used as the probe light and the other is used as the characteristic measurement light.

The optical switch section may include an optical switch having one port on one side and n ports (n is an integer of 2 or more) on the other side. The optical switch optically connects one port on one side and one port selected from n ports on the other side. Light for characteristic measurement is input to the n-th port on the other side of the optical switch, and an optical fiber to be measured is connected to at least one of the first to (n-1) ports on the other side of the optical switch. be done.

Further, the interference section is provided at a branching section that branches the input measurement light into a first optical path and a second optical path, and at either one of the first optical path and the second optical path, and provides a frequency shift to the propagating light. A frequency shifter section, a delay section that is provided on either the first optical path or the second optical path and delays the propagating light, and combines the lights received through the first optical path and the second optical path to produce interference light. It is also possible to use a self-delay type heterodyne interferometer having a multiplexing section that generates .

Moreover, the filter characteristic measuring method of the present invention can be implemented with the above-mentioned optical fiber sensor, and includes a signal strength acquisition process and a transmission frequency acquisition process.

In the signal strength acquisition process, the frequency of the characteristic measuring light is sequentially changed over a range including the frequency transmitted by the filter section, and the signal strength of the combined light at each frequency of the characteristic measuring light is acquired.

In the transmission frequency acquisition process, the frequency of the characteristic measuring light at which the signal intensity of the combined light becomes maximum is acquired as the transmission frequency of the filter section.

The signal strength acquisition process and the transmission frequency acquisition process are preferably performed periodically and infrequently during the period when the selected optical fiber is being measured.

According to the optical fiber sensor filter characteristic measuring method of the present invention, it is possible to detect the characteristics of an FBG filter. As a result, it is also possible to prevent a desired component of Brillouin scattered light from being unable to be extracted.

It is a typical block diagram showing a 1st optical fiber sensor. FIG. 3 is a schematic diagram for explaining a filter characteristic measuring method. FIG. 3 is a schematic diagram for explaining monitoring of filter characteristics. It is a typical block diagram showing a 2nd optical fiber sensor. It is a typical block diagram showing a 3rd optical fiber sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings, but each drawing is only schematically shown to the extent that the invention can be understood. Further, although preferred configuration examples of the present invention will be described below, these are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes and modifications can be made that can achieve the effects of the present invention without departing from the scope of the configuration of the present invention.

(First embodiment)
Referring to FIG. 1, an optical fiber sensor (hereinafter also referred to as a first optical fiber sensor) using SDH-BOTDR according to a first embodiment of the present invention will be described. FIG. 1 is a schematic block diagram of a first optical fiber sensor.

The first optical fiber sensor includes a light source section 100, an optical switch section 300, a filter section 600, an interference section 800, and a signal processing section 900.

The light source section 100 generates probe light. The light source section 100 includes, for example, a wavelength tunable light source 110 and an optical pulse generator 130.

The wavelength tunable light source 110 can be constructed from any suitable conventionally known wavelength tunable laser. The wavelength tunable light source 110 generates continuous light of a predetermined wavelength and sends it to the optical pulse generator 130.

Optical pulse generator 130 may be constructed using any suitable conventionally known acousto-optical (AO) or electro-optical (EO) modulator. The optical pulse generator 130 generates a rectangular optical pulse from continuous light in response to an externally input electric pulse. The repetition period of this optical pulse is set to be longer than the time required for the optical pulse included in the probe light output from the light source to travel back and forth through the optical fiber under test (FUT) 400, which is the optical fiber to be measured. be done.

The optical pulses generated by the optical pulse generator 130 are sent to the circulator 200 as probe light. Note that the light source section 100 may further include an amplifier, an optical filter, etc., but illustration and description thereof are omitted here.

Circulator 200 has first to third input/output ports. Light input to the first input/output port is output from the second input/output port, and light input to the second input/output port is output from the third input/output port. The probe light sent from the light source section 100 to the circulator 200 is input to the first input/output port and output from the second input/output port. The probe light output from the second input/output port of the circulator 200 is sent to the optical switch section 300.

Note that an optical coupler may be used instead of the circulator 200.

The optical switch section includes a reference point fiber 320, a characteristic measurement light source 310, and an optical switch 330. The characteristic measurement light source 310 can generate continuous light of a predetermined wavelength, similarly to the variable wavelength light source 110 included in the light source section 100. The optical switch 330 has one port on one side and n ports (n is an integer of 2 or more) on the other side. Here, one port of the optical switch 330 is referred to as port 0 (port-0), and the ports on the other side are referred to as first to nth ports (port-1 to port-n).

The optical switch 330 selects port 1 from n ports (port-1 to port-n) on the other side, and selects the port (port-0) on one side and the selected k-th port on the other side. (k is an integer greater than or equal to 1 and less than or equal to n) is optically connected to a port (port-k). In the optical switch 330, light input to port-0 is output from port-k, and light input to port-k is output from port-0.

Here, we will explain the case where n is 2, that is, the case where the optical switch 330 has two ports on the other side: a first port (port-1) and a second port (port-2). .

On one side of the optical switch 330, a reference point fiber 320 is connected to port 0 (port-0). Furthermore, on the other side of the optical switch 330, the FUT 400 is connected to port 1 (port-1), and the light source 310 for measuring characteristics is connected to port 2 (port-2).

The FUT 400 is installed in large buildings such as bridges and tunnels, plants, and locations where landslides are likely to occur, and is used to measure temperature and strain.

The probe light is backscattered while propagating through FUT 400. Backscattered light is sent from FUT 400 to optical switch 330.

Note that when the optical switch 300 has three or more ports on the other side, the light source 310 for characteristic measurement is connected to the n-th port (port-n), and the light source 310 for characteristic measurement is connected to the The FUT 400 may be connected to each of ports (1 to port-(n-1)). In this case, measurements of multiple systems can be performed simply by switching the optical switch 330.

The first optical fiber sensor is used to measure temperature, distortion of a structure, and the like. The reference point fiber 320 is provided at a location that is not affected by temperature change or strain of the measurement target, and is used as a reference for measuring temperature or strain. One end of the reference point fiber 320 is optically connected to the second input/output port of the circulator 200. Further, the other end of the reference point fiber 320 is optically connected to port-0 of the optical switch 330. In the optical switch section 300 , the probe light received from the light source section 100 is propagated through the reference point fiber 320 and sent to the optical switch 330 .

Note that if it is sufficient to know relative changes in each position of the FUT 400 with respect to temperature and strain, a configuration may be adopted in which the reference point fiber 320 is not provided. Further, a reference point fiber 320 may be provided between the optical switch 330 and the FUT 400. In these cases, the second input/output port of the circulator 200 and port-0 of the optical switch 300 are optically connected without passing through the reference point fiber 320.

The characteristic measurement light source 310 is connected to the n-th port (port-n) of the optical switch 330 via an optical attenuator (ATT) 340 and an optical isolator (ISO) 350, and in the configuration example shown in FIG. connected to.

Here, the characteristic measuring light, which is a continuous light generated by the characteristic measuring light source 310, is input to port-n of the optical switch 330, passes through the same path as the backscattered light, and is electrically processed by the signal processing unit 900. converted into a signal. In the following description, the backscattered light and characteristic measurement light may be referred to as measurement light.

The intensity of the Brillouin scattered light is generally small, and conversion into an electrical signal is performed using, for example, an avalanche photodiode (APD) as described below. If excessively intense light is input to the APD, the APD may be damaged. Therefore, the ATT 340 is used to reduce the difference in intensity from the Brillouin scattered light. Note that if the light intensity of the continuous light generated by the characteristic measurement light source 310 can be reduced to an extent that does not damage the APD, the ATT 340 may not be provided.

When the optical switch 330 selects port-n as the port on the other side, the characteristic measurement light is input to port-n of the optical switch 330, and the probe light is input from port-n of the optical switch 330. It is being output. It is preferable that an ISO 350 be provided to block the probe light outputted from port-n so that the probe light is not input to the characteristic measurement light source 310 or the like. ISO350 transmits light in the forward direction and blocks light in the reverse direction.

The measurement light, which is backscattered light or characteristic measurement light, is sent from the optical switch section 330 to the signal processing section 900 via the circulator 200, the filter section 600, and the interference section 800. The signal processing unit 900 converts the measurement light into an electrical signal, and then obtains the BFS and the temperature and strain based on the BFS.

The measurement light is input to the second input/output port of the circulator 200 and output from the third input/output port. The measurement light output from the third input/output port of the circulator 200 is sent to the optical amplifier 500.

The optical amplifier 500 is composed of, for example, an erbium-doped optical fiber amplifier (EDFA). Optical amplifier 500 amplifies measurement light. The measurement light amplified by the optical amplifier 500 is transmitted through the filter section 600 and sent to the 1:99 coupler 700.

The filter section 600 includes a first filter 610 and a second filter 620 that are connected in series. The measurement light sent to the filter section 600 passes through the first filter and the second filter in order. The first filter and the second filter are similarly configured, and in this configuration example, are narrowband FBG filters. By providing FBG filters with similar configurations in series, the transmission characteristics of the filter section 600 can be sharpened.

First, the first filter 610 will be explained. The first filter 610 includes a circulator 612 and an FBG 614. The first input/output port of the circulator 612 included in the first filter 610 is connected to the optical amplifier 500, the second input/output port is connected to one end of the FBG section 614, and the third input/output port is connected to the optical amplifier 500. 2 filter 620. Further, the other end of the FBG section 614 is preferably terminated at a termination section 616.

The measurement light sent to the first filter 610 is input to the first input/output port of the circulator 612 and output from the second input/output port. The backscattered light output from the second input/output port is reflected by the FBG 614 at a wavelength that satisfies the diffraction conditions, and is input to the second input/output port of the circulator 612 . The measurement light input to the second input/output port of the circulator 612 is output from the third input/output port and sent to the second filter 620.

The second filter 620, like the first filter 610, includes a circulator 622 and an FBG 624. The first input/output port of the circulator 622 included in the second filter 620 is connected to the third input/output port of the circulator 612 included in the first filter 610, and the second input/output port is connected to one end of the FBG 624. , the third input/output port is connected to a 1:99 coupler 700. Further, the other end of the FBG 624 is preferably terminated at a termination portion 626.

The measurement light sent to the second filter 620 is input to the first input/output port of the circulator 622 and output from the second input/output port. The backscattered light output from the second input/output port is reflected by the FBG 624 at a wavelength that satisfies the diffraction condition, and is input to the second input/output port of the circulator 622 . The measurement light input to the second input/output port of the circulator 622 is output from the third input/output port and sent to the 1:99 coupler 700.

The 1:99 coupler 700 branches the measurement light sent from the second filter 620 into two, sends one to the interference section 800, and outputs the other to the outside from the monitor port. The light outputted to the outside from this monitor port can be used, for example, when measuring the difference in actual frequencies when the variable wavelength light source 110 and the characteristic measurement light source 310 are set to the same value.

The interference section 800 is configured as a self-delayed heterodyne interferometer including a branching section 810, a frequency shifter section 820, a delay section 830, and a multiplexing section 840.

The branching section 810 receives the measurement light through the 1:99 coupler 700 and branches it into a first optical path and a second optical path.

The frequency shifter section 820 is provided in one of the first optical path and the second optical path, in this example, the first optical path, and applies a frequency shift of the beat frequency Δf to the light propagating in the first optical path. The beat frequency Δf is given by an electrical signal generated by a local electrical signal source. The local electric signal source can have a conventionally known configuration, so illustration and description thereof will be omitted.

The delay unit 830 is provided in either the first optical path or the second optical path, in this example, the second optical path, and provides a delay of time τ to the light propagating in the second optical path.

The multiplexer 840 multiplexes the lights propagating through the first optical path and the second optical path to generate interference light. The interference light is sent to the signal processing section 900.

The signal processing section 900 includes two APDs 912 and 914 and a receiving electric circuit 920. The two APDs 912 and 914 constitute a balanced photodiode and convert interference light into an electrical signal. The electrical signals obtained by the two APDs 912 and 914 are sent to a receiving electrical circuit 920. The receiving electric circuit 920 includes an analog-to-digital converter, a mixer, a low-pass filter, a digital signal processing section, and the like.

Analog electrical signals generated by APDs 912 and 914 are converted into digital signals by ADC. Thereafter, a mixer and a low-pass filter obtain a beat signal of the digital signal and the electric signal generated by the local electric signal source. The beat signal is sent to a digital signal processing section.

The digital signal processing unit includes, for example, a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and a storage unit. When the CPU executes a program stored in the ROM, calculation of BFS, calculation of temperature and strain obtained based on BFS, and acquisition of filter characteristics are performed.

The configuration and operation of the interference section 800 and the configuration and operation of the signal processing section 900 can be realized by conventionally known techniques disclosed in Patent Document 1, for example, so illustrations and detailed explanations are omitted here. do.

(Method of measuring filter characteristics)
A filter characteristic measuring method will be explained with reference to FIG. FIG. 2 is a schematic diagram for explaining a filter characteristic measuring method. FIG. 2(A) is a diagram showing filter characteristics, in which the horizontal axis represents the frequency, and the vertical axis represents the transmittance of the first filter. FIG. 2B is a diagram showing the output of the characteristic measuring light source 310, with the frequency plotted on the horizontal axis and the light intensity plotted on the vertical axis. FIG. 2C is a diagram showing the electric signal strength measured by the receiving electric circuit, with the horizontal axis representing the frequency and the vertical axis representing the voltage as the electric signal strength.

When performing normal measurement, the first port (port-1) is selected in the optical switch 330 and backscattered light at the FUT 400 is measured. On the other hand, when measuring the filter characteristics, the second port (port-2) is selected in the optical switch 330, and the characteristic measurement light generated by the characteristic measurement light source 310 is measured.

In the measurement of filter characteristics, in the signal strength acquisition process, while gradually changing (sweeping) the frequency setting value of the characteristic measurement light source 310 in a range including the filter bands of the first filter 610 and the second filter 620, A circuit 920 obtains the amplitude value. For example, from the lower frequency side (the left side of FIG. 2(B)) to the higher frequency side (the right side of FIG. 2(B)), the amount of change is smaller than the width of the transmission frequency band of the filter section 600. Change the frequency setting value gradually at equal intervals. Thereafter, in the transmission frequency acquisition process, the frequency of the characteristic measuring light at which the signal intensity of the interference light becomes maximum is acquired as the transmission frequency of the filter section.

As a result, filter characteristics are obtained from the frequency setting value and the intensity of the electrical signal, as shown in FIG. 2(C). The filter characteristics obtained here are transmission characteristics that indicate the relationship between the frequency of light input to the filter and the intensity of light transmitted through the filter. Furthermore, when the filter is an FBG, the filter characteristics are also referred to as FBG characteristics. To gradually change the frequency setting value, a frequency sweep mechanism included in the light source can be used. In this case, in order to obtain the frequency setting value, it is preferable to be able to calculate the frequency setting value corresponding to the received light in advance from the reception time of the received light.

It is also considered effective to reduce noise by measuring multiple times and averaging each frequency setting value.

In addition, by inserting the measurement of port-2 at low frequency in a time-division multiplexing manner to the normal measurement of port-1, FBG Characteristics can be monitored.

With reference to FIG. 3, monitoring of FBG characteristics will be explained. FIG. 3 is a schematic diagram for explaining monitoring of FBG characteristics. FIG. 3(A) is a schematic diagram of an optical switch when n is 3. FIG. 3(B) is a schematic diagram showing a time series of normal measurement with FUT and characteristic measurement of FBG. FIG. 3(C) shows switching of the optical switch 331 during normal measurement, and FIG. 3(D) shows switching of the optical switch 331 during characteristic measurement.

Note that, although it is preferable to measure the FBG characteristics at all times, normal measurements cannot be performed while the FBG characteristics are being measured. Therefore, the frequency and time of measuring the characteristics of the FBG should be determined to such an extent that the influence on normal measurements can be ignored. That is, it is preferable to measure the characteristics periodically and infrequently.

As shown in FIG. 3(B), the time required for characteristic measurement is set to be shorter than the time required for normal measurement. During normal measurement, as shown in FIG. 3(C), port-1 and port-2 are switched, and two systems of measurement are always performed. On the other hand, when measuring characteristics, measurements are performed by switching ports-1 to port-3, as shown in FIG. 3(D).

(Second optical fiber sensor)
The second optical fiber sensor will be described with reference to FIG. 4. FIG. 4 is a schematic diagram of the second optical fiber sensor.

The first optical fiber sensor includes a characteristic measurement light source 310 in the optical switch section 300, while the second optical fiber sensor uses the characteristic measurement light source 310, ATT 340, and ISO 350 as the characteristic measurement light source section 301. It is installed outside the fiber sensor. The configuration other than this is the same as that of the first optical fiber sensor, so redundant explanation will be omitted.

This second optical fiber sensor requires a connector for connecting the ISO 350 of the characteristic measurement light source section 301 to port-n of the optical switch 330. Therefore, the characteristics may deteriorate due to Fresnel reflection caused by the connector connection at port-n. Further, an interface used to control the characteristic measurement light source 310 must be separately prepared.

However, it has the advantage that it can be implemented as an extension of the conventional configuration.

(Third optical fiber sensor)
The third optical fiber sensor will be described with reference to FIG. 5. FIG. 5 is a schematic diagram of the third optical fiber sensor.

The first optical fiber sensor includes a characteristic measurement light source 310 in the optical switch section 300, while the third optical fiber sensor uses the variable wavelength light source 110 included in the light source section 103 as a characteristic measurement light source. Therefore, the light source section 103 of the third optical fiber sensor further includes a 1:99 coupler 120, an ATT 140, and an ISO 150 in addition to the configuration of the light source section of the first light source section.

The continuous light generated by the wavelength tunable light source 110 is split into two by a 1:99 coupler 120, one of which is sent to an optical pulse generator 130 and used as probe light. The other one is sent to port-n of the optical switch 330 included in the optical switch section 300 via the ATT 140 and the ISO 150. There is a concern that the characteristics of the third optical fiber sensor may deteriorate compared to the first optical fiber sensor due to insertion loss and crosstalk of the 1:99 coupler 120. Furthermore, since the frequency of the wavelength tunable light source 110 is gradually changed when measuring the characteristics, the wavelength setting in the normal measurement must be redone every time the characteristics are measured, which increases the time required to measure the characteristics.

On the other hand, since there is no need to separately provide a light source for measuring characteristics, manufacturing costs can be reduced. Furthermore, since the same variable wavelength light source is used for normal measurement and characteristic measurement, there is no difference between the two light sources. Therefore, it is not necessary to measure the actual frequency difference when the variable wavelength light source 110 and the characteristic measurement light source 310 are set to the same value, which is done by the first optical fiber sensor or the like.

As described above, according to the first to third optical fiber sensors and filter characteristic measuring method of the present invention, it is possible to detect the characteristics of an FBG filter. If the characteristics of the FBG filter change as a result of measuring the filter characteristics, it is possible to prevent the desired Brillouin scattered light component from being unable to be extracted by, for example, changing the frequency of the variable wavelength light source.

In addition to periodically measuring the characteristics, the frequency settings and actual output frequencies of the wavelength tunable light source and the light source for measuring characteristics are also periodically measured using the output of the 1:99 coupler 700. It is desirable to monitor the situation regularly.

100, 103 light source section 110 variable wavelength light source 120, 700 1:99 coupler 130 optical pulse generator 140, 340 optical attenuator (ATT)
150, 350 optical isolator (ISO)
200, 612, 614 Circulator 300 Optical switch section 310 Light source for characteristic measurement 320 Reference point fiber 330, 331 Optical switch 400 Optical fiber to be measured (FUT)
500 Optical amplifier 600 Filter section 610 First filter 614, 624 FBG
616, 626 terminating section 620 second filter 800 interference section 810 branching section 820 frequency shifter section 830 delay section 840 multiplexing section 900 signal processing section 912, 914 avalanche photodiode (APD)
920 Receiving electrical circuit

Claims (8)

Comprising a light source section, an optical switch section, a filter section, an interference section, and a signal processing section,
The light source section includes a variable wavelength light source and generates probe light of a predetermined frequency,
The optical switch section selects backward Brillouin scattered light generated in the optical fiber to be measured by the probe light and characteristic measurement light of a predetermined frequency, and sends the selected light to the filter section as measurement light;
The filter section transmits a specific frequency component of the measurement light,
The interference unit generates interference light by self-delay interference of the measurement light from which a specific frequency component has been extracted;
The optical fiber sensor is characterized in that the signal processing section acquires the transmission characteristic of the filter section based on the signal intensity of the interference light and the frequency of the characteristic measurement light. 2. The optical fiber sensor according to claim 1, wherein the frequency of the characteristic measuring light can be changed over a range including a frequency band transmitted by the filter section. The optical fiber sensor according to claim 1 or 2, further comprising a characteristic measuring light source that generates the characteristic measuring light. 3. The optical fiber sensor according to claim 1, wherein the continuous light generated by the variable wavelength light source is split into two branches, one of which is used as the probe light and the other as the characteristic measuring light. The optical switch section is
Has 1 port on one side,
An optical switch having n (n is an integer of 2 or more) ports on the other side,
The optical switch optically connects one port on the one side and one port selected from the n ports on the other side,
the characteristic measurement light is input to the n-th port on the other side;
3. The optical fiber sensor according to claim 1, wherein an optical fiber to be measured is connected to at least one of the first to (n-1) ports on the other side. The interference part is
a branching section that branches the input measurement light into a first optical path and a second optical path;
a frequency shifter section provided in either the first optical path or the second optical path and giving a frequency shift to the propagating light;
a delay section that is provided on either the first optical path or the second optical path and delays the propagating light;
According to claim 1 or 2, the interferometer is a self-delaying heterodyne interferometer, comprising a multiplexing section that multiplexes light received through the first optical path and the second optical path to generate interference light. Optical fiber sensor as described. A method for measuring filter characteristics in an optical fiber sensor according to claim 1, comprising:
a signal strength acquisition step of sequentially changing the frequency of the characteristic measuring light over a range including the frequency transmitted by the filter section, and acquiring the signal strength of the combined light at each frequency of the characteristic measuring light;
A method for measuring filter characteristics, comprising: obtaining a transmission frequency of the characteristic measurement light when the signal strength of the multiplexed light reaches a maximum as a transmission frequency of the filter section. The signal strength acquisition process and the transmission frequency acquisition process include:
8. The filter characteristic measuring method according to claim 7, wherein the filter characteristic measuring method is performed periodically and infrequently during a period when the selected optical fiber is being measured.

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