JP7775809B2 - Optical fiber characteristic measuring device and optical fiber characteristic measuring method - Google Patents

Description

The present invention relates to an optical fiber characteristic measuring device and an optical fiber characteristic measuring method.

An optical fiber characteristic measuring device is a device that injects continuous or pulsed light into the optical fiber under test, detects the scattered or reflected light generated within the optical fiber under test, and measures the temperature distribution, strain distribution, and other characteristics along the length of the optical fiber under test. In this optical fiber characteristic measuring device, the detected scattered or reflected light changes depending on the physical quantities (e.g., temperature and strain) that affect the optical fiber under test, so the optical fiber under test itself is used as a sensor.

One such optical fiber characteristic measurement device is an OFDR (Optical Frequency Domain Reflectometry) type. This OFDR type optical fiber characteristic measurement device splits frequency-swept light into measurement light and reference light, inputs the measurement light into one end of the optical fiber under test, and detects the interference between the Rayleigh scattered light emitted from the end of the optical fiber under test and the reference light. The resulting detection signal is then analyzed to measure the characteristics of the optical fiber under test along its length. For more information on OFDR type optical fiber characteristic measurement devices, please see, for example, Non-Patent Document 1 below.

Brian J. Soller et al., “High resolution optical frequency domain reflectometry for characterization of components and assemblies”, Optics Express Vol.13, No.2, p.666-674

Conventional OFDR optical fiber characteristic measurement devices use a frequency-tunable light source to obtain frequency-swept light. This frequency-tunable light source has the disadvantage of having nonlinear frequency sweep characteristics and a wider linewidth than light sources that output light of a fixed frequency, resulting in large measurement errors. Furthermore, conventional OFDR optical fiber characteristic measurement devices split the frequency-swept light into measurement light and reference light, and then detect the interference between the Rayleigh scattered light obtained by injecting the measurement light into one end of the optical fiber being measured and the reference light, which limits the measurement distance.

The present invention was made in consideration of the above circumstances, and aims to provide an optical fiber characteristics measurement device and method that have smaller measurement errors and can measure longer distances than conventional devices.

In order to solve the above problem, the optical fiber characteristic measuring device (1, 2) according to the first aspect of the present invention comprises a light source (11) that outputs light of a constant frequency, a first branching unit (12) that branches the light output from the light source into measurement light (LM) and reference light (LR), a frequency shifting unit (13) that sequentially shifts the frequency of the measurement light using a first signal (S10) whose frequency sequentially changes, and an optical fiber (FUT) that inputs the measurement light whose frequency is sequentially shifted into one end of the optical fiber (FUT) to be measured, and It comprises a second branching unit (15) that outputs the generated Rayleigh scattered light (LS), a detection unit (17) that detects interference light between the reference light and the Rayleigh scattered light, a frequency conversion unit (18, 18A, 18B) that converts the frequency of the detection signal (S1) output from the detection unit using second signals (S20, S20a, S20b) whose frequency changes sequentially, and a measurement unit (19, 19A) that measures the characteristics of the optical fiber under test using signals (S2, S2a, S2b) output from the frequency conversion unit.

Furthermore, the optical fiber characteristic measuring device according to the second aspect of the present invention is the optical fiber characteristic measuring device according to the first aspect of the present invention, wherein the frequency change rate of the first signal and the frequency change rate of the second signal are the same, and the frequency range of the second signal is wider than the frequency range of the first signal.

Furthermore, an optical fiber characteristic measuring device according to a third aspect of the present invention is the optical fiber characteristic measuring device according to the first or second aspect of the present invention, wherein the first signal is a signal whose frequency change ends in a predetermined reference time (t _scan ), and the optical fiber characteristic measuring device further comprises a pulsing unit (14) that converts the measurement light output from the frequency shift unit into an optical pulse having a pulse width equal to the reference time.

Furthermore, in the optical fiber characteristic measuring device according to a fourth aspect of the present invention, in the optical fiber characteristic measuring device according to the third aspect of the present invention, the measurement unit comprises an extraction unit (21, 21a, 21b) that extracts a portion corresponding to a measurement point set on the optical fiber under test from the signal output from the frequency conversion unit, and a Fourier transform unit (22, 22a, 22b) that performs a Fourier transform on the portion extracted by the extraction unit to determine the magnitude of the frequency component corresponding to the measurement point.

Furthermore, the optical fiber characteristic measuring device according to the fifth aspect of the present invention is the optical fiber characteristic measuring device according to the fourth aspect of the present invention, wherein the portion cut out by the cutout unit has the same time width as the reference time.

Furthermore, the optical fiber characteristic measuring device according to a sixth aspect of the present invention is the optical fiber characteristic measuring device according to the fourth aspect of the present invention, wherein the frequency conversion unit includes a signal generator (18a) that generates the second signal, and the signal generator is capable of adjusting the timing at which generation of the second signal begins.

Furthermore, the optical fiber characteristic measuring device according to a seventh aspect of the present invention is the optical fiber characteristic measuring device according to the sixth aspect of the present invention, in which a plurality of frequency conversion units are provided, the measurement unit is provided with a plurality of cutout units and Fourier transform units corresponding to the plurality of frequency conversion units, and the signal generators provided in the plurality of frequency conversion units start generating the second signal at different timings.

Furthermore, the optical fiber characteristic measurement method according to the first aspect of the present invention comprises the following steps: a first step of outputting light of a constant frequency; a second step of splitting the light into measurement light (LM) and reference light (LR); a third step of sequentially shifting the frequency of the measurement light using a first signal (S10) whose frequency is sequentially changed; a fourth step of inputting the measurement light whose frequency is sequentially shifted into one end of an optical fiber under test (FUT) and outputting Rayleigh scattered light (LS) generated within the optical fiber under test; a fifth step of detecting interference light between the reference light and the Rayleigh scattered light; a sixth step of converting the frequency of the detection signal (S1) detected in the fifth step using second signals (S20, S20a, S20b) whose frequency is sequentially changed; and a seventh step of measuring the characteristics of the optical fiber under test using the signals (S2, S2a, S2b) whose frequency is converted in the sixth step.

The present invention has the advantage of reducing measurement errors and enabling longer measurement distances compared to conventional methods.

1 is a block diagram showing the configuration of a main part of an optical fiber characteristic measuring device according to a first embodiment of the present invention; 3A to 3C are diagrams for explaining the operation of the optical fiber characteristic measuring device according to the first embodiment of the present invention. 5A and 5B are diagrams for explaining the operation of the optical fiber characteristic measuring device according to the second embodiment of the present invention. FIG. 10 is a block diagram showing the configuration of a main part of an optical fiber characteristic measuring apparatus according to a third embodiment of the present invention. 10A and 10B are diagrams for explaining the operation of the optical fiber characteristic measuring device according to the third embodiment of the present invention.

The following describes in detail an optical fiber characteristic measuring device and an optical fiber characteristic measuring method according to embodiments of the present invention, with reference to the drawings. First, an overview of the embodiments of the present invention will be provided, followed by a detailed description of each embodiment of the present invention.

〔overview〕
An embodiment of the present invention is an optical fiber characteristic measuring device that reduces measurement errors and extends measurement distances compared to conventional devices. Specifically, in an OFDR-type optical fiber characteristic measuring device, measurement errors are reduced by using a light source that outputs light of a constant frequency with a narrower linewidth than a frequency-variable light source. Then, by devising a method for changing the frequency of measurement light incident on the optical fiber under test and a method for detecting Rayleigh scattered light, measurement distances are extended compared to conventional devices.

Conventional OFDR optical fiber characteristic measurement devices use a frequency-tunable light source to obtain frequency-swept light. This frequency-tunable light source has nonlinearity in its frequency sweep characteristics, resulting in measurement errors. Methods have been devised to compensate for this nonlinearity in the frequency sweep characteristics, but the conditions under which sufficient compensation can be achieved are limited, making it difficult to sufficiently reduce measurement errors.

Furthermore, conventional OFDR optical fiber characteristic measurement devices split the light output from a frequency-tunable light source (frequency-swept light) into measurement light and reference light, and then detect the interference between the Rayleigh scattered light obtained by injecting the measurement light into one end of the optical fiber under test and the reference light. As a result, the time width during which the Rayleigh scattered light generated near the other end of the optical fiber under test interferes with the reference light becomes shorter, and the spectral width after Fourier transform becomes wider, making it difficult to achieve measurement accuracy. To avoid this, the length of the optical fiber under test must be shortened, which limits the measurement distance.

Furthermore, the frequency-tunable light sources used in conventional OFDR optical fiber characteristic measurement devices often have a wider linewidth and a shorter coherent time than light sources that output light of a fixed frequency. This puts a strain on the measurement distance and measurement accuracy after Fourier transform.

In this embodiment, light of a constant frequency output from a light source is first split into measurement light and reference light. Next, the frequency of the measurement light is sequentially shifted using a first signal whose frequency sequentially changes, and the measurement light whose frequency sequentially shifts is incident on one end of the optical fiber under test, outputting Rayleigh scattered light generated within the optical fiber under test. Next, interference light between the reference light and Rayleigh scattered light is detected, and the frequency of the detected detection signal is converted using a second signal whose frequency sequentially changes. The frequency-converted signal is then used to measure the characteristics of the optical fiber under test. This reduces measurement error and enables longer measurement distances than conventional methods.

First Embodiment
Optical fiber characteristic measuring device
Fig. 1 is a block diagram showing the main components of an optical fiber characteristic measuring apparatus according to a first embodiment of the present invention. As shown in Fig. 1, the optical fiber characteristic measuring apparatus 1 of this embodiment includes a light source 11, an optical branching device 12 (first branching device), a frequency shifting unit 13, an optical switch 14 (pulsing unit), an optical circulator 15 (second branching device), an optical multiplexer 16, a photodetector 17 (detecting unit), a frequency conversion unit 18, and a measuring unit 19. Such an optical fiber characteristic measuring apparatus 1 measures the characteristics (e.g., temperature distribution, strain distribution, etc.) of the optical fiber FUT to be measured in the length direction (z direction) of the optical fiber FUT.

The light source 11 emits light with a constant frequency f0 and high coherence. This light source 11 can be, for example, a semiconductor laser that emits laser light with a wavelength (e.g., 1.55 μm) that is poorly absorbed by the optical fiber FUT under test. It is desirable that the light emitted from the light source 11 have as narrow a linewidth (spectral linewidth) as possible to increase the measurement accuracy of the optical fiber characteristic measuring device 1.

The optical splitter 12 splits the light emitted from the light source 11 into measurement light LM and reference light LR. The intensity ratio between the measurement light LM and reference light LR is, for example, 1:1. However, the intensity ratio between the measurement light LM and reference light LR is not limited to 1:1 and can be any intensity ratio.

The frequency shift unit 13 includes a frequency generator 13a and an optical frequency shifter 13b, and sequentially shifts the frequency of the measurement light LM. The frequency generator 13a outputs a signal S10 (first signal) whose frequency changes sequentially. Specifically, the frequency generator 13a outputs the signal S10 whose frequency changes sequentially at a constant rate within a frequency range from f11 to f12 (e.g., a range of 250 to 350 MHz). The time required for the frequency generator 13a to change the frequency of the signal S10 from f11 to f12 is t _scan (reference time). In other words, the signal S10 is a signal whose frequency change ends in time t _scan , and such frequency changes are repeated.

The optical frequency shifter 13b sequentially shifts the frequency of the measurement light LM using the signal S10 output from the frequency generator 13a. Since the frequency of the light emitted from the light source 11 is f0, the optical frequency shifter 13b sequentially shifts the frequency of the measurement light LM from f0 + f11 to f0 + f12. For example, a commercially available AO (Acousto-Optics) element can be used as this optical frequency shifter 13b.

The optical switch 14 opens or closes the optical path between the frequency shifter 13 and the optical circulator 15. The optical switch 14 is provided in order to pulse the measurement light LM. Specifically, the optical switch 14 opens the optical path between the frequency shifter 13 and the optical circulator 15 only while the frequency of the measurement light LM is being sequentially shifted by the frequency shifter 13. In other words, the optical switch 14 converts the measurement light LM output from the frequency shifter 13 into an optical pulse having a pulse width of the time t _scan . If necessary, an optical amplifier may be provided between the optical switch 14 and the optical circulator 15.

The optical circulator 15 has a first port, a second port, and a third port. The first port is connected to the optical switch 14. The second port is connected to the optical fiber FUT under test. The third port is connected to the optical multiplexer 16. The optical circulator 15 outputs the measurement light LM input from the first port to the second port. The optical circulator 15 also outputs the Rayleigh scattered light LS from the optical fiber FUT under test, which is input from the second port, to the third port.

The optical multiplexer 16 multiplexes the Rayleigh scattered light LS output from the optical circulator 15 with the reference light LR branched by the optical brancher 12. An optical coupler, for example, can be used as this optical multiplexer 16. The photodetector 17 performs optical heterodyne detection of the light combined by the optical multiplexer 16 and outputs a detection signal S1.

Here, the Rayleigh scattered light LS is the measurement light LM incident on the optical fiber FUT under test that is elastically scattered (scattered without a change in wavelength). Therefore, the Rayleigh scattered light LS generated at each point in the optical fiber FUT under test has a frequency that changes sequentially from f0+f11 to f0+f12 over a fixed time (time t _scan ). This Rayleigh scattered light LS is combined with the reference light LR and optically heterodyne detected, resulting in a component whose frequency changes sequentially from f11 to f12. The detection signal S1 output from the photodetector 17 is due to the Rayleigh scattered light LS generated at each point in the optical fiber FUT under test, and includes a component whose frequency changes sequentially from f11 to f12.

The frequency conversion unit 18 includes a frequency generator 18a (signal generator) and a frequency converter 18b, and converts the frequency of the detection signal S1 output from the photodetector 17. The frequency generator 18a outputs a signal S20 (second signal) whose frequency changes sequentially. Specifically, the frequency generator 18a outputs a signal S20 whose frequency changes sequentially within a frequency range from f21 to f22 (for example, a range from 360 to 560 MHz).

Here, the rate of frequency change of the signal S20 generated by the frequency generator 18a is the same as the rate of frequency change of the signal S10 generated by the frequency generator 13a provided in the frequency shift unit 13. Furthermore, the frequency range of the signal S20 generated by the frequency generator 18a is wider than the frequency range of the signal S10 generated by the frequency generator 13a provided in the frequency shift unit 13. In this embodiment, the frequency range of the signal S20 is twice as wide as the frequency range of the signal S10. Therefore, the time required for the frequency generator 18a to change the frequency of the signal S20 from frequency f21 to frequency f22 is longer than the time t _scan required by the frequency generator 13a to change the frequency of the signal S10 from frequency f11 to frequency f12. In other words, the signal S20 is a signal whose frequency change is completed in a time longer than the time t _scan , and in which such frequency changes are repeated.

Frequency converter 18b uses signal S20 output from frequency generator 18a to convert the frequency of detection signal S1 output from photodetector 17. Specifically, frequency converter 18b converts the frequency of detection signal S1 so that the components caused by Rayleigh scattered light LS generated at each point in the measured optical fiber FUT have different frequencies. For example, the frequency of detection signal S1 is converted so that the component caused by Rayleigh scattered light LS generated at point z1 shown in Figure 1 has a constant frequency f31, and the component caused by Rayleigh scattered light LS generated at point z2 has a constant frequency f32.

The measurement unit 19 includes a cutout unit 21, a Fourier transform unit 22, and a signal processing unit 23, and measures the characteristics (for example, temperature distribution and strain distribution in the longitudinal direction) of the optical fiber FUT under test using the signal S2 output from the frequency conversion unit 18. The cutout unit 21 cuts out a portion from the signal S2 output from the frequency conversion unit 18 that corresponds to a measurement point (a point at which characteristics are to be measured) set in the optical fiber FUT under test. Here, the portion cut out by the cutout unit 21 has the same time width as the time t _scan , regardless of the position of the measurement point.

The Fourier transform unit 22 performs a Fourier transform on the portion of the signal S2 output from the frequency conversion unit 18 that has been extracted by the extraction unit 21, to determine the magnitude of the frequency component corresponding to the measurement point. The signal processing unit 23 performs predefined signal processing using the magnitude of the frequency component determined by the Fourier transform unit 22, thereby measuring the characteristics of the optical fiber FUT under test (e.g., temperature distribution and strain distribution in the longitudinal direction).

<Optical fiber characteristic measurement method>
When measurement begins, light of a constant frequency f0 is emitted from light source 11 (step 1). The light emitted from light source 11 is split into measurement light LM and reference light LR by optical splitter 12 (step 2). The measurement light LM split by optical splitter 12 enters frequency shifter 13, where its frequency is sequentially shifted (step 3). Specifically, the frequency of measurement light LM is sequentially changed at a constant rate from f0 + f11 to f0 + f12 by optical frequency shifter 13b provided in frequency shifter 13, using signal S10 output from frequency generator 13a provided in frequency shifter 13.

The frequency-shifted measurement light LM is pulsed by the optical switch 14. Specifically, the optical switch 14 turns the optical path between the frequency shifter 13 and the optical circulator 15 into a conductive state only while the frequency is being sequentially shifted, thereby making the measurement light LM into an optical pulse having a pulse width of the time t _scan .

The pulsed measurement light LM enters the optical fiber FUT under test from one end thereof via the optical circulator 15. As the pulsed measurement light LM propagates through the optical fiber FUT under test, Rayleigh scattered light LS is generated at each point in the optical fiber FUT under test. A portion of the Rayleigh scattered light LS generated at each point in the optical fiber FUT under test enters the optical circulator 15 from one end of the optical fiber FUT under test and is output to the optical multiplexer 16 (fourth step).

Figure 2 is a diagram illustrating the operation of the optical fiber characteristic measuring device according to the first embodiment of the present invention. Figure 2(a) shows the measurement light LM and Rayleigh scattered light LS traveling through the optical fiber FUT under test. Figure 2(b) shows the detection signal S1 output from the photodetector 17, Figure 2(c) shows the signal S20 output from the frequency generator 18a, and Figure 2(d) shows the signal S2 output from the frequency conversion unit 18. The graph shown in Figure 2(a) has time on the horizontal axis and position along the length of the optical fiber FUT under test on the vertical axis. The graphs shown in Figures 2(b) to 2(d) all have time on the horizontal axis and frequency on the vertical axis.

2A, it is assumed that measurement light LM with a frequency of f0+f11 is incident on the optical fiber FUT under test at time 0, and measurement light LM with a frequency of f0+f12 is incident on the optical fiber FUT under test at time ts. Note that time ts is the time when a time t _scan has elapsed since time 0. For ease of explanation, the measurement light LM incident on the optical fiber FUT under test at time 0 is referred to as measurement light LM1, and the measurement light LM incident on the optical fiber FUT under test at time ts is referred to as measurement light LM2.

As the measurement light LM1 incident on the optical fiber FUT propagates through the optical fiber FUT, Rayleigh scattered light LS is generated at each point in the optical fiber FUT. For example, when the measurement light LM1 reaches point z1 shown in Figure 1 at time t1, Rayleigh scattered light LS11 is generated. This Rayleigh scattered light LS11 propagates in the opposite direction to the propagation direction of the measurement light LM1 and reaches one end of the optical fiber FUT at time t2. Furthermore, when the measurement light LM1 reaches point z2 shown in Figure 1 at time t3, Rayleigh scattered light LS12 is generated. This Rayleigh scattered light LS12 propagates in the opposite direction to the propagation direction of the measurement light LM1 and reaches one end of the optical fiber FUT at time t6.

Similarly, as the measurement light LM2 incident on the optical fiber FUT propagates through the optical fiber FUT, Rayleigh scattered light LS is generated at each point in the optical fiber FUT. For example, when the measurement light LM2 reaches point z1 shown in Figure 1 at time t7, Rayleigh scattered light LS21 is generated. This Rayleigh scattered light LS21 propagates in the opposite direction to the propagation direction of the measurement light LM2 and reaches one end of the optical fiber FUT at time t8. Furthermore, when the measurement light LM2 reaches point z2 shown in Figure 1 at time t9, Rayleigh scattered light LS22 is generated. This Rayleigh scattered light LS22 propagates in the opposite direction to the propagation direction of the measurement light LM2 and reaches one end of the optical fiber FUT at time t10.

The Rayleigh scattered light LS that has reached one end of the optical fiber FUT under test is incident on the optical multiplexer 16 via the optical circulator 15, where it is multiplexed with the reference light LR branched by the optical brancher 12 and then optically heterodyne detected by the photodetector 17 (step 5). In other words, the interference light between the Rayleigh scattered light LS that has reached one end of the optical fiber FUT under test and the reference light LR branched by the optical brancher 12 is detected by the photodetector 17.

As described above, the Rayleigh scattered light LS is the measurement light LM incident on the optical fiber FUT that has been elastically scattered (scattered without a change in wavelength). Therefore, the Rayleigh scattered light LS generated at each point in the optical fiber FUT that has been measured changes in frequency sequentially from f0+f11 to f0+f12 over a fixed time (time t _scan ). This Rayleigh scattered light LS is combined with the reference light LR and optically heterodyne detected, resulting in a component whose frequency sequentially changes from f11 to f12. The detection signal S1 output from the photodetector 17 includes a component whose frequency sequentially changes from f11 to f12, due to the Rayleigh scattered light LS generated at each point in the optical fiber FUT that has been measured.

For example, the component obtained by multiplexing Rayleigh scattered light generated near one end of the optical fiber FUT with the reference light LR and performing optical heterodyne detection is a component A0 whose frequency changes sequentially from f11 to f12 between times 0 and ts (during time t _scan ) in Figure 2(b). Also, the component obtained by multiplexing Rayleigh scattered light generated at point z1 shown in Figure 1 with the reference light LR and performing optical heterodyne detection is a component A1 whose frequency changes sequentially from f11 to f12 between times t2 and t8 (during time t _scan ) in Figure 2(b). Also, the component obtained by multiplexing Rayleigh scattered light generated at point z2 shown in Figure 1 with the reference light LR and performing optical heterodyne detection is a component A2 whose frequency changes sequentially from f11 to f12 between times t6 and t10 (during time t _scan ) in Figure 2(b).

The detection signal S1 output from the photodetector 17 is input to the frequency converter 18, where its frequency is converted (step 6). Specifically, the detection signal S1 is frequency converted by the frequency converter 18b using the signal S20 output from the frequency generator 18a so that the components resulting from the Rayleigh scattered light LS generated at each point in the optical fiber FUT under test have different frequencies.

As mentioned above, the frequency change rate of signal S20 generated by frequency generator 18a is the same as the frequency change rate of signal S10 generated by frequency generator 13a provided in frequency shift unit 13. As mentioned above, Rayleigh scattered light LS is elastically scattered (scattered without a change in wavelength) measurement light LM incident on the optical fiber FUT under test, so the frequency change rate of the Rayleigh scattered light LS generated at each point within the optical fiber FUT under test is the same as the frequency change rate of signal S10.

Therefore, as shown in Figures 2(b) and 2(c), the frequency change rate of signal S20 is the same as the frequency change rate of the component included in detection signal S1 and attributed to Rayleigh scattered light LS occurring at each point in the measured optical fiber FUT. Therefore, when detection signal S1 shown in Figure 2(b) is frequency-converted using signal S20 shown in Figure 2(c), signal S2 shown in Figure 2(d) is obtained. For example, component B0 included in signal S2 and having a constant frequency f30 between times 0 and ts (during time t _scan ) is the frequency-converted component A0 shown in Figure 2(b). Furthermore, component B1 included in signal S2 and having a constant frequency f31 between times t2 and t8 (during time t _scan ) is the frequency-converted component A1 shown in Figure 2(b). Furthermore, component B2 included in signal S2 and having a constant frequency f32 between times t6 and t10 (during time t _scan ) is the frequency-converted component A2 shown in Figure 2(b).

The signal S2 obtained by the frequency conversion unit 18 is input to the measurement unit 19. The measurement unit 19 then performs a process to measure the characteristics of the optical fiber FUT under test (e.g., temperature distribution and strain distribution in the longitudinal direction) (7th step). When the signal S2 is input to the measurement unit 19, the extraction unit 21 first extracts a portion of the signal S2 corresponding to the measurement point set on the optical fiber FUT under test.

For example, when the measurement point is set at point z1 shown in Fig. 1, the clipping unit 21 clips out the portion of the signal S2 between times t2 and t8 shown in Fig. 2(d). When the measurement point is set at point z2 shown in Fig. 1, the clipping unit 21 clips out the portion of the signal S2 between times t6 and t10 shown in Fig. 2(d). Note that the portion clipped out by the clipping unit 21 has the same time width as time t _scan , regardless of the position of the measurement point.

Next, the Fourier transform unit 22 performs a Fourier transform on each of the portions of the signal S2 input to the measurement unit 19 extracted by the extraction unit 21 to determine the magnitude of the frequency components corresponding to each measurement point. Next, the signal processing unit 23 performs predefined signal processing using the magnitudes of the frequency components determined by the Fourier transform unit 22 to measure the characteristics (e.g., temperature, strain, etc.) of the optical fiber FUT under test at each measurement point. By repeating the above-described processing, the characteristics of the optical fiber FUT under test from one end to point z2, for example, are continuously measured.

As described above, in this embodiment, light of a constant frequency output from the light source 11 is first split into measurement light LM and reference light LR. Next, the frequency of the measurement light LM is sequentially shifted using a signal S10 whose frequency changes sequentially, and the measurement light LM whose frequency is sequentially shifted is incident on one end of the optical fiber FUT to be measured, outputting Rayleigh scattered light LS generated within the optical fiber FUT. Next, interference light between the reference light LR and the Rayleigh scattered light LS is detected, and the frequency of the detected detection signal S1 is converted using a signal S20 whose frequency changes sequentially. The frequency-converted signal S2 is then used to measure the characteristics of the optical fiber FUT to be measured.

In this embodiment, the signal S10 output from the frequency generator 13a is used to shift the frequency of the measurement light LM. In general, it is easy to generate an electrical signal whose frequency changes with high precision and linearity. In this embodiment, the frequency of the measurement light LM is shifted using such an electrical signal (signal S10) whose frequency changes with high precision and linearity, thereby reducing measurement errors.

Furthermore, in this embodiment, the acquisition time of the Rayleigh scattered light LS generated at the measurement point on the other end side of the measured optical fiber FUT can be made the same length as the acquisition time of the Rayleigh scattered light LS generated at the measurement point on one end side of the measured optical fiber FUT. Note that the "acquisition time" here refers to the time width during which the Rayleigh scattered light LS and the reference light LR interfere. This makes it possible to improve measurement accuracy and increase the measurement distance.

Furthermore, this embodiment uses constant-frequency light output from the light source 11. A light source that emits constant-frequency light has a narrower linewidth of the emitted light than a frequency wavelength light source, which allows for higher frequency accuracy after Fourier transform. In addition, by appropriately selecting the frequencies of the signal S10 used in the frequency shift unit 13 and the signal S20 used in the frequency conversion unit 18, the frequencies of the detection signal S1 of the photodetector 17 and the signal S2 output from the frequency conversion unit 18 can be made easier to handle.

As such, this embodiment allows for smaller measurement errors and longer measurement distances than conventional methods. For example, conventional OFDR-based optical fiber characteristic measuring devices have extremely high spatial resolution of around several tens of microns, but are limited to measurement distances of around several tens of meters. In contrast, the optical fiber characteristic measuring device 1 of this embodiment has inferior spatial resolution compared to conventional devices, but can achieve measurement distances of 1 km or more.

Second Embodiment
Next, a second embodiment of the present invention will be described. The optical fiber characteristic measuring device of this embodiment has basically the same configuration as the optical fiber characteristic measuring device 1 shown in FIG. 1. Therefore, a detailed description of the configuration and operation of the optical fiber characteristic measuring device will be omitted. In the optical fiber characteristic measuring device of this embodiment, a frequency generator 18a provided in a frequency conversion unit 18 is capable of adjusting the timing at which the signal S2 starts to be generated. The timing of the signal S20 output from the frequency generator 18a is made different from that of the first embodiment, thereby shifting the measurement range of the optical fiber FUT to be measured.

Figure 3 is a diagram illustrating the operation of an optical fiber characteristic measuring device according to a second embodiment of the present invention. Note that Figures 3(a) to 3(d) are similar to Figures 2(a) to 2(d). That is, Figure 2(a) is a diagram illustrating the measurement light LM and Rayleigh scattered light LS traveling through the optical fiber FUT under test. Figure 3(b) is a diagram illustrating the detection signal S1 output from the photodetector 17, Figure 3(c) is a diagram illustrating the signal S20 output from the frequency generator 18a, and Figure 3(d) is a diagram illustrating the signal S2 output from the frequency conversion unit 18. Note that in Figures 3(a) to 3(d), the same components as those shown in Figures 2(a) to 2(d) are designated by the same reference numerals.

As shown in Figure 3(a), at time 0, measurement light LM (measurement light LM1) with a frequency of f0 + f11 is incident on the optical fiber FUT under test, and at time ts, measurement light LM (measurement light LM2) with a frequency of f0 + f12 is incident on the optical fiber FUT under test. This is the same as in the first embodiment.

As the measurement light LM1 incident on the optical fiber FUT under test propagates through the optical fiber FUT, Rayleigh scattered light LS is generated at each point in the optical fiber FUT under test. For example, when the measurement light LM1 reaches point z1 shown in Figure 1, Rayleigh scattered light LS11 is generated; when it reaches point z2 shown in Figure 1, Rayleigh scattered light LS12 is generated (time t3); and when it reaches point z3, which is further away from point z2 shown in Figure 1, Rayleigh scattered light LS13 is generated. Note that point z3 is a point farther away from point z2 than the distance from one end of the optical fiber FUT under test to point z1. These Rayleigh scattered lights LS11, LS12, and LS13 propagate in the opposite direction to the propagation direction of the measurement light LM1. Then, Rayleigh scattered light LS11 reaches one end of the measured optical fiber FUT at time t2, Rayleigh scattered light LS12 reaches one end of the measured optical fiber FUT at time t6, and Rayleigh scattered light LS13 reaches one end of the measured optical fiber FUT at time t31.

Similarly, as the measurement light LM2 incident on the optical fiber FUT propagates through the optical fiber FUT, Rayleigh scattered light LS is generated at each point in the optical fiber FUT. For example, when the measurement light LM2 reaches point z1 shown in FIG. 1, Rayleigh scattered light LS21 is generated; when it reaches point z2 shown in FIG. 1, Rayleigh scattered light LS22 is generated (time t9); and when it reaches point z3, Rayleigh scattered light LS23 is generated. These Rayleigh scattered lights LS21, LS22, and LS23 propagate in the opposite direction to the propagation direction of the measurement light LM2. Rayleigh scattered light LS21 reaches one end of the optical fiber FUT at time t8, Rayleigh scattered light LS22 reaches one end of the optical fiber FUT at time t10, and Rayleigh scattered light LS23 reaches one end of the optical fiber FUT at time t32.

The Rayleigh scattered light LS that has reached one end of the optical fiber FUT under test is incident on the optical multiplexer 16 via the optical circulator 15, where it is multiplexed with the reference light LR and then optically heterodyne detected by the photodetector 17, resulting in the detection signal S1 shown in Figure 3(b). Component A1 contained in the detection signal S1 is a component resulting from the Rayleigh scattered light that occurred at point z1 shown in Figure 1. Component A2 contained in the detection signal S1 is a component resulting from the Rayleigh scattered light that occurred at point z2 shown in Figure 1. Component A3 contained in the detection signal S1 is a component resulting from the Rayleigh scattered light that occurred at point z3, which is farther away than point z2 shown in Figure 1.

The detection signal S1 output from the photodetector 17 is frequency-converted by the frequency converter 18b in the frequency conversion unit 18 using the signal S20 output from the frequency generator 18a. In this embodiment, as shown in FIG. 3(c), the timing at which the signal S20 is output from the frequency converter 18b is set to time t2. That is, output of the signal S20 begins at time t2, when the Rayleigh scattered light LS11 generated when the measurement light LM1 reaches point z1 shown in FIG. 1 reaches one end of the optical fiber FUT under test. The frequency of the signal S20 is f21 at time t2 and f22 at time t32. Time t32 is the time after time t10, i.e., the time from time 0 to time t2, has elapsed.

When the detection signal S1 shown in FIG. 3(b) is frequency-converted using the signal S20 shown in FIG. 3(c), the signal S2 shown in FIG. 3(d) is obtained. For example, the component B1 having a constant frequency f30 between times t2 and t8 (during time t _scan ) contained in the signal S2 is the frequency-converted component A1 shown in FIG. 3(b). Furthermore, the component B2 having a constant frequency f31 between times t6 and t10 (during time t _scan ) contained in the signal S2 is the frequency-converted component A2 shown in FIG. 3(b). Furthermore, the component B3 having a constant frequency f32 between times t31 and t32 (during time t _scan ) contained in the signal S2 is the frequency-converted component A3 shown in FIG. 3(b).

The signal S2 obtained by the frequency conversion unit 18 is input to the measurement unit 19. Then, the same processing as in the first embodiment is performed to measure the characteristics of the optical fiber FUT under test at each measurement point (e.g., temperature distribution and strain distribution in the longitudinal direction). By repeating the processing described above, the characteristics of the optical fiber FUT under test are continuously measured, for example, from point z1 to point z3.

As described above, in this embodiment, as in the first embodiment, constant-frequency light output from the light source 11 is first split into measurement light LM and reference light LR. Next, the frequency of the measurement light LM is sequentially shifted using a signal S10 whose frequency changes sequentially, and the measurement light LM whose frequency is sequentially shifted is incident on one end of the optical fiber FUT to be measured, outputting Rayleigh scattered light LS generated within the optical fiber FUT to be measured. Next, interference light between the reference light LR and the Rayleigh scattered light LS is detected, and the frequency of the detected detection signal S1 is converted using a signal S20 whose frequency changes sequentially. The frequency-converted signal S2 is then used to measure the characteristics of the optical fiber FUT to be measured. As in the first embodiment, this allows for smaller measurement errors and longer measurement distances than conventional methods.

Furthermore, in this embodiment, the frequency generator 18a provided in the frequency conversion unit 18 can adjust the timing at which the signal S2 starts to be generated without changing the frequency range of the signal S20. In this embodiment, the measurement range of the optical fiber FUT to be measured can be shifted simply by changing the timing of the signal S20 output from the frequency generator 18a from that in the first embodiment. Specifically, while the measurement range of the optical fiber characteristic measuring device 1 in the first embodiment was, for example, from one end of the optical fiber FUT to point z2, the measurement range of the optical fiber characteristic measuring device in this embodiment is, for example, from point z1 to point z3.

Third Embodiment
Optical fiber characteristic measuring device
Fig. 4 is a block diagram showing the main configuration of an optical fiber characteristic measuring apparatus according to a third embodiment of the present invention. In Fig. 4, components similar to those shown in Fig. 1 are designated by the same reference numerals. As shown in Fig. 4, the optical fiber characteristic measuring apparatus 2 of this embodiment differs from the optical fiber characteristic measuring apparatus 1 shown in Fig. 1 in that frequency conversion units 18A, 18B, and 18C are provided instead of the frequency conversion unit 18, and a measuring unit 19A is provided instead of the measuring unit 19. The optical fiber characteristic measuring apparatus 2 has a longer measurement distance than the optical fiber characteristic measuring apparatus 1 shown in Fig. 1. It should be noted that in this embodiment, the length of the optical fiber FUT to be measured is approximately three times the length of the optical fiber FUT to be measured in the first embodiment.

Frequency conversion units 18A, 18B, and 18C have the same configuration as frequency conversion unit 18 shown in Figure 1. That is, frequency conversion units 18A, 18B, and 18C are equipped with frequency generator 18a and frequency converter 18b, and convert the frequency of detection signal S1 output from photodetector 17. Here, to distinguish between signal S2 output from frequency conversion unit 18A and signal S2 output from frequency conversion unit 18B, the former is referred to as "signal S2a" and the latter as "signal S2b." Similarly, signal S2 output from frequency conversion unit 18C is referred to as "signal S2c."

Furthermore, to distinguish between the signal S20 output from the frequency generator 18a provided in the frequency conversion unit 18A and the signal S20 output from the frequency generator 18a provided in the frequency conversion unit 18B, the former will be referred to as "signal S20a" and the latter will be referred to as "signal S20b." Similarly, the signal S20 output from the frequency generator 18a provided in the frequency conversion unit 18C will be referred to as "signal S20c." Signals S20a, S20b, and S20c are all signals whose frequencies change sequentially within the frequency range of f21 to f22 (for example, a range of 360 to 560 MHz). However, as will be described in detail later, the generation start timing of signal S20a, signal S20b, and signal S20c are adjusted to be different from one another.

The measurement unit 19A includes cutout units 21a, 21b, and 21c, Fourier transform units 22a, 22b, and 22c, and a signal processing unit 23A, and measures the characteristics of the optical fiber FUT under test using signals S2a, S2b, and S2c output from the frequency conversion unit 18. In other words, the measurement unit 19A includes cutout unit 21a and Fourier transform unit 22a corresponding to frequency conversion unit 18A, cutout unit 21b and Fourier transform unit 22b corresponding to frequency conversion unit 18B, and cutout unit 21c and Fourier transform unit 22c corresponding to frequency conversion unit 18C.

Cutting units 21a, 21b, and 21c are similar to cutting unit 21 shown in FIG. 1, and Fourier transform units 22a, 22b, and 22c are similar to Fourier transform unit 22 shown in FIG. 1. Therefore, detailed description of these units will be omitted. Signal processing unit 23A basically measures the characteristics of the optical fiber FUT under test (e.g., temperature distribution and strain distribution in the longitudinal direction) by performing the same processing as signal processing unit 23 shown in FIG. 1. However, signal processing unit 23A differs from signal processing unit 23 in that it performs the above processing using the magnitude of the frequency components determined by each of Fourier transform units 22a, 22b, and 22c.

<Optical fiber characteristic measurement method>
FIG. 5 is a diagram illustrating the operation of the optical fiber characteristic measuring apparatus according to the third embodiment of the present invention. FIG. 5(a) shows the measurement light LM and Rayleigh scattered light LS traveling through the optical fiber FUT under test. FIG. 5(b) shows the detection signal S1 output from the photodetector 17. FIG. 5(c) shows the signal S20a output from the frequency generator 18a provided in the frequency conversion unit 18A, and FIG. 5(d) shows the signal S2a output from the frequency conversion unit 18A. FIG. 5(e) shows the signal S20b output from the frequency generator 18a provided in the frequency conversion unit 18B, and FIG. 5(f) shows the signal S2b output from the frequency conversion unit 18B. FIG. 5(g) shows the signal S20c output from the frequency generator 18a provided in the frequency conversion unit 18C, and FIG. 5(h) shows the signal S2c output from the frequency conversion unit 18C. In FIGS. 5(a) and 5(b), the same components as those shown in FIGS. 2(a) and 2(b) are denoted by the same reference numerals.

As shown in Figure 5(a), at time 0, measurement light LM (measurement light LM1) with a frequency of f0 + f11 is incident on the optical fiber FUT under test, and at time ts, measurement light LM (measurement light LM2) with a frequency of f0 + f12 is incident on the optical fiber FUT under test. This is the same as in the first and second embodiments.

As the measurement light LM1 incident on the optical fiber FUT propagates through the optical fiber FUT, Rayleigh scattered light LS is generated at each point in the optical fiber FUT. For example, when the measurement light LM1 reaches point z2 shown in FIG. 4, Rayleigh scattered light LS12 is generated (time t3), when it reaches point z4 shown in FIG. 4, Rayleigh scattered light LS14 is generated (time t6), and when it reaches point z6 shown in FIG. 4, Rayleigh scattered light LS16 is generated. These Rayleigh scattered lights LS12, LS14, and LS16 propagate in the opposite direction to the propagation direction of the measurement light LM1. Rayleigh scattered light LS12 reaches one end of the optical fiber FUT at time t6, Rayleigh scattered light LS14 reaches one end of the optical fiber FUT at time t41, and Rayleigh scattered light LS16 reaches one end of the optical fiber FUT at time t61.

Similarly, as the measurement light LM2 incident on the optical fiber FUT propagates through the optical fiber FUT, Rayleigh scattered light LS is generated at each point in the optical fiber FUT. For example, when the measurement light LM2 reaches point z2 shown in Fig. 4, Rayleigh scattered light LS22 is generated (time t9), when the measurement light LM2 reaches point z4 shown in Fig. 4, Rayleigh scattered light LS24 is generated (time t10), and when the measurement light LM2 reaches point z6, Rayleigh scattered light LS26 is generated. These Rayleigh scattered lights LS22, LS24, and LS26 propagate in the opposite direction to the propagation direction of the measurement light LM2. Then, Rayleigh scattered light LS22 reaches one end of the optical fiber FUT at time t10, and Rayleigh scattered light LS24 reaches one end of the optical fiber FUT at time t42. The Rayleigh scattered light LS26 reaches one end of the optical fiber FUT to be measured at time t62, which is the time t _scan after time t61.

The Rayleigh scattered light LS that has reached one end of the optical fiber FUT under test is incident on the optical multiplexer 16 via the optical circulator 15, combined with the reference light LR, and then optically heterodyne detected by the photodetector 17, resulting in the detection signal S1 shown in Figure 5(b). Component A0 contained in the detection signal S1 is a component resulting from Rayleigh scattered light generated near one end of the optical fiber FUT under test. Component A2 contained in the detection signal S1 is a component resulting from Rayleigh scattered light generated at point z2 shown in Figure 4. Component A4 contained in the detection signal S1 is a component resulting from Rayleigh scattered light generated at point z4 shown in Figure 4. Component A6 contained in the detection signal S1 is a component resulting from Rayleigh scattered light generated at point z6 shown in Figure 4.

The detection signal S1 output from the photodetector 17 is input to frequency conversion units 18A, 18B, and 18C and frequency converted. Specifically, the detection signal S1 input to frequency conversion unit 18A is frequency converted by frequency converter 18b using signal S20a output from frequency generator 18a. Similarly, the detection signal S1 input to frequency conversion unit 18B is frequency converted by frequency converter 18b using signal S20b output from frequency generator 18a. Similarly, the detection signal S1 input to frequency conversion unit 18C is frequency converted by frequency converter 18b using signal S20c output from frequency generator 18a.

As shown in Figures 5(c), (e), and (g), the signal S20a used in the frequency conversion unit 18A, the signal S20b used in the frequency conversion unit 18B, and the signal S20c used in the frequency conversion unit 18C are adjusted to be generated at different times. As shown in Figure 5(c), the signal S20a used in the frequency conversion unit 18A is adjusted to be generated at time 0. As shown in Figure 5(e), the signal S20b used in the frequency conversion unit 18B is adjusted to be generated at time t6. Time t6 is the time when the Rayleigh scattered light LS12 generated when the measurement light LM1 reaches point z2 reaches one end of the optical fiber FUT under test. Similarly, as shown in Figure 5(g), the signal S20c used in the frequency conversion unit 18C is adjusted to be generated at time t41. Time t41 is the time when the Rayleigh scattered light LS14 generated when the measurement light LM1 reaches point z4 reaches one end of the optical fiber FUT under test.

When the detection signal S1 shown in Fig. 5(b) is frequency-converted using the signal S20a shown in Fig. 5(c), the signal S2a shown in Fig. 5(d) is obtained. For example, the component B10 contained in the signal S2a and having a constant frequency f41 between times 0 and ts (during time t _scan ) is the frequency-converted component A0 shown in Fig. 5(b). Furthermore, the component B12 contained in the signal S2a and having a constant frequency f42 between times t6 and t10 (during time t _scan ) is the frequency-converted component A2 shown in Fig. 5(b).

When the detection signal S1 shown in Fig. 5(b) is frequency-converted using the signal S20b shown in Fig. 5(e), the signal S2b shown in Fig. 5(f) is obtained. For example, the component B22 having a constant frequency f41 between times t6 and t10 (during time t _scan ) contained in the signal S2b is the frequency-converted component A2 shown in Fig. 5(b). Furthermore, the component B24 having a constant frequency f42 between times t41 and t42 (during time t _scan ) contained in the signal S2b is the frequency-converted component A4 shown in Fig. 5(b).

When the detection signal S1 shown in Fig. 5(b) is frequency-converted using the signal S20c shown in Fig. 5(g), the signal S2c shown in Fig. 5(h) is obtained. For example, a component B34 having a constant frequency f41 between times t41 and t42 (during time t _scan ) contained in the signal S2c is a frequency-converted version of the component A4 shown in Fig. 5(b). Furthermore, a component B36 having a constant frequency f42 between times t61 and t62 (during time t _scan ) contained in the signal S2c is a frequency-converted version of the component A6 shown in Fig. 5(b).

Signal S2a obtained by frequency conversion unit 18A, signal S2b obtained by frequency conversion unit 18B, and signal S2c obtained by frequency conversion unit 18C are input to measurement unit 19A. Signal S2a is extracted and Fourier transformed by extraction unit 21a and Fourier transform unit 22a, signal S2b is extracted and Fourier transformed by extraction unit 21b and Fourier transform unit 22b, and signal S2c is extracted and Fourier transformed by extraction unit 21c and Fourier transform unit 22c. The magnitudes of the frequency components corresponding to the measurement points determined by Fourier transform units 22a, 22b, and 22c are then input to signal processing unit 23A, which measures the characteristics of the optical fiber FUT under test at each measurement point (e.g., temperature distribution and strain distribution in the longitudinal direction). By repeating the above-described process, the characteristics of the optical fiber FUT under test from one end to point z6 are continuously measured, for example.

As described above, in this embodiment, as in the first embodiment, constant-frequency light output from the light source 11 is first split into measurement light LM and reference light LR. Next, the frequency of the measurement light LM is sequentially shifted using a signal S10 whose frequency changes sequentially. The measurement light LM whose frequency is sequentially shifted is then incident on one end of the optical fiber FUT under test, and Rayleigh scattered light LS generated within the optical fiber FUT under test is output. Next, interference light between the reference light LR and the Rayleigh scattered light LS is detected, and the frequency of the detected detection signal S1 is converted using a signal S20 whose frequency changes sequentially. The frequency-converted signal S2 is then used to measure the characteristics of the optical fiber FUT under test. As in the first embodiment, this allows for smaller measurement errors and longer measurement distances than conventional methods.

In addition, this embodiment provides three frequency conversion units 18A, 18B, and 18C, and provides a measurement unit 19A equipped with a cutout unit 21a and a Fourier transform unit 22a corresponding to frequency conversion unit 18A, a cutout unit 21b and a Fourier transform unit 22b corresponding to frequency conversion unit 18B, and a cutout unit 21c and a Fourier transform unit 22c corresponding to frequency conversion unit 18C. The generation start timing of signal S20a used in frequency conversion unit 18A, the generation start timing of signal S20b used in frequency conversion unit 18B, and the generation start timing of signal S20c used in frequency conversion unit 18C are made different. This allows the measurement distance to be longer than in the first and second embodiments without changing the frequency ranges of signals S20a, S20b, and S20c.

The above describes an optical fiber characteristics measuring device and an optical fiber characteristics measuring method according to embodiments of the present invention, but the present invention is not limited to the above embodiments and can be freely modified within the scope of the present invention. For example, in the first to third embodiments described above, sensitivity decreases if the polarization planes of the Rayleigh scattered light LS and the reference light LR do not match, so it is desirable to match the polarization planes of the Rayleigh scattered light LS and the reference light LR using polarization diversity, etc.

Furthermore, in the first to third embodiments, an A/D converter may be provided between the photodetector 17 and the frequency conversion unit 18 (frequency conversion units 18A, 18B, 18C) to convert the detection signal S1 from the photodetector 17 into a digital signal. In this configuration, the frequency conversion unit 18 (frequency conversion units 18A, 18B, 18C) and the measurement unit 19 (measurement unit 19A) are configured with digital circuits. Alternatively, an A/D converter may be provided between the frequency conversion unit 18 (frequency conversion units 18A, 18B, 18C) and the measurement unit 19 (measurement unit 19A) to convert the signal S2 (signals S2a, S2b, S2c) output from the frequency conversion unit 18 (frequency conversion units 18A, 18B, 18C) into a digital signal. In this configuration, the measurement unit 19 (measurement unit 19A) is configured with a digital circuit.

Furthermore, in the above-described first to third embodiments, the frequency range of the signal S20 (signals S20a, S20b, S20c) used in the frequency conversion unit 18 (frequency conversion units 18A, 18B, 18C) is constant (frequencies f21 to f22). However, the frequency range of the signal S20 (signals S20a, S20b, S20c) may be expanded independently of the time t _scan required to change the frequency of the signal S10 used in the frequency shift unit 13 from f11 to f12. Expanding the frequency range of the signal S20 (signals S20a, S20b, S20c) also makes it possible to increase the measurement distance.

Furthermore, in the above third embodiment, an example was described in which the length of the optical fiber FUT under test ranges, for example, from one end of the optical fiber FUT under test to point z6. However, even after time t62, the measurement distance range can be further extended by repeatedly generating signals S20a, S20b, and S20c at the timings shown in Figures 5(c), (e), and (g).

1, 2 Optical fiber characteristic measuring device 11 Light source 12 Optical branching device 13 Frequency shift unit 14 Optical switch 15 Optical circulator 17 Photodetector 18 Frequency conversion unit 18A, 18B, 18C Frequency conversion unit 18a Frequency generator 19, 19A Measuring unit 21 Cutting unit 21a, 21b, 21c Cutting unit 22 Fourier transform unit 22a, 22b, 22c Fourier transform unit 23, 23A Signal processing unit FUT Optical fiber to be measured LM Measurement light LR Reference light LS Rayleigh scattered light S1 Detection signal S2 Signal S2a, S2b, S2c Signal S10 Signal S20 Signal S20a, S20b, S20c Signal t _Scan time

Claims (8)

a light source that outputs light of a constant frequency;
a first branching unit that branches the light output from the light source into measurement light and reference light;
a frequency shifter that sequentially shifts the frequency of the measurement light using a first signal whose frequency sequentially changes;
a second branching section that inputs the measurement light, the frequency of which is sequentially shifted, into one end of the optical fiber under test and outputs Rayleigh scattered light generated within the optical fiber under test;
a detection unit that detects interference light between the reference light and the Rayleigh scattered light;
a frequency conversion unit that converts the frequency of the detection signal output from the detection unit using a second signal whose frequency changes sequentially;
a measurement unit that measures the characteristics of the optical fiber under test using the signal output from the frequency conversion unit;
An optical fiber characteristic measuring device comprising: a frequency change rate of the first signal and a frequency change rate of the second signal are the same;
the frequency range of the second signal is wider than the frequency range of the first signal;
2. The optical fiber characteristic measuring device according to claim 1. the first signal is a signal whose frequency change ends at a predetermined reference time,
3. The optical fiber characteristic measuring device according to claim 1, further comprising a pulsing unit that converts the measurement light output from the frequency shift unit into an optical pulse having a pulse width equal to the reference time. the measurement unit includes a cutting unit that cuts out a portion of the signal output from the frequency conversion unit according to a measurement point set in the optical fiber to be measured;
a Fourier transform unit that performs a Fourier transform on the portion cut out by the cutout unit to obtain the magnitude of the frequency component corresponding to the measurement point;
4. The optical fiber characteristic measuring device according to claim 3, comprising: An optical fiber characteristic measuring device as described in claim 4, wherein the portion cut out by the cutout unit has the same time width as the reference time. the frequency conversion unit includes a signal generator that generates the second signal;
the signal generator is capable of adjusting a timing at which the second signal starts to be generated;
5. The optical fiber characteristic measuring device according to claim 4. The frequency conversion unit is provided in plurality,
the measurement unit includes a plurality of the extraction units and the Fourier transform units corresponding to the plurality of the frequency conversion units,
the signal generators provided in the plurality of frequency conversion units have different generation start timings of the second signals;
7. The optical fiber characteristic measuring device according to claim 6. a first step of outputting light of a constant frequency;
a second step of splitting the light into a measurement light and a reference light;
a third step of sequentially shifting the frequency of the measurement light using a first signal whose frequency is sequentially changed;
a fourth step of inputting the measurement light whose frequency is sequentially shifted into one end of an optical fiber under test and outputting Rayleigh scattered light generated in the optical fiber under test;
a fifth step of detecting interference light between the reference light and the Rayleigh scattered light;
a sixth step of converting the frequency of the detection signal detected in the fifth step using a second signal whose frequency is sequentially changed;
a seventh step of measuring the characteristics of the optical fiber under test using the signal whose frequency has been converted in the sixth step;
An optical fiber characteristic measuring method comprising: