Optical fiber characteristic measuring device and optical fiber characteristic measuring method

Abstract

To provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method that have a smaller measurement error than before and can extend a measurement distance.SOLUTION: An optical fiber characteristic measuring device 1 comprises: an optical divider 12 that divides light with a constant frequency output from a light source 11 into measurement light LM and reference light LR; a frequency shift unit 13 that causes the frequency of the measurement light LM to sequentially shift by using a signal S10 whose frequency sequentially changes; an optical circulator 15 that makes the measurement light LM incident from one end of an optical fiber to be measured FUT, and outputs Rayleigh scattered light LS generated in the optical fiber to be measured FUT; a photodetector 17 that detects interference light of the reference light LR and the Rayleigh scattered light LS; a frequency conversion unit 18 that converts the frequency of a detection signal S1 output from the photodetector 17 by using a signal S20 whose frequency sequentially changes; and a measuring unit 19 that measures the characteristics of the optical fiber to be measured FUT by using the signal S2 output from the frequency conversion unit 18.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] An optical fiber characteristic measuring device is a device that irradiates continuous light or pulsed light into an optical fiber under test, detects scattered light 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 light or reflected light changes depending on 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.

[0003] One such optical fiber characteristic measuring device is an OFDR (Optical Frequency Domain Reflectometry) type. This OFDR type optical fiber characteristic measuring 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 obtained detection signal is then analyzed to measure the characteristics of the optical fiber under test in the longitudinal direction. For more information on OFDR type optical fiber characteristic measuring devices, see, for example, Non-Patent Document 1 below. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] 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 Summary of the Invention [Problem to be solved by the invention]

[0005] 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 a nonlinear frequency sweep characteristic and a wider linewidth than a light source that outputs 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 under test and the reference light, which limits the measurement distance.

[0006] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical fiber characteristic measuring device and an optical fiber characteristic measuring method which have smaller measurement errors and can measure longer distances than conventional devices. [Means for solving the problem]

[0007] In order to solve the above problems, an optical fiber characteristic measuring device (1, 2) according to a first aspect of the present invention includes 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 a frequency shifting unit (13) that inputs the measurement light whose frequency is sequentially shifted into one end of an optical fiber under test (FUT) and shifts the measurement light in the optical fiber under test. The optical fiber measuring device includes a second branching section (15) that outputs the generated Rayleigh scattered light (LS), a detection section (17) that detects the interference light between the reference light and the Rayleigh scattered light, a frequency conversion section (18, 18A, 18B) that converts the frequency of a detection signal (S1) output from the detection section using second signals (S20, S20a, S20b) whose frequency changes sequentially, and a measurement section (19, 19A) that measures the characteristics of the optical fiber to be measured using signals (S2, S2a, S2b) output from the frequency conversion section.

[0008] Furthermore, an optical fiber characteristic measuring device according to a 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.

[0009] Furthermore, an optical fiber characteristic measuring apparatus according to a third aspect of the present invention is the optical fiber characteristic measuring apparatus according to the first or second aspect of the present invention, wherein the first signal is transmitted at a predetermined reference time (t scan ) is a signal in which one frequency change ends at the reference time, and a pulsing unit (14) is provided that converts the measurement light output from the frequency shift unit into an optical pulse having a pulse width equal to the reference time.

[0010] Furthermore, an optical fiber characteristic measuring device according to a fourth aspect of the present invention is the optical fiber characteristic measuring device according to the third aspect of the present invention, wherein the measuring unit comprises a cutting unit (21, 21a, 21b) that cuts out a portion corresponding to a measurement point set on the optical fiber to be measured 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 cut out by the cutting unit to determine the magnitude of the frequency component corresponding to the measurement point.

[0011] Furthermore, an optical fiber characteristic measuring apparatus according to a fifth aspect of the present invention is the optical fiber characteristic measuring apparatus 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.

[0012] Furthermore, an 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.

[0013] Furthermore, an 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, wherein a plurality of the frequency conversion units are provided, the measuring unit is provided with a plurality of the cutout units and the Fourier transform unit corresponding to the plurality of the frequency conversion units, and the timings at which the second signals start to be generated in the signal generators provided in the plurality of the frequency conversion units are different from one another.

[0014] Furthermore, an optical fiber characteristic measuring method according to a first aspect of the present invention includes 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 from one end of an optical fiber under test (FUT) and outputting Rayleigh scattered light (LS) 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 (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. [Effects of the Invention]

[0015] According to the present invention, there are advantages that the measurement error is smaller than in the past and the measurement distance can be increased. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a block diagram showing the configuration of a main part of an optical fiber characteristic measuring apparatus according to a first embodiment of the present invention. [Figure 2] FIG. 2 is a diagram for explaining the operation of the optical fiber characteristic measuring apparatus according to the first embodiment of the present invention. [Figure 3] 5A and 5B are diagrams for explaining the operation of the optical fiber characteristic measuring apparatus according to the second embodiment of the present invention. [Figure 4] 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. [Figure 5] FIG. 10 is a diagram for explaining the operation of the optical fiber characteristic measuring apparatus according to the third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an optical fiber characteristic measuring device and an optical fiber characteristic measuring method according to embodiments of the present invention will be described in detail with reference to the drawings. First, an overview of the embodiments of the present invention will be described, followed by a detailed description of each embodiment of the present invention.

[0018] 〔overview〕 The present invention provides an optical fiber characteristic measurement device that reduces measurement errors and extends measurement distances compared to conventional devices. Specifically, in an OFDR optical fiber characteristic measurement device, measurement errors are reduced by using a light source that outputs light of a constant frequency with a narrower linewidth than a frequency-tunable light source. Furthermore, the present invention provides a longer measurement distance compared to conventional devices 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.

[0019] 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, which causes 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.

[0020] In addition, conventional OFDR optical fiber characteristic measuring devices split the light (frequency-swept light) output from a frequency-tunable light source 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 short, and the spectral width after Fourier transform becomes wide, 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.

[0021] Furthermore, the frequency-tunable light source used in conventional OFDR optical fiber characteristic measurement devices often has a wider linewidth and a shorter coherence time than a light source that outputs light of a fixed frequency, which is disadvantageous in terms of measurement distance or measurement accuracy after Fourier transform.

[0022] In this embodiment, first, light of a constant frequency output from a light source is 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 the 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 allows for smaller measurement errors and longer measurement distances than conventional methods.

[0023] [First embodiment] Optical fiber characteristic measuring device Fig. 1 is a block diagram showing the configuration of the main parts 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 section), a frequency shifting section 13, an optical switch 14 (pulsing section), an optical circulator 15 (second branching section), an optical multiplexer 16, a photodetector 17 (detecting section), a frequency converting section 18, and a measuring section 19. Such an optical fiber characteristic measuring apparatus 1 measures the characteristics (e.g., temperature distribution, strain distribution, etc.) in the length direction (z direction) of the optical fiber FUT to be measured.

[0024] The light source 11 emits light with a constant frequency f0 and high coherence. For example, a semiconductor laser that emits laser light with a wavelength (e.g., 1.55 μm) that is less absorbed by the optical fiber FUT to be measured can be used as this light source 11. It is desirable that the light emitted from the light source 11 has as narrow a linewidth (spectral linewidth) as possible to improve the measurement accuracy of the optical fiber characteristic measuring device 1.

[0025] 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 the reference light LR is, for example, 1:1. However, the intensity ratio between the measurement light LM and the reference light LR is not limited to 1:1 and can be any intensity ratio.

[0026] The frequency shift unit 13 includes a frequency generator 13a and an optical frequency shifter 13b, and sequentially shifts the frequency of the measuring 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 of f11 to f12 (for example, 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). That is, the signal S10 is scan This is a signal in which one frequency change ends at this point, and this frequency change is repeated.

[0027] The optical frequency shifter 13b sequentially shifts the frequency of the measuring 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 measuring 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.

[0028] The optical switch 14 cuts or connects 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 connects 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 pulses the measurement light LM output from the frequency shifter 13 for a period of time t scanIf necessary, an optical amplifier may be provided between the optical switch 14 and the optical circulator 15.

[0029] 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.

[0030] The optical multiplexer 16 multiplexes the Rayleigh scattered light LS output from the optical circulator 15 and the reference light LR branched by the optical brancher 12. For example, an optical coupler can be used as this optical multiplexer 16. The photodetector 17 performs optical heterodyne detection of the light multiplexed by the optical multiplexer 16 and outputs a detection signal S1.

[0031] Here, the Rayleigh scattered light LS is the measurement light LM that is incident on the optical fiber FUT under test and 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 is scattered over a certain period of time (time t scan ), the frequency of which changes sequentially from f0+f11 to f0+f12. When this Rayleigh scattered light LS is combined with the reference light LR and optically heterodyne detected, it becomes a component whose frequency changes sequentially from f11 to f12. The detection signal S1 output from the photodetector 17 is caused by this Rayleigh scattered light LS generated at each point in the measured optical fiber FUT, and includes a component whose frequency changes sequentially from f11 to f12.

[0032] 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 the signal S20 whose frequency changes sequentially within a frequency range of frequencies f21 to f22 (for example, a range of 360 to 560 MHz).

[0033] Here, the rate of change of frequency of signal S20 generated by frequency generator 18a is the same as the rate of change of frequency of signal S10 generated by frequency generator 13a provided in frequency shift unit 13. Furthermore, the frequency range of signal S20 generated by frequency generator 18a is wider than the frequency range of signal S10 generated by frequency generator 13a provided in frequency shift unit 13. In this embodiment, the frequency range of signal S20 is twice as wide as the frequency range of signal S10. Therefore, the time required for frequency generator 18a to change the frequency of signal S20 from frequency f21 to frequency f22 is longer than the time t required for frequency generator 13a to change the frequency of signal S10 from frequency f11 to frequency f12. scan That is, the signal S20 is longer than the time t scan It is a signal in which one frequency change is completed in a time longer than the normal frequency, and such frequency changes are repeated.

[0034] The frequency converter 18b uses the signal S20 output from the frequency generator 18a to convert the frequency of the detection signal S1 output from the photodetector 17. Specifically, the frequency converter 18b converts the frequency of the detection signal S1 so that the components caused by the Rayleigh scattered light LS generated at each point in the measured optical fiber FUT have different frequencies. For example, the frequency converter 18b converts the frequency of the detection signal S1 so that the component caused by the Rayleigh scattered light LS generated at point z1 shown in Figure 1 has a constant frequency f31, and the component caused by the Rayleigh scattered light LS generated at point z2 has a constant frequency f32.

[0035] 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 length 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 the characteristics are to be measured) set on the optical fiber FUT under test. Here, the portion cut out by the cutout unit 21 is a constant value over time t scan has the same time span as

[0036] 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 cut out by the cutout 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 to be measured (for example, temperature distribution, strain distribution, etc. in the longitudinal direction).

[0037] <Optical fiber characteristic measurement method> When measurement begins, light of a constant frequency f0 is emitted from light source 11 (first step). The light emitted from light source 11 is split into measurement light LM and reference light LR by optical splitter 12 (second step). The measurement light LM split by optical splitter 12 enters frequency shifter 13, where its frequency is sequentially shifted (third step). 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.

[0038] The frequency-shifted measuring light LM is pulsed by the optical switch 14. Specifically, the measuring light LM is pulsed for a time t scan The pulse width is then reduced to a light pulse having a width equal to the

[0039] The pulsed measurement light LM is incident on 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 of the optical fiber FUT under test. A portion of the Rayleigh scattered light LS generated at each point of the optical fiber FUT under test is incident on the optical circulator 15 from one end of the optical fiber FUT under test and output to the optical multiplexer 16 (fourth step).

[0040] FIG. 2 is a diagram for explaining the operation of the optical fiber characteristic measuring apparatus according to the first embodiment of the present invention. FIG. 2(a) is a diagram showing the measurement light LM and Rayleigh scattered light LS traveling through the optical fiber FUT under test. FIG. 2(b) is a diagram showing the detection signal S1 output from the photodetector 17, FIG. 2(c) is a diagram showing the signal S20 output from the frequency generator 18a, and FIG. 2(d) is a diagram showing the signal S2 output from the frequency conversion unit 18. The graph shown in FIG. 2(a) has time on the horizontal axis and position in the longitudinal direction of the optical fiber FUT under test on the vertical axis. The graphs shown in FIGS. 2(b) to 2(d) all have time on the horizontal axis and frequency on the vertical axis.

[0041] As shown in Fig. 2(a), it is assumed that the measurement light LM with a frequency of f0+f11 is incident on the optical fiber FUT under test at time 0, and the 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 from time 0 to time t scan For convenience of explanation, the measurement light LM incident on the optical fiber FUT under test at time ts 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.

[0042] As the measurement light LM1 incident on the optical fiber FUT under test propagates through the optical fiber FUT under test, 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 FIG. 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 under test at time t2. When the measurement light LM1 reaches point z2 shown in FIG. 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 under test at time t6.

[0043] Similarly, as the measurement light LM2 incident on the optical fiber FUT under test propagates through the optical fiber FUT under test, Rayleigh scattered light LS is generated at each point in the optical fiber FUT under test. For example, when the measurement light LM2 reaches point z1 shown in FIG. 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 under test at time t8. Furthermore, when the measurement light LM2 reaches point z2 shown in FIG. 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 under test at time t10.

[0044] The Rayleigh scattered light LS that has reached one end of the optical fiber FUT to be measured is incident on the optical multiplexer 16 via the optical circulator 15, and is multiplexed with the reference light LR that has been branched by the optical brancher 12, and then optically heterodyne detected by the photodetector 17 (fifth step). That is, the photodetector 17 detects the interference light between the Rayleigh scattered light LS that has reached one end of the optical fiber FUT to be measured and the reference light LR that has been branched by the optical brancher 12.

[0045] Here, as described above, the Rayleigh scattered light LS is the measurement light LM that is incident on the optical fiber FUT under test and is elastically scattered (scattered without a change in wavelength). For this reason, the Rayleigh scattered light LS generated at each point in the optical fiber FUT under test is scattered over a certain period of time (time tscan ), the frequency of which changes sequentially from f0+f11 to f0+f12. When this Rayleigh scattered light LS is combined with the reference light LR and optically heterodyne detected, it becomes a component whose frequency changes sequentially from f11 to f12. The detection signal S1 output from the photodetector 17 is caused by this Rayleigh scattered light LS generated at each point in the measured optical fiber FUT, and includes a component whose frequency changes sequentially from f11 to f12.

[0046] For example, the component obtained by optical heterodyne detection of Rayleigh scattered light generated near one end of the optical fiber FUT under test after being combined with the reference light LR is shown in FIG. 2(b) between time 0 and ts (time t scan 2(b), the frequency of the component A0 changes from f11 to f12 in sequence between times t2 and t8 (time t scan 2(b), the component A1 has a frequency that changes sequentially from f11 to f12 during the time period from t6 to t10 (time t scan The frequency component A2 changes sequentially from f11 to f12 in the period between f11 and f12.

[0047] The detection signal S1 output from the photodetector 17 is input to the frequency converter 18, where the frequency is converted (sixth step). 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 measured optical fiber FUT have different frequencies.

[0048] As described above, the rate of change of frequency of signal S20 generated by frequency generator 18a is the same as the rate of change of frequency of signal S10 generated by frequency generator 13a provided in frequency shift unit 13. As described above, the Rayleigh scattered light LS is the measurement light LM that is incident on the optical fiber FUT under test and is elastically scattered (scattered without a change in wavelength), and therefore the rate of change of frequency of Rayleigh scattered light LS generated at each point in the optical fiber FUT under test is the same as the rate of change of frequency of signal S10.

[0049] 2(b) and (c), the frequency change rate of the signal S20 is the same as the frequency change rate of the component included in the detection signal S1 and caused by the Rayleigh scattered light LS generated at each point in the optical fiber FUT under test. Therefore, when the detection signal S1 shown in FIG. 2(b) is frequency converted using the signal S20 shown in FIG. 2(c), the signal S2 shown in FIG. 2(d) is obtained. For example, the frequency change rate of the component included in the signal S2 between time 0 and ts (time t scan 2(b) is obtained by converting the frequency of the component A0 shown in FIG. 2(b). scan 2(b) is obtained by converting the frequency of the component A1 shown in FIG. 2(b). scan Component B2, which has a constant frequency f32 between (between (a) and (b)), is obtained by converting the frequency of component A2 shown in FIG. 2(b).

[0050] The signal S2 obtained by the frequency conversion unit 18 is input to the measurement unit 19. Then, the measurement unit 19 performs a process of measuring the characteristics of the optical fiber FUT to be measured (for example, temperature distribution and strain distribution in the length direction) (seventh step). When the signal S2 is input to the measurement unit 19, first, a process of cutting out a portion of the signal S2 corresponding to the measurement point set on the optical fiber FUT to be measured is performed by the cutting unit 21.

[0051] For example, when the measurement point is set at point z1 shown in Fig. 1, the cutout unit 21 cuts out the portion of the signal S2 shown in Fig. 2(d) between times t2 and t8. When the measurement point is set at point z2 shown in Fig. 1, the cutout unit 21 cuts out the portion of the signal S2 shown in Fig. 2(d) between times t6 and t10. Note that the portion cut out by the cutout unit 21 is the portion of the signal S2 shown in Fig. 2(d) between times t6 and t10, regardless of the position of the measurement point. scan has the same time span as

[0052] Next, a 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 component corresponding to each of the measurement points. Subsequently, a signal processing unit 23 performs a process of measuring the characteristics (for example, temperature, strain, etc.) of the optical fiber FUT to be measured at each of the measurement points by performing a predetermined signal processing using the magnitude of the frequency component determined by the Fourier transform unit 22. By repeating the process described above, for example, the characteristics of the optical fiber FUT to be measured from one end to point z2 are continuously measured.

[0053] As described above, in this embodiment, first, light of a constant frequency output from the light source 11 is 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 sequentially changes, and the measurement light LM whose frequency sequentially shifts is made incident on one end of the optical fiber FUT to be measured, and Rayleigh scattered light LS generated within the optical fiber FUT to be measured 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 sequentially changes. The frequency-converted signal S2 is then used to measure the characteristics of the optical fiber FUT to be measured.

[0054] In this embodiment, the signal S10 output from the frequency generator 13a is used to shift the frequency of the measurement light LM. Generally, it is easy to generate an electrical signal whose frequency changes with high accuracy 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 accuracy and linearity, thereby reducing measurement errors.

[0055] 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.

[0056] Furthermore, in this embodiment, constant frequency light is used, 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, and therefore the accuracy of the frequency after Fourier transform can be increased. In addition, by appropriately selecting the frequencies of the signal S10 used in the frequency shifter 13 and the signal S20 used in the frequency converter 18, the frequencies of the detection signal S1 of the photodetector 17 and the signal S2 output from the frequency converter 18 can be made easy to handle.

[0057] As described above, in this embodiment, the measurement error is smaller and the measurement distance can be longer than in the past. For example, a conventional optical fiber characteristic measurement device using OFDR has an extremely high spatial resolution of about several tens of μm, but the measurement distance is limited to about several tens of meters. In contrast, the optical fiber characteristic measurement device 1 of this embodiment has an inferior spatial resolution compared to the past, but can achieve a measurement distance of 1 km or more.

[0058] 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, the frequency generator 18a provided in the frequency conversion unit 18 is capable of adjusting the timing at which the signal S2 starts to be generated. Then, by making the timing of the signal S20 output from the frequency generator 18a different from that of the first embodiment, the measurement range of the optical fiber FUT to be measured is shifted.

[0059] FIG. 3 is a diagram for explaining the operation of the optical fiber characteristic measuring apparatus according to the second embodiment of the present invention. Note that FIGS. 3(a) to 3(d) are similar to FIGS. 2(a) to 2(d). That is, FIG. 2(a) is a diagram showing the measurement light LM and the Rayleigh scattered light LS traveling through the optical fiber FUT under test. FIG. 3(b) is a diagram showing the detection signal S1 output from the photodetector 17, FIG. 3(c) is a diagram showing the signal S20 output from the frequency generator 18a, and FIG. 3(d) is a diagram showing the signal S2 output from the frequency conversion unit 18. Note that in FIGS. 3(a) to 3(d), the same components as those shown in FIGS. 2(a) to 2(d) are denoted by the same reference numerals.

[0060] 3(a), it is assumed that measurement light LM (measurement light LM1) with a frequency of f0+f11 is incident on the optical fiber FUT under test at time 0, and measurement light LM (measurement light LM2) with a frequency of f0+f12 is incident on the optical fiber FUT under test at time ts, as in the first embodiment.

[0061] As the measurement light LM1 incident on the optical fiber FUT under test propagates through the optical fiber FUT under test, 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 Fig. 1, Rayleigh scattered light LS11 is generated, when it reaches point z2 shown in Fig. 1, Rayleigh scattered light LS12 is generated (time t3), and when it reaches point z3, which is farther away from point z2 shown in Fig. 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, the Rayleigh scattered light LS11 reaches one end of the optical fiber FUT under test at time t2, the Rayleigh scattered light LS12 reaches one end of the optical fiber FUT under test at time t6, and the Rayleigh scattered light LS13 reaches one end of the optical fiber FUT under test at time t31.

[0062] Similarly, as the measurement light LM2 incident on the optical fiber FUT under test propagates through the optical fiber FUT under test, Rayleigh scattered light LS is generated at each point in the optical fiber FUT under test. For example, when the measurement light LM2 reaches point z1 shown in FIG. 1, Rayleigh scattered light LS21 is generated, when the measurement light LM2 reaches point z2 shown in FIG. 1, Rayleigh scattered light LS22 is generated (time t9), and when the measurement light LM2 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. Then, Rayleigh scattered light LS21 reaches one end of the optical fiber FUT under test at time t8, Rayleigh scattered light LS22 reaches one end of the optical fiber FUT under test at time t10, and Rayleigh scattered light LS23 reaches one end of the optical fiber FUT under test at time t32.

[0063] The Rayleigh scattered light LS that has reached one end of the optical fiber FUT to be measured is incident on the optical multiplexer 16 via the optical circulator 15, and is multiplexed with the reference light LR, and then optically heterodyne detected by the photodetector 17, thereby obtaining the detection signal S1 shown in Fig. 3(b). The component A1 included in the detection signal S1 is a component caused by the Rayleigh scattered light generated at point z1 shown in Fig. 1. The component A2 included in the detection signal S1 is a component caused by the Rayleigh scattered light generated at point z2 shown in Fig. 1. The component A3 included in the detection signal S1 is a component caused by the Rayleigh scattered light generated at point z3, which is farther away than point z2 shown in Fig. 1.

[0064] 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, the 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. Note that time t32 is the time after time t10, i.e., the time from time 0 to time t2 has elapsed.

[0065] 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 period between times t2 and t8 (time t scan Component B1 having a constant frequency f30 between times t6 and t10 (time t scan Component B2 having a constant frequency f31 between times t31 and t32 (time t scan Component B3, which has a constant frequency f32 between (between (a) and (b)), is obtained by converting the frequency of component A3 shown in FIG. 3(b).

[0066] 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 (for example, temperature distribution, strain distribution, etc. in the length direction). By repeating the processing described above, for example, the characteristics of the optical fiber FUT under test from point z1 to point z3 are continuously measured.

[0067] As described above, in this embodiment, as in the first embodiment, first, constant-frequency light output from the light source 11 is 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 is sequentially changed, and the measurement light LM whose frequency is sequentially shifted is incident on one end of the optical fiber FUT under test, outputting Rayleigh scattered light LS generated within the optical fiber FUT under test. 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 is sequentially changed. The characteristics of the optical fiber FUT under test are then measured using the frequency-converted signal S2. As in the first embodiment, this allows for smaller measurement errors and longer measurement distances than conventional methods.

[0068] 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, the measurement range of the optical fiber characteristic measuring apparatus 1 in the first embodiment was, for example, from one end of the optical fiber FUT to point z2, but the measurement range of the optical fiber characteristic measuring apparatus in this embodiment is, for example, from point z1 to point z3.

[0069] 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 assigned 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.

[0070] The frequency conversion units 18A, 18B, and 18C have the same configuration as the frequency conversion unit 18 shown in FIG. 1. That is, the frequency conversion units 18A, 18B, and 18C include a frequency generator 18a and a frequency converter 18b, and convert the frequency of the detection signal S1 output from the photodetector 17. Here, to distinguish between the signal S2 output from the frequency conversion unit 18A and the signal S2 output from the frequency conversion unit 18B, the former will be referred to as "signal S2a" and the latter will be referred to as "signal S2b." Similarly, the signal S2 output from the frequency conversion unit 18C will be referred to as "signal S2c."

[0071] 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 the "signal S20a" and the latter will be referred to as the "signal S20b." Similarly, the signal S20 output from the frequency generator 18a provided in the frequency conversion unit 18C will be referred to as the "signal S20c." The signals S20a, S20b, and S20c are all signals whose frequencies change sequentially within a 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 timings of the signals S20a, S20b, and S20c are adjusted to be different from one another.

[0072] 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 to be measured using signals S2a, S2b, and S2c output from the frequency conversion unit 18. That is, the measurement unit 19A includes the cutout unit 21a and Fourier transform unit 22a corresponding to the frequency conversion unit 18A, the cutout unit 21b and Fourier transform unit 22b corresponding to the frequency conversion unit 18B, and the cutout unit 21c and Fourier transform unit 22c corresponding to the frequency conversion unit 18C.

[0073] The extracting units 21a, 21b, and 21c are similar to the extracting unit 21 shown in Fig. 1, and the Fourier transforming units 22a, 22b, and 22c are similar to the Fourier transforming unit 22 shown in Fig. 1. Therefore, detailed description thereof will be omitted. The signal processing unit 23A basically measures the characteristics of the optical fiber FUT to be measured (for example, temperature distribution and strain distribution in the length direction) by performing the same processing as the signal processing unit 23 shown in Fig. 1. However, the signal processing unit 23A differs from the signal processing unit 23 in that it performs the above processing using the magnitudes of the frequency components determined by each of the Fourier transforming units 22a, 22b, and 22c.

[0074] <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) illustrates the measurement light LM and Rayleigh scattered light LS traveling through the optical fiber FUT under test. FIG. 5(b) illustrates the detection signal S1 output from the photodetector 17. FIG. 5(c) illustrates the signal S20a output from the frequency generator 18a provided in the frequency conversion unit 18A, and FIG. 5(d) illustrates the signal S2a output from the frequency conversion unit 18A. FIG. 5(e) illustrates the signal S20b output from the frequency generator 18a provided in the frequency conversion unit 18B, and FIG. 5(f) illustrates the signal S2b output from the frequency conversion unit 18B. FIG. 5(g) illustrates the signal S20c output from the frequency generator 18a provided in the frequency conversion unit 18C, and FIG. 5(h) illustrates the signal S2c output from the frequency conversion unit 18C. In addition, 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.

[0075] 5(a), it is assumed that measurement light LM (measurement light LM1) with a frequency of f0+f11 is incident on the optical fiber FUT under test at time 0, and measurement light LM (measurement light LM2) with a frequency of f0+f12 is incident on the optical fiber FUT under test at time ts, as in the first and second embodiments.

[0076] As the measurement light LM1 incident on the optical fiber FUT under test propagates through the optical fiber FUT under test, 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 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. Then, Rayleigh scattered light LS12 reaches one end of the optical fiber FUT under test at time t6, Rayleigh scattered light LS14 reaches one end of the optical fiber FUT under test at time t41, and Rayleigh scattered light LS16 reaches one end of the optical fiber FUT under test at time t61.

[0077] Similarly, as the measurement light LM2 incident on the optical fiber FUT under test propagates through the optical fiber FUT under test, Rayleigh scattered light LS is generated at each point in the optical fiber FUT under test. For example, when the measurement light LM2 reaches point z2 shown in FIG. 4, Rayleigh scattered light LS22 is generated (time t9), when it reaches point z4 shown in FIG. 4, Rayleigh scattered light LS24 is generated (time t10), and when it reaches point z6, Rayleigh scattered light LS26 is generated. These Rayleigh scattered light 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 under test at time t10, and Rayleigh scattered light LS24 reaches one end of the optical fiber FUT under test at time t42. The Rayleigh scattered light LS26 propagates from time t61 to time t scan At time t62, after the passage of time t61, the optical fiber FUT reaches one end thereof.

[0078] 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, and is multiplexed with the reference light LR. After that, the optical heterodyne detection is performed by the photodetector 17, resulting in the detection signal S1 shown in FIG. 5(b). The component A0 included in the detection signal S1 is a component resulting from the Rayleigh scattered light generated near one end of the optical fiber FUT under test. The component A2 included in the detection signal S1 is a component resulting from the Rayleigh scattered light generated at point z2 shown in FIG. 4. The component A4 included in the detection signal S1 is a component resulting from the Rayleigh scattered light generated at point z4 shown in FIG. 4. The component A6 included in the detection signal S1 is a component resulting from the Rayleigh scattered light generated at point z6 shown in FIG. 4.

[0079] The detection signal S1 output from the photodetector 17 is input to frequency converters 18A, 18B, and 18C and frequency-converted. Specifically, the detection signal S1 input to frequency converter 18A is frequency-converted by frequency converter 18b using signal S20a output from frequency generator 18a. The detection signal S1 input to frequency converter 18B is frequency-converted by frequency converter 18b using signal S20b output from frequency generator 18a. Similarly, the detection signal S1 input to frequency converter 18C is frequency-converted by frequency converter 18b using signal S20c output from frequency generator 18a.

[0080] As shown in Figures 5(c), 5(e), and 5(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 timing of 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 timing of 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 timing of 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 the point z4 reaches one end of the optical fiber FUT to be measured.

[0081] 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 period between time 0 and time ts (time t scan Component B10 having a constant frequency f41 between times t6 and t10 (time t scan Component B12 having a constant frequency f42 between (between) is obtained by converting the frequency of component A2 shown in FIG. 5(b).

[0082] 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 period between times t6 and t10 (time t scan Component B22 having a constant frequency f41 between times t41 and t42 (time t scanComponent B24 having a constant frequency f42 between (between) is obtained by converting the frequency of component A4 shown in FIG. 5(b).

[0083] 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, the period between times t41 and t42 (time t scan Component B34 having a constant frequency f41 between times t61 and t62 (time t scan Component B36 having a constant frequency f42 between (between) is obtained by converting the frequency of component A6 shown in FIG. 5(b).

[0084] The signal S2a obtained by the frequency conversion unit 18A, the signal S2b obtained by the frequency conversion unit 18B, and the signal S2c obtained by the frequency conversion unit 18C are input to the measurement unit 19A. The signal S2a is cut out and Fourier transformed by the cutout unit 21a and the Fourier transform unit 22a, the signal S2b is cut out and Fourier transformed by the cutout unit 21b and the Fourier transform unit 22b, and the signal S2c is cut out and Fourier transformed by the cutout unit 21c and the Fourier transform unit 22c. The magnitudes of the frequency components corresponding to the measurement points determined by the Fourier transform units 22a, 22b, and 22c are then input to the signal processing unit 23A, and the characteristics of the optical fiber FUT under test at each measurement point (e.g., temperature distribution, strain distribution, etc. in the length direction) are measured. By repeating the above-described processing, for example, the characteristics of the optical fiber FUT under test from one end to point z6 are continuously measured.

[0085] As described above, in this embodiment, as in the first embodiment, first, constant-frequency light output from the light source 11 is 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 is sequentially changed, and the measurement light LM whose frequency is sequentially shifted is incident on one end of the optical fiber FUT under test, outputting Rayleigh scattered light LS generated within the optical fiber FUT under test. 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 is sequentially changed. The characteristics of the optical fiber FUT under test are then measured using the frequency-converted signal S2. As in the first embodiment, this allows for smaller measurement errors and longer measurement distances than conventional methods.

[0086] Furthermore, in this embodiment, three frequency conversion units 18A, 18B, and 18C are provided, and a measurement unit 19A is provided which includes 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.

[0087] Although the optical fiber characteristic measuring device and the optical fiber characteristic measuring method according to the embodiments of the present invention have been described above, 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, if the polarization planes of the Rayleigh scattered light LS and the reference light LR do not match, sensitivity decreases, so it is desirable to match the polarization planes of the Rayleigh scattered light LS and the reference light LR by polarization diversity or the like.

[0088] 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.

[0089] In the above 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 time t required to change the frequency of the signal S10 used in the frequency shift unit 13 from f11 to f12 is scan Independently of this, the frequency range of the signal S20 (signals S20a, S20b, S20c) may be widened. Widening the frequency range of the signal S20 (signals S20a, S20b, S20c) also makes it possible to increase the measurement distance.

[0090] In the third embodiment, the length of the optical fiber FUT under test is, for example, in the range 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 the signals S20a, S20b, and S20c at the timings shown in Figures 5(c), (e), and (g). [Explanation of symbols]

[0091] 1,2 Optical fiber characteristic measuring device 11 Light source 12 Optical splitter 13 Frequency shift section 14 Optical Switch 15 Optical Circulator 17 Photodetector 18 Frequency conversion section 18A, 18B, 18C Frequency conversion section 18a frequency generator 19,19A Measuring section 21 Cutout part 21a, 21b, 21c Cutout part 22 Fourier transform section 22a, 22b, 22c Fourier transform section 23, 23A Signal processing section FUT Optical fiber under test 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

1. 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:

2. 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.

3. 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.

4. 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:

5. 5. The optical fiber characteristic measuring device according to claim 4, wherein the portion cut out by said cutting unit has the same time width as said reference time.

6. 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.

7. 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.

8. 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:

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