JP6280445B2 - Coherent optical frequency domain reflectometry measurement system - Google Patents

Description

  The present invention relates to a coherent optical frequency domain reflectometry measurement apparatus, and more specifically, an optical frequency domain reflection measurement capable of measuring reflected light or backscattered light from an optical component or an optical transmission line with high spatial resolution. Relates to the device.

  Conventionally, optical frequency domain reflection (C-OFDR: Coherent Optical Frequency Domain Reflectometry) using coherent light as a method capable of measuring reflected light and backscattered light from optical components and optical transmission lines with high spatial resolution. There is a measurement method (for example, see Non-Patent Document 1). In this C-OFDR measurement method, coherent light that has been swept in frequency is incident on a measurement object, and reflected light and backscattered light from the measurement object and pre-branched reference light are coherently detected, and measurement obtained thereby This is a technique that enables the identification of the loss distribution and the failure point of the measurement object by obtaining the reflected light and the backscattered light intensity at an arbitrary position in the measurement object by performing frequency analysis of the beat signal.

  FIG. 1 shows the measurement principle of an optical frequency domain reflection measurement apparatus (C-OFDR). In the C-OFDR 100 shown in FIG. 1, the output optical frequency from the laser light source 12 is swept linearly with respect to time. The frequency-swept light wave A is branched into reference light C and test light B (also referred to as signal light) by a branching unit 14 such as an optical coupler. The test light B is incident on a fiber under test (FUT) 50 through the circulator 16. The test light D backscattered inside the FUT is incident on the optical frequency domain reflection measurement apparatus 100 again, guided to the coupling / branching unit 18 such as an optical coupler via the circulator 16, and multiplexed with the reference light C. Heterodyne detection is performed in the light receiving unit 20 and the analyzer 22. Here, the frequency of the beat signal generated from the reference light C and the test light D is determined by the delay time of the signal light with respect to the reference light. Since the delay time between these two light waves is proportional to the scattered light generation distance inside the FUT of the test light, frequency analysis (Fourier transform) of this beat signal causes the axial distribution of the reflected light intensity in the FUT (FUT Longitudinal distribution) can be measured.

Focusing on the reflected light at a certain point z in the FUT, the relationship between the beat frequency f _b and z generated by the reflected light from that point uses the optical frequency sweep speed γ, the refractive index n of the optical fiber, and the speed of light c. Is expressed as the following equation (1).

However, the frequency sweep speed γ is γ = ΔF / T using the frequency sweep width ΔF and the frequency sweep time T. The spatial resolution Δz is expressed as the following equation (2) using the frequency resolution ΔB.

Since the frequency resolution ΔB is limited to about the reciprocal of the sweep time, the expression (2) becomes the following expression (3).

Thus, in the C-OFDR, when the frequency sweep speed γ is not constant (when the frequency sweep is nonlinear), the spatial resolution is degraded due to the spread of the beat frequency. Furthermore, it is necessary to widen the frequency sweep width ΔF for high spatial resolution measurement. Since the beat frequency is measured using the coherency of the light wave, the long-distance measurement is realized by using the light source coherence length L _c (a value indicating the degree of coherence of light) with respect to the optical path length difference between the reference light and the reflected light. ) Must be long enough. Thus, for long-range high spatial resolution measurement by C-OFDR, it is necessary to use a light source that has a long coherence length and is linear with respect to time and capable of frequency sweeping over a wide range.

  As a method for realizing a frequency swept light source used for C-OFDR, there are a wavelength variable light source method and an external modulation method. Since the wavelength tunable light source has a wide wavelength tunable width, high spatial resolution measurement is possible, but the light source coherence length is short and is not suitable for long distance measurement. On the other hand, an optical frequency sweep light source system using a high coherent light source and an external modulator can realize optical frequency sweep without impairing the high coherence of the light source.

  In a configuration using an external modulator, a modulation sideband is generated with respect to the carrier frequency of the light source by applying sinusoidal modulation to the modulator, and the modulation side is swept linearly with respect to time. Sweep the waveband. At this time, if a plurality of modulation sidebands are generated, the beat signal spectrum generated by each modulation sideband overlaps on the frequency axis, so that a correct backscattered light distribution cannot be obtained. Therefore, as an external modulator, a carrier-suppressed double sideband (DSB-SC) modulator (see, for example, Non-Patent Document 2) or a carrier-suppressed single sideband (SSB-SC) is used. A method has been adopted in which a measurement system is configured using only a ± 1st order modulation sideband using a suppressed carrier modulator (see, for example, Non-Patent Document 3).

W. Eickhoff et al., “Optical frequency domain reflectometry in single-mode fiber,” Appl. Phys. Lett., American Institute of Physics, vol.39, no.9, pp.693-695, 1981 Yukihiro Tsuji 3 others, “Coherent OFDR using a two-electrode driven Mach-Zender modulator”, 1997 Communications Society Conference, IEICE, pp.546-547, 1997 Y. Koshikiya et al., “Long range and cm-level spatial resolution measurement using coherent optical frequency domain reflectometry with SSB-SC modulator and narrow linewidth fiber laser,” Journal of Lightwave Technology, IEEE, vol.26, issue 18, pp .3287-3294, Sep. 2008

  Conventionally, when frequency sweeping is performed using only the ± 1st order modulation sideband using an external modulator, the voltage applied to the modulator must be set to the operating point in order to prevent generation of unnecessary high order sidebands. Only used in regions that show a linear response at the center. For this reason, the power efficiency of the modulation itself is poor, and the intensity of light undergoes a large loss by passing through the modulator. For example, when an SSB-SC modulator is used as a C-OFDR external modulator, a loss of 20 dB or more is usually generated.

  When performing OFDR measurement, it is necessary to secure a certain amount of incident power in order to prevent the weak reflected light from the measured fiber from being buried in noise. For this reason, it is necessary to install an EDFA after the external modulator to compensate for the optical power loss caused by the modulator.

  However, in a phase insensitive amplifier (PIA) such as EDFA, the noise figure becomes 3 dB or more before and after light amplification even in an ideal operation state, and new noise is added. There is. This newly added noise is necessary to preserve the exchange relationship between the two before and after amplification when simultaneously amplifying two quadrature phase amplitudes of electromagnetic fields that are quantum conjugate. It is known to be inevitable noise. As a result, both the intensity noise and the phase noise increase after passing through the EDFA. In C-OFDR using the coherence of light, phase noise becomes a major factor limiting the performance of measurement. As the phase noise increases, fluctuations occur in the beat frequencies of the reference light and the reflected light, and the beat spectrum spreads. For this reason, the spatial resolution of the measurement is deteriorated. In addition, since the maximum loss that can be measured decreases due to an increase in intensity noise, the dynamic range also deteriorates.

  On the other hand, a phase sensitive optical amplifier (PSA) is known as an optical amplifier capable of avoiding a new increase in noise before and after amplification like the above-described PIA. PSA has a characteristic of giving a gain to one of two quadrature amplitudes and an attenuation to the other. FIG. 2 is a diagram showing an image on the constellation map of PIA and PSA amplification. From these characteristics, PSA can amplify light intensity without generating excessive noise in principle, and has an effect of reducing phase noise.

  The present invention has been made in view of such problems, and the object of the present invention is to use a phase-sensitive optical amplifier (PSA) as an optical amplifier for C-OFDR loss compensation using an external modulator. To provide a C-OFDR configuration method that can realize a high spatial resolution of the reflectance distribution of a measurement target and obtain a high dynamic range as compared to the case of using a conventional EDFA. It is in.

  In order to achieve such an object, a first aspect of the present invention is a coherent optical frequency domain reflectometry measurement apparatus that measures a reflectance distribution in the propagation direction of light in a measurement target. The coherent optical frequency domain reflectometry measuring device includes a branching unit that splits light from a light source that emits coherent light into first light and second light, and the optical frequency of the first light linearly with respect to time. A modulation means for sweeping and outputting the third light, a phase sensitive amplifier for amplifying the third light using the second light as excitation light, and the amplified third light as the fourth light and the second light 5th light, the 2nd branch means which branches to 5 light, the reflected light of the 4th light which injects into the measurement object as 4th light as measurement light, and was reflected by the measurement object, and 5th light A multiplexing means for combining the reference light, a means for generating an interference wave between the reflected light and the reference light, a means for optically detecting the interference wave to obtain an interference signal, and a frequency analysis of the interference signal Analyzing means. The coherent optical frequency domain reflectometry measurement apparatus is characterized in that the phase sensitive amplifier is a phase sensitive optical amplifier that utilizes the effect of parametric amplification.

In one embodiment, the coherent optical frequency domain reflectometry measurement apparatus may be configured such that the modulation means suppresses the carrier wave and generates a first-order double sideband centering on the carrier spectrum. The coherent optical frequency domain reflectometry measurement apparatus may further include an optical filter that separates either one of the double sidebands between the modulation unit and the second branching unit. Further, the coherent optical frequency domain reflectometry measurement apparatus may comprise a modulator that utilizes the acoustic-optic effects the path reference light between the second branching means and multiplexing means propagates.

In another embodiment, the phase-sensitive optical amplifier that uses the effect of parametric amplification may be a second-order nonlinear optical material in which a medium that performs parametric amplification is periodically poled. For example, a second-order nonlinear optical material in which a parametric amplification medium is periodically poled is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTiOPO _4. Or LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTiOPO ₄ with at least one of Mg, Zn, Fe, Sc and In as an additive It may be a contained material.

  As described above, according to the present invention, in a C-OFDR using an external modulator, a phase sensitive optical amplifier capable of amplifying light intensity with extremely low noise and reducing phase noise can be modulated. By using it in the subsequent stage of the device, it is possible to realize loss evaluation of the fiber to be measured and the optical component with high spatial resolution and high sensitivity over a wide range.

It is a figure which shows the measurement system of C-OFDR. It is a figure which shows the mode of the output of the conventional optical amplifier and a phase sensitive amplifier on a constellation map. It is a figure which shows the basic composition of a phase sensitive amplifier. It is a figure which shows the structure of the phase sensitive amplifier using the secondary nonlinear optical element concerning one Example of this invention. It is a figure which shows the structure of C-OFDR using PSA concerning one Example of this invention. It is a figure which shows the measurement result of C-OFDR using PSA concerning one Example of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Before describing an optical fiber reflectance distribution measurement system (C-OFDR system) using a phase sensitive optical amplifier (PSA) according to an embodiment of the present invention, that is, a coherent optical frequency domain reflectometry measurement apparatus, its components The PSA that is

  FIG. 3 shows a basic configuration of a phase sensitive optical amplifier (PSA) 300. This optical amplifier 300 includes an optical branching unit 304-1 that branches input signal light, an excitation light source 302, an excitation light phase control unit 303, and an optical branching unit 304-2 that combines excitation light and input signal light. The phase-sensitive light amplifying unit 301 that receives the combined excitation light and the input signal light, and the optical branching unit 304-3 that branches the signal light from the phase-sensitive light amplifying unit 301 are provided. In this optical amplifier 300, the phase sensitive light amplifying unit 301 amplifies the signal light when the phase of the input signal light and the excitation light match (that is, the input signal light 310 is amplified), and the phase of both is 90 degrees. When the phase relationship is shifted, the signal light is attenuated (that is, the input signal light 310 is attenuated). If the phase between the pumping light and the signal light is matched so that the amplification gain is maximized by utilizing this characteristic, the spontaneous emission light having the quadrature phase with the signal light is not generated, that is, the SN ratio is not deteriorated. Signal light can be amplified.

  The optical branching unit 304-1 branches the input signal light 310 into signal light E and signal light F. The signal light F is supplied to the excitation light source 302.

  The optical branching unit 304-3 branches the signal light I output from the phase sensitive light amplification unit 301 into the signal light J and the output signal light 312. The signal light J is supplied to the excitation light phase control unit 303.

  The signal light G from the excitation light source 302 is supplied to the excitation light phase control unit 303 and output as the excitation light 311.

  The pumping light phase control unit 303 is synchronized with the phase of the signal light E branched from the input signal light 310 by the light branching unit 304-1 in order to achieve phase synchronization between the input signal light 310 and the pumping light 311. The phase of the signal light G from the excitation light source is controlled, and the excitation light 311 synchronized with the phase of the signal light E is output. The pumping light phase control unit 303 detects the signal light J, which is part of the signal light I output from the phase sensitive light amplification unit 301 branched by the light branching unit 304-3, with a narrowband detector, and outputs it. The phase of the excitation light 311 is controlled so that the signal becomes maximum. The optical branching unit 304-2 combines the signal light E and the excitation light 311 and outputs the signal light H. As a result, the signal light H obtained by combining the signal light E and the pumping light 311 that are phase-synchronized with each other is input to the phase-sensitive light amplification unit 301, and optical amplification without degradation of the SN ratio can be realized. The pumping light phase control unit 303 may be configured to directly control the phase of the pumping light source 302 in addition to the configuration of controlling the phase of the pumping light on the output side of the pumping light source 302 as shown in FIG.

  Next, a specific configuration of the phase sensitive amplifier (PSA) used in the C-OFDR measurement system according to the present embodiment will be described. In the phase sensitive amplifier according to the present embodiment, in order to obtain sufficient power to obtain the nonlinear optical effect from the weak laser light after passing through the external modulator, a part of the input light is used by using a fiber laser amplifier (EDFA). Amplify. The amplified input light is incident on the first second-order nonlinear optical element, and a second harmonic of the input light is generated inside the first second-order nonlinear optical element. Phase-sensitive amplification can be performed by performing a parametric amplification by entering a part of the input light and the second harmonic generated by the first second-order nonlinear optical element into the second second-order nonlinear optical element. .

  FIG. 4 shows a specific configuration of a phase sensitive amplifier (PSA) used in the C-OFDR measurement system according to the present embodiment. The PSA 400 shown in FIG. 4 includes a polarization controller 402, a first optical branching unit 403, a phase modulator 416, an optical fiber stretcher 417 using a piezoelectric transducer (PZT), and an erbium-doped fiber laser amplifier. (EDFA) 404, first second-order nonlinear optical element 405, polarization maintaining fiber 408, second second-order nonlinear optical element 409, second light branching unit 413, photodetector 414, A phase-locked loop (PLL) circuit 415.

  The polarization controller 402 adjusts the polarization of part of the signal light incident on the PSA 400.

  The first second-order nonlinear optical element 405 includes a first PPLN waveguide 406, a first spatial optical system disposed in front of the first PPLN waveguide 406, and the first PPLN waveguide 406. And a second spatial optical system including a first dichroic mirror 407 disposed in the subsequent stage.

  The second second-order nonlinear optical element 409 includes a second spatial optical system including a second PPLN waveguide 411 and a second dichroic mirror 410 disposed in front of the second PPLN waveguide 411. And a fourth spatial optical system 412 including a third dichroic mirror 412 disposed downstream of the second PPLN waveguide 411.

  The first spatial optical system couples light input from the input port of the first second-order nonlinear element 405 to the first PPLN waveguide 406.

  The second spatial optical system couples the light output from the first PPLN waveguide 406 to the output port of the first second-order nonlinear optical element 405 via the first dichroic mirror 407.

  The third spatial optical system couples the light input from the input port of the second second-order nonlinear optical element 409 to the second PPLN waveguide 411 via the second dichroic mirror 410.

  The fourth spatial optical system couples the light output from the second PPLN waveguide 411 to the output port of the second second-order nonlinear optical element 409 via the third dichroic mirror 412.

  In the example shown in FIG. 4, signal light (input light) 410 that is incident on the PSA 400 and whose polarization is adjusted is branched into an optical signal K and an optical signal L by the optical branching unit 403, and the optical signal K is a second signal. The optical signal L is phase-controlled via a phase modulator 415 and an optical fiber stretcher 417 and enters the EDFA 404. In order to obtain sufficient power to obtain a nonlinear optical effect from weak laser light used for optical communication, the EDFA 404 amplifies the incident signal light L (excitation fundamental wave light), and the amplified signal light M is amplified. 1 is incident on the second-order nonlinear optical element 405. In the first second-order nonlinear optical element 405, the second harmonic signal light N is generated from the incident signal light M and is incident on the second second-order nonlinear optical element 409 via the polarization maintaining fiber 408. The second second-order nonlinear optical element 409 performs phase-sensitive amplification by performing degenerate parametric amplification with the signal light K and the signal light N branched from the input light 510 and outputs the signal light (output signal light) P. To do.

  In the present embodiment, the function of the phase sensitive amplifier (PSA) 400 for amplifying 1.56 μm input light will be described, but the wavelength of the input light is not limited to this. In the PSA 400, a part of the input light is input to the polarization controller 402 and the polarization is adjusted, and then emitted as the input light 401. The input light 401 is branched into signal light K and signal light L by an optical branching unit 403. The signal light L is combined with first pumping light (not shown), then enters an erbium-doped fiber laser amplifier (EDFA) 404, is amplified, and is emitted as signal light M. The signal light M is incident on the first second-order nonlinear optical element 405. The second-order nonlinear optical element 405 of this embodiment includes an optical waveguide 406 made of lithium niobate (PPLN) that is periodically poled. Periodic polarization inversion that satisfies the quasi-phase matching condition that enables the second harmonic generation of the input light is formed.

  The first dichroic mirror 407 has a characteristic of reflecting light having a wavelength near 1.56 μm and transmitting light having a wavelength near 0.78 μm. Therefore, in the first dichroic mirror 407, the excitation light N having a wavelength of 0.78 μm emitted from the first PPLN waveguide 406 is transmitted, and the ASE associated with the excitation fundamental light having a wavelength of 1.56 μm. Since the light is reflected, it is separated from the excitation light. The 0.78 μm excitation light N transmitted through the dichroic mirror 407 is guided to the second second-order nonlinear optical element 409 via the polarization maintaining fiber 408 having a single mode propagation characteristic at this wavelength of 0.78 μm. Yes. At this time, excitation light and ASE light in the vicinity of a wavelength of 1.56 μm that could not be completely removed by the first dichroic mirror 407 are also incident on the polarization maintaining fiber 408, but are in a single mode at 0.78 μm. Since this fiber 408 has a weak light confinement with respect to light having a wavelength of 1.56 μm, it is possible to effectively attenuate such unnecessary light by propagating a length of about 1 m.

  The excitation light N guided to the second second-order nonlinear optical element 409 by the polarization maintaining fiber 408 and the signal light K having a wavelength of 1.56 μm (a part of the input light 401) using the second dichroic mirror 410. Combined. Similar to the first dichroic mirror 407, the second dichroic mirror 410 has a feature of transmitting light having a wavelength in the vicinity of 0.78 μm. Then, the second dichroic mirror 410 is emitted from the first PPLN waveguide 406 to reflect only the input light, and is transmitted / propagated through the first dichroic mirror 407 and the polarization maintaining fiber 408 (can be completely removed). The residual components of the excitation input light and the ASE light in the vicinity of the wavelength of 1.56 μm can be effectively removed.

  The signal light K combined with the excitation light N is incident on the second PPLN waveguide 411. The second PPLN waveguide 411 has the same performance and phase matching wavelength as the first PPLN waveguide 406, and can input phase-sensitive amplification of the input light by parametric amplification. In the present embodiment, the two PPLN waveguides 406 and 411 are each controlled to have a constant temperature by individual temperature controllers. It is conceivable that the phase matching wavelengths do not match at the same temperature due to manufacturing errors of the two PPLN waveguides, but even in such a case, the phase matching wavelengths of both must be matched by individually controlling the temperatures. Can do. The signal light emitted from the second PPLN waveguide 411 is converted into excitation light N (0.78 μm wavelength) and signal light P (output light (1.56 μm wavelength)) amplified by the third dichroic mirror 412. To be separated. Also at this time, since the wavelengths of the pump light and the amplified output light are completely different, unnecessary second harmonic components in the output can be effectively removed.

  In the phase sensitive amplification (PSA), it is necessary to synchronize the phases of the pumping light and the signal light. However, in this configuration, a part of the output amplified signal light P is branched by the optical branching unit 413 and is detected by the photodetector 414. After receiving the light, phase synchronization is performed by a phase locked loop circuit (PLL) 415. The phase modulator 416 is used to apply weak phase modulation to the excitation light using a sine wave. The optical detector 414 and the PLL circuit 415 detect the phase shift of the phase modulation, and feed back to the drive voltage of the optical fiber stretcher 417 and the bias voltage of the phase modulator 416 by PZT. Absorbs fluctuations in the optical phase due to vibrations and temperature fluctuations, enabling stable phase-sensitive amplification.

  FIG. 5 shows a specific configuration of an optical fiber reflectance distribution measurement system (C-OFDR) using a phase sensitive amplifier (PSA) according to an embodiment of the present invention, that is, a coherent optical frequency domain reflectometry measurement apparatus. A C-OFDR 500 shown in FIG. 5 includes a light source 501, a DSB-SC modulator 503, a PSA 504, a light source 501 connected to the input, and a branching unit 502 connected to the DSB-SC modulator 503 and PSA 504 on the output. With. The branching unit 502 has, for example, one input and two outputs, and one output is connected to the input of the DSB-SC modulator 503 and the other output is connected to one of the two inputs of the PSA 504. The other of the two inputs of the PSA 504 is connected to the output of the DSB-SC modulator 503.

  The C-OFDR 500 includes a trigger unit 514 and a signal generator 515.

  Further, the C-OFDR 500 includes a filter 505 connected to the output of the PSA 504, a branching unit 506 such as an optical coupler A connected to the output of the filter 505, and a circulator connected to one of the two outputs of the branching unit 506. And a branching unit 509 such as an optical coupler B connected to the other of the two outputs of the branching unit 506. One of the two outputs of the circulator is connected to an input to which the output of the branch unit 506 is not connected, of the two inputs of the branch unit 509. The other of the two outputs of the circulator is connected to a fiber under test (FUT) via a connector 508.

  Furthermore, the C-OFDR 500 includes a light receiver 510 connected to the two outputs of the branching unit 509, an AD converter 512 that AD converts the output of the light receiver, and an analysis device connected to the AD converter 512. And a computer 513. The trigger unit 514 supplies the AD converter 512 and the signal generator 515 with a trigger for matching the optical frequency sweep timing in the DSB-SC modulator 503 with the sampling timing in the AD converter 512. As a result, the AD converter 512 can supply the computer 513 with the sampling result of only the portion where the light frequency is swept.

  In this embodiment, a narrow line width fiber laser (FL) having a spectral line width of 2 kHz is used as the light source 501. This line width corresponds to a coherence length of about 30 km. The laser beam emitted from the FL 501 is split into two at the branching unit 502, one of the laser beams is used to generate excitation light at the PSA 504, and the other laser beam is driven by an arbitrary signal generator 515. Is incident on the DSB-SC modulator 503.

  The DSB-SC modulator 503 sweeps the optical frequency of the ± first-order modulation sideband by sweeping the sine wave frequency from an arbitrary signal generator 515 linearly with respect to time from 2 to 12 GHz for 1 second. To do. This corresponds to a frequency sweep width ΔF = 10 GHz and a frequency sweep speed γ = 10 GHz / s, and the theoretical formula of spatial resolution is 1 cm from the equation (3).

  The phase-sensitive amplification (PSA) 504 is normally realized by using degenerate parametric amplification, but a pair of light that is symmetrically separated from the optical frequency corresponding to the pumping light wavelength by the optical frequency difference is in phase synchronization with each other. Is known to be capable of phase-sensitive amplification by non-degenerate parametric amplification. In the C-OFDR 500 of this embodiment, in order to sweep the frequency of the light source 501 using the modulator 503, the frequency of the laser light used for the excitation light in the PSA 504 and the laser amplified in the PSA 504 after passing through the modulator 503 are used. It is different from the frequency of light. For this reason, in the present embodiment, a DSB-SC modulator is used as the modulator 503, so that a pair of light with a fixed phase relationship is formed and phase sensitive amplification is performed by non-degenerate operation.

  One sideband of the light after passing through the PSA 504 is removed by the filter 507. This is because when both sidebands are simultaneously incident on the fiber to be measured, the intensity of the reflected light fluctuates due to the beat caused by the interference between the reflected lights of the modulation sidebands. In this embodiment, one modulation sideband is removed by the optical filter 507, but an AO (Acoust Optical) modulator is connected between the reference optical path (the optical coupler A (506) and the optical coupler C (509)). It is possible to separate the beat due to the interference between the reflected light in the modulation sidebands from the measurement result by using it in the optical path).

  Light that has been frequency-modulated and passed through the PSA 504 and the filter 507 is branched by an optical coupler A (506), one of which enters the fiber under test (FUT) having a length of 5 km via the circulator 507, and the other as a reference light path as a reference light. It was incident on. The reflected light and scattered light from the FUT were combined with the reference light by the optical coupler B509 to obtain a beat signal depending on the distance from the reflection point. FIG. 6 shows the reflectance distribution of the FUT measured.

  As shown in FIG. 6A, a Fresnel reflection peak and Rayleigh backscattered light at the FUT end were observed. When the resolution of the Fresnel reflection peak at the fiber end was measured, as shown in FIG. 6B, a resolution of 3.1 cm could be achieved in the configuration of this example using PSA. When the same FUT was measured with the configuration using the conventional EDFA and the resolution was measured in the same manner, the resolution was 5.2 cm as shown in FIG. From the above, it can be seen that the resolution can be improved by configuring the C-OFDR using PSA.

  The PSA in this embodiment uses lithium niobate (LiNbO3) doped with Zn as a second-order nonlinear optical material whose polarization is periodically inverted. However, the PSA is not limited to lithium niobate, and lithium tantalate ( LiTaO3), mixed crystals of lithium niobate and lithium tantalate (LiNb (x) Ta (1-x) O3 (0 ≦ x ≦ 1)), potassium niobate (KNbO3), potassium titanyl phosphate (KTiOPO4), etc. The same effect can be obtained if the second-order nonlinear optical material is used. Further, the additive of the second-order nonlinear optical material is not limited to Zn, and Mg, Zn, Sc, In, Fe may be used instead of Zn, or no additive may be added. Good.

DESCRIPTION OF SYMBOLS 12,302 Light source 14,18,304,403,413,502,506,509 Optical coupler 16,507 Circulator 20 Light-receiving part 22 Analysis apparatus 50 Fiber to be measured (FUT)
100 Optical frequency domain reflection measurement device (C-OFDR)
300,400,504 Phase-sensitive optical amplifier (PSA)
301 Phase Sensitive Light Amplifier 303 Pumping Light Phase Controller 402 Polarization Controller 404 Erbium-Doped Fiber Laser Amplifier (EDFA)
405 Second-order nonlinear optical element (harmonic generation (SHG) module)
406,411 PPLN waveguide 407,410,412 Dichroic mirror 408 Polarization maintaining fiber 409 Second-order nonlinear optical element (optical parametric amplification (OPA) module)
414 Photodetector 415 PLL
417 Optical fiber stretcher 500 Optical fiber reflectance distribution measurement system using phase sensitive amplifier 501 Light source, narrow linewidth fiber laser (FL)
503 DSB-SC modulator 508 connector 510 light receiver 512 AD converter 513 computer 514 trigger unit 515 signal generator

Claims (5)

Branching means for branching light from a light source that emits coherent light into first light and second light;
Modulation means for sweeping the optical frequency of the first light linearly with respect to time and outputting a third light;
A phase sensitive amplifier that amplifies the third light using the second light as excitation light;
Second branching means for branching the amplified third light into fourth light and fifth light;
Combines the reflected light of the fourth light that is incident on the measurement target as the measurement light and reflected by the measurement target, and the reference light that is the fifth light Means for generating an interference wave between the reflected light and the reference light; and
Means for optically detecting the interference wave to obtain an interference signal;
A coherent optical frequency domain reflectometry measuring device for measuring a reflectance distribution in the propagation direction of light in the measurement target, comprising an analysis means for analyzing the frequency of the interference signal,
The phase sensitive amplifier Ri phase sensitive optical amplifier der utilizing the effect of the parametric amplification,
The coherent optical frequency domain reflectometry measurement apparatus characterized in that the modulation means suppresses a carrier wave and generates a first-order double sideband centering on the carrier spectrum . Between the second branching means and the modulating means, a coherent optical frequency domain reflectometry measurement apparatus according to claim 1, characterized in that it comprises an optical filter to separate either of said side band . Coherent optical frequency domain according to claim 1, further comprising a modulator in which the reference beam between said multiplexing means and said second branching means utilizes an acoustic-optic effects in the path of propagation Reflectometry measuring device. Phase sensitive optical amplifier utilizing the effect of the parametric amplification is second-order nonlinear optical material medium which performs parametric amplification are periodically poled, according to any one of claims 1 to 3, characterized in that Coherent optical frequency domain reflectometry measurement device. The second-order nonlinear optical material in which the parametric amplification medium is periodically poled is LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTiOPO ₄ , or , LiNbO ₃ , KNbO ₃ , LiTaO ₃ , LiNb _x Ta _1-x O ₃ (0 ≦ x ≦ 1) or KTiOPO ₄ with a material containing at least one of Mg, Zn, Fe, Sc and In as an additive The coherent optical frequency domain reflectometry measuring device according to claim 4 , wherein

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