Optical fiber property measuring device and optical fiber property measuring method

There are proposed an optical fiber property measuring device and an optical fiber property measuring method which can enhance spatial resolution more than before. In the present invention, in synchronization with frequency modulation applied to x-polarized light, intensity modulation is also applied to the x-polarized light by an intensity modulation means. This makes it possible to increase or decrease the intensity of the x-polarized light at a specific frequency, thereby allowing the effective length of a Brillouin dynamic grating formed by the x-polarized light to be adjusted. As a result, the shape of the reflection spectrum obtained when y-polarized light is reflecting by the Brillouin dynamic grating can also be adjusted optimally, which leads to enhancement of spatial resolution with the y-polarized light.

TECHNICAL FIELD

The present invention relates to an optical fiber property measuring device and an optical fiber property measuring method. The present invention more particularly relates to an optical fiber property measuring device and an optical fiber property measuring method for sensing distribution of strain and temperature applied to a polarization maintaining-type optical fiber under test as a measurement target by using a stimulated Brillouin scattering phenomenon and a Brillouin dynamic grating phenomenon generated in this optical fiber under test.

BACKGROUND ART

Brillouin scattering in an optical fiber is changed depending on a strain applied to the optical fiber. There has conventionally been devised a technique for measuring distributed strain along the optical fiber by using this phenomenon. This measuring technique enables the level of strain to be measured by measurement of a frequency change in Brillouin scattered light, and also enables distorted part of the optical fiber to be identified by measurement of the time until the Brillouin scattered light returns. Accordingly, when optical fibers are wired in all directions on the constructions such as bridges and bridge piers, buildings, and dams, and/or on the materials such as wings and fuel tanks of an airplane, the distribution of strain applied to these constructions and/or materials can be revealed. Based on such distribution of strain, deterioration and/or secular change in materials and structures are revealed. Accordingly, this measurement technique is attracting attention as a technique useful for disaster and/or accident prevention (see, for example, Patent Literatures 1 and 2).

A description is now given of the principle of the Brillouin scattering. When light is incident on a general optical fiber, glass molecules in the material of the optical fiber thermally oscillate and generate ultrasonic waves, which include an ultrasonic wave having a wavelength half the wavelength of the incident light. Periodic change in a refractive index of the glass caused by the ultrasonic wave function as a Bragg diffraction grating for the incident light, and reflects the light backward. This is how the Brillouin scattering phenomenon works. While the reflected light is Doppler-shifted depending on the velocity of the ultrasonic wave, the amount of frequency shift varies depending on expansion and contraction strain applied to the optical fiber. Accordingly, the strain can be detected by measuring the shift amount.

As a typical technique to measure the distribution of such Brillouin scattering along a length direction of the optical fiber, a Brillouin optical correlation domain analysis (BOCDA) method is known as disclosed in Patent Literature 1 and the like.

However, the Brillouin scattering in the optical fiber depends not only on strain but also on temperature. At measurement sites where temperature changes, precise measurement is not available. Accordingly, in order to solve such a problem, an optical fiber property measuring device has been proposed which applies the above-stated BOCDA method to an optical fiber under test having a polarization retention property and which also measures a spectrum of reflected light generated by the Brillouin dynamic grating which is a phenomenon relevant to the Brillouin scattering at the same time, so that both variation of the temperature and the strain can precisely be measured based on the result of these two measurements (see, for example, Non Patent Literature 1).

CITATION LIST

Patent Literature

Non Patent Literature

SUMMARY OF INVENTION

Technical Problem

In the conventional optical fiber property measuring device having such configuration, the amount of change in the temperature and strain can precisely be measured based on the measurement results, respectively. However, there is a problem that it has a limited spatial resolution, which is not sufficient for diagnosing the distribution of strain and the distribution of temperature applied to the constructions and/or the materials.

Moreover, to measure a birefringence itself of an optical fiber with the Brillouin dynamic grating, the above-stated conventional optical fiber property measuring device has limited spatial resolution, which causes a problem of insufficient spatial resolution.

Accordingly, the present invention has been made in consideration of the above-stated points, and an object of the present invention is to propose an optical fiber property measuring device and an optical fiber property measuring method which may enhance spatial resolution more than before.

Solution to Problem

In order to solve the above-stated problem, first and seventh aspects of the present invention includes: shifting a frequency of frequency-modulated first polarized light and making the light enter from one end of an optical fiber under test as probe light, the optical fiber under test having a polarization retention property; making the first polarized light enter from the other end of the optical fiber under test as pump light; making frequency-modulated second polarized light enter from the other end of the optical fiber under test as readout light; measurement means for detecting a reflection spectrum of the readout light and measuring a property of the optical fiber under test, the readout light being reflected by a Brillouin dynamic grating formed by the pump light and the probe light inside the optical fiber under test; and applying intensity modulation to the first polarized light by intensity modulation means in synchronization with frequency modulation applied to the first polarized light.

Advantageous Effects of Invention

According to the present invention, in synchronization with frequency modulation applied to the first polarized light, intensity modulation is also applied by the intensity modulation means. This makes it possible to increase or decrease the intensity of the first polarized light at a specific frequency, thereby allowing the effective length of a Brillouin dynamic grating formed by the first polarized light to be adjusted. As a result, the shape of the reflection spectrum obtained when the light is reflecting by the Brillouin dynamic grating can also be adjusted optimally, which leads to enhancement of spatial resolution.

DESCRIPTION OF EMBODIMENTS

FIG. 1illustrates an optical fiber property measuring device1according to a first embodiment of the present invention, which includes an optical fiber under test28constituted of a polarization maintaining fiber (PMF). Here, reference numeral2designates a light source constituted of a signal generator3and a semiconductor laser4. As the semiconductor laser4, a small-size distributed feedback laser diode (DFB-LD) is used which emits laser light of a narrow spectral bandwidth, for example. The signal generator3outputs a desired modulation signal into the semiconductor laser4as an injection current in order to repeatedly perform frequency modulation (including phase modulation) of continuous laser light emitted from the semiconductor laser4in a sinusoidal shape.

Reference numeral5designates an optical intensity modulator (IM1) that modulates the intensity of light output from the semiconductor laser4to generate, for example, laser light having a frequency component of ±two dozen [GHz] added thereto. The optical intensity modulator5outputs the intensity-modulated laser light to a wavelength selection reflector10via an optical branch device8. The wavelength selection reflector10is a fiber Bragg grating (FBG) in which a periodic change is formed in the refractive index of an optical fiber core. The wavelength selection reflector10may reflect only the laser light of a negative-side frequency component, which is formed by a periodic refractive index change and which satisfies Bragg reflection condition, as x-polarized light, and send out the x-polarized light to a probe light/pump light generating optical path. Contrary to this, the wavelength selection reflector10may pass only the laser light of a positive-side frequency component as y-polarized light, and send out the y-polarized light to a readout light generating optical path.

The x-polarized light as first polarized light and the y-polarized light as second polarized light herein refer to two linearly polarized light components which oscillate in right angle direction with each other (x-axis direction and y-axis direction) inside a plane vertical to a light travel direction. Reference numeral9designates a tuneable bandpass filter (TBF) provided in the probe light/pump light generating optical path. The tuneable bandpass filter (TBF)9may remove excessive light components so that only the x-polarized light reflected from the wavelength selection reflector10may enter into a subsequent optical intensity modulator (IM2)13.

Reference numeral13designates an optical intensity modulator (IM2) used as an intensity modulation means for modulating the intensity of the x-polarized light reflected by the wavelength selection reflector10in synchronization with the frequency modulation applied to the semiconductor laser4. The optical intensity modulator13herein has a function of being able to modulate the intensity of the x-polarized light input from the semiconductor laser4via the wavelength selection reflector10upon reception of a sync signal from the signal generator3that is equivalent to an input signal. Specifically, the optical intensity modulator13is implemented by an electrooptical modulator (EOM). One of the characteristic configurations of the present embodiment is the optical intensity modulator13added to the optical fiber property measuring device using the BOCDA method, the optical intensity modulator13applying intensity adjustment to the x-polarized light. Another configuration of the intensity modulation means will be described later.

In the probe light/pump light generating optical path, the x-polarized light whose frequency and intensity are both modulated by the optical intensity modulator13is amplified by an erbium-doped optical fiber amplifier (hereinafter referred to as EDFA)15. The x-polarized light is then branched by an optical branch device16into two parts in an intensity ratio of 50 to 50, for example. Out of two branched parts, one x-polarized light passes an optical delay device17, and the frequency thereof is lowered by about 11 [GHz] by a single-sideband modulator (SSBM) (hereinafter referred to as a SSB modulator)18. This light is used as x-probe light, which is made to enter into one end of an optical fiber under test28having a polarization retention property (hereinafter referred to as a polarization maintaining fiber-type optical fiber under test (PMF FUT)). The optical delay device17is configured to set specified delay time between x-probe light and x-pump light (described later). The delay time can properly be adjusted by changing the optical fiber length.

The SSB modulator18uses a microwave and precise DC bias control so that out of two primary sideband waves, a higher frequency component can be suppressed while maintaining a stable frequency difference Δν which is a difference in frequency from x-pump light. A lower frequency sideband wave different in frequency by Δν from the input light is output as x-probe light. The lower frequency sideband wave is equal to a microwave frequency.

The other x-polarized light beam branched by the optical branch device16is intensity-modified by an optical intensity modulator24having a reference signal generator23, and then is amplified by an EDFA25. Then, the x-polarized intensity-modulated light which is amplified in the EDFA25passes an optical branch device26and a polarization beam splitter (PBS)29, and enters from the other end of the PMF FUT28as x-pump light. As illustrated inFIG. 2A, the x-probe light and the x-pump light propagate facing each other inside the polarization maintaining fiber-type optical fiber under test28.

As a result, x-polarized light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28is obtained. As illustrated inFIG. 1, the x-polarized light is captured into a photodetector (PD1)34avia the polarization beam splitter29, the optical branch device26, and a tuneable bandpass filter (TBF)33a, and the power thereof is measured in the photodetector34a. The detected output from the photodetector34apasses a lock in amplifier35a, where synchronous detection of the output is performed at the modulation frequency of the x-pump light. Accordingly, a Brillouin gain of the probe light relating to a guidance Brillouin phenomenon is captured into a data processor37as final data at a specified sampling rate. The data processor37serves as a Brillouin gain measurement means constituted of a personal computer, for example.

InFIG. 1, part of x-pump light subjected to reflection and backscattering is also made to exit from the polarization maintaining fiber-type optical fiber under test28. In order to remove such part of the x-pump light, the tuneable bandpass filter33ais provided prior to the photodetector34a.

In addition, the y-polarized light which passed the wavelength selection reflector10passes the optical delay device30, and is amplified by an EDFA31. The y-polarized light amplified by this EDFA31passes an optical branch device32and the polarization beam splitter29, and then enters as y-readout light from the other end of the polarization maintaining fiber-type optical fiber under test28. In this case, as illustrated inFIG. 2B, the x-probe light and the x-pump light propagate facing each other under optimum conditions so that sound waves are formed inside the polarization maintaining fiber-type optical fiber under test28. In the present invention, a periodic refractive index structure formed by such sound waves is referred to as a Brillouin dynamic grating, in which the sound waves excited by the x-probe light and the x-pump light that are incident on an x-polarized plane reflects y-readout light incident on a y-polarized plane, so that y-reflected light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28may be generated.

As illustrated inFIG. 1, the y-polarized light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28is captured into a photodetector (PD2)34bvia the polarization beam splitter29, the optical branch device32, and a tuneable bandpass filter (TBF)33b, and a reflection spectrum (also referred to as a Brillouin dynamic grating spectrum) thereof is measured in the photodetector34b. The detected output from the photodetector34bpasses a lock in amplifier35b, where synchronous detection of the output is performed at the modulated frequency of the pump light. Accordingly, the reflection spectrum of the y-reflected light relating to a Brillouin dynamic grating phenomenon is captured into the data processor37serving as a measurement means as final data at a specified sampling rate. InFIG. 1, in order to remove light other than the y-reflected light, the tuneable bandpass filter33bis provided prior to the photodetector34b.

The optical intensity modulator24provided inside the optical path for the x-pump light is constituted of, for example, an electrooptical modulator like the optical intensity modulator13. As the optical branch devices8,26, and32, circulators, beam splitters, half mirrors and the like may be used. In still another modification, the light source2as a light source unit may emit output light independently for each of the x-probe light and x-pump light, and the y-readout light. For example, when separate light sources are provided, the light sources each include an optical intensity modulator5that synchronizes with frequency modulation of each of the light sources.

In the present embodiment, the optical delay device17, the SSB modulator18, the EDFA20, and the isolator21constitute a probe light generation means for generating x-probe light from the output light of the light source2. The optical intensity modulator24, the EDFA25, and the optical branch device26constitute a pump light generation means for generating x-pump light from the output light of the light source2. The wavelength selection reflector10, the optical delay device30, the EDFA31, and the optical branch device32constitute a readout light generation means for generating y-readout light from the output light of the light source2.

In the present embodiment, the tuneable bandpass filter33a, the photodetector34a, the lock in amplifier35a, and the data processor37detect a Brillouin gain of the x-probe light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28while sweeping a frequency difference between the x-pump light and the x-probe light. Separately from this detection, the tuneable bandpass filter33b, the photodetector34b, the lock in amplifier35b, and the data processor37detect a reflection spectrum of y-reflected light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28.

In the present embodiment, the Brillouin gain of the x-probe light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28and the reflection spectrum of the y-reflected light exiting from the other end of the polarization maintaining fiber-type optical fiber under test28each independently depend on temperature and strain. Therefore, a change in temperature and strain can precisely be calculated based on these two measurement results.

Since the technique to calculate such two physical values each having different dependence on temperature and strain based on such measurement results is disclosed in detail in Non Patent Literature 1 mentioned in the prior art, the description thereof is omitted herein.

The optical fiber property measuring device1according to the present invention also conforms to the basic principle of the BOCDA method. That is, the light source2emits continuous oscillation light, the oscillation frequency of which is changed with a sinusoidal repetitive waveform by the signal generator3, while the SSB modulator18changes a center frequency fprobeof x-probe light so that a difference between the center frequency fprobeof the x-probe light and the center frequency fpumpof x-pump light approximates a Brillouin frequency νB. At almost all the positions, frequency modulation of the x-pump light and the x-probe light is asynchronous and stimulation is suppressed. However, at a correlative position where the frequency modulation of the x-pump light and the x-probe light is in synchronization, stimulated Brillouin scattering occurs. By moving this correlative position, it becomes possible to measure distribution of strain and temperature with the stimulated Brillouin scattering.

When the stimulated Brillouin scattering occurs, the sound waves of a wavelength about half the wavelength of incident light excites relatively strongly inside the polarization maintaining fiber-type optical fiber under test28, and the sound waves form a Brillouin dynamic grating inside the polarization maintaining fiber-type optical fiber under test28. The Brillouin dynamic grating formed by the x-polarized light (periodic structure of refractive index formed by sound waves) functions as a diffraction grating also for y-polarized light orthogonal to the x-polarized light. More specifically, in the polarization maintaining fiber-type optical fiber under test28, when y-readout light, which is y-polarized light orthogonal to x-polarized light, is incident while a frequency difference between the x-pump light and the x-probe light is maintained to be a Brillouin frequency shift, y-reflected light with a reflection spectrum (Brillouin dynamic grating spectrum) having a peak at a specific frequency can be observed.

Here, the present invention is characteristic in the following point. That is, x-polarized light involving generation of x-pump light and x-probe light is subjected to intensity modulation performed by the optical intensity modulator13in synchronization with the frequency modulation of the light source2. Accordingly, the spectrum of the x-polarized light is changed and thereby the effective length of the Brillouin dynamic grating is shortened, so that the spatial resolution for acquiring the spectrum of y-reflected light can be enhanced.

In this connection, since modulation applied to the x-probe light and the y-pump light is periodic, the correlative position by the stimulated Brillouin scattering periodically appears along the polarization maintaining fiber-type optical fiber under test28interposed in between the isolator21and the polarization beam splitter29. Accordingly, the delay amount of the optical delay device17and the frequency modulation frequency fmapplied to the semiconductor laser4are adjusted so that only one correlation peak is locationally present in the polarization maintaining fiber-type optical fiber under test28. Furthermore, in order to expand the measuring range of the x-polarized light while maintaining high spatial resolution Δz with the x-polarized light, it is necessary to increase amplitude Δf of frequency modulation applied to the semiconductor laser4, within the range where the spectrums of the x-probe light and the x-pump light do not overlap.

Next, the operation of the device illustrated inFIG. 1will be described. When laser light that is frequency-modulated by an injection current from the signal generator3is emitted from the semiconductor laser4, the laser light is reflected as x-polarized light which turns into x-pump light and x-probe light by the wavelength selection reflector10. In response to a sync signal output from the signal generator3to the optical intensity modulator13, the x-polarized light is intensity-modulated in synchronization with the frequency modulation in the optical intensity modulator13.

The x-polarized light whose frequency and intensity are both modulated is branched at a specified intensity ratio in the optical branch device16, and one light is input into the SSB modulator18. The SSB modulator18performs SSB modulation of the modulated x-polarized light, and generates a sideband wave having a frequency difference Δν (about 11 GHz) which is different from the center frequency of the modulated light and is close to Brillouin frequency νB. The sideband wave passes the EDFA20and the isolator21, and is made to enter into one end of the polarization maintaining fiber-type optical fiber under test28as x-probe light.

Meanwhile, the other modulated x-polarized light branched from the optical branch device16is input into the optical intensity modulator24, where the intensity thereof is modulated based on the frequency of a reference signal generated from the reference signal generator23. The modulated x-polarized wave chopped by the intensity modulation is amplified in the subsequent EDFA25, and passes the optical branch device26and the polarization beam splitter29before entering into the other end of the polarization maintaining fiber-type optical fiber under test28as x-pump light.

In this way, the x-probe light and the x-pump light propagate in directions opposite from each other in the polarization maintaining fiber-type optical fiber under test28. Consequently, part of x-pump light subjected to reflection and backscattering is made to exit from the polarization maintaining fiber-type optical fiber under test28. At the same time, part of the x-probe light increased by stimulated Brillouin scattering is superimposed on the continuous x-probe light and is made to exit from the polarization maintaining fiber-type optical fiber under test28. These exiting light beams are detected by the photodetector34a, and synchronous detection is performed at the intensity modulation frequency of the x-pump light by the lock in amplifier35a. As a result, only the increased part of the x-probe light generated in synchronization with chopping of the x-pump light is extracted and is amplified before being output, while frequency components other than the increased part are removed.

Upon reception of an output signal from the lock in amplifier35a, the data processor37determines in which frequency the peak of the stimulated Brillouin scattering spectrum is present at the correlation peak position.

In addition, the y-polarized light which passed the wavelength selection reflector10at this time passes the optical delay device30, the EDFA31, the optical branch device32, and the polarization beam splitter29, and enters into the other end of the polarization maintaining fiber-type optical fiber under test28as y-readout light. In this way, when y-readout light enters into the polarization maintaining fiber-type optical fiber under test28during occurrence of the stimulated Brillouin scattering, the y-readout light is reflected by the Brillouin dynamic grating formed by the stimulated Brillouin scattering, and is made to exit from the other end of the polarization maintaining fiber-type optical fiber under test28as y-reflected light. When the exiting light is detected by the photodetector34band then is subjected to synchronous detection at the intensity modulation frequency of the x-pump light by the lock in amplifier35b, only the increased part of the y-reflected light generated in synchronization with chopping of the x-pump light is extracted and is amplified before being input, while frequency components other than the increased part is removed.

Upon reception of an output signal from the lock in amplifier35b, the data processor37determines in which frequency the peak of the Brillouin dynamic grating spectrum is present, and calculates a difference (a peak shift amount) fxy between the peak frequency of the Brillouin dynamic grating spectrum and the original frequency of the light source2. Finally, the data processor37obtains the result of measuring distribution of temperature and strain in the polarization maintaining fiber-type optical fiber under test28based on the peak shift amount of the stimulated Brillouin scattering spectrum and the peak shift amount fxy of the Brillouin dynamic grating spectrum (see Non Patent Literature 1).

Here, the intensity of the Brillouin scattering that occurs at a certain position is expressed by the beat power spectrum of the x-pump light and the x-probe light. In the BOCDA method, appropriate frequency modulation is applied to x-pump light and x-probe light so that the beat power spectrum indicates a correlation peak at a specific position, where a stimulated Brillouin phenomenon is locally generated. The position of the correlation peak is shifted by changing a modulation parameter, and thereby distribution measurement is performed.

Spatial resolution at the time of acquiring a Brillouin gain spectrum (BGS) using the BOCDA method has theoretically and experimentally been confirmed. The resolution as high as 1.6 mm is implemented. However, it was experimentally indicated that the spatial resolution at the time of acquiring the Brillouin dynamic grating (BDG) spectrum with y-readout light did not match with the theoretical spatial resolution of a Brillouin gain spectrum and that the spatial resolution was lower than the theoretical spatial resolution. A simulation was conducted to find out the reason thereof. As a result, it was theoretically found out that the Brillouin dynamic grating spread to the region wider than the region of the spatial resolution for acquiring the Brillouin gain spectrum.

This finding is described with reference toFIGS. 3A and 4A.FIG. 3Ais a time change in the optical frequency of modulated light obtained from the light source2. The optical frequency is modulated into a sinusoidal shape. In this case, since the frequency is formed into a sinusoidal shape, the waveform stays in a maximum displacement portion made up of a peak part and a valley part of the frequency for relatively long time. Accordingly, a time average power spectrum intensity of the frequency corresponding to that portion becomes higher. A left side graph inFIG. 4Ais a schematic view illustrating such a waveform illustrated inFIG. 3Aas a time average spectrum. As described above, the time average power spectrum intensity of the frequency corresponding to the peak part and the valley part of the frequency is higher, and a shoulder portion on both sides of the peak part rises high.

The beat power spectrum formed from the x-pump light and the x-probe light based on x-polarized light having such a time average power spectrum includes, as illustrated in a right graph view ofFIG. 4A, relatively high side lobes S1formed on both sides of a main peak P1in the length direction of an optical fiber. The Brillouin dynamic grating (BDG) formed by such a beat power spectrum spreads in a region wider than the spatial resolution for acquiring the Brillouin gain spectrum (BGS) (expressed by “Long BDG” inFIG. 4A). As a result, the spatial resolution for acquiring the Brillouin dynamic grating of the y-reflected light obtained based on the Brillouin dynamic grating is lowered.

Accordingly in the present invention, as illustrated in the left graph view inFIG. 4B, intensity modulation is applied to the x-polarized light by the optical intensity modulator13, so that the shoulder portion having a high time average power spectrum intensity is adjusted to have an intensity characteristic of a smooth protrusion and the time average power spectrum intensity on both the sides of the peak is suppressed. Accordingly, in the present invention, as illustrated in the right graph view inFIG. 4B, the side lobe S1(FIG. 4A) in a coherence function representing the beat power spectrum can be suppressed. As a result, the Brillouin dynamic grating generated at positions other than the correlation peak can be suppressed, so that the effective length of a Brillouin dynamic grating can be shortened (indicated by “Short BDG” inFIG. 4B), and the spatial resolution for acquiring the Brillouin dynamic grating spectrum of the y-reflected light obtained based on the Brillouin dynamic grating can be enhanced.

InFIG. 4A, the graph view is schematically illustrated to plainly explain that the direct frequency modulation in the light source2with a sinusoidal shape causes the time average power spectrum intensity to concentrate on the shoulder portion on both the sides of the peak. However, in this optical fiber property measuring device1, the waveform as illustrated in area ER1ofFIG. 1is obtained. In this case, as illustrated in an enlarged view of the area ER1inFIG. 5A, the shoulder portion on both the side of the peak does not rise high but has a swelled shape as if there is a corner portion.

The optical intensity modulator13not only applies frequency modulation to the x-polarized light having such a time average spectrum but also applies intensity modulation in synchronization with this frequency modulation. As a result, as illustrated inFIG. 5Bdepicting an enlarged view of an area ER2inFIG. 1, the time average power spectrum intensity of the shoulder portion on both the sides of the peak is suppressed to be smooth. By applying such intensity modulation to the x-polarized light, the effective length of the Brillouin dynamic grating formed in the polarization maintaining fiber-type optical fiber under test28can be shortened, and the spatial resolution for acquiring the spectrum of the y-reflected light obtained based on the Brillouin dynamic grating can be enhanced.

Here,FIG. 3Billustrates a waveform representing optical transmittance in the optical intensity modulator13. The optical intensity modulator13applies intensity modulation to the modulated light illustrated inFIG. 3A, in which a minimum transmittance in the optical transmittance illustrated inFIG. 3Bis aligned with the maximum displacement portion made up of the peak part and the valley part of the frequency, so that a time average spectrum as illustrated inFIG. 5Bcan be generated. As a result, in the optical fiber property measuring device1, the side lobe S1(FIG. 3B) in the beat power spectrum can be suppressed. As a result, the effective length of a Brillouin dynamic grating can be shortened, and the spatial resolution for acquiring a Brillouin dynamic grating spectrum of y-reflected light obtained based on the Brillouin dynamic grating can be enhanced.

As described in the foregoing, in the present invention, in synchronization with frequency modulation applied to x-polarized light, intensity modulation is also applied to the x-polarized light by the optical intensity modulator13. This makes it possible to increase or decrease the intensity of the x-polarized light at a specific frequency, thereby allowing the effective length of a Brillouin dynamic grating formed by the x-polarized light to be adjusted. As a result, the shape of the reflection spectrum obtained when y-polarized light is reflecting by the Brillouin dynamic grating can also be adjusted optimally, which can achieve enhancement of spatial resolution with the y-polarized light.

In the frequency of output light as illustrated inFIG. 3A, the intensity of the light is made closer to a maximum value as the frequency approximates to the center of variation, and the intensity thereof is made closer to a minimum value as the frequency of the output light approximates to an upper limit and a lower limit. This makes it possible to alleviate the situation in which variation in the frequency of the output light from the light source2causes the intensity of the output light to concentrate and to be biased in the vicinity of the upper and lower limits of the frequency. As a result, the side rope of the beat power spectrum can be suppressed and the effective length of a Brillouin dynamic grating can be shortened.

Next, the optical fiber property measuring device1having the above-described configuration was used to examine an influence exerted by applying intensity modulation to x-polarized light by the optical intensity modulator13in synchronization with the frequency modulation of the light source2. In this experimental example, part of the polarization maintaining fiber-type optical fiber under test28was soaked with cool water. A correlation peak was set in the center, and the length of the fiber soaked with water (hereinafter referred to as length of the soaked fiber) was set to 10 [cm], 40 [cm], 70 [cm], and 100 [cm]. The Brillouin dynamic grating spectrum obtained in each fiber length was examined.

During the examination, the Brillouin dynamic grating spectrum before applying intensity modulation to the x-polarized light by the optical intensity modulator13in synchronization with the frequency modulation of the light source2was examined. The result thereof was as illustrated inFIG. 6A. The Brillouin dynamic grating spectrum after applying the intensity modulation to x-polarized light by the optical intensity modulator13in synchronization with the frequency modulation of the light source2was also examined. The result thereof was as illustrated inFIG. 6B.

As illustrated inFIG. 6A, before applying intensity modulation to the x-polarized light in synchronization with the frequency modulation of the light source2, only the peak corresponding to room temperature (indicated by “Room temperature”) could be observed when the length of the soaked fiber was as short as 10 [cm]. Therefore, insufficient spatial resolution could be confirmed. Before applying intensity modulation to the x-polarized light in synchronization with the frequency modulation of the light source2, the peaks corresponding to both cool water temperature (indicated by “5° C. water”) and room temperature were observed even when the length of the soaked fiber was gradually increased up to 100 [cm]. Therefore, insufficient spatial resolution could be confirmed.

Contrary to this, after applying intensity modulation to the x-polarized light by the optical intensity modulator13in synchronization with the frequency modulation of the light source2, the peaks corresponding to both the cool water temperature and room temperature were observed when the length of the soaked fiber was 10 [cm] as illustrated inFIG. 6B, so that the spatial resolution was insufficient. However, the peak corresponding to cooling water was observed when the length of the soaked fiber length was lengthened, and therefore sufficient spatial resolution could be confirmed.

Next, the relationship between the observed peaks corresponding to the cooling water temperature and the room temperature and the soaked fiber length was summarized. The result was as illustrated inFIGS. 7A and 7B. Here, the length of the soaked fiber wherein observed peaks corresponding to the cooling water temperature and the room temperature are identical in intensity was estimated as spatial resolution. As a result, before applying intensity modulation to the x-polarized light in synchronization with the frequency modulation of the light source2, the spatial resolution was 75 [cm] as illustrated inFIG. 7A. As compared with this, after applying intensity modulation to the x-polarized light in synchronization with the frequency modulation of the light source2, the spatial resolution improved up to 17 [cm] could be confirmed as illustrated inFIG. 7B.

FIG. 8illustrates an optical fiber property measuring device41in the second embodiment of the present invention. InFIG. 8, an optical filter42having an appropriate transmission spectral characteristic is arranged in place of the optical intensity modulator13in the probe light/pump light generating optical path for output light from the light source2. In this case, the optical filter42as an intensity modulation means substantially performs intensity modulation in synchronization with the frequency modulation of the output light from the light source2, so that the spectrum distribution of the output light can appropriately be adjusted. When the optical filter42is used, the filtering characteristic of the optical filter42itself enables the intensity of output light to be adjusted in accordance with the frequency of the output light. This makes it extremely easy to suppress the Brillouin dynamic grating generated at the position other than the correlation peak without the necessity of a sync signal from the signal generator3, and makes it possible to enhance the spatial resolution for acquiring the Brillouin dynamic grating spectrum.

In still another configuration of the intensity modulation means, an optical fiber property measuring device45may be applied as illustrated inFIG. 9. In the optical fiber property measuring device45, the optical intensity modulator13of external modulation system in the first embodiment is replaced with a signal generator47of direct modulation system which modulates the frequency of output light from the light source46with a repetitive waveform other than the sinusoidal wave. The signal generator47in the optical fiber property measuring device45illustrated as the third embodiment has a function of modulating the frequency of the output light from the semiconductor laser4with use of a triangular repetitive waveform, for example.

Here,FIGS. 10A to 10Deach illustrate a frequency modulation waveform and a time average spectrum shape calculated based on the frequency modulation waveform in the case of modulating the frequency of output light with a sinusoidal repetitive waveform and the case of modulating the frequency of output light with a repetitive waveform other than the sinusoidal waveform.FIG. 10Aillustrates a frequency modulation waveform obtained by changing the frequency of output light of the light source2into the sinusoidal wave shape. In this case, the waveform stays in the maximum displacement portion of the changing frequency for relatively long time. Accordingly, as illustrated inFIG. 10B, the spectrum intensity (power) has a strong bias on both ends in the vicinity of the upper and lower limits of the frequency.

Contrary to this, when the frequency of the output light of the light source46is changed with the triangular waveform illustrated inFIG. 10C, the waveform stays in each frequency for the same time period. As a result, a uniform spectrum intensity is obtained as illustrated inFIG. 10D. In the third embodiment, the above-stated intensity modulation means is constituted of a signal generator47that modulates the frequency of the light output from the light source46with a repetitive waveform other than the sinusoidal waveform. The intensity modulation applied to the output light is implemented by the signal generator47that modulates the frequency of the light output from the light source46with a repetitive waveform other than the sinusoidal waveform. Thus, the frequency modulation waveform of output light is changed into a waveform other than the sinusoidal wave shape using the signal generator47. This simple operation makes it extremely easy to suppress the Brillouin dynamic grating generated at the position other than the correlation peak, and makes it possible to enhance the spatial resolution for acquiring the Brillouin dynamic grating spectrum, as in the case of applying intensity modulation to the output light.

The above-stated embodiments have been described in the case of being applied to the optical fiber property measuring device which measures the Brillouin gain spectrum and the reflection spectrum of y-reflected light and measures the distribution of temperature and strain. However, without being limited thereto, the present invention may be applied to an optical fiber property measuring device which measures a birefringence of the polarization maintaining fiber-type optical fiber under test28. In this optical fiber property measuring device, stimulated Brillouin scattering is generated with x-polarized light in the polarization maintaining fiber-type optical fiber under test28. However, the Brillouin gain spectrum obtained as a result is not measured and only the Brillouin dynamic grating spectrum is measured. As a result, it becomes possible to measure the birefringence of the polarization maintaining fiber-type optical fiber under test28. In such an optical fiber property measuring device, the spatial resolution for measuring the birefringence of the polarization maintaining fiber-type optical fiber under test28can be enhanced.

In the embodiments disclosed in the foregoing, optical fibers of various configurations may be applied as the optical fiber having a polarization retention property such as the polarization maintaining fiber-type optical fiber under test. For example, optical fibers such as general polarization maintaining optical fibers provided with a substance whose physical stress on the x-polarized plane is different from that on the y-polarized plane, and lead glass optical fibers with a small birefringence, may be applied. In addition to the above, general optical fibers may also be applied if used under the condition that first polarized light and second polarized light do not cross so that the optical fibers gain the polarization retention property.

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