JP2812049B2 - Measurement method and apparatus using OTDR - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to the characteristics of an optical fiber by irradiating a laser beam into an optical waveguide such as an optical fiber or an optical waveguide, and detecting reflected and scattered light of an optical signal transmitted through the optical waveguide. OT for inspection, defect detection, temperature distribution detection, etc.
The present invention relates to a measurement device using DR (Optical Time Domein Reflectmetry).

[0002]

2. Description of the Related Art There are various types of measuring devices using an OTDR.
Utilizing the fact that scattered light is generated by inputting a short pulse light signal having a width of nsec and a peak power of W into the fiber from the end of the optical fiber, and that the intensity of the scattered light at each point of the optical fiber changes with temperature. Thus, temperature information along the longitudinal direction of the optical fiber is obtained.

FIG. 8 is a characteristic diagram of the Rayleigh scattered light and the Raman scattered light. Since the Raman scattered light is weak, but sensitive to a temperature change, an optical fiber type distributed temperature sensor using the Raman scattered light is not available. In recent years, the method has been attracting attention and is being developed as a method for obtaining temperature information of the entire area where the optical fiber is laid in combination with the use of the optical fiber. For example, JP-A-1-240828, JP-A-2-14
0632, JP-A-2-145933, JP-A-2-2-2
23833 and JP-A-3-218372.

FIG. 9 is an explanatory view of the structure of a temperature sensor having an optical fiber type distribution utilizing Raman scattered light disclosed in these publications. In the figure, 71 is an optical coupler,
Reference numeral 72 denotes a light source, 73 denotes an optical signal detector including a spectral unit and an O / E converter, and 74 denotes an optical fiber cable for measurement. In this measuring apparatus, an input port of light and an output port of reflected scattered light are respectively configured using an optical coupler 71, a light source 72 and an optical signal detector 73 are connected to the input port, and a sensor optical fiber is connected to the output port. 74 are connected. That is, when the optical coupler 71 has 2 × 2 ports,
A light source 72 and an optical signal detector 73 are connected to two ports on the input side, a measurement optical fiber cable 74 is connected to one port on the output side, and the other ports are subjected to an end face treatment to prevent Fresnel reflection. Open end.

[0005] A signal input method that has been often used in the measurement technology using OTDR represented by an optical fiber type distributed temperature sensor is a so-called short pulse optical signal input method. In this method, an optical signal is input from a light source to an optical fiber, and the returned light is detected.

[0006]

However, the prior art has the following problems. (1) In the conventional short-pulse optical signal input method, a high-output laser such as a Q-switch laser is required, which has led to an increase in size and cost of the apparatus. In addition to this, there are still problems in the measurement temperature range, the distance resolution, the measurable distance, and the measurement time in this method, and this has limited the practical range. (2) In the measuring device using the conventional optical coupler, the following problem occurs, which has a significant adverse effect on the measuring accuracy, the measuring time, and the like. Ports that do not require an optical coupler are prevented from being reflected by end face treatment or the like, but cannot be ignored because the Raman scattered light is extremely weak with respect to incident light, hindering sensitivity improvement. In the case of a 2 × 1 port structure having no unnecessary port, the isolation light is incomplete, so that the incident light leaks from the fusion processing section or the like to the photodetection section, thereby lowering the sensitivity. This problem hinders the adoption of a signal input type in which the peak power of the optical signal from the light source is small. That is, in this case, high-precision measurement cannot be performed due to disturbance due to reflected light. When an optical coupler is used, for example, in the case of a 2 × 2 port, about 1/2 of the light amount is attenuated in the path transmitted from the light source to the optical fiber, and about 1/2 in the path transmitted from the optical fiber to the optical signal detector. I will. Such attenuation lowers the S / N and causes an increase in detection accuracy, measurement processing time, and the like.

[0007] To solve the above problem (2), there has been reported a measuring device that does not use an optical coupler. FIG.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory diagram of a temperature measuring device disclosed in Japanese Patent Application Publication No. In this device, the Stokes light and the anti-Stokes light are separated with low loss by using the reflection two-color mirror having the characteristic shown in FIG. 11 and the separation two-color mirror having the characteristic shown in FIG. In the figure, 81 is a light source,
82 is an optical fiber sensor unit, 83 is a reflecting two-color mirror, 8
Reference numeral 4 denotes a separation two-color mirror, and reference numerals 85 and 86 denote light receiving units. However, in the measuring device having such a configuration, unnecessary light that becomes a disturbance cannot be removed, so that not only a separate cut filter and the like need to be installed, but also the number of times the Stokes light and the anti-Stokes light pass through the mirror. Due to the difference, an accurate intensity ratio of the scattered light cannot be obtained, and the measurement accuracy decreases. Further, either one of Stokes light and anti-Stokes light must pass through at least two or more types of mirrors, so it is not necessarily an optimal method for processing weak light, and it is not suitable for long-distance measurement. It was inappropriate.

[0008] When actually examining this temperature measuring device, it is as follows. That is, before the light receiving unit 85 receives the laser beam, the light passes through the transmission of the reflection two-color mirror 83 and the reflection of the separation two-color mirror 84, and before the light receiving unit 86 receives the laser light, the two-color reflection. Through the transmission of the mirror 83 and the separation two-color mirror 84. Here, the loss of each part is assumed as follows. Loss L1 between the light source 81 and the optical fiber sensor unit 82 L1 = -0.5 dB Loss L2 between the optical fiber sensor unit 82 and the light receiving unit 85 L2 = (-0.04) + (-0.04) =- 0.08d
B Loss L3 between the optical fiber sensor unit 82 and the light receiving unit 85 L3 = (− 0.04) + (− 0.5) = − 0.54 dB Total loss L2a and L3 from the light receiving units 85 and 86
a is as follows. L2a = (− 0.5) + (− 0.08) = − 0.58d
B (12.5%) L3a = (− 0.5) + (− 0.54) = − 1.04d
B (21.3%) Since the ratio of attenuation between the Stokes light and the anti-Stokes light is extremely different as described above, when the temperature distribution is obtained from the intensity ratio, the measurement accuracy must be reduced. Was.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and it is possible to improve the measurement accuracy by minimizing the loss of laser light, reducing the variation in the attenuation of Stokes light and anti-Stokes light, and improving the OTDR. An object of the present invention is to provide a measurement method and apparatus using OTDR including a suitable signal processing technique.

[0010]

According to one aspect of the present invention, there is provided a measuring method using an OTDR, wherein two pseudo-random signals having the same signal pattern and slightly different frequencies are generated, and one of the generated pseudo-random signals is generated. A step of modulating the laser light source with a random signal to enter the optical fiber, and a light component extraction step of extracting Stokes light and anti-Stokes light by Raman scattering from backscattered light from the optical fiber,
The method includes a step of multiplying the extracted Stokes light and anti-Stokes light by the other pseudo-random signal and then band limiting, and a step of obtaining various characteristics of the optical fiber based on the band-limited signal.

In a measuring method using an OTDR according to another aspect of the present invention, the light component extracting step is an optical separation step using an optical element having a step of extracting only Stokes light and a step of extracting only anti-Stokes light. Therefore, the light emitted from the laser light source does not pass through the optical element until the light enters the optical fiber.

A measuring apparatus using an OTDR according to another aspect of the present invention provides various distribution characteristics of an optical fiber based on the intensity of Stokes light and anti-Stokes light due to Raman scattering among the backscattered light emitted from the optical fiber. OT seeking
In a measurement apparatus using DR, a first two-color mirror composed of an optical filter that transmits and extracts one of Stokes light or anti-Stokes light, and an anti-Stokes light or Stokes light that does not pass through the first two-color mirror are used. A second dichroic mirror composed of an optical filter for transmitting and extracting the light, and a light splitting unit for guiding the optical fiber from the light source to the optical fiber via the first dichroic mirror and the second dichroic mirror. .

According to another aspect of the present invention, there is provided a measuring apparatus using an OTDR, wherein the first two-color mirror comprises an optical filter for extracting one of Stokes light or anti-Stokes light from the backscattered light and reflecting the rest. , The second two-color mirror,
The optical filter comprises an optical filter that transmits and extracts, out of the light reflected by the first two-color mirror, the light that is not extracted by the first two-color mirror out of anti-Stokes light or Stokes light.

A measuring apparatus using an OTDR according to another aspect of the present invention provides various distribution characteristics of an optical fiber based on the intensity of Stokes light and anti-Stokes light due to Raman scattering among the backscattered light emitted from the optical fiber. OT seeking
In a measuring device using DR, a third two-color mirror composed of an optical filter that reflects and extracts one of Stokes light or anti-Stokes light, and an anti-Stokes light or Stokes light that is not reflected by the third two-color mirror is used. A fourth two-color mirror composed of an optical filter that reflects and extracts the light, and a light splitting unit that guides light from the light source to the optical fiber via the third two-color mirror and the fourth two-color mirror.

According to another aspect of the present invention, there is provided a measuring apparatus using an OTDR, comprising: means for generating a first pseudo-random signal based on a first clock signal; and a second means having a frequency slightly different from the first clock signal. Means for generating a second pseudo-random signal based on the first clock signal, means for generating a laser beam amplitude-modulated by the first pseudo-random signal, and inputting the laser light to the optical fiber through the spectroscopic unit; Means for multiplying the random signal by the second pseudo-random signal;
Means for multiplying the second pseudo-random signal by the Stokes light and anti-Stokes light extracted by the spectroscopic unit out of the scattered light generated in the optical fiber, and means for band limiting each multiplied signal, Calculating means for obtaining various characteristics of the optical fiber based on the band-limited signal.

[0016]

The principle of wavelength separation and extraction in the measuring method and apparatus using the OTDR of the present invention will be described with reference to FIG. FIG. 2 is a diagram for explaining the principle of wavelength separation and extraction when two types of two-color mirrors are used. 2, two-color mirror 3 is transmitted through only the inner wavelength lambda ₊ Stokes light of the Raman scattered light other light reflected, the two-color mirror 4 wavelength lambda _- by transmitting only the anti-Stokes light of other Are reflected, and each is constituted by a band-pass filter. The characteristics of the two-color mirrors 3 and 4 are as shown in FIG. 3 or FIG. In FIG.
The filter 3 is composed of an optical filter that transmits and extracts Stokes light, and the optical filter 4 is composed of an optical filter that transmits and extracts anti-Stokes light.
FIG. 4 shows that the filter 3 is composed of an optical filter that extracts Stokes light from the backscattered light and reflects the rest, and the filter 4 is composed of an optical filter that transmits and extracts anti-Stokes light. That is, the characteristic in FIG. 4 indicates that no wavelength other than the target light is transmitted to the light receiver side. In any case, desirably, filter 3,
4 satisfies both the transmission characteristics of FIG. 3 and the reflection characteristics of FIG.

The incident light of wavelength λ ₀ from the laser light source 2 enters from the input port 7 and goes to the two-color mirror 4. Wavelength λ
The light of ₀ is transmitted to the two-color mirror 4 and the two-color mirror 3 in the splitting spectroscope 1.
After being reflected by (1) and (2), the light is input to the sensor optical fiber 11 through the sensor connection port 8. Part of the scattered light generated by the sensor optical fiber 11 is reflected back to the light source side, and then enters the branching spectroscope 1 again from the sensor connection port 8. The two-color mirror 3 transmits only the Stokes light of the returned scattered light, and reflects the other light toward the two-color mirror 4. The two-color mirror 4 transmits only anti-Stokes light and reflects other light. Therefore, Stokes light is output from the Stokes light output port 9 and input to the O / E converter 12a, and anti-Stokes light is output from the anti-Stokes light output port 10 and input to the O / E converter 12b. .

Most of the loss of light passing through the above paths depends on the characteristics of the two-color mirrors 3 and 4. However, today, the transmittance in the transmission wavelength band is 90% or more, and the reflectance in the reflection wavelength band is 98%.
% Of the two-color mirror is relatively easy to obtain, so that the loss in reflection can be almost neglected. It is possible to reduce the loss.

As described above, the branching spectroscope according to the present invention comprises:
Without using a conventional optical coupler, the light source and detector ports are separated by a combination of two-color mirrors, and there is no unnecessary port on the optical fiber side for the sensor. Leakage can be greatly reduced. Such a configuration is particularly effective when pulsed light from a light source having a small peak power, such as a pseudo-random signal, is incident, and the detection performance can be dramatically improved.

The incident light from the laser light source 2 is input to the optical fiber for sensor without passing through the two-color mirrors 3 and 4, and both the generated Stokes light and anti-Stokes light pass through one kind of two-color mirror. Since the light is only transmitted, detection can be performed with low loss and under the same conditions. In addition, since only the required wavelengths are selectively transmitted by the respective two-color mirrors, they are not affected by unnecessary disturbance light, and the wavelength can be effectively improved as compared with the conventional method combining the reflection two-color mirror and the separation two-color mirror. Separation can be performed.

Next, the principle of wavelength separation and extraction in a measuring apparatus using an OTDR according to another embodiment of the present invention will be described with reference to FIG. FIG. 5 is a principle diagram showing an example suitable for effectively removing disturbance such as Rayleigh scattered light when an optical filter is used for a two-color mirror. Two-color mirror 3
a is an optical filter that reflects only Stokes light of wavelength λ + of Raman scattered light or does not transmit only Stokes light, and the two-color mirror 4a is a wavelength λ of Raman scattered light.
And-an optical filter that reflects only anti-Stokes light or does not transmit only Stokes light. The optical characteristics of the filter in each case are shown in FIGS. It is desirable that each two-color mirror simultaneously satisfies the optical characteristics shown in FIGS.

The incident light of wavelength λ ₀ from the laser light source 2 enters from the input port 7 and travels to the two-color mirror 3a. The light having the wavelength λ ₀ passes through the two-color mirrors 3 a and 4 a in the splitting spectroscope 1, and is input to the sensor optical fiber 11 via the sensor connection port 8. Optical fiber for sensor 11
A part of the scattered light generated in step (1) is reflected back to the light source side, and enters the branching spectroscope 1 from the sensor connection port 8 again. The two-color mirror 4a reflects only the inward Stokes light of the returned scattered light, and the other light is transmitted in the direction of the two-color mirror 3a.
The two-color mirror 3a reflects only Stokes light and transmits other light. Therefore, Stokes light is output from the Stokes light output port 9 and input to the O / E converter 12a, and anti-Stokes light is output from the anti-Stokes light output port 10 and input to the O / E converter 12b. . According to the configuration of FIG. 5, the number of times of reflection and transmission of the backscattered light through the optical element to the light receiver is the same as that of the prior art shown in FIG. However, unlike the prior art, since no Rayleigh scattered light is reflected to the light receiver side, effective wavelength separation can be performed without disturbance, which contributes to improvement of measurement accuracy.
On the other hand, the light from the laser light source is transmitted twice, which is not preferable. However, this point can be solved by adopting the pseudo-random signal input method described below.

In a measuring apparatus using OTDR according to another aspect of the present invention, the light source is modulated by pseudo-randomness. From the first pseudo-random signal generator and the second pseudo-random signal generator driven by the first clock signal generator and the second clock signal generator, two signal patterns having the same signal pattern and slightly different frequencies are obtained. A pseudo random signal is repeatedly generated. The laser light is modulated by the first pseudo-random signal, input to the optical fiber for the sensor, receives the reflected and scattered light from the optical fiber, and obtains a detection signal.

When this detection signal is multiplied by the second pseudo-random signal, the result of the multiplication is that the first and second pseudo-random signals used for modulating the laser light have the same signal pattern. Since the frequency is slightly different, 2
The phases of the two signals gradually shift. If the patterns of the two pseudo-random signals match at a certain point in time, the result of the multiplication is a continuous positive signal, but the pattern of the two pseudo-random signals deviates with time, and the result of the multiplication is positive / negative. , And after a lapse of time, the patterns of the two pseudo-random signals match again, and the multiplication result becomes a continuous positive signal again. That is, a time-series signal having a constant pattern and period is obtained as a result of the multiplication. If the time-series signal obtained as a result of the multiplication is band-limited by a low-pass filter having a time constant longer than the period of the pseudo-random signal, a repetitive pulse signal having a high value when the patterns of the two pseudo-random signals match. Is obtained as a detection signal.

A repetitive pulse signal obtained as a detection signal by multiplication of two pseudo-random signals and band limitation is a signal obtained by expanding the auto-correlation function of the pseudo-random signal on the time axis. Due to the band limitation of the signal, a noise component included in the detection signal is removed, and the reflected scattered light from the sensor optical fiber is converted into S / S.
N can be measured well. The magnification of the detection signal on the time axis is determined by the frequency of the two clock signals driving the pseudo-random signal and the frequency difference between them. The frequency of the clock signal is represented by f ₁ and f ₂ , and the frequency difference is represented by Δf = f _1.
Assuming −f ₂ , the magnification is f ₁ / Δf times.

The time delay due to the signal propagation of the optical signal in the sensor optical fiber is also enlarged on the time axis. Therefore, the time delay of the optical signal in the sensor optical fiber is measured by measuring the time delay of the detection signal. , And various characteristics of the position corresponding to the time delay, for example, measurement such as temperature measurement, defect detection, and characteristic inspection can be performed. In addition, by performing such signal processing, for example, a high S / N ratio can be measured with high sensitivity, so that the power of the light source can be reduced, and in practice, the use of a semiconductor laser is sufficient. Measurement becomes possible.

In the present invention, the detection signal is separated into a Stokes signal and an anti-Stokes signal, and the Stokes signal and the anti-Stokes signal processed as described above are obtained. The temperature at the position corresponding to the delay can be measured.

[0028]

FIG. 1 is a block diagram showing a configuration of a distributed temperature sensor according to an embodiment of the present invention. In FIG. 1, a sensor optical fiber 11 is connected to the branching spectroscope 1.
Outputs of the clock signal generators 15 and 16 are supplied to pseudo random signal generators 17 and 18, respectively. The output of the pseudo-random signal generator 17 modulates the laser light source 2,
The laser light source 2 includes a semiconductor laser and a laser driver, and has a structure in which external modulation can be applied. A semiconductor laser has a wavelength λ ₀ = 1310 nm,
A DFB laser having an output P = 5 mW is used, and an isolator for avoiding the influence of reflected light is incorporated. Furthermore, in order to prevent a change in wavelength due to temperature, a device having a built-in electronic cooling element (cooler) and a thermistor is used so that the device can be operated at a constant temperature. O / O connected to the output ports 9 and 10 of the branching spectrometer
The E converters 12a and 12b use avalanche photodiodes as detection elements.

Each of the input / output ports 7 to 10 is provided with an FC connector so that an optical fiber is connected thereto.
In the splitting spectroscope 1, the collimator lens 13 forms and collects a parallel light beam. In addition, two-color mirror 3,
Reference numeral 4 has peaks of transmission wavelengths at 1390 nm and 1240 nm, respectively, and each of them is constituted by a dielectric multilayer filter having a transmittance of 90% or more. The half bandwidth of the transmission band is 4
0 nm, 5 mm from the center wavelength of the transmission band.
The reflectance at the wavelength shifted by 0 nm is 98% or more. Since the Raman scattered light has an intensity about one thousandth of that of the Rayleigh light, in this embodiment, two dichroic mirrors 3 and 4 are used in parallel on the optical axis.

In this embodiment, the clock signal generator 15,
16, the frequency is f ₁ = 250.000 MH
_z, generates two clock signals of _f 2 = 250.001MH _z, drives the pseudo random signal generator 17 and 18. The pseudo-random signal generators 17 and 18 are each composed of a shift register having a feedback loop.
An M-sequence signal having the same pattern with a code length of 1023 is generated from a sequence signal generator. Therefore, the pseudo random signal generators 17 and 18 are hereinafter referred to as M-sequence signal generators 17 and 1
8 shall be referred to.

The laser light source 2 receives an M-sequence signal output from the M-sequence signal generator 17 and generates a laser light intensity-modulated by the M-sequence signal. The modulated light from the laser light source 2 enters the splitting spectroscope 1 from the input port 7 via the optical fiber 22. In the splitting spectroscope 1, the incident light forms a parallel light beam by the collimator lens 13a, and is reflected by the two-color mirrors 4 and 3, respectively, and then condensed again by the collimator lens 13b to reach the sensor connection port 8. The light that has exited the branching spectroscope 1 through the sensor connection port 8 enters the sensor optical fiber 11. The optical signal that has entered the optical fiber for sensor 11 is scattered at each position in the optical fiber for sensor 11, and a part of the scattered light returns to the direction of the incident end and enters the branching spectroscope 1 again. The scattered light incident from the sensor connection port 8 of the branching spectroscope 1 is formed into a parallel light flux by the collimator lens 13b, and then the light of 1390 nm is emitted by the two-color mirror 3.
Only the Stokes light having a peak in the vicinity is transmitted, and the other light is reflected and enters the two-color mirror 4. The two-color mirror 4 transmits only anti-Stokes light having a peak near 1240 nm, and most of the other light is reflected toward the laser light source 2. Light transmitted through the two-color mirrors 3 and 4 is output through output ports 9 and 10 via collimator lenses 13c and 13d, respectively.
And enters the O / E converters 12a and 12b. O
The optical signals incident on the / E converters 12a and 12b are converted into electric signals proportional to the Raman scattered light intensity, and the electric signals are amplified by the amplifiers 14a and 14b.

Each of the multipliers 19a, 19b and 19c is composed of a double balanced mixer. The multiplier 19a receives the amplifier 14a and the M-sequence signal output from the M-sequence signal generator 18 and multiplies them. . The multiplier 19b inputs the amplifier 14b and the M-sequence signal and performs multiplication of the two. Outputs of the multipliers 19a and 19b are band-limited by low-pass filters 20a and 20b, respectively, and a scattered signal from the optical fiber is detected. The detection signal is A / D converted and sampled by the computer 21. The M-sequence signals generated by the M-sequence signal generators 17 and 18 are input to a multiplier 19c, multiply the two signals, and separately form a reference signal band-limited by a low-pass filter 20c. It is D-converted and sampled and input to the computer 21 as a reference signal. The temperature distribution in the sensor optical fiber 11 is determined from the time delay of the detection signal with respect to the reference signal and the electric signal proportional to the intensity of the Raman scattered light (Stokes light, anti-Stokes light).

Next, the loss of light in the distributed temperature sensor will be discussed. For example, assuming that the reflection loss Lr of the two-color mirror 3 is -0.04 dB and the transmission loss Lt is -0.5 dB, the reflection loss Lr of the two-color mirror 4 is -0.04 dB and the transmission loss Lt is -0.5 dB. The total loss is as follows. Loss L1 between light source 2 and sensor optical fiber 11 L1 = (− 0.04) + (− 0.04) = − 0.08d
B Loss L between the sensor optical fiber 11 and the light receiving section 12a
2 L2 = (− 0.04) + (− 0.5) = − 0.54 dB Loss L between the sensor optical fiber 11 and the light receiving unit 12a
3 L3 = −0.5 dB Overall loss L2a up to the light receiving units 12a and 12b,
L3a is as follows. L2a = (− 0.08) + (− 0.54) = − 0.62
dB (13.3%) L3a = (− 0.08) + (− 0.5) = − 0.58d
B (12.5%) Accordingly, the loss of light received by the light receiving units 12a and 12b is smaller than that in the conventional case, and the variation in attenuation between Stokes light and anti-Stokes light is small. I have.

Although the configuration of the spectroscopy unit in the present embodiment is based on the principle of FIG. 2, the signal processing method using the pseudo-random signal described here is applied to the spectroscopy unit of the configuration based on the principle of FIG. Even if it is applied, it is clear that the advantages of the signal processing method are exhibited. Although the present embodiment relates to a temperature measurement technique, the present invention is not limited to only temperature measurement. As long as the OTDR technique is used, it can be applied to any measurement technique involving defect detection and characteristic inspection of an optical fiber. In this embodiment, only one example of the configuration of the two-color mirror is shown. However, other optical filters having the same optical characteristics (transmission characteristics and reflection characteristics) as the derivative multilayer film or having the same functions as the derivative multilayer film are used. The invention is not excluded. Further, in the present embodiment, the case where an M-sequence signal is used as the pseudo-random signal has been described. However, the present invention is not limited to this.
A complementary sequence code, a gold code, or the like can be used.
Further, in this embodiment, two clock signal generators are used for signal transmission, but it is possible for one clock signal generator to have equivalent functions. That is, a first pseudo-random signal is generated by a clock signal generated from one clock signal, and the frequency of the clock signal is slightly changed by a frequency divider provided separately. Can be adopted. Such improvements do not depart from the scope of the present invention.

[0035]

As described above, according to the present invention, the following effects can be obtained because the spectroscopic section is used without using the optical coupler in coupling the laser light source and the O / E converter. . Since there is no unnecessary port, Fresnel reflection, which is a disturbance, can be significantly reduced. Since the loss of light in the spectroscopic unit is extremely small, S / N
Is greatly improved. In addition, since the branching filter section is composed of only two-color mirrors, erroneous detection due to disturbance light and the like, and a decrease in detection accuracy due to uneven transmission / reflection conditions are prevented, compared to a system in which a two-color mirror and a separation mirror are combined. And low-loss separation and extraction.

According to the present invention, noise is reduced by performing a correlation operation on reflected and scattered light from the optical waveguide using laser light modulated by a pseudo-random signal as a signal input to the optical waveguide. By reducing the S / N and improving the S / N, and further increasing the code length, highly sensitive signal detection becomes possible, and the signal processing time by averaging processing can be reduced. For example, by using an M-sequence signal having a code length of 1023, it is possible to measure a signal attenuated by -120 dB or more. Further, measurement in one cycle of the detection signal becomes possible, and time required for averaging a plurality of calculated values can be reduced.

Further, according to the present invention, it is possible to obtain a detection signal which is temporally expanded with respect to the actual propagation time of an optical signal for performing signal processing using two pseudo-random signals having different frequencies. In addition, signal processing such as measurement of time delay of reflected scattered light and intensity detection can be performed at a low speed. For example, if the code length of the M-sequence signal is 4095 and the clock frequency is 250
MHz and the difference between the clock frequencies are 10 kHz, the period of the detection signal is 0.4 sec, and the magnification of the detection signal is 25.
000 times (250 MHz / 10 kHz), and when the detection signal is wampled in 0.1 msec, the conversion distance becomes 0.4 m, and high resolution can be realized even at low sampling speed. In this case, even when the S / N is increased by the averaging process, the average number of times is reduced as compared with the conventional OTDR device, and
With the TDR device, it has become possible to reduce the measurement signal processing time, which took several tens of seconds, to several seconds.

Further, according to the present invention, the incidence of the conventional high-power laser beam is not necessarily required, so that the size of the apparatus, especially the laser light source, can be reduced and the cost can be reduced. In particular, if the optical element is not transmitted at all in the optical path from the laser light source to the incident end of the optical fiber, the laser light before entering the optical fiber is hardly attenuated. To contribute.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a distributed temperature sensor according to one embodiment of the present invention.

FIG. 2 is a diagram illustrating the principle of wavelength separation and extraction when a bandpass filter is used.

FIG. 3 is a characteristic diagram of transmittance of a bandpass filter.

FIG. 4 is a characteristic diagram of the reflectance of a bandpass filter.

FIG. 5 is a diagram illustrating the principle of wavelength separation and extraction when a band cut filter is used.

FIG. 6 is a characteristic diagram of transmittance of a band cut filter.

FIG. 7 is a characteristic diagram of the reflectance of the band cut filter.

FIG. 8 is a characteristic diagram of Rayleigh scattered light and Raman scattered light.

FIG. 9 is a diagram illustrating the configuration of a conventional optical fiber distribution type temperature sensor.

FIG. 10 is a diagram illustrating the configuration of a conventional temperature measurement device.

11 is a characteristic diagram of a reflective two-color mirror of the device of FIG.

FIG. 12 is a characteristic diagram of a separation two-color mirror of the apparatus of FIG. 10;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Branch spectrometer 2 Laser light source 3, 4, 3a, 4a Two-color mirror 7 Input port 8 Sensor connection port 9 Stokes light output port 10 Anti-Stokes light output port 11 Optical fiber for sensor 12 O / E converter (photodetector 13a, 13b, 13c, 13d Collimator lens 14a, 14b Amplifier 15, 16 Clock generator 17, 18 Pseudo-random signal generator 19a, 19b, 19c Multiplier 20a, 20b, 20c Low-pass filter 21 Computer 22 Optical fiber

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Takeo Yamada 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (58) Field surveyed (Int. Cl. ⁶ , DB name) G01M 11/00 -11/02

Claims (6)

(57) [Claims] A step of generating two pseudo-random signals having the same signal pattern and slightly different frequencies, a step of modulating a laser light source with one of the generated pseudo-random signals and causing the laser light source to enter an optical fiber; A light component extraction step of extracting Stokes light and anti-Stokes light by Raman scattering from the backscattered light from the optical fiber, and a step of band limiting after multiplying the extracted Stokes light and anti-Stokes light by the other pseudorandom signal; Obtaining a variety of characteristics of the optical fiber based on the band-limited signal. 2. The light component extraction step is an optical separation step by an optical element having a step of extracting only Stokes light and a step of extracting only anti-Stokes light, wherein the light emitted from the laser light source is 2. The method according to claim 1, wherein the light is not transmitted through the optical element until the light enters the optical fiber. 3. Obtaining various distribution characteristics of the optical fiber based on the intensities of Stokes light and anti-Stokes light due to Raman scattering among the backscattered light emitted from the optical fiber.
In a measuring apparatus using TDR, a first two-color mirror composed of an optical filter that transmits and extracts one of Stokes light or anti-Stokes light;
A second filter comprising an anti-Stokes light which does not transmit through the two-color mirror or an optical filter which transmits and extracts the Stokes light.
A measuring apparatus using an OTDR, comprising: a color mirror; and a spectroscopic unit that guides an optical fiber from a light source to the optical fiber via the first two-color mirror and the second two-color mirror. 4. The first two-color mirror includes an optical filter that extracts one of Stokes light or anti-Stokes light from backscattered light and reflects the rest, and the second two-color mirror includes the second filter. From the light reflected from the two-color mirror,
4. An optical filter according to claim 3, further comprising an optical filter that transmits and extracts the light that is not extracted by the first two-color mirror out of the anti-Stokes light or the Stokes light.
Measurement device using TDR. 5. A method for obtaining various distribution characteristics of an optical fiber based on the intensity of Stokes light and anti-Stokes light due to Raman scattering among the backscattered light emitted from the optical fiber.
In a measuring apparatus using TDR, a third two-color mirror composed of an optical filter that reflects and extracts one of Stokes light or anti-Stokes light;
And a fourth two-color mirror composed of an optical filter that reflects and extracts anti-Stokes light or Stokes light that is not reflected by the two-color mirror, and outputs light from a light source to the third two-color mirror and the fourth A measuring device using an OTDR having a spectroscopic section for guiding to an optical fiber via a two-color mirror. 6. A means for generating a first pseudo-random signal based on a first clock signal, and generating a second pseudo-random signal based on a second clock signal having a frequency slightly different from the first clock signal. Means for generating, a means for generating laser light amplitude-modulated by the first pseudo-random signal, and inputting the laser light to the optical fiber through the spectroscopic unit; and a means for generating the first pseudo-random signal and the second pseudo-random signal. Means for multiplying the second pseudo-random signal by the Stokes light and anti-Stokes light extracted by the spectroscopic unit out of the scattered light generated in the optical fiber. And means for calculating various characteristics of the optical fiber based on the band-limited signal.
A measuring device using the OTDR according to any one of the above.