JP5802895B2 - Optical code modulation pulse correlation processing method - Google Patents

Description

  The present invention relates to a technique for measuring a system response using an optical signal, and more particularly to a technique for improving a distance measurement accuracy by applying a correlation code to improve an S / N ratio of a received signal.

  The response function of the system can be evaluated by inputting a single pulse with a short time width and measuring its output as a function of time. The shorter the pulse width, the higher the frequency characteristics of the system and the more accurate the distance measurement. In this measurement method, the peak power of the pulse signal is increased in order to prevent the S / N ratio of the received signal from being lowered due to the decrease in the energy of the received signal or the increase in noise accompanying the expansion of the reception band. However, when the output power of the transmitter has an upper limit or the maximum input power to the system may be limited, in this case, a code-modulated pulse signal is used instead of a short pulse. By performing correlation processing or pulse compression on a code-modulated signal at the time of reception, the S / N ratio can be improved as compared with the case of using a single pulse. Such a technique is applied in ranging fields such as radar using microwaves and LIDAR (Light Detection and Ranging) using optical signals. Recently, it has been actively applied to distributed optical fiber sensing technologies such as OTDR (Optical Time Domain Reflectometry), Raman OTDR, BOTDR (Brillouin OTDR), and BOTDA (Brillouin Optical Time Domain Analysis).

  The problem with receiving a code-modulated signal and correlating the received signal (pulse compression) is that the waveform of the received signal is distorted, which increases the time sidelobe in the correlation characteristics between it and the original code. It is to end. When the time side lobe becomes large, the accuracy of distance measurement and the distance resolution deteriorate. The mechanism by which the waveform of the received signal is distorted depends on the physical phenomenon involved. In the following, the problem to be solved by the present invention will be described by taking BOTDA and OTDR as an example.

  First, BOTDA will be described as an example. BOTDA is a technique for measuring the distribution of strain or temperature by applying Brillouin amplification in an optical fiber.

  As shown in FIG. 1, in BOTDA, pulse pump light enters from one end face FE1 of an optical fiber 1 used as a sensor, and continuous probe light enters from the other end face FE2. The pulse pump light obtained by pulse-modulating the continuous light output from the laser 2 with the pulse signal generated by the pulse signal generator 4 by the optical modulator 3, and the continuous probe light output from the laser 5. Interferes at the meeting position in the optical fiber 1 and generates a sound wave in the optical fiber 1 by the electrostrictive effect. Due to the diffraction effect of the sound wave, a part of the pulse pump light is frequency-shifted and scattered backward, and is received by the optical receiver 7 via the coupler 6.

  When the frequency difference between the pulse pump light and the continuous probe light coincides with a frequency amount called Brillouin frequency shift specific to the material of the optical fiber 1, the backscattered light overlaps with the continuous probe light in phase, Power increases. That is, the pulse pump light amplifies the continuous probe light via sound waves. This is called Brillouin amplification.

  It is known that the Brillouin frequency shift changes linearly with changes in optical fiber strain and temperature. Therefore, the amplified signal is observed with BOTDA, and the distortion of the optical fiber 1 and the change in temperature can be measured from the change in the frequency difference between the pulse pump light and the continuous probe light at that time. The position can be calculated from the time from when the pulse pump light is incident on the end face FE1 of the optical fiber 1 until the amplified signal of the continuous probe light reaches the end face FE1 of the optical fiber 1.

  In order to increase the distance resolution of BOTDA, the width of the pulse pump light may be shortened. However, at that time, as described above, the SN ratio of the received signal decreases due to the decrease in the energy of the received signal and the increase in noise accompanying the expansion of the reception band. Furthermore, in the case of Brillouin amplification, if the width of the pulse pump light is set to about 10 ns (time called the life of a sound wave) or less, the sound wave contributing to the amplification does not grow sufficiently, and the Brillouin amplification degree rapidly decreases. Also occurs.

As a way to solve these problems,
1) Phase shift pulse BOTDA (PSP-BOTDA)
2) Encoded PSP-BOTDA
Have been proposed (see Non-Patent Document 1 (especially pp. 207-208), Non-Patent Document 2 (especially pp. 33-35)).

  As shown in FIG. 2, the PSP-BOTDA uses a pulsed light for measuring pump light (second pulsed light) and a pulsed light for sound wave excitation (first pulsed light) positioned in front of it in time. And a pump light (0 shift pump light (FIG. 2 (a))) in which the phase of the measurement pulse with respect to the pulse light for sonic excitation is 0, and a pump light (π This is a method of measuring using both of the shift pump light (FIG. 2 (b)).

  By making the width of the sound wave excitation pulse longer than 10 ns, it is possible to sufficiently grow the sound wave involved in the amplification, and to contribute the sound wave to the amplification by the measurement pulse. As a result, it is possible to avoid a rapid decrease in the Brillouin amplification that has conventionally occurred in BOTDA. Further, by subtracting the measurement data by the π shift pump light from the measurement data by the 0 shift pump light, the Brillouin amplification signal by the sound wave excitation pulse is canceled and only the Brillouin amplification signal by the measurement pulse is doubled and extracted. It becomes possible.

  The encoded PSP-BOTDA is obtained by applying a code to the measurement pulse of the pulse pump light of the PSP-BOTDA to form an encoded pulse light train. An example is shown in FIG. As the code, a complementary correlation code (Golay code), a Barker code, an M-sequence code, or the like can be used. With the help of the sound wave generated by the sound wave excitation pulse (first pulse), the encoded pulse light train that is the measurement pulse (second pulse) efficiently Brillouin amplifies the continuous probe light. The Brillouin amplified continuous probe light is received, and correlation processing between the received signal and the original code is performed. As a result, the time width of the received signal is compressed to a pulse width of 1 bit (pulse compression). Therefore, the distance resolution of measurement is improved. Further, the amplitude of the correlated signal is proportional to or approximately proportional to the code length, and the noise amplitude is proportional to or approximately proportional to the square root of the code length. As a result, the signal-to-noise ratio (converted into optical signal power) of the correlated signal can be improved in proportion to or approximately in proportion to the square root of the code length.

  However, when the total time width of the encoded pulse beam becomes equal to or longer than the acoustic wave lifetime of 10 ns, the influence of attenuation of the sound wave generated by the sound wave excitation pulse cannot be ignored. That is, as shown in FIG. 4, the front pulse light train of the encoded pulse light train interacts with a high intensity sound wave, but the rear pulse light train interacts with an attenuated sound wave. The amplified signal becomes a pulse train attenuated with time. As a result, when the Brillouin amplified continuous probe light is received and correlation processing (pulse compression) between the received signal and the original code is performed, the amplitude of the correlation processed signal does not increase in proportion to the code length. Even when the code length is increased, there is almost no change. Furthermore, the noise signal in the time side lobe becomes large, and the distance resolution characteristics deteriorate.

  Next, the problem to be solved by the present invention will be described using OTDR as an example.

  In OTDR, as shown in FIG. 5, the pulsed light obtained by pulse-modulating the continuous light output from the laser 2 with the pulse signal generated by the pulse signal generator 4 by the optical modulator 3 is supplied to the optical fiber 1. An optical signal that is incident from one end face FE1 and part of the pulse light is Rayleigh backscattered in the optical fiber 1 and reaches the end face FE1 is received by the optical receiver 7 via the coupler 6. The scattering position can be calculated from the time from when the pulsed light is incident until the Rayleigh backscattered light is received. Therefore, the optical fiber loss between the two received signal levels from any two points can be evaluated. Further, it is possible to calculate the position of the end of the optical fiber 1 or the break position from the time when the received signal disappears.

  In OTDR, a study to improve the S / N ratio of a received signal by using a pulsed light train in which pulsed light is encoded has been conducted from an early stage, and there is an example of commercialization as a measuring device (Non-Patent Document 3 (particularly pp.24-26). )reference).

  In order to further increase the measurement distance or improve the distance resolution over these OTDRs, it is conceivable to use an optical amplifier (OA). There are many research examples in which OA is applied to OTDR using a single pulse light, but there are few research examples in which OA is applied to OTDR using an encoded pulsed light train. One of the reasons is that, as shown in FIG. 6, when the encoded pulse optical train is amplified by OA, the amplification at the head portion of the encoded pulse optical train is large, but the amplification decreases as it approaches the tail portion. However, the amplified waveform is distorted. As a result, when correlation processing (pulse compression) between the received signal and the original code is performed, the amplitude of the correlation-processed signal does not increase in proportion to the code length, and hardly changes even when the code length is increased. . Furthermore, the noise signal in the time side lobe becomes large, and the distance resolution characteristics deteriorate.

  As described above, the problem when the OA is applied to the encoded OTDR has been described. However, the same problem to be solved exists when the OA is applied to the encoded BOTDR, the encoded BOTDA, and the encoded Raman OTDR.

  An object of the present invention is to increase noise in time side lobes in correlation characteristics even when a received signal waveform is distorted when a code-modulated signal is received and the received signal is subjected to correlation processing (pulse compression). Rather, it is to improve the SN ratio of the received signal and improve the distance measurement accuracy.

In the present invention,
A correlation code sequence signal generator for generating a correlation code sequence signal C (t) as a function of time t;
An optical modulator that modulates light with a signal including the correlation code sequence signal C (t);
When the speed of light is v and the time width of the 1-bit pulse (sub-pulse) of the correlation code sequence signal C (t) is τ, the length is approximately equal to or less than v / (2Γ). The optical signal modulated by the optical modulator is applied to an optical system in which the step response function from the section is h (t) and the inverse function of h (t) is g (t) = 1 / h (t). An optical receiver that receives light output from the optical system and outputs a reception signal RC (t) when input;
A function obtained by multiplying the function g (t) and the correlation code sequence signal C (t) is defined as C (t) g (t), and the received signal RC (t) or the code of the correlation code sequence signal C (t) is obtained. signal obtained by inverting the polarity of the sequence of elements C ^- (t) RC a received signal when using the - ^(t) and the RC from RC (t) when - the signal obtained by subtracting the ^(t), the function And a correlation processor for performing correlation processing with C (t) g (t).

Alternatively, the present invention
A correlation code sequence signal generator for generating a correlation code sequence signal C1 (t) and a reverse correlation code sequence signal C2 (t) in which the code elements are arranged in reverse order as a function of time t;
An optical modulator that modulates light with a signal including the correlation code sequence signal C1 (t) and a signal including the correlation code sequence signal C2 (t);
An optical receiver that receives light output from the optical system when an optical signal modulated by the optical modulator is input to the optical system and outputs a received signal RC1 (t) and a received signal RC2 (t); ,
The received signal RC1 (t) or the correlation code sequence signal C1 (t) of the code sequence of the signal obtained by inverting the polarity of the elements C1 - ^(t) the received signal when using RC1 - when a ^(t) to RC1 (t) from the RC1 - correlation processing a signal obtained by subtracting the ^(t), the correlation code sequence signal C1 (t), the and the received signal RC2 (t) or the correlation code sequence signal C2 (t) signal obtained by inverting the polarity of the elements of the code sequence of C2 - from RC2 (t) RC2 when the ^(t) - - a signal obtained by subtracting the ^(t), the received signal RC2 when using ^(t) A correlation processor for performing correlation processing with the correlation code sequence signal C2 (t);
A synthesis processor that adds the correlation processing result output from the correlation processor with the correlation code sequence signal C1 (t) and the correlation processing result with the correlation code sequence signal C2 (t) and outputs the result. It is characterized by having.

Alternatively, the present invention provides
A correlation code sequence signal generator for generating a correlation code sequence signal C1 (t) and a reverse correlation code sequence signal C2 (t) in which the code elements are arranged in reverse order as a function of time t;
An optical modulator that modulates light with a signal including the correlation code sequence signal C1 (t) and a signal including the correlation code sequence signal C2 (t);
When the speed of light is v and the time width of the 1-bit pulse (subpulse) of the correlation code sequence signal C1 (t) and the correlation code sequence signal C2 (t) is τ, the length is v / (2Γ ) Is a step response function from a short interval substantially equal to or less than h), a linear function with respect to time t is F (t) = β (1−αt), and F (t) is h When an optical signal modulated by the optical modulator is input to an optical system whose function divided by (t) is G (t) = F (t) / h (t), the optical system outputs the optical signal. An optical receiver that receives light and outputs a received signal RC1 (t) and a received signal RC2 (t);
A function obtained by multiplying the function G (t) and the correlation code sequence signal C1 (t) is defined as C1 (t) G (t), and the function G (t) and the correlation code sequence signal C2 (t) are multiplied. the function and C2 (t) G (t), the received signal RC1 (t) or the correlation code sequence signal C1 (t) of the code sequence of elements of the polarity signal obtained by inverting the C1 - when using the ^(t) of the received signal RC1 - RC1 (t) from RC1 when a ^(t) - correlation of a signal obtained by subtracting the ^(t), the function C1 and (t) G (t), and the received signal RC2 signal obtained by inverting the polarity of the elements of the code sequence (t) or the correlation code sequence signal C2 (t) C2 - a reception signal when using the ^(t) RC2 - when a ^(t) RC2 (t ) And RC2 ⁻ (t) subtracted from the function C2 (t) G (t)
A synthesis processor for adding the correlation processing result output from the correlation processor with the function C1 (t) G (t) and the correlation processing result with the function C2 (t) G (t) and outputting the result. It is characterized by comprising.

  As the correlation code, an arbitrary correlation code such as a complementary correlation code (Golay code) and a Barker code is used.

  According to the present invention, since the correlation processing method of the optical code modulation pulse is configured as described above, even when the waveform of the reception signal is distorted, the noise of the reception signal is increased without increasing the noise in the time side lobe in the correlation characteristics. The SN ratio can be improved, and the distance measurement accuracy can be improved.

Configuration diagram showing an example of BOTDA Explanatory drawing showing an example of pump light of PSP-BOTDA Explanatory drawing which shows an example of the pump light of PSK code modulation BOTDA Illustration of waveform distortion of Brillouin signal due to attenuation of sound wave Configuration diagram showing an example of OTDR Illustration of waveform distortion of optical amplification signal The block diagram which shows 1st thru | or 3rd Embodiment and the modification 1 of the correlation processing system of the optical code modulation pulse of this invention Explanatory drawing of SN ratio improvement rate SNIR1, SNIR2, SNIR3 of 1st thru | or 3rd embodiment Explanatory drawing which shows an example of the pump light of ASK code modulation BOTDA The block diagram which shows 4th Embodiment of the correlation processing system of the optical code modulation pulse of this invention Explanatory drawing which shows an example of PSK code modulation probe light or pump light 5 is a block diagram showing fifth to seventh embodiments of the correlation processing method for optical code modulation pulses according to the present invention and a second modification. Explanatory drawing which shows an example of ASK code modulation probe light or pump light The block diagram which shows the modification 3 and 4 of the correlation processing system of the optical code modulation pulse of this invention FIG. 5 is a block diagram showing modified examples 5 and 9 of the correlation processing method of the optical code modulation pulse of the present invention. FIG. 6 is a block diagram showing modified examples 6 to 8 of the optical code modulation pulse correlation processing method of the present invention. Explanatory drawing which shows an example of the pump light of PSK discrete code modulation BOTDA Explanatory drawing which shows an example of the pump light of ASK discrete code modulation BOTDA

[First Embodiment]
FIG. 7 shows an example applied to the correlation processing method of an optical code modulation pulse of the present invention, here, PSP-BOTDA. In the figure, the same components as those of the conventional example are represented by the same reference numerals. 1 is an optical fiber, 2 is a laser, 3 is an optical modulator, 6 and 11 are couplers, 7 is an optical receiver, 12 is an optical frequency shifter, 13 is a correlation code sequence signal generator, 14 is a correlation processor, 15 Is a synthesis processor.

  First, a first embodiment of the present invention will be described along with its operation.

  The laser 2 uses a light source having a narrow oscillation line width and high coherence. The continuous light that is the output of the laser 2 is branched into two by the coupler 11 and used as pump light and probe light, respectively. The light used as the probe light is input to the optical frequency shifter 12, and the optical frequency is downshifted by a frequency similar to the Brillouin frequency shift of the optical fiber 1. The frequency-shifted continuous light enters the optical fiber 1 from the end face FE2 of the optical fiber 1. On the other hand, the light used as the pump light is input to an optical modulator 3 which is an optical modulation means (OM). An output signal from the correlation code sequence signal generator 13 is input to an electric signal input terminal of the optical modulator 3, and code-modulated pump light is generated by the electric signal. The code-modulated pump light passes through the coupler 6 and enters the optical fiber 1 from the end face FE1 of the optical fiber 1.

  Now, it is assumed that a certain part of the optical fiber 1 is distorted and the Brillouin frequency shift of the part changes from the value of the peripheral part. Then, assuming that the frequency difference between the pump light and the continuous probe light is the same as the Brillouin frequency shift of the portion where the distortion occurs, the portion where the distortion is caused by the code-modulated pump light according to the principle of Brillouin amplification The probe light is amplified by Brillouin when passing through. The Brillouin amplified probe light passes through the coupler 6, is input to the optical receiver 7, and is converted into an electric signal. The electrical signal is input to the correlation processor 14. The correlation processor 14 performs a cross-correlation process between the signal based on the output signal of the correlation code sequence signal generator 13 and the output signal of the optical receiver 7 and outputs a pulse compression signal. When there are a plurality of pulse compression signals output from the correlation processor 14, the synthesis processor 15 outputs a pulse compression signal obtained by synthesizing them.

  Hereinafter, the principle of the present invention will be described.

  A case where the correlation code to be used is a complementary correlation code (Golay code) will be described. The complementary correlation code is composed of two code sequences A and B, and A and B satisfy the following properties.

  Here, L is the length of the code, Ai, Bi (i = 1, 2,... L) are elements of the code sequences A and B, and takes a binary value of 1, -1. Accordingly, when the correlation characteristic of the pulse train code-modulated based on the complementary correlation code is processed based on the equation (1), the main lobe (n = 0) takes a value twice the code length L, and the time side lobe ( n ≠ 0) is 0. Therefore, the complementary correlation code has optimum characteristics for distribution measurement.

  Now, the complementary correlation code sequences A1 and B1 expressed as a function of time with a time width of 1 bit of the code as τ are referred to as correlation code sequence signals A1 (t) and B1 (t). Consider a case where the pump light is code-modulated by a signal based on A1 (t) and B1 (t). In an embodiment of the present invention to be described later, since a reverse code sequence in which the order of elements of the code sequence is reversed is also used, the code sequences A1 and B1 are referred to as forward code sequences A1 and B1 here. To do. Further, the length of the portion where the optical fiber 1 is distorted is assumed to be a length vτ / 2 corresponding to a time width τ of a sub-pulse which is a pulse of 1 bit (where v is a length in the optical fiber). speed of light). Then, it is assumed that the probe light is Brillouin amplified by the code-modulated pump light and received by the optical receiver 7 in the portion where the distortion occurs. The received electrical signals are represented by RA1 (t) and RB1 (t).

  Until now, the inventors have reported several kinds of code modulation methods. Here, the method shown in FIG. 3 is assumed. In the method of FIG. 3, pump light is composed of a first pulse for sound wave excitation and a second pulse for measurement. Further, the second pulse is equally divided into L code elements, which is the code length, to create L sub-pulses (L = 4 in the example of FIG. 3). A code is assigned to each sub-pulse. In the example of FIG. 3, code elements 1 and −1 are assigned to phase shifts 0 and π.

  By doing so, it is possible to sufficiently grow a sound wave that contributes to Brillouin amplification by the first pulse, and each sub-pulse can efficiently amplify the probe light using the grown sound wave. . Further, since the sound wave generated by the first pulse interacts coherently with the subpulse, the probe light is used when the phase of the first pulse and the phase of the phase code modulated subpulse are in phase (phase difference 0). On the other hand, the power of the probe light is attenuated when the phase of the first pulse and the phase of the phase-code modulated sub-pulse are opposite in phase (phase difference π). As previously assumed, the length of the portion in which the optical fiber 1 is distorted is the length corresponding to the pulse width of 1 bit of the code, so that the optical receiver 7 receives the change in the probe light power due to the subpulse. The received electrical signal is similar to the code sequence signals A1 (t) and B1 (t).

However, the amplified electric signal generated by the first pulse itself for sound wave excitation overlaps with the received electrical signal. Therefore, the PSP-BOTDA, the code and the elements of the code sequence A1 was replaced with other element sequences A1 ^-, code sequence B1 that further elements of the code sequence B1 was replaced with other elements ^- is also used.

For example, phase complementary correlation code sequence of code length L = 4 is A1 = {1,1,1, -1}, B1 = {1,1, -1,1}
When the phase complementary correlation code sequence replace the element 1 was replaced with -1, obtained by replacing the 1 -1,
A1 ⁻ = {− 1, −1, −1,1}, B1 ⁻ = {− 1, −1,1, −1}
become that way.

The first pulse is kept unchanged, the second pulse code A1 ^- signal based on A1 - generates pump light that is phase code modulation in ^(t) (see FIG. 3 (b)), whereby the probe light A signal obtained by amplifying the signal is also measured. The received electrical signal RA1 - a ^(t). Similarly, the first pulse is kept unchanged, the second pulse code B1 ^- signal based on the B1 - ^(t) in generating the pump light phase code modulation, also thereby signal Brillouin amplified probe light measurement To do. The received electrical signal is RB1 ⁻ (t).

The Brillouin amplified signal by the first pulse light included in RA1 ⁻ (t) and RB1 ⁻ (t) measured in this way is the same as the signal included in RA1 (t) and RB1 (t), but the phase The Brillouin amplified signal by the code-modulated sub-pulse light has a value of reverse polarity. Consequently, their difference signal, ΔRA1 (t) = RA1 ( t) -RA1 - (t) and ΔRB1 (t) = RB1 (t ) -RB1 - (t) , respectively, A1 (t) and B1 ( Except for t) and amplitude, the same waveform is assumed.

  Therefore, the sum of the cross-correlation between A1 (t) and ΔRA1 (t) and the cross-correlation between B1 (t) and ΔRB1 (t),

Ask for. Here, Ψ (nτ) = Ψ (n), A1 (mτ) = A1 _m , ΔRA1 ((m + n) τ) = ΔRA1 _{m + n} , B1 (mτ) = B1 _m , ΔRB1 ((m + n ) τ) = ΔRB1 _{m + n} (hereinafter, the same notation is used).

  In the same way as Expression (1), Expression (2) takes a large value only for the main lobe, and the side lobe becomes zero. That is, it is possible to evaluate a portion where the optical fiber 1 is distorted with a distance resolution vτ / 2 determined by the width τ of the sub-pulse obtained by dividing the second pulse.

  However, in reality, the lifetime of the sound wave generated by the first pulsed light is about 10 ns in the case of the silica-based optical fiber, so that the width Lτ of the second pulsed light is equal to or longer than that. The amplification factor or attenuation factor of each code-modulated sub-pulse is greatly reduced in the second half compared to the first half of the second pulse light (see FIG. 4). As a result, the value of the main lobe of the correlation function of Equation (2) decreases and noise occurs in the side lobe.

  Therefore, it is considered to correct the attenuation of sound waves when performing correlation processing.

The attenuation coefficient Γ of the amplitude of the sound wave involved in Brillouin amplification or Brillouin scattering is given by Γ = πΔν _b where Δν _b is the full width at half maximum of the Brillouin gain spectrum . Therefore, the value of Γ can be calculated by measuring Δν _{b of the} optical fiber 1. At this time, when the speed of light in the optical fiber is v, the step response function of the Brillouin amplification by the measurement pulse (second pulse) from the section whose length is approximately equal to or shorter than v / (2Γ) is Expressing the time when the measurement pulse reaches that short section as a reference,
h (t) = exp (−Γt) (3)
Given in.

Now, the length Δz of the portion where the optical fiber 1 is distorted is Δz ≦ v / (2Γ), and when the pump light modulated by the code sequence A1, A1 ⁻ is used, the length of the code is 1 bit. It is assumed that Δz ≦ vτ / 2, where τ is the time width of the sub-pulse. At this time, in the portion where the distortion has occurred, the change in RA1 (t) of the probe light power amplified by Brillouin by the code-modulated pump light, and the signal received by the optical receiver 7 and converted into an electrical signal are: It is proportional to A1 (t) h (t). The same applies to the difference signal ΔRA1 (t). If the proportionality factor is omitted for simplicity,
ΔRA1 (t) = A1 (t) h (t) (4)
It is. Similarly, the difference signal ΔRB1 (t) when the code sequence B1, B1 ⁻ is used is
ΔRB1 (t) = B1 (t) h (t) (5)
It can be expressed as

  Now, let the inverse function of the step response function h (t) be g (t) = 1 / h (t), and cross-correlate A1 (t) with ΔRA1 (t) g (t) corrected for sound wave attenuation. When the calculation process of the sum of cross-correlation between B1 (t) and ΔRB1 (t) g (t) corrected for sound wave attenuation is performed, the following is obtained.

_{Here, h (nτ) = h n} , g (nτ) = g n and marked (hereinafter, using the same notation).

  That is, by performing the cross-correlation process expressed by Equation (6) that corrects the attenuation of the sound wave, the signal can be detected without reducing the main lobe signal and without causing side lobe noise. It becomes possible.

  However, in actual measurement, as assumed above, there is not one section where Brillouin amplification occurs, and there are a plurality of sections distributed in the length direction of the optical fiber. For this reason, g (t), which is an inverse function of the step response function h (t) expressed with reference to the time at which a measurement pulse reaches a specific short section, cannot be used as it is in the general case. That is, the cross-correlation between a function ΔRA1 (t) g (t) obtained by multiplying the difference signal ΔRA1 (t) by the inverse function g (t) and the sign function A1 (t), and the difference signal ΔRB1 (t) The cross-correlation between the function ΔRB1 (t) g (t) multiplied by g (t) and the sign function B1 (t) cannot be calculated.

  Therefore, in the present invention, attention is paid to the fact that the sound wave attenuates exponentially as shown by the equation (3), and not the “attenuation” of the sound wave but the “attenuation rate” of the sound wave is corrected. Therefore, as shown by the following formula (7), the cross-correlation between A1 (t) g (t) and ΔRA1 (t) and B1 (t) A calculation process of the sum of the cross-correlation between g (t) and ΔRB1 (t) is performed.

  By doing in this way, even when the section where Brillouin amplification occurs is distributed in the length direction of the optical fiber, the main lobe signal is not reduced, and the side lobe noise is not generated. It can be seen that the signal can be detected ideally. Further, as can be seen from a comparison between the equations (6) and (7), in the present invention, when the correlation value of the side lobe at the time shift nτ is obtained, the weight exp (−Γnτ) is applied, so that the code modulation is performed. Even if there is some variation in the amplitude of the optical signal, the effect is suppressed.

[Second Embodiment]
In the present embodiment, the present invention is applied to PSP-BOTDA as in the first embodiment. The basic configuration of the correlation processing method of the optical code modulation pulse according to the second embodiment of the present invention is the same as that of FIG. 7 showing the configuration of the first embodiment, but the correlation code sequence signal generator 13 and the correlation processor 14 are the same. And the function of the synthesis processor 15 is different from that of the first embodiment.

  Correlation code sequence signal generator 13 generates forward complementary correlation code sequence signals A1 (t) and B1 (t) and reverse complementary correlation code sequence signals A2 (t) and B2 (t ) Is generated.

For example, the forward complementary correlation code sequence with a code length L = 4 is A1 = {1,1,1, −1}, B1 = {1,1, −1,1}
Then, the reverse complementary correlation code sequence is
A2 = {-1,1,1,1}, B2 = {1, -1,1,1}
It is.

The correlation code sequence signal generator 13 further generates A1 ⁻ (t), B1 ⁻ (t), A2 ⁻ (t), and B2 ⁻ (t) with the elements of the code exchanged. Then, like the first embodiment, the code sequence signal A1 (t), B1 (t ), A2 (t), B2 (t), A1 - (t), B1 - (t), A2 - (t) , B2 ⁻ (t) Consider a case where the pump light is code-modulated. Similarly to the case of the first embodiment, it is assumed that the probe light is Brillouin amplified by the code-modulated pump light and received by the optical receiver 7 in the portion where the distortion of the optical fiber 1 occurs. Further, the received electrical signals are made to correspond to the code sequences A1, B1, A2, B2, A1 ⁻ , B1 ⁻ , A2 ⁻ , B2 ⁻ , RA1 (t), RB1 (t), RA2 (t), RB2 ^{(t), RA1 - (t} ), RB1 - (t), RA2 - (t), RB2 - and represented by (t). Furthermore, ΔRA1 (t) = RA1 ( t) -RA1 - (t), ΔRB1 (t) = RB1 (t) -RB1 - (t), ΔRA2 (t) = RA2 (t) -RA2 - (t), ΔRB2 (t) = RB2 (t ) -RB2 - and (t).

In the present embodiment, as shown by the following equation (8), the correlation processor 14 causes the cross-correlation Φ _A1 (n) between A1 (t) and ΔRA1 (t), A2 (t), and ΔRA2 ( t) cross-correlation Φ _A2 (n), B1 (t) and ΔRB1 (t) cross-correlation Φ _B1 (n), B2 (t) and ΔRB2 (t) cross-correlation Φ _B2 (n) ). Then, the synthesis processor 15 performs a calculation process for synthesizing the total sum Φ (n).

Here, using the relationship of A2 _i = A1 _{L−i + 1} ,

  The exponential function in the right side {} of Equation (9) can be approximated by a linear function when the argument is small, and the value in {} does not depend on m. as a result,

It becomes. Similarly,

Therefore,

  From the above, by using the forward code and the reverse code thereof, the correlation processing is performed by the correlation processor 14 according to the equation (8), and the correlation processing signal is synthesized by the synthesis processor 15. Even when sections where Brillouin amplification occurs are distributed in the length direction of the optical fiber, the signal is detected without greatly reducing the main lobe signal and without causing significant side lobe noise. It turns out that it becomes possible.

  Next, the SN ratios of the first embodiment and the second embodiment of the present invention are compared. Since the side lobe noise is 0 or almost 0, only the random noise of the receiving circuit is considered. In order to simplify the calculation, the discrete value is replaced with a continuous value.

  At this time, the power PS1, noise power PN1, and signal-to-noise ratio SNR1 of the received signal of the first embodiment are as follows.

Here, σ ² is the noise power when data is sampled at intervals of time τ.

On the other hand, when the measurement pulse after the sound wave excitation pulse is measured with pump light that is not an encoded pulse but only one sub-pulse having a time width τ, the signal-to-noise ratio SNR0 is SNR0 = 1 / σ ^2.
Given in.

  Accordingly, the signal-to-noise ratio improvement rate SNIR1 of the first embodiment is given by the following equation.

  On the other hand, the received signal power PS2, noise power PN2, signal-to-noise ratio SNR2, and signal-to-noise ratio improvement rate SNIR2 of the second embodiment are as follows.

The approximate value of the attenuation coefficient Γ can be calculated if the wavelength of the optical fiber to be used and the light source are determined. For example, in the case of a silica glass optical fiber, the full width at half maximum Δν _b of the Brillouin gain spectrum is about 35 MHz at a wavelength of 1.55 μm.

It is.

FIG. 8 shows the calculation results of SNIR1 and SNIR2 when τ = 0.3 ns and Γ = 0.1 Gs ⁻¹ . In the first embodiment, random noise is also increased as the signal is corrected. Therefore, when the code pulse time Lτ becomes very long, SNIR1 decreases. On the other hand, in the second embodiment, since signal correction is not performed, the amount of increase in random noise is moderate as compared to the first embodiment. As a result, SNIR2 of the second embodiment is larger than SNIR1 of the first embodiment. However, in the second embodiment, it should be noted that noise due to the time side lobe is generated although it is suppressed.

[Third Embodiment]
In the present embodiment, the present invention is applied to PSP-BOTDA as in the first embodiment. The basic configuration of the correlation processing method of the optical code modulation pulse according to the third embodiment of the present invention is the same as that of FIG. 7 showing the configuration of the first embodiment, and the correlation code sequence signal generator 13 and the synthesis processor The function of 15 is the same as that of the second embodiment, but the function of the correlation processor 14 is different from that of the second embodiment.

As in the second embodiment, the correlation code sequence signal generator 13 includes forward complementary correlation code sequence signals A1 (t) and B1 (t) and reverse complementary correlation code sequence signals A2 (t) and B2 (t ) Is generated. Further, the code sequence signal their code elements were replaced with other elements ^{A1 - (t), B1 -} (t), A2 - (t), B2 - (t) also generate. The code sequence signal A1 (t), B1 (t ), A2 (t), B2 (t), A1 - (t), B1 - (t), A2 - (t), B2 - pump by (t) Consider a case where light is code-modulated. Further, as in the first and second embodiments, it is assumed that the probe light is Brillouin amplified by the code-modulated pump light and received by the optical receiver 7 in the portion where the distortion of the optical fiber 1 occurs. Further, the received electrical signals are made to correspond to the code sequences A1, B1, A2, B2, A1 ⁻ , B1 ⁻ , A2 ⁻ , B2 ⁻ , RA1 (t), RB1 (t), RA2 (t), RB2 ^{(t), RA1 - (t} ), RB1 - (t), RA2 - (t), RB2 - and represented by (t). Furthermore, ΔRA1 (t) = RA1 ( t) -RA1 - (t), ΔRB1 (t) = RB1 (t) -RB1 - (t), ΔRA2 (t) = RA2 (t) -RA2 - (t), ΔRB2 (t) = RB2 (t) −RB2 ⁻ (t)

In the present embodiment, the response function of the Brillouin amplified signal is further corrected so that the corrected response function becomes a linear function F (t) = β (1−αt) at time t. for that reason,
G (t) = g (t) F (t) (13)
As shown in the following equation (14), the correlation processor 14 calculates the cross-correlation Ψ _A1 (n) between A1 (t) G (t) and ΔRA1 (t) and A2 (t) G (t ) And ΔRA2 (t), Ψ _A2 (n), B1 (t) G (t) and ΔRB1 (t), Ψ _B1 (n), and B2 (t) G (t) Calculation processing is performed on the cross-correlation Ψ _B2 (n) with ΔRB2 (t). Then, the synthesis processor 15 performs a calculation process for synthesizing the total sum Ψ (n).

Here, using the relationship of A2 _i = A1 _{L−i + 1} ,

In the present invention, since the function F (t) is a linear function, F _m + F _{L− (m + n) +1} on the right side of the above expression is a value independent of m. as a result,

It becomes. Similarly,

  Therefore,

  From the above, the correlation processing is performed by the correlation processor 14 according to the equation (14), and the signal subjected to the correlation processing is synthesized by the synthesis processor 15, so that the section where the Brillouin amplification occurs is the length of the optical fiber. It can be seen that, even when distributed in the direction, the signal can be detected ideally without significantly reducing the signal of the main lobe and without causing side lobe noise.

  Next, the SN ratio of the third embodiment of the present invention is calculated. Since the sidelobe noise is 0 as described above, only the random noise of the receiving circuit is considered. In order to simplify the calculation, the discrete value is replaced with a continuous value.

  At this time, the power PS3, noise power PN3, and signal-to-noise ratio SNR3 of the received signal of the third embodiment are as follows.

FIG. 8 shows the calculation results of SNIR3 when τ = 0.3 ns, Γ = 0.1 Gs ⁻¹ , and α = 0.08 Gs ⁻¹ . It can be seen that the SNIR3 peak value of the third embodiment is higher than the SNIR2 peak value of the second embodiment by appropriately setting the value of α, which is a parameter of the linear function F (t). Further, as already described, the time side lobe is not completely suppressed to 0 in the second embodiment, but in principle it can be set to 0 in the third embodiment. That is, the third embodiment is superior to the second embodiment in both random noise characteristics and sidelobe noise characteristics.

[Modification 1]
The present modification is a case where amplitude shift modulation (ASK) is used as code modulation instead of phase shift modulation (PSK). So far, in the first embodiment, the second embodiment, and the third embodiment, phase shift modulation (PSK) that shifts the phase of the optical signal by 0 and π with respect to the code elements 1 and −1 has been used. . However, the present invention is not limited to PSK but can be applied to ASK.

Specifically, in the first embodiment, the second embodiment, and the third embodiment so far, the amplitude (intensity) of the sub-pulse of the second pulse of the pump light corresponding to the elements 1 and −1 of the code. ) May be intensity-modulated in proportion to 1, 0, respectively. The correlation code sequence signal generator 13 outputs a signal for generating the first pulse of the pump light and a signal for ASK modulating the second pulse, and drives the optical modulator 3 with the output signal. . As an example, FIG. 9 shows a pump light waveform when A1 = {1, 1, 1, −1} and A1 ⁻ = {− 1, −1, −1, 1}.

As in the case of PSK, the difference signal of the received electrical signal when the ASK ΔRA1 (t) = RA1 ( t) -RA1 - (t), ΔRB1 (t) = RB1 (t) -RB1 - (t), ... Is defined. The difference signals ΔRA1 (t), ΔRB1 (t),... When ASK is used are half the size when PSK is used. However, as in the case of PSK, the difference signals ΔRA1 (t), ΔRB1 (t),. It becomes the same waveform as the series A1 (t), B1 (t),. Therefore, in the first embodiment, the second embodiment, and the third embodiment described above, when the second pulse of the pump light is ASK modulated, the equations (7), (8), and (14) are respectively followed. Then, cross-correlation processing and signal synthesis may be performed.

[Fourth Embodiment]
FIG. 10 shows a fourth embodiment of the correlation processing method of the optical code modulation pulse of the present invention, here an example applied to BOTDR instead of PSP-BOTDA. In the figure, the first to third embodiments are shown. The same components as those of Modification 1 are denoted by the same reference numerals. 1 is an optical fiber, 2 is a laser, 3 is an optical modulator, 6, 11 and 21 are couplers, 13 is a correlation code sequence signal generator, 14 is a correlation processor, 22 is an optical receiver, 23 is a local oscillator, Reference numeral 24 is a mixer, 25 is a square processor, and 26 is an averaging processor.

  The output continuous light from the laser 2 is branched into two by the coupler 11. One of the lights is input to the optical modulator 3. An output signal from the correlation code sequence signal generator 13 is input to an electric signal input terminal of the optical modulator 3, and a code-modulated probe light is generated by this electric signal. The code-modulated probe light passes through the coupler 6 and enters the optical fiber 1 from the end face FE1 of the optical fiber 1.

  A part of the incident probe light is scattered backward by a sound wave thermally excited in the optical fiber. This scattered light is called natural Brillouin scattering. Hereinafter, for brevity, it is called Brillouin scattering. Brillouin scattering is separated from incident light by the coupler 6 and guided toward the optical receiver 22. On the other hand, the other light branched by the coupler 11 is used as local light for heterodyne detection of Brillouin scattering.

  Brillouin scattered light and local light merge at the coupler 21. The output of the coupler 21 is input to the optical receiver 22. The optical receiver 22 is normally a balanced receiver that inputs two interference signals in order to reduce intensity noise and increase sensitivity. However, a general optical receiver that inputs only one interference signal may be used. Absent.

  Since the Brillouin scattered light and the local light are separated from each other by the Brillouin frequency shift of the optical fiber 1, a beat signal whose frequency is the Brillouin frequency is output from the optical receiver 22. The beat signal and the output signal of the local oscillator 23 are input to the mixer 24, and a baseband signal is obtained as the output of the mixer 24. The correlation processor 14 performs correlation processing between the baseband signal and the correlation code sequence signal output from the correlation code sequence signal generator 13. The output signal of the correlation processor 14 is squared by the square processor 25. The squared signals are sequentially added by the averaging processor 26.

  An example of code-modulated probe light used in this BOTDR is shown in FIG. In this example, phase modulation is performed with a 4-bit code (code length L = 4). In PSP-BOTDA, a Brillouin amplified signal was measured using a sound wave excited by a pulse for sound wave excitation, but in BOTDR, it is scattered by a sound wave that is naturally excited and thermally excited in an optical fiber. Measure the light. Therefore, as shown in FIG. 11, the pulse for sound wave excitation is not used, and only the code-modulated pulse that is a measurement pulse is used.

  A sound wave that is naturally excited by thermal excitation is attenuated with time by an attenuation coefficient Γ, similarly to a sound wave excited by a pulse for excitation. Therefore, when the total pulse width Lτ of probe light (τ is the time width of a 1-bit sub-pulse) is approximately equal to or less than 1 / Γ, the coherence of the Brillouin scattered light is maintained.

Now, let C be a correlation code sequence, and C (t) be a correlation code sequence signal generated by the correlation code sequence signal generator 13 based on the correlation code sequence. Also, a BOTDR baseband signal measured using the correlation code sequence C is RC (t). As can be seen from the above description, RC (t) is proportional to the product of C (t) and h (t) in equation (3). For simplicity, if the proportionality factor is 1,
RC (t) = cosθC (t) h (t) (18)
It is. Here, θ is the phase difference between the Brillouin scattered light and the local light.

  In the same manner as described above, in order to correct the sound wave attenuation rate, the inverse function of h (t) is set to g (t) = 1 / h (t), and C (t) g (t) When cross-correlation calculation processing with RC (t) is performed, the following results.

  Further, this output is squared by the square processor 25. This measurement is repeated many times. Then, these values are added by the averaging processor 26. The phase difference θ varies statistically randomly between multiple measurements. Therefore, the statistical average value is

So

It becomes.

  From the above, it can be seen that by performing the correlation process shown in Equation (19), the correlation calculation process inherent to the code C can be performed without being affected by the attenuation of the sound wave.

  As the code C, a Barker code or an M-sequence code can be used. Further, even if one of the two series of complementary correlation codes is used, a correlation characteristic having a small side lobe value can be obtained.

[Fifth Embodiment]
FIG. 12 shows an example in which the present invention is applied to an optical code modulation pulse correlation processing system according to a fifth embodiment, here, an optical amplifier type coherent OTDR for measuring Rayleigh scattering. The same components as those in the third embodiment, the first modification, and the fourth embodiment are denoted by the same reference numerals. 1 is an optical fiber, 2 is a laser, 3 is an optical modulator, 6, 11 and 21 are couplers, 13 is a correlation code sequence signal generator, 14 is a correlation processor, 15 is a synthesis processor, and 22 is an optical receiver. , 23 is a local oscillator, 24 is a mixer, 25 is a square processor, 26 is an averaging processor, and 31 is an optical amplifier.

  The output continuous light from the laser 2 is branched into two by the coupler 11. One of the lights is input to the optical modulator 3. An output signal from the correlation code sequence signal generator 13 is input to an electric signal input terminal of the optical modulator 3, and a code-modulated probe light is generated by this electric signal. The code-modulated probe light is amplified by the optical amplifier 31. As the optical amplifier 31, an optical fiber type amplifier to which a rare earth element is added or a semiconductor type optical amplifier using a semiconductor is used. The amplified probe light passes through the coupler 6 and enters the optical fiber 1 from the end face FE1 of the optical fiber.

  Part of the incident probe light is scattered backward due to the refractive index fluctuation of the optical fiber. This scattered light is called Rayleigh scattering. Rayleigh scattering is separated from incident light by the coupler 6 and guided toward the optical receiver 22. On the other hand, the other light branched by the coupler 11 is used as local light for heterodyne detection of Rayleigh scattering.

  Rayleigh scattered light and local light merge at the coupler 21. The output of the coupler 21 is input to the optical receiver 22. As the optical receiver 22, a balanced receiver is usually used to reduce intensity noise and increase sensitivity.

  Here, when the light modulator 3 is a component that shifts the frequency of the probe light when modulating light (for example, an acousto-optic light modulator), the frequencies of Rayleigh scattered light and local light are: A beat signal is output from the optical receiver 22 because the frequency shift is away. This is called heterodyne detection. This beat signal is input to the output signal of the local oscillator 23 and the mixer 24, and a baseband signal is obtained as the output of the mixer 24.

  When the optical modulator 3 is a component that does not shift the frequency of the probe light when modulating light (for example, an electro-optic effect type optical modulator using lithium niobate), the Rayleigh Since the scattered light and the local light have the same frequency, the output from the optical receiver 22 is a baseband signal. This is called homodyne detection. At this time, the local oscillator 23 and the mixer 24 shown in FIG. 12 are not necessary.

  Heterodyne detection and homodyne detection are called coherent detection. In both cases of heterodyne detection and homodyne detection, the correlation processor 14 performs correlation processing between this baseband signal and the correlation code sequence signal that is the output signal of the correlation code sequence signal generator 13. The output signal of the correlation processor 14 is synthesized by the synthesis processor 15. The synthesized signal is squared by the square processor 25. The squared signals are sequentially added by the averaging processor 26.

  Now, a combination of the optical amplifier 31 and the optical fiber 1 is regarded as one optical system. Also, consider the measurement of Rayleigh scattering from one segmented optical fiber divided in the length direction. The time width of the 1-bit pulse is τ, and the length of one section of the divided fiber is vτ / 2 or less. At this time, the waveform of the Rayleigh scattered light is the same as that of the code sequence signal except for the amplitude.

  However, as the amplification degree of the optical amplifier 31 is increased, the front portion of the probe light is greatly amplified, and accordingly, most of the electrons in the active medium of the optical amplifier 31 are lower than the energy level of the excited state. The degree of amplification at the rear portion of the probe light is small (see FIG. 6). As a result, the waveform of Rayleigh scattered light also becomes a waveform that decays exponentially with time.

If the time decay rate coefficient for the electric field strength of the amplified signal at this time is γ, the step response of this optical system is
h _OA (t) = exp (−γt) (20)
It can be approximated as follows. This function is in the form of an exponential function in the same manner as the step response of the Brillouin amplified signal related to the attenuation of the sound wave considered in the first to third embodiments of the present invention. Therefore, it can be said that both can be treated mathematically in the same way. Therefore, the concept of the correlation processing method of the optical code modulation pulse shown in the first to third embodiments and the modification example 1 is also effective in the optical amplifier type coherent OTDR.

  Now, the complementary correlation code sequences A1 and B1 expressed as a function of time with a time width of 1 bit of the code as τ are referred to as correlation code sequence signals A1 (t) and B1 (t). Consider a case where the probe light is code-modulated with a signal based on A1 (t) and B1 (t). FIG. 11 shows a waveform example of probe light that is PSK modulated with a 4-bit code A1 = {1, 1, 1, −1}.

It is assumed that Rayleigh scattered light scattered from the one section of the optical fiber 1 is received by the optical receiver 22 by the probe light that is code-modulated using the code sequence A1 and optically amplified. The detected baseband electric signal RA1 (t) is proportional to A1 (t) h _OA (t). If the proportionality factor is omitted for simplicity,
RA1 (t) = cosθA1 (t) h _OA (t) (21)
It is. Similarly, the baseband electric signal RB1 (t) when the code sequence B1 is used is RB1 (t) = cos θB1 (t) h _OA (t) (22)
It can be expressed as However, (theta) is a phase difference of Rayleigh scattered light and local light. Here, it is assumed that the frequency and phase of the laser 2 and the Rayleigh scattered light do not fluctuate in the time interval in which the probe light is modulated by A1 (t) and B1 (t).

Now, referring to the first embodiment, the function of the inverse of the step response function h _OA (t) is g _OA (t) = 1 / h _OA (t), and the “attenuation rate” of the optical amplification signal is corrected. . Therefore, as expressed by the following equation (23), the cross-correlation between A1 (t) g _OA (t) and RA1 (t) and B1 (t) g _OA (t) and RB1 (t) Calculate the sum of cross-correlation. Cross-correlation is performed by the correlation processor 14, and calculation processing of the sum of cross-correlations is performed by the synthesis processor 15. The sum of the cross correlations is as follows:

  Further, the output of the synthesis processor 15 is squared by the square processor 25. These measurements are repeated many times. Since the phase difference θ varies statistically randomly during multiple measurements, the statistical average value is

So

It becomes.

  That is, by performing the cross-correlation process and the conjugation process expressed by the equation (23) that corrects the attenuation rate of the optical amplification signal, the main lobe signal is not reduced and the side lobe noise is not generated. It becomes possible to detect the Rayleigh scattered signal.

[Sixth Embodiment]
In the present embodiment, as in the fifth embodiment, the present invention is applied to an optical amplifier type coherent OTDR that measures Rayleigh scattering. As mentioned in the text of the description of the fifth embodiment, the concept of the correlation processing method of the optical modulation pulse shown in the second embodiment is also effective in the sixth embodiment of the present invention. The basic configuration of the correlation processing method of the optical code modulation pulse according to the sixth embodiment of the present invention is the same as that of FIG. 12 showing the configuration of the fifth embodiment, but the correlation code sequence signal generator 13 and the correlation processor are the same. 14 and the composition processor 15 are different from those of the fifth embodiment.

  Correlation code sequence signal generator 13 generates forward complementary correlation code sequence signals A1 (t) and B1 (t) and reverse complementary correlation code sequence signals A2 (t) and B2 (t ) Is generated.

  As in the fifth embodiment, consider a case where the probe light is code-modulated by the code sequence signals A1 (t), B1 (t), A2 (t), and B2 (t). Similarly to the fifth embodiment, it is assumed that the code-modulated probe light is Rayleigh scattered, received by the optical receiver 22 and converted into a baseband signal in a partial section of the optical fiber 1. The baseband signal is represented by RA1 (t), RB1 (t), RA2 (t), and RB2 (t) in correspondence with the code sequences A1, B1, A2, and B2.

Further, according to the present invention, as shown in the following equation (24), the correlation processor 14 includes the cross-correlation Φ _A1 (n) between A1 (t) and RA1 (t), A2 (t) and RA2 (t ) Cross-correlation Φ _A2 (n), B1 (t) and RB1 (t) cross-correlation Φ _B1 (n), B2 (t) and RB2 (t) cross-correlation Φ _B2 (n) Perform the calculation process. Then, the synthesis processor 15 performs calculation processing of the total sum Φ (n).

Here, using the relationship of A2 _i = A1 _{L−i + 1} ,

  However, (theta) is a phase difference of Rayleigh scattered light and local light. Here, it is assumed that the frequency and phase of the laser 2 and Rayleigh scattered light do not fluctuate during the time when the probe light is modulated by A1 (t), B1 (t), A2 (t), and B2 (t).

  The exponential function in {} on the right side of Expression (25) can be approximated by a linear function when the argument is small, and the value in {} does not depend on m. as a result,

It becomes. Similarly,

Therefore,

  Further, the output of the synthesis processor 15 is squared by the square processor 25. These measurements are repeated many times. Since the phase difference θ varies statistically randomly during multiple measurements, the statistical average value is

So

It becomes.

  That is, by performing the cross-correlation processing expressed by Equation (24) in which the attenuation factor of the optical amplification signal is corrected, the main lobe signal is not reduced and the side lobe noise is reduced, and the Rayleigh scattered signal is reduced. It becomes possible to detect.

[Seventh Embodiment]
In the present embodiment, as in the fifth embodiment, the present invention is applied to an optical amplifier type coherent OTDR that measures Rayleigh scattering. As mentioned in the text of the description of the fifth embodiment, the concept of the optical modulation pulse correlation processing method shown in the third embodiment is also effective in the seventh embodiment of the present invention. The basic configuration of the correlation processing method of the optical code modulation pulse according to the seventh embodiment of the present invention is the same as that of FIG. 12 showing the configuration of the fifth embodiment, but the correlation code sequence signal generator 13 and the correlation processor are the same. 14 and the composition processor 15 are different from those of the fifth embodiment.

  Correlation code sequence signal generator 13 generates forward complementary correlation code sequence signals A1 (t) and B1 (t) and reverse complementary correlation code sequence signals A2 (t) and B2 (t ) Is generated.

  As in the sixth embodiment, consider a case where the probe light is code-modulated by the complementary correlation code sequence signals A1 (t), B1 (t), A2 (t), and B2 (t). Further, as in the fifth and sixth embodiments, it is assumed that the code-modulated probe light is Rayleigh scattered, received by the optical receiver 22 and converted into a baseband signal in a partial section of the optical fiber 1. . The baseband signal is represented by RA1 (t), RB1 (t), RA2 (t), and RB2 (t) in correspondence with the code sequences A1, B1, A2, and B2.

  A combination of the optical amplifier 31 and the optical fiber 1 is regarded as one optical system. Also, consider the measurement of Rayleigh scattering from one segmented optical fiber divided in the length direction. The time width of the 1-bit pulse is τ, and the length of one section of the divided fiber is vτ / 2 or less.

  If the time attenuation factor coefficient related to the electric field strength of the amplified signal at this time is γ, the step response of this optical system can be approximated as follows.

h _OA (t) = exp (−γt)
The inverse function is g _OA (t) = 1 / h _OA (t).

In the present invention, the response function of the Brillouin amplified signal is further corrected so that the corrected response function becomes a linear function F (t) = β (1−αt) at time t. for that reason,
G _OA (t) = g _OA (t) F (t)
As shown in the following equation (27), the correlation processor 14 calculates the cross-correlation Ψ _A1 (n) between A1 (t) G _OA (t) and RA1 (t) and A2 (t) G _OA. (t) and RA2 (t) cross-correlation Ψ _A2 (n), B1 (t) G _OA (t) and RB1 (t) cross-correlation Ψ _B1 (n), and B2 (t) G _OA The cross-correlation Ψ _B2 (n) between (t) and RB2 (t) is calculated. Then, the synthesis processor 15 performs calculation processing of the total sum Ψ (n).

Here, using the relationship of A2 _i = A1 _{L−i + 1} ,

  Here, θ is a phase difference between Rayleigh scattered light and local light. Here, it is assumed that the frequency and phase of the laser 2 and Rayleigh scattered light do not fluctuate during the time when the probe light is modulated by A1 (t), B1 (t), A2 (t), and B2 (t).

In the present invention, since the function F (t) is a linear function, F _m + F _{L− (m + n) +1} on the right side of the above expression is a value independent of m. as a result,

It becomes. Similarly,

  Therefore,

  Further, the output of the synthesis processor 15 is squared by the square processor 25. These measurements are repeated many times. The phase difference θ varies statistically randomly between multiple measurements. So the statistical average is

So

It becomes.

  That is, by performing the cross-correlation processing expressed by the equation (27) in which the attenuation factor of the optical amplification signal is corrected, the main lobe signal is not reduced, and the side lobe noise is not generated, and the Rayleigh scattered signal is not generated. Can be detected.

[Modification 2]
In this modification, amplitude shift modulation (ASK) is used as code modulation of probe light of an optical amplifier type coherent OTDR instead of phase shift modulation (PSK).

  Up to now, in the fifth embodiment, the sixth embodiment, and the seventh embodiment, in the optical amplifier type coherent OTDR, the phase of the optical signal with respect to the code elements 1 and −1 is respectively used as the code modulation method of the probe light. Phase shift modulation (PSK) with 0, π shift was used. However, the present invention is also applicable to optical amplifier type coherent OTDR and direct detection OTDR using ASK.

First, a method for applying the present invention to an optical amplifier type coherent OTDR will be described. Specifically, in the fifth embodiment, the sixth embodiment, and the seventh embodiment thus far, the amplitude (intensity) of the sub-pulse of the probe light pulse is respectively corresponding to the elements 1 and −1 of the code. The intensity may be modulated in proportion to 1,0. As an example, FIGS. 13A and 13B show pump light waveforms when A1 = {1, 1, 1, −1} and A1 ⁻ = {− 1, −1, −1, 1}. .

Received electrical signal RA1 when the ^{ASK (t), RA1 - (} t), RB1 (t), RB1 - (t), ... with respect to the difference signal ΔRA1 (t) = RA1 (t ) -RA1 - ( t), ΔRB1 (t) = RB1 (t) -RB1 - (t), is intended to define the .... The difference signals ΔRA1 (t), ΔRB1 (t),... When ASK is used are half the size when PSK is used. However, as in the case of PSK, the difference signals ΔRA1 (t), ΔRB1 (t),. The same waveform as the series A1 (t), B1 (t),... Is obtained. Therefore, in the fifth embodiment, the sixth embodiment, and the seventh embodiment described above, even when the pulse of the probe light is ASK modulated, according to the equations (23), (24), and (27), respectively, Correlation processing and signal synthesis may be performed. However, at that time, RA1 (t), RA2 (t), RB1 (t), and RB2 (t) in Expression (23), Expression (24), and Expression (27) are respectively ΔRA1 (t) = RA1 ^{(t) -RA1 - (t)} , ΔRB1 (t) = RB1 (t) -RB1 - (t), ΔRA2 (t) = RA2 (t) -RA2 - (t), ΔRB2 (t) = RB2 (t ) -RB2 ^- shall be deemed to be replaced in (t).

[Modification 3]
FIG. 14 is a third modification of the correlation processing method of the optical code modulation pulse of the present invention, in which ASK is used as the code modulation of the probe light of the optical amplifier type coherent OTDR, and the received signal is squared in the correlation processing. An example performed later will be described, and in the figure, the same components as those in the fifth to seventh embodiments and the modified example 2 are denoted by the same reference numerals. 1 is an optical fiber, 2 is a laser, 3 is an optical modulator, 6, 11 and 21 are couplers, 13 is a correlation code sequence signal generator, 14 is a correlation processor, 15 is a synthesis processor, and 22 is an optical receiver. , 23 is a local oscillator, 24 is a mixer, 25 is a square processor, 26 is an averaging processor, and 31 is an optical amplifier.

  The output from the optical receiver 22 is converted into a baseband signal by the mixer 24, and the baseband signal is squared by the square processor 25. The squared signal is cross-correlated by the correlation processor 14 and then synthesized by the synthesis processor 15.

  In Modification 2, the output from the mixer 24 is correlated in the optical amplifier coherent OTDR, and then squared. The condition that allows such correlation processing is that not only between code bits but also the coherence of the signal between the codes is maintained when a plurality of codes are used. However, when the time interval between code bits or the time interval between codes is long, the coherence of the signal is deteriorated. In such a case, a method is used in which the correlation process is performed after the output from the mixer 24 is squared. At this time, since information on the phase of the signal cannot be used, ASK is used as the code modulation method of the probe light. Accordingly, the waveform of the code-modulated probe light is as shown in FIG.

  The cross-correlation process and the signal synthesis in this modification correspond to the fifth embodiment, the sixth embodiment, and the seventh embodiment, respectively, the expressions (23), (24), and Follow (27).

  However, in this modification, in order to correct the attenuation factor of the optical amplifier output signal and to correlate the squared signal, each variable in Equation (23), Equation (24), and Equation (27) is It is necessary to read as follows (“→” means to replace the left side of “→” with the right side of “→”).

・ Γ → 2γ
(Where 2γ corresponds to the power attenuation factor of the optical amplifier output signal)
・ G _m = exp (Γmτ) → exp (2γmτ)
RA1 _m , RA2 _m , RB1 _m , and RB2 _m are read as signals that are not a mixer output signal but a square of the mixer output signal.

RA1 _m → ΔRA1 _m = RA1 _m −RA1 ⁻¹ _m , RA2 _m → ΔRA2 _m = RA2 _m −RA2 ⁻¹ _m , RB1 _m → ΔRB1 _m = RB1 _m −RB1 ⁻¹ _m , RB2 _m → ΔRB2 _m = RB2 _m −RB2 ⁻¹ _{m
· G m = g m F (} t) = exp (Γmτ) F (t) → exp (2γmτ) F (t)
To read as

[Modification 4]
In this modification, ASK is used as the code modulation of the probe light of the optical amplifier type BOTDR, and the correlation processing is performed after the received signal is squared.

  The difference between this modified example and the modified example 3 is that the signal to be measured is scattered by the optical fiber 1 is Rayleigh scattering (modified example 3) or Brillouin scattering (modified example). Is the difference. Therefore, the basic configuration of the optical amplifier type BOTDR of the present modification is basically the same as FIG. 14 showing the configuration of the optical amplifier type coherent OTDR of the third modification. Since the frequency difference between the local light and the scattered light is different, only the band characteristics of the optical receiver 22 and the frequency of the output signal of the local oscillator 23 are different. Therefore, the cross-correlation process and the signal combining method in the third modification may be performed as they are in this modification.

[Modification 5]
FIG. 15 shows a modification 5 of the correlation processing method of the optical code modulation pulse of the present invention, here an example applied to the optical amplifier type direct detection method OTDR. In the figure, the first to third embodiments and The same components as those of the first modification and the fifth to seventh embodiments and the second modification are denoted by the same reference numerals. 1 is an optical fiber, 2 is a laser, 3 is an optical modulator, 6 is a coupler, 7 is an optical receiver, 13 is a correlation code sequence signal generator, 14 is a correlation processor, 15 is a synthesis processor, and 26 is an addition. An average processor 31 is an optical amplifier. Note that ASK is used as the code modulation method of the probe light.

  The difference between the optical amplifier type coherent OTDR of the modification 3 and the optical amplifier type direct detection system OTDR of this modification is only the optical signal detection system.

  Actually, FIG. 15 is obtained by replacing the set of the couplers 11 and 21, the optical receiver 22, the local oscillator 23, the mixer 24, and the square processor 25 of FIG. Therefore, the cross-correlation process and the signal combining method in the modification 3 are changed to the present modification by replacing the output signal of the square processor 25 in the modification 3 with the output signal of the optical receiver 7 in the modification. Just follow the example.

  In FIG. 15, the light modulator 3 is used as the light modulation means and the light is externally modulated. However, when the laser 2 is a semiconductor laser or the like, a semiconductor laser drive circuit is prepared as the light modulation means. Direct modulation may be used.

[Modification 6]
FIG. 16 shows a sixth modification of the correlation processing method of the optical code modulation pulse of the present invention, here an example applied to the optical amplifier type ASK code modulation BOTDA. In the figure, the first to third embodiments and The same components as those of the first modification and the fifth to seventh embodiments and the second modification are denoted by the same reference numerals. 1 is an optical fiber, 2 is a laser, 3 is an optical modulator, 6 and 11 are couplers, 7 is an optical receiver, 12 is an optical frequency shifter, 13 is a correlation code sequence signal generator, 14 is a correlation processor, 15 Is a synthesis processor, and 31 is an optical amplifier.

  The configuration of this modification is not significantly different from that of the first embodiment of FIG. The difference lies in the use of the optical amplifier 31 and the configuration of the code-modulated pump light.

  By using the optical amplifier 31, the pump light waveform obtained as an output thereof is distorted exponentially as shown in FIG. Further, in this modification, the pump light is not configured by combining the sound wave excitation pulse and the measurement pulse, but is configured only by the code-modulated measurement pulse. The code modulation method is not PSK but ASK.

  That is, in this modification, by inputting the output signal of the correlation code sequence signal generator 13 to the optical modulator 3, the ASK code modulated pump light as shown in FIG. obtain. Therefore, the cross-correlation process and the signal synthesis in the present modification may be performed according to Expression (7), Expression (8), and Expression (14), respectively, corresponding to Modification 1.

However, in this modification, in order to correct the attenuation rate of the optical amplifier output signal, not the attenuation rate of the sound wave, each variable in the equations (7), (8), and (14) is
・ Γ → 2γ
(Where 2γ corresponds to the power attenuation factor of the optical amplifier output signal)
・ G _m = exp (Γmτ) → exp (2γmτ)
_{· G m = g m F (} t) = exp (Γmτ) F (t) → exp (2γmτ) F (t)
It is necessary to read as follows.

[Modification 7]
The present modification is a case where the present invention is applied to an optical amplifier type PSK discrete coding BOTDA. The configuration can be expressed in FIG. 16 as in the sixth modification. Further, the configuration of this modification is not significantly different from that of the first embodiment of FIG. The difference is that the optical amplifier 31 is used, the pump light waveform is distorted exponentially by the use thereof, and the pump light is code-modulated.

  In this modification, code-modulated pump light is configured by combining sound wave excitation pulses for each measurement pulse to which one bit of the code is assigned (see FIG. 17). Therefore, the code bits are discretely arranged at the time interval t3. In such a code modulation method, the waveform of the Brillouin amplified signal does not become an exponential decay waveform due to sound wave attenuation. The distortion of the waveform of the Brillouin amplified signal here is in the optical amplifier.

  Therefore, in the same way as the previous modification of the present invention, the cross-correlation processing and the signal synthesis in this modification are considered by considering the difference in the cause of waveform distortion and the difference in the method of code-modulating the pump light. Corresponding to the first embodiment, the second embodiment, and the third embodiment may be performed according to the equations (7), (8), and (14), respectively. However, at that time, the following replacement is performed.

The cause of the signal distortion is not the attenuation of the sound wave but the attenuation of the amplified signal by the optical amplifier 31.
・ Γ → 2γ
(Where 2γ corresponds to the power attenuation factor of the optical amplifier output signal)
・ G _m = exp (Γmτ) → exp (2γmτ)
_{· G m = g m F (} t) = exp (Γmτ) F (t) → exp (2γmτ) F (t)
Replace the reading.

In addition, since the time distance between adjacent code elements is not τ but t3 as shown in FIG. 17, in the function representing the cross-correlation process,
・ Φ (nτ) = φ (n) → φ (nt ₃ ) = φ (n)
Φ (nτ) = Φ (n) → Φ (nt ₃ ) = Φ (n), etc. Ψ (nτ) = Ψ (n) → Ψ (nt ₃ ) = Ψ (n), etc. ΔRA1 (nτ) = ΔRA1 _n → ΔRA1 (nt ₃ ) = ΔRA1 _n etc. h (nτ) = h _n → h (nt ₃ ) = h _n
G (nτ) = g _n → g (nt ₃ ) = g _n
Read as follows.

[Modification 8]
This modification is that the code modulation method in Modification 7 is changed from discrete PSK to discrete ASK. An example of pump light used in this modification is shown in FIG. The code elements 1 and -1 are assigned ASK 1 and 0 values.

As in the case of PSK, the difference signal of the received electrical signal when the ASK ΔRA1 (t) = RA1 ( t) -RA1 - (t), ΔRB1 (t) = RB1 (t) -RB1 - (t), ... Is defined. The difference signals ΔRA1 (t), ΔRB1 (t),... When ASK is used are half the size when PSK is used. However, as in the case of PSK, the difference signals ΔRA1 (t), ΔRB1 (t),. The same waveform as the series A1 (t), B1 (t),... Is obtained. Therefore, the same cross-correlation processing and signal synthesis as in the modified example 7 may be performed.

[Modification 9]
This modification applies the present invention to an optical amplifier type Raman OTDR.

  The configuration of the optical amplifier type Raman OTDR that measures Raman scattered light is substantially the same as the configuration of the optical amplifier type direct detection method OTDR that measures Rayleigh scattered light described in the fifth modification (FIG. 15). Raman OTDR can sense temperature distribution by utilizing the temperature dependence of Raman scattered light power. In Raman OTDR, in order to extract and measure Raman scattering that is weaker than Rayleigh scattering, an optical filter 32 that blocks the Rayleigh scattering and passes the Raman scattering is installed just before the optical receiver 7. Different.

  Therefore, in this modification, the cross-correlation process and the signal synthesis method described in the modification 5 may be performed as they are.

DESCRIPTION OF SYMBOLS 1: Optical fiber 2, 5: Laser 3: Optical modulator 4: Pulse signal generator 6, 11, 21: Coupler 7, 22: Optical receiver 12: Optical frequency shifter 13: Correlation code sequence signal generator 14: Correlation processing 15: Synthesis processor 23: Local oscillator 24: Mixer 25: Square processor 26: Addition averaging processor 31: Optical amplifier 32: Optical filter FE1, FE2: End face of optical fiber

Horiguchi Tsuneo, Muroi Ryosuke, Iwasaka Ayako, Wakao Koji, Miyamoto Yuki, “BOTDA Using Phase Shift Pulse”, IEICE Transactions B, VOL.J.91-B, NO.2, pp.207-216 , February 2008 Daisuke Uchiyama, Tsuneo Horiguchi, Hiroshi Ando, Yoko Okumoto, Satoshi Kato, Takashi Sasaki, Yu Sawai, "SNR Improvement Rate in Coded PSP-BOTDA", 2010-01-21 Optical Fiber Application Technology Study Group, VOL.109 , NO.377, pp.33-38, OFT2009-68, January 2010 M. Nazarathy, SA Newton, RP Giffard, DS Moberly, F. Sischka, WR Trutna and S. Foster, "Real-Time Long Range Complementary Correlation Optical Time Domain Reflectometer", J. Lightwave Tech., Vol. 7, no. 1, pp. 24-38, 1989

Claims (4)

A correlation code sequence signal generator for generating a correlation code sequence signal C (t) as a function of time t;
An optical modulator that modulates light with a signal including the correlation code sequence signal C (t);
When the speed of light is v, the attenuation coefficient of the amplitude of the sound wave involved in Brillouin amplification or Brillouin scattering is Γ, and the time width of the 1-bit pulse (sub-pulse) of the correlation code sequence signal C (t) is τ. H (t) is the step response function from a short interval whose length is approximately equal to or less than v / (2Γ), and the inverse function of h (t) is g (t) = 1 / h (t An optical receiver that receives the light output from the optical system and outputs the received signal RC (t) when the optical signal modulated by the optical modulator is input to the optical system,
A function obtained by multiplying the function g (t) and the correlation code sequence signal C (t) is defined as C (t) g (t), and the received signal RC (t) or the code of the correlation code sequence signal C (t) is obtained. signal obtained by inverting the polarity of the sequence of elements C ^- (t) RC a received signal when using the - ^(t) and the RC from RC (t) when - the signal obtained by subtracting the ^(t), the function A correlation processing method for optical code modulation pulses, comprising a correlation processor for performing correlation processing with C (t) g (t). A correlation code sequence signal generator for generating a correlation code sequence signal C1 (t) and a reverse correlation code sequence signal C2 (t) in which the code elements are arranged in reverse order as a function of time t;
An optical modulator that modulates light with a signal including the correlation code sequence signal C1 (t) and a signal including the correlation code sequence signal C2 (t);
An optical receiver that receives light output from the optical system when an optical signal modulated by the optical modulator is input to the optical system and outputs a received signal RC1 (t) and a received signal RC2 (t); ,
The received signal RC1 (t) or the correlation code sequence signal C1 (t) of the code sequence of the signal obtained by inverting the polarity of the elements C1 - ^(t) the received signal when using RC1 - when a ^(t) to RC1 (t) from the RC1 - correlation processing a signal obtained by subtracting the ^(t), the correlation code sequence signal C1 (t), the and the received signal RC2 (t) or the correlation code sequence signal C2 (t) signal obtained by inverting the polarity of the elements of the code sequence of C2 - from RC2 (t) RC2 when the ^(t) - - a signal obtained by subtracting the ^(t), the received signal RC2 when using ^(t) A correlation processor for performing correlation processing with the correlation code sequence signal C2 (t);
A synthesis processor that adds the correlation processing result output from the correlation processor with the correlation code sequence signal C1 (t) and the correlation processing result with the correlation code sequence signal C2 (t) and outputs the result. An optical code modulation pulse correlation processing system characterized by comprising: A correlation code sequence signal generator for generating a correlation code sequence signal C1 (t) and a reverse correlation code sequence signal C2 (t) in which the code elements are arranged in reverse order as a function of time t;
An optical modulator that modulates light with a signal including the correlation code sequence signal C1 (t) and a signal including the correlation code sequence signal C2 (t);
The velocity of light is v, the attenuation coefficient of the amplitude of the sound wave involved in Brillouin amplification or Brillouin scattering is Γ, and the 1-bit pulse (subpulse) of the correlation code sequence signal C1 (t) and the correlation code sequence signal C2 (t) ) Is a step response function from a short interval whose length is approximately equal to or less than v / (2Γ), h (t), and a linear function for time t is F ( t) = β (1-αt), and an optical system in which the function obtained by dividing the F (t) by the h (t) is G (t) = F (t) / h (t) An optical receiver that receives the light output from the optical system when the optical signal modulated in step S1 is input and outputs the received signal RC1 (t) and the received signal RC2 (t);
A function obtained by multiplying the function G (t) and the correlation code sequence signal C1 (t) is defined as C1 (t) G (t), and the function G (t) and the correlation code sequence signal C2 (t) are multiplied. the function and C2 (t) G (t), the received signal RC1 (t) or the correlation code sequence signal C1 (t) of the code sequence of elements of the polarity signal obtained by inverting the C1 - when using the ^(t) of the received signal RC1 - RC1 (t) from RC1 when a ^(t) - correlation of a signal obtained by subtracting the ^(t), the function C1 and (t) G (t), and the received signal RC2 signal obtained by inverting the polarity of the elements of the code sequence (t) or the correlation code sequence signal C2 (t) C2 - a reception signal when using the ^(t) RC2 - when a ^(t) RC2 (t ) And RC2 ⁻ (t) subtracted from the function C2 (t) G (t)
A synthesis processor for adding the correlation processing result output from the correlation processor with the function C1 (t) G (t) and the correlation processing result with the function C2 (t) G (t) and outputting the result. An optical code modulation pulse correlation processing method characterized by comprising: In the correlation processing method of the optical code modulation pulse according to any one of claims 1 to 3,
The correlation code is either a complementary correlation code, a Barker code, or an M-sequence code. An optical code modulation pulse correlation processing method.

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