JP6602689B2 - Optical line characteristic analyzer and signal processing method - Google Patents

Description

  The present invention relates to an optical line characteristic analyzer for measuring loss distribution of an optical line and a signal processing method thereof.

  In an optical communication device or an optical fiber sensing device using an optical line such as an optical fiber, a Brillouin time domain analysis method (BOTDA: Brillouin Optical Time Domain Analysis) is a technique for obtaining characteristic distributions such as loss, temperature, and strain of the optical fiber. Is used (see Non-Patent Documents 1-3). In BOTDA, pump light and probe light are incident on the fiber to be measured, and the probe light that has undergone Brillouin amplification by the pump light is analyzed to obtain the loss from the intensity of the Brillouin gain spectrum and the temperature and strain from the frequency shift of the Brillouin gain spectrum. To do. Since this Brillouin gain spectrum depends on the pump light power, when the loss of the optical line is large (for example, the long end optical line or the far end of the branched optical line having a splitter), the Brillouin gain spectrum is about 1% -0.01%. It is necessary to obtain a minute gain. Further, in loss measurement, since it is necessary to measure a minute amount of change in Brillouin gain, higher sensitivity measurement is required.

T. T. et al. Horiguchi et al. , BOTDA-Nondestructive measurement of single-mode optical attention characteristics using Brillouin interaction: Theory, 170, -1, V. Vol. H. Takahashi et al. , “Individual loss distribution measurement in 32-branched PON using pulsed pump-probe Brillouin Analysis”, Optics Express, Vol. 21, no. 6, 6739, (2013). C. Kito et al. “Robust and high-sensitivity Brillouin time-domain sensing with branched-fiber configuration”, J. et al. Lightwave Technol. Vol. 33, no. 20, pp. 4291-4296, 2015.

  In general, when a Brillouin gain spectrum is acquired, probe light subjected to Brillouin amplification is optically / electrically converted by a PD or the like, and then an analog electric signal is converted into a digital signal by an A / D converter and analyzed by a computer. At this time, noise in probe light analysis includes PD thermal noise / shot noise and A / D converter quantization noise. Since the thermal noise / shot noise of PD is white noise, high sensitivity measurement can be performed by measuring multiple times and averaging. On the other hand, since the signal buried in the quantization noise is not improved by the averaging process, there is a problem that the sensitivity of BOTDA is limited by the quantization noise. Therefore, in an optical line with a large loss, the characteristic distribution of the optical line could not be obtained even if the averaging process was repeated.

  Accordingly, an object of the present invention is to provide an optical line characteristic analyzing apparatus and a signal processing method capable of reducing the BOTDA sensitivity limitation due to quantization noise in order to solve the above-described problems.

  In order to achieve the above object, the optical line characteristic analyzing apparatus and signal processing method according to the present invention performs Brillouin amplification from a measurement signal received from probe light subjected to Brillouin amplification before converting an analog signal into a digital signal. It was decided to remove a part of the probe light component that was not received.

Specifically, an optical line characteristic analyzing apparatus according to the present invention is an optical line characteristic analyzing apparatus that analyzes characteristics of an optical fiber to be measured by a Brillouin time domain analysis method (BOTDA: Brillouin Optical Time Domain Analysis),
Test light incident means for injecting at least one of a probe light pulse and a pump light pulse into an incident end of the measured optical fiber;
Light receiving means for photoelectrically converting return light returning to the incident end of the optical fiber to be measured;
Analog signal processing means for performing analog signal processing using an analog electrical signal photoelectrically converted by the light receiving means;
A / D conversion means for converting an analog electrical signal photoelectrically converted by the optical receiving means or an analog electrical signal subjected to the analog signal processing by the analog signal processing means into a digital electrical signal;
Arithmetic processing means for calculating a digital electric signal converted by the A / D converter and analyzing characteristics of the optical fiber to be measured;
With
The analog signal processing means, as the analog signal processing,
A probe light pulse and a pump light pulse having an arbitrary optical frequency difference are incident on the incident end of the optical fiber to be measured at an arbitrary time difference, and the optical receiving means photoelectrically returns the return light that returns to the incident end of the optical fiber to be measured. Before the A / D converter converts the converted analog measurement signal into a digital signal,
Only the probe light pulse is incident on the optical fiber to be measured, and the optical pulse reflected by the far end of the optical fiber to be measured and returned to the incident end is less than 1 in the analog reference signal photoelectrically converted by the light receiving means. An adjustment signal multiplied by a positive number is calculated, and the adjustment signal is subtracted from the analog measurement signal.

Specifically, a signal processing method according to the present invention is a signal processing method for analyzing characteristics of an optical fiber to be measured by a Brillouin time domain analysis method (BOTDA: Brillouin Optical Time Domain Analysis),
An analog measurement signal in which a probe light pulse and a pump light pulse having an arbitrary optical frequency difference are incident on an incident end of the optical fiber to be measured with an arbitrary time difference, and a return light returned to the incident end of the optical fiber to be measured is photoelectrically converted. Before converting to a digital signal
Only the probe light pulse is incident on the optical fiber to be measured, and an analog reference signal obtained by photoelectric conversion of the optical pulse reflected at the far end of the optical fiber to be measured and returning to the incident end is multiplied by a positive number less than 1. An adjustment signal is calculated, and an analog signal processing procedure for subtracting the adjustment signal from the analog measurement signal is performed.

  Normally, BOTDA performs A / D conversion on a signal obtained by amplifying a measurement signal obtained by photoelectrically converting the return light (probe light subjected to Brillouin amplification) from the measured fiber by several percent. The signal to be acquired (the signal corresponding to the Brillouin amplification) is amplified but is about several percent, and most of the measurement signal intensity is occupied by the probe photocurrent (probe light component in the measurement signal). Since this probe photocurrent becomes an offset, the A / D converter cannot convert the Brillouin amplified signal to digital by making full use of the vertical range, and quantization noise is generated.

  In order to convert such a signal into digital with high accuracy, it is necessary to use an A / D converter having a high vertical resolution, but the sampling rate is limited in the high resolution A / D converter. Therefore, the spatial resolution of BOTDA is deteriorated.

  Therefore, in the present invention, a part of the probe photocurrent that is an offset is deleted from the measurement signal before the analog measurement signal is converted to digital. Since the amplitude of the signal to the A / D converter can be effectively increased by reducing the offset, the vertical range of the A / D converter can be utilized and the quantization noise can be reduced.

  Therefore, the present invention can provide an optical line characteristic analyzing apparatus and a signal processing method that can reduce the BOTDA sensitivity limitation due to quantization noise.

  In the optical line characteristic analyzing apparatus and the signal processing method according to the present invention, when the optical fiber to be measured is a branched optical line branched into a plurality of branched optical fibers by an optical branching device at a subsequent stage of the incident end, In the test light incident means, the probe light pulse has a pulse width that does not superimpose any return light reflected by the far end of the branch optical fiber by the optical branching unit, and in the analog signal processing means, the branch optical fiber The analog signal processing is performed every time.

  The optical line characteristic analyzing apparatus and the signal processing method according to the present invention can analyze the characteristic of the optical fiber of each branch optical line.

  INDUSTRIAL APPLICABILITY The present invention can provide an optical line characteristic analyzing apparatus and signal processing method that can reduce the BOTDA sensitivity limitation due to quantization noise.

It is a figure explaining the optical-line characteristic analyzer based on this invention. It is a conceptual diagram explaining operation | movement of the analog signal processing means of the optical-line characteristic analyzer based on this invention. It is a figure explaining the effect of the optical-line characteristic analyzer based on this invention. It is a figure explaining the analog signal which the optical receiving means of the optical-line characteristic analyzer based on this invention outputs. (A) is an analog measurement signal, (b) is an analog reference signal. It is a figure explaining the adjustment signal which the analog signal processing means of the optical-line characteristic analyzer based on this invention outputs. (A) is a case where an analog signal processing procedure is not performed. The quantization noise is (minimum quantization step) / (signal amplitude) = 1 / (10−7) = 1/3. (B) is a case where A = 1 in the analog signal processing procedure. The quantization noise is (minimum quantization step) / (signal amplitude) = 1/10. (C) is a case where A = 1/2 in the analog signal processing procedure. The quantization noise is (minimum quantization step) / (signal amplitude) = 1/5. (D) is a case where A = 3/4 in the analog signal processing procedure. The quantization noise is (minimum quantization step) / (signal amplitude) = 1 / 7.5. It is the example of a measurement of the to-be-measured optical fiber which the optical-line characteristic analyzer based on this invention measured.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments. In the present specification and drawings, the same reference numerals denote the same components.

(Claim 1 Posting Department)
FIG. 1 is a block diagram showing the configuration of the optical line characteristic analyzing apparatus of this embodiment. The optical line characteristic analyzing apparatus is an optical line characteristic analyzing apparatus for analyzing characteristics of an optical fiber to be measured by a Brillouin time domain analysis method (BOTDA: Brillouin Optical Time Domain Analysis),
Test light incident means for injecting at least one of a probe light pulse and a pump light pulse into an incident end of the measured optical fiber;
Light receiving means for photoelectrically converting return light returning to the incident end of the optical fiber to be measured;
Analog signal processing means for performing analog signal processing using an analog electrical signal photoelectrically converted by the light receiving means;
A / D conversion means for converting an analog electrical signal photoelectrically converted by the optical receiving means or an analog electrical signal subjected to the analog signal processing by the analog signal processing means into a digital electrical signal;
Arithmetic processing means for calculating a digital electric signal converted by the A / D converter and analyzing characteristics of the optical fiber to be measured;
With
The analog signal processing means, as the analog signal processing,
A probe light pulse and a pump light pulse having an arbitrary optical frequency difference are incident on the incident end of the optical fiber to be measured at an arbitrary time difference, and the optical receiving means photoelectrically returns the return light that returns to the incident end of the optical fiber to be measured. Before the A / D converter converts the converted analog measurement signal into a digital signal,
Only the probe light pulse is incident on the optical fiber to be measured, and the optical pulse reflected by the far end of the optical fiber to be measured and returned to the incident end is less than 1 in the analog reference signal photoelectrically converted by the light receiving means. An adjustment signal multiplied by a positive number is calculated, and the adjustment signal is subtracted from the analog measurement signal.

  1 includes a first test light output means 11, a second test light output means 12, an incident time controller (15, 16), a pulsing means (13, 14), a multiplexing element. 20, a circulator 21, an optical receiver 26, an analog signal processor 29, an A / D converter 27, and an arithmetic processing unit 28. Here, the first test light output means 11, the second test light output means 12, the incident time controller (15, 16), the pulsing means (13, 14), and the multiplexing element 20 are included in the test light incident means. Equivalent to.

  The first test light output means 11 and the second test light output means 12 output continuous light. The continuous light output from the first test light output means 11 and the second test light output means 12 is referred to as probe light and pump light, respectively. The probe light and the pump light have optical frequency differences that differ by about the Brillouin frequency shift.

The pulsing means (13, 14) includes an acousto-optic switch that drives the acousto-optic element in pulses. The pulsing means (13, 14) may include a waveguide switch that drives the electro-optic element using LiNbO ₃ or a semiconductor optical switch that drives the semiconductor optical amplifier. Hereinafter, an acousto-optic modulator composed of an acousto-optic switch, a LiNbO ₃ modulator composed of an electro-optic switch, or a semiconductor optical amplifier composed of a semiconductor optical switch is referred to as an optical device. The optical device pulses the probe light and the pump light with the time driven by the electric pulse from each of the incident time control means (15, 16).

  At this time, when the optical fiber 22 is not included in the measured fiber 100, the pulse width of the probe light pulse is not limited. On the other hand, when measuring a branched optical line including the optical splitter 22 in the fiber 100 to be measured, the pulsing means (13, 14) sets the pulse width of the probe light pulse to 2nΔL / c or less. Here, ΔL is the minimum value of the difference in length between the branched optical fibers included in the optical fiber to be measured. c is the speed of light in vacuum. n is the refractive index of the optical fiber.

  The incident time control means (15, 16) has a function capable of changing the modulation time of the driving electric pulse supplied to the pulsing means (13, 14). The incident time control means (15, 16) controls the timing at which the optical pulse is generated by changing the timing at which the pulsing means (13, 14) modulates the pulse.

  The multiplexing element 20 combines the probe light pulse and the pump light pulse. The probe light pulse and the pump light pulse combined by the multiplexing element 20 pass through the circulator 21 and enter the measured optical fiber 100. As a result, it is possible to give a time difference between the time during which the probe light pulse is incident on the measured optical fiber 100 and the time during which the pump light pulse is incident on the measured optical fiber 100. One of the probe light pulse and the pump light pulse is always set to zero by setting the voltage of one of the electric pulse in the probe light pulsing means 13 or the electric pulse in the pump light pulsing means 14 to zero. It is also possible to enter only the optical fiber 100 to be measured.

  The measured optical fiber 100 includes an optical splitter 22 and a branched optical fiber 23 that are optically coupled to one end of the backbone optical fiber 50. Here, a reflection type optical filter 24 is disposed at the end (far end) of each branch optical fiber 23. These reflective optical filters 24 have a characteristic of reflecting light of at least the wavelength of the probe light pulse (or both of the probe light pulse and the pump light pulse) and transmitting light of other wavelengths.

  The optical splitter 22 branches the supplied probe light pulse and pump light pulse into N (for example, N = 8). The probe light pulse and the pump light pulse branched by the optical splitter 22 interact in the branch optical fiber 23. Back-scattered light of stimulated Brillouin scattering is generated by the interaction between the probe light pulse and the pump light pulse. The stimulated Brillouin backscattered light and the probe light pulse and the pump light pulse reflected by the reflective optical filter 24 return to the circulator 21, pass through the circulator 21, and are output to the light receiving means 26.

  The light receiving means 26 receives the output light of the circulator 21 that is return light from the optical fiber 100 to be measured. The optical receiver 26 converts the received return light into an analog voltage signal. The analog signal processing unit 29 performs desired signal processing as described below by an analog circuit on the analog voltage signal from the optical receiving unit 26 and outputs the signal to the A / D converter 26. The A / D converter 27 converts the analog voltage signal output from the analog processing means 29 into a digital signal. The A / D converter 27 outputs a digital signal to the arithmetic processing device 28.

  The arithmetic processing unit 28 performs arithmetic processing as will be described later on the input voltage value to obtain a loss distribution with respect to the distance for the optical fiber 100 to be measured.

[Analog signal processing procedure]
FIG. 2 is a conceptual diagram of an analog signal processing procedure performed by the analog signal processing means 29. In the analog signal processing procedure, a probe light pulse having an arbitrary optical frequency difference and a pump light pulse are incident on the incident end of the optical fiber 100 to be measured with an arbitrary time difference, and the return light returns to the incident end of the optical fiber 100 to be measured. Before converting the analog measurement signal photoelectrically converted into a digital signal,
Only the probe light pulse is incident on the optical fiber 100 to be measured, and is reflected by the far end of the optical fiber 100 to be measured and returned to the incident end. The analog reference signal obtained by photoelectric conversion is multiplied by a positive number less than 1. The adjustment signal is calculated, and the adjustment signal is subtracted from the analog measurement signal.

The analog signal processing means 29 first enters only the probe light pulse into the optical fiber 100 to be measured, and acquires the voltage peak V _N of the probe light pulse obtained by the light receiving means 26. Subsequently, the analog signal processing means 29 enters the probe light pulse and the pump light pulse into the optical fiber 100 to be measured, and obtains the voltage peak V ′ _N of the probe light pulse subjected to Brillouin amplification obtained by the light receiving means 26. The differential circuit outputs the difference V _{N_out} between these voltage peaks.
(Formula S) V _N — _out = V ′ _N −AV _N
Here, N is the branch core number of the optical fiber 100 to be measured, and A is a positive number less than 1.

  4 and 5 are conceptual diagrams illustrating an analog signal processing procedure. FIG. 4 is a diagram for explaining an analog signal output from the optical receiver 26. FIG. 4A is an “analog measurement signal” in which the probe light pulse and the pump light pulse are incident on the optical fiber 100 to be measured and the analog signal obtained by the light receiving means 26 is obtained. There is an intensity peak due to stimulated Brillouin scattering at a frequency difference between a certain lobe light pulse and pump light pulse. In FIG. 4B, only the probe light pulse is incident on the optical fiber 100 to be measured, and the analog signal obtained by the light receiving means 26 is an “analog reference signal”, and the analog measurement signal in FIG. The offset for the analog reference signal included in is shown. The light receiving means 26 outputs an analog reference signal or an analog measurement signal according to a light pulse incident on the optical fiber 100 to be measured.

  FIG. 5 is a diagram for explaining an “adjustment signal” output from the analog signal processing means 29 to the A / D converter 27. 5A shows a case where A = 0 in the formula S (conventional case where no analog signal processing procedure is performed), FIG. 5B shows a case where A = 1 in the formula S, and FIG. When A = 1/2 in the formula S, FIG. 5D is an adjustment signal when A = 3/4 in the formula S. In FIG. 5, the vertical axis indicates the quantization step in the A / D converter 27.

  The quantization noise of the A / D converter 27 is generated when the discrete voltage deviates from the analog voltage when the quantization of the A / D converter 27 is coarse with respect to the signal amplitude. Therefore, the analog signal processing means 29 reduces the offset of the analog measurement signal, so that the A / D converter 27 can effectively increase the signal amplitude and reduce the quantization noise (FIG. 5B, FIG. 5 (c), FIG. 5 (d)). Conventionally, when a fiber loss is calculated from a Brillouin gain, a measurement signal (a probe light pulse and a pump light pulse are incident on a measured optical fiber by an arithmetic processing unit 28 and an analog signal (FIG. 5 (A)) is a direct A / D converted signal) to a reference signal (a signal obtained by direct A / D conversion of the analog signal obtained by the optical receiving means with only the probe light pulse incident on the measured optical fiber). The subtraction is an operation with a lot of quantization noise. In this embodiment, quantization noise is reduced by cutting (subtracting) an analog reference signal (offset) from an analog measurement signal before A / D converting the signal.

Specifically, when A is set to ½, the offset voltage is halved with respect to the Brillouin gain. Therefore, the output voltage is appropriately adjusted to the vertical range of the A / D converter and measured. The required vertical resolution can be reduced by 1 bit. When A is set to 3/4, V _{N_out} becomes 1/4 and can be relaxed by 2 bits. It is ideal that A = 1, but due to noise (thermal noise / shot noise) caused by the PD of the light receiving means 26, the intensity of the signal receiving the return light is affected by the noise and fluctuates. For this reason, it is difficult to accurately measure the signal intensity when A = 1, and therefore it is preferable to set A to a value as close to 1 as possible.

For example, in a single line (25 dB line) without a splitter having a total loss of 25 dB as the optical fiber to be measured, the Brillouin gain peak when the optical frequency difference between the probe light and the pump light is shifted to the Brillouin frequency is about 0.02%. If the optical frequency difference between the probe light and the pump light deviates even slightly from the Brillouin frequency shift, the Brillouin gain is further reduced. Figure 3 is a diagram for explaining the intensity of stimulated Brillouin scattering when varying the incident time difference t ₁ to 25dB line between the probe light pulse and the pump light pulse was measured to distance direction (calculation result of the arithmetic processing unit 28) It is. FIG. 3A shows the result of performing the analog signal processing procedure (A≈19 / 20), and FIG. 3B shows the result of not performing the analog signal processing procedure (A = 0 in Formula S). FIGS. 3C and 3D are enlarged views of the vicinity of the position where the stimulated Brillouin scattering occurs (the 130th measurement point). This position is a position where the Brillouin interaction is received by the previous line (after being reflected by the reflection type optical filter 24) which has received a loss of 25 dB from the light incident end of the optical fiber to be measured.

  The Brillouin gain when the analog arithmetic processing procedure is performed is 1.00338, and a Brillouin signal of 0.3% is obtained, whereas the Brillouin gain when the analog arithmetic processing procedure is not performed is 0.014. Only% Brillouin gain is obtained. This is because when the analog arithmetic processing means 29 is not provided, the offset voltage is reduced to 1/20 by using the analog arithmetic processing means 29 set to A≈19 / 20 for the signal that is not obtained by being buried in the quantization noise. This is a result of effective use of the vertical resolution of the A / D converter 27. In the case of this result, the required vertical resolution can be relaxed by 4-5 bits.

[Operation procedure after A / D conversion]
The arithmetic processing unit 28 enters only the probe light pulse into the measured optical fiber 100 based on the probe light intensity when both the probe light pulse and the pump light pulse are incident on the measured optical fiber 100 with the incident time difference t ₁ . The induced Brillouin gain is obtained by subtracting the probe light intensity in the case. Next, the arithmetic processing unit 28 changes the incident time difference t ₁ repeated measurement of the induced Brillouin gain, to calculate the induced Brillouin gain with respect to the distance. The arithmetic processing unit 28 obtains the loss / temperature / strain distribution with respect to the distance for the single line or the branched optical line that is the measured optical fiber 100 from the induced Brillouin gain with respect to the distance. FIG. 6 is a measurement example of the optical fiber 100 to be measured calculated by the arithmetic processing unit 28. The induced Brillouin gain can be obtained from the loss.

  Note that only the probe light pulse is applied to the optical fiber 100 to be measured from the intensity of the probe light when the analog signal processing means 29 enters both the probe light pulse and the pump light pulse into the optical fiber 100 to be measured as shown in Equation S. Since the probe light intensity (multiplied by A) at the time of incidence is subtracted, the arithmetic processing unit 28 uses the probe light intensity when both the probe light pulse and the pump light pulse are incident on the optical fiber 100 to be measured. The probe light intensity (multiplied by 1-A) when only the probe light pulse is incident on the optical fiber 100 to be measured is subtracted.

  Further, since the arithmetic processing unit 28 also requires the probe light intensity when only the probe light pulse is incident on the optical fiber 100 to be measured, the analog signal processing means 29 performs A / D conversion on the analog reference signal that does not perform any operation. The arithmetic processing unit 28 holds the reference signal after A / D conversion.

[Measurement conditions for optical line characteristic analysis]
Next, the operation of the optical line characteristic analyzing apparatus of this embodiment will be described.
First, the optical frequencies of the probe light and the pump light, the pulsing means (13, 14), the optical receiving means 26, and the A / D converter 27 must satisfy the following conditions.
(Condition 1) The frequency difference between the probe light and the pump light is equal to the Brillouin frequency shift of the optical fiber 100 to be measured.
(Condition 2) When the measured optical fiber 100 is a branched optical line, the pulse width τ of the probe light pulse output from the pulsing means 13 is the return light from the reflective optical filter 24 at the end of the branched optical fiber. The time difference is narrower than 2nΔL / c.
(Condition 3) The band of the optical receiver 26 and the band of the A / D converter 27 are bands that can receive the pulse width τ.

  Conditions 1 to 3 have the following meanings.

  Condition 1 is a condition necessary for the probe light pulse and the pump light pulse to cause stimulated Brillouin scattering.

  Condition 2 is a condition necessary when the optical fiber 100 to be measured includes a branch, and is a condition for preventing the stimulated Brillouin scattered light for each branch optical fiber from overlapping. When the pulse width τ of the pulsing means 13 is wider than the minimum value 2nΔL / c of the time difference of the return light from the reflection type optical filter 24 at the end of each branch optical fiber, the stimulated Brillouin scattered light for each branch optical fiber overlaps. In this case, the stimulated Brillouin scattered light for each branch optical fiber cannot be separated in terms of time.

  Condition 3 is a condition for accurately measuring an optical pulse having a pulse width τ. That is, it means that the band of the optical receiving means 26 and the band of the A / D converter 27 need to be wider than 1 / τ.

[BOTDA measurement principle]
The measurement principle of the optical line characteristic analyzer of this embodiment that satisfies the above conditions will be described.
The first test light output means 11 outputs probe light, and the second test light output means 13 outputs pump light. Here, it is assumed that the optical frequencies of the probe light and the pump light are f ₁ and f ₂ . f _B is the optical frequency shift amount due to the measured optical fiber 100 stimulated Brillouin backscattering.

First, the optical line characteristic analyzing apparatus causes a probe light pulse to enter the optical fiber 100 to be measured. Then, the branched optical line characteristic analyzing apparatus makes the pump light pulse incident on the measured optical fiber 100 after t ₁ second from the incidence of the probe light pulse.

  The probe light pulse and the pump light pulse incident on the measured optical fiber 100 are reflected by the reflection optical filter at the far end and collide with each other in the measured optical fiber 100. When the measured optical fiber 100 has the optical splitter 22, the probe light pulse and the pump light pulse are branched into N (for example, N = 8) by the optical splitter 22, and then the far end. The light is reflected by the reflective optical filter 24 and collides in the optical fiber 100 to be measured.

  When the probe light and the pump light collide, stimulated Brillouin scattering occurs, and the probe light acquires information on loss, temperature, and strain at the collision point. Hereinafter, the loss will be described.

ここで、ｚ_１は、分岐光ファイバの入射端から、プローブ光パルスとポンプ光パルスとがインタラクションした位置までの距離である。α_Ｂ（ｚ_１，ｆ）は、入射端からの位置ｚ_１、かつプローブ光／ポンプ光間の周波数差がｆのときの誘導ブリルアン利得である。ｇ_Ｂ（ｆ）は、プローブ光／ポンプ光間の周波数差がｆの場合の誘導ブリルアン散乱係数である。Ｉ_ｐｕｍｐ（ｚ）は、光ファイバの入射端から距離ｚだけ離れた位置におけるポンプ光パルスの強度である。Dｚ_ｐｕｍｐは空間分解能である。 If the frequency difference between the measuring probe light and pumping light line loss of (a) the measured optical fiber 100 is f _B, when the probe light pulse and the pump light pulse impinges, Brillouin diffraction grating is generated, the diffracted When the pump light is reflected by the grating, the probe light pulse is amplified by the expression (1).

Here, z ₁ is the distance from the incident end of the branch optical fiber to the position where the probe light pulse and the pump light pulse interact. α _B (z ₁ , f) is the induced Brillouin gain when the position z ₁ from the incident end and the frequency difference between the probe light and the pump light is f. g _B (f) is a stimulated Brillouin scattering coefficient when the frequency difference between the probe light and the pump light is f. I _pump (z) is the intensity of the pump light pulse at a position away from the incident end of the optical fiber by a distance z. Dz _pump is the spatial resolution.

The loss coefficient of the branch optical fiber (#i) is α _i , the total loss when reciprocating through the branch optical fiber (#i) is 2L _i , the Brillouin frequency shift is f, and the incident probe power is I _probe (0). After the probe light pulse is reflected by the terminal reflection type optical filter (#i), it collides with the pump light pulse at a distance z ₁ from the incident end. The intensity I _probe (2L _i , z ₁ , f) when the probe light pulse after the collision returns to the incident end of the branch optical fiber (test optical fiber 100) is expressed by Expression (2).

From equation (2), the intensity I _probe (2L _i , z, f) of the probe light at the incident end of the branched optical fiber is a function of g _B (f) and I _pump (z). Here, I _pump (z) is expressed by Expression (3), where I _pump (0) is the incident pump power.

Further, the reflected probe light intensity I _ref (2L _i ) that returns to the incident end of the branch optical fiber when only the probe light is incident is expressed by Expression (4).

上記（５）式より、誘導ブリルアン散乱光の利得は、インタラクションした場所までの損失と誘導ブリルアン散乱係数との積を空間分解能で積分した値となる。つまり、上記式（５）は、

のみの関数となる。そこで、以下の演算を行うことで、ある地点ｚ_０からの損失分布を取得することができる。

よって、誘導ブリルアン散乱光の特性を解析すれば、ある地点ｚ_０を基準にした被測定光ファイバ１００の線路損失を測定することができる。 Therefore, Formula (2) is expressed as Formula (5) when Formula (3) and Formula (4) are used.

From the above equation (5), the gain of the stimulated Brillouin scattered light is a value obtained by integrating the product of the loss up to the interaction location and the stimulated Brillouin scattering coefficient with a spatial resolution. That is, the above equation (5) is

Is only a function. Therefore, by performing the following calculation, it is possible to obtain the loss distribution from a point z _0.

Therefore, by analyzing the characteristics of the stimulated Brillouin scattered light, it is possible to measure the line loss of the optical fiber under test 100 relative to the certain point z _0.

となる。また、プローブ光パルスが被測定光ファイバ１００に入射されてから光スプリッタにより分岐され、分岐光ファイバの終端の反射型光フィルタ（＃ａ）で反射されて、被測定光ファイバ１００の入射端からの距離ｌ_ｘ１に到達する時間ｔは、

である。 (B) Measurement of Brillouin scattered light distribution with respect to the distance of the branched optical fiber From the incident end of the measured optical fiber 100 to the end of the branched optical fiber (#a) (1 ≦ a ≦ N, here N = 8) _{Let the} length be La. The probe light pulse is reflected by a reflection type optical filter (#a) installed at the end of the branch optical fiber (#a). Here, the distance from the end of the branch optical fiber l, the refractive index of the optical fiber to be measured 100 n, when the speed of light in a vacuum to is c, the reflected probe light pulses t _1/2 seconds after the l = c / N × t _1/2 so that the distance from the incident end of the optical fiber 100 to be measured is l _x1 , the distance l _x1 is

It becomes. Further, after the probe light pulse is incident on the optical fiber 100 to be measured, it is branched by the optical splitter, reflected by the reflection type optical filter (#a) at the end of the branched optical fiber, and from the incident end of the optical fiber 100 to be measured. The time t to reach the distance l _x1 of

It is.

The time when the pump light enters the optical fiber 100 to be measured is t ₁ second after the probe light is incident. Assuming that the distance from the incident end of the optical fiber 100 to be measured that the pump light reaches after t seconds is l _x2 , the distance l _x2 is expressed by Expression (9).

From the expressions (7) and (9), the probe light pulse and the pump light pulse interact at the position of c · t ₁ / 2n. Also, time for interaction is t _1/2 seconds after the time that is reflected by the reflection type optical filter at the end of the branch optical fiber (#a). That is, the position where the probe light and the pump light interact can be controlled by changing the time difference t _{1 at} which the probe light pulse and the pump light pulse are incident on the optical fiber 100 to be measured. For this reason, the characteristic distribution of stimulated Brillouin scattering with respect to the distance can be obtained.

ここで、他の分岐光ファイバ（＃ｂ）（１≦ｂ≦Ｎの整数、ここではＮ＝８）から戻ってきたプローブ光パルスが光受信器に到達する時間ｔ_ｄｂは、式（１１）で表される。

よって、光受信器に戻る時間差は、式（１２）で表される。

(C) Measurement of time for probe light pulse reflected by branch optical fiber (#a) to reach optical receiver When measured optical fiber 100 is a branch optical line, the following requirements are required.
Let t _da be the time for the probe light pulse to reach the optical receiver. The probe light pulse is reflected by the reflection type optical filter (#a) at the end of the branch optical fiber and returns to the optical receiver. Therefore, the arrival time is expressed by the equation (10).

Here, the time t _db when the probe light pulse returned from the other branch optical fiber (#b) (integer of 1 ≦ b ≦ N, here N = 8) reaches the optical receiver is expressed by the equation (11). It is represented by

Therefore, the time difference for returning to the optical receiver is expressed by Expression (12).

のとき（条件３）、分岐光ファイバ（＃１）〜（＃８）から戻ったプローブ光パルスは光スプリッタで重ならない。そのため、光受信器の到達時間を測定することで、分岐光ファイバ（＃１）〜（＃８）のうちどの分岐光ファイバから出力されたプローブ光パルスであるかを時間的に切り分けることができる。 When L _a ≠ L _b, the time to reach the optical receiver is different. Here, when the pulse width of the probe light pulse is τ,

(Condition 3), the probe light pulses returned from the branch optical fibers (# 1) to (# 8) do not overlap with the optical splitter. Therefore, by measuring the arrival time of the optical receiver, it is possible to temporally determine which of the branched optical fibers of the branched optical fibers (# 1) to (# 8) is the probe light pulse. .

  By the above (a) and (b), the optical line characteristic analyzer can measure the loss distribution of each optical fiber. Further, by performing (c), it is possible to cope with a branched optical line.

  Further, the optical line characteristic analyzing apparatus according to the present invention can relax the requirement of the vertical resolution of the A / D converter 27 by including the analog signal processing means 29. And it can be set as high sensitivity by performing the averaging process of several measurement.

  In the above embodiment, the case where the optical line characteristic analyzing apparatus includes a circulator has been described as an example. However, it is not limited to this. For example, the branching optical line characteristic analyzing apparatus may include a coupler instead of the circulator.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Further, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. For example, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine suitably the component covering different embodiment.

[Appendix]
The following summarizes the characteristics of the optical line characteristic analyzing apparatus of the present embodiment.
An object of the present invention is to provide an optical test apparatus capable of testing loss distribution / temperature / strain distribution of an optical fiber with high sensitivity in an optical line having a large loss.

  The optical line characteristic analyzing apparatus of this embodiment uses BOTDA, and the first test is performed with a predetermined time difference from the peak intensity of the first return light when only the first test light pulse is incident on the optical fiber to be measured. The peak intensity of the second return light when both the light pulse and the second test light pulse are incident on the optical fiber to be measured is held in analog form, and a positive constant of 1 or less is set as the peak intensity of the first return light. An analog calculation process for subtracting the multiplied value from the peak intensity of the second return light is performed, and the obtained analog signal is converted into a digital signal.

  As described above, the optical line characteristic analyzing apparatus of the present embodiment converts the probe light into an analog signal by PD, converts it to a desired signal by an analog circuit, and then converts it to a digital signal by an A / D converter. As a result, the influence of quantization noise of the A / D converter can be reduced / removed. Therefore, the optical line characteristic analyzer of this embodiment can measure the characteristic distribution of an optical line with a large loss with high sensitivity.

11: First test light output means 12: Second test light output means 13, 14: Optical pulse forming means 15, 16: Incident time control means 20: Multiplexing element 21: Circulator 22: Optical splitter 23: Branch optical line 24 : Light reflection filter 26: light receiving means 27: A / D converter 28: arithmetic processing unit 29: analog signal processing means 50: backbone optical fiber 100: measured optical fiber

Claims (4)

An optical line characteristic analyzing apparatus for analyzing characteristics of an optical fiber to be measured by a Brillouin time domain analysis method (BOTDA: Brillouin Optical Time Domain Analysis),
Test light incident means for injecting at least one of a probe light pulse and a pump light pulse into an incident end of the measured optical fiber;
Light receiving means for photoelectrically converting return light returning to the incident end of the optical fiber to be measured;
Analog signal processing means for performing analog signal processing using an analog electrical signal photoelectrically converted by the light receiving means;
A / D conversion means for converting an analog electrical signal photoelectrically converted by the optical receiving means or an analog electrical signal subjected to the analog signal processing by the analog signal processing means into a digital electrical signal;
Arithmetic processing means for calculating the digital electrical signal converted by the A / D conversion means and analyzing the characteristics of the optical fiber to be measured;
With
The analog signal processing means, as the analog signal processing,
A probe light pulse and a pump light pulse having an arbitrary optical frequency difference are incident on the incident end of the optical fiber to be measured at an arbitrary time difference, and the optical receiving means photoelectrically returns the return light that returns to the incident end of the optical fiber to be measured. Before the A / D conversion means converts the analog measurement signal, which is the voltage peak of the probe light pulse subjected to Brillouin amplification obtained by conversion, into a digital signal,
Only the probe light pulse is incident on the optical fiber to be measured, the optical pulse reflected by the far end of the optical fiber to be measured and returned to the incident end is obtained by photoelectric conversion by the optical receiving means . An optical line characteristic analyzing apparatus characterized in that an adjustment signal obtained by multiplying an analog reference signal that is a voltage peak by a positive number less than 1 is calculated, and the adjustment signal is subtracted from the analog measurement signal. When the optical fiber to be measured is a branched optical line branched into a plurality of branched optical fibers by an optical branching device at a stage after the incident end,
The test light incident means has the probe light pulse with a pulse width that does not superimpose any return light reflected by the far end of the branch optical fiber at the optical branch,
2. The optical line characteristic analyzing apparatus according to claim 1, wherein the analog signal processing means performs the analog signal processing for each of the branched optical fibers. A signal processing method for analyzing characteristics of an optical fiber to be measured by a Brillouin optical time domain analysis (BOTDA),
A probe light pulse and a pump light pulse having an arbitrary optical frequency difference are incident on the incident end of the optical fiber to be measured with an arbitrary time difference, and the return light returning to the incident end of the optical fiber to be measured is obtained by photoelectric conversion. Before converting the analog measurement signal that is the voltage peak of the probe light pulse subjected to Brillouin amplification into a digital signal,
An analog that is a voltage peak of the probe light pulse obtained by photoelectrically converting the light pulse that is incident only on the optical fiber to be measured, reflected at the far end of the optical fiber to be measured, and returned to the incident end. A signal processing method comprising: calculating an adjustment signal obtained by multiplying a reference number of 1 by a positive number less than 1 and subtracting the adjustment signal from the analog measurement signal. When the optical fiber to be measured is a branched optical line branched into a plurality of branched optical fibers by an optical branching device at a stage after the incident end,
The probe light pulse has a pulse width such that any return light reflected by the far end of the branch optical fiber is not superimposed by the optical branching device,
The signal processing method according to claim 3, wherein the analog signal processing procedure is performed for each branch optical fiber.

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