JP6263426B2 - Branch optical line characteristic analysis system, branch optical line and manufacturing method thereof - Google Patents

Description

  The present invention relates to a branched optical line characteristic analysis system, a branched optical line, and a method of manufacturing the same, for individually measuring the characteristics of a plurality of branched optical branched lines in, for example, a PON (Passive Optical Network) type optical line.

  Various optical sensing systems using optical fibers have been proposed as techniques for monitoring distortion and temperature changes around the structure. For example, Brillouin OTDR (Optical Time Domain Reflectometer) described in Non-Patent Document 1 and BOCDA (Brillouin Optical Correlation-domain analysis) described in Cited Document 2 are examples. As another technique, a method of arranging an FBG (Fiber Bragg Grating) described in Non-Patent Document 3 has also been proposed.

  In the field of optical communication that aims at FTTH (Fiber to the Home), there is a need to individually monitor the characteristics (loss distribution, etc.) of a branched optical line in a PON (Passive Optical Network) type network. However, in the techniques of Non-Patent Documents 1 and 2, the individual characteristic distribution measurement of the branch line is not realized. Further, the technique of Non-Patent Document 3 cannot obtain a characteristic distribution in the longitudinal direction of the line.

  On the other hand, in a branched optical line in which an upstream optical fiber and a downstream optical fiber are connected by an optical splitter, a probe light pulse and a pump light pulse are incident on the upstream optical fiber, and the two optical pulses collide at the collision position. There is a technique for analyzing the Brillouin gain and individually measuring the characteristic distribution of the branched optical line (see, for example, Non-Patent Document or Non-Patent Document 5). These distributed optical fiber sensing technologies having branches in the form of the line to be measured are advantageous in that the sensor head can be configured with robustness and a flexible topology as compared with the one-stroke optical fiber sensing technology.

H. Ohno, H. Naruse, M. Kihara, and A. Shimada, “Industrial Applications of the BOTDR Optical Fiber Strain Sensor,” Optical Fiber Technology 7, 45-64, (2001). K. Hotate and T. Hasegawa, “Measurement of Brillouin gain spectrum distribution along an optical fiber using a correlation-based technique-Proposal, experiment and simulation-,” IEICE Trans. Electron. E83-C, 405, (2000). Y.J.Rao, “Recent progress in applications of in-fibre Bragg grating sensors,” Optics and Lasers in Engineering 31, 297-324, (1999). H. Takahashi, F. Ito, C. Kito, and K. Toge, “Individual loss distribution measurement in 32-branched PON using pulsed pump-probe Brillouin Analysis,” Optics Express, Vol. 21, No. 6, 6739, ( 2013). H. Takahashi, K. Toge, C. Kito, and F. Ito, “Individual PON Monitoring Using maintenance Band Pulsed Pump-Probe Brillouin Analysis,” 2013 18th OECC, ThP1-4, (2013).

  However, in the techniques shown in Non-Patent Document 4 and Non-Patent Document 5, after a pair of probe light pulses and pump light pulses (hereinafter referred to as pulse pairs) are incident on the optical fiber to be measured, You must wait for the optical fiber to be measured to exit. That is, after the pulse pair of the first wave is incident on the optical fiber to be measured, the pulse pair after the second wave cannot be incident on the optical fiber to be measured until the pulse pair disappears from the optical fiber to be measured. This is because if the probe light and the pump light collide with each other in the optical fiber, accurate characteristic distribution information cannot be acquired from the Brillouin gain peak. Therefore, it takes a lot of time for one measurement.

  Moreover, in the techniques shown in Non-Patent Document 4 and Non-Patent Document 5, the average value is obtained by repeating measurement in the order of tens of thousands of times in order to improve the SN ratio (signal-to-noise ratio). The time required for measurement becomes longer and longer. Furthermore, in cases where the line to be measured is long or cases where the number of measurement points is increased to improve the resolution, the time required to obtain the measurement result is further increased, and thus drastic measures are required.

  The present invention has been made paying attention to the above circumstances, and its purpose is to perform measurement by shortening the measurement interval while maintaining the measurement accuracy when repeatedly performing the measurement on the branched optical line multiple times. An object of the present invention is to provide a branching optical line characteristic analysis system, a branching optical line, and a method for manufacturing the same, which are intended to increase the speed.

  In order to achieve the above object, a first aspect of the present invention relates to a branched optical line in which a first optical fiber and a plurality of second optical fibers are connected by an optical branching device, and the branched optical line. Stimulated Brillouin is generated by the interaction of the probe light pulse and the pump light pulse in the plurality of second optical fibers by entering a probe light pulse and a pump light pulse having a predetermined frequency difference from the first optical fiber. Scattering is generated, and the return light is received through the first optical fiber to obtain a Brillouin gain spectral distribution, and the characteristics of the plurality of second optical fibers are individually determined based on the Brillouin gain spectral distribution. And a branching optical line characteristic analyzing system comprising: an analyzing device for analyzing the first optical fiber and the second optical fiber, wherein the first optical fiber and the second optical fiber are Brillouin. In which the difference in wavenumber shift amount is configured to be the bandwidth greater than the Brillouin gain.

The first aspect of the present invention is characterized by comprising the following aspects.
In the first aspect, the first optical fiber and the second optical fiber control the Brillouin frequency shift amount by controlling at least one of a refractive index profile of a fiber cross section and an additive doping rate with respect to a core of the fiber. Is set to be larger than the Brillouin gain band.

  According to a second aspect, in the analysis apparatus, a pulse width of the probe light pulse, a minimum value of a difference in length between the plurality of second optical fibers, ΔL, a high speed in vacuum c, a second light When the refractive index of the fiber is n, it is set to a value smaller than the minimum value of the time difference 2nΔL / c of the return light from the plurality of second optical fibers.

In a third aspect of the present invention, in the analysis apparatus, when the pulse width of the probe light pulse is τ _probe , the band of the receiving system that receives the return light is set wider than 1 / τ _probe. is there.

  According to the first aspect of the present invention, the difference in the Brillouin frequency shift amount between the first optical fiber and the second optical fiber in the branch optical line is set to a value larger than the Brillouin gain band. Therefore, even if the probe light and the pump light used in the current measurement collide with the pump light and the probe light of the previous measurement reflected by the second optical fiber in the first optical fiber, Brillouin amplification does not occur in this optical fiber. For this reason, it is possible to shorten the measurement interval without impairing the measurement accuracy of the characteristic information in each measurement, and thereby it is possible to speed up the measurement while maintaining the measurement accuracy.

  According to the first aspect, the difference in the Brillouin frequency shift amount between the first optical fiber and the second optical fiber is determined by calculating at least one of the refractive index profile of the fiber cross section and the additive doping rate with respect to the fiber core. By controlling, the value is set to be larger than the Brillouin gain band. For this reason, it can be easily realized simply by applying existing manufacturing techniques.

  According to the second aspect, in the analyzing apparatus, the pulse width of the probe light pulse is ΔL as the minimum value of the length difference between the plurality of second optical fibers, c as the high speed in vacuum, and the second optical fiber. Is set to a value smaller than the minimum value of the time difference 2nΔL / c of the return light from the plurality of second optical fibers. For this reason, it is possible to prevent the return light (stimulated Brillouin scattered light) of each of the second optical fibers from overlapping in the analyzer, that is, causing interference, thereby ensuring that the second optical fiber is temporally reliable. It becomes possible to measure by dividing.

According to the third aspect, when the pulse width of the probe light pulse is τ _probe , the band of the receiving system that receives the return light is set wider than 1 / τ _probe . For this reason, a sufficient dynamic range is ensured in the receiving system in the analyzing apparatus, and thereby, it is possible to accurately analyze the return light.

  That is, according to the present invention, a branching optical line characteristic analysis system that shortens the measurement interval and speeds up the measurement while maintaining measurement accuracy when performing measurements on the branching optical line a plurality of times. An optical line and a manufacturing method thereof can be provided.

1 is a block diagram showing a configuration of a branched optical line characteristic analysis system according to an embodiment of the present invention. The time interval between the probe light pulse and the pump light pulse transmitted in the first measurement and the probe light pulse and the pump light pulse transmitted in the second measurement, and the distance between the probe light pulse and the pump light pulse. The figure for demonstrating. The figure which shows the time position in the branch lower optical fiber of the return light of the probe light pulse and pump light pulse which were transmitted in the 1st measurement, and the probe light pulse and pump light pulse which were transmitted in the 2nd measurement.

Embodiments according to the present invention will be described below with reference to the drawings.
[One Embodiment]
FIG. 1 is a block diagram showing a configuration of a branched optical line characteristic analysis system according to an embodiment of the present invention.
The branched optical line characteristic analysis system of this embodiment includes an analysis device 1 and a branched optical fiber sensor head 2 as a branched optical line to be measured. Then, test light is incident on the branch optical fiber sensor head 2 from the analysis device 1 and the return light is analyzed by the analysis device 1 to analyze the optical line characteristics of the branch optical fiber sensor head 2. The optical line characteristics that can be measured by the branch optical fiber sensor head 2 are, for example, the optical attenuation with respect to the distance, the position of the bending obstacle, the degree of bending, the position of the disconnection obstacle, and the temperature change with respect to the distance.

The branch optical fiber sensor head 2 includes a branch upper optical fiber F ₀ as a first optical fiber and a plurality of lower branch optical fibers F _{1 to} F _N as second optical fibers. It is connected via a splitter SP. Each of the branched lower optical fibers F _{1 to} F _N has a different line length, and light reflection filters R _{1 to} R _N are arranged at the ends. Note that the light reflection filter may be omitted if the end surface has sufficient reflectance.

On the other hand, the analysis device 1 has a function of generating test light. There are two types of test light, one is called probe light and the other is called pump light. The frequency difference between the probe light and the pump light is set so as to coincide with the BFS amount f _B of each branched lower optical fiber F _{1 to} F _N of the branched optical fiber sensor head 2.

The probe light and the pump light are each pulsed and made incident on the branched upper optical fiber F ₀ of the branched optical fiber sensor head 2. The pulsed probe light collides with the pulsed pump light in the branched lower optical fibers F _{1 to} F _N in the process of being reflected and returning at the end of each branched lower optical fiber F _{1 to} F _N. Brillouin amplification and return to the observation apparatus 1 via the branch upper optical fiber F. This Brillouin amplified probe light pulse is referred to as a return light pulse.

Hereinafter, the configuration of the analysis apparatus 1 will be described in detail.
Reference numeral 11 denotes a laser light source that generates continuous light having an optical frequency f _0. The continuous light output from the laser light source 11 is branched into two systems by the branch element 12. One of the branched continuous lights is incident on the optical frequency controller 13, and the optical frequency f ₀ is shifted by the Brillouin frequency shift (BFS) amount f _{B of the} branched lower optical fibers F _{1 to} F _N. As the optical frequency controller 13, an external modulator having a function of changing the frequency of the modulation sideband according to the signal frequency of the sine wave generated by the sine wave generator 14 can be used. More specifically, an optical frequency shifter such as a phase modulator, an amplitude modulator, or an SSB (Single Side Band) modulator using LiNbO ₃ can be used.

The probe light and the pump light whose frequency is shifted are incident on the optical pulse generators 15 and 16, respectively, and are pulsed with a pulse width of 2nΔL / c or less. Here, ΔL is the minimum length difference between the branched lower optical fibers F _{1 to} F _N , c is the light transmission speed (light speed) in vacuum, and n is the refractive index of the optical fiber to be measured. The optical pulse generators 15 and 16 are, for example, an acousto-optic modulator using an acousto-optic switch that drives the acousto-optic element or a waveguide switch that drives an electro-optic element using LiNbO ₃ composed of LiNbO ₃ modulator using.

In the optical pulse generators 15 and 16, a time difference is given between the probe light pulse and the pump light pulse. This can be realized by changing the modulation time of the electric pulse that drives the optical pulse generators 15 and 16 under the control of the incident time controllers 17 and 18. Specifically, an optical device using an acousto-optic modulator or a LiNbO ₃ modulator is modulated with an electric pulse, and the timing of modulation with the electric pulse is changed, thereby controlling the timing of the optical pulse. If one of the electrical pulses applied to the optical pulse generators 15 and 16 is set to zero, only one of the probe light pulse or the pump light pulse can be incident on the branched optical fiber sensor head 2. .

The probe light pulse and the pump light pulse emitted from the optical pulse generators 15 and 16 are amplified to levels necessary for measurement by the optical amplifiers 20 _pr and 20 _pu , respectively, and are incident on the multiplexing element 19. Then, after being multiplexed by the multiplexing element 19, it enters the branched upper optical fiber F ₀ of the branched optical fiber sensor head 2 through the optical circulator 21. Thus, the probe light pulse and the pump light pulse whose pulse width and time difference are controlled are incident on the branch optical fiber sensor head 2 as test light.

When the probe light pulse and the pump light pulse are incident, in the branch optical fiber sensor head 2, the probe light pulse and the pump light pulse are branched by the optical splitter SP and are respectively incident on the branch lower optical fibers F _{1 to} F _N. The Then, the incident probe light pulse is reflected respectively by the light reflection filter R ₁ to R _N at the end position of the branch lower optical fiber F ₁ to F _N, returns the process branched lower optical fiber F ₁ to F in _N And collide with the pump light pulse, thereby being amplified by Brillouin. Then, the Brillouin amplified probe light pulse is incident on the analysis device 1 as return light through the branch upper optical fiber F ₀ .

  In the analysis device 1, the return light is incident on the optical filter 22 through the optical circulator 21. The optical filter 22 has a characteristic of allowing only the probe light pulse necessary for analysis to pass, and only the probe light pulse is incident on the optical receiver 23. The optical receiver 23 photoelectrically converts the received probe light pulse and inputs an electric signal corresponding to the probe light pulse to the A / D converter 24. The A / D converter 23 performs analog / digital conversion on the electrical signal to generate digital data. This digital data is input to the signal processor 25. The signal processing device 25 is composed of, for example, a personal computer (PC), and based on the input digital data of the return light, the optical line characteristics of the branched optical fiber sensor head 2, for example, the optical attenuation with respect to the distance, the position of the bending failure Analyzing the degree of bending, the position of the disconnection obstacle, and the temperature change with respect to the distance.

By the way, the following three conditions are mentioned as conditions necessary for realizing the branched optical line characteristic analysis system according to the present embodiment.
(1) Condition 1
In the branched optical fiber sensor head 2, the branched upper optical fiber F ₀ and the branched lower optical fibers F _{1 to} F _N are different in their Brillouin frequency shift (BFS) amounts f _B ′ and f _B (f _B ′ −f _B ) is set to be larger than the Brillouin gain band Δf _B. This generates Brillouin amplification only in the lower branch optical fibers F _{1 to} F _N , and prevents Brillouin amplification from occurring even if the probe light pulse and the pump light pulse collide in the upper branch optical fiber F ₀ . It is a condition for.

The difference between the Brillouin frequency shift (BFS) amounts f _B ′ and f _B (f _B ′ −f _B ) can be controlled, for example, by changing the refractive index profile of the optical fiber cross section or the Ge doping rate of the core. Is possible. For specific control methods of BFS, see Takahashi, Kunihiro Toge, Fumihiko Ito, Chihiro Kito, “Branch monitoring technology by Brillouin gain analysis of far-end reflection,” Proc. 51th Meeting on Lightwave Sensing Technol., It is described in detail in LST51-18, (2013).

(2) Condition 2
In the optical pulse generators 15 and 16, the pulse width τ _probe of the probe light pulse is ΔL for the minimum difference in length of each of the branched lower optical fibers F _{1 to} F _N , c for the high speed in vacuum, When the refractive index of the optical fiber is n, it is set to a value smaller than the minimum value of the time difference 2nΔL / c of the return light from the end of each of the branched lower optical fibers F _{1 to} F _N. Note that c / n is the speed of light ν in the optical fiber.

Condition 2 is necessary to prevent the return light (stimulated Brillouin scattered light) for each of the branched lower optical fibers F _{1 to} F _{N from} overlapping in the optical receiver 23, that is, preventing interference. If no condition 2 is satisfied, return light from the branching lower optical fiber F ₁ to F _N (stimulated Brillouin scattered light) interfere in the optical receiver 23, each branch bottom optical fibers F ₁ to F _N Can not be separated in time. That is, it becomes impossible to distinguish the branched lower optical fibers F _{1 to} F _N.

(3) Condition 3
The operation bands of the optical receiver 23 and the A / D converter 24 are set wider than the band in which the pulse width τ _probe of the probe light pulse can be received. In general, in order to accurately measure an optical pulse having a pulse width τ _probe, the bandwidth of the optical receiver 23 and the A / D converter 24 needs to be wider than 1 / τ.

Next, the test light standby time, that is, the interval time from the transmission of the nth probe light to the transmission of the (n + 1) th probe light will be described. In the system of this embodiment, as described in Condition 3, in the branched optical fiber sensor head 2, the Brillouin frequency shift (BFS) between the branched upper optical fiber F ₀ and the branched lower optical fibers F _{1 to} F _N is performed. amount f _B ', the difference between _{f B (f B' -f B} ) is set to be a value larger than the Brillouin gain bandwidth Delta] f _B.

Therefore, if a plurality of test lights are not simultaneously present in the branch lower optical fibers F _{1 to} F _N , even if a plurality of test lights exist simultaneously in the branch upper optical fiber F ₀ and a collision occurs, Brillouin. No amplification occurs. For this reason, since the (n + 1) th test light can be incident after the nth test light is incident, the test light standby time T is T = 4L _b / ν
It becomes. Incidentally, L _b is the longest fiber length of the branched lower optical fiber F ₁ ~F _N, ν denotes the speed of light in the optical fiber.

Incidentally, in the conventional branch optical fiber sensor head, a plurality of test lights are allowed to exist in the fiber at the same time not only in the lower branch optical fibers F _{1 to} F _N but also in the upper branch optical fiber F ₀ . Therefore, the waiting time of the test light after the n-th test light is incident until the n + 1-th test light is incident is T _c = 2 (L _a + 2L _b ) / ν
I had to set it.

That is, according to this embodiment,
T = 2L _a / ν
Only the test light waiting time per average can be reduced. In other words, it is possible to shorten one average integration time required for measurement in _{(L a + 2L b) /} 2L b min of 1. Here, L _a is the length of the branched upper optical fiber, L _b is the longest branched lower optical fiber length, and ν is the speed of light in the optical fiber.

Usually, since the measurement results of 10,000 times or more are averaged, the effect of reducing the measurement time becomes very large in proportion to the number of averaging. Further, the waiting time does not depend on the branched upper optical fiber length L _a, superiority distance from the analyzing device 1 to the branch point of the branch optical fiber sensor head 2 as compared with longer prior art increases.

2 and 3 show an example of the incident timing of the probe light pulse and the pump light pulse with respect to the branch optical fiber sensor head 2 according to the present embodiment. In the figure, only the nth and (n + 1) th test lights are shown for simplicity, but the subsequent test lights are actually transmitted. In FIG. 2, Δl indicates an amount obtained by converting the incident time difference Δt for controlling the collision position of the probe light pulse and the pump light pulse in the branched lower optical fibers F _{1 to} F _N to the distance, and 0 <Δl < it is a 2L _b.

That is, the time between sending out the first time of the probe light pulses (1) before sending a second round of the probe light pulses (2) With 4L _b, branched lower light as shown in FIG. 3 The first test light and the second test light are not simultaneously present in the fibers F _{1 to} F _N.

On the other hand, in the branch upper optical fiber F ₀ , as described above, the amount of Brillouin frequency shift (BFS) between the branch upper optical fiber F ₀ and each of the branch lower optical fibers F _{1 to} F _N f _B ′, difference _{f B (f B '-f B} ) is, since it is set to a value larger than the Brillouin gain bandwidth Delta] f _B, the first probe light (pump light) the second pump light ( Brillouin amplification does not occur even when the probe light collides.

で表される増幅を受ける。 Next, the characteristic analysis process of the branched optical fiber sensor head 2 by the arithmetic processing unit 25 of the analysis apparatus 1 will be described.
(I) Measurement of stimulated Brillouin scattering by probe light pulse train and pump light pulse When the probe light pulse and pump light pulse included in the probe light pulse train collide in the branch lower optical fibers F _{1 to} F _N , the stimulated Brillouin is caused by interaction. Scattering occurs. When the frequency difference between the probe light pulse train and the pump light pulse is f _B , the probe light pulse is caused by stimulated Brillouin scattering.

The amplification represented by

Here, α _B (z, f _B ) in Equation (1) indicates the gain due to the induced Brillouin when collision occurs at a position z from the incident end and the Brillouin frequency difference is f _B. g _B (f _B ) represents the stimulated Brillouin scattering coefficient when the frequency difference between the probe light pulse and the pump light pulse is f _B. z represents the distance from the incident end of the branched lower optical fibers F _{1 to} F _N to the position where the probe light pulse and the pump light pulse collide. I _pump (z) indicates the intensity of the pump light pulse at a position away from the incident end of the branched lower optical fibers F _{1 to} F _{N by} a distance z. τ _pump indicates the pulse width of the pump light pulse.

で表される。 Now, for example, let α _{i be} the loss coefficient of the lower branch optical fiber #i, and let L _{t be} the total loss of the optical pulse that reciprocates through the lower branch optical fiber #i. Then, the intensity I _pump (2L _i , z) at the incident end of the probe light pulse that is incident on the branched lower optical fiber #i, reflected at the terminal end, and collides with the pump light pulse at a position z from the incident end is ,

It is represented by

で表される。 As shown in Equation (2), the intensity I _pump (2L _i , z, f _B ) of the probe light pulse at the incident end of the branched lower optical fiber #i is g _B (f _B ) and I _pump (z ) Function. However, I _pump (z) is

It is represented by

で示される。 The reflected probe light pulse intensity I _ref (2L _i ) returning to the incident end when only the probe light pulse is incident is

Indicated by

のように表される。 Therefore, when equation (2) is transformed using equations (3) and (4),

It is expressed as

By analyzing the gain of the Brillouin scattered light, based on Equation (5), the value obtained by integrating the loss up to the location where the probe light pulse and the pump light pulse collide with the pump light pulse width τ _pump and the Brillouin scattering coefficient The product can be calculated. Accordingly, it is possible to obtain Brillouin gain strength taking account of loss in the branched optical fiber sensor head.

(Ii) Measurement of Brillouin scattered light distribution with respect to the distance of the branched lower optical fiber For simplicity, only the case where the _first probe light pulse in the probe light pulse train is incident on the pump light pulse first by time t ₁ will be described. To do. For other probe light pulses included in the probe optical pulse train also, the time t ₁ to t _2, t _3, ..., a similar explanation holds if t _x.

が得られる。 Branched lower optical fiber #a from the incident end of the branch optical fiber sensor head 2 (a is a natural number of 1 ≦ a ≦ N) to the length to the end of the L _a. The probe light pulse is reflected by the light reflection filter at the end of the branched lower optical fiber #a. Now, the distance from the end of the branch lower optical fiber and l, the refractive index of the optical fiber is n, when the speed of light in a vacuum is c, the reflected probe light pulses t _1/2 seconds after the l = c / Since it travels by n × t _1/2 , if the distance from the incident end is l _x1 ,

Is obtained.

で表される。 After the probe light pulse is incident, the time t from entering the branched lower optical fiber #a, reflected by the light reflection filter and reaching l _x1 is

It is represented by

で表される。 It is assumed that the pump light pulse is incident t ₁ second after the probe light pulse is incident. The distance l _x2 from the incident end where the pump light pulse reaches after t seconds is

It is represented by

The pump light pulse collides (interacts) with the probe light pulse at a position where l _x1 = l _x2 , that is, a position where Expression (6) = Expression (8). The timing of the interactions the probe light pulse by the light reflection filter is t _1/2 seconds after being reflected. That is, by changing the time difference Δt at which the probe light pulse and the pump light pulse are incident on the optical fiber to be measured, the position where the probe light pulse and the pump light pulse interact can be controlled. Thereby, the characteristic distribution of the stimulated Brillouin scattering with respect to the distance can be obtained.

According to the above (i) and (ii), for each of the branched lower optical fibers, the individual Brillouin gain spectrum distribution is obtained by dividing (L _a + 2L _b ) / 2L _b by the existing technology (particularly Non-Patent Document 4).
It becomes possible to shorten the waiting time. It is of course possible to calculate the characteristic distribution of differences in temperature, strain, and fiber structure parameters from the distribution of Brillouin gain peaks measured by the technique disclosed in Non-Patent Document 6.

As described above in detail, in the embodiment, in the branched optical fiber sensor head 2, the amount of Brillouin frequency shift (BFS) f between the branched upper optical fiber F ₀ and each of the branched lower optical fibers F _{1 to} F _N is f. _B ', the difference between f _B (f _B' the -f _B), is set to be a value larger than the Brillouin gain bandwidth Delta] f _B. The pulse width τ _probe of the probe light pulse is ΔL for the minimum value of the length difference between the branched lower optical fibers F _{1 to} F _N , c for the high speed in vacuum, and n for the refractive index of the second optical fiber. Is set to a value smaller than the minimum value of the time difference 2nΔL / c of the return light from the end of each of the branched lower optical fibers F _{1 to} F _N. Furthermore, the operation bands of the optical receiver 23 and the A / D converter 24 are set wider than the band in which the pulse width τ _probe of the probe light pulse can be received.

Therefore, Brillouin amplification is generated only in the branch lower optical fibers F _{1 to} F _N , and Brillouin amplification is not caused even if the probe light pulse and the pump light pulse collide in the branch upper optical fiber F ₀ . Can do.
Further, the return light (stimulated Brillouin scattered light) for each of the branched lower optical fibers F _{1 to} F _N can be prevented from interfering in the optical receiver 23, thereby ensuring the second optical fiber in terms of time. It becomes possible to measure by dividing.
Further, a sufficient dynamic range is ensured in the receiving system in the analyzing apparatus 1, which makes it possible to accurately analyze the return light.

This embodiment is particularly effective when the length of the branched upper fiber is increased. For example, when a branch strain sensor is used for monitoring a structure such as a tunnel, considering the cost of the measuring instrument, the measuring instrument should be installed at a location away from the monitoring target and connected to multiple branch sensor heads as appropriate for monitoring. Can be considered. At this time, the branched upper fiber extends from the measuring device installation position to the vicinity of the monitoring object, and is assumed to be longer than 10 km. In the conventional sensor head composed of a single BFS optical fiber, the standby time is enormous, whereas according to the branched optical fiber sensor head 2 according to the present embodiment, the measurement time is measured from the branched upper optical fibers F ₁ to F ₁ . Without depending on the length of F _N , the test light standby time occupying the measurement time can be greatly reduced.

[Other Embodiments]
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. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. For example, some configurations may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different example embodiments may be combined as appropriate.

DESCRIPTION OF SYMBOLS 11 ... Light source, 12 ... Branch element, 13 ... Optical frequency controller, 14 ... Sine wave generator, 15, 16 ... Optical pulse generator, 17, 18 ... Incident time controller, 19 ... Multiplexing element, 20a, 20b ... optical amplifier, 21 ... optical circulator, 22 ... optical filter, 23 ... optical receiver, 24 ... A / D converter, 25 ... processing unit, F ₀ ... branch upper optical fiber, SP ... optical splitter, F ₁ ~ F _N ... lower branch optical fiber, R _{1 to} R _N ... light reflection filter.

Claims (7)

A branched optical line in which a first optical fiber and a plurality of second optical fibers are connected by an optical splitter;
A probe light pulse and a pump light pulse having a predetermined frequency difference are incident on the branch optical line from the first optical fiber, and the probe light pulse and the pump light pulse are transmitted in the plurality of second optical fibers. Stimulated Brillouin scattering is caused by the interaction of the two, the return light is received through the first optical fiber to obtain a Brillouin gain spectral distribution, and the plurality of second Brillouin gains are obtained based on the Brillouin gain spectral distribution. An analysis device for individually analyzing the characteristics of the optical fiber,
The branched optical line characteristic analysis system, wherein the first optical fiber and the second optical fiber are set such that a difference in Brillouin frequency shift amount is larger than the Brillouin gain band.   The first optical fiber and the second optical fiber control at least one of a refractive index profile of a fiber cross-section and an additive doping rate with respect to a core of the fiber, so that a difference in the Brillouin frequency shift amount is the Brillouin gain. 2. The branched optical line characteristic analysis system according to claim 1, wherein the branching optical line characteristic analysis system is set to a value larger than the first band.   The analyzing apparatus sets a pulse width of the probe light pulse, a minimum value of a difference in length between the plurality of second optical fibers, ΔL, a high speed in vacuum c, and a refractive index of the second optical fiber n. 3. The branched optical line characteristic analysis system according to claim 1, wherein a value smaller than a minimum value of a time difference 2nΔL / c of return light from the plurality of second optical fibers is set. 2. The analysis apparatus according to claim 1, wherein when the pulse width of the probe light pulse is τ _probe , a band of a receiving system for receiving the return light is set wider than 1 / τ _probe. 4. The branching optical line characteristic analysis system according to any one of 3 above. A branched optical line in which a first optical fiber and a plurality of second optical fibers are connected by an optical branching device, and a probe light pulse having a predetermined frequency difference from the first optical fiber with respect to the branched optical line And pump light pulses are incident to cause stimulated Brillouin scattering in the plurality of second optical fibers due to the interaction between the probe light pulse and the pump light pulse, and the return light passes through the first optical fiber. A branching optical line characteristic analysis system comprising: an analysis device that obtains a spectral distribution of Brillouin gain by receiving the light and analyzes the characteristics of the plurality of second optical fibers individually based on the spectral distribution of the Brillouin gain; The branch optical line used,
A branched optical line, wherein a difference in Brillouin frequency shift between the first optical fiber and the second optical fiber is set to a value larger than the Brillouin gain band.   The first optical fiber and the second optical fiber control at least one of a refractive index profile of a fiber cross-section and an additive doping rate with respect to a core of the fiber, so that a difference in the Brillouin frequency shift amount is the Brillouin gain. The branched optical line according to claim 5, wherein the branching optical line is set to a value larger than the bandwidth of the optical fiber. A branched optical line in which a first optical fiber and a plurality of second optical fibers are connected by an optical branching device, and a probe light pulse having a predetermined frequency difference from the first optical fiber with respect to the branched optical line And pump light pulses are incident to cause stimulated Brillouin scattering in the plurality of second optical fibers due to the interaction between the probe light pulse and the pump light pulse, and the return light passes through the first optical fiber. A branching optical line characteristic analysis system comprising: an analysis device that obtains a spectral distribution of Brillouin gain by receiving the light and analyzes the characteristics of the plurality of second optical fibers individually based on the spectral distribution of the Brillouin gain; A method of manufacturing the branched optical line used,
By controlling the difference in the Brillouin frequency shift amount between the first optical fiber and the second optical fiber of the branch optical line, by controlling at least one of the refractive index profile of the fiber cross section and the additive doping rate with respect to the core of the fiber, A method of manufacturing a branched optical line, wherein a value larger than the Brillouin gain band is set.

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