JP2014159987A - Characteristic analyzer for optical branch line and characteristic analysis method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To individually measure the optical line loss characteristic of an existing outside facility at a high speed.SOLUTION: A probe light pulse line formed from X number of successive light pulses of different frequencies and a predetermined pulse width, and a pump light pulse line are generated. The frequency difference between the n-th light pulse in the probe light pulse line and the n-th light pulse of the pump light pulse line is set to a frequency difference (ν) that causes a Brillouin interaction. Both the pulse lines are sequentially made incident on with a predetermined incident time difference. While the incident time difference is changed, a light pulse line having a frequency equal to the probe light pulse line returned from a branch lower optical fiber is extracted. From the light reception timing of the extracted light pulse line, it is determined that from which branch lower optical fiber the light pulse line returned. Also, Brillouin gain is measured for each light pulse composing the light pulse line, thereby obtaining Brillouin gain characteristic distribution for each branch lower optical line.

Description

  The present invention relates to an optical branch line characteristic analysis apparatus and a characteristic analysis method for measuring the characteristic of each optical branch line branched by an optical splitter in an optical line such as an optical fiber.

  In an optical communication system using an optical line such as an optical fiber, an optical pulse line monitoring device is used to detect breakage of the optical line or to specify the breakage position. This optical pulse line monitoring apparatus utilizes the fact that backscattered light having the same wavelength as the light is generated and propagates in the reverse direction as the light propagates through the optical line.

  That is, when an optical pulse is incident on the optical line as test light, backscattered light continues to be generated until the optical pulse reaches the breaking point, and the end face of the optical line on which the return light having the same wavelength as the test light is incident. It is emitted from. By measuring the duration of the backscattered light, the break position of the optical line can be specified. A typical monitoring device based on this principle is an OTDR (Optical Time Domain Reflectometer).

  However, with regard to the PON (Passive Optical Network) type optical branch line, in the OTDR installed in the communication facility equipment building, a branch optical fiber (hereinafter referred to as “branch lower part”) located on the user equipment side (lower side) from the optical splitter. It has been difficult to individually identify loss information of optical devices connected to an optical line such as an optical fiber)) or an optical device (reflection filter), an optical splitter, or a fiber connection component.

  On the other hand, as a technique that can individually identify the loss information of the lower branch optical fiber, two test light pulses, a pump light pulse and a probe light pulse, are incident and the Brillouin gain at the collision position of both test lights is analyzed. Thus, a technique for measuring the individual loss distribution of the splitter lower core wire is disclosed (Non-Patent Document 1). However, according to the principle of this method, the measurement time becomes enormous for the lengthening of the optical line to be measured.

  That is, in the technique of Non-Patent Document 1, since only one set of probe-pump light pulse pairs (hereinafter referred to as “pulse pairs”) is used for test light, the first wave pulse pair is incident on the optical fiber to be measured. Until the pulse pair of the first wave is ejected from the optical fiber to be measured, the pulse pair after the second wave cannot enter the optical fiber to be measured. This is because if the first and second pulse pairs are incident before the first pulse pair is ejected from the optical fiber to be measured, the probe light and the pump light collide with each other in a multiple manner. This is because it becomes impossible to obtain the correct loss distribution information. In addition, the technique of Non-Patent Document 1 repeats measurement on the order of tens of thousands of times and performs an averaging process to improve the signal-to-noise ratio. Therefore, the longer the measured optical line, the longer the test light occupies the total measurement time. The waiting time until the light enters is long, and it is difficult to shorten the measurement time of the long-distance line.

  Therefore, in the PON type optical branch line, the optical device and the optical line configuration are newly changed when monitoring the optical device from the optical splitter to the branch lower optical fiber on the user apparatus side, the optical splitter, the fiber connection component, and the like. (Without cost by changing existing equipment), it is possible to individually measure the optical line loss characteristics of existing off-site equipment (branch lower fiber with different fiber length), and waiting time for test light incidence Therefore, there is a need for a technology that can significantly reduce the measurement time.

H. Takahashi, et al., "Individual Loss Distribution Measurement in 4-branched PON Using Pulsed Pump-Probe Brillouin Gain Analysis", ONC2012 Technical Digest, paper OTh3I.2 (2012).

  As described above, when monitoring the optical device of the optical branch line, it is possible to individually measure the optical line loss characteristics of the existing off-site equipment without changing the optical device or the optical line configuration. In addition, there is a need for a technique that significantly reduces the measurement time by reducing the test light incident standby time.

  The present invention has been made paying attention to the above situation, and without changing the optical device or optical line configuration newly, while maintaining the optical fiber identification resolution and the characteristic distribution measurement resolution, It is an object of the present invention to provide an optical branch line characteristic analysis apparatus and its characteristic analysis method capable of individually measuring optical line loss characteristics at high speed.

The optical branch line characteristic analyzer according to the present invention has the following configuration.
(1) An optical branch line characteristic analyzing apparatus for analyzing individual optical line characteristics of an optical branch line having N branch lower optical fibers from a branch point by an optical splitter to the far end of the optical branch line, and having a wavelength The first test light and the second test light having different frequencies are modulated in time to modulate to X (X is a natural number) frequencies, and the test light frequency modulation means synchronizes with the test light modulation means, and the different X Test light pulsing means for pulsing the probe light and the X pump lights with an arbitrary time difference, and the probe light pulse train and the X second light pulses by the pulsed X first test lights A multiplexing means for multiplexing the pump light pulse train by the test light, the combined probe light pulse train and the pump light pulse train are sequentially incident on the fiber to be measured, and the branched optical fiber below the optical splitter. A circulator for extracting the test light returned from the bar, a filter means for extracting frequency components of X first test lights from the test light extracted by the circulator, and a first test light extracted by the filter means The optical / electrical conversion means for receiving the light and converting it into a current, the analog / digital conversion means for converting the first test light converted into the current into a digital signal, and the control information at the time of decoding and generating the digital signal The frequency separation means for identifying which digital signal of the first test light incident on the lower branch optical fiber from the light receiving timing, and separating the signal for each of X frequency components from the identified digital signal; The Brillouin gain of each of the X frequency components is analyzed, and the above measurement is repeated while changing the incident time difference between the probe light pulse train and the pump light pulse train. And a signal calculation processing means for obtaining a Brillouin gain characteristic distribution of each of the lower optical fibers of the optical splitter from the measurement results obtained by repeated measurement, and the test optical frequency modulation means is configured to perform a first operation at predetermined time intervals. A means for frequency modulating the test light and the second test light, the frequency modulation for the first test light and the second test light so as to cause a Brillouin interaction only between the specific first test light and the second test light. The characteristic analysis apparatus shares the synchronization signal between the test light frequency modulation means and the test light pulse conversion means, and grasps the frequency and frequency modulation time as output states of the first test light and the second test light at an arbitrary time. It is set as the aspect which is.

  (2) In (1), the signal processing means measures in advance the return time at the beginning of the probe light pulse train from the N branched lower optical fibers, and the return time at the head of the received probe light pulse train. The return time of each of the X probe optical frequencies included in the probe optical pulse train is specified from the information of the frequency modulation time by the frequency modulation means, and the X probe optical frequencies included in the probe optical pulse train are determined. The modulation frequency to be received is specified from the return time to the frequency modulation time, and the time change of the received signal intensity is Fourier-transformed within the frequency modulation time from the return time of each of the X probe optical frequencies included in the probe optical pulse train. The frequency separation is performed to extract the light amplitude intensity of the probe light frequency component to be received at the time, and the X probe lights are A manner of analyzing independently Brillouin gain that is.

(3) In (1), the test optical frequency modulation means is realized by a modulated electric signal of a single sideband modulator or LN phase modulator and an arbitrary waveform signal generator.
(4) In (1), the test optical frequency modulation means installs a single sideband modulator, an LN phase modulator, or an acoustooptic modulator adjusted to the specific modulation frequency in an optical fiber ring, A mode in which optical modulation is performed step by step at a specific modulation frequency for each revolution.

(5) In (1), the signal processing means applies a window function to the time change of the current signal due to the received probe light pulse train, and extracts the specific signal.
(6) In (1), the optical line characteristic is at least one of an optical attenuation amount with respect to a distance, a position of a bending obstacle, a degree of bending, a position of a disconnection obstacle, and a temperature change amount with respect to the distance.

Moreover, the characteristic analysis method of the optical branch line according to the present invention has the following configuration.
(7) In the optical branch line characteristic analysis method in (1), an individual optical line characteristic is analyzed for an optical branch line having N branch lower optical fibers from the branch point by the optical splitter to the far end of the optical branch line. The first test light and the second test light having different wavelengths are temporally controlled to be modulated to X (X is a natural number) frequency, and synchronized with the test light modulation, The probe light and the X pump lights are pulsed with an arbitrary time difference, and the probe light pulse train of the X X first test lights and the pump light pulse train of the X second test lights are combined. Then, the combined probe light pulse train and pump light pulse train are sequentially incident on the fiber to be measured, the test light returned from the branch optical fiber below the optical splitter is extracted, and the extracted light is extracted. Extract frequency components of X first test lights from the test light, receive the extracted first test lights and convert them into current, and convert the first test lights converted into the current into digital signals The digital signal is decoded, the control information at the time of generation and the light reception timing are used to identify which digital optical signal of the first test light is incident on the lower branch optical fiber, and X digital signals are identified from the identified digital signal. The signal is separated for each frequency component, the Brillouin gain of each of the X frequency components is analyzed, the above measurement is repeated while changing the incident time difference between the probe light pulse train and the pump light pulse train, and the light is obtained from the repeated measurement results. The Brillouin gain characteristic distribution of each of the branched lower fibers is acquired, and the test optical frequency modulation is performed with the specific first test light and the second test light at predetermined time intervals. Frequency modulation is performed on the first test light and the second test light so as to cause Brillouin interaction only between the test lights. In the characteristic analysis, a synchronization signal of the test light frequency modulation and the test light pulse is shared, It is assumed that the frequency and the frequency modulation time are grasped as the output states of the first test light and the second test light in time.

  As described above, according to the present invention, in the individual loss measurement of the optical splitter lower line, it is possible to perform parallel measurement by using test optical pulse pairs having different frequencies, and compared with the conventional technique, the branched optical fiber identification resolution and the characteristic distribution It is possible to provide an optical branch line characteristic analysis device and a characteristic analysis method that achieve high-speed measurement by reducing measurement time without degrading resolution.

It is a block diagram which shows the structure of the characteristic analyzer of the optical branch line which concerns on the 1st Embodiment of this invention. In 1st Embodiment, it is a figure which shows the arrival time difference to the optical receiver of the probe light which returns from the branch line of 3 systems. In a 1st embodiment, it is a figure showing signs that a part of probe light of a probe light pulse train which returns from a branch line of three systems arrives at an optical receiver over time. It is a flowchart which shows the flow of a process of the arithmetic processing unit used for 1st Embodiment. It is a block diagram which shows the structure of the characteristic analyzer of the optical branch line which concerns on the 2nd Embodiment of this invention.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
First, to outline the present invention, an optical branch line characteristic analysis apparatus according to the present invention is applied and configured in the following manner.

(Measured line conditions)
The measured line to which the present invention is applied is that the optical fiber splits one optical fiber from the first to the Nth optical fiber by an optical splitter, and the N branched lower optical fibers from the branch point by the optical splitter to the optical filter. In the optical branch line, the minimum difference in length is equal to or higher than the optical fiber identification resolution of the device of the present invention.

(Outline of device configuration)
The test light of the present invention generates a pair of first test light (probe light) and second test light (pump light) having a frequency difference that causes Brillouin interaction. The signal is modulated to X different frequencies (X is a natural number) by a frequency modulator operating in response to the signal.

  Here, the frequency-modulated electrical signal is a frequency-modulated electrical signal programmed in advance in the arbitrary waveform generator. The arbitrary waveform generator operates in a temporally controlled manner by a trigger signal from a pulse pattern generator (PPG) or a function generator (FG).

As the frequency modulator, a single sideband modulator or an LN phase modulator can be used.
The trigger signal for the PPG or FG is shared with the trigger signal for recording the received signal, so that the transmission side and the reception side of the modulation frequency can be synchronized.
The X probe lights and X pump lights having different frequencies are pulsed with a relatively arbitrary time difference by a pulsing device synchronized by a PPG or FG trigger signal shared with the frequency modulator. It becomes a pulse train and a pump light pulse train. Here, an acousto-optic modulator, an LN intensity modulator, or an optical switch can be used as the pulsing device. The test optical pulse train is transmitted after the previously transmitted test optical pulse train is discharged from the optical fiber to be measured.

The above-described test light pulse train generation means can be realized by the following apparatus configuration.
The optical pulsing device operates while being temporally controlled by a trigger signal from a pulse pattern generator (PPG) or a function generator (FG), and pulses the first test light and the second test light. Here, the time interval for pulsing must be equal to or longer than the round trip time of the optical fiber to be measured for the second test light.

  The pulsed first test light and second test light are applied to an optical fiber ring provided with a single sideband modulator, an LN phase modulator, or an acousto-optic modulator that is adjusted to a specific modulation frequency by a multiplexing element. Incident light is shifted by a specific frequency for each turn, and output from the optical fiber ring as a test optical pulse train in which the frequency is sequentially modulated stepwise.

In order to compensate for the loss of the optical fiber ring and the frequency shifter, an optical amplifier may be installed in the optical fiber ring.
With the above configuration, the first test light and the second test light incident on the line to be measured change at a predetermined frequency interval every predetermined time interval, and form a pair with a frequency difference that matches the Brillouin frequency shift. In addition, the frequency of the X probe light and the X pump light becomes a test light pulse train in which the frequencies do not match.

Also, since the frequency modulation electrical signal, pulsing device, and reception signal recording start operation share one trigger signal, the receiving side characteristic analysis device sets the transmission side output state (frequency, presence / absence of output) at an arbitrary time. I can grasp it.
The probe light pulse train and the pump light pulse train are combined by a multiplexing element and sequentially enter the line to be measured. The X probe lights reflected by the test light reflection filter at the far end of the measured line are subjected to Brillouin gain by causing a Brillouin interaction in the measured line with only the pump light paired with the Brillouin gain frequency difference. Conversely, since no Brillouin interaction occurs between pulses that are not paired with a Brillouin gain frequency difference, the same probe light does not receive multiple Brillouin gains in the measured line.

Test light which has returned is extracted by the optical circulator, only further frequency components of the X number of the probe light is extracted by the optical filter.
The probe light extracted by the optical filter is received by a photo detector and converted into current, and the probe light converted into current is converted into a digital signal by analog / digital conversion, the digital signal is decoded, and the test light is generated. The control information and the light reception timing specify which optical signal of the probe light is incident on the lower branch optical fiber, and the specified digital signal is separated into X frequency components by arithmetic processing such as Fourier transform. Then, the Brillouin gain of each frequency component is analyzed, and line characteristic information up to the collision point of the probe light and the pump light is output.

  Furthermore, the Brillouin gain characteristic of each of the lower branch fibers of the optical splitter is determined by repeating the above measurement while changing the collision point of the probe light and the pump light by changing the incident time difference between the probe light and the pump light. Output the distribution.

(Characteristic items to be measured)
The optical line characteristic is at least one of 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.
(Calculation processing)
The arithmetic processing means enters the probe light pulse train in advance without entering the pump light pulse train, measures the return times t ₁ to t _N at the head of the probe pulse train from the N branched lower optical fibers, respectively, From the return time at the head of the pulse train and the information on the frequency modulation time by the frequency modulation means, the return times of the X probe light frequencies included in the probe light pulse train are specified, and X pieces of light contained in the probe light pulse train are identified. The modulation frequency to be received from the return time of each probe optical frequency to the frequency modulation time is specified, and the time variation of the received signal intensity within the frequency modulation time from the return time of each of the X probe optical frequencies included in the probe optical pulse train Is subjected to frequency separation to extract the probe light frequency component to be received at the time, and the frequency signal component The reference probe optical power is used.

  Subsequently, the probe light pulse train and the pump light pulse train are incident at a predetermined time difference, and the probe light signal power subjected to the Brillouin gain by the pump light is measured in the same manner as the measurement of the reference probe light power. Then, the Brillouin gain obtained by the probe light frequency component is independently analyzed from the ratio of the intensity of the frequency component of the probe light signal power and the reference probe light power.

In addition, regarding the arithmetic processing means, it is possible to reduce the crosstalk of adjacent frequency components when a specific frequency signal is extracted by applying a window function to the time change of the current signal due to the received probe light pulse train.
With the above processing configuration, in the individual loss measurement of the optical splitter bottom line, it is possible to perform parallel measurement using test optical pulse pairs with different frequencies, reducing the measurement time without degrading the branching optical fiber identification resolution and characteristic distribution resolution. An apparatus and method for analyzing characteristics of an optical branch line that realizes possible high-speed measurement can be provided.

(First embodiment)
FIG. 1 is a block diagram illustrating a configuration of an optical branch line characteristic analyzing apparatus according to the first embodiment. The apparatus shown in FIG. 1 (all other components excluding the optical fiber to be measured surrounded by a dotted line) is opposed to X pump lights having different frequencies, each of which has a pair of X probe lights having different frequencies. The characteristic distribution of the Brillouin gain received in the measured fiber 23 by propagation can be obtained.

In the first embodiment shown in FIG. 1, the continuous light output from the laser light source 11 is branched into two systems by the branch element 12.
One of the lights branched by the branch element 12 is a first test light (probe light) and the other is a second test light (pump light). The first test light is shifted in frequency by the Brillouin frequency shift amount ν _B by the optical frequency shifter 13. Specifically, the optical frequency shifter 13 may be any external modulator having a function of frequency of the modulation sidebands is changed according to the signal frequency from the sine wave generator (not shown), a LiNbO ₃ The used phase modulator (LN phase modulator), amplitude intensity modulator, or SSB modulator can be used.

The first test light frequency-shifted by the optical frequency shifter 13 is branched into two systems by the branch element 14, one being probe light and the other being local light. The probe light is incident on the optical frequency modulation device 15 and becomes light whose frequency is modulated in the X stage controlled temporally.
Further, the probe light becomes a probe light pulse train arranged in time with different frequencies of the X wave by pulsed modulation by the optical pulse device 16.

On the other hand, the second test light branched by the branching element 12 is similarly frequency-modulated by the optical frequency modulation device 17 and further arranged temporally with different X-wave frequencies by pulsed modulation by the optical pulse device 18. The pump light pulse train thus obtained is obtained. Of the X probe lights and pump lights, the first wave probe light and pump light, the second wave probe light and pump light, and the X wave probe light and pump light have a frequency difference of ν _B from each other. A probe-pump pair. Only the probe-pump pair satisfies the condition for causing the Brillouin interaction, and both the probe light and the pump light have different frequencies.

Here, when ν ₀ is an unmodulated optical frequency of the laser light source 11, ν _B is an optical frequency shift amount due to Brillouin backscattering, Δν _B is a half-value width of the Brillouin gain band, and A is a constant, a laser light source (frequency ν ₀ ) The gain coefficient g _B of stimulated Brillouin scattering given by 11 is expressed by the following equation.

（1）式より、本発明手法においてポンプ光パルス列に内包されるｉ番目のポンプ光の光周波数はν₀-ν_iとし、プローブ光パルス列に内包されるｉ番目のプローブ光の周波数はプローブ-ポンプ対において互いにν_Bの周波数差を持つようν₀-ν_B-ν_iに設定することでプローブ光はポンプ光から最大の利得を得る。また、プローブ光パルス列およびポンプ光パルス列に内包される各周波数成分について、周波数空間で隣接する成分同士をΔν_Bより十分大きく離して配置することで、ペア間以外でのブリルアン相互作用の発生を回避できる。

From equation (1), the optical frequency of the i-th pump light included in the pump light pulse train in the method of the present invention is ν ₀ −ν _i, and the frequency of the i-th probe light included in the probe light pulse train is the probe- By setting ν ₀ −ν _B −ν _i so as to have a frequency difference of ν _{B in} the pump pair, the probe light obtains the maximum gain from the pump light. In addition, for each frequency component included in the probe light pulse train and the pump light pulse train, the adjacent components in the frequency space are arranged sufficiently far apart from Δν _B to avoid the occurrence of Brillouin interaction other than between the pairs. it can.

As the optical pulse device 16 and the optical pulse device 18, specifically, an amplitude modulator or an acousto-optic modulator using LiNbO ₃ driven with an arbitrary waveform can be used. Here, the optical pulsing means intensity-modulating the continuous probe light and pump light to temporally separate the m-th (m is a natural number) test light and the (m−1) -th test light. That is.

The order of the optical frequency modulation by the optical frequency modulators 15 and 17 and the optical pulse modulation by the optical pulse generators 16 and 18 are arbitrary.
However, a modulation frequency controller 19 for controlling the modulation frequency of the optical frequency modulators 15 and 16, an optical pulse controller (not shown) in the optical pulse generators 16 and 18, and a signal processor (PC) 28 described later. The analog / digital (A / D) converter 27 connected to is synchronized with the trigger signal of the pulse pattern generator (PPG) or the function generator (FG) 20, and is connected to the optical frequency modulators 15 and 17 and the optical signal. The temporal frequency arrangement of the pulsed test light that has passed through the pulsing devices 16 and 18 is in a state that the PC 28 knows.

  A probe light pulse train using probe lights having different X frequencies (wavelengths) and a pump light pulse train using pump lights having different X frequencies (wavelengths) are combined by a multiplexer 21 and pass through a circulator 22 to be measured light. The light is sequentially incident on the fiber 23. The optical fiber to be measured 23 is a test optical reflection filter installed at each of the trunk optical fiber 230, the N-branch optical splitter 231, the lower branch optical fiber 232i (i is a natural number of 1 to N), and the end of the lower branch optical fiber 232i. 233i. The probe light pulse train and the pump light pulse train that are N-branched by the optical splitter 231 cause Brillouin interaction only in the probe-pump pair in the lower branch optical fiber 232i, and the probe light is Brillouin only once during propagation through the optical fiber to be measured. Undergo amplification. Each probe light included in the probe light pulse train subjected to the Brillouin amplification reaches the optical circulator 22 and is guided to the optical filter 24 by the optical circulator 22. Similarly, the pump light pulse train is guided to the optical filter 24. The optical filter 24 transmits only the probe light pulse train, and the probe light pulse train transmitted here is an unmodulated local light branched by the branch element 15. The light is multiplexed by the multiplexing element 25, subjected to heterodyne detection, and received by the optical receiver 26.

The output current from the optical receiver 26 is converted into a digital signal by the A / D converter 27 and then input to the signal processing device 28. The signal processing device 28 performs arithmetic processing described later on the input current value.
Next, the operation of the above-described optical line characteristic analyzing apparatus of the present embodiment will be described. The optical frequency modulation devices 15 and 17, the optical pulse devices 16 and 18, the optical receiver 26, and the optical fiber 23 to be measured need to satisfy the following conditions.

(Condition 1) The frequency ν _rn of the n-th probe light and the frequency ν _pi of the n-th pump light included in the probe light pulse train and the pump light pulse train form a probe-pump pair having a frequency difference of Brillouin frequency shift ν _B. Thus, the optical frequency modulation devices 1 and 2 operate (ν _pn -ν _rn = ν _B ), and the paired probe light and pump light are made into an optical pulse so as to enter the measured optical fiber at an arbitrary time difference. Device 1 and 2 operate.

(Condition 2) When a test light pulse train composed of a probe light pulse train and a pump light pulse train is sequentially incident on the optical fiber to be measured, the second group of test lights is ejected from the optical fiber to be measured. The test light pulses incident on the optical fiber 232i to be measured and the test light pulses included in the test light pulse train have different frequencies from each other (the frequencies of the n-th and m-th probe light and the pump light are different). ν _rn ≠ ν _rm , ν _pn ≠ ν _pm ).

(Condition 3) The signal processing device (PC) 28 for the received signal includes the frequency of the probe light pulse train and the pump light pulse train that are incident on the measured optical fiber 23 at an arbitrary time (temporal arrangement of frequencies), and the pump light with respect to the probe light. You know the time delay.
Here, Conditions 1 to 3 have the following meanings.

  Condition 1 is a condition for the probe light to undergo Brillouin amplification by the pump light. Condition 2 is a condition for acquiring Brillouin gain information only at an arbitrary position z. Condition 3 is a condition necessary for separating and identifying signals collectively received by the optical receiver as signals having characteristic information of each branch lower optical fiber depending on the frequency.

An optical line characteristic analysis method using the present embodiment when these conditions are satisfied will be described.
FIG. 2 shows the relationship between the basic optical fiber (F ₀ ) and each lower branch optical fiber (F _{1 to} F _N ) of the measured optical fiber 23 and the time required for the probe light to reach the optical receiver 26. Here, a case is shown in which the branch optical fiber has three fibers (three systems, N = 3). Assume that the length of the three-branch optical fibers has the relationship of equation (2).

すると、プローブ光の光受信器に到達する時間は、式(3)の関係が成り立つ。

Then, the time of the probe light reaching the optical receiver satisfies the relationship of Expression (3).

ここで、変調周波数制御器１９の制御信号により光周波数変調装置１５，１７が同一周波数を継続出力する時間を周波数変調時間τとすると、

Here, when the time during which the optical frequency modulators 15 and 17 continuously output the same frequency by the control signal of the modulation frequency controller 19 is defined as the frequency modulation time τ,

のとき、各分岐下部光ファイバから戻ったプローブ光は光受信器２６への到達時間に差が生じるため、どの心線から戻ったプローブ光であるかを時間的に切り分けることができる。つまり、周波数の異なるＸ個のプローブパルス列を順次入射した場合にも、光受信器２６に同一周波数のプローブ光が到達することはない。

In this case, since the probe light returned from each branch lower optical fiber has a difference in arrival time to the optical receiver 26, it can be determined in time which probe line the probe light has returned from. That is, even when X probe pulse trains having different frequencies are sequentially incident, probe light having the same frequency does not reach the optical receiver 26.

[Signal processing method]
Next, a method for separating gain information when X probe light pulse trains having different frequencies are used as test light and sequentially incident on the optical fiber to be measured will be described first.
The time width of a probe light pulse train composed of X probe lights having different pulse time width (= frequency modulation time) τ is Xτ. In FIGS. 3B to 3E, as shown in FIG. 3A, the time width 4τ composed of four types of probe lights having a pulse time width τ and frequencies ν ₁ , ν ₂ , ν ₃ , and ν ₄ is shown. A state in which the probe pulse train reaches the optical receiver 26 at times t _d1 , t _d2 , and t _{d3 when} the probe light pulse train is incident on the measured optical fiber 23 having the three-branch optical splitter is shown.

Here, the optical receiver arrival times t _d1 , t _d2 , and t _d3 of the probe optical pulse train have been measured and are assumed to be known amounts. In addition, the probe lights included in the probe light pulse train from the core wire # 1 are named # 1-1, # 1-2,. Similarly, probe light from core wire # 2 is # 2-1, # 2-2, ... # 2-4, probe light from core wire # 3 is # 3-1, # 3-2, ... # 3 Set to -4.

Each probe light pulse train is Fourier-transformed in each grid (Fourier window) divided in time for each frequency modulation time τ according to the number of probe lights included with reference to arrival times t _d1 , t _d2 , t _d3 Then, the light receiving power of the probe light having the frequency to be extracted is detected.
In FIG. 3, t _d1 −t _d2 > 4τ, and the optical receiver 26 does not receive the probe light from the other cores simultaneously while receiving the probe pulse train from the core line # 1.
On the other hand, t _d2 −t _d3 <4τ, and the optical receiver 26 simultaneously receives a part of the probe pulse train from the core wire # 2 and a part of the probe pulse train from the core wire # 3. In this case, the reception method of # 2-3 is described. The frequency component ν ₁ of # 3-1 also exists in the grid to receive the frequency component ν ₃ of # 2-3. Since the arithmetic processing unit 28 receives a trigger signal that synchronizes the optical frequency modulators 15 and 17 and the pulse generators 16 and 18, the temporal arrangement (pulse width, modulation) of the frequencies included in the transmission probe optical pulse train Frequency).

Therefore, the arithmetic processing unit 228 determines the start time of each probe light grid based on the optical receiver arrival times t _d1 , t _d2 , and t _d3 of the probe light pulse train, and the frequency of the probe light to be analyzed included in the grid. It is known ((start time of n-th probe light grid) = (light receiver arrival time of probe light pulse train) + (n−1) × (frequency modulation time τ)). For example, for the probe light pulse train from the core line # 1, the start time of the nth grid is represented by t _d1 + (n−1) τ.

Using this grid as a Fourier window, Fourier transform is performed, and the probe light of the frequency component to be analyzed (here, ν ₃ ) from the power spectrum that overlaps the frequency component ν _{3 of} # 2-3 and the frequency component ν _{1 of} # 3-1 Get power. Thus, signal components can be separated even when probe light from other branch lower lines is received simultaneously.

  The above processing can be performed by the signal processor 28 grasping the frequency transmitted at an arbitrary time and its time arrangement, and measuring the arrival time of the probe light pulse train in advance. Paradoxically speaking, if the signal processing device 28 does not know the time sequence of the frequencies at the time of transmission, the frequency components to be extracted cannot be determined by setting the Fourier window and Fourier transform, so that an appropriate calculation is performed on the receiving side. Processing becomes impossible.

[Use of window function]
In addition, in order to reduce crosstalk of frequency components in signal processing, it is possible to improve frequency resolution by introducing various window functions such as Blackman-Harris window, Hanning window, Hamming window instead of rectangular grid in the Fourier window. it can.
When the above frequency signal separation procedure is followed and conditions 1 to 3 are satisfied, according to this embodiment, individual loss distribution measurements in N branched lower optical fibers can be performed in parallel using test optical pulse pairs with different frequencies. Compared with the prior art, it is possible to perform high-speed measurement that can shorten the measurement time without degrading the branching optical fiber identification resolution and the characteristic distribution resolution.

[Signal processing flow]
The signal processing device 28 performs signal processing according to the flowchart shown in FIG. 4 based on the above measurement.
The temporal frequency arrangement (frequency, frequency modulation time) of the probe light pulse train is recorded in advance in the signal processing device 28 (step S1). As a result, the signal processing device 28 is in a state where the probe optical frequency at an arbitrary time is known.

Only the probe light pulse train is incident, and the return time at the beginning of the probe light pulse train is recorded for each branch core (step S2).
From the information of the head return time and the modulation time, the Fourier window from which each probe light included in the probe light pulse train is to be extracted is determined. The frequency component to be extracted from the nth Fourier window can be determined from the temporal frequency arrangement of the probe light pulse train recorded in advance in the signal processing device 28 (step S3).

  Reference is made to the amplitude intensity of each frequency component under conditions where Brillouin gain is not received (pump light is not incident), and recorded as probe light intensity (step S4), where Brillouin gain is received (pump light is incident) Is recorded as the probe light intensity (step S5), and the Brillouin gain information is obtained by comparing the reference probe light intensity recorded in steps S4 and S5 with the signal probe light intensity (step S6). ).

All the above processes are controlled by a computer.
(Second Embodiment)
FIG. 5 shows an apparatus configuration for carrying out the second embodiment. Compared to the first embodiment, the second embodiment differs in the method of generating the test optical pulse train. In the second embodiment, pulse trains having different frequencies are generated by introducing pulses (time width τ) to the optical fiber rings 32 and 34 having frequency shifters by the multiplexing elements 31 and 33.

Here, an acousto-optic modulator, an LN phase modulator, or a single sideband modulator can be used as the frequency shifter.
When a pulse is introduced into the optical fiber rings 32 and 34 having the frequency shifter, the pulse is output from the optical fiber rings 32 and 34 because the frequency is shifted step by step by the frequency shift amount set in the frequency shifter every turn. light may form a pulse train having a frequency different stages, the pulse interval to be included in the pulse train corresponds to the circulation time of the light t _r of the optical fiber ring 32, 34.

The start time of each probe light grid during reception signal processing can be determined by the following equation.
(Start time of the grid of the nth probe light)
= (Optical receiver arrival time of probe optical pulse train) + (n−1) × (circulation time _tr )
The time width of each grid is equal to the time width τ of each pulse. Compared with the first embodiment, the number of devices that require synchronization is reduced, and the device configuration is simplified. In addition, since a broadband modulation frequency controller is not required, the apparatus becomes inexpensive.

  As described above, according to the first and second embodiments of the present invention, probe-pump light pulse pairs that are paired by a frequency difference that differs by the Brillouin frequency are arranged in terms of time while changing the frequency, respectively. The test light pulse train is made into a pulse train and a pump light pulse train. The test light pulse train is incident on the optical line to be measured with an arbitrary incident time difference, and the reflected light of the probe pulse train incident first and the pump light pulse train incident later are guided in opposite directions. Generates Brillouin scattered light. Then, the scattered light is received by the optical receiver, and the output current of the optical receiver is measured in advance for the return time of the test light so that the first to Nth branched optical fibers can be subjected to stimulated Brillouin scattering. The Brillouin gain analysis is performed by performing Fourier transform in the Fourier window provided with the head of the pulse train as a reference to extract the amplitude intensity of a predetermined frequency component, and the incident time difference between the probe light pulse train and the pump light pulse train is obtained. The characteristic distribution for every branch optical fiber is calculated | required by changing continuously.

Therefore, according to the first and second embodiments, the optical fiber identification resolution and the characteristic distribution measurement resolution are both deteriorated without newly changing the optical device or optical line configuration and compared with the existing technology. Therefore, it becomes possible to individually measure the optical line loss characteristics of the existing off-site equipment at high speed.
The stimulated Brillouin scattering can measure distortion due to light attenuation, temperature, bending, etc. in the optical medium. Here, the optical line characteristics include light attenuation with respect to distance, position of reflection peak, bending This is the temperature change with respect to the position of the obstacle, the degree of bending, the position of the disconnection obstacle, and the distance.

  The stimulated Brillouin scattering can measure distortion due to optical attenuation, temperature, bending, etc. in the optical medium. The optical line characteristics here are the optical attenuation with respect to the distance, the position of the reflection peak, and the bending obstacle. It is the amount of temperature change with respect to the position, the degree of bending, the position of the disconnection obstacle, and the distance.

  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 ... Laser light source, 12 ... Branch element, 13 ... Optical frequency shifter, 14 ... Branch element, 15, 17 ... Optical frequency converter, 16, 18 ... Optical pulse device, 19 ... Modulation frequency controller, 20 ... PPG or FG, 21 ... Multiplexing element, 22 ... Optical circulator, 23 ... Optical fiber to be measured, 230 ... Main optical fiber, 231 ... N-branch optical splitter, 2321 to 232N ... Lower branch fiber, 2331 to 233N ... Light reflection filter, 24 DESCRIPTION OF SYMBOLS ... Optical filter, 25 ... Multiplexing element, 26 ... Optical receiver, 27 ... A / D converter, 28 ... Signal processing apparatus.

Claims (7)

An optical branch line characteristic analysis apparatus for analyzing individual optical line characteristics of an optical branch line having N branch lower optical fibers from a branch point by an optical splitter to the far end of the optical branch line,
Test optical frequency modulation means for temporally controlling the first test light and the second test light having different wavelengths and modulating them to X (X is a natural number) frequencies;
Synchronizing with the test light modulation means, test light pulsing means for pulsing X probe lights and X pump lights having different frequencies with an arbitrary time difference;
Combining means for combining the pulsed probe light pulse train of X first test lights and the pump light pulse train of X second test lights;
A circulator for sequentially injecting the combined probe light pulse train and pump light pulse train into the fiber to be measured and extracting test light returned from the branch optical fiber under the optical splitter;
Filter means for extracting frequency components of X first test lights from the test light extracted by the circulator;
Photoelectric conversion means for receiving the first test light extracted by the filter means and converting it into current;
Analog-to-digital conversion means for converting the first test light converted into the current into a digital signal;
The digital signal is decoded, the control information at the time of generation and the light reception timing are used to identify which digital optical signal of the first test light is incident on the lower branch optical fiber, and X frequencies are identified from the identified digital signal. Frequency separation means for separating signals for each component;
The Brillouin gain of each of the X frequency components is analyzed, the above measurement is repeated while changing the incident time difference between the probe light pulse train and the pump light pulse train, and each of the lower optical fibers branched from the optical splitter is measured repeatedly. Signal processing means for obtaining a Brillouin gain characteristic distribution,
The test light frequency modulation means is a means for frequency-modulating the first test light and the second test light at predetermined time intervals, and the Brillouin interaction is performed only between the specific first test light and the second test light. Performing frequency modulation on the first test light and the second test light so as to generate
The characteristic analysis apparatus shares a synchronizing signal between the test light frequency modulation means and the test light pulse means, and grasps the frequency and frequency modulation time as the output states of the first test light and the second test light at an arbitrary time. An optical branch line characteristic analyzer. The signal processing means includes
Measure the return time at the beginning of the probe light pulse train from each of the N branched lower optical fibers in advance,
From the return time of the head of the received probe optical pulse train and the information of the frequency modulation time by the frequency modulation means, specify the return time of each of the X probe optical frequencies included in the probe optical pulse train,
Specify the modulation frequency to be received from the return time of each of the X probe optical frequencies included in the probe optical pulse train to the frequency modulation time;
The probe optical frequency component to be received at the time by performing Fourier transform on the time change of the received signal intensity within the frequency modulation time from the return time of each of the X probe optical frequencies included in the probe optical pulse train and performing frequency separation Extract the light amplitude intensity of
2. The optical branch line characteristic analyzer according to claim 1, wherein the Brillouin gains obtained from the X probe lights are independently analyzed.   2. The optical branch line characteristic analysis apparatus according to claim 1, wherein the test optical frequency modulation means is realized by a modulated electric signal of a single sideband modulator or an LN phase modulator and an arbitrary waveform signal generator.   The test optical frequency modulation means includes a single sideband modulator, an LN phase modulator, or an acoustooptic modulator adjusted to the specific modulation frequency in an optical fiber ring, and at a specific modulation frequency for each turn. 2. The optical branch line characteristic analyzing apparatus according to claim 1, wherein the optical modulation is performed step by step.   2. The optical branch line characteristic analysis apparatus according to claim 1, wherein the signal processing means extracts a specific signal by applying a window function to a time change of the current signal by the received probe optical pulse train.   2. The optical branch line according to claim 1, wherein the optical line characteristic is at least one of an optical attenuation with respect to a distance, a position of a bending obstacle, a degree of bending, a position of a disconnection obstacle, and a temperature change amount with respect to the distance. Characteristic analysis device. An optical branch line characteristic analysis method for analyzing individual optical line characteristics of an optical branch line having N branch lower optical fibers from a branch point to an optical branch line far end by an optical splitter,
The first test light and the second test light having different wavelengths are temporally controlled to be modulated to X frequencies (X is a natural number),
In synchronization with the test light modulation, X probe lights and X pump lights having different frequencies are pulsed with an arbitrary time difference from each other,
Combining the pulsed probe light pulse train by the X first test lights and the pump light pulse train by the X second test lights,
The combined probe light pulse train and pump light pulse train are sequentially incident on the measured fiber, and the test light returned from the branch optical fiber under the optical splitter is extracted,
Extracting frequency components of X first test lights from the extracted test light;
Receiving the extracted first test light and converting it into a current;
Converting the first test light converted into the current into a digital signal;
The digital signal is decoded, the control information at the time of generation and the light reception timing are used to identify which digital optical signal of the first test light is incident on the lower branch optical fiber, and X frequencies are identified from the identified digital signal. Separate the signal for each component,
Analyzing the Brillouin gain of each of the X frequency components, repeating the above measurement while changing the incident time difference between the probe light pulse train and the pump light pulse train, and repeatedly measuring the Brillouin gain characteristics of each of the optical branching lower fibers Suppose you want to get the distribution
The test light frequency modulation performs frequency modulation on the first test light and the second test light so as to cause a Brillouin interaction only between the specific first test light and the second test light at predetermined time intervals. ,
In the characteristic analysis, the synchronization signal of the test light frequency modulation and the test light pulse is shared, and the frequency and the frequency modulation time are grasped as the output states of the first test light and the second test light at an arbitrary time. Characteristic analysis method for optical branch line.

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