JP2011164075A - Optical pulse testing method and optical pulse testing device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reduce a measurement time required for improving a dynamic range. <P>SOLUTION: The light combining back-scattered light from an optical fiber 18 to be tested and testing light (light emitted from station) is received by a balanced optical receiver 20 to provide a current output. The current is separated for each frequency by frequency separation sections 21-23 to be numerically processed by a numerical processor 24, and the reflectance distribution of the back-scattered light from the optical fiber 18 to be tested by a plurality of frequency components of the testing light is determined by an arithmetic processing unit 25. For example, if Y pieces of frequencies of the testing light are varied for a given frequency interval over a given time interval, additive averaging processing per a single measurement can be performed Y times as many as the conventional OTDR measurement method. More specifically, the measurement time required for obtaining a dynamic range the same as the conventional method can be reduced to 1/Y. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an optical pulse test method (Optical Time Domain Reflectometry, hereinafter referred to as OTDR) for testing an optical loss distribution evaluation and a break point position of an optical fiber line which is a transmission medium of an optical communication signal in the field of an optical communication system and its Relates to the device.

  The OTDR sends a test light pulse to an optical fiber under test (hereinafter referred to as FUT), and receives and analyzes the reflected light from the FUT and Rayleigh backscattered light (hereinafter simply referred to as backscattered light). This is a method and apparatus for measuring the light reflectance distribution (hereinafter referred to as OTDR waveform) at each point of the FUT. Since this technique can test the loss distribution evaluation from one end of the optical fiber, it is an important technique from the viewpoint of maintenance and operation of the installed optical fiber.

  The maximum measurement distance of OTDR is determined by the optical loss value of FUT and the dynamic range of OTDR. Here, the dynamic range is the maximum loss value allowed in the measurement. As a method of expanding the dynamic range, mainly a method of increasing the peak power of the test light pulse incident on the FUT, a method of improving the minimum reception sensitivity of the receiving system, a test light pulse repeatedly incident, and averaging the measurement results There is a method for improving the signal to noise ratio (hereinafter referred to as SNR) of the measurement signal. As a method for improving the minimum reception sensitivity, there is a coherent detection technique, and OTDR (Coherent OTDR, hereinafter referred to as C-OTDR) using this coherent detection technique has been put into practical use as a long-distance measurement method (Non-patent Document 1). reference).

  In this conventional C-OTDR, output light from a light source that emits coherent light is split into test light and local light by an optical directional coupler. The branched test light is amplified by an optical amplifier and then incident on an acousto-optic device, which is pulsed with a frequency shift. The pulsed test light passes through the optical circulator and enters the FUT. Backscattered light generated in the FUT is directed only to the optical receiver side by the optical circulator, and is combined with the local light by the optical directional coupler and then received by the balanced optical receiver. Thereby, an interference beat signal generated by interference between local light and backscattered light is detected as a signal current. The signal current is mixed with a sine wave current having the same frequency as that of the acoustooptic device in a subsequent mixer, and then low-pass filtered by a low-pass filter (hereinafter referred to as LPF). The current that has become a baseband signal by mixing is digitized by an A / D (Analog / Digital) converter, and then subjected to a square addition averaging process by a numerical arithmetic processor. An OTDR waveform can be obtained by logarithmically displaying the processed numerical sequence.

  A one-way dynamic range (hereinafter referred to as SWDR) in C-OTDR is formulated as follows.

ここで、P₀ は試験光パルスのピークパワー、R_rは後方散乱光反射係数、P_min は最小受信感度、ＳＮＩＲは加算平均処理による被測定信号のＳＮＲ改善量である。この内、ｘ回の加算平均処理を行った際のＳＮＩＲは次式で表される。

Here, P ₀ is the peak power of the test light pulse, R _r is the backscattered light reflection coefficient, P _min is the minimum reception sensitivity, and SNIR is the SNR improvement amount of the signal under measurement by the averaging process. Among these, SNIR when x-time averaging processing is performed is expressed by the following equation.

式(1),(2)より加算平均処理数をＸ倍にした際にはＳＷＤＲは5log√ｘ(dB)改善される。
ここで、加算平均処理数を増やすためには、必然的に測定を繰り返し行う必要がある。一般に、測定を繰り返し行う際には、試験光パルスの繰り返し周期をFUTの長さに応じて設定しなければならない。これは一つの試験光パルスによるＦＵＴ遠端からの後方散乱光が測定器に戻る前のタイミングで二つ目の試験光パルスがＦＵＴに入射された際には、一つ目の試験光パルスと二つ目の試験光パルスによる後方散乱光が重なり合ってしまい、正しい測定結果が得られなくなるからである。このように加算平均処理数を増やしてダイナミックレンジを向上させようとすると、ある所定の周期で繰り返し測定する必要があるため、測定時間が長延化するという課題がある。

From the equations (1) and (2), when the number of addition average processing is multiplied by X, SWDR is improved by 5 log√x (dB).
Here, in order to increase the number of addition average processing, it is inevitably necessary to repeat the measurement. In general, when the measurement is repeated, the repetition period of the test light pulse must be set according to the length of the FUT. This is because when the second test light pulse is incident on the FUT at the timing before the backscattered light from the far end of the FUT due to one test light pulse returns to the measuring instrument, This is because the backscattered light from the second test light pulse overlaps and a correct measurement result cannot be obtained. Thus, if it is going to increase the number of addition average processes and improve a dynamic range, since it is necessary to repeatedly measure with a certain predetermined period, there exists a subject that measurement time prolongs.

H. Izumita et al, “The Performance Limit of Coherent OTDR Enhanced with Optical Fiber Amplifiers due to Optical Nonlinear Phenomena” JLT., Vol. 12, no. 7, pp. 1230-1238 (1994)

As described above, a technique for measuring the reflectance distribution of light at each point by sending a test light pulse to the conventional optical line under test (FUT) by OTDR and receiving and analyzing the backscattered light from the FUT. For the purpose of expanding the dynamic range of OTDR, OTDR (C-OTDR) using coherent detection has been put into practical use as a long-distance measurement method. C-OTDR is based on the interference beat signal generated by the interference between the backscattered light from the FUT and the test light (local light), the signal current is digitized by the A / D converter, and then the numerical arithmetic processor. A square addition averaging process is performed to obtain a waveform.
However, in order to increase the number of addition average processing, it is necessary to repeatedly perform the measurement. However, when the number of addition average processing is increased to improve the dynamic range, there is a problem that the time required for measurement increases accordingly.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical pulse test method and apparatus capable of reducing the measurement time required for improving the dynamic range.

The optical pulse test method according to the present invention has the following configuration.
(1) In the optical pulse test method for measuring the reflectance distribution of the reflected light and backscattered light from the optical fiber under test, the output light from the light source is split into two to be local light and test light, and the frequency of the test light Are changed at predetermined intervals for each predetermined time interval, optically amplified, converted into an optical pulse, passed through the optical circulator, repeatedly incident on the optical fiber under test, and output from the optical circulator at each point of the optical fiber under test The backscattered light generated by reflection or scattering is combined with the local light and received to obtain a current signal, the current signal is separated into a plurality of frequency components, and the test signal is subjected to a plurality of frequency components. The reflectance distribution of each of the reflected light from the test optical fiber and the backscattered light is obtained.

Moreover, the optical pulse test apparatus according to the present invention has the following configuration.
(2) A light source that emits coherent light, a branching unit that splits output light from the light source into two to generate local light and test light, and a frequency of the test light that changes by a predetermined interval every predetermined time interval An optical frequency control means for optically amplifying and optically pulsing the output of the optical frequency control means to generate an optical pulse signal, the optical pulse signal being incident on the optical fiber under test, An optical circulator that captures and outputs backscattered light generated by reflection or scattering at each point of the optical fiber, optical coupling means for optically coupling the backscattered light and the local light, and optical reception of the optically coupled optical signal Light receiving means for acquiring current signals, frequency separating means for separating the current signals into a plurality of frequency components, and reflected light from the optical fiber under test by the plurality of frequency components of the test light A manner that and a processing means for obtaining the fine backscattered light each reflectance distribution.

  (3) In the configuration of (2), the frequency separation unit includes a local signal source that generates a signal current, a mixer that mixes an output current from the optical reception unit with a current from the local signal source, A filter for low-pass filtering the output current of the mixer; and a digitizing means for digitizing the output current from the filter, wherein the calculation processing means calculates the current value output from the digitizing means. Processing is performed so that frequency separation is performed.

(4) In the configuration of (3), when the current value digitized by the digitizing means is i (n), the arithmetic processing means I (k, m) obtained by Fourier transforming i (n) By calculating the current value, the current value is frequency-separated.
(5) In the configuration of (2), the frequency separation means separates the output current from the optical receiving means into a plurality of systems, and separates the distributed currents for each frequency component. And an analog circuit.

(6) In the configuration of (2), the optical frequency control means is a phase modulator.
(7) In the configuration of (2), the optical frequency control means is a carrier suppressed optical single sideband modulator.
(8) In the configuration of (2), the optical frequency control means is an amplitude modulator.

(9) In the configuration of (2), the optical pulse unit is an acousto-optic element.
(10) In the configuration of (2), an aspect is provided that includes optical frequency shift means that is installed on the optical path of the local light and shifts the optical frequency of the local light by a certain width.

  According to the present invention, the output light from the light source is branched into two, one of the branched lights is the local light, the other is the test light, and the frequency of the test light is changed by a predetermined interval every predetermined time interval. The test light is converted into an optical pulse, repeatedly incident on the optical fiber to be tested, and the backscattered light generated by reflection or scattering at each point of the optical fiber to be tested is combined with the local light, and the combined light is received. Then, the current is separated for each frequency, and the reflectance distribution of the reflected light and the backscattered light from the optical fiber under test due to a plurality of frequency components of the test light is obtained. Therefore, when the frequency of the test light is changed by Y at a predetermined frequency interval every predetermined time interval, by measuring with frequency separation, the conventional method can acquire only one OTDR waveform per measurement, In this method, Y OTDR waveforms can be acquired, so that the number of addition average processing per measurement can be increased by Y times. When measurement is performed in the same time as the conventional optical pulse test method, the equation (2 ), The SWDR can be improved by 5 log√Y (dB). This also means that the measurement time can be shortened by 1 / Y in order to obtain the same dynamic range as the conventional method.

  Therefore, according to the present invention, it is possible to provide an optical pulse test method and apparatus capable of shortening the measurement time required for improving the dynamic range.

The block block diagram which shows 1st Embodiment of the measuring apparatus which employ | adopted the optical pulse measuring method which concerns on this invention. The wave form diagram which shows the time change of the frequency and voltage of the electric current which are output from the sine wave generator shown in FIG. The wave form diagram which shows the power spectrum of the signal of the center frequencies 100 kHz and 200 kHz and 300 kHz whose frequency band of the signal output from the sine wave generator shown in FIG. 1 is bkHz. In 1st Embodiment, (a) OTDR waveform by multiple frequency component, (b) The schematic diagram which shows the OTDR waveform which performed the final process, respectively. The block block diagram which shows 2nd Embodiment of the measuring apparatus which employ | adopted the optical pulse measuring method which concerns on this invention. The block block diagram which shows 3rd Embodiment of the measuring apparatus which employ | adopted the optical pulse measuring method which concerns on this invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIG. 1 is a block diagram showing an optical pulse test apparatus according to a first embodiment that employs the optical pulse measurement method of the present invention. The optical pulse test apparatus shown in the figure can determine the reflectance distribution of reflected light and backscattered light from the optical fiber under test for each frequency component of the test light.

  The output light from the light source 11 that emits coherent light is branched into two systems by the branch element 12. One of the branched lights is used as local light, and the other is incident on the optical frequency controller 13 as test light. The optical frequency controller 13 shifts the frequency of the test light by a predetermined frequency width at predetermined time intervals.

Here, the branch element 12 is specifically composed of an optical coupler or a half mirror. The optical frequency controller 13 may be an external modulator having a function of changing the frequency of the modulation sideband in accordance with the signal frequency from the first sine wave generator 14 serving as a drive source. It is well known that phase modulators, amplitude modulators, and SSB-SC (carrier wave suppressed optical single sideband) modulators using LiNbO ₃ have their functions.

The test light subjected to frequency control by the optical frequency controller 13 is amplified by the optical amplifier 15 and then optically pulsed by the optical pulse processor 16. Specifically, the optical pulse processor 16 is composed of an acousto-optic switch in which an acousto-optic element is pulse-driven or a waveguide switch in which an electro-optic element using LiNbO ₃ is pulse-driven.

The optical frequency controller 13 and the optical pulsing processor 16 are synchronized with each other so that the time width for optical pulsing is equal to the time width obtained by shifting all the frequencies by the optical frequency controller 13.
The test light that has been optically pulsed by the optical pulse processor 16 passes through the optical circulator 17 and enters the FUT 18. In the FUT 18, backscattered light is generated by the test light pulse. This backscattered light is incident on the optical circulator 17 and does not return to the direction of the optical pulse processor 16 but goes only to the optical receiver side. The backscattered light output from the optical circulator 17 is combined with the local light and the coupling element 19. The output light from the coupling element 19 is received by the balanced optical receiver 20 and becomes a current signal. The current signal output from the balanced optical receiver 20 is mixed with the current signal from the second sine wave generator 22 by the mixer 21, and then the cut-off frequency is about the sampling rate of the digitizer 24 at the subsequent stage. A high frequency component is blocked by a set LPF (low-pass filter) 23. The current signal output from the LPF 23 is digitized by the numerical processing unit 24 and then input to the arithmetic processing unit 25.

The arithmetic processing unit 25 performs arithmetic processing as described below on the input current value, and obtains the reflectance distribution of the backscattered light from the FUT 18 by a plurality of frequency components of the test light.
Next, the operation of the optical pulse test apparatus according to this embodiment configured as described above will be described.

The method of separating the frequency of the output light from the light source 11 by the optical frequency controller 13 and the arithmetic processing unit 25 needs to satisfy the following conditions.
(Condition 1) The line width of the output light from the light source 11 is smaller than the reciprocal of the time during which the optical frequency controller 13 maintains the predetermined frequency.
(Condition 2) The frequency shift interval by the optical frequency controller 13 is a natural number multiple of the reciprocal of the predetermined time interval for sustaining the frequency.
(Condition 3) The total predetermined frequency shift amount by the optical frequency controller 13 is ½ or less of the sampling rate of the numerical processor 24.
(Condition 4) The natural number times the frequency resolution of the Fourier transform by the arithmetic processing is the frequency shift interval by the optical frequency controller 13.

  The above condition 1 requires that a light source having a line width smaller than the reciprocal of the sustained time width of the light amplitude and frequency corresponding to the distance resolution required for the optical pulse test apparatus is prepared as the capability of the light source 11. It means that there is. This is because the signal-to-noise ratio of the signal decreases when the line width of the light source is larger than the modulation bandwidth determined from the reciprocal of the duration of the frequency.

  For example, when the required distance resolution is 1 [km], it is necessary to maintain the frequency of the test light for a time of 10 μs by the optical frequency changing means, which becomes a signal band of 100 [kHz]. Therefore, in order to reduce the influence of the phase noise, the line width of the light source needs to be set to be as small as 1/10 or less of the signal band and to 10 [kHz] or less.

  Condition 2 is necessary for separating and measuring a backscattered light signal of a plurality of frequency components for each frequency component. This is because the power of backscattered light with one frequency cannot be measured unless the power spectrum components of the shifted frequency components are set to be orthogonal to each other.

  In general, the power spectrum of the backscattered light signal to be measured takes the form of the square of the SINC function, and the zero point interval of the SINC function is the reciprocal of a predetermined time interval in which the frequency is maintained by the optical frequency controller 13. By setting the frequency shifted by the optical frequency controller 13 at this zero point position, the power spectrum components by each frequency become orthogonal to each other.

  This means that it is necessary to prepare the first sine wave generator 14 having the function described below. FIG. 2 is a waveform diagram showing temporal changes in the frequency and voltage of the current signal output from the first sine wave generator 14. The period for outputting the voltage is for repeated measurement, and is set according to the length of the FUT 18 as in the conventional optical pulse test apparatus. The time width W for outputting the voltage is set by the product of the frequency duration and the frequency shift number within a range shorter than the period.

さらに、出力する正弦波の周波数は、以下の式に表すように、所定の時間間隔毎に所定周波数だけステップ状にシフトするように設定する。

Further, the frequency of the sine wave to be output is set so as to shift in a stepped manner by a predetermined frequency at predetermined time intervals, as represented by the following expression.

ここで、iはi=1,2,…,lを満たす自然数で、lは周波数のシフト数、ν₀は初期周波数、Δνは所定時間間隔Δt毎に変わる周波数シフト量である。これより、所定周波数シフト総量はlΔνとなる。
周波数間隔Δνは周波数持続時間Δtに対して、以下の関係式を満足させる必要がある。

Here, i is a natural number satisfying i = 1, 2,..., L is a frequency shift number, ν ₀ is an initial frequency, and Δν is a frequency shift amount that changes at every predetermined time interval Δt. Thus, the total predetermined frequency shift amount is lΔν.
The frequency interval Δν needs to satisfy the following relational expression with respect to the frequency duration Δt.

ここで、aは自然数であり、後述する条件３より、以下の範囲を満たす必要がある。

Here, a is a natural number, and it is necessary to satisfy the following range from Condition 3 described later.

ここで、f_s は数値化処理器２４のサンプリングレートである。
図３に帯域100[kHz] 、中心周波数がそれぞれ100[kHz]と200[kHz]、300[kHz]の信号のパワースペクトルを示す。式を満たしている場合には、光周波数変更手段でシフトさせた周波数は、SINC関数の2乗の形をとるパワースペクトルのゼロ点位置にそれぞれ配置されるため、それぞれのパワースペクトル成分が直交する。

Here, f _s is the sampling rate of the digitizer 24.
FIG. 3 shows a power spectrum of a signal having a bandwidth of 100 [kHz] and center frequencies of 100 [kHz], 200 [kHz], and 300 [kHz], respectively. If the equation is satisfied, the frequency shifted by the optical frequency changing means is placed at the zero point position of the power spectrum taking the square shape of the SINC function, so that the power spectrum components are orthogonal to each other. .

The frequency duration Δt is determined from the distance resolution ΔZ _min required for measurement.

例えば、1[km]の測定分解能を設定する際には10[μs]で設定すればよい。
条件３は演算処理装置２５で光周波数の分離を実行するためには、ミキサー２１でミキシングする第２の正弦波発生器２２の出力信号の周波数を調整した後、数値化処理器２４に入力される複数の周波数成分を持った電流信号の最小周波数と最大周波数が以下の関係式を満たすようにする必要がある。

For example, when setting the measurement resolution of 1 [km], it may be set at 10 [μs].
Condition 3 is that the arithmetic processing unit 25 performs optical frequency separation by adjusting the frequency of the output signal of the second sine wave generator 22 to be mixed by the mixer 21 and then input to the numerical processing unit 24. It is necessary that the minimum frequency and the maximum frequency of a current signal having a plurality of frequency components satisfy the following relational expression.

式(6)を満たした場合、光周波数制御器１３で所定の時間間隔毎に所定周波数だけステップ上にシフトさせた分の周波数範囲は全て、数値化処理器２４のサンプリングレートの1/2以内になる。このため、エイリアシングを発生させることなく全ての電流信号を数値化することができ、演算処理装置２５で個々の周波数成分を分解して処理することができる。

When the expression (6) is satisfied, the frequency range for which the optical frequency controller 13 is shifted up the predetermined frequency every predetermined time interval is all within 1/2 of the sampling rate of the numerical processor 24. become. Therefore, all current signals can be digitized without causing aliasing, and the individual frequency components can be decomposed and processed by the arithmetic processing unit 25.

The condition that the maximum current frequency is within half the sampling rate of the digitizer 24 is a request from the well-known Nyquist sampling theorem.
Condition 4 means that in order to separate the signal current having a plurality of frequency components for each frequency component by the optical frequency controller 13, it is necessary to perform the processing described below in the arithmetic processing unit 25. .

This is because the signal power of each frequency component obtained by the measurement is not equal to each other unless the frequency shift interval by the frequency controller 13 and the natural number multiple of the frequency resolution by the arithmetic processing match.
An amplitude value for each frequency can be calculated from the signal which has become a discrete value by the digitizing processor 24 by a discrete Fourier transform described later. When the window length of the window function when executing the discrete Fourier transform is L, the frequency resolution of the obtained frequency spectrum is f _s / L. In order to make the frequency shift interval equal to the frequency resolution, the following conditional expression is required.

この関係を満足するようにＬは設定する必要がある。ここで、ｂは自然数であり、前述した条件３より、以下の範囲を満たす必要がある。

L needs to be set so as to satisfy this relationship. Here, b is a natural number, and it is necessary to satisfy the following range from Condition 3 described above.

上述した４つの条件が満足される場合には、本発明の光パルス試験装置によって、一回の測定で試験光の複数の周波数成分によるＦＵＴ１８からの後方散乱光の反射率分布が正しく検出されることを以下に説明する。
まず、局発光の電界分布を次式で表す。

When the above four conditions are satisfied, the optical pulse test device of the present invention correctly detects the reflectance distribution of the backscattered light from the FUT 18 due to a plurality of frequency components of the test light in one measurement. This will be described below.
First, the local electric field distribution is expressed by the following equation.

ここで、E_L0は局発光の電界分布、νは光周波数、ｊは虚数単位である。周波数制御器１３を用いず、音響光学素子によりパルス化した試験光パルスをＦＵＴ１８に入射した場合、その時刻を起点（t=0）として、ｔ秒後に入射端に戻ってくる後方散乱光の電界分布は、次式で表される。

Here, E _L0 is the electric field distribution of local light, ν is the optical frequency, and j is an imaginary unit. When the test light pulse pulsed by the acousto-optic device is incident on the FUT 18 without using the frequency controller 13, the electric field of the backscattered light returning to the incident end after t seconds from the time (t = 0). The distribution is expressed by the following equation.

ここで、E_B(t)は後方散乱光の電界振幅、ν_A は音響光学素子による周波数シフト量である。後方散乱光と局発光が結合素子により合波され、合波した光を受信したバランス型光受信器２０から出力される電流信号は次式で表される。

Here, E _B (t) is the electric field amplitude of the backscattered light, and ν _A is the frequency shift amount by the acoustooptic device. The backscattered light and the local light are combined by the coupling element, and the current signal output from the balanced optical receiver 20 that receives the combined light is expressed by the following equation.

ここで、ηはバランス型光受信器の量子効率、eは電子の電荷、hはプランク定数を表す。この電流信号を周波数ν_Aの正弦波信号とミキシングし、ＬＰＦ２３で高周波成分を除去することで、ベースバンドの電流信号とする。
この時の電流信号は、次式で表される。

Here, η is the quantum efficiency of the balanced optical receiver, e is the charge of the electrons, and h is the Planck constant. This current signal is mixed with a sine wave signal having a frequency ν _A and a high-frequency component is removed by the LPF 23 to obtain a baseband current signal.
The current signal at this time is expressed by the following equation.

ベースバンド信号となった電流を、数値化処理器２４に入力し、得られた電流値を演算処理装置で２乗する。この結果、次式で表されるパワーが得られる。

The current that has become the baseband signal is input to the digitizing processor 24, and the obtained current value is squared by the arithmetic processing unit. As a result, the power represented by the following equation is obtained.

ここで、E_B(t)² は後方散乱光のパワーであるので、式（15）より、ＦＵＴ１８中の後方散乱光の反射率分布、即ち、ＯＴＤＲ波形が得られることになる。
これに対し、条件２を満たした上で、周波数制御器１３で周波数を所定の時間間隔毎に所定の周波数幅だけシフトさせた試験光を音響光学素子でパルス化し、ＦＵＴ１８に入射した場合には、その時刻を起点（t=0）として、ｔ秒後に入射端に戻ってくる後方散乱光の電界分布は、次式で表される。

Here, since E _B (t) ² is the power of the backscattered light, the reflectance distribution of the backscattered light in the FUT 18, that is, the OTDR waveform is obtained from the equation (15).
In contrast, when the condition light 2 is satisfied, the test light whose frequency is shifted by a predetermined frequency width at a predetermined time interval by the frequency controller 13 is pulsed by the acoustooptic device and incident on the FUT 18. Starting from that time (t = 0), the electric field distribution of backscattered light returning to the incident end after t seconds is expressed by the following equation.

ここで、E_Bi ^m(t)は周波数ν＋ν_A ±ｍ(ν₀ ＋Δν)の試験光パルスによる後方散乱光の電界振幅である。mはm=0,1,2,….の値をとる整数である。一般に、本実施形態で挙げた外部変調器は±m次の変調側波帯を生じさせる。

Here, E _Bi ^m (t) is the electric field amplitude of the backscattered light by the test light pulses at a frequency _{ν + ν A ± m (ν} 0 + Δν). m is an integer having values m = 0, 1, 2,. In general, the external modulator described in this embodiment generates ± m-order modulation sidebands.

  In this case, after the backscattered light and the local light are combined by the coupling element, the current output from the received balanced optical receiver is expressed by the following equation.

この電流を周波数|ν_A −ν₀ |の正弦波信号とミキシングし、遮断周波数が(l+1)Δνに設定されているＬＰＦ２３によって高周波成分を遮断する。ＬＰＦ２３から出力される電流は、次式で表される。

This current is mixed with a sine wave signal having a frequency | ν _A −ν ₀ |, and the high frequency component is cut off by the LPF 23 whose cut-off frequency is set to (l + 1) Δν. The current output from the LPF 23 is expressed by the following equation.

この電流を、前述した条件３を満たした上で数値化処理器２４に入力し、得られた電流値i(n)を演算処理装置２５にて以下に説明する演算処理を施す。
ここで、n=f_stはサンプリング指標であり、n=0,1,2,...,N-1の範囲の整数値をとる。また、Nはサンプリング総数であり、N=f_sTとなる。ここで、Tは測定の繰り返し周期である。

This current is input to the numerical processor 24 after satisfying the above-described condition 3, and the obtained current value i (n) is subjected to arithmetic processing described below by the arithmetic processing unit 25.
Here, n = f _s t is the sampling index, n = 0,1,2, ..., takes an integer value in the range of N-1. N is the total number of samplings, and N = f _s T. Here, T is the measurement repetition period.

  For the current value i (n) obtained by the digitization processor, the arithmetic processing unit 25 first performs a discrete Fourier transform process represented by the following equation.

ここで、k=0,1,2,…,L-1は周波数スペクトル指標であり、mはm=0,1,2..,n/Lの範囲の整数である。また、Lは方形波の窓関数w[n]の窓長であり、次式で表される。

Here, k = 0, 1, 2,..., L-1 is a frequency spectrum index, and m is an integer in the range of m = 0, 1, 2., n / L. L is the window length of the square wave window function w [n] and is expressed by the following equation.

条件4を満足した際には、次式が成立する。

When the condition 4 is satisfied, the following equation is established.

このI[k,m]に対して、絶対値をとった後、２乗する演算処理を施すと、次式が得られる。

If an arithmetic process of taking a square after applying an absolute value to this I [k, m] is performed, the following equation is obtained.

このパワースペクトルの時間変化|I(k,m)|² は(i-1)Δtの遅延時間があるL個のＯＴＤＲ波形を意味している。ただし、L個の内、独立な波形はフーリエ変換の以下に表す振幅スペクトルの対称性のため、半分のL/2になる。

This time change | I (k, m) | ² of the power spectrum means L OTDR waveforms having a delay time of (i−1) Δt. However, the L independent waveforms are half L / 2 due to the symmetry of the amplitude spectrum shown below in the Fourier transform.

図４（ａ）に式（22）より得られる複数周波数成分によるＯＴＤＲ波形の模式図を示す。
遅延時間については数値演算処理を用いて、(i-1)Δtだけそれぞれ時間をずらす操作を行うことで補正することができる。これよりL/2個の反射率および時間について等価なＯＴＤＲ波形を一回の測定で得ることが出来る。
以上の演算処理で得られたL/2個のＯＴＤＲ波形をすべて足し合わせ、L/2で割ることにより加算平均処理を行う。
図４（ｂ）に以上の処理を施したＯＴＤＲ波形の模式図を示す。

FIG. 4A shows a schematic diagram of an OTDR waveform based on a plurality of frequency components obtained from the equation (22).
The delay time can be corrected by performing an operation of shifting the time by (i−1) Δt using numerical calculation processing. Accordingly, an OTDR waveform equivalent to L / 2 reflectances and time can be obtained by one measurement.
The L / 2 OTDR waveforms obtained by the above arithmetic processing are added together and divided by L / 2 to perform the averaging process.
FIG. 4B shows a schematic diagram of an OTDR waveform subjected to the above processing.

Finally, a series of measurement and calculation processes are repeatedly performed, the obtained results are subjected to an averaging process, and the processed numerical sequence is logarithmically displayed to obtain an OTDR waveform having a final high dynamic range.
(Second Embodiment)
FIG. 5 is a block configuration diagram showing the configuration of the optical pulse measurement device according to the second embodiment of the present invention. 5, the same parts as those in FIG. 1 are denoted by the same reference numerals, and different parts will be described here. In FIG. 5, the current signal obtained by the balanced optical receiver 20 is distributed to the c-system mixers 211 to 21c by the signal distributor 26, and the sine wave current signals from the second sine wave generators 221 to 22c, respectively. Then, the high-frequency components are removed by the LPFs 231 to 23c and sent to the numerical processing unit 24. The digitizing processor 24 digitizes the current signals of all systems and sends them to the arithmetic processing unit 25.

  In this embodiment, instead of the frequency separation processing by the arithmetic processing executed in the first embodiment, the frequency separation is performed using an analog circuit described below, so that the condition 3 and the condition required in the first embodiment are obtained. 4 is no longer necessary. However, compared with the first embodiment, the cost and size of the apparatus are increased.

The operation of the second embodiment will be described below.
The configuration until the light is received by the balanced optical receiver 20 is processed in the same manner as in the first embodiment, and the signal output from the balanced optical receiver 20 is distributed to the c system by the signal distributor 26. Is done. Here, c is a natural number having values of c = 1, 2, 3,. After distribution, each line of the signal currents in the respective mixer _{211~21c ν A + ν 0 + Δν} , ν A + ν 0 + 2Δν, ..., a second sine which is set to a frequency of ν _A + ν ₀ + cΔν It is mixed with the current from the wave generators 221 to 22c. The high frequency components of the output currents from the mixers 211 to 21c are cut off by the LPFs 221 to 22c set to cut off frequencies that are approximately the reciprocal of the pulse width. Here, each signal current from which the high frequency component has been removed is input to the digitizing processor 24 and digitized. The c current values that have been digitized are squared by the arithmetic processing unit 25, repeatedly measured, and added and averaged. As a result, c OTDR waveforms can be obtained. Finally, the obtained c number of OTDR waveforms are corrected for delay time, subjected to addition averaging processing, and the obtained numerical sequence is displayed logarithmically, and finally an OTDR waveform having a high dynamic range can be obtained.

(Third embodiment)
FIG. 6 is a block diagram showing the configuration of the third embodiment of the optical pulse testing apparatus according to the present invention. This embodiment replaces the process of mixing the current signal obtained by the balanced optical receiver 20 with the current from the second sine wave generator 22 and the mixer 21, which has been executed in the first embodiment. The gist is to use an optical frequency shifter 27 having the same frequency shift amount as the sine wave generator 22 on the local light emission side. The advantage of this embodiment is that the processing described below is performed at the optical signal stage, so that the electric circuit configuration of the first embodiment can be reduced. The disadvantage is that there are many device components for optical signals.

The operation of the third embodiment will be described below.
The processing for the test light is performed in the same manner as in the first embodiment, and the frequency shift amount of | ν _A −ν ₀ | is the same as that of the second sine wave generator 22 of the first embodiment on the local light emission line. An optical frequency shifter 27 provided with Specifically, the optical frequency shifter 27 is an acoustooptic shifter in which the acoustooptic element is DC driven. Backscattered light from the FUT 18 due to the test light pulse is input to the optical circulator 17 and does not return to the direction of the optical pulse processor 16 but only toward the optical receiver. The backscattered light output from the optical circulator 17 is combined with local light and a coupling element. Output light from the coupling element is received by the balanced optical receiver 20. The output current from the balanced optical receiver 20 is cut off from high frequency components by the LPF 22 whose cut-off frequency is set to be about the sampling rate of the digitizing processor 24 at the subsequent stage. The output current from the LPF 22 is digitized by the digitizing processor 24 and then input to the arithmetic processing unit 25.

The arithmetic processing unit 25 performs the same processing as in the first embodiment. Therefore, according to the configuration of the present embodiment, an OTDR waveform having a high dynamic range can be finally obtained.
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 constituent elements disclosed in the above embodiments. For example, some components 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 ... 1st sine wave generator, 15 ... Optical amplifier, 16 ... Optical pulse processor, 17 ... Optical circulator, 18 ... FUT, 19 ... Coupling Elements: 20 ... balanced optical receiver, 21 ... mixer, 22 ... second sine wave generator, 23 ... LPF (low-pass filter), 24 ... digitization processor, 25 ... arithmetic processing unit, signal distributor 27: Optical frequency shifter.

Claims (10)

In the optical pulse test method for measuring the reflectance distribution of reflected light and backscattered light from the optical fiber under test,
The output light from the light source is split into two to produce local light and test light.
The frequency of the test light is changed by a predetermined interval every predetermined time interval, optically amplified, converted into an optical pulse, passed through an optical circulator, and repeatedly incident on the optical fiber under test,
The backscattered light generated by reflection or scattering at each point of the optical fiber under test output from the optical circulator and the local light are combined and received to obtain a current signal,
Separating the current signal into a plurality of frequency components;
An optical pulse test method characterized in that a reflectance distribution of each of reflected light and backscattered light from an optical fiber under test by a plurality of frequency components of the test light is obtained. A light source that emits coherent light;
Branching means for branching the output light from the light source to generate local light and test light;
An optical frequency control means for changing the frequency of the test light by a predetermined interval every predetermined time interval;
An optical pulse generating means for optically amplifying an output of the optical frequency control means to generate an optical pulse signal; and
An optical circulator that enters the optical pulse signal into the optical fiber under test, and captures and outputs backscattered light generated by reflection or scattering at each point of the optical fiber under test;
An optical coupling means for optically coupling the backscattered light and the local light;
Optical receiving means for optically receiving the optically coupled optical signal to obtain a current signal;
Frequency separation means for separating the current signal into a plurality of frequency components;
An optical pulse testing apparatus comprising: arithmetic processing means for obtaining reflectance distributions of reflected light and backscattered light from the optical fiber under test by a plurality of frequency components of the test light. The frequency separation means includes
A local signal source for generating a signal current;
A mixer for mixing an output current from the optical receiving means with a current from the local signal source;
A filter for low-pass filtering the output current of the mixer;
Quantification means for quantifying the output current from the filter,
3. The optical pulse test apparatus according to claim 2, wherein the arithmetic processing unit performs arithmetic processing on the current value output from the digitizing unit to perform frequency separation.   When the current value digitized by the digitizing means is i (n), the arithmetic processing means calculates the frequency of the current value by calculating I (k, m) obtained by Fourier transform of i (n). The optical pulse test apparatus according to claim 3, wherein separation is performed. The frequency separation means includes
Signal distribution means for distributing the output current from the optical receiving means to a plurality of systems;
3. An optical pulse test apparatus according to claim 2, further comprising an analog circuit that separates the distributed currents of each system for each frequency component.   3. The optical pulse test apparatus according to claim 2, wherein the optical frequency control means is a phase modulator.   The optical pulse test apparatus according to claim 2, wherein the optical frequency control means is a carrier-suppressed optical single sideband modulator.   3. The optical pulse test apparatus according to claim 2, wherein the optical frequency control means is an amplitude modulator.   The optical pulse testing apparatus according to claim 2, wherein the optical pulse forming means is an acousto-optic element.   3. The optical pulse test apparatus according to claim 2, further comprising: an optical frequency shift unit that is installed on the optical path of the local light and shifts the optical frequency of the local light by a certain width.

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