JPH0560648A - Light pulse tester using heterodyne detection of light - Google Patents

Abstract

PURPOSE:To shorten the measuring time by successively allowing a light pulse signal to be incident on an optical fiber before the time when back scattered light ceases to return from the optical fiber and separately measuring back scattered lights. CONSTITUTION:A light pulse tester is equipped with a pulse control signal generator 3 generating a burst signal 13 that let sine waves with (n) kinds of frequencies DELTAf last only for a finite time, and a light modulation part 4 receiving the input of signal light 11 and converting the signal light 11 to a light pulse signal 14 wherein the light frequency of the signal light 11 is shifted by the aforementioned frequencies DELTAf only for the certain time of the burst signal 13 and further equipped with (n) band-pass filters 8 filtering (n) kinds of frequencies DELTAf set by the pulse control signal generator 3 and a signal processing part 9 taking out the outputs of (n) band-pass filters 8 to average them.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pulse tester for measuring loss and break point position of an optical fiber by utilizing optical heterodyne light reception.

[0002]

2. Description of the Related Art Next, the structure of a conventional optical pulse tester will be described with reference to FIG. In FIG. 2, heterodyne light reception is used. In FIG. 2, 1 is a coherent light source, 2 is an optical splitter, 23 is a pulse control signal generator, 4 is an optical modulator,
5 is an optical branching device, 6 is an optical multiplexer, 7 is a photodetector, 28 is a bandpass filter, 29 is a signal processing unit, and 10 is an optical fiber to be measured.

The light source 1 continuously emits coherent light having a stable optical frequency, and the optical splitter 2 splits the emitted light of the light source 1 into a signal light 11 and a local light 12. The pulse control signal generator 23 generates a burst signal 21 in which a sine wave having a frequency Δf is continued for a finite time. The optical modulator 4 changes the optical frequency of the signal light 11 by Δf for a certain time of the burst signal 21.
Only the optical pulse signal 22 is shifted. Optical splitter 5
Splits the backscattered light 15 generated by making the optical pulse signal 22 incident on the optical fiber 10 to the optical multiplexer 6.

The optical multiplexer 6 includes a back scattered light 15 and a local light 1
2 is multiplexed, and the photodetector 7 performs optical heterodyne detection on the signal light multiplexed by the optical multiplexer 6. The bandpass filter 28 filters a frequency equal to the frequency Δf from the signal photoelectrically converted by the photodetector 7, and inputs the frequency to the signal processing unit 29. The signal processing unit 29 takes out the magnitude of the amplitude of this signal, averages it over the outputs of a large number of optical pulse signals 22, and displays it.

[0005]

The backscattered light 15 shown in FIG.
Corresponds to the magnitude of the amplitude of the signal filtered by the bandpass filter 28. More specifically, the magnitude of the backscattered light 15 generated in each part of the optical fiber 10 is determined by the time when the optical pulse signal 22 is emitted.
It is measured from the fact that the amplitude of the signal of the signal processing unit 9 appears at the time when the time for the light to reciprocate from the incident end to each part has passed.

Therefore, in order to average each part of the optical fiber 10 over the outputs of a large number of optical pulse signals 22, the optical pulse signals 22 output up to immediately before are averaged.
After all of the backscattered light 15 caused by the noise has not returned from the optical fiber 10, that is, the signal processing unit 29.
It is necessary to emit the next optical pulse signal 22 after there is no signal input to. When the next optical pulse signal 22 is emitted earlier than the signal input to the signal processing unit 29 disappears, a signal in which the backscattered light 15 generated at a different portion of the optical fiber 10 due to the previous optical pulse signal 22 is mixed is generated. It will appear in the signal processor 9, which cannot be separated.

In reality, the backscattered light 15 is generated by the optical fiber 1
When it returns to the incident end of 0, a part of it may be reflected here and may be incident on the optical fiber 10 again. This re-incident light also produces backscattered light 15,
It will appear as a signal in the signal processing unit 29. Due to this phenomenon, in FIG. 2, the time interval for emitting the optical pulse signal 22 is set to be twice or more the time for the light to make a round trip to the far end of the optical fiber 10.

According to the present invention, before the time when the backscattered light 15 due to the optical pulse signals output up to immediately before does not return from the optical fiber 10, the optical pulse signals are successively incident to make the optical fiber 10 incident. Backscattered light generated in each part 1
It is an object of the present invention to provide an optical pulse tester in which the measurement time can be shortened by making it possible to measure 5 separately.

[0009]

To achieve this object, in the present invention, a light source 1 for emitting continuous coherent light is provided.
And the coherent light from the light source 1 to the signal light 11 and the local light 12
Optical branching device 2 for branching into n and n (n ≧ 2) frequencies Δ
Burst signal 13 that continues the sine wave of f for a finite time
Pulse signal generator 3 and signal light 11
As an input, the optical modulator 4 that changes the optical frequency of the signal light 11 into the optical pulse signal 14 that is shifted by the frequency Δf for a certain time of the burst signal 13, and the optical pulse signal 14 is incident on the optical fiber 10. An optical splitter 5 for extracting the backscattered light 15 generated in the fiber 10, an optical multiplexer 6 for multiplexing the backscattered light 15 split by the optical splitter 5 and the local light 12 split by the optical splitter 2. , A photodetector 7 for heterodyne detecting the light multiplexed by the optical multiplexer 6, and a photodetector 7
Of the band-pass filter 8 and the output of the band-pass filter 8 that receives the detection signal 16 optically-electrically converted by the input signal and filters the n types of frequencies Δf set by the pulse control signal generator 3, respectively. Signal processing unit 9 for taking out and averaging
With.

[0010]

Next, the structure of the optical pulse tester according to the present invention will be described with reference to FIG. Reference numeral 3 in FIG. 1 is a pulse control signal generator, 8 is a bandpass filter, 9 is a signal processing unit, and the others are the same as those in FIG. That is, in FIG. 1, instead of the pulse control signal generator 23 of FIG. 2, a variable pulse control signal generator 3 of Δf of n types (n ≧ 2) is adopted,
Instead of the one band filter 28 in FIG. 2, n band filters 8 that filter frequencies equal to n kinds of Δf set in the pulse control signal generator 3 are used. Further, instead of the signal processing unit 29 of FIG. 2, a signal processing unit 9 that averages the respective outputs of the n band-pass filters 8 separately and synthesizes these respective signals is used.

In FIG. 1, when a large number of optical pulse signals 14 are emitted one after another, Δf of the pulse control signal generator 3 is sequentially changed to n different frequencies, and this is repeated. The principle of the present invention is as follows. Even if the backscattered light 15 due to the plurality of optical pulse signals 14 output up to immediately before returns from the optical fiber 10 at the same time, the optical pulse signals 14 having the same optical frequency are output every n times. As for the time interval of, if the backscattered light 15 caused by one light pulse signal 14 is set to be larger than the time at which the photodetector 17 becomes sufficiently small, each of the components of the frequency equal to Δf of all n kinds is backscattered. The light 15 always contains only backscattered light due to one light pulse signal. Therefore, by separating the detected signal 16 into n types of Δf and processing the Δf, the magnitude of the backscattered light 15 at each portion of the optical fiber 10 can be separately averaged and measured.

Next, the operation of FIG. 1 will be described. Optical splitter 2
The optical frequency of the signal light 11 branched from is shifted by the frequency Δf of the burst signal 13 by the optical modulator 4.
The burst signal 13 passes through the optical branching device 5 and enters the optical fiber 10 as the optical pulse signal 14 for a certain time. The backscattered light 15 generated in each part of the optical fiber 10 passes through the optical branching device 5 and is input to one side of the optical multiplexer 6. The light multiplexed by the optical multiplexer 6 enters the photodetector 7.

The detection signal 16 photoelectrically converted by the photodetector 7 contains a component of frequency Δf equal to the frequency Δf shifted by the generation of the optical pulse signal 14. The frequency Δf of the burst signal 13 is set by the pulse control signal generator 3 one after another for each of n types of frequencies. These n types of frequencies are respectively represented by Δfa, Δfb,
..., Δfn.

The detected signal 16 contains a component of frequency Δfa due to the backscattered light 15 caused by the optical pulse signal 14 shifted by Δfa, and the backscattered light 15 caused by the optical pulse signal 14 shifted by Δfb. , The frequency of the backscattered light 1 due to the optical pulse signal 14 shifted by Δfn in the same manner.
5 includes a component of frequency Δfn.

The detected signal 16 has a total of n types of frequencies Δ.
.., .DELTA.fn are input to n band-pass filters 8 for filtering fa, .DELTA.fb ,. This is sequentially performed by the bandpass filters 8a and 8
b, ..., 8n. The magnitude of the output of the bandpass filter 8 is the optical pulse signal 14 shifted by the frequency Δf.
Corresponds to the size of the backscattered light 15 due to. In other words, the output of the bandpass filter 8a that filters the frequency Δfa corresponds only to the magnitude of the backscattered light 15 caused by the optical pulse signal 14 shifted by Δfa. Frequency Δ
The output of the band-pass filter 8b that filters fb is the backscattered light 1 caused by the optical pulse signal 14 that is shifted by Δfb.
It corresponds only to the size of 5. Similarly, the output of the band-pass filter 8n that filters the frequency Δfn corresponds only to the magnitude of the backscattered light 15 caused by the optical pulse signal 14 shifted by Δfn.

All the outputs of the n band-pass filters 8 are input to the signal processing section 9. In the signal processing unit 9, the frequency Δ
fa, Δfb, ..., Δfn amplitude changes over time are averaged, and the time axes are all aligned and combined with the time from the time when the corresponding optical pulse signal 14 is emitted. Display as measurement data.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described with reference to FIG. If the frequency of the burst signal 13 of the pulse control signal generator 3 is n = 4 types, the number of band-pass filters 8 is n = 4. 4 of burst signal 13 of pulse control signal generator 3
Types of frequencies and band filters 8a, 8b, 8c, 8d
The filtering frequency is, for example, Δfa = 75 MHz, Δfb = 90.
MHz, Δfc = 105 MHz, and Δfd = 120 MHz.

FIG. 3A is a waveform diagram of the burst signal 13 of the pulse control signal generator 3, and the horizontal axis represents time. The frequencies of the burst signal 13 are Δfa, Δfb, Δfc, Δ
It is output in the order of fd, and this is repeated. ta is a time interval between the burst signals 13 and to is a time interval at which the burst signals 13 having the same frequency are output.

FIG. 3A is a waveform diagram of the optical pulse signal 14, and the horizontal axis represents the same time as in FIG. 3A. Optical pulse signal 14
The optical frequency of the light source is f + Δf using the optical frequency f of the light source 1.
a, f + Δfb, f + Δfc, and f + Δfd are output in this order, and this is repeated. ta and to are the same time intervals as in FIG.

To is set to be longer than the time interval at which the return of the backscattered light 15 from the optical fiber 10 due to a certain optical pulse signal becomes sufficiently small. The relationship between to and ta is
Since ta = to / 4, the time interval between the respective optical pulses can be made shorter than the time interval at which the backscattered light due to a certain optical pulse signal becomes sufficiently small at the photodetector 17.

Next, FIG. 4 shows a state in which the backscattered light 15 caused by a certain optical pulse signal becomes sufficiently small.

FIG. 3C shows the optical pulse signal 1 as shown in FIG.
4 is a waveform diagram showing the change over time in the intensity of the backscattered light 15 returning to the incident end of the optical fiber 10 upon which 4 is incident, for each optical frequency. In reality, light that is a combination of these optical frequencies returns. The horizontal axis is the same time as in Fig. For example, at time A, the optical frequencies of the backscattered light returning to the incident end of the optical fiber 10 are f + Δfa, f + Δfb, and f.
Although + Δfc and f + Δfd are mixed, there is only one optical pulse signal that causes backscattering existing at this time for any optical frequency. Among the optical pulse signals output before the optical pulse signal, the backscattered light caused by the optical pulse signals having the same optical frequency is sufficiently small. This is true at any time.

FIG. 3D shows the backscattered light 15 as shown in FIG.
After the local light 12 and the local light 12 are multiplexed by the optical multiplexer 6, the photodetector 7
2 shows the time change of the envelope of the amplitude of the detection signal 16 which is photoelectrically converted into electrical signals by frequency. In reality, these frequencies are the synthesized signal. The horizontal axis represents the same time as in FIG. At any time, the detected signal 16 has a mixed frequency of Δfa, Δfb, Δfc, and Δfd.
Further, at any frequency, the signals are signals corresponding to the time change of the backscattered light caused by one optical pulse signal 14, respectively, and time Ba at which equal time tb has passed from the time when the corresponding optical pulse signal 14 is emitted. , Bb, B
In c and Bd, each frequency Δ of the detection signal 16
The amplitude magnitudes of fa, Δfb, Δfc, and Δfd correspond to the magnitude of the backscattered light generated at the same portion of the optical fiber 10.

FIG. 3E is a diagram showing the outputs of the bandpass filters 8a, 8b, 8c and 8d to which the detection signal 16 shown in FIG. 3D is input. Frequency Δfa, Δfb, Δf
The components of c and Δfd are filtered. At times Ba, Bb, Bc, and Bd, which are equal times tb from the times at which the corresponding optical pulse signals 14 are emitted, the optical fiber 10 is
It shows the magnitude of the backscattered light generated at the same portion of.

The signal processing unit 9 averages the outputs of the band-pass filters 8 and then, at the times Ba, Bb, Bc, and Bd, which are equal times tb from the times at which the corresponding optical pulse signals 14 are emitted. The data is moved so that the signals match on the time axis, and n = 4 types of data are averaged to obtain measurement data.

When the measurement time of FIG. 1 is Tn and the measurement time of FIG. 2 is To, and Tn and To are compared at the same average number M, the time interval tn of the optical pulse signal 14 of FIG. 1 and the light of FIG. The time interval to of the pulse signal 22 is tn = to / n
Therefore, the measurement time required for the average number M times is
Tn = M × tn and To = M × to. As a result, T
Since n = To / n, n optical pulse signals can be emitted during the time when the backscattered light due to the optical pulse signal before n emission is sufficiently small, and the measurement time is shortened to 1 / n. It

Here, an embodiment of the pulse control signal generator 3, the optical modulator 4, and the signal processor 9 will be described. An ultrasonic light modulator is used for the light modulator 4. The burst signal 13 is input to the ultrasonic light modulator. Assuming that the optical frequency of the signal light 11 is f, the burst of the signal light 11 incident on the ultrasonic optical modulator is a light whose optical frequency is shifted by Δf for a certain period of time and is extracted from the emission port.

Next, a configuration example of the pulse control signal generator 3 will be described with reference to FIG. The pulse control signal generator 3 includes n oscillators 31, a switch 32, and a pulse shaper 33. The oscillator 31 has frequencies Δfa and Δ, respectively.
A continuous sine wave signal of fb, ..., Δfn is output.
The switch 32 receives the n sine wave signals as an input and switches the sine wave signals to be taken out at a constant time. Switch 32
The sine wave signal extracted in step 3 is input to the pulse shaper 33, and the pulse shaper 33 cuts out the sine wave signal for a fixed time and outputs a pulse with a rectangular envelope. This output becomes the burst signal 13 shown in FIGS.

Next, a configuration example of the signal processing unit 9 will be described with reference to FIG. The output of the n band-pass filters 8 is the rectifier 91.
The smoothing filter 92 produces a signal corresponding to the amplitude of the original input signal. Then, the A / D converter 93
As a result, digital data is input to the processor 94. The processor 94 is a microprocessor or a digital signal processor. The processor 94 averages the corresponding data for a large number of the optical pulse signals 14 having the same optical frequency, synthesizes the data by aligning the times with reference to the emission time of the corresponding optical pulse signals 14, and measuring the data. Data.

[0030]

According to the present invention, the measurement time is shortened to 1 / n as compared with the optical pulse tester of FIG.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an optical pulse tester according to the present invention.

FIG. 2 is a block diagram of a conventional optical pulse tester.

FIG. 3 is a diagram showing a time change of each signal of FIG.

FIG. 4 is a diagram showing a state in which backscattered light becomes smaller with time.

5 is a diagram showing a configuration example of a pulse control signal generator 3 of FIG.

FIG. 6 is a diagram showing a configuration example of a signal processor 9 of FIG.

[Explanation of symbols]

 1 light source 2 optical splitter 3 pulse control signal generator 4 optical modulator 5 optical splitter 6 optical multiplexer 7 photodetector 8 bandpass filter 9 signal processor 10 optical fiber

Claims (1)

[Claims] 1. A light source for emitting continuous coherent light
(1), a first optical branching device (2) for branching the coherent light of the light source (1) into a signal light (11) and a local light (12), and n kinds (n ≧ 2) of sine of frequency Δf A pulse control signal generator (3) that generates a wave as a burst signal (13) that continues for a finite time and a signal light (11) are input, and the signal light (11) is input for a certain time of the burst signal (13). An optical modulator (4) that changes the optical frequency of the optical pulse signal (14) into the optical pulse signal (14) that is shifted by the frequency Δf, and the optical pulse signal (14) enters the optical fiber (10) to be measured, The second optical branching device (5) for taking out the backscattered light (15) generated at the first backscattering light (15) split by the second optical branching device (5) and the first
The optical multiplexer (6) that combines the local oscillator light (12) that is split by the optical splitter (2) and the photodetector (7) that heterodyne-detects the light that is multiplexed by the optical multiplexer (6). And the detection signal (16) photo-electrically converted by the photodetector (7) are input, and n bands that respectively filter the n kinds of frequencies Δf set by the pulse control signal generator (3) are input. Filter (8)
And a signal processing unit (9) that takes out the output of n band-pass filters (8) and performs averaging, and outputs the frequency every time the burst signal (13) is output from the pulse control signal generator (3). An optical pulse tester using heterodyne light reception, characterized in that the optical fiber (10) is measured while changing Δf.