JPH0543050B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (a) Technical Field of the Invention The present invention involves inputting a burst of coherent light, that is, a coherent light pulse, into an optical fiber to be measured, and optical heterodyne detection of the return light from the optical fiber to be measured. Alternatively, a coherent OTDR (Optical
This relates to Time Domain Reflectometry (Time Domain Reflectometry) equipment.

(b) Prior art and problems The OTDR device, which injects a light pulse into an optical fiber under test and detects return light such as Fresnel reflected light and backscattered light from the optical fiber under test, does not directly receive the return light. A direct detection type OTDR device that detects the waves,
There is a coherent OTDR device that performs heterodyne detection or homodyne detection using local light.
In these OTDR devices, the Fresnel reflected light generated at the near end of the optical fiber under test has a level higher than the backscattered light by about 30 dB to 50 dB, and when such Fresnel reflected light enters the light receiving element, it damages the light receiving element and it. There was a problem in that the light receiving system including the sensor became saturated, making accurate measurements impossible. Direct detection method
In order to solve this saturation of the light receiving system, the OTDR device connects the light source, the optical fiber to be measured, and the light receiving system with a turnover type optical switch, and only receives the return light from the desired observation position of the optical fiber to be measured. Although OTDR devices with a configuration that leads to the system have been proposed,
Since it is a direct detection method, heterodyne and
Compared to the homodyne detection method, the signal-to-noise ratio (SNR) of the light receiving system is low, and even if the incident optical pulse is linearly polarized, the direction of the polarization plane of the returned light is not constant, so the optical switch , there was a constraint that it had to be polarization-independent.

(c) Purpose of the Invention In view of the above problems, the purpose of the present invention is to prevent the signal-to-noise ratio (SNR) of the light receiving system from decreasing, that is, to prevent the received signal from being attenuated. Instead of putting the switch directly into the light path, I controlled the return light. In addition, a polarization-dependent optical switch can also be used, and a direct detection method can be used.
Coherent, superior to OTDR equipment
The present invention provides a coherent OTDR device that can prevent saturation of the light receiving system without significantly reducing the signal-to-noise ratio (SNR) of the light receiving system of the OTDR.

(d) Means for solving the problem An OTDR device using optical heterodyne detection or optical homodyne detection inputs a burst of first coherent light into an optical fiber to be measured, and detects the return light from the optical fiber to be measured. (signal light) is combined with sufficiently strong second coherent light, that is, local light, by a coupling means such as an optical directional coupler. Here, if the frequency of the first coherent light is fs, the frequency of the local light is fs+f in the heterodyne detection method, and fs in the homodyne detection method.
There is a relationship. Then, the combined signal light and local light are incident on a photodetector, but the present invention provides an optical switch that passes or blocks only the local light that is incident on the photodetector, and a desired observation position of the fiber to be measured. The present invention is characterized in that it includes a control means for controlling the optical switch so that only the return light returned by the light source is combined with the local light. That is, when the signal light power is P _S and the local light power is P _L , the current i output from the light receiving element is expressed as i=eη/hν( _PL + P _S +2√ _{L S} COSωt) (1). Here, e is the electronic charge, η is the quantum efficiency of the light-receiving element, h is Planck's constant, ν is the frequency of the local light (however, the frequency of the local light ≒ the frequency of the signal light), and ω is the difference between the local light and the signal light. is the angular frequency of the difference (ω=2πf). Even in the case of excessive Fresnel reflection that occurs at the near end, P _L > P _S. Therefore, when unnecessary signal light such as excessive Fresnel reflected light is incident, if the local light is blocked (that is, P _L = 0). ), the light-receiving element and the light-receiving system including it will not be saturated.

Also, the root mean square value _N ² of the noise current after detection is:
When the local light power P _L is sufficiently large, shot noise due to the local light is dominant, and _N ² =2e(eη/hν) P _L B (2). Here, B is the frequency bandwidth of the light receiving system.

On the other hand, the root mean square value _se ² of the detection signal current is derived from equation (1), but the first and second terms on the right side of equation (1) are removed by a filter, and _se ² = 2(eη/ hν) ² P _L P _S (3).

Therefore, the signal-to-noise ratio (SNR) is: SNR=2(eη/hν) ² P _L P _S /2e(eη/hν) P _L B
It is expressed as = ηP _S /hνB(4).

As can be seen from the above equation, the noise current after detection is 2
In the region where the root mean value _N ² is dominated by shot noise caused by local light, the SNR depends only on the signal optical power P _S , so even if there is insertion loss in the optical switch that passes and blocks the local light, , the SNR does not decrease due to the insertion loss.

Furthermore, since the direction of the polarization plane of the local light can be made constant, it can also be used as a polarization plane dependent optical switch.

(e) Embodiments of the invention Examples of the invention will be described below with reference to the drawings.

FIG. 1 is a block diagram of a first embodiment according to the present invention. In the figure, 1 is a coherent light source;
2 is an optical splitter, 3 and 8 are intermittent optical switches, 4 is an optical coupler, 5 is an optical fiber to be measured, 6 is a pulse generator, 7 and 10 are optical switch drivers, 9 is a pulse delay controller, 11 is a photodetector, 12 is a signal processing device, and 13 is a display device. Correlent light source 1
The continuous wave from is split by the optical splitter 2,
One of the continuous waves is turned into an optical pulse by an intermittent optical switch 3, passes through an optical coupler 4, and enters an optical fiber 5 to be measured. The intermittent optical switch 3 is operated by an optical switch drive signal from an optical switch driver 7 which is generated in synchronization with a pulse generator 6.

The other continuous wave branched by the optical splitter 2 becomes local light, passes through the intermittent optical switch 8, enters the optical coupler 4, and is combined with the return light from the fiber under test 5. be combined.

At this time, the electric pulse generated by the pulse generator 6 is transferred to a pulse delay controller 9 so as to combine the local light with the return light from the desired observation position.
The delay and pulse width are arbitrarily controlled by the switch driver 10, which converts the signal into a switch drive signal to turn the intermittent optical switch 8 on and off.

The combined light waves are then turned into electrical signals by a photodetector 11, subjected to signal processing such as amplification, filtering, logarithmic operations, and A/D conversion by a signal processing device 12, and then displayed on a CRT or the like by a display device 13. Ru.

As the intermittent optical switches 3 and 8, LiNb03,
Materials with electro-optic effect such as LiTa03, KDP, ADP, PLZT or LiNb03, LiTa03,
This can be achieved by applying an electric field or harmonic wave of appropriate strength as an optical switch drive signal to a material with an acousto-optic effect such as Te02, PbMn04, Ge, GaAs, GaP, tellurite glass, or chalcogenide glass.

FIG. 2 is a waveform diagram for explaining the operation of the first embodiment. Waveform (a) is a continuous wave output from a coherent light source, waveforms (b) and (c) are continuous waves branched by optical splitter 2, and waveform (d) is a continuous wave output from optical switch driver 7. The intermittent optical switch 3 is in a passing state while the driving signal is being outputted. The waveform (e) is
The optical pulse and waveform (f) are incident on the optical fiber 5 to be measured.
indicates an observed waveform displayed on the display device 13. Waveform (g) is an optical switch drive signal output from the optical switch driver 10, and while the drive signal is output, the intermittent optical switch 8 is in a passing state. Waveform (h) shows the waveform of the local light that is combined with the return light from the optical fiber 5 to be measured. From the same figure, waveform (d)
When the intermittent optical switch 3 is operated with the switch drive signal of Without switch 8, waveform (f)
Fresnel reflected light with a high level appears from the input end and the terminal end of the optical fiber 5 to be measured, shown by the broken line,
This may cause waveform sagging, etc., making accurate measurement difficult. Therefore, if the intermittent optical switch 8 is operated using the switch drive signal shown in waveform (g), and the local light is passed through and blocked as shown in waveform (h), the high-level Fresnel reflection will not be detected, so the light receiving system will is not saturated, and the broken line portion of waveform (f) is not observed, allowing accurate measurement.

FIG. 3 is a block diagram of a second embodiment according to the present invention. Components having the same structure as in FIG. 1 are designated by the same reference numerals, 14 is a turnover type optical switch, and 15, 16, and 17 are ports. In this configuration, the optical splitter 2 and the intermittent optical switch 3 shown in FIG.
Instead, a turnover type optical switch 14 is used. In the turnover type optical switch 14, ports 15 and 16 are electrically connected while a drive signal is being outputted from the optical switch driver 7, and when no drive signal is being outputted, the port 15 is connected to the port 17. Conductive. The difference from the first embodiment is that the local light is not output while the optical pulse is emitted to the optical fiber 5 to be measured.
This means that there is no branching loss due to the optical splitter 2. The turnover type optical switch 14 is realized by applying ultrasonic waves of appropriate intensity as an optical switch drive signal to a material having an electro-optic effect or an acousto-optic effect, similar to the intermittent type optical switches 3 and 8. can.

FIG. 4 is a waveform diagram for explaining the operation of the second embodiment. Port 15 of turnover type optical switch 14 by optical switch drive signal of waveform (d)
By switching on and off the port 16, an optical pulse of waveform (e) is input to the optical fiber to be measured. Additionally, along with this, a light wave of waveform (i) is output from the port 17.

Then, by operating the intermittent optical switch 8 with the optical switch drive signal having the waveform (g) and outputting the local light having the waveform (h), the same effect as in the first embodiment can be obtained.

FIG. 5 is a block diagram of a third embodiment according to the present invention. Components having the same configuration as those in FIGS. 1 and 3 are designated by the same reference numerals, and 18 is a 1×3 optical switch, and 19, 20, 21, and 22 are ports.

In this configuration, the turnover type switch 14 and the intermittent type optical switch 8 shown in FIG.
An optical switch 18 is used. The 1×3 optical switch 18 operates between ports 19 and 2 while the optical switch drive signal is output from the optical switch driver 7.
0 is conductive and while the optical switch drive signal is being output from the optical switch driver 10, the port 19 is conductive with the port 22. Furthermore, when both optical switch drive signals from both optical switch drivers are not output, port 19 is electrically connected to port 21. Furthermore, the difference between this embodiment and the second embodiment is that the number of parts is reduced and the device becomes more compact.

The 1×3 optical switch 18 is made of a material having an electro-optic effect or an acousto-optic effect, similar to the intermittent optical switches 3 and 8 and the turnover optical switch 14, by applying an electric field or ultrasonic wave of appropriate strength to the material. This can be achieved by applying it as a switch drive signal.

The operation of this embodiment will be explained with reference to FIG. 2. When the optical switch drive signal of waveform (d) is used to connect and disconnect ports 19 and 20 of the 1×3 optical switch 18,
A light pulse as shown in waveform (e) is input into the optical fiber to be measured. When the ports 19 and 19 of the 1.times.3 optical switch 18 are connected to each other using the optical switch drive signal having the waveform (g), the same effects as in the first and second embodiments can be obtained.

Figure 6 shows a coherent image using conventional technology.
FIG. 2 is a configuration diagram of an OTDR device. Figure 1, Figure 3,
Components with the same configuration as in FIG. 5 are shown with the same symbols, but since there is no intermittent optical switch 8 that passes and cuts off the local light, the return light from the optical fiber 7 to be measured saturates the light receiving system. There is a problem in that all of the reflected light, including the return light that causes it, is detected.

Figure 7 shows the direct detection method using conventional technology.
FIG. 2 is a configuration diagram of an OTDR device. Figure 1, Figure 3,
Components having the same structure as those in FIGS. 5 and 6 are designated by the same reference numerals. Further, 23 is a pulsed light source, 24
is an optical switch driver. The pulse light source 23 emits light pulses in synchronization with the electric pulses generated by the pulse generator 6. At that time, the optical switch driver 2
Ports 15 and 17 of the turnover type optical switch 14 are made conductive by the optical switch drive signal No. 4,
The light pulse is made to enter the optical fiber 5 to be measured.

Then, the electric pulse is delayed and pulse width controlled by the pulse delay controller 9 so that the return light from the desired observation position is incident, the ports 15 and 16 are electrically connected, and the return light is guided to the photodetector 11. The turnover type optical switch 14 is driven by the optical switch driver 10.

The returned light is converted into an electric signal by a photodetector 11, processed by a signal processing device 12, and then converted into an electric signal by a display device 13.
Displayed on CRT, etc.

Saturation of the light receiving system can be prevented by this configuration, but there is a restriction that the turnover type optical switch 14 must be polarization independent, and since it is a direct detection method, it is less efficient than a coherent detection method. It also has the disadvantage of low signal-to-noise ratio (SNR).

(f) Effects of the Invention As explained in detail above, in the coherent OTDR device of the present invention, (i) the control means (8, 9) allows only the return light to return at the desired observation position of the optical fiber to be measured; Since it can be detected, accurate measurements can be made without the negative effects of saturation caused by excessive returned light.

(ii) Since the return light is not controlled and local light unrelated to the signal line is passed through and blocked by an optical switch, the excellent signal-to-noise ratio (SNR) of coherent OTDR detection is not degraded at all. . That is, even if there is a loss in the local light, there is no loss in the signal light (return light), so the weak return light is not attenuated.

(iii) Since the direction of the polarization plane of local light can be kept constant,
The optical switch has the advantage that a polarization dependent type can also be used.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the present invention, FIG. 2 is a waveform diagram for explaining the first embodiment of the present invention, and FIG. 3 is a diagram of a second embodiment of the present invention.
FIG. 4 is a waveform diagram for explaining the second embodiment of the present invention, FIG. 5 is a block diagram of the third embodiment of the present invention, and FIG. 6 is a diagram of the conventional example. Configuration diagram of coherent OTDR device by technology,
Figure 7 shows a direct detection OTDR using conventional technology.
It is a block diagram of a device. In the figure, 1 is a coherent light source, 2 is an optical demultiplexer,
3 and 8 are intermittent optical switches, 4 is an optical coupler, 5 is an optical fiber to be measured, 6 is a pulse generator, 7 and 10
is an optical switch driver, 9 is a pulse delay controller, 1
1 indicates a photodetector.

Claims (1)

[Claims] 1. A first means (1, 2, 3) for emitting burst-shaped first coherent light having a constant plane of polarization; second means (1, 2) for emitting a certain coherent second light, the first light is incident on the optical fiber to be measured, and the second means (1, 2) for emitting a certain coherent second light is provided; A coherent OTDR device is equipped with four ports and detects a signal by optical heterodyne detection or optical homodyne detection to measure the loss and break point position of the optical fiber under test. The first optical signal received from the first port is emitted as it is from the second port to the optical fiber under test, and the optical signal from the second port and the optical signal from the third port are combined. an optical means (4) for outputting to a fourth port; and a light detecting means (11) connected to the fourth port.
and receiving the return light from the optical fiber to be measured from the second port so that only the desired return light can be detected by the light detection means, and controlling passage and blocking of the second light for a desired time. A coherent OTDR device characterized by comprising: control means (8, 9) for controlling. 2. The control means (8, 9) causes a burst of coherent light to be emitted to the optical fiber to be measured in order to send only desired returning light from the optical fiber to be measured to the optical detection means. 2. The coherent device according to claim 1, wherein the control signal is transmitted after a predetermined period of time has elapsed.
OTDR equipment.