JPH033175B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical fiber break point detection device that is also capable of detecting a near-end break point.

To detect the break point of an optical fiber, pulse light is input into the optical fiber, the reflected pulse light from the break point is detected, and the position of the break point is determined based on the time from the input of the pulse light to the detection of the reflected pulse light. It is common to calculate A device for detecting such a break point of an optical fiber has conventionally had the configuration shown in FIG. In the figure, 1 is a signal processing section;
2 is a pulse generator, 3 is a laser device, 4 is an optical coupler, 5 is an optical connector, 6 is an optical fiber to be measured, 7
8 is a photodetector made of an avalanche photodiode or the like, and 9 is an amplifier.

Based on the timing signal from the signal processing section 1, the pulse generator 2 applies a pulse signal to the laser device 3,
Generates laser pulse light. This laser pulse light enters the optical fiber 6 to be measured via the optical coupler 4 and the optical connector 5. The laser pulse light is reflected at the breaking point 7 and is sent to the photodetector 8 by the optical coupler 4.
The reflected pulsed light is added to the

The output of the photodetector 8 is amplified by an amplifier 9 and applied to the signal processing section 1. In the signal processing section 1,
The distance L to the breaking point 7 is calculated from the time from generation of the laser pulse light to detection of the reflected pulse light. This laser pulse light is repeatedly generated at predetermined intervals, and by performing averaging processing, the signal processing section 1 can calculate the distance with high accuracy.

The ratio of the reflected pulsed light when the breaking point is within 100 to 200m and the reflected pulsed light when it is 5km is 0.87×10 ³
Therefore, it is common to increase the gain of the amplifier 9 so that a breaking point at a far end such as 5 km can be detected. Therefore, when detecting the near-end break point, the amplifier 9 becomes saturated due to the influence of back scattering. That is, the component of the reflected pulsed light from the near-end break point is buried in the back scattering component and cannot be identified.

For this reason, it has been impossible to detect the near-end break point in the past, so for the near-end break point,
By taking measurements from the other end of the optical fiber,
It was detected as a far-end break point.

An object of the present invention is to enable easy detection of the near-end break point. Examples will be described in detail below.

FIG. 2 is a block diagram of an embodiment of the present invention, in which 11 is a signal processing section, 12 is a pulse generator, 1
3 is a laser device, 14 is an optical coupler, 15 is an optical connector, 16 is an optical fiber to be measured, 17 is a breaking point,
18 is a photodetector, 19 is an amplifier, 20 is an optical switch that is composed of electro-optical elements, etc., and whose light passing loss changes depending on the applied voltage; 21 is a function generator;
22 is a delay circuit.

Laser pulse light from the laser device 13 is incident on the optical fiber 16 to be measured via the optical coupler 14 and the optical connector 15, and the reflected pulse light from the break point 17 and the reflected light due to back scattering are as follows.
The signal is separated by an optical coupler 14 and applied to an optical switch 20. A function generator 2 is connected to this optical switch 20.
1 is delayed by a delay circuit 22 and added to control the amount of light applied to the photodetector 18. That is, the optical switch 20 acts as a time-varying optical attenuator for the reflected light from the proximal end.

FIG. 3 is an explanatory diagram of the operation. When the pulse shown in FIG. generated. This laser pulse light causes reflected light from the optical connector 15 and optical fiber 1 to be measured.
Scattered light (back scatter) due to Rayleigh scattering at 6 and reflected pulsed light from the breaking point 17 are input to the optical switch 20 by the optical coupler 14.

Figure 3c shows an example of each of the above-mentioned reflected lights.The reflected pulsed light at the far end breaking point is indicated by RF', while the reflected pulsed light at the near end breaking point is indicated by RF. . When such reflected light is directly detected by the photodetector 18 and amplified by the amplifier 19, the amplified output becomes as shown in FIG. 3d due to saturation of the amplifier 19. That is, the detection signal of the reflected pulsed light RF at the near-end break point is lost.

Therefore, the optical switch 20 is controlled to prevent saturation in the amplifier 19, and the reflected pulsed light at the near-end break point is
This allows the RF detection signal to be amplified correctly, and the control signal applied to the optical switch 20 is as shown in FIG. 3e.

The time t ₁ is a delay time given by the delay circuit 22 to remove the influence of the reflected light from the optical connector 15, and is given by t ₁ =L _F ·N/C+Tw (1). However, C=speed of light (=3×10 ⁸ m),
N=group refractive index of the optical fiber (≈1.459), L _F =total length of the guide optical fiber, and Tw=pulse width of the laser pulse light. Note that the guide optical fiber refers to a portion of the optical fiber other than the optical fiber 16 to be measured.

Further, the rising edge of the control signal is generated by the function generator 21, The waveform is as follows. However, α = cumulative loss of the optical fiber, and V _H is the switch voltage.

The optical switch 20 can switch light by applying a voltage, and various electro-optical elements can be used. Figure 4 shows an example of the characteristics of the voltage applied to the optical switch 20 and the passing loss. When the voltage is zero, the loss is maximum and the switch is off for optical signals, and as the voltage increases, the loss increases. As a result, the passing loss decreases, and the loss becomes almost zero due to the switch voltage _VH . Therefore, the third
By applying the control signal shown in FIG.

If the time t ₂ is, for example, 3 μS, a loss change will be given to the reflected light up to 300 m from the optical fiber 16 to be measured, and if a loss change is given to the reflected light up to 200 m, then It is sufficient to select 2 μS.

Therefore, from the optical switch 20 to the optical detector 18,
Since the reflected light shown in FIG. 3f is added, the amplifier 19 can amplify the detection signal of the reflected pulsed light RF without saturating it. By controlling the rise characteristic of the optical switch 20, the near-end loss of the optical fiber 16 under test appears to have changed, but the signal processing section 11 can perform correction processing, thereby making it possible to change the near-end loss as shown in FIG. Since the detection signal of the reflected pulsed light at the near-end break point can be easily identified, the near-end break point within 200 m can be easily detected by the calculation function of the signal processing section 1. Furthermore, the breaking point 17
This includes not only complete rupture but also cases where cracks have occurred.

As explained above, the present invention generates a control signal from the function generator 21 in synchronization with the generation of pulsed light to be input to the optical fiber 16 under test, and delays the control signal by a predetermined time by the delay circuit 22. In addition to the optical switch 20, this control signal exponentially reduces the passage loss of the optical switch 20,
The pulsed light reflected from the optical fiber under test is applied to the photodetector 18 via this optical switch 20 to detect the break point, and the pulsed light reflected from the near-end break point has a large light transmission loss. The light passes through the state optical switch 20 and enters the photodetector 18.

As a result, high-level reflected light and backscattered light from the optical connector 15 are attenuated by the optical switch 20, so that the optical detector 18
This makes it possible to prevent saturation of the amplifier 19 that amplifies the output signal, and it becomes possible to reliably amplify and output the detection signal that has detected the reflected pulsed light at the near-end break point. Therefore, there is an advantage that the near-end break point can be easily detected. Note that the far end breaking point can be detected using a detection signal of reflected pulsed light, as in the conventional example.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional optical fiber break point detection device, Fig. 2 is a block diagram of an embodiment of the present invention, Fig. 3 is an explanatory diagram of operation, and Fig. 4 is an explanatory diagram of characteristics of an optical switch. be. 11 is a signal processing unit, 12 is a pulse generator, 13
1 is a laser device, 14 is an optical coupler, 15 is an optical connector, 16 is an optical fiber to be measured, 17 is a breaking point, 1
8 is a photodetector, 19 is an amplifier, 20 is an optical switch, 21 is a function generator, and 22 is a delay circuit.

Claims (1)

[Claims] 1. In an apparatus for detecting a break point of an optical fiber by injecting pulsed light into an optical fiber to be measured and detecting reflected pulse light from the break point, an optical switch whose transmission loss is controlled; a photodetector which applies the reflected pulsed light through the optical switch; an amplifier which amplifies the output of the optical detector; a function generator that generates the control signal that changes almost exponentially to reduce the pulsed light almost exponentially; An optical fiber break point detection device comprising a delay circuit for delaying the time.