JPS62204138A - Optical fiber measuring instrument - Google Patents

Abstract

PURPOSE:To measure distance with high resolution over a wide measuring range by inputting a detection signal from a light receiving means and varying an amplification factor corresponding to the delay time of return light. CONSTITUTION:A pulse generating circuit 34 generates pulses with specific pulse width according to the measurement distance of an optical fiber to excite a laser diode 11. this impulsive light is incident on the optical fiber 2 to be measured through a directional coupler 12 and its return light is incident on the coupler 12, but its intensity falls as the signal arrives from a more distant place and has a longer delay time. The output of the coupler 12 is detected by a photodetecting element 13, whose output is amplified 31 and then inputted to a variable gain amplification part 32. The switch SW0 of the amplification part 32 is turned on in the starting specific section with a control signal 35. This signal is delayed by delay circuits D1-D3 in specific following sections and switches SW1-SW3 are turned on in order. Consequently, the return light is amplified with a larger amplification factor as the delay time becomes longer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to improving the resolution of an optical fiber measuring instrument using the 0TDR method.

(Prior art) 0TDR (0ptical
ime [)omain Reflectom
eter). In 0TDR, since the backscattered light is weak, the S/distance of the output from a light receiving element such as an avalanche photodiode is low.

To improve this, averaging processing is performed, but generally a method is used in which the received signal is subjected to high-speed AD conversion and processed by a computer.

(Problems to be Solved by the Invention) However, as the measurement distance increases, the signal from a distance becomes weaker, and in extreme cases, it becomes less than the resolution of the A/D converter. Therefore, if the amplification degree of the circuit at the front stage input to the AD converter is increased, a problem arises in that signals from a short distance become saturated and cannot be observed.

The present invention was made in order to solve the above problems, and an object of the present invention is to realize an optical fiber measuring instrument that can measure a wide range of measurement distances with high resolution.

(Means for Solving the Problems) The present invention provides optical fiber measurement in which the state of the fiber under test is observed by inputting light from a light source into the fiber under test and detecting the return light from the fiber under test using a light receiving means. It is characterized by an amplification means that inputs the detection signal from the light receiving means and changes the degree of amplification in accordance with the delay time of the returned light, and a signal processing means that inputs the output of this amplification means. It has the following features.

(Example) The present invention will be explained in detail below using the drawings.

FIG. 1 is a block diagram showing an embodiment of an optical fiber measuring device according to the present invention. Reference numeral 1 denotes an optical transceiver section, 2 an optical fiber to be measured to which optical pulses are applied from the optical transceiver section 1, and 3 an electronic circuit that controls the optical transceiver section 1 and amplifies the return light from the optical fiber to be measured 2. Section 4 is a signal processing section that controls the electronic circuit section 3 and processes its output.

In the optical transmitter/receiver 1, 11 is a laser diode constituting a light source, 12 is an optical directional coupler that inputs the output light of the laser diode 11 and outputs it to the optical fiber 2 to be measured, and 13 is a component from the optical fiber 2 to be measured. This is a light-receiving element that receives the returned light from the optical directional coupler 12 through the optical directional coupler 12. In the electronic circuit section 3, 31 is a preamplifier that amplifies the detection output of the light receiving element 13, 32 is a variable gain amplification section that inputs the output of this preamplifier 31, and 33 is a variable delay that controls the gain of this variable gain amplification section 32. A circuit section 34 is a pulse generator 35 that drives the diode laser 11.
This pulse generator 34 and the variable delay circuit section 33
This is a control circuit that controls the In the variable delay circuit section 33, D1 to D3 are delay circuits that each delay the control output of the control circuit 35 by a predetermined time. In the variable gain amplification section 32, Δ2 is an amplifier (for example, an operational amplifier) that amplifies the output of the preamplifier 31, RO to R4 are voltage dividing resistors connected in series between the output terminal of this amplifier Δ2 and the common, and SWo ~S W s is this resistance Ro ~ R4
and the other inverting input terminal of the amplifier A2. Switch SW
The output of the control circuit 35 and the output of the delay circuits D1 to D3 serve as driving inputs of the o'' SW3, respectively.

In the signal processing section 4, 41 is an A/D converter that inputs the output of the amplifier A2, and 42 is a microprocessor that inputs the output of this A/D converter 41 and controls the control circuit 35. This is a signal processing circuit.

The operation of the optical fiber measuring instrument having such a configuration will be explained next. The pulse generation circuit 34 generates a predetermined pulse width (for example, 100nsec to 1μ5e) depending on the measurement distance of the optical cable.
The laser diode 11 is excited by generating the pulse c). The pulsed light output from the laser diode 11 is turned into parallel light by a lens and enters the directional coupler 12, and the output light is condensed by the lens and connected to the optical fiber 2 to be measured.
incident on . The return light from the optical fiber 2 to be measured is converted into parallel light by a lens, and then enters the optical directional coupler 12.

The output of the optical directional coupler 12 is sent to the light receiving element 1 via a lens.
Detected at 3. The output of the light receiving element 13 is sent to the preamplifier 31.
After being amplified, the signal is input to the variable gain amplification section 32.

FIG. 2 is a time chart showing the time change of the backscattered light from the fiber 2 to be measured, in which the intensity of the returned light decreases as the signal comes from a distance with a longer delay time. The variable gain amplification section 32 turns on only the switch S W o in response to a control signal from the control circuit 35 during the OA interval. This signal is A-B.

In the sections B-C and CD, the switches SW+, SW2, and SW are delayed by delay circuits D1 and D3, respectively.
Turn on 3 (if each section is equally spaced). As a result, as the delay time becomes longer, the returned light is amplified to a greater degree. In the figure, 71+T21T3 are respectively delay circuits D1.02. [)3 is the delay time that occurs.

The output of the amplifier 2 is converted into a digital signal by an A/D converter 41 and then sent to a signal processing circuit 42 .

In addition to signal processing such as averaging processing, the signal processing circuit 42 performs processing to make the values at boundaries A, B, and C of each section equal.

According to an optical fiber measuring instrument with such a configuration, since the amplification of the light receiving system is increased as the measurement distance becomes longer, it is possible to measure a wide range of measurement distances with high resolution.For example, point D in Figure 2 In the vicinity, if the gain in the 0- section remains unchanged, the output of amplifier △2 will be below the resolution of the A/D converter, but in the above example, a large gain for the C-D section is used. This allows us to know the details of the failure.This also makes effective use of the number of pits in the A/D converter.

Furthermore, the signal does not become saturated in the OA section, allowing observation over a wide dynamic range.

In the above embodiment, the delay time is divided into four sections and the gain is switched, but any number of sections can be used. Further, the length of each section can be arbitrarily selected.

Further, although the embodiment shown in FIG. 1 shows an example in which the feedback m of the amplifier is changed, an attenuator may be changed in the stage before the amplifier.

Alternatively, the gain may be continuously increased in accordance with the delay time of the returned light.

(Effects of the Invention) As described above, according to the present invention, an optical fiber measuring instrument capable of measuring a wide range of measurement distances with high resolution can be realized with a simple configuration.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing an embodiment of the optical fiber measuring device according to the present invention, and the second factor is a characteristic curve diagram for explaining the operation of the device shown in FIG. 2... Fiber under test, 4... Signal processing means, 11
. . . light source, 13 . . . light receiving means, 32 . . . amplifying means.

Claims (1)

[Claims] (1) In an optical fiber measuring instrument that observes the state of the fiber under test by inputting light from a light source into the fiber under test and detecting the return light from the fiber under test with a light receiving means, the detection signal from the light receiving means is detected. An optical fiber measuring instrument comprising: amplification means that changes the degree of amplification in accordance with the delay time of input and returned light; and signal processing means that inputs the output of the amplification means.