JP2001099750A - Optical pulse tester - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical pulse tester having an optical detection means of accurate sensitivity. SOLUTION: An optical pulse tester is provided with a light emitting means 1 radiating an optical pulse to an optical fiber 7 to be measured, an optical coupler 2 for separating the back-scattered light from the optical fiber 7 to be measured, a light detection means 4 detecting the back-scattered light separated by the optical coupler 2, and a signal processing means 5 processing the detected signals. An incident end mounted on the optical coupler 2 is connected to the light emitting means 1 having known and stable light intensity, thereby the sensitivity of the light emitting means 4 can be measured directly and adjusted to the desired sensitivity.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to an optical pulse tester.

[0002]

2. Description of the Related Art An optical pulse tester (also known as an optical time domain reflectometer,
"OTDR" is used for the acronym of English name). The optical pulse tester applies a fault point search (referred to as TDR) by a pulse method, which has been conventionally used for a coaxial cable or a parallel pair cable, to optical fiber measurement.

A conventional optical pulse tester will be described below with reference to FIG.
This will be described with reference to FIG. FIG. 3 shows a configuration diagram of a conventional optical pulse tester. In the figure, a conventional light pulse tester includes a light emitting means 21, an optical coupler 22, an emission end 23, and a light detecting means 2.
4, the signal processing means 25 and the measured optical fiber 27.

Next, the operation will be described. A short-time light pulse is emitted into the measured optical fiber 27 by the light emitting means 21 of the light pulse tester. As the light pulse travels through the optical fiber to be measured, a part thereof is reflected by impurities or Brownian motion of molecules of the optical fiber to be measured and becomes Rayleigh scattered light. Part of the Rayleigh scattered light scattered omnidirectionally is confined in the optical fiber to be measured, travels in the optical fiber to be measured in a direction opposite to the traveling direction of the optical pulse, and returns to the optical pulse tester. This light is called "backscattered light". When an optical pulse reaches a discontinuous point (connection point, break point, end point, attenuation point, etc.) of the measured optical fiber, reflected light is generated,
A discontinuous decrease in Rayleigh scattered light occurs due to a decrease in light pulse intensity.

[0005] When the state of the optical fiber to be measured is constant, the backscattered light also decreases with the attenuation of the light pulse traveling through the optical fiber to be measured. From the attenuation rate of the backscattered light per unit time, the passage loss of the measured optical fiber at the point where the backscattered light is generated can be estimated. The state of the measured optical fiber at the discontinuous point can be estimated by observing the discontinuous change in the reflected light or the backscattered light intensity at the discontinuous point of the measured optical fiber.
An optical pulse tester is a measuring instrument that obtains the intensity of backscattered light and reflected light in time series starting from the emission of an optical pulse and can observe the state inside the measured optical fiber from one end of the measured optical fiber. is there.

[0006] Since the backscattered light is a part of the transmission loss of the optical fiber to be measured, its light intensity is very small. When an optical pulse having a pulse width of 1 microsecond, which is generally used as a condition for measuring a long-distance optical fiber with an optical pulse tester, is incident on a single-mode measured optical fiber, the backscattered light intensity is It is only a ratio of minus 50 to minus 60 dB of the pulse intensity. The optical pulse travels through the measured optical fiber and attenuates due to the passage loss of the measured optical fiber. The backscattered light returns to the pulse tester from the point of occurrence back through the measured optical fiber. The backscattered light generated after the light pulse has traveled to some extent in the measured optical fiber is greatly attenuated compared to the light intensity of the backscattered light generated in the measured optical fiber at a short distance.
The amount corresponds to the reciprocation of the optical pulse measuring instrument and the optical fiber to be measured to the backscattered light generation point. The transmission loss of the present single mode optical fiber is 0.35 dB / km in the 1.31 micrometer wavelength band, and the wavelength is 1.3.
It is about 0.2 dB / km in the 55-micrometer band. In order to observe optical fibers from several tens to hundreds of kilometers, the optical pulse tester
We must be able to observe very weak optical signals.

When an optical pulse tester measures a short-distance fiber with the far end of the optical fiber to be measured opened, the intensity of the emitted optical pulse is reflected by about minus 17 dB. The optical pulse tester must be able to detect this reflection without saturating it. Therefore, the light detecting means has a range switch and can change the sensitivity.
The optical pulse tester can obtain a wide range of measurement results by connecting the results of multiple measurements while switching the range.

An element for detecting a weak optical signal is an avalanche photodiode (hereinafter referred to as "APD").
This applies the electron avalanche phenomenon and can multiply the free electrons generated by a weak optical signal. An APD is used for the light-to-electricity conversion means in the light detection means of the light pulse test apparatus. In order to generate the avalanche phenomenon, it is necessary to apply a voltage near the breakdown voltage of the APD. In the case of InGaAs APD which is sensitive at the wavelength used for communication,
The applied voltage is around 70 volts.

The rate at which the APD multiplies the free electrons generated by the photoelectric effect by the electron avalanche phenomenon is called the multiplication degree (M is used for the symbol). Since this is a ratio, there is no unit. The optical pulse tester increases the multiplication factor when detecting weak backscattered light. When detecting strong reflected light, the multiplication factor is reduced. The multiplication factor increases rapidly just before the breakdown voltage of the APD. When the multiplication factor is increased, the voltage applied to the APD is set to a voltage slightly smaller than the breakdown voltage of the APD. That value is over 95% of the breakdown voltage of the APD. The breakdown voltage of APD has a negative temperature coefficient. When the applied voltage is constant, the degree of multiplication changes depending on the temperature of the APD, and greatly deviates from a desired value. APD
If the degree of multiplication is not the desired value, the above, when trying to obtain a wide range of measurement results by joining the measurement results, the magnification of the measurement values to be joined do not match, a step occurs at the seam The problem of poor connection occurs.

[0010] In the optical pulse tester, in order to keep the multiplication degree of the APD at a desired value, roughly two measures have been adopted.
One was to put the APD in a thermostat to stabilize the working temperature. The other is to measure the temperature in the vicinity of the APD in order to maintain the desired value, and to estimate the applied voltage for obtaining the desired multiplication factor by calculation. The estimated voltage was applied to the APD to indirectly adjust the multiplication factor. However, a thermometer requires an expensive measuring instrument. Further, since a large amount of power is used for temperature control, it cannot be applied to a portable device using a battery. In the temperature correction, a temperature difference easily occurs between the temperature detection unit and the APD. The temperature coefficient of the APD is not unique, and its value varies with the operating temperature.
Since the APD itself also varies from element to element, the multiplication factor cannot be kept constant with a unique correction coefficient. As a problem common to both, a power supply for applying a voltage to the APD must be stable. As the degree of multiplication is kept high, the applied voltage approaches the breakdown voltage, and it becomes difficult to keep the degree of multiplication constant.

[Problems to be solved by the invention]

However, in the conventional optical pulse tester, since the sensitivity of the light detecting means is not directly measured at the time of sensitivity adjustment, there is a problem that the sensitivity of the light detecting means deviates from a desired value. Therefore, there is a need for an optical pulse tester in which the sensitivity of the light detection means is accurate. An object of the present invention is to solve the above-mentioned problem and to provide an optical pulse tester with accurate sensitivity.

[0012]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, according to the present invention, the configuration of an optical pulse tester is as follows. 1. Light emitting means for emitting a light pulse to the optical fiber to be measured, an optical coupler for separating backscattered light from the optical fiber to be measured, and light detection having a multiplication effect for detecting the backscattered light separated from the optical coupler. Means, a signal processing means for processing the detection signal, in the optical pulse tester, the light intensity is known at the incident end of the optical coupler, and connected to a stable light emitting means, the sensitivity of the light detection means Until the desired sensitivity is obtained, the sensitivity is adjusted by repeatedly performing direct measurement while changing the degree of multiplication of the light detecting means (claim 1). 2. The light detecting means converts an optical signal from the optical coupler (2) into an electric signal, and a light detecting element (11) having a multiplication effect.
And an amplifier (13) for amplifying an electric signal from the photodetector (11) and outputting the amplified signal to the signal processing means 5, and a variable voltage source (12) for applying a voltage to the photodetector. (Claim 2); The light detecting element converts the converted electric signal into a variable voltage source.
Photodetector (11) that multiplies by the multiplication effect generated by the applied voltage of (12) and outputs the multiplied electric signal to amplifier (13)
(Claim 3). 4. The light detecting element is an avalanche photodiode (claim 4).

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the optical pulse tester according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a diagram showing a configuration of an embodiment of an optical pulse tester according to the present invention. In FIG. 1, the optical pulse tester comprises a light emitting means 1, an optical coupler 2, an emitting end 3, a light detecting means 4, a signal processing means 5, and a light emitting means 6, and tests an optical fiber 7 to be measured. It is.

Next, the operation of this embodiment will be described. All operations of the optical pulse tester of FIG. 1 are controlled by the signal processing means 5. The light emitting means 1 generates a light pulse based on a control signal from the signal processing means 5. Generally, the light emitting means 1 can set several values of the pulse width from 1 nanosecond to several tens of microseconds. A repetition period of several microseconds to several tens of milliseconds is used. The light pulse exiting the light emitting means 1 passes through the optical coupler 2 and is emitted from the emission end 3 to the optical fiber 7 to be measured. Backscattered light and reflected light generated by the measured optical fiber 7 are split by the optical coupler 2 through the emission end 3 into the light emitting means 1 side and the light detecting means 4 side. The backscattered light and reflected light output to the light emitting means 1 are reflected or absorbed by the light emitting means 1 and disappear. The backscattered light and reflected light output to the light detecting means 4 are subjected to light-to-electric conversion by the light detecting means 4, and then are electrically amplified and output to the signal processing means 5. The signal processing unit 5 discretely acquires the signal of the light detection unit 4 and holds the signal as a time-series measurement value. The data acquired at this time starts from the time when the light pulse is generated. Signal processing means 5
Has a function of processing, recording and displaying the stored time-series measurement values. The main processes of the signal processing means 5 are listed below. (A) Averaging with a measurement result measured a plurality of times to remove noise. (B) Logarithmic conversion for display. (C) The plurality of measurement results are joined.

Next, the operation of adjusting the sensitivity of the light detecting means 4 will be described. When the optical pulse tester starts measurement or switches the range, the signal processing unit 5 measures the sensitivity of the light detection unit 4 and adjusts the sensitivity. FIG. 2 shows the internal configuration of the light detecting means 4. In the following description, FIG. 2 will be used in conjunction with FIG. The APD 11 in FIG. 2 converts an optical signal from the optical coupler 2 into an electric signal. The APD 11 multiplies the converted electric signal by a multiplication effect generated by the voltage applied to the variable voltage source 12. The APD 11 outputs the multiplied electric signal to the amplifier 13. The amplifier 13 is an APD 11
And amplifies the electric signal from the signal processor and outputs it to the signal processing means 5.
The variable voltage source 12 outputs a voltage set by a control signal sent from, for example, a sensitivity setting unit (not shown) in the processing in the signal processing unit 5. The sensitivity setting means measures the output of the light detecting means 4 in the processing in the signal processing means 5, compares the sensitivity with the set sensitivity, and when the set sensitivity is reached, sets the sensitivity of the light pulse tester. Then, a control signal is sent to the variable voltage source 12. As a result, the output voltage of the variable voltage source 12 can be made variable.

The light emitting means 6 shown in FIG. 1 is a light source capable of outputting a light signal whose light intensity is known and stable. Signal processing means 5
Causes the light emitting means 6 to emit light and output an optical signal to the optical coupler 2. The optical signal output from the light emitting means 6 is divided and output by the optical coupler 2 to the light emitting means 1 side and the light detecting means 4 side. The optical signal output to the light emitting means 1 is reflected or absorbed and disappears. The optical signal output to the light detecting means 4 is optically-electrically converted by the light detecting means 4, and then electrically amplified and output to the signal processing means 5.

The light emitting means 6 has a known and stable light intensity. The passage loss in the path from the light emitting means 6 to the light detecting means 4 is also known and stable. Therefore, the signal processing means 5 can know the sensitivity of the light detecting means 4 by measuring the electric signal intensity of the light detecting means 4. The signal processing means 5
If the sensitivity of the light detecting means 4 is not a desired value, the output voltage of the variable voltage source 12 in FIG. 2 is changed, and the sensitivity of the light detecting means 4 is changed by changing the multiplication factor of the APD 11 in FIG. The signal processing unit 5 measures the output of the light detection unit 4 again to determine whether the sensitivity of the light detection unit 4 has been adjusted to a desired value. If the sensitivity of the light detection means 4 is not a desired value, the output voltage of the variable voltage source 12 in FIG.
The multiplication factor of the APD 11 in FIG. 2 is changed. The signal processing means 5 adjusts the sensitivity of the light detection means 4 to a desired value by repeating the above operation.

[0018]

According to the optical pulse tester of the present invention, a light emitting means for emitting an optical pulse to an optical fiber to be measured, an optical coupler for separating backscattered light from the optical fiber to be measured, and an optical coupler In a light pulse tester having a light detecting means having a multiplication effect of detecting more separated backscattered light and a signal processing means for processing a detection signal, a light intensity is known at an incident end of the optical coupler, and Connecting a stable light-emitting means, and repeatedly adjusting the sensitivity by directly measuring while changing the degree of multiplication of the light detection means until the sensitivity of the light detection means reaches a desired sensitivity; Means are
The optical signal from the optical coupler (2) is converted into an electric signal, and
A photodetector (11) having a multiplication effect, and an amplifier for amplifying an electric signal from the photodetector (11) and outputting the amplified signal to the signal processing means 5
(13), the photodetector applied voltage variable voltage source (12),
Wherein the light detection element multiplies the converted electric signal by a multiplication effect generated by a voltage applied to the variable voltage source (12), and outputs the multiplied electric signal to the amplifier (13). Since the element (11) is an avalanche photodiode, the sensitivity of the light detection means can be adjusted more finely until the sensitivity becomes a desired value.
An optical pulse tester that can obtain more accurate measurement results can be provided. In addition, since the backscattered light or reflected light output to the light detecting means is converted from light to electricity by the light detecting means, electrically amplified and output to the signal processing means, the main processing of the signal processing means is performed. The following processes (A), (B), and (C) can be executed. (A) Averaging with a measurement result measured a plurality of times to remove noise. (B) Logarithmic conversion for display. (C) The plurality of measurement results are joined. Furthermore, an optical pulse tester that can obtain a measurement result in which a step does not occur at the joint by the joining process (C) can be provided.

[Brief description of the drawings]

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

FIG. 2 is an internal configuration diagram of a light detecting unit in the optical pulse tester according to the embodiment of the present invention.

FIG. 3 is a configuration diagram of a conventional optical pulse tester.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1, 21 Light emitting means 2, 22 Optical coupler 3, 23 Outgoing end 4, 24 Light detecting means 5, 25 Signal processing means 6 Light emitting means 7, 27 Optical fiber to be measured 11 APD 12 Variable voltage source 13 Amplifier

Claims (4)

[Claims] 1. A light emitting means for emitting a light pulse to an optical fiber to be measured, an optical coupler for separating backscattered light from the optical fiber to be measured, and a multiplier for detecting the backscattered light separated from the optical coupler. An optical pulse tester having a light detecting means having an effect and a signal processing means for processing a detection signal, wherein a light emitting means having a known light intensity and a stable light emitting means is connected to an incident end of the optical coupler; An optical pulse tester, wherein the sensitivity is adjusted by repeatedly performing direct measurement while changing the multiplication factor of the light detecting means until the sensitivity of the detecting means reaches a desired sensitivity. 2. The photodetector converts an optical signal from an optical coupler (2) into an electric signal, and has a multiplication effect between a photodetector (11) and a photodetector (11). 2. The optical pulse tester according to claim 1, further comprising an amplifier for amplifying an electric signal and outputting the amplified signal to the signal processing means, and a variable voltage source for applying a voltage to the photodetector. . 3. A light detecting element for multiplying a converted electric signal by a multiplication effect generated by a voltage applied to a variable voltage source, and outputting the multiplied electric signal to an amplifier. 3. The optical pulse tester according to claim 2, wherein the optical pulse tester is a detection element. 4. The photodetector according to claim 3, wherein the photodetector is an avalanche photodiode.
An optical pulse tester according to any one of the above.