JPH0354775B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a calibrator for an optical fiber tester (OTDR) used for detecting the propagation characteristics and fault location of a communication system using optical fiber as a transmission medium.

Optical fiber is significantly smaller, lighter, lower priced, and has a higher communication capacity than conventional metal cables such as copper wire, and is also highly flexible. ) optical fiber communication systems for use with optical fibers have recently become rapidly popular. In addition to the above points, optical fiber cables have the advantage that they have less transmission loss and are less susceptible to external electromagnetic fields than metal cables. An OTDR was recently developed to evaluate this optical fiber. OTDR is an optical fiber tester that measures the transmission loss characteristics of optical fiber, detects failures or abnormalities in optical fiber communication systems, and determines their locations. OTDR
To quantitatively measure transmission loss characteristics using
It is necessary that its gain linearity be accurately calibrated. Measuring the transmission loss characteristics of very long optical fibers requires that the OTDR's gain linearity be calibrated over the entire range of input signal levels. Calibrating a wide range OTDR requires the use of extremely long fibers, which is not only very expensive but also impractical. A highly accurate measurement of the time axis, ie the distance of the fibers, is likewise required.

[Purpose of the invention]

The present invention was made in view of the above-mentioned circumstances, and uses a relatively short optical fiber.
The purpose is to easily and quickly perform wide range gain and time calibration of OTDR.

[Summary of the invention]

By connecting a loop of optical fiber with known transmission characteristics to the optical fiber input terminal of the OTDR through a directional coupler, the light from the OTDR can be circulated within this loop to determine its attenuation characteristics and loop length obtained in the OTDR. and the gain due to
This makes it possible to calibrate linearity and time or distance with high precision.

The present invention eliminates the need for a calibration device that includes an extremely long optical fiber, and enables compact calibration of gain and time linearity over the entire range of effective input signal levels of an OTDR.

[OTDR]

Prior to explaining the present invention, an overview of a conventional OTDR will be explained with reference to FIG. Optical pulse generator 1
0 generates a constant periodic infrared light pulse train in response to electrical pulses from a delay circuit 12 operating under the control of a processor 14. This pulse train is transmitted via optical fiber 16 to one port of a three-port directional coupler 18 which directs the optical pulses (i.e., the incident pulses) to an optical output connector 20 from which Although not shown, the signal is also transmitted to the optical fiber under test. Optical connector 20 and coupler 18 reflect a portion of the incident light beam, and the reflected light pulses are transmitted to input stage 22, which includes a photodetector, preamplifier, and logarithmic amplifier.

During each operating cycle of the OTDR, an incident optical pulse enters one end of the optical fiber under test, travels in one direction (hereinafter referred to as the forward direction) within the fiber, and
The light is reflected in the opposite direction (hereinafter referred to as "reverse direction") at a discontinuity point such as a defect or the other end of the fiber in the transmission path. When incident light travels within the fiber, a weak optical signal travels in the opposite direction due to Rayleigh scattering. This backscattered (BS) signal decreases exponentially with the traveling distance of the traveling pulse. Using this BS signal, we will measure the transmission loss characteristics of the optical fiber using OTDR.

The reflected pulse and the BS signal (hereinafter collectively referred to as reflected signals) are transmitted in the opposite direction to the output connector 20 and led to the input stage 22 by the directional coupler 18, where the reflected signals are converted into electrical signals and converted into electrical signals. is amplified logarithmically. This logarithmically compressed signal is applied to a sampler 24 that includes an analog-to-digital (A-D) converter. The sampler 24 converts the instantaneous amplitude of one point of the signal determined by the delay circuit 12 operating under the control of the processor 14 into a digital signal. This digital signal is accumulated and stored in a display memory 26 that includes a display controller. In the next cycle of the OTDR, the instantaneous amplitude of the logarithmically compressed signal at a later point is digitally converted and similarly stored. The above operations are repeated to obtain the number of samples required to reproduce the logarithmically compressed signal.

Next, the data stored in the display memory 26 is continuously read out by the display controller, converted into an analog signal, and applied to the vertical deflection amplifier of the deflection amplifier circuit 28. In addition, in response to this, the display controller generates a horizontal deflection ramp wave and a blanking (Z-axis) signal for return beam blanking. These signals are also supplied to the deflection amplifier circuit 28. The deflection amplifier circuit 28 outputs the vertical and horizontal deflection signals and the Z-axis (blanking) signal to a cathode ray tube (CRT).
30 to display the reflected wave signal waveform on its screen.

Figure 2 shows a defect-free optical fiber.
An example of the display on a CRT screen when tested with an OTDR is shown. The vertical and horizontal axes represent signal amplitude and time, respectively. Reference symbols A, B, and C represent the coupler 1 of the incident pulse generated by the optical pulse generator 10, respectively.
8 and the first reflected pulse (due to partial reflection at connector 20), the BS signal, and the reflected pulse produced at the other end of the fiber cable. Since the interval from the emission of the incident pulse to the first reflected pulse can be ignored in this case, the first reflected pulse is assumed to be practically the same as the incident pulse. The amplitude of the first reflected pulse, represented by pulse A, is significantly smaller than the amplitude of the incident optical pulse itself since it is only a partial reflection within coupler 18 and connector 20. As mentioned above, the BS signal decays exponentially with time (or the traveling distance of the traveling pulse), but the electrical signal is logarithmically amplified by the logarithmic amplifier in the input stage 22 to simplify the reading of the degree of attenuation and to reduce the dynamism. Since we are aiming to expand the range, the BS signal trace slope is approximately linear as shown, and the vertical scale of the CRT is in dB (decibel) units. Attenuation (dB) between any two points of the waveform on the CRT screen
can be read directly from the vertical scale between the two points.

If the fiber transmission loss of the optical fiber under test is L (dB), then the vertical distance between both ends of the inclined trace B is 2L because the reflected wave is reciprocating in the optical fiber. If the vertical distance between the ends of trace B, as measured on the CRT screen, is not 2L, the gain of the vertical deflection amplifier must be adjusted to compensate for the error.

The horizontal axis of the CRT scale is in units of time. The propagation times of the incident and reflected pulses C are proportional to the length of the optical fiber. If the propagation velocity and optical fiber length are known, the propagation time can be determined by calculation. If the one-way propagation time of the optical fiber is T, then the time between the incident pulse (the leading edge of the first reflected pulse A) and the leading edge of the reflected pulse C is 2T. Measure the horizontal distance between the leading edges of these two pulses A and C on the CRT screen to be 2T.
If not, the horizontal deflection amplifier must be adjusted to correct the error.

Tilt trace B displayed on CRT screen
The linearity of depends on several factors, including the gain linearity of the logarithmic amplifier in input stage 22, the time linearity of the horizontal amplifier in deflection circuit 28, and the uniformity of the backscattering strength along the optical fiber cable.
When non-linearity occurs in the displayed waveform, it is difficult to know which of these factors is due to the non-linearity. Calibration of gain and time linearity is not easy.
If the time linearity is correct and the scattering is uniform over the length of the fiber cable, the gain linearity of the logarithmic amplifier can be calibrated. However, it has been found that the optical fiber must be extremely long to calibrate gain linearity over the full range of signal levels.

〔Example〕

Next, referring to FIG. 3, an OTDR according to the present invention will be described.
An example of a calibrator for use will be described. Calibrator 34 includes optical fibers 36 and 38 and a three-port directional coupler 40. Connect one end of the optical fiber 36 to the output connector 20 of the OTDR 32,
The other end is connected to the first port of the directional coupler 40. Both ends of optical fiber 38 are connected to second and third ports of directional coupler 40, respectively.

When the incident pulse passes through the coupler 40, a reflected pulse (hereinafter referred to as the second reflected pulse) is generated by a discontinuity within the coupler and is transmitted through the fiber 36.
Coming back to OTDR32. Directional coupler 40 allows the incident pulse from fiber 36 to pass only to the second port of coupler 40. As the incident pulse travels in the forward direction through fiber 38, a backscattered signal is generated. This backscattered signal is transmitted to the OTDR 3 through the fiber 36 within the coupler 40.
2, and an indirect BS signal that is coupled into the fiber 38 through the third port, passes through the fiber 38 in the opposite direction, and is partially transmitted into the fiber 36 by the coupler. . When the incident pulse reaches the third port of coupler 40,
It is coupled back into fiber 38 via a second port. At the same time, two reflected pulses result from the discontinuity in coupler 40. One of the reflected pulses, called the direct pulse, passes through the coupler 40 and fiber 36 without passing through the fiber 38 to the OTDR 32.
transmitted directly to. Another reflected pulse, called an indirect pulse, passes through fiber 38 in the opposite direction and is transmitted to OTDR 32.

As the incident pulse 38 circulates within the fiber 38, a backscattered signal is constantly generated and both reflected pulses (hereinafter referred to as subsequent reflected pulses) are generated each time they pass through the coupler 40. The incident pulse is coupler 40
It loses energy each time it passes through. Furthermore, since the incident pulse is continually attenuated, the power of subsequent reflected pulses and backscattered signals will be attenuated as well.

FIG. 4 shows an example of the reflected signal waveform displayed on the CRT screen when the calibrator 34 is connected to the OTDR 32. Pulse A represents the first reflected pulse (substantially simultaneous with the emission of the incident pulse), pulses C ₁ -Cn represent the second and subsequent reflected pulses generated by coupler 40, and slope traces B ₁ -Bn represent the backscattered signals. .

If the propagation time in the optical fiber 36 is T
Then, the second reflected pulse C ₁ is detected at time 2T. Assuming that the time for a light pulse to go around the optical fiber 38 once (loop delay) is P, and the propagation time within the coupler 40 is negligible compared to T and P, the first direct pulse is the first direct pulse at time 0. It is detected at time 2T+P after the detection of the reflected pulse. Also, each subsequent direct pulse is detected at a time interval P. The first indirect pulse is detected at 2T+2P (ie simultaneously with the second direct pulse), and subsequent indirect pulses are detected at successive time intervals P. Thus, the second and each subsequent direct pulse is detected simultaneously with the first and each subsequent indirect pulse. Pulse C ₂ shown in FIG. 4 is the first direct pulse, and each subsequent pulse C ₃ to Cn is a direct and indirect pulse detected simultaneously.

From the detection of the first reflected pulse A to the second reflected pulse
During the period up to C ₁ , OTDR 32 receives backscattered signals generated as the incident pulse passes through fiber 36 , as shown by trace B ₁ . Pulse C ₁ and
The OTDR connects the incoming pulse to the fiber 38 between _C2
receives the backscattered signal generated by passing through the At the time of detection of the first direct pulse (2T+P), the detected backscattered signal is transmitted from the OTDR 32 to V(2T+P).
is a direct BS signal generated at a distance of P)/2.
where V is the propagation velocity of the optical pulse in fibers 36 and 38. As the incident pulse passes through the fiber 38, the second direct backscatter signal is superimposed on the first pass of the incident pulse through the fiber 38. The incident pulse strikes the fiber 38 at 2
It also receives indirect backscattered signals during round transmission. Therefore, the slope trace B ₂ is the backscattered signal produced during the first pass of the incident pulse through the middle of the fiber 38, and the subsequent traces B ₃ to Bn
is the sum of the two direct backscattered signals caused by two consecutive rotations of the incident pulse and the indirect backscattered signal caused by the previous rotation of the incident pulse.

The horizontal distance between the leading edges of the series of reflected pulses C ₁ to Cn depends on the loop delay P, which is constant. The vertical distance between the ends of each inclined trace B3 _- Bn depends on the transmission loss Q of the optical fiber 38, which is also constant. The vertical distance between the bottom of each slope trace B3 _- Bn and the top of the immediately following reflected pulse depends on the reflection height R of the directional coupler 40, which is also constant. The vertical distance between adjacent pulses _C3 to Cn is also constant. Finally, coupler 40
The step loss S within is constant and is expressed as the vertical distance between the ends of adjacent traces _B3 to Bn. These relationships allow the OTDR 32 to be properly calibrated using the calibrator 34.

To calibrate the OTDR 32, the vertical amplifier is used to ensure that the vertical distance across each sloped trace _B3 to Bn, measured in dB, is equal to the known fiber loss Q of the optical fiber 38, and the horizontal amplifier is used to The time between the leading edges of pulses C ₁ to Cn is adjusted to be equal to the known propagation time P of optical fiber 38 . If the vertical distance between the ends of trace Bi, measured in dB, is not Q, even though the time between reflected pulse Ci−1 and the leading edge of Ci is P, then at the power level of trace Bi, Gain nonlinearity will exist.
Similarly, if the distance between the leading edges of pulses Ci−1 and Ci is not P even though the vertical distance between both ends of trace Bi is Q, then nonlinearity in the time axis exists. Become. Therefore, the calibrator of the present invention can independently recognize both gain nonlinearity and time nonlinearity. Gain linearity can be calibrated by adjusting the logarithmic amplifier, and time linearity can be calibrated by adjusting the delay of delay circuit 12 or the ramp generator of the display controller. Of course, linearity can also be calibrated using a calibration table. The reflection height R and the step loss S can also be used as an analyzer (analyzing device) of the characteristics of the directional coupler 40.

[Change transformation]

Although the present invention has been described above with reference to one preferred embodiment, it goes without saying that the present invention is not limited to this one embodiment. Various changes and modifications can be made without departing from the spirit of the invention. For example, it is not essential to use fiber loss Q to calibrate gain linearity. As noted above, other quantities that are constant may be used. In addition, one or more sets of optical fibers 36, 38 or directional couplers 40 are prepared and used selectively depending on the calibration range of the OTDR 32, but optical fibers of different lengths and losses can be selectively connected to a common coupler. It is also possible to use it as a calibrator.

〔Effect of the invention〕

According to the present invention, there are three ports, and the first port sends a signal only to the second port, the second port sends a signal to the first and third ports, and the third port sends a signal only to the second port. A directional coupler for transmitting a signal and a relatively short optical fiber with known characteristics connected between the second and third ports of the coupler constitute an optical fiber loop, and the optical pulse is circulated in this loop. . As a result, the sum of the direct reflected pulse of the nth circulating pulse of the optical pulse and a part of the indirect reflected pulse of the (n-1)th circulating pulse is transmitted through the first port.
returned to the OTDR, and in the period between each return pulse,
The sum of the direct backscattered signals of two successive circulating pulses and the indirect backscattered signals of the first of these circulating pulses is configured to return to the OTDR via the first port. As a result, reflected pulses and backscattered signals are returned that are attenuated at a constant rate determined by the propagation characteristics of the fiber at regular intervals determined by the length of the fiber, and these returned signals can be used to accurately calibrate the time and vertical axes of the OTDR. It can be used for Furthermore, since the test pulses circulate through the fiber loop, the short fiber has the same function as an infinite length fiber, and the entire range of the OTDR can be calibrated, yet miniaturization is extremely easy.

[Brief explanation of drawings]

Figure 1 is the block diagram of OTDR, Figure 2 is
A display example of an OTDR, FIG. 3 shows a diagram in which the calibrator of the present invention is connected to an OTDR, and FIG. 4 shows a diagram explaining the calibration of the OTDR calibrator. 18 and 40 are directional couplers, 20 is an optical connector, and 36 and 38 are optical fibers.

Claims (1)

[Claims] 1. An optical fiber tester for calibrating an optical fiber tester that supplies a test light pulse to one end of an optical fiber and detects a reflected pulse returned to one end of the optical fiber to test the characteristics of the optical fiber. A calibrator for a fiber tester, comprising: a first optical fiber having one end connected to the output end of the test light pulse of the optical fiber tester; and a first port connected to the other end of the first optical fiber; The signal received at the first port is outputted only to the second port, the signal received at the second port is divided and outputted to the first and third ports, and the signal received at the third port is outputted to the first and third ports. A directional coupler outputting to a second port; and a second optical fiber connected between the second and third ports of the directional coupler and having known length and signal propagation characteristics. Calibrator for optical fiber testers.