JPS61129549A - Calibrator for optical fiber tester - Google Patents

Abstract

PURPOSE:To calibrate easily and speedily a gain and time over a wide range by connecting an optical fiber which is looped and has known characteristics to the input terminal of a tester through a directional coupler. CONSTITUTION:A calibrating device 34 includes optical fibers 36 and 38 and a three-port directional coupler 40. One terminal of the optical fiber 36 is connected to the connector of the optical fiber tester (OTDR)32 and the other terminal is connected to the 1st port of the directional coupler 40. Both terminals of the optical fiber 38, on the other hand, are connected to the 2nd and the 3rd ports of the directional coupler 40. Then, light from the OTDR is circulated in this loop to constitute a substantially infinite-length optical transmission line by using only the short optical fibers and a directional coupler and know quantities (reflection, attenuation, and delay time) obtained by such constitution are used as standard values, so the calibrating device which is small in size on the whole and calibrates both the gain and time of the OTDR simultaneously with high precision over a wide range is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a calibrator for optical fiber test W (referred to as OTDR) used for detecting propagation characteristics, fault locations, etc. of a communication system using optical fiber as a transmission medium.

Compared to conventional metal cables such as copper wire, optical fibers are significantly smaller, lighter, lower priced, and have a higher communication capacity, and are highly flexible.
Optical fiber communication systems for use with light emitting diodes (CLED) and light emitting diodes (CLED) 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. 0TDR was recently developed to evaluate this optical fiber.

0TDR 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. To quantitatively measure transmission loss characteristics using 0TDR, it is necessary that its gain linearity be accurately calibrated. It must be calibrated over an extremely long range. Calibration of a wide range 0TDR requires the use of extremely long fibers, which is not only very expensive but also impractical. Similarly, highly accurate measurements of the time axis, ie the distance of the fiber, are also required.

[Purpose of the invention]

The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to easily and quickly calibrate 0TDR wide and high gain and time by fully using relatively short optical fibers.

[Summary of the invention]

By connecting a loop of optical fiber with known transmission characteristics to the optical fiber input terminal of the 0TDR through a directional coupler, the light from the 0TDR is circulated within this loop, and the attenuation characteristics and loop length obtained in the 0TDR are determined. This makes it possible to calibrate gain, linearity, and time or distance with high precision.

The present invention eliminates the need for a calibration device that includes extremely long optical fibers, and allows compact calibration of gain and time linearity over the entire range of effective input signal levels of 0TDR.

C0TDR) Prior to explaining the present invention, an overview of the conventional 0TDR will be explained with reference to FIG. Optical pulse generator 10 generates a constant periodic infrared light pulse train in response to electrical pulses from delay circuit 12 operating under the control of processor 14 .

This pulse train is transmitted via an optical fiber 16 to one port of a directional cube 2 (coupler) 18 having three ports, and this coupler 18 receives the optical pulse (i.e., the incident pulse).
is guided to the optical output connector 20, from where it is transmitted to the optical fiber under test (not shown). Optical connector 20
and coupler 18 reflect a portion of the incident light beam, and the reflected light pulses are transmitted to an input stage 22 that includes a photodetector, a preamplifier, and a logarithmic amplifier.

During each operation cycle of the 0TDR, 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 detects defects in the transmission path of this fiber or defects such as the other end. It is reflected in the opposite direction (hereinafter referred to as the opposite direction) at consecutive points. When incident light travels within the fiber, a weak optical signal travels in the opposite direction due to Rayleigh scattering. This backscatter (BS)
The signal decreases exponentially with the distance traveled by the traveling pulse. Using this BS signal, the transmission loss characteristics of the optical fiber by 0TDR are measured.

This 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 an analog-to-digital (A-D) converter 24. 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 transmitted to the display memory 2 including the display controller.
Accumulate and store in 6. In the next cycle of 0TDR, 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 controls the ramp wave for horizontal deflection and the blanking (Z-axis) for return beam blanking.
Generate a signal.

These signals are also supplied to the deflection amplifier circuit 28.

The deflection amplifier circuit 28 supplies vertical and horizontal deflection signals and a two-axis (blanking) signal to a cathode ray tube (CRT) 30 to display the reflected wave signal waveform on its screen.

FIG. 2 shows an example of a display on a CRT screen when a defect-free optical fiber is tested at 0 TDR. The vertical and horizontal axes represent signal amplitude and time, respectively. Reference sign A
, B and C are the first reflected pulse (due to the partial reflection of the incident pulse generated by the optical pulse generator 10 at the turnip 518 and the connector 20), the BS signal, and the first reflected pulse produced at the other end of the fiber cable, respectively. is 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 has a time (
However, the electrical signal is amplified logarithmically by the logarithmic amplifier in the input stage 22 to make it easy to read the degree of attenuation and to expand the dynamic range. The trace slope of the BS signal is approximately linear as shown, and the vertical scale of the CRT is d
The unit is B (decibel). The degree of attenuation (dB) between any two points of the waveform on the CRT screen can be directly read from the vertical scale between the two points.

If the transmission loss of the optical fiber under test is L (dB), then the vertical distance between both ends of the slope trace B is Ji!, since the reflected wave is traveling back and forth in the optical fiber. It becomes rL. If the vertical distance between the ends of trace B, measured on the CRT screen, is not -fL, then
It is necessary to adjust the gain of the vertical deflection amplifier to correct the error.

The horizontal axis of the CRT scale is in units of time. Incident No. 1
The propagation time of the Rust reflected pulse C can be determined by calculation if the length of the optical fiber is known. 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 .The leading edge of both pulses A, !:C is If the horizontal distance on the CRT screen is not @T, the horizontal deflection amplifier must be adjusted to correct the error.

The linearity of the slope trace B displayed on the CRT screen depends on the gain linearity of the logarithmic amplifier in the input stage 22 and the deflection circuit 28.
depends on several factors, including the time linearity of the horizontal amplifier, and the uniformity of the backscatter intensity 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 turns out that the optical fiber must be extremely long to calibrate gain linearity over the full range of signal levels.

〔Example〕

Next, an embodiment of the 0TDR calibrator according to the present invention will be described with reference to FIG. Calibrator 34 includes optical fibers 36 and 38 and a three-port directional coupler 40. One end of the optical fiber 36 is connected to the output connector 20 of the 0TDR 32, and the other end is connected to the first end of the directional coupler 40.
Connect to a port.

Directional couplers 40 connect both ends of the optical fiber 38 to each other.
Connect to the second and third boats of

As the incident pulse 40 passes through the coupler 40, a reflected pulse (hereinafter referred to as a second reflected pulse) is generated by a discontinuity in the coupler and returns to the 0TDR 32 via the fiber 36. Directional coupler 40 allows only the incident pulse from fiber 36 to pass to the second port of coupler 40. As the incident signal travels forward through fiber 38, a backscattered signal is produced. This backscattered signal is coupled within coupler 40 to the direct BS signal transmitted via fiber 36 to 0TDR 32 and the third boat. via fiber 3
8, passes through fiber 38 in the opposite direction, and is split by a coupler into an indirect BS signal which is partially transmitted into fiber 36. When the incident pulse reaches the third port of coupler 40, it is coupled back into fiber 38 through the second port. At the same time, two reflected pulses result from the discontinuity in coupler 40. One reflected pulse, called the direct pulse, is transmitted directly to 0TDR 32 through coupler 40 and fiber 36 without passing through fiber 38.

Another reflected pulse, called an indirect pulse, passes through fiber 38 in the opposite direction and is transmitted to 0TDR 32.

As the incident pulse 38 circulates within the fiber 38, a backscattered signal is constantly generated and a double reflected pulse (hereinafter referred to as a subsequent reflected pulse) is generated each time it passes through the coupler 40.

The incident pulse loses energy each time it passes through coupler 40. Furthermore, since the incident pulse is continually attenuated, the power of subsequent reflected pulses and backscattered signals will be attenuated as well.

Figure 4 shows C when the calibrator 34 is connected to 0TDR32.
An example of a reflected signal waveform displayed on the RT screen is shown.

Pulse A is the first reflected pulse (substantially simultaneous with the emission of the incident pulse), and pulses C7 to Cn are the second reflected pulse generated by coupler 40.
The reflected pulse and the subsequent reflected pulse, also the slope trace B,
thru Bn indicate backscattered signals.

If the propagation time in the optical fiber 36 is T, the second reflected pulse C1 is detected at time 2T. Assuming that the time for the optical pulse to go around the optical fiber 38 once (loop delay) is tP, and the propagation time within the coupler 40 is negligible compared to T and P, the first direct pulse is the first reflection at time 0. It is detected at time 2T+P after the pulse is detected. 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 the subsequent indirect pulse is detected at the front corner P for a stubborn time period. Thus, the second and each subsequent direct pulse is detected simultaneously with the first and each subsequent indirect pulse.

Pulse C2 shown in FIG. 4 is the first direct pulse, and each subsequent pulse C3 to Cn is a direct and indirect pulse detected simultaneously.

During the period from detection of the first direct pulse A to the second reflected pulse C1, the 0TDR 32 receives backscattered signals generated as the incident pulse passes through the fiber 36, as shown by trace B1. Between pulses CI and C2, 0TDR receives a backscattered signal caused by the passage of the incident pulse through fiber 38. Detection time of first direct pulse (2T+P)
, the detected backscattered signal is from 0TDR32 to V(2
The direct BS signal occurs at a distance of T+P)/2.

Here, ■ is the propagation velocity of the optical pulse in the 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.

An indirect backscattered signal is also received when the incident pulse is transmitted through the fiber 38 for the second time. Therefore, slope trace B2
is the backscattered signal generated when the incident pulse passes through the middle of the fiber 38 for the first time, and the subsequent traces B to Bn are the two direct backscattered signals generated by the incident pulse that rotated twice continuously, and the incident pulse is the sum of the indirect backscatter signal caused by the previous rotation of .

The horizontal distance between the leading edges of the series of reflected pulses C1 to Cn depends on the loop delay P, which is constant. Each slope trace B,
The vertical distance between both ends of the optical fiber 38
depends on the transmission loss Q, which is also constant. The vertical distance between the lower end of each sloped trace B3-Bn and the upper end 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 4
The step loss S within 0 is constant and is expressed as the vertical distance between the ends of adjacent traces B3 to Bn. These relationships allow the 0TDR 32 to be correctly calibrated using the calibrator 34.

When calibrating the 0TDR 32, the vertical amplifier calculates the known fiber loss Q of the optical fiber 38 by measuring the vertical distance between the ends of each sloped trace B3 through Bn in dB years.
and the horizontal amplifier is equal to successive pulses C
The time between the leading edges of 1 to Cn is adjusted to be equal to the known propagation time P of the optical fiber 38. if,
The vertical distance between the ends of trace Bi, measured in dB years, is
If the time between the reflected pulse C1-1 and the leading edge of Ci is P, but not Q, then there will be gain nonlinearity in the power level of trace Bi. Similarly,
If the distance between the pulse C1-1 and the leading edge of Ci is not P even though the vertical distance between both ends of the trace Bi is Q, non-linearity of the time axis exists.

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 calibrator. The reflection height R and the step loss S can also be used as an analyzer (analysis device) of the characteristics of the directional force grapher 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 force graphers 40 are prepared and used selectively according to the calibration range of 0TDR32, but optical fibers of different lengths and losses are 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, a substantially infinite length optical transmission line is constructed simply by using a sufficiently short optical fiber and a directional coupler, and the known quantities (reflection, attenuation, delay time) obtained from this are used as standards. , the entire calibrator can be extremely miniaturized, and a calibrator can be obtained which can perform both the gain and time of 0TDR simultaneously and over the entire range with high precision. Furthermore, a remarkable practical effect is obtained in that the degree of linearity over the entire range can be easily determined.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of the 0TDR, FIG. 2 is a display example of the 0TDR, FIG. 3 is a diagram in which the calibrator of the present invention is connected to the 0TDR, and FIG. 4 is an explanatory diagram of the calibration of the 0TDR calibrator. 18.40 is a directional coupler, 2o is an optical connector, 36.
38 indicates an optical fiber.

Claims (1)

[Claims] A calibrator for optical fiber testers that connects a looped optical fiber with known characteristics to the input end of the optical fiber tester via a directional coupler.