JPH11174267A - Nonlinear digital filter for optical pulse tester - Google Patents

Abstract

To provide a nonlinear digital filter for an optical pulse tester for reducing noise superimposed on an OTDR waveform. SOLUTION: A light pulse is incident on an optical fiber connecting a near end and a far end from the near end, and the light intensity of backscattered light consisting of Rayleigh scattering and Fresnel reflection generated in the optical fiber is detected. Is a non-linear digital filter for use in an optical pulse tester that discretizes the signal on the time axis and outputs it. The peak level of the internal noise of the optical pulse tester observed after the fiber end and the backscattered light at the position where the noise is reduced Ε calculating means for calculating a threshold ε of the amplitude of the ε-separated nonlinear digital filter from the difference from the level,
And an ε-separated nonlinear digital filter constituted by the ε value calculated by the ε calculation means, which receives the output signal of the optical pulse tester as an input, removes noise components from the signal, and outputs the signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical pulse tester.
The present invention relates to a technique for reducing noise of an OTDR waveform obtained by an (OTDR: Optical Time Domain Reflectometer), and particularly to a configuration of a nonlinear digital filter therefor.

[0002]

2. Description of the Related Art FIG. 1 is a diagram showing an example of an OTDR waveform. The OTDR 1 detects a connector connection point 3, a fusion splicing point 4, and a fiber end 5 on the optical fiber 2. OTD
The R waveform is superimposed on a linear Rayleigh scattering waveform indicating optical fiber loss, and a step corresponding to the fusion splicing point and a rectangular Fresnel reflection waveform corresponding to the connector splicing point and the fiber end. . On the OTDR waveform, thermal noise generated in the OTDR receiving section is superimposed as noise.

[0003] By analyzing such OTDR waveforms, it is possible to determine the position, loss, reflection amount, optical fiber section loss, and the like of the fusion splicing point and the connector connection point.
In addition, if a normal OTDR waveform is stored in advance, a failure location can be easily detected by comparing the abnormal OTDR waveform when the abnormality has occurred.

[0004]

However, in such measurement using OTDR, as the measurement distance becomes longer, the noise width superimposed on the OTDR waveform becomes larger toward the far end, so that the accuracy of measurement decreases. There is a problem. Also, a method of expanding the pulse width of the OTDR,
Although the noise width to be superimposed on the OTDR waveform can be reduced by a method of increasing the averaging time or the like, there are problems such as a decrease in distance resolution and an increase in the time required for measurement. Further, a method of processing the measured OTDR waveform on software, for example, a method of taking a moving average in a distance direction (Shigeo Minami "Waveform data processing for scientific measurement", CQ Publishing Company, 1986) can be considered.
Such a method has a disadvantage that the waveforms of the fusion splicing portion and the connector connecting portion are distorted, and there is a problem that accurate measurement is difficult.

[0005] The present invention has been made in view of the above circumstances, and has as its object to provide a nonlinear digital filter for an optical pulse tester for reducing noise superimposed on an OTDR waveform.

[0006]

SUMMARY OF THE INVENTION In order to achieve the above object, a nonlinear digital filter for an optical pulse tester according to the present invention has a function of inputting an optical pulse from a near end to an optical fiber connecting a near end and a far end. A non-linear digital filter for use in an optical pulse tester that detects the light intensity of backscattered light consisting of Rayleigh scattering and Fresnel reflection generated in an optical fiber and discretizes it on the time axis and outputs it Ε calculating means for calculating the threshold ε of the amplitude of the ε-separated nonlinear digital filter from the difference between the peak level of the internal noise of the optical pulse tester observed after the end and the backscattered light level at the position where the noise is reduced. , And ε
Ε constituted by the ε value calculated by the calculation means
-It has a separation nonlinear digital filter, receives an output signal of the optical pulse tester, removes noise components from the signal, and outputs the signal.

First, the principle of noise removal using an ε-separated nonlinear digital filter (hereinafter referred to as “ε-filter”) will be described. For the ε-filter itself, see, for example, the paper “Statistical Analysis of ε-Separable Nonlinear Digital Filters” by Arakawa, Harashima, and Miyakawa (IEICE (A), J66-A, 1, pp3).
2-39, 1983).

Generally, in a non-recursive linear low-pass digital filter, an input signal in which a relatively high-amplitude signal component is added to a relatively large-amplitude high-frequency noise is x _n
When, output y _n is

(Equation 1) It is represented by In the equation (1), a _k is a coefficient of a non-recursive linear low-pass digital filter.

(Equation 2) It is determined to be. F (x) is

(Equation 3) Is a nonlinear function whose absolute value is limited to ε or less. Therefore, equation (1) gives the difference x between the input signal and the output signal.
It indicates that the _n -y _n is for smoothing the signal while suppressing within ± epsilon.

A feature of the ε-filter is that it removes high-frequency noise components having a small amplitude less than or equal to the amplitude ε contained in the observed signal without damaging the edge with respect to a steep change in the observed signal. Is to be able to do it. That is, the present invention processes the output of the OTDR using an ε-filter so that the linearized Rayleigh scattering waveform can be obtained without impairing the step caused by fusion splicing and the characteristics of connector connection and Fresnel reflection from the fiber end. The focus was on reducing the noise superimposed on the image.

The most important factor in using an ε-filter to reduce the noise of the OTDR waveform is the coefficient ε. Coefficient ε
If is too large, the waveform at the connection is distorted and accurate measurement cannot be performed. Conversely, if the coefficient ε is too small, the effect of noise reduction cannot be expected. The noise width to be superimposed on the OTDR waveform is ±
Assuming that x ′ _n and the peak level of the noise inside the OTDR observed after the fiber end are x _peak , the relationship between the two is

(Equation 4) (See Nakamura, Tomita, Takasugi, and Sano, "Automatic Analysis Method for Optical Pulse Test Waveform Data," IEICE Spring Spring 1990, B-899.)

[0011] x _n of F (x) of equation (1) by using x _'n obtained in Equation (4), from equation (3), a width of less proximal direction noise superimposed in Value ε is small,
Conversely, in the far end direction where the width of the superimposed noise is large, ε takes a large value. That is, by adaptively changing ε according to the backscattered light level, it is possible to effectively smooth the optical fiber section while minimizing the waveform distortion at the connection point. In the present invention, this ε
From the difference between the peak level of the internal noise of the optical pulse tester observed after the end of the fiber and the backscattered light level at the position where noise reduction is performed. .

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, as an embodiment of the present invention, an OTDR using an ε-filter realized on software will be described.
An example in which the noise reduction processing has been performed will be described. The OTDR measurement conditions are as follows: pulse width is 500 ns, reading resolution is 1 m,
The number of averaging is 2 to the 15th power, and the number of measurement points is 5,000. The noise peak level x _peak is 28 dB, the widths N and M for smoothing are 5 points, and the nonlinear function F
The function shown in FIG. 2 was used as (x). From the difference between the peak level of the internal noise of the optical pulse tester observed after the end of the fiber and the backscattered light level at the position where noise reduction is performed, ε is calculated by the ε calculation means using the equations (4) and (3).
Was calculated.

FIG. 3 shows a comparative example when the noise reduction processing is not performed, and FIG. 4 shows a measurement result when the noise reduction processing according to the present invention is performed. That is, in this case, since the reading resolution is 1 m, one point corresponds to 1 m. Also,
The lack of data at the near end is due to masking to eliminate the effects of reflections occurring at the OTDR and the optical fiber connection. As is clear from the comparison between the two figures, it can be seen that the nonlinear digital filter for an optical pulse tester of the present invention can reduce fine noise superimposed on a linear Rayleigh scattering waveform. Also, it can be seen that there is almost no waveform distortion generated at the fusion connection point and at the Fresnel reflection portion at the connector connection point.

In the above embodiment, the ε-filter realized on software is used, but it is of course possible to configure this in hardware during OTDR. Furthermore, OT
GP-1B or RS-232C between DR and PC
The OTDR waveform can be transferred to a personal computer, and processed using an ε-filter configured by software on the personal computer.

[0015]

As described above, according to the present invention,
Using an ε-separated nonlinear digital filter, an amplitude threshold ε constituting the ε-separated nonlinear digital filter
Can be reduced from the difference between the peak level of the internal noise of the optical pulse tester observed after the fiber end and the backscattered light level at the position where noise reduction is performed, thereby reducing the noise superimposed on the OTDR waveform. it can.

[Brief description of the drawings]

FIG. 1 is a diagram showing a waveform example of OTDR.

FIG. 2 is a diagram illustrating a non-linear function in an embodiment of the present invention.

FIG. 3 is a diagram illustrating a measurement example when noise reduction processing is not performed.

FIG. 4 is a diagram illustrating a measurement example when the noise reduction processing of the present invention is performed.

[Explanation of symbols]

 1 OTDR 2 Optical fiber 3 Connector connection point 4 Fusion connection point 5 Fiber end

Claims (1)

[Claims] An optical pulse is incident on an optical fiber connecting the near end and the far end from the near end, and the light intensity of backscattered light generated in the optical fiber, which is composed of Rayleigh scattering and Fresnel reflection, is detected. In the nonlinear digital filter used to reduce the noise used in the optical pulse tester, which outputs the discretized data on the time axis, the peak level of the internal noise of the optical pulse tester observed after the end of the fiber and the position at which the noise is reduced Calculation means for calculating the threshold value ε of the amplitude of the ε-separated nonlinear digital filter from the difference with the backscattered light level of the ε-separated nonlinear digital filter; A nonlinear signal for an optical pulse tester, comprising a filter, receiving an output signal of the optical pulse tester as an input, removing a noise component from the signal, and outputting the signal. Digital filter.