JPH1172756A - Light signal waveform deterioration compensation apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to compensate the waveform deterioration of the light signals caused by multi-stage transmission of optical filters by passing the light signals through the optical filters for compensation. SOLUTION: The optical filters 2 for demultiplexing the light signals of the respective wavelengths to respective transmission nodes are arranged in a wavelength multiplex transmission system connected in cascade with plural transmission nodes 1-1 to 1-n. In such a case, the optical filters 2-1 to 2-n having the transmission band characteristics to allow the transmission of the light signals of a central frequency (f) are eventually connected in cascade to n-stages. The effective transmission band characteristics by the n-stage transmission are narrowed in the flat transmission characteristics and, therefore, the optical filter 3 for compensation having such a transmission band characteristic as to convert the effective transmission band characteristic to a flat characteristic near the central frequency (f) is arranged behind the optical filter 2-n. As a result, the compensation of the waveform deterioration of the light signals caused by the multistage transmission of the optical filters is made possible without widening the transmission band characteristic of the optical filters.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength division multiplexing transmission system in which a plurality of transmission nodes are connected in cascade. The present invention relates to a signal waveform deterioration compensating device.

[0002]

2. Description of the Related Art A wavelength division multiplexing transmission system for multiplexing and transmitting optical signals of different wavelengths onto one optical fiber not only increases the transmission capacity per fiber, but also establishes a route at a transmission node using wavelengths. Has become possible.
In the path setting based on the wavelength, an optical filter is used to demultiplex and multiplex the optical signal of each wavelength. Therefore, if an optical signal passes through a plurality of transmission nodes, it passes through a plurality of optical filters.

Here, the transmission band characteristic of the optical filter needs to be set narrow enough to prevent the adjacent wavelength light from leaking, wider than the modulation spectrum of the transmitted optical signal, and even flat. This is because leakage of adjacent wavelength light deteriorates reception characteristics as crosstalk light, and a narrow transmission band blocks required high frequency components and causes waveform distortion.

[0004]

By the way, in a wavelength division multiplexing transmission system, even if each optical filter has a transmission band characteristic that does not cause crosstalk or waveform distortion, an optical signal passing through a plurality of stages of optical filters is not affected. It is known that the effective transmission band narrows as the number of stages increases. Therefore, when the number of stages of the optical filter increases, the effective transmission band becomes narrower than the modulation spectrum spread, and the signal waveform is distorted.

The above situation is shown in FIGS. here,
The 3dB transmission bandwidth of each optical filter is
3.5 times, and a case having a Gaussian transmission band characteristic like an arrayed waveguide diffraction grating filter. FIG. 4 is a calculation result of an eye pattern when an optical signal passes through a single-stage optical filter. FIG. 5 shows a calculation result of an eye pattern when an optical signal passes through a 20-stage optical filter. A good eye pattern can be obtained with one-stage transmission, but 20
It can be seen that the shape is distorted when the light passes through the steps.

In order to suppress the waveform deterioration due to the multi-stage transmission of the optical filter, there is a method of setting a wide transmission band characteristic per stage. Causes the influence of light of adjacent wavelengths. That is, it cannot be applied to a wavelength division multiplexing transmission system in which the number of stages of optical filters transmitted by optical signals is different.

An object of the present invention is to provide an optical signal waveform deterioration compensating device capable of compensating for a waveform deterioration of an optical signal caused by multi-stage transmission of an optical filter without widening a transmission band characteristic of each optical filter. Aim.

[0008]

An optical signal waveform deterioration compensating device according to the present invention comprises a compensating optical filter disposed after an optical filter connected in multiple stages, and an optical signal generated by transmitting the optical filters in multiple stages. Is compensated by the compensation optical filter. This compensation optical filter has a transmission band characteristic for converting a transmission band characteristic due to multi-stage transmission of the optical filter into a flat characteristic near the center frequency f. Further, the compensation optical filter has a transmission band characteristic in which the transmittance becomes minimum at the center frequency f.

[0009]

FIG. 1 shows the principle configuration of an optical signal waveform deterioration compensating apparatus according to the present invention. In the figure, in a wavelength division multiplexing transmission system in which a plurality of transmission nodes 1-1 to 1-n are connected in cascade, an optical filter 2 for demultiplexing an optical signal of each wavelength is arranged at each transmission node. Here, focusing on the optical signal of the center frequency f, the optical filters 2-1 to 2-n having a transmission band characteristic for transmitting the optical signal of the center frequency f are connected in cascade of n stages.

At this time, the effective transmission band characteristic due to the n-stage transmission has a narrow flat transmission band as shown in FIG. Therefore, a compensation optical filter 3 having a transmission band characteristic that converts an effective transmission band characteristic into a flat characteristic near the center frequency f is disposed at a stage subsequent to the optical filter 2-n. Note that the compensation optical filter 3 may be arranged inside or outside the transmission node 1-n.

FIG. 2 shows an example of improving the transmission band characteristic by the compensating optical filter 3. In FIG. 2, the horizontal axis is the optical frequency normalized by the bit rate frequency with reference to the center frequency f of the optical signal, and the vertical axis is the amplitude transmittance. In general, power transmittance is used in the transmission band characteristic of an optical filter, but this is a square of the amplitude transmittance. The dotted line shows the effective transmission band characteristics when 20 stages of the optical filter 2 whose 3 dB transmission bandwidth is 3.5 times the bit rate frequency are transmitted. The dashed line indicates the transmission band characteristic of the compensation optical filter 3. The solid line indicates the transmission band characteristic compensated by the compensation optical filter 3.

As described above, when the effective 3 dB transmission bandwidth (in terms of amplitude) becomes substantially equal to the beat rate frequency due to the multi-stage transmission of the optical filter 2, high frequency components of the optical signal are blocked, and FIG. The waveform distortion as shown occurs. This optical signal is input to a compensating optical filter 3 having a transmission band characteristic as indicated by a dashed line.

In this embodiment, a Mach-Zehnder optical filter having a period of 1.2 times the bit rate frequency and a peak-to-bottom power transmittance ratio of 2: 1 is used as the compensation optical filter 3. The period can be set by the optical length difference between the two optical paths of the Mach-Zehnder, and the transmittance ratio can be set by the branching ratio of the optical coupler in the Mach-Zehnder.
Transmission band characteristics of the compensating optical filter 3 (dotted line)
And the transmission band characteristic (dotted line) of the optical filter 2 due to the 20-stage transmission, as shown by the solid line. That is, the peak portion of the transmission band characteristic indicated by the dotted line is reduced, and the transmission band near the center frequency f is flat. Thereby, the waveform distortion of the optical signal can be compensated.

FIG. 3 shows an eye pattern of an optical signal transmitted through the compensation optical filter 3. Compared to FIG. 5, it can be seen that the waveform distortion is compensated and the eye is wide open. In addition,
Although the power of the optical signal is reduced by the transmission of the compensating optical filter 3, this can be compensated by using an optical amplifier or the like.

In this embodiment, an example in which a Mach-Zehnder optical filter is used as the compensation optical filter 3 has been described.
The present invention is not limited to such an optical filter having periodicity, but may be any filter having a transmission band characteristic such that an effective transmission band characteristic is compensated for a flat characteristic near the center frequency f.

[0016]

As described above, the optical signal waveform deterioration compensating device of the present invention can compensate for the waveform deterioration of an optical signal caused by multi-stage transmission of an optical filter by passing the signal through a compensation optical filter. .

[Brief description of the drawings]

FIG. 1 is a diagram showing a principle configuration of an optical signal waveform deterioration compensating device of the present invention.

FIG. 2 is a diagram for explaining an example of improving a transmission band characteristic by a compensating optical filter 3;

FIG. 3 is a diagram showing an eye pattern of an optical signal transmitted through a compensation optical filter 3;

FIG. 4 is a diagram showing an eye pattern of an optical signal transmitted through a single-stage optical filter.

FIG. 5 is a diagram showing an eye pattern of an optical signal transmitted through a 20-stage optical filter.

[Explanation of symbols]

 Reference Signs List 1 transmission node 2 optical filter 3 optical filter for compensation

Claims (2)

[Claims] 1. An optical filter having a transmission band characteristic for transmitting an optical signal of a predetermined wavelength from a wavelength-multiplexed optical signal and blocking an optical signal of another wavelength is connected in multiple stages, and each optical filter is transmitted in multiple stages. An optical signal waveform degradation compensator for compensating for the waveform distortion of an optical signal having a center frequency f, wherein the compensation has a transmission band characteristic for converting a transmission band characteristic due to multi-stage transmission of each optical filter into a flat characteristic near the center frequency f An optical signal waveform deterioration compensating device, comprising: an optical filter for use, wherein an optical signal transmitted through the optical filters in multiple stages is passed through the optical filter for compensation. 2. The optical signal waveform deterioration compensating apparatus according to claim 1, wherein the compensating optical filter has a configuration having a transmission band characteristic in which the transmittance becomes a minimum at a center frequency f.