JPS6128238A - Optical signal receiving and identifying device with mode distribution noise reduction - Google Patents

Abstract

PURPOSE:To increase the repeater distance or to improve the error rate characteristic in a transmission system where mode distribution noise is dominant by using an integration value of a reception signal subjected to the effect of mode distribution noise to identify the noise thereby eliminating the effect to the mode distribution noise and using a reset circuit to eliminate inter-code interference. CONSTITUTION:After a reception signal subjected to photoelectric conversion by a photodiode 3 is amplified by a broad band amplifier 4, the result is integrated by an integration circuit 8 for a 1 time slot time, discrimination of code 0, 1 is conducted by an identification device 7 depending on the output value and the timing signal in this case is obtained by a timing signal extraction circuit 6. A timing signal via a delay line 10 is fed to a reset circuit 9 to reset an output of the integration circuit 8 to zero just after. That is, integration is executed in one time slot in this device and the output is reset to zero at each time slot to eliminate inter-code interference, then the input waveform to the identification device 7 is a trapezoidal wave. A difference of the time Td between an identification timing signal and reset timing signal by the delay line 10, the integration circuit 8 is reset just after the identification and it is repeated afterward.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field of the Invention) The present invention relates to an optical signal reception discriminator used in a digital transmission system using optical fibers.

(Prior art) In a digital optical fiber transmission system, (0,
A case in which the RZ multiplied signal of 1) is used will be explained. The semiconductor laser 1, which is a light source, normally oscillates in several longitudinal modes, and each oscillation mode fluctuates with each optical pulse, but the sum of these longitudinal modes, that is, the total amount of light, is considered to be constant. There is. When such an optical pulse is transmitted through an optical fiber 2 having structural dispersion and material dispersion, the received optical pulse will fluctuate due to the propagation time difference between each longitudinal mode.
Degrades code error rate characteristics. This noise is mode distribution noise, and is one of the major factors that limit the relay distance of optical relay transmission systems.

FIG. 2 shows the configuration of a conventional reception discriminator. The received signal photoelectrically converted by the photodiode 3 is amplified by a wideband amplifier 4 to a level sufficient for identification, and then sent to an equalizer 5.
For example, Gaussian equalization, cosine squared equalization, etc. are performed, and retiming and discrimination are performed using the discriminator 7 based on the timing signal of the timing wave extraction circuit 6. At this time, the relationship between the timing signal and the equalized waveform is as shown in FIG.

Waveform equalization is designed to remove high-frequency components such as thermal tones, M tones, and mode distribution noise, and to suppress SN deterioration due to code amount interference. I am doing it. However, since mode distribution noise has a property that the noise increases in proportion to the signal component, the shoe sound power reaches its maximum at the discrimination point, as shown in FIG. For this reason, when attempting to increase the relay interval, there is a drawback that the relay distance is limited by the dominant mode distribution sound.

(Object of the Invention) The present invention aims to reduce mode distribution shoe noise (1 of C code).
The present invention provides an optical signal reception discriminator for reducing mode distribution noise, which performs discrimination based on the integral value within a time slot and eliminates code amount interference, and resets the integrating circuit to zero after discrimination. be.

(Structure and effects of the invention) The present invention will be explained in detail below.

FIG. 4 shows an embodiment of the present invention, in which 3 is a photodiode, 4-digit wideband amplifier, 6 is a timing signal extraction circuit,
7 is a discriminator, 8 is an integration circuit, 9 is a reset circuit, and 10 is a delay element or delay line.

Next, its operation will be explained. The received signal photoelectrically converted by the photodiode 3 is amplified by the broadband amplifier 4, and then integrated by the integrating circuit 8 for one time slot, and the discriminator 7 determines the sign of 0.1 based on the output value. There is one row left. The timing signal at this time is obtained from the timing signal extraction circuit 6. Immediately after this, a timing signal via a delay line 10 is applied to the reset circuit 9 in order to reset the output of the integration circuit 8 to zero.

A specific example of the integrating circuit 8 and the reset circuit 9 is shown in FIG.

Further, a time chart at this time is shown in FIG. When a rectangular transmission signal is sent out as shown in FIG. 6, the received signal has a shape different from the transmitted waveform due to the influence of the mode distribution noise mentioned above. However, the integral value of the received optical pulse, that is, the optical energy, remains constant as long as the loss of the fiber is constant within the wavelength of the longitudinal mode that can oscillate (the spectral width of the semiconductor laser). Here, the current (2) flowing through the photodiode 3 during photoelectric conversion is proportional to the power of light according to the following equation.

ηe I = -P (1) hν where η° photodiode efficiency.

e: electron charge, h blank constant.

C: Frequency of light; P: Power of light. Therefore, when this current I is integrated, its value becomes a constant value. This principle makes it possible to identify received signals that are disturbed by mode distribution noise.

In the present invention, (1) integration is performed within a time slot and reset to zero for each time slot to eliminate code amount interference, so the input waveform of the discriminator 7 becomes a trapezoidal wave as shown in FIG. When the signal is zero, the output of the integrating circuit is naturally zero. The identification timing signal and the reset timing signal have a difference of time T by the delay line 10,
Immediately after the identification is completed, the integration circuit 8 is reset, and this process is repeated thereafter. The eye pattern of this circuit is as shown in FIG. 7, which is significantly different from the conventional example shown in FIG. 3.

(Effects of the Invention) As explained above, the present invention eliminates the influence of mode distribution noise while performing discrimination based on the integral value of the received signal affected by mode distribution noise, and also eliminates the influence of mode distribution noise by using a reset circuit. Since the amount of interference is eliminated, it has the advantage that it is possible to increase the relay interval and improve error rate characteristics in a transmission system where mode distribution noise is dominant.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram for explaining the mode distribution sound generation mode, FIG. 2 is a block diagram showing the configuration of a conventional optical receiver discriminator, and FIG. 3 is a diagram of the conventional receiver shown in FIG. A time chart showing the relationship between equalized waveforms and timing signals, FIG. 4 is a block diagram showing an embodiment of the present invention, and FIG. 5 shows a specific example of the integrating circuit and its reset circuit in the embodiment of FIG. 4. The circuit diagram, FIG. 6, shows each of the embodiments of the present invention shown in FIG. 4: 1... semiconductor laser, 2... optical fiber, 3...
...Photodiode, 4...Broadband amplifier, 5
...Equalization circuit, 6.. Timing signal extraction circuit, 7.. Discriminator, 8.. Integrating circuit, 9..
- Reset circuit, 10...delay element or delay line. Patent Applicant Nippon Telegraph and Telephone Public Corporation Agent Haku Mizu Tsune Male 1 Mature 4 Figure Male 50 1 JLJ Month 60 Mature 70 171 Lotus, Mouth, I

Claims (1)

[Claims] In a digital optical transmission system using an optical fiber, an integrating circuit for integrating a digital signal converted from an optical signal to an electrical signal, a reset circuit for setting the state of the integrating circuit to an initial value, and the above-mentioned The circuit includes a circuit that identifies the code based on the output value of the integrator, and the integrator circuit performs integration for each code of the received digital code string for one time slot, and when the integration is completed, the identification circuit is provided with a timing signal. A signal is input and identification is performed, and immediately after that, a timing signal delayed by a minute time from the identification timing signal is input to the reset circuit, and the integration circuit is set to its original state. 1. A mode distribution noise reducing optical signal reception discriminator, characterized in that the above process is repeated to perform discrimination.