JPH0369466B2 - - Google Patents

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, a case will be described here in which an RZ signal of {0, 1} is used. 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 optical pulses are transmitted through the optical fiber 2 having structural dispersion and material dispersion, the received optical pulses will fluctuate due to the propagation time difference between each longitudinal mode, deteriorating the bit 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 subjected to Gaussian equalization, cosine squared equalization, etc. by an equalizer 5, and then sent to a timing wave extraction circuit. Retiming and discrimination are performed using the discriminator 7 based on the timing signal 6. At this time,
The relationship between the timing signal and the equalized waveform is shown in FIG. Waveform equalization is designed to remove high-frequency components such as thermal noise, shot noise, and mode distribution noise, and to suppress SN deterioration due to intersymbol interference.Identification is performed when the eye pattern is widest. There is. However, since mode distribution noise has a property that the noise increases in proportion to the signal component, the noise power is 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 noise.

(Object of the Invention) The present invention performs identification based on the integral value within one time slot of a code in order to reduce mode distribution noise, and resets the integration circuit to zero after identification in order to eliminate intersymbol interference. The present invention provides a mode distribution noise-reducing optical signal reception discriminator.

(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 is a broadband 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 It is a delay line.

Next, its operation will be explained. Photodiode 3
The received signal that has been photoelectrically converted by There is. 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 integrating circuit 8 to zero.

A concrete example of the integrating circuit 8 and the reset circuit 9 is shown in FIG. A time chart at this time is shown in Fig. 6. When a rectangular rectangular signal is transmitted 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 described 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 be oscillated (the spectral width of the semiconductor laser). Here, the current I flowing through the photodiode 3 during photoelectric conversion is proportional to the power of light according to the following equation.

I=ηe/hνP (1) Here, η: Photodiode efficiency, e: Electron charge, h: Planck's constant, ν: Light frequency, P: Light power. Therefore, this current I is integrated. Then, the 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, integration is performed within one time slot and reset to zero for each time slot to eliminate intersymbol 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 are made to differ by a time _Td by a delay line 10, and immediately after the identification is completed, the integrating 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, since the present invention performs identification based on the integral value of the received signal affected by mode distribution noise, the influence of mode distribution noise is removed,
In addition, since the reset circuit eliminates intersymbol interference, it has the advantage that it is possible to increase the relay interval or 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 noise generation mode, FIG. 2 is a block diagram showing the configuration of a conventional optical reception discriminator, and FIG. 3 is a diagram of the conventional receiver shown in FIG. 2. 4 is a block diagram showing an embodiment of the present invention, and FIG. 5 is a circuit showing a specific example of the integrating circuit and its reset circuit in the embodiment of FIG. 4. 6 is a time chart showing an example of signals of each part of the embodiment of the present invention shown in FIG. 4, and FIG. 7 is an eye pattern of the optical signal reception discriminator of the present invention. 1... Semiconductor laser, 2... Optical fiber, 3...
...Photodiode, 4...Broadband amplifier, 5...
... Equalization circuit, 6 ... Timing signal extraction circuit, 7
...discriminator, 8...integrator circuit, 9...reset circuit, 10...delay element or delay line.

Claims (1)

[Claims] 1. 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 A circuit is provided for identifying the code based on the output value of the integrating circuit, and the integrating circuit performs integration for each symbol of the received digital code string for one time slot, and when the integration is completed, the identifying circuit A timing signal is input and identification is performed. Immediately after that, a timing signal delayed by a minute amount of time than the identification timing signal is input to the reset circuit to set the integration circuit to its original state. A mode distribution noise-reducing optical signal reception discriminator characterized in that the above process is repeated to perform the discrimination.