JPH06268593A - Optical communications system - Google Patents

Abstract

PURPOSE:To reduce power consumption and to improve the transmission quality when an electrical-to-optical transducer is subject to intensity modulation with plural electric signals whose levels differ from each other. CONSTITUTION:A sender side 1 uses an electrical-to-optical transducer 7 to convert an electric signal into an optical signal and the optical signal is sent to a receiver side through an optical transmission line and an optical-to-electrical transducer 9 on the receiver side 2 converts the optical signal into an electric signal. In the optical communication system as above mentioned, the sender side 1 is provided with a level detection means 5 detecting a level of the electric signal, a level correction means 4 correcting a level of an electric signal of the sender side so that a minimum level detected by the level detection means 5 is a prescribed value or over, a frequency detection means 6 detecting a frequency of the electric signal, and a bias control means 8 applying variable control to a bias current of the electrical-to-optical transducer 7 so that the relation between the level of each electric signal detected by the level detection means 5 and a frequency of each electric signal detected by the frequency detection means 6 satisfies a desired relation to obtain a constant ratio of signal to electric power distortion.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication system for converting an electric signal into an optical signal by an electric / optical converter and transmitting the optical signal through an optical fiber. Especially, when the intensity of the electric / optical converter is modulated by a plurality of electric signals having different levels, the transmission quality thereof is improved.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a configuration of a conventional optical communication system. The frequency-multiplexed electrical signal is input to the input terminal of the transmission side 1. The frequency-multiplexed signal is converted into an optical signal by the electrical / optical converter 7 and the optical fiber 3
Is transmitted to the receiving side 2. The bias current of the electric / optical converter 7 is set to a constant value by the fixed bias circuit 12. Then, on the receiving side 2, the received optical signal is converted into an electric signal by the optical / electrical converter 9, and is output to each of the electric signal output terminals 1 ′ to N ′ by the distributor 10.

As described above, conventionally, when frequency-multiplexed or one electric signal is converted into an optical signal by the electric / optical converter and transmitted by an optical fiber, the input level of the electric signal to the electric / optical converter is It was constant and the bias current of the electro-optical converter was set to a constant value.

[0004]

However, in the conventional method of setting the bias current of the electric / optical converter to a fixed value, when the LD or the like is intensity-modulated by a multi-carrier signal and transmitted, the current and the light intensity of the LD are increased. There is a problem in that the intermodulation distortion falls into the band due to the odd-order distortion and the transmission quality is deteriorated because the relationship with is not linear. There is a means for increasing the bias current of the LD in order to reduce this distortion, but there is a new problem that the power consumption increases due to the increase in the current.

An object of the present invention is to solve these problems, and it is an object of the present invention to provide an optical communication system capable of keeping power consumption low and suppressing distortion below a certain value.

[0006]

In order to solve the above problems, the present invention converts an electric signal into an optical signal by an electric / optical converter on the transmitting side and transmits the optical signal to the receiving side on an optical transmission line. In an optical communication system for transmitting and converting an optical signal into an electric signal by an optical / electrical converter on the receiving side, a level detecting means for detecting the level of the electric signal on the transmitting side and a minimum level detected by the level detecting means Level correction means for correcting the level of the electric signal on the transmission side so that the level becomes a predetermined value or more, frequency detection means for detecting the frequency of the electric signal, and level and frequency detection of each electric signal detected by the level detection means Bias control for variably controlling the bias current of the electro-optical converter so that the relationship with the frequency of each electric signal detected by the means satisfies a desired relationship with which a constant signal-distortion power ratio is obtained. It is characterized in that a stage.

[0007]

According to the present invention having the above-described structure, in order to improve the quality of the electric signal at a low level and reduce the power consumption of the electric / optical converter, the electric signal to the electric / optical converter is supplied. In order to correct the input level and at the same time reduce the intermodulation distortion that occurs between multiple electrical signals due to the non-linearity of the electrical / optical converter, the frequency relationship between the multiple electrical signals and the bias current of the electrical / optical converter are determined. Change.

Therefore, by utilizing the fact that the optical modulation degree changes in accordance with the bias current value of the electric / optical converter and the input level of the electric signal to the electric / optical converter, the electric signal having a low level is converted into the electric / optical signal. When inputting to the converter, the optical modulation level can be increased by increasing the input level of the electric signal to improve the transmission quality.

Further, by utilizing the fact that the intermodulation distortion characteristic changes depending on the bias current value of the electrical / optical converter, when the intermodulation distortion from a plurality of high level electrical signals interferes with the low level electrical signals, the In order to reduce the modulation distortion, the bias current is increased to prevent the deterioration of quality. On the other hand, in other cases, the bias current is set to a small value in order to reduce the power consumption of the electro-optical converter. This makes it possible to economically and with high quality transmit an electric signal through an optical fiber. The same applies to the case of transmitting one electric signal.
The transmission quality can be improved by correcting the input level to the optical converter and changing the bias current.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention. In this embodiment, the signal transmission between the transmitting side 1 and the receiving side 2 in the optical communication system is shown by the optical fiber 3.

The frequency-multiplexed electric signal is input to the input terminal of the transmission side 1, and the frequency-multiplexed signal is input to the level corrector 4, the level detection circuit 5, and the frequency detection circuit 6, respectively. The level detector 4 measures the level of each wave of the multi-carrier signal, which is an input signal, by the level detector 5, and inputs the lowest level of the electric signal of the measurement result. The level corrector 4 accordingly corrects the level so that the lowest level of the wave becomes a certain value or more so that the electric signal satisfies a desired optical modulation degree, and outputs the electric signal to the electric / optical converter 7. This is to prevent being buried in noise when the level is too low.

The bias control circuit 8 includes a level detection circuit 5
Whether the frequency relationship between the total level of the electric signal after level correction and the frequency of each wave of the input multi-carrier signal detected from the frequency detection circuit 6 causes a problem of cross modulation. A signal as a result of determining whether or not is input. When the frequency relationship of the signals detected by the frequency detection circuit 6 causes a problem of cross modulation, for example, in the case of a two-wave multicarrier signal, odd-wave cross-modulation distortion does not pose any problem, but three-wave etc. In the case of the interval signal, the odd-wave cross-modulation distortion drops in the middle signal, which is a problem. The bias control circuit 8 sets the bias current of the electro-optical converter 7 so that the electric signal at a low level and affected by the intermodulation distortion satisfies the desired quality.

On the receiving side 2, the optical signal from the transmitting side 1 is returned to an electric signal by an optical / electrical converter 9, and then frequency-divided by a distributor 10 and output to output terminals 1'-N '. .

FIG. 2 is a characteristic diagram showing the relationship between the signal-to- (noise + distortion) ratio and the electric / optical converter input level (optical modulation degree). The wavy line in the figure shows the relationship between the signal-to-noise ratio and the input level of the electro-optical converter, and the signal-to-noise ratio increases linearly with respect to the input level of the electro-optical converter. Each dotted line in the figure shows the relationship between the signal-to-third-order intermodulation distortion ratio with respect to the bias currents A and B of the electric-to-optical converter and the electric-to-optical converter input level. The ratio degrades second-order with respect to the electro-optical converter input level. Also U
₁ is the electro-optical converter input X, the signal-to-third-order intermodulation distortion ratio for the bias current A, U ₂ is the electro-optical converter input Y,
The signal-to-intermodulation distortion ratio for the bias current A, U ₃ indicates the intermodulation distortion ratio for the electro-optical converter input Y and the bias current B, respectively.

FIG. 3 is a characteristic diagram showing the relationship between the intermodulation distortion and the bias current. Considering, for example, third-order distortion as the intermodulation distortion, when signals of frequencies f ₁ and f ₂ commonly pass (amplify, modulate) through a non-linear element, 2f ₁ −f ₂ , 2f ₂
This is a phenomenon in which a distortion component occurs at the position of -f ₁ .
Each solid line shows the characteristics for the electro-optical converter inputs X and Y. For example, when the bias current is A, electricity /
Increasing the optical converter input from X to Y degrades the intermodulation distortion from U ₁ to U ₂ . When the bias current is changed from A to B when the electric / optical converter input is Y, the intermodulation distortion can be reduced from U ₂ to U ₃ . As can be seen from the figure, since the intermodulation distortion has a bias current dependence characteristic, the signal-to- (noise + distortion) ratio changes with the bias current even when the input level of the electric / optical converter is constant.

FIG. 4 is a conceptual diagram showing the improvement of the transmission quality due to the level correction of the electric signal and the deterioration of the transmission quality due to the intermodulation distortion. (1) shows the number of input electric signals is 3 and the frequency relationship is at regular intervals. It is the figure which showed the spectrum when only the level of the electric signal of frequency 3 is low. Assuming that the broken line in the figure is the lower limit of the desired signal-to-noise ratio, the electrical signal of frequency 3 does not satisfy the desired signal-to-noise ratio because of its low level. At this time, a desired signal-to-noise ratio can be satisfied by correcting the level of the electric signal so that the optical modulation degree increases. (2) is a diagram showing that intermodulation distortion increases and transmission quality deteriorates when level correction is performed as in (1). That is, by performing the level correction, the intermodulation distortion U ₁ generated from the electric signals of frequencies 1 and 2 increases to U ₂ . Therefore, the ratio D / U ₂ between the intermodulation distortion U ₂ after level correction and the maximum level D of the electric signal of frequency 3 does not satisfy the desired D / U ratio. (3) is a diagram showing that the transmission quality is improved by increasing the bias current. In order to satisfy the desired D / U ratio, the bias current value is increased from A to B and the intermodulation distortion is reduced to U ₃ , so that the desired D / U ratio is satisfied. When the level difference between signals is small, the maximum input level to the electro-optical converter is X, and the bias current is reduced to A. Note that the bias current may be set continuously or in steps (stepwise).

[0017]

As described above, according to the present invention,
In an electrical / optical conversion technology for converting a plurality of or one electrical signal having different frequencies into an optical signal and transmitting the optical signal, the electrical / optical conversion is performed in order to improve the noise resistance characteristic of a low level electrical signal and reduce distortion. It is possible to realize an economical and high-quality optical communication system that can achieve low power consumption of the device and distortion at a certain value or less.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of the present invention.

FIG. 2 is a characteristic diagram showing a relationship between a signal-to- (noise + distortion) ratio and an electric / optical converter input level.

FIG. 3 is a characteristic diagram showing the relationship between bias current and intermodulation distortion.

FIG. 4 is a characteristic diagram showing the relationship between intermodulation distortion and signal level.

FIG. 5 is a block diagram showing a configuration of a conventional example.

[Explanation of symbols]

 1 transmitting side 2 receiving side 3 optical fiber 4 level corrector 5 level detecting circuit 6 frequency detecting circuit 7 electric / optical converter 8 bias control circuit 9 optical / electrical converting circuit 10 distributor 11 fixed bias circuit

Claims (2)

[Claims] 1. An electric signal is converted into an optical signal by an electric / optical converter at a transmitting side, the optical signal is transmitted to a receiving side through an optical transmission line, and the optical signal is converted into an electric signal by an optical / electrical converter at the receiving side. In the optical communication system for converting into a signal, the transmitting side has a level detecting means for detecting the level of the electric signal, and the electric signal of the transmitting side is controlled so that the lowest level detected by the level detecting means becomes a predetermined value or more. There is a level correction means for correcting the level, a frequency detection means for detecting the frequency of the electric signal, and a relationship between the level of each electric signal detected by the level detection means and the frequency of each electric signal detected by the frequency detection means. A bias control means for variably controlling a bias current of the electric / optical converter so as to satisfy a desired relation that a constant signal-distortion power ratio is obtained. The optical communication system characterized in that the input level of the electric signal is corrected by the level corrector, and the bias current of the electric / optical converter is varied according to the correction of the level of the electric signal. 2. When a plurality of electric signals having different frequencies are used, the electric signal level lower than a predetermined level among the electric signal levels detected by the level detecting means is set to the predetermined level. An increase correction is performed by the level correction means, and the bias current of the electric / optical converter is increased by the bias control means when the transmission quality is deteriorated due to intermodulation distortion in each electric signal when the correction is performed. Optical communication system described.