JPH0851397A - Distortion compensated light transmitter - Google Patents

Abstract

PURPOSE:To realize the distortion compensated optical transmission of high reliability by setting the electrical length of a negative feedback loop or adjusting the phase of an electric signal to be negatively fed back so that the phase of the electric signal to be fed back becomes an opposite phase to intermodulation distortion produced in an electricity-light converter. CONSTITUTION:An optical signal outputted from a laser diode LD9 is inputted to a light-electricity converter 10, and here, a signal level is detected. Then, a bias control circuit 11 compares an electric signal level detected by the light- electricity converter 10 with the electric signal level corresponding to desired average optical power set beforehand, and controls a bias circuit 8 so that the desired average optical power is obtained. Besides, a phase shift circuit 12 changes the phase of the feedback signal so as to cancel the intermodulation distortion produced in LD9, and supplies this signal to an adder 13. Thus, in the adder 13, a signal for reducing or removing the intermodulation distortion is added to the electric signal supplied from an input terminal 14. Accordingly, the optical signal of the desired average optical power is obtained in LD9, and the intermodulation distortion is reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used as an optical communication type transmitter for converting an electric signal into an optical signal by an electric / optical converter and transmitting the optical signal through an optical fiber, and realizes high transmission quality. The present invention relates to a possible distortion compensation optical transmitter.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a schematic configuration of an example of an electric / optical converter 5 provided in a conventional optical communication type transmitter. The electric / optical converter 5 converts a single or frequency-multiplexed electric signal input from the input terminal 14 into an optical signal and inputs the optical signal into the optical fiber 6. The optical signal incident on the optical fiber 6 is transmitted to the receiving side.

In the electric / optical converter 5, 8 is a bias circuit connected to the input terminal 14, 9 is a laser diode (hereinafter referred to as LD) provided in the subsequent stage of the bias circuit 8, and the bias circuit 8 is An optical signal having an average optical power corresponding to the generated bias current is incident on the optical fiber 6. Reference numeral 10 denotes an optical / electrical converter provided as necessary, which converts the optical signal output from the LD 9 into an electric signal and detects the level of this electric signal. A bias control circuit 11 compares the electric signal level detected by the optical / electrical converter 10 with the electric signal level corresponding to the desired average optical power, and applies a bias so that the desired average optical power is obtained. Control the circuit 8.

With such a configuration, the electric signal input to the LD 9 via the input terminal 14 and the bias circuit 8 is converted into an optical signal and is incident on the optical fiber 6.
At the same time, the optical signal output from the LD 9 is input to the optical / electrical converter 10, where it is converted into an electrical signal and its level is detected. Since the bias control circuit 11 controls the bias circuit 8 according to the electric signal level detected by the optical / electrical converter 10, the bias current is adjusted so that the average optical power of the optical signal output from the LD 9 becomes constant. Is adjusted. As described above, the electric / optical converter 5 having the configuration shown in FIG. 5 employs an optical modulation method in which intensity modulation is performed around a certain bias current.

[0005]

However, an optical modulation method for intensity-modulating a bias current (for example,
In SCM (Subcarrier Multiplexing), noise (RIN: Relative Intensity Noi) generated in an electrical / optical converter is used.
se) and the intermodulation distortion caused by the non-linearity of the electric / optical converter occur outside the determined frequency band, and there is a problem that the standard of the unnecessary radiation cannot be satisfied.

Further, when the negative feedback loop is constructed by a device different from the electro-optical converter module, its electrical length becomes long, which makes it difficult to adjust the phase at a high frequency and cannot exert its effect. It was The present invention has been made in view of the above circumstances and provides a distortion-compensated optical transmission device that is economical and highly reliable, and that can improve noise characteristics of low-level electric signals and reduce distortion. The purpose is to provide.

[0007]

According to a first aspect of the present invention, an optical signal is converted into an optical signal at a transmitting side and transmitted to an optical transmission line, and an optical signal output from the optical transmission line at a receiving side. A distortion-compensated optical transmitter used on the transmitting side of an optical communication system for converting a signal into an electric signal and converting an electric signal into an optical signal by an electric / optical converter, wherein the electric / optical converter comprises:
An electric / optical conversion means for converting an electric signal into an optical signal and sending it to the optical transmission line; and an optical / electric converter for converting an optical signal outputted from the electric / optical conversion means into an electric signal and outputting the electric signal. Means for negatively feeding back an electric signal output from the optical / electrical converter to an input side of the electric / optical converting means, and a negative feedback to a preset level and an input side of the electric / optical converting means. Means for controlling the level of the electric signal input to the electric / optical conversion means so that the average optical power of the optical signal output from the electric / optical converter becomes constant based on the level of the electric signal. , The electric length of the negative feedback loop is set so that the phase of the negatively fed back electric signal is opposite to the phase of the intermodulation distortion generated in the electric / optical converter, or the phase of the negatively fed back electric signal is set. It is characterized by adjusting.

According to a second aspect of the present invention, the transmitting side converts an electric signal into an optical signal and sends the optical signal to the optical transmission line, and the receiving side converts the optical signal output from the optical transmission line into an electric signal. A distortion-compensating optical transmitter used on the transmitting side of an optical communication system for converting an electric signal into an optical signal by an electric / optical converter, wherein the electric / optical converter converts the electric signal into an optical signal. And an electric / optical conversion means for sending the light to the optical transmission line,
An optical / electrical converter that converts an optical signal output from the electric / optical conversion unit into an electric signal and outputs the electric signal, and an input side of the electric / optical conversion unit that outputs the electric signal output from the optical / electrical conversion unit. Means for negatively feeding back, and an average of the optical signals output from the electrical / optical converter based on a preset level and the level of the electrical signal negatively fed back to the input side of the electrical / optical converting means. Means for controlling the level of the electric signal input to the electric / optical conversion means so that the optical power is constant, and the phase of the negatively fed back electric signal is opposite to the noise generated in the electric / optical converter. It is characterized in that the electric length of the negative feedback loop is set to have a phase, or the phase of the negatively fed back electric signal is adjusted.

According to a third aspect of the present invention, in the first aspect of the present invention, the electric / optical converter is an intermodulation device in which the phase of the negatively fed back electric signal is generated in the electric / optical converter. It is characterized in that the electrical length of the negative feedback loop is set so that the phase reduces distortion and noise, or the phase of the negatively fed back electrical signal is adjusted.

[0010]

According to the above construction, the optical signal output from the electric / optical converter is intermodulation distortion generated between a plurality of electric signals due to the non-linearity of the electric / optical converter, or the electric / optical converter. The resulting signal has reduced noise. Further, the average optical power of the optical signal output by the electric / optical converter substantially matches the preset value. Therefore, it is economical and highly reliable, and the improvement of the noise resistance characteristic of the low level electric signal and the reduction of the distortion are realized.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1A is a block diagram showing a schematic configuration of a distortion compensation optical transmitter according to an embodiment of the present invention. The distortion compensation optical transmitter shown in this figure converts, for example, an electrical signal into an optical signal, It is used as a transmitter of an optical communication system that transmits this optical signal by entering it into an optical fiber.

In the distortion compensation optical transmitter shown in FIG.
3 to 3 are input terminals to which electric signals to be transmitted are applied, 4 is a combiner for combining electric signals supplied through the input terminals 1 to 3, and 5 is provided in the subsequent stage of the combiner 4, It is an electro-optical converter that intensity-modulates the output electric signal of the device 4 and converts it into an optical signal, and is generally composed of a laser diode (hereinafter referred to as LD). Further, in this figure, 6 is an optical fiber through which the output optical signal of the electro-optical converter 5 propagates, 7 is provided on the receiving side, and converts the optical signal transmitted through the optical fiber 6 into an electrical signal. It is an optical / electrical converter that outputs as an output, and is configured by, for example, a pin photodiode.

By the way, when the distortion compensating optical transmitter having such a configuration is used in a two-way radio system in which an optical communication system is applied between a radio device and an antenna, a radio signal is transmitted to a mobile terminal on the receiving side. At the time of transmission, a wireless modulator is connected to the input terminals 1 to 3, and an antenna is connected to the subsequent stage of the optical / electrical converter 7. On the other hand, when receiving the radio signal from the mobile terminal, the antenna is the electric / optical converter 5
Connected to the front stage of. That is, from the input terminals 1 to 3,
The received wireless signal is output.

The electro-optical converter 5 (see FIG. 1 (a)) will be described with reference to FIG. 1 (b). FIG.
(B) is a block diagram showing a configuration example of the electro-optical converter 5, and in this figure, portions common to the respective portions of FIG. 5 are denoted by the same reference numerals, and description thereof will be omitted. Figure 1 (b)
5 is different from that shown in FIG. 5 in that an adder 13 is provided in the preceding stage of the bias circuit 8 and a signal based on the electric signal level detected by the bias control circuit 11 is added. The phase circuit 12 for reducing or eliminating the intermodulation distortion generated in the LD 9 is provided.

The principle of reducing or eliminating the intermodulation distortion generated in the LD 9 by the phase circuit 12 will be described with reference to FIG. The circuit shown in FIG. 2 is a circuit that converts an input electrical signal into an optical signal by an LD and outputs the optical signal, which constitutes a negative feedback loop and has a phase of a signal (intermodulation distortion) having a frequency of ω ₁ to be canceled. The feedback signal is input to the adder 13 so as to be reversed.

According to such a configuration, the levels of the signals of the frequencies ω ₂ and ω ₃ output from the LD 9 do not decrease so much, and only the level of the signal of the frequency ω ₁ significantly decreases.
This is because, as shown in the conceptual diagrams of FIGS. 3A and 3B, the feedback signal has a reverse phase at the frequency ω ₁ but does not have a reverse phase at the frequencies ω ₂ and ω ₃ . Is. In principle, equal amplitude and
When a signal of opposite phase is input to LD9, the intermodulation distortion generated in LD9 is completely removed. Further, if the electric length of the negative feedback loop is equal to the opposite phase with respect to the intermodulation distortion, it is not necessary to provide the phase circuit 12. The phase circuit 12 operates to reduce or eliminate intermodulation distortion based on the principles described above.

With such a configuration, when an electric signal is input through the input terminal 14, the adder 13 and the bias circuit 8 in FIG. 1B, the LD 9 outputs the electric signal as an optical signal. And is incident on the optical fiber 6. At the same time, the optical signal output from the LD 9 is input to the optical / electrical converter 10 and the signal level thereof is detected.

The bias control circuit 11 compares the electric signal level detected by the optical / electrical converter 10 with the electric signal level corresponding to a preset desired average optical power,
The bias circuit 8 is controlled so that a desired average optical power can be obtained. Further, the phase circuit 12 changes the phase of the feedback signal so as to cancel the intermodulation distortion generated in the LD 9, and supplies the feedback signal to the adder 13. As a result, in the adder 13, a signal for reducing or removing intermodulation distortion is added to the electric signal supplied from the input terminal 14. Therefore, in the LD 9, an optical signal having a desired average optical power is obtained, and intermodulation distortion is reduced or eliminated.

Now, with reference to FIGS. 4A and 4B, a change in transmission quality according to this embodiment will be described.
FIG. 4A is a conceptual diagram showing the deterioration of the transmission quality due to the intermodulation distortion, and shows the level of each electric signal when transmitting three input electric signals whose frequencies are arranged at equal intervals. In this figure, it can be seen that the electric signal level at the third frequency is low and is greatly affected by distortion. Here, if the broken line in the drawing is the allowable value of distortion, the transmission quality of the electric signal of the third frequency becomes extremely low and the standard is not satisfied if this is left as it is.

Here, when signal transmission is performed using the distortion compensation optical transmitter according to the present embodiment, intermodulation distortion is reduced as shown in FIG. 4 (b). Therefore, FIG.
The transmission quality of the electric signal of the third middle frequency is improved. This means that the noise (R
IN) is the same, and if the noise generated in the optical transmission line is dominant, a signal having a phase opposite to that of the noise is added to the input of the electrical / optical converter 5 in the negative feedback loop (to the adder 13). Input) reduces the noise observed on the receiving side as a result.

As described above, according to the above-described embodiment, an optical signal having a desired average optical power can be obtained, and intermodulation distortion can be reduced or eliminated. Therefore, it is possible to improve the noise resistance and reduce the distortion, especially in the case of a low-level electric signal. In addition, the noise (RI
N) can be reduced or eliminated. Further, since the negative feedback loop is incorporated in the electric / optical converter 5, the electric length can be set short, the circuit can be configured at low cost, and highly reliable phase adjustment can be performed. be able to. In the above-described embodiment, the noise and the intermodulation distortion are described independently, but a synergistic effect that both can be simultaneously reduced by the optimum phase adjustment can be expected.

[0022]

As described above, according to the present invention,
In the electrical / optical conversion technology that converts a single or frequency-multiplexed electrical signal into an optical signal and transmits it, it is possible to improve the noise resistance characteristics of a low-level electrical signal and reduce distortion, and combine it with optical output control. It is possible to provide a negative feedback loop in the electric / optical converter, and it is possible to realize an economical and highly reliable distortion compensation optical transmitter. Further, for example, when the present invention is applied between the wireless device and the antenna, the present invention improves the noise or distortion characteristics of the optical transmission line, and the dynamic range required for the wireless system can be satisfied also in the optical transmission line. In particular, there is an effect that the minimum allowable reception level from the mobile terminal or the like can be further reduced. Further, there is an effect that the distance of the optical transmission line (optical fiber) can be increased.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a distortion-compensating optical transmitter according to an embodiment of the present invention, where (a) is a schematic configuration of the same distortion-compensating optical transmitter and (b) is a distortion-compensating optical transmitter. 1 shows a schematic configuration of an electric / optical converter 5.

FIG. 2 is a diagram for explaining the principle of reducing or eliminating intermodulation distortion generated in the LD 9 by the phase circuit 12.

3A and 3B are conceptual diagrams for explaining the same principle, where FIG. 3A shows an intermodulation distortion signal, and FIG. 3B shows a compensation process for another signal.

FIG. 4 is a conceptual diagram showing deterioration of transmission quality due to intermodulation distortion, in which (a) shows a relationship between an input signal and intermodulation distortion, and (b) shows a conventional optical transmitter and one of the present invention. The difference in effect from the distortion-compensating optical transmitter according to the embodiment is shown.

FIG. 5 is a block diagram showing a schematic configuration of an example of an electrical / optical converter 5 used in a conventional optical transmitter.

[Explanation of symbols]

 1-3, 14 Input terminal 4 Coupler 5 Electric / optical converter 6 Optical fiber (optical transmission line) 7 Optical / electrical converter 8 Bias circuit 9 Laser diode (electrical / optical converting means) 10 Optical / electrical converter 11 Bias control circuit 12 Phase circuit 13 Adder

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Kozo Morita 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (3)

[Claims] 1. A transmission side of an optical communication system in which a transmission side converts an electric signal into an optical signal and sends it out to an optical transmission line, and a reception side converts an optical signal output from the optical transmission line into an electric signal. A distortion-compensating optical transmitter for converting an electric signal into an optical signal by an electric / optical converter, wherein the electric / optical converter converts the electric signal into an optical signal and sends the optical signal to the optical transmission line. And an optical / electrical converter for converting the optical signal output from the electric / optical converter into an electric signal and outputting the electric signal, and an electric signal output from the optical / electrical converter. Electricity/
Means for performing negative feedback to the input side of the optical conversion means, and an output from the electric / optical converter based on a preset level and the level of the electric signal negatively fed back to the input side of the electric / optical conversion means Means for controlling the level of the electric signal input to the electric / optical conversion means so that the average optical power of the electric signal to be supplied is constant, and the phase of the negatively fed back electric signal is the electric / optical converter. Distortion compensating optical transmitter, characterized in that the electrical length of the negative feedback loop is set so as to have an opposite phase to the intermodulation distortion generated in (1) or the phase of the negatively fed back electric signal is adjusted. 2. A transmission side of an optical communication system in which a transmission side converts an electric signal into an optical signal and sends it out to an optical transmission line, and a reception side converts an optical signal output from the optical transmission line into an electric signal. A distortion-compensating optical transmitter for converting an electric signal into an optical signal by an electric / optical converter, wherein the electric / optical converter converts the electric signal into an optical signal and sends the optical signal to the optical transmission line. And an optical / electrical converter for converting the optical signal output from the electric / optical converter into an electric signal and outputting the electric signal, and an electric signal output from the optical / electrical converter. Electricity/
Means for performing negative feedback to the input side of the optical conversion means, and an output from the electric / optical converter based on a preset level and the level of the electric signal negatively fed back to the input side of the electric / optical conversion means Means for controlling the level of the electric signal input to the electric / optical conversion means so that the average optical power of the electric signal to be supplied is constant, and the phase of the negatively fed back electric signal is the electric / optical converter. Distortion compensating optical transmitter, wherein the electrical length of the negative feedback loop is set so as to have a phase opposite to that of the noise generated in 1. or the phase of the negatively fed back electrical signal is adjusted. 3. The electrical length of the negative feedback loop in the electrical / optical converter is such that the phase of the negatively fed back electrical signal is a phase that reduces intermodulation distortion and noise generated in the electrical / optical converter. Or the phase of the negatively fed back electric signal is adjusted.