JPH0936802A - Optical communication equipment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain stable distortion and noise characteristics even if the condition of an optical path is changed and to realize a wide dynamic range by superimposing the auxiliary signal of the level of an input electric signal or the level which does not depend on a frequency on an optical signal by the frequency which is different from the frequency band of the input electric signal. SOLUTION: This device is provided with an optical fiber 16 transmitting an optical signal between a transmission side and a reception side, an electrooptical converter 14 which is provided on the transmission side, converts an input electric signal into an optical signal and transmits the signal to an optical fiber 16 and an optic/electric converter 17 which is provided on the reception side and converts the optical signal from the optical fiber 16 into the electric signal. Further, an auxiliary signal oscillator 12 and an adder 13 are provided as a means superimposing the auxiliary signal of the level of the input electric signal and the level which does not depend on the frequency on the optical signal that the electrooptical converter 14 outputs by the frequency which is different from the frequency band of the input electric signal. As a result, the changes of noise and distortion characteristic can be mitigated and a wide dynamic range can be obtained as the whole of a system.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical communication device for converting an electric signal into an optical signal and transmitting the optical signal through an optical fiber or other optical transmission path. In particular, the present invention relates to a technique for maintaining high signal transmission quality even when a large number of light reflection points are present in an optical transmission line.

[0002]

2. Description of the Related Art FIG. 5 is a block diagram showing a conventional optical communication device for converting an electric signal into an optical signal for transmission.
This optical communication device has input terminals 1 to N, a coupler 111 and an electric / optical converter 114 on the transmitting side, and an optical / electric converter 117, a distributor 118 and output terminals 1p to N on the receiving side.
p, and the transmission side and the reception side are connected by an optical transmission line formed of an optical fiber 116. Single or frequency-multiplexed electrical signals are input to the input terminals 1 to N, and these electrical signals are combined by the coupler 111 and input to the electrical / optical converter 114. As the electric / optical converter 114, for example, a Fabry-Perot type laser diode (F
P-LD), distributed feedback laser diode (DFB-
LD) or a super luminescent diode (SLD) is used to convert an input electric signal into an optical signal. This optical signal is transmitted to the optical / electrical converter 117 on the receiving side via the optical fiber 116, converted into an electric signal, and distributed by the distributor 118 to the output terminals 1p to Np.

[0003]

However, the DFB-LD and FP, which have been conventionally considered to be used for this purpose, have been investigated.
-LD has good noise characteristics and distortion characteristics of a single element, but has a high coherence due to its narrow emission spectrum width,
Depending on the coupling state to the optical fiber, the reflected light at the optical connector, or the number of connection points (number), and the length and loss of the optical fiber, there is a problem that it is easily affected by fluctuations in distortion characteristics, deterioration in noise characteristics, and other factors. there were.

FIG. 6 is a diagram for explaining the problem of the conventional example.
The optical signal output from the electrical / optical converter 114 causes multiple reflection by the optical connector, which causes the electrical / optical converter 1
14 and the mechanism that affects the optical / electrical converter 117 are shown. As described above, the multiple reflection causes a beat between the direct light and the reflected wave, which is one of the causes for deteriorating the noise characteristic.

FIG. 7 shows a DFB-LD as an electric / optical converter.
It is a figure which shows the change of the noise characteristic by fiber length (the number of optical connector connection points) when using SLD and SLD, respectively. The triangles and squares in the figure represent measured values, and the dotted line and alternate long and short dash line represent theoretical values. From this figure, as the number of optical connector connections increases, the emission spectrum width becomes narrower.
The noise characteristic of D is greatly deteriorated. Further, the noise characteristics of the SLD having a wide emission spectrum width hardly change even when the fiber length is changed. However, when the SLD is used, the noise characteristic of the element itself is poor, and the noise characteristic of the entire system is inferior.

For this reason, the DF has a good noise characteristic of the element itself.
When a B-LD or FP-LD is used, distortion may occur depending on the conditions of the optical transmission line such as the number of connectors, the reflectance, the number of connection points, and the loss, and the level of the input electric signal input to the electric / optical converter 114. There is a problem that the noise characteristic changes, and particularly the noise characteristic deteriorates. On the other hand, the spectrum width is 4-5%
The SLD, which has a wide range and low coherence, exhibits stable noise and distortion characteristics regardless of the conditions of the optical transmission line due to its characteristics, so that the system design is easy, but the noise characteristics of the single element is D
Since it is much inferior to FB-LD and the like, there is a problem that it cannot be applied to a system that requires a wide dynamic range.

In other words, if the emission spectrum width is controlled by selecting an element, the noise characteristic does not depend on the fiber length if the emission spectrum width is wide, but the noise characteristic of the element alone deteriorates. On the contrary, if the emission spectrum width is narrow, the noise characteristic is good, but since the dependency of the noise characteristic on the fiber length is large, it is difficult to design the system, and it is not possible to satisfy both the wide spectrum width and the low noise.

The present invention solves such a problem, and uses an electric / optical converter composed of a light emitting element having low noise and low distortion, and a stable distortion, even if the conditions of the optical transmission line are changed, It is an object of the present invention to provide an optical communication device that can obtain noise characteristics and has a wide dynamic range.

[0009]

An optical communication apparatus according to the present invention comprises:
An optical transmission line that transmits an optical signal between the transmitting side and the receiving side,
An electric / optical converter provided on the transmitting side for converting an input electric signal into an optical signal and sending it to an optical transmission line, and an optical / electric conversion provided on the receiving side for converting an optical signal from the optical transmission line into an electric signal In an optical communication device including a receiver, an auxiliary signal having a level different from the frequency band of the input electric signal to the optical signal output from the electric / optical converter or a level independent of the frequency of the input electric signal on the transmission side. Is provided.

That is, using an electric / optical converter having a narrow emission spectrum width, which is composed of a light emitting element having low noise and low distortion,
At frequencies outside the frequency band of the input electrical signal on the transmitter side,
An auxiliary signal having a level that does not depend on the level or frequency of the input electric signal is used to expand the spectrum width of the optical signal. As the spectrum width of the optical signal becomes wider, noise due to reflection can be reduced, noise due to conditions such as fiber length and the number of connection points, and changes in distortion characteristics can be mitigated, and a wide dynamic range can be realized for the entire system. it can.

In order to superimpose the auxiliary signal, the auxiliary signal may be added to the input electric signal at the input of the electric / optical converter, and the optical signal output from the electric / optical converter using the external optical frequency modulator. May be frequency-modulated by the auxiliary signal. In the former case, the level of the total input electric signal is increased and the spectrum width of the optical signal can be expanded. In the latter case, since the optical signal is frequency-modulated, the spectrum width of the optical signal is similarly expanded.

It is preferable that the receiving side monitors the quality of the received signal on the receiving side and transmits the monitoring result to the transmitting side to control the frequency and level of the auxiliary signal. By controlling the level and frequency of the auxiliary signal according to the bit error rate and other reception qualities, it is possible to obtain the optimum improvement effect for the conditions such as the input signal condition, the fiber length, and the connection point.

It is also possible to extract an auxiliary signal by performing optical heterodyne detection on the optical signal from the optical transmission line. This allows
An output proportional to the spectral width of the optical signal is obtained, which makes it easier to control the noise characteristic.

A modulated wave obtained by modulating a carrier wave with a control signal or communication information may be used as the auxiliary signal. As a result, the auxiliary signal can be used not only for extending the optical spectrum but also as a carrier signal for control and communication.

In particular, the present invention is used in a radio system for receiving a plurality of radio signals, optically transmitting them to another base station as they are, detecting the optical signals and then detecting the radio signals, and realizing a wide dynamic range. The signal transmission quality can be improved.

[0016]

1 is a block diagram showing a first embodiment of the present invention. This optical communication device includes an optical fiber 16 for transmitting an optical signal between a transmitting side and a receiving side, and an electrical / optical conversion provided for the transmitting side to convert an input electric signal into an optical signal and send the optical signal to the optical fiber 16. And an optical / electrical converter 17 provided on the receiving side for converting an optical signal from the optical fiber 16 into an electric signal. Here, the feature of the present embodiment is that the optical signal output from the electrical / optical converter 14 has a level on the transmission side that is different from the frequency band of the input electrical signal and that does not depend on the frequency of the input electrical signal. The auxiliary signal oscillator 12 and the adder 13 are provided as means for superimposing the auxiliary signal of 1 above, and the reception quality detector 19 is provided as means for monitoring the quality of the received signal at the receiving side. The control circuit 15 is provided as control means for controlling the frequency and level of the auxiliary signal transmitted to the transmitting side.

The electric signals input to the input terminals 1 to N are
Combined by the combiner 11. An auxiliary signal S from an auxiliary signal oscillator 12 composed of an oscillator or modulator of a constant level is added to the combined electric signal by an adder 13, and the added electric signal is input to an electric / optical converter 14. Obtain an intensity-modulated optical signal. This optical signal is transmitted to the receiving side by the optical fiber 16, converted into an electrical signal by the optical / electrical converter 17, and each electrical signal is distributed to the output terminals 1p to Np and Sp via the distributor 18 and received. It outputs through the quality detector 19.

The reception quality detector 19 detects the quality of the received signal and sends it to the control circuit 15. Control circuit 15
Changes the frequency and level of the auxiliary signal oscillator 12 according to the reception quality such as C / N ratio and bit error.

FIG. 2 shows an example of a change in the emission spectrum width of the electric / optical converter 14, and FIG. 3 shows the relationship between the emission spectrum width of the electric / optical converter 14 and the amount of noise deterioration due to the state of the optical transmission line. Show. By changing the frequency or level of the auxiliary signal, the emission spectrum width emitted from the electrical / optical converter 14 changes, and if the emission spectrum width is widened, the amount of noise deterioration due to the optical transmission line (optical fiber 16) is small. Become. Therefore, by changing the frequency or level of the auxiliary signal oscillator 12 by the control circuit 15 according to the reception quality, the optimum reception quality can be efficiently maintained.

The electrical / optical converter 14 is an FP-LD.
, DFB-LD, etc. have a narrow emission spectrum, low noise,
A low distortion light emitting element is used. A PIN photodiode or an avalanche photodiode is used as the optical / electrical converter 17.

If the auxiliary signal is used as a carrier wave, a control signal and a communication signal can be transmitted together.

FIG. 4 is a block diagram showing a second embodiment of the present invention. The first embodiment of this optical communication device is to superimpose the auxiliary signal at the optical signal stage, not at the electrical signal stage, and to perform optical heterodyne detection on the receiving side of the auxiliary signal separately from the transmission signal. Different from the form.

The electrical signals input to the input terminals 1 to N are combined by the coupler 11 and input to the electrical / optical converter 14 to obtain an intensity-modulated optical signal. This optical signal is input to the external optical frequency modulator 21 and the auxiliary signal S from the auxiliary signal oscillator 12 composed of an oscillator or modulator of a constant level is used as a modulation signal of the external optical frequency modulator 21.
Enter The optical signal from the electrical / optical converter 14 is optical frequency modulated via the external optical frequency modulator 21, and the spectrum width of the optical signal is expanded according to the modulation degree. This optical signal is transmitted to the receiving side by the optical fiber 16, is split into two by the spectroscope 22, one is received by the optical / electrical converter 17, and the respective electrical signals are distributed by the distributor 18, and the reception quality detector It outputs to the output terminals 1p-Np via 19.

With respect to the other optical signal branched by the spectroscope 22, the light output from the light emitter 23 is combined by the multiplexer 24, received by the optical / electrical converter 25, and subjected to optical heterodyne detection. . The detected electrical signal is output to the output terminal Sp via the reception quality detector 19. The reception quality detector 19 detects the quality of the reception signal and sends it to the control circuit 15. The control circuit 15 changes the frequency and level of the auxiliary signal oscillator 12 according to the reception quality. By this means, as with the first embodiment, it is possible to efficiently maintain optimum reception quality.

In this embodiment, since the auxiliary signal is detected by the optical heterodyne detection, an output proportional to the spectral width of the optical signal can be obtained and the noise control can be further facilitated.

Also in the case of this embodiment, as in the first embodiment, the FP-LD or DFB- is used as the electro-optical converter 14.
A light emitting element such as an LD having a narrow emission spectrum, low noise, and low distortion is used. Further, as the optical / electrical converter 17, P
An IN photodiode or an avalanche photodiode is used.

[0027]

As described above, according to the present invention,
In an optical communication device that converts a single or frequency-multiplexed electrical signal into an optical signal for transmission, adds an input electrical signal to a low-noise, low-distortion light emitting element and newly superimposes an oscillator or modulator signal of a certain level. By doing, the state of the optical transmission line,
Particularly, stable noise and distortion characteristics can be realized regardless of the reflection characteristics. This makes it possible to realize an economical optical communication device having a wide input dynamic range.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the relationship between the frequency and level of an auxiliary signal and the optical spectrum width.

FIG. 3 is a diagram showing a relationship between an emission spectrum width and a noise deterioration amount due to a state of an optical transmission line.

FIG. 4 is a block diagram showing a second embodiment of the present invention.

FIG. 5 is a block configuration diagram showing a conventional optical communication device.

FIG. 6 is a diagram showing a mechanism of a reflected wave.

FIG. 7 is a diagram showing an example of changes in noise characteristics due to reflected waves.

[Explanation of symbols]

 1-N input terminal 1p-Np output terminal 11,111 Coupler 12 Auxiliary signal oscillator 13 Adder 14,114 Electric / optical converter 15 Control circuit 16,116 Optical fiber 17,25,117 Optical / electrical converter 18, 118 distributor 19 reception quality detector 21 external optical frequency modulator 22 spectroscope 23 light emitter 24 combiner

Claims (6)

[Claims] 1. An optical transmission line for transmitting an optical signal between a transmitting side and a receiving side, and an electric / optical device provided on the transmitting side for converting an input electric signal into an optical signal and transmitting the optical signal to the optical transmission line. An optical communication device comprising a converter and an optical / electrical converter provided on the receiving side for converting an optical signal from the optical transmission line into an electric signal, wherein the transmitting side has an electric / optical converter An optical communication device comprising means for superimposing an auxiliary signal having a level that is different from the frequency band of the input electric signal or a level that does not depend on the frequency of the input electric signal on the output optical signal. 2. The optical communication apparatus according to claim 1, wherein the superposing means includes means for adding the auxiliary signal to the input electric signal at an input of the electric / optical converter. 3. The optical communication device according to claim 1, wherein the superposing means includes an external optical frequency modulator that frequency-modulates the optical signal output from the electrical / optical converter with the auxiliary signal. 4. The reception side comprises: means for monitoring the quality of the received signal; and control means for transmitting the output of this monitoring means to the transmission side to control the frequency and level of the auxiliary signal. 4. The optical communication device according to any one of 3 to 3. 5. The optical communication device according to claim 1, further comprising means for extracting the auxiliary signal by optical heterodyne detection of the optical signal from the optical transmission line on the receiving side. 6. The optical communication device according to claim 1, wherein the auxiliary signal is a modulated wave obtained by modulating a carrier wave with a control signal or communication information.