JPH02235449A - Optical communication method - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an optical communication system, and in particular, on a transmission path or
The present invention relates to a communication method in an optical communication system in which an optical amplifier is provided at the end of a transmission line.

[Prior Art] In an optical communication system that uses an optical fiber as an optical transmission line or uses optical space propagation as an optical transmission line, the attenuation of the optical signal in the transmission line is determined by the signal-to-noise power ratio (S) at the optical receiver. /N), degrades the quality of the received signal, and ultimately impairs the reliability of optical transmission, which cannot be ignored.

Therefore, a method is being considered in which an optical fiber with low attenuation of the optical signal is used as an optical transmission line, an optical amplifier is provided in the middle of the optical fiber, and the signal light is amplified and transmitted.

An example of a conventional optical communication system equipped with such an optical amplifier is shown in FIG.

As shown in FIG. 3, in the conventional optical communication system, a transmission digital signal 5 to be transmitted is input to a laser drive circuit 12, and a semiconductor laser l3 is turned on/off according to the "l"/"o" values of the signal. be done. Semiconductor laser l3 is O
The laser light signal emitted at the time of N is transmitted through the optical fiber 41, and the optical amplifier 43 amplifies the light intensity attenuated by the transmission through the optical fiber 41 to ensure the necessary light intensity. is transmitted to the photodetector 21
incident on . The incident light is converted by the photodetector 21 into an electrical signal corresponding to the intensity of the incident light. This optical reception signal is amplified by an amplifier 22 and output as a reception signal 6.

Note that FIG. 3 also shows an example of the transmission signal waveform.

[Problems to be Solved by the Invention] However, the optical amplifier 43 used in the conventional optical communication system not only optically amplifies the signal light but also outputs high-level spontaneous emission light due to DC excitation. This leads to deterioration of the S/N ratio and extinction ratio due to spontaneous emission, and good transmission quality cannot be obtained.

For this reason, the signal light output from the optical amplifier 43 has: (1) an optical modulation degree (on-off ratio) deteriorated compared to that of the input light;

(2) So-called beat noise occurs due to interference between human signal light and high-level spontaneous emission light caused by DC excitation, resulting in S/N
will deteriorate.

There was a drawback.

[Means for Solving the Problems] The present invention has been made for the purpose of solving the above-mentioned problems, and includes the following configuration as one means for solving the above-mentioned problems.

That is, in an optical communication system in which an optical amplifier is provided at at least one of the optical transmitting end, the transmission path, and the optical receiving end of an optical transmission line, it is possible to pre-modulate the transmitted signal with a subcarrier signal having a higher frequency than that of the transmitted signal. a modulation means, an optical output means disposed at an optical transmission end for emitting the modulated signal from the pre-modulation means onto a transmission path as a corresponding optical output, and converting the light received from the transmission path into a corresponding electrical signal. comprising a receiving means, a filtering means for filtering the received signal received by the receiving means, with a subcarrier signal frequency modulated by the modulating means as approximately the filtering center frequency, and a demodulating means for demodulating the filtered signal by the filtering means. .

[Function] In the above configuration, by performing optical communication using an optical signal modulated at a subcarrier frequency higher than that of the transmission signal during optical signal transmission, the degree of modulation of the signal light is reduced by the spontaneous emission light generated from the optical amplifier, and This makes it possible to remove S/H deterioration and obtain a received signal of good quality.

[Example] Hereinafter, an example according to the present invention will be described in detail with reference to the drawings.

FIG. 1 is a configuration diagram of an embodiment that best represents the features of the present invention. In the figure, 1 is an optical transmitter, 2 is an optical receiver,
3 is an optical amplifier, 5 is a transmission signal, 6 is a reception signal, 41.4
2 is an optical fiber which is an optical transmission line.

In the optical transmitter l, 10 is a mixer, l1 is a local oscillator that generates a subcarrier signal that is a predetermined high frequency signal,
12 is a laser drive circuit that turns on/off the semiconductor laser l3 in response to the input signal "l"/"0", l3 is a semiconductor laser, and the light emitted from the semiconductor laser 13 is transmitted into an optical fiber 41. and is sent to the optical amplifier 3.

In the optical receiver 2, 21 is a photodetector made of, for example, a PIN type photodiode, and converts the incident light from the optical fiber 42 into a corresponding voltage signal. 22 is an amplifier for amplifying the electric signal from the photodetector 2l, 23 is a bandpass filter having the same center frequency as the frequency of the subcarrier signal generated by local oscillator 11, and 24 is a wave detector.

The optical amplifier 3 of this embodiment includes, for example, the Japanese Patent Publication No. 57-671
A traveling wave type semiconductor amplifier disclosed in No. 5 is preferably used.

The operation of this embodiment having the above configuration will be explained below with reference to FIG.

FIG. 2 shows A, B, and C in FIG. FIG. 2 is a conceptual diagram showing an example of the waveform of an electrical signal or an optical signal at each portion indicated by D and E. FIG.

For example, as shown in FIG.
is input. This transmission signal 5 is a predetermined frequency (high frequency) generated by the local oscillator 11 by the mixer 12.
The subcarrier signal is mixed with the subcarrier signal, converted into the modulated signal shown in FIG. 2B, and input to the laser drive circuit 12. The laser drive circuit 12 drives the semiconductor laser 1 according to this modulation signal.
3 on/off to modulate the optical signal output.

As is well known, the semiconductor laser 13 emits light only when a voltage is applied in the forward direction, so the optical output from the semiconductor laser 13 is as shown in FIG. 2C. Emitted light from the semiconductor laser 13 propagates through the optical fiber 41 and is eventually sent to the optical amplifier 3. The light is then amplified to a predetermined intensity by the optical amplifier 3 and then output to the optical fiber 42. The light then propagates through the optical fiber 42 and reaches the photodetector 21 of the optical receiver 2. The light incident on the photodetector 2l is converted here into an electrical signal corresponding to the intensity of the received light. The signal is then amplified by an amplifier 22 and then sent to a bandpass filter 23. As mentioned above, the bandpass filter 23 is a bandpass filter for a predetermined frequency band whose center frequency is the modulation frequency on the transmitting side (the frequency of the subcarrier signal generated by the local oscillator 11), and removes unnecessary signals other than the subcarrier signal. Noise components etc. are removed. Then, only this subcarrier signal is sent to the detector 24, where it is detected and demodulated into the original frequency signal. This demodulated signal is output as a received signal 6 that is substantially similar to the transmitted signal shown by E in FIG.

In this embodiment as well, spontaneous emission light is superimposed on the output of the optical amplifier 3, and this influence also extends to the output of the photodetector 21. As a result, the modulation degree and S/N ratio of the output of the optical amplifier 22 are deteriorated compared to the transmission signal 5. However, the electrical frequency spectrum of the spontaneously emitted light from the optical amplifier 3 is in the form of direct current superimposed with white noise, and even after this optical signal is converted into an electrical signal, it has the same frequency spectrum. Therefore, the noise signal power caused by the spontaneous emission hardly passes through the bandpass filter 23.

On the other hand, the optical signal transmitted from the optical transmitter l is modulated by the subcarrier signal from the local oscillator 11, and the center frequency of its spectrum matches the center frequency of the bandpass filter 23. Therefore, most of this modulated signal power can pass through the bandpass filter 23.

As a result, the output signal from the bandpass filter 23 has good quality as shown in FIG. 2D, with the influence of spontaneous emission removed, and the signal detected from this signal also has a high quality signal as shown in FIG. 2E. Modulation depth and S/N that closely match the input signal to transmitter 1
Both ratios can be excellent.

[Other Embodiments] Although one embodiment of the present invention has been described above based on FIG. 1, the present invention is not limited to the above-described embodiment.

Although FIG. 1 illustrates an example of an optical fiber communication system using an optical fiber as an optical transmission path, the present invention can also be applied to space propagation optical communication. In this case, the optical amplifier may be provided at at least one of the output of the optical transmitter and the input of the optical receiver.

In addition, in the embodiment shown in Fig. 1, a one-way communication system was considered in which an optical transmitter was installed at one end of the transmission line and an optical receiver was installed at the other end. Of course, the present invention can also be applied to a bidirectional communication system in which both receivers are provided and optical signals are branched and merged at the end of the transmission line.

Furthermore, in the above embodiment, communication between two points was taken as an example, but by providing a transmission path between three or more points and exchanging etc., any two
The present invention is also effective in so-called optical communication networks that perform point-to-point communication or arbitrary multi-point communication.

Furthermore, although the above embodiment describes an optical communication system in which the optical transmitter is provided with only one light source, the optical transmitter is provided with multiple light sources with different wavelengths and a combiner, and the optical receiver is It is clear that the present invention can also be applied to a so-called wavelength division multiplexing optical communication system in which a branching filter is provided on the side of the optical fiber to simultaneously communicate a plurality of signals.

As explained above, according to this embodiment, in an optical communication system using an optical amplifier, an input signal is superimposed on a subcarrier signal, pre-modulated, and then transmitted, and the received signal is made to have a frequency substantially equal to that of the subcarrier signal. With a simple configuration in which the wave is filtered by a bandpass filter with a center frequency of There is an effect that can be obtained. [Effects of the Invention] As explained above, according to the present invention, it is possible to eliminate deterioration in modulation degree and S/N caused by spontaneous emission light generated by an optical amplifier, and to obtain good transmission quality. .

[Brief explanation of the drawing]

Fig. 1 is a conceptual diagram of an optical communication system according to an embodiment of the present invention, Fig. 2 is a diagram showing waveforms of electrical signals and optical signals at each part in Fig. 1, and Fig. 3 is a diagram showing conventional optical communication. It is a conceptual diagram of the method. 42... Optical fiber.

Claims (1)

[Claims] In an optical communication system in which an optical amplifier is provided at least one of an optical transmission end, a transmission path, and an optical reception end of an optical transmission line, the transmission signal is converted into a subcarrier signal having a higher frequency than the signal. a modulating means for modulating the signal, a light outputting means for emitting the modulated signal from the modulating means onto the transmission path as a corresponding optical output, and a receiving means for converting the received light from the transmission path into a corresponding electrical signal. , comprising a filtering means for filtering a received signal received by the receiving means using a subcarrier signal frequency modulated by the modulating means as approximately the filtering center frequency, and a demodulating means for demodulating the filtered signal by the filtering means, An optical communication system characterized by optical communication using optical signals modulated at a subcarrier frequency higher than that of the transmitted signal during signal transmission.