JPS63169131A - Optical digital link - Google Patents

Abstract

PURPOSE:To improve reception sensitivity and an error rate by converting a digital electrical signal to an optical signal after superposing the signal on a carrier wave, then transmitting the optical signal, and eliminating the carrier wave, on the reception side, after converting the optical signal to an electrical signal to reproduce the digital electrical signal. CONSTITUTION:In a transmitter 1 in an optical digital link, a carrier wave generator 2 to generate a carrier wave of a prescribed frequency, a modulator 3 to modulate the carrier wave with a digital electrical signal, and an electro- optical converter 4 to convert an output signal from the modulator 3 to an optical intensity, are provided. In a receiver 6, an optical-electrical converter 7 to convert the optical signal signal to an electrical signal, and a demodulator 9 to eliminate the carrier wave from the electrical signal from the converter 7 and demodulate the original digital signal, are provided. Since such a modulation-demodulation system is employed, the noise is frequencies other than those close to carrier wave frequency can easily be eliminated by such as frequency-selective amplifiers 8, 12. As a result, S/N and the reception sensitivity can be improved, and the error rate can be lowered.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an optical digital link that once converts a digital electrical signal into an optical signal, transmits it via an optical transmission line, and restores it back to a digital electrical signal on the receiving side.

[Conventional technology]

An optical digital link consists of a transmitter that converts a digital electrical signal into an optical signal and transmits it, an optical transmission line that transmits the transmitted optical signal, and a receiver that receives the transmitted optical signal and converts it into a digital electrical signal. Conventionally, the optical intensity is usually directly modulated using a digital signal. Therefore, if waveform distortion or noise occurs during transmission, the received waveform will be distorted and the probability of transmission errors will increase; conventionally, automatic threshold control has been used to deal with this problem. . In other words, after converting a received optical signal into an electrical signal, when reproducing the original digital signal by shaping the waveform using a predetermined threshold value, if the threshold value is fixed, as shown in Figure 2. When the received signal is small compared to when the received signal is large, the stability of the reproduced digital signal is impaired.As shown in Figure 3, the threshold value is automatically set according to the peak of the received signal. This allows stable reproduction of digital signals and reduces the error rate.

[Problems to be solved by the invention]

However, although such automatic threshold control can certainly contribute to improving the error rate when the received signal increases or decreases overall, when noise of the same level as the received signal is mixed in, this noise The problem is that it cannot be removed. Furthermore, when the received optical signal is extremely weak, such as when the transmission distance is long, the magnitude of the electrical signal after photoelectric conversion is also small, so an amplifier with a very large gain is required. Moreover, since this amplifier needs to operate with a large gain over a wide band, it is not easy to manufacture and is also a factor in increasing costs. An object of the present invention is to provide an optical digital link that is resistant to noise, can be expected to further improve the error rate, is easy to manufacture, and can be manufactured at low cost.

[Means to solve the problem]

According to this invention, an optical digital link transmitter includes a carrier wave generator that generates a carrier wave of a predetermined frequency, a modulator that modulates this carrier wave with a digital electrical signal, and converts the output signal of this modulator into optical intensity. The receiver is equipped with an opto-electric converter that converts the received optical signal into an electrical signal, and a carrier wave is removed from the electrical signal from the opto-electric converter to generate the original digital signal. It is equipped with a demodulator that demodulates the information.

[For production]

In the optical digital link of this invention, instead of directly converting a digital electrical signal into an optical intensity signal, the digital electrical signal is placed on a carrier wave, converted to an optical signal, and transmitted, and the receiving side converts the optical signal into an electrical signal. After conversion, the carrier wave is removed to reproduce the digital electrical signal. Since this modulation-demodulation method is used, noise can be easily removed. That is, noise at frequencies other than the vicinity of the carrier frequency can be easily removed using a frequency selective amplifier or the like. Since the S/N ratio can be increased in this way, reception sensitivity can be improved and error rate can be improved. In addition, since the amplifier on the receiving side (the amplifier after the signal is converted into an electrical signal) has a narrow frequency bandwidth, it is easy to manufacture one with high gain and good stability, which leads to cost reduction. .

【Example】

In FIG. 1, a transmitter 1 includes an oscillator 2 and a modulator 3.
and an electro-optical converter 4. The oscillator 2 generates a carrier wave of a constant frequency, and this carrier wave is modulated by a digital electrical signal in the modulator 3. The modulation scheme may be FM or AM or other modulation scheme. The thus modulated signal is sent to the electro-optical converter 4 and converted into an optical intensity signal. This optical signal is transmitted through an optical transmission line 5 such as an optical fiber (or space),
The signal is sent to the receiver 6. In the receiver 6, the received optical signal is first converted into an electrical signal by an opto-electrical converter 7, and after this electrical signal is amplified by an amplifier 8, it is sent to a demodulator 9. Here, among the electrical signals output from the opto-electrical converter 7, only the frequency component of the carrier wave contains a signal component, so the amplifier 8 is a narrow band tuning amplifier that amplifies only the frequency of this carrier wave. It is preferable that The demodulator 9 is, for example, an oscillator 10
, a mixer 11 , an amplifier 12 , and a detector 13 . In this case, the output signal of the amplifier 8 and the oscillator 8
A mixer 11 mixes the signal with a predetermined frequency from the two, to obtain a signal with a frequency corresponding to the difference between the two frequencies, and this signal is amplified with an amplifier 12. Preferably, this amplifier 12 is also a narrow band tuned amplifier that amplifies only the frequency of the above difference. By detecting the output signal of this amplifier 12 with a detector 13 and removing the frequency component of the above difference, the original digital electric signal can be reproduced. In this embodiment, the S/N ratio can be increased by using a frequency selective amplifier as the amplifier 8.12. This also means that the amplifier does not have to be wideband, making it possible to easily obtain an amplifier with high stability and high gain, and to reduce costs.

【Effect of the invention】

In the optical digital link of the present invention, noise can be easily removed and the S/N ratio can be increased, so reception sensitivity can be improved and error rate can be improved. Further, the amplifier on the receiving side may have a narrow frequency bandwidth, and even high gain amplifiers with good stability can be easily manufactured, and costs can be reduced.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of an embodiment of the present invention, and FIGS. 2 and 3 are waveform diagrams for explaining a conventional example. DESCRIPTION OF SYMBOLS 1... Transmitter, 2.10... Oscillator, 3... Modulator, 4... Electro-optical converter, 5... Optical transmission line, 6...
-Receiver, 7...Optoelectrical converter, 8.12...Amplifier, 9...Demodulator, 11...Mixer, 13...
Detector.

Claims (1)

[Claims] (1) A transmitter that converts a digital electrical signal into an optical signal and transmits it, an optical transmission line that transmits the transmitted optical signal, and a receiver that receives the transmitted optical signal and converts it into a digital electrical signal. In the optical digital link, the transmitter includes a carrier wave generator that generates a carrier wave of a predetermined frequency, a modulator that modulates this carrier wave with a digital electrical signal, and an electric generator that converts the output signal of this modulator into optical intensity. The receiver includes a photoelectric converter that converts the received optical signal into an electrical signal, and a carrier wave is removed from the electrical signal from the photoelectric converter to demodulate the original digital signal. An optical digital link characterized in that it includes a demodulator and a demodulator.