JPH0435185A - Video signal optical transmission system - Google Patents

Abstract

PURPOSE:To send a video signal with high quality without use of an expensive isolator by sending a pulsized analog signal subject to LD modulation through a non-reflection type single mode fiber whose receiving end employs an oblique connector. CONSTITUTION:A PFM modulator 20 receives an input signal 50 and converts it into a PFM signal 61. The PFM signal 61 has a constant pulse width and has a pulse frequency which varies with a signal amplitude of the input signal 50. A semiconductor laser drive section 10 receives the PFm signal 61 to drive a semiconductor laser 11 and outputs an optical pulse signal. As a result, the optical pulse signal has a waveform with very small jitter like a signal 63 at a reception end. The optical pulse signal 63 is converted into a reproduced pulse signal 64 by a photodetector 14 and an opto/electric conversion section 15 and demodulated into an original video signal (output signal) 53 by a PFM demodulator section 21.

Description

[Detailed Description of the Invention] The present invention is applicable to the industrial field of application.
Furthermore, the present invention relates to a video signal optical transmission system in which a semiconductor laser is used to convert the signal into an optical pulse signal and transmit the signal.

2. Description of the Related Art One method for optically transmitting video signals is an analog method. Figure 5 shows a block diagram of the analog system, and Figure 6 shows its main waveform diagram. In the analog system, the video baseband signal or RF signal 50 input to input terminal 1 is
A semiconductor laser drive unit 10 and a semiconductor laser (LD) 11 are used to convert the signal into an optical signal and connect it to a single mode fiber (SM).
F)]2. Here, the point where the refractive index changes in the middle of the single mode fiber 12 (for example, at a connector, etc.)
If there is, a reflection phenomenon of the optical signal occurs at that point, and the generated reflected light returns to the light emitting element (LD) and I side. L
The reflected light that returns to DII is called reflected return light) 4%
If there is reflected return light, the reflected return light overlaps the optical signal output from the LD 11, and the signal becomes a signal with a lot of noise and distortion as shown at 51, significantly degrading the signal quality.

Measures against reflected light are extremely important in analog transmission. Specific measures include (friend 1) inserting an isolator 30 between the LD and the fiber to prevent reflected return light, 2) direct reading of the fiber using a splice or a connector with high return loss, and 3) connecting the light receiving element to the fiber. Diagonal connectors 3 are used in the joints. An optical signal propagated through a single-mode fiber with the measures described below (as shown in Figure 52) generates almost no noise or distortion. It is converted into an electrical signal (output signal) 53 and output from the output terminal 2.

In the analog system described above, the circuit configuration is simple (the noise and distortion generated by the noise directly degrades the transmission quality), and the use of expensive isolators as a countermeasure against reflected light returns (1) to 3 above. ) conditions are required.

On the other hand, in the pulsed analog system, the video signal is changed in advance to the repeating pulse signal frequency, pulse 1 low or pulse position, and then converted into an optical pulse signal by the LD and transmitted. In the receiving section, the received optical pulse signal is amplified into a pulse signal with a constant amplitude, and then it is identified (sliced) at a certain level, and the u H'''L'' of the pulse signal is
and demodulates the original video signal. Therefore, 11 HI+ or IILI+ of the optical pulse signal to be transmitted
Noise and distortion superimposed on the level can be removed by the identification section of the receiver. Therefore, the pulsed analog method (as compared to the pulsed analog method) has the advantage of being less susceptible to the effects of noise and distortion generated in the transmission system. It is also a relatively simple and short lesson.
It is often used for medium-distance (about 10 to 20 km) video signal transmission.

Problems to be Solved by the Invention Since the pulsed analog system described above is not easily affected by noise and distortion generated in the transmission system, no particular attention has been paid to the influence of reflected return light on transmission quality. However, in this method, the repetition frequency, pulse width, and pulse position of the optical pulse signal, in other words, the time of rise or fall of the pulse, are modulated in an analog manner, and the video signal is transmitted. If this happens, fluctuations will occur at the rising or falling edges of the optical pulse signal, and this fluctuation will degrade the transmission quality of the transmitted video signal as noise.

The present invention has been made in view of the above, and an object of the present invention is to provide a video signal optical transmission system that can transmit high-quality video signals by using a non-reflective single mode fiber and a diagonal connector. It is said that

Means for Solving the Problems The present invention solves the above problems by 1) converting a pulsed analog signal modulated by a video signal into an optical pulse signal using a semiconductor laser (LD), and converting the optical pulse signal into an optical pulse signal. Non-reflective single mode fiber (S
MF), the optical pulse signal is input to a light-receiving element using a diagonal connector, the light-receiving element converts the optical pulse signal into a pulsed analog signal, and demodulates the original video signal. and 2) In the signal transmission method described in 1) above, a component video signal consisting of a luminance signal and two color difference signals is subjected to pulse frequency modulation (PFM) on the luminance signal, and its P
The pulse width of the FM signal is alternately pulse width modulated (PWM) with the two color difference signals to make the component video signal one pulse train signal (PFWM), and the PFW
This is a video signal optical transmission system that converts the M signal into an optical pulse signal using a semiconductor laser (LD).

Effect of the Invention The present invention uses the method described above to suppress fluctuations in the rise or fall times of optical pulse signals caused by reflection, and can stably and accurately transmit high-quality video signals.

Embodiment As an example of a pulsed analog transmission system, the case of pulse frequency modulation (PFM) is shown in FIG. In FIG. 1, 20 is a PFM modulation plate, and 21 is a PFM demodulation section. Also, the same components as in FIG. 4 are designated by the same numbers and their explanations will be omitted.

In the following, using Figure 2 which shows the waveforms of the main parts of the same circuit,
The details of the operation will be explained.

The PFM modulator 20 converts the input signal 50 into an input LPFM signal 6
Convert to 1. In the PFM signal 61, the pulse width is constant,
The pulse repetition frequency varies proportionally to the signal amplitude of the input signal 50. +;! in the semiconductor laser drive unit 10; , P.F.
The M signal 61 is input to drive the semiconductor laser 11 and output an optical pulse signal.

Here, if there is a connector etc. in the single mode fiber 12 of the transmission line and there is reflected return light from the connector etc. to the LDII, the optical pulse signal output from the LDII and the reflected return light overlap, and at the receiving end, the optical signal 62 is "Stand like that."
This results in a waveform with fluctuating double or fall times (shaded area in the figure).

Such fluctuations in the rise or fall times of pulse signals (usually called ``jitter'') are noise in pulsed analog transmission systems and dominate the signal-to-noise ratio of the transmitted video signal.

Therefore, when connecting the single mode fiber 12,
By making the fiber non-reflective by using fusion bonding or by using a diagonal connector with a large return loss, and by using the diagonal connector 13 for coupling with the light-receiving element 14, reflected return light is prevented. The pulsed analog method is less susceptible to the effects of non-linear distortion in the transmission system than the analog method, so there is no need for an expensive isolator as a countermeasure against reflected return light, and a diagonal connector is used at the receiving end of the non-reflective fiber. This provides practically sufficient characteristics.

As a result, at the receiving end, the optical pulse signal has a waveform with extremely small jitter, such as signal 63.

The optical pulse signal 63 is converted into a reproduced pulse signal 64 by the light receiving element 14 and the optical/electrical converter 15, and then the original video signal (output signal) 53 is demodulated by the PFM demodulator 21. That is, the jitter due to reflection is By preventing this, high-quality video signals can be transmitted using a pulsed analog method.

As another example, FIG. 3 shows a PFWM modulation unit 6 that multiplexes component video signals into one pulse train.
FIG. 4 shows a waveform diagram of the main part of the waveform.

In the PFWM method (of the component video signal consisting of a luminance signal and two color difference signals, the luminance signal is subjected to pulse frequency modulation (PFM)), and the pulse width of the PFM signal is alternately pulse width modulated ( PWM) to multiplex component signals into one pulse train and transmit it.

The PFWM modulator 60 modulates the component video signals Y, Pb, and Pr, respectively, and multiplexes them to output one PFWM signal 70. Semiconductor laser drive unit 1
0 (Yo) Input the PFWM signal 70 and turn on the semiconductor laser 1.
1 is a drive unit that outputs an optical pulse signal 71;

The optical pulse signal 71 uses both the rising and falling edges of the pulse to transmit the component video signal. The waveform distortion caused by jitter deteriorates the linearity of each component video signal and the leakage (crosstalk) characteristics between component video signals.Therefore, in the PFWM method, the transmission path is constructed using a non-reflection single mode fiber and a diagonal connector. This is extremely effective in improving transmission quality as a countermeasure against reflected return light.

Effects of the Invention As described above, according to the present invention, an expensive isolator is used by transmitting a pulsed analog signal modulated by an LD through a non-reflection type single mode fiber with a diagonal connector at the receiving end. Therefore, it is possible to provide a video signal optical transmission system that is extremely useful in practice and can transmit high-quality video signals without any interference.

[Brief explanation of drawings]

Figure 1 shows the blocks of an embodiment of the circuit used in the video signal optical transmission system of the present invention. Figure 2 shows the main waveforms of the circuit. Figure 3 shows the blocks +21 of an embodiment of the circuit used in the PFWM transmission system. ．． Figure 4 shows waveforms of the main parts of the same circuit. Figure 5 shows block 1 of the circuit used in the analog system. Figure 6 shows waveforms of the main parts of the circuit. 0... Semiconductor laser drive unit, ■... Semiconductor laser, 2... Non-fouling single mode fiber λ3.
...Diagonal connector 4 ... Light receiving element, 5 ... Optical/electrical converter \ 21 ... PFM demodulator 20 ...)) FM Modulation section, 40... PFWM modulation section θ Name of agent Patent attorney Shigetaka Awano There is already one person in the figure and the figure is θ

Claims (2)

[Claims] (1) Convert a pulsed analog signal modulated by a video signal into an optical pulse signal using a semiconductor laser, transmit the optical pulse signal through a non-reflective single mode fiber, and then connect it to a light receiving element using a diagonal connector. 1. A video signal optical transmission system characterized in that the optical pulse signal is input to a light receiving element, and a light receiving element converts the optical pulse signal into a pulsed analog signal to demodulate the original video signal. (2) In the signal transmission method according to claim 1, a component video signal consisting of a luminance signal and two color difference signals is subjected to pulse frequency modulation (PFM), and the luminance signal is subjected to pulse frequency modulation (PFM).
The pulse width of this pulse frequency modulation signal is alternately pulse width modulated with the two color difference signals to convert the component video signal into one pulse train signal, and this pulse train signal is converted into an optical pulse signal using a semiconductor laser. A video signal optical transmission system characterized by: