JPS58121843A - Signal transmission system with optical fiber - Google Patents

Abstract

PURPOSE:To obtain signals with high quality, by providing a means controlling the gain of a variable gain amplifier with an output of a differential amplifier, and obtaining a signal being reduced interference noise component from the amplifier. CONSTITUTION:An output signal of a photo diode 9 is led to a branching filter 12, an output from an output terminal 12 of the filter 12 is led to the variable gain amplifier 17, the output from an output terminal 14 of the filter 12 is led to the differential amplifier 16 to a reference terminal 15 of which a positive level DC voltage is applied, and the output of the amplifier 16 is given to a gain control input terminal 18 of the amplifier 17. That is, the degree of interference caused in the transmission system is detected by checking the component around DC of the reception signal and the disturbance of transmission signals is corrected by using this method.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal transmission system using an optical fiber, and more particularly to a signal transmission system using an optical fiber suitable for obtaining a wide band and high quality signal.

Conventionally, when converting a wideband signal, for example, a V) (F broadcast TV multiplex signal) into an optical signal and transmitting it through an optical fiber,
The signal transmission system has the configuration shown in FIG. That is, after a human-powered electric signal inputted to the terminal 1 is guided to the drive circuit 2 and amplified, the laser diode 3 is driven by the signal. In this case, the bias point of the laser diode 3 and the amplitude of the human power signal are appropriately set so that the light emission output from the laser diode 3 has a linear relationship with the human power electric signal. The light emission output from the laser diode 3 is connected to the coupling part 4
, the optical signal from the optical fiber 6 is received by the photodiode 9 via the connector 7 and the coupling part 8, and the optical signal is converted into optical power. This current is amplified by an amplifier 10, and an electrical signal of a predetermined level is extracted from a terminal 11.

However, the signal transmission system using optical fibers described above has the following major drawbacks. That is, the optical signal entering the optical fiber 6 from the laser diode 3 travels backwards due to the reflection phenomenon at the coupling part 4 and the optical connector 5.
The light enters the laser diode 3 again, thereby disturbing the light emitting function and changing the light emitting wavelength and light emitting output. As a result, intensity modulation is applied to the light emission output.

Now, if the human electric signal to terminal 1 is (1+mf(t)l(2)ω.t), where m: video signal modulation degree f(t): video signal ω.: carrier angular frequency, then the optical signal is CI +K (1+mf (t)) - ω, t)oo
s(, 1st, where k: optical modulation degree ωe: carrier angular frequency) If the operation of the laser diode 3 becomes unstable for the reasons mentioned above, (1+n(t)) becomes The sum of the numbers becomes (1+n(t))(1+k(1+mf(t))cog,
. t,) cos, et Here, n (t): It comes to be expressed as a noise signal, and when this is optically detected, (1+
n(t))(:1+k(1+mf(t))cosωo
t ) = 1 + n (t ) 1k (1+ m
f (t) ) aysto (, t+kn(tH1+
mf(t))cosω.

Then, it becomes (1). The first term "1" on the right side of equation (1) is a DC component, the second term r](t) is a noise component, and the third term k(1+m f
(t) cnsto ot is the signal component, the fourth term kn
(t>(1+mf(t))□□□ωo1 is an interference component between noise and a signal.

Here, considering the nature of n(t), its frequency spectrum becomes as shown in Figure 2, and in the low range where the angular frequency ω is small, the power P(P-b/ω2+a0.1.b The higher the frequency (electronic number) is strong and ω is large, the weaker the power P is.

According to experimental results, most of the energy falls within a frequency range of several MHz.

The problem here is a multiplex transmission system for TV signals in the so-called VHF band of several tens of MI (2 or more), and the second and sixth terms on the right side of equation (1) can be separated by a high-pass filter. However, the fourth term is kn(t)(1+mf(
t)), which originally has a strong low frequency component (
1+mf(t)), the low-frequency interference signal ends up remaining.

The present invention has been made in view of the above, and an object of the present invention is to provide a signal transmission system using optical fibers that can reduce interference noise components and obtain high quality signals.

The first feature of the present invention is that the output of a photodiode that converts an optical signal from an optical fiber into a current signal is passed through a demultiplexer to separate it into a low frequency component and a high frequency component. The above low-frequency component is manually powered by a variable gain amplifier, and a positive level DC voltage is applied to the above low-frequency component to a differential amplifier, and the output of this differential amplifier controls the gain of the above variable gain amplifier, and the above variable gain amplifier interferes with the interference. The present invention is configured to include means for obtaining a signal with reduced noise components. The second feature is that the laser diode that generates the optical signal is further provided with a means for giving a signal that is the sum of a human-powered electric signal and a pilot sine wave signal with a different frequency band, and is provided before and after the upper splitter. A high-pass filter is provided, and an amplitude detection circuit is provided between the duplexer and the differential amplifier.

The present invention will be described below with reference to the embodiments shown in FIGS. 3 and 5 and the embodiments shown in FIG.
This will be explained in detail using FIG.

FIG. 3 is a block diagram showing an embodiment of the optical fiber signal transmission system of the present invention. The same parts as in FIG. In FIG. 3, the output signal of the photodiode 9 is guided to a branching filter 12, the output from the output terminal 16 of the branching filter 12 is guided to a variable gain amplifier 17, and the output signal from the output terminal 14 of the branching filter 12 is guided to a variable gain amplifier 17. The output of the differential amplifier 16 is such that a DC voltage of a positive level is applied to the reference terminal 15, and the output of the differential amplifier 16 is outputted from the differential amplifier 16VC.

Figure 4 is a waveform diagram 1 to explain the operation of Figure 3.
Then, the signal 21 to be transmitted in FIG. 4(a) is connected to the terminal 1.
is applied, the output power of the photodiode 9 is (1
), the original signal k(1+mf(t)lemθ, , t
, interference signal N+n(t)) near DC, kn(t)(1'+mf(t)cosω.t) occurring near the original signal.
It is the sum of the interference signals. Therefore, first, in order to take out the interference signal n, (by using the difference in the band), the output signal of the photodiode 9 is guided to the duplexer 12, and from the duplexer 12 k(1+n(t))N+mf( t)) is obtained and this output is led from the terminal 16 to the variable gain amplifier 17, while an output shown as [1+n(t)) is obtained and this output is led from the terminal 14 to the differential amplifier 16. give to Since a positive level DC voltage is applied to the differential amplifier 16, the output of the differential amplifier 16 is as shown in FIG. 4(C).
A voltage proportional to i(1+n(t)l) is obtained as indicated by 23. Since this output voltage is applied to the gain control input terminal 18 of the variable gain amplifier 17, the amplification gain of the variable gain amplifier 17 changes and the terminal 11, k(1+n''(t)1(1+mf(t))aos shown at 24 in FIG. 4(d)
ω. A signal t is output.

By the way, since 1n(t)1<:1 generally, the output of the variable gain amplifier 17 is yBemwω. Indicated by t.

In other words, the degree of interference that occurs in the transmission system is determined by the DC of the received signal.
This is detected by examining nearby components, and this is used to correct disturbances in the transmitted signal. Therefore, according to the embodiment of the present invention, a high quality signal with reduced interference noise signals can be obtained.

FIG. 5 is a configuration diagram showing another embodiment of the present invention, and FIG.
The same parts as in FIG. 3 are indicated by the same reference numerals. The key point of the present invention is to extract the interference signal n(t) generated in the transmission system using the difference in band from the main signal (1+m f (t) ) (IJOt), and use this to perform correction. Particularly, in FIG. 3, the duplexer 12 is used to extract the interference signal, but in FIG.
7 and (1+m f(t) l・(2)ω(,1
A pilot sine wave signal 2 shown as mp(2)ωpt which has a different frequency band from the input electric signal 25 shown as
6 is added together and inputted to the drive circuit 2 via the output terminal 1 of the adder 27, and then the laser diode 6, the coupling part 4, the optical connector 5, the optical fiber 6, the original connector 7, the coupling part 8, the photodiode 9, etc. Through the optical transmission system, as the output of the photodiode 9, (1+n(t)l(1+k(1+mf(t)coaa+
gt-1-mpaosa+pt)). This signal is (1)D
Component near O 1+n(t) (2) Component near signal spectrum (1+n(t))kmf(t)aoi+ω. t(3) component near the high lot a spectrum (1+n(t))km
It consists of three types: pco[la+pt, but each band has a different ω. , ωp so that each of the components (1) to (3) can be separated by a filter, and the output of the photodiode 9 is first passed through the high-pass filter 28 to remove the component (1) above. , then (2), (
The components of 6) were separated. And in this (
The output 30 resulting from the component (2) is guided to the variable gain amplifier 17, and the output 31 resulting from the component (3) is guided to the amplitude detection circuit 32.
to obtain an interference wave component (1-1-r>(t)), give this interference wave component to the differential amplifier 16, and as in FIG. It is applied to the gain control input terminal 18, and the amplification gain of the variable gain amplifier 17 is changed to obtain the component (1+n(t)) near the signal spectrum.
)kmf(t)cma+ot and the interference wave n(t)Q is removed. In addition, in an actual VHF multiplex transmission system, as shown in FIG.
Since 12-channel transmission is performed using the band, the pilot signal is preferably about 200 MHz. The same effect as in the case of FIG. 3 can be obtained by using ramie as shown in FIG. 5.

As explained above, according to the present invention, interference noise components can be reduced and high quality signals can be obtained, and furthermore, since no special circuit is used, the present invention can be easily realized.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional signal transmission system using optical fibers, Fig. 2 is a diagram showing the noise spectrum generated in the transmission path, and Fig. 3 is an embodiment of the signal transmission system using optical fibers of the present invention. 4 is a waveform diagram for explaining the operation of FIG. 3, FIG. 5 is a configuration diagram showing another embodiment of the present invention, and FIG. 6 is a diagram showing the pilot arrangement in FIG. 5. It is. 2: Drive circuit, 6: Laser diode, 4.8 = Coupling unit, 5, 7: Optical connector, 6: Optical fiber, 9: Photodiode, 12.29: Duplexer, 16: Differential amplifier, 1
7: Variable gain amplifier, 27: Adder, 28: High frequency filter
Router, 32: Amplitude detection circuit. f l 圀rzEJ' ω 3 m ノ5t6 枦 4

Claims (1)

[Claims] 1. An optical signal modulated by a human-powered electric signal from a laser diode is input into an optical fiber and transmitted, and the optical signal from the optical fiber is converted into a current signal using a photodiode. In the device, the output of the photodiode is passed through a duplexer to separate it into a low frequency component and a high frequency component, the high frequency component is inputted to the variable gain amplifier, and the low frequency component is means for applying a positive level DC voltage to a differential amplifier, controlling the gain of the variable gain amplifier with the output of the differential amplifier, and obtaining a signal with reduced interference noise components than the variable gain amplifier. A signal transmission system using optical fiber, characterized by comprising: 2. An optical signal from a laser diode modulated by a human-powered electric signal is transmitted by entering an optical fiber, and the optical signal from the optical fiber is converted into a current signal using a photodiode, means for providing a signal obtained by adding the human-powered electric signal and a pilot sine wave signal having a different frequency band to the laser diode, and passing the output of the photogate through a high-pass filter and then a splitter to separate low-frequency components. The high-frequency component is input to a variable gain amplifier, and the low-frequency component is passed through an amplitude detection circuit and then applied with a positive level DC voltage to be applied to a differential amplifier. A signal transmission system using an optical fiber, comprising: means for controlling the gain of the variable gain amplifier at the output to obtain a signal with reduced interference noise components from the variable gain amplifier.