JPH06152521A - Frequency division multiplex light transmission system - Google Patents

Abstract

PURPOSE:To prevent the degradation of a C/N and distortion characteristic due to the decrease of the number of channels for light transmission. CONSTITUTION:A level adjustment is executed by a detector circuit 15, a differential amplifier 14, and a PIN variable attenuator 13, so that the total sum of the signal level in the entire transmission band of a signal outputted by an amplifier 12 can be equal to a value indicated by a prescribed voltage Vr1. Therefore, the level of the signal inputted to an amplifier 11 is decreased according as the number of the channels is decreased, and the attenuating amount of the PIN diode variable attenuator 13 is decreased so that the total sum of the signal level outputted by the amplifier 12 can be equal to the value indicated by the reference voltage Vr1. On the other hand, at a reception side, the level at the time of demodulation is adjusted by a band pass filter 24, a detector circuit 26, a differential amplifier 27, and a PIN variable attenuator 22, so that the level of the pilot signal of a prescribed frequency can be turned to a prescribed value. The connection of an optical fiber 3 with a coaxial cable can be attained without deteriorating the C/N and the distortion characteristic.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to frequency division multiplexing (FD).
M) Optical transmission system. In particular, the present invention relates to a system that prevents a decrease in C / N ratio due to a change in the number of channels used.

[0002]

2. Description of the Related Art Recently, an FDM signal optical transmission system has been proposed in which a laser beam is directly amplitude-modulated by a frequency-multiplexed signal and sent out to an optical transmission line. This method has excellent compatibility with the conventional CATV system using a coaxial cable, and
Since the non-relay transmission distance can be long, the number of relay amplifiers can be reduced, the transmission equipment cost per channel is low, and it is suitable for the CATV network trunk line.

[0003]

However, the CAT
In the case of the V system, the number of channels used is not fixed and varies with time. That is, the number of carriers in the entire transmission band is not constant, and the number of carriers increases in proportion to the number of channels used. On the other hand, when the oscillation light of the laser diode is amplitude-modulated by the frequency-multiplexed electric signal, the frequency modulation is simultaneously performed. This phenomenon is called chirping, and the spectrum line width when chirped becomes wider as the number of carriers of the frequency multiplexed signal increases. As a result, the C / N ratio in the optical transmission line decreases as the number of carriers decreases. That is, if the level of the modulation signal input to the laser diode is set in a state where all channels are used, there is a problem that the C / N ratio decreases as the number of channels used decreases.

The present invention has been made to solve the above problems, and an object thereof is to prevent the C / N ratio from being lowered even if the number of channels used changes in an optical transmission system. Is.

[0005]

The structure of the invention for solving the above-mentioned problems is, in an FDM optical transmission system, a first conversion device for converting an electric signal into an optical signal and an electric signal of a first electric transmission line. A variable gain amplifier for amplification, a level detector for detecting the sum of signal levels in all transmission bands, a deviation of the total sum of signal levels detected by the level detector from a predetermined level is calculated, and the deviation signal is used for the variable gain amplifier. , Which is a variable gain signal output to a variable gain amplifier, and an electrical / optical conversion element that amplitude-modulates the laser light by the electrical signal output from the variable gain amplifier to convert it into an optical signal. The second conversion device for converting into a signal,
An optical / electrical conversion element that amplitude-demodulates an optical signal transmitted through an optical transmission line to convert the optical signal into an electric signal; a variable gain amplifier that amplifies the electric signal demodulated by the optical / electrical conversion element; A predetermined frequency level detector that detects the signal level of a signal of a predetermined frequency among the electric signals that are output, and a deviation of the level detected by the predetermined frequency level detector from the predetermined level are calculated, and the deviation signal is output to the gain variable amplifier. It is characterized by comprising a comparator which outputs a variable gain signal to a variable gain amplifier.

[0006]

The laser light is amplitude-modulated by the frequency-multiplexed electric signal, and the sum of the signal levels in the entire transmission band of the frequency-multiplexed electric signal, which is the modulation signal, becomes a predetermined constant level. Adjusted by a variable gain amplifier.
Therefore, if the number of used channels decreases, the sum of the signal levels in all transmission bands is adjusted to be constant, so that the signal level of the used channels increases. In this state, even if the laser light is amplitude-modulated and the amplitude-modulated light is transmitted through an optical transmission path such as an optical fiber, the C / N ratio does not decrease.

On the other hand, when the optical signal is demodulated into an electric signal, the variable gain amplifier adjusts so that the signal level in the used channel of the demodulated electric signal becomes a predetermined level. As a result, the frequency-multiplexed electric signal transmitted through the second electric transmission line is adjusted so that the signal level of each channel becomes a predetermined level. Therefore, C by the coaxial transmission line
The transmission standard such as ATV system can be sufficiently satisfied, and the distortion characteristics and the C / N ratio are not deteriorated.

[0008]

1 is a diagram showing a configuration of an optical transmission system according to a specific embodiment of the present invention. Coaxial cable (first electric transmission path) 4 for transmitting frequency-multiplexed electric signals
Are connected to the first converter 1. The first conversion device 1 includes an amplifier 11 and a PIN diode variable attenuator 1
3, amplifier 12, differential amplifier 14, detection circuit 15, DF
And a B laser diode 16. Of these, the amplifier 11, the PIN diode variable attenuator diode 13, and the amplifier 12 constitute a variable gain amplifier. The first conversion device 1 is a device that converts a frequency-multiplexed electrical signal into a frequency-multiplexed optical signal and sends it to an optical fiber (optical transmission line) 3.

The receiving end of the optical fiber 3 is connected to a second converter 2 for converting the frequency-multiplexed optical signal into a frequency-multiplexed electric signal. The second conversion device 2 is
IN photodiode 28, amplifier 21, PIN diode variable attenuator 22, amplifier 23, bandpass filter 2
4, an amplifier 25, a detection circuit 26, and a differential amplifier 27. The second converter 2 converts the frequency-multiplexed electric signal obtained by the conversion into a coaxial cable (second electric transmission line) 5
It is a device for sending to.

Next, the operation of this system will be described. The frequency multiplexed electric signal (CATV signal) transmitted by the coaxial cable 4 is amplified by the amplifier 11,
It is attenuated by the PIN diode variable attenuator 13 and amplified by the amplifier 12. The signal output from the amplifier 12 is input to the DFB laser diode 16, and the single mode laser is amplitude-modulated (luminance-modulated) by the signal,
It is converted into a frequency multiplexed optical signal. This frequency-multiplexed optical signal propagates through the optical fiber 3 toward the second conversion device 2.

On the other hand, a part of the output signal of the amplifier 12 is branched by the branching device 17 and input to the detection circuit 15. The detection circuit 15 is specifically composed of a diode and detects signals in the entire transmission band. The level of the output signal of the detection circuit 15 represents the sum of the signal levels in the entire transmission band of the frequency-multiplexed electric signal output from the amplifier 12. The output signal of the detection circuit 15 is input to one terminal of the differential amplifier 14. The reference voltage Vr1 is input to the other terminal of the differential amplifier 14. Therefore, the level of the output signal of the differential amplifier 14 represents the deviation of the level of the output signal of the detection circuit 15 from the reference voltage Vr1.

The signal output from the differential amplifier 14 is input to the PIN diode variable attenuator 13. The amount of attenuation is controlled by this signal. That is, the level is adjusted so that the sum of the signal levels of the signals output from the amplifier 12 in the entire transmission band becomes equal to the predetermined reference voltage Vr1. Therefore, as the number of used channels decreases, the amplifier 11
Although the total sum of the signal levels in the entire transmission band of the signal input to is reduced, the PIN diode variable attenuation is performed so that the total sum of the signal levels in the entire transmission band of the signal output from the amplifier 12 becomes equal to the value indicated by the reference voltage Vr1. The attenuation amount of the container 13 becomes small.

The level-adjusted frequency-multiplexed electric signal output from the amplifier 12 is input to the DFB laser diode 16 to amplitude-modulate the laser light to obtain a frequency-multiplexed optical signal. This frequency-multiplexed optical signal is transmitted through the optical fiber 3 and is transmitted to the second conversion device 2. Second conversion device 2
Then, the PIN photodiode 28 receives the frequency-multiplexed optical signal and demodulates it into the frequency-multiplexed electrical signal. The frequency-multiplexed electric signal is input to the amplifier 21, amplified, input to the PIN diode variable attenuator 22 for level adjustment, input to the amplifier 23 and amplified, and then transmitted to the coaxial cable (second electric transmission). Route 5).
A part of the output signal of the amplifier 23 is branched by the branching device 29, the pilot signal of a predetermined frequency is extracted by the bandpass filter 24, amplified by the amplifier 25, and then detected by the detection circuit 26 to obtain the level of the pilot signal. Is detected. The output signal of the detection circuit 26 is the differential amplifier 2
Type in 7. The reference voltage Vr2 is input to the differential amplifier 27, and the output of the differential amplifier 27 becomes a signal representing the deviation of the level of the pilot signal output from the detection circuit 26 from the reference voltage Vr2. This signal is input to the PIN diode variable attenuator 22 to change the attenuation amount of the attenuator 22.

In this way, when the number of channels decreases, the sum of the signal levels in all transmission bands is made constant, so that the signal level of each channel increases.
On the receiving side, a pilot signal of a predetermined frequency is detected, and PI is adjusted so that the level of the pilot signal becomes a predetermined value.
The electric signal is adjusted after the N photodiode.
Therefore, the signal level of the frequency-multiplexed electric signal output to the coaxial cable 5 becomes the operation level of the predetermined coaxial transmission system again.

In the above embodiment, the number of channels used was changed and the C / N ratio deviation of the frequency-multiplexed electric signal converted by the second converter 2 was measured. C / N ratio deviation is 4
It is expressed based on the C / N ratio when 0 channel is used. For comparison, the same experiment was performed to measure the C / N ratio deviation when the level of the modulated signal was not adjusted in the first converter 1. The result is shown in FIG. Figure 2
As can be seen from the above, when the level adjustment is performed as in the present embodiment, the C / N ratio is conversely improved with the decrease in the number of channels as compared with the case where the level adjustment is not performed. I understand.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of an optical transmission system according to a specific example of the present invention.

FIG. 2 is a measurement diagram for measuring the relationship between the C / N ratio deviation at the receiving end and the number of channels used in the optical transmission system.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... 1st converter 2 ... 2nd converter 3 ... Optical fiber (optical transmission line) 4 ... Coaxial cable (1st electric transmission line) 5 ... Coaxial cable (2nd electric transmission line) 13 ... PIN diode variable attenuator ( Variable gain amplifier 12 ... Amplifier (gain variable amplifier) 14 ... Differential amplifier (comparator) 15 ... Detection circuit (level detector) 16 ... DFB laser diode (electric / optical conversion element) 22 ... PIN diode variable attenuator ( Variable gain amplifier 23 ... Amplifier (variable gain amplifier) 24 ... Band pass filter (predetermined frequency level detector) 26 ... Detection circuit (predetermined frequency level detector) 27 ... Differential amplifier (comparator) 28 ... PIN photodiode ( Optical / electrical conversion element)

Claims (1)

[Claims] 1. A system in which multi-channel signals are transmitted by frequency division multiplexed signals, wherein a first electric transmission line for transmitting the frequency division multiplexed signals as electric signals and laser light by the electric signals are used. A first conversion device for amplitude-modulating and transmitting a modulated optical signal, an optical transmission line for transmitting the optical signal modulated by the first conversion device, and an optical signal for receiving the optical signal from the optical transmission line to generate electrical signals. An optical transmission system comprising: a second conversion device that amplitude-demodulates a signal and sends out a demodulated electric signal; and a second electric transmission line that transmits an electric signal output by the second conversion device. The conversion device includes a variable gain amplifier that amplifies an electric signal of the first electric transmission path, a level detector that detects a sum of signal levels of all transmission bands of the variable gain amplifier, and a level detector that detects the level. A deviation of the total sum of the signal levels with respect to a predetermined level and outputting the deviation signal to the variable gain amplifier as a variable gain signal of the variable gain amplifier; and a laser based on the electric signal output from the variable gain amplifier. And an electric / optical conversion element for amplitude-modulating light and converting the light into an optical signal, wherein the second conversion device performs amplitude demodulation of the optical signal transmitted through the optical transmission line to convert the optical signal into an electric signal. A conversion element, a variable gain amplifier that amplifies the electric signal demodulated by the optical / electrical conversion element, and a predetermined frequency level detector that detects a signal level of a signal of a predetermined frequency among the electric signals output by the variable gain amplifier. And a deviation of the level detected by the predetermined frequency level detector with respect to a predetermined level is calculated, and the deviation signal is used as a gain variable signal of the gain variable amplifier. And a comparator which outputs the variable gain amplifier to the variable gain amplifier as a frequency division multiplexing optical transmission system.