JPH06152525A - Electrical-to-optical transducer for frequency division multiplex light transmission system - Google Patents

Abstract

PURPOSE:To prevent the deterioration of C/N accompanied with the fluctuation of the number of channels by increasing the amplification factor of a gain variable amplifier according as the number of channel data are decreased. CONSTITUTION:In a first transducer (electrical-to-optical transducer) 1, the gain variable amplifier is constituted of an amplifier 11, a PIN diode variable attenuator 13, and an amplifier 12. One part of the output signal of the amplifier 11 is branched, and demodulated by a demodulator 14. The data are the number of channel data transmitted from a center device 6. A relation between the number of channels and the gain of the gain variable amplifier is stored in a memory 15. The relation is set so that the amplification factor can be increased in inverse proportion to the decrease of the number of the channels. The attenuation of a PIN diode variable attenuator 13 is controlled by a gain value read from the memory 15. That is, a level adjustment is executed so that the level of a signal outputted by the amplifier 12 is increased in inverse proportion to the decrease of the number of the channels.

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 total sum of signal levels in all transmission bands is not constant, and the total sum of signal levels 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, C / N in the optical transmission line
The ratio decreases as the number of carriers decreases. That is,
If the level of the modulation signal to be 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]

SUMMARY OF THE INVENTION The structure of the invention for solving the above-mentioned problems is to transmit a frequency division multiplexed signal to an electric transmission line and to electrically transmit channel number data indicating the number of channels used of the frequency division multiplexed signal. Electrical / optical conversion used in a system for transmitting to a data channel of a transmission line, amplitude-modulating laser light by a frequency division multiplexed signal received from an electric transmission line, and transmitting the modulated optical signal to the optical transmission line A variable gain amplifier for amplifying an electric signal on an electric transmission line, a demodulation device for demodulating channel number data from a data channel on the electric transmission line, and a small value for the channel number data demodulated by the demodulation device The amplitude of the laser light is modulated by the control device that controls the gain of the variable gain amplifier to increase and the electric signal output from the variable gain amplifier. Characterized by providing an electrical / optical conversion element for converting an optical signal.

[0006]

From the center, the frequency-multiplexed electric signal is sent out to the electric transmission line. At this time, the number of channels used of the frequency-multiplexed electric signal, that is, the number-of-carriers data indicating the number of carrier waves is also transmitted. It is sent to the data channel of the electric transmission line. This channel number data is received from the data channel and demodulated by the demodulator. Then, the control device controls so that the smaller the channel number data is, the larger the amplification factor of the variable gain amplifier is. Therefore, if the number of channels used decreases, the amplification factor increases, and the signal level of the channels used 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.

[0007]

1 is a diagram showing a configuration of an optical transmission system according to a specific embodiment of the present invention. Center device 6
Is a transmitter 64 for frequency-multiplexing a television reception wave and an independent broadcast wave in a predetermined frequency arrangement and sending them to the coaxial cable (first electric transmission path) 4, an oscillator 61 for generating a pilot signal, and a scan for the entire transmission band. Then, it comprises a carrier wave detector 62 for measuring the number of carriers in the entire transmission band and a modulator 63 for modulating numerical data into an RF signal of a data channel. The coaxial cable 4 is connected to the first conversion device 1. The first conversion device 1 includes an amplifier 11, a PIN diode variable attenuator 13, and an amplifier 1.
2, demodulator 14, memory 15, D / A converter 17, DF
And a B laser diode 16. Of these, the amplifier 11, the PIN diode variable attenuator 13, and the amplifier 12 constitute a variable gain amplifier, and the memory 15 and D
The / A converter 17 constitutes a control device. 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.

A second conversion device 2 for converting the frequency-multiplexed optical signal into a frequency-multiplexed electrical signal is connected to the receiving end of the optical fiber 3. The second conversion device 2 is
It is composed of an IN photodiode 28, an amplifier 21, a PIN diode variable attenuator 22, an amplifier 23, a demodulator 71, a memory 72, and a D / A converter 73. The second conversion device 2 is a device for sending the frequency-multiplexed electric signal obtained by the conversion to the coaxial cable (second electric transmission path) 5.

Next, the operation of this system will be described. In the center device 6, the carrier detector 62 scans the entire transmission band, detects the image carriers of all CATV channels, and counts the number of carriers. This number is stored as channel number data, modulated into an RF signal of the data channel by the modulator 63, and sent to the coaxial cable (first electric transmission path) 4. The detection of the carrier wave is executed at predetermined time intervals. Further, the received television signal and the independent broadcast wave are multiplexed into a predetermined frequency arrangement by the transmitting device 64 and sent to the coaxial cable 4.

The frequency-multiplexed electric signal (CATV signal) transmitted by the coaxial cable 4 is amplified by the amplifier 11, 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 and 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 11 is branched by the branching device 18, and the demodulator 14 demodulates the RF signal of the data channel into data. This data is the number-of-channels data sent from the center device 6, and is output to the address line of the memory 15. The memory 15 stores the relationship between the number of channels and the gain of the variable gain amplifier in a map format. That is, the gain value corresponding to the number of channels is stored in the address indicated by the channel number data. This relationship is set so that the amplification factor increases proportionally as the number of channels used decreases. The gain value read from the memory 15 is converted into an analog voltage signal by the D / A converter 17, and P
It is input to the IN diode variable attenuator 13.

The amount of attenuation of the PIN diode variable attenuator 13 is controlled by this voltage signal. That is, the level of the signal output from the amplifier 12 is adjusted so as to increase in inverse proportion to the decrease in the number of used channels. 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 signals input to the input terminal is small, the total sum of the signal levels in the entire transmission band output from the amplifier 12 becomes equal to a predetermined value
The attenuation amount of the N diode variable attenuator 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 propagates 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 21 is branched by the branching device 74, the RF signal of the data channel is demodulated by the demodulator 71, and the channel number data is decoded. This channel number data becomes an address signal of the memory 72. Memory 7
In 2, the relationship between the number of channels and the gain of the variable gain amplifier is stored in a map format. That is, the gain value corresponding to the number of channels is stored in the address indicated by the channel number data. This relationship is such that in the first conversion device 1, the amplification factor decreases proportionally as the number of channels decreases in order to return the signal level increased according to the decrease of the number of channels to the original signal level. Set in a relationship. The gain value read from the memory 72 is converted into an analog voltage signal by the D / A converter 73, and P
It is input to the IN diode variable attenuator 22.

The amount of attenuation of the PIN diode variable attenuator 22 is controlled by this voltage signal. As a result, the amplifier 2
With respect to the signal output by the signal No. 3, the signal level in a predetermined use channel becomes equal to a predetermined reference level. 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.

Next, another embodiment will be described. This embodiment is different from the above embodiment only in the second conversion device. The second conversion device 2, as shown in FIG.
Photodiode 28, amplifier 21, PIN diode variable attenuator 22, amplifier 23, bandpass filter 24,
It is composed of an amplifier 25, a detection circuit 26, and a differential amplifier 27. The second conversion device 2 is a device for sending the frequency-multiplexed electric signal obtained by the conversion to the coaxial cable (second electric transmission path) 5.

In the second converter 2, 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 band pass filter 24, and the amplifier 2
After being amplified by 5, the signal is detected by the detection circuit 26 and the level of the pilot signal is detected. Detection circuit 2
The output signal of 6 is input to the differential amplifier 27. The reference voltage Vr2 is input to the differential amplifier 27, and the differential amplifier 27
Is a signal representing the deviation of the level of the pilot signal output from the detection circuit 26 with respect to 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, in this system, the signal level in the entire transmission band increases as the number of channels used decreases, but the receiving side detects a pilot signal of a predetermined frequency and In the device after the PIN photodiode, the level of the electric signal is adjusted so that the level becomes a predetermined value. 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.

[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.

FIG. 3 is a block diagram showing a configuration of an optical transmission system according to another embodiment.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... 1st converter (electric / optical converter) 2 ... 2nd converter 3 ... Optical fiber (optical transmission line) 4 ... Coaxial cable (electrical transmission line) 5 ... Coaxial cable 13 ... PIN diode variable attenuator (gain variable) Amplifier ... 12 ... Amplifier (gain variable amplifier) 14 ... Demodulator (demodulator) 15 ... Memory (control device) 17 ... D / A converter (control device) 16 ... DFB laser diode (electric / optical conversion element) 22 ... PIN diode variable attenuator 23 ... Amplifier 24 ... Band pass filter 26 ... Detection circuit 27 ... Differential amplifier 28 ... PIN photodiode 61 ... Oscillator 62 ... Carrier wave detector 63 ... Modulator 71 ... Demodulator 72 ... Memory 73 ... D / A converter

Claims (1)

[Claims] 1. An electric transmission line used in a system for transmitting a frequency division multiplexed signal to an electric transmission line and transmitting channel number data indicating the number of used channels of the frequency division multiplexed signal to a data channel of the electric transmission line. Amplitude modulation of the laser light is performed by the frequency division multiplexed signal received from the
An optical conversion device, wherein a variable gain amplifier for amplifying an electric signal of the electric transmission line, a demodulation device for demodulating the channel number data from a data channel of the electric transmission line, and the channel demodulated by the demodulation device A control device that controls so that the gain of the variable gain amplifier is increased as the number data becomes smaller, and the laser light is amplitude-modulated by the electric signal output from the variable gain amplifier to be converted into an optical signal. An electric / optical conversion device for a frequency division multiplexing optical transmission system, which is provided with an electric / optical conversion element.