JP2914008B2 - Automatic frequency control method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic frequency control system, and more particularly to an automatic frequency control system for a local oscillation light source of a coherent optical heterodyne receiver.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a conventional automatic frequency control device. In the figure, 1 is a connector for outputting input signal light, 2 is a local oscillation light source for outputting local oscillation light, and 3 is a signal light which is input from the connector 1 and is mixed with local oscillation light output from the local oscillation light source 2. Optical multiplexer,
Reference numeral 4 denotes a photodetector which receives the light mixed by the optical multiplexer 3 and outputs a signal having a difference between the frequency of the signal light and the frequency of the local oscillation light as an intermediate frequency signal.

[0005] Reference numerals 5 and 6 denote a preamplifier and an automatic gain amplifier for amplifying the intermediate frequency signal to a certain level, and 7 a demodulator for converting the output signal of the automatic gain amplifier 6 into a baseband signal and outputting the baseband signal. , 8 is a timing extraction circuit for extracting and outputting a clock signal from the input baseband signal, and 9 is an identification circuit for retiming the baseband signal in response to the input of the clock signal to identify and output data.

A frequency discriminator 10 receives an output signal of the automatic gain amplifier 6 and outputs an error signal, and 11 receives an error signal of the frequency discriminator 10 and outputs an error signal of the frequency discriminator 10 at predetermined time intervals. Is a local oscillation light source control circuit that outputs a control signal for controlling the frequency by changing the ambient temperature or the applied voltage of the local oscillation light source 2 so that is zero.

[0005] Reference numeral 12 denotes a data output terminal, and reference numeral 13 denotes a clock signal output terminal. FIG. 3 is a diagram showing a waveform of an error signal which is a difference between the frequency of the input signal light and the frequency of the local oscillation light. This error signal is an output signal of the frequency discriminator 10.

Next, the operation of the automatic frequency control device having the above configuration will be described. First, the optical multiplexer 3 is connected to the connector 1
And the local oscillation light output from the local oscillation light source 2 are mixed and output to the photodetector 4. The photodetector 4 detects a signal having a difference between the frequency of the signal light and the frequency of the local oscillation light, and outputs an intermediate frequency signal.

The preamplifier 5 and the automatic gain amplifier 6 amplify the intermediate frequency signal to a certain level, and output the amplified signal to the demodulator 7 and the frequency discriminator 10. The frequency discriminator 10 receives the output signal of the automatic gain amplifier 6 and sends an error signal (see FIG. 3), which is a difference between the frequency of the input signal light and the frequency of the local oscillation light, to the local oscillation light source control circuit 11. Output.

The local oscillation light source control circuit 11 receives the error signal, and changes the ambient temperature or the applied current of the local oscillation light source 2 at regular time intervals so that the error signal becomes zero. Can be controlled.

As described above, the frequency of the local oscillation light source is controlled such that the output of the frequency discriminator that outputs the difference between the frequency of the input signal light and the frequency of the local oscillation light as a voltage is always zero.

[0010]

In this conventional automatic frequency control system, when the input signal light is temporarily cut off, the output of the frequency discriminator becomes zero regardless of the frequency of the local oscillation light source. Therefore, the frequency of the local oscillation light source becomes indefinite, and even if a signal is input again, the frequency of the local oscillation light source is extremely separated from the frequency of the input signal light, and the frequency control is not performed normally. There was a point.

[0011]

SUMMARY OF THE INVENTION An automatic frequency control system according to the present invention comprises an output side of a photodetector and an input side of a control circuit.
Connected and detects signal light input break based on intermediate frequency signal
Optical input disconnection detection that outputs and outputs the optical input disconnection signal to the control circuit
Means connected to and output from the control circuit.
Storage means for storing control information for the local oscillation light source
The control circuit controls the optical input when the optical input disconnection signal is input.
The control information immediately before the power-off signal is input is read from the storage means.
And controls the local oscillation light source based on this control information.
Means .

[0012]

According to the present invention, even if the optical input is temporarily interrupted, stable frequency control can be immediately restored when the optical signal is input again.

[0013]

FIG. 1 is a block diagram showing one embodiment of an automatic frequency control system according to the present invention. In FIG. 1, a baseband signal 14 is input to an input terminal 14a of the baseband signal output from the demodulator 7, and a clock signal extracted from the baseband signal is output from an output terminal 14b. This is a timing extraction circuit that outputs an alarm signal used as an optical input disconnection signal from an output terminal 14c and outputs a clock signal from an output terminal 14d. Note that the demodulator 7 and the timing extraction circuit 14
Collectively, it is referred to as an optical input disconnection detecting unit.

Reference numeral 15 denotes an input terminal 15a to which an error signal output from the frequency discriminator 10 is input, an input terminal 15b to which an alarm signal output from an output terminal 14c of the timing extraction circuit 14 is input, and an input terminal 15b to which an alarm signal is input. When a signal is input, a read control signal for reading the contents of the following memory is output from the input terminal 15c, and the read control signal corresponds to the contents of the following memory, that is, the frequency value of the local oscillation light source 2 immediately before the alarm signal is output. A local oscillation light source control circuit for reading the ambient temperature or the applied current value and outputting a control signal for controlling the ambient temperature or the applied current for changing the frequency of the local oscillation light source 2 from the output terminal 15d. That stores the ambient temperature or the applied current value corresponding to the frequency value of the local oscillation light source 2 immediately before detecting It is a (storage unit). Ambient temperature
The degree or applied current value is called control information.

Next, the operation of the automatic frequency control system having the above configuration will be described. First, the optical multiplexer 3 mixes the signal light output from the connector 1 and the local oscillation light output from the local oscillation light source 2 and outputs the mixed light to the photodetector 4. The photodetector 4 detects a signal having a difference between the frequency of the signal light and the frequency of the local oscillation light, and outputs an intermediate frequency signal (intermediate frequency signal) .

The preamplifier 5 and the automatic gain amplifier 6 amplify the intermediate frequency signal to a certain level, and output the amplified signal to the demodulator 7 and the frequency discriminator 10.
The frequency discriminator 10 is connected to the automatic gain amplifier 6
And outputs an error signal (see FIG. 3), which is the difference between the frequency of the input signal light and the frequency of the local oscillation light, to the input terminal 15a of the local oscillation light source control circuit 15.

When an error signal is input to the input terminal 15a of the local oscillation light source control circuit 15, the local oscillation light source control circuit 15 sets the output terminal 15 so that the error signal becomes zero at regular time intervals.
By outputting a control signal from d and changing the ambient temperature of the local oscillation light source 2 or the applied current, the frequency can be controlled.

Next, when the input signal light is temporarily cut off, the output signal of the frequency discriminator 10 becomes zero irrespective of the frequency of the local oscillation light source 2.
Detects that the number of clock components extracted from the baseband signal input to the input terminal 14a is reduced, and outputs an alarm signal used as an optical input disconnection signal from the output terminal 14c to the local oscillation light source control circuit 15. Terminal 1
5b.

For this reason, the local oscillation light source control circuit 15
When an alarm signal is input to the input terminal 15b,
Since the read control signal is output from the output terminal 15c to the memory 16, the ambient temperature or the applied current value corresponding to the frequency value of the local oscillation light source immediately before the stored alarm signal is output from the memory 16 is read out. A control signal for holding the frequency of the local oscillation light source immediately before the output of the alarm signal is output from the output terminal 15d.

For this reason, the frequency of the local oscillation light source 2 is maintained, and even if the input signal is temporarily cut off, the frequency of the local oscillation light is kept at the value immediately before the input cutoff, and the signal is input again. Then, the above-described control can be started.

[0021]

As described above in detail, according to the automatic frequency control system according to the present invention, when the optical input interruption is temporarily detected, the frequency of the local oscillation light source is changed immediately before the optical input interruption is detected. Since the frequency of the local oscillation light source hardly changes before and after the optical input interruption is detected, stable frequency control can be performed immediately when an optical signal is input again. There is.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of an automatic frequency control system according to the present invention.

FIG. 2 is a block diagram showing a conventional automatic frequency control method.

FIG. 3 is a diagram showing an error signal in FIGS. 1 and 2;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Connector 2 Local oscillation light source 3 Optical multiplexer 4 Photodetector 7 Demodulator 9 Identification circuit 10 Frequency discriminator 14 Timing extraction circuit 15 Local oscillation light source control circuit 16 Memory

Claims (1)

(57) [Claims] 1. A local oscillation light source for outputting a local oscillation light, and an intermediate frequency based on a frequency difference between a signal light and the local oscillation light.
A photodetector that outputs a number signal; and a frequency valve that outputs an error signal based on the intermediate frequency signal.
Control and another device, the local oscillation light source so that the frequency difference between the local oscillator light and the basis of the error signal the signal light becomes a constant value
A coherent optical heterodyne receiver having a control circuit for controlling the output side of the photodetector and the input side of the control circuit.
Input of the signal light based on the intermediate frequency signal.
To detect an optical input signal and output an optical input disconnection signal to the control circuit.
Power-off detection means , connected to the control circuit and output from the control circuit;
Storage means for storing control information for said local oscillation light source
And the control circuit, when the optical input disconnection signal is input, the control circuit
The control information immediately before the optical input disconnection signal is input is described above.
From the storage means, and based on the control information,
An automatic frequency control method comprising means for controlling a vibration light source .