JPH10173602A - Frequency-stabilizing circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a frequency-stabilizing circuit (AFC circuit) which can prevent a deadlock, when sudden fluctuation occurs at start up time and in a signal source, which can improve the stability of a device and whose reliability improves. SOLUTION: This frequency stabilizing circuit (AFC circuit) 20 eliminates an intensity modulation signal, which is simultaneously generated when a beat signal is in the blowing band of the AFC circuit 20 and amplifies/detects the intensity modulation signal which is simultaneously generated, when the beat signal is not in the pull-in band of the AFC circuit 20 as the detection mean of the beat signal. Thus, AFC control is started after the beat signal is recognized to be in the pull-in band of the AFC circuit 20 by using a beat signal detection circuit 9 which detects the presence or absence of the beat signal by detecting the presence or absence of the intensity modulated signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an FM modulator using coherent optical communication and optical heterodyne, and more particularly to a frequency stabilizing circuit (AFC circuit) for stabilizing the frequency of a beat signal.

[0002]

2. Description of the Related Art Conventionally, a coherent optical transmission device as shown in FIG. 6 has been proposed (refer to Japanese Patent Laid-Open No. 4-346525, hereinafter referred to as prior art 1). As shown in FIG. 6, the coherent optical transmission device includes a transmitting device 51 and a receiving device 52 each including a semiconductor laser 67 as a first light emitting element.

The receiving device 52 comprises an optical coupler 59, a light receiving circuit 60, a filter 61, a mixer 62, and an electric FM modulation circuit 6.
3, a detection circuit 64, a control circuit 65, and a local oscillation semiconductor laser 66 as a second light emitting element.
Here, the optical coupler 69 is the semiconductor laser 5 of the transmitting device 51.
7 and the signal light of the semiconductor laser 66 for local oscillation are combined. The light receiving circuit 60 includes a light receiving element and converts the beats of the two signal lights output from the optical coupler 59 into electric signals.

A filter 61 separates a carrier component and a first side-wave component from the output signal of the light-receiving circuit 60 when the semiconductor laser 57 (first light-emitting element) converts the optical frequency, and separates them. Extract. The mixer 62 calculates the product of the carrier component and the first side wave component. The detection circuit 64 detects the amplitude of the pilot signal. The control circuit 65 controls the optical frequency of the signal light of the local oscillation semiconductor laser 66 so that the amplitude of the pilot signal is maximized. In addition, electric FM
The demodulation circuit 63 demodulates the angle modulation signal that is the output signal of the mixer. Here, the filter 61, the mixer 62, the detection circuit 64, and the control circuit 65 include a frequency stabilization circuit (A
FC circuit).

[0005]

However, in the coherent optical transmission device of the prior art 1, the first light emitting element and the second light emitting element are sufficiently stable at the time of startup.
The AFC circuit operates before the emission frequency difference falls within the pull-in band of the AFC circuit, and the AFC circuit diverges to turn off the light emitting element, and the device does not start up,
Due to the divergence of the FC circuit, there is a disadvantage that an excessive current is applied to the light emitting element and the element is broken.

When the first light emitting element or the second light emitting element is modulated in the optical frequency domain, light intensity modulation is performed simultaneously with the optical frequency modulation, and the intensity modulation is performed even if the beat signal is not in the band. Since the detected signal is detected, it is not possible to determine the presence or absence of a beat signal even if the output of a light receiving circuit having a light receiving element is detected.

Therefore, a technical problem of the present invention is to prevent deadlock at the time of start-up and when a sudden change occurs in a signal source, improve the stability of the device, and improve reliability. An object of the present invention is to provide an improved frequency stabilization circuit (AFC circuit).

[0008]

According to the present invention, the optical signal of the first light emitting element 1 and the optical signal of the second light emitting element 2 are combined by the optical coupler 3, and the light receiving element 4 performs heterodyne detection. Frequency stabilizing circuit (AFC circuit 20) for keeping the frequency of the beat signal constant using an optical heterodyne method of obtaining an electric signal (beat signal) having a frequency of the difference between the light emitting frequencies of the two light emitting elements 1 and 2 , A splitter 5 for splitting a part of the beat signal for the AFC circuit 20;
A splitter 6 for distributing the beat signal for the C circuit 20 to the frequency discriminator 7 and the beat signal detecting circuit 9; a frequency discriminator 7 for outputting a DC voltage corresponding to the frequency of the beat signal output from the splitter 6; , Beat signal frequency is AFC circuit 2
A beat signal detecting circuit 9 for detecting whether or not the signal is present in the pull-in band of 0, and when the beat signal is detected in the pull-in band by the beat signal detecting circuit 9, receiving an output of the frequency discriminator 7 and receiving a predetermined signal. And a control circuit 8 for controlling the first light emitting element 1 or the second light emitting element 2 so that the frequency of the light emitting element becomes equal to or lower than the pull-in band, and keeping the light emitting element in a free-running state when it is out of the pull-in band. An AFC circuit characterized by the following is obtained.

According to the present invention, a beat signal detecting circuit is provided to prevent the divergence of the AFC circuit 20 by operating the AFC circuit 20 in a state where there is a signal outside the pull-in band of the AFC circuit 20 at the time of startup. 9, the AFC control is performed after confirming that the signal is within the pull-in band of the AFC circuit 20.
A C circuit 20 is obtained.

According to the present invention, the beat signal detecting circuit 9 for detecting a beat signal has the first pass band having the same pass band as the highest frequency of the pull-in band of the AFC circuit 20.
Low-pass filter (LPF) 10 and a first low-pass filter (L
PF) an output amplitude limiting amplifier 11 for limiting and amplifying the output of the output, and a second low-pass filter having the same pass band as the highest frequency of the modulation signal input to the first light emitting element 1 or the second light emitting element 2. (LPF) 12, a detector 14 that determines the voltage based on the output signal of the second low-pass filter (LPF) 12, and a determination circuit 15 that determines the presence or absence of a beat signal from the voltage output from the detector 14. Thus, a beat signal detection circuit 9 characterized by comprising the following is obtained.

Further, according to the present invention, the beat signal detection circuit 9 detects the presence or absence of an intensity modulation component of the light intensity used in the optical heterodyne system and modulated simultaneously with the modulation of the optical frequency. A beat signal detection circuit 9 characterized by detecting the presence or absence of a signal is obtained.

According to the present invention, there is provided an FM comprising any one of the frequency stabilizing circuits 20 described above.
A modulator is obtained.

Further, according to the present invention, there is provided an optical transmission device comprising any one of the frequency stabilizing circuits 20 described above.

In short, the frequency stabilization circuit (AF
C circuit) 20 as means for detecting a beat signal
When the beat signal is within the blowing band of the AFC circuit 20, the simultaneously generated intensity modulation signal is deleted, and when the beat signal is not within the pulling band of the AFC circuit 20, the simultaneously generated intensity modulation signal is amplified and detected. A beat signal detection circuit that detects the presence or absence of a beat signal by detecting the presence or absence of a modulation signal is used.
The AFC circuit 20 starts the AFC control after confirming that it has entered the pull-in band of the FC circuit 20.

[0015]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an FM modulator using an optical heterodyne technology for a frequency stabilizing circuit (hereinafter, referred to as an AFC circuit) according to an embodiment of the present invention.

The FM modulator shown in FIG. 1 comprises a first light emitting element 1, a second light emitting element 2, an optical coupler 3, and a light receiving element 4.
, A first splitter 5, a second splitter 6, a frequency discriminator 7, a control circuit 8, and a beat signal detection circuit 9.

Here, the AFC circuit 20 comprises a second branching device 6, a frequency discriminator 7, a control circuit 8, and a beat signal detecting circuit 9. The second branching device 6 branches a part of an input signal to a circuit for detecting a frequency error to a beat detection circuit. The frequency discriminator 7 detects the frequency of the input signal. The control circuit 8 detects an error between the signal frequency detected by the frequency discriminator 7 and the set frequency, and controls the frequency of the signal source to be equal to the set frequency.

The beat signal detecting circuit 9 has a first low-pass filter 10 having a pass band equal to the pull-in band of the AFC circuit 20, and an output amplitude limiting amplifier for a pass signal of the first low-pass filter 10. An output amplitude limiting amplifier 11, a second low-pass filter 12 for passing up to the highest frequency of a modulated signal frequency-modulated with a beat signal as a carrier,
And a determination circuit 14 for determining the presence or absence of a beat signal from the output of the detector 13, and the presence or absence of a beat signal from the output signal of the splitter 6. Is detected.

FIG. 2 is a diagram for explaining the operation of the frequency discriminator 7 of the AFC circuit 20 of FIG. FIG. 3 shows A
FIG. 4 is a diagram provided for describing an operation of a beat signal detection circuit 9 of the FC circuit 20. FIG. 4 is a diagram provided for explaining the operation of the beat signal detection circuit 9. The operation of the AFC circuit 20 will be described in detail with reference to FIG. 1, FIG. 2, FIG. 3, and FIG.

First, the signal split by the second splitter 6 is input to the frequency discriminator 7. As shown in FIG. 2, the frequency discriminator 7 generates a DC voltage according to the frequency of the input signal. The signal frequency is detected based on this voltage value.

Next, the control circuit 8 compares the output voltage of the frequency discriminator 7 with the voltage corresponding to the set frequency, and controls the voltage for driving the signal source so that the signal frequency becomes equal to the set frequency. At this time, if the beat signal detection circuit 9 has not detected a beat signal, the above control is not performed, and the signal voltage is fixed at the initial value to drive the signal source.

The other signal split by the second splitter 6 is input to a beat signal detection circuit 9.
The signal input to the beat signal detection circuit 9 enters the first low-pass filter 10.

As shown in FIG. 3A, the first low-pass filter 10 blocks a signal outside the pull-in band of the AFC circuit 20. That is, when the beat signal is not within the pull-in band of the AFC circuit 20 (signal waveform (a)), the first low-pass filter 10 cuts off the beat signal 22 and simultaneously modulates the frequency of the signal source. Only the generated intensity modulation component 21 is passed {signal waveform (b)}.

The intensity modulation component 21 is subjected to amplitude-limited amplification by the amplitude-limited amplifier 11 {signal waveform (c)},
The second low-pass filter 12 is entered. Here, the second low-pass filter 12 blocks a signal having a higher frequency than a modulation signal for frequency-modulating the signal source.

Here, as shown in the input waveform (i) and the output waveform (nu) of FIG. 4, in the amplitude limiting amplifier 11, an amplitude exceeding a predetermined magnitude is deleted.

Since the intensity modulation component 21 is the same as the modulation signal, it passes through the second low-pass filter 12 (signal waveform (d)) and enters the detector 13. The detector 13 receives a signal input and generates an arbitrary voltage. Upon receiving this voltage, the determination circuit 14 determines that the beat signal 22 is not within the pull-in band, and sends a signal to the control circuit 8.

As shown in FIG. 3B, the beat signal is A
When the signal is within the pull-in band of the FC circuit 20 (the signal waveform (e)), the first low-pass filter 10 outputs the beat signal 22
And the intensity modulation component 21 (the signal waveform (f)).

Here, as shown in the input waveform (ル) and the output waveform (ヲ) of FIG. 4, in the amplitude limiting amplifier 11, an amplitude exceeding a predetermined magnitude is deleted.
In the amplitude limiting amplifier 11, the intensity modulation component 2 having a small amplitude is used.
1 is deleted, and only the beat signal 22 is amplified. The output signal {signal waveform (g)} of the amplitude limiting amplifier 11 is
Signal is outside the pass band of the low-pass filter
Nothing is output from the second low-pass filter as shown in FIG. Further, no voltage is generated because there is no input to the detector 13. Accordingly, the determination circuit 14 determines that the beat signal is within the pull-in band, and sends a signal to the control circuit 8.

Next, the operation of the FM modulator using the AFC circuit 20 according to the embodiment of the present invention will be described.

Referring again to FIG. 1, the first light emitting element 1
, A modulation signal is input and optically modulated. The frequency-modulated light and the light from the second light emitting element 2 are combined by an optical coupler 3, and optical heterodyne detection is performed by a light receiving element 4. At this time, a beat signal having a frequency equal to the difference between the emission center frequency of the first light emitting element 1 and the emission center frequency of the light emitting element 2 is obtained.
This beat signal is a broadband FM signal whose oscillation center frequency is the difference frequency between the emission center frequency of the first light emitting element 1 and the emission center frequency of the light emitting element 2.

The beat signal is partially branched by the first branching unit 5. The split signal is input to an AFC circuit 20 that controls the center frequency of the FM modulation signal to be constant.

The beat signal input to the AFC circuit 20 is further split by the second splitter 6 and input to the frequency discriminator 7. The frequency discriminator 7 generates a DC voltage according to the frequency of the input signal. The signal frequency is detected based on this voltage value. Next, the control circuit 8 compares the output voltage of the frequency discriminator 7 with the voltage corresponding to the set frequency, and controls the drive current of the second light emitting element 2 so that the signal frequency becomes equal to the set frequency. At this time, if the beat signal detection circuit 9 has not detected a beat signal, the above-described control is not performed, and the drive current is fixed at an initial value to drive the second light emitting element 2.

At this time, the first light emitting element 2 is replaced with the first light emitting element 2.
Even if the light emitting element 1 is controlled, the function of the AFC circuit 20 is not impaired.

The other signal split by the second splitter 6 is input to the beat signal detection circuit 9 and the signal input to the beat signal detection circuit 9 is input to the first low-pass filter 10. to go into. The first low-pass filter 10 blocks signals outside the pull-in band of the AFC circuit 20.

Here, when the beat signal is not within the pull-in band of the AFC circuit 20, the first low-pass filter 10 cuts off the beat signal and simultaneously generates an intensity modulation component generated when the signal source is frequency-modulated. Only let through. This intensity modulation component is subjected to amplitude limiting amplification by the amplitude limiting amplifier 11 and then enters the second low-pass filter 12. Second low-pass filter 1
2 blocks a signal having a higher frequency than a modulation signal for frequency-modulating the signal source. Since the intensity modulation component is the same as the modulation signal, it passes through the second low-pass filter 12 and passes through the detector 13
to go into. Here, the detector 13 receives a signal input and generates an arbitrary voltage. Upon receiving this voltage, the determination circuit 14 determines that the beat signal is not within the pull-in band, and sends a signal to the control circuit 8.

On the other hand, when the beat signal is within the pull-in band of the AFC circuit 20, the first low-pass filter 10 passes both the beat signal and the intensity emphasizing component. Next, in the amplitude limiting amplifier 11, the intensity modulation component having a small amplitude is deleted, and only the beat signal is amplified. Since the output signal of the amplitude limiting amplifier 11 is outside the pass band of the second low-pass filter 12, nothing is output from the second low-pass filter 12. Therefore, no voltage is generated because there is no input to the detector 13. Accordingly, the determination circuit 14 determines that the beat signal is within the pull-in band, and sends a signal to the control circuit 8.

FIG. 5 is a diagram showing a configuration in which an FM modulator having an AFC circuit according to an embodiment of the present invention is applied to a coherent optical communication system using the AFC circuit.

As shown in FIG. 5, the first light emitting element 1 is used on the optical transmitter 16 side, and the optical coupler 17 is used on the optical receiver 17 side.
Are used as receiving means, and the FM modulator portion other than the first light emitting element 1 is used for the receiver.

[0040]

As described above, by using the AFC circuit of the present invention, it is possible to prevent deadlock at the time of start-up and when a sudden change occurs in a signal source, thereby reducing the stability of the device. Can be improved.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of an FM modulator using an AFC circuit according to an embodiment of the present invention.

FIG. 2 is a diagram which is used for describing an operation of a frequency discriminator 7 of the AFC circuit of FIG.

FIG. 3 is a diagram provided for describing an operation of a beat signal detection circuit of the AFC circuit in FIG. 1;

FIG. 4 is a diagram which is used for describing the operation of the amplitude limiting amplifier of the AFC circuit of FIG.

FIG. 5 is a diagram showing a configuration in which the FM modulator according to the embodiment of the present invention is applied to a coherent optical communication system.

FIG. 6 is a block diagram showing a coherent optical communication device according to the related art 1.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 first light emitting element 2 second light emitting element 3 optical coupler 4 light receiving element 5 first splitter 6 second splitter 7 frequency discriminator 8 control circuit 9 beat signal detection circuit 10 first low-pass filtering Device 11 Amplitude limiting amplifier 12 Second low-pass filter 13 Detector 14 Judgment circuit 15 Intermediate frequency amplifier 16 Optical transmitter 17 Optical receiver

Claims (6)

[Claims] 1. An optical signal from a first light emitting element and an optical signal from a second light emitting element are coupled by an optical coupler, and heterodyne detection is performed by a light receiving element to emit light from the first and second light emitting elements. In a frequency stabilizing circuit that keeps the frequency of a beat signal constant using an optical heterodyne method for obtaining a beat signal having a frequency of a frequency difference, a first part of the beat signal is branched for the frequency stabilizing circuit. A splitter, a second splitter for further distributing the split beat signal, and a frequency discriminator for outputting one of the beat signals split by the second splitter as a DC voltage corresponding to a frequency A beat signal detection circuit for detecting whether or not another frequency of the beat signal distributed by the second branching device is within the pull-in band of the frequency stabilization circuit; and a beat signal detection circuit. When a beat signal is detected in the pull-in band by the output circuit, at least one of the first light emitting element 1 and the second light emitting element is controlled so as to receive an output of the frequency discriminator and have a predetermined frequency. And a control circuit for controlling one of the light emitting elements and keeping the light emitting element in a free running state when the light emitting element is out of the pull-in band. 2. The frequency stabilizing circuit according to claim 1, wherein the frequency stabilizing circuit is operated in a state where a signal is outside a pull-in band of the frequency stabilizing circuit at the time of startup. A frequency stabilizing circuit for performing AFC control after confirming that the signal is within the pull-in band by the beat signal detecting circuit in order to prevent divergence of the signal. 3. A beat signal detecting circuit for detecting a beat signal, comprising: a first low-pass filter having the same pass band as the highest frequency of the pull-in band of the frequency stabilizing circuit;
And a second low-pass filter having the same pass band as the highest frequency of the modulation signal input to the first light-emitting element or the second light-emitting element. And a detector for determining the voltage based on the output signal of the second low-pass filter, and a determination circuit for determining the presence or absence of a beat signal from the voltage output from the detector. Beat signal detection circuit. 4. The beat signal detection circuit according to claim 3, wherein the presence / absence of a beat signal is detected by detecting presence / absence of an intensity modulation component of a light intensity used in an optical heterodyne system and modulated simultaneously with modulation of an optical frequency. A beat signal detection circuit for detecting a beat signal. 5. An FM modulator comprising the frequency stabilizing circuit according to claim 1 or 2. 6. An optical transmission device comprising the frequency stabilizing circuit according to claim 1.