JP2002057628A - Frequency stabilization control circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a frequency stabilization control circuit which can surely lead in an intermediate frequency to an operation stabilizing point for controlling the stabilization of intermediate frequency, without producing a malfunction out of band of a frequency discriminator when the intermediate frequency in an optical transmission system is led in. SOLUTION: When an intermediate frequency signal that is obtained by mixing an output light of a modulating laser 1 and that of a local oscillating laser 2 and converting them photoelectrically by a light receiver 4, is led in a predetermined band, a means for outputting an overflow signal (frequency counter 9 and overflow detecting part 10) is attached to a frequency discriminator if an input frequency is higher than a desired frequency, or a frequency higher than the desired one is not measured and a function for keeping the highest frequency is given. Thus, an intermediate frequency can be surely led in to the operation stabilizing point for controlling the stabilization of intermediate frequency without producing a malfunction at an unstable point out of band of the high-frequency side of a preceding part of the frequency discriminator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermediate frequency stabilization control circuit for stabilizing an intermediate frequency in an optical signal transmission system, for example, an optical signal transmitter or receiver using heterodyne detection. About the method.

[0002]

2. Description of the Related Art An optical signal transmitter or receiver using optical heterodyne detection receives a combined light of a signal light and a local oscillation light with a photodetector, and generates a beat corresponding to a frequency difference between the signal light and the local oscillation light. Is the intermediate frequency (IF)
The transmitter is to re-modulate the light with the IF signal and transmit the light again.
The receiver obtains a baseband signal by demodulating the IF signal. In this communication system, the IF fluctuates due to the frequency fluctuation of the signal light and the local oscillation light, and the transmission characteristics or the reception characteristics are deteriorated. Therefore, the IF stabilization control is required.

Since the IF band is much narrower than the frequency shift of the light source, the intermediate frequency stabilization control (AFC:
In order to operate Automatic Frequency Control, it is necessary to sweep the frequency of signal light or local oscillation light in advance to bring the IF into the operating range of the AFC. By the way, when the frequency of the local oscillation light is swept,
The frequency of the IF signal is looped back at zero. That is, depending on whether the frequency of the local oscillation light is higher or lower than that of the signal light, for example, even if the frequency of the IF signal is the same, one becomes the real band IF signal and the other becomes the image band IF signal. And A
The input frequency versus output voltage characteristics of the frequency discriminator used for FC have opposite characteristics between the real band and the image band. Therefore, when the operating point of the AFC is set in the real band, if the IF signal is in the image band, the AFC does not operate.

As a method of avoiding this problem, conventionally,
An IF signal is input to the frequency discriminator, and while sweeping the frequency of the signal light or the local oscillation light, a change rate of the frequency discriminator output voltage with respect to the sweep frequency is obtained. Has often been used. In using this method,
When the input frequency dependence of the output frequency of the frequency discriminator is as shown in FIG. 4, the operation stable point of the AFC in the real band is point E (region B), whereas the output voltage of the frequency discriminator is A region B ′ at point F where the rate of change with respect to the sweep frequency is equal to point E, which is the stable operation point of the AFC, also exists in the image band. Various methods have been devised for the separation from the point F.

The one described in Japanese Patent Application Laid-Open No. 5-227093 is one example. This conventional example will be described with reference to FIG. FIG. 7 is a block diagram showing a configuration of a coherent optical communication receiver to which an intermediate frequency stabilization control method in a conventional optical signal transmission system is applied, which is described in the above publication. In the illustrated configuration, the signal light 24 transmitted by the optical fiber 19 is multiplexed with the local oscillation light 23 emitted from the semiconductor laser 22 by the optical coupler 20 and is incident on the balanced receiver 21. here,
The temperature of the semiconductor laser 22 is kept constant. The IF signal output from the balanced receiver 21 is a bandpass filter (B) having a pass band of 2.5 GHz to 7.5 GHz.
After passing through a PF) 25, it is input to a demodulation circuit (DEM) 26 and a frequency discriminator (DISC) 28.

The output of the demodulation circuit 26 is a first low-pass filter (LPF1) 2 having a cutoff frequency of 2.5 GHz.
7, and a 2.5 Gb / s data signal is taken out. The demodulation circuit 26 is adjusted so that the IF demodulation frequency is 5 GHz and the maximum demodulation efficiency is obtained. The characteristic of the frequency discriminator 28 is a zero cross at 5 GHz, and a control signal for IF stabilization is output in the actual band. The output of the frequency discriminator 28 is a second low-pass filter (LPF2) 2 having a cutoff frequency of 10 kHz.
After 9, the signal is input to the phase comparator 30 and the first A / D converter 33. The first A / D converter 33 samples the output of the frequency discriminator 28 and inputs the data to the microcomputer 34.

[0007] The output of the oscillator 31 having a transmission frequency of 1 kHz passes through a switch 32, and then passes through a phase comparator 30 and an adder 3
7 is input. The output of the phase comparator 30 is the second A /
After being sampled by the D converter 36, it is input to the microcomputer 34. D / A converter 35
Receives a signal from the microcomputer 34 and outputs a current control signal for the semiconductor laser 22. This signal is
After being added to the signal from the oscillator 31 by the adding circuit 37,
The current driver (DRV) 38 of the semiconductor laser 22 is applied. The microcomputer 34 controls the pull-in of the IF from the start of reception of the signal light 24 through the following process.

When the microcomputer 34 receives the reception start command, the microcomputer 34 outputs a current control signal via the D / A converter 35 to sweep the frequency of the local oscillation light 23 from the low frequency side to the high frequency side in a range of 20 GHz. . Also, the switch 32 is closed and the signal of the oscillator 31 is
8 is supplied. As a result, the local oscillation light 23 is subjected to FM modulation with a frequency shift amount of 500 MHz and a modulation frequency of 1 kHz. When the IF signal appears in the frequency discrimination band during the sweep of the local oscillation light 23, the second low-pass filter 3
6 outputs a demodulated 1 kHz FM signal. The output of the phase comparator 30 behaves according to the movement of the IF frequency.

When the IF signal enters a region where the output voltage of the phase comparator 30 becomes 0.5 V or more, the microcomputer 34 stores in the memory the current control signal output to the current driver 38 while passing through this region. When the IF signal has passed through this band, the microcomputer 34 calculates the first injection current sweep width of the semiconductor laser 22 required for passing through this region.

Next, if the output of the phase comparator is-
When passing through the region where the voltage is 0.5 V or less, the microcomputer 34 sets the semiconductor laser 22 required for passing through this region.
The second injection current sweep width is also calculated, and the first and second
If the ratio of the sweep width of the injection current becomes 1.5 or more, the microcomputer 34 stops the sweep of the local oscillation light 23 and the center point of the region where the output voltage of the phase comparator 30 becomes 0.5 V or more. Is read from the memory and output to the current driver 38. At the same time, the microcomputer 34 enters the AFC operation, and thereafter, the first A
A current control signal for IF stabilization is calculated from a signal input from the / D converter 33 and output to the current driver 38. As a result, the IF frequency is reliably stabilized at the actual band of 5 GHz.

That is, here, in order to sweep the oscillation frequency of the light source of the local oscillation light 23 and draw the IF to the AFC operation stable point, the IF signal is input to the frequency discriminator 28 and the injection current of the semiconductor laser 23 is Alternatively, while sweeping the temperature, the rate of change of the output voltage of the frequency discriminator 29 is measured, and the width of the area where the rate of change is higher than the first threshold value (injection current or temperature sweep width) and immediately before or The width of the area where the rate of change immediately after is lower than the second threshold value is compared, and if the former (the latter) is larger than the latter (the former) by a certain percentage or more, the IF is added to the former (the latter) area. It is set to pull in a signal.

[0012]

However, in the conventional frequency stabilization control circuit described above, the problem that the threshold value fluctuates due to the individual difference between the frequency discriminator and the filter, and the problem that the circuit is complicated are described. Had.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and eliminates a malfunctioning point of a frequency discriminator and ensures that an intermediate frequency is set to an operation stable point of the intermediate frequency stabilization control. An object of the present invention is to provide an intermediate frequency stabilization control circuit that can be pulled in.

[0014]

In order to solve the above problem, an intermediate frequency stabilization control circuit according to the present invention outputs an overflow signal to a frequency discriminator to which an intermediate frequency signal is input, when the frequency is higher than an arbitrary frequency. Means for performing With this configuration, the frequency stabilization control circuit can reliably pull the intermediate frequency to the operation stable point of the intermediate frequency stabilization control without malfunctioning outside the high frequency side of the components before the frequency discriminator. Yes, it is possible to eliminate a malfunction point outside the band on the high frequency side of components before the frequency discriminator.

[0015] Further, a means for outputting an overflow signal at a frequency equal to or higher than the arbitrary frequency includes a frequency counter to which the intermediate frequency signal is input, and an overflow signal when the count number of the frequency counter reaches an arbitrary count number. And an overflow detector for outputting. With this configuration, a malfunction point outside the high-frequency band of the component before the frequency counter is eliminated.

The frequency counter further includes an electric coupler for branching the intermediate frequency signal, and a frequency divider for dividing one of the intermediate frequency signals branched by the electric coupler to an operating frequency of the frequency counter. , An intermediate frequency signal output from the frequency divider is supplied, and the frequency counter includes an overflow detection unit. With this configuration, a malfunction point existing outside the high-frequency band of a component such as a frequency divider before the frequency discriminator is deleted.

Further, the frequency discriminator to which the intermediate frequency signal is input is provided with a means for holding the highest frequency without measuring any frequency or more. With this configuration, a frequency stabilization control circuit that can reliably pull the intermediate frequency to the operation stable point of the intermediate frequency stabilization control without malfunctioning at an unstable point outside the high-frequency band of components before the frequency discriminator This makes it possible to eliminate malfunction points outside the high-frequency band of components before the frequency discriminator, thereby simplifying the configuration.

The means for holding at the highest frequency without measuring at or above an arbitrary frequency counts by inputting the intermediate frequency signal, but has a function of holding at the highest frequency without counting at or above an arbitrary frequency. It consists of an added frequency counter. With this configuration, a frequency stabilization control circuit that can reliably pull the intermediate frequency to the operation stable point of the intermediate frequency stabilization control without malfunctioning at an unstable point outside the high-frequency band of components before the frequency discriminator This makes it possible to eliminate malfunction points outside the high-frequency band of components before the frequency discriminator, thereby simplifying the configuration.

The frequency counter further includes an electric coupler for branching the intermediate frequency signal, and a frequency divider for dividing one of the intermediate frequency signals branched by the electric coupler to an operating frequency of a frequency counter. Is configured to be supplied with an intermediate frequency signal output from the frequency divider. Further, the frequency counter has a function of holding at the highest frequency without counting any frequency or more. With this configuration, the intermediate frequency can be reliably pulled into the operation stable point of the intermediate frequency stabilization control without malfunctioning at an unstable point outside the high-frequency band of a component before the frequency discriminator such as the frequency divider. This is a frequency stabilization control circuit, which can eliminate a malfunction point outside a high frequency band of a component such as a frequency divider before a frequency discriminator, thereby simplifying the configuration.

[0020]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to FIGS. (Embodiment 1) FIG. 1 is a block diagram showing a configuration of an optical signal transmitter using heterodyne detection to which a frequency stabilization control circuit can be applied in an optical signal transmission system according to Embodiment 1 of the present invention. . In this figure, the bold line indicates the optical path.

In the optical signal transmitter shown in FIG. 1, light outputted from two lasers (modulation laser 1 and local oscillation laser 2) is multiplexed by an optical coupler 3, and the multiplexed light is received by a photodetector 4. Is photoelectrically converted. The beat signal thus obtained is amplified by the amplifier 5 and then branched by the electric coupler 6, one of the two outputs is input to the transmission laser 7, and the signal converted into an optical signal is transmitted. Electric coupler 6
Is input to the frequency divider 8.

The frequency divider 8 divides the frequency up to the operating frequency of the frequency counter 9 as a frequency discriminator for inputting the intermediate frequency. The frequency divider 8 divides the intermediate frequency output from the electric coupler 6 and outputs the frequency. It is input to the counter 9. The frequency counter 9 is controlled by a microcomputer (microcomputer) unit 11, and its output is digital. The frequency counter 9 starts counting with a start signal from the microcomputer unit 11 and outputs data to the microcomputer unit 11 when the counting is completed. The overflow detecting section 10 outputs an overflow signal to the microcomputer section 11 when the counter 9 reaches an arbitrary count number.

The reset of the frequency counter 9 and the overflow detector 10 is performed by the microcomputer 1
This is executed by inputting the reset signal from No. 1.
The microcomputer unit 11 derives a frequency below an arbitrary frequency in accordance with the output from the frequency counter 9 and the overflow detection unit 10 and determines that the overflow signal continues to be output in a certain region above the arbitrary frequency, thereby indicating that the signal is out of the band. judge.

On the other hand, since the oscillation frequency of the laser fluctuates depending on the bias current and the temperature, the microcomputer unit 11 adjusts the DA converter 12 so as to compensate for the difference between the frequency derived from the output of the frequency counter 9 and the target frequency.
To 15, a temperature control circuit 16, a bias current control circuit 17, a temperature control circuit 18, and a bias current control circuit 1.
By controlling the bias current and the temperature of the modulation laser 1 and the local oscillation laser 2 by outputting a control signal to 9, the intermediate frequency output from the electric coupler 6 is stabilized.

Here, the characteristics of the frequency divider 8 and the frequency counter 9 will be briefly described with reference to FIGS. The frequency divider 8 and the frequency counter 9 have a limited band. The frequency divider operates within the band (f _L <f) for normal operation as shown in FIG.
<In f _H), the input frequency and outputs a frequency divided by prescribed division ratio. Then, it outputs a constant frequency f _d that is when the input frequency (hereinafter f _L) 0 Hz around to be in out-of-band high-frequency side (or f _H).

Further, the frequency counter 9, as shown in FIG. 3, according to the number of bits of the frequency counter 9, if the upper limit value f _H of the frequency, so that the count from 0 Hz. Similarly, the frequency of other components before the frequency divider 8 may be limited.

Considering the above frequency characteristics, the light receiving device 4
As shown in FIG. 4, the output frequency of the frequency counter 9 with respect to the output frequency (input frequency of the frequency counter 9) has a linear characteristic in a band where no overflow signal is output, and various characteristics depending on connection components outside the band. have. Here, the frequency on the horizontal axis is the difference between the oscillation frequency of the modulation laser 1 and the oscillation frequency of the local oscillation laser 2, and the oscillation frequency of the modulation laser 1 is higher than the oscillation frequency of the local oscillation laser 2. Is positive.

When the temperature and the bias current of the modulation laser 1 are controlled to stabilize the intermediate frequency, the temperature,
Considering that the frequency fluctuation with respect to the bias current fluctuation operates in the positive and negative regions in FIG. 4 and the band limitation of the frequency of the parts before the frequency counter 9, the intermediate frequency stabilization control is performed normally. In this case, it is necessary to draw the frequency in advance into the area B in FIG.

Here, a case in which the temperature or the bias current of the modulation laser 1 is swept will be described. The overflow detection unit 10 in FIG.
By setting the overflow signal to be output at a frequency higher than the upper limit of the absolute value of the frequency, the microcomputer unit 11 determines whether the overflow signal continues to be output in a certain region, and determines whether the frequency is out of band. Can be determined. The microcomputer unit 11
While sweeping the temperature or bias current of the modulation laser 1,
The frequency is read in a range where the overflow signal is not output in a certain region and the rate of change of the frequency with respect to the temperature or the bias current is positive, and the temperature or the bias current is stored in a region near the target frequency (region A in FIG. 4). Is set, the frequency can be drawn to the stable point of the intermediate frequency stabilization control.

As described above, according to the first embodiment of the present invention, a circuit for outputting an overflow signal at an arbitrary frequency or higher is attached, so that a part outside the frequency counter on the high frequency side may be unstable. The AFC can be started without fail at the fixed point, without fail, by pulling the intermediate frequency to the operation stable point of the intermediate frequency stabilization control.

(Embodiment 2) FIG. 5 is a block diagram showing a configuration of an optical signal transmitter using heterodyne detection to which a frequency stabilization control circuit in an optical signal transmission system according to Embodiment 2 of the present invention can be applied. FIG. As shown in FIG. 5, the second embodiment is obtained by removing the overflow detector 10 from the configuration of the optical signal transmitter shown in the first embodiment. Other circuit configurations and operations are the same as those described in the first embodiment, and thus detailed description is omitted.

In the second embodiment, the frequency counter 9
Is set so that the counter is held at the upper limit value of the absolute value of the frequency in the regions B and C in FIG.
The frequency reading of the microcomputer unit 11 is as shown in FIG. Here, if the gradient of the frequency with respect to the temperature or the bias current is equal to or more than a certain value, and it is determined that the region B is the region B in FIG. By setting the temperature or the bias current in A), the frequency can be drawn to the stable point of the intermediate frequency stabilization control.

As described above, according to the second embodiment of the present invention, by adding a function of counting at an arbitrary frequency or higher and holding the same at the highest frequency, the components on the high frequency side of the parts before the frequency discriminator are added. It is possible to reliably pull the intermediate frequency to the operation stable point of the intermediate frequency stabilization control and start AFC with a simpler configuration without malfunctioning at an unstable point outside the band.

[0034]

As described above, according to the present invention, the frequency discriminator is provided with a circuit for outputting an overflow signal at an arbitrary frequency or higher, or at the highest frequency without measuring at an arbitrary frequency or higher. By attaching the holding function, the intermediate frequency can be reliably pulled into the operation stable point of the intermediate frequency stabilization control without malfunctioning at the unstable point outside the high frequency band of the parts before the frequency discriminator. The effect is obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an optical signal transmitter using heterodyne detection to which a frequency stabilization control circuit according to a first embodiment of the present invention can be applied;

FIG. 2 is a diagram showing the input frequency dependence of the output frequency of a frequency divider for explaining the operation of an optical signal transmitter using heterodyne detection to which the frequency stabilization control circuit according to the present invention can be applied.

FIG. 3 is a diagram showing the input frequency dependence of the output frequency of a frequency counter for explaining the operation of an optical signal transmitter using heterodyne detection to which the frequency stabilization control circuit according to the present invention can be applied.

FIG. 4 is a diagram showing the dependence of the output frequency of a frequency counter on the output frequency of a light receiver for explaining the operation of an optical signal transmitter using heterodyne detection to which the frequency stabilization control circuit according to the present invention can be applied.

FIG. 5 is a block diagram showing a configuration of an optical signal transmitter using heterodyne detection to which the frequency stabilization control circuit according to the second embodiment of the present invention can be applied;

FIG. 6 is a diagram illustrating the operation of the optical signal transmitter using heterodyne detection to which the frequency stabilization control circuit according to the present invention can be applied. Diagram showing the dependence of the output frequency of the counter on the output frequency of the receiver

FIG. 7 is a block diagram showing a configuration of an optical communication receiver using heterodyne detection to which a conventional frequency stabilization control circuit can be applied;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Modulation laser 2 Local oscillation laser 3 Optical coupler 4 Optical receiver 5 Amplifier 6 Electric coupler 7 Transmission laser 8 Divider 9 Frequency counter 10 Overflow detection unit 11 Microcomputer unit (microcomputer unit) 12, 13, 14, 15 DA converter 16, 18 Temperature control circuit 17, 19 Bias current control circuit

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/14

Claims (6)

[Claims] A frequency stabilization control circuit for mixing an output light of a modulation laser and an output light of a local oscillation laser, and then pulling an intermediate frequency obtained by photoelectric conversion by a light receiver into a predetermined band. The frequency discriminator to which the intermediate frequency signal is input, by providing a means for outputting an overflow signal at an arbitrary frequency or more, by pulling a frequency into a band where the overflow signal is not output, the frequency discriminator before the frequency discriminator. A frequency stabilization control circuit characterized in that an intermediate frequency is drawn to an operation stable point of the intermediate frequency stabilization control without malfunctioning at an unstable point outside a band on a high frequency side of a component. 2. The frequency stabilization control circuit according to claim 1, wherein said means for outputting said overflow signal at a frequency equal to or higher than said arbitrary frequency comprises: a frequency counter to which said intermediate frequency signal is input; And an overflow detection unit that outputs the overflow signal when the count reaches an arbitrary count number. 3. The frequency stabilization control circuit according to claim 2, wherein one of the electrical coupler for branching the intermediate frequency signal and one of the intermediate frequency signals branched by the electrical coupler is divided into an operating frequency of the frequency counter. A frequency stabilization control circuit, comprising: a frequency divider that circulates, and wherein the frequency counter outputs an overflow signal outside a high-frequency band of components before the frequency divider. 4. A frequency stabilization control circuit for mixing an output light of a modulation laser and an output light of a local oscillation laser, and then pulling an intermediate frequency obtained by photoelectric conversion by a light receiver into a predetermined band. The frequency discriminator to which the intermediate frequency signal is input, by providing a means for holding at the highest frequency without measuring any frequency or higher, reads out a frequency in a band equal to or lower than the held highest frequency, and performs the frequency discrimination. A frequency stabilization control circuit characterized in that an intermediate frequency is drawn into an operation stable point of the intermediate frequency stabilization control without malfunctioning at an unstable point outside a high-frequency band of a component before the device. 5. The frequency stabilization control circuit according to claim 4, wherein the means for holding the highest frequency without measuring any frequency higher than the arbitrary frequency inputs and counts the intermediate frequency signal. What has been described above is a frequency stabilization control circuit having a frequency counter to which a function of holding at the highest frequency without counting is added. 6. The frequency stabilization control circuit according to claim 5, wherein one of the electrical coupler for branching the intermediate frequency signal and one of the intermediate frequency signals branched by the electrical coupler is divided into an operating frequency of the frequency counter. A frequency stabilization control circuit, comprising: a frequency divider that circulates, and wherein the frequency counter outputs an overflow signal outside a high-frequency band of components before the frequency divider.