JPH04252527A - Intermediate frequency pull-in method - Google Patents

Abstract

PURPOSE:To stop frequency sweep of an intermediate frequency signal at an operation stable point of an automatic frequency control circuit by stopping the sweeping of an oscillating frequency from a local oscillation light source at stable region of a real band or an image band. CONSTITUTION:The frequency is swept while applying small amplitude FM modulation to a radiation light 5 of a local oscillation light source 4, and a frequency discriminator 10 installed in the reception system demodulates the intermediate frequency signal subject to FM modulation. The demodulation signal and an FM modulation signal before being inputted to the local oscillation light source 4 are multiplied. The time when the multiplied value is at a high or a low level is measured while the oscillating frequency is swept. The band when the time is longer than a predetermined time is discriminated to be a stable region of a real band or an image band. Then the sweeping of the oscillation frequency of the local oscillation light source 4 is stopped at either of the bands.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermediate frequency pull-in method used in a coherent optical communication system.

0002

[Background Art] Optical heterodyne detection communication (coherent optical communication) has significantly higher receiving sensitivity than direct detection communication that modulates the intensity of light, and has high frequency utilization efficiency, so long-distance, high-density transmission is possible. It has the advantage of being possible. In this coherent optical communication, a photodetector receives the signal light sent from the optical transmitter and the light from the local oscillation light source built into the optical receiver. In this case, a beat corresponding to the frequency difference between the signal light and the local oscillation light appears as an intermediate frequency electric signal in the output of the photodetector.
By demodulating this, a baseband signal can be obtained. At this time, if the relative frequency difference between the local oscillation light and the signal light is not kept constant, fluctuations will occur in the intermediate frequency, so an AFC (automatic frequency control circuit) must be installed to stabilize the intermediate frequency. It will be done.

0003

[Problem to be Solved by the Invention] By the way, since the amount of frequency deviation of the light source is much wider than the intermediate frequency band, AF
In order to operate the AFC, it is necessary to sweep the frequency of the local oscillation light in advance to bring the intermediate frequency (IF) into the operating range of the AFC. By the way, when the frequency of the locally oscillated light is swept, the frequency of the IF signal is folded back at a low frequency and moves in the image band. That is, depending on whether the frequency of the local oscillation light is higher or lower than that of the signal light, even if the frequencies of the IF signals are the same, one will be an IF signal in the real band and the other will be in the image band. The input frequency versus output voltage characteristics of the frequency discrimination circuit used in AFC have opposite characteristics in the real band and in the image band. For this reason, when the stable point of AFC operation is set in the real band, if the IF signal arrives within the image band, the AFC will not operate and the IF will become unstable. As a means to avoid this problem, IF
During the IF sweep, small amplitude FM modulation is applied to the local oscillation light source, the FM modulated signal is demodulated by a frequency discriminator installed in the receiver, and the phase state of this extracted modulated signal is monitored to perform the IF sweep. A method was proposed to stop it. By the way, when the modulation signal for applying FM modulation to this local oscillation light and the demodulated FM modulation signal are phase detected using a phase comparator such as a mixer, the DC output during IF sweep is
The component exhibits characteristics as shown in FIG. 2(d) under the influence of the IF band filter. In other words, even if the characteristic of the frequency discriminator is as shown in FIG. 2(a), if it is passed through an IF band filter with the characteristic of FIG. 2(b), the discrimination characteristic will become as shown in FIG. 2(c), and as a result, the phase comparator The IF dependence of the terminal voltage of A is shown in Figure 2(d).
The level becomes high even in a region other than the stable operating point of the FC. This causes a problem in that it is not possible to completely specify the timing at which the IF sweep should be stopped.

The present invention eliminates such drawbacks and improves AFC
An object of the present invention is to provide an intermediate frequency pull-in method that can always stop the frequency sweep of an IF signal at a stable operation point.

[0005]

[Means for Solving the Problems] The first invention of the present application is a method of sweeping the oscillation frequency of a local oscillation light source in a coherent optical communication receiver to draw an intermediate frequency within a predetermined band. The frequency is swept while applying small amplitude FM modulation to the light emitted from the local oscillation light source, and the intermediate frequency signal subjected to the FM modulation is demodulated by a frequency discriminator installed in the receiving system to perform the FM modulation. A signal is extracted, the extracted demodulated signal is multiplied by the FM modulation signal before being input to the local oscillation light source, and the time period during which this multiplied value is at a high level or low level during the sweep of the oscillation frequency. The band in which this time is longer than a predetermined time is determined to be the stable area of the real band or the image band, and the sweeping operation of the oscillation frequency of the local oscillation light source is stopped in one of these bands. It's a joke.

A second invention of the present application is a method of sweeping the oscillation frequency of a local oscillation light source in a coherent optical communication receiver to bring an intermediate frequency within a predetermined band. Sweeping the frequency while applying small amplitude FM modulation to the emitted light, demodulating the FM modulated intermediate frequency signal by a frequency discriminator installed in the receiving system to extract the FM modulated signal; The demodulated signal obtained by the demodulation signal is multiplied by the FM modulation signal before being input to the local oscillation light source, and the multiplied value and the bias current and temperature of the local oscillation light source are stored one by one in a memory while the oscillation frequency is being swept. Then, using this stored data, calculate the time during which the multiplication value is at high level or low level, determine the range where this time is longest as the stable area of the real band or image band, and calculate the range of these ranges. The stored bias current and temperature corresponding to one of them are read from the memory and supplied to the local oscillation light source.

A third invention of the present application provides a method for sweeping the oscillation frequency of a local oscillation light source in a receiver for coherent optical communication to draw an intermediate frequency within a predetermined band. An intermediate frequency signal is input to the frequency discriminator, and this output voltage as well as the bias current and temperature of the local oscillation light source are stored in memory one by one during the sweep of the oscillation frequency, and this stored data is used to Calculate the time differential of the output voltage, find the time during which the calculated differential value becomes high level or low level, determine the range where this time is the longest as the stable region of the real band or image band, and calculate the time within these ranges. The stored bias current and temperature corresponding to one of them are read out from the memory and supplied to the local oscillation light source.

[0008]

[Operation] In the first and second inventions of the present application, the IF sweep should be stopped because the stable region of the real band or image band is determined by using the region where the multiplication value is at the high level or low level for the longest time. The timing can be specified at one point (see FIG. 2). As a method for measuring the time during which the multiplication value is at high level or low level, the first invention uses a method of constantly monitoring the multiplication value and time during the IF sweep. In the second invention, the multiplication value is stored one by one in the memory during the IF sweep, and after the sweep is completed, the stored multiplication value is retrieved to find the time when the level is high or low. There is. In addition, the third invention uses frequency discrimination characteristics and the corresponding bias current and temperature of the local oscillation light source instead of applying FM modulation to the local oscillation light.
Store everything in memory during F sweep. After the IF sweep, the frequency discrimination characteristic data is differentiated with respect to time. This differential value has the same meaning as the multiplication value in the first and second inventions. Therefore, the region in which the differential value remains at high level or low level for the longest time can be determined to be the stable region of the real band or image band. The IF signal is pulled in by reading the bias current and temperature corresponding to these regions from the memory and supplying them to the local oscillation light source.

[0009]

Embodiments FIG. 1 shows the configurations of the first and second embodiments. The first embodiment implements the first invention of the present application at 2.5 Gb/sFS.
This is applied to a K (frequency shift keying) optical heterodyne receiver. The signal light 6 transmitted through the optical fiber 1 is multiplexed with the locally oscillated light 5 emitted from the locally oscillated light source 4 by the optical coupler 2 and is input to the balanced receiver 3 . The IF signal output from the balanced receiver 3 passes through a bandpass filter 7 with a passband of 2.5 GHz to 7.5 GHz, and then is input to a demodulation circuit 8 and a frequency discrimination circuit 10. The output of the demodulation circuit 8 is connected to a first low-pass filter 9 with a cutoff frequency of 2.5GHz,
A b/s data signal is taken out. Note that the demodulation circuit 8 is adjusted so that the IF center frequency is 5 GHz and maximum demodulation efficiency is obtained. The characteristics of the frequency discriminator 10 are shown in Figure 3.
As shown in , there 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 10 is input to a phase comparator 12 and a second switch 18 after passing through a second low-pass filter 11 with a cutoff frequency of 10 kHz. The second switch 18 is controlled by the microcomputer 16, and its output is input to an adder circuit 19. Oscillation frequency 1
The output of the kHz oscillator 13 is input to the phase comparator 12 and the adder circuit 19 after passing through the first switch. The output of the phase comparator 12 is sampled by an A/D converter 15, and then the data is input to a microcomputer 16. The D/A converter 17 receives a signal from the microcomputer 16 and outputs a frequency sweep signal of the local oscillation light source 4. This signal is added to the signal from the second low-pass filter 11 or the signal from the oscillation circuit 13 in the adder circuit 19, and then added to the bias current of the local oscillation light source 4. This microcomputer 16 performs IF pull-in control through the following process from the start of receiving the signal light 6. When the reception start command is input to the microcomputer 16, the D/A converter 17 outputs a 10 Hz sawtooth wave for sweeping the frequency of the local oscillation light source in the 20 GHz range. Further, the first switch 14 is closed and the signal from the oscillation circuit 13 is supplied to the local oscillation light 4. As a result, the local oscillation light 4 has a frequency shift amount of 500MHz and a modulation frequency of 1.
FM modulation of kHz is applied. At this time, the second switch 18 is open. I during the sweep of the local oscillation light 5
When the F signal appears within the frequency discrimination band of Fig. 3(a), the second
The demodulated 1kHz F from the low-pass filter 11 of
An M modulated signal is output. The output of the phase comparator 12 is IF
It behaves as shown in Figure 3(b) depending on the frequency movement. The microcomputer 16 measures in real time the time when the output of the phase comparator 12 becomes 0.5V or more,
At the moment when this time becomes 15 ms or more, the frequency sweep of the local oscillation light 5 is stopped, the first switch 14 is opened, and the second switch is short-circuited. As a result, the IF frequency is stabilized at the actual band of 5 GHz.

The second embodiment is an application of the second invention of the present application, and can be shown using the same system as the first embodiment shown in FIG. The difference from the first embodiment is the operation of the microcomputer, so this operation will be explained below. When the reception start command is input to the microcomputer 16, the D/A converter 17 outputs a signal to sweep the frequency of the local oscillation light source in the range of 20 GHz.
A sawtooth wave of 0Hz is output. Further, the first switch 14 is closed and the reception signal from the oscillation circuit 13 is supplied to the local oscillation light source 4. As a result, the local oscillation light 4 has a frequency deviation of 500
FM modulation with a MHz modulation frequency of 1 kHz is applied. At this time, the second switch 18 is open. When an IF signal appears within the frequency discrimination band shown in FIG. 3A during sweeping of the local oscillation light 5, the second low-pass filter 11 outputs a demodulated 1 kHz FM modulation signal. The output of the phase comparator 12 behaves as shown in FIG. 3(b) depending on the movement of the IF frequency. The microcomputer 16 monitors the movement of the output voltage of the phase comparator 12 and the D/A converter 1.
The output data (current address) to 7 is stored one by one in the memory at a sampling frequency of 100 kHz. Local oscillation light 5
After the frequency sweep, the microcomputer converts the phase comparator 12
The current address corresponding to the center point of this region is output to the D/A converter 17. At the same time, the first switch 14 is opened and the second switch 18 is opened.
short circuit. As a result, the IF frequency is 5GHz, which is the actual band.
It will be stabilized.

FIG. 4 shows the configuration of the third embodiment. The third embodiment is an application of the third invention of the present application. Third
Since this embodiment is a modification of the control section of the first and second embodiments, this portion will be explained below. When the reception start command is input to the microcomputer 16, D
/A converter 17 changes the frequency of the local oscillation light source to 20
A 10Hz sawtooth waveform is output for sweeping in the GHz range. At this time, the second switch 18 is open. When an IF signal appears within the frequency discrimination band shown in FIG. 3(a) during sweeping of the local oscillation light 5, the second low-pass filter 11 outputs a voltage as shown in FIG. The microcomputer 16 captures this voltage via the A/D converter 15 at a sampling frequency of 100 kHz and stores it in memory. At the same time, output data (current address) to the D/A converter 17 is also stored in the memory. After sweeping the local oscillation light, the microcomputer 16 differentiates the stored voltage with respect to time. Then, the current address at the center of the region where the value after differentiation is 0.5 V or more is read from the memory and output to the D/A converter 17. At the same time, the second switch 18 is shorted and the IF frequency is stabilized at 5 GHz, which is the actual band.

[0012] In addition to the above-described embodiments, various modifications of the invention of the present application can be considered. Although the oscillator 13 is provided independently in the first and second embodiments, it is also possible to output a 1 kHz sine wave from the microcomputer 16 itself via the D/A converter 17. Further, in the embodiment, the AFC signal is supplied to the local oscillation light source via the second switch 18, but the A/D converter 15 supplies the AFC signal to the local oscillation light source via the second low-pass filter 1.
Read the output voltage of 1 and send this value to microcomputer 1.
The data may be output to the D/A converter 17 after being processed in the D/A converter 17. As a result of the above, the adder circuit 19 becomes unnecessary. In the embodiment, the temperature of each local oscillation light source 4 is kept constant, but this temperature may be swept during frequency sweeping. In this case, it is necessary for the microcomputer 16 to simultaneously store this temperature in the memory during frequency sweep, and at the time of drawing the IF to a stable point, read out the temperature corresponding to this frequency from the memory and apply it to the local oscillation light source. It is possible to facilitate identification of high levels by dividing each data by the minimum value of the output voltage of the phase comparator 12 stored in the second embodiment.

[0013]

As described in detail above, by using the present invention, an intermediate frequency pull-in method that can always stop the frequency sweep of the IF signal at the stable point of AFC operation has been realized.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram showing a first embodiment of the present invention.

FIG. 2 is a characteristic diagram of a circuit that implements a conventional intermediate frequency pull-in method.

FIG. 3 is a frequency discrimination characteristic and a phase comparator output characteristic diagram in the second and third embodiments.

FIG. 4 is a configuration diagram showing a third embodiment.

FIG. 5 is a diagram showing frequency discrimination characteristics in a third example.

[Explanation of symbols] 1 Optical fiber 2 Optical coupler 3 Balanced receiver 4 Local oscillation light source 5 Local oscillation light 6 Signal light 7 Bandpass filter 8 Demodulation circuit 9 First low-pass filter 10 Frequency discrimination circuit 11 Second low-pass filter 12 Phase comparator 13 Oscillator 14 First switch 15 A/D converter 16 Microcomputer 17 D/A converter 18 Second switch 19 Adder circuit

Claims (3)

[Claims] 1. A method of sweeping the oscillation frequency of a local oscillation light source in a receiver for coherent optical communication to bring the intermediate frequency within a predetermined band, wherein a small-amplitude FM is applied to the light emitted from the local oscillation light source. Sweeping the frequency while applying modulation, demodulating the FM-modulated intermediate frequency signal using a frequency discriminator installed in the receiving system to extract the FM modulated signal, and combining this extracted demodulated signal with the FM modulated signal. Multiply by the FM modulation signal before input to the local oscillation light source, measure the time during which this multiplied value is at a high level or low level during the sweep of the oscillation frequency, and determine that this time is a predetermined time. An intermediate frequency pulling method characterized in that a longer band is determined to be a stable region of an actual band or an image band, and the sweeping operation of the oscillation frequency of the local oscillation light source is stopped in one of these bands. 2. A method of sweeping the oscillation frequency of a local oscillation light source in a receiver for coherent optical communication to bring an intermediate frequency within a predetermined band, wherein a small amplitude FM is applied to the light emitted from the local oscillation light source. Sweeping the frequency while applying modulation, demodulating the FM-modulated intermediate frequency signal using a frequency discriminator installed in the receiving system to extract the FM modulated signal, and combining this extracted demodulated signal with the FM modulated signal. The FM modulation signal before being input to the local oscillation light source is multiplied by the FM modulation signal, and while the oscillation frequency is being swept, this multiplication value and the bias current and temperature of the local oscillation light source are stored in memory one by one. Using the data, calculate the time during which the multiplication value is at high level or low level, determine the range where this time is longest as the stable area of the real band or image band, and set it to either of these ranges. An intermediate frequency pulling method characterized in that the corresponding stored bias current and temperature are read from the memory and supplied to the local oscillation light source. 3. In a method of sweeping the oscillation frequency of a local oscillation light source in a receiver for coherent optical communication to draw an intermediate frequency within a predetermined band, a frequency discriminator installed in the receiver Input the intermediate frequency signal,
The output voltage of the frequency discriminator as well as the bias current and temperature of the local oscillation light source are stored in memory one by one during the sweep of the oscillation frequency, and the time differential of the output voltage is calculated using this stored data. , find the time at which the calculated differential value becomes high level or low level, determine the range where this time is longest as the stable region of the real band or image band, and store the memory corresponding to either of these ranges. An intermediate frequency pulling method characterized in that the bias current and temperature are read out from the memory and supplied to the local oscillation light source.