JPH08181666A - Automatic frequency control circuit for polarized wave diversity optical receiver - Google Patents

Abstract

PURPOSE: To allow the circuit to operate stably even against fast fluctuation of a polarized wave by selecting one intermediate frequency signal in two polarized components in response to an output signal of a hysteresis circuit and controlling an optical local oscillator based on the selected signal. CONSTITUTION: When a polarized state of a signal light fluctuates with a frequency sufficiently higher than a reply frequency of automatic frequency control, an output signal of a comparator circuit 33 fluctuates fast in response to a polarized wave fluctuation. The fluctuation is absorbed by a filter circuit 34 limiting a frequency band and an output of the circuit 34 in which the fluctuation is suppressed is given to a hysteresis circuit 35. The output of the circuit 35 is made to be stable and a switch circuit 36 selects only one intermediate frequency signal continuously. Thus, even when high speed polarized wave fluctuation is in existence, the intermediate frequency signal with higher power is obtained from the circuit for each half period of the polarized wave fluctuation and stable automatic frequency control is conducted based on the intermediate frequency signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applicable to lightwave communication, which is expected to be applied to future long distance and large capacity communication.
The present invention relates to an automatic frequency control circuit used in a polarization diversity receiver that employs a polarization diversity reception method as a polarization fluctuation countermeasure and enables stable reception.

[0002]

2. Description of the Related Art Lightwave communication systems include intensity modulation / direct detection systems and coherent optical communication systems, and coherent optical communication systems include homodyne systems and heterodyne detection systems.

Of these, the heterodyne detection method
4 shows the input signal light and the optical local oscillator.
After combining the local oscillation light generated in 2 in the mixing circuit 1 '
Then, the photodetector circuit 3 performs photodetection to obtain an intermediate frequency signal.
Demodulate using the intermediate frequency signal. At this time, the intermediate frequency
Automatic frequency control that keeps the center frequency of the signal constant (AFC
Control) is required. In AFC control, the intermediate frequency signal
A part is branched by the branch circuit 5, and the frequency is discriminated by the frequency discrimination circuit 17.
After distinguishing several outputs, the output is converted into a current by the voltage-current conversion circuit 18.
Then, the signal is fed back to the optical local oscillator 2 and, as a result, the intermediate frequency signal is transmitted.
Intermediate frequency f _IFTo maintain a constant
Wave number f_LIs used.

In lightwave communication, signal light may cause polarization fluctuation in which the plane of polarization changes in a propagation path such as an optical fiber. As a countermeasure against the polarization fluctuation of the signal light, a polarization diversity receiving method is effective. Is. In the polarization diversity receiving method, optical heterodyne detection is performed using two receivers corresponding to the two polarization states of the signal light, horizontal and vertical, and two intermediate frequency signals are output and demodulated. A configuration that adds both is used.

FIG. 5 shows an example of the configuration of a receiving apparatus using such a polarization diversity receiving system. As shown in the figure, in the mixing circuit 1, the signal light is split into two horizontal / vertical polarizations, and the local oscillation light of the optical local oscillator 2 is split into two horizontal / vertical polarizations so that the horizontal polarization components And vertical polarization components are respectively mixed to obtain intermediate frequency component light of two polarizations of horizontal / vertical, and this is received by the receiver on the side of horizontal polarization (optical detection circuit 3H,
Amplification circuit 4H, branch circuit 5H, demodulation circuit 6H) and vertical polarization side receiver (optical detection circuit 3V, amplification circuit 4V, branch circuit 5V, demodulation circuit 6V) demodulate each and add the demodulated signals respectively. The circuit 8 adds the values.

As the AFC control in this polarization diversity receiving system, as shown in FIG. 6, the power of the intermediate frequency signal of two horizontal and vertical polarization planes branched by the branch circuits 5H and 5V is detected by the power detector. 10H, 10
Each of them is detected by V and compared with each other by the comparison circuit 12, the comparison circuit 12 judges the intermediate frequency signal of the larger power, and the switch drive circuit 16 makes the switch circuit 11
Is used to select the intermediate frequency signal having the larger power, and then the frequency discrimination circuit 17 discriminates the frequency of the selected intermediate frequency signal to feed back to the optical local oscillator 2. That's the method.

In this comparison and selection method, the powers of two intermediate frequency signals are detected and compared with each other, and then the comparison circuit 12
Output signal of the hysteresis circuit to prevent chattering 1
5, the switch circuit 11 is input to control the switch circuit 11 to select an intermediate-frequency signal having a large power, thereby switching the two intermediate-frequency signal powers with respect to minute fluctuations in the magnitude relationship. The circuit 11 is prevented from switching unnecessarily frequently.

[0008]

In the AFC control using the comparison and selection method described above, stable operation is required even with high-speed polarization fluctuation. When the polarization state of the signal light fluctuates at high speed, the response time of the analog control circuit from the power detection to the control of the switch circuit 11 is so large that it cannot be ignored, compared to the cycle of the power fluctuations of the two intermediate frequency signals. Then, the operation timing of the switch circuit 11 is delayed with respect to the polarization fluctuation, and it becomes impossible for the switch circuit 11 to always select an intermediate frequency signal having a large power. Particularly, in the worst case where the phase is shifted by 180 degrees, the switch circuit 11 always selects the intermediate frequency signal having a small power.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an automatic polarization diversity optical receiver capable of operating stably even with high-speed polarization fluctuations. It is to provide a frequency control circuit.

[0010]

FIG. 1 is a diagram illustrating the principle of the present invention. An automatic frequency control circuit of a polarization diversity receiver according to the present invention includes a power detection circuit 31, which detects the powers of intermediate frequency signals of two polarization components,
32, and a comparison circuit 33 for comparing the magnitudes of the powers of the two intermediate frequency signals detected by the power detection circuits 31, 32,
A filter circuit 34 for limiting the frequency band of the comparison result signal of the comparison circuit 33, and a hysteresis circuit 35 for giving a hysteresis characteristic to the output signal of the filter circuit 34.
And a switch circuit 36 that selects one of the intermediate frequency signals of the two polarization components according to the output signal of the hysteresis circuit 35, and controls the optical local oscillator based on the signal selected by the switch circuit 36. It is composed of.

The filter circuit 34 can be constructed by a low-pass filter, and the cut-off frequency f of this low-pass filter.
_LPF is f _AFC <f _LPF <f _HIS and f _{A2 / A1} <f _HIS , where f _AFC is the response frequency of automatic frequency control, f _HIS
Is preferably set to be a response frequency of the hysteresis circuit, and f _{A2 / A1} is a frequency of the low-pass filter whose normalized voltage amplitude matches the hysteresis ratio A ₂ / A ₁ of the hysteresis circuit.

Further, a phase compensating circuit 37 for compensating for a signal delay generated in the filter circuit 34 is provided in the subsequent stage of the filter circuit 34, and an output signal of the filter circuit 34 is inputted to the hysteresis circuit 35 through the phase compensating circuit 37. Can be configured as.

[0013]

When the polarization state of the signal light fluctuates at a frequency sufficiently higher than the response frequency f _AFC of the automatic frequency control, the output signal of the comparison circuit 33 also fluctuates at high speed in response to the polarization fluctuation. However, this fluctuation is absorbed by the filter circuit 34 that limits the frequency band, and the filter circuit 3
The fluctuation of the output signal of 4 is suppressed and the hysteresis circuit 35
Is input to Therefore, the output signal of the hysteresis circuit 35 is stable and does not fluctuate with respect to the polarization fluctuation, and therefore the switch circuit 36 does not perform the switching operation and continuously selects only one intermediate frequency signal. If only one of the intermediate frequency signals is continuously selected for the high-speed polarization fluctuation, the switching operation phase of the high-speed polarization fluctuation, especially the polarization fluctuation, may be shifted by 180 °. Even if there is a polarization fluctuation, the automatic frequency control circuit can obtain an intermediate frequency signal on the high power side every half cycle of the polarization fluctuation, and perform stable automatic frequency control based on this intermediate frequency signal. It is possible.

The filter circuit 34 can be constituted by a low-pass filter, and its cutoff frequency f _{LP F} is set so that f _AFC <f _LPF <f _HIS and f _{A2 / A1} <f _HIS . In this way, when the frequency f _{A2 / A1} is smaller than the response frequency f _HIS , the switching operation of the switch circuit 36 is 1 even if the polarization state of the signal light fluctuates at high speed.
Even if it happens once, it will not happen more than once.

[0015]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 shows a polarization diversity type receiver using an automatic frequency control circuit as an embodiment of the present invention. In FIG. 2, the mixing circuit 1 mixes the horizontal / vertical polarization components of the signal light transmitted through the optical transmission line with the local oscillation light output from the optical local oscillator 2. The optical local oscillator 2 oscillates local oscillation light. Optical detection circuit 3H,
3V optically detects the optical signals of the horizontal / vertical polarization components output from the mixing circuit 1, performs optical / electrical conversion, and outputs an intermediate frequency signal. The amplifier circuits 4H and 4V are the photodetector circuit 3
The intermediate frequency signals output from H and 3V are amplified respectively. Branch circuits 5H and 5V are amplifier circuits 4H and 4V
A part of the output signal of the above is branched and given to the AFC circuit. The demodulation circuits 6H and 6V respectively demodulate the intermediate frequency signals of the horizontal / vertical polarization components to obtain baseband signals. Adder circuit 8
Adds the baseband signals output from the demodulation circuits 6H and 6V, respectively. Baseband circuit 7 is addition circuit 8
The clock CLK is extracted and identified from the baseband signal output from.

In the AFC circuit, the branch circuit 9
H and 9V further branch a part of the intermediate frequency signal branched by the branch circuits 5H and 5V, respectively. Power detector 10
H and 10V detect the power of the intermediate frequency signal branched by the branch circuits 9H and 9V, respectively. The comparison circuit 12 compares the power levels of the power detection signals of the two intermediate frequency signals from the power detectors 10H and 10V. The low-pass filter (low-pass filter circuit) 13 applies frequency band limitation to the signal output from the comparison circuit 12. The phase compensation circuit 14 compensates the phase delay of the signal generated in the low pass filter 13. The hysteresis circuit 15 gives the signal output from the phase compensation circuit 14 the hysteresis characteristic shown in FIG. The switch drive circuit 16 drives the switch circuit 11 to switch using the signal output from the hysteresis circuit 15. The switch circuit 11 is a branch circuit 9H, 9V
One of the two intermediate frequency signals branched by is selected. The frequency discriminating circuit 17 discriminates the frequency of the signal output from the switch circuit 11. The voltage-current conversion circuit 18 feeds back the frequency output from the frequency discrimination circuit 17 to the injection current of the local oscillation light in the optical local oscillator 2.

The difference between this AFC circuit and the AFC circuit of FIG. 6 is that a low pass filter 13 and a phase compensation circuit 14 are provided after the comparison circuit 12, and the comparison circuit 1
The output signal of 2 is the low-pass filter 13 and the phase compensation circuit 1
4 is input to the hysteresis circuit 15.

Cutoff frequency f _LPF of the low-pass filter 13
The setting method of will be described below with reference to FIG. In FIG. 3, the vertical axis represents the normalized voltage amplitude, the horizontal axis represents the frequency, and (a) is the frequency characteristic of AFC control,
(B) shows the frequency characteristic of the low-pass filter 13, and (C) shows the frequency characteristic of the hysteresis circuit 15.

Now, the response frequency of AFC control is f _AFC ,
When the response frequency of the hysteresis circuit 15 is f _HIS , the cutoff frequency f _LPF of the low-pass filter 13 is set so that f _AFC <f _LPF <f _HIS and f _{A2 / A1} <f _HIS . However, the response frequency f of the hysteresis circuit 13
It is assumed that the switch drive circuit 16 and the switch circuit 11 respond sufficiently faster than the _HIS . f _{A2 / A1} is
The low-pass filter 1 corresponding to the point where the normalized voltage amplitude matches the hysteresis ratio A ₂ / A ₁ shown in FIG.
It is a frequency on the characteristic (b) of 3. The above A ₂ is the maximum fluctuation width of the input signal amplitude to the hysteresis circuit, and A ₁ is the width of the hysteresis.

Thus, the frequency f _{A2 / A1} is the response frequency f
When it is smaller than _HIS , the polarization state of the signal light fluctuates at high speed, and even if the output signal of the comparison circuit 12 follows it and fluctuates at high speed, the fluctuation is absorbed by the low-pass filter 13, Since the fluctuation width of the signal input to the hysteresis circuit 15 is within the width A ₂ of the hysteresis, the switch operation by the switch circuit 11 may be performed once but not twice or more.

That is, as shown in FIG. 3B, when the amplitude of the input signal to the hysteresis circuit 15 is A ₃ ,
If the input signal amplitude | A ₃ | <A ₂ , the hysteresis circuit 1
Since the output signal of 5 has a constant value, the output signal of the comparison circuit 12 is low-pass filtered by the low-pass filter 13 so as to have the above-mentioned relationship with respect to high-speed polarization fluctuation.

In this way, the response frequency f of the AFC control is
_When the polarization state of the signal light fluctuates at a frequency sufficiently higher than _AFC , control is performed so that the switching operation of the switch circuit 11 is not performed and only one intermediate frequency signal is selected. In this way, if only one of the intermediate frequency signals is selected, the intermediate frequency signal can be obtained for each half cycle of the polarization fluctuation. Therefore, even in the worst case where the phase shifts by 180 ° as described above, one of the intermediate frequency signals can be obtained. By continuously selecting the frequency signal, a signal having power is extracted from the intermediate frequency signal in every half cycle, and the frequency discrimination circuit 17
Can be entered.

[0023]

As described above, according to the present invention, the automatic frequency control circuit in the polarization diversity optical receiver can be stably operated even with high-speed polarization fluctuation. .

[Brief description of drawings]

FIG. 1 is a diagram illustrating the principle of the present invention.

FIG. 2 is a diagram showing a polarization diversity optical receiver including an automatic frequency control circuit as an embodiment of the present invention.

FIG. 3 is a diagram illustrating characteristics of a low-pass filter and a hysteresis circuit in the device of the embodiment.

FIG. 4 is a diagram showing a conventional optical receiver having an AFC control function.

FIG. 5 is a diagram showing a conventional polarization diversity optical receiver.

FIG. 6 is a diagram showing a configuration example of an AFC circuit used in a conventional polarization diversity optical receiver.

[Explanation of symbols]

 1, 1'mixing circuit 2 optical local oscillator 3, 3H, 3V optical detection circuit 4, 4H, 4V amplification circuit 5, 5H, 5V, 9H, 9V branch circuit 6, 6H, 6V demodulation circuit 7 baseband circuit 8 adder 10H, 10V Power detection circuit 11 Switch circuit 12 Comparison circuit 13 Low-pass filter 14 Phase compensation circuit 15 Hysteresis circuit 16 Switch drive circuit 17 Frequency discrimination circuit 18 Voltage-current conversion circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H04B 10/04 10/06 10/152 10/142 H04B 9/00 L

Claims (3)

[Claims] 1. A power detection circuit (31, 32) for detecting the power of two intermediate frequency signals of two polarization components, and a comparison for comparing the magnitudes of the powers of the two intermediate frequency signals detected by the power detection circuit. A circuit (33), a filter circuit (34) for limiting the frequency band of the comparison result signal of the comparison circuit, a hysteresis circuit (35) for giving a hysteresis characteristic to an output signal of the filter circuit, and a hysteresis circuit of the hysteresis circuit. A switch circuit (3 that selects one of the intermediate frequency signals of the two polarization components according to the output signal
6) and an automatic frequency control circuit of a polarization diversity optical receiver configured to control an optical local oscillator based on a signal selected by the switch circuit. 2. The filter circuit comprises a low pass filter.
And its cutoff frequency f _LPFBut f_AFC<F_LPF<F_HIS And f_{A2 / A1}<F_HIS However, f_AFCIs the response frequency of automatic frequency control, f_HIS
Is the response frequency of the hysteresis circuit, f_{A2 / A1}Is the standard
The generated voltage amplitude is the hysteresis of the hysteresis circuit.
Ratio A₂/ A₁The frequency of the low-pass filter that matches
2. The bias according to claim 1, wherein the bias is set as follows.
Wave diversity optical receiver automatic frequency control circuit. 3. A phase compensation circuit (3) for compensating a signal delay generated in the filter circuit after the filter circuit.
7. The automatic frequency control circuit of the polarization diversity optical receiver according to claim 1, wherein the output signal of the filter circuit is provided to the hysteresis circuit through the phase compensation circuit.