JPH06104834A - Automatic gain control circuit - Google Patents

Abstract

PURPOSE:To prevent deterioration in a code error rate characteristic by suppressing noise generated from an optical receiver to which no optical signal is led because of a polarized wave ratio. CONSTITUTION:A transmitted optical signal is divided by a polarized wave separation coupler 103 depending on a polarized wave ratio and led to photo detection circuits 105, 106, in which the signal is converted into an intermediate frequency signal and amplified and demodulated at variable gain amplifiers 107, 108 and demodulators 109, 110 at the poststage. Part of the intermediate frequency signal is fed to detectors 112, 113, in which the signal is subjected to envelope detection. When all of the optical signals are led to the photo detection circuit 105 regardless of the polarized wave ratio, the demodulation circuit 109 outputs a demodulation signal and noise from the photo detection circuit 106 is outputted from the opposite demodulation circuit 110 and a code error rate characteristic is deteriorated. Then a voltage by the output of the detector 112 is added to a control voltage of the variable gain amplifier 107 to which the optical signal is led at an adder 121 to increase the gain, and conversely the control voltage of the variable gain amplifier 108 of the opposite side is unchanged and the gain is suppressed thereby preventing the noise from being invaded in the demodulation signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic gain control circuit for an intermediate frequency amplifier in a polarization diversity optical heterodyne receiver of a transmission device via an optical fiber.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram of an example of a conventional automatic gain control circuit. The optical signal frequency-modulated by the transmitter 301 according to the digital signal is transmitted to the fiber 3
It is transmitted in 02 and guided to the polarization separation coupler 303.
The polarization separation coupler 303 guides the optical signal to the optical detection circuit 305 or 306 according to the polarization ratio of the optical signal transmitted through the optical fiber 302 (hereinafter referred to as the ratio of S wave: P wave). For example, when the polarization ratio of the optical signal after transmission is O (P wave), it is guided to the optical detection circuit 305, and when it is 1 (S wave), it is guided to 306. Assuming that the polarization ratio of the optical signal is O (P wave), the optical signal that has passed through the polarization separation coupler 303 and the output light of the local oscillation light source 304 provided in the optical receiver and the optical detection circuit 305. Is combined and detected at and converted to an intermediate frequency signal. Thereafter, the intermediate frequency signal is demodulated by the demodulation circuit 309.
The signal is demodulated at and output from the multiplexer 311 as a demodulated signal.

A part of the intermediate frequency signal is a variable gain amplifier 30.
7 is branched at the latter stage, and envelope detection is performed by the detectors 312 and 313. The output passes through the adder 314 and returns to the variable gain amplifier 307 to become a gain control signal, which is controlled so that the output of the intermediate frequency signal is always constant. The output of the adder 314 is the variable gain amplifier 308 of the other optical receiver.
The gain control is performed in the same manner.

FIG. 4 is a correlation diagram showing the operating characteristics of the respective parts of FIG. When the polarization ratio changes, the output power of the photodetector circuits 305 and 306 changes like (L) and (m) according to the polarization ratio. Since the control signals (p) to the variable gain amplifiers 307 and 308 are constant at V ₂ regardless of the polarization state, the outputs of the detectors 312 and 313 are as shown in (n) and (o), respectively. The output of 314 is (m) + (o) = V ₂
And the loop is closed. The gain control of the intermediate frequency amplifier is performed by the above operation.

[0005]

In this conventional automatic gain control circuit, the control voltage (p) to the variable gain amplifier is constant as V ₂ regardless of the polarization ratio, so that the polarization ratio is 0, for example.
Even when the output (m) of the optical detection circuit 306 is (longitudinal wave) and is 0, the gain of the variable gain amplifier 308 is equivalent to 307. At this time, the output (m) is 0, but in actuality, shot noise, thermal noise, etc. due to the local oscillation light from the photodetector circuit 306 are amplified by the variable gain amplifier 308 and output through the combiner 311 together with the demodulated signal. To be done. As a result, there is a problem that the code error rate characteristic is deteriorated.

[0006]

SUMMARY OF THE INVENTION An automatic gain control circuit according to the present invention comprises a gain control circuit for the first and second intermediate frequency amplifiers in an optical heterodyne detection receiver of the polarization diversity receiving system, and an intensity of an input optical signal. Is applied to the gain control circuits of the first and second intermediate frequency amplifiers, respectively. The difference between the first control output proportional to the first control output and the second control output proportional to the polarization separation ratio of the input signal light is applied to the gain control circuits of the first and second intermediate frequency amplifiers, respectively. And

Further, the automatic gain control circuit of the present invention includes means for adding two outputs of the envelope detection of the polarization diversity optical heterodyne receiver and outputting the gain control signal, and an optical signal according to the polarization ratio. And a means for increasing the gain of the guided intermediate frequency amplifier on the optical receiver side and suppressing the gain of the intermediate frequency amplifier on the optical receiver side where the optical signal is not guided.

[0008]

The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram of an embodiment of the present invention, and FIG. 2 is a correlation diagram showing operation characteristics of each part of the present embodiment.

In this embodiment, the transmitted optical signal is divided by the polarization separation coupler 103 according to the polarization ratio, and is guided to each of the optical detection circuits 105 and 106. Here, the signal is converted into an intermediate frequency signal and amplified and demodulated by the variable gain amplifiers 107 and 108 and demodulators 109 and 110 in the subsequent stages. A part of the intermediate frequency signal is applied to the detectors 112 and 113 and envelope detection is performed. If all the optical signals are guided to the optical detection circuit 105 side by the polarization ratio, the demodulation circuit 109 outputs the demodulation signal, while the opposite demodulation circuit 110 outputs the noise from the optical detection circuit 106. The code error rate characteristics deteriorate. Therefore, the voltage corresponding to the output of the detector 112 is added to the repeater 121 to the control voltage of the variable gain amplifier 107 on the side where the optical signal is guided to increase the gain, and conversely, the control voltage of the variable gain amplifier 108 on the opposite side is The gain is suppressed without changing, and the noise from the photodetector circuit 106 is prevented from being mixed with the demodulated signal.

Next, the operation of this embodiment will be described. The optical signal separated by the polarization separation coupler 103 is guided to the optical detection circuit 105 or 106, and the local oscillation light source 10
The output light of FIG. 4 is combined and detected to be an intermediate frequency signal, which is output as shown in FIGS. 2A and 2B according to the polarization ratio. This intermediate frequency signal is supplied to the demodulation circuits 109 and 110 and the detectors 112 and 113 through the variable gain amplifiers 107 and 108. The outputs of the detectors 112 and 113 are shown in FIG.
As shown in (f) and (g), adders 114, 115 and 1
18 and is provided to inverting amplifiers 114 and 115.
The gains of the inverting amplifiers 114 and 115 are selected to be 1.
The output is shown in FIGS. 2 (f ') and (g').

(F ') =-(f') (g ') =-(g') These outputs (f ') and (g') and the outputs (f ') and (g' of the detectors 112 and 113). ) Are added by the adders 116 and 117 to obtain (h) and (i), respectively.

The outputs (h) and (i) are the common-mode amplifier 11
Given to 9,120. Here, the common-mode amplifiers 119, 1
The gain of 20 is selected to be 0.5, and the outputs are (j) and (k).

(J) = (h) × 0.5 (k) = (i) × 0.5 On the other hand, the output (c) of the adder 118 is the detectors 112 and 113.
Is the sum of the outputs (f) and (g) of (c) = (f) +
(G).

The value of (c) is constant at V ₁ regardless of the polarization ratio. In-phase amplifiers 119 and 120 outputs (j) and (k) are output (c) and adders 121 and 122.
And control voltages (d) and (e) to the variable gain amplifiers 107 and 108 are obtained. (D) = (c) + (j) (e) = (c) + (k) Now, Assuming that the polarization ratio of the optical signal is 0 (longitudinal wave), the output power (a) of the photodetector circuit 105 is 1.0 and the output power (b) of the opposite photodetector circuit 106 is 0. 1, in which an optical signal is guided accordingly, automatic gain control circuits (f), (g '), (h) of the optical receiver on the upper half side,
The values of (j) are as follows.

Output of detector 112 (f) = 1.0 Output of inverting amplifier 114 (g ′) = 1.0 Output of adder 116 (h) = 2.0 Output of common-mode amplifier 119 (j) = 1.0 where , The output (c) of the adder 118 is V independent of polarization.
_{Since 1} is constant, the adder 1 to the variable gain amplifier 107
The control voltage (d) of 21 becomes (d) = V ₁ +1.0.

On the other hand, the automatic gain control circuits (g), (f ') of the optical receiver on the lower half side where the optical signal is not guided,
The values of (j) and (k) are as follows.

Output of detector 113 (g) = 0 Output of inverting amplifier 115 (f ′) = 0 Output of adder 117 (i) = 0 Output of common-mode amplifier 120 (k) = 0 Then, adder to variable gain amplifier 108 The control voltage (e) of 122 becomes (e) = V ₁ +0.

As described above, when the polarization ratio is 0 (longitudinal wave),
The gain control voltage (d) to the optical receiver side through which the optical signal is guided becomes V1 +1.0, and the gain of the variable gain amplifier 107 increases. Further, the gain control voltage (e) to the optical receiver on the opposite side becomes V ₁ +0, and the gain of the variable gain amplifier 108 is suppressed. When the polarization ratio of the optical signal is 1 (transverse wave), as shown in FIG.
All the values of (k) are inverted. Therefore, the control signals to the variable gain amplifiers 107 and 108 of each optical heterodyne receiver are also inverted, resulting in (d) = V1 + 0, and the side where the optical signal is guided in the same way as in the case where the polarization ratio is 0 (longitudinal wave). The gain of the optical receiver is increased and the gain on the opposite side is suppressed.

When the polarization ratio gradually changes from 0 to 1, the gain control signal changes according to the value of the polarization ratio as shown in FIGS. 2 (d) and 2 (e). Similarly, the gain of the gain amplifier changes, and when the polarization ratio becomes 0.5, the two gains have the same value.

By doing so, shot noise, thermal noise, etc. due to the locally oscillated light from the photodetector circuits 105, 106 can be suppressed according to the polarization ratio, so that the deterioration of the code error rate characteristic can be prevented.

[0021]

As described above, the automatic gain control circuit of the present invention suppresses the gain of the intermediate frequency amplifier of the optical heterodyne receiver on the side where the optical signal is not guided in accordance with the polarization ratio, thereby demodulating the signal. In addition, it is possible to prevent the shot noise, the thermal noise, and the like due to the local oscillation light from the photodetector circuit from being mixed, and it is possible to eliminate the deterioration of the code error rate characteristic.

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of the present invention.

FIG. 2 is a correlation diagram showing characteristics of each part of this embodiment.

FIG. 3 is a block diagram of an example of a conventional automatic gain control circuit.

FIG. 4 is a block diagram of an example of a conventional automatic gain control circuit.

[Explanation of symbols]

 101, 301 Transmitter 102, 302 Optical fiber 103, 303 Polarization separation coupler 104, 304 Local oscillation light source 105, 106, 305, 306 Photodetector circuit 107, 108, 307, 308 Variable gain amplifier 109, 110, 309, 310 Demodulation circuit 111, 311 Combiner 112, 113, 312, 313 Detector 114, 115 Inverting amplifier 116, 117, 118, 314 Adder 119, 120 In-phase amplifier 121, 122 Adder

Claims (2)

[Claims] 1. A gain control circuit for the first and second intermediate frequency amplifiers in an optical heterodyne detection receiver of a polarization diversity receiving system, and a first proportional to the intensity of an input optical signal.
Gain control circuit of the first and second intermediate frequency amplifiers, respectively, applying a difference between the control output of the first control signal and the second control output proportional to the polarization separation ratio of the input signal light to the gain control circuits of the first and second intermediate frequency amplifiers, respectively. circuit. 2. A means for adding two outputs of envelope detection of a polarization diversity optical heterodyne receiver and outputting as a gain control signal, and an optical receiver side to which an optical signal is guided according to a polarization ratio. An automatic gain control circuit comprising means for increasing the gain of the intermediate frequency amplifier and suppressing the gain of the intermediate frequency amplifier on the side of the optical receiver to which the optical signal is not guided.