JPS60172842A - Controller of optical reception circuit - Google Patents

Abstract

PURPOSE:To obtain a reception output signal with high S/N by detecting directly the polarized state of an input signal from a detection signal to attain control. CONSTITUTION:In taking the distribution ratio of a signal power PS and a station light emitting power PL to a photodetector 6a in a polarized separator 5 respectively as AS, AL and the distribution ratio to a photodetector 6b as BS, BL, then the ratio is controlled so as to be AS=AL and BS=BL, an output of the controller 9 is applied to an optical modulator 3 as a modulation signal and the separated linearly polarized light is detected respectively by the photodetectors 6a, 6b, synthesized differentially by a differential synthesizer 7 and a received output signal is obtained at an output terminal 8.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a control device that maximizes and keeps the detection output constant in an optical heterodyne receiving circuit without depending on the polarization state of received light.

Background> Figure 1' shows the configuration of an optical receiving circuit proposed in Japanese Patent Application No. 58-155089. Input signal light from an optical transmission line 1 such as an optical fiber is multiplexed by a multiplexer 4 with local light that has passed through an optical modulator 3 from a local light source 2 .

This combined light is separated into mutually orthogonal linearly polarized lights by the polarization separator 5, and these separated linearly polarized lights are sent to the optical detector 6a,
6b, respectively. These optical detection outputs are differentially combined by a differential combiner 7 to obtain a received output signal at an output terminal 8.

At this time, the condition that the received output signal remains constant regardless of the polarization state of the input signal light is the distribution ratio of the signal light powers P and 8 in the polarization splitter 5 and the local light power PL to the optical detector 6a side. As, Az,, if the distribution ratios to the photodetector 6b side are Bs and BL, respectively, As = A
x, rBS=BL, and at this time the received output signal at terminal 8 becomes maximum. According to this optical receiver circuit, A
By setting 8=AL I Bs =BL, a constant output can be obtained regardless of the polarization state of the input signal light.

In order to control the received output signal so that it has a maximum value and is constant without depending on the polarization state of the input signal light, it is generally necessary to detect an error signal that indicates the magnitude and direction of fluctuation in the input signal light. It is conceivable to perform control such that the error signal reaches zero. In order to detect this error signal, it is common to forcibly modulate one of the local light sources with the input signal light and extract the modulated components. Therefore, the signal-to-noise ratio of the received output signal at the terminal 8 is degraded due to the mixing of the modulation component for detecting the error signal and the loss of the error signal detection circuit.

<Summary of the Invention> In order to eliminate these drawbacks, the present invention performs control by directly detecting the polarization state of an input signal from a detection signal, and thus performs modulation without detecting an error signal. There is no mixing of modulation components.

There is no loss in the error signal detection circuit, and high S
An object of the present invention is to provide an optical receiving circuit control device that makes it possible to obtain a reception ratio signal of N ratio.

<Embodiment> As shown in FIG. 1, a control device 9 of the present invention includes a photodetector 6
Connected to the output sides of a and 6b. This control device 9 is configured as shown in FIG. 2, for example. That is, the photodetector 5
Each output of a and 6b is a linear detector 11a.

11b, and the square root of these detection outputs is taken by square root characteristic devices 12a and 12b.7 The high frequency signal from the signal generator 13 is sent to a splitter 14 into two signals whose phases are 90 degrees different from each other. These two signals are then split into amplitude modulators 16a, respectively.

At 16b, the amplitude is modulated by each output of the square root characteristic device 12a, 12b. Amplitude modulator 16a, 1
The outputs of 6b are combined by a combiner 17. Signal generator 13
The phase of the high-frequency signal is adjusted by a phase shifter 18, and the output of the synthesizer '17 is phase-detected by a phase detector 19. This phase detection output is linearized by an inverse function linearizer 21, and the linearized output is supplied as a modulation signal to the optical modulator 3 in FIG. The state is changed, that is, the local light power P
Distribution ratio destination of L optical detectors 6a' and 6b.

BL is made to change.

Now, of the output light of the multiplexer 4 in FIG. 1, the signal light power is P8+the local light power is PL. Among the components separated by the polarization separator 5, the electric field direction on the photodetector 6a side is set to 0 degrees, the polarization state of the signal light is 082, and the polarization state of the local light is θL
Then, the distribution ratio As + Ar- on the photodetector 6a side and the distribution ratio Bs l BL on the photodetector 6b side are AB = cos2θs, A for the signal light and local light, respectively.
L=width 2θL + Bs :l:5l12θs + B
t, = sin2θL. Now, a photodetector (IA
, 5b is the detection efficiency of optical detectors 6a, 6.
The output power Pl of b, P2 is P1=A8ALP8PLD, P+=BsBLPsPt
, D-(1). This output power P 1 + P 2
The voltages E+ and E2 after being linearly detected by the linear detectors 11a and llb and then passing through the square root characteristic devices 12a and 12b are as follows, assuming that the constant term is 1, E+=W-Kl+E2-J-BsBr,- Kt-
...(2) becomes. On the other hand, the high frequency signals ca and wet generated by the signal generator 13 are converted to the signal ccos by the splitter 14.
It is split into wct and csin Wet. The demultiplexed signals coswct and slnwct are processed by amplitude modulators 15a and 16b and square root characteristic units 12a and 12.
b (D is amplitude modulated by each output E11E2. Output signal E3 + of amplitude modulator 16a, 16b
E4 becomes each when the constant term is set to 2. These two output signals E3. E4 is combined by the combiner 17 to become a signal. This signal E5 is phase-detected by a phase detector 19 using a signal from the signal oscillator 13 and a reference phase signal whose phase is adjusted by a phase shifter 18. Its detection output signal E
6 is as follows.

This signal E6//i is linearized by the inverse function linearizer 21,
The output signal E7 is as follows. Here, in a steady state (a state where the input signal light two-station light is not fluctuating), the signal light distribution ratio and the local light distribution ratio are controlled to be equal, so As=AL IBs=&, E7 is, The polarization state θ8 of the input signal is shown. Here, optical modulator 3
The half-wave voltage (voltage that rotates linearly polarized light by 90 degrees) is V
When π is assumed, when the output voltage E8 of the amplifier 22 is As = 1, K Eg "" O, and when As = O, KE8 = V
If the offset and gain are given so that If you want to do it like this, AL
The condition of = As + BL = Bs is satisfied.

Now, suppose that the polarization state θB of the input signal light changes and the distribution ratio A8 of the input signal light changes from ASI to Ag3.
Distribution ratio AL of local light determined by E7 obtained from equation (6)
is always between Ast and Ag3, and never exceeds Ag3. This indicates that the distribution ratio AL is controlled too much beyond A8□, and the knob is controlled too much to prevent vibration. In addition, since it is possible to control As+BS by controlling AsALzBs&, even if the distribution ratio AL+BL of the local light varies due to disturbance in the control system, the control can be performed against the variation. will be carried out.

Note that when the input level of the phase detector 19 fluctuates, the detected output fluctuates 3. Therefore, for example, as shown by the dotted line, the output of the synthesizer 17 is branched and supplied to the level fluctuation detector 23 to detect the difference from the reference level. Detects the detection output and sends a signal □
The level controller 24 inserted between the signal generator 13 and the duplexer 14 is controlled to control the level of the high frequency signal input to the duplexer 14, so that the output of the synthesizer 17 reaches the reference level. All you have to do is make it happen. In this way, even if the level of the input signal light fluctuates, it will not be affected by this, and the received output signal can always be obtained at the maximum value regardless of the polarization state of the input signal light.

FIG. 3 shows a main part of another embodiment of the present invention, in which the inverse linearization amount 2 in FIG. 2 is omitted.

In this configuration, the output signal of the phase detector 19 is not linearized,
Since the sine function is applied to the optical modulator 3 as it is, an error occurs in the set local light distribution ratio AL + BL. FIG. 4 shows the characteristics obtained by calculating the setting error due to the configuration shown in FIG. The horizontal axis is time, the vertical axis in FIG. 2A is the error in the output power of the differential combiner 7 in FIG. 2, the vertical axis in FIG. Curve 26 is equation (8)
The calculated distribution ratio AL of the local light, curve 27 is the error in the output power, and points 28 and 29 are the error in the distribution ratio AL and output power when the distribution ratio AL of the local light fluctuates due to disturbance (noise) in the control system. This shows that As can be understood from FIG. 4, even if the output signal of the phase detector 19 is not linearized, in a steady state, that is, in a state where the polarization θS of the input signal light does not vary, the steady error in the output power is It can be stabilized within 0.02 dB, and this error is negligible, and stabilization is achieved in response to disturbances in the control system.

<Effects> Compared to commonly thought error detection methods, the control device according to the present invention directly detects and controls the magnitude and direction of fluctuations in the polarization state of the input signal light, so the received output signal has no effect on the received output signal. Fluctuations caused by error signal detection are not added. Furthermore, when conventional control techniques are applied, problems such as oscillation and overshoot occur depending on the characteristics of the control system, but these problems do not occur with the control device of the present invention.

Since this control device does not perform error detection, there is no proportional, integral, or differential operation for the amount of erroneous deposition, so the response characteristics are determined by the band of the control system and high-speed response is possible.

Further, the polarization state of the input signal light is not detected by directly branching the input signal light, and no loss is caused to the input signal light.

By adding the control device according to the present invention to the optical receiving circuit shown in FIG. 1, a maximum and constant detection output can be obtained regardless of the polarization state of the signal light, and the configuration of the receiving circuit can be simplified.
This has advantages such as preventing deterioration of the N ratio.

The optical receiving circuit is not limited to the basic configuration shown in FIG. 1, but the present invention can be applied to various types of optical receiving circuits as shown in Japanese Patent Application No. 58-155089.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the configuration of an optical receiving circuit using the optical receiving circuit control device of the invention, FIG. 2 is a block diagram showing an embodiment of the invention, and FIG. 3 is a block diagram showing another embodiment of the invention. FIG. 4 is a block diagram showing the main parts, and FIG. 4 is a characteristic diagram showing an example of the results of calculating the characteristics of the present invention according to the embodiment of FIG. 1: Optical transmission line, 2: Local light source, 3: Optical modulator, 4: Multiplexer, 5: Polarization splitter, 5a, 6b: Optical detector, 7
: differential synthesizer, 8: reception output terminal, 9: control device of the present invention, 11a, 11b=linear detector, 12a, 12b=
Square root characteristic device, 13: Signal generator, 14: Demultiplexer, 16
a, 16b = amplitude modulator, 17 dual combiner, 18: phase shifter, 19: phase detector, 21: inverse function linearizer, 22: amplifier. Patent Applicant Nippon Telegraph and Telephone Public Corporation Agent Takuo Kusano 1 Kuchioki 20 (To

Claims (2)

[Claims] (1) The signal light input from the optical transmission line and the local light from the local light source are combined after passing through the optical modulator. The combined light is separated into two orthogonal linearly polarized lights, and these two linearly polarized lights are divided into the first . A first linear detection circuit that detects the output signal of the first photodetector in an optical signal circuit that performs optical detection using a second photodetector and differentially combines and outputs the outputs of these photodetections. a first square root characteristic device for obtaining the square root of the linear detection output signal thereof, a second linear detector for detecting the output signal of the second photodetector, and obtaining the square root of the linear detection output signal. A second square i-characteristic generator, a signal generator that generates a high-frequency signal, and a first square i-characteristic generator that generates the high-frequency signal with a phase difference of 90 degrees from each other. a demultiplexer that demultiplexes the first signal into a second signal; a first amplitude modulator that amplitude modulates the first signal with the output signal of the first square root characteristic device; a second amplitude modulator that performs amplitude modulation with the output signal of the square root characteristic generator; a synthesizer that combines the output signals of the second amplitude modulator; a phase shifter that adjusts the phase of the signal from the signal generator; and a phase detector that detects the phase of the output signal of the synthesizer using the output of the phase shifter. 1. An optical receiver circuit control device comprising: a phase detector; and means for controlling the modulation of the optical modulator using an output signal from the phase detector to control the polarization state of the output light. (2) Claim 1, characterized in that an inverse function linearizer for linearizing the phase detection output signal of the phase detector is added between the phase detector and the amplifier. optical receiver circuit control device.