JP2000206474A - Optical transmission circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical transmission circuit which realizes optimum transmission characteristic according to a transmission line by stabilizing an operational point of an optical modulator and also by controlling the operational point. SOLUTION: This method varies the operational point at a stable operation time in the optical transmission circuit, constituted of a variable gain amplifier 4 amplitude modulating a low frequency signal with an inputted data signal and superimposing the low frequency signal on the data signal, an optical modulator 2 given the data signal superimposed with the low frequency signal and a bias voltage by the variable gain amplifier 4 and performing optical modulation and a bias control circuit for extracting low frequency signal components from the output beam of the optical modulator 2 and for controlling the bias voltage, based on the error information of the operational point of the optical modulator incorporated in the low frequency signal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission circuit for stabilizing an output optical modulation signal.

[0002]

2. Description of the Related Art Mach-Zehnder type optical intensity modulators (hereinafter, referred to as MZ modulators) are used in an ultrahigh-speed optical transmission system which is currently being put into practical use as an electric-optical conversion means for a transmitted data signal. In order to use this MZ modulator in a transmission system, the MZ modulator needs to operate stably with respect to temperature fluctuations and aging fluctuations. However, in a normal MZ modulator alone, the bias voltage at the operating point fluctuates with respect to temperature and time (hereinafter, referred to as “bias voltage”).
Therefore, it is necessary to stabilize the operating point by some kind of control circuit. For example, a method of modulating the amplitude of a low-frequency signal with an input data signal and performing control using the low-frequency signal component (Japanese Patent Laid-Open Publication No. 3-251815
No. "Optical transmitter, optical modulator control circuit and optical modulation method"). FIG. 9 shows a configuration example of this method. That is, as shown in FIG.
The gain of the data signal input to the variable gain amplifier 4 is controlled by the low frequency signal output from the low frequency oscillator 3, and the output of the variable gain amplifier 4 is removed by the capacitor 5 to remove the low frequency component. After that, it is input to the optical modulator 2. At this time, the input signal to the optical modulator 2 is a signal obtained by amplitude-modulating a low-frequency signal using a data signal as a carrier (see FIG. 10). In the optical modulator 2, the output light of the light source 1 is intensity-modulated by the optical modulator input signal from the capacitor 5 and output to the optical branching circuit 6. In the optical branching circuit 6, the output light of the optical modulator 2 is branched and input to the photodetector 7. The output of the photodetector 7 is input to the low-frequency amplifier 8, and a low-frequency component is extracted. The low-frequency component extracted by the low-frequency amplifier 8 is synchronously detected with the output signal of the low-frequency oscillator 3 by the phase detector 9 to extract an error signal. The error signal extracted by the phase detector 9 is input to the integrator 12. The output signal of the integrator 12 is input to a bias tee 13 having a terminating resistor 14, and the optical modulator 2
Is controlled. At this time, since a value obtained by temporally integrating the error from the operating point is output to the output of the integrator 12, a feedback loop is formed and the operating point is stabilized. In this case, the operating point of the modulator is uniquely determined, and the operating point is controlled such that the cross point of the eye pattern of the optical signal output from the modulator is always at the center of the amplitude level.

[0003]

However, when performing long-distance transmission using an optical fiber, the operating point for obtaining optimum transmission characteristics is not always an eye pattern due to the nonlinearity and dispersibility inherent in the optical fiber. Is not always at the center of the amplitude level of

In addition, when transmission is performed through an optical fiber, it is difficult to obtain optimum transmission characteristics only at a single operating point due to the influence of nonlinearity and dispersibility inherent in the optical fiber. It is necessary to switch the polarity of the chirp characteristic (phase modulation characteristic generated by intensity modulation) of the modulator accordingly.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and stabilizes an operating point of an optical modulator and enables control of the operating point. It is an object of the present invention to provide an optical transmission circuit capable of realizing the above.

Another object of the present invention is to provide an optical transmission circuit having a modulator control circuit capable of controlling an operating point by adding an offset voltage to an error signal detection output of the control circuit.

Further, according to the present invention, a polarity switching circuit is added to the error signal detection output of the control circuit, and a polarity switching circuit is added to the data signal input portion, and these are switched at the same time, so that the output optical signal is output. It is an object of the present invention to provide an optical transmission circuit capable of switching the polarity of a chirp characteristic of a modulator without affecting the polarity.

[0008]

In order to achieve the above object, an optical transmission circuit according to the present invention comprises a circuit for modulating the amplitude of a low-frequency signal with an input data signal and superimposing the low-frequency signal on the data signal; An optical modulator that performs optical modulation by applying a data signal on which a low-frequency signal is superimposed and a bias voltage by this circuit, and extracts the low-frequency signal component from output light of the optical modulator and includes the low-frequency signal component in the low-frequency signal A bias control circuit for controlling the bias voltage based on error information of a modulator operating point, wherein the optical transmitting circuit has means for varying an operating point during stable operation. It is.

The present invention also provides the above optical transmission circuit,
As means for varying the operating point at the time of stable operation, an offset addition circuit is added to the error information detecting unit to vary the operating point at the time of stable operation. According to the present invention, by adding an offset voltage to an error signal detection output of a control circuit, an optical transmission circuit having a modulator control circuit capable of controlling an operating point is enabled.

[0010] The present invention also provides the above optical transmission circuit,
To change the operating point during stable operation, add a polarity switching circuit to the error information detection unit and data signal input unit, and add a function to shift the operating point during stable operation by the half-wavelength voltage of the modulator. It is characterized by the following. In the present invention, by adding a polarity switching circuit to the error signal detection output of the control circuit and adding a polarity switching circuit to the data input unit, an optical transmission circuit capable of switching the polarity of the chirp characteristic of the modulator is provided. It will be realized.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a structural explanatory view showing an embodiment of the present invention.

That is, the data signal is supplied to the variable gain amplifier 4.
The gain of the data signal input to the variable gain amplifier 4 is controlled by the low frequency signal output from the low frequency oscillator 3, and the output of the variable gain amplifier 4 is removed by the capacitor 5 to remove the low frequency component. After that, it is input to the optical modulator 2. At this time, the input signal to the optical modulator 2 is a signal obtained by amplitude-modulating a low-frequency signal using a data signal as a carrier (see FIG. 10). In the optical modulator 2, the output light of the light source 1 is intensity-modulated by the optical modulator input signal from the capacitor 5 and output to the optical branching circuit 6. At this time, assuming that the average amplitude of the optical modulator input signal matches the half-wave voltage of the optical modulator 2, the optical modulator output signal becomes as shown in FIGS. The frequency signal component changes. 2 shows the optimum operating point, FIG. 3 (a) shows the operating point when drifting in the positive direction, and FIG. 3 (b) shows the operating point when drifting in the negative direction. In the optical branching circuit 6, the output light of the optical modulator 2 is branched and input to the photodetector 7. The output of the photodetector 7 is input to the low-frequency amplifier 8, and a low-frequency component is extracted. The low-frequency component extracted by the low-frequency amplifier 8 is synchronously detected with the output signal of the low-frequency oscillator 3 by the phase detector 9 to extract an error signal. The error signal extracted by the phase detector 9 is added to the offset signal supplied from the offset generation circuit 11 in the addition circuit 10 and input to the integrator 12. The output signal of this integrator 12 is
And the bias tee 13 controls the bias voltage of the optical modulator 2. At this time, since a value obtained by temporally integrating the error from the operating point is output to the output of the integrator 12, a feedback loop is formed and the operating point is stabilized. When the operation is stabilized, the input signal to the integrator 12 is 0, and the error signal and the offset signal at this time cancel each other. Therefore, the operating point is controlled by adjusting the offset value. Things become possible. 4A and 4B are diagrams for explaining control of an operating point by offset adjustment. FIG. 4A illustrates a case where there is no offset, FIG.
(C) shows the case of a negative offset voltage.

As described above, in the optical transmission circuit of this embodiment, since the operating point of the optical modulator can be stabilized and the operating point can be controlled at the same time, the optimum transmission characteristics according to the transmission path can be obtained. Realization becomes possible.

FIG. 5 is a structural explanatory view showing another embodiment of the present invention.

That is, the data signal is supplied to the polarity switching circuit 16.
The polarity of the data signal input to the polarity switching circuit 16 is inverted or non-inverted by the polarity switching signal, and then input to the variable gain amplifier 4. The data signal input to the variable gain amplifier 4 has its gain controlled and amplified by the low frequency signal output from the low frequency oscillator 3, and the output of the variable gain amplifier 4 is removed after the low frequency component is removed by the capacitor 5. The signal is input to the modulator 2. At this time, the input signal to the optical modulator 2 is a signal obtained by amplitude-modulating a low-frequency signal using a data signal as a carrier (see FIG. 10). In the optical modulator 2, the output light of the light source 1 is intensity-modulated by the optical modulator input signal from the capacitor 5 and output to the optical branching circuit 6. At this time, assuming that the average amplitude of the optical modulator input signal is equal to the half-wave voltage of the optical modulator 2, the optical modulator output signal becomes as shown in FIGS. The frequency signal component changes. 6 shows the optimum operating point, FIG. 7 (a) shows the operating point when drifting in the positive direction, and FIG. 7 (b) shows the operating point when drifting in the negative direction. In the optical branching circuit 6, the output light of the optical modulator 2 is branched and input to the photodetector 7. The output of the photodetector 7 is input to the low-frequency amplifier 8, and a low-frequency component is extracted. The low-frequency component extracted by the low-frequency amplifier 8 is synchronously detected with the output signal of the low-frequency oscillator 3 by the phase detector 9 to extract an error signal.
The error signal extracted by the phase detector 9 is supplied to a polarity switching circuit 15.
After the polarity is inverted or non-inverted by the polarity switching signal, the signal is input to the integrator 12. The output signal of the integrator 12 is input to a bias tee 13 having a terminating resistor 14, and the bias tee 13 controls a bias voltage of the optical modulator 2. At this time, since a value obtained by temporally integrating the error from the operating point is output to the output of the integrator 12, a feedback loop is formed and the operating point is stabilized. As shown in FIG. 8, the polarity of the error signal is inverted at the time of the polarity inversion, so that the operating point is shifted by a half-wavelength voltage from the non-inversion time, so that the polarity of the optical signal is inverted.
Since the data signal is also inverted at the same time, the logical relationship between the output optical signal and the input data signal is constant regardless of the polarity switching of the chirp characteristic.

As described above, in the optical transmission circuit of this embodiment, the operating point of the optical modulator can be stabilized, and at the same time, the polarity of the chirp characteristic of the modulator can be switched. Transmission characteristics can be realized.

[0018]

As described above, according to the present invention, the operating point of the optical modulator is stabilized, and at the same time, the operating point can be controlled. An optical transmission circuit that can be realized can be provided.

[Brief description of the drawings]

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

FIG. 2 is an explanatory diagram showing an example of an optical modulator output signal according to an embodiment of the present invention.

FIG. 3 is an explanatory diagram illustrating another example of an optical modulator output signal according to an embodiment of the present invention.

FIG. 4 is an explanatory diagram showing control of an operating point by offset adjustment according to an embodiment of the present invention.

FIG. 5 is a configuration explanatory view showing another embodiment of the present invention.

FIG. 6 is an explanatory diagram showing an example of an optical modulator output signal according to another embodiment of the present invention.

FIG. 7 is an explanatory diagram showing another example of an optical modulator output signal according to another embodiment of the present invention.

FIG. 8 is an explanatory diagram showing polarity switching of chirp characteristics according to another embodiment of the present invention.

FIG. 9 is a configuration explanatory diagram showing a conventional optical transmission circuit.

FIG. 10 is an explanatory diagram showing an optical modulator input signal of a conventional optical transmission circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Optical modulator 3 Low frequency oscillator 4 Variable gain amplifier 5 Capacitor 6 Optical branch circuit 7 Photodetector 8 Low frequency amplifier 9 Phase detector 10 Addition circuit 11 Offset generation circuit 12 Integrator 13 Bias tee 14 Terminating resistor 15 Polarity Switching circuit 16 Polarity switching circuit

Claims (3)

[Claims] 1. A circuit for modulating the amplitude of a low-frequency signal with an input data signal and superimposing the low-frequency signal on the data signal, and a data signal on which the low-frequency signal is superimposed and a bias voltage are provided by the circuit. An optical modulator that performs optical modulation;
A bias control circuit that extracts the low-frequency signal component from the output light of the optical modulator and controls the bias voltage based on error information of a modulator operating point included in the low-frequency signal. An optical transmission circuit, comprising: means for varying an operating point during a stable operation in the transmission circuit. 2. The light according to claim 1, wherein an offset addition circuit is added to the error information detecting section as means for changing the operating point during the stable operation, and the operating point during the stable operation is made variable. Transmission circuit. 3. As a means for varying the operating point during stable operation, a polarity switching circuit is added to the error information detecting section and the data signal input section to shift the operating point during stable operation by a half-wavelength voltage of the modulator. The optical transmission circuit according to claim 1, wherein a function is added.