JPH04109686A - Optical output compensation circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical output compensation circuit that stabilizes the output of a light source such as a semiconductor laser used in an optical transmitter.

[Conventional technology]

Conventionally, as this type of optical output compensation circuit, for example, there is a circuit shown in FIG. 4 used in an optical transmitter of a digital optical communication system. This circuit is a circuit that performs feedback stabilization on the output power of a laser diode (LD), not shown. The rearview mirror light from the LD is received by the p1n photodiode 1, and the light emission output of the LD is monitored. A DC component of the optical output emitted from the LD is detected by a resistor 2 and a capacitor 3, and is applied to a non-inverting input of a differential amplifier circuit 4. The operating point of this amplifier circuit 4 is set by a resistor 5. On the other hand, an electrical input signal (digital signal) for driving the LD is applied to an inverting input of the differential amplifier circuit 4 via a resistor 6. Therefore, the optical output level of the LD is compared with the electrical input signal pattern by the differential amplifier circuit 4. Based on this comparison result, the transistor 7 that determines the bias current of the LD is driven.

According to this feedback stabilization circuit, the average level of the light emission output of the LD is stabilized. Moreover, while the digital input signal "0" continues for a long time or while there is no input signal on the channel,
This prevents the feedback circuit from erroneously increasing the bias current of the LD.

[Problem to be solved by the invention]

However, the conventional optical output compensation circuit described above is configured to keep only the average value of the DC component of the optical output of the LD constant. Therefore, the alternating current component of the optical output of the LD, that is, the signal component to be transmitted, fluctuates.

As a result, the extinction ratio fluctuated, and the reception sensitivity of the optical receiver that receives the optical signal emitted from the LD was unstable.

[Means to solve the problem]

The present invention has been made to solve these problems, and includes a second load that detects an alternating current component of optical output using a photocurrent generated in a light-receiving element; The second negative feedback circuit stabilizes the alternating current component of the light output emitted from the light source based on the alternating current component of the force.

[Effect]

The alternating current component of the optical output of the light source is detected by the second load of the light receiving element, and the variation of the optical signal emitted from the light source is detected by the second load.
is canceled by the negative feedback circuit.

〔Example〕

FIG. 1 shows the circuit configuration of a digital optical transmitter to which an optical output compensation circuit according to an embodiment of the present invention is applied.

The optical transmitter includes a light source 11 that generates an optical signal to be transmitted in an optical communication system, and an optical output compensation circuit 12 that stabilizes the optical output emitted from the light source 11. The light source 11 is equipped with an LDlB.
The current flowing through is switched by transistor Q1 whose load is LDlB and transistor Q2 whose load is resistor 14. A digital electric signal is applied to the base of each transistor Ql, Q2 via an input buffer 15, and switching of the current flowing to LDlB is performed based on this digital signal. Also, each transistor Q
The current flowing through I and Q2 is made constant by the transistor Q3 and the emitter resistor 16. By switching the applied current, the light emission output of LDlB is intermittent, and an optical signal is generated.

The optical output compensation circuit 12 includes a pin photodiode (PD) 16 built into the LD module that receives light emitted from the rear of the LDlB.

In the PD 16, a resistor 17 that detects the DC component of the optical output emitted from the I and D13 is used as a first load, and a resistor 18 that detects the AC component of the optical output emitted from the LDlB is used as a second load. The resistance value R1 of the resistor 17 constituting the first load is set larger than the resistance value R2 of the resistor 18 constituting the second load (R1>>R2). Therefore, the time constant due to the resistor 17 and the parasitic capacitance of the PD 16 is larger than the time constant due to the resistor 18 and the parasitic capacitance of the PD 16. Therefore, the resistor 17 detects the DC component of the optical output, and the resistor 18 detects the AC component of the optical output. Note that the capacitor 19 is for grounding the AC component of the previous output.

In this way, a voltage proportional to the alternating current component of the optical signal emitted from LDlB appears between the terminals of resistor 18. Therefore, when digital modulation is applied to LDlB, the optical output current shown in FIG. 2(a), for example, is generated. The horizontal axis in FIG. 2A represents time t, and the vertical axis represents current i, and the current zero level of the optical output current is slightly higher than the current zero level shown on the vertical axis. This optical output current is converted into a voltage by the resistor 18, and becomes a voltage signal with an amplitude v1 shown in FIG. 2(b). In the same figure (b), the horizontal axis is time t2, and the vertical axis is voltage V.
The zero voltage level of the voltage signal is slightly higher than the zero voltage level shown on the vertical axis. Further, the rise and fall of the voltage signal waveform are slow due to the parasitic capacitance of the PD 16.

The voltage signal appearing across the terminals of resistor 18 is applied to the base of transistor Q5 shown in FIG.
and is amplified by an amplifier circuit constituted by transistor Q5. The amplified voltage signal has its DC component removed by a capacitor 28, and is further connected to resistors 29, 30 .
31 and capacitor 32, it is converted to an appropriate bias voltage value for transistor Q6. Here, the capacitor 32 functions to ground the alternating current component of the voltage signal.

Transistor Q6. The resistor 33 and capacitor 34 are
A peak detection circuit is configured to detect the peak value of the AC component of the optical output. That is, transistor Q6 is driven with a voltage proportional to the alternating current component of the optical output, and its freptor current is integrated by resistor 33 and capacitor 34. Therefore, the peak voltage value detected by this detection circuit is proportional to the amplitude v1 of the voltage signal shown in FIG. 2(b), and is proportional to the magnitude of the signal component of the optical output emitted from the LD 13. . Furthermore, the LD modulation current determined by the base potential of transistor Q3 is also proportional.

The second peak voltage is applied to the differential amplifier circuit 36 via the resistor 35.
is applied to the inverting input of A feedback resistor 37 and a capacitor 3 are connected between the inverting input and the output terminal of the differential amplifier circuit 36.
8 is connected, and a reference voltage Vrl for setting the operating point of the amplifier circuit is applied to the non-inverting input. Therefore, the peak voltage of the AC component of the optical output and the reference voltage Vr
The voltage difference between the voltage and the voltage is inverted and amplified and output. The inverted and amplified signal is applied to the base of transistor Q3 via three resistors. Therefore, the differential amplifier circuit 36 adjusts the base voltage of the transistor Q3, that is, the modulation current of the LD13, so as to cancel out the variation in the peak voltage. As a result, the peak voltage value output from the peak detection circuit constituted by the transistor Q6 becomes constant, and the magnitude of the alternating current component of the optical output proportional to this peak voltage value becomes constant.

Note that the transistor Q5. Q6 and differential amplifier circuit 36
The circuit constituted by the above forms a second negative feedback circuit.

On the other hand, an optical output average voltage shown in FIG. 2(c) appears between the terminals of the resistor 17 due to the optical output current generated in the PD 16. In the figure, the horizontal axis represents time t, and the vertical axis represents voltage V. Since the resistance value R1 of the resistor 17 is sufficiently larger than the resistance value R2 of the resistor 18, the optical output current shown in FIG. Ru.

The amplitude v2 of this optical power average voltage is proportional to the average value of the DC component of the optical output emitted from the LD 13, and the optical output average voltage is applied to the inverting input of the differential amplifier circuit 20 via the resistor 21.

A feedback resistor 22 and a capacitor 23 are connected between the inverting input and the output terminal of the differential amplifier circuit 20, and a reference resistor 22 and a capacitor 23 are connected to the non-inverting input of the differential amplifier circuit 20 to set the operating point of the amplifier circuit. A voltage Vr2 is applied. Therefore, the difference voltage between the optical output average voltage and the reference voltage Vr2 is inverted and amplified and output.

The inverted and amplified voltage signal is applied to the base of transistor Q4 via resistor 24. The collector of the transistor Q4 is connected to the cathode of the LD13, and the emitter is connected to the resistor 25. The transistor Q4 is driven by the inverted output voltage of the differential amplifier circuit 20, and the LD
The bias current 13 is adjusted by this transistor Q4 so as to cancel out the variation in the optical output average voltage. As a result, the average value of the DC component of the optical output of the LD 13 becomes constant.

Note that the circuit composed of the differential amplifier circuit 20 and the transistor Q4 forms a first negative feedback circuit.

The optical output waveform emitted from the LD 13 is shown in FIG. 2(d). In the figure, the horizontal axis represents time t, and the vertical axis represents light intensity P, and the optical output zero level of the optical output waveform is higher by Pl than the optical output zero level shown on the vertical axis. Furthermore, the peak value of the optical output waveform is Pz, and the magnitude of the optical signal component is Pz.
z is (P2P1), and the average light output value P2v is (P1+P2
)/2. Optical output compensation circuit 1 according to the above embodiment
2, a first load for detecting a DC component and a first
The average optical output value Pav is kept constant by the negative feedback circuit. Furthermore, the magnitude Pz of the optical signal component is made constant by the second load that detects the AC component and the second negative feedback circuit. Therefore, the six values of Pl and Pz become constant, and the extinction ratio P2/PI, which is the ratio of Pz to Pl, becomes constant.

Therefore, the reception sensitivity of the optical receiver that receives the optical signal generated by the LDlB is stabilized.

Next, using the optical transmitter according to the embodiment described above, L
The results of observing the light output emitted from D will be explained. This observation was carried out under the measurement system shown in FIG.

That is, the pseudo pulse signals A and B are applied from the signal generator 42 to the optical transmitter 41 to which this embodiment is applied.

By applying these pseudo pulse signals A and B to the bases of transistors Ql and Q2 constituting the current switching type drive circuit, the light emission output of LDlB is intermittent and an optical signal is generated. The optical signal generated by the LDlB is transmitted to the photoelectric converter 44 through the fiber 43 and converted into an electrical signal. This photoelectric converter 44 corresponds to an optical receiver in an optical communication system. The output of the photoelectric converter 44 is further input to an oscilloscope 45, and the optical signal output from the LDlB is observed.

The optical output waveform of LDlB based on this observation is similar to the waveform shown in Fig. 2(d), and the optical output zero level P of the optical output waveform is
1 and the peak value P2 did not fluctuate and were confirmed to be constant. It was also confirmed that even if the ambient temperature of the optical transmitter 41 was changed, the magnitude Pz of the signal component of the previous output emitted from the PD 13 and the average optical output level Pav were kept constant. Therefore, according to the optical transmitter according to this embodiment, the extinction ratio P 2 /P 1 is always kept constant.

〔Effect of the invention〕

As described above, according to the present invention, the alternating current component of the optical output of the light source is detected by the second load of the light receiving element, and the variation in the optical signal emitted from the light source is canceled by the second negative feedback circuit. Therefore, the optical signal component and the average optical output level output from the light source are stabilized, and the extinction ratio becomes constant. As a result, the reception sensitivity of the optical receiver that receives the optical signal emitted from the light source becomes stable.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing the configuration of an optical transmitter to which an optical output compensation circuit according to an embodiment of the present invention is applied, FIG. 2 is a signal waveform diagram in each part of the circuit shown in FIG. 1, and FIG. FIG. 1 is a block diagram showing a measurement system for observing an optical signal generated from the optical transmitter shown in FIG. 1, and FIG. 4 is a circuit diagram showing the configuration of a conventional optical output compensation circuit. 11... Light source, 12... Optical output compensation circuit, 13...
・Semiconductor laser diode, 15...input buffer,
16... p1n photodiode, 17... resistor (first load), 18... resistor (second load), Q1 to Q
6...Transistor, 20.36...Differential amplifier circuit. Representative Patent Attorney Yoshiki Hase
Salt 1) Tatsuya Example M4 generation 1 diagram 5 skin type 2 manga 1 fixed thread diagram 3

Claims (1)

[Claims] a first load of the light receiving element that detects a direct current component of the light output by a photocurrent generated in the light receiving element; a second load of the light receiving element that detects an AC component of the optical output; and a first negative that stabilizes the DC component of the optical output based on the DC component of the optical output detected by the first load. An optical output compensation circuit comprising: a feedback circuit; and a second negative feedback circuit that stabilizes an AC component of the optical output based on an AC component of the optical output detected by the second load.