JPH04245688A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To make it possible to stabilize the average output of an optical signal output and the amplitude of the signal and further reduce the generation of waveform distortion in a semiconductor laser drive circuit. CONSTITUTION:This device comprises a photodiode PD which receives an optical signal output from a semiconductor laser LD and outputs a monitor signal, a peak value detection circuit 4 which receives the peak of the monitor signal, a bottom detection circuit 5 which detects a lower signal level of the peak value, and a feedback control function which carries out feedback control of an average level of modulation current to be applied to the semiconductor laser and bias current and the pulse width of the modulated current pulse.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive circuit. More specifically, the present invention relates to a novel structure of a semiconductor laser drive circuit used in optical communication systems and the like, which has a function of automatically maintaining the quality of its output optical signal.

0002

[Background Art] In an optical transmitter used for optical communications, etc., an optical signal corresponding to the data to be transmitted is generated by a semiconductor laser drive circuit that supplies a semiconductor laser with a modulated current that is modulated in accordance with the data to be transmitted. are doing. On the receiving side, this modulated optical signal is demodulated and the data is reproduced.
Here, the extinction ratio of the optical signal is a characteristic that deeply affects the sensitivity on the receiving side. Extinction ratio means the ratio of two types of light intensities corresponding to each binary signal.In order to effectively detect an optical signal consisting of two types of intensities on the receiving side, the light output on the transmitting side must be It is extremely important that the extinction ratio of the signal is stable.

[0003] Therefore, a semiconductor laser drive circuit that can always output an optical signal with a stable extinction ratio by compensating for the temperature characteristics of the semiconductor laser has already been proposed in Japanese Patent Laid-Open No. 2-58387.

The semiconductor laser drive circuit proposed here uses a photodiode to monitor the optical signal output from the semiconductor laser, and calculates the average output and extinction of the output optical signal based on the monitor signal output from the photodiode. The bias current and modulation current supplied to the semiconductor laser are controlled so that the ratio is stable.

[0005]

[Problem to be Solved by the Invention] However, in the conventional semiconductor laser drive circuit as described above, when the temperature of the semiconductor laser increases, the cross point of the waveform of the output optical signal shifts, causing waveform distortion. There is a problem.

FIG. 4 is a diagram showing a change in the output signal waveform of an optical signal output from the semiconductor laser driving circuit as described above between room temperature and high temperature.

As shown in the figure, the cross point located near the center of the signal amplitude at room temperature shifts as the temperature rises.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a novel semiconductor laser drive circuit that can solve the problems of the prior art and effectively correct waveform distortion caused by the temperature characteristics of a semiconductor laser. It is said that

[0009]

[Means for Solving the Problems] According to the present invention, a semiconductor laser and a drive circuit that supplies a current modulated in accordance with an input signal to the semiconductor laser are provided, and the drive circuit is bypassed and the semiconductor laser is A semiconductor laser drive circuit configured to supply a bias current to the laser includes a light receiving element that receives a part of the output optical signal of the semiconductor laser and outputs a monitor signal, and a light receiving element on the side where the signal level of the monitor signal is high. a peak value detection circuit for detecting a peak; a modulation current control circuit for feedback controlling a modulation current supplied to the semiconductor laser based on the output of the peak value detection circuit; a bottom value detection circuit for detecting a bottom value; a bias current control circuit for feedback controlling a bias current supplied to the semiconductor laser based on the output of the bottom value detection circuit; and a bias current control circuit for feedback controlling the bias current supplied to the semiconductor laser based on the output of the bottom value detection circuit; a pulse width adjustment circuit that controls the modulation current control circuit so as to perform feedback control of the pulse width of the modulation current based on the feedback control circuit; Provided is a semiconductor laser drive circuit characterized in that the semiconductor laser drive circuit is also stabilized.

0010

[Operation] The semiconductor laser drive circuit according to the present invention is configured to control not only the DC component level and bias current of the modulation current supplied to the semiconductor laser, but also the pulse width of the modulation current pulse. This is the main feature.

That is, in the conventional semiconductor laser drive circuit,
The quality of the output optical signal was improved by controlling the amount of current supplied to the semiconductor laser. However, such control cannot compensate for the deterioration of the signal waveform of the output optical signal.

On the other hand, in the semiconductor laser drive circuit according to the present invention, the pulse width of the modulated current modulated by the input signal is feedback-controlled based on the monitor signal from the output optical signal. This effectively compensates for pulse width distortion.

A semiconductor laser drive circuit according to the present invention includes a photodiode that receives an output optical signal from the semiconductor laser, and further includes a peak value detection circuit that detects the highest value of a monitor current output from the photodiode, and a monitor and a bottom value detection circuit that detects the lowest value of current.

The pulse width of the modulated current modulated by the input signal is feedback-controlled by referring to the output of the bottom value detection circuit. That is, when the minimum value of the output optical signal from the semiconductor laser decreases, the pulse width of the modulation current supplied to the semiconductor laser is increased. Through such an operation, the pulse width of the output optical signal is corrected, and the cross point of the optical signal does not shift.

Furthermore, as in the conventional semiconductor laser drive circuit, the average optical output of the output optical signal is stabilized by feedback controlling the modulation current supplied to the semiconductor laser with reference to the output of the peak value detection circuit. can be made into Furthermore, by referring to the output of the bottom value detection circuit and performing feedback control on the bias current supplied to the semiconductor laser, it is possible to stabilize the extinction ratio of the output optical signal.

[0016] The present invention will now be described in more detail with reference to Examples, but the following disclosure is merely an example of the present invention and is not intended to limit the technical scope of the present invention in any way.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram showing the basic configuration of a semiconductor laser driving circuit according to the present invention.

As shown in the figure, this semiconductor laser drive circuit includes a pulse width control circuit 1 that adjusts the modulation current pulse width of the semiconductor laser LD, and a bias current control circuit 2 that supplies a bias current to the semiconductor laser LD. , a photodiode PD that monitors the optical signal output from the semiconductor laser LD, a peak value detection circuit 4 that extracts the peak from the monitor signal output from the photodiode PD, and a peak on the lower side of the signal level from the monitor signal. It mainly consists of a bottom value detection circuit 5 that extracts the bottom value. Further, the modulation current control circuit 3 receives an input signal via the pulse width adjustment circuit 1.

In the semiconductor laser drive circuit configured as described above, the average optical output of the output optical signal is controlled by feedback-controlling the modulation current supplied to the semiconductor laser with reference to the output of the peak value detection circuit. It can be stabilized. Furthermore, by referring to the output of the bottom value detection circuit and performing feedback control on the bias current supplied to the semiconductor laser, it is possible to stabilize the extinction ratio of the output optical signal.

Furthermore, this semiconductor laser drive circuit is configured to feedback-control the pulse width of the modulation current modulated by the input signal with reference to the output of the bottom value detection circuit. That is, when the minimum value of the output optical signal from the semiconductor laser decreases, the pulse width control circuit 1 increases the pulse width of the modulation current supplied to the semiconductor laser, and conversely increases the output optical signal from the semiconductor laser. The device is configured to change the pulse width of the modulation current supplied to the semiconductor laser when the minimum value increases.

FIG. 2 is a circuit diagram showing a more specific example of the configuration of the semiconductor laser drive circuit shown in FIG. 1.

As shown in the figure, this semiconductor laser drive circuit includes a drive circuit a that supplies a modulation current to the semiconductor laser LD, and a pulse width control circuit that generates an input signal to the drive circuit a and controls its pulse width. A circuit b, a monitor circuit c that receives the output optical signal of the semiconductor laser LD and outputs a monitor signal, a bottom hold circuit d and a peak hold circuit e that receive the monitor signal, and is driven by receiving the output of the bottom hold circuit d. It mainly consists of a bias current control circuit f that supplies a bias current to the circuit a, and a drive current control circuit g that receives the output of the peak hold circuit e and supplies a modulation current to the drive circuit a. .

In the drive circuit a, a pair of transistors Q1 and Q2 whose emitters are connected in common, and a semiconductor laser L connected as a load to the transistor Q1.
It is composed of D.

The pulse width adjustment circuit b also includes an operational amplifier IC1 that generates mutually complementary input signals from the DATA signal input to its positive phase input. A pair of complementary signals output by operational amplifier IC1 are applied as input signals to the bases of transistors Q1 and Q2. Further, a reference voltage defined by the transistor Q3 is applied to the negative phase input of the operational amplifier IC1. Transistor Q3 is connected in series with resistor R1 and resistor R2 between the power supply voltage and ground,
The operational amplifier IC1 is configured to apply a voltage generated between the resistor R1 and the transistor Q3 to the negative phase input of the operational amplifier IC1 in accordance with the voltage applied to its base. Furthermore, a resistor R3 is connected to the base of the transistor Q3.
The output of a non-inverting amplifier constituted by a resistor R4 and an operational amplifier IC2 is applied, and the output of a bottom hold circuit d, which will be described later, is connected to the input of this non-inverting amplifier. A predetermined reference voltage ref. 1 is applied.

The monitor circuit c includes a photodiode PD that receives an optical signal output from the semiconductor laser LD, and a resistor R.
The operational amplifier IC3 operates as a current-voltage conversion amplifier with its input and output short-circuited by a circuit 5, and its output is inputted to a bottom hold circuit d and a peak hold circuit e, respectively, which will be described later.

The bottom hold circuit d is an operational amplifier IC that receives the monitor signal output from the monitor circuit c at its positive phase input.
4, a diode D1 whose anode is connected to the output of the operational amplifier IC4, and a capacitor C1 that connects the cathode of the diode D1 to the ground, and the cathode of the diode D1 and the negative phase input of the operational amplifier IC4 are short-circuited. ing. The bottom hold circuit d receives the output of the previous stage and outputs a signal having a value corresponding to the peak of the monitor signal output from the photodiode PD.

The second peak hold circuit e includes an operational amplifier IC5 which receives the monitor signal output from the monitor circuit c at its positive phase input, a diode D2 whose cathode is connected to the output of the operational amplifier IC5, and an anode of the diode D2 and ground. It is equipped with a capacitor C2 that connects the anode of the diode D2 and the operational amplifier IC5.
It is short-circuited with the negative phase input of . This peak hold circuit e outputs a signal having a value corresponding to the peak on the lower side of the signal level of the monitor signal output from the photodiode PD.

The bias current control circuit f includes an operational amplifier IC6 that receives the output of the bottom hold circuit d at its positive phase input.
and a transistor Q4 whose base receives the output of the operational amplifier IC6. Transistor Q4
Its emitter is connected to ground via a resistor R6, and its collector is connected to the connection point between the semiconductor laser LD and the transistor Q1 in the drive circuit a. A predetermined reference voltage ref.
2 is applied.

The drive current control circuit g includes an operational amplifier IC7 that receives the output of the peak hold circuit e as a positive phase input;
The transistor Q5 receives the output of the operational amplifier IC7 at its base. The emitter of transistor Q5 is connected to ground via resistor R7,
The collector is the transistor Q1 in the drive circuit a,
Commonly connected to the emitter of Q2. A predetermined reference voltage ref. 3 is applied.

The semiconductor laser drive circuit configured as described above operates as follows.

[0031] When the DATA signal is input to the input terminal,
A pair of complementary input signals corresponding to the DATA signal are generated by operational amplifier IC1 and applied to the bases of transistors Q1 and Q2, respectively. Here, when the input signal applied to the transistor Q1 is at "H" level, the transistor Q1 becomes conductive and the transistor Q2 becomes non-conductive. Therefore, the semiconductor laser LD
A driving current flows through the semiconductor laser LD, and the semiconductor laser LD outputs an optical signal. Conversely, when the input signal applied to the transistor Q1 is at the "L" level, the transistor Q1 becomes non-conductive and the drive current flowing to the semiconductor laser LD is cut off. Therefore, the semiconductor laser LD is extinguished.

The photodiode PD of the monitor circuit c is
Receives a part of the optical signal output by the semiconductor laser LD,
A current corresponding to the intensity of the received optical signal is output as a monitor signal. This monitor signal is amplified by a current-voltage conversion amplifier constituted by a resistor R5 and an operational amplifier IC3, and then input to a bottom hold circuit d and a peak hold circuit e.

The bottom hold circuit d extracts the bottom value of the input signal. However, as described above, the bottom value output from the bottom hold circuit d is the peak value on the side where the signal level of the monitor signal output from the photodiode PD is low. The bottom value output from the bottom hold circuit d is input to the operational amplifier IC2 of the input signal control circuit b.
and is input to the operational amplifier IC6 of the bias current control circuit f.

The operational amplifier IC2, whose anti-phase input and output are short-circuited by the resistor R3, generates a voltage proportional to the input peak value (bottom value). Since this voltage is applied to the base of the transistor Q3, a voltage that is inversely proportional to the change in the bottom value of the monitor signal output from the photodiode PD is applied to the negative phase input of the operational amplifier IC1.

Here, the operational amplifier IC1 generates an input signal to the drive circuit a from the DATA signal input to the positive phase input, using its negative phase input as a reference value. Therefore, when the bottom value of the output optical signal of the semiconductor laser LD decreases, the voltage applied to the negative phase input of the operational amplifier IC1 decreases, and the pulse width of the input signal output from the operational amplifier IC1 increases. Conversely, when the bottom value of the output optical signal of the semiconductor laser LD increases, the operational amplifier I
The voltage applied to the negative phase input of C1 increases, and the pulse width of the input signal output from operational amplifier IC1 becomes smaller.

Also, the operational amplifier IC6 of the bias current control circuit f which also receives the output of the bottom hold circuit d
is the bottom value input to the positive phase input and the predetermined reference voltage r
ef. The voltage corresponding to the difference between
applied to the base of.

Therefore, when the bottom value of the output optical signal decreases, the bias current increases. On the other hand, when the bottom value of the output optical signal increases, the bias current decreases. The above operation stabilizes the extinction ratio of the output optical signal.

On the other hand, the operational amplifier IC7 of the drive current control circuit g receiving the output of the peak hold circuit e receives the peak value inputted to the positive phase input and a predetermined reference voltage ref. 3
A voltage corresponding to the difference between Q5 and Q5 is applied to the base of transistor Q5.

That is, when the peak value of the output optical signal decreases, the modulation current increases. On the other hand, when the peak value of the output optical signal increases, the modulation current decreases. The above operation stabilizes the output optical power of the output optical signal.

As explained above, in the semiconductor laser drive circuit according to the present invention, due to its unique configuration, all of the output optical power, extinction ratio, and pulse width of the output optical signal can be maintained regardless of changes in temperature, etc. stable.

FIG. 3 is a diagram showing another specific example of the configuration of the semiconductor laser drive circuit according to the present invention. In this figure, the main difference from the semiconductor laser drive circuit shown in FIG. 2 is the configuration of the pulse width adjustment circuit, and other common components are given common reference numerals and explained. omitted.

As shown in the figure, this semiconductor laser drive circuit has the same configuration as the circuit shown in FIG. 1 except for the configuration of the pulse width control circuit b2. Therefore, a part of the output optical signal is monitored by the monitor circuit c, and the bottom hold circuit d and the peak hold circuit e are output from the monitor signal.
The bias current control circuit f and the drive current control circuit g feedback-control the bias current and drive current of the drive circuit a based on the bottom value and peak value detected by the circuit shown in FIG. Perform the exact same action.

On the other hand, the pulse width adjustment circuit b2 has a unique configuration and includes three inverters IC11 and IC12.
, a circuit that directly handles input signal pulses constituted by IC13, and the reverse phase input and output are short-circuited by resistor R3, and the output of bottom value detection circuit d is connected to inverter IC13.
It mainly consists of an operational amplifier IC2 for transmitting data to However, the operational amplifier IC2 of the circuit shown in FIG.
Unlike, here, the output of the bottom detection circuit d is connected to the anti-phase input of the operational amplifier IC2, and the operational amplifier IC2 operates as an inverting amplifier.

The inverter IC11 receives the input signal SD and its inverted signal at its non-inverting input and inverting input, and has an inverting output and a non-inverting output, respectively. The non-inverting output of inverter IC11 is connected to the non-inverting input of inverter IC12, and the inverting output of inverter IC11 is connected to the non-inverting input of inverter IC13. The inverter IC13 also receives a reference voltage ref. 0 is applied to its inverting input, and its inverting output is connected to the non-inverting input of inverter IC12.

Therefore, the non-inverting output of inverter IC11 and the inverting output of inverter IC13 are connected to the non-inverting input of inverter IC12 via the wired OR formed at node N. Inverter IC
A reference voltage also supplied from the reference voltage generating circuit G is applied to the inverting input of No. 12, and its complementary outputs are connected to the bases of transistors Q1 and Q2 of the drive circuit a.

Here, the circuit b2 inputs the input data signal SD and its inverted signal to the inverter IC11. The inverted output of the inverter IC11 is input to the non-inverted input of the inverter IC13, and the logical sum (worded OR) of the inverted output of the inverter IC13 and the non-inverted output of the inverter IC11 is inputted to the non-inverted input of the inverter IC12. The inverter IC13 converts the inverted signal of the input signal SD to the reference voltage ref. 0, the inverted signal is added to the input signal SD by wired OR, and is input to the inverter IC12. Inverter I is connected to the inverting input of inverter IC12.
The same reference voltage as the inverting input of C13 is applied, and the inverter IC12 shapes the waveform of the input signal SD using the reference voltage.

Therefore, the output signals from the pair of mutually complementary outputs of the inverter IC12 correspond to the input signal SD, but the waveform of this output signal is different from the pulse width of the input signal. The signal has a pulse width on which the delay in is superimposed.

As described with respect to the circuit shown in FIG. 1, the bottom hold circuit d outputs a voltage that is inversely proportional to fluctuations in the monitor current output from the photodiode PD. This voltage is further inverted by the operational amplifier IC2 and applied to the non-inverting input of the inverter IC13. Therefore, when the output of the bottom hold circuit d increases, that is, when the lowest value of the output optical signal of the semiconductor laser LD decreases, the current flowing through the transistor Q3 decreases, the delay time due to the inverter IC12 increases, and the drive circuit a
The pulse width of the input signal supplied to the input signal becomes wider. Conversely, when the lowest value of the output optical signal of the semiconductor laser LD increases, the output of the bottom hold circuit d, which is the bottom value, decreases and the current flowing through the transistor Q3 increases. Therefore, the delay time caused by the inverter IC12 is reduced, and the pulse width of the input signal supplied to the drive circuit a is narrowed.

By the above-described operation, the semiconductor laser L
The pulse width distortion of the optical signal output by D can be compensated for.

[0050]

As explained above, the semiconductor laser drive circuit according to the present invention can suppress characteristic fluctuations of an output optical signal due to temperature fluctuations, etc., by adjusting not only the output optical power and extinction ratio of the output optical signal but also the signal waveform. It has a function that automatically stabilizes up to . Therefore, it can be used particularly advantageously in applications where a wide operating temperature range is required in the field of optical communications.

Further, the above-described functions of the semiconductor laser drive circuit according to the present invention can be realized with a simple configuration as specifically explained in the embodiments, and the cost of the optical communication device can be reduced for implementation. It is not necessary to raise the level of

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the basic configuration of a semiconductor laser drive circuit according to the present invention.

FIG. 2 is a diagram showing a more specific configuration example of the circuit shown in FIG. 1;

FIG. 3 is a diagram showing another specific configuration example of the circuit shown in FIG. 1;

FIG. 4 is a signal waveform diagram for explaining fluctuations in an output optical signal due to temperature changes in a conventional semiconductor laser drive circuit.

[Explanation of symbols]

Claims (1)

[Claims] 1. A semiconductor laser, and a drive circuit that supplies a current modulated in accordance with an input signal to the semiconductor laser, the drive circuit being bypassed to supply a bias current to the semiconductor laser. The semiconductor laser drive circuit configured includes a light receiving element that receives a part of the output optical signal of the semiconductor laser and outputs a monitor signal, and a peak value detection circuit that detects a peak on the higher side of the signal level of the monitor signal. , a modulation current control circuit that feedback-controls the modulation current supplied to the semiconductor laser based on the output of the peak value detection circuit; a bottom value detection circuit that detects a peak on the side where the signal level of the monitor signal is low; a bias current control circuit that feedback-controls the bias current supplied to the semiconductor laser based on the output of the bottom value detection circuit; and a bias current control circuit that receives the input signal and controls the pulse width of the modulation current based on the output of the bottom value detection circuit. a pulse width adjustment circuit that controls the modulation current control circuit so as to perform feedback control; an optical output power of an optical signal output from the semiconductor laser;
A semiconductor laser drive circuit characterized in that both the extinction ratio and pulse width are stabilized.