JP2910280B2 - Laser diode drive circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode driving circuit. More specifically, the present invention provides a laser including: a differential amplifier circuit that outputs a complementary modulation signal; and a switching circuit that switches a drive current supplied to a laser diode by the modulation signal output by the differential amplifier circuit. The present invention relates to a novel configuration of a diode drive circuit.

[0002]

2. Description of the Related Art In an optical transmitter used for optical communication or the like, an optical signal corresponding to data to be transmitted is generated by a laser diode driving circuit for supplying a driving current modulated according to data to be transmitted to a laser diode. doing.

FIG. 5 is a diagram showing a typical configuration of a conventional laser diode drive circuit.

As shown in FIG. 1, the laser diode driving circuit mainly includes a differential amplifier circuit A and a switching circuit B.

A differential amplifier circuit A receives external input signals IN and IN ^* at its input, and a pair of complementary outputs OU
T ^* and OUT are output. On the other hand, switching circuit B
, The transistor Q ₂ to which the laser diode LD and a transistor Q ₁ which the connected other end connected to a constant current source as a load, one end the other end is grounded is connected to a constant current source in common with the transistor Q ₁ to one end , And the anode of the laser diode LD is grounded. The control terminal of the transistor Q ₁ is the one of the output OUT ^* of the differential amplifier circuit A, the other output OUT of the differential amplifier circuit A is applied to the control terminal of the transistor Q _2.

In the semiconductor laser drive circuit configured as described above, when one of the inputs IN to the differential amplifier circuit A transitions to a low level, the output OUT ^* transitions to a high level and the output OUT transitions to a low level. I do. Therefore, the drive current flows through the laser diode LD becomes conductive transistor to Q ₁ switching circuit B. On the other hand, when one input IN to the differential amplifier circuit A transitions to a high level, the output OUT ^* goes to a low level and the output OUT goes to a high level. Thus, while the transistor Q ₂ is turned, the transistor Q ₁ is the laser diode LD becomes nonconductive quenches.

[0007]

In order to speed up the operation of the laser diode driving circuit constructed as described above, the output signals OUT and OU of the differential amplifier circuit A are required.
It is necessary to speed up the state transition of T ^* . That is, it is necessary to shorten the rise time and the fall time of the output signal of the differential amplifier circuit A.

FIG. 6 shows the signal waveform of the drive current I supplied to the laser diode LD in the laser diode drive circuit shown in FIG. 5 and the signal waveform of the optical signal P output from the laser diode LD corresponding to the waveform. FIG.

As shown in FIG. 6A, the transistor Q ₁
Drive current I at the same time as the predetermined rise time _tr
And then goes to a steady state at a high level. Further, the transistor Q ₁ is shifted to the non-conducting state, the drive current I reaches a steady state at a low level after reduction at a predetermined fall time t _f.

Here, when the change of the drive current as described above is appropriate, the signal waveform of the optical signal output P of the laser diode LD is substantially similar to the signal waveform of the drive current I.
However, if the rise time t _r of the drive current I is particularly short, as shown in FIG. 6 (b), at the rising edge of the optical signal output P, and cause disturbance of the waveform, called a relaxation oscillation at the edge of the signal waveform output light The signal becomes unstable. For this reason, it has been difficult to increase the speed of the laser diode drive circuit while maintaining the signal quality.

It is an object of the present invention to solve the above-mentioned problems of the prior art and to provide a novel laser diode drive circuit capable of improving the operation speed without causing relaxation oscillation in an output optical signal. For that purpose.

[0012]

According to the present invention, an input signal is applied to a control terminal, a first transistor forming a first current path together with a first load, and an inverted signal of the input signal is applied to a control terminal. An inverting amplifier including a second transistor that forms a second current path together with a second load; a laser diode; and a control terminal configured to control the modulation signal output from the first current path of the inverting amplifier using the laser diode as a load. And a switching circuit including a third transistor connected to one end of the third transistor and having a control terminal receiving a modulation signal output from the second current path of the inverting amplifier. In the diode drive circuit, the first and second loads are:
An active load including a first or second FET, respectively, and a gate width of the first FET is equal to the second F
A laser diode drive circuit is provided which is configured to be smaller than the gate width of the ET.

[0013]

The main feature of the laser diode drive circuit according to the present invention is that the rise characteristic and the fall characteristic of the output signal of the differential amplifier circuit for generating the modulation signal can be individually set. And

That is, in the conventional laser diode driving circuit, the rising characteristic and the falling characteristic of the modulation signal change simultaneously. Therefore, when the operation speed is increased, it is inevitable that the rising characteristic of the drive current becomes steep.

On the other hand, in the laser diode drive circuit according to the present invention, the load forming the differential amplifier circuit is set as the active load, and the load forming the active load is set to the F level.
By individually setting the gate width of the ET, it is possible to set the rising characteristic and the falling characteristic of the modulation signal output from the differential amplifier circuit to be different from each other.

The differential amplifier circuit used in the laser diode drive circuit is constituted by a pair of current paths each formed from a transistor receiving an input signal at a control terminal and a load connected to the transistor. In the circuit according to the present invention, the loads used here are active loads using FETs, and the gate widths of the FETs constituting each active load are different from each other. That is, the gate width of the active load FET inserted into the current path that outputs the modulation signal that rises when the laser diode emits light is set to be smaller than the gate width of the active load FET of the other current path.

The rising and falling characteristics of the output signal of the differential amplifier circuit described above are defined by the charging time or discharging time for the parasitic capacitance that is parasitic on the current path constituting the circuit. Therefore, as described above, at the output terminal connected to the current path including the FET having the narrow gate width, the amount of current that can flow through the FET is small, and the rise of the modulation current is slow. At this time, the other output terminal of the differential amplifier circuit outputs a signal complementary to this output.

On the other hand, when the laser diode generates a modulation signal to be extinguished, the current flows rapidly through the FET having a wide gate connected to the other output, so that the differential signal driven by the constant current source is used. In the amplifier circuit, the falling characteristic of the modulation signal on the falling side is steeper than the rising characteristic.

With such an operation, in the laser diode driving circuit according to the present invention, a modulation signal having a slow rise when the laser diode emits light and a sharp fall when the laser diode extinguishes directly drives the drive current of the laser diode. Supplied to the controlling transistor. Therefore, when the speed of the modulation signal is increased, the relaxation oscillation generated at the time of rising of the output optical signal can be effectively suppressed. In other words, even if the rising characteristic of the drive signal is limited to a range where relaxation oscillation does not occur in the output optical signal, only the falling characteristic of the signal can be speeded up, so that the signal speed as a whole can be improved. Can be.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the following disclosure is merely an example of the present invention and does not limit the technical scope of the present invention.

[0021]

FIG. 2 is a diagram showing a specific configuration example of a differential amplifier circuit used in a laser diode drive circuit according to the present invention. The characteristic of the laser diode driving circuit according to the present invention lies mainly in the configuration of the differential amplifier circuit. The basic configuration of the entire laser diode driving circuit is the same as that of the conventional laser diode driving circuit shown in FIG. It is.

As shown in the figure, this differential amplifier circuit
Formed between the ground and the voltage source, a first current path formed by the load Z ₁ and transistor Q _3, a load Z
And a second current path formed by the _second and the transistor Q _4. One ends of the loads Z ₁ and Z ₂ are commonly grounded via a diode D. or,
One end of the transistor Q ₃ and Q ₄ are connected in common to a current source transistor Q _5. Incidentally, the differential amplifier circuit, the control terminals of the transistors Q _3, Q ₄ as input, the transistor Q _3, Q ₄ and the load Z _1, output Z ₂ and the connection points respectively OUT ^*, is set to OUT.

In the differential amplifier circuit shown in FIG.
₁ and z ₂ are the FETs Q ₆ and Q ₇ and the diode groups D ₁ and D
₂ and is configured as an active load. That is, the loads Z ₁ and Z ₂ are connected to the transistors Q ₃ ,
Q ₄ and diodes inserted between the as D, connected to a diode group D _1, D ₂ comprises a plurality of diodes connected in series with each other, the terminals at both ends of the diode groups D _1, D ₂ The control terminal is connected to transistors Q ₃ and Q
It is constituted by FETs Q ₆ and Q ₇ which are short-circuited on the ₄ side. The FET Q ₆ is configured such that its gate width is smaller than the gate width of the FET Q ₇ .

FIG. 2 is a conceptual circuit diagram for explaining the operation of the differential amplifier circuit configured as described above.

FIG. 3 is a diagram showing signal waveforms of an input signal and an output signal with respect to the differential amplifier circuit shown in FIG.

As shown in FIG. 2A, each of the outputs OUT and OUT ^* of the differential amplifier circuit has a parasitic capacitance C due to wiring and the like that actually constitute the circuit. Accordingly, for the square wave input signals IN and IN ^* as shown in FIGS. 3A and 3B, the transition of the signal output by the differential amplifier circuit involves charging or discharging the parasitic capacitance C. it is necessary to rise time t _r and fall time t _f required.

[0027] Here, as described above, the gate width of the FETs Q ₆ is set smaller than the gate width of the FETs Q _7. Therefore, when the transistor Q ₃ has shifted from the conductive state to a non-conductive state, the current flowing through to charge the parasitic capacitance C is reduced in response to a narrow gate width, FIG. 3
rise time t _r of the output OUT ^* as shown in (c) becomes longer. Therefore, as shown in FIG.
^The fall time of the output OUT that outputs a signal complementary to ^* becomes longer.

On the other hand, when the transistor Q ₃ changes from the non-conductive state to the conductive state, the transistor Q ₄ simultaneously changes from the conductive state to the non-conductive state. Accordingly, the current flowing through the current path including the load z ₂ and the transistor Q ₄ is rapidly increased, the current flowing through the load z ₁ and the transistor Q current path configured by ₃ decreases rapidly. Accordingly, as shown in FIG. 3C, the output OUT ^* falls rapidly. The output OUT that outputs a signal complementary to the output OUT ^* also rises rapidly.

The modulation signal of the modulation signal as described above, is applied to the transistor Q ₁ which is directly connected to the laser diode LD of the switching circuit shown in FIG. 2 (b) (FIG. 3 (c)
2), the differential amplifier circuit shown in FIG. 2A has a longer rise time _{tr and a} longer fall time t _f.
Is output. FIG. 4 shows the signal waveform of the drive current I when such a modulation signal is applied to the switching circuit shown in FIG. 2B, and the optical signal waveform P generated by the laser diode LD by the drive current I. FIG.

As shown in FIG. 4A, the signal waveform of the drive current supplied to the laser diode LD in the switching circuit also has a rise time corresponding to the signal waveform of the drive current supplied to the switching circuit. is longer than t _r is the fall time t _f. As shown in FIG. 4B, the laser diode LD driven by such a drive current does not cause relaxation oscillation at the rising edge of the signal waveform.

[0031]

As described above, the laser diode drive circuit according to the present invention has a unique configuration, and the rise time of the modulation signal applied to the transistor directly connected to the laser diode is shorter than the fall time. Is also longer. Therefore, even if the operating speed of the circuit is increased, the occurrence of the relaxation oscillation caused by the sharp rise of the driving current is suppressed.

[Brief description of the drawings]

FIG. 1 is a diagram showing a specific configuration example of a differential amplifier circuit which is a characteristic part of a laser diode drive circuit according to the present invention.

FIG. 2 is a diagram for explaining the operation of the circuit shown in FIG.

FIG. 3 is a signal waveform diagram for explaining an operation of the differential amplifier circuit shown in FIG. 2;

FIG. 4 is a signal waveform diagram for explaining an operation of the switching circuit shown in FIG. 2;

FIG. 5 is a diagram showing a typical configuration of a conventional laser diode drive circuit.

FIG. 6 is a signal waveform diagram for explaining an operation of the circuit shown in FIG. 5;

[Explanation of symbols]

A differential amplifier circuit, B switching circuit, C parasitic capacitance, D diode, D _1, D ₂ diode group, LD laser diode, Q ₁ to Q ₅ transistors, Q _6, Q ₇ FET

Claims (1)

(57) [Claims] An input signal is applied to a control terminal to form a first current path together with a first load, and an inverted signal of the input signal is applied to a control terminal to generate a second current path together with a second load. An inverting amplifier including a second transistor, a laser diode, a third transistor having the laser diode as a load, receiving a modulation signal output from the first current path of the inverting amplifier at a control terminal, and a third transistor. A switching circuit including: three transistors; and a fourth transistor having one end commonly connected and receiving a modulation signal output from the second current path of the inverting amplifier at a control terminal. The second load is an active load including the first or second FET, respectively, and the gate width of the first FET is the second load. The laser diode driving circuit, characterized in that it is configured to be smaller than the gate width of the ET.