JPH01204537A - Semiconductor laser driving circuit - Google Patents

Abstract

PURPOSE:To correct the waveform distortion due to the delay in the lighting time of a semiconductor laser by providing a relative level difference between the noninverting and inverting output of a 1st waveform shaping device, using a differential signal of a 2nd waveform shaping device after level adjustment as an output and using the noninverting and inverting outputs as control signals to supply to a current changeover switch. CONSTITUTION:A level adjustment means varying the cross point is provided, which provides a relative level difference between the noninverting and inverting outputs of the 1st waveform shaping device 1 receiving the differential signal, and the 2nd waveform shaping device 2 receivers the differential signal after level adjustment and supplyes the noninverting and inverting outputs to current changeover switches Q1, Q2 as the control signal. The pulse width of the output of the 2nd waveform shaping device 2 is varied by the deviation of the cross point to modulate a semiconductor laser LD. Thus, the distortion of the optical output waveform caused due to the lighting time delay of the semiconductor laser LD is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a drive circuit for a semiconductor laser used in an optical transmitter as a light source for optical communication.

[Conventional technology]

Conventionally, a circuit as shown in FIG. 5 has been used as this type of circuit. In order to increase the speed, a current changeover switch is used, and the electrical input signal SD and its inverted signal SD are input to the waveform shaping circuit 1, and the positive phase and negative phase outputs are used to control the transistors Q1. Q2 is turned on and off in a complementary manner. The semiconductor laser LD is equipped with an automatic optical output compensation circuit (Auto Power Control
(reult) 3, a bias current is constantly flowing through the semiconductor laser LD, and only when the transistor Q1 is turned on and the signal amplitude currents from the current source 4 overlap, a current that exceeds the transmission threshold current flows through the semiconductor laser LD, causing optical output. arise. The automatic optical output compensation circuit 3 has a function of adjusting the value of the bias current according to changes in ambient temperature and changes over time, and compensating for these changes to keep the optical output power of the semiconductor laser LD constant. There is.

[Problem to be solved by the invention]

In the conventional configuration described above, a method is used to modulate the semiconductor laser LD with a digital signal with a duty of 50%, but the optical output waveform (eye pattern) does not correspond to a pulse waveform with a duty of 50%. There was a problem that distortion occurred. This is because the semiconductor laser LD has an inherent light emission time delay.

[Means to solve the problem]

The semiconductor laser drive circuit of the present invention connects a second waveform shaper in cascade to a first waveform shaper that inputs a differential signal, and connects the positive-phase and negative-phase outputs of the first waveform shaper. The second waveform shaper receives the level-adjusted differential signal and sends its positive phase and negative phase outputs as control signals to the current selection switch. This is what I did.

[Effect]

The level adjustment means shifts the cross point of the normal phase output signal and the negative phase output signal downward by, for example, shifting the level of the negative phase output among the outputs from the first waveform shaper downward. As a result, the signal output through the second waveform shaper has the pulse width of the positive phase output expanded from its original value, for example, 50%. If this increase in pulse width is made to account for the light emission time delay inherent to the semiconductor laser, when the semiconductor laser is made to emit light using this positive phase output, the distortion due to the light emission time delay of the semiconductor laser will be corrected, and the original duty will be reduced. 50% light output is obtained.

〔Example〕

Hereinafter, one embodiment of the present invention will be described with reference to the attached FIGS. 1 to 4. In addition, in the description of the drawings, the same elements are given the same reference numerals, and redundant description will be omitted.

FIG. 1 is a circuit diagram of a laser drive circuit according to an embodiment of the present invention. In the figure, a second waveform shaping circuit 2 is cascade-connected to a first waveform shaping circuit 1.
The configuration is such that transistors Ql and Q2 constituting a current changeover switch are turned on and off by the positive-phase and negative-phase outputs of the waveform shaping circuit 2. These first and second
Pull-down resistors R1°R2 and R3 are connected between the waveform shaping circuits 1 and 2, but due to the action of the resistor R1 inserted in series with the negative phase output line and the resistor R3 connected to this, The level of the negative phase output output from the first waveform shaping circuit 1 is shifted lower than the level of the positive phase output. Next, this operation will be explained in detail with reference to FIG.

The first waveform shaping circuit 1 receives an input signal SD.

A pulse signal as shown in FIG. 2(a) is input as SD. After this pulse signal passes through the first waveform shaping circuit 1 and further passes through the resistor R1 (point A).

In B), as a result of shifting down the level of the negative phase output, S
The cross point between D and SD is shifted downward.

As a result of such a pulse signal being input to the second waveform shaping circuit 2, as its output, points C and D are shown in FIG. 2(c).
A waveform like the one shown is obtained.

That is, the time T' when the signal SD obtained as the positive phase output of the second waveform shaping circuit 2 reaches the high level (011gh)
is the amount by which the cross point between the signals SD and SD shifts in FIG. is twice as long as the time ΔT corresponding to T' −T+2Δ
It becomes T.

Therefore, the signals SD and SD input as control signals to the current changeover switch are as shown in FIGS. 3(a) and (b), respectively.
As shown in the figure, the sD pulse width is expanded.

Here, if the increase 2ΔT in the pulse width of the signal SD is set to correspond to the light emission time delay inherent to the semiconductor laser LD, the pulse width distortion due to the light emission time delay of the semiconductor laser LD will be just corrected, and as shown in FIG. A duty as shown in (C) and a light output of 50% can be obtained. This improves the waveform distortion on the eye pattern that has been observed in the past.

This effect can be confirmed using a measurement system as shown in FIG. In the figure, 11 is a pattern generator that generates the differential signals SD and SD as shown in FIG. 2(a), and 12 is the drive circuit shown in FIG. 1. If the optical output generated by the semiconductor laser LD is received by the optical/electrical converter 14 via the optical fiber 13 and the electrical signal obtained as the output is observed with an oscilloscope, the result will be as shown in FIG. 3(C). , a distortion-corrected waveform is obtained.

This invention is not limited to the above embodiments, and various modifications are possible.

For example, although transistors Ql and Q2 whose emitters are commonly connected are used as current changeover switches, other switching means may be used. Further, it goes without saying that the means for lowering the level of the negative phase output of the waveform shaping circuit 1 is not limited to the configuration using the illustrated resistors.

〔Effect of the invention〕

As explained above, according to the present invention, a level adjustment means for changing the cross point by giving a relative level difference between the positive phase and negative phase outputs of the first waveform shaper is provided, and after the level adjustment, The pulse width of the output of the second waveform shaper that receives the differential signal of Distortion of the optical output waveform due to time delay can be corrected. Therefore, when used in an optical transmitter in an optical communication system, it helps improve the receiving sensitivity of the optical receiver and is very effective.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIGS. 2 and 3 are waveform diagrams for explaining its operation, FIG. 4 is a block diagram showing its optical output measurement system, and FIG. The figure is a circuit diagram showing a conventional example. 1.2... Waveform shaping circuit, LD... Semiconductor laser,
Ql, Q2...Transistor, R1, R2...Resistor. Figure 4

Claims (1)

[Claims] In a semiconductor laser drive circuit equipped with a waveform shaper that receives differential signals as input, and a current changeover switch that uses the positive-phase and negative-phase outputs of this waveform shaper as control signals and has a semiconductor laser connected to the load, differential a first waveform shaper inputting a signal; a level adjustment means for changing a cross point by giving a relative level difference between a positive phase output and a negative phase output of the first waveform shaper; 1. A semiconductor laser drive circuit comprising: a second waveform shaper that receives an adjusted differential signal as an input and sends positive-phase and negative-phase outputs as control signals to a current selection switch.