JPH03169155A - Drive circuit for laser diode - Google Patents

Abstract

PURPOSE:To attain the correction at pulse drive in matching with an element characteristic of a laser diode by varying a leading differentiating time constant and a trailing differentiating time constant of an input signal so as to differentiate a drive pulse, generating the leading correction current and subtracting the current from the drive current. CONSTITUTION:One time constant is used both for a differentiating circuit charge characteristic and a discharge characteristic in a conventional drive circuit, and when a time constant is selected to correct a long overshoot, in the case of a short off-pulse, the correction current is in mismatching with the overshoot of the pulse. Then the time constant is made different from the leading and the trailing of a differentiating circuit 21 to select the correction time constant effecting on the time of the overshoot of a correction circuit 22 and the correction time constant with respect to the pulse pattern effect differently. Thus, in matching with the long overshoot for the leading time constant, since the correction current is matched with the pulse overshoot after short-off by means of the trailing time constant, the pulse drive in matching with the characteristic of the laser diode is corrected.

Description

[Detailed description of the invention] [Table of contents] Overview Industrial field of application Prior art (Figures 4 to 6) Means for solving the problems to be solved by the invention (Figure 1) Working examples (a) Description of one embodiment (Figures 2 and 3) (b)
Description of other embodiments Effects of the invention [Summary] Regarding a drive circuit for a laser diode used as a light emitting source in a laser printer, etc., the purpose of the present invention is to perform correction during pulse drive in accordance with the element characteristics of the laser diode. A signal current source that generates a drive current according to the drive pulse, and a pulse that generates a rising correction current according to the rise of the drive pulse and subtracts it from the drive current to adjust the drive current of the laser diode. In a laser diode drive circuit having a characteristic correction circuit, the pulse characteristic correction circuit includes a differentiating circuit that differentiates the drive pulse by changing a differential time constant of the rising edge and a differential time constant of the falling edge of the input signal; It has a correction circuit that generates a rising correction t current from the differential output of the circuit and subtracts it from the drive current.

[Industrial application field]

The present invention relates to a driving circuit for a laser diode used as a light emitting source in a laser printer or the like.

Laser diodes are widely used as light emitting sources in laser printers and the like.

Generally, a laser diode is driven to emit light by a pulse signal such as a video signal, but when driven by a pulse, even if the laser diode is driven with a constant current due to the temperature characteristics of the element, the optical output can have a transient response of overshoot to the forward current.

It is desired to correct this pulse response characteristic and obtain light with a constant output.

[Conventional technology]

FIG. 4 is a composition diagram of the prior art, and FIGS. 5 and 6 are explanatory diagrams of the prior art.

As shown in Figure 6 (A), when a laser diode is driven with a pulse signal such as a video signal, a forward current I flows through the laser diode, and the optical output exhibits a transient response that overshoots at the rise of the pulse signal. (The shaded area in the figure) appears. This transient response causes the laser diode to emit extra light, and in the laser printer, the density in that area becomes high, causing uneven shading.

For this reason, corrections as shown in FIG. 5 have been performed using a drive circuit as shown in FIG.

That is, as shown in Fig. 4, a signal current source 1 is provided in the laser diode LD, and under the condition that the alarm signal is not raised, the video signal is passed through an AND gate, the level is adjusted with resistors r4 and r5, and the processing is performed. Once the lid is connected to the base of the transistor Qc which is grounded to the resistor r3, a constant current Ic is applied.
flow.

On the other hand, a pulse characteristic correction circuit 2 is provided to correct the pulse characteristics, and an inversion circuit 20 for inverting the video signal.
, a differentiation circuit 21 that differentiates an inverted signal, and a correction circuit 22 that generates a correction current 1b based on the differential signal.

The video signal is NANDed with the high level input input via the resistor R● by the NAND gate NAND of the inverting circuit 20, and becomes an inverted signal Va, which is generated by the resistors Rl, R! The level is adjusted by , and the signal is input to the differentiation circuit 21 .

The differentiating circuit 21 consists of a capacitor CI and a resistor R3,
When the inverted signal Va is at a high level, discharge is performed based on the reference voltage Vcc, and when the inverted signal Va is at a low level, the discharge is performed based on the reference voltage Vcc.
It is further charged and the potential becomes Vx.

During this charging, a correction current 1b, which is the same as the charging current, flows through the transistor QI of the correction circuit 22 in the direction of the diode D1 through the resistor R4.

This correction t current 1b is added to the drive current I of the laser diode LD and becomes a constant current Ic flowing to the signal current source 1, so that the drive current 1 is changed to the correction current r as shown in FIG.
It is obtained by subtracting the b-minute constant voltage vLI from c. As a result, the driving current (2) is smoothed by the rising edge of the video signal, so that overshoot of the optical output P at the rising edge of the video signal can be prevented as shown in FIG.

Note that the light from the laser diode LD is received by a photodiode PD connected to a resistor r2, amplified by an amplifier AMP, and output as a monitor output vH, which is provided for automatic power control of the laser diode LD (not shown).

In this way, since the transient response of the overshoot of the laser diode LD has a differential shape, the current is corrected using the differential output correction current rb using the differentiating circuit 21.

Therefore, the magnitude and shape of the correction current 1b are determined by the differentiating circuit 2.
It was uniquely determined by the time constants Cl and R3 of 1.

(Problem B to be solved by the invention) By the way, the magnitude of the overshoot of the laser diode LD changes depending on the temperature characteristics of the laser diode LD, etc., and the lower the temperature of the laser diode LD, the higher the optical output becomes. The higher the value, the lower the light output.

Therefore, if the video signal is turned on immediately after being turned off (for example, pulses C and D in FIG. 6(A)), the amount of overshoot will be large, and the off period of the video signal will be When a long and then on signal is input (for example, at pulse b (!:C) in FIG. 6(A)), the amount of overshoot will be small.

The correction current 1b is determined by the differential time constants C1 and R3, but assuming a relatively long pulse like pulse C in FIG. 6(A) and FIG. 5, and therefore a long overshoot, the correction time In order to make the time constant 01 and R3 of the differentiating circuit 2■ longer, it was necessary to increase the time constant 01 and R3.

In this way, in the case of pulse d in FIG. 5, in which an on signal is input immediately after the video signal is turned off, in the case of pulse d in FIG.
), the potential Vx after turning off the pulse C becomes larger than Vcc and tries to settle down to Vcc with the time constant of CiRa, but before it settles down to Vcc, the pulse d arrives at a high potential and is swung below Vcc, Since the amount of level oscillated at this time is constant, there is a problem that the correction current 1b becomes small as shown in FIG. 5, and the amount of overshoot of the optical output P becomes large.

Conversely, if the time integer is determined to compensate for overshoot when the pulse interval is short, there is a problem in that long overshoot cannot be fully compensated for.

That is, in the conventional technology, if the time constants CI and R3 are determined by a certain pulse pattern, the amount of correction becomes small due to the pattern effect for fast-changing or long pulse patterns, making it difficult to sufficiently prevent overshoot. There was a problem that it could not be corrected.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a laser diode drive circuit that can perform correction during pulse drive in accordance with the device characteristics of the laser diode.

[Means to solve the problem]

FIG. 1 is a diagram of the principle of the present invention.

As shown in FIG. 1(A), in order to drive a laser diode LD, the present invention includes a signal current source 1 that generates a drive current in response to a drive pulse, and a signal current source 1 that generates a rise correction current in response to the rise of the drive pulse. In the laser diode drive circuit, the pulse characteristic correction circuit 2 includes a pulse characteristic correction circuit 2 that generates a pulse, subtracts it from the drive current, and adjusts the drive current of the laser diode LD. and a differentiating circuit 21 that differentiates the driving pulse by changing the differential time constant of the falling edge, and a correcting circuit 22 that generates a rising edge correction current from the differential output of the differentiating circuit 21 and subtracts it from the driving current. .

[Effect]

Conventionally, both the charging and discharging characteristics of the differentiating circuit 21 are determined by one time constant, and for this reason, the discharging characteristics cannot be controlled with respect to the pulse pattern interval, and at the rise of the next pulse, the discharge characteristics are determined according to the pattern effect. The peak during charging was uniquely determined by the pattern interval. For this reason, as mentioned above, when the time constant was adjusted to correct a long overshoot, a phenomenon occurred in which the correction current did not match the overshoot of the pulse in the case of a short post-off pulse. Therefore, in the present invention, the rise and fall time constants of the differentiating circuit 21 are changed to change the correction time constant that affects the overshoot time and the correction time constant for the pulse pattern effect.

As a result, as shown in Fig. 1 (B), for example, the rise time constant can be adjusted to a long overshoot, and the fall time constant can be used to adjust the correction current to a short overshoot of the pulse after turning off. Ta.

Therefore, it has become possible to make corrections during pulse driving that match the characteristics of the laser diode element.

〔Example〕

(al　Explanation of one embodiment FIG. 2 is a diagram showing the structure of an embodiment of the present invention.

In the figure, the same parts as those shown in FIG. 4 are indicated by the same symbols.

In the structure shown in FIG. 2, a circuit in which a resistor Rs and a diode D2 are connected in series in parallel with the resistor R3 is added to the differentiating circuit 21.

Therefore, charging of the capacitor C1 of the differentiating circuit 21 is performed through the resistor R3, and discharging is performed through the resistor R3, the diode D1, and the resistor R5.

As a result, the charging time constant is determined by CiR3,
The discharge time constant is determined by R9, Rs, and Cl. FIG. 3 is an explanatory diagram of an embodiment of the present invention.

If the time constants ci, R3 during charging are set to correct the long overshoot of pulse C, the time constants CI, R3, R5 during discharging correct the overshoot of pulse d, which turns on after a short off. It can be adjusted as follows.

That is, as shown in FIG. 3, the time constants C1, R3, and R5 during discharging
to make the discharge characteristic (attenuation characteristic) steeper, to make the charging peak value Vx at the rising edge of pulse d smaller, and to make the correction current I
The correction current 1b' is controlled so as to increase the peak of b' and correct the amount of overshoot when the pulse interval is short.

As a result, even if the charging time constant is adjusted to a long overshoot such as pulse C, the correction current Ib has a shape that can compensate for the overshoot of a pulse that turns on after a short turn off, such as pulse d, by changing the discharge time constant. ' can be generated.

Therefore, as shown in FIG. 3, no overshoot is observed in the optical output P, and both the overshoot of long pulses and short-interval pulses can be corrected. (bl Description of Other Embodiments In the above embodiment, the overshoot correction time is lengthened and the amount of correction for the pattern effect of the video signal is changed in accordance with the pulse C. By reversing the polarity, it is possible to shorten the correction time for overshoot and lengthen the time constant for correction for video signal pattern effects.

Further, in the above-described embodiment, the video signal is inverted and differentiated, but the drive signal (pulse) is not limited to the video signal and does not need to be inverted.

Although the present invention has been described above using examples, the present invention can be modified in various ways according to the gist of the present invention, and these are not excluded from the present invention.

〔Effect of the invention〕

As explained above, according to the present invention, by changing the correction time constant for pattern effects and the correction time constant for overshoot time, it is possible to perform correction during pulse driving that matches the overshoot characteristics of the laser diode. It has the effect of becoming.

4

[Brief explanation of the drawing]

Figure 1 is a diagram of the principle of the present invention. FIG. 2 is a schematic diagram of an embodiment of the present invention; FIG. 3 is an explanatory diagram of one embodiment of the present invention; Figure 4 is a structural diagram of the conventional technology. FIGS. 5 and 6 are explanatory diagrams of the prior art. In the figure, 2 2 L 1-signal current source; 2.-pulse characteristic correction circuit, 1-differential circuit, 2-・Correction circuit, D・---Laser diode.

Claims (1)

[Claims] In order to drive a laser diode (LD), a signal current source (1) that generates a drive current according to a drive pulse; a signal current source (1) that generates a rising correction current according to the rising edge of the driving pulse;
Subtracted from the drive current, the laser diode (LD)
In a laser diode drive circuit having a pulse characteristic correction circuit (2) that adjusts the drive current of the input signal, the pulse characteristic correction circuit (2) changes the differential time constant of the rising edge and the differential time constant of the falling edge of the input signal. A differentiating circuit (21) for differentiating the driving pulse using the differentiating circuit (21), and a correcting circuit (22) for generating a rising correction current from the differential output of the differentiating circuit (21) and subtracting it from the driving current. Diode drive circuit.