JPH0525262Y2 - - Google Patents

Description

[Detailed explanation of the idea]

Technical field of the invention This invention relates to a laser diode drive circuit.
The aim is to achieve a constant optical output and to prevent damage to the laser diode due to surges and the like. Background of the Technology A laser diode is driven by a circuit as shown in FIG. In this figure, 10 is a laser diode, 1
2 is a current driver that receives an input signal V _P and generates a drive current I _P ; 14 is a power supply that supplies a bias current I _b ; 16 is a current limiter that limits the bias current within a predetermined value; 18 is a choke coil; and 20 is a differential a reference value from a reference voltage generator 22, and
The signals indicating the laser diode light output detected by the photodiode 24 are compared, and the bias current I _b output by the power supply 14 is adjusted so that the difference becomes zero. 26 is an amplifier for the output of the photodiode 24. As is known, the driving current vs. optical output characteristic of a laser diode exhibits an L-shaped characteristic with the left and right sides reversed.When the driving current is small, there is almost no optical output, and when the driving current exceeds a certain value (threshold), the optical output suddenly decreases. begins to occur. The bias current I _b is a current corresponding to this threshold value, and when this is added to the signal current I _P , the signal-optical output characteristic becomes linear. The threshold value changes depending on the temperature, so the diode 24 detects the optical output and changes the bias current I _b so that it becomes constant (assuming I _P is constant). The series circuit of resistor R and capacitor C is for surge absorption, and absorbs excessive high frequency voltage applied to laser diode 10. D is for reverse voltage prevention, and prevents reverse voltage from being applied to the laser diode 10. Prior Art and Problems In such a laser diode drive circuit, if R and C are removed and the signal voltage V _P is made into a rectangular wave as shown in Figure 2, the optical output P of the laser diode 10 will initially be as shown in the figure. It exhibits a characteristic of attenuating after a large value. This is because the light output after attenuation is rather normal, and the initial large light output is the result of adding up the rest. Therefore, when the space period is larger than the mark period, the large output portion becomes even larger. FIGS. 4 and 5 are diagrams showing this. FIG. 4 shows the case where the light emission period is short, and FIG. 5 shows the case where the light emission period is long. Assuming the case shown in FIG. 4, the laser has no choice but to operate at less than the rated output due to the limitations of the peak. When RC is added, the optical output P generally becomes smaller in the first part as shown in FIG. When a laser diode is used in a printer or the like, the required frequency ranges from 0 to several MHz, and the ratio of marks and spaces varies considerably. Therefore R,
Without C, the optical output fluctuates greatly, which is a problem. There is also the problem that when RC is added, the beginning part is missing. In Figure 1 R, C series circuit, the third
It is impossible to make all of the optical outputs P in FIGS. 4, 4, and 5 constant, that is, a rectangular wave. For example, if P in Figure 4 is set to RC which makes it a square wave, Figure 5 will not become a rectangular wave, but Figure 5 will become a rectangular wave.
If RC is used, the capacitor capacity will be too large and the waveform in Figure 4 will be distorted. Figures 6 and 7 are graphs showing this. Figure 6 shows an example in which RC is adjusted to a short emission period, and Figure 7 is an example in which RC is adjusted to a long emission period. The optical output of b is attenuated, and in the latter case, the optical output becomes small in the case of a short light emission period (the driving period ends before the laser diode is not sufficiently driven). In addition, in the method shown in FIG. 1 in which the reverse surge is released by diode D, when the space period is short as shown in FIG. 8, the correction for the next light emission is not sufficiently performed, and a peak still occurs at the beginning of the light output. Purpose of the invention The present invention was devised in view of the above points, and is a laser diode drive that can maintain constant optical output over a wide frequency range and protect the laser diode from forward and reverse surges. It is intended to provide a circuit. Structure of the Invention The present invention provides a laser diode drive circuit having means for supplying a drive current for driving a laser diode according to an input signal V _P , the invention including a compensation circuit for adjusting the drive current for driving the laser diode. the compensation circuit includes a resistor R1 and a capacitor C1, and a differentiation circuit that is charged and discharged in accordance with a signal * _Vp whose polarity is inverted from the input signal _VP ;
A transistor Q is driven by the charging voltage B of the differentiating circuit, and the drive current is biased by the current controlled by the transistor Q. This will be explained with reference to Embodiment of the invention Figure 9 is a diagram showing the principle of constant optical output. A current source I _C is connected in parallel to the laser diode 10, and the current I _f flowing through the laser diode is set to I _f = I _b + I _P - I _C. The aim is to improve the pulse characteristics (constant light output). I _c is the compensation current for this purpose. I _c 11th
When selected as shown in the figure, the optical output becomes as shown by the dotted line in Figure 10a, and can be made constant. Such compensation current
A circuit for generating I _c is shown in FIG. In this diagram R ₁
~R ₄ is a resistor, C ₁ is a capacitor, and Q is a transistor. A series circuit of resistor R ₄ and transistor Q is connected in parallel to the laser diode 10, and the resistor
A differentiating circuit including R ₁ and capacitor C ₁ controls the base of transistor Q. *V _p is a signal obtained by inverting the polarity of the input signal V _p input to the amplifier 12. FIG. 13 shows the waveforms of each part of this circuit when the light emission period is long, and FIG. 14 shows the waveforms when the light emission period is short. As shown in Figure 13, when the light emission period is long, the capacitor _C1 is sufficiently charged, and when the non-light emission period begins, the potential at point B rises to a level higher than the power supply voltage +V and discharge begins, but the period is short. Therefore, the discharge is small. Before it is fully discharged, the next light emitting period begins, and it is pulled toward negative *V _p , but since there is a residual charge, the potential at point B does not become so negative. In the end, the potential at point B does not become strongly negative, but changes as shown in the figure, with a long negative period. As a result, _the transistor Q is turned on for a relatively long time from the beginning of the light emitting period, and a compensation current I _c having a waveform as shown in the figure flows. P becomes a rectangular wave. When the light emitting period is short, the capacitor _C1 stops charging before it is sufficiently charged, and is sufficiently discharged, so that the potential at point B eventually becomes strongly negative and has a waveform similar to *V _p as shown in the figure. Therefore, the compensation current I _c and _the laser diode drive current If become as shown in the figure, and the optical output P becomes a rectangular wave. The dotted line is before correction. In applications such as laser printers, light output is the second
The results are shown in the figure, and the measured data is shown in the table below, and the observed waveform is shown in Fig. 15. In this table, P _{1 (nw)} is the light output at the start of light emission, P _{2 (nw)} is the light output at the start of light emission, and ΔP _s is P ₁ - P ₂ .

[Table] Figure 15a shows the observed waveform for the No. 1 case, and Figure 15b shows the observed waveform for the No. 4 case. Effects of the Invention As is clear from the above explanation, in the present invention, a compensation current is caused to flow according to the marks and spaces of the input signal, and the optical output can be made into a rectangular wave similar to the rectangular wave input signal. Further, when a reverse voltage is applied to the laser diode, the transistor Q is turned on to protect the laser diode.

[Brief explanation of the drawing]

Figure 1 is a conventional laser diode drive circuit diagram, Figures 2 to 8, Figures 10, 11, and 1.
3 and 14 are waveform diagrams for explaining operation, FIG. 9 is a principle diagram of the present invention, FIG. 12 is a circuit diagram showing an embodiment of the present invention, and FIG. 15 is a waveform showing an example of observed waveform of optical output. It is a diagram. In the drawing, 10 is a laser diode, 12 is a signal current source, 14 is a bias current source, I _p is a signal current,
I _b is the bias current, R ₁ to R ₄ , C ₁ , Q are the compensation circuits,
Q is a transistor, R ₁ and C ₁ are differentiating circuits, and *V _p is the inversion voltage of the signal voltage.

Claims (1)

[Claims for Utility Model Registration] A laser diode drive circuit having means 12 for supplying a drive current for driving a laser diode 10 according to an input signal _VP , the circuit comprising: adjusting the drive current for driving the laser diode 10; The compensation circuit includes a resistor R1 and a capacitor C1, and the compensation circuit includes a resistor R1 and a capacitor C1, and
It consists of a differentiating circuit that is charged and discharged according to the signal *V _P in which the polarity of V _P is inverted, and a transistor Q that is driven by the charging voltage B of the differentiating circuit, and the current that is controlled by the transistor Q. A laser diode drive circuit, characterized in that the drive current is biased by.