JP2003060289A - Semiconductor laser drive circuit and imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-speed and accurate semiconductor laser drive circuit, and an imaging apparatus. SOLUTION: A drive current, which is made flow into a semiconductor laser 10, is composed of a sum current from a bias current source 12, a threshold current source 11, and a modulation current source 13. In this case, the bias current source 12 should be set to be approximately 1 mA to several mA. The threshold current source 11 is the current source of a threshold, where a semiconductor laser 10 emits light. Since the bias current source 12 flows, the threshold current source 11 may be a current where the current value is subtracted (a threshold current less a bias current). Additionally, the modulation current source 13 is a current source that is modulated according to an input signal, thus controlling the light emission of the semiconductor laser 10.

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 and an image forming apparatus, and more particularly, to a semiconductor laser of a semiconductor laser used in a laser printer, an optical disk device, a digital copying machine, an optical communication device or the like. The present invention relates to a drive circuit and an image forming apparatus.

[0002]

2. Description of the Related Art Conventional semiconductor laser drive circuits are roughly classified into a non-bias type and a bias type. In the non-biased method, the bias current of the semiconductor laser is set to 0,
This is a method of driving a laser diode (hereinafter referred to as "LD") with a pulse current corresponding to an input signal. In the biased method, the bias current of the semiconductor laser is set to the threshold current of the semiconductor laser, and the bias is always applied. This is a method of driving a LD by applying a pulse current corresponding to an input signal to the bias current while flowing a current.

[0003]

By the way, when a semiconductor laser having a large threshold current is driven by a non-biased method,
Even if a drive current corresponding to the input signal is applied to the LD, it takes some time until carriers with a concentration capable of laser oscillation are generated, and a time delay occurs until light emission. As a result, there is no problem if the input signal is sufficiently larger than the light emission delay time and the light emission delay amount can be ignored, but if the semiconductor laser in a laser printer, an optical disk device, a digital copying machine, etc. is to be driven at high speed, it is desirable. Only pulses smaller than the pulse width can be obtained.

Therefore, in order to reduce the time delay until the laser emission, a biased method in which the oscillation threshold current of the semiconductor laser is supplied in advance has been proposed. In this biased method, since the oscillation threshold current of the semiconductor laser is supplied in advance, the light emission delay time is eliminated, but even when light is not emitted, light is always emitted near the oscillation threshold (usually 200).
.mu.W to 300 .mu.W), the extinction ratio becomes small in the case of optical communication, and causes a background stain in the case of a laser printer, an optical disk device, a digital copying machine and the like.

In order to solve such a problem, in the field of optical communication, in JP-A-4-283978, JP-A-9-83050, etc., a bias-free method is basically used and immediately before light emission. A configuration has been proposed in which an oscillation threshold current is passed through. However, recently, in laser printers, optical disk devices, digital copying machines, etc.,
In pursuit of higher resolution, a 650 nm red LD,
Furthermore, a system using a 400 nm ultraviolet LD or the like has begun to be put into practical use. These semiconductor lasers are conventional 1.
Compared with LDs of 3 μm, 1.5 μm, and 780 nm band, it has a characteristic that it takes a longer time to generate carriers with a concentration capable of laser oscillation, and even in the above method, it is smaller than the desired pulse width. There is a problem that only the pulse width can be obtained.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a high-speed and highly-accurate semiconductor laser drive circuit and an image forming apparatus.

[0007]

In order to solve the above problems, the present invention employs means for solving the problems having the following features.

According to a first aspect of the present invention, there is provided bias current generating means for constantly supplying a minute bias current to the semiconductor laser, threshold current generating means for supplying a threshold current to the semiconductor laser, and the bias current generating means according to an input signal. A driving current generating means for driving the semiconductor laser to emit light, the threshold current generating means generates a threshold current from the input signal, and the on-period of the threshold current generated by the threshold current generating means is Driving the semiconductor laser with a sum current of three currents of the bias current, the drive current, and the threshold current, which is a period longer than the light emission period for emitting light by the drive current generating means and includes the light emission period. Is a semiconductor laser driving circuit.

According to the invention described in claim 1, a bias current generating means for constantly supplying a minute bias current to the semiconductor laser, a threshold current generating means for supplying a threshold current to the semiconductor laser, and an input signal according to the input signal. Drive current generating means for driving the semiconductor laser to emit light, the threshold current generating means generating a threshold current from the input signal, and turning on the threshold current generated by the threshold current generating means. The period is longer than the light emission period of light emission by the drive current generating means and includes the light emission period, and the semiconductor laser is driven by the sum current of the three currents of the bias current, the drive current, and the threshold current. By doing so, a high-speed and highly accurate semiconductor laser drive circuit can be provided.

According to a second aspect of the invention, in the semiconductor laser drive circuit according to the first aspect, the current generated by the bias current generating means is several mA or less.

According to the second aspect of the present invention, the current generated by the bias current generating means is several mA or less, so that the extinction ratio can be sufficiently secured and high speed / high speed
A highly accurate semiconductor laser drive circuit can be provided.

According to a third aspect of the present invention, in the semiconductor laser drive circuit according to the first aspect, a delay signal generating means for delaying the input signal for a predetermined time and a delay signal delayed by the delay signal generating means are provided. Drive current generating means for driving the semiconductor laser based on the above, and the threshold current generating means for generating a logical sum of the input signal and the delay signal, the threshold current generated by the threshold current generating means is the drive It is characterized in that it does not turn off before the current turns off.

According to the third aspect of the present invention, the threshold current generated by the threshold current generating means is not turned off before the drive current is turned off. It is possible to provide an LD drive circuit that can accurately emit light from an LD according to an input signal without being turned off.

According to a fourth aspect of the present invention, in the semiconductor laser drive circuit according to the first aspect, the drive current generating means has an initialization means that operates when the power is turned on or when reset is released. According to another aspect of the present invention, the amount of light emitted from the semiconductor laser is set to a predetermined value.

According to a fifth aspect of the present invention, in the semiconductor laser drive circuit according to the fourth aspect, the initialization means includes a current or a voltage when the light intensity of the semiconductor laser is a predetermined value, and the semiconductor laser drive circuit. It is characterized in that a difference from a current or a voltage when the light quantity is smaller than a predetermined value is detected, and the light quantity when the semiconductor laser emits light is set to a predetermined value.

According to a sixth aspect of the present invention, in the semiconductor laser drive circuit according to the fourth aspect, the initialization means sets the current or voltage when the light amount of the semiconductor laser is a predetermined value and the semiconductor laser. It is characterized in that a difference from a current or a voltage in the case of offset light emission is detected, and the light amount of the semiconductor laser at the time of light emission is set to a predetermined value.

According to a seventh aspect of the present invention, in the semiconductor laser drive circuit according to the fourth aspect, the initialization means is a current or voltage when the light amount of the semiconductor laser is a predetermined value, and the semiconductor laser driving circuit. The amount of light is 1 / N (N
Is characterized by detecting a difference from a current or a voltage in the case of a natural number of 2 or more) and setting the light amount of the semiconductor laser at the time of light emission to be a predetermined value.

According to an eighth aspect of the present invention, in the semiconductor laser drive circuit according to any one of the fifth to seventh aspects, the initialization means includes a timing generation section and a detection section for detecting the difference. A current setting unit that sets a light amount when the semiconductor laser emits light, a value detected by the detection unit based on the timing generated by the timing generation unit, and a value set by the current setting unit correspond to each other. It is characterized in that it is configured with a comparison unit that performs successive comparison.

According to the invention described in claims 4 to 8,
A semiconductor laser drive circuit capable of high-speed and high-accuracy pulse output, in which the initial amount of light emitted from the semiconductor laser is set to a predetermined value by the initialization means, overshoot, etc. does not occur with a simple configuration. Can be provided.

According to a ninth aspect of the present invention, in the semiconductor laser drive circuit according to any one of the first to eighth aspects, a light receiving section for detecting the optical output of the semiconductor laser, and the light receiving section detect the light output. It is characterized by further comprising current control means for controlling a current supplied to the semiconductor laser based on a light receiving signal proportional to an optical output of the semiconductor laser.

The invention described in claim 10 is claim 9
In the semiconductor laser drive circuit described above, the current control means generates a control signal by comparing the magnitude of the received light signal with a predetermined value, and controls the threshold current generation means by this control signal. To do.

According to the invention described in claim 9 or 10, the control signal is generated based on the light receiving signal and the control signal having a predetermined value, and the threshold current generating means and the like are controlled by this control signal. Therefore, even if there is a change due to temperature,
A semiconductor laser drive circuit with stable output can be provided.

The invention described in claim 11 is claim 1
0 in the semiconductor laser drive circuit, the current control means samples the control signal when the drive current generation means is in an ON state, and controls the threshold current generation means based on the sampled value. Characterize.

According to the eleventh aspect of the present invention, by sampling the control signal when the drive current generating means is in the ON state and controlling the threshold current generating means based on the sampled value. For example, not only the light amount adjustment is performed only outside the image writing area, but also in the writing area, if the LD is turned on, by sampling and controlling each time, even if there is a change due to temperature, A semiconductor laser drive circuit with stable output can be provided.

The invention described in claim 12 is the same as claim 1.
11. The semiconductor laser drive circuit according to claim 11, further comprising means for detecting a voltage of an output section for driving the semiconductor laser and controlling a voltage of a power supply supplied to the semiconductor laser based on the detected value. Characterize.

According to the twelfth aspect of the present invention, there is provided means for detecting the voltage of the output section for driving the semiconductor laser and controlling the voltage of the power supply to be supplied to the semiconductor laser based on the detected value. It is possible to realize an LD drive circuit that consumes less power and can drive a large number of LDs.

According to a thirteenth aspect of the present invention, a semiconductor laser whose output is modulated by an image modulation signal, and a scanning means for scanning the rotating photoconductor with the light of the semiconductor laser,
An image forming apparatus for forming an electrostatic latent image on the rotating photoconductor according to the image modulation signal, wherein the semiconductor laser driving circuit according to any one of claims 1 to 12 drives the semiconductor laser. A characteristic image forming apparatus.

According to the thirteenth aspect of the present invention, since the semiconductor laser drive circuit according to any one of the first to twelfth aspects is used, an image forming apparatus capable of high speed and high precision pulse output is provided. Can be realized.

The invention described in claim 14 is claim 1
In the image forming apparatus described in 3, shading correction is performed by changing the full-scale value of the drive current generating means according to the scanning of the scanning means.

According to the fourteenth aspect of the present invention, the shading correction is surely performed by performing the shading correction by changing the full scale value of the drive current generating means in accordance with the scanning of the operating means. It can be carried out.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings.

In the present invention, attention is paid to the characteristics of the LD, and the semiconductor laser is driven by the sum current of the bias current, the oscillation threshold current and the light emission current. Here, the bias current is a very small amount of current, unlike the bias current of the bias type. The characteristics of the semiconductor laser are that the impedance is considerably large in the non-biased state, and even if a threshold current is flown from this state, it does not go to a level where the light emission delay time immediately disappears due to the influence of the inductance component in the semiconductor laser. If a current is applied to the semiconductor laser even at about 1 mA, the impedance of the semiconductor laser is considerably lowered. Therefore, if a threshold current is applied from this state,
Since the light emission delay time is easily eliminated, a very small bias current is effective. Further, in this case, since the light emission amount of the semiconductor laser is at a sufficiently small level, the extinction ratio in the case of optical communication, which is a problem in the biased system, does not decrease, and the laser printer, the optical disc, and the like. It is possible to provide a semiconductor laser drive circuit and an image forming apparatus that do not cause a background stain in an apparatus, a digital copying machine, or the like. (Basic Structure of the Present Invention) A basic conceptual diagram (basic structure) of the present invention is shown in FIG. In FIG. 1, the drive current source to be passed through the semiconductor laser 10 is composed of the sum current from the three current sources of the bias current source 12, the threshold current source 11, and the modulation current source 13. Of these, the bias current source 12 is a current source which allows a current of about 1 mA to flow at most about several mA. The threshold current source 11 is a current source that supplies a threshold value at which the semiconductor laser 10 emits light. In FIG. 1, since there is the bias current source 12, a current value obtained by subtracting the current value (threshold current-bias current) may be used. The modulation current source 13 is a current source that is modulated according to the input signal, and the emission of the semiconductor laser 10 is controlled by this.

Here, regarding the bias current source, as shown in FIG.
This will be described with reference to FIG. 2 and 3 show the output P (μW) and the LD drop voltage VL when a small current is passed through a certain LD.
The actual measurement result of DDOWN is shown. Looking at FIG. 2 and FIG.
LD drop voltage VLDDOWN has LD current ILD of 250
About 1.4V has already been generated at μA, and gradually increases as ILD increases. Since LD has a DC resistance component, VLDDOWN gradually increases as ILD increases. When ILD is 0, VLDDOWN is 0, while when ILD is 250 μA, VLDDOWN is
It can be seen that N of 1.4 V is generated, and the impedance of the LD is sufficiently small only by applying 250 μA to the ILD. Accordingly, it can be predicted that the response characteristic is sufficiently improved by passing the threshold current after passing 250 μA. That is, for example, it can be seen that by allowing a minute bias current of about 1 mA to flow through the LD, the LD drop voltage change is small and the LD responds at high speed. Also,
The light emission output of the LD at this time is 1.26 μW even at 1 mA, and considering that the normal LD light emission amount is 1 mW or more, it is about 0.1%, and the extinction ratio in optical communication,
It can be seen that the level is such that the scumming does not occur in the laser printer and the digital copying machine. In addition, even if one LD (photodiode) and many LDs are provided like the LD array, the other LD emits a slight light emission of about 1 μW in response to the control of the light amount of one LD. It doesn't matter. In addition, in FIG. 2 and FIG.
It is explained as an example by actually measuring the characteristics of
However, it shows the same characteristics. (First Configuration: Generation of Control Signal of Threshold Current Source) FIG. 4 shows a first configuration example of the present invention. FIG. 4 shows a configuration example for generating a control signal for the threshold current source. Since the threshold current source 11 of the LD largely changes depending on the temperature, the threshold current source is controlled constantly or by using a sample hold circuit. In the figure, the output voltage of the PD 20 that detects the optical output of the semiconductor laser 10 is compared with the emission control voltage, and the threshold current source 11 is controlled so that the output of the PD 20 and the emission control voltage match.

On the other hand, the bias current source 12 may be a fixed minute current, and the modulation current source 13 is L at the time of initial setting.
If the characteristic peculiar to D is measured and set, since there is little change due to temperature, a fixed current may be used as well.

With such a structure, it is possible to provide a high-speed and highly accurate semiconductor laser drive circuit. (Second Configuration) FIG. 5 shows a second configuration example of the present invention. In FIG. 5, a switch circuit is connected in series to the threshold current source and the modulation current source. When the threshold ON signal is applied to the switch circuit 31, the current of the threshold current source 11 is LD
Supplied to 10. Similarly, when the modulation signal is applied to the switch circuit 32, the current of the modulation current source 13 is supplied to the LD 10. 6 and 7 show examples of the timings of the modulation signal and the threshold value ON signal.

FIG. 6 shows a light emission command signal (A), a light emission command signal delay pulse (B), a modulation signal (C), and a threshold value ON signal.
(D), LD drive current (E) and LD output optical waveform (F)
It is shown. Although not shown in FIG. 6, the light emission command signal (A) input from the outside is delayed by the delay circuit to become the modulation signal (C), and the logical sum of the light emission command signal (A) and the delay signal (B) is obtained. It becomes the threshold value ON signal (D).

At this time, the LD drive current is applied to the LD 10.
(E) is supplied. The LD drive current (E) is the sum of the bias current + threshold current + modulation current and three currents, as shown in FIG. 6 (E). Further, the threshold current is 1 to 1 when the modulation current is on.
It is turned on 10 ns ago and turned off at the same time as the modulation current. The time difference between this threshold current and the modulation current is
Shorter is better, but assuming an actual laser printer or digital copying machine, if there is about 1 dot or less, even if there is a weak light emission due to the threshold current, the light emission is small and slightly There is no problem as it is only dirty. When a red LD or an ultraviolet LD is used, it takes a longer time to generate a carrier having a concentration capable of laser oscillation than an infrared LD, and therefore, depending on the LD, In some cases, it may be necessary to pass the threshold current about 10 ns before the LD driver,
In the case of a replication specific IC), an LD driver ASIC compatible with a wide variety of LDs can be realized as long as it has a function of externally controlling the delay time.

FIG. 7 shows an example of the timings of the modulation signal and the threshold value ON signal different from those of FIG. Similar to FIG. 6, in FIG. 7, the light emission command signal (A), the delay pulse (B) of the light emission command signal, the modulation signal (C), the threshold ON signal (D), the LD drive current (E), and the LD output are used. An optical waveform (F) is shown.

FIG. 7 shows an example of the timing at which the threshold current is turned off after the modulation signal for the threshold on signal is turned off. Normally, even if it is said that the threshold on signal is turned off at the same time as the modulation signal, it is difficult to turn off at the same timing at high speed, so the threshold on signal is turned off after confirming that the modulation signal is off. However, the difference is at most a few ns, and even if a laser printer or a digital copying machine is assumed, the background stain is generated within only a few ns, so there is no problem on the image. With this configuration, it is possible to realize an LD drive circuit that can accurately output a pulse without turning off the threshold value ON signal before the modulation signal. (Detection of differential quantum efficiency of semiconductor laser and initial setting of modulation current source) When power supply voltage is turned on (or LD off is released), L
The differential quantum efficiency of D (the slope characteristic (differential characteristic) of the output power with respect to the LD current) is detected, and the modulation current is initialized based on the characteristic. FIG. 8 shows a configuration relating to detection of the differential quantum efficiency of the semiconductor laser and initial setting of the modulation current source at that time. FIG. 8 includes a timing generation unit 51, a differential quantum efficiency detection unit 52, and a D / A (digital / analog conversion) unit 53.

The initial setting of the modulation current source is as follows:
Applied to the LD10 at the time of power (maximum power)
Voltage VLD_FULL(The current flowing in the LD at that time
Flow I_OPAnd when a threshold current flows.
VLD applied to the LD10 of the_TH(then
The current flowing through the LD is I_THAnd)
The difference between these voltages is VLD_DAnd the difference between these currents
I_DAnd This VLD _DOr I_DUsing the
Set the current value of the adjusting current source to (I_OP-I_TH). So
As a result, when the modulation signal is on, the current of the threshold current source 11
Combined with, the current that the LD emits at full power,
It can be supplied from the modulation current source 13. Also the modulation
The current supplied from the current source 13 is the current of the threshold current source 11.
Combined with the current, the LD emits light at full power and is almost critical
Since this is a typical current value, the consumption of modulation current is minimized.
You can In addition, the current of the threshold current source 11 and the bypass current
Supplying excess current to LD even when combined with ass current source
And the life of the LD can be extended.
It

Here, two examples of setting methods will be described.

The first method will be described with reference to FIGS. 9 and 10. FIG. 9 is a diagram showing a differential quantum efficiency characteristic of an LD. When the current supplied to the LD increases and reaches a current value I _TH , the LD starts to emit light (the voltage applied to the LD at that time is VLD. _TH , and the output power of the LD at that time is P _TH ). The current that outputs the maximum power P _O determined by the standard is I _OP (the voltage applied to the LD at that time is VLD _OP ).

FIG. 10A shows the timing signal LVCO from the timing generator 51, and FIG. 10B shows the LD.
10C is a current supplied to the D / A and is a digital value output from the D / A. Note that the values in FIG. 10C are examples and are not limited thereto.

The timing generator 51 operates only at the time of initialization, and the timing signal LVC shown in FIG.
O is supplied to the differential quantum efficiency detection unit 52. The differential quantum efficiency detection unit 52, based on the timing signal LVCO,
10 timings from = 0 to T = 9 are generated. The differential quantum efficiency detection unit 52 performs processing according to each timing, and outputs an 8-bit value to the D / A unit 53 in synchronization with the timing from the timing generation unit 51. D
/ A section 53, for example, 1, 0.5, 0.25, 0.
Values such as 125 are output in descending order.

The differential quantum efficiency detector 52, at T = 0,
The LD is forcibly turned on (full power lighting), the LD is caused to perform offset light emission (near I _TH ) at T = 1, and the LD is turned off (at a bias current of about 1 mA) at T = 9. Also, T =
At 1, the difference of I _OP −I _TH is held. On the other hand, in accordance with the timing from the timing generation unit 51, the D / A unit 53 causes T = 0 to T = 9 to be 1, 0.5,
Values such as 0.25, 0.125, etc. are output in order. Here, for example, the difference of I _OP −I _TH is 0.7 mA, and the value of the D / A unit 53 is 1, 0.5, 0.25, 0.1.
25 etc. is 1 mA, 0.5 mA, 0.25 mA, 0.1
The following description will be given on the assumption that the modulation current source 13 is controlled so as to correspond to 25 mA or the like.

At T = 2, the output of 1 is applied from the D / A section 53 to the modulation current source 13, and the modulation current source 13 outputs 1 mA.
Current flows. This current is supplied to the differential quantum efficiency detector 52.
Is detected and compared with the held 0.7 mA. As a result, since 1 mA> 0.7 mA, the differential quantum efficiency detection unit 52 ignores “1” and prepares for the next timing.

At T = 3, an output of 0.5 is applied from the D / A section 53 to the modulation current source 13, and a current of 0.5 mA flows from the modulation current source 13. The differential quantum efficiency detection unit 52 detects this current and holds it at 0.7 mA.
Compare with. As a result, since 0.5 mA <0.7 mA, the differential quantum efficiency detection unit 52 sets “0.5” and prepares for the next timing.

At T = 4, an output of 0.25 is applied from the D / A section 53 to the modulation current source 13, and a current of 0.25 mA flows from the modulation current source 13. This current is detected by the differential quantum efficiency detection unit 52, and “0.
0.75mA summed with "0.5mA" corresponding to "5"
Is compared with the held 0.7 mA. As a result, 0.75 mA> 0.7 mA, so the differential quantum efficiency detection unit 52 ignores "0.5" and prepares for the next timing.

At T = 5, from the D / A section 53, 0.125
Is applied to the modulation current source 13, and a current of 0.125 mA flows from the modulation current source 13. The differential quantum efficiency detection unit 52 detects this current, and adds it to “0.5 mA” corresponding to “0.5” previously set to 0.6.
25 mA is compared with the held 0.7 mA. As a result, since 0.625 mA <0.7 mA, the differential quantum efficiency detection unit 52 sets “0.125” and prepares for the next timing.

When T = 6, the D / A section 53 outputs 0.062.
5 is applied to the modulation current source 13 and the modulation current source 13
A current of 0.0625 mA flows. This current is detected by the differential quantum efficiency detection unit 52, and "0.625" corresponding to the previously set "0.5" and "0.125" is detected.
0.6875 mA summed with “mA” is compared with the held 0.7 mA. As a result, 0.6875m
Since A <0.7 mA, the differential quantum efficiency detection unit 52
Sets "0.0625" to prepare for the next timing.

At T = 7, from the D / A section 53, 0.031
The output of 25 is applied to the modulation current source 13,
A current of 3 to 0.03125 mA flows. The differential quantum efficiency detection unit 52 detects this current, and sets “0.5”, “0.125”, and “0.0625” set in advance.
0.718 combined with "0.6875 mA" corresponding to
Compare 75 mA with the held 0.7 mA. As a result, since 0.71875 mA> 0.7 mA, the differential quantum efficiency detection unit 52 displays “0.03125”.
And ignore it. In this way, the initial setting current value of the modulation current source is set to (I _OP −I _TH ).
In this example, the differential quantum efficiency detection unit 52 sets the set values of “0.5”, “0.125”, and “0.0625” as the output value of the D / A unit 53 and corresponds to this output value. A current of “0.6875 mA” that flows is supplied from the modulation current source 13.

The above numerical values are examples. It can also be a rounded value. Further, although the example of FIG. 10 is a case where the D / A has an 8-bit configuration, the required timing number is changed depending on the number of bits configuring the D / A.

Further, in the case of this example, in order to obtain the threshold current I _TH at the time of initialization, the current source which operates only at the time of initialization is set so that the desired offset light emission value can be obtained from the external terminal. It may be provided. Further, the timing signal LVCO may be configured so that its timing can be adjusted by an external terminal.

The second method will be described with reference to FIGS. 11 and 12. Differs from the first method, as a value held in the differential quantum efficiency detection unit _52, instead of using the (I OP _{-I TH),} is that it uses an _{(I OP / 2-I TH} ). Therefore, in the differential quantum efficiency detection unit 52, a value obtained by doubling the held (I _OP / 2-I _TH ) and a current corresponding to the value of the D / A unit 53 from T = 2 to T = 9 are obtained. Compare. Other than that, since it is the same as the first method, description thereof will be omitted.

[0055] In the above description, the current value as a value held in the differential quantum efficiency detection unit 52 _(I OP _{-I TH)}
Or _{(I OP / 2-I TH} ) a was used, the current value _(I O P _{-I TH)} or _{(I OP / 2-I TH} ) to the corresponding LD voltage _(VLD FULL -VLD _TH) or ( V
LD _FULL / 2-VLD _TH ) may be used to initialize the modulation current source by the LD voltage.

In the first method, the D / A section 53 drives the LDs in descending order such as 1, 0.5, 0.25, 0.125, etc., and compares them. ing. As a result, since the initial value is large, there are cases where the LD is drastically exceeded the standard, and this may cause damage to the LD or shorten the life of the LD. However, in the second method, instead of using (I _OP −I _TH ), (I _OP /
2- _ITH ) does not cause such a problem. However, in the second method, (I _OP / 2-
Since I _TH ) is compared with a doubled value, the control accuracy is lower than that of the first method. (Third Configuration: Generation of Control Signal of Threshold Current Source) FIG.
An example of a configuration for generating a control signal of a threshold current source different from that of FIG. 4 is shown. FIG. 13 shows a configuration example in which the timing for controlling the threshold current is sampled when the modulation signal is on and held when the modulation signal is off. In the case of this configuration, for example, not only the light amount adjustment is performed only outside the image writing area, but L
If D is turned on, it is possible to perform sampling and control each time. (Fourth Configuration) FIG. 14 shows a fourth configuration example of the present invention.
FIG. 14 shows an example of a specific configuration example of the timing diagrams shown in FIGS. 6 and 7 when it is configured by the one-chip ASIC 50. The light emission command signal becomes a modulation signal through the delay unit 54. Also, the light emission command signal (A) and the delay signal (B)
Becomes a threshold ON signal (D) via the threshold signal generator 55.
The modulation signal (C) drives the switch of the modulation current, and the threshold ON signal drives the switch of the threshold current.
An LD drive current (E) as shown in FIGS. 6 and 7 is generated.
Further, the threshold current is sampled when the modulation signal is on and is held when the modulation signal is off, as in FIG. The D / A unit that determines the modulation current is shown in FIGS.
The modulation current source 13 is controlled in accordance with the timing shown in (3) so that a desired light amount can be obtained from the LD 10.
(Fifth Configuration) FIG. 15 shows a fifth configuration example of the present invention.
FIG. 15 shows a configuration example having a shading correction function and an LD power source (VLD) control function in addition to the same parts as in FIG.

First, the shading correction function will be described. The differential quantum efficiency of the LD detected when the power is turned on or when the reset is released is set to D / A. This D / A
When the current or voltage that determines the full scale of the current is input from the external terminal and the full scale is changed, the light emission amount of the LD can be changed. For example, in an LD writing system that performs a raster scan, the energy density of the central portion is high, so that the scanning end has a large light emission amount and the central portion has a small light emission amount so that the LD light emission amount is inversely corrected. Correction (shading correction). The speed of this correction may be slow, as long as the change follows the time during which the LD scans one line. Shading correction is performed by changing the current value of the D / A by a signal that changes the amount of light emission as described above according to the time for scanning one line from the outside.

Next, the LD power source (VLD) control function will be described. The purpose of controlling VLD is to drive the LD drive unit to the ASIC.
Drive current of the LD is 10 depending on the LD.
Since it is necessary to pass a large current of about 0 mA, ASI
The power consumption as C is affected. For example, with a 5V power supply,
Assuming that the voltage drop of the LD is 2 V, the ASIC requires 100 mA at 3 V, that is, 300 mW with the LD current alone. When the number of LDs to drive is 2, 600mW only by the LD current, and when the number of LDs to drive is 4, L
The D current alone would require 1200 mW. Since it becomes difficult to drive a large number of LDs as they are, VLD is controlled. In the above example, for example, since the cathode portion of the LD has a voltage of 3V, the power consumption has increased, but if the cathode portion of the LD can be controlled to about 1V, the LD
The current consumption of the ASIC is 1/3. VLD
Detection of the threshold ON signal or the modulation signal detects the cathode potential of the LD when it is ON, and outputs a VLD control signal to the outside of the ASIC so that it becomes a certain desired voltage (for example, 1 V). VL
The D control signal is input to, for example, the base of the power transistor, and if the emitter of the power transistor is connected to the LD power source, VLD can be controlled. Since this control speed may be a speed sufficiently slower than the modulation speed of the LD, any power transistor may be used as long as it can supply a sufficient current to the LD. By having such a VLD control function, an LD drive circuit that consumes less power and can drive a large number of LDs can be realized.

[0059]

As described above, according to the present invention, it is possible to provide a high-speed and highly accurate semiconductor laser drive circuit and image forming apparatus.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a basic conceptual diagram (basic configuration) of the present invention.

FIG. 2 is a diagram for explaining a bias current source of an LED.
(Part 1).

FIG. 3 is a diagram for explaining a bias current source of an LED.
(Part 2).

FIG. 4 is a diagram for explaining a first configuration example of the present invention.

FIG. 5 is a diagram for explaining a second configuration example of the present invention.

FIG. 6 is a diagram for explaining an example (part 1) of timings of a modulation signal and a threshold value ON signal.

FIG. 7 is a diagram for explaining an example (part 2) of timings of a modulation signal and a threshold value ON signal.

FIG. 8 is a diagram for explaining a configuration example for initial setting of a modulation current source.

FIG. 9 is a diagram (No. 1) for explaining the first method of initial setting of the modulation current source.

FIG. 10 is a diagram (No. 2) for explaining the first method of initial setting of the modulation current source.

FIG. 11 is a diagram (No. 1) for explaining the second method of initial setting of the modulation current source.

FIG. 12 is a diagram (No. 2) for explaining the second method of initial setting of the modulation current source.

FIG. 13 is a diagram for explaining a configuration example for generating a control signal for a threshold current source.

FIG. 14 is a diagram illustrating a fourth configuration example of the present invention.

FIG. 15 is a diagram illustrating a fifth configuration example of the present invention.

[Explanation of symbols]

10 Laser diode (LD) 11 Threshold current source 12 Bias current source 13 Modulated current source 20 Photodiode (PD) 21 Differential amplifier 31, 32 switch circuit 41 Sample and hold circuit 50 ASIC 51 Timing generator 52 Differential quantum efficiency detector 53 D / A section 54 Delay unit 55 Threshold signal generator 61 VLD controller 62 VLD detector

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Nihei Yasushi             1-3-3 Nakamagome, Ota-ku, Tokyo Stocks             Company Ricoh F-term (reference) 2C362 AA03 AA53 AA55 AA56 AA57                       AA61                 5C051 AA02 CA07 DB02 DB07 DC03                       DE03 DE17 FA01                 5C072 AA03 BA03 HA02 HB01 XA05                 5D119 AA23 AA24 HA03 HA12 HA44                 5F073 BA01 BA06 BA07 FA01 GA02                       GA03 GA12

Claims (14)

[Claims] 1. A bias current generating means for constantly supplying a minute bias current to a semiconductor laser, a threshold current generating means for supplying a threshold current to the semiconductor laser, and a drive for causing the semiconductor laser to emit light according to an input signal. Driving current generating means, the threshold current generating means generates a threshold current from the input signal, the threshold current generated by the threshold current generating means ON period, the drive current generating means emits light. The semiconductor laser drive circuit is a period longer than the light emission period and including the light emission period, and drives the semiconductor laser with a sum current of three currents of the bias current, the drive current, and the threshold current. . 2. The semiconductor laser drive circuit according to claim 1, wherein the current generated by the bias current generating means is several mA or less. 3. The semiconductor laser drive circuit according to claim 1, wherein the input signal is delayed by a predetermined time, and a drive for driving the semiconductor laser based on the delay signal delayed by the delay signal generation means. A threshold current generating means for generating a logical sum of the input signal and the delay signal, wherein the threshold current generated by the threshold current generating means is turned off before the drive current is turned off. A semiconductor laser drive circuit characterized in that 4. The semiconductor laser drive circuit according to claim 1, wherein the drive current generating means has an initialization means that operates when power is turned on or reset is released, and the initialization means emits light from the semiconductor laser. A semiconductor laser drive circuit, wherein the amount of light at time is set to a predetermined value. 5. The semiconductor laser drive circuit according to claim 4, wherein the initialization unit is a current or a voltage when the light quantity of the semiconductor laser is a predetermined value, and a current or voltage when the light quantity of the semiconductor laser is smaller than a predetermined value. Detects the difference from the current or voltage,
A semiconductor laser drive circuit, characterized in that the amount of light emitted from the semiconductor laser is set to a predetermined value. 6. The semiconductor laser drive circuit according to claim 4, wherein the initialization means is a current or a voltage when the light intensity of the semiconductor laser is a predetermined value and a current or a voltage when the semiconductor laser is caused to emit an offset light. A semiconductor laser drive circuit, wherein a difference from a voltage is detected and the amount of light emitted from the semiconductor laser is set to a predetermined value. 7. The semiconductor laser drive circuit according to claim 4, wherein the initialization means is a current or voltage when the light intensity of the semiconductor laser is a predetermined value, and the light intensity of the semiconductor laser is 1 / N of the predetermined value. A semiconductor laser driving circuit, wherein a difference from a current or a voltage in the case of (N is a natural number of 2 or more) is detected, and the light amount of the semiconductor laser at the time of light emission is set to a predetermined value. 8. The semiconductor laser drive circuit according to claim 5, wherein the initialization unit includes a timing generation unit, a detection unit that detects the difference, and a light amount when the semiconductor laser emits light. A current setting unit and a comparison unit that performs successive comparison so that the value detected by the detection unit based on the timing generated by the timing generation unit and the value set by the current setting unit correspond to each other. A semiconductor laser drive circuit characterized in that it is configured. 9. The semiconductor laser drive circuit according to claim 1, wherein a light receiving section for detecting a light output of the semiconductor laser and a light output of the semiconductor laser detected by the light receiving section are in proportion to each other. A semiconductor laser drive circuit comprising a current control means for controlling a current supplied to the semiconductor laser based on the received light receiving signal. 10. The semiconductor laser drive circuit according to claim 9, wherein the current control means compares the magnitude of the received light signal with a predetermined value to generate a control signal, and the control signal generates the threshold current. A semiconductor laser drive circuit characterized by controlling means. 11. The semiconductor laser drive circuit according to claim 10, wherein the current control unit samples the control signal when the drive current generation unit is in an ON state, and the threshold current is based on the sampled value. A semiconductor laser drive circuit characterized by controlling a generation means. 12. The semiconductor laser drive circuit according to claim 1, wherein a voltage of an output section that drives the semiconductor laser is detected, and a voltage of a power supply supplied to the semiconductor laser based on the detected value. A semiconductor laser drive circuit having means for controlling the semiconductor laser. 13. A semiconductor laser whose output is modulated by an image modulation signal, scanning means for scanning the rotating photoconductor with the light of the semiconductor laser, and an electrostatic latent image on the rotating photoconductor according to the image modulation signal. An image forming apparatus for forming an image, wherein the semiconductor laser is driven by the semiconductor laser drive circuit according to any one of claims 1 to 12. 14. The image forming apparatus according to claim 13, wherein a shading correction is performed by changing a full scale value of the drive current generating means in accordance with scanning by the scanning means. apparatus.