JP2008200872A - Semiconductor laser driving method for electrophotographic apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem that the laser light emitting time corresponding to one dot is changed by changing the response speed of a semiconductor laser caused by a change in temperature and change with time in an electrophotographic apparatus, to deteriorate the quality of printing. <P>SOLUTION: From the amount of adjustment of laser feeding electric current when APC (Auto Power Control) is executed which has been generally used as a driving method of the semiconductor laser, the laser ON/OFF time corresponding to one dot is adjusted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a semiconductor laser driving method for an electrophotographic apparatus.

  Semiconductor lasers are highly temperature dependent, and as the temperature rises, the threshold current increases and the slope efficiency decreases. As a means of obtaining a constant light output, the laser light output is detected by a photodiode, and this detection is performed. APC (Auto Power Control) control for controlling the laser drive current based on the result is used. When a semiconductor laser is modulated at high speed, a method is generally used in which a bias current less than a threshold current value is supplied in advance and modulation is performed by superimposing the modulation current on the bias current.

  With reference to FIG. 1, the change in the light output when the laser characteristic changes will be described. FIG. 1 shows an example of laser current-light output characteristics. When a laser having the characteristic IP-ref is driven with a constant bias current Ib and modulation current ILD_ref, the optical output decreases to P ′ when the laser characteristic changes to IP, but the modulation current is reduced by APC control. The light can be emitted with a desired light output P by increasing by ΔILD. However, the time required for the light output to emit light at the desired value P is shortened by the time required to supply the increase ΔILD of the modulation current.

  As the printing speed and resolution of the electrophotographic apparatus increase, and the one-dot emission time becomes shorter, the variation in the one-dot emission time due to the change in laser characteristics greatly affects the printing result. As a method for making the size of one dot uniform by driving control of a semiconductor laser, for example, there are Patent Document 1 and Patent Document 2.

JP 2003-154705 A JP 2002-211035 A

  In an electrophotographic apparatus, the laser emission time corresponding to one dot changes due to a change in the response speed of the semiconductor laser caused by a change in temperature and a change with time, and the print quality deteriorates.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a semiconductor laser driving method for an electrophotographic apparatus that eliminates the drawbacks of the prior art and can print high-definition images.

  To achieve the above object, the present invention provides a semiconductor laser, means for supplying current to the semiconductor laser, means for detecting the optical output of the semiconductor laser, means for adjusting the current supplied to the semiconductor laser, A means for turning on / off the current supply to the semiconductor laser according to the image data signal and a means for adjusting the on-time of the semiconductor laser are provided. The current supplied to the semiconductor laser is adjusted to control the optical output constant. In the semiconductor laser driving apparatus, the laser on time is adjusted based on the adjustment amount of the current supplied to the semiconductor laser.

  According to the present invention, the size of one dot is uniform, and high-definition images can be printed.

  The configuration of the present invention will be described with reference to FIG. The semiconductor laser 1 is supplied with a current necessary for laser emission by a bias current supply circuit 4 and a modulation current supply circuit 3 that can vary the amount of supply current. The optical output of the semiconductor laser 1 is detected by the photodiode 2, amplified by the optical output detector 5, and sent to the APC controller 6.

  The APC control unit 6 calculates the value of the modulation current ILD that makes the detection value equal to the reference value based on the difference between the light output detection value sent from the light output detection unit 5 and the light output reference value. The modulation current set value is sent to the supply circuit 3 and the ON time control unit 7. Here, the light output reference value is set to a value output by the light output detector 5 when the semiconductor laser 1 emits light with a desired light output.

  The modulation current supply circuit 3 increases or decreases the modulation current ILD supplied to the semiconductor laser 1 based on the modulation current set value sent from the APC control unit 6.

  The ON time control unit 7 calculates the correction amount of the laser ON time based on the difference between the modulation current setting value sent from the APC control unit 6 and the modulation current reference value corresponding to ILD_ref, and sends the correction amount to the ON time adjustment unit 8. In response, the ON time set value is sent. The ON time adjusting unit 8 outputs a signal M1 obtained by correcting the ON time of the laser modulation signal Mref according to the ON time setting. The ON / OFF timing of the modulation current supply circuit 3 is controlled according to the corrected laser modulation signal M1.

  The effect of the present invention will be described with reference to FIGS.

  FIG. 3 shows an optical output obtained when the laser modulation signal Mref before ON time correction, the laser modulation signal M1 corrected in the direction of extending the ON time, the laser supply current Iop, and the laser having the characteristic IP_ref are driven by the modulation signal Mref. The optical output P0 obtained when driving the laser of Pref and characteristic IP with the modulation signal Mref, and the time change of the optical output P1 obtained when driving the laser of characteristic IP with the modulation signal M1 are shown.

  First, the optical output Pref when the laser having the characteristic IP_ref is driven by the pre-correction modulation signal Mref will be described.

  The laser supply current Iop begins to increase at the timing (1) when the modulation signal Mref is turned ON, the laser emission starts at the timing (2) when Iop reaches the threshold value Ith_ref, and the optical output Pref begins to increase. When the laser supply current Iop continues to increase and reaches Ib + ILD_ref (timing (4)), the light output Pref starts to emit light at the desired light output P.

  When the laser supply current Iop starts decreasing at the timing (6) when the modulation signal Mref is turned OFF, the optical output Pref also starts decreasing without delay.

  That is, the time Tp_ref in which the laser emits light at the desired light output P is shorter than the ON time Ton_ref of the modulation signal Mref by the time Tth_ref and the time Tslp_ref. Here, the light emission time Tp_ref is an ideal light emission time obtained when there is no change in the laser characteristics, and the object of the present invention is to emit light at a desired light output P even when the characteristics of the laser change. Is equal to Tp_ref.

  Second, the optical output P0 when the laser having the characteristic IP is driven by the pre-correction modulation signal Mref will be described.

  The laser supply current Iop begins to increase at the timing (1) when the modulation signal M0 is turned ON, the laser emission starts at the timing (3) when Iop reaches the threshold value Ith, and the optical output P0 begins to increase. When the laser supply current Iop continues to rise and reaches Ib + ILD (timing (5)), the light output P0 starts to emit light at the desired light output P. When the laser supply current Iop starts decreasing at timing (6) when the modulation signal Mref is turned OFF, the optical output P0 also starts decreasing without delay.

When the light emission time Tp_ref when there is no laser characteristic change and the light emission time Tp0 when there is a laser characteristic change are compared, the light emission time Tp0 when there is a laser characteristic change by the time Td is shorter. Td is generated because the laser supply current required to emit light with the light output P increases due to the increase in threshold current value and the decrease in slope efficiency.
Td = (Tth + Tslp) − (Tth_ref + Tslp_ref)
= Tp_ref-Tp0 Formula (1)
= (Time required for laser supply current to increase by ΔILD) Equation (2)
It can be expressed as.

  Third, the optical output P1 when the laser having the characteristic IP is driven by the corrected modulation signal M1 will be described.

  The laser supply current Iop starts increasing at the timing (1) when the modulation signal M1 is turned ON, the laser emission is started at the timing (3) when Iop reaches the threshold value Ith, and the optical output P1 starts increasing.

  When the laser supply current Iop continues to increase and reaches Ib + ILD (timing (5)), the light output P1 starts to emit light at the desired light output P. When the laser supply current Iop starts decreasing at the timing (7) when the modulation signal M1 is turned OFF, the optical output P1 also starts decreasing without delay.

  As described above, the change in the light output P1 is the same as the change in the light output P0 when the laser is driven by the uncorrected modulation signal Mref except for the timing (7) at which the light output starts to decrease.

The time difference ΔTon between the timing (6) at which the optical output P0 starts to decrease and the timing (7) at which the optical output P1 starts to decrease is the ON time correction amount of the laser modulation signal. At this time, the time for the laser to emit light at the desired light output P is extended by ΔTon.
Tp1 = Tp0 + ΔTon (3)
It is. Moreover, from the equation (1),
Tp_ref = Tp0 + Td (4)
It is.

  Comparing equation (3) and equation (4), it can be seen that the actual light emission time Tp1 and the target light emission time Tp_ref are equal when ΔTon and Td are equal.

  Here, from equation (2), the laser ON time reduction amount Td depends on the increment ΔILD of the laser modulation current ILD by APC control. In the present invention, the correction amount ΔTon of the laser ON time is determined based on ΔILD. Therefore, the ON time correction amount ΔTon can be controlled to be equal to the ON time reduction amount Td.

  That is, it is possible to control the actual light emission time Tp1 with the laser characteristic change to be equal to the light emission time Tp_ref when there is no laser characteristic change.

  Up to this point, the case where the light emission time is shortened due to the increase in the laser threshold current value and the slope efficiency decrease has been described. Even in this case, it is possible to control the emission time to be equal to the emission time Tp_ref when there is no change in the laser characteristics by shortening the ON time of the laser modulation signal Mref according to the value of ΔILD.

  As described above, according to the present invention, since one dot emission time can be made constant regardless of the laser characteristics (threshold current value and slope efficiency), high-definition images can be printed.

It is explanatory drawing of the electric current-light output characteristic of a semiconductor laser. It is explanatory drawing which showed the Example of this invention. It is explanatory drawing which showed the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Photodiode 3 Modulation current supply circuit 4 Bias current supply circuit 5 Optical output detection part 6 APC control part 7 ON time control part 8 ON time adjustment part

Claims (3)

  A semiconductor laser; means for supplying current to the semiconductor laser; means for detecting the optical output of the semiconductor laser; means for adjusting the current supplied to the semiconductor laser; and In the semiconductor laser driving apparatus, which has means for turning on / off the current supply and means for adjusting the on-time of the semiconductor laser, and adjusts the current supplied to the semiconductor laser to control the optical output constant, the semiconductor laser A method for driving a semiconductor laser of an electrophotographic apparatus, comprising adjusting a laser on time based on an adjustment amount of a current supplied to the electrophotographic apparatus.   2. A multi-beam electrophotographic apparatus having a plurality of semiconductor lasers, wherein each semiconductor laser is driven by the method according to claim 1.   2. A multi-beam electrophotographic apparatus having a laser diode array, wherein each laser element is driven by the method according to claim 1.

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