JP5016805B2 - Semiconductor laser drive device - Google Patents

Description

  The present invention relates to a semiconductor laser driving apparatus in which current control (hereinafter referred to as APC) is performed so that the amount of light emitted during driving of a semiconductor laser becomes a preset value. In particular, the present invention relates to a semiconductor laser driving apparatus that enables high-speed driving in a laser driving system that controls PWM modulation and power modulation in combination.

  As a conventional light amount control method, there are a PWM method for controlling the laser light emission pulse width in a dot and a power modulation method for controlling the laser power. When the image clock speed is increased to cope with high resolution or the like, in the PWM method, the laser light amount is insufficient in the low density portion (the region having a narrow pulse width), and the linearity of the laser light amount becomes poor and the gradation is deteriorated. On the other hand, in the light quantity control method combining PWM and power modulation, the linearity defect can be improved by optimizing the laser power in the low density portion where the gradation is deteriorated. FIG. 1 shows a state in which the amount of light is insufficient in the low density portion due to the increase in speed, and laser light emission when light amount addition is performed by power modulation. FIG. 2 shows the linearity in comparison. In the light quantity control method that combines power modulation with PWM, the light quantity is added in a narrow pulse width region where the light quantity is insufficient, and linearity degradation can be improved. For this reason, a light quantity control system that combines PWM and power modulation is used as a means for increasing the speed of the image clock.

FIG. 3 is a circuit configuration diagram of a light quantity control method combining PWM and power modulation, as disclosed in Patent Document 1. In FIG. In the configuration of FIG. 3, switching circuits 307 and 309 are connected to current sources 308 and 310, respectively. Assuming that the current generated by the current source 308 is I ₁ and the current generated by the current source 310 is I ₂ , the laser driving current causes the switching circuits 307 and 309 to operate independently, whereby I ₁ , I ₂ , and I 3 patterns of ₁ + I ₂ can be made. Vin1 and Vin2 instructing driving of each switching circuit are logical sums of the signals S1 and S2 selectively output for each dot from the power switching circuit 302 and the PWM signal. For example, if the S1 signal is Hi, the PWM signal passes through the AND gate 303 as it is to drive the switching circuit 307, and the current I ₁ flows to the laser 311 following the PWM signal. If S1 is Low, the switching circuit 307 is not PWM driven and I ₁ does not flow to the laser 311. By switching S1 and S2 for each dot, the drive current value (I ₁ , I ₂ , or I ₁ + I ₂ ) to be used for each dot can be selected and PWM driven. The PWM signal and the S1 and S2 signals are generated by the PWM circuit 301 and the power switching circuit 302 based on the input Video signal.

  Patent Document 2 discloses a semiconductor laser driving circuit having a switching circuit that performs switching for each of a plurality of constant current sources and a constant current circuit, selectively driving the constant current source, and performing intensity modulation. Here, a plurality of signal lines connecting each switching circuit and the control unit have substantially the same length, and the signal lines are provided with a load adjustment circuit, thereby adjusting the floating capacitance of the signal lines and switching timing. A technique for reducing the deviation is shown.

Japanese Patent Laid-Open No. 4-77050 Japanese Patent Laid-Open No. 10-173270

  However, in the semiconductor laser driving circuit having the above-described configuration, the input signals to the plurality of current switches driven following the PWM are likely to cause a phase shift, and this phase shift causes distortion in the light emission pulse, resulting in deterioration of the image. There is a problem that it ends up. When the PWM signal generated from the PWM generation unit is transmitted to the input unit of each current switch means via the logic gate as in the configuration of FIG. 3, a propagation delay occurs when the PWM signal passes through the logic gate. The propagation delay time of the logic gate varies depending on the element, and the signals Vin1 and Vin2 after passing through the selector circuit are likely to cause a phase shift due to the propagation delay time difference.

FIG. 4 shows the state of the input signal shift caused by the delay time difference and the distortion of the light emission pulse. t ₁ and t ₂ indicate the delay time with respect to the PWM signal, and the light emission pulse is distorted due to the difference in the delay time. In particular, the influence of an input signal shift is large in a thin pulse, and a laser power shortage is likely to occur. For this reason, in the conventional method, when power modulation is used in a higher speed region, an image defect occurs due to distortion of the light emission pulse. Further, the phase shift of the input signal is caused not only by the difference in the propagation delay time but also by the difference in the stray capacitance of the signal line. In order to completely eliminate the phase shift, special consideration is required for the routing of signal lines, the thickness and length of the lines, and adjustment becomes difficult as the laser drive speeds up, resulting in an increase in cost.

  The object of the present invention is to eliminate the distortion of the light emission pulse due to the phase shift of the input signal to the current switch means, and to drive the semiconductor laser by combining power modulation and PWM even when the laser is driven at a higher speed. Is to provide a device.

The apparatus of the present invention configured purpose to achieve a plurality of current sources currents each outputs is controlled to be a predetermined current value, the switch group connected to each of the current source, And a switching means connected between the switch group and the semiconductor laser, and the switching means and the switch group switch output currents of the plurality of current sources to supply a driving pulse to the semiconductor laser. In the driving device, the switch group includes a plurality of current sources commonly connected at one end and a plurality of current switches commonly connected at one end, and the switch means includes the switch unit, A current switch connected between a common connection end of a plurality of current switches and the semiconductor laser, and an output current from the switch group is the plurality of current switches. Et current is summed at the common connection end of said plurality of current switches, current switches each included in the switch group is selectively each of the plurality of current sources are driven by a first signal based on the image information and off to set the output current from the plurality of current sources, said switch means, connected together added at end together the said output of the second signal of the plurality of current switches is driven by based on the image information said drive pulse by turning on and off the current is set to a pulse width determined by said second signal is supplied to the semiconductor laser, the first signal, said a portion corresponding to a predetermined pixel of the first signal first Two signals are input to the plurality of current switches in synchronization with the second signal so that a portion corresponding to the predetermined pixel is synchronized, and the first signal is input to the plurality of currents. The period for turning on, the second signal is characterized to include a period for supplying the drive pulse to the switching means.

  In the semiconductor laser driving apparatus having the above-described configuration, the current switches that generate current pulses for laser driving are switched by the same PWM signal input, and no input signal deviation occurs between the current switches. For this reason, when superimposing the pulse current, the waveform distortion caused by the input signal deviation is eliminated, and the light amount control combining the power modulation and the PWM can be stably performed even during high-speed driving.

  In addition, by controlling the phase and pulse width of the control signal instructing switching of the laser driving current for each image information, it becomes possible to control the switching of the laser driving current so that it always occurs when the laser is OFF. Waveform distortion caused by switching of the laser drive current can be prevented.

  As described above, in the semiconductor laser driving device according to the present invention, it is possible to stably perform light amount control combining power modulation and PWM in a higher speed driving region. For this reason, it is possible to improve the gradation and the number of gradations against the gradation deterioration caused by the high-speed laser drive due to high resolution or the like.

(Embodiment 1)
FIG. 5 is a block diagram showing Embodiment 1 of the semiconductor laser driving circuit of the present invention. The semiconductor laser drive circuit of this embodiment is provided in an image forming apparatus that scans a semiconductor laser to form an image, and controls the light amount of the semiconductor laser by combining PWM and power modulation.

In FIG. 5, the semiconductor laser 506 is driven to emit light by a bias current I _B and a switching current I _SW . Bias current I _B is the output current of the bias current source 513, the current value is set by LED emission region. In the present invention, the laser beam intensity is modulated by switching the current value of the switching current _ISW for each pixel. Note that the current value of each current source is controlled by the APC so as to have a light amount value set at the time of laser emission.

In the configuration of this embodiment, the on-off of the current switch 510, 511, and 512 is a plurality of current switches, for switching the current value of the switching current I _SW for each pixel. The laser light emission pulse width is controlled by the current switch 520 connected between the common connection ends of the current switches 510, 511, and 512 and the semiconductor laser 506. Details of the operation of each switch will be described below. The current sources 507, 508, and 509, which are a plurality of current sources, are current-controlled so that the amount of laser light at the time of output becomes a preset value. The current switches 510, 511, 512 corresponding to the current sources 507, 508, 509 are driven by control signals output from the power modulation control unit 502, and output currents I _SW0 , I _SW1 , _Turn I _{SW2 on} and off. The power modulation control unit 502 generates a control signal based on the image information in each pixel and drives each current switch independently.

The output currents of the current switches 510, 511 and 512 are added at the common connection ends of the current switches 510, 511 and 512, and the added currents are switched by a current switch 520 driven by a PWM signal. The PWM signal is generated in the PWM signal generation unit 503 based on the same image information input to the power modulation control unit 502 and generates a pulse instructing driving of the current switch 520. The output pulse current current obtained by superimposing (switching current I _SW) to bias current I _B from the current switch 520, the semiconductor laser 505 emits light.

  Switching of the laser drive current is controlled so as to always be performed when the laser is OFF. In this system in which the laser emission intensity is modulated by switching the drive current, a stable light quantity cannot be obtained if the laser drive current is switched during laser emission. For this reason, it is necessary to switch the laser drive current before and after laser emission so that the laser drive current is stabilized at the set current value during laser emission. In the present embodiment, control is performed so that the laser drive current is always switched when the laser is OFF by controlling the pulse width and phase of the control signal to set values for the image information. FIG. 6 shows an example of the input timing of the PWM signal and the control signal. In the example of FIG. 6, synchronization is performed so that the center of the PWM signal and the control signal comes to the center of the pixel, and the pulse width is controlled so that the control signal has a wider pulse than the PWM signal. ing. When the PWM is always grown from the center of the pixel, the control signal may be switched for each pixel. When the PWM is grown from the end of the pixel, the control signal controls the phase and pulse width so that the control signal protrudes from the one pixel region to the adjacent pixel region for the switching time required for current switching.

  The plurality of current sources and the plurality of current switches are not limited to the three illustrated. The present invention can also be applied to a circuit configuration in which three or more series circuits of current sources and current switches are connected in parallel.

In the present embodiment, a configuration having three current sources is described. In this case, the laser drive current can have 23 ³ patterns by combining them as I _SW0 , I _SW0 + I _SW1 , I _SW0 + I _SW2 . When N stages of current sources and current switches are provided, 2 ^N laser drive current patterns can be obtained, and the number of stages can be increased as necessary to increase the current value pattern of the laser drive current. it can.

  In this embodiment, since the added current is switched by the current switch 520, a phase shift of the input signal does not occur and a light emission failure caused by the phase shift does not occur compared to a method in which power modulation is performed by adding pulse currents. Has the advantage of not happening. In addition, since it can be stably driven by a laser without being affected so much by the capacity of the signal line, the routing, and the variation in the switching speed of each current switch, it is relatively easy to drive at a higher speed. . Compared with the second embodiment, the number of current switches can be reduced, so that the cost can be reduced.

(Embodiment 2)
FIG. 7 is a block diagram showing Embodiment 2 of the semiconductor laser driving circuit of the present invention. As in the first embodiment, the semiconductor laser drive circuit according to the present embodiment performs light amount control of the semiconductor laser by combining PWM and power modulation.

7, similarly to Embodiment 1, the semiconductor laser 706 emit light driven by the bias current I _B and the switching current I _SW. Bias current I _B is the output current of the bias current source 713, the current value is set by LED emission region. In the present invention, the laser beam intensity is modulated by switching the current value of the switching current _ISW for each pixel. Note that the current value of each current source is controlled by the APC so as to have a light amount value set at the time of laser emission.

In the configuration of this embodiment, the on-off of the current switch 710,711,712 a first plurality of current switches, for switching the current value of the switching current I _SW for each pixel. In the configuration of the present embodiment, the laser emission pulse width is controlled by the current switches 720, 721, and 722 which are the second plurality of current switches. Details of the operation of each switch will be described below. Note that the power modulation control unit and the PWM generation unit have the same functions and operations as those described in the first embodiment, and a detailed description thereof is omitted here. The current sources 507, 508, and 509, which are a plurality of current sources, are current-controlled so that the amount of laser light at the time of output becomes a preset value. The current switches 510, 511, 512 corresponding to the current sources 507, 508, 509 are driven by control signals output from the power modulation control unit 502, and output currents I _SW0 , I _SW1 , _Turn I _{SW2 on} and off. Here, it is assumed that the above-described control signal is generated for each pixel based on the image information.

The lower ends of the current switches 720, 721, 722 are commonly connected, and the common connection end is connected to the semiconductor laser 706. The current switches 720, 721, and 722 instructed to be driven by the PWM signal turn on / off the output currents of the current switches 710, 711, and 712, and follow the PWM from the respective output currents at the common connection ends. Output current pulses. Output pulse currents from the current switches 720, 721, and 722 are added together at the common connection end, and the added pulse current (switching current I _SW ) is superimposed on the bias current I _B to drive the semiconductor laser 705. Make it emit light. The PWM signal is generated by the PWM signal generation unit 703 from the image information, and is input from the output unit of the PWM signal generation unit 703 to the signal input unit of the current switches 720, 721, and 722 via the same signal line. Switch at the same timing.

  The plurality of current sources, the first plurality of current switches, and the second plurality of current switches are not limited to the three illustrated. The present invention can also be applied to a circuit configuration in which three or more series circuits of a current source and two current switches are connected in parallel.

  In this embodiment, by having a current switch that drives following the PWM corresponding to each current source, the current flowing through each current switch is constant even if the laser drive current is changed for each pixel. Compared to the first embodiment, less current flows through each current switch. Each current switch independently generates a pulse current and superimposes it to generate a pulse current. For this reason, even when the switching speed of the current switch is slow and the current does not rise up to the set value, the current can be raised to the set value without increasing the pulse width. However, laser light emission is possible. For this reason, in a high resolution machine, a sharper latent image can be formed and the resolution can be increased.

  Since the current switch for performing the PWM drive is connected to the signal output unit of the PWM signal generation unit through a common signal line, the laser emission pulse is not distorted due to the phase shift of the input signal. However, the added current pulses may be distorted due to differences in switching speed between the current switches. Therefore, it is preferable to design the current switches 720, 721, and 722 as close to the semiconductor laser as possible and to minimize the wiring to the semiconductor laser. Further, it is preferable to select the elements so that the delay time at the time of switching and the individual difference of the switching speed are as small as possible between the elements of each current switch that performs PWM driving. The speed of each current switch may be adjusted by a speed-up capacitor or the like.

It is a figure which shows the mode of the laser light emission which generate | occur | produces in the conventional PWM system, and the mode of the laser light emission improved by this invention. It is a figure which shows the mode of the linearity defect which generate | occur | produces in the conventional PWM system, and the linearity improvement by this invention. FIG. 11 is a configuration diagram of a semiconductor laser driving circuit that performs light amount control combining PWM and power modulation disclosed in Patent Document 1. It is a figure which shows the mode of the input signal shift | offset | difference which generate | occur | produces with the conventional semiconductor laser drive circuit, and the laser emission defect. It is a block diagram which shows Embodiment 1 of the semiconductor laser drive circuit of this invention. It is a figure which shows the mode of the control signal in Embodiment 1 of this invention, a PWM signal, and laser emission. It is a block diagram which shows Embodiment 2 of the semiconductor laser drive circuit of this invention.

Explanation of symbols

301 PWM circuit 302 Power switching circuits 303 and 304 AND elements 305 and 306 Signal lines 307 and 309 Current switches 308 and 310 Current sources 311 Semiconductor lasers 501 and 701 Controllers 502 and 702 Power modulation controllers 503 and 703 PWM signal generator 504 , 704 APC circuit 505, 705 Photodiode 506, 706 Semiconductor laser 507, 508, 509, 707, 708, 709 Current source 510, 511, 512, 710, 711, 712, 720, 721, 722 Current switch 513, 713 Bias Current source

Claims (3)

A plurality of current sources currents each outputs is controlled to be a predetermined current value, each connected switch group of the current source, and the switches and connected to switch means between said semiconductor laser In the semiconductor laser driving apparatus that supplies output pulses to the semiconductor laser by switching output currents of the plurality of current sources by the switch means and the switch group,
The switch group includes a plurality of current sources commonly connected at one end and a plurality of current switches commonly connected at one end, and the switch means includes the plurality of current switches. Current switch connected between the common connection end and the semiconductor laser, the output current from the switch group, the current from the plurality of current switches are added together at the common connection end of the plurality of current switches,
Each current switch included in the switch group is set said output current from said plurality of current sources are selectively turned on and off each of the plurality of current sources are driven by a first signal based on the image information ,
It said switch means, connected together added at end combined pulse width determined the drive pulse by turning on and off the output current in the second signal of the second signal of the plurality of current switches is driven by based on the image information Set to be supplied to the semiconductor laser ,
The plurality of current switches are synchronized with the second signal so that a portion of the first signal corresponding to the predetermined pixel and a portion of the second signal corresponding to the predetermined pixel are synchronized with each other. Entered in
The period during which the first signal turns on the plurality of current sources includes a period during which the second signal supplies the driving pulse to the switch means.
A semiconductor laser driving device. A plurality of current sources each controlled to have a predetermined current value; a switch group connected to each of the current sources; and a switch means connected between the switch group and the semiconductor laser. A semiconductor laser driving apparatus for supplying drive pulses to the semiconductor laser by switching output currents of the plurality of current sources by the switch means and the switch group;
The switch group includes the plurality of current sources each having one end commonly connected, and a plurality of first current switches each having one end connected to the other end of the plurality of current sources. Is composed of a plurality of second current switches each having one end commonly connected, the common connection end of the plurality of second current switches being connected to the semiconductor laser, and the other end of the plurality of second current switches Is connected to the other ends of the plurality of first current switches, and currents from the plurality of second current switches are added together at the common connection end,
Each first current switch included in the switch group is driven by a first signal based on image information to selectively turn on / off each of the plurality of current sources to set output currents from the plurality of current sources. And
The switch means is driven by a second signal to turn on and off output currents from the plurality of current sources to set the drive pulse to a pulse width determined by the first signal and supply the semiconductor laser to the semiconductor laser;
The second signal is generated based on the image information by a second signal generation unit, and is input to the plurality of second current switches from the second signal generation unit via the same signal line, Two current switches are switched at the same timing,
The plurality of current switches are synchronized with the second signal so that a portion of the first signal corresponding to the predetermined pixel and a portion of the second signal corresponding to the predetermined pixel are synchronized with each other. Entered in
The period during which the first signal turns on the plurality of current sources includes a period during which the second signal supplies the driving pulse to the switch means.
A semiconductor laser driving device. 3. The semiconductor laser driving device according to claim 1 , wherein a phase and a pulse width of the second signal are set for each of the image information .

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