JPH0883950A - Laser drive device - Google Patents

Abstract

PURPOSE: To reduce a fluctuation in the quantity of light due to an impedance change generated at the time of switching a semiconductor laser as a result of dispersion of threshold values of a semiconductor laser of a laser printer. CONSTITUTION: In addition to a switching circuit 21 driving a semiconductor laser LD according to an picture signal, a bias current part 24 feeding bias current is connected to a semiconductor laser. The light emission quantity of the semiconductor laser LD which is made to emit light in a natural light emission region is detected by a photo diode PD. An APC voltage control 25 for bias current computes a threshold value of the semiconductor laser in terms of changes in the light emission quantity of the semiconductor laser in response to a voltage increase in a drive current fed to the semiconductor laser. A computed value is processed to the level of a value which does not create a fog in image, and the value obtained in this processing is used to set the voltage of the bias current. The bias current part 24 feeds the set bias current to the semiconductor laser when the picture signal is off.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser driving device for a laser printer (laser beam printer).

[0002]

2. Description of the Related Art In a conventional laser printer, recording paper is fed from a recording paper storage unit such as a paper cassette in accordance with a printing command from an external device such as a computer, and a recording paper feeding timing and a video are fed by a synchronous feeding means such as registration rollers. Images are recorded by conveying them in synchronization with the timing of sending image information from the controller.

FIG. 11 shows the structure of a conventional laser printer, and FIG. 12 shows its operation. Laser printer 1101
Video controller unit 1128 of RDY (ready)
If you confirm that the signal is TRUE,
The NT (print) signal is set to TRUE.

The print controller 1126 starts driving the main motor 1123 and the polygon motor 1114 when the PRINT signal becomes TRUE. When the main motor 1123 is driven, the photosensitive drum 1117, the fixing roller 1109, and the paper discharge roller 1111 rotate. After that, the light amount control and the primary charging 1119, the developing device 1120,
The high voltage driving of the transfer charger 1121 is also sequentially performed.

When the rotation of the polygon motor 1114 is in a steady state, the print controller 1126 turns on the T ₁ second sheet feeding clutch 1124 in FIG. 12 to drive the sheet feeding roller 1105, and the recording sheet S to the registration roller pair 1106. To feed. Then, the print controller 1126 causes the recording paper S
VS at the timing when the tip of the roller reaches the registration roller pair
The REQ signal is sent to the video controller unit 1128, the paper feed roller clutch 1124 is turned off, and the driving of the paper feed roller 1105 is stopped.

When the video controller unit 1128 finishes developing the image information into a dot image and is ready to output the VDO signal, it confirms that the VSREQ signal is TRUE, sets the VSYNC signal to TRUE, and synchronizes this. Then, after T _V seconds, the output of the VDO signal as image data for one page is started.

At this time, the print controller 1126 makes the VS
The registration roller clutch 1125 is turned on after T ₃ seconds from the rise of the YNC signal to drive the registration roller pair. The registration roller pair 1106 is driven by the time T until the trailing edge of the recording sheet S passes through the registration roller pair 1106.
_It takes ₄ seconds. During this time, the print control unit 1126
Sends the HSYNC signal to the video controller unit 1128 at a predetermined timing synchronized with the laser scanning, and modulates the laser light scanning the photosensitive drum 1117 based on the VDO signal.

When printing the next page,
The PRINT signal is set to TRUE again after T ₅ seconds. After that, the same operation as the first page is performed.

Due to the operations of the print controller 1126 and the video controller 1128, the recording paper S
Is a paper feed roller 1105, a registration roller pair 1106,
Image recording unit 1108, fixing device 1109, paper discharge roller 11
The image is sequentially conveyed to 11 and an image is recorded.

Next, the conventional laser diode drive circuit 1113 of the above laser printer will be described. Although not shown, this laser drive circuit is composed of a constant current circuit, a switching circuit, and an amplifier circuit. The constant current circuit is a voltage-current converter, and supplies a current I ₁ according to the light amount signal from the control device. A circuit for switching this current I ₁ by the laser lighting signal is a switching circuit. The laser diode emits light according to the operation of the switching circuit. The amount of emitted light is taken out as a current value by a photodiode and converted into a voltage signal by a resistor. The light emission amount extracted as a voltage value is amplified by an amplifier circuit and becomes a light emission amount signal. The control device drives the laser diode by monitoring the light emission amount signal and raising the level of the light amount signal until the specified light amount is reached.

FIG. 13 shows a circuit configuration of the above switching circuit. The switching circuit 132 mainly includes a differential amplifier circuit (not shown) that generates modulation signals complementary to each other.
And bias circuit 133 for improving switching characteristics
And a first transistor Q1 and a first transistor Q1 having a laser diode LD136 as a load.
And a second transistor Q2 whose one ends are commonly connected. Then, as shown in FIG. 14, this differential amplifier circuit turns on / off the transistors Q1 and Q2 in response to a laser lighting signal 141 from a control device (not shown).
F is complementarily driven, and a current value 142 obtained by adding a current (laser drive current) according to the light amount signal 143 from the control device and a bias current is passed to the laser diode 136 to emit laser light.

[0012]

However, as is well known, since the threshold current of a semiconductor laser device varies greatly depending on the individual device and the temperature of the operating environment, as shown in FIG. The light intensity output from the element varies greatly depending on the individual elements even if the driving current is the same, and is extremely unstable with respect to temperature.
Therefore, the bias current suitable for each element cannot be driven, and the improvement of switching and the reduction of impedance change are insufficient, resulting in density difference and density unevenness.

Further, in the above-mentioned conventional example, when the laser drive circuit is driven, variations in the threshold value of the laser diode are not taken into consideration, and only a fixed bias current can be supplied to each laser diode. It was For this reason, the light quantity variation due to impedance change is not optimally suppressed in each laser diode, and a vertical and horizontal density difference and density unevenness are generated during image formation, and image quality is degraded.

In view of the above points, an object of the present invention is to
It is an object of the present invention to provide a laser driving device for a laser printer that reduces the fluctuation of the light amount due to the impedance change generated at the time of switching (starting up) the semiconductor laser and improves the quality of the image.

[0015]

In order to achieve the above object, the present invention provides a laser driving device of an image forming apparatus for forming an image using a semiconductor laser, wherein the semiconductor laser is made to emit light in a spontaneous emission region. Detecting means for detecting the amount of emitted light, and calculation for calculating the threshold value of the semiconductor laser based on the change in the amount of emitted light of the semiconductor laser detected by the detecting means with respect to the voltage increase of the drive current supplied to the semiconductor laser. Means, an arithmetic means for arithmetically processing the value calculated by the calculating means to a value that does not cause fogging of the image, and a bias current of a voltage set on the basis of the value obtained by the arithmetic means to the semiconductor laser. And a bias current supply unit for supplying the bias current.

Further, the present invention is preferably characterized in that the bias current supply means supplies the bias current to the semiconductor laser when the image signal applied to the semiconductor laser is turned off.

Further, the present invention is preferably characterized in that the calculating means detects a change in the light emission amount by using a differentiating circuit.

Further, the present invention is preferably characterized in that the calculating means detects a change in the light emission amount by using an amplifier circuit.

Further, the present invention is preferably characterized in that the bias current supply means has a constant voltage circuit composed of a differential circuit.

Further, the present invention is preferably characterized in that the bias current supply means has a constant voltage circuit composed of one transistor.

[0021]

According to the present invention, the optimum bias current can be supplied to each semiconductor laser when the VDO signal is OFF, and only the laser drive current can be supplied when the VDO signal is ON. As a result, it is possible to minimize the influence of the light quantity variation due to the impedance variation of the semiconductor laser, and it is possible to obtain high-quality image formation.

[0022]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) FIG. 1 shows the overall schematic configuration of Embodiment 1 of the present invention. In FIG. 1, reference numeral 11 is a laser light source as an exposing means for forming an image using the laser light of a semiconductor laser. Reference numeral 12 is a laser light detecting means for monitoring the output of the laser light via a photodiode. Thirteen
Is a laser light amount control means for controlling the laser light source 11 so that a predetermined light output level is obtained based on the output of the laser light detection means 12. 16 is a laser light amount control means 13
The timing of the laser light amount control means 13 is set such that the main scanning direction is executed in the area excluding the maximum recordable area and the sub-scanning direction is executed in the image guarantee areas at the front and rear ends in the recording medium. It is a timing control means for controlling. The laser light amount control means 13 has a light output level determination means 14 and a light output level setting means 15.

FIG. 2 shows details of the laser light amount control means 13 of FIG. In FIG. 2, reference numeral 21 indicates that the laser drive current is ON.
A switching circuit unit for turning on / off, a laser driving APC voltage control unit 22 for setting a current value to be supplied to the laser diode LD, a laser lighting signal 23 for controlling laser emission, and a bias current 24 for the laser diode LD. Is a bias current section for supplying a bias current, 25 is a bias current APC voltage control section for setting a bias current value, and 26 is a light quantity control section.

FIG. 3 shows the details of the laser light detection means 12 and the light output level control means 14 of FIG. This circuit has a photodiode (PD) 31, variable resistors 32 and 33, a differentiation circuit 35 including a capacitor and a differential amplifier. FIG.
Shows the waveform of the output voltage of the circuit of FIG.

FIG. 5 shows the operation procedure of the first embodiment of the present invention. First, the laser lighting signal 23 is input to the differential input section of the bias current section 24, and the bias current AP for forced lighting is input.
C is started (S51). The PD signal 42 at that time is passed through the differentiating circuit 35 in the light emission of the region where the natural light emission is surely performed (n-th step: see FIG. 4). The output signal 43 of the differentiating circuit 35 is recorded in the internal memory as a differential value at the time of spontaneous light emission (S52) and set (S53).

Next, the APC (automatic power control) voltage value 41
Is increased by 1 step and the differential value at the time of 1 step UP is recorded in the internal memory (S54). The differential value recorded this time is compared with the previously set differential value (S55), and if they are equal, the process returns to S54 to raise the value by one step. If the differential value this time is larger, the laser diode LD
The APC voltage value of the threshold current is recorded and set (S56). The recorded APC voltage value is arithmetically processed to a value that does not cause fogging in the image (S57), and the arithmetic processing result is used as the bias current APC.
The voltage value is set (S53).

By the above processing, as shown in the waveform of FIG. 6, it is possible to set the bias current according to the threshold characteristics of each laser diode and the ambient temperature, and VD
It is possible to supply the laser drive current 61 when the O signal (laser lighting signal) 23 is ON, and the optimum bias current 62 when it is OFF. As a result, as shown by the laser beam 63, the fluctuation of the light amount due to the impedance change of the laser diode LD is suppressed to the minimum limit.

As described above, according to this embodiment, the bias current is controlled to obtain the laser light 63 with a small variation in the light quantity, so that it is possible to form a high-quality, high-definition and high-quality image.

(Second Embodiment) Next, a second embodiment of the present invention will be described with reference to FIGS.

The entire structure of the second embodiment and the structure of the laser light amount control means are similar to those of the first embodiment shown in FIGS. FIG. 7 shows the configuration of the laser light detection means 12 and the light output level determination means 14 of the second embodiment, and FIG. 8 shows the waveform of the output voltage of the circuit of FIG. The circuit of FIG. 7 includes a photodiode (PD) 71, variable resistors 72 and 73, an operational amplifier 75.
And resistors 76 and 78.

The flowchart of FIG. 9 shows the second embodiment of the present invention.
The control procedure of is shown. First, the laser lighting signal 23 is input to the differential input section of the bias current section 24 to start the bias current APC by forced lighting (S91). In light emission in two areas (n, n + 1 step: see FIG. 8) where natural light emission is surely performed, the PD signal 82 at that time is passed through the amplifier circuit 75. Based on the levels of the two output signals 83 of the amplifier circuit 75, the light amount variation value for one step during spontaneous light emission is recorded in the internal memory (S92), and the setting is performed (S92).
93).

Next, the APC voltage value 81 is increased by one step, and the value of the light amount variation for one step UP is calculated and recorded (S94). The value recorded this time is compared with the previously set light intensity fluctuation value (S95), and if they are equal, S94
Then, if the light quantity fluctuation value at this time is larger, the time is recognized as the threshold value of the laser diode LD, and the APC voltage value of the threshold current is recorded (S96).

The recorded APC voltage value is arithmetically processed to a value that does not cause fogging of the image (S97), and the arithmetic processing result is set as the APC voltage value of the bias current (SS).
98).

By the above processing, the threshold current of each laser diode or the bias current according to the environmental temperature can be set as in the first embodiment, and when the laser lighting signal (VDO signal) is turned on. Laser drive current, O
An optimal bias current can be supplied during FF. As a result, the fluctuation of the light amount due to the impedance change of the laser diode LD can be suppressed to the minimum limit.

As described above, according to this embodiment, the bias current can be controlled in the same manner as in the first embodiment to obtain the laser light with a small fluctuation in the light quantity. Therefore, high quality, high definition and high quality image formation can be achieved. Is possible.

(Third Embodiment) FIG. 10 shows a circuit configuration of a third embodiment of the present invention. In FIG. 10, 101 is a switching circuit unit for turning on / off the laser drive current, and 102
Is a laser driving APC voltage control unit for setting a current value supplied to the laser diode, 103 is a laser lighting signal for controlling laser emission, 104 is a bias current circuit unit for supplying a bias current to the laser diode, and 105 is a bias The bias current APC voltage control unit for setting the current, 106 is a light amount control unit.

The difference between the third embodiment and the first and second embodiments is that
That is, the bias current section 04, and the input section of the laser lighting signal 103 is not a differential circuit but is composed of one transistor.

The operation principle is the same as in the first and second embodiments, the bias current APC is started by forced lighting, and the threshold value of the laser diode LD is different in the differential value (the differentiating circuit 35 of FIG. 3 is used. In this case) or the difference in the light amount fluctuation value for one step (when the amplifier circuit 75 of FIG. 7 is used), it is recognized and recorded as the APC voltage value of the threshold current. The recorded value is arithmetically processed to a value that does not cause fogging of the image, and is set as the APC voltage value of the bias current.

By the above processing, the same operation and effect as those of the first and second embodiments can be obtained.

[0041]

As described above, according to the present invention, V
It becomes possible to drive the optimum bias current when the DO signal is ON and only the laser drive current when the VDO signal is OFF, which leads to a change in impedance of the laser diode and a reduction in relaxation oscillation. This leads to a decrease in light amount fluctuation when the laser diode is driven, and has an effect that a vertical or horizontal density difference or density unevenness is small, and a high-quality, high-definition image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an overall configuration of a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing details of laser light amount control means according to the first embodiment of the present invention.

FIG. 3 is a circuit diagram showing details of a laser light detection means and a light output level determination means according to the first embodiment of the present invention.

4 is a timing chart showing a waveform of an output voltage in the circuit of FIG.

FIG. 5 is a flowchart showing a control procedure according to the first embodiment of the present invention.

FIG. 6 is a waveform diagram showing the control result of the first embodiment of the present invention.

FIG. 7 is a circuit diagram showing details of laser detection means and optical output level determination means according to a second embodiment of the present invention.

8 is a timing chart showing a waveform of an output voltage in the circuit of FIG.

FIG. 9 is a flowchart showing a control procedure according to the second embodiment of the present invention.

FIG. 10 is a circuit diagram showing details of a laser light amount control procedure according to the third embodiment of the present invention.

FIG. 11 is a cross-sectional view showing an internal configuration of a conventional laser printer.

12 is a timing chart showing the operation of the laser printer of FIG.

FIG. 13 is a circuit diagram showing a configuration of a conventional switching circuit.

FIG. 14 is a waveform diagram showing a control result in a conventional example.

FIG. 15 is a graph showing an example of current-light intensity characteristics of a semiconductor laser device.

[Explanation of symbols]

11 Laser Light Source 12 Laser Light Detecting Means 13 Laser Light Amount Controlling Means 14 Optical Output Level Determining Means 15 Optical Output Level Setting Means 16 Timing Controlling Means 17 Photosensitive Body 21, 101 Switching Circuits 22, 102 Laser Driving APC Voltage Controlling Means 23, 103 Laser Lighting Signal 24, 104 Bias Current Section 25, 105 Bias Current APC Voltage Control Section 26, 106 Light Quantity Control Section 35 Differentiation Circuit 75 Amplification Circuit 1101 Laser Printer 1113 Laser Diode Drive Circuit 1114 Polygon Motor 1117 Photosensitive Drum 1126 Print Control Section 1127 signal 1128 video controller 1129 general-purpose interface (Centro, RS232
C etc.) 1130 External device

Claims (6)

[Claims] 1. A laser driving device of an image forming apparatus for forming an image using a semiconductor laser, the detecting means detecting a light emission amount of the semiconductor laser emitted in a spontaneous light emission region, and supplying to the semiconductor laser. Calculating means for calculating the threshold value of the semiconductor laser based on the change in the light emission amount of the semiconductor laser detected by the detecting means with respect to the increase of the driving current voltage, and the value calculated by the calculating means is applied to the image. And a bias current supply means for supplying to the semiconductor laser a bias current having a voltage set on the basis of the value obtained by the calculation means. Laser drive device. 2. The laser drive device according to claim 1, wherein the bias current supply means supplies the bias current to the semiconductor laser when an image signal applied to the semiconductor laser is turned off. 3. The laser driving device according to claim 1, wherein the calculating means detects a change in the light emission amount by using a differentiating circuit. 4. The laser driving device according to claim 1, wherein the calculating unit detects a change in the light emission amount by using an amplifier circuit. 5. The laser driving device according to claim 1, wherein the bias current supply means has a constant voltage circuit composed of a differential circuit. 6. The laser drive device according to claim 1, wherein the bias current supply means has a constant voltage circuit composed of one transistor.