JP2003258371A - Laser diode driving unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a laser diode driving unit which is capable of providing a high-quality image with a little unevenness in density by reducing a fluctuation in light quantity of an LD due to a temperature, in an imaging device having a multibeam laser. <P>SOLUTION: When conducting an automatic current control, the LD driving unit 1 of the imaging device using a multibeam laser controls an order in which driving currents are output from an LD driver 14 to make LDs 171-174 emit light in a prescribed order. Consequently, the automatic current control can be done with thermal influences from adjacent light emitting points being suppressed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode driving device for controlling the light amount of a plurality of laser beams in an image forming apparatus having a plurality of laser beams.

[0002]

2. Description of the Related Art In recent years, image forming apparatuses such as laser beam printers are provided with a plurality of light emitting points in order to obtain high speed output and high resolution without increasing the number of revolutions of the polygon mirror or the number of faces of the polygon mirror. Multi-beam lasers are used. In such an image forming apparatus, a laser diode (hereinafter, referred to as “LD”) used as a light emitting source.
Has a characteristic that the threshold current increases due to temperature rise or deterioration.

FIG. 6 is a diagram showing a drive current-optical output characteristic curve of an LD. As shown in FIG. 6, when the ambient temperature of the LD is changed from T _A to T _B, because the threshold current varies in I _thB from I _thA, in order to always obtain a constant amount of light, current corresponding to this variation Need to control. In other words, in constant current operation, the light output fluctuates and problems such as a constant print density cannot be obtained.Therefore, the amount of light output from the semiconductor laser is monitored by a photodiode, and the received light output voltage is used as a reference voltage. In comparison, the automatic current control (APC: Automatic Po
An LD drive device that performs a wer control circuit) is required.

Here, since the multi-beam laser has different drive current-light output characteristics for each light emitting point, it is necessary to perform automatic current control for each light emitting point. Since the control is performed by a single photodiode, it is necessary to make a plurality of light emitting points emit light independently. Note that this automatic current control is performed in a non-image forming period in which image formation is not performed, for example, a period in which image formation for one page ends and a transition to the next page occurs, or a blank period between one main scan and one main scan. To be executed.

[0005]

By the way, when the light emitting points are made to emit light independently, since the adjacent light emitting points are not thermally insulated from each other, the heat generated by the light emission of one light emitting point is generated by the light emission of the other light emitting point. Raise the temperature. In particular, during the automatic current control, the time for lighting the light emitting points is longer than the image forming period, so that the light emitting points are likely to be thermally affected by the adjacent light emitting points. Therefore, if the automatic current control is performed while being thermally affected by the adjacent light emitting points, there is a problem in that the current cannot be controlled accurately and the light amount cannot be constantly controlled during the image forming period.

In view of this, Japanese Patent Laid-Open No. 2000-190563 calculates the amount of heat generated by a plurality of light sources, predicts the temperature changes of the plurality of light sources based on the amount of heat, and calculates a plurality of temperature changes based on the temperature changes. A technique for correcting the drive current so that the light amount is constant is disclosed. However, according to the present invention, since many processing steps are included in the automatic current control, there are problems that the cost is increased and the processing speed is decreased due to the higher function of the apparatus.

An object of the present invention is to provide a laser diode drive device in an image forming apparatus equipped with a multi-beam laser, which is capable of providing a high-quality image with less unevenness in image density by reducing fluctuations in the amount of light due to the temperature of the LD. That is.

[0008]

In order to solve the above-mentioned problems, the invention according to claim 1 is to provide a plurality of laser diodes arranged in parallel, detection means for respectively monitoring the output light amounts of the plurality of laser diodes, and Automatic current control means for performing current feedback control so that the light emission amounts of the plurality of laser diodes are constant according to the output light amount detected by the detection means, and the plurality of the plurality of current control means according to the control of the automatic current control means. In a laser diode drive device including a current applying unit that applies a current to a laser diode, when the current applying unit applies a current independently to the plurality of laser diodes, among the plurality of parallel laser diodes, Timing control to control the timing to apply current in the order that adjacent laser diodes are not continuous It is characterized in that it comprises a stage.

Therefore, when performing automatic current control on a plurality of parallel laser diodes, the adjacent laser diodes can be made to emit light in a non-continuous order, so that the thermal influence of the light emission of the laser diodes on the adjacent laser diodes is caused. It has no effect. That is, as the temperature around the laser diode that emitted light rises, the temperature of other laser diodes rises,
It is possible to prevent the output light amount of other laser diodes from decreasing. Therefore, the automatic current control can be performed in a state where the light output characteristics are constant, so that it is possible to prevent the density unevenness by keeping the output light amount constant in the image forming period, and to provide a high-quality image. it can.

According to a second aspect of the invention, in the first aspect of the invention, when the timing control means applies a current to the four parallel laser diodes by the current applying means, two timing diodes are not located at the ends. It is characterized in that either one of the laser diodes is selected first to control the timing of applying the current.

Here, for example, when four parallel laser diodes are arranged in order from the left, the second from the left,
4th, 1st, 3rd order, or 3rd from left, 1
By applying the current in the order of the second, fourth, and second, it is possible to prevent the adjacent laser diodes from continuously emitting light. Since the example described above is an example, the arrangement of the four laser diodes in parallel is not limited to this, and may be a configuration in which various laser diodes are arranged in parallel.

Therefore, in a laser diode driving device provided with four laser diodes arranged in parallel, it is possible to prevent adjacent laser diodes from continuously emitting light in accordance with the number of laser diodes. In addition, it is possible to prevent the output light amount from decreasing due to the heat generation of other laser diodes.

The invention according to claim 3 is the invention according to claim 1 or 2.
In the invention described above, the current applying unit includes a bias current applying unit that applies a current to each of the plurality of laser diodes in the vicinity of a threshold current or less emitted by the plurality of laser diodes in parallel, and the timing control unit is When applying a current to the laser diode by the bias current applying means, the timing of applying the current is controlled in an order in which adjacent laser diodes among the plurality of parallel laser diodes are not continuous. .

Therefore, when only the bias current is applied to the laser diode and the threshold current of the laser diode is measured, the thermal effect caused by applying the bias current to the adjacent laser diode can be reduced, An accurate threshold current can be measured for each laser diode. As a result, a bias current corresponding to the variation in the characteristics of the laser diode can be applied to each laser diode, and the laser diode can efficiently emit light and can emit light with a more accurate output light amount. It is possible to secure the print density and provide good image quality.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. However, the scope of the invention is not limited to the illustrated example. Here, the laser diode drive device according to the present invention and the LD drive device 1 according to the present embodiment
The correspondence between each component in and is specified. That is, the laser diode of the laser diode driving device of the present invention corresponds to the LD array 17 of the LD driving device 1 in the present embodiment, and the detecting means corresponds to the PD 16.
Further, the automatic current control means of the present invention is the same as the A of the present embodiment.
It corresponds to the PC circuit 15, and the current applying means corresponds to the LD driver 14. Further, the timing control means of the present invention corresponds to the CPU 11 of the present embodiment, and the bias current applying means corresponds to the bias current supply circuit 142.

The configuration will be described below. FIG. 1 is a block diagram showing a main configuration of an LD drive device 1 according to this embodiment. The LD array 17 shown in FIG. 1 has a configuration including four LDs 171 to 174, as an example.
The number of LDs is not limited to this. As shown in FIG.
The drive device 1 is a CPU (Central Processing Unit) 1
1, PWM (Pulse Width Modulation) 12, PM (Po
wer Modulation) 13, LD (Laser Diode) driver 14, APC circuit 15, PD (Photo Diode) 16, L
It is configured to include the D array 17 and the like.

The CPU 11 has a built-in ROM (Read Only Me).
The various system programs stored in the memory are read and executed to drive and control the respective parts of the LD drive device 1.
Specifically, the CPU 11 sends to the LD driver 14 a control signal S1 for controlling the light emitting order of the LDs 171 to 174 so that the adjacent light emitting points do not continuously emit light during the non-image forming period, that is, during the automatic current control. Output. Also, C
The PU 11 converts the image data input for each line into image data of 4 lines in the image forming period,
It outputs to PMW12 with an image clock signal.

The PWM 12 performs pulse width modulation on the image data input from the CPU 11 and outputs it to the PM 13 together with the image clock signal. The PM 13 power-modulates the image data input from the PWM 12 and outputs it to the LD driver 14 together with the image clock signal.

The LD driver 14 is based on a control signal input from the CPU 11 during the non-image forming period.
The individual light emitting points are turned on in a predetermined order, and the feedback control (feedback control) input from the APC circuit 15 corrects the light amount fluctuations of the LDs 171 to 174. In addition, the LD driver 14 is configured to use the PM driver during the image forming period.
Based on the image data input from 13 and the image clock signal, the LDs 171 to 174 are driven to form an image.

The APC circuit 15 converts the monitor current input from the PD 16 for detecting the light emission amounts of the LDs 171 to 174 into a corresponding voltage, compares the monitor current with the reference voltage, and compares the voltage corresponding to the monitor current with the reference voltage. L so that and match
Feedback control is performed on the D driver 14, and LDs 171 to 174 are performed.
The light emission amount of is controlled to be a predetermined light amount.

Now, referring to FIG. 2, the LD driver 1
4 and the configuration of the APC circuit 15 will be described in more detail.
FIG. 2 shows the LD driver 14, the APC circuit 15, and the PD1.
6 is a block diagram showing a circuit configuration of the LD array 17. As shown in FIG. 2, the LD driver 14 includes a signal power supply circuit 141 and a bias current supply circuit 142, and the APC circuit 15 has a current-voltage conversion circuit 1.
51, a comparator 152, and a reference voltage supply circuit 153.

The signal power supply circuit 141 generates a drive current for causing the LDs 171 to 174 to emit light with a predetermined light emission amount based on a control signal input from the CPU 11 in the non-image forming period, and the LDs 171 to 174. Apply to. Here, the control signal S1 input from the CPU 11
Is an LD so that adjacent light emitting points do not emit light continuously.
The light emission order of 171 to 174 is controlled, and the signal power supply circuit 141 controls the LD 17 according to the control signal S1.
A drive current is applied to 1-174. Further, the signal power supply circuit 141 applies current based on the feedback control from the comparator 152 so as to keep the output light amount of the LDs 171 to 174 constant.

The bias current supply circuit 141 includes an LD 17
The threshold current I _th with the light emission start currents of 1 to 174 as the threshold
A current is applied to the LDs 171 to 174 near the following. That is, in the image forming period, by applying bias current to each light emitting point, it is possible to emit light with a more accurate light amount, and it is possible to reduce fluctuations in the light amount due to heat generation at other light emitting points. Further, the bias current supply circuit 141 keeps the LD 171 in the non-image forming period.
To 174, a threshold current I _th is measured by applying a bias current, and feedback control from the comparator 152 applies a bias current according to the optical output characteristics of the LDs 171 to 174.

The reason why the amount of light emitted from the LD fluctuates is as follows.
This is because the threshold temperature I _th changes due to the change in the ambient temperature caused mainly by the heat generated by the light emission of the LD (see FIG. 6). Therefore, the bias current supply circuit 141 needs to apply a bias current corresponding to the change in the threshold current I _th to the LD. When a plurality of LDs are provided, L
Since the threshold current I _th differs depending on the individual difference of D, the bias current applied to each LD differs. Therefore, in the non-image forming period, the bias current supply circuit 1
42 applies a bias current to each of the LDs 171 to 174 to accurately measure the threshold current I _th ,
It is necessary to control the bias current according to the individual difference of 174.

That is, in the non-image forming period, in a state where it is not affected by the temperature change of the adjacent light emitting points,
It is necessary to measure the threshold current I _th , and the bias current supply circuit 142 uses the control signal S input from the CPU 11.
Based on 1, the bias current is applied in a predetermined order that is not thermally affected by the adjacent light emitting points.

The PD 16 monitors the light amount of the LDs 171 to 174 and outputs a monitor current corresponding to the light emission amount to the current-voltage conversion circuit 151. Current-voltage conversion circuit 151
Causes the monitor current to flow through the monitor resistor, thereby generating a monitor voltage and outputting the monitor voltage to the comparator 152.

The comparator 152 is a current-voltage conversion circuit 151.
Monitor voltage input from the reference voltage supply circuit 153
The reference voltage input from the above is compared, and feedback control of the signal power supply circuit 141 and the bias current supply circuit 142 is performed based on the obtained comparison value.

The LD array 17 includes four LDs 171 to 1
74 are provided, and four LDs are arranged in parallel on a substantially straight line. The LDs 171 to 174 are the signal current supply circuit 1
The current applied from 41 and the bias current supply circuit 1
Light is emitted with a predetermined amount of light emission according to the bias current applied from 42.

Next, the order in which the current is applied to the LDs 171 to 174 based on the control signal S1 output from the CPU 11 will be described with reference to FIG. As shown in FIG. 3, it is assumed that the LDs 171 to 174 are linearly arranged in order from the left side. Here, LD171-174
When the order of the drive currents applied to the LDs is the order of the case 1 shown in FIG.
The drive current is applied in the order of LD171 → LD174 → LD172, and the LD171 to 174 can emit light without the emission points adjacent to each other being continuous.

Further, subsequently, when a bias current is applied, a predetermined rest period is provided and the LD 173 is used.
By applying the bias current in the order of: LD171 → LD174 → LD172, the bias current can be supplied without the light emitting points adjacent to each other being continuous.
In addition, by applying a current to the LDs 171 to 174 in the order of case 2 to case 4 shown in the chart of FIG. 3, heat generation by the adjacent light emitting points does not affect other light emitting points, and automatic current control is performed. , And the threshold current can be measured.

As a result, it is possible to reduce the influence of light emission and heat generation due to bias current application on the adjacent light emitting points, and it is possible to perform more accurate automatic current control and measurement of the threshold current I _th . The order in which the currents shown in FIG. 3 are applied is the case 1 in which the same light emitting point is not continuous when applying the bias current after performing the automatic current control.
The order of case 4 is more preferable.

Next, the operations of the signal current circuit 141 and the bias current supply circuit 142 will be described with reference to the timing charts shown in FIGS. FIG. 4 is a timing chart conceptually showing the index signal and the signals output from the signal power supply circuit 141 and the bias current supply circuit 142, and the horizontal axis represents time. Note that FIG. 4 is a timing chart collectively showing four signals output to the LDs 171 to 174.
As shown in FIG. 4, in the non-image forming period, the signal power supply circuit 141 outputs a signal for performing automatic current control, and thereafter, the bias current supply circuit 142 measures the threshold current I _th. Signal is output. Further, when the index signal is output and the image forming period starts, the image data signal is output from the signal power supply circuit 141.

Next, with reference to FIG. 5, a detailed timing chart at the time of automatic current control during the non-image forming period and at the time of applying the bias will be described. FIG. 5 is a timing chart exemplifying a case where automatic current control and bias energization are performed in the order shown in Case 1 of FIG. 3, and the horizontal axis indicates time. In addition, during automatic current control, LD
Drive currents for causing the light emitting devices 171 to 174 to emit light
4 and LD 171 to 174 when bias is applied.
Bias currents applied to B are shown as B1 to B4.

As shown in FIG. 5, during the non-image forming period, the drive currents I3 and I are supplied from the signal power supply circuit 141.
LD1, I4, and I2 are sequentially output, so that LD1
73, LD171, LD174, and LD172 emit light. After the rest period, the bias current B3,
LD is output by sequentially outputting B1, B4, and B2.
The threshold currents I _th of 171 to 174 are measured. Then, based on the results of the automatic current control and the measurement of the threshold current I _th , the drive current required to obtain the light amount required by the APC 15 is obtained, and the light emission amounts of the LDs 171 to 174 are controlled.

As described above, in the present embodiment, the LD driving device 1 of the image forming apparatus using the multi-beam laser has the signal power supply circuit 14 when performing the automatic current control.
LD17 is controlled by controlling the order of the drive currents output from the LD17.
By making the light emitting order of 1 to 174 emit light in a predetermined order, the automatic current control is performed in a state in which the thermal influence of the adjacent light emitting points is suppressed. In addition, when the threshold current I _th is measured and an accurate bias current is set, the bias currents output from the bias current supply circuit 142 are applied to the LDs 171 to 174 in a predetermined order so that they are adjacent to each other. LD171 without being affected by the thermal effects of LD171-174
The threshold current I _th can be measured based on the individual difference of ˜174.

Thus, accurate automatic current control can be performed, and the LDs 171 to 174 can always be made to emit light with a constant light amount during the image forming period. Therefore, it is possible to form a high-quality image without density unevenness. Further, by applying a bias current and measuring the threshold current I _th of the LDs 171 to 174 during the non-image forming period, the light quantity can be controlled more accurately, so that the light quantity is stable and the print density is stable. Can be constant.

Further, by performing the automatic current control and the measurement of the threshold current I _th , the light amount difference due to the difference in the current-light output characteristic of each light emitting point peculiar to the image forming apparatus using the multi-beam laser is corrected. Therefore, it is possible to realize a constant print density and provide good image quality.

Note that the description in this embodiment described above
Is an example of a suitable LD drive device 1 according to the present invention,
It is not limited to this. For example, LD drive device
Although the number of LDs provided in 1 has been described as an example of four,
The number of LDs is not limited to this, and may be at least 4 or more.
Good. In this case, the LD drive device 1 is connected to the LDs in parallel.
In addition, due to the order in which LDs adjacent to each other are not continuous,
Drive current or bias current is applied to automatically control the current.
Control or threshold current I _thIt suffices if the configuration is such that
In addition, in the present embodiment, the LDs 171 to 174 are
The description has been given by taking the configuration in which one row is arranged in parallel as one row.
The arrangement of D171 to 174 is not limited to this. However,
LDs 171 to 174 are arranged on a straight line or on a curved line.
Preferably there is.

In addition, the detailed configuration and the detailed operation of each terminal or component part of the LD drive device 1 according to the present embodiment can be appropriately changed without departing from the spirit of the present invention.

[0040]

According to the invention described in claim 1, when performing automatic current control for a plurality of parallel laser diodes, it is possible to cause adjacent laser diodes to emit light in a non-consecutive order. There is no thermal influence of the laser on the adjacent laser diode. That is, it is possible to prevent the temperature of the other laser diode from increasing due to the temperature increase around the laser diode that emitted light, and the decrease of the output light amount of the other laser diode. Therefore, the automatic current control can be performed in a state where the light output characteristics are constant, so that it is possible to prevent the density unevenness by keeping the output light amount constant in the image forming period, and to provide a high-quality image. it can.

According to the invention described in claim 2, in addition to the effect of the invention described in claim 1, in a laser diode drive device including four laser diodes arranged in parallel, adjacent laser diodes are provided according to the number of laser diodes. It is possible to prevent the diode from continuously emitting light, and it is possible to prevent a decrease in the output light amount due to heat generation of another laser diode when performing automatic current control.

According to the third aspect of the present invention, in addition to the effect of the first or second aspect of the present invention, when only the bias current is applied to the laser diode and the threshold current of the laser diode is measured, it is adjacent to the laser diode. The thermal effect caused by applying the bias current to the laser diode can be reduced, and the accurate threshold current can be measured for each laser diode. As a result, a bias current corresponding to the variation in the characteristics of the laser diode can be applied to each laser diode, and the laser diode can efficiently emit light and can emit light with a more accurate output light amount. It is possible to secure the print density and provide good image quality.

[Brief description of drawings]

FIG. 1 is a block diagram showing a main configuration of an LD drive device 1 according to an embodiment to which the present invention is applied.

2 is an LD driver 14 and an APC circuit 15 shown in FIG.
3 is a block diagram showing a detailed configuration of FIG.

FIG. 3 shows LDs 171 to 17 from a signal power supply circuit 141.
4 is a chart showing an example of the order of applying a current to No. 4;

4 is a timing chart conceptually showing operations of a signal current circuit 141 and a bias current supply circuit 142 of FIG.

5 is a timing chart showing in detail the timing chart shown in FIG.

FIG. 6 is a graph showing a correspondence relationship between an LD drive current and a light output characteristic.

[Explanation of symbols]

1 LD drive 11 CPU 12 PWM 13 PM 14 LD driver 141 signal power supply circuit 142 Bias current supply circuit 15 APC 151 Current-voltage conversion circuit 152 comparator 153 Reference voltage 16 PD 17 LD array 171-174 LD

Claims (3)

[Claims] 1. A plurality of laser diodes arranged in parallel, detection means for respectively monitoring output light quantities of the plurality of laser diodes, and light emission quantities of the plurality of laser diodes according to the output light quantities detected by the detection means. Automatic current control means for feedback control of current so as to be constant,
In a laser diode drive device comprising a current applying unit that applies a current to the plurality of laser diodes according to the control of the automatic current control unit, the current applying unit independently applies a current to the plurality of laser diodes. In this case, the laser diode drive device is provided with a timing control means for controlling the timing of applying the current in the order in which the adjacent laser diodes among the plurality of parallel laser diodes are not continuous. 2. The timing control means selects first one of the two laser diodes which are not located at the ends when the current application means applies a current to the four parallel laser diodes. 2. The timing for applying an electric current is controlled.
The laser diode driving device described. 3. The current applying means comprises bias current applying means for applying a current to each of the plurality of laser diodes in the vicinity of a threshold current or less emitted by the plurality of laser diodes arranged in parallel, and the timing control means. When applying a current to the plurality of laser diodes by the bias current applying means, the timing of applying the current is controlled in an order in which adjacent laser diodes among the plurality of parallel laser diodes are not continuous. The laser diode drive device according to claim 1 or 2.