JPH04356986A - Laser recorder - Google Patents

Abstract

PURPOSE:To improve frequency characteristic, thermal characteristic, etc., and to reduce an irregularity in a picture density by maintaining a light emission current constant, and controlling a bias current to be applied to a semiconductor laser. CONSTITUTION:A current for allowing a semiconductor laser 1 to radiate a light, is generated by a light emission constant-current source 24, and supply of the generated current to the laser 1 is ON/OFF-controlled by a switching circuit 21. A predetermined bias current is supplied by a bias constant-current source 22 to the laser irrespective of the operating state of the circuit 21. A light amount of the laser 1 is detected by a current/voltage converter 25, and the output light amount of the laser is so controlled by a controller 26 as to become a predetermined value while the light emission current is maintained constant by monitoring the output of the converter 25. Thus, an irregularity in a picture density can be suppressed, and the bias current is supplied to improve frequency characteristic, thermal characteristic.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser recording apparatus such as a laser printer, a digital copying machine, and a laser facsimile that performs exposure using a semiconductor laser.

0002

2. Description of the Related Art Conventionally, in laser recording devices such as laser printers, digital copying machines, and laser facsimile machines, images are recorded by exposing and scanning a photoreceptor with a laser beam modulated according to input image information. is formed. A semiconductor laser (LD) is used as a laser generating means in a laser recording device. In laser recording devices, in order to reproduce halftones, a method is known in which one dot of the laser beam is pulse width modulated (that is, the laser lighting time is changed) to express multiple gradations of light and shade with one pixel. It is being In this case, the density of the image is determined by the integral value of the energy of the laser beam, so in a laser recording device using pulse width modulation, the image density is determined by the lighting time of the laser and the peak light amount of the laser beam.

[0003] Semiconductor lasers suffer from variations in elements, temperature,
The emission threshold current and slope efficiency change due to changes over time. Therefore, in a laser recording apparatus, in order to maintain good image quality, it is necessary to control the current flowing through the semiconductor laser and stably control the light amount to a predetermined value.

A common method for controlling a semiconductor laser is to detect the amount of laser light in a non-image area, control the applied current to a predetermined amount of light, and maintain the current in the image area. Furthermore, when a bias current is applied to the semiconductor laser when it is not emitting light, the light emitting current to be switched is reduced, the switching time of the switching transistor is reduced, and the light emission frequency characteristics of the semiconductor laser are improved. This contributes to stabilizing the optical pulse width. Furthermore, the droop characteristic, which is a thermal characteristic of the semiconductor laser, is improved. This has the advantage that more precise control is possible than controlling the light amount with one current source, and the bias current has a great advantage in terms of stability of image density. In addition, since the loss of the switching transistor is reduced, there is an advantage that transistors with low allowable loss can be used.

[0005] Therefore, laser recording apparatuses employing various bias current control methods have been proposed. The following methods are typical. (1) The bias current is fixed and the light emission current is changed to control the amount of light. (2) Controlling the amount of light by changing both the bias current and the light emitting current.

FIG. 12 is a characteristic diagram showing the laser emission operation according to the method (1) above. In this method, a predetermined optical output can be obtained by constantly flowing a constant bias current and controlling the light emitting current.

FIG. 13 is a characteristic diagram showing the laser emission operation according to the method (2) above. In this method, a predetermined optical output can be obtained by controlling the bias current to be close to the emission threshold and further controlling the emission current.

[0008]

[Problems to be Solved by the Invention] However, in the above-mentioned conventional technology, if the slope efficiency decreases or the operating current increases due to a rise in the temperature of the semiconductor laser or changes over time, the value of the emission current increases. This increase lengthens the switching time, narrows the pulse width of the optical waveform, and reduces the average energy of the laser beam. Therefore, there is a problem in that the image density varies. In particular, variations in low concentration areas with narrow pulse widths were a major problem. Further, in the method (2), since two current sources are controlled individually, there is a problem in that the processing takes time. Furthermore, it is necessary to have a circuit such as a D/A converter for each of the two current sources, leading to an increase in cost. An object of the present invention has been made in view of the actual state of the prior art described above, and it is possible to improve frequency characteristics, thermal characteristics, etc. by keeping the light emitting current constant and controlling the bias current applied to the semiconductor laser, thereby improving the image quality. An object of the present invention is to provide a laser recording device that can reduce variations in density. Also,
Another object of the present invention is to provide a laser recording device that can shorten control time at low cost. Furthermore, another object of the present invention is to provide a laser recording device that can eliminate excessive light emission.

[0009]

Means for Solving the Problems In order to achieve the above object, the present invention provides a laser recording apparatus that exposes a photoreceptor to light by modulating and driving a semiconductor laser based on image data. a light-emitting constant current source that generates a current to cause light emission; a switching circuit that turns on/off supply of the light-emitting current generated by the light-emitting constant current source to the semiconductor laser; a bias constant current source that supplies a constant bias current to the semiconductor laser; a light amount detection means for detecting the light amount of the semiconductor laser; and a light amount detection means that monitors the output of the light amount detection means while keeping the light emitting current constant. A control means for controlling the amount of light to a predetermined value is provided. In order to simplify the circuit configuration, the light emitting constant current source can be configured to generate only a fixed current value. In addition, in order to prevent over-emission, the value of the light emitting current is set such that the relationship between the minimum value SEmin of the slope efficiency of the semiconductor laser, the operating light amount PO, and the minimum value Iopmin of the operating current is
It is desirable that /SEmin<(emission current value)<Iopmin.

0010

[Operation] According to the above-mentioned means, the semiconductor laser is driven at a constant current value by the light emitting constant current source, and this constant current is switched and modulated by the switching circuit according to the gradation signal. A predetermined bias current is applied to the semiconductor laser by a bias constant current source. Therefore, constant current driving suppresses variations in switching time, thereby suppressing variations in image density, and supplying bias current improves frequency characteristics and thermal characteristics. In addition, since the light emitting constant current source only needs to output a fixed current value, only one arithmetic unit is required to control the current value, resulting in low cost.Furthermore, since only one current source is required, control It becomes possible to shorten the time. Furthermore, by satisfying the relationship PO /SEmin < (emission current value) < Iopmin, the laser will not oscillate when the lighting signal is "L" and over-emission will not occur when no bias current is applied. .

[0011]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit diagram showing an embodiment of a laser recording apparatus according to the present invention. Further, FIG. 2 is a perspective view showing the configuration of a writing optical system to which the laser recording apparatus according to the present invention is applied. Furthermore, FIG. 3 is a front cross-sectional view showing the configuration of a laser recording apparatus using the writing optical system of FIG. 2. In FIG.

First, the configuration shown in FIG. 2 will be explained. 1 is a semiconductor laser; 2 is a polygon mirror to which a laser beam from the semiconductor laser 1 is irradiated; and 3 is a photosensitive drum to which exposure is performed by the laser beam scanned by the polygon mirror 2. A condensing lens 4 and a cylindrical lens 5 are sequentially arranged on the optical path between the semiconductor laser 1 and the polygon mirror 2, an fθ lens 6 is arranged on the output optical path of the polygon mirror 2, and a mirror 7
is installed. Furthermore, a photodetector 8 used to keep the writing start position constant is provided on the output optical path of the polygon mirror 2.

Next, the configuration of FIG. 3 will be explained. Reference numeral 9 denotes a writing optical system unit which is a unit of the writing optical system shown in FIG. A dustproof glass 10 is disposed at the beam emitting portion of the writing optical system unit 9, and the inside of the writing optical system unit 9 has a sealed structure. A photoreceptor drum 3 is disposed on the output optical path of the mirror 7, a charger 11 for uniformly charging the surface of the photoreceptor drum 3 is provided before the exposure position, and a charger 11 is provided after the exposure position. A developing device 12 is provided for developing a latent image formed by exposure with toner. A transfer charger 13 is located downstream of this developing device 12.
A transport path 14 is provided to introduce the transfer paper into the transfer section, and a paper feed cassette 15 is provided adjacent to the entrance of the transport path 14. A conveyor belt 17 for horizontally conveying the sheet on which transfer has been completed is provided downstream of the transfer charger 13, and a fixing device 18 is provided downstream of the transfer charger 13. In addition, in order to remove toner remaining on the surface of the photoreceptor drum 3 after transfer,
A cleaning device 16 is provided.

In the configurations shown in FIGS. 2 and 3, the semiconductor laser 1 is modulated in accordance with image data, and the laser beam passes through a condenser lens 4 and a cylindrical lens 5 to one of the mirror surfaces of a polygon mirror 2 that rotates at high speed. It is incident on one. As the polygon mirror 2 rotates, the emitted light is continuously scanned in the circumferential direction of the polygon mirror 2, and is irradiated in the axial direction of the photoreceptor drum 3 via the fθ lens 6 and mirror 7 to perform exposure. be exposed. The photosensitive drum 3 rotates in the direction of the arrow shown in the figure, and its surface is charged in advance by a charger 11. The latent image formed by exposure by the writing optical system unit 9 reaches a position facing the developing device 12 as the photosensitive drum 3 rotates, and is developed with toner to form a developed image. This image is developed by the transfer charger 1 as the photoreceptor drum 3 further rotates.
3, one sheet of transfer paper is supplied from the paper feed cassette 15 via the conveyance path 14 at the same timing, and the toner image on the photoreceptor drum 3 is transferred by the electrostatic attraction force of the transfer charger 13. is transferred onto the transfer paper. The transferred image is sequentially sent to the fixing device 18 via the conveyor belt 17 from the transferred portion, and the transferred image is fixed onto the paper.

Next, a pulse width modulation (hereinafter referred to as PWM) circuit for modulating a laser beam will be explained. This PWM circuit is for modulating a laser lighting signal into a pulse width corresponding to a grayscale signal from a control section.

FIG. 4 is a block diagram of a configuration in which 3-bit parallel signal lines are assigned to gradation signals and outputs of 8 gradations including white and black are obtained using 3 bits. In the figure, D1 to D6 are delay lines, G1, G2, G3 are AND gates, G4,
G5 and G6 are OR gates, and S is a selector. AND
Gate G1 connects pixel clock CLK and delay line D1
The AND gate G2 performs a logical product between the pixel clock CLK and the output of the delay line D2, and the AND gate G3 performs a logical product between the pixel clock CLK and the output of the delay line D3. OR gate G4 takes the logical sum of the pixel clock CLK and the output of the delay line D4, OR gate G5 takes the logical sum of the pixel clock CLK and the output of the delay line D5, and OR gate G6 takes the logical sum of the pixel clock CLK and the output of the delay line D5. A logical OR is performed with the output of D6. The output of each gate G1 to G6 is
It is input to selector S. Also, selector S A, B,
A gradation signal is input to each terminal of C, and a laser lighting signal is output from its output terminal.

The PWM circuit shown in FIG. 4 uses a pixel clock C
LK and a signal obtained by delaying this CLK by an arbitrary time using a delay line. Prepare as many gray levels as the number of gray levels (in this example, 6 types), and convert them into gray level signals using selector S. An output with a corresponding pulse width is obtained.

FIGS. 5 and 6 are circuit diagrams and operation waveform diagrams for obtaining an arbitrary pulse width output from the pixel clock CLK using a delay line and an AND gate. Also, Figure 7
FIG. 8 is a circuit diagram and an operation waveform diagram for obtaining an arbitrary pulse width output from the pixel clock CLK using a delay line and an OR gate. As shown in FIG. 5, by inputting the pixel clock CLK and the delay signal b passed through the delay line D (delay time ΔT1) to the AND gate G,
As shown in FIG. 6, a signal c having a pulse width of (T-ΔT1) can be obtained. Furthermore, as shown in FIG. 7, the pixel clock CLK and the delay line D (delay time ΔT2
) is input to the OR gate G, it is possible to obtain a signal e having a pulse width of (T+ΔT2) as shown in FIG.

The circuits having the configurations shown in FIGS. 5 and 7 are provided for the number of gradations, and the signals outputted from these circuits are input to the selector S, and the gradation is determined by the gradation signal instructed from the control section. It is possible to obtain a signal output with a pulse width corresponding to the Although FIG. 4 shows an example of output of 8 gradations,
By increasing the number of circuits and gradations, it is possible to obtain output with even more gradations. For example, by preparing a circuit for setting 16 types of pulses and a 4-bit gradation signal, output of 16 gradations can be obtained.

Next, the laser recording apparatus shown in FIG. 1 will be explained. The semiconductor laser 1 has a built-in photodiode (PD) 1a for monitoring the emission intensity. The anode of the semiconductor laser 1 is connected to a +5V power supply, and the cathode thereof is connected to a bias constant current source 22 (comprised of an operational amplifier 22a and a transistor 22b). In order to drive the semiconductor laser 1, a switching circuit 21 consisting of two FETs (field effect elements) 21a, 21b and a driver 21c is connected, and a PWM circuit 23 is connected to the driver 21c of this switching circuit 21. Further, the switching circuit 21 is connected to a light emitting constant current source 24 (comprised of an operational amplifier 24a and a transistor 24b) for causing a constant light emitting current ISW to flow through this circuit. Furthermore, photodiode 1a
A current-voltage conversion circuit 25 for converting the light-receiving current into a voltage value is connected to the control unit 26, and its output voltage is applied to the control section 26 as an APC (automatic power control) voltage. The control unit 26 includes an analog input port, an output port, a gradation signal output terminal, a data terminal, and a pixel clock terminal, and outputs a gradation signal to the PWM circuit 23. A latch 2 is connected to the data terminal and output port of the control unit 26.
7 is connected, and this latch 27 has a D/A (digital/
An analog) converter 28 is connected. This D/
A bias current signal converted into an analog signal by the A converter 28 is applied to the bias constant current source 22.

The bias constant current source 22 supplies a bias current IB to the semiconductor laser 1 according to the voltage value of the bias current signal.
flow. The current value is detected by the photodiode 1a, and the detected current is converted into a voltage value by the current-voltage conversion circuit 25 and applied to the control section 26. The control unit 26 monitors the APC voltage and stops incrementing the data when the amount of light emitted from the semiconductor laser 1 reaches the set amount of light PO. Then, the gradation signal is set to (1, 1, 1) and the laser lighting signal is set to "L". In this way, the light amount is controlled and the printable state is set.

Then, a light emitting current ISW switched by a laser turn-on signal which is an image signal is added to the bias current IB that is constantly applied to the semiconductor laser 1, and the semiconductor laser 1 blinks in accordance with the current. This forms an image.

FIG. 9 is a characteristic diagram showing the relationship between forward current and optical output P of a semiconductor laser. In the figure, Ith is the light emission threshold current, PO is the set light amount, and IOP is the operating current at the set light amount. Further, SE is slope efficiency, which is expressed by the following equation. SE=ΔP/ΔI As is clear from FIG. 9, the light output is the light emission threshold current Ith
It increases rapidly when it exceeds.

FIG. 10 shows an example of the characteristics of a semiconductor laser, in which the minimum, standard, and maximum values were measured by changing the conditions of the emission threshold current Ith, the set light intensity PO, and the operating current IOP at the set light intensity. It shows the value.

Next, the APC operation of the laser recording apparatus having the configuration shown in FIG. 1 will be explained. First, the control unit 26 activates the laser lighting signal by setting the gradation signal to (1, 1, 1), and further turns on the switching circuit 21 to turn on the semiconductor laser 1 using the light emitting constant current source 24. A predetermined light emitting current ISW is applied. At this time,
Data controlling the bias current sent to the latch 27 and the D/A converter 28 is set to 0. Light emitting current ISW
The value of is set to prevent laser oscillation when the laser lighting signal is "L", and to prevent excessive light emission when only the light emission current ISW is applied without applying the bias current IB. In the case of the semiconductor laser 1 having the above characteristics, the following conditions are satisfied. PO/SEmin<ISW<Iopmin5/0.
20<ISW<40(mA) 25<ISW<40(mA)

Normally, the semiconductor laser 1 is PO /SEmi
Since n < Iopmin, it is possible to set the light emitting current ISW that satisfies the above conditions. However, if the light emitting current ISW is set to a value close to PO /SEmin,
Even when the laser lighting signal is “L”, the slope efficiency SE
is the minimum, the semiconductor laser 1 has a light emission threshold current Ith
A current with a value close to is applied. It is known that when a current having a value close to the emission threshold current Ith is applied to the semiconductor laser 1, the semiconductor laser 1 is not sufficiently quenched and emits light of Pth as shown in FIG. Furthermore, if the light emitting current ISW is set to a too large value, the bias current IB will be reduced by that amount, and the advantage of applying the bias current will be reduced. In this embodiment, the light emitting current ISW is set to about 30 mA. Next, the control unit 26 controls the latch 2
7 and the data sent to the D/A converter 28, thereby increasing the bias current IB. Then, the APC voltage output from the current-voltage conversion circuit 25 is monitored, and data increment is stopped when the amount of light emitted from the semiconductor laser 1 reaches the set amount of light PO. Next, the gradation signal is set to (1, 1, 1) and the laser lighting signal is set to "L".

FIG. 11 is a characteristic diagram showing the laser emission operation according to the present invention. In this case, the light emitting current is kept constant and the bias current is controlled to obtain a predetermined light output. According to this method, even if the slope efficiency or operating current changes due to temperature rise or aging of the semiconductor laser 1,
The light emitting current does not change. Therefore, variations in switching time are suppressed, the pulse width of the optical waveform is stabilized, and variations in image density can be suppressed.

[0028]

[Effects of the Invention] Since the present invention is constructed as described above, it achieves the following effects. In the laser recording device according to claim 1, the laser recording device exposes a photoconductor by modulating and driving a semiconductor laser based on image data, and a light emission constant current source that generates a current that causes the semiconductor laser to emit light; , a switching circuit that turns on/off supply of the light emitting current generated by the light emitting constant current source to the semiconductor laser, and a bias constant current that supplies a constant bias current to the semiconductor laser regardless of the operating state of the switching circuit. a light amount detection means for detecting the light amount of the semiconductor laser; and a control means for controlling the output light amount of the semiconductor laser to a predetermined value while keeping the light emitting current constant while monitoring the output of the light amount detection means. As a result, variations in switching time are suppressed, and as a result, variations in image density are suppressed, and by supplying a bias current, frequency characteristics and thermal characteristics can be improved. In the laser recording apparatus according to the second aspect of the present invention, since the light emitting constant current source generates only a fixed current value, cost reduction can be achieved and control time can be shortened. In the laser recording device according to the third aspect, the value of the light emitting current is the minimum value SEmin of the slope efficiency of the semiconductor laser, and the operating light amount P==
, the minimum value of the operating current Iopmin is expressed as PO /SEmin <(emission current value) <Iopm
Since it is made to be in, over-emission will not occur.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a laser recording device according to the present invention.

FIG. 2 is a perspective view showing the configuration of a writing optical system to which a laser recording device according to the present invention is applied.

FIG. 3 is a front sectional view showing the configuration of a laser recording apparatus using the writing optical system of FIG. 2;

FIG. 4 is a block diagram of a configuration in which 3-bit parallel signal lines are assigned to gradation signals and outputs of 8 gradations including white and black are obtained using 3 bits.

FIG. 5 is a circuit diagram for obtaining an arbitrary pulse width output from a pixel clock CLK using a delay line and an AND gate.

FIG. 6 is a waveform diagram showing the operation of the circuit of FIG. 5;

FIG. 7 is a circuit diagram for obtaining an arbitrary pulse width output from a pixel clock CLK using a delay line and an OR gate.

8 is an operation waveform diagram showing the operation of the circuit of FIG. 7. FIG.

FIG. 9 is a characteristic diagram showing the relationship between forward current and optical output P of a semiconductor laser.

FIG. 10 is an explanatory diagram showing an example of characteristics of a semiconductor laser.

FIG. 11 is a characteristic diagram showing the laser emission operation in the present invention.

FIG. 12 is a characteristic diagram showing a laser emission operation using a method in which the bias current is fixed and the emission current is changed to control the amount of light.

FIG. 13 is a characteristic diagram showing a laser emission operation in which a predetermined optical output is obtained by constantly flowing a constant bias current and controlling the emission current.

[Explanation of symbols]

1 Semiconductor laser 1a Photodiode 3 Photoreceptor drum 9 Writing optical system unit 21 Switching circuit 22 Bias constant current source 23 PWM circuit 24 Light emitting constant current source 25 Current voltage conversion circuit 26 Control section 27 Latch 28 D/A converter

Claims (3)

[Claims] 1. A laser recording device that exposes a photoconductor by modulating and driving a semiconductor laser based on image data, comprising: a light emission constant current source that generates a current that causes the semiconductor laser to emit light; and a light emission constant current source that generates a current that causes the semiconductor laser to emit light. a switching circuit that turns on/off supply of a light emitting current generated in a source to the semiconductor laser; a bias constant current source that supplies a constant bias current to the semiconductor laser regardless of the operating state of the switching circuit; A light amount detection means for detecting the light amount of the laser, and a control means for controlling the output light amount of the semiconductor laser to a predetermined value while keeping the light emitting current constant while monitoring the output of the light amount detection means. A laser recording device featuring: 2. The laser recording apparatus according to claim 1, wherein the light emitting constant current source generates only a fixed current value. 3. The value of the light emission current has a relationship between the minimum value SEmin of the slope efficiency of the semiconductor laser, the operating light amount PO , and the minimum value Iopmin of the operating current such that PO /SEmin <(light emission Current value)
<A laser recording device characterized by being Iopmin.