JP2015103680A - Semiconductor-laser control device, image formation device, and method of controlling semiconductor laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor-laser control device that allows stably outputting a desired light waveform from a semiconductor laser.SOLUTION: An LD control circuit 23, which is a semiconductor-laser control device supplying a pulse-shaped driving current to an LD 14, includes: an overshoot-current supplying section supplying an overshoot current to the LD 14 at the time of rising the driving current; and an undershoot-current supplying section supplying an undershoot current to the LD 14 at the time of falling the driving current. The overshoot-current supplying section adjusts the overshoot current according to changes of the driving current, and the undershoot-current supplying section adjusts the undershoot current according to changes of the driving current.

Description

  The present invention relates to a semiconductor laser control device, an image forming apparatus, and a semiconductor laser control method. More specifically, the present invention relates to a semiconductor laser control device that supplies a pulsed drive current to a semiconductor laser, and an image forming apparatus including the semiconductor laser control device. And a semiconductor laser control method for controlling a semiconductor laser.

  2. Description of the Related Art Conventionally, there is known a semiconductor laser driving device that sets an overshoot current and an undershoot current to be supplied to a semiconductor laser in accordance with a set value of a bias current and / or a switching current to be supplied to the semiconductor laser (for example, Patent Documents). 1).

  However, the semiconductor laser driving device disclosed in Patent Document 1 cannot stably output a desired optical waveform from the semiconductor laser.

  The present invention provides a semiconductor laser control device that supplies a pulsed drive current to at least one semiconductor laser, an overshoot current supply unit that supplies an overshoot current to the semiconductor laser when the drive current rises, and the drive current At least one of an undershoot current supply unit that supplies an undershoot current to the semiconductor laser at the fall of the overshoot current, the overshoot current supply unit adjusts the overshoot current according to a change in the drive current, The undershoot current supply unit adjusts the undershoot current according to a change in the drive current.

  According to the present invention, a desired optical waveform can be stably output from a semiconductor laser.

1 is a diagram illustrating a schematic configuration of a laser printer according to an embodiment. It is a figure for demonstrating the optical scanning device in FIG. It is a block diagram for demonstrating LD control circuit. 6 is a timing chart of applied current, LD potential, and optical waveform of Comparative Example 1. 10 is a timing chart (No. 1) of an applied current, an LD potential, and an optical waveform of Comparative Example 2. 12 is a timing chart (No. 2) of an applied current, an LD potential, and an optical waveform of Comparative Example 2. 3 is a timing chart of applied current, LD potential, and optical waveform of Example 1. 10 is a timing chart of an applied current, an LD potential, and an optical waveform of Comparative Example 3. 6 is a timing chart of applied current, LD potential, and optical waveform of Example 2. It is a flowchart for demonstrating the procedure of the initial setting of an overshoot electric current and an undershoot electric current. 10 is a timing chart of applied current, LD potential, and optical waveform for explaining an example (Example 3) of adjusting an undershoot current according to a change in drive current. 12 is a timing chart of applied current, LD potential, and optical waveform for explaining an example (Example 4) of adjusting an undershoot current and an overshoot current according to a change in drive current. It is a graph which shows the relationship between the change of a drive current, and the change of an undershoot current. It is a graph which shows the relationship between the change of a drive current, and the change of an overshoot current. It is a table which shows the drive current, optimal undershoot current, and optimal overshoot current corresponding to each light quantity. It is a figure which shows the relationship between the variation | change_quantity of the drive current corresponding to the variation | change_quantity of light quantity, the optimal undershoot current adjustment amount, and the optimal overshoot current adjustment amount. It is a figure for demonstrating an example of the production | generation method of an applied current. It is a figure for demonstrating the other example of the production | generation method of an applied current. FIG. 19A is a graph showing the relationship between the applied current and the optical output, and FIG. 19B is a graph showing the relationship between the elapsed time and the current waveform. It is a figure for demonstrating the relationship between a driver (LD control circuit), total parasitic capacitance C, and LD. 1 is a diagram illustrating a schematic configuration of a color printer.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. FIG. 1 shows a schematic configuration of a laser printer 1000 according to an embodiment.

  The laser printer 1000 includes an optical scanning device 1010, a photosensitive drum 1030, a charging charger 1031, a developing roller 1032, a transfer charger 1033, a charge eliminating unit 1034, a cleaning unit 1035, a toner cartridge 1036, a paper feeding roller 1037, a paper feeding tray 1038, A registration roller pair 1039, a fixing roller 1041, a paper discharge roller 1042, a paper discharge tray 1043, a communication control device 1050, a printer control device 1060 that comprehensively controls the above-described units, and the like are provided. These are housed in predetermined positions in the printer housing 1044.

  The communication control device 1050 controls bidirectional communication with a host device (for example, a personal computer) via a network or the like.

  The photosensitive drum 1030 is a cylindrical member, and a photosensitive layer is formed on the surface thereof. That is, the surface of the photoconductor drum 1030 is a scanned surface. The photosensitive drum 1030 rotates in the direction of the arrow in FIG.

  The charging charger 1031, the developing roller 1032, the transfer charger 1033, the charge removal unit 1034, and the cleaning unit 1035 are each disposed in the vicinity of the surface of the photosensitive drum 1030. Then, along the rotation direction of the photosensitive drum 1030, the charging charger 1031 → the developing roller 1032 → the transfer charger 1033 → the discharging unit 1034 → the cleaning unit 1035 are arranged in this order.

  The charging charger 1031 uniformly charges the surface of the photosensitive drum 1030.

  The optical scanning device 1010 scans the surface of the photosensitive drum 1030 charged by the charging charger 1031 with laser light modulated based on image information (image data) from the host device, and the surface of the photosensitive drum 1030 is scanned. An electrostatic latent image corresponding to the image information is formed. The electrostatic latent image formed here moves in the direction of the developing roller 1032 as the photosensitive drum 1030 rotates. The configuration of the optical scanning device 1010 will be described later.

  The toner cartridge 1036 stores toner, and the toner is supplied to the developing roller 1032.

  The developing roller 1032 causes the toner supplied from the toner cartridge 1036 to adhere to the electrostatic latent image formed on the surface of the photosensitive drum 1030 to visualize the image information. Here, the electrostatic latent image to which the toner is attached (hereinafter also referred to as “toner image” for the sake of convenience) moves in the direction of the transfer charger 1033 as the photosensitive drum 1030 rotates.

  Recording paper 1040 is stored in the paper feed tray 1038. A paper feed roller 1037 is disposed in the vicinity of the paper feed tray 1038, and the paper feed roller 1037 takes out the recording paper 1040 one by one from the paper feed tray 1038 and conveys it to the registration roller pair 1039. The registration roller pair 1039 temporarily holds the recording paper 1040 taken out by the paper supply roller 1037, and in the gap between the photosensitive drum 1030 and the transfer charger 1033 according to the rotation of the photosensitive drum 1030. Send it out.

  A voltage having a polarity opposite to that of the toner is applied to the transfer charger 1033 in order to electrically attract the toner on the surface of the photosensitive drum 1030 to the recording paper 1040. With this voltage, the toner image on the surface of the photosensitive drum 1030 is transferred to the recording paper 1040. The recording sheet 1040 transferred here is sent to the fixing roller 1041.

  In the fixing roller 1041, heat and pressure are applied to the recording paper 1040, whereby the toner is fixed on the recording paper 1040. The recording paper 1040 fixed here is sent to the paper discharge tray 1043 via the paper discharge roller 1042 and is sequentially stacked on the paper discharge tray 1043.

  The neutralization unit 1034 neutralizes the surface of the photosensitive drum 1030.

  The cleaning unit 1035 removes the toner remaining on the surface of the photosensitive drum 1030 (residual toner). The surface of the photosensitive drum 1030 from which the residual toner has been removed returns to the position facing the charging charger 1031 again.

  Next, the configuration of the optical scanning device 1010 will be described. As shown in FIG. 2 as an example, the optical scanning device 1010 includes an LD 14 (laser diode) as a light source, a polygon mirror 13, a scanning lens 11, a PD 12 (photo detector) as a photodetector, a scanning control device 15, and the like. I have. These are assembled at predetermined positions in a housing (not shown).

  In the following, for convenience, the direction corresponding to the main scanning direction is abbreviated as “main scanning corresponding direction”, and the direction corresponding to the sub scanning direction is abbreviated as “sub scanning corresponding direction”.

  The LD 14 is also called an edge emitting laser, and emits laser light from one end surface toward the deflecting / reflecting surface of the polygon mirror 13. On the other end surface side of the LD 14, a PD 30 (photodetector) (see FIG. 3) as a photodetector for detecting light emitted from the LD is disposed.

  As an example, the polygon mirror 13 has a hexahedral mirror having an inscribed circle radius of 18 mm, and each mirror serves as a deflecting reflection surface. The polygon mirror 13 deflects the laser light from the LD 14 while rotating at a constant speed around an axis parallel to the sub-scanning corresponding direction.

  An optical system (also called a pre-deflector optical system) that forms an image of the laser light emitted from the LD 14 in the vicinity of the deflection reflection surface of the polygon mirror 13 in the sub-scanning corresponding direction is provided between the LD 14 and the polygon mirror 13. May be. Examples of the optical element constituting the pre-deflector optical system include a coupling lens, an aperture member, a cylindrical lens, and a reflection mirror.

  The scanning lens 11 is disposed on the optical path of the laser light deflected by the polygon mirror 13. Then, the laser light passing through the scanning lens 11 is irradiated (condensed) on the surface of the photosensitive drum 1030 to form a light spot. This light spot moves in the longitudinal direction of the photosensitive drum 1030 as the polygon mirror 13 rotates. That is, the photoconductor drum 1030 is scanned. The moving direction of the light spot at this time is the “main scanning direction”. The rotation direction of the photosensitive drum 1030 is the “sub-scanning direction”.

  The optical system disposed on the optical path between the polygon mirror 13 and the photosensitive drum 1030 is also called a scanning optical system. In the present embodiment, the scanning optical system is composed of a scanning lens 11. The scanning optical system may have a plurality of scanning lenses. Further, at least one folding mirror may be disposed on at least one of the optical path between the scanning lens 11 and the photosensitive drum 1030.

  The PD 12 is arranged on the optical path of the laser beam that is deflected by the polygon mirror 13 and passes through the scanning lens 11. The PD 12 may be disposed on the downstream side in the scanning direction with respect to the photosensitive drum 1030, or may be disposed on the upstream side in the scanning direction.

  Therefore, the laser beam deflected by the polygon mirror 13 enters the PD 12 every time one line is scanned. When the PD 12 detects the laser beam, the PD 12 outputs a detection signal (output signal) to the phase synchronization circuit 25 described later.

  As an example, the scanning control device 15 includes an image processing unit 21, an LD control circuit 23, a phase synchronization circuit 25, a clock generation circuit 27, and the like.

  The phase synchronization circuit 25 generates an image clock with phase synchronization for each line based on the output signal from the PD 12. The phase synchronization circuit 25 receives a high-frequency clock signal from the clock generation circuit 27, thereby achieving phase synchronization of the pixel clock. The pixel clock generated by the phase synchronization circuit 25 is supplied to the image processing unit 21 and the LD control circuit 23.

  The image processing unit 21 generates a write control signal based on the image data (image information) from the host device and the pixel clock from the phase synchronization circuit 25 and supplies it to the LD control circuit 23. The write control signal is a signal for controlling the light emission level (light emission intensity) of the LD 14 and the lighting / extinguishing timing (writing timing).

  The LD control circuit 23 drives and controls the LD 14 based on the pixel clock from the phase synchronization circuit 25 and the write control signal from the image processing unit 21.

  As a result, the laser light from the LD 14 is deflected by the rotating polygon mirror 13 and is irradiated onto the photosensitive drum 1030 which is a scanned medium via the scanning lens 11. The irradiated laser beam becomes a light spot on the photosensitive drum 1030, and an electrostatic latent image corresponding to image information is formed on the photosensitive drum 1030.

  In this embodiment, in order to simplify the description, the light source is a single LD (laser diode) as an example of a semiconductor laser, but includes a plurality of LDs arranged one-dimensionally or two-dimensionally. A laser diode array (LDA) may be used, or a VCSELA (surface emitting laser array) including a single VCSEL (vertical cavity surface emitting laser) or a plurality of VCSELs (surface emitting lasers) arranged one-dimensionally or two-dimensionally. ).

  By the way, as a light source used in an image forming apparatus or the like, a semiconductor laser such as an LD or a VCSEL, a semiconductor laser array in which a plurality of the semiconductor lasers are integrated, a VCSEL (surface emitting laser), etc. are currently widely used. It has various light response characteristics due to its structure, wavelength characteristics, output characteristics, etc.

  For example, when a semiconductor laser and a laser drive circuit (laser driver) are mounted on a substrate housed in a package, parasitic capacitance and inductance in the wiring between the semiconductor laser and the laser drive circuit and in the package of the laser itself, There are multiple factors that affect the response characteristics of light, such as resistance components. In particular, when the number of lasers increases, the number of packages on which lasers and the like are mounted increases, and as a result, factors causing fluctuations in optical response characteristics such as an increase in parasitic capacitance in the package occur.

  For example, compared to an infrared 780 nm band semiconductor laser, a red 650 nm band semiconductor laser generally has a large differential resistance, so that it always has a high response speed as an optical waveform depending on the configuration of a circuit to be driven or a substrate. In some cases, the waveform becomes dull. Even for infrared semiconductor lasers, VCSELs (surface emitting lasers) have a differential resistance (several hundreds of ohms) that is different from that of edge emitting lasers due to structural differences from edge emitting lasers (several to several tens of ohms). Very large compared to For this reason, the time constant of CR that is larger than the LD due to the parasitic capacitance including the terminal capacitance and package capacitance of the VCSEL itself, the parasitic capacitance in the wiring of the substrate on which the VCSEL is mounted, the terminal capacitance of the driver, etc. and the differential resistance of the VCSEL Even when the VCSEL itself has an element characteristic that can be modulated at high speed and a cutoff frequency Ft, the VCSEL itself cannot obtain a high-speed response waveform when it is mounted on the substrate.

  In addition, when there are multiple semiconductor lasers that have such factors that change the response characteristics of light, variations in response characteristics between lasers increase due to variations in wiring length and laser parasitic capacitance, resulting in oscillation delays and There is a problem that density variation and color misregistration in image formation are caused by a difference in transition time of light quantity fluctuation.

  When considering the current and light output characteristics of the semiconductor laser, the LED region (spontaneous emission region) below the threshold current and the LD region (stimulated emission region) above the threshold current emit light with respect to the amount of current change. Since the amount of fluctuation in the amount of light (emission intensity) varies greatly, the light emission level in the LED area is increased when the image forming apparatus or the like is driven by increasing the current from the state where a bias current equal to or lower than the threshold current is applied to the light emission level. The low value may cause oscillation delay, variation in lighting time, pulse narrowing, etc. at the rise of the waveform with respect to the drive signal.

  In semiconductor lasers with such fluctuation factors, especially when multiple lasers are used, the variation in response characteristics between the lasers increases, resulting in density variations in image formation due to differences in oscillation delay and transition time of fluctuations in the amount of light. Cause color shift.

  Next, as a condition at the time of turning on the laser, consider a case in which the pulse is continuously turned on from a state where the light extinction period lasts long in the configuration of the laser driving circuit that constantly applies a bias current to the LD.

  When the LD is turned on, an LD potential corresponding to the light emission level of the LD is generated at the LD terminal, but at the timing when the LD is turned off, the lighted LD potential is turned off, that is, a bias potential (only bias current is supplied). Transition to the LD potential).

  When the optical waveforms of the first pulse and the next pulse are compared, if the LD potential at the time of rising is the same extinction LD potential level, a current for lighting from the same potential to the light emission level is applied. In both cases, the same rising waveform can be obtained.

  On the other hand, when the next pulse emission occurs within the transition time from the lighting state to the extinguishing state, the light emission current of the next pulse emission is applied without the LD potential being lowered to the extinguishing state. Changes.

  In order to stabilize the rising characteristics of the optical waveform, by applying an undershoot current (hereinafter also referred to as a UD current) for setting a bias potential at the timing of extinction, a stable optical waveform regardless of the extinction time A response can be realized. In addition, the overshoot current (hereinafter also referred to as OV current) is adjusted with the undershoot current optimized, and the rise response of the light waveform is improved, thereby improving the rise response during lighting and the light emission pulse width. It is possible to control the optical waveform to realize the stabilization.

  However, in the system in which the above adjustment has been made, when the drive current changes due to fluctuations in the amount of light due to aging or temperature changes, the UD current and OV current deviate from the optimum values. Problems such as dullness and excessive light emission may occur.

  The LD control circuit 23 will be described below with reference to FIG. Here, each component constituting the LD control circuit 23 including the CPU 50 is mounted on the same substrate.

  The CPU 50 generates a light emission level setting signal for setting the light emission level (drive current amplitude) of the LD 14 based on the light emission level of the LD 14 and the write control signal for controlling the lighting / light-off timing, and is driven through the operational amplifier OPA. Send to current source OPS.

  The drive current source OPS generates a drive current based on the light emission level setting signal from the CPU 50. The drive current source OPS includes, for example, a plurality of transistors.

  The CPU 50 also turns the pulse-like modulation signal (turns on / off the LD 14) based on the writing control signal from the image processing unit 21 and the pixel clock whose phase is set for each line from the phase synchronization circuit 25. A timing signal to be controlled) is generated and sent to the switch SW1 (for example, a transistor).

  The switch SW1 is turned on when the modulation signal is at a high level and turned off when the modulation signal is at a low level.

  Therefore, when the switch SW1 is ON, a closed circuit including the drive current sources OPS and LD14 is configured, and a pulsed drive current flows from the drive current source OPS to the LD14.

  Further, the CPU 50 outputs a bias ON signal to the switch SW2 (for example, a transistor). The switch SW2 is ON while the bias ON signal is input.

  Therefore, when the switch SW2 is ON, a closed circuit including the bias current sources BAS and LD14 is configured, and a bias current flows from the bias current source BAS to the LD14. The value of the bias current is preferably in the vicinity of the threshold current of the LD 14.

  Further, the CPU 50 sends an initial value or adjustment value of the OV current to the overshoot current source OVS via the DAC 31 (digital / analog conversion circuit), and sends an initial value or adjustment value of the UD current to the undershoot current via the DAC 31. Send to source UDS. Each of the overshoot current source OVS and the undershoot current source UDS includes, for example, a plurality of transistors. The initial values and adjustment values of the UD current and OV current will be described in detail later.

  Further, the CPU 50 sends a plurality of signals including a modulation signal (light emission command signal) to the selector SEL1 for setting the supply timing of the UD current based on the write control signal. Further, the CPU 50 sends a plurality of signals including a modulation signal (light emission command signal) for setting the supply timing of the OV current to the selector SEL2 based on the write control signal.

  The selector SEL1 generates an undershoot ON signal for setting the supply timing of the UD current using a plurality of signals from the CPU 50, and outputs the signal to the switch SW3 (for example, a transistor).

  The switch SW3 is ON while the undershoot ON signal is input.

  Therefore, when the switch SW3 is ON, a closed circuit including the undershoot current sources UDS and LD14 is configured, and for example, a rectangular pulsed UD current flows from the undershoot current source UDS to the LD14. The undershoot current source UDS generates a UD current based on the initial value or adjustment value of the amplitude of the UD current from the CPU 50.

  The selector SEL2 generates an overshoot ON signal that sets the supply timing of the overshoot current using a plurality of signals from the CPU 50, and outputs it to the switch SW4 (for example, a transistor).

  The switch SW4 is ON while the overshoot ON signal is input.

  Therefore, when the switch SW4 is ON, a closed circuit including the overshoot current sources OVS and LD14 is configured, and for example, a rectangular pulsed OV current flows from the overshoot current source OVS to the LD14. The overshoot current source OVS generates an OV current based on an initial value or an adjustment value of the amplitude of the OV current from the CPU 50.

  When the PD 30 detects the light from the LD 14, it converts the detected light amount into an electrical signal and outputs it to the CPU 50 via the LPF 32 (low-pass filter) and ADC (analog-digital converter) 34.

  Therefore, the CPU 50 sets the amplitude of the drive current based on the target light amount level (light emission level), and outputs the set amplitude as the light emission level setting signal to the drive current source OPS via the operational amplifier OPA.

  Further, the CPU 50 adjusts the UD current and the OV current according to the change in the amplitude of the drive current due to the passage of time or the temperature change, and sets the adjustment value of the UD current and the adjustment value of the OV current through the DAC 31 respectively. Send to UDS, overshoot current source OVS. Here, the UD current and the OV current are adjusted to values according to the value after the change of the amplitude of the drive current. It is preferable that the adjustment amounts of the UD current and the OV current are the same as the amplitude change amount of the drive current.

  As can be understood from the above description, an undershoot current supply unit that supplies the undershoot current to the LD 14 includes the CPU 50, the DAC 31, the undershoot current source UDS, the selector SEL1, and the switch SW3. Further, an overshoot current supply unit for supplying an overshoot current to the LD 14 is configured including the CPU 50, the DAC 31, the overshoot current source OVS, the selector SEL2, and the switch SW4.

  The configurations of the undershoot current supply unit and the overshoot current supply unit are not limited to those described above, and can be changed as appropriate.

  Here, the LD 14 as a light source includes a drive current source OPS for supplying a drive current to the LD 14, an overshoot current source OVS for supplying an OV current to the LD 14, and a bias current source for supplying a bias current to the LD 14. Consider a case where the LD 14 is driven with a specific lighting pattern (one pixel ON, one pixel OFF), for example, using a plurality of current sources including an undershoot current source UDS for supplying BAS and UD currents to the LD 14. It is assumed that the OV current and the UD current can be set digitally by a DAC code.

  The light emitted from the LD 14 enters the PD 30 that is a light receiving element for monitoring the light amount, and a PD current corresponding to the light amount flows through the PD 30. At this time, for example, a resistor is used to convert the PD current into a PD voltage, and the detected signal is integrated by the LPF 32 to detect a signal as a DC level signal. The ADC 34 digitizes the amount, and the CPU converts the amount into a signal. to decide. The CPU 50 compares the target light amount level (light emission level) of the write control signal with the detection signal level. If the detection signal is smaller than the target light amount, the DAC code is increased to increase the overshoot charge amount, and the signal detection and the CPU 50 are performed. Compare the signals at. The overshoot charge amount is a product of the amplitude of the OV current and the supply time of the OV current.

  When driving the semiconductor laser, as in Comparative Example 1 shown in FIG. 4, the optical waveform when the applied current (current supplied to the semiconductor laser) is normally applied (supplied) is the semiconductor laser and the laser driver, A waveform affected by an oscillation delay or a dull rise due to a parasitic capacitance added in parallel to the semiconductor laser by a package, substrate, wiring, or the like on which they are mounted. The degree of the effect appears remarkably in elements such as red LD and VCSEL having a large differential resistance. In addition, since the oscillation delay of the optical waveform with respect to the applied current signal has a value in the order of ns, the degree of influence becomes higher in a system that requires a high-speed pulse with a pulse width of 10 ns or less.

  Also, when generating a continuous pulsed optical waveform, there may be a difference in the rising response of the optical waveform between when the turn-off time before turning on the semiconductor laser is long and when it is short.

  FIG. 4 shows an application in the case where the semiconductor laser is controlled to emit light by supplying a driving current to the semiconductor laser only at the lighting timing, that is, by supplying a pulsed driving current with a bias current always applied. A timing chart of current, LD potential, and optical waveform is shown.

  In the left and center optical waveforms in FIG. 4, the extinction time T1 is sufficiently long, and the rising responses of the optical waveforms after the extinction time T1 are equivalent. This is because the LD potential becomes the bias potential Vbi when a sufficient time elapses after the light is turned off when the LD potential immediately before lighting is compared, and when the drive current is applied from the same LD potential (here, the bias potential Vbi). This is because the rising response of the optical waveform is equivalent.

  On the other hand, in the right optical waveform in FIG. 4, the extinction time T2 before lighting is very short compared to the extinguishing time T1, and the next pulse comes before the voltage level drops from the level at lighting to the bias potential Vbi (parasitic). When the next lighting starts before the capacity is completely discharged), the level of the LD potential at the rise becomes high.

  As a result, when the light extinction time before lighting is long and the LD potential at the rise of the optical waveform is the bias potential Vbi, the light extinction time before lighting is short and the LD potential at the rise of the optical waveform is higher than Vbi. In some cases, even if the same charge amount is supplied to the semiconductor laser when it is turned on, the response and shape of the optical waveform are different.

  In a state where the LD potential at the time of rising is higher than the bias potential Vbi, a faster response waveform with a reduced oscillation delay than in the prior art is obtained. However, in this case, even if the input data width is the same, the width and shape of the output optical waveform are different. For example, when this LD control circuit is applied to an image forming apparatus, the stability of the image may be lowered. is there.

  Next, the problem of Comparative Example 2, which is an example of adding an undershoot current to the drive current, will be described with reference to FIGS. 5 and 6 show timing charts of the applied current, LD potential, and optical waveform for the LD shown in FIG. 4 under two conditions in which the amount of undershoot charge supplied at the falling timing of the applied current is different. An example is shown. The undershoot charge amount is a product of the amplitude of the UD current and the supply time of the UD current.

  FIG. 5 shows an example in which pulsed light emission (pulse lighting) is repeatedly performed at the lighting time T1 and the turn-off time T2. FIG. 6 shows an example in which pulse light emission is repeatedly performed at the lighting time T1 and the light-off time T3 (<T2). 5 and 6 indicate the case where the undershoot charge amount is smaller than the optimum value, and the alternate long and short dash line in FIGS. 5 and 6 indicates the case where the undershoot charge amount is larger than the optimum value.

  Here, it is assumed that an optimal (ideal) undershoot charge amount is applied and the LD potential after application reaches the bias potential Vbi in the shortest time. When the undershoot charge amount is smaller than the optimum value, the LD potential immediately after the application of the undershoot current is not lowered to the bias potential Vbi. Therefore, the next pulse is applied before the LD potential is lowered to the bias Vbi. As a result, particularly when the turn-off time is short as shown in FIG. 6, the rising response of the optical waveform of the pulse becomes too sharp and there is a risk of over-emission.

  Assuming that the amount of light corresponding to the lighting time at this time is P1a in FIG. 5 and P1b in FIG. 6, the relationship is P1a <P1b, and the turn-off time despite the same turn-on time T1. The shorter light emission of FIG. 6 results in a larger total light emission amount.

  On the other hand, when the undershoot charge amount is larger than the optimum value, the LD potential immediately after the application of the undershoot current is lower than the bias potential Vbi, so that the next pulse is applied before it rises to the bias potential Vbi. As a result, particularly when the turn-off time is short as shown in FIG. 6, there is a possibility that an oscillation delay or a waveform rounding may occur.

  If the amount obtained by integrating the amount of light corresponding to the lighting time at this time is P2a in FIG. 5 and P2b in FIG. 6, the relationship P2a> P2b is established, and even though the lighting time T1 is the same, FIG. In FIG. 5, which has a longer turn-off time, the total light emission amount becomes larger, which is opposite to the case where the undershoot charge amount is insufficient.

  Such a difference in total light emission occurs because the next lighting starts while the LD potential after application of the undershoot current transitions from a potential different from the bias potential Vbi that is stable at the time of extinction to a bias potential. This is because the response at the rise of the optical waveform is different from the optimum state.

  Therefore, by applying an optimal undershoot charge amount at which the LD potential after application of the undershoot current is equivalent to the bias potential Vbi, a stable light waveform and a light waveform having an integrated amount of light amount can be obtained regardless of the turn-off time. realizable.

  By the way, the undershoot current application time tud, which is the time for applying (supplying) the undershoot current, needs to be set shorter than T2 or T3, which is the turn-off time. This is because when the tud is longer than T2 and T3, the UD current is applied at the timing of the rise of the next lighting waveform, and the rise characteristic of the waveform is distorted.

  Here, as a lighting pattern in the case of adjusting the UD current, it is necessary to pay attention to the response time of the potential change that changes due to factors such as parasitic capacitance in the system including the LD 14 and the LD control circuit 23. For example, when the extinguishing time is T2, the bias potential Vbi is set regardless of the amount of undershoot charge. Therefore, even if the undershoot current is changed in this state, the rise response does not change, so that the rise response of the waveform is stabilized. There is no need to adjust the undershoot charge amount.

  On the other hand, when the turn-off time is short as shown in FIG. 6, for example, the amount of undershoot charge affects the rising characteristics of the LD potential (optical waveform), so it is necessary to adjust the amount of undershoot charge.

  FIG. 7 shows a first embodiment which is an example in which the undershoot charge amount is optimized. That is, FIG. 7 shows the current at the fall timing (at the fall) of the drive current in the case where the applied current is configured including the bias current, drive current, and undershoot current shown in FIGS. The case where the optimum value of the subtracted undershoot charge amount (optimum undershoot charge amount) is applied is shown.

  By applying the optimal undershoot charge amount at the falling timing of the optical waveform (at the time of falling), the waveform response caused by the parasitic capacitance caused by the wiring, driver, LD14, etc. is stabilized, and the variation in delay amount is reduced. It becomes possible to do. In addition, when the optimum undershoot charge amount is applied, it is possible to make a transition to the bias potential Vbi at a high speed immediately after that, and to set the LD potential to the bias potential Vbi until the next lighting signal after turning off is input. Thus, an optical waveform having an optimum (ideal) rising characteristic can be realized regardless of the turn-off time.

  Next, the problem of the comparative example 4, which is an example of applying the overshoot current, will be described with reference to FIG.

  In Comparative Example 4, as shown in FIG. 8, the semiconductor laser is driven using an applied current including a bias current, a drive current, and an overshoot current.

  By the way, in general, when a semiconductor laser is lit only with a drive current, a delay or a dullness occurs in the rising response waveform due to various factors caused by a drive system including a laser driver for driving the semiconductor laser.

  Therefore, in Comparative Example 4, waveform delay and dullness can be improved by adding an overshoot current that is a large current instantaneously when the drive current rises.

  However, in Comparative Example 4, the rising response is improved, but the response characteristic varies depending on the LD potential immediately before the drive current is applied.

  For example, in the left and center waveforms in FIG. 8, the extinguishing time T1 is set long, and the LD potential at the rise becomes the bias potential Vbi.

  On the other hand, the light waveform on the right side of FIG. 8 has a short extinction time T2 before lighting, and the LD potential when the central waveform is extinguished is not lowered to the bias potential Vbi, and the overshoot current from a certain potential higher than the bias potential Vbi. Is applied, and there is a risk of excessive light emission at the start-up compared to the left and center optical waveforms.

  As described above, in Comparative Example 4, the rise response of the waveform varies depending on the length of the turn-off time, and there is a risk of deteriorating the laser life due to excessive light emission.

  Therefore, by applying the optimum undershoot charge amount, it is possible to always set the LD potential at the time of light emission (at the time of lighting) to the same level and obtain a stable photoresponse waveform regardless of the extinguishing time.

  In order to achieve both the method of improving the rise response of the waveform by applying an overshoot current in this way and the method of realizing a stable rise response regardless of the turn-off time by applying an undershoot current, Need to be adjusted based on the target optical waveform.

  As the method, the first method of adjusting the undershoot current first, and then adjusting the overshoot current, and the second method of adjusting the overshoot current first and then adjusting the undershoot current are described. Two methods are conceivable.

  First, consider the first method, that is, the case of adjusting the undershoot current first. As described above, it is necessary to adjust the undershoot current in the case of a light emission pattern with a short turn-off time in which a difference occurs in the rising response of the waveform depending on the amount of undershoot charge. In this case, if the undershoot current is changed, the rising shape changes with the falling shape of the waveform, so how to set the target optical waveform and its integrated light quantity when no overshoot current is applied. It is difficult to determine whether or not it should be.

  Next, consider the second method, that is, the case of adjusting the overshoot current first. In order to adjust the overshoot current, the LD potential before rising needs to be stable at the bias potential. The purpose of applying the overshoot current is to compensate for the amount of decrease in the integrated light quantity from the ideal optical waveform due to the dullness at the rising portion of the optical waveform, by causing the overshoot current to emit excessive light.

  Further, when the optimum value of the undershoot current was described in Comparative Example 2, it was stated that the optimum value is determined by the parasitic capacitance of the system including the light source (for example, LD) and the light source control device (LD control circuit). Therefore, if the substrate configuration on which the light source and the light source control device are mounted is determined, the parasitic capacitance can be roughly estimated and will not vary greatly.

  Therefore, in the configuration in which the above system is determined, by applying an initial value of the undershoot current corresponding to the expected parasitic capacitance in advance, the optimum undershoot current required for stabilizing the rising response of the waveform Can be set to a small value (a value approximate to the undershoot current).

  Consider a state where an initial value of the undershoot current is applied and an overshoot current is further applied. Due to the initial value of the undershoot current, the LD potential after extinguishing converges to the bias potential Vbi in a corresponding short time, if not as much as the optimum undershoot current.

  In this case, the target integrated light amount (target) is adjusted by adjusting the overshoot current so that the integrated light amount (time integrated value of the light amount) becomes the target waveform in the light emission pattern having a turn-off time longer than the convergence time. The overshoot charge amount can be adjusted with respect to the integrated light amount).

  Next, let us consider a case where undershoot current adjustment is performed while an optimum overshoot current determined by overshoot current adjustment is applied. Here, at the time of adjusting the overshoot current, it is desirable that the rising waveform and the integrated light amount are adjusted with the LD potential in a state where the optimum undershoot current is applied so that the LD potential before the rise becomes the bias potential Vbi.

  Therefore, by adjusting the undershoot current with a light emission pattern (lighting pattern) that repeatedly turns on with a light-off time shorter than the time of overshoot current adjustment and with a light-off time short enough to cause a change in LD potential due to parasitic capacitance, By setting the integrated light intensity that does not change the rise response as the target value and performing the undershoot current adjustment, the ideal rise response waveform obtained by the overshoot current adjustment is maintained and stable regardless of the turn-off time. It is possible to adjust the undershoot current to realize the rising response.

  FIG. 9 shows a flowchart of applied current, LD potential, and optical waveform when the overshoot current and undershoot current are optimally adjusted.

  As a result, after adjusting the overshoot current, each current can be optimized by adjusting the undershoot current, improving the rising characteristics of the optical waveform, and rising the optical waveform regardless of the turn-off time. Reproducibility can be stabilized.

  Hereinafter, an example of a method for setting initial values of the OV current and the UD current will be described with reference to FIG. FIG. 10 is a flowchart showing a procedure for performing OV current adjustment and UD current adjustment on at least one LD. This flowchart is based on a processing algorithm executed by the CPU 50 and corresponds to not only a single LD but also a plurality of LDs. In addition, the case where there are a plurality of LDs means, for example, a case where an LD corresponding to each color for forming a color image is provided or a plurality of lights from an LDA including a plurality of LDs on at least one photosensitive drum. The case where it scans simultaneously is mentioned.

  In the first step S1, it is determined whether or not there are a plurality of LDs. If the determination in step S1 is affirmed, the process proceeds to step S2. On the other hand, if the determination in step S1 is negative, the process proceeds to step S3.

  In step S2, one LD to be adjusted is set from the plurality of LDs. When step S2 is executed, the process proceeds to step S3.

  In step S3, the LD is lit with an OV current adjustment pattern that repeats the pulses of the lighting time T1 and the turn-off time T2 (step S2). Therefore, the light amount of each light pulse from the LD is detected by the PD, smoothed (integrated) by the LPF, and the average value of the integrated light amounts of the plurality of light pulses is acquired by the ADC.

  In the next step S4, the OV current is adjusted. Specifically, the OV current is gradually increased from a small value. Increasing the OV current also increases the integrated light quantity. Therefore, the CPU compares the average integrated light quantity acquired by the ADC in step S3 with the target integrated light quantity.

  In the next step S5, the adjustment value of the OV current is stored. Specifically, when the average value is smaller than the target integrated light amount, the overshoot current is further increased, and the overshoot current at the time when the average value becomes equal or larger is set as the optimum overshoot current (OV current adjustment value). Save to memory (not shown).

  In the next step S6, the UD current adjustment pattern that repeats the pulses of the lighting time T1 and the turn-off time T3 (<T2) is turned on. Therefore, the light amount of each light pulse from the LD is detected by the PD, smoothed (integrated) by the LPF, and the average value of the integrated light amounts of the plurality of light pulses is acquired by the ADC.

  In the next step S7, the UD current is adjusted. Specifically, the initial value of the undershoot current is set slightly larger than the parasitic capacitance of the system including the LD and the LD control circuit, and the undershoot current is gradually reduced. When the undershoot current is reduced, the response of the falling portion of the optical waveform is deteriorated, and the integrated light quantity at the falling portion is increased. On the other hand, at the rising portion of the optical waveform, the amount of integrated light decreases because the undershoot current is excessive, that is, the LD potential approaches the bias potential from a state lower than the bias potential. Therefore, the CPU compares the average integrated light quantity acquired by the ADC in step S6 with the target integrated light quantity.

  In the next step S8, the UD adjustment value is stored. Specifically, the undershoot current is decreased, and the undershoot current at the time when the average value is equal to or smaller than the target integrated light amount is stored in a memory (not shown) as the optimum undershoot current (UD current adjustment value). To do. When step S8 is executed, the adjustment of one LD is completed.

  In the next step S9, it is determined whether there is an unadjusted LD. If there is an unadjusted LD, the determination in step S9 is affirmed, and the process returns to step S2. If there is no unadjusted LD, the determination in step S9 is denied and the flow ends.

  In other words, when there are a plurality of LDs, the initial value of the overshoot current and the initial value of the undershoot current are optimally set for all the LDs by repeating the series of steps S2 to S9 for each LD. It is possible to reproduce an optical waveform that is stable regardless of the turn-off time and that has a target integrated light quantity.

  Next, with reference to FIG. 11, an example (Example 3) of a method for adjusting the undershoot current according to the change in the drive current will be described. Here, the “change in drive current” includes a change due to a change in environmental temperature and a change due to a change over time in the light emission characteristics of the laser. Therefore, in view of the change in the environmental temperature over time, the “change in drive current” may be referred to as the “change in drive current over time”.

  Here, a method for adjusting the undershoot current when the drive current Iop is changed will be described using a timing chart of an applied current to the LD 14, an LD potential, and an optical waveform. Let the amount of change in Iop be ΔIop.

  First, the UD current is set to an initial value Iud1, which is an optimal UD current with respect to the initial value of the drive current Iop set according to the target light amount level.

  After the UD current is set to the initial value Iud1, the light amount (light emission level) of the light from the LD 14 slightly changes from the target light amount level due to factors such as a change in laser characteristics with time and a change in environmental temperature, that is, the drive current Iop is initial. A slight change from the value causes variations in the rising response of the waveform. This is because the optimum value of the UD current deviates from the initial value Iud1 due to the change in the drive current Iop.

  Therefore, in this embodiment, the undershoot current change amount ΔIud is set according to the drive current change amount ΔIop according to the light emission level change amount ΔP, thereby optimizing the undershoot current Iud with respect to the change in the drive current Iop. Can be performed simply.

  FIG. 13 is a graph showing the relationship between the drive current Iop and the optimum undershoot current with respect to the drive current Iop (hereinafter also referred to as optimum Iud).

  Then, when outputting an optical waveform, consider a case where a bias current Ibi is applied when the light is turned off and a combined current = bias current Ibi + drive current Iop is applied when the light is turned on, as shown in FIG. In this case, the optimum undershoot current is a current for quickly transitioning the potential from the potential Vop determined by the combined current = bias current Ibi + drive current Iop to the bias potential Vbi.

  Here, the optimal undershoot current when the combined current at the light amount P1 is Ibi + Iop1 is Iud1, the optimal undershoot current when the combined current at the light amount 2 is Ibi + Iop2 is Iud2, and the difference between Iop2 and Iop1 is ΔIop. Assuming that the difference between Iud2 and Iud1 is ΔIud, ΔIop and ΔIud are substantially proportional when the application time of the undershoot current is constant.

  That is, as shown in FIG. 13, when the drive current changes from Iop1 to Iop2, the optimum undershoot current changes from Iud1 to Iud2, and ΔIud / ΔIop takes a substantially constant value regardless of the amount of light. Therefore, when the drive current changes by ΔIop, setting the optimal undershoot current change amount ΔIud corresponding to ΔIop makes it possible to set the optimal undershoot current with a simple configuration without going through the initial adjustment procedure. Become.

  As a result, according to the present embodiment, even after the initial setting of the UD current, even when the amount of light from the LD 14 fluctuates due to the influence of the process and the optical system over time, the process of the initial setting of the UD current is not performed. It is possible to correct the UD current after the light amount variation to an optimum value by a simple procedure, and a stable optical waveform response can be realized regardless of the turn-off time of the turn-on / off pulse.

  Below, an example (Example 4) which adjusts about both an undershoot electric current and an overshoot electric current is demonstrated using FIG.

  Here, in the LD control circuit that emits light with the light amount P1, the drive current Iop1, and the LD potential Vop1, and turns on and off the bias current Ibi and the bias potential Vbi when the light is turned off, the undershoot current is set to the initial value Iud1. Consider the case where the rising response of the waveform is stabilized.

  When the light emission level becomes P1 + ΔP1 due to the light amount fluctuation ΔP1 over time, the drive current becomes Iop + ΔIop1, and when the UD current is maintained at the initial value Iud1, the LD potential can be lowered to the bias potential by the increase in the LD potential. Disappear. In other words, when the UD current is insufficient with respect to the optimum value and the turn-off time is short in the optical waveform response, the waveform rises steeply.

  Accordingly, by changing the UD current to ΔIud1 and setting the optimum value with respect to the change amount ΔIop1 of the drive current, a stable optical waveform response can be realized regardless of the turn-off time.

  In FIG. 12, when the undershoot current application time is tud and the overshoot current application time is tov, the UD current minute change ΔIud and the OV current minute change amount with respect to the minute change ΔIop of the drive current Iop. Based on the idea of optimizing the change in the LD potential, ΔIov is set to a current amount that makes the applied charge amount determined by the application time and the applied current equal to each other. It should be possible to set the amount of shoot current. Therefore, by setting the undershoot charge amount and overshoot charge amount such that tud × ΔIud = tov × ΔIov, a stable optical waveform can be formed regardless of the turn-off time. In this case, the undershoot current change amount ΔIud and the overshoot current change amount ΔIov may be made equal, the undershoot current application time tud and the overshoot current application time tov may be made equal, ΔIud and ΔIov are made different, and Tud and tov may be different.

  FIG. 14 is a graph showing the relationship between the drive current and the optimum overshoot current with respect to the drive current (hereinafter also referred to as optimum Iov). As can be seen from FIG. 14, since the change ΔIop of the drive current Iop and the change ΔIov of the overshoot current Iov have a substantially proportional relationship, an appropriate change amount of Iov according to the change amount of Iop can be obtained. High-precision optical waveform correction can be realized without initializing the OV current.

  That is, by adding the change amount ΔIud of the UD current to the UD current and adding the change amount ΔIop of the OV current to the OV current, the initial setting process requiring a set time is not performed, and a highly accurate and simple configuration is achieved. Optical waveform response can be realized.

  FIG. 15 shows an example of a table indicating values of the drive current Iop, the optimum Iud, and the optimum Iov for each of the light amounts P1 to P4.

  This table is created by obtaining drive currents (Iop1 to Iop4), optimum Iud (Iud1 to Iud4), and optimum Iov (Iov1 to Iov4) for each light quantity, and arranging them in association with each other.

  When the drive current changes (the amount of light changes), the UD current and the OV current are selected by selecting the optimum Iud and the optimum Iov according to the values after the change in the drive current based on the values in the table. By adjusting, it is possible to form a stable optical waveform with a simplified correction value without going through the initial setting process of the UD current and the OV current.

  When the table of FIG. 15 is used, for example, when the value after the change of the drive current is less than Iop1, the value is regarded as Iop1, and when the value after the change of the drive current is Iop1 to Iop2, the value Is regarded as a more approximate value of Iop1 and Iop2, and when the value after the change in the drive current is Iop2 to Iop3, the value is regarded as a more approximate value of Iop2 and Iop3, and the change in the drive current When the later value is Iop3 to Iop4, the value is regarded as a more approximate value of Iop3 and Iop4, and when the value after the change of the drive current is larger than Iop4, the value is regarded as Iop4. good.

  In the table of FIG. 15, the light amount, the drive current, the optimum Iud, and the optimum Iov are displayed in four steps, but may be displayed in three steps or less or five steps or more.

  Further, the table of FIG. 15 includes both the optimum value of the UD current and the optimum value of the OV current. In short, it is sufficient that at least one of them is included.

  FIG. 16 shows a method in which correction coefficients of Iud and Iov are obtained with respect to the light quantity range (P1 to P2), and when the light quantity changes, a current amount proportional to Iop is applied based on the coefficient. A table is shown. Here, when the proportional coefficient of the UD current is A and the proportional coefficient of the OV current is B, A and B are values that change according to the application time of the UD current, the application time of the OV current, and the like.

  In this case, when the drive current changes (the amount of light changes), ΔIop × A, which is a value obtained by multiplying the change amount ΔIop of the drive current by A, is applied as a UD current, and B is applied to the change amount ΔIop of the drive current Iop. By applying ΔIop × B, which is a multiplied value, as an OV current, a stable optical waveform can be generated regardless of the extinguishing time and the amount of light, compared to the case of maintaining the initial setting state.

  Note that the table in FIG. 16 includes both the UD current change amount and the OV current change amount, but in short, it is sufficient that at least one of them is included.

  FIG. 17 shows an example of a process of generating a modulation current (applied current) based on a light emission command signal (modulation signal), an overshoot ON signal, and an undershoot ON signal corresponding to image data from the host device. Yes.

  First, a delay pulse 1 delayed by t1 with respect to the light emission command signal is generated, and an overshoot ON signal and an undershoot ON signal are generated from the light emission command signal and the delay pulse 1. The overshoot ON signal and the undershoot ON signal are input to the corresponding switches, and the current from the corresponding current source is turned on and off, whereby the waveform of the modulation current (applied current) is obtained.

  More specifically, in FIG. 17, the light emission command signal is delayed to become a delay pulse 1, and an overshoot ON signal can be generated by ANDing the light emission command signal and the inversion of the delay pulse 1. For example, this delay may be simply performed by an inverter array or a buffer array, or a signal that has been subjected to waveform shaping after being delayed by a low-pass filter including a resistor and a capacitor may be used. In either case, the delay amount can be easily changed by changing the number of stages or the filter constant.

  At this time, the modulation current is the sum of the drive current and the OV current. The overshoot ON signal is turned on for a short time from the timing when the drive current is turned on, for example, 0.4 ns to 5 ns. However, it is preferable to set the time when the gradation reproducibility is the best. The magnitude of the OV current (drive current × C) is preferably set in consideration of the characteristics of the LD, the sensitivity characteristics of the photosensitive drum, etc., but is usually set to about C = 0.1-1. The This is because if it is larger than this, there is a high possibility that the amount of light exceeds the rating of the LD, and there is a high risk of destroying the LD, which causes a problem in terms of the life and safety of the LD.

  FIG. 18 shows an example in which the application time of the OV current and the UD current is made different by using the delay pulse 1 and the delay pulse 2 having a delay time different from that of the delay pulse 1. As can be seen from FIG. 18, by generating a plurality of delay pulses from the light emission command signal (modulation signal), the application time t1 of the OV current and the application time t2 of the UD current can be easily changed as necessary. . For example, it is conceivable that the application time of the UD current is reduced as the minimum value of the turn-off times of the plurality of pulses is reduced.

FIG. 19A is a graph showing IL characteristics (current-light output characteristics) of the semiconductor laser. For the sake of explanation, FIG. 19 shows abbreviations of the currents as follows.
Iop: Laser operating current (drive current)
Ith: Laser threshold current Idrv: Iop: Laser drive current driven at high speed Idrv = Ild + Iled
Ild: current corresponding to the LD region of the laser Iled: current corresponding to the LED region of the laser Vop: operating voltage during light emission Vth: voltage when threshold current is applied

  FIG. 19B shows an example of an applied current waveform when the semiconductor laser is turned on. In FIG. 19B, the rising characteristics of the drive current are shown by linear approximation for the sake of simplicity. Here, it rises from time t1, becomes Iled at time t2, and becomes Iop = Iled + Ild at time t3.

  As can be seen from FIGS. 19A and 19B, the increase rate of the light output with respect to the increase in current is very small because of the LED region of the semiconductor laser until time t2 of the applied current waveform. On the other hand, after t2, since it becomes the LD region of the semiconductor laser, the increase rate of the optical output with respect to the increase in current is large and appears as the rising characteristic of the optical waveform.

  In FIG. 19A, the IL characteristic of the LD in a certain state is indicated by a solid line (a), and the IL characteristic when it is assumed that the maximum optical output PO of the LD is reached at the maximum current value Iop_max in the specification is indicated by a broken line (b ).

  By the way, when a plurality of semiconductor lasers and a driver (LD control circuit) are mounted on a substrate on which the two are connected by wiring, as shown in FIG. 20 as an example, the parasitic capacitance of a package that accommodates the semiconductor lasers. There are a plurality of parasitic capacitances such as a parasitic capacitance of a package in which the driver is accommodated and a parasitic capacitance due to wiring. In this case, when the semiconductor laser is turned on, a current (Iled + Ild) that should be given to each semiconductor laser flows after charging (Ic) to the total parasitic capacitance C including these parasitic capacitances. Due to variations in the total parasitic capacitance C between lasers, time differences such as response characteristics and oscillation delay times occur between semiconductor lasers. Even when the light is turned off, the discharge of the total parasitic capacitance C affects the falling characteristics and the next rising response.

  Therefore, the charge amount corresponding to the total parasitic capacitance C is discharged at high speed by the UD current, and the LD potential is rapidly changed to the bias potential, thereby improving the reproducibility of the rising response waveform at the next light emission (lighting time). be able to.

  The LD control circuit 23 of the present embodiment described above is a semiconductor laser control device that supplies a pulsed drive current to the LD 14, and includes an overshoot current supply unit that supplies an overshoot current to the LD 14 when the drive current rises, and The undershoot current supply unit supplies an undershoot current to the LD 14 when the drive current falls, and the overshoot current supply unit changes the drive current (for example, a change due to a change in ambient temperature or a change in light emission characteristics of the LD 14 over time). The overshoot current is adjusted according to the undershoot current, and the undershoot current supply unit adjusts the undershoot current according to the change in the drive current.

  In this case, even if the overshoot current and the undershoot current deviate from the appropriate values corresponding to the values after the change of the drive current due to the change of the drive current, it can be adjusted to the appropriate values.

  As a result, a desired optical waveform can be stably output from the LD 14.

  By providing both the overshoot current supply unit and the undershoot current supply unit, both the OV current and the UD current can be adjusted to appropriate values according to changes in the drive current, and the reproducibility of the optical waveform can be improved. Can be improved.

  Further, the value obtained by multiplying the adjustment amount of the undershoot current by the time during which the undershoot current is supplied is approximately equal to the value obtained by multiplying the adjustment amount of the overshoot current and the time during which the overshoot current is supplied. By making them equal, a stable optical waveform can be output regardless of the turn-off time.

  In addition, at least one of the undershoot current and the overshoot current includes a plurality of adjustment values (for example, Iud1 to Iud4, Iov1 to Iov4) individually determined according to a plurality of values (for example, Iop1 to Iop4) of the drive current. By adjusting based on the table, the adjustment operation can be simplified and the adjustment time can be shortened.

  Further, by adjusting at least one of the undershoot current and the overshoot current based on a proportional constant (for example, A, B) determined in advance according to the change ΔIop of the drive current, the adjustment operation can be simplified and the adjustment time can be reduced. Can be shortened.

  Also, by starting at least one of the adjustment of the undershoot current and the adjustment of the overshoot current based on at least one of the ambient temperature change and the elapsed time after the start of startup, for example, the amount of light from the LD 14 is referred to. Even without this, at least one of the undershoot current and the overshoot current can be adjusted in accordance with the change in the drive current. Specifically, a table showing a change in driving current corresponding to at least one of an ambient temperature change and an elapsed time after start of startup may be created in advance, and adjustment may be performed based on the table. An example of the adjustment based on the elapsed time after the start of activation includes a case where the adjustment is performed every predetermined time (periodically) after the start of activation. As an example of adjusting based on a change in ambient temperature, for example, a case where a predetermined temperature is used as a reference value and the temperature is adjusted when the temperature exceeds or falls below the reference value can be cited.

  In addition, by detecting a change in the amount of light from the LD 14 and obtaining a change in the drive current based on the change in the amount of light, the change in the drive current can be detected with high accuracy, thereby adjusting the undershoot current and the overshoot current. It can be performed with high accuracy.

  Further, the LD control circuit 23 controls a plurality of semiconductor lasers (for example, LDs), so that variation in rising characteristics between the semiconductor lasers can be suppressed.

  Further, the laser printer 1000 includes a photosensitive drum 1030 and an LD 14, and an optical scanning device 1010 that scans the photosensitive drum 1030 with light from the LD 14, and a pulsed drive current corresponding to image information to the LD 14. And an LD control circuit 23 to be supplied.

  In this case, the LD control circuit 23 can stably output a desired optical waveform from the LD 14, so that an image can be stably and accurately formed on the photosensitive drum 1030.

  As can be seen from the above description, the semiconductor laser control device of the present invention is characterized by the light quantity, temperature, and time passage after the initial setting of at least one of the overshoot current and the undershoot current in order to solve the problems of the prior art. When a change occurs, each current has a function to set the overshoot current and undershoot current based on a table created in advance and a conversion formula for the optimum undershoot current and overshoot current for the drive current. It is possible to realize a highly accurate and stable optical waveform while reducing the adjustment time.

  As a result, overshoot current adjustment and undershoot current adjustment functions that improve the pulse reproducibility of the optical waveform, even if the drive current changes due to fluctuations in the characteristics of the semiconductor laser or environmental temperature after the initial setting, the changes By changing the overshoot current and undershoot current according to the amount, a stable light waveform can be obtained regardless of the pixel lighting cycle (semiconductor laser lighting cycle) and extinguishing time, and image formation with high pixel reproducibility A device can be realized.

  In the above-described embodiment, the LD control circuit 23 includes both the overshoot current supply unit and the undershoot current supply unit. However, the present invention is not limited to this. That is, supply and adjustment of overshoot current and supply and adjustment of undershoot current may be performed, supply and adjustment of overshoot current may be performed, or supply and adjustment of undershoot current may be performed only. Also good.

  In the above embodiment, the bias current Ibi is applied, but it is not always necessary to apply it.

  In the above embodiment, the drive current is adjusted based on the light amount from the LD 14, that is, the output signal from the PD 30, and at least one of the UD current and the OV current is adjusted based on the adjustment amount (change amount). good.

  In the above embodiment, an LD (laser diode: edge emitting laser) is used as the semiconductor laser, but a surface emitting laser (VCSEL) may be used instead. Since the surface emitting laser has a lower rise characteristic than the LD, the effect of the present invention can be further expected.

  In the above embodiment, the laser printer 1000 is employed as the image forming apparatus of the present invention, but the present invention is not limited to this. For example, the image forming apparatus of the present invention may be a color printer 2000 including a plurality of photosensitive drums as shown in FIG. 21 as an example.

  The color printer 2000 is a tandem multicolor printer that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow), and is a black station (photosensitive drum K1, charging device K2). , Developing device K4, cleaning unit K5, and transfer device K6), cyan station (photosensitive drum C1, charging device C2, developing device C4, cleaning unit C5, and transfer device C6), and magenta station ( The photosensitive drum M1, the charging device M2, the developing device M4, the cleaning unit M5, and the transfer device M6), and the yellow station (the photosensitive drum Y1, the charging device Y2, the developing device Y4, the cleaning unit Y5, and the transfer device Y6). ), Optical scanning device 2010, and transfer belt 2 80, and a fixing unit 2030.

  Each photoconductive drum rotates in the direction of the arrow in FIG. 21, and a charging device, a developing device, a transfer device, and a cleaning unit are arranged around each photoconductive drum along the rotation direction. Each charging device uniformly charges the surface of the corresponding photosensitive drum. The surface of each photosensitive drum charged by the charging device is irradiated with laser light by the optical scanning device 2010, and a latent image is formed on each photosensitive drum. Then, a toner image is formed on the surface of each photosensitive drum by a corresponding developing device. Further, the toner image of each color is transferred onto the recording paper on the transfer belt 2080 by the corresponding transfer device, and finally the image is fixed on the recording paper by the fixing unit 2030.

  The optical scanning device 2010 includes an LD control circuit having the same configuration as the LD control circuit 23, which has the same LD as the LD 14 of the above embodiment for each color and controls each LD. Thus, the same effect as that of the optical scanning device 1010 can be obtained, and the occurrence of color misregistration can be suppressed. In addition, since the color printer 2000 includes the optical scanning device 2010, the same effect as the laser printer 1000 can be obtained.

  By the way, in the color printer 2000, color misregistration may occur due to manufacturing error or position error of each component. Even in such a case, since the optical scanning device 2010 includes an LD control circuit that controls each LD, the occurrence of the color misregistration can be prevented.

  In the color printer 2000, the case where the optical scanning device is integrally configured has been described. However, the present invention is not limited to this. For example, an optical scanning device may be provided for each image forming station, or an optical scanning device may be provided for every two image forming stations.

  In the color printer 2000, the case where there are four photosensitive drums has been described. However, the present invention is not limited to this. For example, five or more photosensitive drums may be provided.

  In addition, the image forming apparatus of the present invention may be an image forming apparatus that directly irradiates laser light onto a medium (for example, paper) that develops color with laser light.

  The image forming apparatus of the present invention may be an image forming apparatus using a silver salt film as an image carrier. In this case, a latent image is formed on the silver salt film by optical scanning, and this latent image can be visualized by a process equivalent to a developing process in a normal silver salt photographic process. Then, it can be transferred to photographic paper by a process equivalent to a printing process in a normal silver salt photographic process. Such an image forming apparatus can be implemented as an optical plate making apparatus or an optical drawing apparatus that draws a CT scan image or the like.

  In addition to image forming apparatuses such as laser printers, color printers, and digital copying machines, the present invention also includes semiconductor lasers such as optical disk apparatuses, optical communication apparatuses, and laser radar apparatuses, and semiconductor laser control that controls the semiconductor lasers. The present invention can be applied to equipment including the device.

  DESCRIPTION OF SYMBOLS 14 ... LD (semiconductor laser), 23 ... LD control circuit (semiconductor laser control apparatus), 1000 ... Laser printer (image forming apparatus), 1010 ... Optical scanning apparatus, 1030 ... Photosensitive drum, 2000 ... Color printer (Image forming apparatus) ), 2010... Optical scanning device.

JP 2011-198877 A

Claims (11)

In a semiconductor laser control device for supplying a pulsed drive current to at least one semiconductor laser,
At least one of an overshoot current supply unit that supplies an overshoot current to the semiconductor laser when the drive current rises and an undershoot current supply unit that supplies an undershoot current to the semiconductor laser when the drive current falls ,
The overshoot current supply unit adjusts the overshoot current according to a change in the drive current,
The semiconductor laser control device according to claim 1, wherein the undershoot current supply unit adjusts the undershoot current according to a change in the drive current.   The semiconductor laser control device according to claim 1, comprising both the overshoot current supply unit and the undershoot current supply unit.   A value obtained by multiplying the adjustment amount of the undershoot current and the time during which the undershoot current is supplied, and a value obtained by multiplying the adjustment amount of the overshoot current and the time during which the overshoot current is supplied are approximately The semiconductor laser control device according to claim 2, wherein the semiconductor laser control devices are equal to each other.   The at least one of the undershoot current and the overshoot current is adjusted based on a table including a plurality of adjustment values individually determined according to a plurality of values of the drive current. The semiconductor laser control apparatus as described in any one of -3.   4. The apparatus according to claim 1, wherein at least one of the undershoot current and the overshoot current is adjusted based on a proportional constant determined in advance according to a change in the drive current. The semiconductor laser control device described.   6. At least one of the adjustment of the undershoot current and the adjustment of the overshoot current is started based on at least one of an ambient temperature change and an elapsed time after the start of startup. A semiconductor laser control device according to claim 1. Detecting a change in the amount of light from the semiconductor laser;
The semiconductor laser control device according to claim 1, wherein a change in the driving current is obtained based on the change in the light amount.   The semiconductor laser control device according to claim 1, wherein the semiconductor laser is a surface emitting laser. A photosensitive drum;
An optical scanning device having at least one semiconductor laser and scanning the photosensitive drum with light from the semiconductor laser;
An image forming apparatus comprising: the semiconductor laser control device according to claim 1, wherein a pulsed drive current corresponding to image information is supplied to the semiconductor laser. Supplying a pulsed drive current to the semiconductor laser;
At least one of supplying an overshoot current to the semiconductor laser at the time of rising of the drive current, and supplying an undershoot current to the semiconductor laser at the time of falling of the drive current,
In the step of supplying the overshoot current, the overshoot current is adjusted according to a change in the drive current, and the adjusted overshoot current is supplied to the semiconductor laser.
In the step of supplying the undershoot current, the undershoot current is adjusted in accordance with a change in the drive current, and the adjusted undershoot current is supplied to the semiconductor laser.   11. The semiconductor laser control method according to claim 10, comprising both a step of supplying the overshoot current and a step of supplying the undershoot current.