Light-emitting element driving device

In a semiconductor laser driving device using a voltage drive system, a drive current of a semiconductor laser is controlled by a drive current control circuit so that a light intensity of light beam emitted from the semiconductor laser is equal to a predetermined light intensity. In a drive voltage control circuit, the drive voltage applied to the semiconductor laser at the time of switching off the semiconductor laser LD is set on the basis of the detected voltage (terminal voltage).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element driving device large in internal resistance (series resistance) and particularly to a light-emitting element driving device preferably used for driving a light-emitting element such as a laser element used in laser xerography.

2. Description of the Related Art

In a field of laser xerography using a laser element as a light source, the demand for higher resolution and higher speed has been intensified. The on/off control speed (hereinafter referred to as modulating speed) for controlling the laser element to be driven according to input image data is limited. When one laser light beam is used, the modulating speed cannot but be sacrificed if resolution in a sub scanning direction as well as resolution in a main scanning direction needs to be improved. Therefore, the number of laser light beams cannot but be increased if resolution in a sub scanning direction needs to be improved while the modulating speed is not changed. When, for example, four laser light beams are used, resolution both in the main scanning direction and in the sub scanning direction can be improved to twice on the assumption that the modulating speed is the same as that in the case where one laser light beam is used.

Incidentally, semiconductor lasers are roughly classified into edge emission type laser elements (hereinafter referred to as edge emission lasers) and surface emission type laser elements (hereinafter referred to as surface emission lasers). The edge emission laser has a structure in which laser light is taken out in a direction parallel to an active layer. The surface emission laser has a structure in which laser light is taken out in a direction perpendicular to an active layer. Heretofore, in laser xerography, the edge emission laser has been generally used as a laser light source.

From the point of view of increasing the number of laser light beams, the surface emission laser is however structurally preferred to the edge emission laser for the purpose of increasing the number of laser light beams because the edge emission laser is regarded as having a technical difficulty. For this reason, in the recent years, there has been advanced the development of a device using a surface emission laser capable of emitting a large number of laser light beams as a laser light source to satisfy the demand for higher resolution and higher speed in the field of laser xerography.

Semiconductor laser drive systems are roughly classified into voltage drive systems and current drive systems. Assuming now that a light-emitting element large in internal resistance such as a GaN (gallium nitride) blue laser or a single mode surface emission laser needs to be driven, then a time constant τ(=R·C) decided on the basis of internal resistance R and parasitic capacitance C of wiring increases if the light-emitting element is driven by the current drive system. As a result, the leading and trailing edges of a drive current waveform become very slow so that the modulating speed is reduced. Accordingly, if the modulating speed cannot be improved greatly, there is no merit in use of the surface emission laser as a laser light source particularly in laser xerography.

From this point of view, the voltage drive system is more advantageously used as a system for driving a light-emitting element large in internal resistance than the current drive system. Heretofore, in a driving device using the voltage drive system, the light-emitting element has been driven by a voltage by an element lower in output impedance than the light-emitting element (e.g., see JP-A-2001-036186) in order to increase the modulating speed. In the voltage drive type laser driving device, feedback control is performed so that a drive voltage Voncorresponding to the set intensity of light is applied to the semiconductor laser when the semiconductor laser is switched on, and a bias voltage Vbiasnot higher than an emission threshold is applied to the semiconductor laser when the semiconductor laser is switched off.

On the other hand, in a device for driving laser elements (multi-beam laser) capable of emitting a large number of laser light beams, a bias current Ibiasflowing at the time of switching off has been heretofore set to be common to the plurality of semiconductor lasers in order to attain reduction in cost (e.g., see JP-A-09-272223). In the current drive type multi-beam laser driving device, variations in characteristic in the plurality of semiconductor lasers are considered so that the maximum current value selected from variations in current is set as the bias current Ibiascommon to all the semiconductor lasers when the modulating speed is given preference, and the minimum current value selected from variations in current is set as the bias current Ibiaswhen prevention of abnormal lighting is given preference.

Incidentally, the drive current I of a semiconductor laser is generally given by the expression:
I=Is * [exp{q(V−IR)/kT}−1]

in which Is is a backward saturation current, q is an elementary electric charge (charge of an electron), V is a drive voltage, R is the internal resistance of the semiconductor laser, k is a Boltzmann constant, and T is an absolute temperature.

In a low light intensity region in which the voltage drop by the internal resistance R is low, it is obvious from the characteristic graph shown inFIG. 11that the drive current I changes exponentially according to the drive voltage V. Accordingly, in the voltage drive type driving device, because the drive current I, that is, the intensity of emitted light changes exponentially according to the change quantity ΔV of the drive voltage V if negative feedback control is performed to control the terminal voltage when the semiconductor laser is switched on, there is a problem that the gain of negative feedback control changes widely so as to make it difficult to perform stable control.

Even when the semiconductor laser is switched off, a voltage needs to be applied to the semiconductor laser in order to quickly switch off the semiconductor laser large in internal resistance R. The switching-off of the semiconductor laser can be achieved most simply if the applied voltage is made zero. In the edge emission laser, it is however necessary to supply a bias current Ibiasnear the emission threshold current to the semiconductor laser continuously for the purpose of high-speed modulation even at the time of switching off the semiconductor laser. Accordingly, a voltage corresponding to the current near the emission threshold current must be applied to the semiconductor laser when the semiconductor laser is switched off.

On the other hand, in the surface emission laser, it is unnecessary to supply the bias current Ibiasfor the purpose of high-speed modulation because the volume of a resonator is small. It is however preferable that a bias voltage enough to avoid laser oscillation is applied to the semiconductor laser when the semiconductor laser is switched off because the modulating speed can be increased from the point of view of circuitry as the amplitude of the drive voltage decreases. In the bias voltage applied at the time of switching off the semiconductor laser like at the time of switching on the semiconductor laser, it is however difficult to set the drive voltage because it is obvious fromFIG. 11that the drive current changes widely as the drive voltage changes slightly.

Even if the drive voltage can be set, it is difficult to keep the bias current Ibiasproper on the basis of only the drive voltage without consideration of the current because the current varies according to temperature change when the bias voltage is set fixedly. As described above, it is preferable that, in order to quickly drive the semiconductor laser large in internal resistance, a drive voltage not higher than the threshold and near the ON voltage is applied to the semiconductor laser when the semiconductor laser is switched off. Controllability like that at the time of switching on the semiconductor laser however becomes an issue if the voltage needs to be directly controlled.

Variation in the intensity of emitted light according to temperature change is a more significant issue in the voltage drive system. The intensity of light emitted from the semiconductor laser is basically proportional to the drive current. The emission threshold current makes a large contribution to temperature compensation. Accordingly, when the semiconductor laser is driven by a current sufficiently larger than the emission threshold current, variation in the intensity of emitted light according to the temperature change is small enough to be negligible.

In the case of the voltage drive system, because the terminal voltage of the semiconductor laser has a negative temperature coefficient for the temperature when the current is kept constant, it is necessary to reduce the drive voltage according to the temperature change so that the intensity of light does not vary in spite of the temperature rise in the condition that the semiconductor laser is switched on. Conversely, in the condition that the semiconductor laser is switched off, the voltage at the time of switching on the semiconductor laser must be increased to be higher than the voltage at the time of switching off the semiconductor laser in accordance with the temperature fall so that the semiconductor laser can be switched on again with the same intensity of light because the temperature decreases at the time of switching off the semiconductor laser.

As described above, when the intensity of light is to be controlled by the voltage drive system with accuracy equal to that obtained by the current drive system, the drive voltage for the semiconductor laser must be changed according to the temperature of the semiconductor laser.

In the related-art current drive system suitable for driving a plurality of semiconductor lasers (multi-beam laser), when, for example, a bias current Ibiasto flow in common to all the semiconductor lasers at the time of switching off the semiconductor lasers is set according to the semiconductor laser having the largest emission threshold current Ith, the bias current Ibiasexceeds the emission threshold current Ithso that some semiconductor laser may be switched on continuously if variation in characteristic of the semiconductor lasers is large. It is therefore necessary to suppress variation in characteristic of the semiconductor lasers strictly. However, if the specification for the semiconductor laser is stringent, the situation that the semiconductor laser cannot be provided may inevitably occur because of increase in cost of the semiconductor laser and reduction in yield of the semiconductor lasers according to circumstances.

On the other hand, in the related-art voltage drive system suitable for driving GaN blue edge emission lasers or single mode surface emission lasers, it is necessary to provide voltage sources according to elements because the optimal values of the bias voltages applied at the time of switching off the semiconductor lasers are different according to the elements. In this case, it is necessary to provide capacitors of high capacitance according to the elements in order to produce voltage sources with impedance kept low up to a high frequency. Particularly when the driving device is assumed to be formed as an IC, it is necessary to provide such capacitors of high capacitance in the outside of the IC because provision of the capacitors in the inside of the IC causes increase in cost. It is however a matter of course that increase in cost is brought about because the capacitors provided in the outside of the IC are required.

Therefore, when one common voltage source is to be used in the same manner as one common current value set in the current drive system, the bias voltage must be controlled according to the maximum or minimum value of the thread current. When a common voltage value is to be set according to the maximum or minimum value in the same manner as in the current drive system, it is necessary to suppress variation in characteristic of the semiconductor lasers strictly. That is, when a multi-beam laser driving device is to be formed, a drive circuit for one semiconductor laser needs to be simplified as much as possible in order to attain reduction in cost. As is obvious from the above description, the performance of the semiconductor lasers is however sacrificed when the related-art voltage drive system is used.

SUMMARY OF THE INVENTION

In order to solve the above described problem, according to one aspect of the invention, there is provided a light-emitting element driving device including: a first control unit adapted to control a drive voltage applied from a voltage source to a light-emitting element; a second control unit adapted to control a drive current supplied from a current source to said light-emitting element; a test current supply unit adapted to supply a test current to the light-emitting element when the light-emitting element is switched off; and a bias voltage setting unit adapted to set a bias voltage applied to the light-emitting element on the basis of a terminal voltage of the light-emitting element supplied with the test current from the test current supply unit when the light-emitting element is switched off, wherein, the first control unit and the second control unit controls an light intensity of a light beam emitted from the light-emitting element.

In the light-emitting element driving device configured as described above, a voltage drive and a current drive (voltage drive->current drive) are used in combination. For setting of the bias voltage, a current expected to flow in the light-emitting element at the time of switching off the light-emitting element is actually supplied as a test current from the test current supply unit to the light-emitting element in a period in which neither light intensity control nor modulation is performed. Thus, a terminal voltage of the light-emitting element supplied with the test current is detected. Then, the bias voltage setting unit sets the bias voltage applied to the light-emitting element at the time of switching off the light-emitting element on the basis of the detected voltage. Accordingly, the bias voltage applied to the light-emitting element at the time of switching off the light-emitting element can be easily set to be a target value compared with the related art in which it was difficult to control the current because the current varied widely according to slight voltage change.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

First Embodiment

FIG. 1is a circuit diagram showing an example of configuration of a light-emitting element driving device according to a first embodiment of the invention. In the first embodiment, for example, a semiconductor laser, particularly, a GaN blue laser or a single mode surface emission laser high in internal resistance is used as a light-emitting element to be driven. These are generically referred to as semiconductor laser LD. For example, the semiconductor laser LD has a cathode grounded, and an anode provided as a drive end.

The light-emitting element driving device according to the first embodiment includes: a drive current control circuit11for controlling a drive current of the semiconductor laser LD to make the intensity of light emitted from the semiconductor laser LD coincident with set light intensity; a drive voltage control circuit12for controlling a drive voltage applied to the semiconductor laser LD at the time of switching on the semiconductor laser LD on the basis of a terminal voltage of the semiconductor laser LD when the light intensity is controlled to be equal to the set light intensity by the drive current control circuit11; a test current supply circuit13for supplying a test current to the semiconductor laser LD; a bias voltage setting circuit14for setting a bias voltage applied to the semiconductor laser LD at the time of switching off the semiconductor laser LD on the basis of the terminal voltage of the semiconductor laser LD supplied with the test current from the test current supply circuit13; a light intensity detection circuit15for detecting the intensity of light emitted from the semiconductor laser LD; and an error detection circuit16for detecting an error voltage between a voltage Vdetdetected by the light intensity detection circuit15and a reference voltage Vrefand outputting the error voltage as a light intensity control voltage Vcont.

The drive current control circuit11includes an inverter111, a current source112, a capacitor C11, and switches SW11and SW12. The light intensity control voltage Vcontis supplied from the error detection circuit16to the drive current control circuit11through the drive voltage control circuit12. The light intensity control voltage Vcontpasses through the inverter111and the switch SW11and serves as a control input of the current source112. The capacitor C11is connected between a power-supply line L of a power-supply voltage VDD and an output terminal of the switch SW11. One end of the current source112is connected to the power-supply line L. One end of the switch SW12is connected to the other end of the current source112. The other end of the switch SW12is connected to one end of a switch SW24.

In the drive current control circuit11configured as described above, the light intensity control voltage Vcontsupplied from the error detection circuit16through the drive voltage control circuit12is input as a control voltage of the current source112via the inverter111and the switch SW11to control the drive current supplied from the current source112to the semiconductor laser LD. As a result, in the drive current control circuit11, the drive current of the semiconductor laser LD is controlled so that the intensity of light emitted from the semiconductor laser LD coincides with set light intensity (target light intensity) decided on the basis of the reference voltage Vrefof the error detection circuit16.

The drive voltage control circuit12includes four switches SW21to SW24, two capacitors C21and C22, and an operational amplifier211. One end of the switch SW21is connected to an output side of the error detection circuit16. The other end of the switch SW21is connected to a non-inverted input terminal of the operational amplifier211. The capacitor C21is connected between the other end of the switch SW21and the ground to thereby operate with the switch SW21to form a sample-and-hold circuit. The operational amplifier211has the non-inverted (+) input terminal connected to the other end of the switch SW21.

The switch SW22has one end connected to an output terminal of the operational amplifier211. The capacitor C22is connected between the other end of the switch SW22and the ground to thereby operate with the switch SW22to form a sample-and-hold circuit. The switch SW23has one end connected to the other end of the switch SW22. The switch SW24has one end connected to the other end of the switch SW23, and the other end connected to an anode of the semiconductor laser LD. An inverted (−) input terminal of the operational amplifier211is connected to a common junction between the other end of the switch SW23and one end of the switch SW24.

In the drive voltage control circuit12configured as described above, the switches SW22and SW23are turned off and the switches SW21and SW24are turned on when the drive current of the semiconductor laser LD is controlled, that is, when light intensity is controlled. When the semiconductor laser LD is switched off, the switches SW22and SW23are turned on and the switches SW21and SW24are turned off. As a result, the operational amplifier211forms a voltage follower buffer, so that the output voltage of the operational amplifier211is held by the capacitor C22. The hold voltage is a terminal voltage of the semiconductor laser LD when the light intensity has a set value. When the semiconductor laser LD is switched on, the switches SW23and SW24are turned on so that the hold voltage of the capacitor C22is applied as a drive voltage to the semiconductor laser LD through the switches SW23and SW24.

The test current supply circuit13includes a current source131, a switch SW31, and a test current setting portion132. The current source131has one end connected to the power-supply line L. The switch SW31has one end connected to the other end of the current source131, and the other end connected to the anode of the semiconductor laser LD. The test current setting portion132sets a current of the current source131, that is, a test current.

In the test current setting circuit13configured as described above, in order to decide the bias voltage Vbiasapplied to the semiconductor laser LD at the time of switching off the semiconductor laser LD, the switch SW31is turned on at the time of switching off the semiconductor laser LD in a period in which neither light intensity control nor modulation is performed as will be described later. As a result, the test current set by the test current setting portion132is supplied from the current source131to the semiconductor laser LD through the switch SW31. In the test current setting portion132, a current corresponding to the bias voltage Vbiasto be applied to the semiconductor laser LD at the time of switching off the semiconductor laser LD is set as the test current. Generally, a voltage lower than the oscillation threshold voltage of the semiconductor laser LD, preferably a voltage slightly lower than the oscillation threshold voltage of the semiconductor laser LD is set as the bias voltage Vbiasin order to increase the modulating speed.

The bias voltage setting circuit14includes switches SW41and SW42, capacitors C41and C42, and a voltage follower buffer141. The switch SW41has one end connected to the anode of the semiconductor laser LD. The switch SW42has one end connected to the anode of the semiconductor laser LD, and the other end connected to a non-inverted input terminal of the buffer141. The capacitor C41is connected between the other end of the switch SW42and the ground to thereby operate with the switch SW42to form a sample-and-hold circuit. An output terminal of the buffer141is directly connected to an inverted input terminal of the buffer141and further connected to the other end of the switch SW41. The capacitor C42is connected between the output terminal of the buffer141and the ground.

In the bias voltage setting circuit14configured as described above, the switch SW42is turned on when the test current is supplied from the test current supply circuit13to the semiconductor laser LD, and further turned off. As a result, the terminal voltage of the semiconductor laser LD is held by the capacitor C41while the test current flows in the semiconductor laser LD. The hold voltage is held by the capacitor C42through the voltage follower buffer141. When the switch SW41is turned on at the time of switching off the semiconductor laser LD, the hold voltage is applied as a bias voltage Vbiasto the semiconductor laser LD.

For example, the light intensity detection circuit15uses a photo diode PD as a photo detector for detecting laser light emitted from the semiconductor laser LD. The photo diode PD has a cathode connected to the power-supply line L. One end of a resistor R is connected to an anode of the photo diode PD. The other end of the resistor R is grounded. Assuming now that the photo diode PD detects laser light emitted from the semiconductor laser LD, then a current corresponding to the intensity of the light flows in the resistor R. As a result, a detection voltage corresponding to the intensity of light emitted from the semiconductor laser L is generated between the opposite ends of the resistor R.

The detection voltage is supplied to a non-inverted input terminal of an amplifier151. An output terminal of the amplifier151is directly connected to an inverted input terminal of the amplifier151. The output voltage of the amplifier151, that is, the detection voltage of the light intensity detection circuit15is supplied to the drive current control circuit11. That is, a feedback system is formed so that the detection voltage of the light intensity detection circuit15is fed back to the drive current control circuit11to thereby perform automatic power control (hereinafter referred to as APC) for controlling the laser power of the semiconductor laser LD to a value decided by the reference voltage Vref.

The error detection circuit16includes a differential amplifier161, switches SW61and SW62, and a capacitor C61. The differential amplifier161has a non-inverted input terminal to which the reference voltage Vrefis input, and an inverted input terminal to which the detection voltage Vdetsupplied from the light intensity detection circuit15via the switch61is input. Incidentally, the reference voltage Vrefis set by a voltage value corresponding to the target light intensity (laser power) of the semiconductor laser LD. The switch SW62and the capacitor C61are series-connected between the inverted input terminal of the differential amplifier161and an output terminal of the differential amplifier161.

In the error detection circuit16configured as described above, the differential amplifier161compares the detection voltage Vdetcorresponding to the light intensity of the semiconductor laser LD detected by the light intensity detection circuit15, with the reference voltage Vrefset according to the target light intensity of the semiconductor laser LD and outputs the difference (error voltage) between the two voltages as a light intensity control voltage Vcont.

Incidentally, the switches SW11and SW12of the drive current control circuit11, the switches SW21to SW24of the drive voltage control circuit12, the switch SW31of the test current supply circuit13, the switches SW41and SW42of the bias voltage setting circuit14and the switches SW61and SW62of the error detection circuit16are controlled to be turned on/off by the control circuit17. In the semiconductor laser driving device according to the first embodiment, the control circuit17is configured so that voltage drive control is always performed at the time of turning on/off the semiconductor laser LD in a modulation period in which the semiconductor laser LD is repeatedly driven to be turned on/off according to the input data (pulse data).

The circuit-operation of the light-emitting element driving device, that is, the semiconductor laser driving device according to the fist embodiment configured as described above will be described below.

When powered on, the device gets into an APC mode. In the APC mode, the switches SW61and SW62in the error detection circuit16are turned on, the switches SW21and SW24in the drive voltage control circuit12are turned on, and the switches SW11and SW12in the drive current control circuit11are turned on. As a result, the current of the current source112flows as a drive current in the semiconductor laser LD via the switches SW12and SW24, so that the semiconductor laser LD is switched on.

When the semiconductor laser LD is switched on, the photo diode PD of the light intensity detection circuit15receives laser light emitted from the semiconductor laser LD. As a result, a current corresponding to the intensity of the light flows in the photo diode PD. The current flowing in the photo diode PD is converted into a voltage by the resistor R. The voltage is amplified by the amplifier151, so that the amplified voltage is output as a detection voltage Vdetcorresponding to the laser power (light intensity) of the semiconductor laser LD.

The detection voltage Vdetis supplied to the error detection circuit16so as be provided as an inverted input of the differential amplifier161via the switch SW61. The differential amplifier161amplifies the difference (error voltage) between the detection voltage Vdetand the reference voltage Vrefand outputs the error voltage as a light intensity control voltage Vcont. The light intensity control voltage Vcontis supplied as a control voltage to the current source112via the switch SW21, the operational amplifier211., the inverter111and the switch SW11. The drive current of the semiconductor laser LD is controlled on the basis of the light intensity control voltage Vcont, so that the light intensity of the semiconductor laser LD is controlled.

The control of the negative feedback loop according to the detection voltage Vdetof the light intensity detection circuit15by the error detection circuit16and the drive current control circuit11converges the detection voltage Vdetso that the detection voltage Vdetfinally coincides with the reference voltage Vrefset according to the set light intensity. As a result, the light intensity of the semiconductor laser LD becomes equal to the set light intensity. The aforementioned series of feedback control is APC (automatic power control). The APC operation may be performed once or may be repeated by a plurality of times.

After completion of the APC operation, the switch SW11of the drive current control circuit11and the switch SW62of the error detection circuit16are turned off and the switches SW22and SW23of the drive voltage control circuit12which were turned off at the time of APC are turned on. As a result, the output voltage of the differential amplifier161just before that, that is, the light intensity control voltage Vcontat the time of controlling the light intensity of the semiconductor laser LD to the set light intensity is held by the capacitors C61and C21. Further, the control voltage for setting the drive current at the time of controlling the light intensity of the semiconductor laser LD to the set light intensity is held by the capacitor C11. Further, the capacitor C22of the drive voltage control circuit12is charged with the light intensity control voltage Vcontheld by the capacitor C21. On this occasion, the voltage held by the capacitor C11forms a control voltage for setting the drive current at the time of driving the semiconductor laser LD to emit light with the set light intensity while the voltages held by the capacitors C21and C22form a terminal voltage of the semiconductor laser LD having the set light intensity.

When the APC operation is completed, the device gets into a modulation mode in which driving the semiconductor laser LD is controlled to be turned on/off according to the input data. In the modulation mode, the switch SW24of the drive voltage control circuit12is turned on when the semiconductor laser LD is switched off. The hold voltage of the capacitor C22, that is, the terminal voltage of the semiconductor laser LD having the set light intensity is applied to the anode of the semiconductor laser LD through the switches SW23and SW24. Further, because the switch SW12of the drive current control circuit11is turned on, the drive current is supplied from the current source112to the semiconductor laser LD through the switch SW12.

In the case of an edge emission laser, a bias current Ibiasnear the emission threshold current needs to flow in the semiconductor laser LD continuously for obtaining high-speed modulation even at the time of switching off the semiconductor laser LD. Accordingly, a bias voltage Vbiascorresponding to the current near the emission threshold current needs to be applied to the semiconductor laser LD when the semiconductor laser LD is switched off. In the case of a semiconductor laser high in internal resistance, the amplitude of the drive voltage needs to be reduced at the time of switching on/off the semiconductor laser. Accordingly, such a voltage that the semiconductor laser is not switched on is preferably applied to the semiconductor laser.

In the first embodiment, therefore, the test current supply circuit13and the bias voltage setting circuit14operate so that the bias voltage Vbiasapplied to the semiconductor laser LD at the time of switching off the semiconductor laser LD is set as follows. That is, the switch SW31is turned on at the switching off the semiconductor laser LD in a period in which neither APC nor modulation is performed. As a result, the current to flow in the semiconductor laser LD at the time of switching off the semiconductor laser LD is supplied as a test current from the current source131to the semiconductor laser LD through the switch SW31by the test current setting portion132.

On the other hand, in the bias voltage setting circuit14, the switch SW42is turned on when the test current is supplied from the test current supply circuit13to the semiconductor laser LD, and then turned off. As a result, the terminal voltage of the semiconductor laser LD supplied with the test current is held by the capacitor C41. Then, the hold voltage of the capacitor C41is held by the capacitor C42through the buffer141. When the switch SW41is then turned on at the time of switching off the semiconductor laser LD, the hold voltage of the capacitor C42is applied as a bias voltage Vbiasto the semiconductor laser LD via the switch SW41.

As described above, in the semiconductor laser LD driving device according to the first embodiment, that is, in the voltage drive type driving device, in order to set the bias voltage Vbias, the current to flow in the semiconductor laser LD at the time of switching off the semiconductor laser LD is actually supplied as a test current to the semiconductor laser LD in a period in which neither APC nor modulation is performed. The terminal voltage of the semiconductor laser LD supplied with the test current is detected. The bias voltage Vbiasto be applied to the semiconductor laser LD at the time of switching off the semiconductor laser LD is set on the basis of the detected voltage. Accordingly, the bias current to flow in the semiconductor laser LD at the time of switching off the semiconductor laser LD can be easily set at the target value compared with the related art in which the current varied widely according to the slight change of the voltage so that it was difficult to control the current. That is, when the current is controlled to obtain the drive voltage so that the voltage is driven on the basis of the voltage value of the drive voltage, the voltage drive can be made with controllability equal to that of the current drive.

FIG. 2is a circuit diagram showing an example of configuration of import part of the light-emitting element driving device, that is, the semiconductor laser driving device according to a first modification of the first embodiment. InFIG. 2, parts the same as those inFIG. 1are denoted by the same reference numerals as those inFIG. 1. InFIG. 2, the test current supply circuit13, the bias voltage setting circuit14, the light intensity detection circuit15and the error detection circuit16are the same in configuration and operation as those inFIG. 1.

The semiconductor laser driving device according to the first modification of the first embodiment has a feature in that the output voltage of the error detection circuit16, that is, the light intensity control voltage Vcontis directly input to an analog inverter111in the drive current control circuit11so that the drive current of the current source112is directly controlled on the basis of the light intensity control voltage Vcont. The drive voltage control circuit12includes switches SW21and SW22, capacitors C21and C22, and a voltage follower buffer211. The drive voltage control circuit12has the same circuit configuration as the bias voltage setting circuit14.

In the semiconductor laser driving device according to the first modification of the first embodiment, the drive current is supplied from the current source112to the semiconductor laser LD at the time of APC because the switches SW12and SW13of the drive current control circuit11are turned on. On this occasion, the reference voltage and a light output voltage corresponding to the intensity of emitted light are input to non-inverted input and inverted input terminals of the operational amplifier161of the error detection circuit16. As a result, a voltage proportional to the difference between the two voltages is input as a drive current control voltage to the current source112through the analog inverter111, so that the current is set to obtain the set light intensity.

In the drive voltage control circuit12, because the switch SW21is turned on and the switch SW22is turned off, the terminal voltage of the semiconductor laser LD is held by the capacitor C21. Accordingly, the voltage follower buffer211operates so that the capacitor C22is charged with the voltage held by the capacitor C21.

When APC is completed, the switches SW12and SW13of the drive current control circuit11and the switch SW21of the drive voltage control circuit12are switched off so that the voltage of the capacitor C12serves as a control voltage for setting the drive current to make the semiconductor laser LD emit light with the set light intensity. Accordingly, the voltage held by the capacitors C21and C22serves as a terminal voltage of the semiconductor laser LD for obtaining the set light intensity.

As described above, in the semiconductor laser LD driving device according to the first modification of the first embodiment, that is, in the voltage drive type driving device, the drive current of the semiconductor laser LD is controlled to make the light intensity of the semiconductor laser LD equal to the set light intensity. The terminal voltage of the semiconductor laser LD is detected when the intensity of light emitted from the semiconductor laser LD is equal to the set light intensity. The drive voltage applied to the semiconductor laser LD at the time of switching on the semiconductor laser LD is set on the basis of the detected voltage (terminal voltage). Accordingly, because the drive current of the semiconductor laser LD is substantially proportional to the light intensity, the gain of the negative feedback loop for APC can be kept constant. Accordingly, stable control can be performed compared with the related-art case where negative feedback is performed by controlling the terminal voltage at the time of switching on the semiconductor laser LD.

FIG. 3is a circuit diagram showing an example of configuration of important part of the light-emitting element driving device, that is, the semiconductor laser driving device according to a second modification of the first embodiment. InFIG. 3, parts the same as those inFIG. 1are denoted by the same reference numerals as those inFIG. 1. InFIG. 3, the test current supply circuit13, the bias voltage setting circuit14, the light intensity detection circuit15and the error detection circuit16are the same in configuration and operation as those inFIG. 1.

In the semiconductor laser driving device according to the second modification of the first embodiment, the drive voltage control circuit12includes four switches SW21, SW22, SW23and SW24, capacitors C21and C22, and a buffer211. The switch SW21has one end connected to an output side of the error detection circuit16, and the other end connected to a non-inverted input terminal of the buffer211. The capacitor C21is connected between the other end of the switch SW21and the ground to thereby operate with the switch SW21to form a sample-and-hold circuit. The buffer211is made of an operational amplifier having an output terminal and an inverted input terminal connected to each other.

The switch SW22has one end connected to the output terminal of the buffer211. The capacitor C22is connected between the other end of the switch SW22and the ground to thereby operate with the switch SW22to form a sample-and-hold circuit. The switch SW23has one end connected to the other end of the switch SW22, and the other end connected to an anode of the semiconductor laser LD. The switch SW24has one end connected to the other end of the switch SW23and to the anode of the semiconductor laser LD, and the other end connected to the output terminal of the buffer211.

In the drive voltage control circuit12configured as described above, at the time of APC, the switches SW22and SW23are turned off and the switches SW21and SW24are turned on. As a result, the output voltage of the error detection circuit16, that is, the light intensity control voltage Vcontis applied to the semiconductor laser LD via the switch SW21, the buffer211and the switch SW24so that a feedback loop is formed.

When the semiconductor laser LD is switched off, the switch SW22is turned on and the switches SW21, SW23and SW24are turned off. As a result, the output voltage of the buffer211, that is, the terminal voltage of the semiconductor laser LD at the time of completion of the light intensity control is held by the capacitor C22. When the switch SW23is then turned on at the time of switching on the semiconductor laser LD, the hold voltage of the capacitor C22is applied as a drive voltage to the semiconductor laser LD through the switch SW23.

The drive current control circuit11includes an operational amplifier114, switches SW11and SW12, a capacitor C11, and a current source112. The operational amplifier114has an inverted input terminal connected to the one end of the switch SW24and to the output terminal of the buffer211, and a non-inverted input terminal connected to the anode side of the semiconductor laser LD. The capacitor C11is connected between a power-supply line L of a power-supply voltage VDD and an output side terminal of the switch SW11. The current source112has one end connected to the power-supply line L. The switch SW12has one end connected to the other end of the current source112, and the other end connected to the anode side of the semiconductor laser LD.

In the drive current control circuit11configured as described above, the switches SW11and SW12are turned on at timing independent of both light intensity control and modulation. On this occasion, the switch SW24of the drive voltage control circuit12is turned off. As a result, the drive current control circuit11supplies the drive current to the semiconductor laser LD so that the terminal voltage of the semiconductor laser LD coincides with the output voltage of the buffer211(i.e., the voltage of the capacitor C21for sampling and holding the terminal voltage at the time of light intensity control). Then, the switch SW11is turned off so that the control voltage for supplying the drive current is held by the capacitor C11. When modulation lighting is performed, the switch SW12turned on so that a compensating current for compensating the current driving capacity of the voltage source is supplied to the semiconductor laser LD.

As described above, in the semiconductor laser LD driving device according to the second modification of the first embodiment, the voltage source of low output impedance is controlled to perform voltage drive control so that the influence of a time constant decided on the basis of the internal resistance and capacitance of the light-emitting element can be reduced compared with the case of current drive control though a long time is required for convergence of light intensity into a target value because of the influence of the time constant if the current source is controlled to perform current drive control at the time of light intensity control when the internal resistance of the semiconductor laser LD is very high. Accordingly, the time required for convergence in light intensity control can be shortened.

Generally, the temperature of a light-emitting element such as a semiconductor laser varies according to the drive current. In the case of constant voltage drive, variation in current, that is, variation in light intensity is caused by the temperature change of the semiconductor laser when the semiconductor laser is driven. Although such variation in light intensity can be neglected in the case of ordinary drive control, it is preferable that the variation in light intensity caused by the temperature change is eliminated in order to achieve more excellent drive control.

The light intensity of the semiconductor laser is basically proportional to the drive current. The emission threshold current makes a great contribution to the temperature. Accordingly, the drive voltage of the semiconductor laser can be controlled on the basis of detection of the temperature change of the semiconductor laser so that constant light intensity can be always provided. Various methods have been heretofore proposed as the method for detecting the temperature of the semiconductor laser. One of the methods for detecting the temperature of the semiconductor most directly is a method in which the laser temperature is predicted on the basis of variation in the terminal voltage of the semiconductor laser.

Particularly when the semiconductor laser is in a constant current state, the temperature of the semiconductor laser can be detected accurately because the terminal voltage is proportional to the temperature. Therefore, when control is shifted from constant voltage drive to constant current drive after the semiconductor laser is driven with a constant voltage at the leading edge/trailing edge for switching on/off the semiconductor laser, the temperature can be detected on the basis of the change of the terminal voltage of the semiconductor laser in the period of constant current drive.

Upon such circumstances, the voltage drive system and the current drive system may be used in combination so that a function for correcting the variation in light intensity caused by the temperature change (ΔT) in the period of constant current drive can be given to the light-emitting element driving device as will be described as a second embodiment of the invention.

Second Embodiment

FIG. 4is a circuit diagram showing an example of configuration of the light-emitting element driving device according to a second embodiment of the invention. InFIG. 4, parts the same as those inFIG. 1are denoted by the same reference numerals as those inFIG. 1. InFIG. 4, the light intensity detection circuit15and the error detection circuit16are dispensed with. The light-emitting element driving device according to the second embodiment, that is, the semiconductor laser driving device is configured so that a correction circuit for correcting variation in light intensity caused by the temperature change (ΔT) is provided as well as the voltage drive system and the current drive system are used in combination.

To achieve the current drive, there are provided a bias current supply circuit18for supplying a bias current Ibiasto the semiconductor laser LD at the time of switching off the semiconductor laser LD and a bias current setting portion19for setting the bias current Ibias. The bias current supply circuit18includes a current source181, and a switch SW81. The current source181has one end connected to the power-supply line L. The switch SW81has one end connected to the other end of the current source.181, and the other end connected to the anode of the semiconductor laser LD.

The bias current setting portion19sets the bias current Ibiassupplied to the semiconductor laser LD at the time of switching off the semiconductor laser LD on the basis of the bias voltage Vbiasset based on the terminal voltage of the semiconductor laser LD supplied with the test current in the bias voltage setting circuit14. Specifically, for example, the bias current setting portion19includes a differential amplifier191, a switch SW71, and a capacitor C71.

The differential amplifier191receives the bias voltage Vbiasset by the bias voltage setting circuit14and the terminal voltage of the semiconductor laser LD as an inverted input and a non-inverted input respectively and sets the current of the current source181, that is, the bias current Ibiason the basis of such negative feedback control that the terminal voltage of the semiconductor laser LD at the time of switching off the semiconductor laser LD coincides with the bias voltage Vbiasset by the bias voltage setting circuit14. The switch SW71and the capacitor C71form a sample-and-hold circuit. The output of the differential amplifier191, that is, a voltage value corresponding to the bias current Ibiasto be set is held by the capacitor C71. When the configuration of negative feedback control is used, control can be performed by simple configuration so that the terminal voltage of the semiconductor laser LD at the time of switching off the semiconductor laser LD coincides with the set bias voltage Vbias.

Further, a correction circuit20for correcting variation in light intensity caused by the temperature change (ΔT) includes an error amplifier201with a gain of 1, a switch SW91, and a capacitor C91. The error amplifier201has a non-inverted input terminal supplied with the terminal voltage of the semiconductor laser LD. Assuming now that a constant current flows in the semiconductor laser LD, then the terminal voltage varies according to the temperature of the semiconductor laser LD. Specifically, the terminal voltage decreases as the temperature of the element increases. Accordingly, when the terminal voltage of the semiconductor laser LD is detected, the temperature of the semiconductor laser LD can be monitored.

Incidentally, the technique for monitoring the temperature of the semiconductor laser LD is not limited to the technique of detecting the terminal voltage of the semiconductor laser LD. For example, a technique using the detection output of a temperature detection unit such as a thermistor disposed near the semiconductor laser LD may be used. The technique of detecting the terminal voltage of the semiconductor laser LD has however an advantage in that the temperature of the semiconductor laser LD can be monitored more speedily and more accurately.

The switch SW91and the capacitor C91form a sample-and-hold circuit. That is, the terminal voltage of the semiconductor laser LD is sampled by the switch SW91, so that the sampled voltage is held by the capacitor C91. The hold voltage of the capacitor C91is used as a reference voltage given to the inverted input terminal of the error amplifier201. An output terminal of the error amplifier201is connected to one end (open end) of the capacitor C21of the drive voltage control circuit12.

In the correction circuit20, the switch SW91is turned on under control of the control circuit17, for example, at timing before the aforementioned APC period to thereby sample the terminal voltage of the semiconductor laser LD. In this manner, because sampling is performed at timing before the APC period, the terminal voltage stable before the increase in the temperature of the semiconductor laser LD can be sampled. The sampled voltage is used for the following correction process.

That is, the error amplifier201receives the terminal voltage of the semiconductor laser LD as a non-inverted input, compares the terminal voltage with the hold voltage of the capacitor C91, that is, with the reference voltage sequentially and supplies the error amplified voltage to the open end of the capacitor C21of the drive voltage control circuit12. On this occasion, because the gain of the error amplifier201is set to be 1, the hold voltage of the capacitor C21shifts by the error amplified voltage. That is, the error amplified voltage is superposed as a correction value on the hold voltage of the capacitor C91. The corrected voltage is applied to the semiconductor laser LD when the semiconductor laser LD is driven to be switched on (to emit light).

Changeover control of the respective switches is performed by the control circuit17to thereby switch the voltage drive and the current drive over to each other. The changeover control by the control circuit17will be described specifically with reference toFIG. 5, which is a time chart of waveforms. In a modulation period, the “H”-level period of input data (A) is the period in which the semiconductor laser LD is switched on (emits light), and the “L”-level period of input data (A) is the period in which the semiconductor laser. LD is switched off.

At the beginning of the “H”-level period of input data (A), the control circuit17turns on the switches SW23and SW24of the drive voltage control circuit12. As a result, the voltage held by the capacitor C22is applied to the semiconductor laser LD through the switches SW23and SW24. As a result, the semiconductor laser LD is driven with a constant voltage in a predetermined period of the leading edge of input data (A).

After the passage of the voltage drive period, the control circuit17turns off the switch SW23of the drive voltage control circuit12and turns on the switch SW12of the drive current control circuit11. As a result, a drive current corresponding to the voltage held by the capacitor C11(seeFIG. 1) is output from the current source112so that the drive current is supplied to the semiconductor laser LD through the switch SW12. As a result, the semiconductor laser LD is driven with a constant current after the passage of the voltage drive period.

Next, at the beginning of the “L”-level period of input data (A), the control circuit17turns on the switch SW41of the bias voltage setting circuit14. As a result, the voltage held by the capacitor C42is applied as a bias voltage Vbiasto the semiconductor laser LD through the switch SW41. The semiconductor laser LD supplied with the bias voltage Vbiasis driven with a constant voltage in a predetermined period of the trailing edge of input data (A).

After the passage of the voltage drive period, the control circuit17turns off the switch SW41of the bias voltage setting circuit14and turns on the switch SW81of the bias current supply circuit18. As a result, a constant bias current Ibiasset by the bias current setting portion19is output from the current source181so that the bias current Ibiasis supplied to the semiconductor laser LD through the switch SW81. As a result, the semiconductor laser LD is driven with a constant current after the passage of the voltage drive period.

As described above, in the semiconductor laser driving device using a constant voltage drive and a constant current drive in combination, when the constant voltage drive is shifted to the constant current drive after the semiconductor laser LD is driven with a constant voltage in the period of the leading/trailing edge for switching on/off the semiconductor laser LD, the temperature of the semiconductor laser LD can be detected on the basis of the change of the terminal voltage of the semiconductor laser LD in the constant current drive period. Further, when the drive voltage of the semiconductor laser LD is controlled on the basis of the detection of the temperature of the semiconductor laser LD so that the intensity of light can be always kept constant, control of the light intensity of the semiconductor laser LD can be achieved by the voltage drive with accuracy equal to that of the current drive. The light intensity control based on the detection of the temperature is performed by a system including the correction circuit20.

A correcting operation in the system including the correction circuit20is performed in a laser switching-off period in which a bias current Ibiasslightly lower than the emission threshold current of the semiconductor laser LD is supplied to the semiconductor laser LD to make the current drive after the voltage drive is made in response to the “L”-level of input data (A). As a result, variation in light intensity caused by the temperature change ΔT is corrected.

Specifically, in the error amplifier201, the terminal voltage of the semiconductor laser LD is compared with the hold voltage of the capacitor C91, so that the error amplified voltage is given to the open end of the capacitor C21. As a result, the error amplified voltage is superposed as a correction value on the hold voltage of the capacitor C21, so that the superposed voltage is applied as a corrected voltage to the semiconductor laser LD. As a result, variation in light intensity caused by the temperature change ΔT is corrected, so that the intensity of light is kept constant even in the case where the temperature change ΔT remains.

Assuming now that a long time is required for shifting the constant voltage drive to the constant current drive when the on (or off) state of the semiconductor laser LD is changed over to the off (or on) state in a semiconductor laser driving device using the constant voltage drive and the constant current drive in combination, then the drive voltage of the semiconductor laser LD cannot be corrected on the basis of the temperature during the shifting time. Particularly in the case of halftone printing in which a short lighting time is repeated in the field of laser xerography using the semiconductor laser LD as a light source, there is a problem that the shifting time may have bad influence on correction of variation in light intensity caused by the temperature change.

On the contrary, in the semiconductor laser driving device according to the second embodiment, a bias current Ibiasis set by the bias current setting portion19on the basis of the bias voltage Vbiasset by the bias voltage setting circuit14, so that the bias current Ibiasis supplied to the semiconductor laser LD at the time of switching off the semiconductor laser LD. Accordingly, the voltage at the voltage drive operation is made substantially equal to the voltage at the current drive operation, so that the width of voltage fluctuation at the time of shifting the voltage drive to the current drive is reduced. Furthermore, because the bias current Ibiasflowing in the semiconductor laser LD is set to be slightly smaller than the emission threshold current, impedance of the semiconductor laser LD at the time of switching off the semiconductor laser LD can be reduced. Accordingly, the time requiring for shifting the constant voltage drive to the constant current drive can be shortened. As a result, the period in which the constant voltage drive is shifted to the constant current drive and in which correction for temperature cannot work can be minimized, so that the first embodiment can make a great contribution to improvement in temperature compensating accuracy.

Incidentally, as described above, in the semiconductor laser LD, the drive current I varies exponentially according to the drive voltage V (seeFIG. 11). As shown inFIG. 6, the V-I characteristic (voltage-current characteristic) varies according to the temperature change. Accordingly, if a bias voltage Vbias1is continuously applied to the semiconductor laser LD in spite of increase in temperature of the semiconductor laser LD in the condition that the bias voltage Ibiasis set one the basis of the bias voltage Vbias1when, for example, the temperature is low, the bias current Ibiasincreases according to the variation in V-I characteristic. If it comes to the worst case, the bias current Ibiaswill exceed the threshold current Ithso that the semiconductor laser LD may emit light.

Therefore, the setting of the bias voltage Vbiasby the bias voltage setting circuit14and the setting of the bias current Ibiasby the bias current setting portion19may be performed at intervals of a predetermined cycle, for example, a cycle of APC (light intensity control). For example, in the V-I characteristic shown inFIG. 6, a constant test current (approximated as IbiasinFIG. 6) is supplied to the semiconductor laser LD so that the bias voltage is readjusted to a bias voltage Vbias2on the basis of increase in temperature. Accordingly, even in the case where the V-I characteristic of the semiconductor laser LD varies according to the temperature change, the bias current Ibiasflowing in the semiconductor laser LD can be kept constant.

Incidentally, in the first embodiment, in the voltage drive type semiconductor laser driving device, the drive current of the semiconductor laser LD is controlled so that the light intensity of the semiconductor laser LD is made equal to the set light intensity, and the terminal voltage of the semiconductor laser LD with the set light intensity is detected so that the drive voltage applied to the semiconductor laser LD at the time of switching on the semiconductor laser LD is set on the basis of the detected voltage (terminal voltage). The setting of the drive voltage can be also applied to the voltage drive in the semiconductor laser driving device according to the second embodiment.

Further, the second embodiment is the same as the first embodiment in that the terminal voltage of the semiconductor laser LD supplied with the test current is detected so that the bias voltage Vbiasapplied to the semiconductor laser LD at the time of switching off the semiconductor laser LD is set on the basis of the detected voltage.

FIG. 7is a circuit diagram showing an example of configuration of important part of the light-emitting element driving device, that is, the semiconductor laser driving device according to a first modification of the second embodiment. InFIG. 7, parts the same as those inFIG. 4are denoted by the same reference numerals as those inFIG. 4.

The semiconductor laser driving device according to the second embodiment is configured so that the test current supply circuit13and the bias current supply circuit18include the current sources131and181respectively. On the contrary, the semiconductor laser driving device according to the first modification of the second embodiment is configured so that a current source131is used in common to the two current supply circuits13and18. Specifically, as is obvious fromFIG. 7, the current source131of the test current supply circuit13serves also as the current source181(seeFIG. 4) of the bias current supply circuit18.

The reason why the current source131can be used in common to the two current supply circuits13and18is as follows. That is, the bias current supply circuit18needs to operate only in the period in which the semiconductor laser LD is switched off, whereas the test current supply circuit13needs to operate only in a partial period except the APC period and the modulation period. Accordingly, the operating period of the test current supply circuit13and the operating period of the bias current supply circuit18can be set so as not to overlap each other.

Because the current source131is used in common to the test current supply circuit13and the bias current supply circuit18in this manner, one current source181can be dispensed with. Accordingly, consumed electric power can be reduced as well as simplification in circuit configuration and reduction in cost can be attained. It is however necessary to provide a switch SW32between the current source131and the test current setting portion132and a switch SW82between the current source131and the bias current setting portion19as shown inFIG. 7so that the respective outputs of the test current setting portion132and the bias current setting portion19are not given to the current source131continuously.

Although the first modification of the second embodiment has shown the case where the current source131is used in common to the test current supply circuit13and the bias current supply circuit18, the invention may be also applied to the case where the current source131of the test current supply circuit13is used to serve also as the current source (the current source for the drive current at the time of switching off the semiconductor laser LD)113of the drive current control circuit11. The use of the current source131in common to the test current supply circuit13and the drive current control circuit11can be also applied to the semiconductor laser driving device according to the first embodiment.

Although the first and second embodiments have been described upon the case where each embodiment is applied to a driving device for driving a single light-emitting element, specifically for driving a single semiconductor laser LD, the invention is not limited to the driving device for driving a single light-emitting element. For example, the invention may be also applied to a driving device for driving a plurality of light-emitting elements such as a surface emission laser (multi-laser) having a large number of light-emitting portions emitting laser beams respectively. This is a light-emitting element driving device according to a third embodiment of the invention as will be described below.

FIG. 8is a block diagram showing an example of configuration of important part of the light-emitting element driving device, that is, the multi-laser driving device according to the third embodiment of the invention. InFIG. 8, parts the same as those inFIG. 4are denoted by the same reference numerals as those inFIG. 4.

The case where the configuration of the semiconductor laser driving device according to the second embodiment is used as a basic configuration will be described by way of example here but it is also possible to use the configuration of the semiconductor laser driving device according to the first embodiment as a basic configuration. The light intensity detection circuit15shown inFIG. 1and the drive current control circuit11, the control circuit17and the correction circuit20shown inFIG. 3are not shown inFIG. 8. The large number of light-emitting portions in the multi-laser (surface emission laser) are represented by two light-emitting portions LD1and LD2(hereinafter referred to as semiconductor lasers LD1and LD2) for the sake of simplification of the drawing.

InFIG. 8, test current supply circuits13-1and13-2, bias current supply circuits18-1and18-2and bias current setting portions19-1and19-2are provided in correspondence with the two semiconductor lasers LD1and LD2. A test current setting portion132and a bias voltage setting circuit14are provided in common to the two semiconductor lasers LD1and LD2. Incidentally, the test current setting portion132maybe provided for each of the test current supply circuits13-1and13-2in the same manner as in the first or second embodiment. The provision of the test current setting portion132in common to the test current supply circuits13-1and13-2has however an advantage in that reduction in cost is attained.

In the multi-laser driving device according to the third embodiment, a result of an arithmetic operation for the respective terminal voltages of the two semiconductor lasers LD1and LD2is used for setting the bias voltage Vbiasby the bias voltage setting circuit14. Therefore, an arithmetic circuit21is provided as a stage in front of the bias voltage setting circuit14.

The arithmetic circuit21makes an arithmetic operation, for example, of an average value (V1+V2)/2 by using the respective terminal voltages V1and V2of the semiconductor lasers LD1and LD2which are supplied with test currents smaller than the emission threshold currents, preferably, slightly smaller than the emission threshold currents from the test current supply circuits13-1and13-2respectively. Incidentally, calculation of a voltage value is not limited to calculation of such an average value. For example, a voltage value between the minimum and maximum values of the respective terminal voltages and except the minimum and maximum values may be used. Specifically, a value as center as possible in variation in characteristic of the semiconductor lasers, such as a median, a mode, etc., may be preferably used.

The voltage value (V1+V2)/2 which is a result of the arithmetic operation by the arithmetic circuit21is given to the bias voltage setting circuit14. In the bias voltage setting circuit14, the voltage value (V1+V2)/2 is sampled by the switch SW42and held by the capacitor C41. The voltage held by the capacitor C41is applied as a bias voltage Vbiasto the anodes of the semiconductor lasers LD1and LD2when the switches SW41-1and SW41-2are turned on at the time of switching off the semiconductor lasers LD1and LD2.

The voltage held by the capacitor C41is also supplied to the bias current setting portions19-1and19-2. Each of the bias current setting portions19-1and19-2is formed like the bias current setting portion19shown inFIG. 4. That is, for example, each of the bias current setting portions19-1and19-2includes a differential amplifier, and a sample-and-hold circuit. The respective currents of the current sources181-1and181-2, that is, bias currents Ibiasare set on the basis of such negative feedback control that the terminal voltages V1and V2of the semiconductor lasers LD1and LD2coincide with the bias voltage Vbiasset by the bias voltage setting circuit14when the semiconductor lasers LD1and LD2are switched off. When the configuration of negative feedback control is used, control can be performed by a simple configuration so that the terminal voltages of the semiconductor lasers LD1and LD2at the time of switching off the semiconductor lasers LD1and LD2coincide with the calculated common bias voltage Vbias.

Voltage-current drive (voltage drive->current drive) can be performed as follows. When the switches SW41-1and SW41-2are turned on, the bias voltage Vbiaswith a voltage value (V1+V2)/2 set by the bias voltage setting circuit14is applied to the anodes of the semiconductor lasers LD1and LD2through the switches SW41-1and SW41-2(Voltage Drive). Then, when the switches SW41-1and SW41-2are turned off and the switches SW81-1and SW81-2of the bias current supply circuits18-1and18-2are turned on, the bias currents Ibiasset on the basis of the bias voltage Vbiasby the bias current setting portions19-1and19-2are supplied from the current sources181-1and181-2to the semiconductor lasers LD1and LD2respectively (Current Drive).

In the voltage-current drive, because the voltage value of the bias voltage Vbiascorresponds to the current value of the bias current Ibiasset on the basis of the bias voltage Vbias, the voltage value V1/V2of the terminal voltage of each semiconductor laser LD1/LD2converges rapidly into the voltage value for the current drive when the voltage drive is shifted to the current drive.

As described above, in the multi-laser driving device according to the third embodiment, a test current is supplied to each semiconductor laser when a plurality of semiconductor lasers (e.g., two semiconductor lasers in the third embodiment) are switched off. An arithmetic operation for the terminal voltages of the semiconductor lasers supplied with the test current is carried out. A bias voltage Vbiasapplied to each semiconductor laser at the time of switching off the semiconductor laser is set on the basis of the calculated voltage value. Accordingly, one bias voltage setting circuit14can be provided in common to the plurality of semiconductor lasers. In other words, bias voltage setting circuits14each including a capacitor C41need not be provided for the plurality of semiconductor lasers respectively. Accordingly, simplification in circuit configuration and reduction in cost can be attained.

Particularly when a voltage value between the minimum and maximum values of the respective terminal voltages of the plurality of semiconductor lasers and except the minimum and maximum values, such as an average, a median, a mode, etc., is calculated so that the bias voltage Vbiasis set by the calculated voltage value, the voltage value of the bias voltage Vbiascan be set to be near the center value of variation in characteristic of the semiconductor lasers even in the case where characteristic of the semiconductor lasers varies. Accordingly, the drive condition common to all the semiconductor lasers can be prevented from being prejudiced extremely as if the bias voltage Vbiaswas set on the basis of the maximum or minimum of variation in characteristic of the semiconductor lasers.

If variation in characteristic of the semiconductor lasers is large, there is a possibility that several beams may be emitted slightly or the modulation speed may become slow. In laser xerography using a laser array, because the influence of one laser on image quality is divided by the number of lasers, this is insignificant compared with a single laser. Accordingly, specifications of the semiconductor lasers can be loosened. This makes a great contribution to improvement in yield.

When bias currents Ibiasare set on the basis of the bias voltage Vbiasso that the bias currents Ibiasare supplied to the plurality of semiconductor lasers respectively at the time of switching off the semiconductor lasers, the voltage at the voltage drive operation can be made substantially equal to the voltage at the current drive operation. Accordingly, the width of voltage fluctuation is reduced when the voltage drive is shifted to the current drive. Accordingly, the time required for shifting the constant voltage drive to the constant current drive can be shortened. As a result, the period for shifting to the constant current operation in which correction for temperature cannot work can be minimized. This can make a great contribution to improvement in temperature compensating accuracy described in the second embodiment.

FIG. 9is a circuit diagram showing an example of configuration of the light-emitting element driving device, that is, the semiconductor laser driving device according to a first modification of the third embodiment. InFIG. 9, parts the same as those inFIG. 8are denoted by the same reference numerals as those inFIG. 8.

The first modification of the third embodiment is configured on the same gist as that of the modification of the second embodiment. That is, the current sources131-1and131-2of the test current supply circuits13-1and13-2are used so as to serve as the current sources.181-1and181-2of the bias current supply circuits18-1and18-2respectively.

Because the current sources131-1and131-2are used in common to the test current supply circuits13-1and13-2and the bias current supply circuits18-1and18-2in this manner, one current source per semiconductor laser can be dispensed with. Accordingly, consumed electric power can be reduced as well as simplification in circuit configuration and reduction in cost can be attained. The effect increases as the number of semiconductor lasers increases.

FIG. 10is a block diagram showing an example of configuration of important part of the light-emitting element driving device, that is, the multi-laser driving device according to a fourth embodiment of the invention. InFIG. 10, parts the same as those inFIG. 8are denoted by the same reference numerals as those inFIG. 8.

Although the multi-laser driving device according to the third embodiment is configured so that a single voltage value (V1+V2)/2 is calculated by a single arithmetic circuit21, the multi-laser driving device according to the fourth embodiment is configured so that a plurality of voltage values (e.g., three voltage values) are calculated by a plurality of arithmetic circuits (e.g., three arithmetic circuits21-1,21-2and21-3). The other configuration of the multi-laser driving device according to the fourth embodiment is the same as that of the multi-laser driving device according to the third embodiment. One voltage value is selected from the calculated three voltage values by the switch SW43and given to the bias voltage setting circuit14.

One21-1of the three arithmetic circuits21-1,21-2and21-3calculates a voltage value (2V1+V2)/3. Another arithmetic circuit21-2calculates a voltage value (V1+V2)/2. The last arithmetic circuit21-3calculates a voltage value (V1+2V2)/3. Incidentally, these calculated values are provided not for limitation but for exemplification. For example, four or more optional voltage values between V1and V2(the maximum and minimum values) may be calculated.

As described above, in the multi-laser driving device according to the fourth embodiment, a plurality of voltage values are calculated by arithmetic operations using the respective terminal voltages of the plurality of semiconductor lasers supplied with the test current. A voltage value is selected from the plurality of voltage values and applied as a bias voltage Vbiasto the plurality of semiconductor lasers at the time of switching off the semiconductor lasers. Bias currents Ibiassupplied to the plurality of semiconductor lasers respectively at the time of switching off the semiconductor lasers are set on the basis of the selected voltage value. Accordingly, the optimal bias voltage Vbiasand bias current Ibiascan be selected from several options in accordance with the distribution of variation in characteristic of the plurality of semiconductor lasers. As a result, more accurate drive control can be achieved.

Although the embodiments have been described, byway of example, upon the case where a GaN blue laser or a single mode surface emission laser is used as a light-emitting element to be driven, the invention is not limited to application to drive of these laser elements and may be also applied to voltage drive for general light-emitting elements high in internal resistance such as EL (Electro-Luminescence) elements.

As described above, according to a first aspect of the invention, there is provided a light-emitting element driving device including: a first control unit adapted to control a drive voltage applied from a voltage source to a light-emitting element; a second control unit adapted to control a drive current supplied from a current source to said light-emitting element; a test current supply unit adapted to supply a test current to the light-emitting element when the light-emitting element is switched off; and a bias voltage setting unit adapted to set a bias voltage applied to the light-emitting element on the basis of a terminal voltage of the light-emitting element supplied with the test current from the test current supply unit when the light-emitting element is switched off, wherein, the first control unit and the second control unit controls an light intensity of a light beam emitted from the light-emitting element.

According to the first aspect of the invention, a voltage drive and a current drive (voltage drive->current drive) are used in combination. For setting of the bias voltage, a current expected to flow in the light-emitting element at the time of switching off the light-emitting element is actually supplied as a test current from the test current supply unit to the light-emitting element in a period in which neither light intensity control nor modulation is performed. Thus, a terminal voltage of the light-emitting element supplied with the test current is detected. Then, the bias voltage setting unit sets the bias voltage applied to the light-emitting element at the time of switching off the light-emitting element on the basis of the detected voltage. Accordingly, the bias voltage applied to the light-emitting element at the time of switching off the light-emitting element can be easily set to be a target value compared with the related art in which it was difficult to control the current because the current varied widely according to slight voltage change.

According to a second aspect of the invention, in addition to the first aspect of the invention, the second control unit controls the drive current when the light intensity of light beam emitted from the light-emitting element is in control, and the first control unit sets the drive voltage in accordance with a voltage generated in the light-emitting element as a result of the control of the drive current by the second control unit.

According to the second aspect of the invention, the drive current of the light-emitting element is controlled by the second control unit so that the light intensity of light beam emitted from the light-emitting element is equal to predetermined intensity of light. Thus, the terminal voltage of the light-emitting element is detected when the intensity of light emitted from the light-emitting element is equal to the predetermined intensity of light. The first control unit sets the voltage value of the voltage applied from the voltage source to the light-emitting element at the time of switching on the light-emitting element on the basis of the detected voltage (terminal voltage). Accordingly, because the drive current of the light-emitting element, particularly a semiconductor laser is substantially proportional to the intensity of light, the gain of a negative feedback loop for controlling the intensity of light can be kept constant. Accordingly, stable control can be achieved.

According to a third aspect of the invention, in addition to the first aspect of the invention, the first control unit controls the drive voltage when the light intensity of light beam emitted from the light-emitting element is in control, and the second control unit sets the drive current in accordance with a current flowing in the light-emitting element as a result of the control of the drive voltage by the first control unit.

According to the third aspect of the invention, when the internal resistance of the light-emitting element is very high, a long time is required for convergence into the target light intensity because of the influence of a time constant decided on the basis of the internal resistance and capacitance of the light-emitting element if the current source is controlled to drive the current at the time of controlling the light intensity. In this case, however, the time of convergence in the light intensity control can be shortened when the voltage source with low output impedance is controlled to drive the voltage.

According to a fourth aspect of the invention, there is provided a light-emitting element driving device for controlling a drive voltage applied from a voltage source to a light-emitting element while controlling a drive current supplied from a current source to the light-emitting element to thereby control an light intensity of a light beam emitted from the light-emitting element, the light-emitting element driving device including: a bias current supply unit adapted to supply a bias current to the light-emitting element; a test current supply unit adapted to supply a test current to the light-emitting element when the light-emitting element is switched off; a bias voltage setting unit adapted to set a bias voltage applied to the light-emitting element on the basis of a terminal voltage of the light-emitting element supplied with the test current from the test current supply unit when said light-emitting element is switched off; and a bias current setting unit adapted to set the bias current supplied from the bias current supply unit to the light-emitting element on the basis of the bias voltage set by the bias voltage setting unit when the light-emitting element is switched off.

According to the fourth aspect of the invention, a voltage drive and a current drive (voltage drive->current drive) are used in combination. The bias current to flow in the light-emitting element by the current drive at the time of switching off the light-emitting element is set by the bias current setting unit on the basis of the bias voltage set by the bias voltage setting unit. The bias current is supplied from the bias current supply unit to the light-emitting element when the light-emitting element is switched off. As a result, the voltage at the voltage drive operation becomes substantially equal to the voltage at the current drive operation, so that the change width of the voltage at the time of transition from the voltage drive to the current drive is reduced. Accordingly, the time required for transition from the constant voltage drive to the constant current drive can be shortened.

According to a fifth aspect of the invention, in addition to the fourth aspect of the invention, the bias current setting unit sets the bias current by performing negative feedback control to make the terminal voltage of the light-emitting element coincides with the bias voltage set by the bias voltage setting unit.

According to the fifth aspect of the invention, when, for example, negative feedback control using a differential amplifier is used, control can be performed by simple configuration so that the terminal voltage of the light-emitting element at the time of switching off the light-emitting element coincides with the set bias voltage.

According to a sixth aspect of the invention, in addition to the fourth aspect of the invention, the setting of the bias voltage by the bias voltage setting unit and the setting of the bias current by the bias current setting unit are performed at intervals of a predetermined cycle to thereby readjust the bias current in accordance with voltage-current characteristic change of the light-emitting element due to temperature change.

According to the sixth aspect of the invention, when the setting of the bias voltage and the setting of the bias current are performed at intervals of a predetermined cycle, for example, a light intensity control cycle to thereby readjust the bias current in accordance with voltage-current characteristic change of the light-emitting element due to temperature change, the bias current to flow in the light-emitting element can be kept constant even in the case where the voltage-current characteristic of the light-emitting element varies according to the temperature change.

According to a seventh aspect of the invention, in addition to the fourth aspect of the invention, a current source for the bias current supply unit serves also as a current source for the test current supply unit.

According to the seventh aspect of the invention, because a current source can be used in common to the bias current supply unit and the test current supply unit, one current source can be saved. Accordingly, consumed electric power can be reduced as well as simplification of circuit configuration and reduction in cost can be attained.

According to an eighth aspect of the invention, there is provided a light-emitting element driving device for controlling a drive voltage applied from a voltage source to a plurality of light-emitting elements while controlling a drive current supplied from a current source to the plurality of light-emitting elements to thereby control the light intensity of light beams emitted from the plurality of light-emitting elements, the light-emitting element driving device including: a test current supply unit adapted to supply a test current to the light-emitting element when the light-emitting element is switched off; an arithmetic unit adapted to carry out an arithmetic operation for respective terminal voltages of the plurality of light-emitting elements supplied with the test current from the test current supply unit; and a bias voltage setting unit for adapted to set a bias voltage applied to the plurality of light-emitting elements on the basis of a voltage value calculated by the arithmetic unit when the plurality of light-emitting elements are switched off.

According to the eighth aspect of the invention, one bias voltage setting circuit can be provided in common to the plurality of light-emitting elements. Accordingly, simplification of circuit configuration and reduction in cost can be attained.

According to a ninth aspect of the invention, in addition to the eighth aspect of the invention, the arithmetic unit calculates the voltage value between a maximum value and a minimum value of the respective terminal voltages of the plurality of light-emitting elements and except the maximum value and the minimum value.

According to the ninth aspect of the invention, when a voltage value between the maximum value and the minimum value of the respective terminal voltages of the plurality of light-emitting elements and except the maximum and the minimum values, such as an average, a median, a mode, etc., is calculated and set as the bias voltage, the voltage value of the bias voltage can be set to be near the center value of variations in characteristic of the light-emitting elements even when there are variations in characteristic of the light-emitting elements. Accordingly, the drive condition common to all the light-emitting elements can be prevented from being prejudiced extremely.

According to a tenth aspect of the invention, in addition to the eighth aspect of the invention, the arithmetic unit calculates a plurality of voltage values and selects the voltage value of the bias voltage from the plurality of voltage values.

According to the tenth aspect of the invention, a plurality of voltage values are calculated by the arithmetic operation on the basis of the terminal voltages of the plurality of light-emitting elements supplied with the test current. A voltage value selected from the calculated voltage values is set as a bias voltage applied to the plurality of light-emitting elements at the time of switching off the plurality of light-emitting elements. Accordingly, an optimal bias voltage can be selected from several options according to the distribution of variations in characteristic of the plurality of light-emitting elements. As a result, drive control can be achieved with higher accuracy.

According to an eleventh aspect of the invention, in addition to the eighth aspect of the invention, the test current supply unit includes a unit common to the plurality of light-emitting elements for setting a current value of the test current.

According to the eleventh aspect of the invention, because the unit for setting the current value of the test current is used in common to the plurality of light-emitting elements, there is an advantage for attaining reduction in cost.

According to a twelfth aspect of the invention, there is provided a light-emitting element driving device for controlling a drive voltage applied from a voltage source to a plurality of light-emitting elements while controlling a drive current supplied from a current source to the plurality of light-emitting elements to thereby control the light intensity of light beams emitted from the plurality of light-emitting elements, the light-emitting element driving device including: a bias current supply unit adapted to supply bias currents to the plurality of light-emitting elements respectively; a test current supply unit adapted to supply a test current to the plurality of light-emitting elements when the plurality of light-emitting elements are switched off; an arithmetic unit adapted to carry out an arithmetic operation for respective terminal voltages of the plurality of light-emitting elements supplied with the test current from the test current supply unit; a bias voltage setting unit adapted to set a bias voltage applied to the plurality of light-emitting elements on the basis of a voltage value calculated by the arithmetic unit when the plurality of light-emitting elements are switched off; and a bias current setting unit adapted to set the bias currents supplied from the bias current supply unit to the plurality of light-emitting elements on the basis of the bias voltage set by the bias voltage setting unit when the plurality of light-emitting elements are switched off.

According to the twelfth aspect of the invention, a voltage drive and a current drive (voltage drive->current drive) are used in combination for each of the light-emitting elements. The bias currents to flow in the plurality of light-emitting elements by the current drive at the time of switching off the plurality of light-emitting elements are set by the bias current setting unit on the basis of the bias voltage set by the bias voltage setting unit. The bias currents are supplied from the bias current supply unit to the plurality of light-emitting elements when the plurality of light-emitting elements are switched off. As a result, the voltage at the voltage drive operation becomes substantially equal to the voltage at the current drive operation, so that the change width of the voltage at the time of transition from the voltage drive to the current drive is reduced. Accordingly, the time required for transition from the constant voltage drive to the constant current drive can be shortened for each of the light-emitting elements.

According to a thirteenth aspect of the invention, in addition to the twelfth aspect of the invention, the bias voltage setting unit calculates a plurality of voltage values and selects the voltage value of the bias voltage from the plurality of voltage values.

According to the thirteenth aspect of the invention, a plurality of voltage values are calculated by the arithmetic operation on the basis of the terminal voltages of the plurality of light-emitting elements supplied with the test current. A voltage value selected from the calculated voltage values is used as a bias voltage applied to the plurality of light-emitting elements at the time of switching off the plurality of light-emitting elements. Bias currents supplied to the plurality of light-emitting elements respectively at the time of switching off the plurality of light-emitting elements are set on the basis of the selected voltage value. Accordingly, an optimal bias voltage and optimal bias current can be selected from several options according to the distribution of variations in characteristic of the plurality of light-emitting elements. As a result, more accurate drive control can be achieved.

According to a fourteenth aspect of the invention, in addition to the twelfth aspect of the invention, the bias current supply unit sets the bias currents by performing negative feedback control to make each of the terminal voltages of the plurality of light-emitting elements coincides with the bias voltage set by the bias voltage setting unit.

According to the fourteenth aspect of the invention, when, for example, negative feedback control using a differential amplifier is used, control can be performed by simple configuration so that the terminal voltage of each of the plurality of light-emitting elements at the time of switching off the plurality of light-emitting elements coincides with the calculated common bias voltage.

According to a fifteenth aspect of the invention, in addition to the twelfth aspect of the invention, the setting of the bias voltage by the bias voltage setting unit and the setting of the bias currents by the bias current setting unit are performed at intervals of a predetermined cycle to thereby readjust the bias currents in accordance with voltage-current characteristic change of the plurality of light-emitting elements due to temperature change.

According to the fifteenth aspect of the invention, when the setting of the bias voltage and the setting of the bias currents are performed at intervals of a predetermined cycle, for example, a light intensity control cycle to thereby readjust the bias currents in accordance with voltage-current characteristic change of the light-emitting element due to temperature change, the bias currents to flow in the plurality of light-emitting elements respectively can be kept constant even in the case where the voltage-current characteristic of the plurality of light-emitting element varies according to the temperature change.

According to a sixteenth aspect of the invention, in addition to the twelfth aspect of the invention, a current source for the bias current supply unit serves also as a current source for the test current supply unit.

According to the sixteenth aspect of the invention, because a current source can be used in common to the bias current supply unit and the test current supply unit, one current source per light-emitting element can be dispensed with. Accordingly, consumed electric power can be reduced as well as simplification of circuit configuration and reduction in cost can be attained. The effect increases as the number of light-emitting elements increases.

According to a seventeenth aspect of the invention, in addition to the twelfth aspect of the invention, the test current supply unit includes a unit common to the plurality of light-emitting elements for setting a current value of the test current.

According to the seventeenth aspect of the invention, because the unit for setting the current value of the test current is used in common to the plurality of light-emitting elements, there is an advantage for attaining reduction in cost.

According to a eighteenth aspect of the invention, there is provided a light-emitting element driving device including: a first control unit adapted to control a drive voltage applied from a voltage source to a light-emitting element; and a second control unit adapted to control a drive current supplied from a current source to the light-emitting element, wherein the first control unit and the second control unit controls an light intensity of a light beam emitted from the light-emitting element, wherein the first control unit sets a voltage of the voltage source in accordance with a voltage generated in the light-emitting element as a result of the control of the drive current by the second control unit.

According to the eighteenth aspect of the invention, the drive current of the light-emitting element is controlled by the second control unit so that the intensity of light emitted from the light-emitting element is equal to set light intensity. The terminal voltage of the light-emitting element is detected when the intensity of light emitted from the light-emitting element is equal to the set light intensity. The second control unit sets the drive voltage applied to the light-emitting element at the time of switching off the light-emitting element on the basis of the detected voltage (terminal voltage). Accordingly, because the drive current of the light-emitting element, particularly a semiconductor laser is substantially proportional to the intensity of light, the gain of a negative feedback loop for controlling the intensity of light can be kept constant. Accordingly, stable control can be achieved.