JPS58107693A - Semiconductor laser driving device - Google Patents

Abstract

PURPOSE:To improve the output intensity of emitted laser light, in the laser driving device, by using an FET for current control. CONSTITUTION:A laser ON OFF control circuit 11 is connected to a switching FET Q, and the FET 20 is connected for the current control. The FET 20 and resistors RF and RD operate as a current limiter for a laser 2. A current control circuit 12 minutely adjusts the current flowing the FET 20, and controls the current flowing the laser 2. Said control circuit 12 has a switch 22. The switch 22 is connected to the side of a reference voltage until the emitted light output becomes a specified value, and makes a laser current IQ constant. When the output of the laser 2 has reached the specified value, the switch 22 is connected to the side of a sample and hold circuit 21. In this constitution, the current controlling FET 20 is controlled to the current value corresponding to the output of the laser 2, which is detected by a light output detecting element 9.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a driving device for controlling switching of a semiconductor laser.

Semiconductor lasers (hereinafter simply referred to as lasers) are used in laser printers, communications, and other applications. To explain the laser printer with reference to the blocks and diagrams in FIG. 1, 1 is a laser driving device that outputs a signal for controlling on/off of a laser 2, 3 is a collimator lens, and 4 is a beam expander. Reference numeral 5 denotes a polygon mirror having a rotating polygon mirror attached to a polygon motor shaft, which scans the laser beam that has passed through the collimator lens 3 and the beam expander 4 at high speed. Reference numeral 6 denotes an fθ lens that projects the laser beam scanned by the polygon mirror 5 onto the rotating photosensitive drum 7. Reference numeral 8 denotes a light receiving element for generating a synchronization signal, which is provided near the end of the photoreceptor drum 7 and receives the laser beam to synchronize the rotation of the photoreceptor drum 7 with the scanning of the laser beam.

In addition, 9 is a light output detection element, which is built into the laser 2 or installed near the laser 2, receives the laser beam of the laser 2, and returns the signal to the laser driving device @1 to detect the laser 2. Try to stabilize the output.

The operation of laser 2 in the laser printer shown in Figure 1 is explained in the second diagram.
This will be explained using the waveform diagram in the figure. When the laser beam starts scanning by rotating the polygon mirror 5 in the direction of the arrow, the laser beam enters the light receiving element 8 and a synchronizing signal is generated from the light receiving element 8. This synchronizing signal causes a control circuit (not shown) to be activated. The laser enable signal from
The time until the laser beam reaches the printing position on the photosensitive drum 7 is counted. When exposing a character portion on a photoreceptor (negative-positive), the laser 2 is also turned off during the period when the laser enable signal is turned off. When the time comes for the laser beam to reach the printing position, the laser enable signal is turned on and the laser 2 starts operating. While the laser beam scans the image printing area, the laser driving device 1 controls the laser 2 according to the image data. On/on control.

When the scanning of the image printing area is completed, the laser enable signal is once turned off and then turned on again at a preset light time, and the laser 2 is also turned on. During this period when the laser 2 is on, the laser beam scan returns to the starting position (the left end in the figure), the photodetector 8 generates a synchronization signal again, and the above printing operation is executed on the next line. . As described above, in the case of d-positive, in which the operation timing of the laser 2 is synchronized every time the laser beam scans one line on the photoreceptor drum 7, the on-off of the laser is opposite to the above.

Also, number one? , during the timing of A in the figure, the optical output of the laser 2 is detected by the optical output detection element 9, and the signal is fed back to the laser driving device 1 to stabilize the laser emission output.

Conventionally, a laser driving device 1 used in this type of laser printer, etc., has a laser 2 connected to the collector side of a differential amplifier including transistors Q, Q2, etc., as shown in FIG. , transistor Q,
On/off control of the laser 2 is performed by the laser on/off signal inputted to the base of the transistor q3, and the optical output signal fed back from the laser light output detection element 9 is connected to the base of the transistor q3 via the amplifier AMP. Devices configured to control the drive current are known.

In such a conventional circuit, a transistor Q□ is used to control on/off of the laser 2 at high speed.

It is desirable that q2 operates in the active region, and usually,
A minute current flows through the laser 2, and the laser 2 always emits weak light (LED light emission). Therefore, when this circuit is applied to a laser printer, image fogging occurs, which is a practical problem.

For this reason, it is conceivable to use a high-speed switching element such as a high-speed transistor or a field-effect transistor (hereinafter referred to as FET) in place of the above-mentioned differential amplifier type switching circuit. When the circuit is schematically shown in FIG. 3, a new problem arises. That is, the assumed circuit is, as shown in FIG. 3, a circuit including a negative current control transistor 10 and a fixed resistor R1 in parallel between a series circuit of the laser 2 and a high-speed switching transistor Q. A laser on/off control circuit 11 is connected to the switching transistor q, and a current control circuit 12 is connected to the current control transistor 10.

In this laser driving device, when the switching transistor Q is turned on, a current iq flows through the laser 2 and emits light. The current iq is made up of the sum of the current iF flowing through the current control transistor 10 and the current iD flowing through the fixed resistor R[), and by controlling the current iF by the current control circuit 12, the light emission output of the laser 2 is stabilized. keeping.

However, in such a drive device, when the switching transistor q is turned on and off, it takes time for minority carriers to be injected into that region or to escape from the base region, which also affects the load resistance R1-. , the on/off characteristics of the laser 2 itself deteriorates.

An object of the present invention is to improve the switching characteristics of the laser emission output intensity in such a laser driving device. That is, the present invention aims to achieve the above object by using an FET for current control in a laser driving device.

Hereinafter, the present invention will be explained in detail using one example.

FIG. 5 is a circuit diagram showing an embodiment of the present invention, in which the parallel circuit provided between the laser 2 and the switching FET Q' is different from the conventional example shown in FIG. ) is a parallel circuit with switching F
A laser on/off control circuit 11 is connected to E T Q′, and a current control circuit 12 is connected to the current control FET 20 .

For example, J107 (manufactured by Siliconix Corporation) can be used as the FET 20 for current control.

In this embodiment, the current control circuit 12 includes the optical output detection element 9
a sample-and-hold circuit 21 that samples and holds an amplified output signal; and a switch 2, such as an analog switch, that selects either this output signal or a reference voltage depending on the magnitude of the output signal of this sample-and-hold circuit 21.
2, and a circuit that controls the current control FET 20 based on the output signal of the switch 22.

In this embodiment, the FET 20 and the resistors RF and RD act as current limiting resistors for the laser 2. When FET20 is connected as shown in Figure 3, this! ET20 can be treated as a pure resistance element.

If the resistance that appears when FET 20 is turned on is T(, then the combined resistance of this parallel circuit is RD-(RF 1r() R=-1-11111111111-kD +' p
+ r( , and the current IQ flowing through the laser 2 is RD + RF
+r( , and by controlling the resistance r( of FET2Q),
It can be seen that fq is controlled. Here, ■cc is the power supply voltage, ■F is the forward voltage of the laser 2, and VQ is the FET2
The saturation voltage is 0.

Since the relationship between the gate voltage of FET2Q and the electric wire iF is expressed, for example, as shown in FIG. 6, the reference voltage r2 may be set so that iF is at the center of the controlled range.

In the embodiment shown in FIG. 5, which is set in this way, the laser on/off control circuit 11 starts from the state where the laser 2 is off.
turns on the switching FETQ'. At this time, since the signal from the optical output detection element 9 is zero or very small, the output signal of the sample and hold circuit 21 is equal to the reference voltage v.
r3, therefore, the state of the signal from the comparator 23 connects the switch 22 to the reference voltage r1 side.

As a result, currents corresponding to the reference voltages ■ and □ flow through the FET 20, and the sensor 2 emits light at a preset current value.

Next, the light emission output of the laser 2 reaches a predetermined value, and the output signal of the siple hold circuit 21 reaches the reference voltage v,3. Then, the switch 22 is connected to the sample and hold circuit 21 side. In this state, when the emission output of the laser 2 increases, the gate voltage of the FET 20 (
The absolute value of (negative) increases, increasing the resistance r( of FET 20, decreasing the current iQ of laser 2, and reducing the light emission output of laser 2. Conversely, when the light emission output of laser 2 decreases, acts in the opposite way to the above and increases the light emission output of the laser 2. Also, when the light emission output of the laser 2 decreases below a predetermined value, the switch 22 changes the reference voltage vr
FE switches to the □ side so that the current iQ reaches the set value.
T20 is controlled.

Switching F as in the printing operation of a laser printer
When ETQ' is repeatedly turned on and off, the switch 22 remains connected to the sample and hold circuit 21 due to the sampled and held signal.

Switching FE by laser on/off control circuit 11
When TQ was turned on and off, the emission output intensity of laser 2 rose as shown by the solid line in Figure 4 Tal during low frequency switching, and had a good fall as shown in the same figure during high frequency switching. As shown by the solid line in (b), the contrast or resolution also improved.

In this embodiment, the current i of the laser 2 is controlled by switching the switch 22 until the emission output of the laser 2 reaches a constant value.
Since it is configured to keep the Q constant, the sample and hold circuit 2
It is possible to prevent an accident in which an overcurrent flows through the FET 20 and damages the laser 2 if sampling of the voltage to the FET 1 is not performed or if the voltage drops extremely.

Nao, in the embodiment of FIG.
' may be the switching transistor q shown in FIG. Further, the positions of the parallel circuit including the FET 20 and the switching FET Q' (or the switching transistor Q) may be exchanged.

As described above, the present invention uses FETs with fast rise and fall speeds for current control to stabilize the light emission output of the laser. Laser emission output switch,
A laser drive device with excellent printing characteristics can be provided, and when the laser drive device of the present invention is used in a laser printer, a printed image with excellent resolution can be obtained.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a block circuit diagram showing a laser printer as an example to which the present invention is applied, FIG. 2 is a waveform diagram showing the operating state of the laser in the laser printer of FIG. 1, and FIG. The figure is a block circuit diagram showing a conventional laser drive device, FIG. 4 is a laser emission waveform diagram comparing the laser emission state in an embodiment of the present invention and a conventional laser drive device, and FIG. A block circuit diagram showing one embodiment, FIG. 6 is an FE
FIG. 3 is a diagram showing gate voltage versus load current characteristics of T. DESCRIPTION OF SYMBOLS 1... Laser drive device, 2... Laser, 9... Optical output detection element, 10... Current control transistor, 1
DESCRIPTION OF SYMBOLS 1...Laser on/off control circuit, 12...Current control circuit, 20...FET for current control, 21...Sample hold circuit, 22...Switch, RD...Fixed resistance, Q.・・Switching transistor, q′・・
・Switching FETo
. Patent Applicant Minolta Camera Co., Ltd. Agent Patent Attorney Aoyama Sogai 8 No. 155 Figure 2 Leh 7 --- 1 --- 1 ■ Town curt ---
------- U-Yu, Yu, Eng, --- Third cause, Figure 4, 1- Procedural amendment (spontaneous) January 29, 1980 Director General of the Patent Office Indication of the case 1982 Patent Application No. 206580 2 Name of the invention Semiconductor laser drive device 3 Relationship with the person making the amendment Patent applicant Osaka Vhi IV Quadutemachi Address Nobobanike Daihan International Building 4, 2-chome, Azuchi-cho, Higashi-ku, Osaka
Date of amendment order by agent 5 Motto 7. Contents of amendment (1) The word "the" in line 12 of page 6 of the specification has been changed to [
The base of the current control transistor 10 is corrected. (2) On the 3rd line of page 8, replace “Figure 8” with “Figure 5”
and correct it. (3) Delete the phrase "Laser...has gotten better." from lines 12 to 18 on page 10. (4) Delete the word "conventional" in the 5th line of page 12. (5) On page 12, lines 5 and 6, it says “indicates”.
amend it to ``to explain the disadvantages of''. (6) On the 6th to 8th lines of page 12, the phrase [Figure 4 is a waveform diagram] has been replaced with ``Figure 4 is a circuit diagram showing an example of a conventional laser device.'' to correct.

Claims (2)

[Claims] (1) A high-speed switching means is connected in series to the semiconductor laser, a parallel circuit including a circuit including a field effect transistor and a fixed resistor is connected in series to the semiconductor laser, and a current control circuit is connected to the field effect transistor. A semiconductor laser driving device characterized in that the current flowing through the semiconductor laser is controlled by finely adjusting the current flowing through the connected field effect transistor. (2) The current control circuit causes a constant current to flow through the field effect transistor until the light emission output of the semiconductor laser reaches a constant value, and after the light emission output of the semiconductor laser reaches a certain value, the current control circuit controls the current to flow in accordance with the light emission output of the semiconductor laser. 2. The semiconductor laser driving device according to claim 1, further comprising a protection circuit that changes the current flowing through the field effect transistor.