JPH03288870A - Semiconductor laser driving circuit - Google Patents

Abstract

PURPOSE:To hold the light emitting intensity of a semiconductor laser always at a constant level by successively switching plural constant current sources and controlling the current supply of respective constant current sources so that the detection output of the light emitting intensity of the semiconductor laser becomes a prescribed value. CONSTITUTION:The semiconductor laser driving circuit is provided with a detection means 16 for detecting the light emitting intensity of the semiconductor laser 10 and a current control means 4 for successively switching plural constant current sources A, B and controlling the current supply of respective current sources A, B. The means 16 detects the light emitting intensity of the laser 10 and supplies its detection output to the means 4. When the light emitting intensity of the laser 10 is changed in accordance with a change in peripheral temperature, the means 4 successively switches the sources A, B and controls the current supply of respective sources A, B so that respective detection outputs of the means 16 become respective prescribed values. Thus, the light emitting intensity of the laser 10 is held at the constant value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser drive circuit used in an image forming apparatus such as a laser beam printer or a laser facsimile that uses a semiconductor laser as a signal modulation light emitting element.

[Conventional technology]

2. Description of the Related Art In recent years, digital copying apparatuses that read original images using charge-coupled device (CCD) line sensors or the like and output the read image signals to a laser printer or the like for recording have become popular.

Documents that can be used in such devices include those with gradations such as photographs and paintings, but when reading images with such gradations and outputting the image data to a laser printer, Gradation is expressed using the conventionally known dither matrix method.

That is, one pixel is configured as a matrix of, for example, 4×4 blocks, and by performing binary control for each block, the shading of one pixel is expressed as 4×4=16 gradations. If it is desired to obtain even more gradations, this can be achieved by increasing the matrix size (8×8 matrix for expression of 64 gradations).

However, in the conventional multi-gradation expression method such as that disclosed in Japanese Patent Application Laid-open No. 61-189574, it is necessary to increase the matrix size as the number of gradations increases. The problem was that the image became rough and the resolution deteriorated.

Further, in order to improve this drawback, a pulse width modulation method is also used to control the lighting time of a semiconductor laser device (LD) for the purpose of multi-level expression of a laser beam.

This method can realize multi-value expression, but if the desired number of multi-value levels increases, the minimum pulse width must be reduced accordingly.

For example, if you want to perform 10-value modulation with a pixel period of 100 nsec, the minimum pulse width is generally 10 nsec.
sec is required. Such short pulses require careful consideration in the design of the signal transmission means, switching elements, and their mounting, which may lead to increased complexity and cost of the device.

Further, when the intensity of the manual pulse current is kept constant, the density characteristic of the recorded image with respect to the pulse width W of the semiconductor laser exhibits a nonlinear characteristic, as shown in the graph shown in FIG. In the figure, It and It(1!>l,) represent different input pulse current intensities of the semiconductor laser, respectively. As is clear from the figure, the current intensity is constant N or 1
In the case of 1), if you try to record a low-density image with good gradation, the pulse width must be made smaller, and at the same time, the characteristic curve becomes steeper in the low-density area, so the accuracy of controlling the pulse width also becomes smaller. High prices are required. Therefore, if the pixel period has to be set short as described above, the above-mentioned problems become even more severe.

Therefore, by modulating the output by combining the pulse width gradation and the intensity gradation of the modulation current, it is possible to improve the image quality of low-density images, which was not sufficient with multiple image output using only the conventional pulse width modulation method. .

For example, the different input terminal strengths shown in FIG. 12, Iz
By modulating the output of the semiconductor laser by combining the modulation with different pulse widths, three levels of current intensity I can be achieved.
Output modulation combining I, 12 and I, +l, and pulse width modulation becomes possible.

FIG. 13 is a graph showing the relationship between the pulse width W and the image density at each current intensity in correspondence with the density gradation.

In the figure, at points a and b where the concentration is high, the current intensity is 1, +I
If the circuit design is such that the current intensity is modulated by t, the current intensity is 11 at points c and d with medium density, and the current intensity is 1□ at points e and f with low density, the low density of the image can be reduced. Gradation control in the density region becomes easy, and the drawbacks of the prior art, such as the complexity of the circuit and the need for high pulse stability, can be avoided. That is, the current intensity is a constant value II +
In the case of 11, in this figure, in order to achieve density levels 1 to 6, it is necessary to set the gradation with a pulse width corresponding to each point a, b, g, h, l, and j. Ta.

Since the pulse widths of points j, t, and h that give densities 1 to 3 are very close to each other, it is not easy to control the gradation in low-density areas of the image, and when considering the stability of the pulse widths, the image quality However, 1. By switching to different current intensity levels such as , Iz, etc. and modulating the output, the constraints on pulse width approaches are reduced as shown by points a to f, which not only prevents deterioration of image quality but also facilitates circuit design and reduces costs. Effects such as reduction can be obtained.

[Problem to be solved by the invention]

Incidentally, the optical output of a semiconductor laser used in an image forming apparatus largely depends on temperature. FIG. 14 is a graph showing the relationship between the input terminal 1 and the optical output P of the semiconductor laser when the temperature changes. As shown in the figure, the optical output P of the semiconductor laser is the dark value input terminal ■ relative to the input terminal I.
. It has non-linear characteristics that rapidly increase after
It shows a temperature dependence that shifts in the direction. That is, low temperature (T1
), even if the optical output P is obtained for the input current ■1, if the temperature is high <('ri), the optical output decreases and only the optical output of P can be obtained. Images recorded with conventional laser printers experience significant changes in shading due to temperature changes.

The present invention has been made to improve these points, and includes a semiconductor laser drive circuit that selects two or more constant current sources to control the gradation of the modulation current of a semiconductor laser.
The object of the present invention is to provide a drive circuit whose optical output is stable against temperature changes.

[Means to solve the problem]

In order to solve the above problems, the present invention includes a first modulating means that modulates the pulse width of a pulse signal according to image data;
A second device comprising a plurality of constant current sources and modulating the amplitude of the pulse signal by switching the constant current sources according to image data to change the current injected into the semiconductor laser.
A semiconductor laser drive circuit comprising a modulation means and a drive pulse signal modulated by the first and second modulation means, a detection means for detecting the emission intensity of the semiconductor laser, and a plurality of constant current sources. Current control means is provided for sequentially switching and controlling the supply current of each constant current source so that the detection output of the detection means at that time becomes a predetermined value.

[Effect]

The detection means detects the emission intensity of the semiconductor laser and supplies the detected output to the current control means. When the emission intensity of the semiconductor laser changes due to a change in ambient temperature, the current control means sequentially switches the plurality of constant current sources. Then, by controlling the supply current of each constant current source so that the detection output of the detection means at that time becomes a predetermined value,
The emission intensity of the semiconductor laser is kept constant.

〔Example〕

Hereinafter, one embodiment of the present invention will be described based on the accompanying drawings.

FIG. 2 is a schematic configuration diagram of a laser printer suitable for applying the present invention, FIG. 3 is a schematic circuit diagram of a laser printer,
FIG. 4 is a schematic block diagram of a human-powered image processing apparatus for reading images, and FIG. 5 is a block diagram of an image signal system circuit of a copying machine composed of the human-powered image processing apparatus and a laser printer.

In Figure 2, lO is a semiconductor laser diode (L
D), the laser beam generated by the LDIO is made into a parallel beam by the collimator lens 11, deflected by the optical scanning device 12 consisting of a rotating polygon mirror, and the scanning light is passed through the condensing lens 13 to form an image. The spot is a rotating polygon mirror 12
As the photoreceptor 14 is rotated by the motor M, it is repeatedly scanned in the direction of the arrow X, and at the same time, the photoreceptor 14 is rotationally driven. The first photodetector I5 is provided outside the information writing area, and is located outside the rotating polygon mirror 1.
2 detects the deflected laser beam and generates a synchronization signal. The laser beam emitted backward from the LDIO is incident on the second photodetector 16, and the light intensity of the LDIO is detected.

As shown in FIG. 3, the signal processing circuit 2 outputs an image signal (video data) to the LD drive circuit 3, and its timing is controlled by a synchronization signal from the first photodetector 15. The LD drive circuit 3 drives the LDIO in response to the image signal from the signal processing circuit 2, and a laser beam modulated by the image signal is irradiated onto the photoreceptor 14 to form an electrostatic latent image (see Fig. 2). reference). This electrostatic latent image is developed by a developing device (not shown) and transferred onto a recording paper or the like by a transfer device (also not shown).

The current control circuit IO controls the laser drive circuit 3 according to the output signal of the second photodetector 16 to keep the output light amount of the LDIO constant. This method is advantageous because it does not reduce the light intensity of the laser beam that can actually be used, unlike the method of guiding a portion of the laser beam emitted forward to the photodetector.

On the other hand, as shown in FIG. 4, the image reading device uses a light source 17 to illuminate the document d, and a lens 18 disposed opposite to the document d to transfer the document image onto the light-receiving surface of the CCD line sensor 19. The optical image is photoelectrically converted by a CCD line sensor 19 to obtain image data.

In the copying machine, as shown in FIG. 5, the CCD reading section 1 processes image information output from the CCD line sensor 19 and outputs an analog pixel signal, and this signal is input to the signal processing circuit 2. Vi from the signal processing circuit 2
A deo signal is output.

In the signal processing circuit 2, the input analog pixel signal is subjected to density conversion by the analog logarithmic conversion circuit 20, and the A/D
The signal is converted into a 6- to 8-bit digital signal by a converter 21, and further subjected to shading correction by a shading correction circuit 22. The shading-corrected image information is γ-converted by the T-conversion circuit 23 and then output to the LD drive circuit 3 as a V1deo signal.

The LD drive circuit 3 drives the LDI based on the V1deo signal.
0 to generate a signal-modulated light beam.

FIG. 1 is a specific circuit block diagram of an LD drive circuit 3 that is an embodiment of the present invention.
Current switching circuit 31, AND gate 34, 35, A, B
It is composed of two switching circuits 32 and 33.

When image data Video is manually input to the PWM circuit 30, a modulated pulse signal MP having a multi-value pulse width corresponding to the gray level of the pixel is generated. This modulated pulse signal MP is input to AND gates 34 and 35. On the other hand, image data V
ideo is also input to the current switching circuit 31, and current switching signals S,, st (ON
, OFF signal). Current switching signal S,,S,
If the current switching circuit 31 is configured so that the switching period is for each pixel, then, for example, the LD drive signal v, , 1, which is the logical product of the current switching signal Sl and the modulation pulse signal MP, is as follows.
When the current switching signal S1 is turned on, the modulation pulse signal MP is turned on only during the period when the modulation pulse signal MP is turned on. LD drive signal viR
The same applies to 2.

FIG. 6 is a detailed circuit diagram of switching circuit A or B, which includes two FETs, 'rat (Tt+), Tt
Differential amplifier by z('r!) and transistor T1(
It is composed of a constant current source based on T2>. In the figure,
323 (333) is a buffer amplifier, 321 (
331), 322 (332) are inverters.

Composite modulation of the LDIO light emission pulse width and light emission intensity is performed as follows. First, when the LD drive signal vI, 11 (or V, n2) is given to the switching circuit A (B), 1 (Hi
gh level) 10 (Lo-level) according to the signal, 2
FETs Tz (Tz+) and Ttz (Tzz) are turned on. For example, when the LD drive signal Vinl is 1, the LDIO side FETs, T, (T□
> is turned on, and the it current j flowing through the transistor T1,
flows to LDIO. Furthermore, when the LD drive signal V, wa2 is 1, the current iz flowing through the transistor Tt
However, when the LD drive signals vlfi1, 2, 2 become 1 at the same time, the LDIO operates so that a current of i, +it flows through the LDIO. The period during which these currents flow is the pulse width time of the modulated pulse signal MP.

Furthermore, the light emission intensity of the LDIO is determined by the amount of current flowing through it. If the emission intensities of the LDIO when the currents are t,, iz and 11+iz are PI-Pt and P1*1, the LDIO emits light only during the period when the modulation pulse signal MP is on and the current switching signal S,, S, is on. Emission intensity Pr, Pz or Pll according to current switching signals S, ,S,
, it will emit light. The currents 11 and i2 are the control voltages V, , V and the load resistance rl+'! Therefore, it is only necessary to select these and set the currents 11 and 12 that give an appropriate gray level of the image.

In this embodiment, the switching circuit has a configuration including a differential amplifier as described above, and the LD drive signal V i ,
When l is 1, LD side FET, Tz (Tz+)
However, when the LD drive signal V in is O, the F on the non-LD side
ET and 'I'+z (Ttz) are each turned on, and the other FET is turned off. Therefore, the constant current source uses the LD drive signal v1. , (vI,, 1. ■!, , 2) causes constant current to continue to flow without interruption. The switching circuit 32.33 can be realized with a circuit other than differential type, but the control voltage V (Vi, Vz) can be realized without turning off the transistors ('r, , T!).
Since the collector current continues to flow in accordance with the current, the transistors (T+, T!) are driven in the active region, and it is possible to prevent the switching time from decreasing due to one accumulation time peculiar to transistors.

Next, a specific circuit diagram of the PWM circuit 30 is shown in FIG. In the figure, 301 is a D latch register, 302, 3
03 is a delay element, 304 is a pig selector, 305,
306 is an AND circuit, and 307 and 308 are OR circuits. Further, FIG. 8 is a timing chart of the main human output signals of the PWM circuit 30.

In this example, for ease of understanding, the image data is a 3-bit digital signal DO to D2, and five levels of values including 0 (off) are selected by the D latch register 301. DO represents dot on/off; when it is 0, the dot is off, and when it is l, the dot is on. Further, only when the dot is on, four pulse widths are selected using the two bits Di and D2.

We will explain how to create four different pulse widths. Three types of signals are generated using the data clock CLK and the delayed clocks C1 and C2 obtained through the delay elements 302 and 303.

CLK and CI pass through an AND circuit 305 to obtain a pulse signal PLO of approximately 1/4 duty. CLK itself becomes a pulse signal PLI with a duty of approximately 50%.

Further, C1 and CLK pass through an OR circuit 307 to become a pulse signal PL2 of 3/4 duty, and CLK and C2 pass through an OR circuit 308 to become a pulse signal P of approximately 90% duty.
It becomes L3.

Pulse signals PLO to PL3 having different pulse widths are input to the data selector 304 and are used as data DI.

One of them is selected by D2. The data DO and the selected pulse signal PLn (n=0 to 3) are input to an AND circuit 306, and the pulse width is modulated by the pulse signal PLn only when the data DO is 1. When DO is 0, 0 is output regardless of DI and D2. In this way P
The WM circuit 30 responds to the manually input video data.
It is possible to output the output pulse signal MP with five different pulse widths including 0.

The pulse signal MP is ANDed with the current switching signals S, , S by AND circuits 34 and 35, and the LD drive signal V 1
g is manually applied to switching circuits A and B (see Fig. 1).

Since the delay time of the delay elements 302 and 303 can be arbitrarily selected, the set value of the pulse width can also be freely varied. However, in this example, the level of PLO and the level of PL2 are not independent, and PLI is also fixed at 50% (CLK duty).

FIG. 9 shows a circuit diagram of a main part of a specific example of a PWM circuit 30 that has further improved this point and can set each level completely independently, showing delay elements 311, 312, . = 313. Generation of delayed CLK delayed by 314 is an or game to generate an output pulse with a duty larger than CLK) 317, 31
In order to generate an output pulse with a small duty, AND gates 315 and 316 are used to perform a logical AND with CLK. Incidentally, if a delay element whose delay time can be selected by tap switching is used, each level can be easily set.

Next, the operation of the current control circuit 4 will be explained.

The light output emitted from the LDIO changes depending on the temperature as described above. The current control circuit 4 operates two switching circuits A32. based on the photodetection output of the second photodetector 16 of the laser beam emitted from the LDIO. The constant current source of B33 is controlled independently. That is, when controlling the constant current source of switching circuit A, only the constant current source of switching circuit A is turned on, the constant current source of switching circuit B is turned off, and the current injected into the LDIO is controlled by the constant current source of switching circuit A. Let us assume only the current l supplied from the current source. The output of the laser light emitted from the LDIO at this time is detected by the second photodetector 16, and based on this detected value, the L
The constant current control circuit 4 controls the current value 11 of the constant current source of the switching circuit A so that the optical output of the DIO becomes a predetermined set value.

In this way, the t current value 1 of the constant current source of switching circuit A
1 is controlled to a predetermined value, only the constant current source of switching circuit B is turned on, the constant current source of switching circuit A is turned off, and in the same way as the constant current source of switching circuit A is controlled, the constant current source of switching circuit B is turned on. Controls the current value supplied from a constant current source.

In this embodiment, the current control circuit 4 is configured to control the switching circuits A32 and B33, but current control circuits may be provided independently for each of the switching circuits A32 and B33.

Furthermore, during the above control operation period, the LDIO remains lit, so the above control is preferably executed outside the optical image scanning area in order to avoid adverse effects on the recorded image.

FIG. 1O is a detailed diagram of the current control circuit 4, and FIG. 11 is an explanatory diagram showing the state of the optical output P when the t* control circuit 4 controls the optical output of the LDIO.

The current control circuit 4 controls the current value of the current source of the LD drive circuit 3 so that the output voltage from the amplifier 5 has a constant value with reference to the reference voltage. In FIG. 10, the detection output vpti, which is proportional to the intensity P of the laser light detected by the second photodetector 16 and amplified by the amplifier 5, is compared with the reference voltage V ref by the comparator 43, and depending on the result, output a high-level or low-level signal. Vp
When d<Vref, the level is low, and when Vpd≧Vref, the level is high.

If the output of the comparator 43 is at a high level, that is, LD
When the photodetection output Vpd of the IO is higher than the reference voltage Vref, when the timing signal PCHK permits the counting operation of the up/down counter (UDC) 41, the UDC 41 operates as a down counter.

The output of the UDC 41 is converted into an analog output by a digital-to-analog (D/A) converter 40, and the intensity of the current from the LD drive circuit 3 to the LDIO is changed depending on the output. In this case, the drive current of the LDIO decreases, and the photodetection output voltage Vpd of the amplifier 5 decreases.

When the output of the comparator 43 is inverted from high level to low level, the counting operation of the UDC 41 is prohibited.

If the output of the comparator 43 is at a low level, that is, LDI
When the photodetection output Vpd of O is smaller than the reference voltage V ref, when the timing signal PCHK permits the counting operation of the UDC 41, the UDC 41 operates as an up counter, the drive current of the LDIO increases, and the output of the amplifier 5 increases. Output voltage Vpd increases.

By stopping the counting operation when the output of the comparator 43 reverses from high level to low level as described above, the optical output P of the LDIO becomes a value slightly lower than the reference value as shown in FIG. 11(A). Retained. On the other hand, if the counting operation is stopped when the output of the comparator 43 is reversed from a low level to a high level, the optical output P of the LDIO will be maintained at a value slightly higher than the reference value, as shown in FIG. 11(B). become.

In other words, when the photodetection output Vpd and the reference voltage Vref are compared and the count of the UDC 41 is stopped when the high and low voltages change, when Vpd is pd≧Vref and when Vpd<Vre
At the time of f, a difference occurs in the voltage in the stable state.

Therefore, when the output of the comparator 43 is inverted from a low level to a high level, the UDC 41 is set to operate as a down counter. In this way, when Vpd<Vref, the low level is reversed to high level, and when the high level is reversed again to low level, the counting operation is stopped, and as shown in Figure 11 (C), the count operation is stopped. The light output is kept at a slightly lower level. That is, the photodetection output Vpd and the reference voltage V
Regardless of whether the ref is high or low, the LDIO drive current is maintained at a constant value as in the state shown in FIG. 11(A).

On the other hand, when the photodetection output Vpd of LDIO decreases and exceeds the reference value Vref, the UDC 41 does not prohibit the counting operation, but when the photodetection output Vpd of LDIO increases and exceeds the reference value (11!Vref) Even if the count operation is prohibited, the holding current of the LDIO can be kept constant in exactly the same way.

The edge detection circuit 42 is a circuit that detects a change in the comparison output from the comparator 43 and allows or prohibits the counting operation of the LIDC 41.

〔Effect of the invention〕

As described above, the present invention sequentially switches a plurality of constant current sources in a semiconductor laser drive circuit, and switches each constant current source so that the detection output of the emission intensity of the semiconductor laser at that time becomes a predetermined value. Since the present invention is equipped with a current control means for controlling the supply current of the constant current source, even if the emission intensity of the semiconductor laser changes due to a change in the ambient temperature, the current control means controls the supply current of the constant current source, so that the semiconductor laser always maintains a constant supply current. Laser emission intensity can be maintained.

[Brief explanation of drawings]

FIG. 1 is a block diagram of an LD drive circuit according to an embodiment of the present invention, FIG. 2 is a schematic configuration diagram of a laser printer, FIG. 3 is a schematic circuit diagram of a laser printer, and FIG. 4 is a schematic configuration of an image input device. Figure 5 is a block diagram of the image signal system circuit of the copying machine, Figure 6 is a detailed circuit diagram of the switching circuit, and Figure 7 is a detailed circuit diagram of the switching circuit.
The figure shows a specific circuit diagram of the PWM circuit, Fig. 8 is a timing chart of input/output signals of the PWM circuit, Fig. 9 is a main part circuit diagram of another specific example of the PWM circuit, and Fig. 10 shows the current control circuit. Detailed diagram: Figure 11 is an explanatory diagram showing the state of the optical output of the LD, Figure 12 is a characteristic diagram of the recorded image density with respect to the pulse width of the LD, and Figure 13 is a diagram showing the relationship between the pulse width and image density at the density gradation. The graph shown in FIG. 14 corresponds to the input current and L
It is a graph which shows the relationship of the optical output of D. l...CCD reading section, 2...signal processing circuit 3...LD drive circuit, 4...current control circuit, 5
...Amplifier, IO...LD, 12...Optical scanning device, 14...Photoconductor, 16...Second photodetector, 30...PWM circuit, 31 ...Current switching circuit, 32゜33...Switching circuit, 41...
...UDC0 Figure 2 Figure 4 Figure 5 Figure 6 Figure 10 Figure 11 Figure Vpd ≧ Vref Vpd < Vref Vpd < Vref Figure 13 Figure 14

Claims (1)

[Claims] The semiconductor laser is provided with a first modulating means that modulates the pulse width of the pulse signal according to image data, and a plurality of constant current sources, and by switching the constant current sources according to the image data. a second modulating means for modulating the amplitude of the pulse signal by switching the injection current to the first and second modulating means;
A semiconductor laser drive circuit in which a drive pulse signal is modulated by a modulation means, comprising: a detection means for detecting the emission intensity of the semiconductor laser;
A semiconductor laser drive characterized by comprising current control means for sequentially switching the plurality of constant current sources and controlling each supply current so that the detection output of the detection means at that time becomes a predetermined value. circuit.