JPH02151455A - Intensity-of-light modulation circuit of semiconductor laser - Google Patents

Abstract

PURPOSE:To control the intensity of light with good accuracy by individually providing constant current sources for driving semiconductor laser having the current value corresponding to the intensity of light of each step and controlling a switching circuit on the basis of an intensity-of-light modulation signal and selecting the constant current source for driving the semiconductor laser. CONSTITUTION:A plurality of constant current sources 2, 3 and switching circuits 4, 5 for connecting the constant current sources to semiconductor laser are provided to the semiconductor laser 1 and a monitoring photodetector 7 for monitoring the intensity of the laser beam of the semiconductor laser 1 is provided. Intensity-of-light control and holding means 9, 10 equipped with a comparing means 15 for comparing the monitor output with a reference value, a feedback drive means 13 for feeding back the comparing result to the respective constant current sources 2, 3 to drive said current sources in a coincidence state and holding means 12, 12 for holding the driving current values of the constant current sources in the coincidence state are provided at every constant current sources 2, 3. A timing alteration circuit 18 for performing the operation of the one for a low intensity-of-light level among the intensity-of-light control and holding means 9, 10 at a cycle later than that of the one for a high intensity-of-light level is provided and a band alteration circuit 19 for lowering the detection band of the one for the low intensity-of-light level as compared with that of the one for the high intensity-of-light level is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to light intensity modulation of a semiconductor laser in a laser printer, etc., which is capable of recording with multi-gradation by changing the light intensity of laser light from a semiconductor laser. Regarding circuits.

Conventional technology Conventionally, in laser recording devices that record halftone images by exposing and scanning laser light from a semiconductor laser onto a photoreceptor, the driving current-light output characteristics of the semiconductor laser are determined by the spontaneous emission region and laser oscillation. It has been difficult to obtain stable light output because the light output varies drastically depending on the region. Furthermore, since the sensitivity of the photoreceptor etc. is not linear with respect to the input optical power, it has been difficult to form a highly accurate halftone image on the photoreceptor.

In consideration of these points, there is a conventional laser recording apparatus capable of recording halftones, such as the one disclosed in Japanese Patent Application Laid-open No. 102544/1983. This is done in order to change the nonlinearity of the recording optical system (characteristics that vary drastically between the natural light emission region and the laser oscillation region mentioned above) so that the light emission level command signal of the semiconductor laser and the optical output have a linear relationship. The signal is corrected into a linear relationship using a correction table, and the corrected input signal is converted into analog by a D/A converter to drive a semiconductor laser.

In addition, the light intensity modulation signal for the semiconductor laser has multiple levels, and in order to stably output light at each light intensity level, multiple constant current sources are switched via a modulation switching element. A method has also been proposed by Suito Ibon, in which the semiconductor laser is connected to a laser and the light intensity output of the semiconductor laser is changed in accordance with the connected constant current source. More specifically, one of the multiple constant current sources is set near the bias current of the semiconductor, and the rest are set so that their current values have a 2':1 relationship. has been done. As a result, by commonly using one of the former semiconductor lasers for bias, the linear part of the semiconductor laser's drive current-optical output characteristic that is greater than or equal to the bias current can be used to reduce the number of current sources and increase the input signal. A light output that varies linearly with respect to the angle is obtained.

Problems to be Solved by the Invention In the case of the former Japanese Patent Application Laid-Open No. 63-102544, due to the combination of a correction table and a D/A converter, due to quantization errors during processing in the D/A converter for driving the semiconductor laser, This may cause an error in the optical output power of the semiconductor laser.

Also, in the case of the latter multiple constant current source switching method, the output power is linear with respect to the input signal.

In reproducing a halftone image, the halftone reproducibility of the recorded image deteriorates due to nonlinear parts such as the sensitivity of the photoreceptor.

Therefore, a plurality of constant current sources each having a different drive current value are provided together with a switching circuit, the switching circuit is switched by a light intensity modulation signal, and the semiconductor laser is driven by the constant current source corresponding to the light intensity modulation signal. A method has been proposed that improves the reproducibility of halftones in recorded images. In this case, since the output characteristics of the semiconductor laser can vary depending on temperature and the like, the light intensity of the semiconductor laser is monitored by a photodiode or the like, and the corresponding constant current source is feedback-controlled to maintain a predetermined light intensity.

However, when controlling the power of laser light with a low light intensity level, the S/N of the output signal of a light receiving element such as a photodiode that monitors the laser light of the semiconductor laser deteriorates because the light intensity level is low. Therefore, in such a case, it is not possible to accurately control the low light intensity level.

Means for Solving the Problem A plurality of constant current sources are set with different driving current values for the semiconductor laser, and each constant current source is connected in series and switched according to a light intensity modulation signal to generate a corresponding constant current source. A switching circuit connected to the semiconductor laser is provided, a monitoring light receiving element is provided for monitoring the light intensity of the laser light emitted from the semiconductor laser, and the monitor output by the monitoring light receiving element is used as a reference for each light intensity level. A light intensity comprising a comparison means for comparing the values, a feedback driving means for feeding back the comparison results to each constant current source to drive it to a matching state, and a holding means for holding the driving current value of the constant current source in a matching state. A control holding means is provided for each constant current source, and a timing change circuit is provided for causing the light intensity control holding means for low light intensity levels to operate at a slower cycle than the one for high light intensity levels; A band changing circuit is provided in which the detection band for a low light intensity level in the light intensity control holding means is lower than that for a high light intensity level.6 The operating light intensity is variably controlled at each level from high to low. In semiconductor lasers, when the light intensity level is low, the detection signal from the monitor light receiving element becomes weak and has a poor S/N ratio, but low signals can be obtained by lowering the detection band through a band changing circuit. Even if the signal is weak, it has a good S/N ratio. In addition, when the timing change circuit is low, the control and holding operations of the light intensity control holding circuit are performed at low speeds, making it possible to sufficiently follow even small signals, resulting in highly accurate control. can. Therefore, not only the opening current value for a high light intensity level but also the drive current value for a low light intensity level can be accurately feedback controlled and the current value maintained.

Example An embodiment of the present invention will be described based on the drawings.

First, with reference to FIG. 2, a method of varying the light intensity of a semiconductor laser, which is the premise of this embodiment, will be explained.

FIG. 2 shows the driving current-light output characteristic of a semiconductor laser, and the light intensity changes depending on the driving current. Looking at this characteristic in more detail, as is well known, it can be divided into two linear regions, a spontaneous emission region and a laser oscillation region, with the bias current line H as the boundary. In the figure, the solid line indicates the temperature t.

The dashed lines indicate the characteristics at different temperatures.

As can be seen from the comparison between the two, in this characteristic, the bias current ITH changes depending on the temperature as shown in ITH8pITK, but the linear slope relationship between the drive current and the light intensity hardly changes even if the temperature changes. have characteristics.

In order to stabilize the light intensity using a semiconductor laser with such characteristics, as a plurality of steps to vary the light intensity of the semiconductor laser, a current value for driving the semiconductor laser corresponding to the light intensity of each step is determined. If a current source is provided individually, a switching circuit is controlled by a light intensity modulation signal, and a constant current source is selected to drive the semiconductor laser, the semiconductor laser will emit light at a predetermined light intensity based on the drive current value. Become.

For example, in FIG. 2, in order to obtain the light intensity Pn, it is sufficient to flow a constant current with an opening current value of In to the semiconductor laser, and in order to obtain the light intensity PI11, a constant current with a driving current value of In is passed through the semiconductor laser. For example, if such light intensities Pn and Pm are required, it is sufficient to prepare two constant current sources with current values of In and Im and select 4t by a switching circuit. More specifically, the light intensity modulation of the semiconductor laser 1 is controlled by a circuit configuration as shown in FIG. First, constant current sources 2 and 3 (each driving current value is In,
In) is provided. These constant current sources 2,
3 are individually connected to the semiconductor laser 1 via switching circuits g4 and 5 connected in series. Each of the switching circuits 4.5 includes a pair of parallel transistors QiL-Q connected to the source Vce and the semiconductor laser 1, respectively.
iz, Q □pQz□, voltage dividing resistor R1□, R1,
, R21, and R22.

1-transistor Q iZ+ Qzz supplies current from Vce to the corresponding constant current source 2.3, and also supplies current from Vce to the corresponding constant current source 2.3. Q□, the corresponding constant current source 2゜3
is selectively connected to the semiconductor laser 1. A light intensity modulation signal is input to these transistors QAEyQz□ via a modulation decoder 6.

As a result, by controlling the transistor Q1□tQz□ on and off using the optical intensity modulation signal, the constant current source 2
, 3 are both disconnected from the semiconductor laser 1, only the constant current source 2 is connected to the semiconductor laser 1 (driven with the drive current value In), and only the constant current source g3 is connected to the semiconductor laser 1. (driven with drive current value In), constant current ig
3.4 are both connected to the semiconductor laser 1 (driven with the drive current value In+Im), resulting in four types of states.

Four-value (including output O) optical intensity modulation is possible.

Here, since the output characteristics of the semiconductor laser 1- may fluctuate due to temperature, etc., in order to cope with such fluctuations,
A monitor feedback control system is added to the constant current source 2.3. First, a photodiode 7 is provided as a monitoring light-receiving element that monitors a portion of the laser light emitted from the semiconductor laser 1. An amplifier 8 for amplifying the monitor output is connected to the photodiode 7. Light intensity control and holding circuits 9 and 10 for respective constant current sources 2 and 3 are connected to the output side of this amplifier 8. First, the light intensity control holding circuit 9 includes a comparator 11 as a comparing means, an up/down counter 12 as a holding means, and a D/A converter 13 as a feedback driving means, and is connected to the constant current source 2 for feedback. has been done. First, the photodiode output (monitor output) amplified by the amplifier 8 is converted to a reference voltage V as a reference value in the comparator 11. It is compared with PIw+ax. Here, the reference voltage VxytvLmmx+ is a voltage value that is equal to the output of the photodiode 7 when the light emission amount is Pn (i.e., Lmax) in the drive current-light intensity output characteristic of the semiconductor laser 1 shown in FIG. . The comparison result by the comparator 11 is input to the up/down counter 12 as an H level or L level digital binary signal. This up/down counter 12 counts the output from the comparator 11 (up or down) according to the timing of the clock signal from the clock generator 14. The output from this counter 12 is converted into analog by the D/A converter 13. After that, it is fed back to the constant current source 2,
A driving current is applied to the semiconductor laser 1 by a constant current source 2 in accordance with the analog conversion value. In such feedback control, by stopping the counting operation by the up/down counter 12 when the polarity of the output from the comparator 11 is reversed, the drive current value of the constant current source 2 changes from the initial In to the drive when the control is stopped. The current value is held at In'. Therefore, even if the output fluctuates due to temperature etc., the semiconductor laser 1
The actual light intensity is controlled to be two intensities and stabilized.

On the other hand, the light intensity control holding circuit 10 is also similar, and consists of a comparator 15 as a comparing means, an up/down counter 16 as a holding means, and a D/A converter 17 as a feedback driving means. Feedback is connected.

The drive current value for the constant current source 3 is controlled and maintained in the same manner. Note that the photodiode output amplified by the amplifier 8 is compared with a reference voltage Vi+++rim+al as a reference value in a comparator 15, and the reference voltage V□Ffm1m) at this time is equal to that of the semiconductor laser 1 shown in FIG. This is a voltage value that is equal to the output of the photodiode 7 when one light emission amount Ptn (that is, La1n) is determined in the drive current-light intensity output characteristic.

By the way, in the operation on the side of the intensity control and holding circuit 10 for light intensity Pm, since the level of light intensity Pn+ is smaller (lower level) than the level of light intensity Pn, the control/holding photo The output of the diode 7 also decreases and is weak, resulting in poor S/N. Regarding this point 1, in this embodiment, the lower the light intensity level, such as the light intensity Pm, the lower the light intensity level, the slower the control/holding operation is performed. A frequency divider 18 as a timing change circuit is added to the input. In other words, the up/down counter 16 uses a frequency-divided clock signal obtained by dividing the clock signal from the clock generator 14 by the frequency divider 18.
By operating the up/down counter 12 side, the counting operation is performed at a lower speed than the up/down counter 12 side. Therefore, even if the output from the photodiode 7 is small, there is sufficient counting time and highly accurate control is possible. Furthermore, in this embodiment, a resistor R is provided between the amplifier 8 and the comparator 15 in order to improve the problem of poor S/N when the light intensity level is low.
, a low-pass filter LPF19 as a band changing circuit using a capacitor C is inserted. By lowering the detection band of the detection system using the LPF 19, even if the signal from the amplifier 8 is weak, the signal-to-noise ratio is improved, and the signal is subjected to comparison processing by the comparator 15.

As shown in FIG. 1, the up/down counters 12.16 are supplied with signals C0NTl, 2 from the counting control circuits 2o, 21 instructing them to start counting. That is, when the above-described control operation is started by the signals C0NT1 and C0NT2 and the polarities of the counters 12 and 16 are reversed, the counting control circuits 20 and 21 stop the counting operations of these counters 12 and 16, and the semiconductor laser 1
is controlled so that a maintained constant current (active current value) flows. By the way, in this example.

For example, when applied to a laser printer, the output control of the semiconductor laser 1 when the light intensity level is high is performed at the cycle of each main scanning of the laser beam, and the output control of the semiconductor laser 1 when the light intensity level is low is performed. The other one is slower and is performed at a cycle for each page.

Therefore, C0UTI,2 is given as a timing signal as shown in FIG. 3, for example.

First, (a) in the same figure shows an output signal from a photodiode that is installed near the scanning start position of the photoreceptor (not shown) and receives scanning laser light, and is output every main scanning (T□: main scanning period). This signal is output as a print timing signal. Based on such a printing timing signal, during a period T3 of several hundred μs outside the printing period T2, the signal C
0NTl becomes 1 (level, up/down counter 1
2 is given an enable signal (see (d) in the same figure) and enters a counting start state. Thereafter, when the signal CON T 1 becomes L level and the polarity of the comparison output from the comparator 11 changes from L level to I-I level as shown in FIG. 3(d), the counter 12 Counting is stopped, the control/holding operation by the light intensity control/holding circuit 9 is completed, and the driving current value is held at the driving current value In during the printing period T8 in the next main scanning period T1.

On the other hand, the period T3 outside the printing period T2 is usually several hundred μs.
If you attempt to control not only the light intensity level Pn but also the light intensity level Pm within this period, the detection band of the photodetection system will become wider, and the
In the m-side control, the S/N ratio of the signal deteriorates. Therefore, control of this lower light intensity level pH is performed on a page-by-page basis. Figure 3 (b
) indicates a page signal, which is H during the printing period T4 of one page.
level. Here, the page signal always has a period T for paper feeding even during continuous printing. This period T5 is from 0 to several seconds depending on the printing speed, and the signal C0NT2 is output within this period T, and the up/down counter 16 is controlled by the low-speed divided clock signal from the frequency divider 18. The light intensity level Pm may be controlled in the same manner as the light intensity level Pn described above.

However, it is possible to perform accurate feedback control and maintain the current value, and it is possible to effectively deal with fluctuations in the characteristics of the semiconductor laser due to heat or the like.

Effects of the Invention Since the present invention is constructed as described above, the light intensity is high to high.
In semiconductor lasers that are variably controlled at various levels over low light intensity levels, when the light intensity level is low, the detection signal from the monitor light receiving element becomes weak and the S/N is poor, but when the light intensity level is low, The monitor output from the monitor light receiving element is detected through the band changing circuit.
'jF Q is lowered, so even a weak signal
/H, and for low light intensity levels, the timing change circuit performs the control and holding operations of the corresponding light intensity control holding circuit at low speed, so even weak signals can be Signal processing time can be secured, and highly accurate control operation is possible. Therefore, the drive current value can be adjusted not only for high light intensity levels but also for low light intensity levels.

[Brief explanation of the drawing]

The drawings show an embodiment of the present invention, and FIG. 1 is a circuit diagram, FIG. 2 is a diagram of driving current-light intensity output characteristics of a semiconductor laser, and FIG. 3 is a timing chart. 1... Semiconductor laser, 2, 3... Constant current source, 4, 5...
...Switching circuit, 7...Monitor light receiving element, 9
゜1o... Light intensity control holding circuit, 11... Comparison means, ]-2... Holding means, ], 3... Feedback driving means, 15... Comparing means, 16... Holding means ,
17... Feedback drive means, 18... Timing change circuit, 19... Band change circuit

Claims (1)

[Claims] A semiconductor laser, a plurality of constant current sources each having a different driving current value set for the semiconductor laser, and a constant current source provided in series for each constant current source and corresponding to a light intensity modulation signal are connected to the semiconductor laser. a monitoring light receiving element for monitoring the light intensity of the laser light emitted from the semiconductor laser, a comparison means for comparing the monitor output from the monitoring light receiving element with a reference value for each light intensity level, and a comparison result. A light intensity control device provided for each constant current source includes a feedback driving means for feeding back the current value to each constant current source to drive it to a matched state, and a holding means for holding the drive current value of the constant current source in a matched state. control holding means; a timing change circuit for causing those of the light intensity control holding means for low light intensity levels to operate at a slower cycle than those for high light intensity levels; 1. A light intensity modulation circuit for a semiconductor laser, comprising a band changing circuit that lowers a detection band for a light intensity level from that for a high light intensity level.