JP2003072147A - Imaging apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize stable laser drive by restraining deterioration of the switching performance due to change of the light amount derived from the laser thermal interference and increase of the switching current in a controlling method for a semiconductor laser unit for emitting a plurality of laser beams. SOLUTION: The bias current flowing in the laser is controlled always to an optimum value by connecting constant current circuit for supplying a constant current to a semiconductor laser and a constant current driver for driving the laser according to the voltage of a connected capacitor, and correcting the voltage of the capacitor by a certain cycle via a comparator and an analog switch.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-beam writing type image forming apparatus such as a laser beam printer which performs optical writing using a plurality of light beams.

[0002]

2. Description of the Related Art In an electrophotographic apparatus such as a copying machine or a laser beam printer, a laser beam is used for image formation. In recent years, as the frequency of a video clock used for driving a laser beam has become higher with the increase in speed and definition, peripheral elements such as a laser driver have become unable to cope with speed. Therefore, simultaneous writing of two lines is performed using a plurality of light sources such as a laser array, and multi-beam scanning in which the video clock is halved is performed. FIG. 9 shows an example of a scanning optical system using multiple beams. In FIG. 9, the two laser beams emitted from the laser array 71 are collimated by the collimator lens 72 and then reflected by the rotating polyhedron 73 to form the lens 7
The image is narrowed down by 4 and scanned on the photoconductor 75 to form a latent image. The reference position detection unit 76 generates the timing for recording the laser light on the photoconductor, and the reference position detection signal is output when the laser light scans the reference position detection unit 76.

On the other hand, a laser is generally controlled by Auto Power Control (APC) in order to keep the laser intensity constant. FIG. 5 shows an example of an APC control circuit that controls the laser light amount to a constant level.
Reference numeral 1 denotes a laser array having two laser emission parts. In FIG. 5, each laser emission part in the laser array 51 is referred to as a laser 1 and a laser 2, respectively, and the AP of the laser 1 is selected.
The C control circuit is shown. In FIG. 5, a part of the two laser beams emitted from the laser array 51 is converted into a photocurrent by a single monitoring photodiode 52, and the photocurrent is further converted into a photovoltage by a resistor 53 to be converted into an analog voltage. It is input to the switch 55. The analog switch 55 is connected to the comparator 54, and switches the optical voltage of the photodiode 52 on / off of the input to the comparator 54. The comparator 54 has a photovoltage of the photodiode 52 input to one terminal and a reference voltage Vre input to the + terminal.
Comparing with f1, if the optical voltage is lower than the reference voltage Vref1, the capacitor 56 is charged, and if the optical voltage is higher than the reference voltage Vref1, the capacitor 56 is discharged. The drive circuit 57 drives each laser in the laser array 51 according to the potential of the capacitor 56. As a result, the amount of laser light is controlled to a value preset by each reference voltage Vref1. By the way, in an apparatus such as a laser printer, when performing APC control, it is necessary for the laser to emit light. However, during printing, extra printing may occur, so the laser cannot be emitted arbitrarily. Therefore, it is general to add a sample / hold circuit to the APC control circuit, provide a sample period in a non-printing area that does not affect printing, and perform APC control within this period. The analog switch 55 and the capacitor 56 in FIG. 5 are an example of a sample / hold circuit that holds a sample operation for performing APC control and a result of performing APC control, and performs a hold operation so that the laser emits light at that value. In FIG.
LD1 SAMPLE P is a signal that controls sample / hold. When LD1 SAMPLE P becomes "1", each switch of the analog switch 55 is closed (ON), the output of the comparator 54 is accumulated in the capacitor 56, and the APC operation is performed. By setting LD1 SAMPLE P to "0" during the printing period, the hold period starts and the analog switch 55 opens (OFF).
During the hold period, the laser 1 in the laser array 1 emits light with an amount of light according to the voltage of the capacitor 56. The same applies to the laser 2 circuit (not shown).

Next, the operation timing of the conventional APC control will be described with reference to FIGS. 6 and 7. FIG. 6 shows an example of a conventional APC control timing circuit. 7 is the AP of FIG.
7 is a timing chart showing the operation of the C control timing circuit. In FIG. 6, a flip-flop (hereinafter, FF) 6
It is assumed that the outputs Q of 6, 67 and 68 are all initial value logic "0". The output A of the reference position detector 61 is input to the timer trigger generation circuit 62. The timer trigger generation circuit 62 generates a trigger signal B based on the reference position detection signal A, and the timer 6 connected to the subsequent stage of the timer trigger generation circuit 62.
3. Start timer 64 and timer 65. At the timing corresponding to the non-printing area, the time-out signal C is first output from the timer 63, FF67 is set, and FF67 is set.
Since the output of is a logical "1", LD1 ON P and LD1 SAM
PLE P becomes logic "1" and puts laser 1 in the sample state. Next, the timer 64 times out after a certain time delay from the time-out of the timer 63, and outputs the time-out signal D. Since the time-out signal D is connected to the reset terminal of FF67, the output of FF67 is set to logic "0" and at the same time, FF68 is set and its output is set to logic "1".
To Therefore, LD1 SAMPLE P becomes logical "0" and laser 1 is in the hold state while LD1 ON P and LD2 ON
Since P is a logical "1", the laser 1 and the laser 2 emit light in the hold state, and the reference position detector 76 in FIG. 9 scans during this period. In general, a laser array has a narrow interval between internal lasers, so that the emission of one laser may affect the intensity of the other laser. In some cases, a circuit that waits for a certain period of time before light emission is added.

The signal B is again output from the timer trigger generating circuit 62 by the output of the next reference position detecting section 61, and the timer 63 and the timer 64 are restarted, and at the same time, the FF 68
To reset. When FF68 is reset, LD1 ON P and LD2 ON P are set until the next timer 63 times out.
Becomes a logical "0". The writing of the image corresponding to the print signal on the photosensitive drum 75 is performed during this period.

The FF 66 alternately repeats the logic "0" and the logic "1" each time the output of the timer trigger generation circuit 62 arrives. The FF 66 is switched to the output E, F of the FF 66 by LD1 SAMPLE P or L.
Select either D2 SAMPLE P to be logical "1". As a result, laser 1 and laser 2 are alternated for each scan.
APC operation will be performed.

The sample / hold operation timing of the laser array 1 is controlled by repeating the above operation.

Next, the bias current of the laser will be described with reference to FIG. FIG. 8 is a diagram showing a relationship between laser current and laser light output, a laser drive current and time, and a relationship between time and laser output. From the relationship between the laser current and the laser light output shown in FIG. 8, when the current passed through the laser is gradually increased, laser light emission is started at a light output proportional to the current from the time when the value called a threshold value is exceeded. Ib1 is a current that is always passed through the laser and is called a bias current. To set the bias current, for example, a constant current circuit 58 is added in FIG. 5 and a reference current Vref2 applied from the outside is set to keep a constant current flowing through the laser. Figure 5
The constant current driver 57 in FIG. 2 switches the laser by LD1 ON P driven according to the image to be printed. The bias current Ib1 and the switching current I
The laser performs switching with a predetermined light amount PO as shown in FIG. 8 by the current on which sw1 is superimposed.

[0009]

In the conventional bias circuit, since the setting of the bias current is adjusted to a fixed value by the reference voltage Vref2 at the beginning, the function is not sufficient depending on the change over time in the laser characteristics in the laser array. There was a problem that it could not be demonstrated. Lasers are generally very sensitive to temperature and have the characteristic that the luminous efficiency decreases as the temperature rises. That is, as shown by the dotted line in FIG. 8, when the ambient temperature rises, the current required for the laser to emit the same amount of light PO tends to increase. In FIG. 8, at normal temperature, the current Ib1 + required to obtain a predetermined light amount PO
In contrast to Isw1, when the ambient temperature is high, the current required to obtain the predetermined light amount PO increases to Ib2 + Isw2. As a result, since the bias current is Ib1 = Ib2 = constant, the switching current Isw2 increases with respect to Isw1 at room temperature. Therefore, since the current to be switched increases within the same time, the conventional bias circuit has a problem that the switching ability is significantly reduced when the ambient temperature rises. The problem is that in a multi-beam laser, the laser emitting portions in the laser array are close to each other, so that an increase in switching current causes thermal mutual interference between lasers in the laser array, which also reduces the switching ability. It was a factor.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above problems and to provide an image forming apparatus having a very small change in light quantity and a stable switching performance.

[0011]

In order to solve the above-mentioned problems, an image forming apparatus according to the present invention is an image forming apparatus using an electrophotographic process for forming an electrostatic latent image on a photoconductor. The detection means for monitoring the optical output of each laser, the calibration means for changing the bias current according to the detection means and the output, and the control means for correcting the bias current at a constant cycle by the signal of the detection means are provided.

[0012]

DETAILED DESCRIPTION OF THE INVENTION The present invention will be described with reference to FIGS.

FIG. 1 shows an example of a laser control circuit according to the present invention. In FIG. 1, the laser array has two laser diodes and photodiodes in one package. In FIG. 1, the laser light emitted from the laser 1 is converted into a photocurrent by the photodiode 2 and converted into a photovoltage by the resistor 3. The optical voltage corresponding to the amount of laser light is input to the resistors 4 and 5 via the analog switches 6 and 7. Analog switch 6,
Reference numeral 7 is for selecting which one of the comparators 4 and 5 the optical voltage is input to. The selection is LD1 SAMPLE which is an external signal.
Determined by P and LD1 BIASSH P. LD1 SAMPL
When EP becomes logic "1", analog switch 6 closes,
The optical voltage is input to the comparator 5, and when LD1 BIAS SH P becomes the logic “1”, the analog switch is closed and the optical voltage is input to the comparator 4.

The constant current driver 10 causes a current corresponding to the voltage of the capacitor 8 to flow through the laser 1 when LD1 ON P becomes logic "1", and the constant current driver 11 causes the capacitor 9 when LD1 BIAS SH P becomes logic "1". The current corresponding to the voltage of 1 is supplied to the laser 1. The constant current circuit 12 is LD1 BI
When AS SH P becomes a logic "1", a certain current flows through the laser 1. As its configuration, for example, a circuit using a current mirror or the like is used.

Next, the operation timing of the laser control circuit will be described with reference to FIGS. FIG. 2 shows an example of a timing circuit for performing APC control of the laser control circuit according to the present invention. The operation of the timing circuit of FIG. 2 will be described below with reference to FIG. In FIG. 2, the reference position detector 2
From 1 the signal a is output for each scan, and the signal a is input to the timer trigger generation circuit 22. The timer trigger generation circuit 22 generates a trigger signal b for starting the timers 23, 24 and 25 from the signal a. The pulse of the signal b from the timer trigger generation circuit 22 is the timer 2
3. When input to the timer 24 and the timer 25, these timers start counting a predetermined time. First, the timer 23 times out and outputs the signal c. Signal c is FF
The output f of 26 is set to logic "1". The FF 29 selects which one of the laser 1 and the laser 2 is controlled. When the initial state is logic "0", the FF is generated by the signal b.
In the output of 29, the signal i is logic "1" and the signal j is logic "0".
Becomes Therefore, by the signals f, i, j, LD1 BIAS SH P
Becomes a logical "1", and LD2 BIAS SH P becomes a logical "0". When LD1 BIAS SH P becomes logic “0”, the constant current circuit 12 of FIG. 1 is driven and a certain current flows through the laser 1,
Further, the analog switch 7 is closed and the photovoltage corresponding to the light amount of the laser 1 is input to the comparator 4. The photovoltage is the comparator 4
When the optical voltage is lower than the reference voltage Vref2, the comparator 4 charges the electric charge of the capacitor 9 and the optical voltage is the reference voltage Vref2.
If it is lower than ref2, the electric charge of the capacitor 9 is discharged. Since the constant current driver 11 drives the laser 1 with a current according to the voltage of the capacitor 9, the output of the laser 1 is periodically calibrated to a value close to the reference voltage Vref2. This calibration is called bias calibration. Constant current circuit 1 in this calibration
No. 2 is added to obtain the laser light amount necessary for bias calibration. Without the constant current circuit 12, bias calibration is performed at an extremely low laser light amount, so that the bias calibration accuracy is also low.

Next, the timer 24 times out, and the signal d
Is output. The signal d resets FF26 and sets FF27. Therefore, the signal f which is the output of the FF 26 becomes the logic "0", and the LD1 BIAS SH P becomes the logic "0",
Also, since the signal g which is the output of the FF 27 becomes a logic "1",
When LD1 SAMPLE P becomes logic "1", the analog switch is closed, so that the photovoltage corresponding to the light quantity of the laser 1 is input to the comparator 5. Since the photovoltage is compared with the reference voltage Vref1 input to the + terminal of the comparator 5, the comparator 5 charges the capacitor 8 if the photovoltage is lower than the reference voltage Vref1, and the photovoltage is the reference voltage. If it is lower than Vref1, the electric charge of the capacitor 8 is discharged. Since the constant current driver 10 drives the laser 1 with a current according to the voltage of the capacitor 8, the output of the laser 1 is periodically calibrated to a value close to the reference voltage Vref1. Next, when the timer 25 times out,
Since the FF 27 is reset by the output e, the signal g becomes the logic "0", and the LD1 SAMPLE P becomes the logic "0". When LD1 SAMPLE P becomes logic "0", the analog switch 8 is released, and thereafter, when LD1 ON P becomes logic "1", the laser 1 is driven with the light amount according to the voltage of the capacitor 8. Writing of image data in the electrophotographic apparatus is performed during this period.

The effect of bias calibration will be described with reference to FIG. FIG. 4 is an explanatory diagram showing the relationship between the laser current and the laser light amount, the laser drive current and the time, and the time and the laser light amount in the laser control circuit according to the present invention. In the relationship between the laser current and the laser light amount, the solid line portion is a graph showing the current-laser light amount characteristic at room temperature. At room temperature, the laser 1 is driven by the bias current Ib1 determined by bias calibration and the switching current Isw1 determined by light amount correction. In FIG. 4, the dotted line is an example showing the current vs. laser light quantity characteristic when the environmental temperature of the laser is high. When the ambient temperature is high, the laser current required to obtain the same predetermined amount of light PO increases,
The bias calibration sets Ib2 to a larger current than Ib1. On the other hand, the switching current is set to almost the same value as at room temperature because the bias current increases due to the bias calibration. Therefore, even if the environmental temperature of the laser changes, the switching current of the laser does not change and stable switching performance can be obtained. Further, since the switching current does not change, thermal interference in the laser array can be minimized.

[0018]

According to the present invention, since the bias current of the laser is calibrated at a constant cycle, the bias current can always be kept at an optimum value even when the characteristics of the laser array change due to temperature or the like. , Mutual interference in the laser array can be reduced, and stable light emission can be achieved.

[Brief description of drawings]

FIG. 1 is a schematic diagram illustrating a laser drive circuit of an image forming apparatus that is an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a timing control circuit of an image forming apparatus that is an embodiment of the present invention.

FIG. 3 is a timing chart of the image forming apparatus that is an embodiment of the present invention.

FIG. 4 is a graph illustrating a bias operation of a laser.

FIG. 5 is a schematic diagram of a laser drive circuit in a conventional image forming apparatus.

FIG. 6 is a schematic diagram of a timing control circuit in a conventional image forming apparatus.

FIG. 7 is a timing chart of a conventional image forming apparatus.

FIG. 8 is a graph illustrating a bias operation of a laser.

FIG. 9 is a schematic diagram showing an example of a multi-beam scanning optical system.

[Explanation of symbols]

1 ... Laser array, 4 ... Comparator, 7 ... Analog switch, 9 ... Capacitor, 11 ... Constant current driver, 12 ... Constant current circuit.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) G03G 15/043 H04N 1/04 104Z H04N 1/113 F term (reference) 2C362 AA03 AA16 BA04 BA60 BA67 2H045 AA01 CB42 DA41 2H076 AB06 AB12 AB22 AB34 DA17 DA32 5C072 AA03 BA13 FB27 HA02 HA06 HB04 HB06 UA13 XA05

Claims (5)

[Claims] 1. An image forming apparatus comprising: a semiconductor laser array that is independently modulated according to an image signal; and an optical scanning device that scans a plurality of laser beams emitted from the semiconductor laser onto a recording medium. 2. An image forming apparatus, comprising: a constant current circuit which constantly supplies a current to each of the laser light sources, and a control device which corrects the current of the constant current circuit at a constant cycle near the threshold value of each laser light source. 2. The image forming apparatus according to claim 1, wherein the correction of the constant current circuit is sequentially performed during the scanning of the laser light. 3. The image forming apparatus according to claim 1, wherein the correction of the constant current circuit is sequentially performed for each scanning of the laser light. 4. The image forming apparatus according to claim 1, wherein when the constant current circuit is corrected, a constant current is applied so that the laser emits light at a predetermined light amount or more. 5. The image forming apparatus according to claim 1, wherein the correction of the constant current circuit is performed immediately before the correction of the laser light amount.