JPH0771175B2 - Automatic light output controller - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an automatic light output control device for controlling the output of a light emitting element such as a laser diode or a light emitting diode.

[Conventional technology]

Light-emitting devices such as laser diodes and light-emitting diodes are in the spotlight as optical media for writing an image corresponding to an input image signal or code data on a photoconductor in a laser beam printer, for example.

FIG. 3 is a diagram showing the light emission power characteristic of the laser diode, where the horizontal axis represents the forward current I _F of the laser diode,
The vertical axis represents the light emission output P _O. The curve a shows the emission power characteristic at an ambient temperature of -10 °, the curve b shows the emission power characteristic at an ambient temperature of 25 °, and the curve c shows the emission temperature at 60 ° C.
Shows the emission power characteristic of °.

As you can see from this figure, the emission output of the laser diode
P _O has a characteristic that it greatly changes depending on the ambient temperature.

FIG. 4 is a diagram showing the relative relationship between the threshold current of the laser diode and the ambient temperature, where the horizontal axis represents the ambient temperature T _C (° C.) and the vertical axis represents the threshold current I _TH (mA). .

As can be seen from this figure, the threshold current I _TH of the laser diode has a characteristic that it fluctuates depending on the environmental temperature.

Each of the above characteristics is unique to the device, and the characteristics are not uniform. Therefore, it is very difficult to control the light emission output P _O of the laser diode to be constant.

Therefore, normally, as shown in FIG. 5, a power detector (PD) 302 including a laser diode (LD) 301 and a photodiode for constantly monitoring the light emission output P _O of the laser diode (LD) 301 is provided. It is provided.

FIG. 6 is a circuit diagram showing an example of the automatic light output control device for the laser diode LD shown in FIG. 5, and 11 is a microcomputer, which digitally outputs the current for driving the laser diode LD according to the output of the power detector PD. Control.
12 is a D / A converter converts the digital data 11a (8 bits) output from the output port PB ₀ ~PB ₇ of the microcomputer 11 into an analog value (255 steps). 12a is a constant voltage power supply, and the reference potential 12b is the port of the D / A converter 12.
Supply to V _REF . The reference potential 12b determines the step width of the analog value output from the port _IO . A constant current control circuit 13 controls the drive current applied to the laser diode LD according to the analog value output from the port _IO of the D / A converter 12. A transistor 14 drives a laser diode LD. Reference numeral 15 is a differential amplifier that amplifies the output of the power detector PD. Reference numeral 16 is an A / D converter, which converts the output 15a of the differential amplifier 15 into, for example, 8-bit digital data. The output 16a of the A / D converter 16 is input to the input ports PA _{0 to} PA ₇ of the microcomputer, and the emission output P _O of the laser diode LD is decoded. A drive signal 17 turns on the transistor 18 to turn on the transistor 14 to operate the laser diode LD.

Next, the operation of FIG. 6 will be described with reference to FIGS. 7 and 8 (a) and (b).

FIG. 7 shows the forward current of the laser diode LD shown in FIG.
It is a figure which shows the relationship between I _F and light emission output P _O , the horizontal axis shows forward current I _F (mA), and the vertical axis shows light emission output P _O (mW). Note that I _TH represents the threshold value of the forward current I _F , and I _MIN , I _TYP , I
_MAX indicates the minimum current value of the forward current I _F, the standard current value, the maximum current value, respectively. P _TH is the threshold emission output when the forward current I _F is I _TH , and P _MIN , P _TYP , P _MAX is the forward current I _F when I _MIN , I _TYP , T _MAX. The minimum emission output, standard emission output, and maximum emission output of are shown.

FIGS. 8A and 8B are flowcharts showing an example of the light emission output control operation of the conventional laser diode. It should be noted that (1) to (4) and (11) to (16) indicate respective steps.

First, the drive signal 17 shown in FIG. 6 is raised to set the laser diode LD to a state capable of emitting light (1). Next, the digital value of the digital data 11a (initial value is "00000000") to the D / A converter 12 is increased by one step (2). At this time, the microcomputer 11 decodes the light emission output P _O from the value monitored by the power detector PD, that is, the output 16a. This emission output P _O is the standard emission output
It is determined whether or not it is larger than P _TYP (3), and if NO, the process returns to step (2), the number of steps is increased successively, and the condition of step (3) is satisfied, that is, the light emission output P _O is the standard light emission output. When P _TYP is _exceeded , the digital data 11a is held and the control ends (4).

The flow shown in FIG. 8 (b) is performed at a predetermined time interval after the initial value is set, and first, the drive signal 17 shown in FIG. 6 is raised to set the laser diode LD to a state capable of emitting light (11). ). At this time, the microcomputer 11 decodes the light emission output P _O from the value monitored by the power detector PD, that is, the output 16a. It is determined whether or not this light emission output P _O is larger than the standard light emission output P _TYP (12). If NO, the number of steps is increased sequentially (13) and the process returns to step (12). If YES, D / A Digital data 11a to converter 12
Decrease the digital value of by one step (14). Next, it is determined whether the light emission output P _O is smaller than the standard light emission output P _TYP (15). If NO, the process returns to step (12). If YES, the digital data 11a is held and the control ends (16). ).

[Problems to be solved by the invention]

As described above, the forward current I _F of the laser diode LD is conventionally
The maximum current value I _MAX of the forward current I _F is set to the D / A converter 12 so that the environmental temperature characteristics shown in FIGS.
Since it is divided into 256 steps, one step in the D / A converter 12 becomes considerably rough, the emission output P _O of the laser diode LD cannot be finely controlled, and the emission output P _O cannot be constantly controlled. There was a problem.

The present invention has been made to solve the above problems, and initial light emission output control for roughly adjusting the drive current applied to the light emitting element until the light emission output of the light emitting element exceeds the threshold light emission output, When the threshold light emission output is exceeded, the light emission output control for finely adjusting the drive current applied to the light emitting element and the switching control with the threshold light emission output level of the light emitting element as the boundary are controlled, thereby providing a relatively simple configuration. Without causing a sudden change in the light amount to the light emitting element such as a laser diode,
It is an object of the present invention to provide an automatic light output control device that can adjust the light emission output of a light emitting element roughly until it exceeds a threshold light emission output and then finely adjust it until a desired light emission output.

[Means for solving problems]

In the automatic light output control device according to the present invention, the output of the current varying means is fixed to the lower limit value until the light emitting output P _O of the light emitting element exceeds the threshold light emitting output P _TH, and the current determining means is fixed. Is gradually increased to a first value and applied to the light emitting element, and after the light emission output P _O exceeds the threshold light emission output P _TH , the output of the current determining means is changed to the first value. A value fixed to a value of 1 and the output of the current varying means lowered stepwise from the first value to the second value is applied to the light emitting element, and the light emission output P _O is the threshold value. The setting of the initial value is completed at the point where the light emission output P _TH is less than that, and after the setting of the initial value, the output of the current determining means is fixed to the first value and the output of the current varying means is changed to the above-mentioned value. what graduated from the second value, is applied to the light emitting element, the light-emitting output P _O is out standard emission Exit control at a point below the P _TYP, when said light emission output P _O does not reach the standard light emission output P _TYP is gradually changing the output of said current determining means from the first value, A control in which the output of the current varying unit is gradually changed is applied to the light emitting element, and the current determining unit and the current varying unit are controlled until the light emitting output P _O falls below the standard light emitting output P _TYP. Means are provided.

[Action]

In the present invention, according to the light output of the light emitting element detected by the detecting means, the control means changes the current until the light emitting output P _O of the light emitting element exceeds the threshold light emitting output P _TH at initialization. The output of the means is fixed to the lower limit value, the output of the current determining means is fixed to the first value stepwise, and the output of the current varying means is changed stepwise from the first value to the second value. The reduced value is applied to the light emitting element, the initial value setting is completed when the light emission output P _O falls below the threshold light emission output P _TH , and thereafter, the light emission detected by the detection means at predetermined intervals. When the standard light emission output P _TYP is not reached based on the light emission output P _{O of the} element, the output of the current determining means is changed stepwise from the first value, and the output of the current varying means is changed stepwise. Applied to the light emitting element, and the light emission output P _O The current varying means and the current determining means are programmably controlled so as to fall below the optical output P _TYP .

〔Example〕

FIG. 1 is a circuit diagram for explaining an example of an automatic light output control device showing an embodiment of the present invention, and the same parts as those in FIG. 6 are designated by the same reference numerals.

In this figure, 1 and 2 are latches, and the digital data 11a (8 bits) output from the output ports PB _{0 to} PB ₇ of the microcomputer 11 are used as latch clocks output from the clock ports CLK1 and CLK2 of the microcomputer 11. Latch synchronously. 3 is a D / A converter, latch 1
Digital data latched on 11a (8 bits)
Is converted into an analog value (255 steps). Reference numeral 4 is a D / A converter which constitutes the current varying means of the present invention, and converts the digital data 11a (8 bits) latched by the latch 2 into an analog value (255 steps). 5 is the constant current control circuit 13
The output of the D / A converters 3 and 4 is added by an Op amplifier (combining means) provided in. The unit step widths of the D / A converters 3 and 4 are different, and the step width of the D / A converter 3 is equal to the unit step width (1 step width) of the D / A converter 12, Bowl 4
The range width covered by is not less than the unit step width of the D / A converter 3, and the range width is appropriately set according to the required resolution.

Next, the automatic light output control operation of the laser diode according to the present invention will be described with reference to FIGS. 2 (a) and 2 (b).

2 (a) and 2 (b) are flow charts for explaining the automatic light output control operation of the laser diode showing the embodiment of the present invention. (21)-(29) and (31)-
(48) indicates each step.

First, the drive signal 17 shown in FIG. 1 is raised to set the laser diode LD to a state capable of emitting light (21). Then, the microcomputer 11 digital data 11a from the output port PB ₀ ~PB ₇ (8 bits), i.e., outputs the lowest limit value (128 th) "01000000" (22). Next, the latch 2 latches the digital data 11a in synchronization with the latch clock sent from the clock port CLK2 (23). Subsequently, the digital data 11a is increased by one step (24), and the latch 1 synchronizes with the digital data 11a in synchronization with the latch clock sent from the clock port CKL1.
That is, "01000001" is latched (25). Then
Power detector PD light emission output P _O is threshold light emission output P
_It is judged whether or not it has _{exceeded TH} (26), if NO, the process returns to step (24), if YES, the digital data 11a is lowered by one step (27), in synchronization with the latch clock sent from the clock port CLK2, Latch 2 is digital data 11a
Latch (28). Next, it is determined whether the light emission output P _O of the power detector PD is lower than the threshold light emission output P _TH (29). If NO, the process returns to step (26), and if YES, the initialization flow ends.

The flow shown in FIG. 2 (b) is performed at a predetermined time interval after the initial value is set, and the drive signal 17 shown in FIG. 1 is raised to set the laser diode LD in a state capable of emitting light (3
1). At this time, the microcomputer 11 decodes the light emission output P _O from the value monitored by the power detector PD, that is, the output 16a. This emission output P _O is the standard emission output
It is judged whether or not it is larger than P _TYP (32), and if YES, the digital data 11a is lowered by one step (33), and the latch 2 latches the digital data 11a in synchronization with the latch clock sent from the clock port CLK2. To (3
Four). Next, it is determined whether the light emission output P _O is below the standard light emission output P _TYP (35).
If it is O, it is judged whether or not the digital data 11a is the lower limit value "00000000" (36), and if it is NO, the process returns to step (32) and is repeated until the light emission output P _O falls below the standard light emission output P _TYP.
If S, the digital data 11a for the D / A converter 3 is lowered by one step (37). Then, in synchronization with the latch clock sent from the clock port CLK1, the latch 1 latches the digital data 11a and returns to step (32) (3
8).

On the other hand, if the judgment in step (32) is NO, the digital data 11a is increased by one step (39), and the clock port C
The latch 2 latches the digital data 11a in synchronization with the latch clock sent from LK2 (40). Then, it is judged whether the digital data 11a is the upper limit value (4
If 1) or NO, the process returns to step (32), and if YES, the digital data 11a sent to the D / A converter 3 is increased by one step (42). Next, the latch 1 latches the digital data 11a in synchronization with the latch clock sent from the clock port CLK1 (43). Then, the digital data 10a sent to the D / A converter 4 is set to the minimum lower limit value (44),
In synchronization with the latch clock sent from the clock port CLK2, the latch 2 latches the digital data 11a and returns to step (32) (45).

[Effects of the Invention] As described above, according to the present invention, the light emission output P _O of the light emitting element is set to the threshold value in the initial setting in accordance with the light output of the light emitting element detected by the detection means by the control means. Until the light emission output P _TH is exceeded, the output of the current varying means is fixed to the minimum value, the output of the current determining means is fixed to the first value stepwise, and the output of the current varying means is changed stepwise. Applied to the light emitting element, which is lowered from the first value to the second value,
The initial value setting ends when the light emission output P _O falls below the threshold light emission output P _TH , and thereafter, the standard light emission is performed based on the light emission output P _O of the light emitting element detected by the detection means at predetermined intervals. When the output P _TYP is not reached, the output of the current determining means is changed stepwise from the first value, and the output of the current varying means is changed stepwise and applied to the light emitting element. However, since the current varying means and the current determining means are programmably controlled so that the light emission output P _O is lower than the standard light emission output P _TYP , the light emitting element such as a laser diode or the like emits a sudden light amount with a relatively simple structure. There is an effect that the light emission output of the light emitting element can be coarsely adjusted until it exceeds the threshold light emission output and then finely adjusted to a desired light emission output without causing fluctuation.

[Brief description of drawings]

FIG. 1 is a circuit diagram for explaining an example of an automatic light output control device showing an embodiment of the present invention, and FIGS. 2 (a) and 2 (b) are automatic light output of a laser diode showing an embodiment of the present invention. FIG. 3 is a flowchart illustrating the control operation, FIG. 3 is a diagram showing the emission power characteristic of the laser diode, FIG. 4 is a diagram showing the relative relationship between the threshold current of the laser diode and the ambient temperature,
FIG. 5 is a circuit diagram showing the principle of detecting the light emission output of the laser diode, FIG. 6 is a circuit diagram showing an example of the automatic light output control device for the laser diode LD shown in FIG. 5, and FIG. 7 is shown in FIG. FIG. 8A and FIG. 8B are diagrams showing the relationship between the forward current of the laser diode LD and the light emission output, and FIGS. 8A and 8B are flowcharts showing an example of the light emission output control operation of the conventional laser diode. In the figure, 1 and 2 are latches, 3 and 4 are D / A converters, and 5 is an Op amplifier.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI technical display location B41J 2/455 H01S 3/133 (72) Inventor Yukio Sato 3-30-2 Shimomaruko, Ota-ku, Tokyo No. Canon Inc. (56) Reference JP-A 51-52221 (JP, A) JP-A 60-233614 (JP, A) JP-A 60-171863 (JP, A) JP-A 60-120659 (JP, A)

Claims (1)

[Claims] 1. A current determining means for stepwise determining a drive current of the light emitting element according to a light output of the light emitting element detected by the detecting means, and a light output of the light emitting element detected by the detecting means. The light emission is achieved by combining the current varying means for varying the driving current stepwise finer than the step width of the driving current determined by the current determining means, and the output of the current varying means and the output of the current determining means. An automatic light output control device comprising a combining means for applying a drive current to the element, wherein the output of the current varying means is set until the light emission output P _O of the light emitting element exceeds a threshold light emission output P _TH. The light emission output P _{O exceeded the} threshold light emission output P _TH, which was fixed to the lower limit value and the output of the current determining means was increased stepwise to the first value and applied to the light emitting element. After that,
The output of the current deciding means is fixed to a first value, and the output of the current varying means is stepwise reduced from the first value to the second value and applied to the light emitting element to emit the light. output
The setting of the initial value is completed at the point where P _O falls below the threshold light emission output P _TH , and after the setting of the initial value, the output of the current determining means is fixed to the first value, The output of the current varying means, which is stepwise changed from the second value, is applied to the light emitting element, and the control is ended when the light emission output P _O is below the standard light emission output P _TYP. When the light emission output P _O does not reach the standard light emission output P _TYP , the output of the current determining means is changed stepwise from the first value, and the output of the current varying means is changed stepwise. One is applied to the light emitting element, and the light emission output P _O
The automatic light output control device is provided with a control means for controlling the current determining means and the current varying means until the value falls below the standard light emission output P _TYP .