JPH11330599A - Semiconductor laser driver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a semiconductor laser driver allowing the light quantity of a laser beam to be simply adjusted for a set light quantity. SOLUTION: A monitor circuit 30 comprises a photodiode 31 disposed behind a cleavage plane of a semiconductor laser 10 to output a photo current Ipd proportional to the light quantity of a laser beam, variable resistor 32 for adjusting the light quantity of the laser beam of the semiconductor laser 10 and fixed resistor 33. A voltage Vcc is fed to the cathode of the photodiode 31, a voltage V0 (e.g. 5 V) lower than the voltage Vcc and higher than a reference voltage Vref is fed to one end of the variable resistor 32, and the fixed resistor 33 is commonly inserted between the anode of the photodiode 31 and the other end of the variable resistor 33 to feed back the voltage between both ends as a monitor voltage to a semiconductor laser drive circuit 20.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser driving apparatus for driving a semiconductor laser, and more particularly, to a technique for adjusting a light amount of a laser beam emitted from a semiconductor laser.

[0002]

2. Description of the Related Art For example, in an image forming apparatus such as a laser printer for forming an image by an electrostatographic system, a semiconductor laser is used as a light source for light writing on a photosensitive drum. The surface of the photosensitive drum, which is driven to rotate at a constant speed by a laser beam, is exposed and scanned to form an electrostatic latent image.

[0003] By the way, a semiconductor laser oscillates when a driving current flowing in a forward direction exceeds a threshold current.
A laser beam having an amount of light corresponding to the drive current is emitted from both sides of the cleavage plane, but the amount of the laser beam is easily affected by the operating temperature and changes. For example, if the operating temperature is between 20 ° C and 7
When the temperature rises to 0 ° C., the threshold current increases several times that at 20 ° C., so that even if the drive current is maintained at the same value, the light amount of the laser beam is reduced by a fraction. In particular, in the case of an image forming apparatus, since the toner is fixed on a recording sheet at a high temperature, the change range of the operating temperature due to the heat from the fixing device is large, and the change in the light amount of the laser beam is remarkable. When such a change in light amount occurs, the photosensitive member cannot be reliably exposed, and a stable reproduced image cannot be formed.
Therefore, a semiconductor laser driving device that can keep the light amount of the laser beam constant even when the operating temperature changes has been conventionally considered.

FIG. 3 is a circuit diagram showing a configuration of the conventional semiconductor laser driving device. As shown in the figure, the semiconductor laser drive device comprises a semiconductor laser drive control circuit 200.
And a monitor circuit 300.

[0005] The semiconductor laser drive control circuit 200 includes a monitor voltage V _m and a predetermined reference voltage V _ref is inputted, an amplifier 20 for outputting a signal corresponding to the difference between the voltage V _m, V _ref
1 and both voltages V _m , V based on the output of the amplifier 201.
a semiconductor laser drive circuit 202 for controlling a drive current I _LD flowing through the semiconductor laser 100 so that _ref coincides with each other.

The monitor circuit 300 includes a semiconductor laser 100
Current I proportional to the amount of laser beam emitted from
_It comprises a photodiode 301 for outputting a _PD and a variable resistor 302 for adjusting the light amount of the laser beam of the semiconductor laser 100. A voltage V _cc (for example, 12 V) is supplied to the cathode of the photodiode 301, and a variable resistor 302 (for example, resistance = 0 to 50 kΩ) is connected between the anode and the ground of the photodiode 301. The variable resistor 302 converts the light current I _PD into a light voltage V _PD , and feeds back the light voltage V _PD to the semiconductor laser drive control circuit 200 as a monitor voltage V _m .

According to such a semiconductor laser driving device,
If the operating temperature changes, the amount of laser beam
In this case, the reduced spectral current I_PD, That is, monitor
Pressure V_mDecreases and the monitor voltage V_m And the reference voltage match
Monitor voltage V_m Drive current I_LDIs increased
It is. In contrast, when the amount of laser beam increases
In some cases, the amount of laser beam decreases due to changes in operating temperature
The driving current I_LDIs reduced. did
Therefore, regardless of the change in the operating temperature, the monitor voltage V _m But one
The laser beam intensity is maintained almost constant.
Will be.

By the way, in order to surely expose the photoreceptor, the light amount of the semiconductor laser 100 must be the maximum rating (for example,
It is preferable to set the value as large as possible within a range not exceeding 5 mW). Therefore, before shipping the image forming apparatus in which the semiconductor laser driving device is incorporated, the optical power meter 400 is installed on the surface of the photosensitive drum, and the laser beam is received by the sensor (not shown) of the optical power meter 400. The light amount is measured by the operator, and the variable resistance 302 is set to a resistance value at which a worker can obtain a desired light amount slightly smaller than the maximum rating (for example, the light amount at 4.5 mW, and this light amount is hereinafter referred to as “set light amount”). After operation, the resistance was fixed at this value.

[0009]

However, in the above-described conventional semiconductor laser driving device, the light amount of the laser beam is in inverse proportion to the resistance value of the variable resistor 302, and the resistance value of the variable resistor 302 near the set light amount is small. Since the rate of change of the light amount with respect to the change is large, it is difficult to adjust the set light amount.

In addition, there is a high possibility that the semiconductor laser may be erroneously operated at the time of this adjustment, causing the semiconductor laser to emit light exceeding the maximum rating, thereby destroying the semiconductor laser.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems, and has as its object to provide a semiconductor laser driving device capable of easily adjusting the light amount of a laser beam to a set light amount.

It is another object of the present invention to provide a semiconductor laser driving device which is less likely to damage a semiconductor laser.

[0013]

In order to solve the above-mentioned problems, a semiconductor laser driving device according to the present invention, based on a monitor voltage and a predetermined reference voltage, flows through a semiconductor laser such that both voltages coincide. A semiconductor laser drive device comprising: a semiconductor laser drive control means for controlling a drive current; and a light quantity monitor means for feeding back a monitor voltage obtained based on a light quantity of a laser beam emitted from a semiconductor laser to the semiconductor laser drive control means. A light current output means for supplying a first voltage to one end and outputting a photocurrent proportional to the light quantity of the laser beam emitted when the semiconductor laser is driven; And a variable resistor for changing the resistance value to change the light amount of the laser beam, and the other end of the photocurrent output means and the variable resistor. Interposed in common between the command, characterized in that it comprises a resistor for outputting a voltage across as said monitor voltage.

Further, in the semiconductor laser driving device according to the present invention, the second voltage is smaller than the first voltage,
It is set to a value larger than the reference voltage.

[0015]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a semiconductor laser driving apparatus according to the present invention. FIG.
1 is a circuit diagram showing the overall configuration of the semiconductor laser driving device 1. FIG.

As shown in FIG. 1, a semiconductor laser driving device 1 includes a semiconductor laser driving control circuit 20 and a monitor circuit 3.
0.

The semiconductor laser drive control circuit 20 generates a monitor voltage _Vm and a predetermined reference voltage _Vref (for example, 1.5 V).
: It is input, both voltages V _m, an amplifier 21 for outputting a signal corresponding to the difference between V _ref, flowing through the semiconductor laser 10 as both voltages V _m, V _ref is matched on the basis of the output of the amplifier 21 driving Semiconductor laser drive circuit 2 for controlling current ILD
2 is provided.

The monitor circuit 30 is disposed behind the cleavage plane of the semiconductor laser 10 and outputs a photocurrent _IPD proportional to the light amount of the laser beam. Variable resistance 32 (for example, resistance value = 0 to 5) for adjusting the light quantity of the laser beam
0 kΩ) and a fixed resistor 33. A voltage V _cc (for example, 12 V) is supplied to the cathode of the photodiode 31, and a voltage V ₀ (for example, 5 V) smaller than the voltage V _{cc and} higher than the reference voltage V _ref is supplied to one end of the variable resistor 32. ) Is supplied, and the fixed resistor 33 is commonly interposed between the anode of the photodiode 31 and the other end of the variable resistor 33, and the voltage between both ends, that is, the optical voltage obtained by converting the photocurrent I _PD is obtained. V _PD and voltage V ₀ are adjusted by a variable resistor 3
2 and the voltage V _ref obtained by dividing the voltage with the monitor voltage V _m
As feedback to the semiconductor laser drive circuit 20.

The photodiode 31 is a semiconductor laser 1
It receives the laser beam emitted from the rear side of the cleavage plane of 0, and outputs the photocurrent I _PD corresponding to the light amount of the laser beam. The photocurrent I _PD flows to the ground via the fixed resistor 33. The resistance value of the fixed resistor 33 is R, and the photocurrent I _PD
Assuming that the voltage generated in the fixed resistor 33 is V _PD , Equation (1) is obtained.

V _PD = I _PD · R (1)

On the other hand, the current flowing through the variable resistor 32 is represented by I
_Assuming that V _dev is the voltage generated in the fixed resistor 33 by the current I _dev , Vdev is given by the following equation (2). V _dev = I _dev · R (2) Since the monitor voltage V _m is a voltage between both ends of the fixed resistor 33, it is obtained by the equation (3).

V _m = V _PD + V _dev = I _PD · R + I _dev · R = R · (I _PD + I _dev ) (3)

On the other hand, the current I _dev flowing through the variable resistor 32
Is obtained from Expression (4). I _dev = (V ₀ −V _m ) / R _VR (4) By substituting equation (3) into equation (4) and rearranging I _dev , equation (5) is obtained. I _dev = {V ₀ − (I _PD · R)} / (R _VR + R) (5) When this equation (5) is substituted into equation (3) and I _dev is eliminated, equation (6) is obtained. Can be

V _m = {R · (I _PD · R _VR + V ₀ )} / (R _VR + R) (6) When the resistance value R _VR of the variable resistor 32 is fixed, from this equation (6) the monitor voltage V _m is independent of the current I _dev, a photocurrent I _PD it can be seen that a proportional relationship.

According to such a semiconductor laser driving device,
If the operating temperature changes, the amount of laser beam
In this case, the reduced spectral current I_PD, That is, monitor
Pressure V_mDecreases and the monitor voltage V_m And the reference voltage match
Monitor voltage V_m Drive current I_LDIs increased
It is. In contrast, when the amount of laser beam increases
In some cases, the amount of laser beam decreases due to changes in operating temperature
The driving current I_LDIs reduced. did
Therefore, regardless of the change in the operating temperature, the monitor voltage V _m Is based
Reference voltage V_ref Is maintained at the same constant value as
The light quantity will be kept almost constant.

Next, the adjustment of the light quantity of the semiconductor laser 10 will be described. In order to clarify the difference from the conventional one, a conventional semiconductor laser driving device will first be considered. In the conventional semiconductor laser driving device, the photocurrent I _PD is
02, the optical voltage generated in the variable resistor 302 and the reference voltage V _PD are used as the monitor voltage V _m , and have the relationship of Expressions (7) and (8).

V _PD = I _PD · R _VR (7) V _m = V _PD = V _ref (8)

Here, _assuming that V _ref = 1.5 (V) and the relational expression (9) between I _PD and R _VR is obtained from Expressions (7) and (8), I _PD = V _ref / R _VR = 1.5 / R _VR ≧ 0 (9)

On the other hand, the light quantity and the drive current I _LD are directly proportional,
The amount of light and the photocurrent I _PD are directly proportional. Therefore, the relationship between the resistance R _VR of light intensity and a variable resistor 302, can be applied to Equation (9), the amount of light is inversely varies with the resistance value R _VR of the variable resistor 302 (in FIG. 2 (See graph P1).

Therefore, according to the conventional semiconductor laser driving device, in the vicinity of the set light amount (see α in FIG. 2), the change rate of the light amount with respect to the change in the resistance value of the variable resistor 302 is large. Adjustment was difficult. Further, if the resistance value of the variable resistor 302 is erroneously adjusted to 0Ω, the light amount becomes infinite and the semiconductor laser is destroyed.

Next, adjustment of the light amount of the semiconductor laser 10 according to the present embodiment will be described. V ₀ = 5 (V), V
_ref = 1.5 (V), R = 4 (kΩ), and the photocurrent I _PD
And when determining the relationship between the resistance value R _VR of the variable resistor 32 from equation (6), equation (10) is obtained. I _PD = {V _m (R + R _VR ) −R · V ₀ } / (R · R _VR ) = (V _m / R) + {(V _m −V ₀ ) / R _VR } = (V _ref / R) + {(V _ref −V ₀ ) / R _VR ｛= {1.5 / (4 · 10 ³ )} + {(1.5−5) / R _VR ＝= 0.375 · 10 ⁻³ − (3 .5 / R _VR ) ≧ 0 (10)

On the other hand, the light quantity and the drive current I _LD are directly proportional,
The amount of light and the photocurrent I _PD are directly proportional. Therefore, the relationship between the resistance R _VR of light intensity and a variable resistor 32, it is possible to apply equation (10), the amount of light to the resistance value R _VR of the variable resistor 32 as asymptote rated light amount (5 mW) And negatively proportionally change (see graph P2 in FIG. 2).

When adjusting the light quantity, first of all,
Semiconductor laser driving device 1 with the value set to the minimum value (0Ω)
Turn on the power to. At this time, a fixed voltage is applied to the fixed resistor 33.
Pressure V ₀ Is supplied, the monitor voltage V_m Is high and semi-conductive
The light amount of the body laser 10 is small and substantially “0”. Variable resistor
As the resistance value of anti-32 gradually increases, the monitor voltage
V_m Gradually decreases and the monitor voltage V_m Is the reference voltage V
_ref When the voltage becomes lower, the semiconductor laser drive circuit 22
Style I_LDOf the laser beam from the semiconductor laser 10
Is emitted. When the laser beam is emitted, work
The user measures the light amount of the semiconductor laser 10 with the optical power meter 40.
While monitoring, set the variable resistor 32 until the set light amount is obtained.
Operate to adjust the light intensity. In this case, in the vicinity of the set light amount (Fig.
2), the resistance value of the variable resistor 32 is changed.
The rate of change of the amount of light with respect to conversion is extremely small. Therefore,
According to the semiconductor laser driving device of the present embodiment,
Adjustment of the constant light amount becomes extremely easy.

Further, during the adjustment of the set amount of light, the photodiode 31 outputs a monitor current I _m in accordance with the amount of the laser beam, a semiconductor laser driving circuit 7 matches the monitor voltage V _m is the reference voltage V _ref Semiconductor laser 1
Controlling the 0 of the drive current I _LD, but the voltage V _0, the voltage V _cc
Smaller than the reference voltage V _ref, so that no matter how the variable resistor 32 is operated,
The amount of light can be kept below the maximum rating, and the semiconductor laser 10 can be prevented from being broken.

[0035]

As described above, according to the semiconductor laser driving device of the present invention, the light quantity monitoring means has the first end at one end.
And a photocurrent output means for outputting a photocurrent proportional to the amount of the laser beam emitted when the semiconductor laser is driven. A second voltage is supplied to one end, and the resistance is changed to change the resistance value of the laser beam. A variable resistor for changing the amount of light, and a resistor that is interposed in common between the other end of the photocurrent output means and the variable resistor and ground, and outputs a voltage between both ends as the monitor voltage. However, although the conventional semiconductor laser drive circuit has a simple configuration in which the fixed resistance is increased by one and the connection pattern is partially changed, the larger the light amount of the semiconductor laser, the more the change in the light amount with respect to the change in the resistance value of the variable resistor becomes gentler. And the adjustment of the set light quantity can be made extremely easy.

According to the semiconductor laser driving device of the present invention, the second voltage is set to a value lower than the first voltage and higher than the reference voltage. Regardless of how the resistor is manipulated, the amount of light of the semiconductor laser can be suppressed below the maximum rating, and the semiconductor laser can be completely destroyed.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an example of a semiconductor laser drive control circuit according to the present invention.

FIG. 2 is a graph showing a relationship between a variable resistance value and a light emission output in the semiconductor laser drive control circuit of the present invention and a conventional semiconductor laser drive control circuit.

FIG. 3 is a circuit diagram showing an example of a conventional semiconductor laser drive control circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor laser drive device 10 Semiconductor laser 20 Semiconductor laser drive control circuit 21 Amplifier 22 Semiconductor laser drive circuit 30 Monitor circuit 31 Photodiode 32 Variable resistance 33 Fixed resistance

Claims (2)

[Claims] 1. A semiconductor laser drive control means for controlling a drive current flowing through a semiconductor laser based on a monitor voltage and a predetermined reference voltage so that the voltages match each other, and a light amount of a laser beam emitted from the semiconductor laser. A light amount monitoring means for feeding back a monitor voltage obtained based on the above to the semiconductor laser drive control means, wherein the light amount monitoring means has a first voltage supplied to one end thereof, and a semiconductor laser. A photocurrent output means for outputting a photocurrent proportional to the light quantity of the laser beam emitted at the time of driving; a variable resistor for supplying a second voltage to one end and changing the resistance value to change the light quantity of the laser beam; A resistor commonly interposed between the other end of the photocurrent output means and the variable resistor and ground, and outputting a voltage between both ends as the monitor voltage; A semiconductor laser driving apparatus comprising: a. 2. The semiconductor laser driving device according to claim 1, wherein the second voltage is set to a value lower than the first voltage and higher than the reference voltage.