JP2857459B2 - Optical scanning device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical scanning device, and more particularly, to a convergent spot position (beam waist position) using an electro-optical lens (variable focal length lens) in a laser printer.
The present invention relates to an optical scanning device for correcting fluctuation. For example, the present invention is applied to digital copiers, digital colors, laser faxes, and the like.

Prior art As known documents describing the prior art according to the present invention,
There is a "laser unit" in Japanese Patent Publication No. 1-28381 and an "optical scanning system" in Japanese Patent Application Laid-Open No. 58-57108.

The Japanese Patent Publication No. 1-28381 discloses that the interval between the laser light source and the optical system is not shifted even if there is a temperature change in the apparatus, and the convergent spot position of the laser beam is fixed.
It is disclosed that the wavelength shift due to the temperature change of the semiconductor laser is cooled using a Peltier element to prevent the temperature from rising. However, as the temperature inside the equipment rises,
The semiconductor laser element moves toward the collimating lens due to the thermal expansion of the laser holding member, and the collimating lens moves toward the semiconductor laser element due to the thermal expansion of the holding member of the collimator lens, and the distance between the collimating lens and the semiconductor laser element increases. to shrink. In order to compensate for this reduction, the distance between the semiconductor laser element and the collimator lens is increased by the thermal expansion of the connecting member. It is difficult to compensate. Further, there is a disadvantage that the cost is increased, for example, a Peltier element is used to prevent heat generation of the semiconductor laser.

Japanese Patent Application Laid-Open No. 58-57108 discloses that in a light beam scanning optical system, each lens system can be moved without fixing the positional relationship, and the imaging characteristics of the optical system are complicated and expensive. There is disclosed an optical scanning method which can be improved without any problem. Specifically, an optical scanning method in which a collimator lens, a condensing lens, and the like are moved and controlled by an actuator to improve an imaging characteristic in planar scanning. However, the deviation from the plane scanning generated within one scanning cycle of the laser beam is corrected by controlling the movement of the collimator lens, the condensing lens, etc. in the optical axis direction. In such a method, it is necessary to set precisely. There is a drawback that the optical system, which has to be movable, has a movable part, and the reliability is significantly reduced.

The present invention has been made in view of the above circumstances, and has been described in view of the fact that the imaging optical system is displaced due to a temperature change in the apparatus, and the beam waist position of the convergent beam is displaced from the surface of the medium to be scanned. In order to prevent this, an electro-optic lens with a variable focal length function was used, and the voltage applied to the electro-optic lens was controlled according to the temperature change so that the beam waist position was always constant. Deviations from planar scanning that occur within one scanning cycle are corrected by a high-frequency voltage applied to the electro-optic lens without moving the optical system, and a direct current applied to the electro-optic lens is used for temperature changes in the device. It is an object of the present invention to provide an optical scanning device capable of performing a good plane scan by correcting the bias voltage with a bias voltage.

In order to achieve the above object, the present invention provides (1) a laser light source, an electro-optic lens on which laser light from the laser light source is incident, and a light source for deflecting and scanning a light beam emitted from the electro-optic lens. An optical deflector, an imaging optical system for converging and imaging a laser beam deflected by the optical deflector, a power supply for applying a DC voltage to the electro-optical lens, the laser light source and an electro-optical lens And an optical housing for housing an optical deflector and an imaging optical system, a temperature sensor installed in the optical housing, and a medium to be scanned scanned by the deflected laser beam. By controlling the voltage applied to the electro-optical lens according to the output signal and changing the focal length of the electro-optical lens, the beam waist position of the converged and focused deflection laser beam is adjusted. The position always coincides with the surface of the medium to be scanned with respect to the environmental temperature change.
(2) a power supply for applying a DC bias voltage to the electro-optical lens, and a power supply for applying a high frequency driving voltage;
The DC bias voltage is changed to control the beam waist position to be constant with respect to the environmental temperature change, and the high-frequency driving is performed with respect to the change in the beam waist position occurring within one line scan of the deflected laser beam. It is characterized in that correction is made by a power supply. Hereinafter, a description will be given based on examples of the present invention.

Generally, in a laser beam printer, a laser beam emitted from a laser enters a rotary polygon mirror through an optical modulator and is horizontally scanned in a one-dimensional direction. When using a semiconductor laser as the light source,
No modulator is required for direct modulation. The laser light reflected by the rotating polygon mirror enters the fθ lens. The fθ lens is
The lens is designed so that light reflected in the direction of θ with respect to the optical axis is condensed at a position of f · θ when the focal length of the lens is f.

An electrophotographic photosensitive drum as used in a copying machine is arranged at the focal point, and is uniformly charged immediately before writing. Thereafter, the portion irradiated with the laser beam loses its potential, and the carbon toner adheres thereto. The laser beam is scanned horizontally, while the photosensitive drum moves in a direction perpendicular thereto, so that when the laser beam is modulated, an image and a character pattern are formed as a potential pattern in the same manner as when scanning a television screen. After the toner on the photosensitive drum is transferred to the printing paper, it is fixed by heat or pressure.

1 (a) and 1 (b) are configuration diagrams for explaining an embodiment of an optical scanning device according to the present invention, where FIG. 1 (a) is a cross-sectional view of a scanning optical system, and FIG. 1 (b) is a perspective view. It is. In the figure, 1 is a semiconductor laser, 2 is a collimating lens, 3 is an electro-optic lens, 4 is a cylinder lens, 5 is a temperature sensor, 6 is a polygon mirror, 7 is an fθ lens, 8 is a drive motor, 9 is a housing, 10 is Amplifier, 11 is an A / D converter, 12 is a conversion table, 13 is a D / A converter, 14 is a DC power supply, 15 is a support member, A
Is a medium to be scanned. The divergent beam from the semiconductor laser 1 is converted into a parallel beam by the collimating lens 2 and is incident on the electro-optical lens 3. The electro-optical lens 3 has a function of varying the focal length, and can vary the focal length by applying a voltage value. Further, the outgoing beam subjected to the lens action by the electro-optic lens 3 passes through the cylinder lens 4 and is deflected and reflected by the polygon mirror 6, and
The fθ lens 7 converges and forms an image on the surface to be scanned. The number of the fθ lenses increases as high quality printing is desired. The polygon mirror 6 is rotated by a motor 8.

The temperature inside the device is detected by the temperature sensor 5 attached to the support member 15 in the optical housing 9, and the output signal from the temperature sensor 5 is amplified by the amplifier 10, and the A / D converter 1
The signal is converted into a digital signal by 1 and input to the conversion table 12. The conversion table 12 incorporates in advance data on the relationship between the temperature and the output voltage of the temperature sensor. Conversion table 12
Is converted into an analog signal by the D / A converter 13 and the DC voltage applied to the electro-optical lens 3 is controlled so that the beam waist position always coincides with the surface of the medium 15 to be scanned.

2 (a) to 2 (c) are configuration diagrams of the electro-optical lens, where FIG. 2 (a) is a perspective view, FIG. 2 (b) is a side view as viewed from the incident beam direction in FIG. 2 (a), and FIG. (B) is a side view as viewed from the opposite side, in which 16, 17, 19, 20, 21, and 22 are electrode films, and 18 is a power supply. The electro-optic lens 3 has electrode films 16 and 17 formed on opposing surfaces of an electro-optic medium (electro-optic crystal having no electrodes), and a power supply 18 for applying a voltage to the electrode films 16 and 17 is provided. It is. PLZT (le
Ad (plomb) lanthanum zirconate titanate (electro-optic crystal) is used, and its composition is 9/65/35 (PbZrO ₃ is 65%, PbT
Dope 9% of La to 35% mixed crystal of iO ₃ to obtain transparency)
Is suitable, but other compositions may be used. Optically polished surfaces where laser light enters and exits the PLZT electro-optic crystal, and Au is formed as an electrode film by vacuum evaporation along the optical path on a surface orthogonal to these surfaces. Good.
As a forming method, screen printing using a conductive paste may be used.

The principle of the electro-optic lens will be described. When no voltage is applied to the electro-optic lens, the incident laser beam exits without any change. At this time, it has no lens action. When a voltage is applied, an electric field distribution as shown by the dotted lines in FIGS. 2B and 2C is generated, which becomes strong near the electrode and weak near the center. As a result, a refractive index distribution occurs in the crystal due to the electro-optic effect. The direction of the electric field Z, the direction of the incident laser beam and Y, the incident laser beam is assumed that polarized in the Z direction, when the Z-component of the refractive index when a voltage is applied to the n _2, Becomes n ₀ is the refractive index of the PLZT electro-optic crystal at V = 0, R
₃₃ is a matrix component of the secondary electro-optic constant. Wherein (1) the refractive index change [Delta] n _Z by the electric field E _Z from equation And is proportional to the square of the electric field strength. The refractive index is small in the region where the electric field is strong, and large in the region where the electric field is weak. As a result, the lens action occurs toward the center in the Z direction and the X direction. The lens effect of the electro-optic lens increases as the optical path length increases.

Further, when the laser light linearly polarized in the Z direction is incident on the electro-optical lens as a parallel beam, the lens acts in the Z direction by the voltage applied to the electrode films 16 and 17 formed along the top and bottom of the optical path, and the focal length Becomes f ₀ and converges to the point A.
Then the laser beam is converged to the focal distance f _'0 next to the point A receives the lens action in the X direction by the voltage applied to the electrode film 19, 20, 21, 22 when incident on the area shown in FIG. (C) I do. At point A, a spot converged in both the X and Z directions is obtained. In order to converge to the same point A in both the X and Z directions, the lengths l and l 'of the electrode films may be set according to the convergence performance in each direction.

FIG. 3 shows the measured values of the focal length f with respect to the applied voltage of the electro-optical lens of the present invention. In this case, the focal length f was a distance from the center of the electro-optical lens to the convergence position.

FIG. 4 is a graph in which the fluctuation of the beam waist position in the optical scanning device when the electro-optical lens is not used is plotted with respect to the environmental temperature change. The cause of this variation is that the oscillation wavelength of the semiconductor laser shifts to a longer wavelength side as the temperature rises (for example, about 0.3 nm / ° C), the distance between the lenses due to the thermal expansion of the optical system and the support member of the semiconductor laser, and the semiconductor laser And the change in the distance between the lenses. Particularly, the wavelength shift of the semiconductor laser greatly contributes to the beam wear position fluctuation.

Therefore, in the embodiment shown in FIG. 1, the applied voltage is controlled by the electro-optical lens so that the above-mentioned fluctuation due to the environmental temperature change of the beam waist position is always constant. I do.

In FIG. 3, a current of Vb to the electro-optical lens, the focal distance is f _b. The ambient temperature is room temperature (25
C), Vb is set so that the beam waist position of the converged laser beam is on the surface of the medium to be scanned (Vb = 300 V / m)
m).

FIG. 5 shows the relationship between the environmental temperature and the applied voltage so that the beam waist position is always constant. This temperature change is corrected including the temperature characteristics of the electro-optical lens. Therefore, if a conversion table is prepared in advance so as to output the voltage shown in FIG. 5, the beam waist position can be made constant by the electro-optic lens.

FIG. 6 is a block diagram showing another embodiment of the present invention. Reference numeral 23 denotes a high-frequency power source, and other parts having the same functions as those in FIG. 1 are denoted by the same reference numerals. The difference from the embodiment of the present invention shown in FIG. 1 is that a high-frequency power supply 23 is provided. The high-frequency power supply 23 is configured to perform the correction of the field curvature occurring within one scanning cycle of the laser beam simultaneously with the temperature correction. In this case, the high-frequency voltage applied to the electro-optic lens is applied to the electrode films 16, 17, 19, 20, 21, 22, and 22 of the electro-optic lens.
Or the same amount of correction in the main scanning and sub-scanning directions may be applied, or two high-frequency power sources may be prepared, and different high-frequency voltages may be applied to correct the main scanning and sub-scanning directions. The amount can also be varied.

Effects As is clear from the above description, the present invention has the following effects.

(1) The environmental temperature in the device is measured by a temperature sensor, and the voltage applied to the electro-optical lens is changed by the output signal from the sensor to control the beam waist position of the converged laser beam. It is possible to prevent the shift of the oscillation wavelength of the semiconductor laser, the gap shift due to the thermal expansion of the support of the semiconductor laser and the optical system, and the fluctuation of the beam waist position caused by the change of the focal length of the electro-optical lens itself, and the like. The beam waist position is always on the surface of the medium to be scanned irrespective of the temperature change, and a stable optical scanning device is realized.

(2) In addition to preventing the beam waist position from fluctuating due to environmental temperature changes, the electro-optic lens can be further controlled by a high-frequency power supply to correct the field curvature that occurs within one scanning cycle of the laser beam. Irrespective of this, a stable optical scanning device in which the beam waist position is always on the surface of the medium to be scanned and has no curvature of field is realized.

[Brief description of the drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of an optical scanning device according to the present invention, FIG. 2 is a configuration diagram of an electro-optical lens,
FIG. 3 is a diagram showing measured values of the focal length with respect to an applied voltage of the electro-optic lens, FIG. 4 is a diagram showing a beam waist position variation of the optical scanning device when the electro-optic lens is not used, and FIG. FIG. 6 shows the relationship between the environmental temperature and the applied voltage so that the beam waist position is always constant.
FIG. 6 is a configuration diagram showing another embodiment of the present invention. 1 ... Semiconductor laser, 2 ... Collimate lens, 3 ...
Electro-optic lens, 4 ... Cylinder lens, 5 ... Temperature sensor, 6 ... Polygon mirror, 7 ... Fθ lens, 8 ...
... Motor, 9 ... Housing, 10 ... Amplifier, 11 ... A /
D converter, 12 conversion table, 13 D / A converter, 14
... DC power supply, 15 ... Support member.

Claims (2)

(57) [Claims] A laser light source; an electro-optic lens on which laser light from the laser light source is incident; an optical deflector for deflecting and scanning a light beam emitted from the electro-optic lens; An imaging optical system for converging and forming the focused laser beam, a power supply for applying a DC voltage to the electro-optical lens, the laser light source, an electro-optical lens, an optical deflector, and an imaging optical system are housed. An optical housing, and a temperature sensor installed in the optical housing,
A scanning medium to be scanned by the deflected laser beam, by controlling a voltage applied to the electro-optic lens according to an output signal from the temperature sensor, by changing the focal length of the electro-optic lens, An optical scanning device, wherein a beam waist position of a converged and formed deflected laser beam always coincides with the surface of the medium to be scanned with respect to a change in environmental temperature. 2. A power supply for applying a DC bias voltage to the electro-optical lens, and a power supply for applying a high-frequency drive voltage, wherein the DC bias voltage is changed to change the beam waist position with respect to environmental temperature changes. Control to be constant,
2. The optical scanning device according to claim 1, wherein a change in a beam waist position occurring within one line scanning of the deflected laser beam is corrected by the high frequency driving power supply.