JPH03129318A - Laser beam diameter controller - Google Patents

Abstract

PURPOSE:To enable laser beam diameter conversion with a shutter speed of 10 ns order by providing a 3rd slit on a 2nd polarizing plate on the same optical axis and nearly in the same shape with a 1st slit. CONSTITUTION:An aperture is constituted by superposing a 1st polarizing plate 6, a polarizing element 7, and the 2nd polarizing plate 8 in the traveling direction of a laser beam and the 1st polarizing plate 6 and the 2nd polarizing plate 8 have mutually 90 deg. different planes of polarization. The intermediate polarizing element 7 is provided with electrodes 4 and 4' for voltage application at the opposite end parts of an electrooptic material 3, which has planes of polarization of incident light and projection light rotated by 90 deg. by the application of a voltage, and also provided with a 1st slit 1 extending in a beam scanning direction at the center part, and a light shield plate 9 which is provided with a 2nd slit 2 wider than the range of the 1st slit slightly in a subscanning direction is superposed; and the 2nd polarizing plate 8 is provided with the 3rd slit 5 on the same optical axis and almost in the same shape with the 1st slit 1 of the polarizing element 7. Consequently, the polarizing operation of the electrooptic material by the voltage application is switched at the speed of almost 10 ns order almost simultaneously with the voltage application.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a laser beam diameter control device for a laser optical device.

1 East 1 Laser optical devices, such as laser printers and digital copying machines, use a laser beam to optically write on a photoreceptor to form a latent image on the photoreceptor. An image is formed using do sotos caused by spots as pixels.

In the image formed in this way, the density of the image appears to vary depending on the diameter of the dot. Therefore, by changing the diameter of the dots and changing the ratio of large and small dots, it is possible to adjust the image to have multiple gradations.

A possible method for adjusting multiple gradations of an image is to print by changing the interval at which dots of the same diameter are formed, but if the dots are not multiplied by an integral number, it is difficult to deal with this by simple electrical processing.

Therefore, in order to perform multi-step image adjustment, it is appropriate to change the diameter of the laser beam spot formed on the photoreceptor in accordance with input data.

Now, as a device for changing the laser beam diameter of a laser beam focusing device, for example, in Japanese Patent Application Laid-Open No. 108522, a beam shape or a beam pattern is added to the incident side position of a condensing lens that forms a minute spot on the imaging plane. By arranging a liquid crystal element equipped with an electrode shaped to selectively vary the direction of the laser beam, and controlling the voltage application state of the liquid crystal element using an input signal, the diameter of the laser beam incident on the photoreceptor is changed to create a minute spot. A beam diameter conversion device that converts the diameter is disclosed.

By the way, when trying to adjust one image to multiple gradations, it is desirable to vary the spot diameter, that is, the beam diameter, for each pixel. The distance is 10n
, s, the conversion speed of the laser beam diameter is required to be on the order of 10 ns.

However, in the case of the beam diameter conversion device using the aforementioned liquid crystal element, the operating speed of the liquid crystal element due to voltage application is on the order of ms, so the shutter speed of the liquid crystal element mentioned above does not affect the density adjustment of the entire screen. However, it is inappropriate for the purpose of performing multi-level H adjustment by converting the beam diameter on a pixel-by-pixel basis.

In view of the above-mentioned circumstances, the present invention provides a shutter speed corresponding to the pixel dot formation interval in units of one pixel, that is, Io.
An object of the present invention is to provide a laser beam diameter control device capable of converting a laser beam diameter having a shutter speed on the order of nanoseconds.

In order to solve the above-mentioned problem, the laser beam diameter control device of the present invention for the title 0 focuses a laser beam emitted from a laser diode with a collimating lens and narrows the beam diameter with an aperture. The above-mentioned aperture of the optical device is arranged in front and behind the laser beam traveling direction, with first and second polarizing plates whose shoulder surfaces differ from each other by 90 degrees, and between them, and with a control voltage application terminal, which extends in the beam scanning direction. A light-shielding plate having a first slit and a second slit slightly wider in the sub-scanning direction than the range of the first slit is overlapped, and by applying a voltage, the shoulder surfaces of the incident light and the output light are set at 90 degrees. It consists of a rotating electro-optic material plate, and is arranged so that the direction of the light control surface when no voltage is applied coincides with the shoulder surface of the first polarizing plate. The second optical signal plate is characterized in that a third slit having substantially the same shape as the first slit is provided on the same optical axis.

Visit-1 Since the aperture is configured as described above, when no voltage is applied to the M optical element, there is no polarizing effect of the electro-optic material, so light in a certain polarization axis direction that passes through the first polarizing plate is After passing through the area between the first slit and the second slit, the area outside the third slit is blocked by the second m light beam whose polarization plane differs by 90 degrees, and the light falls into the area of the first slit. Equal third slit range'! t 3ai 'Ii
and becomes the emitted light.

On the other hand, when a voltage is applied to the In optical element, the direction of the polarization plane of the electro-optic material is rotated by 90 degrees, so that the shoulder surface on the incident side coincides with the shoulder surface of the first polarizing plate, and the shoulder surface on the output side coincides with the shoulder surface of the first polarizing plate. is the second
This corresponds to the direction of the polarization plane of the extinction plate. Therefore, the light rays emitted from the first polarizing plate are the same as those of the first polarizing plate. It passes through the range of the second slit, passes through the range corresponding to the third slit of the second polarizing plate, and the second slit outside of the third slit and exits.6 Therefore, when no voltage is applied, the aperture is narrow; The aperture widens when voltage is applied.

The polarization effect of the electro-optic material due to voltage application is switched at the same time as voltage application, approximately on the order of 10 ns.

Circle 11 Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

FIG. 1 is a diagram showing the configuration of a writing optical character device of a laser printer or digital copying machine equipped with a variable aperture 12 according to the present invention.

Divergent light emitted from a laser diode (LD) which is a light source is focused by a kwamate lens 11'''C' and converted into parallel light. The beam enters the cylindrical lens 15 with a limited width, thereby further narrowing down the beam width in the sub-scanning direction.The beam passing through this is incident on the mirror surface of a rotating polygon mirror 16 that is rotated at a constant high speed by a motor 17. The reflected light passes through the F-θ lens 18.19, is reflected by the first mirror 23 and the second mirror 20, and is reflected by the photoreceptor belt 24.
Light is scanned by illuminating a predetermined width for each mirror surface of the rotating polygon mirror. The image plane of the irradiated light is flattened by the action of the F-theta lenses 18 and 19, the scanning speed on the photoreceptor becomes constant, and an image is formed on a straight line. Further, sub-scanning is performed by moving the photoreceptor belt 24 at a constant speed.

A part of the light beam near the end of the deflection range by the rotating polygon mirror 16 is reflected by the second mirror, and then passes through the third mirror 21, the fourth mirror 22, and the cylindrical cuff lens 14 to reach one end of the optical fiber 13. The light enters a photoelectric sensor (not shown) from the other end, and main scanning is synchronized by its output signal.

As can be seen from the above description, the slit width of the aperture 12, together with the cylindrical lens 15, determines the size of the dots formed on the photoreceptor in the sub-scanning direction.

Next, an embodiment of the variable aperture according to the present invention will be described in detail with reference to FIG.

The aperture 12 is constructed by overlapping a first polarizing plate 6, a polarizing element 7, and a second leading plate 8 in the laser beam traveling direction. In FIG. 2, these are shown separated from each other. The first polarizing plate 6 and the second polarizing plate 8 have polarization planes different from each other by 90°.

The intermediate polarizing element 7 is an electro-optic material fi3 that rotates the plane of polarization of the incident light and the incident light by 90' by applying a voltage.
Voltage applying electrodes 4.4' are provided at opposing ends of the electrodes, and a first slit 1 extending in the beam scanning direction is provided in the center, and the beam extends slightly beyond the range of the first slit in the sub-scanning direction (in the width direction of the slit). The light shielding plates 9 provided with the second slits 2 are stacked one on top of the other.

The second light plate 8 has the first slit 1 of the polarizing element 7.
A third slot 5 having approximately the same shape is provided on the same optical axis.

The first slit 2 may be simply cut out, or may be filled with a transparent material that has the same light transmittance as the electro-optic material plate 3 and has no polarizing property.

Therefore, when no voltage is applied between the voltage application electrodes 4 and 4', the electro-optic material plate 3 of the polarizing element 7 does not have a light-spreading effect and functions simply as a transparent plate. The incident light a having a random polarization plane passes through the first polarizing plate 6 so that only the light component with the polarization axis direction b passes through the first slit 1 of the polarizing element 7 and the second slit including the range on both sides in the direction of the burial mound.・Soto 2
The light passes through the range of , becomes light with a polarization plane in the same direction C as b, and enters the second a photoelectric element 8 . Since the groove direction of the second polarizing plate 8 is 90° different from the polarization direction of the first polarizing plate 6, i
The light emitted from the second slit 2 of the optical element 7 is emitted only from a narrow range of the third slit 5.

On the other hand, when a voltage is applied to the voltage application electrode 4.4'a of the first sunlight element 7, the polarization plane of the electro-optic material plate 3 rotates by 90'' between the incident side and the output side, and the light is emitted from the first compensator plate 6. The light beam in the polarization direction S becomes light in the polarization direction d rotated by 90 degrees from the polarization direction C from the range of the second slit 2 including the first slit and the range on both sides of the first slit in the width direction, as shown by the broken line in the figure. and enters the second polarizing plate 8. Since the polarization direction d matches the polarization direction of the 21st and 4th light plates 8, the light emitted from the second slit 2 of the first optical element 7 is directly converted into the first and second II light. The light beam passes through the front plate.Therefore, the width of the light beam becomes wider than when no voltage is applied.

As a result, the beam diameter in the sub-scanning direction changes depending on whether or not a voltage is applied, and the size of the beam spot, or even the image forming dot, can be varied. Adjustment is possible.

As an electro-optic material whose polarization plane rotates by 90° upon application of a voltage used in the present invention, lead lanthanum zirconate titanate (lead lanthanum zirconate titanate) is used.
onate LitanaLe (called PLZT by arranging the initials of the constituent element symbols) is famous. The optical properties of this material change upon application of voltage, mechanical compression, or tension. Other materials may also be used as the electro-optic material used in the present invention.

The change in optical properties caused by voltage application occurs almost instantaneously, making it extremely suitable as a variable aperture for laser optical devices that require a shutter speed on the order of 10 ns.

Circle-1 As described above, according to the present invention, it is possible to obtain dots having different diameters in the sub-scanning direction for each pixel, and it is possible to perform multi-gradation adjustment of an image. According to this method, since a different value can be output in the sub-scanning direction for each dot, there is an advantage that positional deviations are less likely to occur compared to a method using a combination of dots of the same diameter.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing the configuration of an example of an optical device for writing a laser beam image 11 with an aperture to which the present invention is applied;
FIG. 2 is an exploded perspective view showing the structure of an aperture according to an embodiment of the present invention. DESCRIPTION OF SYMBOLS 1... First slit, 2... Second slit, 3... Electro-optic material plate, 4.4'... Voltage application terminal, 5... Third slit. 6... 11th light plate, 7... Polarizing element, 8... Second polarizing plate, 9... Light blocking plate, 10... Laser diode, 11... Collimating lens, 12... Aperture , 16... Rotating polygon mirror, 18.19... F-θ lens, 24... Photoreceptor M 1 Figure

Claims (1)

[Claims] In a laser beam diameter control device for a laser optical device in which a laser beam emitted from a laser diode is condensed by a collimating lens and the beam diameter is narrowed by an aperture, the apertures are arranged in front and rear of the laser beam traveling direction, First and second polarizing plates having polarization planes different by 90 degrees, a control voltage application terminal disposed between them, and a first slit extending in the beam scanning direction are provided, and the first slit A light-shielding plate having a second slit slightly wider in the sub-scanning direction than the range of and a polarizing element arranged such that the direction of the polarization plane of the first slit coincides with the polarization plane of the first polarizing plate, and the second polarizing plate has a slit approximately on the same optical axis as the first slit. A laser beam diameter control device characterized in that a third slit of the same shape is provided.