JPH05294004A - Optical writing device - Google Patents

Abstract

PURPOSE:To make it possible to optically write gradation image having excellently stable gradation by providing a means for applying voltage selectively and a scanning means for scanning and exposing, on a photosensitive body, a laser beam passed through an electro-optical element. CONSTITUTION:A laser beam jetted from a semiconductor laser 1 is converted into a parallel light by a collimator lens 2 and injected into an electro-optical element 4 after it is converged by a cylindrical lens 3. The laser beam jetted from the electro-optical element 4 is converged by a condenser 5 and jetted on a scanning mirror 6. Thereafter, it is scanned and exposed along a direction of axis of rotation of a photosensitive drum 9. At this time, the laser beam is converged in the horizontal and vertical directions as a spot of a predetermined shape on the photosensitive drum 9 by an ftheta image-formation optical system consisting of an ftheta lens 7 and a cylindrical mirror 8. Further, it is scanned and exposed on the photosensitive drum 9 and optical writing corresponding to image information is performed on the photosensitive drum 9.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in an image recording apparatus such as a digital copying machine or a printer, and is intended to form an electrostatic latent image by selectively irradiating a photoconductor with light according to image information. The present invention relates to an optical writing device, in particular, an optical writing device that can obtain good latent image contrast and can express gradation.

[0002]

2. Description of the Related Art Conventionally, as the above-mentioned optical writing device, for example, there is one shown in FIG. In this optical writing device, a laser beam emitted from the semiconductor laser 100 is converted into parallel light by a collimator lens 101, and then is temporarily condensed on a scanning mirror 103 by a cylindrical lens 102 in the sub-scanning direction. Then, the condensed beam is scanned by the scanning mirror 103 along the rotation axis direction of the photoconductor drum 104, and is moved on the photoconductor drum 104 by the fθ imaging optical system including the fθ lens 105 and the cylindrical mirror 106. A spot having a uniform diameter is condensed in the main scanning direction and the sub scanning direction, and an image is written on the photosensitive drum 104.

The exposure profile on the photosensitive drum 104 by the spot is preferably rectangular as shown in FIG. 10 from the viewpoint of obtaining a good electrostatic latent image contrast, and is disclosed in Japanese Patent Application Laid-Open No. 62-257268. In some cases, as in the method disclosed in the publication, this is achieved by using a semiconductor laser having a plurality of light emitting portions and combining them on a focusing surface. However, as shown in FIG. 11, the exposure profile of a normal single laser beam can be approximated by a Gaussian distribution.

Further, in an image recording device using such an optical writing device, gradation recording is performed in order to improve the image quality of the image. A typical method is
As shown in FIG. 12, there is one in which the lighting time of the semiconductor laser is changed by controlling the pulse width to control the exposure area of one pixel for recording by the laser beam.

As such a technique, for example, there is one disclosed in Japanese Patent Laid-Open No. 62-39972.
The image processing apparatus according to Japanese Patent Laid-Open No. 62-39972 is an image processing apparatus which responds to a digital input signal, and includes a raster scanning print unit for generating a series of continuous scanning lines, and a raster scanning printing unit for the apparatus. A means for generating a pulse width modulated signal from the digital input signal; and means for supplying the pulse width modulated signal to the printing portion to generate each scanning line as a series of line segments from the printing portion. The length of the line segment is controlled in accordance with the pulse width modulation signal to generate a screen having a plurality of line segments with variable density from the line segment.

[0006]

However, the above-mentioned prior art has the following problems. That is, the above-mentioned conventional optical writing device has the photosensitive drum 104.
As shown in FIG. 11, the exposure profile of the above focused spot has a shape that can be approximated by a Gaussian distribution, so the potential of the electrostatic latent image formed on the photoconductor drum 104 is also a Gaussian distribution. Can be approximated in an inverted form. Therefore, when writing a thin line or the like having a width close to the spot diameter, there is a problem that the edge of the latent image may not be sharp or the line width may be uneven due to fluctuations in the development level. It was a thing.

Further, as a conventional gradation recording method by exposure time modulation using a beam having a Gaussian profile, as disclosed in the above-mentioned JP-A-62-39972, a reference triangular wave and an image are used. It is known to generate a pulse width modulation signal by comparing gradation signals. The exposure with the pulse-width modulated spot can form a latent image close to an area gradation by binary values which is stable for the electrophotographic process, when the area larger than the spot diameter is exposed. However, when the pulse width becomes extremely short, the exposure by the spot reflects the instability with respect to the response characteristics of the drive circuit of the semiconductor laser which is the light source and the fluctuation near the threshold in the electrophotographic process.
There is a problem in that the intensity modulation behavior is displayed rather than the area modulation behavior, and gradation reproduction becomes unstable. This instability in analog gradation reproduction in the electrophotographic process can be solved by making the exposure spot size infinitely small, but in reality there is a limit to the optical system. It is impossible to do this.

In order to prevent the unevenness of the line width due to the above-mentioned sharpness of the edge of the latent image and the variation of the development level, the semiconductor laser light source having a plurality of light emitting portions as described above is used to synthesize the profile into a rectangular shape. It is possible.

However, in this case, since the position of the light emitting portion formed on the semiconductor laser is constant, once the image forming optical system is determined and aligned, the position of the focused spot on the photosensitive drum is determined. Is also specified. Therefore, there is a new problem that the shape of the composite profile cannot be changed arbitrarily and gradation recording cannot be performed by adjusting the spot width of the composite profile.

Further, as a method for performing gradation recording with high image quality and excellent stability, as disclosed in JP-A-60-68317, JP-A1-262519, and the like, Several techniques for changing the spot diameter of the laser beam have also been proposed.

An optical scanning device for a laser beam printer disclosed in Japanese Patent Laid-Open No. 60-68317 discloses a light source for emitting a divergent laser beam modulated by a video signal, and a divergent laser beam emitted from the light source. A light beam converting means for converting into a parallel laser beam, a deflecting means for deflecting the laser beam from the light beam converting means, an imaging optical system for imaging the laser beam from the deflecting means, and the imaging In an optical scanning device of a laser beam printer including an image carrier on which the laser beam is imaged by an optical system, a beam diameter control means for controlling the diameter of the parallel laser beam incident on the deflection means is provided. It is composed.

Further, the image forming apparatus disclosed in Japanese Patent Laid-Open No. 1-262519 is an image forming apparatus which forms a latent image on a photosensitive member by irradiation of a laser beam, and a predetermined image is formed in the optical path of the laser beam. The movable slit for blocking the optical path is arranged at the cycle.

However, these proposals require a mechanical driving method such as moving the collimator lens or providing a movable slit in the optical path to switch the beam diameter, so that each pixel is modulated. It has a problem that it is not suitable for high-speed operation of several tens of MHz.

Therefore, the present invention has been made to solve the above-mentioned problems of the prior art, and its object is to provide an exposure profile having a sharp edge for obtaining a latent image having good contrast. It is an object of the present invention to provide an optical writing device capable of forming a spot image of the exposure profile at high speed for each pixel and rapidly writing a gradation image having excellent gradation stability.

[0015]

The above objects of the present invention are as follows.
It is achieved by the following configuration.

That is, according to the present invention, optical writing is performed to scan and expose a laser beam on / off controlled by image information on a photoconductor to form an electrostatic latent image on the photoconductor according to the image information. In the device, an electro-optical element having a plurality of electrodes arranged in the optical path of the laser beam, disposed in at least a part of a spatial region through which the laser beam passes, and arranged along the optical path of the laser beam, A voltage applying unit that selectively applies a voltage to a plurality of electrodes according to a gradation signal of image information, and a scanning unit that scans and exposes a laser beam that has passed through the electro-optical element onto a photosensitive member are provided. It is composed.

A voltage grating is applied to the electrode of the electro-optical element in a predetermined pattern by a voltage applying means to form a phase grating in the electro-optical element by utilizing a periodic change in the refractive index.

The beam incident on the electro-optical element acting as the phase grating enters from the end of the electro-optical element to the inside, is totally reflected on the inner surface of the crystal in which the phase grating is formed, and is again outside the element. The beam path is formed so as to exit to.

Further, the beam incident on the electro-optical element acting as the phase grating is guided into the waveguide formed on the substrate of the electro-optical element and passes through the phase grating formed on the waveguide. Alternatively, the beam path may be configured.

[0020]

According to the present invention, the electro-optical element having a plurality of electrodes arranged in the optical path of the laser beam, disposed in at least a part of the spatial region through which the laser beam passes, and arranged along the optical path of the laser beam. And a voltage applying unit that selectively applies a voltage to the plurality of electrodes according to a gradation signal of image information, and a scanning unit that scans and exposes a laser beam that has passed through the electro-optical element onto a photoconductor. As described above, by applying a voltage to the electrode of the electro-optical element in a predetermined pattern by the voltage applying means, a phase grating utilizing a periodic change in the refractive index is formed in the electro-optical element. be able to. for that reason,
The laser beam incident on the electro-optical element is diffracted by the phase grating whose refractive index changes periodically, and the electro-optical element emits multi-order diffracted light and zero-order light beam. The multi-order diffracted light and the zero-order light beam are scanned and exposed on the photoconductor by the scanning means,
Multi-order diffracted light diffracted at a predetermined angle and 0
It is possible to irradiate a laser beam in which a subsequent light beam is superposed, and to form an exposure profile of the beam in a shape with a sharp edge.

Further, by changing the pattern of the voltage applied to the electrodes of the electro-optical element according to the gradation signal of the image information, the spot diameter of the exposure profile is modulated at a high speed for each pixel to obtain the gradation. Optical writing of a gradation image having excellent stability can be performed.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on the illustrated embodiments.

FIG. 1 shows an embodiment of the optical writing device according to the present invention.

In the figure, reference numeral 1 denotes a semiconductor laser as a laser light source.
For example, those having an emission wavelength of about 780 nm are used. The laser beam emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2 and then condensed by the cylindrical lens 3 in the sub-scanning direction of the beam. The condensed laser beam enters from the mirror-polished side surface 4a of the electro-optical element 4 at a certain angle, and is most beamed on the inner surface 4b in the electro-optical element 4 where the phase grating described later is formed. The diameter is reduced, and after being totally reflected by the inner surface 4b, the light is emitted from the other side surface 4c which is also mirror-polished.

The laser beam emitted from the electro-optical element 4 is condensed by the condenser lens 5 and irradiated on the scanning mirror 6, and the scanning mirror 6 scans the photosensitive drum 9 in the direction of the rotation axis. Exposed. that time,
The laser beam is focused as a spot of a predetermined shape on the photosensitive drum 9 in the main scanning direction and the sub-scanning direction by the fθ imaging optical system including the fθ lens 7 and the cylindrical mirror 8, and scanning exposure is performed on the photosensitive drum 9. Then, the optical writing according to the image information is performed on the photosensitive drum 9.

By the way, in this embodiment, a plurality of electrodes disposed in the optical path of the laser beam are arranged in at least a part of the spatial region through which the laser beam passes, and are arranged along the optical path of the laser beam. An electro-optical element having the same, a voltage applying means for selectively applying a voltage to the plurality of electrodes according to a gradation signal of image information, and a laser beam that has passed through the electro-optical element are scanned and exposed on the photoconductor. And scanning means.

That is, the electro-optical element 4 is arranged between the cylindrical lens 3 and the condenser lens 5 in the optical path of the laser beam so as to include all the spatial regions through which the laser beam passes. .. As a material of the electro-optical element 4, for example, a crystal of LiNbO ₃ which is a crystal having a first-order electro-optical effect (Pockels effect) is used. However, in addition to this, the electro-optical element 4
Examples of the material include LiTaO ₃ , BaTiO ₃ , and KH ₂ PO having a first-order electro-optical effect (Pockels effect).
Crystals such as ₄ (KHP), KD ₂ PO ₄ (KDP) and ZnO are used.

The crystal forming the electro-optical element 4 is formed in a relatively thin rectangular parallelepiped shape as shown in FIG. The electro-optical element 4 is arranged so that its longitudinal direction is orthogonal to the optical path of the laser beam.
As described above, the laser beam focused by the cylindrical lens 5 is incident on the electro-optical element 4 from the mirror-polished side surface 4a at an angle.
The beam diameter is most narrowed on an inner surface 4b, which is a region where a phase grating described later is formed, inside the electro-optical element 4, and after the inner surface b is totally reflected, the other side surface c is also mirror-polished.
Is emitted from.

The principal plane orientation of the crystal forming the electro-optical element 4 is the Y plane, as shown in FIG. 3, and on the lower outer surface 4b 'corresponding to the Y plane of the electro-optical element 4, A plurality of comb-shaped aluminum electrodes 10, 10, ... For forming an electric field are formed along the Z-axis direction. These aluminum electrodes 10, 10, ... Are formed along the optical path direction of the laser beam, and are arranged in parallel with each other at a predetermined interval. The aluminum electrodes 10, 10 ...
Are arranged with a width of 10 μm and a pitch of 20 μm, for example. Now, the diameter of the incident laser beam is about 2.5
.. mm, the total number of electrodes 10, 10 ... Is set to 128 in order to match the width of the region forming the phase grating.

The aluminum electrodes 10, 10 ... Are formed by depositing an aluminum thin film to a uniform thickness on a LiNbO ₃ substrate forming the electro-optical element 4, and then patterning the deposited aluminum thin film by photolithography. Then, it is manufactured by etching. The aluminum electrodes 10, 10 ... Produced as described above are independently drawn out via lead lines 11, 11 ... As shown in FIG. 3, so that a voltage can be applied individually. These lead wires 11, 11, ... Are connected to an FPC (Flexible Pr) (not shown) by means such as thermocompression bonding.
An integrated circuit) and is connected to the voltage applying means 12 via an FPC (not shown). A voltage of a pattern according to a gradation signal of image information is selectively applied to each of the aluminum electrodes 10, 10 ...

With the above structure, the optical writing apparatus according to this embodiment performs optical writing on the photosensitive drum as follows. That is, as shown in FIG. 1, the laser beam emitted from the semiconductor laser 1 is converted into parallel light by the collimator lens 2 and then condensed by the cylindrical lens 3 in the beam sub-scanning direction. This condensed beam, after passing through the electro-optical element 4, is condensed by the condenser lens 5 to be scanned by the scanning mirror 6.
It is irradiated to the upper side, and is scanned and exposed by the scanning mirror 6 along the rotation axis direction of the photoconductor drum 9 through the fθ imaging optical system including the fθ lens 7 and the cylindrical mirror 8.
Optical writing according to image information is performed on the photosensitive drum 9.

At this time, as shown in FIG. 2, after the laser beam is incident on the electro-optical element 4 along the X-axis direction of the crystal, aluminum electrodes 10, 10 ... Are formed on the laser beam. Total reflection is performed on the surface 4b, which is just inside one of the surfaces of the crystal that is parallel to the X-Z axis plane. The polarization direction of the beam incident on the crystal forming the electro-optical element 4 is parallel to the Z-axis direction.

The behavior of the laser beam in the electro-optical element 4 will be described below. The angle condition for the laser beam incident on the electro-optical element 4 to be totally reflected on the inner surface 4b thereof is determined by the refractive index (n = 2.2) of LiNbO ₃ . In this case, the internal reflection surface 4 of the electro-optical element 4
The incident angle θ on b is θ = sin ⁻¹ (1 / 2.2) = 27
If it is deg or more, there is no light loss and total reflection occurs. Therefore, the incident angle θ on the internal reflection surface 4b of the electro-optical element 4 is set to 27 deg or more.

The aluminum electrodes 10, 10 ... Formed on the lower outer surface 4b 'of the electro-optical element 4 correspond to the gradation signals of the pixel currently being optically written by the voltage applying means 12, respectively. The voltage of the pattern is selectively applied.

Now, the aluminum electrodes 10, 10 ...
As shown in FIG. 4, when positive (+) and negative (-) voltages are applied alternately, the electro-optical element 4 is arranged in the Z direction at an interval equal to the arrangement pitch P of the aluminum electrodes 10, 10 ,. An electric field E is formed toward. In the portion of the electro-optical element 4 where the electric field E is formed, the refractive index changes along the Z direction due to the first-order electro-optical effect. This change in the refractive index does not occur on the electrode surface where the electric field is not applied, but only between the electrodes, so that the grid of the refractive index change due to the electric field is formed at a cycle according to the pitch of the grid where the electric field is formed. A phase change having this period is generated in the laser beam passing through the. That is, the electro-optical element 4 acts as a phase grating.

Therefore, diffraction occurs in the laser beam that has passed through the electro-optical element 4. When the laser beam is incident in parallel along the electrodes 10, 10, ..., Raman-Nass diffraction occurs, and the laser beam goes on both sides of the 0th-order light that travels straight, the 1st-order light, the 2nd-order light ,. Diffracted light is generated. At that time, when the semiconductor laser 1 is used as the light source, most of the multi-order light is the primary light component. The diffraction angle θ of the m-th order diffracted light has the following relationship with the pitch Λ of the phase grating. sin θ = mλ / Λ (m is the order of light) (1)

Here, λ is the wavelength of the laser light.
Now, assuming that the voltage pattern applied to the electrodes 10, 10, ... Is set so that + voltage and 0 V are alternately applied as shown in FIG. 4, the pitch Λ of the formed phase grating is
The pitch of 0, 10 ... Is equal to 20 μm. When the diffraction angle θ of the primary light at this time is obtained, θ = sin ⁻¹ (0.78 / 20) is obtained. Further, as shown in FIG. 5, when a voltage of + and 0 V are applied alternately to every two adjacent electrodes, the pitch Λ of the formed phase grating is 40 μm, which is twice the electrode pitch. When the diffraction angle θ of the primary light at this time is similarly obtained from the equation (1), it is 1.12 deg.
Similarly, the voltage pattern applied to the electrodes can be controlled to change the pitch of the phase grating formed.

When forming a phase grating having the same pitch in all the regions through which the laser beam passes, specifically, a phase grating having an integer multiple of the electrode pitch can be prepared on the basis of an electrode pitch of 20 μm, and the maximum. , The total number of electrodes 128 is divided into three regions, and + voltage and 0V are alternately assigned to the respective regions, the pitch is 850 μm
Phase gratings up to the degree can be obtained.

In this case, the diffraction angle of the first-order light is found to be about 0.0526 deg from the equation (1). FIG. 6 shows the relationship between the pitch of the phase grating that can be formed in this embodiment and the diffraction angle of the primary light obtained thereby. The phase grating pitch is 8
The change in the diffraction angle when changing by about 20 μm in the vicinity of 50 μm is very subtle. For example, the diffraction angle is 0.0532 deg at a pitch of 840 μm, and the diffraction angle is 0.0520 deg at a pitch of 860 μm. The difference between the diffraction angles is, for example, that the 0th-order light condensed on the photosensitive drum 9 and the 1st-order light
The difference in the spot position of the next light varies depending on the focal length of the condensing system, but is on the order of several μm.
An exposure profile close to a rectangle for obtaining good contrast can be obtained by subtly changing the overlapping manner of the 0th-order light and the 1st-order light.

It is apparent that the diffraction angle of the primary light can be delicately changed digitally by controlling the input voltage pattern to the electrodes, that is, the pitch of the phase grating by the above method.

As a result, on the photosensitive drum 9, 0
The position at which the secondary light and the primary light are focused is determined by the diffraction angle θ of the diffracted light and the focal length of the focusing optical system, as schematically shown in FIG. 7A. At this time, the larger the diffraction angle θ,
On the photoconductor drum 9, the degree of diffraction and the diffraction angle are adjusted so that the combined profile in which the 0th-order light profile and the 1st-order light profile are separated becomes a shape close to a rectangle. Further, according to the gradation signal, as shown in FIGS. 7B to 7E, it is possible to control how the 0th-order light and the 1st-order light overlap with each other to change the spot diameter of the combined exposure profile. At this time, it is possible to obtain stable area gradation gradation expression for the electrophotographic process.

In the electro-optical element 4 of the above-mentioned embodiment, the electrodes 10, 10 ... Are formed on the outer side surface 4b ', and the width of the region functioning as a phase grating is set to 2. It is set to about 5 μm. Therefore, the maximum value of the grating pitch that can be obtained is for all electrodes 1
When 0, 10, ... Are divided into three regions, and are limited to about 850 μm, which is ⅓ of the total width.
When the incident beam is expanded and used by a beam expander or the like, it is also possible to form a device in which the width of the region formed by the electrode is widened to form a phase grating with a larger pitch. In such a case, the diffraction angle of the primary light of the laser beam by the electro-optical element 4 can be greatly reduced.

In the above embodiment, the electrodes 10, 10 ...
The pitch is set to 20 μm, but by further reducing the pitch, the resolution regarding pitch control can be increased, and more delicate control of the diffraction angle can be made possible.

Second Embodiment FIG. 8 shows a second embodiment of the present invention. The same parts as those in the above embodiment are designated by the same reference numerals,
The electro-optical element 4 serving as the phase grating described in the first embodiment causes a beam to enter the bulk LiNbO ₃ single crystal and causes total reflection in a region near the electrode surface where the phase grating is formed. There is a device that causes diffraction, but basically it is not limited to this as long as it is a material and structure of an electro-optical element that can cause diffraction by a phase grating whose grating pitch can be changed.

For example, as shown in FIG. 8, LiTaO ₃
A beam is guided to an optical waveguide 20 formed by diffusing Nb in LiTaO ₃ on a substrate, and a grayscale signal is applied to the interdigital electrodes 10, 10 ... Produced by patterning an aluminum thin film on the optical waveguide by photolithography technology. By applying a voltage with a compliant electrode pattern, the pitch of the phase grating is controlled, a desired diffraction angle is obtained, and the beam diameter of the combined profile of 0th-order light and 1st-order light is changed. good.

The other structure and operation are the same as those of the first embodiment, and therefore the description thereof is omitted.

[0047]

According to the present invention, which has the above-described structure and operation, it is possible to form an exposure profile having a sharp edge for obtaining a latent image having a good contrast, and a spot diameter of this exposure profile. It is possible to provide an optical writing device capable of optically writing a gradation image excellent in gradation stability by modulating each pixel at high speed.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical writing device according to the present invention.

FIG. 2 is a perspective view showing an electro-optical element.

FIG. 3 is a rear view showing the electro-optical element.

FIG. 4 is an explanatory diagram showing the operation of the electro-optical element.

FIG. 5 is an explanatory view showing the operation of the electro-optical element.

FIG. 6 is a graph showing the relationship between the pitch of the phase grating formed in the electro-optical element and the diffraction angle of the primary light.

7 (a) to 7 (e) are an optical path explanatory diagram and an explanatory diagram of a combined profile showing an irradiation state of a laser beam diffracted by an electro-optical element, respectively.

FIG. 8 is a configuration diagram of an electro-optical element showing another embodiment of the optical writing device according to the present invention.

FIG. 9 is a block diagram showing a conventional optical writing device.

FIG. 10 is an explanatory diagram showing an exposure profile.

FIG. 11 is an explanatory diagram showing an exposure profile.

FIG. 12 is an explanatory diagram showing a conventional gradation expression method.

[Explanation of symbols]

1 semiconductor laser, 2 collimator lens, 3 cylindrical lens, 4 electro-optical element, 5 condenser lens,
6 scanning mirror, 7 fθ lens, 8 cylindrical mirror, 9 photosensitive drum, 10 electrodes, 12 voltage applying means.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁵ Identification number Office reference number FI technical display location G02F 1/03 503 H04N 1/04 104 A 7251-5C 1/23 103 B 9186-5C

Claims (1)

[Claims] 1. An optical writing device for scanning and exposing a laser beam on / off controlled by image information onto a photoconductor to form an electrostatic latent image according to the image information on the photoconductor. An electro-optical element having a plurality of electrodes arranged in at least a part of a space region through which the laser beam passes, in the optical path of the laser beam, and the plurality of electrodes. It is characterized by further comprising: a voltage applying means for selectively applying a voltage in accordance with a gradation signal of image information; and a scanning means for scanning and exposing a laser beam having passed through the electro-optical element onto a photoconductor. Optical writing device.