JP3156305B2 - Optical writing device and gradation image recording device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical writing apparatus for selectively irradiating a photosensitive member such as a digital copying machine or a printer with light to form an electrostatic latent image, and in particular, to perform gradation expression. For changing the intensity of a laser beam, and a gradation image recording apparatus using the same.

[0002]

2. Description of the Related Art FIG. 11 is an explanatory view of a schematic configuration of a conventional optical writing device. The beam emitted from the semiconductor laser 1 is collimated by the collimator lens 2 and is condensed on the polygon scanner 7 by the cylindrical lens 3 in the sub-scanning direction. The condensed beam is scanned by the polygon scanner 7 along the rotation axis of the photoreceptor 10, and the fθ imaging optical system (fθ lens 8,
The light is condensed as spots at uniform intervals on the photoreceptor 10 by the cylindrical mirror 9) to form an electrostatic latent image.

In recording an image using such an optical writing device, not only binary recording but also gradation recording is performed to improve the image quality of the image. The method is shown in FIG.
As shown in FIG. 2, one pixel is constituted by a matrix of a plurality of spots having binary outputs, and the number of ON spots in the pixel and the pixel size are changed according to an image to express a pseudo halftone. (See, for example,
As shown in FIG. 13, the emission time of a semiconductor laser is controlled (see, for example, JP-A-62-3997).
No. 2), an example in which the drive current of a semiconductor laser is controlled to change the optical output (Japanese Patent Laid-Open No. 60-208165) is known.

[0004]

These conventional gradation recording methods have the following problems, respectively. First, a method of forming one pixel by a plurality of spots is generally called a dither method or a density pattern method. However, when the number of spots in one pixel is increased to increase the number of gradations, the resolution of an image is increased. Will go down. It is necessary to reduce the spot size in order to make the number of gradations and the resolution compatible, but in order to reduce the spot diameter, the numerical aperture N of the condenser lens must be reduced.
Since A needs to be large, it leads to an increase in the cost of the optical system. In addition, there is a limit from the shape of the far field pattern of the LD of the light source and the restrictions on the light source system space. Also, even if the spot is made smaller and its density is increased, the rotation speed of the polygon scanner and the data transfer speed must also be increased.

A method of controlling the light emission time of a semiconductor laser is to compare a reference triangular wave with an image signal and to modulate a pulse width modulation signal as disclosed in Japanese Patent Application Laid-Open No. 62-39972. What produces it is known.
The spot formed by the pulse-width-modulated beam exhibits an area gradation behavior of exposure on the photoconductor. However, when the pulse width is shortened, the exposure by the spot shows an intensity modulation behavior rather than an area gradation behavior, non-linearity is generated in an output gradation with respect to an input, and an effective number of gradations is reduced. Therefore, the spot size must be as small as possible. However, an optical system for reducing the spot size as much as possible is required, but this also leads to an increase in cost and has technical limitations.

Further, when the light output is changed by controlling the driving current of the semiconductor laser, a delicate control is required because the change in the light output with respect to the current is large due to the characteristics of the semiconductor laser. Fluctuations in output must also be taken into account, making stable gradation control difficult.

As another example, as disclosed in Japanese Patent Application Laid-Open No. 61-62276, there is an example in which a plurality of beams having different outputs are superimposed to express gradation, but the optical system becomes complicated and the number of gradations is increased. In order to increase the number of beams, it is necessary to increase the number of beams, and there is a disadvantage that it is difficult to align the plurality of beams and the cost of the optical system is increased.

The present invention has been made in view of these circumstances, and does not reduce the resolution of writing, does not increase the transfer speed of image signals or the rotation speed of a polygon scanner, and does not increase the optical system. It is an object of the present invention to provide a gradation image optical writing device which is excellent in the number of gradations and the stability of the gradation without requiring an increase in cost, and a gradation image recording device provided with the same.

[0009]

In order to achieve the above object, the invention according to claim 1 is to scan a laser beam whose on / off is controlled by an image signal, focus the laser beam on a photoreceptor, and statically scan the laser beam. In an optical writing device that forms an electrostatic latent image, a phase grating that can selectively control the generation and disappearance of a grating based on an image gradation signal in a part or all of a space area where a laser beam passes is provided in an optical path of the laser beam. And performing intensity modulation of a beam passing through the controlled phase grating. According to a second aspect of the present invention, in the first aspect, the phase grating is a phase grating formed in a crystal having an electro-optic effect and having a periodically changing refractive index. According to a third aspect of the present invention, there is provided a gradation image recording apparatus including the optical writing device according to the first or second aspect.

Also, the laser beam incident on the phase grating is incident on the inside of the crystal from the end of the electro-optic crystal, is totally reflected on the inner surface of the crystal on which the phase grating is formed, and is emitted again outside the crystal. , A laser beam path can be configured.

Further, the beam incident on the phase grating is:
Guided into a waveguide formed on a substrate of electro-optic crystal,
The laser beam path can be configured to pass through a phase grating formed on the waveguide.

As the phase grating used in the present invention, a crystal having an electro-optical effect or a crystal having a magneto-optical effect is used.

[0013]

The beam emitted from the laser light source is collimated and then enters the phase grating. The generation and disappearance of the phase grating can be controlled in accordance with the gradation signal obtained from the image information. Therefore, the beam incident on the phase grating passes through the phase grating generated in accordance with the gradation signal. The beam passing through this phase grating is split into multi-order diffracted light and zero-order light,
The respective intensities are modulated according to the degree of diffraction (that is, the proportion of the area that undergoes a phase change). The intensity-modulated beam is condensed as a spot while scanning the photosensitive member by the scanning means and the condensing means. As a result, writing of an image capable of gradation expression is performed.

The maximum diffraction efficiency can be obtained by injecting the laser beam so that the polarization direction of the laser beam incident on the phase grating is parallel to the diffraction direction.

According to the spatial intensity distribution of the laser beam, the number of electrodes for simultaneously applying a voltage among the electrodes forming the phase grating is changed for each position where the laser beam passes, and the positions of the electrode and the laser beam are changed. The light intensity can be modulated at a uniform rate regardless of the above.

Further, the voltage applied to the electrodes forming the phase grating is changed for each position through which the laser beam passes according to the spatial intensity distribution of the laser beam, and the light intensity is similarly modulated at a uniform rate. Can be.

[0017]

【Example】

Hereinafter, specific embodiments of the present invention will be described with reference to the drawings. Example 1 A device using an electro-optic crystal as a phase grating and applying total reflection of a beam inside the crystal will be described. FIG.
1 shows a schematic configuration diagram of an optical writing device according to the present embodiment.
The laser light source 1 is a semiconductor laser having an emission wavelength of, for example, about 780 nm, and the emitted beam is collimated by the collimator lens 2. The collimated beam is converged in one direction by the cylindrical lens 3. The condensed beam enters at a certain angle from the mirror-polished side surface 4a of the device 4 made of an electro-optic crystal, and the beam diameter is most narrowed on the inner surface 4b corresponding to a region where the phase grating is formed inside the device 4. The light is totally reflected by the inner surface 4b and is emitted from the other side surface 4c which is also mirror-polished. The structure of the device 4 will be described in detail below.

The material of the device 4 is LiNbO ₃ which is a crystal having a primary electro-optic effect (Pockels effect). In addition, as the crystal having the Pockels effect, LiTaO ₃ can also be used.

The principal plane orientation of the crystal of the device 4 is the Y plane, and as shown in FIG. 2A, a cross finger (or comb) may be applied so that an electric field is applied to the Y plane in the Z-axis direction. Is formed with a width of 10 μm and a pitch of 20 μm. Here, the number of the electrodes 12 is 128, and a voltage of 0 volt is applied to every other electrode 12a (64),
A voltage corresponding to the gradation signal of the image is applied to the other electrodes 12b (64). The 64 electrodes 12b to which the voltage is applied are independently drawn out via the lead lines 13 shown in FIG. 2B so that a voltage can be individually applied. Of course, if the voltage is extracted so that a voltage can be individually applied to all 128 electrodes 12, the voltage application pattern is controlled so that an electric field is generated between adjacent electrodes.
A structure in which eight gratings are turned on / off can also be used. Each of the extracted wirings 13 is connected to a flexible printed circuit (FPC) by thermocompression bonding (not shown).
, And a unique gradation signal is input to each.
The electrode 12 was prepared by patterning and etching an aluminum thin film deposited on a LiNbO ₃ substrate by photolithography.

As shown in FIG. 2B, the beam incident along the X-axis direction of the crystal of the device 4 is
2 is totally reflected on the surface just inside one of the surfaces parallel to the XZ axis plane of the crystal in which it is formed. The polarization direction of the beam incident on the crystal is adjusted so as to be parallel to the Z-axis direction.

Hereinafter, the behavior of the beam in the crystal of the device 4 will be described. The angle condition when the incident beam is totally reflected on the inner surface of the crystal is the refractive index of LiNbO ₃ (n =
2.2). In this case, if the incident angle θ ₁ of the crystal internal reflection surface is θ ₁ ≧ sin ⁻¹ (1 / 2.2) = 27 deg, there is no light loss and total reflection occurs.

When a voltage is applied to the electrode 12 corresponding to the gradation signal, an electric field as shown in FIG. 3 is formed in the vicinity of the plane on which the electrode 12 is formed inside the crystal. The portion where the electric field is formed causes a change in the refractive index of the incident light in the Z-axis direction due to the electro-optic effect. This change in the refractive index does not occur near the electrodes 12 where no electric field is applied, but only between the electrodes 12 to which a voltage is applied. Then, the voltage application electrode 12
A grating of refractive index change due to the electric field is formed at a period corresponding to the pitch of b, and a beam passing therethrough causes a phase change having this period. That is, the voltage application electrode 12
b acts as a phase grating.

The beam that has passed through the phase grating causes diffraction. When the beam enters this phase grating at an angle that satisfies the Bragg condition described later, Bragg diffraction occurs, and the outgoing light is split into only the 0th-order light and the 1st-order light (see FIG. 10). Further, the beam is parallel to the X-axis direction which is the same direction as the direction along the electrode 12 (when projected on the XZ-axis plane).
Incident on the surface, Raman-Nass diffraction occurs, and the beam
In addition to the primary light and the primary light, a secondary light or more multi-order light is also generated. The degree of these diffractions is digitally changed by a gradation signal input via the FPC, and the intensity of the beam is modulated.

Hereinafter, a case where Raman-Nass diffraction is generated by the device 4 to modulate the intensity of the laser beam will be described. In this device 4, the efficiency of diffraction is controlled by adjusting the number of electrodes 12b to which a voltage is applied. That is, the beam incident on the vicinity of the electrode 12b to which the voltage is applied is diffracted by the grating generated there, and is divided into a multi-order light component and a zero-order light component.
Since the beam near b passes through without being changed in phase,
Only the zero-order light component is present. The multi-order light component at this time or 0
The next light component is used for modulation. Hereinafter, a specific procedure for the modulation using the zero-order light component will be described.

First, as shown in FIG. 4, a voltage is applied to all the electrodes 12b of the device 4 (every other electrode 12b (64 of the interdigital electrodes 12)), and a voltage is applied between the electrodes. After an electric field is generated and the incident laser beam is diffracted, the 0th-order light and the multi-order light are imaged and separated by the condenser lens 5. Here, the multi-order light is cut by the light shielding member 6 so that only the zero-order light can pass. Next, a voltage at which the light intensity of the zero-order light becomes 0 is set as a drive voltage (several tens of volts). If the number of electrodes 12b to which a voltage is applied is reduced in this state, the intensity of the zero-order light changes in digital steps and increases according to the number.

Incidentally, the spatial intensity distribution of the incident laser beam can be approximated to a normal Gaussian distribution as shown in FIG. Therefore, when the relative position of the beam and the grating changes, the rate of change in the intensity of the beam as a result of diffracting one grating differs.

That is, in the case of the present embodiment, of the 64 electrodes 12b, the voltage applying electrodes 12b at the electrodes 12b located at both ends (for example, near the electrodes No. 0 to 10 and near the electrodes No. 55 to 64). The effect on the beam intensity change is smaller than that of the electrode 12b (for example, near the electrodes Nos. 20 to 44) located at the center. Therefore, as shown in FIG. 0 to 10 and the electrode NO. The change in the light intensity with respect to the change in the phase grating due to the voltage application electrode around 55 to 64 is small, but in other regions, the change in the light intensity with respect to the change in the phase grating due to the voltage application electrode is almost linear.

According to the relationship shown in FIG. 6, the arrangement position and the number of the electrodes 12b to which a voltage is applied to obtain a desired light intensity can be determined in advance, and the light intensity can be adjusted so as to change in equal steps. . That is, the central electrode 12
When the voltage application is adjusted using only b, the light intensity can be changed almost in proportion to the number of electrodes to which the voltage is applied. Further, in order to change the light intensity evenly and stepwise using the electrodes 12b at both ends, it is necessary to change the number of electrodes 12b larger than the number of electrodes 12b at the center. Conversely, in order to slightly change the light intensity, the number of gradation steps can be increased by adjusting the number of electrodes 12b at both ends.

As described above, a desired light intensity can be obtained by applying a voltage to a predetermined electrode group in accordance with the gradation signal.

Embodiment 2 In Embodiment 1, Raman-Nass diffraction is generated, and 0
The intensity was modulated using the next light. Here, intensity modulation using multi-order light will be described as a second embodiment. FIG. 7 schematically shows the configuration. In the same manner, a laser beam is incident on the same device 4 as that described in the first embodiment to generate diffraction. The condensing lens 5 images and separates the 0th-order light and the multi-order light. Here, the 0-order light is blocked by the light shielding member 11.
So that only multi-order light can pass through. The relative intensity of the multi-order light becomes 0 when no voltage is applied to each electrode 12b, increases with the applied voltage, and reaches a maximum at a certain voltage. If the number of electrodes to which a voltage is applied is changed in this state, the intensity of the multi-order light changes in digital steps according to the number of electrodes as in the first embodiment.

The behavior shows a curve relationship with the zero-order light described in the first embodiment, which has exactly the opposite slope. An electrode 1 for applying a voltage to obtain a desired light intensity according to the spatial intensity distribution (Gaussian distribution) of the incident laser beam.
By determining the position and number of 2b and adjusting the light intensity to change in equal steps, a gradation expression with good linearity can be obtained.

The number of electrodes can be reduced by lowering the applied voltage and using the relative intensity below the maximum value and adjusting the voltage applied to each electrode 12b according to the spatial intensity distribution of the beam. The same effect as the changing adjustment method can be obtained. That is, by making the applied voltage of the electrodes 12b at both ends higher than the applied voltage at the center, an applied voltage distribution is provided and the degree of diffraction by the electrodes 12b at both ends is changed by the degree of diffraction of the electrode 12b at the center. By making it larger, the light intensity due to ON per electrode 12b can be changed in uniform steps regardless of the position of the electrode to which the voltage is applied (FIG. 8).

In this case, since the voltage applied to the central electrode 12b is reduced, the diffraction efficiency at this portion is slightly reduced. However, it is not necessary to operate the electrodes 12b in groups simultaneously as in the above-described example. There is an advantage that the number of gradation steps obtained by a device having the same number of electrodes can be increased.

When an image is written using multi-order light, the beam is separated into a plurality of spots when the beam is condensed by a single lens. It is necessary to combine the spots by an optical system using a plurality of lenses for focusing.

Embodiment 3 In the device serving as a phase grating described in Embodiments 1 and 2, a laser beam is incident on a LiNbO ₃ single crystal in a bulk state, and the device is used in a region where the phase grating is formed near an electrode surface. Diffraction is caused by using total reflection, but basically any material and structure of a device that can cause diffraction by a phase grating that can selectively control the generation and extinction of the grating. It is not limited. LiTaO ₃ for example on a LiTaO ₃ substrate as shown in FIG. 9
Guides a beam to an optical waveguide 20 made by diffusing Nb into
By grouping and addressing the interdigital electrodes 12 formed on the optical waveguide 20 into an electrode group according to the gradation signal, the diffraction efficiency is controlled, and a desired zero-order light intensity or primary intensity is obtained. Can be. The interdigital electrode 12 was produced by depositing an aluminum thin film on the surface of the waveguide 20 and patterning the film by photolithography.

In the first and second embodiments, since the total reflection on the side surface parallel to the Z axis of the crystal is used, the area that undergoes a phase change is limited to the reflection point near the electrode 12. In this case, a phase change can be received in a region equal to the length of the electrode 12. Therefore, in this case, it is possible to increase the length of the region where the beam acts on the phase grating, and there is an advantage that the driving voltage can be reduced.

Fourth Embodiment In the first, second and third embodiments, intensity modulation using zero-order light and multi-order light by Raman-Nass diffraction has been described. Example 3
As described in, in the device structure that can lengthen the length of the region where the beam interacts with the phase grating,
When the relationship between the length L of the phase grating formed in the device, the pitch P thereof, and the wavelength λ of the incident beam satisfies the condition of 4π ≦ 2πLλ / nP ² , Bragg diffraction occurs.

As shown in FIG. 10, when the beam is incident on the grating at an angle such that the Bragg angle θ ₂ satisfies θ ₂ = sin ⁻¹ (λ / 2P), the zero-order light remains unchanged. The primary light is deflected at an angle θ ₂ as if reflected by a grating. Since only primary light is generated as multi-order light, when this primary light is used as an intensity-modulated beam, unlike a multi-order light that becomes a plurality of spots due to Raman-Nass diffraction, one point is formed on the photoconductor. There is an advantage that an optical system for condensing light is not required.

[0039]

According to the present invention, the number of gradations can be reduced without lowering the writing resolution, increasing the transfer speed of the image signal and the rotation speed of the polygon scanner, and without increasing the cost of the optical system. Thus, it has become possible to provide a gradation recording method which is excellent in gradation stability without being influenced by the instability of the output of the laser light source.

[Brief description of the drawings]

FIG. 1 is a diagram schematically illustrating the configuration of a gradation image recording apparatus according to the present invention.

FIG. 2 is a diagram illustrating an electrode structure of a phase grating device used in Example 1 of the present invention.

FIG. 3 is a diagram illustrating an electric field formed in the phase grating device used in the first embodiment of the present invention.

FIG. 4 is a diagram when diffraction is generated by the phase grating device according to the first embodiment of the present invention.

FIG. 5 is a diagram illustrating a relationship between an arrangement position of a voltage application electrode and light intensity (Gaussian distribution) according to the first embodiment of the present invention.

FIG. 6 is a diagram illustrating an intensity modulation characteristic of the phase grating device according to the first embodiment of the present invention.

FIG. 7 is a diagram illustrating a second embodiment according to the present invention.

FIG. 8 is a diagram illustrating an intensity modulation characteristic of the phase grating device according to the second embodiment of the present invention.

FIG. 9 is a diagram illustrating a third embodiment according to the present invention.

FIG. 10 is a diagram illustrating a fourth embodiment according to the present invention.

FIG. 11 is a diagram illustrating an outline of a configuration of a conventional optical writing device.

FIG. 12 is a diagram for explaining an exposure position of a spot by a conventional multi-tone recording method (matrix).

FIG. 13 is a diagram for explaining an exposure position of a spot by a conventional multi-tone recording method (pulse width modulation).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser, 2 ... Collimator lens, 3 ... Cylindrical lens, 4 ... Phase grating device, 4a ... Beam incident side end face of phase grating device, 4b ... Beam internal total reflection surface of phase grating device, 4c ... Phase grating device 5: Condensing lens, 6: Multi-order light cutting member, 7: Polygon scanner, 8: fθ lens, 9
... Cylindrical mirror, 10 ... Photoconductor, 11 ... 0th order light cut member, 12 ... Electrode, 20 ... Optical waveguide

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-195688 (JP, A) JP-A-50-17652 (JP, A) JP-A-3-109518 (JP, A) JP-A 57-195 94726 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B41J 2/44 B41J 2/52 G02F 1/03 505 G03G 15/04 H04N 1/113

Claims (3)

(57) [Claims] 1. An optical writing device that scans a laser beam whose on / off is controlled by an image signal and condenses the laser beam on a photosensitive member to form an electrostatic latent image, wherein a spatial region through which the laser beam passes A phase grating capable of selectively controlling the generation and extinction of a grating based on an image gradation signal in part or all of the laser beam is provided in the optical path of the laser beam, and the intensity of a beam passing through the controlled phase grating is modulated. An optical writing device characterized by the above-mentioned. 2. The optical writing apparatus according to claim 1, wherein the phase grating is a phase grating formed in a crystal having an electro-optic effect and having a periodically changing refractive index. 3. A gradation image recording device comprising the optical writing device according to claim 1.