JPH05107883A - Optical write device and multilevel image recorder - Google Patents

Abstract

PURPOSE:To provide a multilevel image recorder with a write device for a multilevel image which is excellent in the number of tones and the stability of the tone without lowering the resolution of write, increasing the transfer speed of an image signal and the rotating speed of a polygon scanner, and raising the cost of the optical system. CONSTITUTION:As to the optical write device and the multilevel image recorder rwith the optical write device, an electrostatic latent image is formed by allowing a laser beam which is on/off controlled according to the image signal to scan and condensing it on a photosensitive body 10. A device 4 provided with a phase grating which can selectively control the forming and the vanishing of the grating based on an image multilevel signal in a spatial area where the beam passes is provided in the optical path of the laser beam, and intensity- modulation is executed to the beam passing through the phase grating. The phase grating is the grating obtained by the periodical change of refractive index formed in crystal having an electrooptical effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical writing device for selectively irradiating light on a photoconductor such as a digital copying machine or a printer to form an electrostatic latent image, and particularly, for expressing gradation. Therefore, the present invention relates to an optical writing device that changes the intensity of a laser beam and a gradation image recording device that uses the optical writing device.

[0002]

2. Description of the Related Art FIG. 11 is an explanatory diagram 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 condensed by the cylindrical lens 3 on the polygon scanner 7 in the sub-scanning direction. Then, the condensed beam is scanned by the polygon scanner 7 along the rotation axis of the photoconductor 10, and the fθ imaging optical system (fθ lens 8,
By the cylindrical mirror 9), it is condensed as spots on the photoconductor 10 at uniform intervals 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 in order to improve the image quality of the image. The method is as shown in FIG.
As shown in 2, one pixel is configured by a matrix of a plurality of spots having binary output, and the pseudo halftone is expressed by changing the number of ON spots and the pixel size in the pixel according to the image. (For example, JP-A-60-
No. 236575), as shown in FIG. 13, the emission time of a semiconductor laser is controlled (for example, Japanese Patent Laid-Open No. 62-3997).
No. 2), an example of controlling the drive current of the semiconductor laser to change the optical output (Japanese Patent Laid-Open No. 60-208165) and the like are known.

[0004]

Each of these conventional gradation recording methods has the following problems. First, a method of forming one pixel with a plurality of spots is generally called a dither method or a density pattern method. However, if the number of spots in one pixel is increased to increase the number of gradations, the image resolution Will fall. It is necessary to reduce the spot size in order to achieve both the number of gradations and the resolution, but in order to reduce the spot diameter, the numerical aperture N of the condenser lens is
Since it is necessary to increase A, the cost of the optical system increases. Further, there is a limit due to the shape of the far-field pattern of the LD of the light source and the space of the light source system. Moreover, even if the spots are made smaller and the density thereof is increased, the rotation speed of the polygon scanner and the data transfer speed must be increased.

A method for controlling the light emission time of a semiconductor laser is to compare a reference triangular wave with an image signal to determine a pulse width modulation signal, as in the example disclosed in Japanese Patent Laid-Open No. 62-39972. It is known to produce.
The spot with the pulse width modulated beam exhibits the area gradation behavior of the exposure on the photoreceptor. However, when the pulse width is shortened, the exposure by the spot exhibits an intensity modulation-like behavior rather than an area-gradation-like behavior, the output gradation with respect to the input is non-linear, and the effective gradation number is reduced. Therefore, the spot size should be as small as possible. However, although an optical system for reducing the spot size as much as possible is required, this also leads to an increase in cost and has a technical limit.

Further, when the drive current of the semiconductor laser is controlled to change the light output, 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. It is difficult to perform stable gradation control because the output variation must be taken into consideration.

In addition, 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 overlapped to perform gradation expression, 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, which has a drawback that it is difficult to align the plurality of beams and the cost of the optical system increases.

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

[0009]

The above object of the present invention can be achieved by the following constitutions. That is, in an optical writing device that scans a laser beam whose on / off is controlled by an image signal and focuses it on a photoconductor to form an electrostatic latent image, in the optical path of the laser beam, An optical writing device which is provided with a phase grating capable of selectively controlling the generation and disappearance of the grating based on an image gradation signal in a part or all of a spatial region through which a laser beam passes, and which modulates the intensity of the beam passing through the phase grating. And a gradation image recording device including the optical writing device. Here, the phase grating is, for example, a phase grating formed in a crystal having an electro-optical effect and having a periodically changing refractive index.

The laser beam incident on the phase grating enters 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 to the outside of the crystal. , The laser beam path can be configured.

The beam incident on the phase grating is
Guided inside the waveguide formed on the 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. Since the generation and disappearance of the phase grating can be controlled according to the gradation signal obtained from the image information, the beam incident on the phase grating passes through the phase grating generated according to the gradation signal. The beam that has passed through this phase grating is split into multi-order diffracted light and zero-order light,
Each intensity is modulated according to the degree of diffraction (that is, the proportion of the area that undergoes the phase change). The intensity-modulated beam is focused as a spot while scanning the photoconductor by the scanning means and the focusing means. As a result, writing of an image capable of expressing gradation 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.

Further, according to the spatial intensity distribution of the laser beam, among the electrodes forming the phase grating, the number of electrodes to which a voltage is simultaneously applied is changed for each position where the laser beam passes, and the positions of the electrodes and the laser beam are changed. The light intensity can be modulated at an equal rate regardless of the above.

Further, according to the spatial intensity distribution of the laser beam, the voltage applied to the electrodes forming the phase grating is changed for each position where the laser beam passes, and the light intensity is similarly modulated at an equal ratio. You can

[0017]

【Example】

Specific embodiments of the present invention will be described below with reference to the drawings. Example 1 A device in which an electro-optic crystal is used as a phase grating and total reflection of a beam inside the crystal is applied will be described. Figure 1
A schematic configuration diagram of the optical writing device according to the present embodiment is shown in FIG.
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 condensed by the cylindrical lens 3 in one direction. The focused beam is incident from the mirror-polished side surface 4a of the device 4 made of an electro-optic crystal at an angle, and the beam diameter is most narrowed at the inner surface 4b corresponding to the region where the phase grating inside the device 4 is formed. The light is totally reflected by the inner surface 4b and 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-optical 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. 2 (a), a cross finger shape (or a comb shape may be used) so that an electric field is applied to the Y plane in the Z axis direction. Aluminum electrodes 12 are formed with a width of 10 μm and a pitch of 20 μm. Here, the number of electrodes 12 is 128, and a voltage of 0 volt is applied to every other electrode 12a (64),
A voltage corresponding to an image gradation signal is applied to the other electrodes 12b (64 lines). The 64 electrodes 12b to which the voltage is applied are independently drawn out via the lead line 13 shown in FIG. 2B so that the voltage can be applied individually. Of course, when 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.
It is also possible to have a structure in which eight grids are turned on / off. Then, the respective wirings 13 drawn out are subjected to FPC (Flexible Printed Circuit) by thermocompression bonding (not shown).
And a unique grayscale signal is input to each.
The electrode 12 was produced by patterning an aluminum thin film deposited on a LiNbO ₃ substrate by photolithography and etching.

As shown in FIG. 2 (b), the beam incident along the X-axis direction of the crystal of the device 4 is generated by these electrodes 1
2 is totally reflected on a surface just inside one surface parallel to the XZ axis plane of the formed crystal. The polarization direction of the beam incident on the crystal is adjusted so as to be parallel to the Z-axis direction.

The behavior of the beam in the crystal of the device 4 will be described below. 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 =
It can be determined from 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 will be no light loss and total reflection will occur.

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 crystal in the vicinity of the plane where the electrode 12 is formed. In the portion where the electric field is formed, the refractive index of the incident light changes in the Z-axis direction due to the electro-optic effect. This change in the refractive index does not occur near the electrodes 12 to which no electric field is applied, but only between the electrodes 12 to which a voltage is applied. The voltage applying electrode 12
A grating whose refractive index changes due to an electric field is formed in a cycle corresponding to the pitch of b, and a beam passing therethrough undergoes a phase change having this cycle. That is, the voltage application electrode 12
b acts as a phase grating.

The beam passing through the phase grating causes diffraction. When the beam is incident on this phase grating at an angle that satisfies the Bragg condition described later, Bragg diffraction occurs and the emitted light is split into only two lights, 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 X-Z axis plane).
When incident on, Raman-Nass diffraction occurs and the beam becomes 0
In addition to the secondary light and the primary light, secondary light equal to or higher than secondary light is also generated. The intensity of the beam is modulated by digitally changing the degree of these diffractions by the gradation signal input through the FPC.

The case where Raman-Nass diffraction is generated by the device 4 to modulate the intensity of the laser beam will be described below. 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 near the electrode 12b to which the voltage is applied is diffracted by the grating generated therein and divided into the multi-order light component and the zero-order light component, but the electrode 12 to which the voltage is not applied.
Since the beam in the vicinity of b passes through without any change in phase,
It is only the 0th-order light component. 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 0th-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 interdigitated electrodes 12) to apply a space between the electrodes. After generating an electric field and diffracting the incident laser beam, the condenser lens 5 images and separates the zero-order light and the multi-order light. 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 0th-order light becomes 0 is set as a drive voltage (several 10V). If the number of electrodes 12b to which a voltage is applied is reduced in this state, the intensity of the 0th-order light changes in digital steps to increase according to the number.

By the way, 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 positions of the beam and the grating change, the rate of change in the intensity of the beam as a result of causing diffraction per grating differs.

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

According to the relationship shown in FIG. 6, the arrangement position and the number of the electrodes 12b to which a voltage is applied in order to obtain a desired light intensity are predetermined, and the light intensity can be adjusted 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 use the electrodes 12b on both ends to uniformly change the light intensity in a stepwise manner, it is necessary to change the number of electrodes 12b larger than the number of the central electrodes 12b used. On the contrary, in order to slightly change the light intensity, it is possible to increase the number of gradation steps by adjusting the number of electrodes 12b used on 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 at that time is generated.
The intensity was modulated using the next light. Here, intensity modulation using multi-order light will be described as a second embodiment. FIG. 7 shows a schematic configuration. A laser beam is similarly incident on the same device 4 as that described in the first embodiment to generate diffraction. The condenser lens 5 images and separates the 0th-order light and the multi-order light. Here, the 0th-order light is shielded from the light 11
Cut 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 becomes maximum at a certain voltage. When 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 exhibits a curve relationship with the 0th-order light described in Example 1 and having exactly the opposite slope. Then, the electrode 1 for applying a voltage to obtain a desired light intensity in accordance with the spatial intensity distribution (Gaussian distribution) of the incident laser beam.
By determining the position and number of 2b and adjusting the light intensity so as to change in equal steps, gradation expression with good linearity can be obtained.

If the applied voltage is lowered and the relative intensity is used below the maximum value and the voltage applied to each electrode 12b is adjusted according to the spatial intensity distribution of the beam, the above-mentioned number of electrodes can be obtained. The same effect as the changing adjustment method can be obtained. That is, the applied voltage to the electrodes 12b on both end sides is made higher than the applied voltage on the central part side to give an applied voltage distribution, and the diffraction degree by the electrodes 12b on both end sides is made equal to that of the electrode 12b on the central part side. By making it larger, the light intensity due to ON per electrode 12b can be changed in equal steps regardless of the electrode position to which the voltage is applied (FIG. 8).

In this case, since the applied voltage to the central electrode 12b is lowered, the efficiency of diffraction at this portion is slightly lowered, but it is not necessary to simultaneously operate the electrodes 12b in groups as in the above-mentioned example. The advantage is that it is possible to increase the number of gradation steps that can be obtained with a device having the same number of electrodes.

When writing an image using multi-order light, the beam is split into a plurality of spots when it is condensed by a single lens. It is necessary to combine the spots by an optical system using a plurality of lenses that collect light.

Example 3 In the phase grating devices described in Examples 1 and 2, a laser beam is made to enter a LiNbO ₃ single crystal in a bulk state, and a phase grating in a region near the electrode surface is formed. Diffraction is caused by using total internal reflection, but basically, if the material and structure of the device that can cause diffraction by a phase grating that can selectively control the creation and disappearance of the grating, this It is not limited. LiTaO ₃ for example on a LiTaO ₃ substrate as shown in FIG. 9
Guide the beam to the optical waveguide 20 made by diffusing Nb into
To obtain the desired 0th-order light intensity or 1st-order intensity by controlling the diffraction efficiency by grouping the interdigital electrodes 12 formed on the optical waveguide 20 into electrode groups according to a gradation signal and performing addressing. You can The interdigital electrode 12 was formed by depositing an aluminum thin film on the surface of the waveguide 20 and patterning it by photolithography.

In the case of Examples 1 and 2, since the total reflection on the side surface parallel to the Z axis of the crystal is utilized, the region subject to the phase change is limited to the reflection portion in the vicinity of the electrode 12. In this case, the phase can be changed 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 phase grating and the beam interact, and there is an advantage that the drive voltage can be lowered.

Fourth Embodiment In the first, second and third embodiments, the intensity modulation using 0th-order light and multi-order light by Raman-Nass diffraction has been described. Example 3
As explained in, in the device structure where the length of the region where the phase grating and the beam interact can be long,
Bragg diffraction occurs when the relationship between the length L of the phase grating formed in the device and its pitch P and the wavelength λ of the incident beam satisfies the condition of 4π ≦ 2πLλ / nP ² .

As shown in FIG. 10, in the Bragg diffraction, when the beam is incident on the grating at an angle at which the Bragg angle θ ₂ satisfies θ ₂ = sin ⁻¹ (λ / 2P), the 0th-order light remains unchanged. Passing through in the direction, the primary light is deflected at an angle θ ₂ as if reflected by the grating. Since only the first-order light is generated as the multi-order light, when this first-order light is used as an intensity-modulated beam, unlike the multi-order light that becomes a plurality of spots due to Raman-Nass diffraction, there is one point on the photoreceptor. There is an advantage that an optical system for condensing the light on the surface is unnecessary.

[0039]

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

[Brief description of drawings]

FIG. 1 is a diagram illustrating an outline of a configuration of a gradation image recording apparatus of the present invention.

FIG. 2 is a diagram illustrating an electrode structure of the 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 Example 1 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 relationship diagram of the arrangement position of the voltage application electrode and the light intensity (Gaussian distribution) according to the first embodiment of the present invention.

FIG. 6 is a diagram showing intensity modulation characteristics by the phase grating device of Example 1 according to the present invention.

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

FIG. 8 is a diagram showing intensity modulation characteristics by the phase grating device of Example 2 according to 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 illustrating exposure positions of spots by a conventional multi-gradation recording method (matrix).

FIG. 13 is a diagram illustrating an exposure position of a spot by a conventional multi-gradation 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 entrance side end surface of phase grating device, 4b ... Beam internal total reflection surface of phase grating device, 4c ... Phase grating device End face of the beam emission side, 5 ... Condensing lens, 6 ... Multi-order light cutting member, 7 ... Polygon scanner, 8 ... fθ lens, 9
... Cylindrical mirror, 10 ... Photoreceptor, 11 ... Zero-order light cutting member, 12 ... Electrode, 20 ... Optical waveguide

Continuation of the front page (51) Int.Cl. ⁵ Identification number Office reference number FI Technical display location G02F 1/03 505 8106-2K H04N 1/04 104 A 7251-5C

Claims (3)

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