# Patent

## Title
Color display device using separate diffracted light and illumination light

## Abstract

A display device is disclosed. The display device includes a condenser unit, an illumination lens system, a plurality of diffractive light modulators, a mirror for separating illumination light from diffracted light, a polarized beam separator, a filter unit and a projection unit.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a color display device and, more particularly, to a color display device in which a simple optical system can be realized by separating diffracted light from illumination light.
2. Description of the Related Art
A light beam scanning apparatus is an apparatus for producing an image by causing a light beam to form spots on a photosensitive medium through scanning in an image forming apparatus such as a laser printer, a display device, a Light Emitting Diode (LED) printer, an electrophoto copying machine or a word processor.
As image forming apparatuses develop toward compactness, high speed and high resolution, light beam scanning apparatuses have been accordingly researched and developed to have the characteristics of compactness, high speed and high resolution.
Light beam scanning apparatuses for image forming apparatuses may be mainly classified into laser scanning type apparatuses using an f·θ lens, and image head printer type apparatuses, according to the light beam scanning method and the construction of a light beam scanning apparatus.
FIG. 1 is a perspective view showing the configuration of a conventional laser scanning apparatus using an f·θ lens.
Referring to FIG. 1, the conventional laser scanning apparatus includes a Laser Diode (LD) 10 that radiates a light beam in response to a video signal, a collimator lens 11 that converts the light beam, which is radiated from the LD 10, into parallel light, a cylinder lens 12 that converts the parallel light, which is passed through the collimator lens 11, into linear light coplanar with a scanning direction, a polygon mirror 13 that reflects the linear light, which is passed through the cylinder lens 12, while moving it at a constant linear velocity, a polygon mirror driving motor 14 that rotates the polygon mirror 13 at a constant velocity, an f·θ lens 15 that has a constant refractive index with respect to an optical axis, deflects the light, which is reflected by the polygon mirror 13 and which has a constant angular velocity, in a principal scanning direction, corrects aberrations of the light and focuses the corrected light on an illumination surface, a reflection mirror 16 that reflects the light beam, which is passed through the f·θ lens 15, in a predetermined direction and forms a dot-shaped image on the surface of a photosensitive drum 17, that is, an image plane, a horizontal sync mirror 18 that reflects the laser beam, which is passed through the f·θ lens 15, in a horizontal direction, and an optical sensor 19 that receives the laser beam, which is reflected by the horizontal sync mirror 18, and performs synchronization.
It is difficult for the above-described laser scanning type light beam scanning apparatus to achieve high-speed printing, due to the low switching speed of the laser diode 10 and the speed problem of the polygon mirror 13.
That is, to increase the scanning speed of the light beam, the polygon mirror 13 must be rotated using a high-speed motor. However, the high-speed motor is expensive, and the motor operating at high speed generates heat, vibration and noise, thus degrading operational reliability, so that a significant improvement in scanning speed cannot be expected.
Another scheme for improving the speed of the light beam scanning apparatus relates to an image head printing type light beam scanning apparatus using a multi-beam type beam formation apparatus.
Such a multi-beam optical scanning apparatus has a plurality of light emitting parts (laser heads) as light sources. The multi-beam optical scanning apparatus optically scans the surface of a recording medium using a plurality of light spots formed on the surface of the recording medium in such a way that a plurality of light beams radiated from the plurality of light emitting parts is focused by an imaging lens through an optical reflector.
In order to accomplish high-speed printing using only a single light spot, the number of times the surface of the recording medium is optically scanned per unit time must be significantly large. Meanwhile, the rotational speed of the optical reflector and the image clock cannot comply with the large number of optical scans. Accordingly, if the number of beam spots that simultaneously scan the surface of the recording medium increases, the rotational speed of the optical reflector and the image clock may be reduced in proportion to the number of beam spots.
In order to form a plurality of beam spots in the most effective manner, a laser element that functions as a light source has a plurality of light emitting points (light emitting parts) that can be independently operated.
Such a laser element having a plurality of light emitting points is commonly called a “monolithic multi-beam laser element.” When the monolithic multi-beam laser element is used, most of optical elements disposed behind the light source can be used for a plurality of light beams, so that the monolithic multi-beam laser element provides significant advantages in terms of cost, process and control.
FIG. 2 is a view illustrating a conventional laser scanning scheme in which laser scanning is performed by a plurality of beams produced by an LED array disposed in an image head.
Referring to FIG. 2, an LED array 21 is disposed in an image head 20 to have LEDs that can cover the width of printing paper, and generates a plurality of beams. Unlike the laser scanning scheme, printing can be performed on a line-at-a-time basis without using a polygon mirror or an f·θ lens, thus significantly improving printing speed.
This monolithic multi-beam laser element includes a so-called surface emitting laser (surface emitting type semiconductor laser).
The surface emitting laser emits light beams parallel to the thickness direction of a silicon layer, whereas a conventional semiconductor laser emits light beams perpendicular to the thickness direction of a silicon layer.
Furthermore, the surface emitting laser has the following characteristics. That is, the conventional semiconductor laser emits divergent light that has an elliptical cross section and considerably varied divergence angles, whereas the surface emitting laser can emit a circular beam that has a stabilized divergence angle.
However, the surface emitting laser has a problem in that an output light beam has an unstable polarization direction. Although the polarization direction can be controlled by the manufacturing method to some degree, it varies depending upon a light emitting point, ambient temperature and output.
The reflectance, transmittance and angle characteristics of the optical elements of an optical scanning apparatus, such as a polygonal mirror like an optical reflector, the scanning lens (f·θ lens) of an optical imaging system, and an echo mirror for changing an optical path, vary depending upon the polarization direction of an input light beam.
For this reason, when the monolithic multi-beam laser element including a surface emitting laser is used as the light source of an optical scanning apparatus, a plurality of beam spots that optically scans the surface of a recording medium have different intensities due to the different polarization directions of light emitting points. Further, the difference in intensity results in irregular pitch in an image, thus considerably degrading image quality.
SUMMARY OF THE INVENTION
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a color display device in which a simple optical system can be realized by separating diffracted light from illumination light.
In order to accomplish the above object., the present invention provides a display device, including a condenser unit for converging polarized light output from a plurality of light sources, and outputting the converged polarized light; an illumination lens system for converting the polarized light, which is output from the condenser unit, into linear parallel light; a plurality of diffractive light modulators for producing diffracted light beams having a plurality of diffraction orders by optically modulating the polarized light for each wavelength; a mirror for separating illumination light and diffracted light, the mirror passing the polarized light, which is output from the illumination lens system, therethrough using slits, the mirror reflecting the diffracted light beams, which are output from the diffractive light modulators, toward a filter system; a polarized beam separator for separating the polarized light, which is output from the mirror for separating illumination light from diffracted light, on a wavelength basis, and then allowing the separated beams to be incident on the diffractive light modulators, and converging the diffracted light beams output from the plurality of the diffractive light modulators and then outputting the converged light to the mirror for separating illumination light from diffracted light; the filter system for selecting the diffracted light beams having desired diffraction orders from among the diffracted light beams, which are output from the diffractive light modulators, using a spatial filter; and a projection system for focusing the diffracted light beams selected by the filter system, and projecting the focused light beams onto a screen.



BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a perspective view showing the construction of a conventional laser scanning apparatus using a single light source and an f·θ lens;
FIG. 2 is a view showing a conventional laser scanning scheme in which laser scanning is performed by a plurality of beams produced by an LED array disposed in an image head;
FIG. 3 shows the configuration of a color display device using separate diffracted light from illumination light according to an embodiment of the present invention;
FIGS. 4A to 4C are a perspective view, a plan view, a side sectional view and a cross section showing an optical path produced through the illumination lens of FIG. 3, respectively;
FIG. 5 is a perspective view illustrating the construction of the diffractive light modulator of FIG. 3;
FIG. 6 is a view illustrating the reflection angle of the diffractive light modulator of FIG. 3;
FIG. 7 is a schematic diagram illustrating diffracted light generated by the diffractive light modulator of FIG. 3;
FIGS. 8A and 8B are sectional views showing optical paths formed through a filter system;
FIG. 9 is a front view illustrating a mirror for separating diffracted light from illumination light, which is shown in FIG. 3; and
FIG. 10 is a front view illustrating the filter unit of FIG. 3.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
The construction of a color display device using separate diffracted light and illumination light according to the present invention is described in detail with reference to the accompanying drawings below.
FIG. 3 shows the configuration of a color display device using separate diffracted light and illumination light according to an embodiment of the present invention.
Referring to FIG. 3, the color display device using separate diffracted light and illumination light according to the embodiment of the present invention includes a light source system 300 composed of a plurality of light sources 301a to 301c, a polarized light separation unit 310 composed of a plurality of Polarized Beam Splitters (PBSs) 311a to 311c, a condenser unit 320 composed of one mirror 321c and a plurality of dichroic mirrors 321a and 321b, an illumination lens system 330 composed of a cylinder lens 331 and a collimator lens 332, a light modulation system 340 composed of a mirror 341 for separating diffracted light and the illumination light, an X-prism 342, and a plurality of diffractive light modulators 345a to 345c, a filter system 350 composed of a Fourier lens 351 and a spatial filter 352, a projection system 360 composed of a pair of Fourier lenses 361 and 363, a speckle remover 362 and a galvano mirror 364, and a screen 370.
The plurality of light sources 300 is composed of, for example, a red light source 301a, a blue light source 301b and a green light source 301c. The plurality of light sources 300 may be light sources that are formed of semiconductor devices such as LEDs or Laser Diodes (LDs). Such semiconductor light sources have many characteristics that are suitable for use in color display apparatuses compared to other light sources.
The polarized light separation unit 310 separate polarized light output from the plurality of the light sources 301a to 301c, and allows the S polarized light of red light, the P polarized light of green light and the S polarized light of blue light to be incident on the condenser unit 320.
That is, the red polarized beam splitter 311a of the polarized light separation unit 310 reflects the S polarized light of red light, which is output from the red light source 301a, toward the condenser unit 320. The green polarized beam splitter 311b reflects the P polarized light of green light, which is output from the green light source 301b, toward the condenser unit 320. The blue polarized beam splitter 311c reflects the S polarized light of blue light, which is output from the blue light source 301c, toward the condenser unit 320.
The reflection mirror 321c of the condenser unit 320 reflects blue P polarized light, which is output from the blue light source 301c, toward the illumination lens system 330. The dichroic mirror 321b passes blue P polarized light, which is incident from the reflection mirror 321c, therethrough, but reflects green P polarized light, which is output from the green light source 301b, toward the illumination lens system 330.
Further, the dichroic mirror 321a passes blue P polarized light and green P polarized light, which are incident from the dichroic mirror 321b at the front stage, therethrough, but reflects red light, which is incident from the red light source 301a, toward the illumination lens system 330.
As described above, blue polarized light, green polarized light and red polarized light are condensed into a multi-beam by the reflection mirror 321c and the plurality of the dichroic mirrors 321a, 321b, thereby forming a single illumination system.
The illumination lens system 330 then converts the resulting multi-beam into linear parallel light, and causes it to be incident on the light modulation system 340.
An example of the cross section of light used in this case is shown in “A” of FIGS. 4a to 4c. As shown in “A” of FIG. 4A to 4C, the cross section of the light is circular. The intensity profile of light has Gaussian distribution, as shown in “B” of FIG. 4A to 4C.
The illumination lens system 330 converts incident light into linear parallel light having an elliptical cross section, and includes a cylinder lens 331 and a collimator lens 332.
That is, the illumination lens system 330 converts the multi-beam, which is condensed by the condenser unit 320, into linear light coplanar with an optical path, and causes it to be incident on the diffractive light modulators 345a to 345c through the X-prism 342.
In this case, in order for the light incident from the condenser unit 320 to be horizontally incident on diffractive light modulators 345a to 345c, which are located horizontally to the direction of the optical path, the cylinder lens 331 converts the parallel light into linear light having a horizontal direction, as shown in “C” of FIG. 4A to 4C, and then allows it to be incident on corresponding diffractive light modulators 345a to 345c through the collimator lens 332.
In this case, the collimator lens 332 converts spherical light, which is incident from the condenser unit 320 through the cylinder lens 331, into parallel light, and then allows it to be incident on corresponding diffractive light modulators 345a to 345c. 
The collimator lens 332 can include, for example, a concave lens 332a and a convex lens 332b, as shown in FIG. 4A to 4C.
The concave lens 332a perpendicularly spreads the linear light incident from the cylinder lens 331, as shown in “D” of FIG. 4A to 4C, and then allows it to be incident on the convex lens 332b. The convex lens 332b converts the incident light incident from the concave lens 332a into parallel light, as shown in “E” of FIG. 4A to 4C, and then outputs the parallel light. FIG. 4A to 4C is a perspective view of an optical system including a light source, a cylinder lens and a collimator lens. FIG. 4B is a perspective view of the optical system, FIG. 4C is a side sectional view of the light.
Meanwhile, the mirror 341 for separating diffracted light from illumination light has an open hole 341a formed at its center, as shown in FIG. 9, and thus passes the illumination light therethrough.
The first diagonal plane 343 of the X-prism 342 reflects blue light, and allows it to be incident on the diffractive light modulator 345c for the blue color, but passes green light and red light therethrough.
The second diagonal plane 344 of the X-prism 342 passes green light therethrough, and allows it to be incident on the diffractive light modulator 345b for the green color. The second diagonal plane 344 also reflects red light, and allows it to be incident on the diffractive light modulator 345a for the red color.
Each of the diffractive light modulators 345a to 345c produces diffracted light beams by diffracting linear light incident from the X-prism 342, and allows the diffracted light to be incident on the X-prism 342 again.
In this case, the diffraction angle of the diffracted light formed by the diffractive light modulators 345a to 345c is proportional to the wavelength. In this case, the diffractive light modulators 345a to 345c can include various types of diffractive light modulators. An open hole-type diffractive light modulator is shown in FIG. 5 as an example.
Referring to FIG. 5, the open hole-type diffractive light modulator includes a silicon substrate 501, an insulating layer 502, a lower micro mirror 503, and a plurality of elements 510a to 510n. Although, in this case, the insulation layer and the lower micro mirror are separately constructed, the insulation layer itself can function as the lower micro mirror if it has a light-reflecting characteristic.
The silicon substrate 501 is provided with a depressed portion to provide air spaces to the elements 510a to 510n. The insulation layer 502 is formed on the silicon substrate 501. The lower micro mirror 503 is deposited on the insulation layer 502 above the depressed portion of the silicon substrate 501. The bottoms of the elements 510a to 510n are attached to both sides of the insulation layer 502 beside the depressed portion of the silicon substrate 501. The silicon substrate 501 can be fabricated of a single material such as Si, Al2O3, ZrO2, quartz or SiO2. The upper and lower layers (divided by dotted lines in FIG. 5) of the silicon substrate 501 can be fabricated of heterogeneous materials.
The lower micro mirror 503 is deposited above the silicon substrate 501, and diffracts incident light by reflecting it. The lower micro mirror 503 can be fabricated of a metallic material such as Al, Pt, Cr or Ag.
The element 510a (although only the element 510a is described herein, the remaining elements have the same construction and operation) has a ribbon shape. The element 510a includes a lower support 511a, both sides of the bottom of which are attached to both sides of the insulation layer 502 beside the depressed portion of the silicon substrate 501, so that the central portion of the lower support 511a is spaced apart from the depressed portion of the silicon substrate 501.
Piezoelectric layers 520a and 520a′ are formed on both sides of the lower support 511a. Driving force is provided to the element 510a by the contraction and expansion of the piezoelectric layers 520a and 520a′. 
The lower support 511a may be fabricated of Si oxide such as SiO2, Si nitride such as Si3N4, a ceramic substrate such as Si, ZrO2 and Al2O3, and Si carbide. However, the lower support 511a may be omitted when necessary.
Each of the piezoelectric layers 520a and 520a′ includes a lower electrode layer 521a or 521a′ configured to provide a piezoelectric voltage, a piezoelectric material layer 522a or 522a′ formed on the lower electrode layer 521a or 521a′ and configured to contract and expand and generate vertical driving force when voltages are applied to both surfaces thereof, and a upper electrode layer 523a or 523a′ formed on the piezoelectric material layer 522a or 522a′ and configured to provide a piezoelectric voltage to the piezoelectric material layer 522a or 522a′. When voltage is applied to the upper electrode layers 523a and 523a′ and the lower electrode layers 521a and 521a′, the piezoelectric material layers 522a and 522a′ contract and expand, thus causing vertical movement of the lower support 511a. 
The electrodes 521a, 521a′, 523a and 523a′ may be fabricated of a material such as Pt, Ta/Pt, Ni, Au, Al or RuO2, and may be deposited by sputtering or evaporation to have a thickness within a range of 0.01 to 3 μm.
Meanwhile, an upper micro mirror 530a is deposited on the center portion of the top of the lower support 511a, and includes a plurality of open holes 531a1 to 531a3. In this case, the open holes 531a1 to 531a3 preferably have a rectangular shape, but may have any closed curve shape such as a circle or an ellipse. When the lower support 511a is fabricated of a light-reflective material, the upper micromirror 530a is not necessary. In this case, the lower support 511a may function as the upper micro mirror.
The open holes 531a1 to 531a3 pass light incident on the element 510a therethrough, and allow the light to be incident on the portion of the lower micro mirror 503 corresponding to the portion where the open holes 531a1 to 531a3 are formed, so that the lower micro mirror 503 and the upper micro mirror 530a can form a pixel.
That is, for example, the portion “A” of the upper micro mirror 530a where the open holes 531a1 to 531a3 are formed, and the portion “B” of the lower micro mirror 503 can form a single pixel.
In this case, the incident light, which passes through the portion where the open holes 531a1 to 531a3 of the upper micro mirror 530a are formed, can be incident on the corresponding portion of the lower micro mirror 503. When the distance between the upper micro mirror 530a and the lower micro mirror 503 is an odd multiple of λ/4, maximally diffracted light is produced. In addition, an open hole-type diffractive light modulator applicable to the present invention is disclosed in U.S. patent application Ser. No. 10/952,573.
Meanwhile, each of the open-hole type diffractive light modulators 345a to 345c produces diffracted light beams by modulating the linear light incident from the illumination lens system 330, and then allows the diffracted light beams to be incident on the mirror 341 for separating the diffracted light from the illumination light using the X-prism 342, as described above.
The reflection angle of the diffracted light formed as described above is shown in FIG. 6. From FIG. 6, it can be understood that the incidence angle and reflection angle of the diffracted light are the same. If the diffracted light is incident on the diffractive light modulators 345a to 345c at an angle of θ°, the reflection angle thereof is also θ°. In the present invention, the incidence angle is 90°, and the reflection angle is also 90°. The diffracted light formed by the diffractive light modulators 345a to 345c is shown in FIG. 7. 0-, and ±1-order diffracted light are formed in a periodic direction of the grating. If the periodic direction of the grating is the same as a horizontal direction of the linear parallel light, diffracted light as shown in FIG. 7 is formed.
In this case, if the diffractive light modulators 345a, 345c are located at a vertical tilt of ±1.41° against the ground, the diffracted light can be separated on a wavelength using the spatial filter 352 so that they have a width of 5.4 mm and a gap between them.
The filter system 350 is preferably composed of the Fourier lens 351 and the spatial filter 352. The Fourier lens 351 separates incident diffracted light on an order basis. The spatial filter 352 transmits only diffracted light of a desired order.
FIGS. 8A and 8B are sectional views showing optical paths formed through a filter system. From the drawings, it can be understood that 0-order diffracted light and ±1-order diffracted light overlap each other. Therefore, the Fourier lens 351 needs to separate the 0-order diffracted light and the ±1-order diffracted light. After the 0-order diffracted light and the ±1-order diffracted light are separated from each other, a mechanical spatial filter 352 can filter only diffracted light having desired orders.
Referring to FIG. 8A, if three beams having different wavelengths are incident on the Fourier lens 351, the Fourier lens 351 converges the beams.
Further, FIG. 8B shows side sectional view of any one of the three beams. The 0-order diffracted light is focused on a specific point after passing through the projection lens 351. In this case, the +1-order diffracted light is focused on a location above a location on which the 0-order diffracted light is focused, and the −1-order diffracted light is focused on a location below the location on which the 0-order diffracted light is focused. If the spatial filter 352 is located at a location near the focal points, only diffracted light having desired orders can be passed through the spatial filter 352. The front view of the spatial filter 352 used in this case is shown in FIG. 10. Since the distance of the +1-order diffracted light from the center of the spatial filter 352 and the distance of the −1-order diffracted light from the center of the spatial filter 352 are different for each wavelength, the +1-order diffracted light and the −1-order diffracted light can be separated from each other using the spatial filter 352.
The projection system 360 includes the pair of the Fourier lens 361 and 363, and the speckle remover 362, and projects incident diffracted light beams onto the screen 370. In other words, the projection system 360 focuses diffracted beams having predetermined diffraction coefficients, which are incident through the filter system 350, on the screen 370, thus forming a spot.
As described above, the present invention is advantageous in that a simple optical system can be constructed using a mirror for separating diffracted light from illumination light and a color image can be implemented using the simple optical system.
Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

## Claims

1. A display device, comprising:
a condenser unit for converging polarized light output from a plurality of light sources, and outputting the converged polarized light;
an illumination lens system for converting the polarized light, which is output from the condenser unit, into linear parallel light;
a plurality of diffractive light modulators for producing diffracted light beams having a plurality of diffraction orders by optically modulating the polarized light;
a mirror for separating illumination light and diffracted light, the mirror passing the polarized light, which is output from the illumination lens system, the mirror reflecting the diffracted light beams, which are output from the diffractive light modulators, toward a filter system;
a polarized beam separator for separating the polarized light, which is output from the mirror for separating illumination light from diffracted light, and then allowing the separated beams to be incident on the diffractive light modulators, and converging the diffracted light beams output from the plurality of the diffractive light modulators and then outputting the converged light to the mirror for separating illumination light from diffracted light;
the filter system for selecting the diffracted light beams having desired diffraction orders from among the diffracted light beams, which are output from the diffractive light modulators, using a spatial filter; and
a projection system for focusing the diffracted light beams selected by the filter system, and projecting the focused light beams onto a display.

2. The display device as set forth in claim 1, wherein the illumination lens system includes:
a cylinder lens for convening the light, which is output from the light source, into the linear light; and
a collimator lens for converting the linear light, which is output from the cylinder lens, into the parallel light.

3. The display device as set forth in claim 2, wherein the polarized beam separator is an X-prism.

4. The display device as set forth in claim 1, wherein the filter system includes:
a Fourier lens for converging the diffracted light, which is output from the mirror for separating illumination light from diffracted light, on an order basis; and
the spatial filter separating the diffracted light, which is converged by the Fourier lens, using a plurality of spatially separated slits.