Abstract:
Disclosed herein is an order separation- and multibeam formation-based printing apparatus using an optical modulator, in which diffracted beams having two or more diffraction numbers, formed by reflecting and diffracting multibeam light, are assigned to respective photosensitive surface sections of a photosensitive drum according to wavelength and diffraction order to form latent images on the surface of the photosensitive drum at an improved resolution.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates, in general, to a printing apparatus using an optical modulator and, more particularly, to an order separation- and multibeam formation-based printing apparatus using an optical modulator, in which diffracted beams formed from multibeam light through diffraction and reflection are radiated onto respective surface sections of a drum so as to form latent images having improved resolution on the surface of the drum.  
         [0003]     2. Description of the Related Art  
         [0004]     Nowadays, printer technology has been developing toward high speed, miniaturization, high resolution, and low cost. A typical laser printer employs a laser scanning scheme of scanning laser beams using a laser diode and an f-θ lens.  
         [0005]     To achieve high-speed printing, an image head scheme taking advantage of a multibeam beamformer has been adopted. With such a scheme, high-speed and high-resolution printing is possible, but a high cost is also incurred because it requires a plurality of light sources.  
         [0006]     With reference to  FIG. 1 , a conventional laser scanning scheme that uses a single light source and an f-θ lens is illustrated.  
         [0007]     As seen in this figure, a conventional laser scanning operation starts with the emission of a light beam from a laser diode (LD)  10  in response to a video signal. The light beam is collimated by a collimator lens  11  into parallel light beams and is further converged on a polygon mirror  13  by a cylinder lens  12 . While passing through the cylinder lens  12 , the parallel light beams are converted into linear light beams that are parallel to a scanning direction.  
         [0008]     Rotating at a constant speed, the polygon mirror  13  driven by a motor deflects the linear light beams incident thereon and scans them in the direction of an f-θ lens  15 .  
         [0009]     While the linear light beams are transmitted through the f-θ lens  15 , their aberrations are corrected. The aberration-corrected linear light beams are reflected by a bend-back mirror  16  and scan a photosensitive drum  17  at a constant velocity due to the constant rotation speed of the polygon mirror  13 .  
         [0010]     Due to problems of a low switching speed of the laser diode  10  and a low scanning speed of the polygon mirror  13 , this laser scanning scheme is difficult to apply to high printing speed implementation.  
         [0011]     For example, an improvement in the scanning speed of the light beam in the laser scanning scheme requires the polygon mirror to rotate at a higher speed, thus requiring a high-speed driving motor. However, a higher speed motor may increases the production cost, and the motor rotating at high speed produces heat, vibration and noise, thus degrading the operational reliability of the apparatus provided therewith.  
         [0012]     As another approach to improving the scanning speed of an optical scanning unit, an image head printing scheme, in which a multi-beam beamformer is utilized, has been suggested.  
         [0013]      FIG. 2  shows an image head used in a conventional laser scanning scheme. As shown in this figure, an image head  20  has an LED array composed of a sufficient number of LEDs to cover the scanning width of a paper to be printed. In contrast to the laser scanning scheme, this image head printing scheme uses neither a polygon mirror nor an f-θ lens and forms multibeam light which allows all of the content of a line to be printed at the same time, thereby significantly enhancing the printing speed.  
         [0014]     However, the image head printing scheme suffers from the disadvantage of having increased production cost because there is a large number of LEDs  22  in the LED array  21  and uniform images are not obtained due to low optical uniformity among LEDs  22  in the array.  
       SUMMARY OF THE INVENTION  
       [0015]     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 printing apparatus using an optical modulator, which is able to form latent images having improved resolution on the surface of a photosensitive drum by dividing multibeam light according to wavelength and diffraction order through reflection and diffraction to form beams having different wavelengths and diffraction orders, and radiating the beams on respective surface sections of a photosensitive drum.  
         [0016]     In accordance with the present invention, the above object could be accomplished by the provision of a printing apparatus using an order separation and multibeam formation based optical modulator, comprising: an illumination lens unit for converting multibeam light incident from a light source unit into linear parallel beams; a diffractive optical modulator for modulating the linear parallel beams emergent from the illumination lens unit to form diffracted multibeam light having a plurality of diffraction orders; a filter unit for separating the diffracted multibeam light according to diffraction order and for selectively passing the resulting separated beams therethrough; and a projection system in which a drum has a surface divided into two or more sections and the diffracted beams are assigned to the respective sections according to wavelength and diffraction order so as to form latent images on the surface of the drum. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]     The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present invention and together with the description serve to explain the principles of the invention. Other objects of the present invention and many attendant advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, in which like reference numerals designate like parts throughout the figures.  
         [0018]      FIG. 1  is a schematic view showing a conventional laser scanning scheme using a single light source and an f-θ lens;  
         [0019]      FIG. 2  is a schematic view showing a conventional laser scanning scheme of performing laser scanning using a plurality of beams emitted from an LED array built in an image head;  
         [0020]      FIG. 3  is a schematic view showing the structure of an order separation- and multibeam formation-based printing apparatus using a diffractive optical modulator in accordance with an embodiment of the present invention;  
         [0021]     FIGS.  4 A˜ 4 C shows optical paths of a light beam passing through an illumination lens unit used in the printing apparatus of  FIG. 3  in perspective view, plan view and side cross sectional view;  
         [0022]      FIG. 5  is a perspective view showing a diffractive optical modulator used in the printing apparatus of  FIG. 3 ;  
         [0023]      FIG. 6  is a schematic view illustrating the angle of reflection of the diffractive modulator of  FIG. 5 ;  
         [0024]      FIG. 7  is a schematic view showing a diffracted light beam formed by the diffractive optical modulator of  FIG. 5 ;  
         [0025]      FIG. 8A and 8B  are a plan view and a cross sectional view, respectively, showing the optical paths of light beams passing through a Fourier lens used in the printing apparatus of  FIG. 3 ;  
         [0026]      FIG. 9A and 9B  are schematic views showing examples of filters useful in the printing apparatus of  FIG. 3 ; and  
         [0027]      FIG. 10  is a schematic view showing the structure of an order separation- and multibeam formation-based printing apparatus using a diffractive optical modulator, in accordance with another embodiment of the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     Reference should be made to the drawings to describe the structure of an order separation- and multibeam formation-based printing apparatus using an optical modulator, in detail. A description will be given of a piezoelectric diffractive optical modulator, below, but it should be understood that the principle of the present invention is applicable to transmissive, reflective, or other diffractive optical modulators.  
         [0029]      FIG. 3  is a diagram that shows the structure of a printing apparatus using an order separation and multibeam formation-based optical modulator, in accordance with an embodiment of the present invention.  
         [0030]     This printing apparatus using an order separation and multibeam formation-based optical modulator, as seen in  FIG. 2 , comprises a light source unit  300 , an illumination lens  310 , a diffractive optical modulator  315 , a Fourier lens  320 , a filter  325 , reflection mirrors  330  and  340  and a drum  350 .  
         [0031]     The light source unit  300  is composed of a plurality of light sources  301   a  and  301   b , which emit beams having wavelengths different from one another, and a dichroic mirror  302 . For the preparation of the light sources  301   a  and  301   b , semiconductor devices such as light emitting diodes (LEDs) or laser diodes (LDs) may be employed. Functioning as a filter that passes light beams having certain wavelengths therethrough but reflects light beams having different wavelengths, the dichroic mirror  32  can focus light beams emergent from the light sources  301   a  and  301   b  of different wavelengths to form multibeam light.  
         [0032]     A cross section of the multibeam light emerging from the light source unit  300  is depicted in (A) of FIGS.  4 A˜ 4 C. The multibeam light emerging from the light source unit  300  has a circular cross section, while its intensity profile forms a Gaussian distribution, as seen in (B) of FIGS.  4 A˜ 4 C.  
         [0033]     Composed of a cylinder lens  311  and a collimator lens  312 , the illumination lens unit  310  converts the incident beam into linear parallel beams with an elliptical cross section as seen in (C) to (E) of FIGS.  4 A˜ 4 C. That is, through the cylinder lens  311  and the collimator lens  312 , the beam emergent from the light source unit  300  is made linear in a direction parallel to the optical direction and thus incident on the diffractive optical modulator  315 , aligned parallel to the optical path.  
         [0034]     When emerging out of the cylinder lens  311 , the incident linear beam is converted into a linear beam parallel to the direction of the optical path.  
         [0035]     Before being incident on the diffractive optical modulator  315 , the linear beam transmitted through the cylinder lens  311  is collimated into parallel beams by the collimator lens  312 .  
         [0036]     In an embodiment of the present invention, the collimator lens  312  may be comprised of a concave lens  312   a  and a convex lens  312   b , as seen in FIGS.  4 A˜ 4 C.  
         [0037]     The concave lens  312   a  allows the linear beam to diverge up and down and be incident on the convex lens  312   b  as seen in (D) of FIGS.  4 A˜ 4 C. After passing through the convex lens  312   b , a parallel beam emerges, as seen in (E) of FIGS.  4 A˜ 4 C.  
         [0038]     Thereafter, the diffractive optical modulator  315  diffracts the light incident from the illumination lens unit  310  to produce diffracted light having a plurality of orders.  
         [0039]     In  FIG. 5 , an example of the diffractive optical modulator  315 , having an open-hole type structure, used in the present invention is depicted. As seen in this figure, the open-hole based diffractive optical modulator comprises a base substrate  501 , an insulation layer  502 , a lower micromirror  503 , and a plurality of elements  510   a  to  510   n . Although being separated from the lower micromirror in this figure, the insulation layer, if reflective, may itself be used as the micromirror.  
         [0040]     The base substrate  501  has a depression, formed in a middle portion, for providing air spaces for the elements  510   a ˜ 510   n , with the insulation layer  502  formed over predetermined areas of the upper surface thereof. The lower micromirror  503  is deposited on the insulation layer  502  within the depression. On each of the opposite banks located beside the depression, an array of elements  510   a ˜ 510   n  is built. The base substrate  501   a  may be made from a single material selected from among Si, Al 2 O 3 , ZrO 2 , quartz, SiO 2 , etc., or may be divided into two parts having materials different from each other (on the basis of the dotted line represented in the figure).  
         [0041]     The micromirror  503 , deposited on the base substrate  501 , functions to reflect an incident light beam for the purpose of diffraction. The lower micromirror  503  is made of metal such as Al, Pt, Cr, Ag, etc.  
         [0042]     Because the elements have the same structure, only one of them will be described below. As seen, the element  510   a  looks like a ribbon and has a lower support  511   a  which spans the depression over a set of opposite banks, at its lowest layer, so that the element  501   a  is spaced apart from the depression of the base substrate  501  at a middle portion.  
         [0043]     Piezoelectric cells  520   a  and  520   a ′ are respectively formed on opposite side portions of the lower support  511   a , and contract or expand to provide the drive power of the element  510   a.    
         [0044]     As a material for the lower support  511   a , Si oxides, such as SiO 2 , Si nitrides, such as (Si 3 N 4 ), and Si carbides may be used. Also, a ceramic substrate, such as Si, ZrO 2  or Al 2 O 3 , may be used as the lower support  511   a . Optionally, the lower support  511   a  may be omitted.  
         [0045]     Each of the piezoelectric cells  520   a  and  520   a ′ disposed on respective side portions of the lower support includes a lower electrode layer  521   a ,  521   a ′ and an upper electrode layer  523   a  and  523   a ′ with a piezoelectric layer  522   a ,  522   a ′ interposed therebetween. When an external electrical field is applied across the lower electrode layer  521   a ,  521   a ′ and the upper electrode layer  523   a ,  523   a ′, the piezoelectric layer  522   a ,  522   a ′ contracts and expands in response to the drive power applied, to cause motion of the lower support  511   a  in a direction perpendicular to its plane.  
         [0046]     For the formation of the electrodes  521   a ,  521   a ′,  523   a ,  523   a ′, a material selected from among Pt, Ta/Pt, Ni, Au, Al, RuO2, etc. may deposited in a thickness from 0.01 to 3 μm by a dry-type method such as sputtering, evaporation, etc.  
         [0047]     In each element, an upper micromirror  530   a  provided with a plurality of open holes  531   a   1 ,  531   a   2  is deposited on a middle portion of the lower support  511   a . The open hole may have any shape. For example, it may be a rectangle, a circle, or an oval, or any other curved shape, preferably a rectangle. The lower support, if formed of a light reflecting material, need not have an upper micromirror deposited thereon if it can function as a mirror itself.  
         [0048]     Upon passing through the open holes  531   a   1 ,  531   a   2  of the upper micromirror  530   a , a light beam is diffracted and incident on corresponding areas of the lower micromirror  503 , whereby a combination of the lower micromirror  503  and the upper micromirror  530   a  can form a pixel.  
         [0049]     For instance, a portion A of the upper micromirror  530   a , in which the open holes  531   a   1 ,  531   a   2  are formed, can form a pixel, in combination with a portion B of the lower micromirror  503 .  
         [0050]     When the distance between the upper micromirror  530   a  and the lower micromirror  503  is odd number multiples of λ/4, the diffractive light beam has maximum intensity.  
         [0051]     The diffractive optical modulator  315  functions to diffract a linear light beam incident thereon and allow the diffracted light beam to be incident on the Fourier lens  320 .  
         [0052]     When reflected in the diffractive optical modulator, the diffracted light beam has the angle of reflection depicted in  FIG. 6 . As seen, the angle of incidence of the diffracted light beam is equal to the angle of reflection. That is, when the light beam is incident at an angle of θ degrees on the optical modulator  315 , it is reflected at an angle of θ degrees.  
         [0053]     Next, referring to  FIG. 7 , the diffracted light that is generated by the diffractive optical modulator  315  is shown. Acting as a diffraction grating, the diffractive optical modulator generates 0 th  and ±1 st  order diffraction beams in the periodical direction of the grating. As seen, light incident on the diffractive optical modulator is split into light beams having a plurality of diffraction orders.  
         [0054]      FIG. 8  shows the function of the Fourier lens  320 . Using the Fourier lens  320 , the diffracted light beams are aligned according to diffraction order and focused on the filter  325 .  
         [0055]      FIG. 8A  is a plan view. As seen in this plan view, the diffracted light, when incident on the Fourier lens  320 , is aligned and focused according to the diffraction order.  
         [0056]      FIG. 8B  is a side cross-sectional view. After passing through the Fourier lens  320 , the 0 th -order diffraction light beam is focused on a predetermined point while the +1 st -order diffraction light beam and the −1 st -order diffraction light beam are respectively focused at positions above and below the point of focus of the 0 th -order diffraction light beam.  
         [0057]     Therefore, the filter  325  performs its function by locating its slot at a position near the focused point of a desired order diffraction light beam. In detail, the 0th order diffraction light beam can be utilized when a slot capable of passing the 0th order light beam therethrough is positioned at the focused point of the 0th diffraction light beam. The same is true of the other order diffraction light beams. Accordingly, the diffracted light beams can be selectively utilized by locating the slots of the filter at appropriate positions.  
         [0058]     Particularly in the present invention, the diffractive optical modulator  315  modulates the light beams incident thereon in a time divisional manner. The optical modulator perform modulation functions on the optical information that is incident on a first drum surface  350   a  during a first predetermined time period, on the optical information that is incident on a second drum surface  350   b  during a second predetermined time period, on the optical information that is incident on a third drum surface  350   c  during a third predetermined time period, and on the optical information that is incident on a fourth drum surface during a fourth predetermined time period. Accordingly, the filter  325  passes only +1 st -order diffraction light beams having a first wavelength therethrough and thus allows a modulated diffracted light beam to be incident on the first drum surface  350   a  during the first time period. Next, the filter  325  allows the passage of only +1 st -order diffraction light beams having a second wavelength to be incident on the second drum surface  350   b  during the second time period. Likewise, the filter  325  passes only −1 st -order diffraction light beams having the second wavelength so as to allow a modulated light beam to be incident on the third drum surface  350   c  during the third time period, and then, −1 st -order light beams having the first wavelength are passed and then incident on the fourth drum surface  350   d  during the next time period. With this structure, the diffractive optical modulator  315  can obtain four times higher resolution than can a conventional optical modulator having the same number of pixels.  
         [0059]     In response to the order-dependant, time-divisional modulation of the diffractive optical modulator  315 , the filter  325  must have a filtering function. In detail, when +1 st -order diffracted light having the first wavelength is passed, the filter  325  must block +1 st -order diffracted light beams having the second wavelength and −1 st -order diffracted light beams having the first and second wavelengths from passing therethrough. The passage of the +1 st -order diffracted light beams requires that the filter not allow the passage of other diffracted light beams, including +1 st -order diffracted light beams having the first wavelength and −1 st -order diffracted light beams having the first and second wavelengths. Likewise, while passing −1 st -order diffracted light beams having the first wavelength therethrough, the filter block the passage of other diffracted light beams, including +1 st -order diffracted light beams and −1 st -order. diffracted light beams having the second wavelength. Also, the passage of −1 st -order diffracted light beams of the second wavelength excludes the passage of the other diffracted light beams, including the +1 st -order diffracted light beams and the −1 st -order diffracted light beam having the first wavelength. In this regard, the filter  325  may be a rotary filter in which slots are designed to be positioned on different axes that cross each other, as depicted in  FIGS. 9A and 9B . Of course, dichroic filters may be used for the selective passage of the appropriate diffracted light beams. If N is an integer, the rotary filter may have 2N+1 slots as seen in  FIGS. 9A and 9B .  
         [0060]     Turning to  FIG. 3 , a combination of a reflection mirror  330   a , a dichroic mirror  340   aa , and a reflection mirror  340   ab  guides the +1 st -order diffracted light beams having the first wavelength onto a first surface area  350   a  of the drum. Through the reflection mirror  330   a  and the dichroic mirror  340   aa , the +1 st -order diffracted light beams having the second wavelength are reflected onto a second surface area  350   b  of the drum while a combination of a reflection mirror  330   b  and a dichroic mirror  340   ba  leads the −1 st -order diffracted light beams having the second wavelength onto a third surface area of the drum. Along a combination of a reflection mirror  330   b , a dichroic mirror  340   ba  and a reflection mirror  340   bb , the 1 st -order diffracted light beams having the first wavelength reach a fourth surface area  350   d  of the drum.  
         [0061]      FIG. 10  depicts the structure of a printing apparatus of an order separation and multibeam formation-based optical modulator, in accordance with an embodiment of the present invention.  
         [0062]     The difference between the printing apparatuses of  FIGS. 10 and 3  is in the light sources used: the printing apparatus of  FIG. 3  uses monochromic light sources while the printing apparatus of  FIG. 10  uses a polychromic light source.  
         [0063]     As described hereinbefore, the printing apparatus using an order-separation and multibeam formation-based diffractive optical modulator in accordance with the present invention is able to form images on a large screen using the lowest possible number of actuating cells, with the concomitant advantage of obtaining high resolution at a low cost.  
         [0064]     Although the preferred embodiments of the present invention have been disclosed for illustrative purpose, 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.