WO1995005944A1

WO1995005944A1 - A printer and a print engine therefor

Info

Publication number: WO1995005944A1
Application number: PCT/US1994/009203
Authority: WO
Inventors: Aram Mooradian
Original assignee: Micracor, Inc.
Priority date: 1993-08-24
Filing date: 1994-08-15
Publication date: 1995-03-02

Abstract

A laser printer is described which uses a print engine for forming an image on a print medium (36). The print engine splits a light beam (16) into a plurality of dispersed light beams (24) of equal intensity. The dispersed beams are collimated and focused onto the print medium (36) to form a pixel image. Pixel information is encoded onto the dispersed beams before or after they are formed.

Description

A PRINTER AND A PRINT ENGINE THEREFORE

Background of the Invention

This invention relates to laser print engines and more particularly to a laser printer using the print engine of the invention. A laser printer consists of a print medium and a print engine. The print medium may, for example, comprise a rotating drum having an exposed cylindrical, photosensitive surface with a uniform charge formed on it, or may comprise a photon sensitive emulsion in which photons remove material and ink adheres to the media depending upon whether the material is removed or not. The print engine typically comprises a laser which emits a beam of light which is modulated with picture element (pixel) information usually in binary, or in a gray scale format. The beam is directed onto a rotating multi-facet scanning mirror rotating at a uniform rate. The laser beam reflecting from the rotating mirror repeatedly transverses along the medium. An image is formed by the print engine on the medium in accordance with the beam modulation. The image may be latent and then developed by a toner to provide a visual image which is then transferred onto paper or may be directly formed on the medium.

The disadvantages of this approach include, among other things, the cost associated with the mechanical scan system. The spatial resolution associated with such a system is generally limited by the mechanical tolerances of the rotating mirror system and the size. Many print machine applications require the need for lower cost, higher spatial resolution and smaller size laser print engines with pixel spot sizes of less than 10 μm and position accuracy of better than 1 μm. Summarv of the Invention

In accordance with the invention, a laser printer is formed of a print medium and a laser print engine for forming an image on the print medium. The laser engine is comprised of a laser source for generating a single output beam, a beam splitter optic for splitting the laser output beam into an array of equal amplitude beams travelling parallel to each other in the same direction and plane; a modulator for encoding the equal amplitude beams with the pixel information to be printed, and optics for focusing each of the modulated beams onto the print medium.

The array of modulated beams can be stepped laterally by mechanical means, such as precise stepping motors, to fill in the image information to any spatial resolution consistent with the focused laser beam spot size and the pixel separation. Alternately, an acousto-optic scanning device could be used to provide this lateral scanning capability. Either one or two-dimensional modulated beam arrays may be used in conjunction with either a mechanical or an acousto-optic scan.

In an alternate embodiment, the wavelength of the single laser source is rapidly changed in time to produce an output beam in the form of a series of pulses of progressively different wavelengths. The train of pulses is then split into separate beams using a grating and are then aligned and focused onto a print medium. A single amplitude modulator placed at the output of the laser source is then used to encode pixel information on each wavelength pulse from the laser source in either a digital or a gray scale intensity format. In the case of a tunable semiconductor diode laser, the diode may be modulated directly, eliminating the need for an external light modulator.

Brief Description of the Drawings

Fig. 1 is a perspective schematic view of a laser printer of the invention.

Fig. 2 is a schematic side view of an alternate embodiment of the laser printer using two reflective modulators arrays.

Fig. 3 is a schematic view of a two-dimensional laser printer using an array of laser sources.

Fig. 4 is a schematic view of a two-dimensional laser printer using a single laser source.

Fig. 5 is a schematic diagram of an alternate embodiment of the invention. Fig. 6 is a timing diagram showing the waveform at various locations in the schematic of Fig. 5.

Detailed Description of the Invention

Referring to Fig. 1, the invention will now be described in detail in connection therewith. A light source such as laser 14 may be used to generate a primary light beam 16. Laser 14 may comprise a semiconductor diode laser, diode laser array, diode pumped solid state laser, solid state laser array or any other appropriate laser. A particularly useful embodiment uses a diode laser array pumped solid state laser. The pump diode laser array 12 is remotely located and its light is directed through a fiber optical delivery system 38 to an input facet 40 of solid state laser 14. Laser 14 may be a gain switched version of the solid state waveguide laser of the type described in co-pending U.S. Patent Application Serial Number 07/952,952 filed September 29, 1992 (Attorney Docket No. MIT-4606ZCA) incorporated herein by reference (See, e.g., Fig. 4). A gain-switched or Q- switched laser system is particularly appropriate for exposing photosensitive materials which have a non-linear response.

The output beam 16 from laser 14 is coupled to an optical multibeam generator consisting of first optical element and a second optical element 22. The first optical element 18 is preferably a binary element which simultaneously splits or divides the primary beam into a plurality (seven, for example are shown) of equal amplitude sub-beams 24. The second optical element 22 is also preferably a binary element which aligns each sub- beam in respective parallel paths propagating toward a random access spatial light modulator 26. In modulator 26, each sub-beam 24, is modulated with pixel information generated by microprocessor 70 which is electrically- coupled by a plurality of wires 53 to individually addressable pads 51 of modulator 26. Binary optical elements of the type described here are readily available from commercial suppliers. The modulated sub-beams 28 are focused, by focusing optics 30, onto a photo-sensitive medium, such as, a print drum or plate 36. Optics 30 may comprise an array of micro-optical lenses or an array of binary optical lenses.

All of the engine components may be mounted on a base 60 which is laterally moveable relative to the print medium by stepping motor Ml mechanically coupled to base 60 by gear mechanism 62 to laterally move the focused sub- bea s 34 along the print medium 36 in response to command from a microprocessor 70.

Alternatively, as will be shown in connection with Fig. 4 the focused sub-beams may be electro-optically scanned relative to the print medium.

Several types of spatial light modulators 26 are contemplated for use in the invention. One is a liquid crystal modulator. Liquid crystal spatial light modulators use an electric field applied to a pneumatic or ferroelectric liquid crystal to change its transmission of polarized light. Liquid crystal modulators have high contrast ratios (>100:1) can handle high power densities (500 W/cm²) , and have small switching energies (-1 nJ) . They can be readily fabricated in one or two dimensional arrays with greater than 5-cm dimensions. The switching speed of liquid crystal modulators is usually limited to speeds of about 20 μs. Recent results have demonstrated individual pixel switching rates of 50 Khz in large area arrays. These modulators would be appropriate for relatively slower speed-writing applications. Higher speed can be achieved by parallel operation of liquid crystal array modulators, for example, in a two- dimensional array format.

An alternate approach is the use of electro- mechanical spatial light modulators which use micro mirrors which are held by a torsional structure and are rotated by the application of a voltage to an electrode on the device. The capacitive force generated causes the micro mirror to rotate and thereby deflect an incident beam. These devices have been fabricated from silicon and/or aluminum. A review of this technology is contained in an article by L.J. Hornbeck of Texas Instruments in the SPIE Proceedings 1150, (1989)86. The maximum typical deflection angles are about 10 degrees before nonlinearity and hysteresis sets in.

An alternate electro-mechanical modulator has been demonstrated at Stanford University by D. Bloom and co- workers. The fastest speed of response demonstrated with this technology is less than 40 ns. The concept involves the use of a deflected membrane which is etched in the form of a grating. By applying a voltage to this grating, electrostatic forces move it relative to a flat surface beneath it. The motion is an integer half wave length and the reflected beam is either not deflected or is scattered out of the direction of the main beam. Contrast ratios 25:1 have been measured for devices having a 25-μm width. Typical drive voltages are presently about 20 volts and could come down to less than 10 volts making them operate in the logic level range. Large areas well in excess of 1 mm² are possible with relatively fast switching speeds using an array of 25 μm devices. A major advantage of this technology is that there is a slow dependence of diffraction with wavelength, in contrast to electro- absorption devices discussed below. These devices presently operate in reflection. Gray scale intensity may be achieved by control of the drive voltage to vary the grating scattering efficiency

A third approach is the use of electro-absorption (EA) light modulators. EA light modulators have the potential to also be used as a low cost array modulator similar in cost to the electro-mechanical devices. (EA) light modulators are based upon the use of the electric field dependence of the energy band gap of a direct band gap semiconductor. When the optical energy of a light beam is above the band gap energy, the light is strongly absorbed and when an electric field is applied, the band gap shifts to higher energies and the material becomes transmissive. Quantum well structures are used to engineer the optical absorption properties of the devices to suit the modulator needs. These quantum well structures e.g. GaAlAs, are typically operated with a pn junction that is reverse biased in order to achieve electric field levels of a few thousand volts/cm required for the necessary band gap shift. These GaAlAs spatial light modulators work with the excellent contrast ratio and speed, but require wavelengths typically in the 800 nm region. Recent work has been carried out on GalnAs quantum well modulators for use with 1.06 μm laser sources, however, the contrast ratios have been less than that for the GaAlAs devices. Contrast ratios at 1.06 μm of 6:1 have been demonstrated in a single reflective GalnAs multi-quantum well modulator having up to 90 wells. More typically, however, contrast ratios have been in the range of 2:1 in earlier work. Higher contrast ratios can be achieved as shown in Fig. 2 if two or more reflective devices 26A and 26B are placed in series in the path of the multiple beams 24' from beam divider 52'. An additional advantage of the technology is the use of logic level voltages to drive the devices which require no additional power supplies other than the direct signals provided by the output command signals from the microprocessor 70. Both the electro-absorption and the electro-mechanical light modulators have an additional major advantage in being relatively insensitive to alignment position in the print engine optical train. For analog gray-scale applications, all modulators would require a linearization circuit to provide the dynamic range requirements.

Another approach is the use of acousto-optic deflectors which are formed from the interaction of an optical wave with an acoustic wave within a crystalline media. A standing acoustic wave causes the refractive index to vary sinusoidally via the photoelastic effect. The sinusoidal reflective index is equivalent to a transmission diffraction grating and an optical wave will be diffracted by an angle θ when the acoustic wave is present.

The diffraction angle is determined by:

θ=—2 (1)

where λ is the wavelength of the light, u, is the acoustic frequency, and V, is the acoustic propagation velocity. The frequency of the acoustic standing wave is determined by lapping the acousto-optic crystal to a precise thickness which precisely defines the diffraction angle. Acousto-optic deflectors can be readily fabricated in two- dimensional arrays and are commercially available with up to 64 elements. An important advantage of acousto-optic deflectors is their modulation rate. The rate at which the diffracted beam can be turned "on" or "off" is principally determined by the time necessary for the acoustic wave to transit the optical beam. For beams on the order of 1 mm in diameter, typical rise times are on the order of 150 ns which is suitable for a laser printer application. Faster modulation rates are obtained with smaller beam diameters. The diffraction efficiency of an acousto-optic deflector is also important to the system performance for a laser printer. The diffraction efficiency can be greater than 75% for some materials.

There are two ways in which an acousto-optic deflector array can be used as a modulator in the laser printer. The first method is to block the undiffracted beam. When no acoustic wave is present the pixel is in the off state and no laser intensity reaches the media. When an acoustic wave is present, the diffracted beam reaches the media. The contrast ratio, or the ratio of the laser intensity reaching the media in the on versus the off state, is high using this method but the intensity uniformity of the pixels may have to be adjusted by individually adjusting the RF power to each acousto-optic element. Gray scale intensities for each pixel can be achieved by varying the RF drive power to each modulator. A second method is to block the diffracted laser intensity. When no acoustic wave is present the pixel is in the on (or write state) . The beam uniformity in this case depends on the uniformity of the beams input to the acousto-optic deflector array. In the off state, the laser intensity is diffracted into a beam block. Since the diffraction efficiency is not 100% the remainder will still be transmitted to the media. Thus, this method will have relatively low contrast ratio. Higher contrast ratios can be obtained by using acousto-optic elements in series.

Preferably, the optics components 18, 22 and 32 are formed from binary optical elements. Such elements are a subset of a class of optics which use diffraction rather than reflection or refraction to control the propagation of radiation. The term binary refers to the fact that the surfaces of these optics are composed of N discrete levels where N = 2, 4, 8, 16... Binary optics can be considered to be a specialized type of diffraction grating.

Conventional optics most often use spherical surfaces to refract light. The surfaces of refractive optics are always continuous and have no discontinuities. It is also possible to direct light by means of diffraction. By blocking the zones of a phase front that would give rise to destructive interference, it is possible to enhance the intensity at a specific point in the image plane. Rather than blocking zones which would give rise to destructive interference, more intensity is obtained by changing the phase of destructive zones by 180°. This is the operative principle of a Fresnel lens. A conventional spherical lens may be "collapsed" into a Fresnel lens by removing integral multiples of λ phase shift from the surface to generate a surface profile equivalent to a conventional lens. The surface of the Fresnel lens is discontinuous which give rise to diffraction. Light passing through the Fresnel lens is diffracted to a focal point with 100% efficiency. Fresnel lenses have been used in limited applications due to the difficulty in fabricating a discontinuous surface profile. The difficulty in making Fresnel optical elements is overcome by using binary optics where a continuous surface profile is approximated by a surface profile consisting of discrete steps. Binary optics are manufactured using semiconductor processing techniques. A mask which describes the N = 2 levels is prepared and used to expose a photoresist on the desired substrate. This pattern is then transferred into the substrate using either ion milling or plasma etching. This process is then repeated for subsequent binary levels where the number of levels is related to the number of etches, E, by N= 2^E. With current technology, binary optics with N = 16 (η_m - 98.7%) are the state of the art.

Binary optics have several advantages over traditional reflective or refractive optics. Since the elements are produced using lithographic techniques common to the semiconductor industry, binary optical devices can be readily mass produced. The cost of fabricating binary optics in production can be quite low. Like the semiconductor industry, many elements can be fabricated on a single substrate. For arrays of optics (such as lenslet arrays) the elements can be fabricated with lithographic positional accuracy (+/-0.1 μm) . This can greatly simplify optical array system design since the complexity of mechanically aligning elements to high accuracy is completely eliminated. This technology can also provide the option of producing square spots in the focal plane with no additional cost or complexity. The spatial positional accuracy is critical to certain applications for printing. The lateral spatial position of the focus will be displaced by an amount given by /ø, where / is the focal length of the lens and ø is the change in the angle of the laser beam incident on the focusing lens. Since all of the beams in the laser array remain parallel to each other independent of the angle with respect to the focusing lens array, the relative positions of the focal spots will remain unchanged to a first order. The parallelism and the beam separations are determined by the lithographic accuracy of the binary optical elements and these tolerances should be maintained in actual operation over the temperature ranges encountered in an actual print machine. This inherent accuracy will provide a significant latitude in the manufacturing assembly tolerances, since the focal spot positions are nearly independent of the relative angle between the laser beam and the binary optical element surfaces. For the printer system of the invention, three different types of binary optical elements are required. The first is beamsplitter element 18 which takes a single laser beam and splits it into N equal intensity beams travelling at different angles from a single point. The second binary optic element 22 takes the "fan" of beams 20 and makes them parallel to one another; this element is essentially a binary optic Fresnel lens. The final optic element 30 is an array of binary optic microlenses 32 which focus the sub-beams 34 onto the medium 36. One major advantage of having binary optical focusing lenses is that the spatial indexing is directly related to the pitch accuracy of the binary optical elements which can be on the order of 0.1 μm. As long as the array of separate sub-beams 34 are parallel, then the focal positions are indexed to at least 0.1 μm accuracy and are independent to a first order from any variation of separation of the beams in the array. Other types of micro-optical lenses that are known in the art form may also be used in place of the binary optical focusing lenses. An increase in processing speed can be obtained by using a two-dimensional array of laser beams and liquid crystal modulators, or other spatial light modulators, such as those described here, as shown in Fig. 4. For example, by having five beams in parallel, a 50 kHz switching speed can be effectively increased to 250 kHz. Fig. 4 illustrates a 5X3 print engine array in which three rows of sub-beams 34A, 34B, 34C are formed by three laser light sources 14A, 14B, 14C, respectively. The three output beams, one from each source, are coupled to respective optical beam splitters 18a, 18b, 18c where the three output beams are each split into five dispersive sub-beams B1-B5. Each of the three sets of five sub-beams are collimated, modulated and focused in respective units 80A, 80B and 80C, as previously described, to produce a 5X3 array of focused modulated sub-beams 90.

Conversely, only one laser need be used to form a two dimensional print engine, as shown in Fig. 4. In this case, a single two-dimensional binary optical element 18' is used to divide the primary beam 16' into an array of equal or nearly equal amplitude beams 20A,20B emanating from the single input beam 16'. This two-dimensional fan of beams is made parallel by binary optical element 22' followed by a two-dimensional version of the spatial light modulators 26' already described. A two-dimensional array of binary optical or other lenses 20' focuses the two- dimensional beams 32A,32B onto the photon-sensitive print media 36'. These arrays may be arranged to provide uniform focal points on a non-flat surface. This may be accomplished, for example, by adjusting the focal lengths and apertures to match the surface contour in order to provide equal laser spot sizes.

Note also that an electro-optical scanner array 90 may be provided adjacent focusing lens array 30' to scan the focused sub-beams 34A and 34B across the print media 36'. Figure 5 illustrates an alternate embodiment of the invention in which the light source 14 comprises one or more wavelength tunable lasers which is/are rapidly tuned in successive time periods by tuner mechanism 96 to produce a light beam 38. Light beam 38 consists of a series of pulses - (see curve A of Fig. 6) of consecutively increasing or decreasing wavelength

Primary beam 38 is amplitude modulated by a single spatial modulator, of the type previously described, to encode each wavelength pulse λ,-)^ with the pixel information from microprocessor 70. For example, as shown in Fig. 6, curve B; λ₂ and λ₅ may be blocked or deleted by modulator 100 and the rest of the wavelength pulses on beam 38 passed onto a wavelength sensitive element 102, such as, a grating 102 which diffracts each discrete wavelength pulse of the beam 38 into a different angle forming successive in time sub- beams B,-B„.

The separate diffracted sub-beams Bι-B_n are collimated by binary optical element 104 and focused by binary optical element 106 onto print media 36. The source 14 may consist of any laser which can be tuned over a broad spectral bandwidth with each wavelength operating with a controlled time delay. An example of such a laser could be an external cavity controlled semiconductor laser. Such lasers can have gain-bandwidths as great as 100 nm for semiconductor laser materials such as GaAlAs. The tuning element may be a grating, a tunable compound etalon, a Lyot filter or an acousto-optic filter controlled by command signals from microprocessor 70. These elements would be placed in the cavity to rapidly shift the output wavelength with time to produce a stream of pulses each having a different wavelength covering the gain bandwidth of the laser. The pulses may be subsequently modulated either digitally or with an intensity or pulse length control for gray scale print applications, as previously described. Alternatively, as shown in dotted lines, the laser may be pulsed (or Q- switched) in a coded fashion by modulator 98 to eliminate the need for external modulator 100 entirely. This approach is particularly simple for a semiconductor diode laser which can be directly amplitude modulated at high speed. To simplify the tuning of the laser, the longitudinal cavity modes may be chosen to provide the required spectral separation.

The number of resolvable spots on the print medium is limited by the resolution of the grating and the spectral separation of each pulse from the laser. With the present technology, it would be possible to have at least 1000 resolvable spots per laser, for example.

Depending on the resolution required, it may not be necessary to use the optical elements 104 which make the beams parallel.

Equivalents

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the following claims.

Claims

CLAIMSWhat is claimed is:

1. A printer comprising a print engine and a print medium and wherein the print engine comprises: a) a light source for generating an output beam of intense light; b) an optical system for dividing the output beam into an array of sub beams travelling generally parallel to each other in the same plane; c) a first modulator for encoding the array of beams with the information to be printed; and d) an optical focusing element for focusing each of the array beams; and wherein the print medium is a device sensitive to the intensity of the focused beams for forming an image of the information to be printed.

2. The printer of Claim 1 in which the light source comprises a device selected from the group comprising laser diodes, semiconductor lasers, solid state lasers and longitudinally optically pumped gain switched waveguide lasers.

3. The printer of Claim 1 including a tuner for rapidly changing the wavelength of the light source in time to produce said output beam having pulses of different wavelength and wherein the first modulator encodes the information onto the pulses before the output beam is separated into said array of sub beams.

4. The printer of Claim 1 wherein the first modulator encodes the information onto the array of sub beams.

5. The printer of Claim 1 wherein the optical system is comprised of a first binary optics element for simultaneously dividing the output beam into an array of equal amplitude beams and a second binary optics element for aligning the equal amplitude beams in parallel.

6. The printer of Claim 1 wherein the focusing element is a binary optic device.

7. The printer of Claim 1 wherein the first modulator is a device taken from the group comprising liquid crystal light modulators, electro-mechanical spacial light modulators, electro-absorption light modulators and acousto-optic light modulators.

8. The printer of Claim 1 wherein the light source comprises a two-dimensional array of laser devices for generating a two dimensional array of laser light beams.

9. The printer of Claim 1 wherein the print medium is a rotating drum with an outer photosensitive surface.

10. The printer of Claim 1 including means for moving the print engine relative to the medium.

11. A print engine comprising: a) a light source for generating an output beam of intense light; b) an optical system for dividing the output beam into an array of parallel light beams; c) a first modulator for encoding each of the light beams with the information to be printed; and d) an optical focusing element for focusing each of the light beams onto a print medium.

12. The engine of Claim 11 in which the light source comprises a device selected from the group comprising laser diodes, semiconductor lasers and solid state lasers and longitudinally optically pumped gain switched waveguide lasers.

13. The print engine of Claim 11 wherein the first modulator encodes the information onto the light beams after they are formed.

14. The print engine of Claim 11 further comprising a tuner for changing the wavelength of the light source to produce an output beam consisting of a series of pulses of different wavelengths and wherein the first modulator modulates the pulses to encode the information.

15. The engine of Claim 14 wherein the optical system is comprised of an optical element for diffracting the output beam into an array of sub beams in accordance with the wavelength of the pulses and a second binary optics element for aligning the diffracted beams in parallel.

16. The engine of Claim 11 wherein the focusing element is a binary optic device.

17. The engine of Claim 11 wherein the modulator is a device taken from the group comprising liquid crystal light modulators, electro-mechanical spacial light modulators, electro-absorption light modulators and acousto-optic light modulators.

18. The engine of Claim 11 wherein the light source comprises a two-dimensional array of lasers for generating a two dimensional array of laser beams.

19. A method of printing comprising the steps of: a) generating an output beam of intense light; b) splitting the output beam into an array of parallel light beams; c) encoding the array of beams with the information to be printed; d) focusing each of the array beams onto a print medium; and e) forming an image on the print medium of the information to be printed.

20. The method of Claim 19 in which the light source comprises a device selected from the group comprising laser diodes, semiconductor lasers, tunable semiconductors for lasers, solid state lasers and longitudinally optically pumped gain switched lasers.

21. The method of Claim 19 wherein the information is encoded before the array of beams is formed.

22. The method of Claim 19 wherein the information is encoded after the array of beams is formed.

23. The method of Claim 19 wherein the beam of intense light is comprised of a series of pulses of different wavelength and wherein the beam is split into an array of sub-beams of different wavelengths by a diffraction grating.

24. The method of Claim 23 wherein the information is encoded by amplitude or pulse length modulating the series of pulses.

25. The method of Claim 19 wherein the modulator is a device taken from the group comprising liquid crystal light modulators, electro-mechanical spacial light modulators, electro-absorption light modulators and acousto-optic light modulators.

26. The method of Claim 19 wherein the light source comprises a two-dimensional array generating a two dimensional array of laser beams.

27. The method of Claim 19 wherein the print medium is a rotating drum with an outer photosensitive surface.

28. The method of Claim 27 including moving the medium relative to the beams.