Abstract:
A system and method for reducing or eliminating the speckle intensity distribution of a laser imaging system. In one embodiment of the invention, a radio frequency signal is injected into a semiconductor laser light source ( 12 ) for a projection system ( 10 ) to create different speckle patterns that blend together on a projection surface ( 19 ). In another embodiment of the invention, optical feedback is used to induce a laser light source for a projection system ( 10 ) to create different speckle patterns that blend together on a projection surface ( 19 ). In another embodiment of the invention, the laser light source wavelength is Doppler shifted to produce different speckle patterns. In another embodiment of the invention, a means of deflection is used to directionally move the beam to reduce noticeable speckle.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Reference is made to commonly-assigned copending U.S. patent application Ser. No. 09/788,862, filed Feb. 20, 2001, entitled A SPECKLE SUPPRESSED LASER PROJECTION SYSTEM USING RF INJECTION, by Roddy et al.; U.S. patent application Ser. No. 09/788,978, filed Feb. 20, 2001, entitled A SPECKLE SUPPRESSED LASER PROJECTION SYSTEM USING A MULTI-WAVELENGTH DOPPLER SHIFTED BEAM now U.S. Pat. No. 6,445,487, by Roddy et al., the disclosures of which are incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to a laser projection system, and more specifically to a laser imaging system incorporating speckle suppression. 
     BACKGROUND OF THE INVENTION 
     Recently, projection systems have been created that utilize lasers as a light source. Two primary forms of laser projection systems exist. One laser projection system uses a laser and a raster technique to write an image pixel by pixel to a projection surface. Another laser projection system uses a laser as an illumination source and a spatial light modulator, such as a LCD, to project an image in its entirety onto a projection surface. 
     Laser projection systems using a raster procedure operate by deflecting a beam of coherent light generated by a laser to form an image. The deflectors include devices such as spinning mirrors and acousto-optic deflectors (AODs). Red, green, and blue light from laser sources can be independently modulated, and then combined and scanned onto a surface using a polygon mirror or galvanometer in a color projection system. 
     Unfortunately, there are a number of problems associated with laser raster projection systems. For example, flicker places an upper limit on the number of pixels displayable. Only one pixel can be displayed at any given moment due to the nature of the deflectors. Furthermore, there is no persistence to the display. Since laser projection systems direct the light onto a diffusion surface, all points to be displayed must be illuminated within a time period less than the critical flicker frequency (CFF) of the human eye. 
     Another problem with laser raster projection systems relates to the generation of color images, which requires the use of multi-colored lasers. Substantial difficulties exist in aligning and synchronizing multiple deflectors so as to simultaneously image different colors at a given pixel location. 
     Liquid Crystal Displays (LCDs) are also used in projection systems that may produce speckled images. Light sources used for LCD projection systems include incandescent lamps, arc lamps, Light Emitting Diodes (LEDs) and lasers. While LEDs emit incoherent light that would not produce speckle patterns in a projected image, they do not output sufficient light for projection systems. Lamps are brighter than LEDs, but not bright enough to be used in projection systems for large screens and lamps generate considerable heat. Lasers can be used as a light source for a LCD projection system since they are capable of outputting more usable light, thus providing a very bright image over a large area. 
     Lasers used as light sources for laser raster or LCD projection systems produce an undesirable speckle pattern in a projected image. Laser speckle is an interference pattern that results from the reflection or transmission of highly coherent light from an optically rough surface, one whose local irregularities in depth are greater than one quarter of a wavelength. For example, if a laser beam is directed at a wall, a bright spot with a surrounding distribution of speckles is observed, rather than a uniformly illuminated spot. Laser light directed at an uneven projection surface is reflected as different phases of light. The human eye perceives these different phases as interference. Thus, the mutual interference of partially coherent beams causes the speckle pattern. 
     Various systems and methods have been attempted in the prior art to address speckle elimination or suppression in a projected image. Speckle reduction techniques relying on mechanical motion can be difficult to implement and are prone to failure. Diffusers are also capable of reducing speckle, but tend to be extremely loose. U.S. Pat. No. 4,011,403 discloses an object-illuminating and imaging system comprised of a laser as a light source and an optical fiber as a light transmitter. A light-flow-disruptive means acts upon the collimated illumination to reduce objectionable speckle effects. In one embodiment, a diffuser such as polytetrafluoroethylene lens or disc may be interposed in the light flow path at a location between the light source and the object viewed to reduce objectionable speckle. Alternatively, speckle effect may be reduced by vibrating one of the elements in the optical path traversed by the light beam. A speckle reduction system that uses a diffusing element, however, will result in a display system with significant optical power losses. 
     U.S. Pat. No. 4,155,630 discloses a process and apparatus for improving image quality by speckle elimination in a coherent light imaging system. Diffused laser light is directed onto a mirror having a rocking motion that will cause the reflected rays to sweep a two-dimensional area. The reflected light is focused through a diffuser before collimating the light for use in image creation. However, the use of both a rocking mirror and a diffuser for speckle elimination in an imaging system results in significant losses of optical power. 
     U.S. Pat. No. 5,272,473 discloses a coherent light projection system and method having reduced speckle. The invention is comprised of a coherent light source and a display screen. The coherent light source generates a plurality of narrow light beams that impinge on the display screen at a plurality of associated points. The display screen has a transducer that generates surface acoustic waves that traverse the associated points. Significant movement of the display screen such as to sufficiently reduce noticeable speckle is difficult, especially when the display screen is large. 
     U.S. Pat. No. 5,274,494 discloses a method and apparatus for reducing or eliminating the speckle intensity of distribution in a highly coherent source. A coherent beam of light is directed into Raman cells to obtain a broad spectral bandwidth beam of light having additional side wavelengths other than the original wavelength. The composite beam having broad spectral bandwidth is capable of forming images that are substantially free of speckle intensity source. It is necessary to compensate for spatial incoherence that the Raman cells cause. Furthermore, the use of multiple Raman cells introduces significant complexity in a speckle reduction system. 
     U.S. Pat. No. 5,313,479 discloses a system and method for a speckle-free display system comprised of at least one coherent light source, a diffusing element located in a plane intercepts coherent light beam, a spatial light modulator for receiving the diffused light beam and for generating an image light beam, and a viewing surface. The movement of the diffusing element causes the speckle interference pattern to move on the viewing surface. The diffusing element can be rotated or vibrated. A speckle reduction system that uses a rotating diffusing element, however, will result in a display system with significant optical power losses. 
     In addition to image projection systems incorporating speckle reduction, lithographic systems have utilized speckle reduction techniques. U.S. Pat. No. 5,453,814 discloses a uniform illumination system and method for microlithography. A light source, having a solid state laser, emits a beam that is separated into a number of segments. Segments are frequency shifted by a different amount such that they do not substantially overlap in the frequency domain. Each segment passes through a short focal length lens element of a fly&#39;s eye array to be dispersed onto a mask plane for uniformly illuminating a mask. The lens element of the fly&#39;s eye array are contained within a small region in comparison to the width of the dispersed beam segments, such that each beam segment contributes illumination to the entirety of a common portion of the mask. The system provides uniform illumination in the deep ultra-violet range without speckles or fringes. The fly&#39;s eye array of frequency shifting elements, each element of which shifts by a different frequency, is an optically complex means of incorporating speckle reduction in an image projection system. 
     It is, therefore, desirable to reduce speckle in a laser projection system by non-mechanical or diffusion means. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a system and method for speckle suppression in laser projection systems. 
     The present invention is directed to overcome the problem set forth above. Briefly summarized according to one aspect of the present invention, a speckle suppressed laser projection system comprises a constant power supply and a semiconductor laser powered by the constant power supply. On optical fiber is positioned in front of an output of the laser wherein the optical fiber induces optical feedback in the laser which causes the laser to produce a multi-wavelength output beam. An optical projection system projects the multi-wavelength beam on a screen. 
     In another embodiment of the present invention, a radio frequency (RF) signal is injected into at least one semiconductor laser that acts as the light source for a projection system. The injection of radio frequency changes the laser emission mode structure rapidly, thus producing multiple longitudinal lasing modes, each of which exhibits a different speckle pattern. Images projected in a projection system comprises of radio frequency injected lasers will suppress the appearance of unwanted speckle, since the differing speckle patterns produced by the different laser operational modes will be superimposed and will blend together on a projection surface. 
     In a further aspect of the invention, the wavelength of a laser light source for a projection system is Doppler shifted to produce different speckle patterns. In one embodiment, the laser projection system is comprised of at least one laser and an acousto-optic modulator (AOM) which changes the laser wavelength by the RF carrier frequency to the AOM. Slewing the RF carrier frequency Doppler shifts the laser wavelength, alters the speckle pattern, and reduces the delectability of the speckle in the image formed on a projection surface. 
     In an additional aspect of the invention, a deflector directionally moves the output beam of a laser projection system to reduce noticeable speckle in an image on a projection surface. The pointing angle of the deflected beam from the AOM is a strong function of the carrier frequency. By slewing the frequency, the beam location on the screen can be changed, thus changing the speckle pattern. The movement of the beam need only be on the order of 1 pixel which does not significantly affect the modulation transfer function (MTF) in the direction of the movement. Beam movement at approximately 45 degrees to the vertical or horizontal can minimize the loss of MTF in the horizontal and vertical directions, and be less perceptible to the eye. 
     The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is a schematic for an image projection system using a multi-longitudinal laser source using a radio frequency (RF) injected semiconductor laser in an image projection system. 
     FIG. 1 b  is a graph of power versus wavelength for a multi-wavelength spectrum from a RF injected semiconductor laser. 
     FIG. 2 a  is a schematic for an image projection system using an optical fiber to induce multi-longitudinal mode operation in a semiconductor laser. 
     FIG. 2 b  is a graph of power versus wavelength for a multi-wavelength spectrum from a semiconductor laser with optical feedback from an optical fiber. 
     FIG. 3 is a schematic for an image projection system that uses a partially reflecting mirror to induce multi-longitudinal mode operation in a laser. 
     FIG. 4 is a schematic for an image projection system that uses a multi-wavelength laser beam by Doppler shifting with a variable frequency acousto-optic modulator (AOM). 
     FIG. 5 is a schematic for an image projection system that uses an angularly varying laser beam by using a variable frequency AOM. 
     FIG. 6 is a schematic for an image projection system that uses an angularly varying beam to illuminate an area array device to project an image. 
     FIG. 7 is a schematic for an image projection system that uses an angularly varying beam in a raster scan laser imaging system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be directed in particular to elements forming part of, or in cooperation more directly with, the apparatus in accordance with the present invention. It is understood that elements not specifically shown or described may take various forms well known to those skilled in the art. 
     FIG. 1 a  shows a laser projection system  10  comprising a radio frequency (RF) oscillator  11  driving a semiconductor laser  12  that acts as the light source for an image projection system  13 . The RF oscillator signal  14 , combined with a DC bias signal  15 , drive the semiconductor laser  12 . A laser beam  16  with optical power output P o  is emitted from the semiconductor laser. At the same time, a monitor beam with optical power output P m , at usually about 3% of P, is emitted from the semiconductor laser towards an internal photodiode  17 . Power output P m  from the semiconductor laser is monitored by the internal photodiode, and provides a signal to the control circuit  18 . The control circuit  18  monitors the variation in the signal from the internal photodiode  17 , and adjusts the DC bias current to the semiconductor laser accordingly so as to maintain constant output power. 
     A multi-wavelength laser beam  16  with power output P o  is emitted from the semiconductor laser towards an optical projection system  13 . The injection of radio frequency changes the laser emission from single mode to an effectively multimode pattern. 
     FIG. 1 ( b ) shows a graph of a multi-wavelength spectrum from an RF injected semiconductor laser. Forced multimode operation by radio frequency injection produces approximately 4 or 5 lasing modes, each of which exhibit a different speckle pattern. Using the multimode beam as input, the optical projection system shown in FIG. 1 ( a ) projects an image onto a projection surface, such as a screen  19 . Images projected in a projection system comprised of radio frequency injected lasers will suppress unwanted speckle patterns, since the differing speckle patterns produced by the different laser operational modes will blend together in an image. 
     FIG.  2 ( a ) shows a schematic for a laser projection system  20  that uses a semiconductor laser system to produce a multi-longitudinal mode output by optical feedback. A constant current supply  21  provides a drive current for the semiconductor laser  12 . An optical fiber  22  is positioned in front of the output beam of the laser. The semiconductor laser and the optical fiber are affixed to a thermoelectric cooler  23  to maintain a constant temperature for both elements. A temperature controller  24  monitors the thermoelectric cooler and provides a drive signal to the thermoelectric cooler. Laser output P o  is directed towards the optical fiber. 
     The laser beam is primarily directed through the optical fiber to the optical projection system  13 , but a portion of the light is reflected off of the optical fiber and back into the semiconductor laser. This induces optical feedback in the laser, which in turn induces mode hopping and multi-longitudinal mode output from the semiconductor laser. For example, semiconductor lasers available from Pointsource use a fiber pigtail to introduce optical feedback. 
     FIG.  2 ( b ) shows the multi-wavelength spectrum from the semiconductor laser when an optical fiber is place in front of the output of the laser. Optical feedback changes the laser emission from single mode to an effectively multimode pattern. Forced multimode operation by optical feedback produces approximately 4 or 5 lasing modes, each of which exhibit a different speckle pattern. Using the multimode beam as input, the optical projection system shown in FIG.  2 ( a ) projects an image onto a projection surface, such as a screen  19 . Images projected using a projection system comprised of induced multimode lasers will suppress unwanted speckle patterns, since the differing speckle patterns produced by the different laser operational modes will blend together in an image. 
     FIG. 3 shows a schematic for a laser projection system  30  with a laser system that produces a multi-longitudinal mode output by optical feedback. A laser  22  is driven by a signal form a power supply  31 . The laser could be a gas laser, a semiconductor laser, or a solid state laser. The optical output power of the laser, P o , is directed towards the surface of a partially reflecting mirror  32 . 
     The laser beam  33  is partially transmitted through the mirror  32  towards the optical projection system  13 . However, the laser beam is also partially reflected back towards the output window of the laser. When this light reenters the laser, it disturbs the laser cavity and causes the laser to output a multi-wavelength beam. Optical feedback changes the laser emission from single mode to an effectively multimode pattern. The laser may be instantaneously single mode, but it is forced to switch its mode structure rapidly. Forced multimode operation by optical feedback produces approximately 4 or 5 lasing modes, each of which exhibit a different speckle pattern. The multi-wavelength beam is directed towards a projection system. Because the projected beam from the optical projection system is multi-wavelength, the image formed on the imaging surface does not exhibit noticeable speckle. The varying wavelengths of the beam reduce the noticeable speckle, since the speckle patterns for each wavelength overlap and blend with each other in an image. 
     FIG. 4 shows a schematic for an image projection  40  that uses a multi-wavelength laser beam  16  by Doppler shifting with a variable frequency acousto-optic modulator (AOM)  41 . An oscillator  42  is used to generate a radio frequency (RF) signal  43 . A modulation oscillator  44  is used to generate a modulation signal. Both the RF signal generated by the oscillator and the modulation signal are directed into a frequency modulator  46 , which combines the signals to form a RF modulated signal  47  and directs the new signal into the AOM. The output of a laser  48  is directed towards the AOM that is receiving the RF modulated signal. 
     Slewing the RF signal frequency shifts the laser wavelength, alters the speckle pattern, and reduces the speckle in the image formed. The multi-wavelength Doppler shifted output beam from the AOM is directed towards an optical projection system  13 . The projection system projects an image onto an imaging surface  19 . Since the input beam to the optical projection system is a multi-wavelength beam, the varying wavelengths of the beam reduce the noticeable speckle because the speckle patterns for each wavelength overlap and blend with each other in an image. The modulation oscillator  44  need not be just a simple sinewave generating device. In order to minimize dwell at any one frequency and, therefore, the appearance of speckle, a triangle waveform is advantageous. 
     FIG. 5 shows a schematic for an image projection system  50  that uses an angularly varying laser beam with a variable frequency acousto-optic modulator (AOM). An oscillator  42  is used to generate a radio frequency (RF) signal  43 . A modulation oscillator  44  is used to generate a modulation signal  45 . Both the RF signal generated by the oscillator and the modulation signal are directed into a frequency modulator  46 , which combines the signals to form a RF modulated signal  47  and directs the new signal into the AOM. The output of a laser is directed towards the AOM  41  that is receiving the RF modulated signal. The AOM outputs a beam with a small angle variation towards an optical projection system. 
     The optical projection system  50  outputs an angularly varying beam towards an imaging surface  19 . By slewing the frequency, the beam location on the screen can be changed, thus changing the speckle pattern. To minimize dwell at any screen location, it is advantageous to have the modulation signal  45  generated by the modulation oscillator  44  be a triangle wave. The movement of the beam need only be approximately 1 pixel so as not to destroy the modulation transfer function (MTF) in the direction of the movement. Since the eye is very sensitive to horizontal and vertical edges, but less sensitive to angles in between, beam movement of approximately 45 degrees can minimize the perceived loss of MTF in the horizontal and vertical directions. However, the beam movement frequency must be high enough to prevent any obvious flicker to the eye. For example, as noted in U.S. Pat. No. 5,272,473, a beam movement frequency between approximately 5 Hz and 60 Hz, depending on light level, would be sufficiently undetectable by the human eye. 
     FIG. 6 shows a schematic for a laser system  60  capable of generating an angularly varying beam that illuminates an area array device to project an image. A laser beam  61  is directed towards an AOM  41 . A variable frequency carrier signal  62  is injected into the AOM. The AOM outputs an angularly varying beam  63  towards expansion optics  64  that expand the beam. The expanded beam is directed towards a beamsplitter cube  65 . On the adjacent side of the beamsplitter cube from the angularly varying beam entry, a spatial light modulator  66  projects an image from image data  67  provided to the spatial light modulator driver  68 . The beamsplitter cube outputs an image through an optional polarizer  69 , and then towards a projection lens  70 . The output of the projection lens directs an angularly varying beam  71  towards a projection surface  19 . 
     The image projected on the projection surface is speckle suppressed, since the angularly varying beam blends the speckle pattern of the beam with the image. As stated in the previous embodiment, the movement of the beam need only be approximately 1 pixel so as not to destroy the modulation transfer function (MTF) in the direction of the movement. Since the eye is very sensitive to horizontal and vertical edges, but less sensitive to angles in between, beam movement of approximately 45 degrees may minimize the loss of MTF in the horizontal and vertical directions. However, the beam movement frequency must be high enough to prevent any obvious flicker to the eye. For example, a beam movement frequency between approximately 5 Hz and 60 Hz would be sufficiently undetectable by the human eye. For the sake of simplicity, FIG. 6 shows only a single beam projection system. Multiple beams, e.g. red, green and blue, can be used from separate sources and spatial light modulators and then combined using an X-cube prism, or other suitable means, for a multicolor projection system. There are a number of examples of multibeam systems in the art of incoherent color projectors. 
     FIG. 7 shows a schematic for a laser system  80  capable of generating an angularly varying beam in a raster scan laser system that projects an image pixel by pixel. The output beam  61  of a laser  48  is directed towards the input of an AOM  41 . The AOM  41  is driven by a pixel modulation signal  72 . The output beam of the AOM  42  is directed towards another AOM  63  directly in front of the output of the first AOM, which is driven by a variable frequency carrier signal  64 . An angularly varying beam is outputted from the variable frequency driven AOM towards a set of optics  65 . The output beam from the optics is directed towards a high speed horizontal deflector, such as a polygon  86 . The reflected beam  88  from the polygon is directed towards a slow speed vertical deflector  89 , such as a galvanometer or mirror. The reflected beam from the vertical deflector is directed towards a projection lens  70 . An angularly varying output beam  90  is outputted from the projection lens towards a projection surface  19 . 
     The image projected on the screen is speckle suppressed, since the angularly varying beam blends the speckle pattern of the beam with the image. As stated in the previous embodiment, the movement of the beam need only be approximately 1 pixel so as not to destroy the modulation transfer function (MTF) in the direction of the movement. Since the eye is very sensitive to horizontal and vertical edges, but less sensitive to angles in between, beam movement of approximately 45 degrees may minimize the loss of MTF in the horizontal and vertical directions. However, the beam movement frequency must be high enough to prevent any obvious flicker to the eye. For example, a beam movement frequency between approximately 5 Hz and 60 Hz would be sufficiently undetectable by the human eye. 
     Thus, it is seen that speckle may be suppressed by the use of RF injection, optical feedback, Doppler shifted wavelength, or a deflection means suppresses speckle in a laser projection system. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     PARTS LIST 
       10 . Laser projection system 
       11 . Radio frequency (RF) oscillator 
       12 . Semiconductor laser 
       13 . Optical projection system 
       14 . RF oscillator signal 
       15 . DC bias signal 
       16 . Multi-wavelength laser beam 
       17 . Internal photodiode 
       18 . Control circuit 
       19 . Screen 
       20 . Laser projection system 
       21 . Constant current supply 
       22 . Optical fiber 
       23 . Thermoelectric cooler 
       24 . Temperature controller 
       25 . Laser 
       30 . Laser projection system 
       31 . Power supply 
       32 . Partially reflecting mirror 
       40 . Image projection system 
       41 . Acousto-optic modulator (AOM) 
       42 . Oscillator 
       43 . Radio frequency (RF) signal 
       44 . Modulation oscillator 
       45 . Modulation signal 
       46 . Frequency modulator 
       47 . RF modulated signal 
       48 . Laser 
       50 . Image projection system 
       60 . Laser projection system 
       61 . Laser beam 
       62 . Carrier signal 
       63 . Varying beam 
       64 . Expansion optics 
       65 . Beamsplitter cube 
       66 . Spatial light modulator 
       67 . Image data 
       68 . Modulator driver 
       69 . Polarizer 
       70 . Projection lens 
       71 . Angularly varying beam 
       72 . Pixel modulation signal 
       80 . Laser projection system 
       86 . Polygon 
       88 . Reflected beam 
       89 . Vertical deflector 
       90 . Output beam