U.S. patent application number 09/788978 was filed with the patent office on 2002-08-22 for speckle suppressed laser projection system using a multi-wavelength doppler shifted beam.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Markis, William R., Roddy, James E..
Application Number | 20020114057 09/788978 |
Document ID | / |
Family ID | 25146181 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020114057 |
Kind Code |
A1 |
Roddy, James E. ; et
al. |
August 22, 2002 |
SPECKLE SUPPRESSED LASER PROJECTION SYSTEM USING A MULTI-WAVELENGTH
DOPPLER SHIFTED BEAM
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. 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.
Inventors: |
Roddy, James E.; (Rochester,
NY) ; Markis, William R.; (Spencerport, NY) |
Correspondence
Address: |
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
25146181 |
Appl. No.: |
09/788978 |
Filed: |
February 20, 2001 |
Current U.S.
Class: |
359/278 ;
348/E5.137; 348/E5.139; 348/E9.026; 359/285; 359/287 |
Current CPC
Class: |
H01S 5/06213 20130101;
H01S 5/0652 20130101; H04N 5/7416 20130101; G02F 1/116 20130101;
H04N 9/3129 20130101; H04N 5/74 20130101 |
Class at
Publication: |
359/278 ;
359/287; 359/285 |
International
Class: |
G02F 001/23; G02F
001/11 |
Claims
What is claimed is:
1. A speckle suppressed laser projection system comprising: a laser
which provides a laser beam; a modulation oscillator which produces
a signal; an oscillator which produces a carrier; a frequency
modulator, wherein said frequency modulator combines said signal
from said modulation oscillator and said carrier from said
oscillator to produce a frequency modulated signal; and an
acusto-optic modulator wherein said modulator receives said
frequency modulated signal and said laser beam, and outputs a
multi-wavelength Doppler shifted beam.
2. A speckle suppressed laser projection system as in claim 1
wherein an optical projection system displays said multi-wavelength
doppler shifted beam on a screen.
3. A speckle suppressed laser projection system as in claim 2
wherein image data is supplied to said optical projection
system.
4. A speckle suppressed laser projection system as in claim 1
wherein said signal produced by the modulated oscillator is a
triangle wave.
5. A speckle suppressed laser projection system comprising: a laser
which provides a laser beam; a modulation oscillator which produces
a signal; an oscillator which provides a carrier; a frequency
modulator, wherein said frequency modulator combines said signal
from said modulation oscillator and said carrier from said
oscillator to produce a frequency modulated signal; an acusto-optic
modulator wherein said modulator receives said frequency modulated
signal and said laser beam to produce a laser beam with a small
angle variation; and an optical projection system wherein said
optical projection system projects said laser beam with small angle
variations on a screen.
6. A speckle suppressed laser projection system as in claim 5
wherein image date is supplied to said optical projection
system.
7. A speckle suppressed laser projection system as in claim 5
wherein the signal produced by the modulation oscillator is a
triangle wave.
8. A speckle suppressed laser protection system as in claim 5
wherein the angular variation is substantially at a 45 degree angle
to the horizontal axis of the screen.
9. A speckle suppressed laser projection system comprising: a laser
which generates a laser beam; a circuit, wherein said circuit
produces a variable frequency career signal; a modulator wherein
said modulator is injected with said variable frequency carrier
signal which modifies said laser beam with a small angular
variation; expansion optics which expands said small angular beam
from the modulator, and the output from said optics is an expanded
angular beam; a spatial light modulator (SLM) driver, wherein said
(SLM) driver receives image data as an input; a spatial light
modulator (SLM), wherein said SLM receives a signal from said SLM
driver and produces an output; a beamsplitter cube, wherein said
cube combines said expanded beam from said expansion optics with
said output from said SLM; and a projection lens, wherein said
projection lens outputs an angularly varying beam.
10. A speckle suppressed laser projection system as in claim 9
wherein said angularly varying beam forms an image with speckle
suppression.
11. A method of speckle reduction, the method comprising the steps
of: generating a signal; modulating said signal to produce a
frequency modulated carrier signal; injecting said frequency
modulated carrier into an acousto-optic modulator (AOM); directing
a laser beam into said AOM, wherein said output of the AOM when it
is injected by said carrier signal is a multi-wavelength Doppler
shifted beam; directing said multi-wavelength beam into an optical
projection system; projecting said multi-wavelength beam onto a
projection surface to form an image, where the multi-wavelength
beam has different speckle patterns that blend together.
12. A method of speckle reduction, the method comprising the steps
of: generating a signal; modulating said signal to produce a
frequency modulated carrier signal; injecting said frequency
modulated carrier into an acousto-optic modulator (AOM); directing
a laser beam into said AOM, wherein said output of the AOM when it
is injected by said carrier signal is a beam with a small angle
variation; directing said beam with small angle variation into an
optical projection system; and projecting said beam with small
angle variation onto a projection surface to form an image, wherein
the speckle patterns of the beam are blended together.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly-assigned copending U.S. patent
application Ser. No. (Attorney Docket No.______ 81 620/NAB), filed
herewith, entitled A SPECKLE SUPPRESSED LASER PROJECTION SYSTEM
USING RF INJECTION, by Roddy et al.; U.S. patent application
No.______ (Attorney Docket No. 82323/NAB), filed herewith, entitled
A SPECKLE SUPPRESSED LASER PROJECTION SYSTEM WITH PARTIAL BEAM
REFLECTION, by Roddy et al., the disclosures of which are
incorporated herein.
FIELD OF THE INVENTION
[0002] This invention relates generally to a laser projection
system, and more specifically to a laser imaging system
incorporating speckle suppression.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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's eye array to be dispersed onto a mask plane
for uniformly illuminating a mask. The lens element of the fly'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'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.
[0015] It is, therefore, desirable to reduce speckle in a laser
projection system by non-mechanical or diffusion means.
SUMMARY OF THE INVENTION
[0016] It is an object of the present invention to provide a system
and method for speckle suppression in laser projection systems.
[0017] The present invention is directed to overcome the problem
set forth above. Briefly summarized according to one aspect of the
present invention, a 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.
[0018] In a further aspect of the 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.
[0019] 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.
[0020] 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
[0021] FIG. 1a 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.
[0022] FIG. 1b is a graph of power versus wavelength for a
multi-wavelength spectrum from a RF injected semiconductor
laser.
[0023] FIG. 2a is a schematic for an image projection system using
an optical fiber to induce multi-longitudinal mode operation in a
semiconductor laser.
[0024] FIG. 2b is a graph of power versus wavelength for a
multi-wavelength spectrum from a semiconductor laser with optical
feedback from an optical fiber.
[0025] 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.
[0026] 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).
[0027] FIG. 5 is a schematic for an image projection system that
uses an angularly varying laser beam by using a variable frequency
AOM.
[0028] 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.
[0029] 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
[0030] 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.
[0031] FIG. 1a 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.sub.o is emitted from the semiconductor laser. At
the same time, a monitor beam with optical power output P.sub.m, at
usually about 3% of P.sub.o, is emitted from the semiconductor
laser towards an internal photodiode 17. Power output P.sub.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.
[0032] A multi-wavelength laser beam 16 with power output P.sub.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.
[0033] 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.
[0034] FIG. 2(a) shows a schematic for an image 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 signal 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.sub.o is directed towards the
optical fiber.
[0035] 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.
[0036] 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.
[0037] FIG. 3 shows a schematic for an image 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 from 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.sub.o, is directed towards the surface of a partially reflecting
mirror 32.
[0038] 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.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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 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.
[0043] 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.
[0044] 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 generated 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.
[0045] 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 is
driven by a pixel modulation signal 62. The output beam of the AOM
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.
[0046] 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.
[0047] 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.
[0048] 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
[0049] 10. Laser projection system
[0050] 11. Radio frequency (RF) oscillator
[0051] 12. Semiconductor laser
[0052] 13. Optical projection system
[0053] 14. RF oscillator signal
[0054] 15. DC bias signal
[0055] 16. Multi-wavelength laser beam
[0056] 17. Internal photodiode
[0057] 18. Control circuit
[0058] 19. Screen
[0059] 20. Image projection system
[0060] 21. Constant current supply
[0061] 22. Optical fiber
[0062] 23. Thermoelectric cooler
[0063] 24. Temperature controller
[0064] 25. Laser
[0065] 30. Image projection system
[0066] 31. Power supply
[0067] 32. Partially reflecting mirror
[0068] 40. Image projection system
[0069] 41. Acousto-optic modulator (AOM)
[0070] 42. Oscillator
[0071] 43. Radio frequency (RF) signal
[0072] 44. Modulation oscillator
[0073] 45. Modulation signal
[0074] 46. Frequency modulator
[0075] 47. RF modulated signal
[0076] 48. Laser
[0077] 50. Image projection system
[0078] 60. Laser projection system
[0079] 61. Laser beam
[0080] 62. Carrier signal
[0081] 63. Varying beam
[0082] 64. Expansion optics
[0083] 65. Beamsplitter cube
[0084] 66. Spatial light modulator
[0085] 67. Image data
[0086] 68. Modulator driver
[0087] 69. Polarizer
[0088] 70. Projection lens
[0089] 71. Angularly varying beam
[0090] 80. Laser projection system
[0091] 86. Polygon
[0092] 88. Reflected beam
[0093] 89. Vertical deflector
[0094] 90. Output beam
* * * * *