WO2012100298A1

WO2012100298A1 - Laser optical engine with means to remove laser speckle

Info

Publication number: WO2012100298A1
Application number: PCT/AU2012/000063
Authority: WO
Inventors: Danny Jung; Peter Rubinshtein
Original assignee: Digislide Holdings Limited
Priority date: 2011-01-28
Filing date: 2012-01-27
Publication date: 2012-08-02

Abstract

A laser optical engine for reducing laser speckle. The engine comprises in series a green laser adapted to emit a beam of light through a prism, first lenslet array, a beam expander, a first and then a second dichroic mirror, a second lenslet array, a lens, a polarising beam splitter, a LCOS panel and a projection lens. When using the optical engine to project the image as displayed in the LCOS panel three lasers are used being in the green, blue and red parts of the electromagnetic spectrum. The blue and red lasers do not pass through the beam expander but rather are introduced into the optical engine by reflection through their respective dichroic mirrors.

Description

Laser optical engine with means to remove laser speckle

FIELD OF THE INVENTION

The present invention relates to an optical engine consisting of an LCOS (Liquid crystal on silicon) panel using multiple lasers to illuminate the panel, a projection lens then relaying the image onto a surface, the optical engine minimizing laser speckle.

BACKGROUND TO THE INVENTION

The use of lasers as a light source for projection has recently gained the attention of inventors and industry alike. Lasers have these distinct advantages over LEDs (Light emitting diodes).

1. The lasers are already polarised and as the LCOS requires polarised light, lasers are a more efficient light source.

2. The entendue of lasers are much less than LED's. Entendue is important because it never decreases in any optical system. A perfect optical system produces an image with the same entendue as the source. Thus for lasers the focal ratio of the optical engine can be high resulting in small light cone angles.

3. The lasers beams have practically collimated shapes which makes

optical design much simpler and more efficient.

4. Existing projections systems such as the MEMS (Micro-Electro- Mechanical Systems) system using scanning lasers do not change the intensity of the laser beam, and thus a hazard to the human eye remains.

The advantages of the small angles are threefold.

1. It gives a large depth of focus allowing focus free operation.

2. The contrast of the projected image can be higher than a design using large cone angles. 3. The small cone angles allows for a simpler lens design resulting in a smaller package.

However, one disadvantage of using lasers as light sources is speckle or noise which can result in grainy images if it were not removed. Speckle has been, and is, the chief limitation of coherent imaging. An image with a speckle problem will have the visual effect of a boiling liquid or of a television set that is way out of tune.

A speckle pattern is a random intensity pattern produced by the mutual interference of a set of wavefronts. This phenomenon has been known and the subject of investigation by scientists for hundreds of years. The issue of speckles however, has become a focus in recent times because of the increase in the use of lasers, and the variety of applications thereof.

The speckle pattern is random, particularly if the laser beam is scattered off a rough surface. Likewise, when the laser is beamed through imperfect optics, or through the atmosphere, speckle forms, which if highly magnified appears like the image of a star.

Speckle is a problem in laser based display systems like the Laser TV. Speckle is usually quantified by the speckle contrast. Speckle contrast reduction is essentially the creation of many independent speckle patterns, so that they average out on the retina/detector. This can be achieved by several methods.

1. Angle diversity - with illumination from different angles.

2. Polarization diversity - using different polarization states.

3. Wavelength diversity - using multiple laser sources with differing wavelengths. 4. Scanning lasers, such as the MEMS system (as above)

Other methods of speckle reduction in coherent optical imaging include rotating diffusers which interfere with the spatial coherence of the laser light, synthetic array heterodyne detection, use of spatial filters and optical heterodyn detection - when signal frequencies are non-linearly mixed with a local oscillator set at a close-by frequency, resulting in a different frequency, which carries amplitude, phase, and frequency modulatioii of the original higher frequency signal, but which is oscillating at a lower frequency.

Any method which includes some form of vibrating component at a frequency too high for the human eye to see the speckle, unfortunately introduces moving components into the design.

Furthermore, the tremendous scientific and medical advancements made possible by laser technology have not come without risk. The use of lasers in consumer devices is that the laser can be very hazardous to the human retina.

The eye is particularly vulnerable to lasers because the radiation is emitted in a well-collimated beam. If a laser beam intersects the eye, radiation is transmitted through the cornea, aqueous humor, lens and vitreous humor and is imaged or focused to a small area on the retina, causing tissue necrosis resulting in visual impairment. The severity of the injury will be dependent on the exposure dose.

Lasers emitting wavelengths approximately 400 to 1,400 nm are in the "retinal hazard spectral region".

In industrial situations severe injuries have been caused by frequent and/or long term exposure to high-density beams and a failure by personnel to follow OHSW procedures and use protective eyewear.

Traditionally, consumer acquired injuries have been very minimal because commercially available lasers have not been potent enough to cause serious or long- term damage. However, the introduction of the laser based projection systems to consumer markets has the potential to change that.

Whilst European Standards, Australian Standards and USA Standards have been documented with regard to consumer devices that contain LED and laser technologies, risk remains. The existing arrangements may be satisfactory for a range of applications, however, they are not the best solution since they do not provide the lowest cost of manufacturing of the projection devices. They are also not robust given that the potential use in miniature hand-held display devices and/or projection systems embedded in common systems such as mobile phones, GPS devices and gaming consoles are subject to frequent dropping or mishandling. Furthermore larger projection device that are in less than ideal conditions require more robust units when used, for example, in developing countries.

The object of this invention is to provide an optical engine that overcomes at least some of the abovementioned problems or provides the public with a useful alternative.

SUMMARY OF THE INVENTION

Therefore in one form of the invention there is proposed A laser optical engine for reducing laser speckle, the apparatus comprising;

a green laser adapted to emit a beam of light through a prism and a beam expander, said apparatus further including a first lenslet array positioned between the prism and the beam expander, said light passing through the beam expander then passing through a first and then a second dichroic mirror and then through a second lenslet array and through a lens before passing through a polarising beam splitter and reflected in a LCOS panel to then pass through a projection lens and be projected onto a surface.

In preference the laser optical further comprises a red and a blue leaser, the light from those lasers entering the optical engine by reflection through respective dichroic mirrors. In preference the lenslet arrays comprise a hexagonal grid.

In preference the first and second lenslet arrays comprise a square grid of at least 16 lenslets at least partially within the beam.

In preference the lenslet arrays comprise fused silica. In preference the beam splitter widens the beam of light thereby dissipating it and reducing its intensity.

In preference the lenslet arrays comprise a first side comprising an array of concave cylindrical surfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing, which is incorporated in and constitutes a part of the specification, illustrates an implementation of the invention and, together with the description, serves to explain the advantages and principles of the invention. In the drawing:

Fig 1 is a schematic drawing illustrating the optical engine.

LIST OF COMPONENTS

01 green laser

2A prism 2B beam expander

03 lenslet array

04 dichroic mirror

05 dichroic mirror

06 lenslet array

07 lens

08 polarising beam splitter

09 LCOS panel

10 projection lens DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

The following detailed description of the invention refers to the accompanying ' drawing. Wherever possible, the same reference numbers will be used throughout the drawing and the following description to refer to the same and like parts.

Dimensions of certain parts shown in the drawing may have been modified and or exaggerated for the purposes of clarity or illustration.

The optical engine described herein may utilize any sized LCOS panel, or similar display panel,, and Red, Green and Blue lasers to illuminate the panel.

The initial prototype has used a 0.29" WVGA monochrome LCOS panel. Other prototypes may include a .21 " LCOS panel, SVGA, WVGA or SVGA SIZE. It is however no intended to limit the invention to either the size or resolution of the panel.

Typically Red, Green and Blue lasers illuminate the panel, however the projection system may use a hybrid of laser and LED light sources. This is well known in prior art.

A projection lens relays the image onto a screen or wall. The three lasers are turned on and off in synchronization with the appropriate Red, Green and Blue image on the LCOS panel to produce full colour images on the screen. Likewise, the basic principles of colour sequential digital projectors are well known. The advantages and disadvantages of the use of lasers have been noted above. As outlined above the disadvantages of using lasers as light sources are speckle and safety issues. The present invention addresses the above issues, as well as having the potential to lower the cost of manufacture.

Because the Green laser has a narrower beam waist diameter of approximately 50 microns (or maximum of .1 mm) it is more prone to speckling issues. It is also more prone to being a hazard to the human eye. Thus, the following description addressed the issues with the Green laser. However, Red and Blue lasers maybe similarly treated, or may beam directly onto dichroic mirrors. Much depends on the width of the laser beam.

The light from the Green laser 1 beams onto a prism 2A. The prism is designed with two functions; to slightly expand the beam diameter and to provide some de- speckling properties.

The beam then passes through a further beam expander 2B with a lenslet array 3 having a hexagonal shape lenslet. This configuration does three things.

It changes the Gaussian profile of the beam into a Top Hat profile thereby illuminating the 16:9 array uniformly. It reduces remaining speckle by overlapping multiple images from each of the lenslet and it allows considerable error in beam alignment from the laser.

The hexagonal lenslet array may also be incorporated into the Red and Blue lasers. As the laser light is made up of a multitude of images from each lenslet the beam intensity is reduced, further reducing eye hazard and further reducing speckle.

A Red, Green and Blue laser illuminates a lenslet array 6 with rectangular lenslets of 16:9 aspect ratio via dichroic mirrors 4 and 5. The light from the Red and Blue lasers are collimated using a collimating lens. The light from the Green laser 1, is expanded using a beam expander 2A and 2B, which incorporates a lenslet array 3 with hexagonal shaped lenslets.

Again, the 1 :9 ratio matches the aspect ratio of the panel ensuring efficient and uniform illumination, and minimal light loss. Again, a different panel aspect ratio will call for a corresponding aspect ratio on the lenslet array. Again, the beam is widened reducing speckle further reducing the hazard to the human eye of the end consumer. The light emanating from the 16:9 array is relayed through a lens 7 onto the polarising beam splitter 8. The polarising beamsplitter acts as an analyser, allows the light to reflect off the LCOS panel, further reducing the laser hazard.

The light reflects onto the LCOS panel 9 which allows the light from "on" pixels to pass to the screen while blocking the "off" pixels to create the image. The image is then passed back through the polarising beamsplitter 8 and out through the projection lens 10 as the full projected image.

It should be noted that the design of the prism and the ratio of the lenslet array may be changed under different designs to match a different aspect ratio of a different panel, to ensure efficient and uniform illumination of a different panel.

The reader will now appreciate the advantages of the present invention. There are no moving parts, which are, by their nature, subject to wear and tear and can be the subject to damage from dropping or mishandling in hand held devices. With no moving parts in a larger projection system it may be used in an environment in which little is known of technology, technical maintenance and may require a more robust feature within the device. The device is less expensive to manufacture and reduces laser hazards for the human eye when projection system is used within a consumer or commercial environment.

Further advantages and improvements may very well be made to the present invention without deviating from its scope. Although the invention has been shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope and spirit of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus. Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of the common general knowledge in this field.

In the summary of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprising" is used in the sense of "including", i.e. the features specified may be associated with further features in various embodiments of the invention.

Claims

1. A laser optical engine for reducing laser speckle, the apparatus comprising; a green laser adapted to emit a beam of light through a prism and a beam expander, said apparatus further including a first lenslet array positioned 5 between the prism and the beam expander, said light passing through the beam expander then passing through a first and then a second dichroic mirror and then through a second lenslet array and through a lens before passing through a polarising beam splitter and reflected in a LCOS panel to then pass through a projection lens and be projected onto a surface.

10.

2. A laser optical engine as in claim 1 further comprising a red and a blue leaser, the light from those lasers entering the optical engine by reflection through respective dichroic mirrors.

3. A laser optical engine as in claim 1 wherein the lenslet arrays comprise a

hexagonal grid.

15 4. A laser optical engine as in claim 1 where the first and second lenslet arrays comprise a square grid of at least 16 lenslets at least partially within the beam.

5. A laser optical engine as in claim 1 wherein the lenslet arrays comprise fused silica.

6. A laser optical engine as in claim 1 wherein the beam splitter widens the beam 0 of light thereby dissipating it and reducing its intensity.

7. A laser optical engine as in claim 1 wherein the lenslet arrays comprise a first side comprising an array of concave cylindrical surfaces.