BACKGROUND OF THE INVENTION

The present invention relates to an illumination device, and more particularly to a laser illumination device based on electrically switchable Bragg gratings that reduces laser speckle.

Miniature solid-state lasers are finding their way into a range of display applications. The competitive advantage of lasers results from increased lifetime, lower cost, higher brightness and improved colour gamut. Although lasers offer much more compact illumination solutions than can be provided with conventional sources such as LED the demand for yet more compressed form factors continues unabated. Classical illumination designs using beam splitters and combiners fail to meet the requirements. It is known that delivering laser illumination via waveguide optics can result in 50% reduction in size compared with conventional lens combiner splitter schemes.

Laser displays suffer from speckle, a sparkly or granular structure seen in uniformly illuminated rough surfaces. Speckle arises from the high spatial and temporal coherence of lasers. Speckle reduces image sharpness and is distracting to the viewer. Several approaches for reducing speckle contrast have been proposed based on spatial and temporal decorrelation of speckle patterns. More precisely, speckle reduction is based on averaging multiple sets of speckle patterns from a speckle surface resolution cell with the averaging taking place over the human eye integration time. Speckle may be characterized by the parameter speckle contrast which is defined as the ratio of the standard deviation of the speckle intensity to the mean speckle intensity. Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast. Traditionally, the simplest way to reduce speckle has been to use a rotating diffuser to direct incident light into randomly distributed ray directions. The effect is to produce a multiplicity of speckle patterns while maintaining a uniform a time-averaged intensity profile. This type of approach is often referred to as angle diversity. Another approach known as polarization diversity relies on averaging phase shifted speckle patterns. In practice neither approach succeeds in eliminating speckle entirely.

It is known that speckle may be reduced by using an electro optic device to generate variations in the refractive index profile of material such that the phase fronts of light incident on the device are modulated in phase and or amplitude. U.S. Pat. No. 8,224,133 with issue date 17 Jul. 2012 entitled LASER ILLUMINATION DEVICE discloses a despeckler based on a new type of diffractive electro optical device known as an electrically Switchable Bragg Grating (SBG).

There is a need for a compact laser illuminator that efficiently combines waveguide optics with a diffractive electro-optical despeckler.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an compact laser illuminator based on a waveguide that provides despeckled light output.

The objects of the invention are achieved in a first embodiment comprising a laser; a waveguide comprising at least first and second transparent lamina; a first grating device for coupling light from the laser into a TIR path in the waveguide; a second grating device for coupling light from the TIR path out of the waveguide; and a third grating device for applying a variation of at least one of beam deflection or phase retardation across the wavefronts of the TIR light. The first second and third grating devices are each sandwiched by transparent lamina.

In one embodiment of the invention the third grating device is electrically switchable. Transparent electrodes are applied to portions of transparent lamina sandwiching the grating device gratings. The electrodes substantially overlap the grating device.

In one embodiment of the invention the third grating device is electrically switchable. Transparent interdigitated electrodes are applied to portions of a transparent lamina overlapping the grating device.

In one embodiment of the invention the optical prescription of the third grating device varies along said waveguide.

In one embodiment of the invention the first and second grating devices are grating lamina.

In one embodiment of the invention the third grating device comprises more than one grating lamina adjacently disposed along the waveguide.

In one embodiment of the invention the third grating device comprises a two dimensional array of SBG elements. Transparent electrodes are applied to overlapping portions of transparent lamina sandwiching the SBG elements. At least on of the electrodes is pixelated into elements substantially overlapping the SBG elements.

In one embodiment of the invention the first and second grating devices are SBGs.

In one embodiment of the invention the third grating device is a SBG.

In one embodiment of the invention the illuminator further comprises a fourth grating device for applying variation of at least one of beam deflection or phase retardation across the wavefronts of the TIR light. The third and fourth grating devices overlap. The third and fourth grating devices have identical prescriptions and are configured in a reciprocal sense.

In one embodiment of the invention the third grating device overlaps the first grating device. In one embodiment of the invention the third grating device overlaps the second grating device.

In one embodiment of the invention the third grating device is disposed along the TIR path between the first and second grating devices.

In one embodiment of the invention the third grating device diffuses light into the direction of the TIR path.

In one embodiment of the invention the at least one of the transparent lamina is wedged.

In one embodiment of the invention at least one end of the waveguide is terminated by a reflector.

In one embodiment of the invention the illuminator further comprises a reflector disposed adjacent to an external surface of the waveguide. The reflector comprises a transmission grating and a mirror.

In one embodiment of the invention the at least one end of the waveguide is terminated by a quarter wave plate and a mirror.

In one embodiment of the invention the second grating device comprises overlapping grating lamina separated by a transparent lamina. One grating lamina diffracts S-polarized light and the other grating lamina diffracts P-polarized light.

In one embodiment of the invention there is provided an illumination device comprising: a laser; a waveguide comprising at least first and second transparent lamina; a first grating device comprising an array of selectively switchable grating elements; and a second grating device for coupling light from the TIR path out of the waveguide. The first and second grating devices are each sandwiched by the transparent lamina. Each grating element encode a beam deflector for coupling light from the laser into a TIR path in the waveguide and a lens having a Fourier plane in proximity to an illumination plane of the illumination device

A more complete understanding of the invention can be obtained by considering the following detailed description in conjunction with the accompanying drawings, wherein like index numerals indicate like parts. For purposes of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail.

