LASER ILLUMINATOR BACKGROUND OF THE INVENTION

This application claims priority to United Kingdom patent application No. GB0516063.5 filed 4 August 2005, The present invention relates to an illumination device, and more particularly to a laser illuminator device based on electrically switchable Bragg gratings.

Miniature solid state lasers are currently being considered for a range of display applications. Earlier laser diodes, called edge-emitting diodes, emit coherent light or infrared energy parallel to the boundaries between the semiconductor layers. More recent technologies such as vertical cavity surface emitting laser (VCSEL) and the Novalux Extended Cavity Surface Emitting Laser (NECSEL) emit coherent energy within a cone perpendicular to the boundaries between the layers. The VCSEL emits a narrow, more nearly circular beam than traditional edge emitters, which makes it easier to extract energy from the device. The NECSEL benefits from an even narrower emission cone angle. Solid-state lasers emit in the infrared. Visible wavelengths are obtained by frequency doubling of the output, Solid-state lasers may be configured in arrays comprising as many as thirty to forty individual dies. The laser die are independently driven and would normally emit light simultaneously, The competitive advantage of lasers in display applications results from increased lifetime, lower cost, higher brightness and improved colour gamut. As lasers are polarized, they are ideally suited to Liquid Crystal on Silicon (LCoS) or High Temperature Poly Silicon (HTPS) projectors. In contrast to incoherent sources, lasers do not result in light from unwanted polarization states being discarded.

Laser displays suffer from speckle, a granular structure arising from the high spatial and temporal coherence of lasers, Speckle reduces image sharpness and is distracting to the viewer. It is well knqwn that speckle can be reduced by applying decorrelation procedures based on combining multiple sets of speckle patterns, or cells, from a given speckle-generating surface 5 during the spatio-temporal resolution of the human eye, Tn the case of laser arrays, speckle reduces as the inverse of the square root of the number of die, For example, in the case of a thirty-six element laser array the speckle contrast is reduced to 1/ √ 36 (or 17%). On this basis, speckle is a significant problem for single die laser sources, However, single die laser illuminators offer the benefits of cheaper packaging an d the potential for higher output powers 10 Combining multiple laser die in tightly packed arrays may present thermal management problems.

There is a requirement for a compact, efficient laser illuminator device that can overcome the problem of laser speckle,

15 SUMMARY OF THE INVENTION

It is an object of the present invention to provide compact, efficient laser illuminator that can overcome the problem of laser speckle.

In a first embodiment of the invention the illuminator comprises a laser source and an 20 Electrically Switcbable Bragg Grating (ESBG) device disposed along the path of the beam emitted by said laser. A variable voltage generator is coupled to the ESBG. The laser and ESBG device form part of an apparatus for illuminating an electronic display to provide a viewable image. The laser source comprises a single laser emitter die. Essentially, the ESBG device is configured to modify the optical characteristics! of incoming light to provide a series of speckle

cells, The voltage applied to the ESBG device is cyclically varied from zero to some specified maximum value at a high frequency. The effect of the varying voltage is to vary the optical effect of the ESBG device on incoming light in a corresponding fashion. In effect, the ESBG device generates a multiplicity of different speckle patterns within the human eye integration time. λ human eye observing the display integrates said patterns to provide a substantially de- speckled final image.

In a second embodiment of the invention the laser source comprises a multiplicity of laser emitter die. The ESBG device comprise a multiplicity of separately controllable ESBG elements. Each laser die has a unique ESBG element disposed along its beam path. Each ESBG element is configured to modify the optical characteristics of incoming light to provide a time series of speckle cells. In one operational embodiment said lasers are operated such that the illumination from the laser die is provided in a time sequence,

In a third embodiment of the invention the laser source comprises a multiplicity of laser emitter die. The ESBG device comprises a multiplicity of separately controllable ESBG elements. Each laser die has a unique ESBG element disposed along its beam path. Each ESBG element is configured to modify the optical characteristics of incoming light to provide a time series of speckle cells, The laser die emit light simultaneously while each ESBG element is configured to provide a series of speckle cells for an incident laser beam.

In a fourth embodiment of the invention the ESBG devices in any of the embodiments described above may be configured using two ESBG layers disposed in sequence. The ESBG layers are operated in tandem with alternating voltages applied across the ESBG layers. The

optical effect of each ESBG device is varied from zero to maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion. Each incremental change in the applied voltage results in a unique speckle phase cell. By having a phase lag between the voltages applied across the ESBGs the average intensity of the speckle phase cells remains substantially constant, thereby satisfying the statistical requirements for speckle reduction, Tn further embodiments of the invention related to said fourth embodiment other types of waveforms might be applied, for example: sinusoidial; triangular; rectangular or other types of regular waveforms. In alternative embodiments of the invention random waveforms may be used.

In a further embodiment of the invention the illuminator comprises a multiplicity of laser emitter die configured as a two dimensional array and an ESBG device comprising a multiplicity of separately controllable ES BG elements configured as a two dimensional array disposed to provide a unique ESBG element along the beam path from each laser element. The illuminator further comprises a multiplicity of ESBG elements configured as a stack. The ESBG array directs light from said laser die towards said ESBG stack. The ESBG stack directs light beams from said laser die towards the viewer.

In any of the above-described embodiment of the invention the ESBG device may be a variable ESBG diffuser.

In any of the above-described embodiments of the invention the ESBG device may be a variable ESBG axicon phase retarder.

In any of the above-described embodiment of the invention the ESBG device may be a variable ESBG sub wavelength grating.

In any of the above-described embodiments of the invention a beam-shaping element disposed along the laser beam paths may be used to shape lhe intensity profile and cross sectional geometry of the illuminator beam,

In any of the above-described embodiment of the invention a micro lens element may be disposed between the laser die and the corresponding ESBG elements.

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,

BRIEF DESCRIPTION OF THE DRAWINGS

FIG.1 is a schematic side elevation view of a first embodiment of the invention, FIG.2 is a schematic side elevation view illustrating second and third embodiments of the invention, FIG.3 A is a schematic side elevation view of a fourth embodiment of the invention,

FIG.3B is a chart showing a first characteristic of the fourth embodiment of the invention. FIG.3C is a chart showing a first characteristic of the fourth embodiment of the invention, FIG4 is a schematic side elevation view of a further embodiment of the invention. FIG, 5 A is a schematic side elevation view of a further embodiment of the invention.

FIG 5B is a front elevation of a first detail of the embodiment of FtG.5 A FTG.6 is a schematic side elevation view of a further embodiment of the invention. FIG.7A is a schematic side elevation view of a prior art device related to one particular embodiment of the invention,

5 FIG.7B is a schematic side elevation view of one particular embodiment of the invention. FIG.7C is a schematic side elevation view of a prior art device related to one particular embodiment of the invention,

FTG.8 is a schematic side elevation view of a further embodiment of the invention, FIG.9 is a schematic side elevation view of a further embodiment of the invention. 10

DETAILED DESCRIPTION OF THE INVENTION

PlG, 1 shows a schematic side elevation view of a first embodiment of the invention. The illuminator comprises a laser source 1 and an Electrically Switchable Bragg Grating (ESBG) device 2, which is disposed along the laser beam path. The laser source 1 comprises a single

15 laser emitter die. The ESBG drive electronics are indicated by 3. The laser and ESBG form part of an apparatus for illuminating an electronic display to provide a viewable image. Light from the illuminator enters a projection optical system generally represented by 4. The projection optical system may comprise an electronic display panel such as an LCD, a projection lens, relay optics for coupling the ESBG device to the display panel, filters, prisms, polarizers and other

20 optical elements commonly used in displays. The final image is projected onto a projection semen 5. A micro lens 7 may be used to convert diverging laser emission light 100 into a collimated beam 200. The collirnated beam is diffracted into a direction 300 by the ESBG, The optical system 4 forms a diverging beam 400, which illuminates the screen 5, The details of the projection optical system 4, screen 5 and microlens 7 do not form part of the invention. The

invention is not restricted to any particular type of display configuration . At least one viewable surface illuminated by the laser light exhibits laser speckle, Said viewable surface may at least one of the projection screen 5 or an internal optical surface within the projection optical system.

⁵ The basic principles of ESBGs are now briefly summarized prior to discussing the basic principles of speckle reduction using said ESBG device.

An Electrically Switchable Holographic Bragg Gratings (BSBGs) is formed by recording a volume phase grating, or hologram, in a polymer dispersed liquid crystal (PDLC) mixture,

10 Typically, ESBG devices are fabricated by first placing a thin film o Pa mixture of photopolymerizable monomers and liquid crystal material between parallel glass plates. Techniques for making and filling glass cells are well known in the liquid crystal display industry. One or both glass plates support electrodes, typically transparent indium tin oxide films, for applying an electric field across the PDTX layer. A volume phase grating is then

15 recorded by illuminating the liquid material with two mutually coherent laser beams, which interfere to form the desired grating structure. During the recording process, the monomers polymerize and the HPDLC mixture undergoes a phase separation, creating regions densely populated by liquid crystal micro-droplets, interspersed with regions of clear polymer. The alternating liquid crystal-rich and liquid crystal-depleted regions form the fringe planes of the

20 grating. The resulting volume phase grating can exhibit very high diffraction efficiency, which may be controlled by the magnitude of the electric field applied across the PDLC layer. When an electric field is applied to the hologram via transparent electrodes, the natural orientation of the LC droplets is changed causing the refractive index modulation of the fringes to reduce and the hologram diffraction efficiency to drop to very low levels. Note that the diffraction efficiency of

the device can be adjusted, by means of the applied voltage, over a continuous range from near 100% efficiency with no voltage applied to essentially zero efficiency with a sufficiently high voltage applied. U.S. Patent 5,942,157 and U.S. Patent 5,751,452 describe monomer and liquid crystal material combinations suitable for fabricating ESBG devices. A publication by Butler et al. ("Diffractive properties of highly birefringent volume gratings: investigation", Journal of the Optical Society of America B, Volume 19 No. 2, February 2002) describes analytical methods useful to design ESBG devices and provides numerous references to prior publications describing the fabrication and application of ESBG devices.

Each ESBG layer is recorded in HPDLC sandwiched between transparent substrates to which transparent conductive coatings have been applied Each ESBG has a diffracting state and a non-diffracting state. Each ESBG diffracts light in a direction substantially parallel to the optical axis when in said active state. However, each ESBG is substantially transparent to said light when in said inactive state. Each ESBG can be designed to diffract at least one wavelength of red, green or blue light.

Speckle reduction is based on averaging multiple (M) sets of speckle patterns from a given speckle surface spatio-resolution cell). The averaging takes place over the human eye integration time. Several approaches for reducing speckle contrast have been proposed based on spatial and temporal dccorrelation of speckle patterns. Under optimal conditions speckle contrast is reduces from unity to the square root of M. The value of M should be as large as possible. However, the value of M is limited by the numerical aperture of the imaging optics ^, In other words the minimum cell size is approximately equal to the laser wavelength divided by the numerical aperture, The set of M phase cells in a resolution cell constitutes a phase pattern

Temporally varying the phase pattern faster than the eye temporal resolution destroys the light spatial coherence, thereby reducing the speckle contrast . The basic statistical properties of speckle are discussed by J. W, Goodman in a first paper entitled "Some Fundamental Properties of Speckle" ( J. Opt, Soc, Am. 66, pp, 1145-1149, J 976) and a second paper entitled "Statistical Properties of Laser Speckle Patterns" (Topics in Applied Physics volume 9,edited by J. C. Dainty, pp, 9-75, Springer- Verlag, Berlin Heidelberg, 1984)

The ESBG device is configured to generate set of unique speckle phase cells by operating on the angular or polarization characteristic of rays propagating through the ESBG device. As will be explained below, in any of the embodiments of the invention the ESBG device may comprise more than one ESBG layer. Furthermore, the ESBG device may be configured in several different ways to operate on one or more of the phase, and ray angular characteristics of incoming light In one implementation of the invention the ESBG device may be configured as a diffuser. In another implementation of the invention the ESBG device may be configured as a phase retarder based on a sub wavelength grating exhibiting form birefringence, Alternatively, the ESBG device may be configured as a lens of the type known as an axicon. Varying the electric field applied across the ESBG device varies the optical effect, of the ESBG device by changing the refractive index modulation of the grating. Said optica! effect could be a change in phase or a change in beam intensity or a combination of both, The optical effect of Ae ESBG device is varied from zero to a predetermined maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion. Said variation may follow sinusoidial, triangular, rectangular or other types of regular waveforms. Alternatively, the waveform may have random characteristics. Each incremental change in the applied voltage results in a unique speckle phase cell. A human eye 5 observing the display integrates speckle

patterns such as those illustrated by 500a, 500b to provide a substantially de speckled final image.

In a second embodiment of the invention shown in the schematic side elevation view of FIG.2, the laser source comprises a multiplicity of laser emitter die configured as a two- dimensional array 10. A microlens array 70 containing elements may bo provided. For example in the array shown in the Figure the lens element 71 converts diverging light 101 from laser element 11 into a collimated beam 201. The microlens array does not form part of the invention. The ESBG device 20 comprises at least one ESBG army where each array contains a multiplicity of separately controllable ESBG elements similar to the one shown in FIG.1 , As shown in FIG.2, a collimated beam 201 propagates through an ESBG array element 21. Each ESBG element is operative to receive light from one laser die. In one operational embodiment of the invention said lasers and said ESBGs are operated such that the illumination from the lasers is provided in a time sequence.

In a third embodiment of the invention, which is also illustrated by FIG.2, the lasers emit light simultaneously. Each ESBG device provides a unique set. of speckle phase cells from its corresponding laser die.

In yet further embodiments of the invention the ESBG devices in any of the embodiments described above may be configured using multiple ESBGs disposed in sequence. For example, referring to the side elevation view of FIG.3 A, it will be seen that the ESBG 2 of FIG.1 has been replaced by the two ESBGs 21 and 22, which are controlled by the ESBG controller 30. The ESBGs 21 and 22 are operated in tandem with alternating voltages applied across the ESBG layers. The optical effect of each ESBG device is varied from zero to maximum value at a high

frequency by applying an electric field that varies in a corresponding varying fashion. Each incremental change in the applied voltage results in a unique speckle phase cell Referring to FTG.3B which is a chart showing voltage versus time applied to the ESBGs 21 and 22 it will be seen that there is a phase lag between the voltages applied across the ESBGs. The effect of applying such waveforms is that the average intensity of the speckle phase cells remains substantially constant, thereby satisfying the statistical requirements for speckle reduction, Other types of waveforms may be applied, for example sinusoidial, triangular, rectangular or other types of regular waveforms. Alternatively, it may be advantageous in statistical terms to use waveforms based on a random stochastic process such as the waveforms illustrated in the chart of FTG.3 C.

In any of the embodiments of the invention beam-shaping element disposed along tine laser beam paths may be used to shape the intensity profile of the illuminator beam. Laser array tend to have emitting surface aspect ratios of that are incompatible with the aspect ratios of common microdisplay devices, FIG.4 shows a side elevation view of an illuminator similar to the embodiment of FIG.1, which further comprises the beam-shaping element 8, The beam- shaping element may be a light, shaping dtffuser such as the devices manufactured by POC Inc. (USA) or a Computer Generated Hologram. Other technologies may be used to provide the light shaping function.

The ESBG device may be configured to perform the additional function of beam steering. This may be advantageous with laser arrays in which the die has large separations. In such a configuration at least one ESBG layer is configured to generate speckle phase cells while a further one or more ESBG layers are configured to diffract incident light into a specified

direction. Desirably, the second BSBG operates according to the basic principles described in United States Patent No. 6 , 115, 152.

Tn an embodiment of the invention shown in the schematic side elevation view ofFlG.Sλ the illuminator comprises a multiplicity of laser emitter die configured as a two dimensional array 10 and an ESBG device comprising a first array of separately controllable ESBG elements 20 and a second array of separately controllable ESBG elements 30. The illuminator further comprises a multiplicity of ESBG elements configured as a stack 40. The first ESBG array operates in a similar fashion to the ESBG device illustrated in FTG.2, However the function of the second ESBG array is to deflect beams from the laser die towards the ESBG stack 40. The illuminator may further comprise the microlens array 70, which does not form part of the present invention. The ESBG stack directs light beams from said laser die towards the viewer, For example the converging light from the die 11 is collimated by the microlens element 71 into the beam direction 201 , The angular or polarization characteristics of the beam are modified by the ESBG element 22, The ESBG element 23 deflects the beam 201 into the beam direction 300, The beam 300 is deflected into the direction 400 by element 41 of the ESBG stack 40, F.IG.5B is a front elevation view of a portion of the microlens array, FTG.5C is a front elevation view of a portion of the laser die array, The configuration of FIG.5A may be used in conjunction with any of the speckle reduction methods disclosed in the present application.

It will be clear that that by eliminating the first ESBG array 20 from the apparatus shown in FIG.5 there is provide a means for combining beams from multiple laser sources into a common direction

In another embodiment of the invention the functions performed by the ESBG arrays 20 and 30 in FIG.5 may be combined in a single ESBG layer,

ESBG devices according to the principles of the present invention can be configured to

5 provide a range of spatio temporal speckle averaging schemes. In any of the embodiments shown in FIGS, 1 -5 the ESBG device could be configured as a variable subwavelength grating. Essentially the ESBG device acts as a variable phases retarder, FIG.6 shows a cross section view of a sub wavelength grating 50. The light regions 51 represent polymer fringes. The shaded regions 52 represent PDLC fringes. The grating pitch must be much larger than the incidence

10 light wavelength, Light 600 incident at an angle θ continues to propagate at the same angle after passing through the grating 601. Sub-wavelength gratings are high spatial frequency gratings such that only the aero order 600, forward diffracted 601 and backward "diffracted" waves 602 propagate. All higher diffracted orders are evanescent Incident light waves cannot resolve the sub-wavelength structures and see only the spatial average of the grating material properties,

15

ESBG device configured as sub wavelength gratings exhibit a property known as form birefringence whereby polarized light that is transmitted through the grating will have its polarization modified, Subwavelength gratings behave like a negative uniaxial crystal, with an optic axis perpendicular to the PDLC planes, The basic principles of sub wavelength gratings

20 discussed is Born and Wolf, Principles of Optics, 5th Ed., New York (1975). It is known that the retardance is related to the net birefringence, which is the difference between the ordinary index of refraction and the extraordinary index of refraction of the sub-wavelength grating,

Where the combined thickness of the PDLC plane and the polymer plane is substantially less than an optical wavelength the grating will exhibit form birefringence. The magnitude of the shift in polarization is proportional to the length of the grating. By carefully selecting the length of the subwavelength grating for a given wavelength of light, one can rotate the plane of polarization, Thus, the birefringence of the material may be controlled by simple design parameters and optimized to a particular wavelength, rather than relying on the given birefringence of any material at that wavelength,

It is known that the effective refractive index of the liquid crystal is a function of the applied electric field, having a maximum when the field is zero and a value equal to that of the polymer at some value of the electric field, Thus, by application of an electric field, the refractive index of the liquid crystal and, hence, the refractive index of the PDLC plane can be altered. When the refractive index of the PDLC plane exactly matches to the refractive index of the polymer plane, the birefringence of the subwavelength grating can be switched o£f. To form a half-wave plate, the retardance of the subwavelength grating must be equal to one-half of a wavelength and to form a quarter-wave plate, the retardance must be equal to one-quarter of a wavelength,

ESBG devices based on sub-wavelength gratings as described above may be operated in tandem with alternating voltages applied across the ESBG layers according to the principles illustrated in FIG.3. The retardance of each ESBG device is varied from zero to maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion, Each incremental change in the applied voltage results in a unique speckle phase cell, The effect of applying waveforms such as those illustrated in FIG.3 is that the average intensity of the

speckle phase cells remains substantially constant thereby satisfying the statistical requirements for speckle reduction.

ESBG devices according to the principles oJPthe present invention can be also configured as variable axicon devices. In such embodiments of the invention the ESBG acts as a variable phase retarder. According to the basic theory of axicons, a uniform plane wave passing through an infinite axicon has a transverse intensity profile represented by a First order Bessel function. The intensity profile is constant along the path giving what is effectively a non diffracting beam, The basic principles of axicons are discussed in an article by J.H. McLeod entitled "Axicons and Their Uses" (JOSA, 50 (2), 1960, p. J 66) and another article by R.M, Herman et T. A. Wiggins entitled "Production and uses of diffraction less beams" (JOSA A, 8 (6), 1991). Practical axicons use coUimated Gaussian input beams and generate output beams that ate referred to in the literature as a Bessel-Gauss beams, Classical axicons are typically conical single element lenses. The transverse intensity distribution at a specific position is created by constructive interference from a small arwmlus of rays incident on the axicon. Beam intensity is characterized by a intense central region encircled by rings of lower intensity. Each ring contain same amount of energy, Axicons have minimal optical power imparting only a small deviation to the incoming beam.

ESBG devices based on axicons as described above may operated in tandem with alternating voltages applied across the ESBG layers according to the principles illustrated in FIG.3, The retardance of each ESBG axicon device is varied from zero Io a predetermined maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion. Each incremental change in the applied voltage results in a unique speckle

phase cell. The effect of applying waveforms such as those illustrated in FIG.3 is that the average intensity of the speckle phase cells remains substantially constant thereby satisfying the statistical requirements for speckle reduction. In one embodiment of the invention ESBG devices based on axicons could be configured in tandem .In such a configuration laser 5 wavefronts will not be diffracted but will only experience phase retardation. The diffracted beams substantially overlay the non-diffracted beams. Both diffracted and non-diffracted beams undergo phase retardation.

FIG.7A is a schematic side elevation view of one configuration 60 of a pair of conical 10 lens axicons 61 ,62. FIG. 7B is a schematic side elevation view of an ESBG device 70 comprising a pair of BSBG layers 7) ,72 having optical characteristics equivalent to conical lens axicons 61 ,62 respectively. FIG.7C is a schematic side elevation view of an alternative arrangement of conical lens axicons 80 comprising a pair of conical lens axicons 81,82 which could be encoded into the ESBG layers 71 ,72 respectively, 15

ESBG devices according to the principles of the present invention can be also configured as variable difmsers. A variable diffuser is provided by recording diffusing characteristics in to an ESBG layer using procedures well known to those skilled in the art of holography. Conventionally, holographic optical element with diffusing characteristics are recorded by using 20 a holographic cross beam recording apparatus with a diffuser inserted into one of the recoding beams, ESBG devices characterised as diffusers may be operated in tandem with alternating voltages applied across the ESBG layers according to the principles illustrated in FIG.3. The transmittance of each ESBG device is varied from zero to a predetermined maximum value at a high frequency by applying an electric field that varies in a corresponding varying fashion, Each

incremental change in the applied voltage results in a unique speckle phase cell. The effect of applying waveforms such as those illustrated in FIG.3 is that the average intensity of the speckle phase celts remains substantially constant thereby satisfying the statistical requirements for speckle reduction. Tn a further embodiment of the invention shown in F1G.9 which is similar to that illustrated in FIG.2 it will be seen that the ESBG array 2 of FIG.2 has been replaced by the two ESBGs 2a and 2b, which are controlled by the ESBG controller 30. The ESBGs 2a,2b may encode axicons, sub-wavelength gratings or diftusers, As discussed in the preceding paragraphs, the ESBGs 2a and 2b are operated in tandem with alternating voltages applied across the ESBG layers, The angular or polarization effect of each ESBG array cell is varied at a high frequency by applying an electric field that varies in a corresponding varying fashion, Each incremental change in the applied voltage results in a unique speckle phase cell. The laser source comprises a multiplicity of laser emitter die configured as a two-dimensional array 10. A microlens array 70 containing elements may be provided. For example in the array shown in the Figure the lens element 71 converts diverging light 101 from laser element 11 into a collimated beam 201. The beam 201 propagates through the ESBG array elements 21a and 21 b in sequence.

The invention may also be used in other applications such as optical telecommunications

In preferred practical embodiments of the invention the ESBG layers would be combined in a single planar multilayer device. The multilayer ESBG devices may be constructed by first fabricating the separate ESBG devices and then laminating the ESBG devices using an optical adhesive. Suitable adhesives are available from a number of sources, and techniques for bonding

optical components are well known. The multilayer structures may also comprise additional transparent members, if needed, to control the optical properties of the illuminator.

For the sake of simplicity the invention has been discussed with reference to monochromatic illumination. It will clear from the above discussion that the invention may be applied to illuminators using red, green and blue laser sources by providing separate ESBG layers for each colour.

While the invention has been discussed with reference to single laser die or rectangular arrays of laser die, it should be emphasized that the principles of the invention apply to any configuration of laser die,

It should be emphasized that the Figures are exemplary and that the dimensions have been exaggerated. For example thicknesses of the ESBG layers have been greatly exaggerated.

The ESBGs may be based on any crystal material including nematic and chiral types.

Although the invention has been described in relation to what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not limited to the disclosed arrangements, but rather is intended to cover various modifications and equivalent constructions included within the spirit and scope of the invention,