Near-eye display with laser diode illumination
A near-eye display has a light-guide optical element (LOE) with two major parallel external surfaces. An image projector introduces image illumination into the LOE so as to propagate by internal reflection. The image projector includes one or more light-generating laser diode operated by a controller. A coupling-out arrangement includes a set of mutually-parallel, partially-reflective surfaces associated with the LOE at an oblique angle to the major external surfaces for coupling the illumination out of the LOE towards the eye of the observer. The light-generating laser diode has a characteristic variation of coherence length of generated light as a function of diode actuation power. The controller actuates the laser diode at a level of diode actuation power that generates light with a coherence length that is less than twice a spacing between adjacent partially-reflective surfaces of the coupling-out arrangement.
1. A near-eye display for projecting an image to an eye of an observer, the near-eye display comprising:
(a) a light-guide optical element (LOE) having first and second major external surfaces that are planar and mutually parallel;
(b) an image projector for projecting illumination corresponding to the image, said image projector being optically coupled to said LOE so as to introduce the illumination into said LOE so as to propagate within said LOE by internal reflection at said first and second major external surfaces, said image projector comprising:
(i) at least one light-generating laser diode, and
(ii) a controller for actuating said laser diode to generate light; and
(c) a coupling-out arrangement comprising a plurality of mutually-parallel, partially-reflective surfaces associated with said LOE at an oblique angle to said first major external surface for coupling the illumination out of said LOE towards the eye of the observer, said coupling-out arrangement having a spacing between adjacent surfaces of said plurality of partially-reflective surfaces,
wherein said light-generating laser diode has a characteristic variation of coherence length of generated light as a function of diode actuation power, and wherein said controller is configured to actuate said laser diode at a level of diode actuation power that generates light with a coherence length less than twice said spacing.
2. The near-eye display of claim 1 , wherein said controller is configured to vary said level of diode actuation power over at least one range of values that excludes at least one level of diode actuation power corresponding to a maximum of said function.
3. The near-eye display of claim 1 , wherein said controller is configured to vary said level of diode actuation power over at least one range of values for which said coherence length lies below a threshold value.
4. The near-eye display of claim 1 , wherein said level of diode actuation power is proximal to a minimum of said function.
5. The near-eye display of claim 1 , wherein said level of diode actuation power is greater than a maximum nominal power rating of said laser diode.
6. The near-eye display of claim 1 , wherein said plurality of partially-reflective surfaces are located internally within said LOE.
7. The near-eye display of claim 1 , wherein said at least one light-generating laser diode is implemented as at least three light-generating laser diodes generating different colors of visible light.
8. The near-eye display of claim 1 , wherein said at least one light-generating laser diode is deployed to illuminate at least part of a spatial light modulator deployed in an image plane of said image projector.
9. The near-eye display of claim 1 , wherein said controller is configured to implement a brightness adjustment, and where at least part of said brightness adjustment is implemented using pulse-width modulation of pulses of light generated by said laser diode.
10. The near-eye display of claim 1 , wherein said image projector further comprises a scanning arrangement configured for scanning illumination from said laser diode across an image plane of said image projector.
11. The near-eye display of claim 10 , wherein said image projector further comprises an electrically-switchable optical attenuator deployed for varying an output intensity of light from said laser diode output from said image projector synchronously with said scanning.
12. The near-eye display of claim 1 , wherein said image projector further comprises a path-multiplying reflector arrangement positioned in a path of light generated by said laser diode, said path-multiplying reflector arrangement comprising a full reflector and at least one partial reflector deployed parallel to said full reflector and spaced from said full reflector by more than half said coherence length.
13. The near-eye display of claim 1 , wherein said image projector further comprises a path-multiplying diffuser arrangement positioned in a path of light generated by said laser diode, said path-multiplying diffuser arrangement comprising a reflector and a diffuser layer deployed parallel to said reflector and spaced from said reflector by more than half said coherence length such that light generated by said laser diode passes through said diffuser layer, is reflected from said reflector, and passes again through said diffuser layer.