Compact projector for head-mounted displays

Abstract

A compact projector for use in a head-mounted display device comprises an illumination section, a relay section, and a numerical aperture expander (NAE). The illumination section includes one or more illumination sources, a scanning device, and a focusing lens that focuses the light onto an image plane. The NAE receives light from the illumination section, expands the mean numerical aperture of the light, and transmits the light to the relay section. The relay section includes optical elements that collimate the light from the image plane to an exit pupil. The projector can also be equipped with horizontal and / or vertical axis stops that prevent stray light from passing through the exit pupil.

Description

[Technical field]

[0001] The present invention relates to displays, and in particular to miniature projectors for use in head mounted displays and augmented reality systems. [Background technology]

[0002] In near-eye or head-up displays, the function of the laser projector is to couple a scanned laser beam into a waveguide, which transmits the illumination to the viewer's eye. Typically, the laser beam is scanned over the image field by a scanning mirror, and pupil imaging is used to maintain the beam coupling into the waveguide.

[0003] To project an image onto a near-eye head-mounted display, the entrance pupil of the waveguide must be a) is fully illuminated to produce a uniform image; and b) It should be perfectly coupled to the waveguide even while scanning the projected field of view.

[0004] For laser projectors, the laser beam is usually focused to a spot on an optical element called a numerical aperture expander (NAE), after which the light is collimated and directed to the exit pupil. The NAE can be implemented, for example, by a diffuser or a microlens array (MLA). After the NAE, the light is collimated and directed to the exit pupil. A scanning device with one or more scanning mirrors is used to steer the laser beam over the projected field of view. The scanning device usually forms the limiting optical aperture of the laser projector.

[0005] In color laser projectors, the light illumination from red, green, and blue lasers is typically combined into a single collimated beam that is scanned by a scanning device and expanded by an NAE. The projector's optical system and its associated mechanical support are often bulky and difficult to implement in a head-mounted display. The imaging performance of prior art laser projectors is primarily limited by spherical, chromatic, and / or field curvature optical aberrations. Spherical aberrations arise when forming a large projection field of view (FOV) because the laser focusing lens must focus the light rays to a large numerical aperture. Chromatic aberrations result from the different ray paths of the red, green, and blue laser illuminations. Field curvature aberrations can be positive, as in the case of refractive optical elements, or negative, as in the case of reflective optical elements. In prior art laser projectors, the above aberrations severely limit the achievable image resolution. Summary of the Invention

[0006] The present invention is an innovative miniature projector that provides high image resolution by at least partially correcting a major source of optical aberration. The projector is comprised of an illumination section, a NAE, and a relay section, and in some embodiments includes an optical stop disposed in close proximity to the exit pupil of the relay section.

[0007] In this application, the term "laser" when used as a noun or adjective is intended to include various illumination sources used in head mounted displays, such as laser diodes, light emitting diodes (LEDs), micro LEDs, and liquid crystal on silicon (LCOS) illumination devices. Additionally, the use of the term "plane" in optical terms such as image plane, conjugate plane, and principal plane is understood to refer to a surface that may or may not be flat in the strict mathematical sense.

[0008] According to one aspect of the subject matter of the present disclosure, there is provided a miniature projector for use in a head mounted display device, comprising: an illumination unit; a relay unit; and a numerical aperture expander (NAE), wherein the illumination unit has one or more illumination sources, a focusing lens that focuses light onto an image plane, and a scanning device disposed between the focusing lens and the image plane, the relay unit includes an optical element that collimates light from the image plane onto an exit pupil, and the NAE is configured to receive light from the illumination unit, where the received light has a first average numerical aperture, and transmit the light to the relay unit, where the transmitted light has a second average numerical aperture greater than the first average numerical aperture by an average magnification factor of the NAE that is greater than 1.

[0009] According to some aspects, the scanning device is illuminated by a focused beam.

[0010] According to some aspects, the illumination portion also includes a field lens disposed between the scanning device and the image plane and proximate to the image plane.

[0011] According to some embodiments, the beam diameter of light propagating from the scanning device to the field lens is reduced by at least a factor of two.

[0012] According to some embodiments, the scanning device comprises a single mirror that rotates about two substantially orthogonal axes, or two mirrors that each rotate about a single axis.

[0013] According to some embodiments, the illumination source comprises an illumination source selected from the group consisting of a laser diode, a parallel laser diode, a light emitting diode (LED), a micro LED, and a liquid crystal on silicon (LCOS) illumination device.

[0014] According to some embodiments, the illumination portion comprises one or more photodetectors for monitoring the illumination power emitted by the one or more illumination sources.

[0015] According to some embodiments, the one or more photodetectors include a spectral filter.

[0016] According to some aspects, the illumination portion includes a reflective lens.

[0017] According to some embodiments, the projector includes at least two illumination sources arranged in a side-by-side configuration, and a first portion of light emitted by each of the illumination sources is transmitted by a scanning device and a focusing lens.

[0018] According to some embodiments, a second portion of the light emitted by each of the illumination sources is reflected towards the photodetector array.

[0019] According to some embodiments, the second portion of the light is emitted along the fast axis of the illumination source having a wide beam divergence.

[0020] According to some embodiments, the spacing between the outermost beams of the side-by-side configuration spans at least 0.1 millimeters.

[0021] According to some embodiments, the surface of the NAE is curved to at least partially correct field curvature aberrations caused by the relay portion and / or the illumination portion.

[0022] According to some embodiments, the NAE is embedded between the optical components with no gaps between the components.

[0023] According to some embodiments, the NAE is implemented as an at least partially transmissive or partially reflective microlens array (MLA) or light diffuser.

[0024] According to some embodiments, the NAE is implemented as a diffuse MLA that includes a relatively low light output diffuser superimposed on the surface of a relatively high light output MLA.

[0025] According to some embodiments, the average magnification value of the NAE is in the range of 2-5.

[0026] According to some aspects, the relay portion includes a refractive collimating lens or a reflective collimating lens.

[0027] According to some embodiments, the relay portion includes one or more polarizing optical elements.

[0028] According to some aspects, the relay portion includes a polarizing beam splitter and a reflective collimator lens.

[0029] According to some aspects, the focusing lens and relay are configured such that the scan plane of the scanning device is an image conjugate of the exit pupil.

[0030] According to another aspect of the subject matter of the present disclosure, there is provided a miniature projector for use in a head mounted display device that includes a relay portion optically coupled to a waveguide, the relay portion having an exit pupil and the waveguide having an entrance pupil.

[0031] According to some embodiments, the horizontal and / or vertical stops are disposed at or proximate to the exit and / or entrance pupil.

[0032] According to some aspects, the relay portion comprises a coupling prism that optically connects the exit pupil to the entrance pupil.

[0033] According to some embodiments, one or more surfaces of the coupling prism include a lateral stop and / or a vertical stop.

[0034] According to another aspect of the subject matter of the present disclosure, there is provided a miniature projector for use in a head mounted display device, comprising: an illumination unit; a relay unit; and a numerical aperture expander (NAE), wherein the illumination unit includes one or more illumination sources and a focusing lens that focuses light onto an image plane, and the relay unit includes an optical element that collimates light from the image plane onto an exit pupil, the NAE being configured to receive light from the illumination unit, where the received light has a first average numerical aperture, and transmit the light to the relay unit, where the transmitted light has a second average numerical aperture greater than the first average numerical aperture by an average magnification factor of the NAE that is greater than 1, and the NAE includes a curved surface whose curvature is configured to at least partially correct field curvature aberrations due to the relay unit and / or the illumination unit.

[0035] According to some embodiments, the average magnification value of the NAE is in the range of 2-5.

[0036] According to another aspect of the subject matter of the present disclosure, there is provided a miniature projector for use in a head mounted display device, comprising: an illumination unit; a relay unit; and a numerical aperture expander (NAE), wherein the illumination unit includes one or more illumination sources, a focusing lens that focuses light onto an image plane, and a scanning device disposed between the focusing lens and the image plane, wherein the scanning device is illuminated by a focused beam; the relay unit includes an optical element that collimates light from the image plane onto an exit pupil; and the NAE is configured to receive light from the illumination unit, where the received light has a first average numerical aperture, and transmit the light to the relay unit, where the transmitted light has a second average numerical aperture greater than the first average numerical aperture by an average magnification factor of the NAE that is greater than 1.

[0037] According to some aspects, the illumination section also includes a field lens disposed between the scanning device and the image plane and proximate to the image plane.

[0038] According to some embodiments, the beam diameter of light propagating from the scanning device to the field lens is reduced by at least a factor of two. [Brief description of the drawings]

[0039] The invention is herein described, by way of example only, with reference to the accompanying drawings, in which like reference numerals are used to indicate similar or similar elements throughout the drawings, in which:

[0040] [Figure 1] 2 is a schematic diagram of the main optical components of a laser projector in accordance with the principles of the present invention; [Figure 2A] FIG. 2 is a diagram of the field curvature adjacent the image plane of FIG. 1. [Figure 2B] FIG. 2 is a diagram of the field curvature adjacent the image plane of FIG. 1. [Diagram 3] 1 is an exploded optical layout of an exemplary laser projector in accordance with a first embodiment of the present invention. [Figure 4] FIG. 4 is a ray tracing diagram for scanning according to the embodiment of FIG. 3. [Diagram 5] 1 is an exploded optical layout of an exemplary laser projector in accordance with a second embodiment of the present invention. [Figure 6] 1 shows structural details of an implanted NAE according to the principles of the present invention. [Figure 7A] FIG. 1 is a diagram of a prior art NAE surface. [Figure 7B] FIG. 2 is a diagram of an embedded NAE surface according to the present invention. [Figure 8A] 2 is a schematic optical layout of an exemplary illumination section for a laser projector having multiple laser beams, in accordance with the present invention. [Figure 8B] 2 is a schematic optical layout of an exemplary illumination section for a laser projector having multiple laser beams, in accordance with the present invention. [Figure 8C] 2 is a schematic optical layout of an exemplary illumination section for a laser projector having multiple laser beams, in accordance with the present invention. [Figure 9A] FIG. 2 is a side view of a portion of a laser projector and associated waveguide in accordance with the present invention. [Figure 9B]1 is a perspective view of a portion of a laser projector illustrating an exemplary rotation of a scanning mirror. [Figure 10A] 1A-1C show an example implementation of vertical and horizontal axis stops configured to prevent stray light from passing through the exit pupil of a projector. [Figure 10B] 1A-1C show an example implementation of vertical and horizontal axis stops configured to prevent stray light from passing through the exit pupil of a projector. [Figure 10C] 1A-1C show an example implementation of vertical and horizontal axis stops configured to prevent stray light from passing through the exit pupil of a projector. [Figure 11A] FIG. 1 illustrates an exemplary stopping arrangement in accordance with the present invention. [Figure 11B] FIG. 1 illustrates an exemplary stopping arrangement in accordance with the present invention. [Figure 11C] FIG. 1 illustrates an exemplary stopping arrangement in accordance with the present invention. [Figure 11D] FIG. 1 illustrates an exemplary stopping arrangement in accordance with the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] 1 shows a schematic diagram of the main optical components of a miniature projector 100 according to the principles of the present invention. The projector is composed of three main sections: an illumination section 100U, an NAE 100M containing one or more optical elements, and a relay section 100L. The illumination section includes one or more illumination sources, which may be, for example, lasers or light emitting diodes (LEDs).

[0042] Light from laser 201 is focused onto image plane 215 by focusing lens 223 which forms a focused beam. The focused beam is steered in two orthogonal directions by scanning device 211 which may consist of, for example, two scanning mirrors 224 and 225 as shown in FIG. 1, or a single scanning mirror with two axes of rotation. By locating scanning device 211 in the focused beam, it is not necessary to place an additional lens after scanning mirror 225 far away from image plane 215. Such additional lenses found in prior art laser projectors have the disadvantage of adding significant weight and complexity.

[0043] The beam passing through field lens 219 is focused onto image plane 215 and then formed into a parallel beam by collimator lens 218. Field lens 219 is preferably close to image plane 215 to have a negligible effect on the focus position and beam divergence, and also to allow pupil imaging between the scanning device and pupil plane 234.

[0044] The light beam entering image plane 215 has a relatively narrow numerical aperture, indicated at 214. For example, the diameter of the focused beam in illumination section 100U may be 1.3 mm at the principal plane of focusing lens 223, 0.8 mm at mirror 224, and only 0.25 mm at the principal plane of field lens 219. At image plane 215, the diameter of the laser spot is typically on the order of 0.01 mm, or 10 microns.

[0045] Without the presence of the NAE100M to expand the numerical aperture, the light entering the collimator lens 218 would form a collimated narrow beam and would not fill the exit pupil 234, as shown by the dashed arrow in FIG. 1. This would result in an incomplete image at the entrance pupil of the waveguide that couples the light to the viewer's eye. The effect of the NAE100M is to expand the numerical aperture, thereby filling the exit pupil 234, as shown by the solid arrow. Any residual scattered light is absorbed by the stop 260.

[0046] The average magnification of the NAE, denoted by "R", has a value equal to or greater than 1. In general, the term "numerical aperture" refers to the half angle of the beam in a direction perpendicular to the beam propagation axis. For beams with a circular cross-sectional shape, a single value of the numerical aperture in degrees is sufficient to characterize the angular width of the beam. For more common beams with non-circular cross-sectional shapes, the average value of the numerical aperture can be calculated by averaging over the solid angle of the beam. In this case, the term "average magnification of the NAE" refers to the ratio in degrees / degree between the average numerical aperture of the beams leaving the NAE and the average numerical aperture of the beams entering the NAE.

[0047] 1, projector 100 essentially has two focusing mechanisms: one that focuses the laser beam onto image plane 215, and one that collimates the beam entering exit pupil 234. Each of these focusing mechanisms contributes to field curvature.

[0048] 2(a) and 2(b) show the field curvature adjacent to the image plane 215, assuming no field curvature correction by the NAE. The field curvature on the side facing the illumination unit 100U is represented by the dotted line 215A. The field curvature on the side facing the relay unit 100L has two possibilities, as shown by the dashed lines 215B1 and 215B2 in FIG. 2(a) and FIG. 2(b), respectively. In FIG. 2(a), the curvatures of the lines 215A and 215B1 have the same orientation, i.e., both are concave when viewed from the relay unit 200L. By implementing a curved surface on the NAE 100M according to the curvatures of the lines 215 and 215B1, it is possible to at least partially correct the net field curvature and optimize the resolution of the projected image.

[0049] In FIG. 2(b), the curvatures of the lines 215A and 215B2 are in opposite directions. In this case, the curved surface of the NAE 100M should be implemented mainly according to the curvature direction of the line 215B2. This will be described in more detail below in connection with the specific optical layout of the transition section.

[0050] FIG. 3 shows an optical layout of an exemplary laser projector 200 according to a first embodiment of the present invention. The projector 200 is composed of three sections: an illumination section 200U, an NAE 200M, and a relay section 200L. The light from the laser 201 passes through a dynamic focusing device 340, such as a Corning® Varioptic® variable focus liquid lens manufactured by Corning Incorporated, under the control of a focus actuator 345. The light is then reflected by a folding mirror 222 and passes through a focusing lens 223, which is preferably a doublet or aspheric lens. The scanning mirrors 224 and 225 direct the beam to a field lens 219, which tilts the beam to generate a pupil image between the conjugate plane of the scanning mirror 224 or 225 and the exit pupil 234. Optionally, mirror 222 may be partially transmissive and a photodetector 235 may be disposed as shown to receive light transmitted through mirror 222. Photodetector 235 may be used to monitor various emission parameters of laser 201, such as the emitted radiation power level.

[0051] The laser focus spacer 227 is used to optimally focus the laser spot on the image plane 215 during alignment of the illumination unit 200U. The NAE 200M includes an NAE carrier 228 and an NAE substrate 229 in close proximity to the image plane 215. The numerical apertures of the beams entering and leaving the NAE 200M are represented by arrows 220U and 220L, respectively. The collimator focus spacer 230 is used to optimize the collimation of the light to the exit pupil 234 during alignment of the relay unit 200L. The space appearing between the NAE substrate 229 and the spacer 230 in the exploded optical layout of FIG. 3 is for clarity of illustration and is not intended to be a gap between components. In fact, it is preferable that the NAE substrate 229 and the spacer 230 are in contact, since gaps between components tend to complicate the mechanical structure and assembly of the projector and may compromise the structural integrity and sealing against moisture and particle contamination.

[0052] Light passing through the collimator focus spacer 230 is reflected by a polarizing beam splitter (PBS) 231 onto a reflective collimator lens 232. The collimated reflected beam passes through the PBS 231 and a polarization manipulator 233 to an exit pupil 234. The polarization manipulator 233 is either a polarization scrambler or an active focus device controlled by a polarization actuator 335, such as a liquid crystal actuator. The use of the polarization manipulator 233 is only possible if the laser 201 has a well-defined polarization.

[0053] In the embodiment of FIG. 3, the field curvature at the image plane 215 is similar to that shown in FIG. 2A, and the curvatures of the lines 215A and 215B1 have the same orientation. The curvature of the line 215A of the illumination unit 200U is mainly generated by the focusing lens 223 illuminated by the converging light, and the scanning mirrors 224 and 225. The curvature of the line 215B1 of the relay unit 200L is mainly generated by the reflecting collimator lens 232. Due to the relatively large numerical aperture of the relay unit, the relay unit imparts field curvature defocusing than the illumination unit, and therefore must be more strictly corrected to prevent image degradation. In order to minimize the laser spot size and maximize the image resolution of the projected image field, the physical curvature of the NAE substrate 229 should be designed to be substantially the same as the curvature of the line 215B1.

[0054] Figure 4 is a ray tracing diagram for scanning according to the embodiment of Figure 3. The scanning mirror 225 is tilted to illuminate different parts of the field of view (FOV) seen by the viewer. The light output of the field lens 219 in the embodiment of Figure 3 is determined so that the scanning plane of mirror 224 or 225 is an image conjugate of the exit pupil 234. In this way, collimated light continues to pass through the exit pupil 234 even during off-axis scanning by the scanning device.

[0055] FIG. 5 shows an optical layout of an exemplary laser projector 300 according to a second embodiment of the present invention. The projector 300 is composed of three parts: an illumination part 300U, an NAE 300M, and a relay part 300L. As in FIG. 3, the scanning mirrors 224 and 225 direct the beam to a field lens 219, which tilts the beam to generate a pupil image between the conjugate plane of the scanning mirror 224 or 225 and the exit pupil 234. The NAE 300M includes an NAE carrier 328 and an NAE substrate 329. The numerical apertures of the beams entering and leaving the NAE 300M are indicated by arrows 320U and 320L. The relay part 300L includes a refractive element 332. The collimated light passes through a polarization manipulator 233 and an exit pupil 234.

[0056] In order to optimize the resolution over the entire imaging field of the laser projector 300, the curvature of the NAE substrate 329 is opposite to that of the NAE substrate 229 of Fig. 3. The reason for this is as follows: in the embodiment of Fig. 5, the field curvature at the image plane 215 is similar to that shown in Fig. 2B, and the curvatures of the lines 215A and 215B2 have opposite directions. The field curvature of the line 215B2 is mainly caused by the refractive element 332 of the relay section 300L.

[0057] The numerical aperture of the relay section at arrow 320L is larger than that of the illumination section at arrow 320U. Since the depth of field (DOF) is inversely proportional to the numerical aperture, the DOF of the relay section is relatively small, typically around 0.1 mm, compared to the DOF of the illumination section, which is typically around 0.5 mm. As a result, the curvature of the NAE substrate 329 is mainly governed by the curvature of the relay section, and its orientation corresponds to the orientation represented by line 215B2. The curvature of the NAE substrate 329 is determined so that the degree of defocus due to field curvature is approximately the same on both sides of the NAE substrate. The optimal surface shape of the NAE substrate 329 may be spherical in some implementations of the laser projector 300 and aspheric in other implementations.

[0058] FIG. 6 shows structural details of an embedded NAE 600 according to the principles of the present invention. Embedded NAEs are less susceptible to moisture and contamination, and are more mechanically robust than prior art NAEs with air gaps. Although the elements of NAE 600 are shown as having flat surfaces, the surfaces may also be curved, as in the case of NAE substrates 229 and 329 of FIGS. 3 and 5, respectively. Elements 350 and 356 represent spacers adjacent to NAE 600, which are similar to spacers 227 and 230 shown in FIG.

[0059] The numerical apertures for light entering and leaving the NAE 600 are indicated diagrammatically by arrows 360 and 362, respectively. The value of the average magnification R of an NAE is generally greater than 1, typically in the range of 2-5. The NAE carrier 352 is bonded to the NAE substrate 354 by applying adhesive 358 to mating surface 378. The refractive index of the NAE carrier is denoted by n(352) and the refractive index of the adhesive is denoted by n(358). If, as in prior art NAEs, the adhesive were replaced by gaps between the components having a refractive index equal to 1, the value of R would be reduced by a factor equal to [n(358)-n(352)] / [n(352)-1]. Thus, the introduction of gaps between the components reduces the average magnification R of the NAE.

[0060] FIG. 7A is a diagram of a prior art NAE surface 376, which can correspond to the surface of a diffuser or microlens array (MLA). An incident ray 368 from a scanning laser strikes the surface 376 at a smooth region, and scattering causes an increase in the numerical aperture, as represented by ray 370. However, an incident ray 369 strikes the surface 376 at a sharp edge region between adjacent diffusers or microlenses. In this case, in addition to the normal scattered ray 372, scattering also produces rays 375 that are diffracted at a wide range of angles. These rays cause multiple scattering, reducing image contrast at some pixels of the projected image.

[0061] FIG. 7B is a diagram of an embedded NAE surface 378 according to the present invention. Surface 378 is free of sharply pointed areas. Incident ray 369 in this case gives rise to rays 372 and 378, all of which contribute to the desired increase in numerical aperture as they leave the NAE. There are no rays diffracted over a wide range of angles as in the prior art of FIG. 7A. As a result, the embedded NAE of the present invention does not suffer from the degradation of image contrast exhibited by prior art diffusers or MLAs.

[0062] When the NAE surface 378 is implemented as an MLA, the full width at half maximum diameter of the illuminated spot on the MLA should be preferably 1-4 times larger than the pitch between adjacent microlenses to avoid diffraction effects.

[0063] Light transmitted through the NAE of FIG. 7B may result in illumination of non-uniform intensity. To correct for the non-uniformity, the NAE may be implemented with a "diffused MLA," in which a diffuser is superimposed on the overall curved surface of the MLA. The light output of the diffuser is preferably small compared to the light output of the MLA. For example, the numerical aperture expansion provided by the diffuser may be only one-fifth to one-third of the numerical aperture of the MLA. In a diffused MLA constructed according to the inventive principles disclosed above, the non-uniformity of the MLA is effectively averaged out, while the non-uniformity of the diffuser is insignificant.

[0064] 8A, 8B and 8C show schematic optical layouts of an exemplary illumination section for a laser projector with multiple laser beams according to the present invention. Laser module 801 emits beams 401, 402 and 403 with wavelengths corresponding to, for example, red, green and blue illumination. Typically, the beams are emitted by three adjacent laser sources arranged in a parallel configuration within laser module 801. The outermost laser beams are typically spaced apart by 0.1 mm or more. The lasers are aligned so that all these beams pass through focusing lens 223 and illuminate scanning mirrors 224 and 225. Real-time monitoring of laser module 801 by photodetector array 412 is essential to prevent power spikes and maintain tight control over the illumination power level and stability of each emitted beam.

[0065] In FIG. 8A, beams 401, 402, and 403 are partially reflected by partially reflecting surface 406A. A portion of the light in each beam is transmitted through surface 406A, then reflected by surface 408A and focused by lens 410 onto photodetector array 412. Lens 410 images the light source planes of the multiple lasers in laser module 801 onto photodetector array 412, such that each photodetector in array 412 receives an optical output corresponding to a single laser. To reduce optical crosstalk between photodetectors, one or more spectral filters are preferably applied to array 412 such that different portions of the photodetector array are sensitive to different laser wavelengths.

[0066] In FIG. 8B, curved reflective surface 408B combines the two functions of reflection and focusing into a single element, which replaces surface 408A and lens 410 of FIG. 8A.

[0067] A laser beam, such as one emitted by a laser diode, is typically characterized by a "fast" axis, which has a relatively wide beam divergence angle, and a "slow" axis, which has a relatively narrow beam divergence angle. Figure 8C shows a schematic optical layout in which the light beam used to monitor the laser output is emitted on the fast axis at a large angle, which does not strike the scanning mirrors 224 and 225.

[0068] 8C, a reflective mirror 414 with a central hole or aperture is positioned so that light used for imaging passes through the central hole and is reflected by the fully (100%) reflective mirror 406C towards the scanning mirror. Light emitted on the fast axis at a large angle is reflected by the mirror 414 and then focused by the lens 410 onto the photodetector array 412. Other shapes of mirror 414 are possible, including those that collect light from only one side of a diverging illumination beam.

[0069] The schematic optical layouts shown in Figures 8A, 8B, and 8C are exemplary, and many alternative configurations are possible. For example, reflective elements 406A, 406B, and 406C, which function as folded reflectors, are shown as being in the plane of the drawing. However, a more compact and light-efficient configuration can often be achieved by disposing the reflective elements outside the plane of the drawing. Additionally, array 412 and lens 410 can be disposed directly behind reflector 222C of Figure 3, thereby eliminating the need for reflective mirror 408.

[0070] 9A is a side view of a portion of a laser projector and associated waveguide, in accordance with the present invention. Insofar as the scanning mirrors 224 and 225 are often the smallest aperture of the projector system, optimal light transmission is achieved when the scanning plane of the scanning device is an image conjugate of the exit pupil 234. The exit pupil also serves as the entrance pupil of the waveguide 450, as shown in FIG. 9A. The size and shape of the optics of the laser projector are determined so that the exit pupil 234 completely overlaps with the entrance pupil of the waveguide 450.

[0071] 9B is a perspective view of a portion of a laser projector showing an exemplary rotation of the scanning mirror. In FIG. 9B, the scanning mirror 225 is tilted at a 90 degree angle relative to the incident laser light and other components of the laser projector. The exit pupil 234 is also rotated so that the scan plane of the mirror 225 remains an image conjugate of the exit pupil.

[0072] 10A, 10B, and 10C show exemplary implementations of vertical and horizontal axis stops configured to prevent stray light from passing through the exit pupil of the projector where it can cause image distortion and degradation of contrast. The optical stops are comprised of vertical axis stop 464 and horizontal axis stop 460, where the vertical axis points in the direction of the thickness of the waveguide as shown in FIG. 9A, and the horizontal axis points in the direction perpendicular to the vertical axis and the direction of propagation of light inside the waveguide.

[0073] In Figure 10A, stray light can be generated by broad angle scattering in the NAE, as represented by arrow 299. Figure 10B is a top view showing a horizontal axis stop 460 attached to a surface of a combining prism 462. Figure 10C is a side view showing a vertical axis stop 464 attached to a different surface of the combining prism 462.

[0074] 10A-10C, stops 460 and 464 are somewhat displaced from the location of exit pupil 234. This displacement is undesirable because it may allow some scattered light to pass through exit pupil 234 and into waveguide 450.

[0075] 11A-11D are diagrams illustrating an exemplary stop configuration according to the present invention, where the stop is disposed proximate to the exit pupil 234. The various stop components are indicated at 460A, 460B, 460C, and 460D.

[0076] 11A, 11C, and 11D, the coupling prism is disposed to the side of the waveguide 450. In FIG. 11C, the horizontal axis part 460C is on the interface between the coupling prism and the waveguide 450. This has the advantage that the vertical axis stop 464 can be very close to the exit pupil. In FIG. 11B, the coupling prism is disposed on top of the waveguide 450. This has the advantage that the horizontal axis stop part 460B can be very close to the exit pupil.

[0077] 11D, an additional vertical axis stop 460D is introduced to block stray light that does not originate from the NAE. Stop 460D is generally wider than exit pupil 234 and can be implemented in combination with stop 460B.

[0078] The configurations of Figures 11A, 11B, and 11C are particularly suited to laser projector embodiments in which the scanning mirrors are substantially separated and small compared to the aperture of the other optical elements in the projector.

[0079] It is understood that the above description is intended to serve as an example only, and many other embodiments are possible within the scope of the present invention as defined by the appended claims. For example, the illumination source of the illumination section of the projector may be a laser, a light emitting diode (LED), a micro LED, and / or a liquid crystal on silicon (LCOS) illumination device. As another example, the coupling prism between the relay section and the waveguide may operate in a reflection mode instead of a transmission mode. Furthermore, many other configurations of the NAE are possible based on the principles disclosed herein, as would be readily apparent to one skilled in the art of optical design, in addition to those explicitly shown in Figures 6 and 7B.

Claims

1. A projector for use with a display device, the projector comprising: an illumination unit; a relay unit; and a numerical aperture expander (NAE); the illumination unit includes one or more illumination sources, a focusing lens that focuses light onto an image plane, and a scanning device interposed between the focusing lens and the image plane; the relay unit includes an optical reflecting lens that collimates light from the image plane onto an exit pupil; the relay unit is configured such that a scan plane is an image conjugate of the exit pupil, the exit pupil serving as an entrance pupil of a waveguide; the NAE is configured to receive light from the illumination portion, the received light having a first average numerical aperture, and transmit light to the relay portion, the transmitted light having a second average numerical aperture greater than the first average numerical aperture by an average magnification factor of the NAE that is greater than 1; A miniature projector, wherein the surface of the NAE is curved to at least partially correct field curvature aberrations caused by the relay section and the illumination section.

2. 10. The projector of claim 1, wherein the scanning device is illuminated by a converging beam.

3. 10. The projector of claim 1, wherein the illumination section further comprises a field lens interposed between the scanning device and the image plane and proximate to the image plane.

4. 4. The projector of claim 3, wherein a beam diameter of light propagating from said scanning device to said field lens is reduced by at least a factor of two.

5. 2. A projector as claimed in claim 1, wherein the scanning device comprises a single mirror rotating about two substantially orthogonal axes, or two mirrors each rotating about a single axis.

6. 10. The projector of claim 1, wherein the one or more illumination sources comprise an illumination source selected from the group comprising laser diodes, parallel laser diodes, light emitting diodes (LEDs), micro LEDs, and liquid crystal on silicon (LCOS) illumination devices.

7. 10. The projector of claim 1, wherein the illumination portion comprises one or more photodetectors for monitoring illumination power emitted by the one or more illumination sources.

8. The projector of claim 7 , wherein the one or more photodetectors include at least one spectral filter.

9. The projector of claim 1 , wherein the illumination portion comprises a reflective lens.

10. 10. The projector of claim 1, comprising at least two illumination sources arranged in a side-by-side configuration, a first portion of light emitted by each of the illumination sources being transmitted by the scanning device and the focusing lens.

11. The irradiation unit comprises two or more photodetectors for monitoring the irradiation power emitted by the one or more irradiation sources; 11. The projector of claim 10, wherein a second portion of the light emitted by each of said illumination sources is reflected towards a photodetector array of a plurality of said photodetectors.

12. 12. The projector of claim 11, wherein the second portion of light is emitted along a fast axis of the illumination source, the fast axis having a wide beam divergence.

13. 11. The projector of claim 10, wherein the spacing between the outermost beams of the parallel arrangement amounts to at least 0.1 millimeters.

14. The projector of claim 1 , wherein a surface of the NAE is curved to at least partially correct field curvature aberrations caused by at least one of the relay section and the illumination section.

15. The projector of claim 1 , wherein the NAE is embedded between optical components with no gaps between the components.

16. The projector of claim 1 , wherein the NAE is implemented as an at least partially transmissive or partially reflective microlens array (MLA) or light diffuser.

17. The projector of claim 1 , wherein the NAE is implemented as a diffuse MLA including a relatively low light output diffuser superimposed on a surface of a relatively high light output MLA.

18. 2. The projector of claim 1, wherein the average magnification value of the NAE is in the range of 2-5.

19. The projector of claim 1 , wherein the relay portion comprises one or more polarizing optical elements.

20. 20. The projector of claim 19, wherein the relay portion includes a polarizing beam splitter and a reflective collimator lens.

21. 1. A miniature projector for use in a head mounted display device, the projector comprising: an illumination unit; a relay unit; and a numerical aperture expander (NAE); the illumination unit includes one or more illumination sources and a focusing lens that focuses light onto an image plane; the relay unit includes an optical reflecting lens that collimates light from the image plane onto an exit pupil; The NAE receives light from the illumination portion, the received light having a first average numerical aperture; and transmits light to the relay portion, the transmitted light having a second average numerical aperture greater than the first average numerical aperture by an average magnification factor of the NAE that is greater than 1; A miniature projector, wherein the NAE comprises a curved surface, the curvature of which is configured to at least partially correct field curvature aberrations resulting from at least one of the relay section and the illumination section.

22. 22. The projector of claim 21, wherein the average magnification value of the NAE is in the range of 2-5.

23. 1. A miniature projector for use in a head mounted display device, the projector comprising: an illumination unit; a relay unit; and a numerical aperture expander (NAE); The illumination section includes one or more illumination sources, a focusing lens for focusing light onto an image plane, and a scanning device interposed between the focusing lens and the image plane, the scanning device being illuminated by a focused beam; the relay unit includes an optical reflecting lens that collimates light from the image plane onto an exit pupil; The NAE is configured to receive light from the illumination section, the received light having a first average numerical aperture, and transmit light to the relay section, the transmitted light having a second average numerical aperture greater than the first average numerical aperture by an average magnification factor of the NAE that is greater than 1.

24. 24. The projector of claim 23, wherein the illumination portion further comprises a field lens interposed between the scanning device and the image plane and proximate to the image plane.

25. 25. The projector of claim 24, wherein a beam diameter of light propagating from the scanning device to the field lens is reduced by at least a factor of two.

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