Patent Publication Number: US-11656467-B2

Title: Compact laser-based near-eye display

Description:
BACKGROUND 
     Near-eye display technology has evolved in recent years into an emerging consumer technology. In head-worn display devices, for example, binocular near-eye display provides 3D stereo vision for virtual-reality (VR) presentation. When implemented with see-through optics, near-eye display provides mixed- or augmented-reality (AR) presentation, where VR elements are admixed into a user&#39;s natural field of view. Despite such benefits, near-eye display technology still faces various technical challenges, including the challenge of providing desired display luminance using compact, light-weight, low-power components. 
     SUMMARY 
     One aspect of this disclosure relates to a near-eye display device comprising a pupil-expansion optic, first and second lasers, a drive circuit coupled operatively to the first and second lasers, a beam combiner, a spatial light modulator (SLM), and a computer. The first and second lasers are configured to emit in respective first and second wavelength bands. The beam combiner is configured to geometrically combine emission from the first and second lasers into a collimated beam. The SLM has a matrix of electronically controllable pixel elements and is configured to receive the collimated beam and to direct the emission in spatially modulated form to the pupil-expansion optic. Coupled operatively to the drive circuit and to the SLM, the computer is configured to parse a digital image, trigger the emission from the first and second lasers by causing the drive circuit to drive a first current through the first laser and a second current through the second laser, and control the matrix of pixel elements such that the spatially modulated form of the emission projects an optical image corresponding to the digital image. 
     This Summary is provided to introduce in simplified form a selection of concepts that are further described in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    shows aspects of an example near-eye display device. 
         FIG.  2    shows aspects of an example monocular system of a near-eye display device. 
         FIG.  3    shows aspects of an example edge-emitting diode laser of a near-eye display device. 
         FIG.  4    shows aspects of an example beam combiner of a near-eye display device. 
         FIG.  5    shows aspects of an example laser enclosure of a near-eye display device. 
         FIG.  6    shows aspects of an example display projector of a near-eye display device, based on a reflective LCOS-type SLM. 
         FIGS.  7 A,  7 B, and  7 C  show aspects of example timing diagrams for illuminator modulation in a display projector of a near-eye display device. 
         FIG.  8    illustrates an example interference fringe that may be observed on a near-eye display device. 
         FIG.  9    is a plot of a Fourier transform of example laser emission overlaid with length ranges corresponding to observed optical path-length differences for an example near-eye display device. 
         FIGS.  10 A,  10 B,  10 C, and  10 D  are illustrative plots of selected emission properties of an example laser as functions of controllable parameters. 
         FIGS.  11 A,  11 B, and  11 C  show aspects of additional example timing diagrams for laser modulation in a near-eye display device. 
         FIG.  12    shows aspects of an example near-eye display method. 
         FIGS.  13 A,  13 B,  13 C, and  13 D  show aspects of an example pupil-expansion optic of a near-eye display device. 
         FIGS.  14 A and  14 B  show aspects of stereoscopic display projection in an example near-eye display device. 
         FIG.  15    shows aspects related to ocular sensing in an example near-eye display device. 
         FIG.  16    shows aspects of an example onboard computer of a near-eye display device. 
     
    
    
     DETAILED DESCRIPTION 
     This disclosure is presented by way of example and with reference to the drawing figures listed above. Components, process steps, and other elements that may be substantially the same in one or more of the figures are identified coordinately and described with minimal repetition. It will be noted, however, that elements identified coordinately may also differ to some degree. It will be further noted that the figures are schematic and generally not drawn to scale. Rather, the various drawing scales, aspect ratios, and numbers of components shown in the figures may be purposely distorted to make certain features or relationships easier to see. 
     As noted above, one challenge facing near-eye display technology is the ability to project high-luminance display imagery using compact, light-weight, low-power components. This is especially true for near-eye display devices in which a spatial-light modulator (SLM) is used to form the display imagery. SLM variants such as liquid-crystal-on-silicon (LCOS) and digital micromirror device (DMD) matrices are capable of high-brightness operation with good spatial and color fidelity. The overall power efficiency of an SLM-based display is limited, however, by the efficiency of illumination of the SLM. Light-emitting diode (LED) emitters, while sufficiently compact for near-eye display, exhibit significant etendue loss and require downstream polarization filtering for SLM illumination. Etendue and polarization losses for LED illumination of an SLM may be about 30% and 50%, respectively. 
     In contrast, the output of a semiconductor laser is intrinsically polarized and etendue-conserving, and some semiconductor lasers provide high brightness and high efficiency. Nevertheless, the spatial and temporal coherence of laser emission may be problematic for near-eye display. At any angle in the user&#39;s field of view, a near-eye display device admits of plural optical paths from the emission source to the user&#39;s anatomical pupil. When coherent light arriving along any two of the optical paths converges at the pupil, such light will interfere constructively or destructively. Accordingly, at angles in the field-of-view where the difference in the optical path lengths matches a longitudinal mode of the coherent emission, the user may see a distracting display artifact in the form of an interference fringe. 
     The solutions herein provide practical ways of using laser emission to illuminate an SLM in a near-eye display device. Among other advantages, they provide high brightness with fewer artifacts of illumination coherence. In this manner, the disclosed solutions, enacted separately or in any combination, enable compact, light-weight, energy-efficient near-eye display. 
     One solution is to illuminate the SLM using plural lasers in one to all of the primary-color channels. The plural lasers of each primary-color channel may differ in cavity length, thereby providing broader (i.e., less monochromatic) emission, with additional longitudinal modes. Thus, for any mode matched to an optical path-length difference and causing an interference fringe, there will be one or more additional modes unmatched to the optical path-length difference. As a result, interference fringes due to any one mode are effectively ‘washed out’—i.e., reduced to a chromatic variation that the user cannot perceive, thus mitigating potentially distracting visual artifacts. 
     Another solution achieves a similar effect but with fewer lasers for each primary-color channel. It leverages the broadened gain spectrum of a semiconductor laser driven by modulated current of sufficient high-frequency content. By modulating the drive current above and below the lasing threshold over predetermined intervals, stimulated emission is achieved over a range of wavelengths (and longitudinal modes) broad enough to wash out the interference fringes as noted above. In some examples, a single drive-modulated laser can simulate the emission spectrum of plural lasers having different cavity lengths. 
     Related to the strategies above is an additional solution of combining, efficiently and compactly, the output of plural semiconductor lasers. State-of-the-art birefringence- or diffraction-based beam combiners may not be scalable to larger numbers of combined beams without exceeding the tight dimensional constraints of a practical near-eye display device. By contrast, the geometric beam combiner disclosed herein is linearly scalable to larger numbers of combined beams. Furthermore, the arrangement of the geometric beam combiner relative to the individual lasers allows the same set of collimation optics to be used to collimate the emission from every beam en route to the SLM. 
     Turning now to the drawings,  FIG.  1    shows aspects of an example near-eye display device  10 . The near-eye display device is configured to be worn and operated by a user and to display still or moving images in the user&#39;s field-of-view. In some examples, the near-eye display device may include or be part of an AR or VR system that presents computer-generated, holographic imagery in the user&#39;s field-of-view. In some examples, user-input componentry of the AR or VR system may enable the user to interact with (e.g., manipulate) such imagery. To support any, some, or all of these functions, inter alia, near-eye display device  10  includes an onboard computer  12  having a processor  14  and associated computer memory  16 . In the illustrated example, near-eye display device  10  takes the form of a head-mounted visor. In other examples, a near-eye display device may take the form of goggles, a helmet, or eyeglasses. In still other examples, a near-eye display device may be a component of a non-wearable display system, such as a display system installed in a vehicle. 
     Near-eye display device  10  is configured for binocular image display. To that end, the near-eye display device includes a right monocular system  18 R that presents a right optical image  20 R in front of the user&#39;s right eye, and a left monocular system  18 L that presents a left optical image  20 L in front the user&#39;s left eye. For stereoscopic display the right and left optical images may be configured with stereo disparity appropriate to display a three-dimensional subject or scene (as described with reference to  FIGS.  14 A and  14 B ). In other examples, binocular display may be provided via a single display projected system akin monocular system  18 , but configured to project the right and left optical images into the right and left eyes, respectively. 
       FIG.  2    shows aspects of an example monocular system  18  of near-eye display device  10 . The monocular system includes a display projector  22  configured to form an optical image  20 . The display projector includes a high-resolution SLM  24  illuminated by a plurality of lasers  26 . Each laser is configured to emit (i.e., lase) in a particular wavelength band—e.g., first laser  26 A is configured to emit in a first wavelength band, second laser  26 B is configured to emit in a second wavelength band, and third laser  26 C is configured to emit in a third wavelength band. In some examples, the plurality of lasers may include at least one laser of each primary color—e.g., red, green, and blue. 
     The primary color red refers herein to light of one or more bands, however narrow, that fall within a range of 625 to 700 nanometers (nm). The primary color green refers to light of one or more bands, however narrow, that fall within a range of 500 to 565 nm. The primary color blue refers to light of one or more bands, however narrow, that fall within a range of 440 to 485 nm. In some examples, the wavelength ranges of the primary colors here noted may be broadened by as much as 10%. In some examples, the ranges of the primary colors here noted may be narrowed by as much as 10%. 
     Any, some, or all of the lasers may take the form of a semiconductor laser, such as a diode laser. In more particular examples, any, some, or all of the lasers may take the form of an edge-emitting diode laser, a double-heterostructure laser, a quantum-well laser, a distributed Bragg-reflector laser, a vertical-cavity surface-emitting laser, and/or an external-cavity laser, as examples. Efficient, compact lasers of virtually any architecture may be used. 
       FIG.  3    shows aspects of an example edge-emitting diode laser  26 A. Laser  26 A includes an elongate optical cavity  28  spanning a gain structure  30  and a reflector structure  32 . The gain structure includes epitaxial layers  34 P and  34 N, which bracket the optical cavity in the epitaxial direction. Epitaxial layer  34 N is an n-doped layer grown on n-type substrate  36  and coupled to electrically conductive (e.g., metal) cathode  38 . Epitaxial layer  34 P is a p-doped layer grown on epitaxial layer  34 N and coupled to electrically conductive anode  40 . Partially reflective aperture  42  is arranged at one end of the optical cavity; reflector structure  32  is arranged at the opposite end. Pumped by electric current flowing from anode  40  to cathode  38 , gain structure  30  amplifies the light reflecting back and forth within the optical cavity via stimulated radiative emission. Reflector structure  32  may comprise a diffractive reflector providing high reflectance and wavelength selectivity. In one example, the reflector structure includes a coated facet of the diode laser with parallel layers of alternating refractive index aligned perpendicular to the optical cavity. Reflections from the interfaces between layers combine constructively to achieve a partially or highly reflective facet in a selected wavelength band. 
     Continuing in  FIG.  3   , the emission from an edge-emitting diode laser diverges maximally in a wide-divergence plane  44 W and diverges minimally in a narrow-divergence plane  44 N, orthogonal to the wide-divergence plane. In some examples, the ‘angle’ of divergence in the wide-divergence plane may be diffraction-limited and Gaussian, at 20 to 40 degrees FWHM; the angle of divergence in the narrow-divergence plane may be about 5 to 10 degrees. 
     Each laser  26  of display projector  22  is coupled operatively to drive circuit  48 . The drive circuit is configured to drive a controlled current through each of the lasers—a first current through first laser  26 A, a second current through second laser  26 B, etc. More particularly, the controlled current is driven through gain structure  30 , from anode  40  to cathode  38 . In some examples, drive circuit  48  is configured to drive a periodic current through the gain structure. This feature supports field-sequential color display, pulse-width modulation for color balance, and spectral broadening as described hereinafter. The drive circuit may include, inter alia, a pulse-width modulator and a transconductance amplifier for each driven laser. 
     In some examples, the plurality of lasers  26  may illuminate SLM  24  via a beam combiner arranged in display projector  22 . The beam combiner may be configured to geometrically combine concurrent and/or sequential emission from each of the lasers into a collimated beam.  FIG.  4    shows aspects of an example beam combiner  50 A. Beam combiner  50 A includes a laser enclosure  52  in which lasers  26  are arranged.  FIG.  5    shows aspects of an example laser enclosure  52 A. 
     Laser enclosure  52 A includes a window  54  configured to transmit the emission from the lasers. In some examples, the atmosphere within the laser enclosure may be substantially depleted of oxygen. Each of the lasers  26  may be oriented in laser enclosure  54 A such that the wide-divergence planes  44 W of the lasers are parallel to each other and orthogonal to base  56  of the laser enclosure. To that end, the lasers may be oriented with mutually parallel optical cavities  28 . In some examples, some or all of the lasers may share an electrode, such as cathode  38 , which is arranged in contact with base  56 . In the illustrated example, the base delimits a flat mount  58  configured to carry heat away from the lasers. While not strictly necessary, any, some, or all of the lasers  26  may be arranged such that narrow-divergence plane  44 N is common to all of the lasers. To that end, the lasers may be arranged such that every optical cavity  28  lies within the same narrow-divergence plane. 
     Generally speaking, the laser enclosure may be configured to redirect (viz., to reflect or refract) the emission from any, some, or all of the lasers out of the narrow-divergence plane. This beam-turning effect contributes to an overall compact configuration of the beam combiner. In the illustrated example, laser enclosure  52 A includes a mirror  60  configured to receive and reflect emission from lasers  26  and thereby achieve this effect. In the illustrated example, mirror  60  is arranged within the laser enclosure, behind window  54 . In some examples the mirror may support one or more high-reflectance coatings—e.g., a different diffractive coating for each primary color, configured to reflect wavelengths corresponding to that primary color. In some examples, the mirror  60  may be a glass mirror. In other examples, the mirror may comprise highly polished and passivated metal, such as aluminum. 
     As shown in  FIG.  4   , the beam combiner may include one or more collimation optics configured to collimate the combined emission from the lasers. In the illustrated example, beam combiner  50 A includes a wide-diameter cylindrical collimation optic  62 W and a narrow-diameter cylindrical collimation optic  62 N. The wide-diameter cylindrical collimation optic has a cylindrical axis  64 W aligned normal to the wide-divergence planes of the lasers. The narrow-diameter cylindrical collimation optic has a cylindrical axis  64 N aligned normal to any plane orthogonal to the wide-divergence planes of the lasers. Accordingly, the wide-diameter cylindrical collimation optic reverses the divergence occurring in wide-divergence planes  44 W, and the narrow-diameter cylindrical collimation optic reverses the divergence occurring in narrow-divergence planes  44 N. In other examples, an engineered aspherical Fresnel optic may be used to collimate the combined emission from lasers  26 . Turning optics  66 A and  66 B of beam combiner  50 A fold the optical axis of laser enclosure  52 , contributing to an overall compact configuration. Beam combiner  50 A includes one or more sensors  68  (e.g., photodiodes) having an output responsive to the concurrent emission of lasers  26 . Output of the sensor can be used to maintain color balance in monocular system  18 , as described further below. 
     Beam combiner  50 A includes a diffuser  70  arranged in series with the one or more collimation optics and configured to diffuse the emission from lasers  26 . The diffuser is configured to homogenize the collimated beam so that the emission from each laser homogeneously illuminates the matrix of pixel elements of SLM  24 . Beam combiner  50  includes a laser despeckler  72  arranged in series with the collimation optics and configured to despeckle the emission from lasers  26 . ‘Speckle’ is observed when a spatially coherent, monochromatic wavefront interacts with a surface rough enough to scatter the light along optical paths that differ on the order of a wavelength and arrive at the same observation point. In the illustrated example, the diffuser is arranged optically downstream of the collimation optics, and the despeckler is arranged optically downstream of the diffuser. 
     A beam combiner may be configured to geometrically combine emission from plural lasers  26  irrespective of the wavelength or polarization state of the emission. For instance, a beam combiner may combine emission from first and second lasers having the same emission spectrum but differing substantially in output power. A first laser of higher output power may be turned when high brightness is required in a given color channel; a second laser of lower output power may be turned on when high-brightness is not required. A beam combiner may also combine emission from lasers having different emission spectra, as described hereinafter. 
     SLM  24  of  FIG.  2    includes a matrix of electronically and independently controllable pixel elements. The particular SLM technology may vary from one implementation to the next. In  FIG.  2   , display projector  22  forms optical image  20  by reflection of laser emission from the SLM. In other examples, an optical image may be formed by transmission of the laser emission through a suitably configured, transmissive SLM. In some examples, the SLM may comprise a liquid-crystal-on-silicon (LCOS) matrix. In other examples, the SLM may comprise a digital light projector (DLP) such as a digital micromirror device (DMD). 
       FIG.  6    shows aspects of an example display projector  22 A of a near-eye display device. Display projector  22 A is based on a reflective LCOS-type SLM  24 A. The display projector includes a PCB mounting  74 . Arranged over the PCB mounting, CMOS layer  76  defines the matrix of pixel elements of the SLM. A high-efficiency reflective coating  78  is arranged over the CMOS layer and configured to reflect the incident beam from beam combiner  50 . The incident beam is spatially modulated via liquid-crystal (LC) layer  80 . The LC layer includes a film of LC molecules (e.g., nematic LC molecules) maintained in quiescent alignment via alignment layer  82 . One or more transparent electrodes  84  are arranged over the alignment layer. The one or more transparent electrodes may include a degenerately doped semiconductor (e.g., indium tin oxide) on a suitable substrate. In other examples, the one or more transparent electrodes may include a microwire mesh or an extremely thin metal film. Cover glass  86  is arranged over the one or more transparent electrodes. In this configuration, the spatially modulated light reflecting from reflective coating  78  is directed back through the stack to exit polarizer  88  and then on to the eyepiece (e.g., pupil-expansion optic) of monocular system  18 . 
     Computer  12  is coupled operatively to drive circuit  48  and to SLM  24 . The computer is configured to parse a digital image, which may comprise plural component images, each associated with a corresponding primary color (e.g., red, green, and blue). The computer is configured to trigger emission from any, some, or all of the lasers  26  by controlling the drive currents supplied to gain structures  30  of the lasers by drive circuit  48 . The computer is also configured to control the matrix of pixel elements of SLM  24 . Such control is enacted synchronously and coordinately, such that the spatially modulated form of the emission emerging from the SLM projects an optical image  20  corresponding to the parsed digital image. In some examples, the computer is configured to coordinately control the drive circuit and the matrix of pixel elements in a time-multiplexed manner to provide field-sequential color display. By repeating such control over a time-indexed sequence of digital images, the computer may cause display projector  22  to project video. 
     Returning again to  FIG.  2   , display projector  22  projects optical image  20  through a physical aperture of finite size. Optics downstream of the display projector focus the optical image onto the anatomical right or left pupil of the user. In doing so, the downstream optics direct the image through an entry pupil, defined as the image of the physical aperture at the anatomical-pupil position. Due to the small size of the physical aperture and other factors, the entry pupil may be too small to align reliably to the user&#39;s anatomical pupil. Accordingly, monocular system  18  includes a pupil-expansion optic  90 . In the illustrated example, SLM  24  is configured to direct the combined emission from lasers  26 , in spatially modulated form, to the pupil-expansion optic. The pupil-expansion optic releases the optical image over an expanded exit pupil, which may be large enough to cover the entire area over which the user&#39;s pupil is likely to be. Such an area is called an ‘eyebox’. 
     Pupil-expansion optic  90  is configured to receive optical image  20  from display projector  22  and to release an expanded form  20 ′ of the optical image toward the pupil position  92 . In the illustrated example, the pupil-expansion optic includes an optical waveguide  94 , an entry grating  96  and an exit grating  98 . The pupil-expansion optic may also include other gratings not shown in  FIG.  2   . It will be understood that the term ‘grating’ is broadened herein to include any kind of diffractive optical element (DOE), irrespective of whether that element includes a pattern of elongate diffractive features. Non-limiting example gratings include a surface-relief type grating comprising a series of closely spaced channels formed on the optical waveguide, or a volume grating or index-modulated grating formed in the optical-waveguide material. 
     Entry grating  96  is a diffractive structure configured to receive optical image  20  and to couple the light of the optical image into optical waveguide  94 . After coupling into the optical waveguide, the display light propagates through the optical waveguide by total internal reflection (TIR) from the front and back faces of the optical waveguide. Exit grating  98  is a diffractive structure configured to controllably release the propagating display light from the optical waveguide in the direction of pupil position  92 . To that end, the exit grating includes a series of light-extraction features arranged from weak to strong in the direction of display-light propagation through the optical waveguide, so that the display light is released at uniform intensity over the length of the exit grating. In this manner, pupil-expansion optic  90  may be configured to expand the exit pupil of display projector  22  so as to fill or overfill the eyebox of the user. This condition provides desirable image quality and user comfort. 
     In some examples, pupil-expansion optic  90  may expand the exit pupil of display projector  22  in one direction only—e.g., the horizontal direction, in which the most significant eye movement occurs. Here, the display projector itself may offer a large enough exit pupil—natively, or by way of a vertical pre-expansion stage—so that vertical expansion within the optical waveguide is not necessary. In other examples, pupil-expansion optic  90  may be configured to expand the exit pupil in the horizontal and vertical directions. In such examples, display light propagating in a first direction within the optical waveguide may encounter a turning grating (not shown in  FIG.  2   ) having a plurality of diffraction features arranged weak to strong in a first direction. The turning grating may be configured such that the light diffracted by the diffraction features is turned so as to propagate in a second direction, having now been expanded in the first direction. Parallel rays of the expanded light then encounter exit grating  98  and are out-coupled from the waveguide as described above. A more detailed example of a pupil-expansion optic employing a turning grating is described hereinafter, in connection to  FIGS.  13 A through  13 D . 
     Despite the utility of diffractive optical elements for coupling light into and out of an optical waveguide, in-coupling and out-coupling optical elements based on reflection, refraction, and/or scattering are envisaged as alternatives to DOEs. In still other examples, a pupil-expansion optic may include, in lieu of an optical waveguide, a series of reflective-refractive interfaces (so-called ‘venetian blinds’) oriented 45 degrees relative to the optical axis. Irrespective of the particular pupil-expansion technology employed, a pupil expansion optic necessarily increases the number of optical path lengths between the emission source and the user&#39;s pupil, thereby increasing the potential for overlap between the optical path lengths and the longitudinal mode spacings of coherent laser emission. 
       FIGS.  7 A,  7 B, and  7 C  show aspects of example timing diagrams for illuminator modulation in a display projector of a near-eye display device. The timing diagram of  FIG.  7 A  illustrates the strategy known as ‘field-sequential color display’, where red, green, and blue illuminators are energized during successive intervals within each image frame. During the interval in which the red-emitting illuminator is energized, the pixel elements of the SLM are biased according to the component digital image corresponding to the red-color channel, and likewise for the green- and blue-emitting illuminators. The required modulation for field-sequential color display is slow on the timescale of illuminator and SLM response but fast on the timescale of the human ocular system. Accordingly, the component red, green, and blue images appear fused to the near-eye display user. 
     For each timing diagram in  FIG.  7 A , ff., the vertical axis represents drive current applied to the red-, green-, or blue-emitting illuminator. In examples in which the illuminator is a laser, the modulation is between below-threshold drive current A and above-threshold drive current B, where ‘threshold’ refers to the laser&#39;s drive-current threshold for stimulated radiative emission. In some examples, a nonzero value of below-threshold drive current A provides decreased power loss and emission latency. 
     The insets in  FIGS.  7 A , ff are plots of emission power as functions of wavelength. The wavelength range for each inset well within and much narrower than the afore-noted wavelength range of the indicated primary color. The inset of  FIG.  7 A  shows an example emission spectrum  102 G 1  of green-emitting diode laser  26 G 1 , using the indicated modulation scheme. The emission spectrum has a relatively narrow FWHM 1 , which corresponds to a sparse longitudinal-mode spacing.  FIG.  8    provides a rough illustration of a display artifact  104  that may be observed through a near-eye display device in which an SLM is illuminated by laser  26 G 1 . As noted hereinabove, the source of the artifact is coincidence between a longitudinal mode of coherent emission and the path-length difference along plural optical paths that carry the coherent emission from the laser to the user&#39;s pupil. 
       FIG.  9    presents data that illustrates this coincidence by way of a non-limiting, example. In particular,  FIG.  9    is a plot of a Fourier transform  106  of green laser emission, such as the emission from laser  26 G 1 , overlaid with plural length ranges  108 . The length ranges corresponding to selected optical path-length differences observable for an example near-eye display device. As expected, the longitudinal mode spacing is approximately two times the optical cavity length (which is the cavity length multiplied by the index of refraction) of the laser. For instance, a blue laser may have a cavity length in the range of 300 to 900 μm; a green laser may have a cavity length in the range of 400 to 1000 μm; and a red laser may have a cavity length in the range of 600 to 2000 μm. 
     More particularly, length range  108 A corresponds to complex 1a DOE1 1a3 1b 1b 01b33 RG plate. Length range  108 B corresponds to complex 1a DOE1 1a3 1b1b 01b33 BG plate. Length range  108 C corresponds to zero-order in glass RG plate. Length range  108 D corresponds to complex 1b DOE1 01b31a1a1a33 RG plate. Length range  108 E corresponds to complex 1bDOE101b31a1a1a33 BG plate. Length range  108 F corresponds to DOE3-2order RG plate. Length range  108 G corresponds to DOE2 order BG plate. Because Fourier transform  106  has peak coherence within length range  108 G, it is expected that this mode will give rise to an interference fringe due to an optical path length passing through the RG plate at second order. 
     While coherent illumination may cause display artifacts in display systems of various kinds, a near-eye display device with a pupil-expansion optic is particularly prone to such artifacts—as the primary function of the pupil expander is to multiply the number of optical paths from the display projector to the user&#39;s pupil. Presented next are various spectral-broadening approaches that may be used in a near-eye display device to wash out the interference fringes caused by the coincidence between longitudinal modes and optical path-length differences. 
     In some examples, a portion of the overall fringe-reduction strategy may include avoidance of longitudinal modes that yield the strongest interference fringes for a given near-eye display configuration. Thus, in a near-eye display device that admits of a plurality of optical path lengths from a laser and through a pupil-expansion optic, where the cavity length of the laser corresponds to a longitudinal mode spacing, the cavity length may be selected to avoid coincidence between the longitudinal mode spacing and any difference in the plurality of optical path lengths. That approach may be practical only for avoidance of the most prominent and/or predictable interference fringes. Accordingly, in scenarios where coincidence between a longitudinal mode of a first laser and an optical path-length difference gives rise to an interference fringe, the cavity length of a second laser of the same primary color may be selected to wash out the interference fringe. The term ‘wash out’ is meant to convey the idea that every combination of optical paths carrying a longitudinal mode that coincides with the path-length difference also carries numerous other longitudinal modes that fail to coincide with the path-length difference. Each of the other modes combines to weaken the brightness contrast of the interference fringe, reducing it to a chromatic variation that the user cannot perceive. 
     Thus, one approach herein is to provide spectral diversity by including, within each primary-color band, emission from plural lasers with offset emission-wavelength bands. Returning briefly to  FIG.  5   , laser enclosure  52 A includes two lasers of each primary color: red-emitting lasers  26 R 1  and  26 R 2 , green-emitting lasers  26 G 1  and  26 G 2 , and blue-emitting lasers  26 B 1  and  26 B 2 . The inset of  FIG.  7 B  represents a first wavelength band  102 G 1  for green-emitting laser  26 G 1  and a second wavelength band  102 G 2  for green-emitting laser  26 G 2 . The second wavelength band is spectrally distinct from the first wavelength band but of the same primary color (green) as the first wavelength band. The plot also shows, in dashed lines, the combined emission profile from both of the green-emitting lasers at equal power. The combined emission profile has a FWHM 1+2 , which is greater than the FWHM of wavelength band  102 G 1  and greater than the FWHM of  102 G 2 . 
     In some examples the peak wavelength of the first wavelength band may exceed the peak wavelength of the second wavelength band by three nanometers or more. More generally, the first and second wavelength bands (and so on) may be selected to provide spectral diversity for fringe mitigation, while still providing desired irradiance in the same primary-color channel. As illustrated in  FIG.  10 A , the peak emission wavelength of a diode laser may increase with increasing cavity length. Accordingly, desired wavelength diversity may result from the combined emission of a first laser  26 G 1  and a second laser  26 G 2 , which differ substantially in cavity length. In other words, the cavity lengths may differ in accordance with an engineering specification, not merely as a result of manufacturing tolerance. In some examples, the cavity length of the first laser may exceed the cavity length of the second laser by five percent or more. 
     The examples above should not be construed to limit the range of variants and alternatives for achieving the desired spectral broadening. The principles illustrated in the drawings for green laser emission apply equally to laser emission of any primary or non-primary color. While  FIG.  5    shows two lasers of each primary color, a given primary-color channel may include more than two lasers, or only one. In any configuration, if spectral diversity sufficient to wash out the red interference fringes cannot be provided by two red-emitting lasers, then a third red-emitting laser may be added. If the interference fringes from one, suitably configured blue-emitting laser are acceptably subtle, then a second blue-emitting laser may be unnecessary. The foregoing configurations enable concurrent operation of selected combinations of lasers. That approach may provide maximum display brightness and a simplified control strategy. Nevertheless, another acceptable approach is to operate the indicated combination of lasers in a time-multiplexed manner and to rely on the latency of the human ocular system to fuse successive fringe-prone image subframes into a fringe-averaged result. This variant is shown in the timing diagram of  FIG.  7 C . 
     As noted hereinabove, any, some, or all of the lasers  26  may include a reflector structure  32  comprising an electrooptical material. By varying the control voltage applied to the reflector structure, the gain spectrum of the laser may be shifted such that the emission-wavelength band of the laser is controllable based on the control voltage.  FIG.  10 B  provides illustrative plot showing an example dependence of peak emission wavelength on control voltage. In examples supporting this variant, drive circuit  48  may be further configured to vary the control voltage based on control signal from the computer, in order to urge the emission-wavelength band toward a predetermined wavelength distribution. This feature can be used to simulate a variable cavity length. Controlled variation of the gain spectrum may be used, for example, to quell fringes that appear under particular operating conditions of a near-eye display device, such as when the user&#39;s gaze is directed to angles at the extrema of the field-of-view. 
     Another way of achieving spectral diversity is to leverage the effect of drive-current transients on the gain spectrum of a semiconductor laser. This tactic may require fewer lasers to achieve a similar effect as the multi-laser configurations above. For some lasers, a drive-current excursion above the lasing threshold triggers stimulated emission over a relatively broad range of wavelengths (and longitudinal modes). With continued above-threshold bias, the emission relaxes to a narrower distribution at the long-wavelength end of the range. By modulating the drive current above and below the lasing threshold over narrow enough intervals, the relaxation stops abruptly. Thus, under steady-state periodic modulation with sufficient high-frequency content, the steady-state emission from the laser is broadened ( FIG.  10 C ) and blue-shifted ( FIG.  10 D ) relative to the emission under direct-current (d.c.) bias. 
     In view of this effect, drive circuit  48  may be configured to drive a periodic current through the gain structure of any laser  26 . Computer  12  may be configured to control the periodic current to drive plural cycles of modulation through the gain structure during projection of a single optical image (e.g., a primary-color component of a digital image). As a result the wavelength band of the emission from the laser may be broader than the wavelength band of emission from the same laser when driven by unmodulated drive current. In some examples the periodic current includes a pulse-modulated current including a train of current pulses. As noted above, the value of the pulse width may influence the gain profile of the laser over a domain of sufficiently short pulse widths. The plot in  FIG.  10 C  provides an illustration of this effect. In more particular examples, the pulse-modulated current may include a train of current pulses having a pulse width of twenty nanoseconds or shorter. 
     The timing diagram of  FIG.  11 A  shows an example pulse train for laser  26 G 1 . The inset of  FIG.  11 A  shows an emission-wavelength band broadened with respect to the emission-wavelength band of the same laser, shown in  FIG.  7 B .  FIG.  11 B  shows train of shorter pulses for the same laser, and the inset illustrates the emission-wavelength band further broadened. 
     As noted hereinabove in the context of cavity-length variation, a portion of the fringe-reduction strategy may include judicious avoidance of emission-wavelength bands that yield the strongest fringes for a given near-eye display configuration. Thus, in a near-eye display device that admits of a plurality of optical path lengths from a laser through a pupil-expansion optic, and wherein the gain profile of the laser corresponds to a longitudinal mode spacing, the pulse width may be selected to avoid coincidence between the longitudinal mode spacing and the plurality of optical path lengths. This can be done, for instance, by engineering a predetermined blueshift in the emission-wavelength band of the laser. In scenarios in which coincidence between the longitudinal mode spacing and the plurality of optical path lengths gives rise to an interference fringe, the pulse width may be increased so as to wash out the interference fringe. In configurations including first and second lasers of the same primary color, the pulse width of periodic modulation of the second laser may be used to wash out an interference fringe caused by emission from the first laser, or vice versa. 
     Generally speaking, the train of current pulses applied to the gain section of a laser defines the average duty cycle of the laser. Computer  12  may be configured to adjust the pulse separation in view of a (predetermined) pulse width, so as to control the average duty cycle. This approach can be appreciated by comparison of  FIGS.  11 B and  11 C , where the emission-wavelength band in  FIG.  11 C  has the same FWHM as that of  FIG.  11 B  but provides only half the power. The computer may control the average duty cycle so as to provide setpoint power in a primary color band, for example. The spectral broadening achievable via pulse-modulation of the drive current is also achievable via continuous-wave (e.g., sinusoidal) modulation with equivalent Fourier spectrum. In some examples, accordingly, the periodic current applied to the gain section may include a radio-frequency modulated current. 
       FIG.  12    shows aspects of an example near-eye display method  110  to be enacted by an onboard computer of a near-eye display device. The method is supported by the configurations herein and by other near-eye display configurations. 
     At  112  of method  110 , the computer parses a digital image. In some examples, the digital image may correspond to a video frame. In some examples, the digital image may be a component image representing display-image content in one of a plurality of color channels. In parsing the digital image, the computer reads a brightness value corresponding to coordinates X i , Y i  of each pixel i of the digital image. 
     At  114  the computer controls a matrix of electronically controllable pixel elements of an SLM of the near-eye display device. As noted hereinabove, the SLM is configured to receive emission from one or more lasers and to direct the emission in spatially modulated form to a pupil-expansion optic. The matrix is controlled such that the spatially modulated form of the emission projects an optical image corresponding to the digital image parsed at  112 . More specifically, the computer geometrically maps each pixel of the parsed digital image to a row and column of the SLM and controls the bias applied to the pixel element at the mapped row-column intersection. The bias is controlled so as to provide the appropriate relative brightness for each locus of the optical image emerging from the SLM. 
     At  116  the computer computes the average duty cycle for the pulse-modulated drive current supplied to a laser in a display projector of the near-eye display device. The average duty cycle may be computed so as to provide color balance for field-sequential color-display where plural lasers are pulse-modulated. In some examples the computer may control the average duty cycle so as to provide setpoint power in a primary-color band, such as a red, green, or blue band. 
     At  118  the computer computes a pulse width and a pulse spacing of the pulse-modulated drive current so as to operate the laser at the duty cycle computed at  116 . The pulse width and pulse spacing may be computed in dependence on various factors. Such factors include (a) the average duty cycle computed at  116 , (b) the required spectral diversity, and/or (c) any of a plurality of use conditions (vide infra) of the near-eye display device. In some examples the pulse width may be fully determined by the required spectral diversity; accordingly the computer may adjust the pulse separation in view of the fully determined pulse width, so as to arrive at the average duty cycle computed at  116 . 
     At  120  the computer controls a drive circuit of the near-eye display device to drive plural cycles of periodic current through a gain structure of a laser while the optical image corresponding to the parsed digital image is projected. For instance, plural cycles of the modulation may be received during a period in which the SLM is set to a given primary-color component. In this example, the periodic current comprises a pulse train having the pulse width and pulse spacing computed at  118 . In some examples the periodic current includes a train of current pulses having a pulse width of twenty nanoseconds or shorter and defining the average duty cycle. 
     At  122  the computer senses the total power provided within the primary-color channel corresponding to the parsed digital image. The power may be sensed via a photodiode sensor arranged in a beam combiner of the near-eye display device, for example. The power sensed in this manner may be used by the computer to iteratively refine the duty-cycle computation of  116 , for example. 
     As noted above, the computer may be configured to control the average duty cycle, pulse width, and/or pulse separation responsive to one or more operating conditions of the near-eye display device. Generally speaking, the pulse width may be reduced under conditions where increased spectral diversity in a given color channel is required to reduce fringing and, to conserve power, increased under conditions where increased spectral diversity is not required. In near-eye display devices equipped with an eye-tracking sensor, the discriminant for whether increased spectral diversity is required may be linked to the angle of the user&#39;s gaze within the field-of-view. In other words, angles at which problematic interference fringes do and do not appear may be predicted based on the physical configuration of the near-eye display components. The computer may be configured to apply more aggressive fringe mitigation when the user&#39;s gaze is directed at angles where interference fringes are most prevalent for a given primary color. Such gaze angles may correspond to a condition in which the laser(s) of that primary color are driven by pulse trains of the shortest pulse widths. In some examples, the pulse width may be shortest when the battery is fully charged and may increase as the battery charge is depleted. In some examples, the pulse width may be shortest under low ambient lighting, when the user is most likely to discern interference fringes, and may increase with increasing ambient brightness. The average duty cycle also may depend on the ambient light level—viz., to project brighter display imagery under brighter ambient lighting. 
     In view of the various ways in which the parameters of the periodic drive current may be controlled pursuant to changes in operating conditions, method  110  includes, at  124 , a step in which the various operating conditions are sensed. Such operating conditions may include battery charge, ambient light level, and the angle of the user&#39;s gaze within the field-of-view, as examples. 
     The following section provides additional non-limiting description of a pupil-expansion optic  90 A with reference to  FIGS.  13 A through  13 D . In these drawings, optical waveguide  94  comprises a transparent (e.g., glass or polymer) slab with a planar entry face  126  and an opposing, planar exit face  128 .  FIG.  13 A  is a plan view of entry face  126 ;  FIG.  13 B  is a view of exit face  128  as seen through the entry face.  FIGS.  13 C and  13 D  are perspective views of the pupil-expansion optic rotated in opposite directions about a horizontal axis aligned to the forward edge. 
     Pupil-expansion optic  90  includes an entry zone  130  where the optical image is received through entry face  126  and an exit zone  132  where the expanded form of the optical image is released through exit face  128 . The pupil-expansion optic also includes an initial-expansion zone  134  that receives the display light from entry zone  130  and expands the display light en route to the exit zone. Pupil-expansion optic  90  includes a plurality of differently configured diffraction gratings arranged in the different zones. 
     In the illustrated example, rightward expansion grating  96 R is arranged on entry face  126 , and leftward expansion grating  96 L is arranged on exit face  128 . The rightward and leftward expansion gratings are entry gratings that extend through initial-expansion zone  134  and overlap in entry zone  130 . Exit grating  98  is arranged on entry face  126 , in exit zone  132 . In other examples, any, some, or all of the diffraction gratings enumerated above may be arranged on the opposite face of the optical waveguide relative to the illustrated configuration. 
     Operationally, low-angle display light is received in entry zone  130 , through entry face  126 . Rightward expansion grating  96 R and leftward expansion grating  96 L cooperate to couple the low-angle display light into optical waveguide  94 . Specifically, leftward expansion grating  96 L diffracts some of the incoming, low-angle display light obliquely rightward and downward at a supercritical angle, such that it now propagates through the optical waveguide in a rightward and downward direction. At each bounce from entry face  126 , the propagating light encounters rightward expansion grating  96 R, which directs successive, increasing portions of the light directly downward. This function expands the display light in the rightward direction and conveys the rightward-expanded display light into exit zone  132 . In a complementary manner, rightward expansion grating  96 R diffracts some of the incoming, low-angle display light obliquely leftward and downward at a supercritical angle, such that it propagates through the optical waveguide in a leftward and downward direction. At each bounce from exit face  128 , the propagating light encounters the leftward expansion grating, which directs successive, increasing portions of the light directly downward. This function expands the display light in the leftward direction and conveys the leftward-expanded display light into exit zone  132 . In the exit zone, the propagating display light at each bounce from entry face  126  encounters exit grating  98 , which directs successive, increasing portions of the rightward- and leftward-expanded display light out of optical waveguide  94 . In this manner, the display light is expanded in the downward direction—i.e., perpendicular to the rightward and leftward expansion effected by the right- and leftward expansion gratings. 
     The following section provides additional non-limiting description of monocular system  18  and near-eye display device  10 . Each optical image formed by monocular system  18  is a virtual image presented at a predetermined distance Z 0  in front of user O. The distance Z 0  is referred to as the ‘depth of the focal plane’ of the optical image. In some monocular systems, the value of Z 0  is a fixed function of the design parameters of display projector  22 , entry grating  96 , exit grating  98 , and/or other fixed-function optics. Based on the permanent configuration of these structures, the focal plane may be positioned at a desired depth. In one example, Z 0  may be set to ‘infinity’, so that each optical system presents a optical image in the form of collimated light rays. In another example, Z 0  may be set to 200 centimeters, requiring the optical system to present each optical image in the form of diverging light. In some examples, Z 0  may be chosen at design time and remain unchanged for all virtual imagery presented by the display system. Alternatively, the optical systems may be configured with electronically adjustable optical power, to allow Z 0  to vary dynamically according to the range of distances over which the virtual imagery is to be presented. 
     A binocular near-eye display device employing a fixed or variable focal plane may be capable of presenting virtual-display imagery perceived to lie at a controlled, variable distance in front of, or behind, the focal plane. This effect can be achieved by controlling the horizontal disparity of each pair of corresponding pixels of the right and left stereo images, as described below with reference to  FIGS.  14 A and  14 B . 
       FIG.  14 A  shows right and left image frames  136 R and  136 L overlaid upon each other for ease of illustration. The right image frame encloses right optical image  20 R, and the left image frame encloses left optical image  20 L. Viewed concurrently through a near-eye display device  10 , the right and left optical images may appear to the user as 3D hologram  138 , comprised of individually rendered loci. Each locus i of the visible surface of the hologram has a depth coordinate Z i  associated with a corresponding pixel (X i , Y i ) of each of the right and left optical images. The desired depth coordinate may be simulated as follows. 
     At the outset, a distance Z 0  to a focal plane F of the near-eye display system is chosen. Then the depth coordinate Z for every locus i of the visible surface of the hologram is set. This is done by adjusting the positional disparity of the two pixels corresponding to locus i in the right and left optical images relative to their respective image frames. In  FIG.  14 B , the pixel corresponding to locus i in the right image frame is denoted R i , and the corresponding pixel of the left image frame is denoted L i . In  FIG.  14 B , the positional disparity is positive—i.e., R i  is to the right of L i  in the overlaid image frames. Positive positional disparity causes locus i to appear behind focal plane F. If the positional disparity were negative, the locus would appear in front of the focal plane. Finally, if the right and left optical images were superposed (no disparity, R i  and L i  coincident) then the locus would appear to lie directly on the focal plane. Without tying this disclosure to any particular theory, the positional disparity D may be related to Z, Z 0 , and to the interpupillary distance (IPD) of the user by 
     
       
         
           
             D 
             = 
             
               IP 
               ⁢ 
               D 
               × 
               
                 
                   ( 
                   
                     1 
                     - 
                     
                       
                         Z 
                         0 
                       
                       Z 
                     
                   
                   ) 
                 
                 . 
               
             
           
         
       
     
     In some examples, computer  12  maintains a model of the Cartesian space in front of the user, in a frame of reference fixed to near-eye display device  10 . The user&#39;s pupil positions are mapped onto this space, as are the image frames  136 R and  136 L, each positioned at the predetermined depth Z 0 . Then, the visible surface of hologram  138  is assembled, with each locus i of the viewable surface of the imagery having coordinates X i , Y i , and Z i , in the common frame of reference. For each locus of the visible surface, two-line segments are constructed—a first line segment to the pupil position of the user&#39;s right eye and a second line segment to the pupil position of the user&#39;s left eye. The pixel R i  of the right optical image, which corresponds to locus i, is taken to be the intersection of the first line segment in right image frame  136 R. Likewise, the pixel L i  of the left optical image is taken to be the intersection of the second line segment in left image frame  136 L. This procedure automatically provides the appropriate amount of shifting and scaling to correctly render the visible surface, placing every locus i at the appropriate distance and with the appropriate perspective. In some examples, the approach outlined above may be facilitated by real-time estimation of the user&#39;s pupil positions. That variant is described hereinafter, with reference to  FIG.  15   . In examples in which pupil estimation is not attempted, a suitable surrogate for the pupil position, such as the center of rotation of the pupil position, or eyeball position, may be used instead. 
     Returning again to  FIG.  2   , monocular system  18  may be configured to vary the focal plane on which virtual display imagery is presented. In the illustrated example, the monocular system includes a variable-focus lens  140  of variable optical power. Computer  12  is configured to control the focusing bias of the variable-focus lens such that the display light is imaged onto a focal plane positioned at a controlled, variable distance from pupil position  92 . In stereoscopic near-eye display devices, this control feature may be enacted in combination with appropriate control of the stereo disparity as described above. Monocular system  18  of  FIG.  2    also includes a fixed-focus lens  142  in series with variable-focus lens  140  and arranged to pre-bias the vergence of the display light released from pupil-expansion optic  90 . 
     Applied in an AR display system, variable-focus lens  140  and/or fixed-focus lens  142  would alter the vergence of the external light received from opposite the user. In  FIG.  2   , accordingly, monocular system  18  further comprises a variable-compensation lens  144  of variable optical power and a fixed compensation lens  146 . In some examples, the fixed optical power of fixed-compensation lens  146  may oppose and substantially reverse the fixed optical power of fixed-focus lens  142 . When controlling the focusing bias such that the display light is imaged onto a focal plane positioned at a controlled, variable distance from user O, computer  12  may also synchronously control the compensation bias of the variable compensation lens such that the external light reaches the user with unchanged vergence. 
       FIG.  15    is provided in order to illustrate schematically how ocular sensing may be enacted in near-eye display device  10 . This approach may be used to sense the user&#39;s pupil positions for highly accurate 3D rendering, to accommodate a range of different users, and/or to support the methods herein. 
     The configuration illustrated in  FIG.  9    includes, for each monocular system  18 , a camera  148 , an on-axis lamp  150 A and an off-axis lamp  150 B. Each lamp may comprise a light-emitting diode (LED) or diode laser, for example, which emits infrared (IR) or near-infrared (NIR) illumination in a high-sensitivity wavelength band of the camera. 
     The terms ‘on-axis’ and ‘off-axis’ refer to the direction of illumination of the eye with respect to the optical axis A of camera  148 . As shown in  FIG.  15   , off-axis illumination may create a specular glint  152  that reflects from the user&#39;s cornea  154 . Off-axis illumination may also be used to illuminate the eye for a ‘dark pupil’ effect, where pupil  156  appears darker than the surrounding iris  158 . By contrast, on-axis illumination from an IR or NIR source may be used to create a ‘bright pupil’ effect, where the pupil appears brighter than the surrounding iris. More specifically, IR or NIR illumination from on-axis lamp  150 A may illuminate the retroreflective tissue of the retina  160 , which reflects the illumination back through the pupil, forming a bright image  162  of the pupil. Image data from the camera is conveyed to associated logic of computer  12 . There, the image data may be processed to resolve such features as one or more glints from the cornea, or the pupil outline. The locations of such features in the image data may be used as input parameters in a model—e.g., a polynomial model—that relates feature position to the apparent center of the pupil. 
     The configuration illustrated in  FIG.  15    may also be used to sense relatively long-timescale pupillary movement associated with changing gaze vector or accommodation (when enacted concurrently in the right and left monocular systems) as well as relatively short-timescale saccadic movement. The configuration illustrated in  FIG.  15    may also be used to sense nictitation. In other configurations, the pupil position may be determined, estimated, or predicted in various other ways—e.g., using an electrooculographic sensor in lieu of ocular imaging. 
     The methods herein may be tied to a computer system of one or more computing devices. Such methods and processes may be implemented as an application program or service, an application programming interface (API), a library, and/or other computer-program product. 
       FIG.  16    provides a schematic representation of a computer  12  configured to provide some or all of the computer-system functionality disclosed herein. Computer  12  may take the form of onboard computer  12 A, while in some examples at least some of the computer-system functionality may be provided by communicatively coupled offboard computer. 
     Computer  12  includes a logic system  14  and a computer-memory system  16 . Computer  12  may optionally include a display system  18 , an input system  164 , a network system  166 , and/or other systems not shown in the drawings. 
     Logic system  14  includes one or more physical devices configured to execute instructions. For example, the logic system may be configured to execute instructions that are part of at least one operating system (OS), application, service, and/or other program construct. The logic system may include at least one hardware processor (e.g., microprocessor, central processor, central processing unit (CPU) and/or graphics processing unit (GPU)) configured to execute software instructions. Additionally or alternatively, the logic system may include at least one hardware or firmware device configured to execute hardware or firmware instructions. A processor of the logic system may be single-core or multi-core, and the instructions executed thereon may be configured for sequential, parallel, and/or distributed processing. Individual components of the logic system optionally may be distributed among two or more separate devices, which may be remotely located and/or configured for coordinated processing. Aspects of the logic system may be virtualized and executed by remotely-accessible, networked computing devices configured in a cloud-computing configuration. 
     Computer-memory system  16  includes at least one physical device configured to temporarily and/or permanently hold computer system information, such as data and instructions executable by logic system  14 . When the computer-memory system includes two or more devices, the devices may be collocated or remotely located. Computer-memory system  16  may include at least one volatile, nonvolatile, dynamic, static, read/write, read-only, random-access, sequential-access, location-addressable, file-addressable, and/or content-addressable computer-memory device. Computer-memory system  16  may include at least one removable and/or built-in computer-memory device. When the logic system executes instructions, the state of computer-memory system  16  may be transformed—e.g., to hold different data. 
     Aspects of logic system  14  and computer-memory system  16  may be integrated together into one or more hardware-logic components. Any such hardware-logic component may include at least one program- or application-specific integrated circuit (PASIC/ASIC), program- or application-specific standard product (PSSP/ASSP), system-on-a-chip (SOC), or complex programmable logic device (CPLD), for example. 
     Logic system  14  and computer-memory system  16  may cooperate to instantiate one or more logic machines or engines. As used herein, the terms ‘machine’ and ‘engine’ each refer collectively to a combination of cooperating hardware, firmware, software, instructions, and/or any other components that provide computer system functionality. In other words, machines and engines are never abstract ideas and always have a tangible form. A machine or engine may be instantiated by a single computing device, or a machine or engine may include two or more subcomponents instantiated by two or more different computing devices. In some implementations, a machine or engine includes a local component (e.g., a software application executed by a computer system processor) cooperating with a remote component (e.g., a cloud computing service provided by a network of one or more server computer systems). The software and/or other instructions that give a particular machine or engine its functionality may optionally be saved as one or more unexecuted modules on one or more computer-memory devices. 
     Machines and engines may be implemented using any suitable combination of machine learning (ML) and artificial intelligence (AI) techniques. Non-limiting examples of techniques that may be incorporated in an implementation of one or more machines include support vector machines, multi-layer neural networks, convolutional neural networks (e.g., spatial convolutional networks for processing images and/or video, and/or any other suitable convolutional neural network configured to convolve and pool features across one or more temporal and/or spatial dimensions), recurrent neural networks (e.g., long short-term memory networks), associative memories (e.g., lookup tables, hash tables, bloom filters, neural Turing machines and/or neural random-access memory) unsupervised spatial and/or clustering methods (e.g., nearest neighbor algorithms, topological data analysis, and/or k-means clustering), and/or graphical models (e.g., (hidden) Markov models, Markov random fields, (hidden) conditional random fields, and/or AI knowledge bases)). 
     When included, display system  18  may be used to present a visual representation of data held by computer-memory system  16 . The visual representation may take the form of a graphical user interface (GUI) in some examples. The display system may include one or more display devices utilizing virtually any type of technology. In some implementations, display system may include one or more virtual-, augmented-, or mixed reality displays. 
     When included, input system  164  may comprise or interface with one or more input devices. An input device may include a sensor device or a user input device. Examples of user input devices include a keyboard, mouse, or touch screen. 
     When included, network system  166  may be configured to communicatively couple computer  12  with one or more other computer. The network system may include wired and/or wireless communication devices compatible with one or more different communication protocols. The network system may be configured for communication via personal-, local- and/or wide-area networks. 
     One aspect of this disclosure is directed to a near-eye display device comprising a pupil-expansion optic, first and second lasers, a drive circuit coupled operatively to the first and second lasers, a beam combiner, a spatial light modulator (SLM), and a computer. The first laser is configured to emit in a first wavelength band. The second laser configured to emit in a second wavelength band. The beam combiner configured to geometrically combine emission from the first and second lasers into a collimated beam. The SLM has a matrix of electronically controllable pixel elements and is configured to receive the collimated beam and to direct the emission in spatially modulated form to the pupil-expansion optic. Coupled operatively to the drive circuit and to the SLM, the computer configured to: parse a digital image, trigger the emission from the first and second lasers by causing the drive circuit to drive a first current through the first laser and a second current through the second laser, and control the matrix of pixel elements such that the spatially modulated form of the emission projects an optical image corresponding to the digital image. 
     In some implementations, the beam combiner includes one or more collimation optics configured to collimate the emission. In some implementations, emission from the first laser diverges maximally in a first plane, and emission from the second laser diverges maximally in a second plane, and the first and second lasers are oriented such that the first and second planes are parallel. In some implementations, the one or more collimation optics include a first cylindrical collimation optic with a cylindrical axis normal to the first and second planes. In some implementations, the one or more collimation optics include a second cylindrical collimation optic with a cylindrical axis normal to a plane orthogonal to the first and second planes. In some implementations, the beam combiner includes a diffuser arranged optically downstream of the one or more collimation optics and configured to diffuse the emission. In some implementations, the beam combiner includes a laser despeckler arranged optically downstream of the one or more collimation optics and configured to despeckle the emission. In some implementations, the first and second lasers are among a plurality of lasers coupled operatively to the drive circuit, the beam combiner is configured to geometrically combine emission from each of the plurality of lasers, and the plurality of lasers includes at least one laser of each primary color. In some implementations, a cavity of the first laser and a cavity of the second laser lie in a plane, and the beam combiner is configured to redirect the emission of the first and second lasers out of the plane. In some implementations, the beam combiner includes a mirror configured to receive the emission of the first and second lasers. In some implementations, the mirror supports a high-reflectance coating for each of a plurality of primary colors. In some implementations, the mirror and the first and second lasers are enclosed in an oxygen-depleted laser enclosure, and the laser enclosure includes a window configured to transmit the emission. In some implementations, the first and second lasers include a shared electrode in contact with a base parallel to the plane. In some implementations, the base delimits a flat mount configured to carry heat away from the first and second lasers. In some implementations, the first laser is configured to provide emission of substantially higher power than the second laser. In some implementations, the beam combiner includes one or more sensors with output responsive to emission of the first and/or second lasers, and the computer is configured to control the first and second currents further based on the output. In some implementations, the digital image is one of a plurality of component digital images parsed by the computer, each associated with a corresponding primary color, and the computer is further configured to: coordinately control the matrix of pixel elements and the drive circuit in a time-multiplexed manner to provide field-sequential color display. 
     Another aspect of this disclosure is directed to a near-eye display device comprising a pupil-expansion optic, first and second lasers, a drive circuit coupled operatively to the first and second lasers, a beam combiner, an SLM, and a computer. The first laser is configured to emit in a first wavelength band. The second laser is configured to emit in a second wavelength band. The beam combiner is configured to geometrically combine emission from the first and second lasers into a collimated beam. The beam combiner includes one or more collimating optics arranged in series with a diffuser, where the diffuser and each of the one or more collimating optics are configured to receive the emission. The SLM has a matrix of electronically controllable pixel elements and is configured to receive the collimated beam and to direct the emission in spatially modulated form to the pupil-expansion optic. Coupled operatively to the drive circuit and to the SLM, the computer is configured to: parse a digital image, trigger the emission from the first and second lasers by causing the drive circuit to drive a first current through the first laser and a second current through the second laser, and control the matrix of pixel elements such that the spatially modulated form of the emission projects an optical image corresponding to the digital image. Here the first and second lasers are among a plurality of lasers coupled operatively to the drive circuit, the beam combiner is configured to geometrically combine emission from each of the plurality of lasers, and the plurality of lasers includes at least one laser of each primary color. 
     Another aspect of this disclosure is directed to a near-eye display device comprising a pupil-expansion optic, first and second lasers, a drive circuit coupled operatively to the first and second lasers, a beam combiner, an SLM, and a computer. The first laser has a first gain structure and is configured to emit in a first wavelength band. The second laser has a second gain structure and configured to emit in a second wavelength band. The beam combiner is configured to geometrically combine emission from the first and second lasers into a collimated beam. The SLM has a matrix of electronically controllable pixel elements and is configured to receive the collimated beam and to direct the emission in spatially modulated form to the pupil-expansion optic. Coupled operatively to the drive circuit and to the SLM, the computer configured to: parse a digital image, trigger the emission from the first and second lasers by causing the drive circuit to drive a first current through the first gain structure and a periodic second current through the second gain structure, and control the matrix of pixel elements such that the spatially modulated form of the emission projects an optical image corresponding to the digital image. Here the periodic second current includes plural cycles of modulation driven through the second gain structure while the optical image is projected. 
     In some implementations, the beam combiner includes one or more collimating optics arranged in series with a diffuser, the diffuser and each of the one or more collimating optics are configured to receive the emission, the first and second lasers are among a plurality of lasers coupled operatively to the drive circuit, the beam combiner is configured to geometrically combine the emission from each of the plurality of lasers, and the plurality of lasers includes at least one laser of each primary color. 
     It will be understood that the configurations and/or approaches described herein are exemplary in nature, and that these specific embodiments or examples are not to be considered in a limiting sense, because numerous variations are possible. The specific routines or methods described herein may represent one or more of any number of processing strategies. As such, various acts illustrated and/or described may be performed in the sequence illustrated and/or described, in other sequences, in parallel, or omitted. Likewise, the order of the above-described processes may be changed. 
     The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various processes, systems and configurations, and other features, functions, acts, and/or properties disclosed herein, as well as any and all equivalents thereof.