TECHNICAL FIELD

The present systems, devices, and methods generally relate to holograms and particularly relate to holograms with controllably broadened bandwidth.

BACKGROUND

Description of the Related Art

Holograms

A hologram is a recording of a light field, with a typical light field comprising a pattern of optical fringes generated by interference between two beams of laser light. The hologram is made up of physical fringes, where physical fringes comprise variations in the refractive index or absorbance of the holographic recording medium.

During hologram playback, at least a portion of the light field used to record a hologram may be recreated by illuminating the hologram with laser light. If the laser light comprises the same wavelength and angle as the first beam of laser light used to record the hologram, and the fringes have not been altered after recording, the holographic medium will diffract laser light with the same angle and pattern as the second beam of laser light used to record the hologram. The intensity of the diffracted light is determined by the efficiency of the hologram, where the efficiency of the hologram is the fraction of the light of the first beam of laser light that is diffracted in the direction of the second beam of laser light; hologram efficiency may be in a range from 0-100%. The efficiency of a hologram depends on both the angle and the wavelength of light used to illuminate the holographic medium. Multiple holograms may be recorded in a single holographic recording medium, the multiple holograms comprising a multiplexed hologram.

Hologram Recording

A pattern of optical fringes may be generated by the interference of two beams of laser light; the two beams of laser light may be created by splitting a single beam of laser light. The two beams of laser light are typically referred to as the object beam and the reference beam. Hologram recording is typically designed such that, during playback, the recorded hologram is illuminated with laser light recreating the reference beam and the object beam is then replicated by the hologram.

Holograms are recorded in a holographic recording medium which may be a silver halide photographic emulsion, dichromated gelatin, photopolymer, or other physical media. Silver halide emulsions record a hologram as a pattern of absorbance and reflectance of light. Dichromated gelatin and photopolymer both record a hologram as a pattern of varying refractive index. Recording a hologram as a pattern of refractive index is advantageous since all of the illuminating laser light may theoretically leave the hologram; no light is necessarily absorbed by the hologram.

Laser Projectors

A projector is an optical device that projects or shines a pattern of light onto another object (e.g., onto a surface of another object, such as onto a projection screen) in order to display an image or video on that other object. A projector necessarily includes a light source, and a laser projector is a projector for which the light source comprises at least one laser. The at least one laser is temporally modulated to provide a temporal pattern of laser light and usually at least one controllable mirror is used to spatially distribute the modulated temporal pattern of laser light over a two-dimensional area of another object. The spatial distribution of the modulated temporal pattern of laser light produces a series of images at or on the other object. In conventional laser projectors, the at least one controllable mirror may include: a single digital micromirror (e.g., a microelectromechanical system (“MEMS”) based digital micromirror) that is controllably rotatable or deformable in two dimensions, or two digital micromirrors that are each controllably rotatable or deformable about a respective dimension, or a digital light processing (“DLP”) chip comprising an array of digital micromirrors.

Wearable Heads-Up Displays

A head-mounted display is an electronic device that is worn on a user's head and, when so worn, secures at least one electronic display within a viewable field of at least one of the user's eyes, regardless of the position or orientation of the user's head. A wearable heads-up display is a head-mounted display that enables the user to see displayed content but also does not prevent the user from being able to see their external environment. The “display” component of a wearable heads-up display is either transparent or at a periphery of the user's field of view so that it does not completely block the user from being able to see their external environment. Examples of wearable heads-up displays include: the Google Glass®, the Optinvent Ora®, the Epson Moverio®, and the Sony Glasstron®, just to name a few.

Eyebox

In near-eye optical devices such as rifle scopes and wearable heads-up displays, the range of eye positions (relative to the device itself) over which specific content/imagery provided by the device is visible to the user is generally referred to as the “eyebox.” An application in which content/imagery is only visible from a single or small range of eye positions has a “small eyebox” and an application in which content/imagery is visible from a wider range of eye positions has a “large eyebox.” The eyebox may be thought of as a volume in space positioned near the optical device. When the eye of the user (and more particularly, the pupil of the eye of the user) is positioned inside this volume and facing the device, the user is able to see all of the content/imagery provided by the device. When the eye of the user is positioned outside of this volume, the user is not able to see at least some of the content/imagery provided by the device.

The geometry (i.e., size and shape) of the eyebox is an important property that can greatly affect the user experience for a wearable heads-up display. For example, if the wearable heads-up display has a small eyebox that centers on the user's pupil when the user is gazing directly ahead, some or all content displayed by the wearable heads-up display may disappear for the user when the user gazes even slightly off-center, such as slightly to the left, slightly to the right, slightly up, or slightly down. Furthermore, if a wearable heads-up display that has a small eyebox is designed to align that eyebox on the pupil for some users, the eyebox will inevitably be misaligned relative to the pupil of other users because not all users have the same facial structure. Unless a wearable heads-up display is deliberately designed to provide a glanceable display (i.e., a display that is not always visible but rather is only visible when the user gazes in a certain direction), it is generally advantageous for a wearable heads-up display to have a large eyebox.

Demonstrated techniques for providing a wearable heads-up display with a large eyebox generally necessitate adding more bulky optical components to the display. Technologies that enable a wearable heads-up display of minimal bulk (relative to conventional eyeglass frames) to provide a large eyebox are generally lacking in the art.

BRIEF SUMMARY

A hologram with controllably broadened bandwidth may be summarized as comprising: a first surface; a second surface opposite the first surface; and a set of fringes disposed between the first surface and the second surface, wherein: the set of fringes possess a first Bragg angle nearest the first surface; the set of fringes possess a second Bragg angle nearest the second surface; and the Bragg angle of the fringes varies between the first surface to the second surface. The hologram may possess an angular bandwidth of at least 18.5 degrees when illuminated with laser light with a wavelength equal to the center wavelength of the hologram.

The set of fringes may possess a first slant angle nearest the first surface; the set of fringes may possess a second slant angle nearest the second surface; and the slant angle of the fringes of the set of fringes may vary between the first surface to the second surface. The set of fringes may possess a first fringe spacing nearest the first surface; the set of fringes may possess a second fringe spacing nearest the second surface; and the fringe spacing of the fringes of the set of fringes may vary between the first surface to the second surface. The hologram may further include: a donor material, wherein the fringes of the set of fringes that are nearest the first surface may include a first concentration of donor material and the fringes of the set of fringes that are nearest the second surface may include a second concentration of donor material. The first concentration of donor material may be less than the second concentration of donor material. The hologram may further include: a photopolymer material, wherein the first concentration of donor material may be less than a concentration of photopolymer material at the first surface. The hologram may include a wavelength-multiplexed hologram. The wavelength-multiplexed hologram may include a red hologram, a green hologram, and a blue hologram.

An apparatus for controllable broadening of the bandwidth of a hologram may be summarized as including: a hologram film comprising hologram fringes; a donor film comprising donor material physically coupled to the hologram film; at least one laser light source, wherein each laser light source is positioned at a respective illumination angle and arranged to illuminate the hologram with a respective beam laser light, wherein each beam of laser light has a respective incident angle at which the respective beam of laser light is incident to the hologram film; at least one light sensor, wherein each light sensor is positioned at a respective playback angle and arranged to measure the intensity of laser light played back by the hologram at each respective playback angle; and a curing lamp positioned and oriented to illuminate the hologram film with light of a wavelength or range of wavelengths capable of curing the hologram film.

Each of the at least one laser light sources may comprise a multiple-wavelength laser light source. Each of the at least one light sensors may comprise a single-wavelength light sensor. The hologram film may comprises a wavelength-multiplexed hologram film. The donor material of the donor film may not be sensitive to the laser light emitted by the at least one laser light source. The apparatus may further comprise a temperature sensor. The apparatus may further comprise a heat source sufficiently adjacent proximate to the hologram film and the donor film to increase the temperature of at least one of: the hologram film and the donor film. The apparatus of claim B5 may further comprise: a cold source sufficiently adjacent proximate to the hologram film and the donor film to decrease the temperature of at least one of: the hologram film and the donor film.

A method for controllable broadening of a bandwidth of a hologram may be summarized as including: physically coupling a donor film to a hologram film, wherein the donor film comprises donor material, the hologram film comprises a hologram, and wherein physically coupling a donor film to a hologram film causes a first amount of donor material to diffuse from the donor film into the hologram film; monitoring a bandwidth of the hologram of the hologram film until a first amount of bandwidth broadening has occurred; in response to achieving the first amount of bandwidth broadening: fixing the first amount of donor material; and physically de-coupling the donor film from the hologram film.

Physically coupling a donor film to the hologram film may include forming an interface between the donor film and the hologram film to cause the first amount of donor material to diffuse from the donor film across the interface into the hologram film. The donor material may comprise curable donor material, and fixing the hologram film may include curing the curable donor material. The hologram film may comprise a first photopolymer film, the donor film may comprise a second photopolymer film, and physically coupling a donor film to the hologram film may include physically coupling the second photopolymer film to the first photopolymer film. Monitoring the bandwidth of the hologram may include illuminating the film with laser light with at least one incident angle and measuring the intensity of the laser light diffracted by the hologram at least one angle.

Monitoring the bandwidth of the hologram may include illuminating the film with laser light with at least one wavelength. Illuminating the film with laser light with at least one wavelength may include illuminating the film with laser light of a wavelength to which the donor material is insensitive. The method may further include: monitoring the intensity of laser light diffracted by the hologram film at least one angle. The method may further include: monitoring the intensity of laser light diffracted by the hologram film at least one wavelength. The hologram film may comprise at least one plane-wave sub-hologram, and monitoring the bandwidth of the hologram within the hologram film may include monitoring the bandwidth of the at least one plane-wave sub-hologram. The method may further include: bleaching the hologram film. The hologram film may comprise a wavelength-multiplexed hologram, and monitoring the bandwidth of the hologram of the hologram film until a first amount of bandwidth broadening has occurred may include monitoring the bandwidth of each wavelength-specific hologram comprising the hologram film until a respective first amount of bandwidth broadening has occurred for each wavelength specific hologram comprising the hologram film.

The method may further include: physically coupling an additional donor film to the hologram film to cause a second amount of donor material to diffuse from the donor film into the hologram film; monitoring the bandwidth of the hologram of the hologram film until a second amount of bandwidth broadening has occurred; in response to achieving the second amount of bandwidth broadening: fixing the second amount of donor material; and physically de-coupling the additional donor film from the hologram film. The method may further include: heating at least one of: the hologram film and the donor film. The method may further include: cooling at least one of: the hologram film and the donor film. The method may further include: monitoring the temperature of at least one of: the hologram film and the donor film.

An eyeglass lens for use in a wearable heads-up display may be summarized as comprising: a hologram comprising: a first surface; a second surface opposite the first surface; and a set of fringes disposed between the first surface and the second surface; and at least one lens portion, wherein each lens portion is physically coupled to the hologram, and wherein: the set of fringes possess a first Bragg angle nearest the first surface; the set of fringes possess a second Bragg angle nearest the second surface; and the Bragg angle of the set of fringes varies between the first surface to the second surface.

The hologram may possess an angular bandwidth of at least 18.5 degrees when illuminated with laser light with a wavelength equal to the center wavelength of the hologram. The set of fringes may possess a first fringe spacing nearest the first surface; the set of fringes may possess a second fringe spacing nearest the second surface; and the fringe spacing of the fringes of the set of fringes may vary between the first surface to the second surface. The eyeglass lens may further comprise: donor material, wherein the fringes of the set of fringes nearest the first surface may comprise a first concentration of donor material and the fringes of the set of fringes nearest the second surface may comprise a second concentration of donor material. The eyeglass lens may further comprise: photopolymer material, wherein the first concentration of donor material may be less than the concentration of photopolymer material at the first surface and wherein the second concentration of donor material may be greater than the concentration of photopolymer material at the second surface. The set of fringes may comprise a wavelength-multiplexed hologram. The wavelength-multiplexed hologram may comprise a red hologram, a green hologram, and a blue hologram.

A wearable heads-up display (WHUD) with an expanded eyebox may be summarized as comprising: a support structure; a projector; and a transparent combiner positioned and oriented to appear in a field of view of an eye of a user when the support structure is worn on a head of the user, the transparent combiner comprising: a hologram comprising: a first surface; a second surface opposite the first surface; and a set of fringes disposed between the first surface and the second surface; and at least one lens portion, wherein each lens portion is physically coupled to the, and wherein: the set of fringes possess a first Bragg angle nearest the first surface; the set of fringes possess a second Bragg angle nearest the second surface; and the Bragg angle of the set of fringes varies between the first surface to the second surface.

The set of fringes may be characterized by a first fringe spacing nearest the first surface; the set of fringes may be characterized by a second fringe spacing nearest the second surface; and a spacing of the fringes of the set of fringes may vary between the first surface to the second surface. The hologram may further comprise donor material, and the fringes of the set of fringes that are nearest the first surface may comprise a first concentration of donor material and the fringes of the set of fringes that are nearest the second surface may comprise a second concentration of donor material. The hologram may further comprise a photopolymer material, and the first concentration of donor material may be less than the concentration of photopolymer material at the first surface and the second concentration of donor material may be greater than the concentration of photopolymer material at the second surface. The set of fringes may comprise a wavelength-multiplexed hologram. The wavelength-multiplexed hologram may comprise a red hologram, a green hologram, and a blue hologram. The hologram may possess a total two-dimensional angular bandwidth of 18.5 degrees when illuminated with laser light with a wavelength equal to a center wavelength of the hologram. The projector may include an optical element arranged to replicate an image generated by the projected into at least two exit pupils, wherein each exit pupil may originate from a respective one of N effective projector positions; each the N effective projector positions may be positioned within a total two-dimensional angular range θ of the hologram; and θ is less than 18.5 degrees. The projector may include an optical element arranged to steer an exit pupil within a total two-dimensional angular range θ of the hologram, and θ is less than 18.5 degrees.

