Patent Description:
Electronic displays are a nearly ubiquitous medium for communicating information to users of a wide variety of devices and products. Among the most commonly found electronic displays are the cathode ray tube (CRT), plasma display panels (PDP), liquid crystal displays (LCD), electroluminescent displays (EL), organic light emitting diode (OLED) and active matrix OLEDs (AMOLED) displays, electrophoretic displays (EP) and various displays that employ electromechanical or electrofluidic light modulation (e.g., digital micromirror devices, electrowetting displays, etc.). In general, electronic displays may be categorized as either active displays (i.e., displays that emit light) or passive displays (i.e., displays that modulate light provided by another source). Among the most obvious examples of active displays are CRTs, PDPs and OLEDs/AMOLEDs. Displays that are typically classified as passive when considering emitted light are LCDs and EP displays. Passive displays, while often exhibiting attractive performance characteristics including, but not limited to, inherently low power consumption, may find somewhat limited use in many practical applications given the lack of an ability to emit light.

To overcome the limitations of passive displays associated with emitted light, many passive displays are coupled to an external light source. The coupled light source may allow these otherwise passive displays to emit light and function substantially as an active display. Examples of such coupled light sources are backlights. Backlights are light sources (often panel light sources) that are placed behind an otherwise passive display to illuminate the passive display. For example, a backlight may be coupled to an LCD or an EP display. The backlight emits light that passes through the LCD or the EP display. The light emitted is modulated by the LCD or the EP display and the modulated light is then emitted, in turn, from the LCD or the EP display. Often backlights are configured to emit white light. Color filters are then used to transform the white light into various colors used in the display. The color filters may be placed at an output of the LCD or the EP display (less common) or between the backlight and the LCD or the EP display, for example.

The present invention provides a diffraction grating-based backlight having controlled diffractive coupling efficiency, the diffraction grating-based backlight comprising: a light guide configured to guide light as a beam of light at a non-zero propagation angle; and a plurality of diffraction gratings at and arranged along a surface of the light guide, a diffraction grating of the plurality being configured to diffractively couple out a portion of the guided light beam through the surface as a coupled-out light beam directed away from the light guide surface at a predetermined principal angular direction, said diffraction gratings having substantially equal size, wherein the diffraction gratings of the plurality comprise diffractive features having a diffractive feature modulation configured to selectively control a diffractive coupling efficiency of individual ones of the diffraction gratings as a function of distance along the light guide surface to compensate for a decrease in light intensity of light guided within the light guide, wherein the diffractive feature modulation comprises modulation of subwavelength gaps in the diffractive features, the subwavelength gaps being configured to modulate an effective local diffractive coupling strength of the diffractive features, wherein the subwavelength gaps are parallel with a propagation direction of the guided light beam.

The present invention further provides a method of electronic display operation, the method comprising: guiding light in a light guide; providing a controlled diffraction coupling efficiency of a plurality of diffraction gratings having substantially equal size at a surface of the light guide by modulating diffractive features of individual ones the diffraction gratings to compensate for a decrease in light intensity of light guided within the light guide, wherein the diffractive feature modulation comprises modulation of subwavelength gaps in the diffractive features, the subwavelength gaps being configured to modulate an effective local diffractive coupling strength of the diffractive features, wherein the subwavelength gaps are parallel with a propagation direction of the guided light beam; and diffractively coupling out a portion of the guided light using the plurality of diffraction gratings according to the controlled diffraction coupling efficiency, diffractively coupling out producing a plurality of light beams directed away from the light guide surface at predetermined principal angular directions, wherein the light beams of the plurality correspond to pixels of the electronic display.

Certain examples and embodiments may have other features that are one of in addition to and in lieu of the features illustrated in the above-referenced figures.

Embodiments in accordance with the principles described herein provide electronic display backlighting using diffractive feature modulation to control diffractive coupling strength or efficiency. In particular, backlighting of an electronic display described herein employs a plurality of diffraction gratings. The diffraction gratings are used to diffractively scatter or couple light out of a light guide and to direct the coupled-out light in a viewing direction of the electronic display. An amount of light coupled out by individual ones of the diffraction gratings is determined by diffractive coupling efficiency or equivalently diffractive coupling strength of the diffraction gratings. Diffractive feature modulation is used to provide selective control of the diffractive coupling efficiency. According to the claimed invention, the selectively controlled diffractive coupling efficiency compensates for a decrease in light intensity of light guided within the light guide. By compensating for the decrease in light intensity, light coupled out of the light guide by the diffraction gratings may be more uniform along a length of the light guide or a backlight employing same, for example.

According to various embodiments, the coupled-out light forms a plurality of light beams that are directed in the viewing direction. Light beams of the plurality may have different principal angular directions from one another, according to various embodiments of the principles described herein. In particular, the plurality of light beams may form or provide a light field in the viewing direction. In some embodiments, the light beams having the different principal angular directions (also referred to as 'the differently directed light beams') may be employed to display three-dimensional (<NUM>-D) information. For example, the differently directed light beams may be modulated and serve as pixels of a 'glasses free' <NUM>-D electronic display. By compensating for changes in light intensity (e.g., the light intensity decrease) using the selectively controlled diffractive coupling efficiency, an electronic display employing diffraction gratings having diffractive feature modulation may exhibit improved uniformity of illumination than is possible otherwise, for example.

<FIG> illustrates a graph of light intensity as a function of distance in a light guide. The illustrated light intensity as a function of distance may be consistent with light intensity observed in a light guide used in a diffraction grating-based backlight, for example. In particular, as light propagates along a length of the light guide from an input end to an end opposite the input end (e.g., a terminal end), portions of the guided light may be coupled out, e.g., by diffraction gratings. As the guided light is coupled out, less light remains in the light guide resulting in a decrease in light intensity along a remaining length of the light guide. Light intensity along the length of the light guide may also be affected by other processes including, but not limited to, absorption loss and various forms of scattering loss. According to various examples, the light intensity may decrease exponentially as a function of distance or length, as illustrated in <FIG>, for example. Embodiments according to the principles described herein may be used to mitigate or compensate for the decrease in light intensity as a function of distance.

Herein, a 'light guide' is defined as a structure that guides light within the structure using total internal reflection. In particular, the light guide may include a core that is substantially transparent at an operational wavelength of the light guide. In various examples, the term 'light guide' generally refers to a dielectric optical waveguide that employs total internal reflection to guide light at an interface between a dielectric material of the light guide and a material or medium that surrounds that light guide. By definition, a condition for total internal reflection is that a refractive index of the light guide is greater than a refractive index of a surrounding medium adjacent to a surface of the light guide material. In some examples, the light guide may include a coating in addition to or instead of the aforementioned refractive index difference to further facilitate the total internal reflection. The coating may be a reflective coating, for example. According to various examples, the light guide may be any of several light guides including, but not limited to, one or both of a plate or slab guide and a strip guide.

Further herein, the term 'plate' when applied to a light guide as in a 'plate light guide' is defined as a piece-wise or differentially planar layer or sheet. In particular, a plate light guide is defined as a light guide configured to guide light in two substantially orthogonal directions bounded by a top surface and a bottom surface (i.e., opposite surfaces) of the light guide. Further, by definition herein, the top and bottom surfaces are both separated from one another and may be substantially parallel to one another in at least a differential sense. That is, within any differentially small region of the plate light guide, the top and bottom surfaces are substantially parallel or co-planar. In some examples, a plate light guide may be substantially flat (e.g., confined to a plane) and so the plate light guide is a planar light guide. In other examples, the plate light guide may be curved in one or two orthogonal dimensions. For example, the plate light guide may be curved in a single dimension to form a cylindrical shaped plate light guide. In various examples however, any curvature has a radius of curvature sufficiently large to insure that total internal reflection is maintained within the plate light guide to guide light.

According to various examples described herein, a diffraction grating (e.g., a multibeam diffraction grating) may be employed to scatter or couple light out of a light guide (e.g., a plate light guide) as a light beam. Herein, a 'diffraction grating' is generally defined as a plurality of features (i.e., diffractive features) arranged to provide diffraction of light incident on the diffraction grating. In some examples, the plurality of features may be arranged in a periodic or quasi-periodic manner. For example, the diffraction grating may include a plurality of features (e.g., a plurality of grooves in a material surface) arranged in a one-dimensional (<NUM>-D) array. In other examples, the diffraction grating may be a two-dimensional (<NUM>-D) array of features. The diffraction grating may be a <NUM>-D array of bumps on or holes in a material surface, for example.

As such, and by definition herein, the 'diffraction grating' is a structure that provides diffraction of light incident on the diffraction grating. If the light is incident on the diffraction grating from a light guide, the provided diffraction or diffractive scattering may result in, and thus be referred to as, 'diffractive coupling' in that the diffraction grating may couple light out of the light guide by diffraction. The diffraction grating also redirects or changes an angle of the light by diffraction (i.e., at a diffractive angle). In particular, as a result of diffraction, light leaving the diffraction grating (i.e., diffracted light) generally has a different propagation direction than a propagation direction of the light incident on the diffraction grating (i.e., incident light). The change in the propagation direction of the light by diffraction is referred to as 'diffractive redirection' herein. Hence, the diffraction grating may be understood to be a structure including diffractive features that diffractively redirects light incident on the diffraction grating and, if the light is incident from a light guide, the diffraction grating may also diffractively couple out the light from light guide.

Further, by definition herein, the features of a diffraction grating are referred to as 'diffractive features' and may be one or more of at, in and on a surface (i.e., a boundary between two materials). The surface may be a surface of a plate light guide, for example. The diffractive features may include any of a variety of structures that diffract light including, but not limited to, one or more of grooves, ridges, holes and bumps at, in or on the surface. For example, the diffraction grating may include a plurality of parallel grooves in the material surface. In another example, the diffraction grating may include a plurality of parallel ridges rising out of the material surface. The diffractive features (e.g., grooves, ridges, holes, bumps, etc.) may have any of a variety of cross sectional shapes or profiles that provide diffraction including, but not limited to, one or more of a sinusoidal profile, a rectangular profile (e.g., a binary diffraction grating), a triangular profile and a saw tooth profile (e.g., a blazed grating).

By definition herein, a 'multibeam diffraction grating' is a diffraction grating that produces coupled-out light that includes a plurality of light beams. Further, the light beams of the plurality produced by a multibeam diffraction grating have different principal angular directions from one another, by definition herein. In particular, by definition, a light beam of the plurality has a predetermined principal angular direction that is different from another light beam of the light beam plurality as a result of diffractive coupling and diffractive redirection of incident light by the multibeam diffraction grating. For example, the light beam plurality may include eight light beams that have eight different principal angular directions. The eight light beams in combination (i.e., the light beam plurality) may represent a light field, for example. According to various examples, the different principal angular directions of the various light beams are determined by a combination of a grating pitch or spacing and an orientation or rotation of the diffractive features of the multibeam diffraction grating at points of origin of the respective light beams relative to a propagation direction of the light incident on the multibeam diffraction grating.

According to various embodiments described herein, the light coupled out of the light guide by the diffraction grating (e.g., a multibeam diffraction grating) represents a pixel of an electronic display. In particular, the light guide having a multibeam diffraction grating to produce the light beams of the plurality having different principal angular directions may be part of a backlight of or used in conjunction with an electronic display such as, but not limited to, a 'glasses free' three-dimensional (<NUM>-D) electronic display (e.g., also referred to as a multiview or 'holographic' electronic display or an autostereoscopic display). As such, the differently directed light beams produced by coupling out guided light from the light guide using the multibeam diffractive grating may be or represent 'pixels' of the <NUM>-D electronic display. Moreover, the differently directed light beams may form a light field, according to various examples.

Herein, a 'light source' is defined as a source of light (e.g., an apparatus or device that produces and emits light). For example, the light source may be a light emitting diode (LED) that emits light when activated. Herein, a light source may be substantially any source of light or optical emitter including, but not limited to, one or more of a light emitting diode (LED), a laser, an organic light emitting diode (OLED), a polymer light emitting diode, a plasma-based optical emitter, a fluorescent lamp, an incandescent lamp, and virtually any other source of light. The light produced by the light source may have a color (i.e., may include a particular wavelength of light), or may be a range of wavelengths (e.g., white light).

In accordance with some embodiments of the principles described herein, a diffraction grating-based backlight is provided. <FIG> illustrates a cross sectional view of a diffraction grating-based backlight <NUM> with modulated diffractive coupling. <FIG> illustrates a cross sectional view of a diffraction grating-based backlight <NUM> with modulated diffractive coupling. As illustrated in <FIG> and <FIG>, diffractive coupling modulation is used to vary or selectively control a diffractive coupling efficiency or diffractive coupling strength of the diffraction grating-based backlight <NUM>. The controlled diffractive coupling efficiency may be used to compensate for or mitigate an effect of a variation in light intensity within the diffraction grating-based backlight <NUM>.

For example, the controlled diffractive coupling efficiency may be used to mitigate or compensate for the effects of an exponential decrease in light intensity along a length of the diffraction grating-based backlight <NUM> due to out-coupling or scattering of light from the diffraction grating-based backlight <NUM>. The out-coupling may be used to form a plurality of light beams <NUM> directed away from a surface of the diffraction grating-based backlight <NUM> (e.g., a to form a light field), for example. In some embodiments, the diffraction grating-based backlight <NUM> may be a light source or 'backlight' of an electronic display. In particular, according to some embodiments, the electronic display may be a so-called 'glasses free' three-dimensional (<NUM>-D) electronic display (e.g., a multiview display or autostereoscopic display) in which the light beams <NUM> correspond to pixels associated with different 'views' of the <NUM>-D display.

In particular, the light beams <NUM> may form a light field in a viewing direction of the electronic display. A light beam <NUM> of the plurality of light beams <NUM> (and within the light field) provided by the diffraction grating-based backlight <NUM> may be configured to have a different principal angular direction from other light beams <NUM> of the plurality, according to some embodiments. Further, the light beam <NUM> may have both a predetermined direction (principal angular direction) and a relatively narrow angular spread within the light field. The principal angular direction of the light beam <NUM> may correspond to an angular direction of a particular view of the <NUM>-D electronic display, for example. As such, the light beam <NUM> may represent or correspond to a pixel of the <NUM>-D electronic display, according to some examples.

In other embodiments, the light beams <NUM> of the plurality may have substantially similar predetermined principal angular directions (not illustrated in <FIG>). The similarly directed light beams <NUM> generally do not form a light field, but instead represent out-coupled light that is substantially unidirectional. The similarly directed light beams <NUM> may be used to backlight a two-dimensional (<NUM>-D) display, for example.

In some embodiments, the light beams <NUM> may be modulated (e.g., by a light valve as described below). The modulation of the light beams <NUM> directed in different angular directions away from the diffraction grating-based backlight <NUM> may be particularly useful for dynamic <NUM>-D electronic display applications, for example. That is, the individually modulated light beams <NUM> directed in a particular view direction may represent dynamic pixels of the <NUM>-D electronic display corresponding to the particular view direction.

As illustrated in <FIG> and <FIG>, the diffraction grating-based backlight <NUM> comprises a light guide <NUM>. In particular, the light guide <NUM> may be a plate light guide <NUM>, according to some embodiments. The light guide <NUM> is configured to guide light from a light source (not illustrated in <FIG>) as guided light <NUM>. For example, the light guide <NUM> may include a dielectric material configured as an optical waveguide. The dielectric material may have a first refractive index that is greater than a second refractive index of a medium surrounding the dielectric optical waveguide. The difference in refractive indices is configured to facilitate total internal reflection of the guided light <NUM> according to one or more guided modes of the light guide <NUM>, for example.

In some embodiments, the light from the light source is guided as a beam of light <NUM> along a length of the light guide <NUM>. Further, the light guide <NUM> may be configured to guide the light (i.e., the guided light beam <NUM>) at a non-zero propagation angle. The guided light beam <NUM> may be guided at the non-zero propagation angle within the light guide <NUM> using total internal reflection, for example.

As defined herein, the non-zero propagation angle is an angle relative to a surface (e.g., a top surface or a bottom surface) of the light guide <NUM>. In some examples, the non-zero propagation angle of the guided light beam <NUM> may be between about ten (<NUM>) degrees and about fifty (<NUM>) degrees or, in some examples, between about twenty (<NUM>) degrees and about forty (<NUM>) degrees, or between about twenty-five (<NUM>) degrees and about thirty-five (<NUM>) degrees. For example, the non-zero propagation angle may be about thirty (<NUM>) degrees. In other examples, the non-zero propagation angle may be about <NUM> degrees, or about <NUM> degrees, or about <NUM> degrees.

In some examples, the light from a light source is introduced or coupled into the light guide <NUM> at the non-zero propagation angle (e.g., about <NUM>-<NUM> degrees). One or more of a lens, a mirror or similar reflector (e.g., a tilted collimating reflector), and a prism (not illustrated) may facilitate coupling light into an input end the light guide <NUM> as the beam of light at the non-zero propagation angle. Once coupled into the light guide <NUM>, the guided light beam <NUM> propagates along the light guide <NUM> in a direction that is generally away from the input end (e.g., illustrated by bold arrows pointing along an x-axis in <FIG>). Further, the guided light beam <NUM> propagates by reflecting or 'bouncing' between the top surface and the bottom surface of the light guide <NUM> at the non-zero propagation angle (e.g., illustrated by an extended, angled arrow representing a light ray of the guided light beam <NUM>).

The guided light beam <NUM> produced by coupling light into the light guide <NUM> may be a collimated light beam, according to some examples. In particular, by 'collimated light beam' it is meant that rays of light within the guided light beam <NUM> are substantially parallel to one another within the guided light beam <NUM>. Rays of light that diverge or are scattered from the collimated light beam of the guided light beam <NUM> are not considered to be part of the collimated light beam, by definition herein. Collimation of the light to produce the collimated guided light beam <NUM> may be provided by the lens or mirror (e.g., tilted collimating reflector, etc.) used to couple the light into the light guide <NUM>, for example.

In some examples, the light guide <NUM> (e.g., as a plate light guide <NUM>) may be a slab or plate optical waveguide comprising an extended, substantially planar sheet of optically transparent, dielectric material. The substantially planar sheet of dielectric material is configured to guide the guided light beam <NUM> using total internal reflection. According to various examples, the optically transparent material of the light guide <NUM> may include or be made up of any of a variety of dielectric materials including, but not limited to, one or more of various types of glass (e.g., silica glass, alkali-aluminosilicate glass, borosilicate glass, etc.) and substantially optically transparent plastics or polymers (e.g., poly(methyl methacrylate) or 'acrylic glass', polycarbonate, etc.). In some examples, the light guide <NUM> may further include a cladding layer on at least a portion of a surface (e.g., one or both of the top surface and the bottom surface) of the light guide <NUM> (not illustrated). The cladding layer may be used to further facilitate total internal reflection, according to some examples.

According to various embodiments, the diffraction grating-based backlight <NUM> further includes a plurality of diffraction gratings <NUM>. The plurality of diffraction gratings <NUM> may be arranged as or represent an array of diffraction gratings <NUM>, for example. As illustrated in <FIG>, the diffraction gratings <NUM> are located at a surface of the light guide <NUM> (e.g., a top or front surface). In other examples (not illustrated), one or more of the diffraction gratings <NUM> may be located within the light guide <NUM>.

A diffraction grating <NUM> of the plurality is configured to scatter or couple out a portion of the guided light beam <NUM> from the light guide <NUM> by or using diffractive coupling (e.g., also referred to as 'diffractive scattering'), according to various embodiments. For example, the portion of the guided light beam <NUM> may be diffractively coupled out by the diffraction grating <NUM> through the light guide surface (e.g., through the top surface of the light guide <NUM>). Further, the diffraction grating <NUM> is configured to diffractively couple out the portion of the guided light beam <NUM> as a coupled-out light beam (e.g., a light beam <NUM>). The coupled-out light beam <NUM> is directed away from the light guide surface at a predetermined principal angular direction, according to various examples. In particular, the coupled-out portion of the guided light beam <NUM> is diffractively redirected away from the light guide surface by the plurality of diffraction gratings <NUM> as a plurality of light beams <NUM>. As discussed above, each of the light beams <NUM> of the light beam plurality may have a different principal angular direction and the light beam plurality may represent a light field, according to some examples. According to other examples, each of the light beams <NUM> of the light beam plurality may have substantially the same principal angular direction and the light beam plurality may represent substantially unidirectional light as opposed to the light field represented by the light beam plurality having light beams with different principal angular directions.

According to various embodiments, each of the diffraction gratings <NUM> comprises a plurality of diffractive features <NUM> that provide diffraction. The provided diffraction is responsible for the diffractive coupling of the portion of the guided light beam <NUM> out of the light guide <NUM>. For example, the diffraction grating <NUM> may include one or both of grooves in a surface of the light guide <NUM> and ridges protruding from the light guide surface that serve as the diffractive features <NUM>. The grooves and ridges may be arranged parallel or substantially parallel to one another and, at least at some point, perpendicular to a propagation direction of the guided light beam <NUM> that is to be coupled out by the diffraction grating <NUM>.

In some examples, the grooves or ridges may be etched, milled or molded into the surface or applied on the surface. As such, a material of the diffraction grating <NUM> may include a material of the light guide <NUM>. As illustrated in <FIG>, for example, the diffraction gratings <NUM> comprise substantially parallel grooves formed in the surface of the light guide <NUM>. In <FIG>, the diffraction gratings <NUM> comprise substantially parallel ridges that protrude from the light guide surface, for example. In other examples (not illustrated), the diffraction gratings <NUM> may be implemented in or as a film or layer applied or affixed to the light guide surface.

The plurality of diffraction gratings <NUM> may be arranged in a variety of configurations at, on or in the surface of the light guide <NUM>, according to various examples. For example, the plurality of diffraction gratings <NUM> may be arranged in columns and rows across the light guide surface (e.g., as an array). In another example, a plurality of diffraction gratings <NUM> may be arranged in groups (e.g., a group of three gratings, each grating in the group being associated with a different color of light) and the groups may be arranged in rows and columns. In yet another example, the plurality of diffraction gratings <NUM> may be distributed substantially randomly across the surface of the light guide <NUM>.

According to various embodiments, the diffractive features <NUM> of the diffraction gratings <NUM> of the plurality comprise a diffractive feature modulation. The diffractive feature modulation is configured to selectively control a diffractive coupling efficiency, or equivalently a diffractive coupling strength, of the diffractive gratings <NUM>. In particular, diffractive coupling efficiency is selectively controlled by the diffractive feature modulation as a function of length along the light guide surface. Moreover, the diffractive feature modulation provides selective control of the diffractive coupling efficiency as a function of length along the plurality of diffractive gratings <NUM> (e.g., arranged as an array). For example, the diffractive feature modulation may be configured to provide an increase (e.g., an exponential increase) in the diffractive coupling efficiency as a function of length along the light guide surface to compensate for an exponential decrease in an intensity of the guided light beam due to the diffractive coupling-out of the guided light beam portion. The exponential increase may be configured to be about inverse to the exponential decrease in an intensity of the guided light beam <NUM> within the light guide <NUM> as a function of the light guide length, for example. In other examples, the diffractive feature modulation may be configured to provide another change in the diffractive coupling efficiency as a function of length including, but not limited to, an exponential decrease, a linear increase or decrease, a quadratic increase or decrease, or a sinusoidal in the diffractive coupling efficiency.

The diffractive feature modulation comprises a modulation of or variation in characteristics of the diffractive features <NUM> of adjacent diffraction gratings <NUM>. As used herein, the term 'characteristic' when applied to diffractive features <NUM> is defined as one or more of a physical size, a shape and an arrangement of the diffractive features <NUM> within the diffraction grating <NUM>. In some embodiments, the diffractive feature modulation may be substantially static or substantially unchanging as a function of time. That is, the diffractive feature modulation represents a change or variation of the diffractive features <NUM> with distance or length but not with time. As such, in some embodiments, the diffractive feature modulation may be referred to as 'DC diffractive feature modulation,' where 'DC' is used in a manner analogous to 'direct current' or 'DC' as is used in electronics to signify a constant value (e.g., of current or voltage) as a function of time.

According to the claimed invention, modulation of diffractive feature characteristics (i.e., diffractive feature modulation) does not include a variation in an overall size of the diffraction grating <NUM>, by definition herein. In particular, while an overall size of the diffraction grating <NUM> may also be used to control diffraction coupling efficiency of a diffraction grating <NUM>, diffraction feature modulation either does not employ or does not exclusively employ diffraction grating size, as used herein. That is, diffraction grating size may be employed in addition to but not instead of diffraction feature modulation to selectively control diffractive coupling efficiency. According to the claimed invention, the plurality of diffractive gratings <NUM> comprises diffraction gratings having substantially equal size and diffractive coupling efficiency is selectively controlled using diffractive feature modulation alone.

According to some embodiments, the diffractive feature modulation comprises modulation of diffractive feature amplitude. In particular, an amplitude of the diffractive features <NUM> may be modulated or varied from one diffraction grating <NUM> to the next as a function of distance to realize the diffractive feature modulation. For example, a depth of grooves or a height of ridges of the diffraction gratings <NUM> (i.e., diffractive feature amplitude) may be varied as a function of distance along the light guide <NUM>. In an example, the groove depth or ridge height of each successive diffraction grating <NUM> may be increased from a light guide end adjacent to the light source (i.e., the input end) to an opposite end of the light guide <NUM> (i.e., the terminal end). Increasing the groove depth or the ridge height of the successive diffraction gratings <NUM> increases the diffractive coupling efficiency or diffractive coupling strength of the diffraction gratings <NUM> as a function of length along the light guide <NUM>.

<FIG> illustrates diffractive feature modulation comprising modulation of groove depth as a function of length or equivalently as a function of distance along the light guide <NUM>. In particular, groove depth of successive diffraction gratings <NUM> increases from a diffraction grating <NUM> adjacent to the input end (left side) of the light guide <NUM> to a diffraction grating <NUM> adjacent to the terminal end (right side) of the light guide <NUM>, as illustrated in <FIG>. <FIG> illustrates diffractive feature modulation comprising ridge height modulation as a function of length along the light guide <NUM>. In particular, the ridge heights of successive diffraction gratings <NUM> increase from the input end to the terminal end (right to left) of the light guide <NUM>, as illustrated in <FIG>.

The groove depth variation or the ridge height variation of diffractive feature amplitude modulation may be provided (e.g., during manufacture) of the diffraction gratings <NUM> using techniques including, but not limited to, grey tone lithography, multiple-level dry etching, and nanoimprint lithography. Grey tone lithography comprises using a resist mask having different feature depths representing the grooves or the ridges of the diffraction gratings <NUM>. Controlling exposure times of the resist, for example, may produce the different feature depths. Multiple-level dry etching may employ a plurality of substantially independent lithography-plus-dry etching steps to achieve a similar plurality of different etch levels or depths to produce the diffractive feature amplitude modulation, for example.

According to some examples (not illustrated), the diffractive feature modulation comprises modulation of a duty cycle of the diffractive features of the diffraction gratings <NUM>. In particular, a ratio of width-to-pitch of the grooves or the ridges may be varied between adjacent diffraction gratings <NUM> of the plurality to provide the duty cycle modulation. According to some examples, the width-to-pitch ratio may be varied around a mean value of about fifty percent (<NUM>%). Varying the width-to-pitch ratio in a vicinity around <NUM>% may minimize production of higher order diffraction components, for example. For example, the duty cycle modulation may comprise a width-to-pitch ratio variation between about thirty percent (<NUM>%) and about seventy percent (<NUM>%). In another example, the width-to-pitch ratio variation may be between about forty percent (<NUM>%) and about sixty percent (<NUM>%) or between about forty-five percent (<NUM>%) and about fifty-five percent (<NUM>%).

According to all embodiments of the claimed invention, an effective density of the individual diffractive features <NUM> may be varied or modulated to provide the diffractive feature modulation. In particular, diffractive feature modulation comprises subwavelength gaps in the diffractive features <NUM>. The subwavelength gaps are configured to modulate an effective density or equivalently an effective local diffractive coupling strength of the individual diffractive features <NUM>. Using the subwavelength gaps is referred to as 'effective density modulation' of diffractive features herein since the number, width and spacing of the subwavelength gaps may change an effective density of individual ones of the diffractive features <NUM> resulting in an effective change in diffractive strength thereof. Note that subwavelength gaps are employed in effective density modulation to avoid creation of additional diffraction orders, according to some embodiments.

<FIG> illustrates a top view of a diffraction grating <NUM> including subwavelength gaps <NUM> in an example, according to an embodiment according to the claimed invention. In particular, <FIG> illustrates a top view of a diffraction grating <NUM> that is a multibeam diffraction grating <NUM>, by way of example and not limitation, which has subwavelength gaps <NUM> between the diffractive features <NUM>. Multibeam diffraction gratings <NUM> are described in more detail below and with reference to <FIG>.

Referring to <FIG>, the subwavelength gaps <NUM> between the diffractive features <NUM> are substantially parallel with a propagation direction of the guided light beam <NUM> (e.g., indicated by a bold arrow labeled <NUM>) along the light guide <NUM>. The guided light beam <NUM> is to be coupled out of the light guide <NUM> by the diffraction grating <NUM>. Varying a one or more of a number (i.e., density), a width, and a spacing of the subwavelength gaps <NUM> of a diffraction grating <NUM> provides the diffractive feature modulation, according to some embodiments.

Diffractive feature modulation may comprise one or more of modulation of diffractive feature amplitude, modulation of a duty cycle of the diffractive features, and modulation of an effective density of subwavelength gaps <NUM> in the diffractive features. In particular, different ones of diffractive feature modulations (i.e., diffractive feature amplitude modulation, diffractive feature duty cycle modulation and diffractive feature effective density modulation) may affect the diffractive coupling efficiency differently as a function of wavelength of the guided light beam <NUM>. For example, combining the various different modulation types may facilitate balancing or tuning of the diffractive coupling efficiency as a function of wavelength (e.g., to fine tune color balancing), for example. Further, according to some embodiments, the diffractive feature modulation may be substantially uniform along the length, while substantially non-uniform diffractive feature modulation may be employed in other embodiments.

According to some embodiments, the plurality of diffraction gratings <NUM> comprises a multibeam diffraction grating <NUM>. For example, all or substantially all of the diffraction gratings <NUM> of the plurality may be multibeam diffraction gratings <NUM>. The multibeam diffraction grating <NUM> is a diffraction grating <NUM> that is configured to couple out the portion of the guided light beam <NUM> as a plurality of light beams <NUM> (e.g., as illustrated in <FIG> and <FIG>), wherein a light beam <NUM> of the plurality has a different principal angular direction from other light beams <NUM> of the light beam plurality. Together, the plurality of light beams <NUM> coupled out by the multibeam diffraction grating <NUM> form a light field, according to various embodiments.

According to various examples, the multibeam diffraction grating <NUM> may comprise a chirped diffraction grating <NUM>. By definition, the 'chirped' diffraction grating <NUM> is a diffraction grating exhibiting or having a diffraction spacing of the diffractive features that varies across an extent or length of the chirped diffraction grating <NUM>, e.g., as illustrated in <FIG> and <FIG>. Herein, the varying diffraction spacing is referred to as a 'chirp'. As a result, the guided light beam <NUM> that is diffractively coupled out of the light guide <NUM> exits or is emitted from the chirped diffraction grating <NUM> as the light beams <NUM> at different diffraction angles corresponding to different points of origin across the chirped diffraction grating <NUM>. By virtue of a predefined chirp, the chirped diffraction grating <NUM> is responsible for the predetermined and different principal angular directions of the coupled-out light beams <NUM> of the light beam plurality.

<FIG> illustrates a cross sectional view of a multibeam diffraction grating-based backlight <NUM> in an example, according to an embodiment consistent with the principles described herein. <FIG> illustrates a perspective view of the multibeam diffraction grating-based backlight <NUM> of <FIG> in an example, according to an embodiment consistent with the principles described herein. As illustrated therein, the multibeam diffraction grating-based backlight <NUM> comprises a multibeam diffraction grating <NUM>. The multibeam diffraction grating <NUM> comprises grooves in a surface of the light guide <NUM>, by way of example and not limitation. For example, the multibeam diffraction grating <NUM> illustrated in <FIG> may be one of the groove-based diffraction gratings <NUM> illustrated in <FIG>.

As illustrated in <FIG> (and also <FIG> by way of example not limitation), the multibeam diffraction grating <NUM> is a chirped diffraction grating. In particular, as illustrated, the diffractive features <NUM> are closer together at a first end <NUM>' of the multibeam diffraction grating <NUM> than at a second end <NUM>". Further, the diffractive spacing d of the illustrated diffractive features <NUM> varies linearly from the first end <NUM>' to the second end <NUM>". In some examples, the chirped diffraction grating <NUM> may have or exhibit a chirp of the diffractive spacing d that varies linearly with distance. As such, the chirped diffraction grating <NUM> may be referred to as a 'linearly chirped' diffraction grating.

In some examples, the light beams <NUM> produced by coupling light out of the light guide <NUM> using the multibeam diffraction grating <NUM> may diverge (i.e., be diverging light beams <NUM>) when the guided light beam <NUM> propagates in the light guide <NUM> in a direction from the first end <NUM>' of the multibeam diffraction grating <NUM> to the second end <NUM>" of the multibeam diffraction grating <NUM> (e.g., as illustrated in <FIG>). Alternatively, converging light beams <NUM> may be produced when the guided light beam <NUM> propagates in the reverse direction in the light guide <NUM>, i.e., from the second end <NUM>" to the first end <NUM>' of the multibeam diffraction grating <NUM>, according to other examples (not illustrated).

In another example (not illustrated), the chirped diffraction grating <NUM> may exhibit a non-linear chirp of the diffractive spacing d. Various non-linear chirps that may be used to realize the chirped diffraction grating <NUM> include, but are not limited to, an exponential chirp, a logarithmic chirp or a chirp that varies in another, substantially non-uniform or random but still monotonic manner. Non-montonic chirps such as, but not limited to, a sinusoidal chirp or a triangle or sawtooth chirp, may also be employed. Combinations of any of these types of chirps may also be employed.

As illustrated in <FIG>, the multibeam diffraction grating <NUM> includes diffractive features <NUM> (e.g., grooves or ridges) in, at or on a surface of the light guide <NUM> that are both chirped and curved (i.e., the multibeam diffraction grating <NUM> is a curved, chirped diffraction grating). The guided light beam <NUM> has an incident direction relative to the multibeam diffraction grating <NUM> and the light guide <NUM>, as illustrated by a bold arrow labeled <NUM> in <FIG>. Also illustrated is the plurality of coupled-out or emitted light beams <NUM> pointing away from the multibeam diffraction grating <NUM> at the surface of the light guide <NUM>. The illustrated light beams <NUM> are emitted in a plurality of predetermined different principal angular directions. In particular, the predetermined different principal angular directions of the emitted light beams <NUM> are different in both azimuth and elevation (e.g., to form a light field), as illustrated. According to various examples, both the predefined chirp of the diffractive features <NUM> and the curve of the diffractive features <NUM> may be responsible for the predetermined different principle angular directions of the emitted light beams <NUM>.

For example, due to the curve, the diffractive features <NUM> within the multibeam diffraction grating <NUM> may have varying orientations relative to an incident direction of the guided light beam <NUM>. In particular, an orientation of the diffractive features <NUM> at a first point or location within the multibeam diffraction grating <NUM> may differ from an orientation of the diffractive features <NUM> at another point or location relative to the guided light beam incident direction. With respect to the coupled-out or emitted light beam <NUM>, an azimuthal component ϕ of the principal angular direction {θ, ϕ} of the light beam <NUM> may be determined by or correspond to the azimuthal orientation angle ϕf of the diffractive features <NUM> at a point of origin of the light beam <NUM> (i.e., at a point where the incident guided light <NUM> is coupled out), according to some examples. As such, the varying orientations of the diffractive features <NUM> within the multibeam diffraction grating <NUM> produce different light beams <NUM> having different principle angular directions {θ, ϕ}, at least in terms of their respective azimuthal components ϕ.

In particular, at different points along the curve of the diffractive features <NUM>, an 'underlying diffraction grating' of the multibeam diffraction grating <NUM> associated with the curved diffractive features <NUM> has different azimuthal orientation angles ϕf. Thus, at a given point along the curved diffractive features <NUM>, the curve has a particular azimuthal orientation angle ϕf that generally differs from the azimuthal orientation angle ϕf at another point along the curved diffractive features <NUM>. Further, the particular azimuthal orientation angle ϕf results in a corresponding azimuthal component ϕ of a principal angular direction {θ, ϕ} of a light beam <NUM> emitted from the given point. In some examples, the curve of the diffractive features (e.g., grooves, ridges, etc.) may represent a section of a circle. The circle may be coplanar with the light guide surface. In other examples, the curve may represent a section of an ellipse or another curved shape, e.g., that is coplanar with the light guide surface.

In other examples, the multibeam diffraction grating <NUM> may include diffractive features <NUM> that are 'piecewise' curved. In particular, while the diffractive feature may not describe a substantially smooth or continuous curve per se, at different points along the diffractive feature within the multibeam diffraction grating <NUM>, the diffractive feature still may be oriented at different angles with respect to the incident direction of the guided light beam <NUM>. For example, the diffractive feature <NUM> may be a groove including a plurality of substantially straight segments, each segment having a different orientation than an adjacent segment. Together, the different angles of the segments may approximate a curve (e.g., a segment of a circle), according to various examples. In yet other examples, the diffractive features <NUM> may merely have different orientations relative to the incident direction of the guided light at different locations within the multibeam diffraction grating <NUM> without approximating a particular curve (e.g., a circle or an ellipse).

The multibeam diffraction grating-based backlight <NUM> may further include the light source (not illustrated in <FIG>), according to some embodiments. The light source may be configured to provide light that, when coupled into the light guide <NUM>, is the guided light beam <NUM>. In various embodiments, the light source may be substantially any source of light including, but not limited to, one or more of a light emitting diode (LED), a fluorescent light and a laser. In some examples, the light source may produce a substantially monochromatic light having a narrowband spectrum denoted by a particular color. In other examples, the light provided by the light source has a substantially broadband spectrum. For example, the light produced by the light source may be white light and the light source may be a fluorescent light.

According to some embodiments of the principles described herein, an electronic display is provided. In various embodiments, the electronic display is configured to emit modulated light beams as pixels of the electronic display. Further, in various examples, the emitted modulated light beams may be preferentially directed toward a viewing direction of the electronic display as a plurality of differently directed light beams. In some examples, the electronic display is a three-dimensional (<NUM>-D) electronic display (e.g., a glasses-free <NUM>-D electronic display). Different ones of the modulated, differently directed light beams may correspond to different 'views' associated with the <NUM>-D electronic display, according to various examples. The different views may provide a 'glasses free' (e.g., autostereoscopic) representation of information being displayed by the <NUM>-D electronic display, for example.

<FIG> illustrates a block diagram of an electronic display <NUM> in an example, according to an embodiment consistent with the principles described herein. In particular, the electronic display <NUM> illustrated in <FIG> is a <NUM>-D electronic display <NUM> (e.g., a 'glasses free' <NUM>-D electronic display) configured to emit modulated light beams <NUM> representing pixels corresponding to different views of the <NUM>-D electronic display <NUM>. The emitted, modulated light beams <NUM> are illustrated as diverging (e.g., as opposed to converging) in <FIG> by way of example and not limitation.

The <NUM>-D electronic display <NUM> illustrated in <FIG> includes a plate light guide <NUM> to guide light. The guided light in the plate light guide <NUM> is a source of the light that becomes the modulated light beams <NUM> emitted by the <NUM>-D electronic display <NUM>. According to some examples, the plate light guide <NUM> may be substantially similar to the light guide <NUM> described above with respect to diffraction grating-based backlight <NUM>. For example, the plate light guide <NUM> may be a slab optical waveguide that is a planar sheet of dielectric material configured to guide light by total internal reflection. The guided light may be guided at a non-zero propagation angle as a beam of light. Further, the guided light beam may be a collimated light beam, according to some embodiments.

The <NUM>-D electronic display <NUM> illustrated in <FIG> further includes an array of multibeam diffraction gratings <NUM>. According to various embodiments, the multibeam diffraction gratings <NUM> of the array have a diffractive feature modulation configured to selectively control a diffractive coupling efficiency of the multibeam diffraction gratings <NUM>. In particular, the selectively controlled diffractive coupling efficiency provided by the diffractive feature modulation is a function of length along the array (or distance along the propagation direction), according to various embodiments.

In some examples, the multibeam diffraction gratings <NUM> may be substantially similar to the multibeam diffraction gratings <NUM> of the diffraction grating-based backlight <NUM>, described above. In particular, the multibeam diffraction gratings <NUM> of the array are configured to couple out a portion of the guided light as a plurality of light beams <NUM>. The multibeam diffraction gratings <NUM> are configured to couple out the guided light portion according to the controlled diffractive coupling efficiency. That is, an amount of the guided light that is coupled out by a given multibeam diffraction grating <NUM> of the array is determined by the controlled diffractive coupling efficiency produced by the diffractive feature modulation. Further, the multibeam diffraction grating <NUM> is configured to direct the light beams <NUM> in a corresponding plurality of different principal angular directions.

Further, in some embodiments, the array of multibeam diffraction gratings <NUM> may include a chirped diffraction grating. In some examples, diffractive features (e.g., grooves, ridges, etc.) of the multibeam diffraction gratings <NUM> are curved diffractive features. For example, the curved diffractive features may include ridges or grooves that are curved (i.e., continuously curved or piece-wise curved) and spacings between the curved diffractive features that vary as a function of distance across the multibeam diffraction gratings <NUM> of the array.

As illustrated in <FIG>, the <NUM>-D electronic display <NUM> further includes a light valve array <NUM>. The light valve array <NUM> includes a plurality of light valves configured to modulate the differently directed light beams <NUM> of the light beam plurality, according to various examples. In particular, the light valves of the light valve array <NUM> modulate the differently directed light beams <NUM> to provide the modulated light beams <NUM> that are or represent pixels of the <NUM>-D electronic display <NUM>. Moreover, different ones of the modulated, differently directed light beams <NUM> may correspond to different views of the <NUM>-D electronic display. In various examples, different types of light valves in the light valve array <NUM> may be employed including, but not limited to, liquid crystal (LC) light valves and electrophoretic light valves. Dashed lines are used in <FIG> to emphasize modulation of the light beams <NUM>, by way of example.

In some examples (e.g., as illustrated in <FIG>), the <NUM>-D electronic display <NUM> further includes a light source <NUM>. The light source <NUM> is configured to provide light that propagates in the plate light guide <NUM> as the guided light. In particular, the guided light is light from the light source <NUM> that is coupled into the edge of the plate light guide <NUM>, according to some examples. In some examples, the light source <NUM> is substantially similar to the light source described above with respect to the diffraction grating-based backlight <NUM>. For example, the light source <NUM> may include an LED of a particular color (e.g., red, green, blue) to provide monochromatic light or a broadband light source such as, but not limited to, a fluorescent light, to provide broadband light (e.g., white light).

According to some examples of the principles described herein, a method of electronic display operation is provided. In particular, the method of electronic display operation comprises controlling diffraction coupling efficiency using modulation of diffractive features of a plurality of diffraction gratings.

<FIG> illustrates a flow chart of a method <NUM> of electronic display operation in an example, according to an embodiment consistent with the principles described herein. As illustrated, the method <NUM> of electronic display operation comprises guiding <NUM> light in a light guide. In some embodiments, the light guide and the guided light may be substantially similar to the light guide <NUM> and guided light beam <NUM>, described above with respect to the diffraction grating-based backlight <NUM>. In particular, in some embodiments, the light guide may guide <NUM> the guided light according to total internal reflection as a beam (e.g., a collimated beam) of light. The light beam may be guided <NUM> at a non-zero propagation angle, for example. Further, the light guide may be a substantially planar dielectric optical waveguide (e.g., a plate light guide), in some embodiments.

The method <NUM> of electronic display operation further comprises providing <NUM> a controlled diffraction coupling efficiency of a plurality of diffraction gratings. The plurality of diffraction gratings may be at a surface of the light guide and may be arranged as an array, for example. According to various embodiments, the controlled diffraction coupling efficiency is provided <NUM> using modulation of diffractive features of the diffraction gratings. According to the claimed invention, the diffractive feature modulation comprises effective density modulation of diffractive features <NUM>, substantially as described above with respect to the diffraction grating-based backlight <NUM>.

The method <NUM> of electronic display operation further includes diffractively coupling out <NUM> a portion of the guided light using the plurality of diffraction gratings according to the controlled diffraction coupling efficiency. That is, the portion of the guided light that is diffractively coupled out <NUM> is determined by the controlled diffraction coupling efficiency provided <NUM> using diffractive feature modulation of the diffraction gratings.

According to various examples, the plurality of diffraction gratings is located at a surface of the light guide. For example, the diffraction gratings may be formed in the surface of the light guide as grooves, ridges, etc. In other examples, the diffraction gratings of the plurality may include a film on the light guide surface. In some examples, the diffraction gratings are substantially similar to the diffraction gratings <NUM> described above with respect to the diffraction grating-based backlight <NUM>. In particular, the diffraction gratings may be multibeam diffraction gratings configured to produce a plurality of light beams from the diffractively coupled out <NUM> portion of the guided light. In other examples, the diffraction gratings are located elsewhere including, but not limited to, within the light guide.

The portion of diffractively coupled out <NUM> guided light of the method <NUM> of electronic display operation produces a plurality of emitted light beams directed away from the surface of the light guide. Each of the emitted light beams of the light beam plurality is directed away from the surface at a predetermined principal angular direction. In particular, when a diffraction grating is a multibeam diffraction grating, an emitted light beam of the light beam plurality may have a different principal angular direction from other emitted light beams of the light beam plurality. According to some embodiments, the light beams of the light beam plurality may correspond to pixels of the electronic display. In particular, the emitted light beams from the multibeam diffraction grating may correspond to pixels of different views of a three-dimensional (<NUM>-D) electronic display.

In some examples, the method <NUM> of electronic display operation further includes modulating <NUM> the light beams of the plurality of emitted light beams using a corresponding plurality of light valves. In particular, the plurality of emitted light beams that is diffractively coupled out <NUM> is modulated <NUM> by passing through or otherwise interacting with the corresponding plurality of light valves. The modulated light beams may form the pixels of the electronic display (e.g., the <NUM>-D electronic display), according to some embodiments. For example, the modulated <NUM> light beams may provide a plurality of views of the <NUM>-D electronic display (e.g., a glasses-free, <NUM>-D electronic display).

In some examples, the plurality of light valves used in modulating <NUM> the plurality of light beams is substantially similar to the light valve array <NUM> described above with respect to the <NUM>-D electronic display <NUM>. For example, the light valves may include liquid crystal light valves. In another example, the light valves may be another type of light valve including, but not limited to, an electrowetting light valve and an electrophoretic light valve.

Claim 1:
A diffraction grating-based backlight (<NUM>, <NUM>) having controlled diffractive coupling efficiency, the diffraction grating-based backlight comprising:
a light guide (<NUM>, <NUM>) configured to guide light as a beam of light (<NUM>) at a non-zero propagation angle; and
a plurality of diffraction gratings (<NUM>, <NUM>) at and arranged along a surface of the light guide, a diffraction grating of the plurality being configured to diffractively couple out a portion of the guided light beam through the surface as a coupled-out light beam (<NUM>, <NUM>) directed away from the light guide surface at a predetermined principal angular direction, said diffraction gratings having substantially equal size,
characterized in that the diffraction gratings of the plurality comprise diffractive features (<NUM>) having a diffractive feature modulation configured to selectively control a diffractive coupling efficiency of individual ones of the diffraction gratings as a function of distance along the light guide surface to compensate for a decrease in light intensity of light guided within the light guide, wherein the diffractive feature modulation comprises modulation of subwavelength gaps in the diffractive features, the subwavelength gaps being configured to modulate an effective local diffractive coupling strength of the diffractive features, wherein the subwavelength gaps are parallel with a propagation direction of the guided light beam.