Detailed Description
In order that the above objects, features and advantages of the invention will be readily understood, a further description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. However, the exemplary embodiments can be embodied in many forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the example embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar structures, and thus a repetitive description thereof will be omitted. The words expressing the positions and directions described in the present invention are described by taking the drawings as an example, but can be changed according to the needs, and all the changes are included in the protection scope of the present invention. The drawings of the present invention are merely schematic representations of relative positional relationships and are not intended to represent true proportions.
Near-eye display means a display device that is worn on the eyes of a user, for example, near-eye display means is typically presented in the form of glasses or a helmet. Near-to-eye display products have advantages of Immersion (Immersion), interactivity (Interaction), imagination (imaging) and the like, and are gradually and widely applied to the civil fields such as film and television, education, medical treatment and the like from being initially applied to the military field.
Currently common near-eye display devices are virtual reality and augmented reality. Virtual Reality (VR) technology presents a totally enclosed Virtual environment, and creates an immersive viewing experience in a three-dimensional environment by means of a display module. The display principle is that the left and right eye screens respectively display left and right eye images, and human eyes can generate stereoscopic impression in the brain after acquiring the information with the difference. Augmented reality (Augmented Reality, AR for short) technology is an enhanced projection way to superimpose a virtual scene in a real environment. The AR display can fuse the external environment with the virtual world on the basis of the VR display, presenting a vivid interactive experience.
Near-eye display technology typically employs a micro-display screen in combination with an optical system to magnify an image and then image the image in the human eye. However, the near-eye display device adopting the geometrical optical system is generally larger in volume and weight, so that the waveguide type near-eye display device is generated, the volume and weight of the near-eye display device can be effectively reduced, but compared with other geometrical optical schemes, the waveguide type near-eye optical scheme is lower in light utilization rate, and the expansion of the exit pupil of the eyepiece system to obtain large exit pupil display can also lead to the reduction of luminous flux of the system in unit area in the exit pupil range, so that the brightness of an output image of the system is lower compared with other near-eye display optical systems, and the wearing experience and the expansion of application scenes are influenced.
Therefore, the embodiment of the invention provides the near-eye display device, which can effectively improve the light efficiency, reduce the light energy loss of a system, effectively improve the brightness of an output image and improve the wearing experience.
Fig. 1 is a schematic side view of a near-eye display device according to an embodiment of the invention.
As shown in fig. 1, the near-eye display device includes: a display screen 1, a waveguide 2, an in-coupling part 3 and an out-coupling part 4.
The display screen 1 serves as an image source for displaying images. In a near-eye display device, a left-eye system and a right-eye system are generally included, wherein the left-eye system and the right-eye system respectively include a display screen, and when a human eye views left-eye and right-eye images, certain parallax is generated, so that a stereoscopic display effect is generated.
The display 1 in a near-eye display device is typically smaller in size, and a higher resolution display may be used to display more image details, providing a finer display image.
The display screen 1 may be one of a liquid crystal display, a light emitting diode display, an organic light emitting diode display, and a liquid crystal on silicon display, which is not limited herein.
The Liquid Crystal Display (LCD) mainly comprises a backlight module and a Liquid crystal display panel. The liquid crystal display panel does not emit light and needs to realize brightness display by means of a light source provided by the backlight module. The display principle of LCD is to place liquid crystal between two pieces of conductive glass, and to drive the electric field between two electrodes to cause the electric field effect of liquid crystal molecule torsion to control the transmission or shielding function of the backlight source, so as to display the image. If a color filter is added, a color image can be displayed. The liquid crystal display technology is mature, and the liquid crystal display screen has lower cost and excellent performance.
A light emitting Diode (LIGHT EMITTING Diode, abbreviated as LED) display is a display screen formed by an LED array, and uses LEDs as display sub-pixels, so that image display can be realized by controlling the display brightness of each LED. The LED display has the characteristics of high brightness, small power consumption, low voltage requirement, small and convenient equipment and the like. The LED display is used as the display screen 1 in the near-eye display device, which is advantageous for realizing miniaturization of the near-eye display device. A Micro light emitting Diode (Micro LIGHT EMITTING Diode, abbreviated as Micro LED) display is that an LED chip is miniaturized, so that more pixels can be arranged in a limited size, and the resolution of a display screen is improved.
An Organic Light-Emitting Diode (OLED) display is also referred to as an Organic laser display, an Organic Light-Emitting semiconductor display. An OLED display belongs to a current type organic light emitting device, which is a phenomenon of emitting light by injection and recombination of carriers, and the light emission intensity is proportional to the injected current. Under the action of an electric field, holes generated by the anode and electrons generated by the cathode of the OLED move, are respectively injected into the hole transport layer and the electron transport layer, and migrate to the light emitting layer. When the two meet at the light emitting layer, an energy exciton is generated, thereby exciting the light emitting molecule to finally generate visible light. The OLED display is a self-luminous display screen, so that a backlight module is not required to be arranged, the whole thickness of the device is small, the miniaturization of the near-to-eye display device is facilitated, and the whole device is more convenient to install.
LCOS (Liquid Crystal on Silicon, LCOS) is a novel reflective display technology with LCD and CMOS integrated circuit combined organically, and has the advantages of high brightness, high resolution and power saving.
The waveguide 2 is located at the light-emitting side of the display screen 1 and is used for conducting total reflection of light. When light is incident from the optically dense medium to the optically sparse medium, the incident angle is larger than the critical angle, and only the reflected light does not refract light. The waveguide 2 utilizes the principle of total reflection, is generally a parallel plate, has a high refractive index, and can conduct total reflection in the waveguide 2 when light rays are incident at an angle larger than a critical angle.
The coupling-in part 3 and the coupling-out part 4 are located on the same side of the waveguide 2, which in an embodiment of the invention may particularly be located on the side of the waveguide 2 facing away from the display screen 1. The coupling-in part is used for receiving emergent light of the display screen 1 and coupling the received light into the waveguide; the coupling-out part is used for coupling out the light rays in the waveguide.
In order to make the light incident on the waveguide at the same angle, a collimator lens group 5 may also be provided between the coupling-in part 3 and the waveguide 2, as shown in fig. 1. The collimator lens group 5 includes at least one lens for collimating the outgoing light of the display panel 1, thereby converting the outgoing light of the display panel 1 into parallel light. The parallel light rays are incident to the waveguide and then are incident to the coupling-in part, and the incident angles of the parallel light rays to the coupling-in part are the same, so that the design difficulty of the coupling-in part can be simplified. In the embodiment of the present invention, in order to simplify the design, the collimated light may be made to perpendicularly enter the waveguide 2.
The working principle of the near-eye display device is as follows: the display screen 1 displays required image information and emits light rays; the collimating lens group 5 converts emergent rays of the display screen 1 into parallel rays; the parallel light passes through the waveguide 2 and enters the waveguide 2 under the coupling action of the coupling-in part; light entering the waveguide 2 propagates from the coupling-in portion side to the coupling-out portion side under the condition that the total internal reflection condition of the waveguide is satisfied, and is output from the waveguide 2 to enter the human eye for imaging under the coupling action of the coupling-out portion.
Fig. 2 is a schematic side view of a coupling portion according to an embodiment of the present invention.
In an embodiment of the present invention, as shown in fig. 2, the coupling portion includes: a first grating 31 and a second grating 32; the first grating 31 is located on the side of the waveguide facing away from the display screen and the second grating 32 is located on the side of the first grating facing away from the waveguide.
In a waveguide near-eye display system, the coupler (including the in-coupling and out-coupling portions) as a core element of the system determines to a large extent the final imaging quality of the system. Among them, prisms, surface gratings, free-form surfaces, oblique gratings, and relief gratings have been selected as couplers for waveguide-type near-eye display devices. But finally, the grating manufactured by utilizing the optical coherence superposition principle, namely the volume holographic grating (Volume holographic grating, VHG for short), is distinguished by higher diffraction efficiency, fewer diffraction orders and strict color selectivity.
The volume hologram gratings may be classified into transmission volume hologram gratings and reflection volume hologram gratings according to the recording modes. The reflective volume hologram grating has a smaller wavelength bandwidth and a larger angular bandwidth, so in the embodiment of the present invention, the first grating 31 adopts the reflective volume hologram grating, and the waveguide system using the volume hologram grating as the coupler is a holographic waveguide system.
Fig. 3 is a diffraction schematic diagram of a volume hologram according to an embodiment of the present invention.
The volume holographic grating is an optical element with a periodic structure, and generally forms interference fringes with light and shade distribution by direct interference in a micron-sized thickness photosensitive polymer film in a double-beam holographic exposure mode, so that the periodic change of the refractive index in the material is caused. As shown in fig. 3, the diffraction pattern is divided into reflected diffraction light Ir and transmitted diffraction light Id after the incident light Io is incident on the volume hologram grating.
In the embodiment of the present invention, the first grating 31 adopts a reflective volume hologram grating, and a certain order of reflected diffraction light is generally selected to optimize the parameters of the volume hologram grating, but the transmitted diffraction light is not fully utilized, resulting in lower light efficiency. Therefore, as shown in fig. 2, in the embodiment of the present invention, the second grating 32 is disposed on the side of the first grating 31 facing away from the waveguide 2, and the first grating 31 adopts a volume holographic grating to reflect and diffract part of the incident light into the waveguide 2, and the second grating 32 can reflect and diffract the transmitted diffracted light of the volume holographic grating back into the waveguide to be utilized, so as to effectively improve the light efficiency, reduce the light energy loss of the system, effectively improve the brightness of the output image, and improve the wearing experience.
In a specific implementation, the first grating 31 is a volume holographic grating, and the second grating 32 may be one of a blazed grating, a rectangular grating, or a sinusoidal grating. In designing the parameters of the second grating 32, it is necessary to make the diffracted light of the first set reflection diffraction order of the first grating 31 and the diffracted light of the second set reflection diffraction order of the second grating 32 parallel to each other. In a specific implementation, the first grating 31 parameter may be optimized by using the diffracted light of the first set reflection diffraction order as the light conducted in the waveguide; the parameters of the second grating 32 are optimized so that the diffracted light of the first set reflection diffraction order of the first grating 31 and the diffracted light of the second set reflection diffraction order of the second grating 32 satisfy the condition of total reflection in the waveguide, so that the light is fully utilized.
The following describes the design process and principle of the grating in the near-eye display device according to the embodiment of the invention in detail. Wherein, the first grating 31 adopts a reflection type holographic grating, and the second grating 32 adopts a blazed grating.
As shown in fig. 3, d is the thickness of the volume hologram grating,The grating tilt angle, θ, is the grating incidence angle. Its relative dielectric constant epsilon can be expressed as:
Wherein ε0 is the average dielectric constant and its value is equal toN0 is the average refractive index of the volume holographic grating, delta epsilon represents the relative dielectric constant amplitude of the volume holographic grating, the value of the relative dielectric constant amplitude is equal to 2n0 delta n, and delta n is the refractive index modulation degree; k represents a grating vector of the volume hologram grating, the value of which is equal to 2pi/Λ, wherein Λ is a grating period of the volume hologram grating, and the direction of K is parallel to the period direction of the grating.
Fig. 4 is a schematic diagram of a working principle of a volume hologram grating according to an embodiment of the present invention.
For a volume holographic grating, its diffraction efficiency is one of the important factors limiting the performance of a near-eye display system. In some embodiments, as shown in fig. 4, the volume hologram grating (first grating 31) -1 st order diffracted light is selected for propagation of light. Wherein, the grating inclination angle of the volume holographic grating isThickness dv, bragg angle θB, period ΛV. When the incident light is incident at θ0, the diffraction angle of the-1 st order diffracted light of the volume hologram grating is θ-1, and the total reflection angle of the-1 st order diffracted light in the waveguide is θr, assuming that the-1 st order diffracted light satisfies the total internal reflection condition of the waveguide. Holographic waveguide systems utilize the characteristic of total reflection within the waveguide for image transmission. Thus, there is a need to satisfy:
Wherein nw represents the refractive index of the slab waveguide.
The geometrical relationship of the angles inside the waveguide 2 is:
In designing an optical system, it is generally necessary to select a center wavelength to design parameters of the optical system, and green light with a wavelength of 550nm is selected as incident light because visible light can be positioned in the middle to be used as a center wavelength to design a holographic waveguide system. To further simplify the design, suitable materials may be chosen to equalize the refractive indices of the waveguide 2 and the volume hologram grating (first grating 31).
For example, dichromated gelatin having a refractive index of 1.52 is used as a recording material of the volume hologram grating, and the waveguide material is BK7G18, and the waveguide refractive index is 1.52 when the wavelength is 550 nm.
When the incident light is vertically incident, i.e., θ0 =0, the grating tilt angle of the volume hologram grating is calculated according to the above formulas (1) and (3)The requirements are satisfied: taking 25 DEG as an initial value of the grating tilt angle, the initial person for the rest parameters is as follows: refractive index modulation degree Δn=0.03; thickness dv=10 μm. After initial parameters of the grating are determined, the volume holographic grating is modeled by utilizing strict coupled wave theory analysis, and each parameter is optimized.
The effect of each parameter on diffraction efficiency is shown in fig. 5 to 7. Fig. 5 is a change curve of diffraction efficiency according to the change of the inclination angle of the grating, fig. 6 is a change curve of diffraction efficiency according to the change of the modulation degree of refractive index, and fig. 7 is a change curve of diffraction efficiency according to the change of thickness. The diffraction efficiency of TE-polarized light and TM-polarized light is also shown in fig. 6 and 7, respectively.
After comprehensively considering all factors, optimizing the volume holographic grating parameters, wherein all structural parameters after optimization are as follows:Δn＝0.03，dv＝12um。
the diffraction efficiency of the optimized volume hologram grating is shown in the following table:
Wherein R, T represents diffraction efficiency of-1 st order reflected light and 0 th order transmitted light of the volume hologram grating, respectively. As can be seen from the above table, when the incident light is incident in TM polarization state, the diffraction efficiency of the 0 th order transmitted light of the volume hologram grating is as high as 14.1%, resulting in a great energy loss.
The results of the above table are obtained when the incident light satisfies the bragg condition, but the incident light tends to deviate from the bragg condition during actual fabrication and use. When incident light deviates from the bragg condition, its diffraction efficiency varies with the angle of incidence and the wavelength. Fig. 8 is a variation curve of diffraction efficiency according to an embodiment of the present invention, and fig. 9 is a variation curve of diffraction efficiency according to a wavelength according to an embodiment of the present invention. As shown in fig. 8 and 9, the light efficiency of the system decreases when the incident light deviates from the bragg condition.
At this time, by adding a blazed grating (second grating 32), the light transmitted by the volume hologram grating (first grating 31) is reused, and the light efficiency is increased.
As shown in fig. 2, in order to eliminate stray light as much as possible, the main diffraction light (-1 st order) of the blazed grating (second grating 32) should be parallel to the-1 st order reflection light of the volume hologram grating (first grating 31), that is, θs,-1＝θv,-1, and the diffraction angle θk is calculated according to the grating equation of the blazed grating:
wherein, the grating equation of blazed grating is: m is the diffraction order, n1 is the refractive index of the incident region, n2 is the refractive index of the grating region, lambda is the wavelength, thetai is the incident angle, lambdas is the blazed grating period.
When the incident light is vertically incident, the diffracted light deviates from the normal direction of the grating surface to the normal direction of the notch surface by a blaze angle; from the geometrical calculation, the blaze angle is now 27 °. For ease of design, assuming that the refractive index of the blazed grating is equal to the average refractive index of the volume holographic grating, the grating formula for the blazed grating is combined: The calculated blazed grating height was 228nm and period 447nm.
The diffraction efficiency of the grating at different polarization states when the incident light is normally incident is shown in the table below.
As can be seen from the above table, compared with the volume hologram grating (first grating 31) alone, the diffraction efficiency of the volume hologram grating combined with the blazed grating (second grating 32) in the TE polarization state is improved by 1.5%, the diffraction efficiency in the TM polarization state is improved by 9.4%, and the diffraction efficiency in the unpolarized light is improved by 5.3%. Therefore, the light efficiency is improved, the light energy loss of the system is reduced, the brightness of an output image is effectively improved, and the wearing experience is improved.
Fig. 10 is a comparison curve of diffraction efficiency with wavelength according to an embodiment of the present invention, and fig. 11 is a comparison curve of diffraction efficiency with incident angle according to an embodiment of the present invention. Here, diffraction efficiencies of TE polarized light and TM polarized light are shown in fig. 10 and 11, respectively, TE1 and TM1 represent diffraction efficiencies of TE polarized light and TM polarized light when the volume hologram grating (first grating 31) is used alone, and TE2 and TM2 represent diffraction efficiencies of TE polarized light and TM polarized light when the volume hologram grating (first grating 31) is combined with the blazed grating (second grating 32).
As shown in fig. 10 and 11, when the incident light deviates from the bragg condition, the diffraction efficiency of using the volume hologram grating in combination with the blazed grating is always higher than that of using the volume hologram grating alone under the same condition. As can be seen from fig. 11, the use of the volume hologram grating in combination with the blazed grating as the coupling portion (corresponding to TE2 and TM2 curves) has a larger range of incidence angles corresponding to diffraction efficiencies greater than a preset value (e.g., 0.5) than the use of the volume hologram grating alone (corresponding to TE1 and TM1 curves) at a range of incidence angles corresponding to the relative diffraction efficiencies, which means that the volume hologram grating in combination with the blazed grating has a larger angular bandwidth as the coupling portion, thereby not only improving the diffraction efficiency of the coupling portion but also enabling the system to have a larger field angle.
As shown in fig. 1, the light efficiently coupled into the waveguide 2 finally needs to be coupled out from the waveguide 2 by the coupling-out portion 4 and then incident to the human eye. In the embodiment of the present invention, the coupling-out portion 4 may include a third grating 41, and diffract the light incident into the third grating 41 by using the diffraction effect of the grating, so that the diffracted light exits along the set direction, and the condition that the light is totally reflected in the waveguide 2 is destroyed, and exits from the waveguide 2.
In a specific implementation, the third grating 41 may also be a reflective grating, and the human eye is located on the side of the waveguide 2 facing away from the third grating 41. Specifically, the third grating 41 may also be a reflective volume hologram grating, where the first grating and the third grating both use reflective volume hologram gratings, and the periodic structure of the third grating may be disposed in mirror symmetry with the periodic structure of the first grating, that is, the periodic widths of the first grating and the third grating are equal, and the tilt angles are disposed in mirror symmetry, so that chromatic dispersion can be effectively eliminated.
Fig. 12 is a schematic diagram of a side view of a near-eye display device according to an embodiment of the invention.
In some embodiments, as shown in fig. 12, the diffraction efficiency of the third grating 41 is distributed in a step-like manner, when light is transmitted from the coupling-in portion 3 to the coupling-out portion 4, a part of the light is output from the waveguide 2 under the diffraction effect of the third grating 41, and the rest of the light continues to be totally reflected and propagates forward, so that the one-dimensional expansion of the exit pupil is realized after repeating the process for a plurality of times.
As shown in fig. 12, the diffraction efficiency of the third grating 41 gradually increases with the direction away from the first grating (i.e., the first direction x), i.e., η1<η2<η3; by controlling the diffraction efficiency of the third grating 41 at different positions, it is ensured that the intensity of the diffracted light in the whole output range is almost the same, i.e. Idiff1＝Idiff2＝Idiff3, after the light has passed through the exit pupil expansion.
For the purpose of extending the exit pupil, the length of the third grating 41 in the first direction x is typically larger, and may be set to 8mm to 10mm, so as to cover the size of the pupil of the human eye under intense light. The first direction x is a connection direction between a center point of the first grating and a center point of the third grating.
Compared with other near-eye display schemes, the holographic waveguide near-eye display scheme can easily expand the exit pupil of the ocular system, so that the large-view-field and large-exit pupil display of the near-eye display device is realized.
Fig. 13 is a schematic top view of a near-eye display device according to an embodiment of the invention.
Further, in some embodiments, as shown in fig. 13, the near-eye display device further includes: an intermediate grating 6; the intermediate grating 6, the third grating 41 and the first grating 31 are both located on the same side of the waveguide, in particular on the side of the waveguide facing away from the display screen. For convenience of description, a line connecting the center point of the first grating 31 and the center point of the third grating 41 is referred to as a first direction x, and a line connecting the center point of the first grating 31 and the center point of the intermediate grating 6 is referred to as a second direction y, as shown in fig. 13, the first direction x is perpendicular to the second direction y. In practice, the parameters of the intermediate grating 6 may be designed according to the actual situation, in which case the first direction x may also be non-perpendicular to the second direction y.
The diffraction efficiency of the intermediate grating 6 may be distributed in a step-like manner as the third grating 41, and the diffraction efficiency of the intermediate grating 6 gradually increases along with the direction away from the first grating 31, so that the intermediate grating 6 can expand light along the second direction y and then emit the light to the third grating 41, and then the third grating 41 expands the light along the first direction x, so that the two-dimensional expansion of the exit pupil can be realized.
Likewise, in order to cover the size of the pupil of the human eye under intense light, the length of the intermediate grating 6 along the second direction y is 8mm to 10mm; the length of the third grating 41 in the first direction x is 8mm to 10mm, and the length in the second direction y is 8mm to 10mm.
The near-to-eye display device provided by the embodiment of the invention can be applied to the fields of AR or VR and the like, and is not limited herein.
The invention also provides a simulation test for irradiance distribution of the near-eye display device provided by the embodiment. Fig. 14 is an irradiance distribution diagram of a near-eye display device provided by an embodiment of the present invention at a central field of view of an exit pupil, fig. 15 is an irradiance distribution diagram of a near-eye display device provided by an embodiment of the present invention at a horizontal field of view of an edge of the exit pupil, fig. 16 is an irradiance distribution diagram of a near-eye display device provided by an embodiment of the present invention at a vertical field of view of an edge of the exit pupil, and fig. 17 is an irradiance distribution diagram of a near-eye display device provided by an embodiment of the present invention at a full field of view of the exit pupil. Wherein, the visual angle range of the edge horizontal visual field is (0 degree, +/-10 degrees), and the angular visual range of the edge vertical visual field is (+ -7.5 degrees, 0 degree).
As can be seen from fig. 14 to 17, the near-to-eye display device provided by the embodiment of the invention can realize 20 ° ×15° field display and has good illuminance uniformity in the exit pupil range of 20mm×20 mm. Meanwhile, the light energy utilization rate of the coupling part is 10.31% by adopting the first grating and the second grating, and is improved by 2.17% compared with a system with only the first grating.
According to a first inventive concept, a near-eye display device includes: the device comprises a display screen, a waveguide, a coupling-in part and a coupling-out part. The display screen displays the required image information and emits light rays; the light passes through the waveguide and enters the waveguide under the coupling action of the coupling-in part; under the condition that the total internal reflection condition of the waveguide is met, light entering the waveguide propagates from one side of the coupling-in part to one side of the coupling-out part, and is output from the waveguide to enter human eyes for imaging under the coupling action of the coupling-out part. The coupling-in part comprises a first grating and a second grating, the first grating can reflect and diffract part of incident light into the waveguide, and the second grating can reflect and diffract the transmitted and diffracted light of the first grating back into the waveguide to be utilized, so that the light efficiency is effectively improved, the light energy loss of a system is reduced, the brightness of an output image is effectively improved, and the wearing experience is improved.
According to a second inventive concept, the first grating employs a reflective volume holographic grating having a smaller wavelength bandwidth and a larger angular bandwidth than a transmissive volume holographic grating. The second grating adopts one of blazed grating, rectangular grating or sinusoidal grating. In designing parameters of the first grating and the second grating, it is necessary to make the diffracted light of the first set reflection diffraction order of the first grating and the diffracted light of the second set reflection diffraction order of the second grating parallel to each other. The diffraction light of the first set reflection diffraction order can be used as the light conducted in the waveguide light, and the parameters of the first grating are optimized; and optimizing parameters of the second grating based on the fact that the diffracted light of the second set reflection diffraction order is parallel to the diffracted light of the first set reflection diffraction order of the first grating, so that the diffracted light of the first set reflection diffraction order of the first grating and the diffracted light of the second set reflection diffraction order of the second grating meet the condition of total reflection in the waveguide, and light is fully utilized.
According to a third inventive concept, the first grating, the second grating, and the waveguide are fabricated by selecting appropriate materials such that refractive indexes of the first grating, the second grating, and the waveguide are equal to simplify the design.
According to a fourth inventive concept, the near-eye display device further comprises a collimating lens group between the display screen and the waveguide, the collimating lens group comprising at least one lens for collimating the outgoing light of the display screen, thereby converting the outgoing light of the display screen into parallel light. The parallel light rays are incident to the waveguide and then are incident to the coupling-in part, and the incident angles of the parallel light rays to the coupling-in part are the same, so that the design difficulty of the coupling-in part can be simplified. To simplify the design, collimated light may be made to enter the waveguide at normal incidence.
According to a fifth inventive concept, the first grating is a volume holographic grating and the second grating is a blazed grating; the combination of the volume hologram grating and the blazed grating as the coupling-in part not only improves the diffraction efficiency of the coupling-in part but also enables the system to have a larger field angle compared with the single use of the volume hologram grating as the coupling-in part.
According to a sixth inventive concept, the coupling-out part includes a third grating, and diffracts light incident into the third grating by diffraction of the grating, so that the diffracted light exits along a set direction, and conditions for total reflection of the light in the waveguide are destroyed, and the diffracted light exits from the waveguide. The third grating can also adopt a reflection type volume holographic grating, the first grating and the third grating both adopt reflection type volume holographic gratings, the periodic structure of the third grating can be arranged in mirror symmetry with the periodic structure of the first grating, namely, the periodic widths of the first grating and the third grating are equal, and the inclined angles are arranged in mirror symmetry, so that chromatic dispersion can be effectively eliminated.
According to a seventh inventive concept, the diffraction efficiency of the third grating is distributed in a step-by-step manner, when light is transmitted from the coupling-in portion to the coupling-out portion, a part of the light is output from the waveguide under the diffraction effect of the third grating, and the rest of the light continues to be totally reflected and transmitted forward, and finally, one-dimensional expansion of the exit pupil is realized after repeating the process for a plurality of times. By controlling the diffraction efficiency of the third grating at different positions, it can be ensured that the intensity of the diffracted light in the whole output range is almost the same after the light passes through the exit pupil expansion.
According to the eighth inventive concept, in order to achieve the purpose of exit pupil expansion, the length of the third grating in the direction away from the first grating is generally larger, and may be set to 8mm to 10mm, so that the size of the pupil of the human eye under intense light may be covered.
According to a ninth inventive concept, the near-eye display device further comprises: an intermediate grating; the middle grating, the third grating and the first grating are all positioned on the same side of the waveguide. The diffraction efficiency of the intermediate grating can be distributed like the third grating in a step-by-step manner, and the diffraction efficiency of the intermediate grating is gradually increased along with the direction away from the first grating, so that the intermediate grating can expand light along the direction away from the first grating and then emit the light to the third grating, and then the third grating expands the light along the direction away from the first grating, so that the two-dimensional expansion of the exit pupil can be realized.
According to the tenth inventive concept, the near-eye display device may be applied to the fields of AR or VR and the like.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. It is therefore intended that the following claims be interpreted as including the preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention also include such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.