Optical waveguide and display device

An optical waveguide and a display device are provided. The optical waveguide includes a waveguide substrate causing light to propagate within it through total internal reflection and at least one grating disposed between and inclined with respect to the first and second surfaces of the waveguide substrate for transmitting and reflecting incident light or for transmitting and diffractively deflecting incident light. The transmitted light is coupled into the waveguide substrate to propagate within it through total internal reflection, and the reflected or diffractively deflected light is coupled out from the waveguide substrate. The grating is configured such that the reflection efficiency or diffraction deflection efficiency when an incidence angle is less than a first angle and greater than a fourth angle is greater than that when the incidence angle is greater than the first angle, wherein the first angle is greater than the fourth angle.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority to Chinese Patent Application No. 202310565756X, filed on May 19, 2023, which is hereby incorporated by reference in its entirety.

BACKGROUND

The present application generally relates to the technical field of optics, and more specifically to an optical waveguide and a display device having the same.

With the development of science and technology, Augmented Reality (AR) technology, as a very intelligent and portable display technology, is slowly moving towards the public. Its main feature is that virtual images are superimposed on a real scene, such that people can watch the real scene while watching the virtual images. It is precisely because of the above characteristics of AR display that the technology is currently more and more widely used in security, education, medical, military, industrial, entertainment and other industries.

AR glasses are one of the important mediums in the field of augmented reality display. The optical waveguide has the advantages of allowing production with high ability for mass production and a light, thin form, and is gradually approved in the field of AR display, being expected to become the mainstream technology development direction of AR field in the future.

The existing optical waveguide solutions are roughly divided into two categories, one is geometric optical waveguide, and the other is diffractive optical waveguide.

The main implementation of the geometric optical waveguide is geometric array optical waveguide, which is mainly composed of a series of semi-transparent and semi-reflective mirrors, wherein the mirrors are surfaces embedded in a glass substrate and form a specific angle with the transmitted light. Each mirror will reflect part of the light out of the waveguide into the human eye. At present, the mirror is mainly implemented by multilayer film, the multilayer film system of each mirror needs to be carefully designed, and the number of film layers usually reaches dozens of pieces.

In the diffractive optical waveguide, in order to achieve a larger FOV (Field of View), better non-uniformity and higher efficiency at the same time, it is typically needed that the coupling-out grating may be designed to have a certain two-dimensional grating region, and a certain one-dimensional grating region on the sides of the two-dimensional grating region to improve the efficiency. However, there is a large difference in coupling-out efficiency between the one-dimensional and two-dimensional regions, and the factor of the difference typically may be more than 3 times, and even reach more than 6 times. This leads to the problem that the coupling-out strength of the two-dimensional region in the middle and the one-dimensional region on its sides are quite different, resulting in clear demarcation for bright to dark to bright areas, which is undesired to the non-uniformity index, and is also undesired for viewing by human eyes.

Therefore, it is necessary to improve the optical waveguide and display device to solve at least one technical problem.

SUMMARY

A series of simplified concepts are introduced into the portion of Summary of the present application, which would be further described in the portion of Detailed Description section. The Summary of the present application does not mean attempting to define the key features and essential technical features of the claimed technical solution, let alone determining the protection scope thereof.

To at least partially solve the above problems, a first aspect of the present application provides an optical waveguide comprising a waveguide substrate and at least one grating. The waveguide substrate has a first surface and a second surface oppositely disposed for totally reflecting light entering the inside of the waveguide substrate and causing the light to propagate within the waveguide substrate through total internal reflection. The at least one grating is disposed in the waveguide substrate and located between and inclined with respect to the first surface and the second surface for transmitting and reflecting incident light or for transmitting and diffractively deflecting incident light. Wherein the transmitted light is coupled into the waveguide substrate to propagate within the waveguide substrate through total internal reflection, and the reflected or diffractively deflected light is coupled out from the waveguide substrate. The grating is configured such that when incidence angles of the incident light on the grating are different, reflection efficiencies or diffraction deflection efficiencies of the grating are different, and the reflection efficiency when the incidence angle is less than a first angle and greater than a fourth angle is greater than the reflection efficiency when the incidence angle is greater than the first angle, or the diffraction deflection efficiency when the incidence angle is less than the first angle and greater than the fourth angle is greater than the diffraction deflection efficiency when the incidence angle is greater than the first angle. Wherein the first angle is greater than the fourth angle.

The optical waveguide according to the present application includes at least one grating disposed in the waveguide substrate, which may transmit and reflect incident light or transmit and diffractively deflect incident light, thereby realizing a semi-reflective and semi-transparent optical function, simple and flexible design, and easy processing. Wherein the reflected light (the diffraction deflected light) is used for human observation, and the reflected light (the diffraction deflected light) may be made to have higher brightness by setting a smaller incidence angle so as to improve user experience.

Optionally, the grating is configured such that the reflection efficiency when the incidence angle is less than or equal to a second angle and greater than or equal to a third angle is more than three times the reflection efficiency when the incidence angle is greater than the first angle; or the diffraction deflection efficiency when the incident angle is less than or equal to the second angle and greater than or equal to the third angle is more than three times the diffraction deflection efficiency when the incidence angle is greater than the first angle. Wherein the second angle is less than or equal to the first angle and greater than the fourth angle, and the third angle is less than the second angle and greater than or equal to the fourth angle.

According to the present application, the reflection efficiency of the incidence angle within a certain range is significantly higher than that of the incidence angle outside the range, or the diffraction deflection efficiency of the incidence angle within a certain range is significantly higher than that of the incidence angle outside the range.

Optionally, the grating is configured such that when the incidence angle is less than or equal to the second angle and greater than or equal to the third angle, a fluctuation coefficient s1 of the reflection efficiency or of the diffraction deflection efficiency is less than or equal to 0.33. Wherein the fluctuation coefficient s1 is calculated according to the following formula:
s1=(Rmax−Rmin)/(Rmax+Rmin)

Wherein Rmaxis a maximum value of the reflection efficiency when the incidence angle is less than or equal to the second angle and greater than or equal to the third angle, Rminis a minimum value of the reflection efficiency when the incidence angle is less than or equal to the second angle and greater than or equal to the third angle; or Rmaxis a maximum value of the diffraction deflection efficiency when the incidence angle is less than or equal to the second angle and greater than or equal to the third angle, and Rminis a minimum value of the diffraction deflection efficiency when the incidence angle is less than or equal to the second angle and greater than or equal to the third angle.

According to the present application, when the incidence angle is within a certain range (less than or equal to the second angle and greater than or equal to the third angle), the grating has high and uniform reflection efficiency or the diffraction deflection efficiency.

Optionally, the difference between the third angle and the second angle is greater than 10°.

According to the present application, the variation range of the incidence angle corresponding to the effect that the grating has the high and uniform reflection efficiency or diffraction deflection efficiency is greater than 10°. When the refractive index of the waveguide substrate is certain, the field of view FOV has a larger range.

Optionally, the first angle is 30° to 35°.

According to the present application, when the incidence angle is below 30° to 35°, the reflected light has higher brightness.

Optionally, the optical waveguide comprises a plurality of the gratings.

According to the present application, the plurality of the gratings may successively reflect incident light, thereby increasing the illuminated area of emitted light and improving user experience.

Optionally, the number of the gratings is N, and N is an integer greater than 1. The N gratings are numbered sequentially according to the direction of an optical path, and the reflection efficiency of the grating with the subsequent number is greater than the reflection efficiency of the grating with the previous number, or the diffraction deflection efficiency of the grating with the subsequent number is greater than the diffraction deflection efficiency of the grating with the previous number.

Optionally, a kthsaid grating has reflection efficiency Rkand transmission efficiency Tkor the kthsaid grating has diffraction deflection efficiency Rkand transmission efficiency Tk, and k is an integer greater than or equal to 1 and less than or equal to N. Each of the gratings has respective efficiency coefficient s2. Wherein the efficiency coefficient s2iof light beam reaching an ithsaid grating is calculated according to the following formula:

Wherein i is an integer greater than 1 and less than or equal to N, j is an integer, and the efficiency coefficient s21of a first said grating is its reflection efficiency or diffraction deflection efficiency R1. The optical waveguide is configured such that a quotient of the difference between the maximum the minimum values of the N efficiency coefficients s2 divided by the sum of the maximum the minimum values of the N efficiency coefficients s2 is less than or equal to 0.33.

According to the present application, the set of the reflection efficiency or diffraction deflection efficiency of each grating may make the uniformity of the emitted light intensity in the field of view better.

Optionally, the plurality of the gratings are substantially parallel to each other and/or distributed at substantially equal intervals.

According to the present application, the plurality of the gratings are substantially parallel to each other and/or distributed at substantially equal intervals, which is beneficial to form an emitted light field with uniform brightness.

Optionally, the grating is constructed as a volume holographic grating having an equal refractive index fringe surface. Wherein a reflecting mirror surface is the equal refractive index fringe surface.

According to the present application, the semi-reflective and semi-transparent functions, simple and flexible design, and easy processing are realized by modulating the refractive index inside the volume holographic grating.

Optionally, the volume holographic grating has a thickness of 1 to 20 microns, and/or the volume holographic grating has an amplitude of refractive index modulation of 0.01 to 0.2.

According to the present application, the volume holographic grating has a smaller thickness and a wider angular spectrum width, and the reflection efficiency or diffraction deflection efficiency of the grating may be adjusted through the amplitude of refractive index modulation.

Optionally, an included angle between the volume holographic grating and the first surface or the second surface is 20° to 30°, and/or the volume holographic grating has a grating period of 2000 to 7000 lp/mm.

According to the present application, the volume holographic grating may be flexibly designed.

Optionally, a thickness direction of the volume holographic grating is parallel to or perpendicular to the equal refractive index fringe surface, and/or the volume holographic grating includes a photosensitive material with an average refractive index of 1.5 to 2.0.

According to the present application, the thickness direction of the volume holographic grating is parallel to or perpendicular to the equal refractive index fringe surface, which can simplify design and processing. In the case of the same optical-mechanical FOV, the selection of a waveguide material with a larger refractive index is beneficial to improve the efficiency uniformity in the field of view.

Optionally, each of the volume holographic gratings is configured to have a nonuniformly distributed amplitude of refractive index modulation.

Optionally, each of the volume holographic gratings is configured such that the amplitude of refractive index modulation at both ends is less than that in the middle part.

Optionally, the volume holographic grating is constructed as an apodized volume holographic grating.

Specifically, each of the volume holographic gratings is configured to have the following refractive index modulation distribution n(x,z):
n(x,z)=n0+n1(x,z)·cos[(xsin ϕ+zcos ϕ)],

wherein
n1(x,z)=nm·sinc[2(z−d/2)/d], or
n1(x,z)=nm·exp[−(z−d/2)2],wherein x is a coordinate along the direction of a surface of the volume holographic grating, z is a coordinate along a thickness direction of the volume holographic grating, n0is an average refractive index of material of the volume holographic grating, nmis a preset constant, ϕ is an inclination angle of the volume holographic grating,is a grating vector of the volume holographic grating determined according to Bragg condition, and the equal refractive index fringe surface is perpendicular to the grating vector.

According to the present application, the set of the nonuniformly distributed amplitude of refractive index modulation or the apodized volume holographic grating may effectively weaken the side lobe of the angular spectral curve of the volume holographic grating, improve the waveguide efficiency and signal-to-noise ratio, and reduce energy waste.

Optionally, each of the volume holographic gratings is configured to have a uniformly distributed amplitude of refractive index modulation.

Optionally, each of the volume holographic gratings is configured to have the following refractive index modulation distribution n(x,z):
n(x,z)=n0+n1·cos[|K|(xsin ϕ+zcos ϕ)],wherein x is a coordinate along the direction of a surface of the volume holographic grating, z is a coordinate along a thickness direction of the volume holographic grating, n0is an average refractive index of material of the volume holographic grating, n1is an amplitude of refractive index modulation of the volume holographic grating, ϕ is an inclination angle of the volume holographic grating,is a grating vector of the volume holographic grating determined according to Bragg condition, and the equal refractive index fringe surface is perpendicular to the grating vector.

According to the present application, the reflection efficiency or diffraction deflection efficiency of the grating is set through the uniformly distributed amplitude of refractive index modulation and the periodic refractive index modulation inside the grating to realize the control of the light field, such that the volume holographic grating may realize the semi-reflective and semi-transparent function, simple and flexible design, and easy processing.

Optionally, the optical waveguide includes a plurality of the volume holographic gratings with the same grating vector, and wherein the plurality of the volume holographic gratings have different thicknesses and/or the plurality of the volume holographic gratings have different amplitudes of refractive index modulation.

According to the present application, the reflection efficiency or diffraction deflection efficiency and diffraction bandwidth or reflection bandwidth of the volume holographic grating may be adjusted by setting the amplitude of refractive index modulation and thickness, and the design is simple.

Optionally, the grating is constructed as a sub-wavelength grating.

According to the present application, the semi-reflective and semi-transparent optical effect may be realized by using the sub-wavelength grating structure, which is simple and flexible in design and easy to process.

A second aspect of the present application provides a display device including an optical machine for emitting light beam and the optical waveguide of the aforesaid technical solution, and wherein the light beam emitted by the optical machine is used as incident light of the optical waveguide.

According to the present application, the optical waveguide of the display device includes at least one grating disposed in the waveguide substrate, which may transmit and reflect incident light or transmit and diffractively deflect incident light, thereby realizing the semi-reflective and semi-transparent optical function, simple and flexible design, and easy processing. Wherein the reflected light or diffractive light is used for human observation, and the reflected light or diffractive light may be made to have higher brightness by setting a smaller incidence angle to improve user experience.

Optionally, the display device further includes a reflector for reflecting the light beam emitted by the optical machine and causing the light beam reflected by the reflector to propagate within the waveguide substrate through total internal reflection.

According to the present application, the reflector may increase the flexibility of the optical machine setting.

Optionally, the display device is a near-eye display device and further includes a lens including the optical waveguide and a frame for holding the lens close to eyes.

Optionally, the display device is an augmented reality display device or a virtual reality display device.

The optical waveguide according to the present application may be used, for example, for AR glasses.

Optionally, the display device is an optical pupil expansion device.

The optical waveguide according to the present application may be used, for example, for a pupil expansion device.

EXPLANATION OF REFERENCE NUMERALS

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the present application. However, it is obvious to those skilled in the art that the present application may be implemented without one or more of these details. Some technical features well-known in the art are not described in other examples in order to avoid confusion with the present application.

In order to thoroughly understand the present application, a detailed description will be provided in the following description. It should be understood that these embodiments are provided so that this disclosure will be thorough and complete, and fully convey the concept of these exemplary embodiments to those of ordinary skill in the art. Obviously, the implementation of the embodiments of the present application is not limited to the specific details familiar to those skilled in the art. The preferred embodiments of the present application are described in detail as follows. However, in addition to these detailed descriptions, the present application may have other embodiments.

Ordinals such as “first” and “second” recited in the present application are merely identifiers but do not have any other meaning, such as a specific order and the like. Moreover, for example, the term “first component” itself does not imply an existence of “second component”, and the term “second component” itself does not imply an existence of “first component.” The use of the words “first”, “second” and “third” does not indicate any order, and these words may be construed as names.

It should be noted that the terms “up”, “down”, “front”, “back”, “left”, “right”, “inside”, “outside” and similar expressions used herein are for illustrative purposes only and are not restrictive.

The first aspect of the present application provides an optical waveguide, which realizes the semi-reflective and semi-transparent functions of a multilayer film mirror surface through a grating, and which is simple and flexible in design, and easy to process.

Now, the exemplary embodiments of the present application will be described in more detail with reference to the accompanying drawings.

As shown inFIG.1, an optical waveguide100according to a first embodiment of the present application includes a waveguide substrate10and at least one grating20. The waveguide substrate10has a first surface11and a second surface12which are oppositely disposed and used to totally reflect the light D1 entering the inside of the waveguide substrate10and cause the light D1 to propagate within the waveguide substrate10through total internal reflection. For example, the light D2 is formed by total reflection of the light D1. The grating20is disposed in the waveguide substrate10. The grating20is located between the first surface11and the second surface12, and is inclined with respect to the first surface11and the second surface12for transmitting and reflecting the light D2 (i.e., D2 or D1 is the incident light of the grating20). Wherein the transmitted light D3 is coupled into the waveguide substrate10to propagate within the waveguide substrate10through total internal reflection, and the reflected light D4 is coupled out from the waveguide substrate10so as to be received by human eyes.

Therefore, when the optical waveguide100includes a plurality of gratings20(for example, the optical waveguide100includes gratings20a,20b,20cand20ddisposed in sequence according to the direction of an optical path) arranged in sequence according to the direction of the optical path, the transmitted light (or the light after total reflection) of the previous grating20becomes the incident light of the next grating20, and the reflected light D4 of each grating20is coupled out from the waveguide substrate10.

The relationship between the reflection efficiency ηRand the incidence angle θ of the grating20of the first embodiment is shown inFIG.3, wherein the images of the relationship between the reflection efficiency ηRand the incidence angle θ of the gratings20a,20b,20cand20dare curves a, b, c and d, respectively. The grating20is configured such that when the incidence angles θ of the incident light D2 on the grating20are different, the reflection efficiencies ηRof the grating20are different. The reflection efficiency ηRwhen the incidence angle θ is less than a first angle θ1 and greater than a fourth angle θ4 is greater than the reflection efficiency ηRwhen the incidence angle θ greater than the first angle θ1. Wherein the first angle θ1 is greater than the fourth angle θ4.

As shown inFIG.2, an optical waveguide200according to a second embodiment of the present application includes a waveguide substrate110and at least one grating120. The waveguide substrate110has a first surface111and a second surface112which are oppositely disposed and used to totally reflect the light S1 (for example, the light S2 is formed by total reflection of the light S1) entering the inside of the waveguide substrate110and cause the light S1 to propagate within the waveguide substrate110through total internal reflection. The grating120is disposed in the waveguide substrate110. The grating120is located between the first surface111and the second surface112, and is inclined with respect to the first surface111and the second surface112for transmitting and diffracting incident the light S2 (i.e., S2 or S1 is the incident light of the grating120). Wherein the transmitted light S3 is coupled into the waveguide substrate110to propagate within the waveguide substrate110through total internal reflection, and the diffractively deflected light S4 is coupled out from the waveguide substrate110so as to be received by human eyes.

Therefore, when the optical waveguide200includes a plurality of gratings120(for example, the optical waveguide200includes gratings120a,120b,120cand120ddisposed in sequence according to the direction of an optical path), arranged in sequence according to the direction of the optical path, the transmitted light (or the light after total reflection) of the previous grating120becomes the incident light of the next grating120, and the diffractively deflected light S4 of each grating120is coupled out from the waveguide substrate110.

The relationship between the diffraction deflection efficiency ηTand the incidence angle θ of the grating120of the second embodiment is shown inFIG.4, wherein the images of the relationship between the diffraction deflection efficiency ηTand the incidence angle θ of gratings120a,120b,120cand120dare curves a, b, c and d, respectively. The grating120is configured such that when the incidence angles θ of the incident light S2 on the grating120are different, the diffraction deflection efficiencies ηTof the grating120are different. The diffraction deflection efficiency ηTwhen the incidence angle θ is less than the first angle θ1 and greater than the fourth angle θ4 is greater than the diffraction deflection efficiency ηTwhen the incidence angle θ is greater than the first angle θ1. Wherein the first angle θ1 is greater than the fourth angle θ4.

Optionally, as shown inFIG.3, the grating20is configured such that the reflection efficiency ηRwhen the incidence angle θ is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3 is more than three times the reflection efficiency ηRwhen the incidence angle θ is greater than the first angle θ1. Wherein the second angle θ2 is less than or equal to the first angle θ1 and greater than the fourth angle θ4, and the third angle θ3 is less than the second angle θ2 and greater than or equal to the fourth angle θ4.

Therefore, when the incidence angle θ of the incident light D2 on the grating20is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3, the reflection efficiency ηRis significantly higher than the reflection efficiency ηRwhen the incidence angle θ is in other ranges.

Optionally, as shown inFIG.4, the grating120is configured such that the diffraction deflection efficiency ηTwhen the incidence angle θ is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3 is more than three times the diffraction deflection efficiency ηTwhen the incidence angle θ is greater than the first angle θ1. Wherein the second angle θ2 is less than or equal to the first angle θ1 and greater than the fourth angle θ4, and the third angle θ3 is less than the second angle θ2 and greater than or equal to the fourth angle θ4.

Therefore, when the incidence angle θ of the incident light S2 on the grating120is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3, the diffraction deflection efficiency ηTis significantly higher than the diffraction deflection efficiency ηTwhen the incidence angle θ is in other ranges.

Optionally, the grating20/120is configured such that when the incidence angle θ is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3, a fluctuation coefficient s1 of the reflection efficiency ηRof the grating20or of the diffraction deflection efficiency ηTof the grating120is less than or equal to 0.33. Wherein the fluctuation coefficient s1 is calculated according to the following formula (1):
s1=(Rmax−Rmin)/(Rmax+Rmin)  (1)wherein Rmaxis a maximum value of the reflection efficiency ηRof the grating20when the incidence angle θ is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3, Rminis a minimum value of the reflection efficiency ηRof the grating20when the incidence angle θ is less than or equal to the second angle θ2 and greater than or30equal to the third angle θ3, or Rmaxis a maximum value of the diffraction deflection efficiency ηTof the grating120when the incidence angle θ is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3, and Rminis a minimum value of the diffraction deflection efficiency ηTof the grating120when the incidence angle θ is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3.

Therefore, when the incidence angle θ of the incident light on the grating is less than or equal to the second angle θ2 and greater than or equal to the third angle θ3, the value of the reflection efficiency ηRor of the diffraction deflection efficiency ηTis relatively uniform.

In the present application, the difference between the third angle θ3 and the second angle θ2 is greater than 10°. For example, it may be 13° to 15°.

Optionally, as shown inFIGS.3and4, the first angle θ1 is 30° to 35°.

Optionally, the optical waveguide100includes a plurality of gratings20. The optical waveguide200includes a plurality of gratings120.

Further optionally, the plurality of gratings20/120are substantially parallel to each other and/or distributed at substantially equal intervals.

Optionally, the number of the gratings20/120is N (N is an integer greater than 1), and the N gratings20/120are numbered sequentially according to the direction of the optical path. The reflection efficiency ηRof the grating20with the subsequent number is greater than the reflection efficiency ηRof the grating20with the previous number, or the diffraction deflection efficiency ηTof the grating120with the subsequent number is greater than the diffraction deflection efficiency ηTof the grating120with the previous number.

As shown inFIG.1, the number of the grating20is set to four, including gratings20a,20b,20cand20d. As shown inFIG.3, the reflection efficiencies ηRof the gratings20a,20b,20cand20dincrease sequentially.

As shown inFIG.2, the number of the gratings120is set to four, including gratings120a,120b,120cand120d. As shown inFIG.4, the diffraction deflection efficiencies ηTof the gratings120a,120b,120cand120dincrease sequentially.

Further, the kthgrating20has reflection efficiency Rkand transmission efficiency Tk, or the kthgrating120has diffraction deflection efficiency Rkand transmission efficiency Tk(k is an integer greater than or equal to 1 and less than or equal to N). Each of the gratings20/120has respective efficiency coefficient s2, wherein the efficiency coefficient s2iof light beam reaching the ithgrating20/120is calculated according to the following formula (2):

s⁢2i=Ri⁢∏j=1i-1Tj(2)wherein i is an integer greater than 1 and less than or equal to N, j is an integer, and the efficiency coefficient s21of the first grating20a/120ais its reflection efficiency or diffraction deflection efficiency R1.

In order to make the emitted light intensities of the plurality of gratings20/120uniform, Optionally, the optical waveguide100/200is configured such that a quotient of the difference between the maximum and minimum values of the N efficiency coefficients s2 divided by the sum of the maximum and minimum values of the N efficiency coefficients s2 is less than or equal to 0.33.

For example, if the transmission efficiencies and reflection efficiencies of the gratings20a,20b,20cand20dare respectively T1, T2, T3and T4, and R1, R2, R3and R4when the light is incident at a small angle (e.g., less than or equal to the second angle θ2 and greater than or equal to the third angle θ3), the proportion of light energy (efficiency coefficient) s2iof a single ray reflected by the grating20a,20b,20cand20dis R1, T1·R2, T1·T2·R3and T1·T2·T3·R4, respectively. The closer these four values are, the more uniform the energy of the light received by human eyes. The preferred values of R1, R2, R3and R4may be obtained by calculation.

Similarly, if the transmission efficiencies and diffraction deflection efficiencies of the gratings120a,120b,120c, and120dare respectively T1, T2, T3and T4, and R1, R2, R3and R4, when the light is incident at a small angle (e.g., less than or equal to the second angle θ2 and greater than or equal to the third angle θ3), the proportion of light energy (efficiency coefficient) s2iof a single ray reflected by the grating120a,120b,120cand120dis R1, T1·R2, T1·T2·R3and T1·T2·T3·R4, respectively. The closer these four values are, the more uniform the energy of the light received by human eyes. The preferred values of R1, R2, R3and R4may be obtained by calculation.

Optionally, the grating20/120is constructed as a volume holographic grating. As shown inFIGS.5and6, the volume holographic grating has an equal refractive index fringe surface21/121, wherein a reflecting mirror surface is the equal refractive index fringe surface21/121. Therefore, the control of the light field may be realized through the periodic refractive index modulation inside the volume holographic grating.

In the present application, as shown inFIG.1, in the embodiment where the reflected light of the grating20is coupled out from the waveguide substrate10, the grating20is referred to as a reflective volume holographic grating20. As shown inFIG.2, in the embodiment where the diffracted light of the grating120is coupled out from the waveguide substrate110, the grating120is referred to as a transmissive volume holographic grating120.

The key parameters of the volume holographic grating20/120include period Λ, inclination angle ϕ, thickness d, and amplitude of refractive index modulation n1, which need to satisfy the Bragg reproduction condition 2Λ cos(ϕ−θ)=m. Wherein the inclination angle ϕ is the included angle between the grating thickness direction z (seeFIGS.5and6) and the grating vector K, the period κ and the inclination angle ϕ mainly determine the diffraction direction of the light beam, while the thickness d and the amplitude of refractive index modulation n1mainly affect the diffraction deflection efficiency ηT(or reflection efficiency ηR) and the diffraction bandwidth (or reflection bandwidth).

For the reflective volume holographic grating20, according to the Kogelnik coupled wave theory, the reflection efficiency ηRmay be calculated according to formula (3) under the Bragg condition:
ηR=tan2v(3)wherein v is the coupling strength.

In the first embodiment, the product of the amplitude of refractive index modulation n1and the thickness d determines the reflection efficiency ηRunder the Bragg condition, and the reflection efficiency ηRchanges periodically with the product. The smaller the thickness d is, the wider the angular spectrum width is. In the case of the same thickness d, the reflection efficiency ηRmay be adjusted by adjusting n1.

For the transmissive volume holographic grating120, according to the Kogelnik coupled wave theory, the diffraction deflection efficiency ηTmay be calculated according to formula (4) under the Bragg condition:
ηT=sin2v(4)wherein v is the coupling strength.

In the second embodiment, the product of the amplitude of refractive index modulation n1and the thickness d determines the diffraction deflection efficiency ηTunder the Bragg condition, and the diffraction deflection efficiency ηTchanges periodically with the product. The smaller the thickness d is, the wider the angular spectrum width is. In the case of the same thickness d, the diffraction deflection efficiency ηTmay be adjusted by adjusting n1.

Optionally, the volume holographic grating20/120has a thickness d of 1 to 20 microns, and/or the volume holographic grating20/120has an amplitude of refractive index modulation n1of 0.01 to 0.2.

Optionally, the included angle between the volume holographic grating20and the first surface11or the second surface12is 20° to 30°, and/or the volume holographic grating20has a grating period A of 2000 to 7000 lp/mm.

Optionally, the included angle between the volume holographic grating120and the first surface111or the second surface112is 20° to 30°, and/or the volume holographic grating120has a grating period A of 2000 to 7000 lp/mm.

Optionally, the thickness direction of the volume holographic grating20is perpendicular to the equal refractive index fringe surface21. As shown inFIG.5, the inclination angle ϕ is 0°.

Optionally, the thickness direction of the volume holographic grating120is parallel to the equal refractive index fringe surface121. As shown inFIG.6, the inclination angle ϕ is 90°.

In some embodiments, the thickness direction of the volume holographic grating20may not be perpendicular to the equal refractive index fringe surface20. In some embodiments, the thickness direction of the volume holographic grating120may not be parallel to the equal refractive index fringe surface121.

Optionally, the volume holographic grating20/120includes a photosensitive material with an average refractive index n0of 1.5 to 2.0. The photosensitive material may be selected from, for example, photopolymer, polymer dispersed liquid crystal (PDLC), and the like. In the case of the same optical-mechanical FOV, the selection of a waveguide material with larger refractive index is beneficial to improve the efficiency uniformity in the field of view. Due to the existence of the refraction effect, within the same angular variation range (FOVout) of the light D1 or S1, the larger the waveguide refractive index is, the smaller the angle range of the incidence angle (FOVin) corresponding to the grating20/120in the waveguide is. That is, it is beneficial to realize the condition that the incidence angle θ of the grating20/120is less than the first angle θ1 and greater than the fourth angle θ4.

Optionally, each of the volume holographic gratings20/120may be configured to have a non-uniformly distributed amplitude of refractive index modulation n1.

Further, each of the volume holographic gratings20/120is configured such that the amplitude of refractive index modulation n1at both ends is less than that in the middle part.

Further, the volume holographic grating20/120is constructed as an apodized volume holographic grating.

Optionally, each of the volume holographic gratings20/120is configured to have the refractive index modulation distribution n(x,z) calculated according to the following formula (5):
n(x,z)=n0+n1(x,z)·cos[|K≡(xsin ϕ+zcos ϕ)]  (5)wherein the value of n1is sinc-shaped apodization, which is calculated as formula (6):
n1(x,z)=nm·sinc[2(z−d/2)/d](6)or, the value of n1is Gaussian-shaped apodization, which is calculated as formula (7):
n1(x,z)=nm·exp[−(z−d/2)2]  (7)wherein x is a coordinate along the direction of a surface of the volume holographic grating20/120, z is a coordinate along a thickness direction of the volume holographic grating20/120, n0is an average refractive index of material of the volume30holographic grating20/120, nm is a preset constant, ϕ is an inclination angle of the volume holographic grating20/120,is a grating vector of the volume holographic grating20/120determined according to the Bragg condition, and the equal refractive index fringe surface is perpendicular to the grating vector.

Therefore, the optical path of the incident light D2 passing through the reflective volume holographic grating20configured as an apodized volume holographic grating is shown inFIGS.7(a) and7(b). As shown inFIG.7(a), when incident at a small angle (e.g., less than or equal to the second angle θ2 and greater than or equal to the third angle θ3), part of the light D4 is coupled out from the waveguide substrate10after being reflected by the grating20, and part of the light D3 is incident on the next grating20after being transmitted by the grating20. As shown inFIG.7(b), when incident at a large angle (e.g., greater than the second angle θ2), the transmittance of the grating20to the incident light D2 is high, which reduces reflected stray light, and may improve waveguide efficiency and signal-to-noise ratio.

The optical path of the incident light S2 passing through the transmissive volume holographic grating120configured as an apodized volume holographic grating is shown inFIGS.8(a) and8(b). When incident at a small angle (e.g., less than or equal to the second angle θ2 and greater than or equal to the third angle θ3), part of the light S4 is coupled out from the waveguide substrate110after being diffractively deflected by the grating120, and part of the light S3 is incident on the next grating120after being transmitted by the grating120or totally reflected within the waveguide substrate110(as shown inFIG.8(a)). When incident at a large angle (e.g., greater than the second angle θ2), the transmittance of the grating120to the incident light S2 is high (as shown inFIG.8(b)), which reduces diffracted stray light, and may improve waveguide efficiency and signal-to-noise ratio.

Optionally, each of the volume holographic gratings20/120may also be configured to have a uniformly distributed amplitude of refractive index modulation n1.

Further, each of the volume holographic gratings20/120is configured to have the refractive index modulation distribution n(x,z) of the following formula (8):
n(x,z)=n0+n1·cos[|(xsin ϕ+zcos ϕ)]  (8)wherein x is a coordinate along the direction of a surface of the volume holographic grating20/120, z is a coordinate along a thickness direction of the volume holographic grating20/120, n0is an average refractive index of material of the volume holographic grating20/120, n1is an amplitude of refractive index modulation of the volume holographic grating20/120, ϕ is an inclination angle of the volume holographic grating20/120,is a grating vector of the volume holographic grating20/120. The grating vectoris determined according to the Bragg condition, and the equal refractive index fringe surface is perpendicular to the grating vector.

Optionally, the optical waveguide100includes a plurality of the volume holographic gratings20/120with the same grating vector, wherein the plurality of the volume holographic gratings20/120have different thicknesses d and/or have different amplitudes of refractive index modulation n1so as to improve the consistency of reflection efficiency (or diffraction deflection efficiency) within the target angle range.

When the working wavelength (incident light wavelength) λ and the angle of incident light are determined, the period A and the inclination angle θ of the volume holographic grating20/120may be determined according to the K-vector circle method.

For example, Table 1 gives a specific example of the relevant parameters of the optical waveguide100including the reflective volume holographic grating20according to the above. Wherein the incidence angle of light D1 entering the waveguide substrate10on the first surface11of the waveguide substrate10is 45°, the included angle α between the grating20and the second surface12of the waveguide substrate10is set to 22.5°, and the number of the grating20is set to four.

Table 1 Example table of parameters of reflective volume holographic grating20

In the embodiment shown in Table 1, the wavelength of the incident light is 530 nm, the reflective volume holographic grating20is configured such that the thickness direction is perpendicular to the equal refractive index fringe surface, the thickness d is 2 μm, and the period is 2455 lp/mm. The gratings20a,20b,20cand20dhave the amplitudes of refractive index modulation n1of 0.04, 0.048, 0.06 and 0.1, respectively. The gratings20a,20b,20cand20dall have the maximum reflection efficiency when the incident light has an angle of 22.5°, and the maximum reflection efficiency increases sequentially.

As another example, Table 2 shows the parameters of a specific example of an optical waveguide200including the transmissive volume holographic grating120. Wherein the incidence angle of light S1 entering the waveguide substrate110on the first surface111of the waveguide substrate110is 45°, the included angle α between the grating120and the second surface of the waveguide substrate is 22.5°, and the number of gratings120is set to four.

Table 2 Example table of parameters of transmissive volume holographic grating120

In the embodiment shown in Table 2, the wavelength of the incident light is 530 nm, the transmissive volume holographic grating120is configured such that the thickness direction is parallel to the equal refractive index fringe surface, the thickness d is 2 μm, and the period is 5927 lp/mm. The gratings120a,120b,120c, and120dhave the amplitudes of refractive index modulation n1of 0.04, 0.05, 0.068 and 0.15, respectively. The gratings120a,120b,120cand120dall have the maximum diffraction deflection efficiency when the incident light has an angle of 22.5°, and the maximum diffraction deflection efficiency increases sequentially.

In another embodiment, the grating20/120may be constructed as a sub-wavelength grating50. The reflection efficiency of the sub-wavelength grating may be controlled by adjusting the parameters such as the period, duty cycle, thickness, and refractive index of the material. As shown inFIGS.9(a) and9(b), the sub-wavelength grating50is configured, for example, with a one-dimensional or two-dimensional hole structure51(or a columnar structure), and the grating material may be a dielectric material or a metal material.

The optical waveguide of the present application may realize the semi-reflective and semi-transparent functions of the multilayer film mirror surface by the grating provided in the waveguide substrate, which is simple and flexible design, and easy to process. By setting the period, inclination angle, amplitude of refractive index modulation, and thickness parameters of the grating, the reflection efficiency and reflection bandwidth, or the diffraction efficiency and diffraction bandwidth of the grating may be adjusted to achieve the effect of less stray light, uniform energy distribution and large FOV.

On the other hand, the present application further provides a display device.

As shown inFIG.10, in the first embodiment, a display device110includes an optical machine30and the optical waveguide100of the first embodiment described in the first aspect. Wherein the optical machine30is used for emitting light beam used as the incident light of the optical waveguide100. Optionally, the optical machine30may project image light to the optical waveguide100, and the light emitted by the optical machine30is visible light. The light (e.g., image light) projected by the optical machine30onto the optical waveguide100is coupled out from the waveguide substrate10and enters the eyes of viewer after total internal reflection of the waveguide substrate10and reflection of the grating20, thereby enabling the viewer to see the image projected by the optical machine30.

As shown inFIG.11, in the second embodiment, a display device210includes an optical machine30and the optical waveguide200of the second embodiment described in the first aspect. Wherein the optical machine30is used for emitting light beam used as the incident light of the optical waveguide200. Optionally, the optical machine30may project image light to the optical waveguide200, and the light emitted by the optical machine30is visible light. The light (e.g., image light) projected by the optical machine30onto the optical waveguide200is coupled out from the waveguide substrate10and enters the eyes of viewer after total internal reflection of the waveguide substrate10and diffraction of the grating120, thereby enabling the viewer to see the image projected by the optical machine30.

Optionally, as shown inFIG.10, the display device110further includes a reflector40for reflecting the light beam emitted by the optical machine30and causing the light beam reflected by the reflector40to propagate within the waveguide substrate10through total internal reflection. Other display devices provided in the present application may also include a reflector40.

The display device provided in the present application may be any device including the aforementioned optical waveguide100/200. For example, the display device is an augmented reality display device or a virtual reality display device, wherein the augmented reality display device includes but is not limited to the devices such as augmented reality (AR) glasses, an automotive head-up display (HUD), or the like. For example, the display device is an optical pupil expansion device.

For example, as shown inFIG.12, the display device may be a near-eye display device300that may include a lens310and a frame320. Wherein the lens includes the optical waveguide100/200described in the first aspect, and the frame320is used to hold the lens310close to eyes. The optical machine330of the near-eye display device300may project an image onto the optical waveguide100/200.

It should be understood that the display device according to the present application includes all features and effects of the optical waveguide according to the present application.

The processes and steps described above in all preferred embodiments are only examples. Unless an adverse effect occurs, the various processing operations may be performed in a different order than the one described above. The sequence of steps in the above processes may also be added, combined or subtracted according to actual needs.

In understanding the scope of the present application, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. This concept also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives.

The term “attached” or “attaching” as used herein includes: configuration(s) in which an element is directly fixed to another element by fixing the element directly to another element; configuration(s) in which element is indirectly fixed to another element by fixing the element to an intermediate member(s) which in turn are fixed to another element; and configuration(s) in which one element is integral with another element, that is, one element is essentially a part of another element. This definition also applies to words having similar meanings such as “connected”, “joined”, “coupled”, “mounted”, “glued”, and “fixed” and their derivatives. Finally, terms of degree such as “substantially”, “about” and “approximately” as used herein mean an amount of deviation of the modified term such that the end result is not significantly changed.

Unless otherwise defined, the technical and scientific terms used herein have the same meanings as commonly understood by those skilled in the technical field of the present application. The terms used herein are only for the purpose of describing specific implementation, and are not intended to limit the present application. Feature(s) described in one embodiment herein may be applied to another embodiment alone or in combination with other feature(s), unless the feature(s) are not applicable in the other embodiment or otherwise stated.

The present application has been described through the above-mentioned embodiments, but it should be understood that the above-mentioned embodiments are only for the purpose of illustration and description, and are not intended to limit the present application to the scope of the described embodiments. Moreover, those skilled in the art could understand that the present application is not limited to the above-mentioned embodiments. More variations and modifications may also be made according to the teachings of the present application, and these variations and modifications fall within the protection scope claimed by the present application.