CN113589524A - Design method of holographic grating optical waveguide planar light-gathering system for LiFi communication - Google Patents

Design method of holographic grating optical waveguide planar light-gathering system for LiFi communication Download PDF

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CN113589524A
CN113589524A CN202111168677.2A CN202111168677A CN113589524A CN 113589524 A CN113589524 A CN 113589524A CN 202111168677 A CN202111168677 A CN 202111168677A CN 113589524 A CN113589524 A CN 113589524A
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holographic grating
optical waveguide
light
grating
holographic
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CN113589524B (en
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吴之旭
唐荣欣
夏勇
王玉暤
赵建波
汤文超
邓辉
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Nanchang University
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B27/00—Optical systems or apparatus not provided for by any of the groups G02B1/00 - G02B26/00, G02B30/00
    • G02B27/0012—Optical design, e.g. procedures, algorithms, optimisation routines
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B19/00—Condensers, e.g. light collectors or similar non-imaging optics
    • G02B19/0004—Condensers, e.g. light collectors or similar non-imaging optics characterised by the optical means employed
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B6/00—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings
    • G02B6/10—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type
    • G02B6/12—Light guides; Structural details of arrangements comprising light guides and other optical elements, e.g. couplings of the optical waveguide type of the integrated circuit kind
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Abstract

The invention discloses a design method of a holographic grating optical waveguide plane condensing system facing LiFi communication, which condenses and deflects incident light through a holographic grating and folds the light path of diffracted light of the holographic grating through an optical waveguide; the method comprises the following steps: s1, selecting different integration modes between the holographic grating and the visible light detector according to the requirements of the plane light-gathering system; s2 designing a holographic grating visible light convergence deflection initial system; s3 designing a holographic grating optical waveguide condensing system; s4, solving the propagation vector and grating period constant of the emergent light of the holographic grating through light tracing; s5 calculating the diffraction efficiency of the holographic grating; s6 records holographic grating and test. The invention replaces the traditional refraction/reflection type light gathering function by the low-cost holographic grating, deflects the incident light and realizes the coupling with the optical waveguide; the ultrathin design of the visible light communication condensing system is realized by repeatedly folding the optical waveguide to the optical path; and the use efficiency of the detector is improved.

Description

Design method of holographic grating optical waveguide planar light-gathering system for LiFi communication
Technical Field
The invention belongs to the technical field of optical system light condensation, and particularly relates to a design method of a holographic grating optical waveguide planar light condensation system for LiFi communication.
Background
Visible light communication (LiFi), a full network two-way wireless solution based on visible light, has been identified as an important component of the future 6G, and is expected to reach 80 billion dollars in the market size of LiFi by 2030. The visible light communication can provide safe high-speed connection for application scenes such as indoor internet surfing, vehicle and underwater communication, and the like, and the technology can simultaneously take advantages of low energy consumption and low cost into account because the wireless connection is provided by utilizing the lighting infrastructure. However, the capacity of the visible light communication system is easily affected by the link blockage and random receiving direction, so that the interference resistance is poor and the error rate is high: first, visible light communication utilizes the infrastructure of the lighting system, the primary function of which is lighting, so the illumination intensity must be within the user's acceptance for lighting; secondly, if there is corresponding stray light (such as sunlight, flashlight, etc.) in the environment, the signal-to-noise ratio of the signal received by the detector is very low, and the error rate is high.
By adding the light condensing system, the illumination intensity received by the detection chip can be improved, the signal-to-noise ratio of the detector can be effectively improved, and the error rate is reduced. The current light condensing systems of visible light communication detectors mainly include refractive light condensing systems and reflective light condensing systems. Wherein the refraction type system utilizes a conventional lens, a Fresnel lens or a super surface system to converge illumination within the aperture range of the lens on a detection chip; reflective systems typically use spherical or aspherical mirrors to focus the light intensity impinging on the mirrors onto a detection chip. The two types of detection and light-gathering systems are usually large in size and difficult to form a planar integrated design with a chip; and as the condensing area increases, the three-dimensional size of the condensing system also increases rapidly. The invention provides a light condensing system based on a holographic grating and an optical waveguide, which can converge large-area light intensity in an ultrathin plane and is very easy to integrate with an optical detection chip; the system can protect the waveguide layer covering the surface of the detector, so that the service life of detection is effectively prolonged; third, the system is low cost.
Disclosure of Invention
Aiming at the defects and difficulties in the prior art, the invention aims to provide a holographic grating optical waveguide plane light-gathering system facing visible light communication.
A design method of a holographic grating optical waveguide planar light condensing system for LiFi communication is provided, wherein the planar light condensing system comprises a holographic grating, an optical waveguide and a visible light detector, and the holographic grating and the visible light detector are integrated on the optical waveguide; the holographic grating is formed by exposing two beams of structural light beams on a photosensitive material film and then developing, and is used for condensing and deflecting incident light; the optical waveguide folds the light path of the diffraction light of the holographic grating, and adopts a flat optical waveguide made of glass and PMMA materials; when the diffraction light of the holographic grating is coupled into the optical waveguide, the coupling angle needs to be larger than the total reflection angle of the optical waveguide.
The recording system of the holographic grating consists of two beams of structural beams, one beam of structural beam is called as a reference beam, and the incident direction of the reference beam is vertical to the photosensitive material; the other beam of the structured light beam is called an object light beam, the included angle between the incident direction of the object light beam and the photosensitive material is larger than the total reflection angle of the optical waveguide, and the incident light beam has focal power.
The design method comprises the following steps:
s1, selecting different integration schemes between the holographic grating and the visible light detector according to the requirements of the plane light-gathering system;
different integration modes between the holographic grating and the visible light detector are realized through the shape and the size of the holographic grating and the spatial position relation between the holographic grating and the visible light detector, and the integration scheme comprises three types: (1) the visible light detector is positioned on the side surface of the optical waveguide, and the planar holographic grating is integrated on the flat optical waveguide; (2) the planar holographic grating and the visible light detector are positioned on the same plane of the flat optical waveguide; (3) the annular holographic grating and the visible light detector are positioned on the same plane of the flat optical waveguide, and the visible light detector is positioned at the circle center of the annular holographic grating.
S2, designing a holographic grating visible light convergence deflection initial system;
and calculating an initial system without the optical waveguide for folding the light according to the spatial position and the light-gathering area of the holographic grating in the integration scheme of the step S1 and the spatial position of the visible light detector.
The center position coordinates of the visible light detector are
Figure 100002_DEST_PATH_IMAGE001
Wherein, in the step (A),
Figure DEST_PATH_IMAGE002
is the abscissa of the position where the visible light detector is placed,
Figure 100002_DEST_PATH_IMAGE003
is the thickness of the optical waveguide;
the holographic grating converges and deflects the incident light in a collimation state
Figure DEST_PATH_IMAGE004
The point is that, among others,
Figure 100002_DEST_PATH_IMAGE005
the number of times of total reflection of the light on the optical waveguide interface is represented by a positive integer;
after incident light is reflected, converged and deflected by the holographic grating, the deflection angles of the edge and the center are respectively
Figure DEST_PATH_IMAGE006
，
Figure 100002_DEST_PATH_IMAGE007
，
Figure DEST_PATH_IMAGE008
The deflection angle is equal to the incident angle of the light incident on the optical waveguide interface, and the smallest incident angle is used for realizing the most efficient optical coupling
Figure 93121DEST_PATH_IMAGE008
The total emission angle of the upper surface and the lower surface of the optical waveguide is required to be larger than
Figure 100002_DEST_PATH_IMAGE009
Wherein
Figure DEST_PATH_IMAGE010
，
Figure 100002_DEST_PATH_IMAGE011
Is the refractive index of air and is,
Figure DEST_PATH_IMAGE012
is the refractive index of the optical waveguide material.
S3, designing a holographic grating optical waveguide light condensing system;
establishing a holographic grating optical waveguide simulation model in ZEMAX according to the initial system designed in the step S2;
s4, solving the propagation vector and grating period constant of the emergent light of the holographic grating through light tracing;
Figure 100002_DEST_PATH_IMAGE013
wherein the content of the first and second substances,
Figure DEST_PATH_IMAGE014
is the incident and diffracted light propagation vectors of the structured beam,
Figure 100002_DEST_PATH_IMAGE015
is the construction beam wavelength;
Figure DEST_PATH_IMAGE016
incident and diffracted light of the reconstructed beamThe vector of the propagation is then transmitted,
Figure DEST_PATH_IMAGE017
is the reconstruction beam wavelength;
Figure DEST_PATH_IMAGE018
is the grating vector perpendicular to the interference fringes of the holographic grating, and has a size of
Figure 100002_DEST_PATH_IMAGE019
Wherein
Figure DEST_PATH_IMAGE020
Is the grating period constant;
to achieve a high diffraction efficiency, the wavelength is
Figure 100002_DEST_PATH_IMAGE021
Constructed beam and wavelength of
Figure DEST_PATH_IMAGE022
According to the Bragg matching condition, the grating period constants of the grating in the x direction and the z direction can be deduced:
Figure 100002_DEST_PATH_IMAGE023
，
Figure DEST_PATH_IMAGE024
wherein
Figure 100002_DEST_PATH_IMAGE025
And
Figure DEST_PATH_IMAGE026
is the angle of the reference beam and the object beam in the constructed beam,
Figure 100002_DEST_PATH_IMAGE027
is the refractive index of the diffraction grating material,
Figure DEST_PATH_IMAGE028
are the wavelengths of the two constructed beams.
S5, calculating the diffraction efficiency of the holographic grating;
the diffraction efficiency calculation formula of the holographic grating is as follows:
Figure 100002_DEST_PATH_IMAGE029
in the formula (I), the compound is shown in the specification,
Figure DEST_PATH_IMAGE030
in order to be the diffraction efficiency of the holographic grating,
Figure 100002_DEST_PATH_IMAGE031
is the refractive index modulation of the holographic grating,
Figure DEST_PATH_IMAGE032
is the thickness of the holographic grating.
S6 recording holographic grating and testing
Laser is divided into object beam and reference beam by beam splitter, and is adjusted by adjusting reflector M1And a mirror M2Obtaining the reference beam angle satisfying the design, and adjusting the reflector M3Obtaining an object beam angle satisfying the design;
the prism system consists of two identical triangular prisms, a fixed interval is reserved between the two triangular prisms and used for placing photosensitive materials, and the prism system is fixedly installed through a support for 3D printing;
cutting glass or PMMA material according to the size of the optical waveguide designed in the step S1 to form a flat waveguide; then, adhering holographic grating photosensitive materials according to the size and the position, and placing the holographic grating photosensitive materials in a fixed gap between the coupling prisms; opening laser, forming interference fringes on the photosensitive material by the object beam and the reference beam, and recording a volume holographic grating; and testing the diffraction efficiency of the volume holographic grating, and comparing the diffraction efficiency with a simulation result.
Furthermore, the photosensitive material adopts dichromated gelatin and silver halide.
Compared with the traditional refraction/reflection type light condensation technology, the invention has the beneficial effects that:
(1) the light gathering function is realized by the holographic grating with low cost, and the method has the advantages of extremely low cost and quick production;
(2) the optical waveguide can realize the miniaturization and the planarization of the light condensing system through multiple folding of the optical path, thereby being easy to integrate with a visible light detection chip;
(3) the optical waveguide can cover the surface of the detector to protect the detector, so that the service life of the detector is prolonged.
Drawings
FIG. 1 is a schematic cross-sectional view of a side integrated light collection system;
FIG. 2 is a 3D schematic of a side integrated light collection system;
FIG. 3 is a schematic cross-sectional view of a coplanar integrated light collection system;
FIG. 4 is a 3D schematic of a coplanar integrated light collection system;
FIG. 5 is a schematic cross-sectional view of an annular integrated light collection system;
FIG. 6 is a 3D schematic of an annular integrated light collection system;
FIG. 7 is a schematic diagram of an initial system for deflecting and converging incident light rays by a holographic grating;
FIG. 8 is a cross-sectional view of a light ray trace of a holographic grating optical waveguide condensing system;
FIG. 9 is a 3D view of a light ray trace of a holographic grating optical waveguide condensing system;
FIG. 10 is a schematic diagram of a holographic grating Bragg condition;
FIG. 11 is a graph of diffraction efficiency of a holographic grating at different deflection angles;
FIG. 12 is a diagram of an optical path of a holographic grating recording system;
fig. 13 is a comparison of the simulation result of the diffraction efficiency of the holographic grating with the experimental result.
Illustration of the drawings: 1-holographic grating; 2-an optical waveguide; 3-a visible light detector; 4-an object beam; 5-a reference beam; 6-a beam splitter; 7-mirror M1(ii) a 8-mirror M2(ii) a 9-mirror M3(ii) a 10-a prism; 11-grating to be recorded.
Detailed Description
The invention will be further described with reference to the accompanying drawings.
The invention discloses a plane light-gathering system based on holographic grating optical waveguide, which is developed for a visible light communication system, and incident light is converged and deflected through a holographic grating; the light is folded through the optical waveguide to realize a plane light condensing system.
The specific design scheme is as follows:
step one, selecting different integration schemes of a light condensing system and a visible light communication detector according to a plane light condensing system.
The holographic grating is provided with a reflection type holographic grating and a projection type holographic grating, and the reflection type holographic grating can realize high diffraction efficiency in a wider wave band, and the optical waveguide covers the surface of the holographic optical waveguide and has the function of protecting the holographic grating, so that the reflection type holographic grating is preferentially adopted to deflect and converge incident light; it is also within the scope of the present application to preferably deflect and concentrate incident light rays with a projective holographic grating.
The light condensing system based on the holographic grating optical waveguide is a planar structure and has a multiple integration mode with a visible light detector, and 3 types of integration schemes are listed in the scheme and are respectively shown in figures 1-6.
Fig. 1 and fig. 2 are a schematic cross-sectional view and a schematic 3D view of a side-integrated light condensing system, respectively, that is, a visible light detector 3 is located on a side surface of an optical waveguide 2, and a planar holographic grating 1 is integrated on the optical waveguide 2 of a flat plate;
fig. 3 and 4 are a schematic cross-sectional view and a schematic 3D view of a coplanar integrated light-gathering system, respectively, that is, the planar holographic grating 1 and the visible light detector 3 are located on the same plane of the flat optical waveguide 2;
fig. 5 and fig. 6 are a schematic cross-sectional view and a schematic 3D view of the annular integrated light condensing system, respectively, that is, the annular holographic grating 1 and the visible light detector 3 are located on the same plane of the flat optical waveguide 2, and the visible light detector 3 is located at the center of the annular holographic grating 1.
Designing a holographic grating visible light convergence deflection initial structure.
And calculating an initial system without the light waveguide folding light according to the spatial position and the light condensing area of the holographic grating in the integration scheme of the first step and the spatial position of the visible light detector. As shown in FIG. 7, the center position of the visible light detector is coordinated with
Figure 100002_DEST_PATH_IMAGE033
Wherein
Figure DEST_PATH_IMAGE034
Is the abscissa of the position where the visible light detector is placed,
Figure 100002_DEST_PATH_IMAGE035
is the thickness of the optical waveguide; the holographic grating converges and deflects the incident light in a collimation state
Figure DEST_PATH_IMAGE036
Points of which
Figure DEST_PATH_IMAGE037
The number of times of total reflection of the light on the optical waveguide interface is represented by a positive integer; after incident light is reflected, converged and deflected by the holographic grating, the deflection angles of the edge and the center are respectively
Figure DEST_PATH_IMAGE038
，
Figure DEST_PATH_IMAGE039
，
Figure DEST_PATH_IMAGE040
The deflection angle is equal to the incident angle of the light incident on the optical waveguide interface, and the smallest incident angle is used for realizing the most efficient optical coupling
Figure 210594DEST_PATH_IMAGE040
The total emission angle of the upper surface and the lower surface of the optical waveguide is required to be larger than
Figure DEST_PATH_IMAGE041
Wherein
Figure DEST_PATH_IMAGE042
，
Figure DEST_PATH_IMAGE043
Is the refractive index of air and is,
Figure DEST_PATH_IMAGE044
is the refractive index of the optical waveguide material.
And step three, designing a holographic grating optical waveguide condensing system.
According to the initial system in step two, a holographic grating optical waveguide simulation model is built in ZEMAX (simulated ray traces are shown in fig. 8 and 9). Adding holographic grating in sequence mode, can use ZEMAX to embed any one of 4 holographic grating surfaces: hologrm 1, Hologrm 2, Toroidal Hologrm, Optically Fabried Hologrm. In the scheme, the Hologram 2 is used, and the corresponding parameters are as follows:
basic parameters Construct X1, Y1, Z1, X2, Y2, Z2, Construct Wave: the source position (X1, Y1, Z1) of the structured light beam I; the source position (X2, Y2, Z2) of the structured light beam II; constructing the wavelength Construct Wave of the light beam;
diffraction order, when the value is 0, the tracing is 0 order diffraction light; 1 st order diffracted light when the value is 1;
defining whether the surface type is a Volume holographic grating or a thin holographic grating, wherein the value of 0 indicates the Volume holographic grating, and the value of 1 indicates the Volume holographic grating;
hologram Thickness, the Thickness of the volume holographic grating, this Thickness is the fictitious Thickness, is only used for calculating the diffraction efficiency;
n1 and n2: n1 are refractive indices of materials in which the object beam of the construction beam is located before entering the volume holographic grating; n2 is the refractive index of the material in which the reference beam of the construction beam is located before entering the volume holographic grating;
n is the average refractive index of the volume holographic grating photosensitive material;
dn is the modulation range of the volume holographic grating refractive index;
the Shrinkage is that at the time of processing, the volume holographic grating can expand or contract, thereby changing the thickness of the volume holographic grating; at a value of 0, there is no shrinkage; if not 0, a thickness scaling value is indicated, e.g., 0.98 indicates a shrinkage of 2%;
index Shift is the change in the average refractive Index of the volume holographic grating after development;
consider Fresnel if set to 1, then Fresnel losses are taken into account, and set to 0, then Fresnel losses are not taken into account.
And step four, solving the propagation vector and the grating period constant of the emergent light of the holographic grating through light tracing.
Figure DEST_PATH_IMAGE045
Wherein
Figure DEST_PATH_IMAGE046
Is the incident and diffracted light propagation vectors of the structured beam,
Figure DEST_PATH_IMAGE047
is the construction beam wavelength;
Figure DEST_PATH_IMAGE048
is the incident and diffracted light propagation vectors of the reconstructed beam,
Figure DEST_PATH_IMAGE049
is the reconstruction beam wavelength;
Figure DEST_PATH_IMAGE050
is the grating vector perpendicular to the interference fringes of the holographic grating, and has a size of
Figure DEST_PATH_IMAGE051
Wherein
Figure DEST_PATH_IMAGE052
Is the grating period constant. Reconstruction beams are a general term in holographic gratings. The holographic grating is formed by two beams of light beamsAfter the recording is finished, one beam of light which is the same as the structural beam is irradiated, and a beam which is similar to the other structural beam is obtained, wherein the irradiated beam is called a reconstruction beam.
To achieve a high diffraction efficiency, the wavelength is
Figure 915899DEST_PATH_IMAGE047
Constructed beam and wavelength of
Figure 514370DEST_PATH_IMAGE049
The reconstructed beam of (2) is subject to the Bragg condition, as shown in FIG. 10K x AndK z is thatKThe components in the x and z directions.
According to the Bragg matching condition, the grating period constants of the grating in the x direction and the z direction can be deduced:
Figure DEST_PATH_IMAGE053
，
Figure 266426DEST_PATH_IMAGE024
wherein the content of the first and second substances,
Figure DEST_PATH_IMAGE054
and
Figure DEST_PATH_IMAGE055
is the angle of the reference beam and the object beam in the constructed beam,
Figure DEST_PATH_IMAGE056
is the refractive index of the diffraction grating material,
Figure DEST_PATH_IMAGE057
are the wavelengths of the two constructed beams.
And step five, calculating the diffraction efficiency of the holographic grating.
The diffraction efficiency calculation formula of the holographic grating is as follows:
Figure DEST_PATH_IMAGE058
in the formula (I), the compound is shown in the specification,
Figure DEST_PATH_IMAGE059
in order to be the diffraction efficiency of the holographic grating,
Figure DEST_PATH_IMAGE060
is the refractive index modulation of the holographic grating,
Figure DEST_PATH_IMAGE061
is the thickness of the holographic grating, the calculated diffraction efficiency of the holographic grating is shown in fig. 11.
And step six, recording the holographic grating and testing.
The holographic grating recording system consists of two beams of structured light, wherein one beam of structured light is called as reference light, and the incident direction of the reference light is vertical to the photosensitive material; the other beam of the structured light beam is called an object light beam, the included angle between the incident direction of the object light beam and the photosensitive material is larger than the total reflection angle of the optical waveguide, and the incident light beam has focal power.
The holographic grating recording system uses the design shown in fig. 12, and the laser is split into an object beam 4 and a reference beam 5 by a beam splitter. By adjusting the mirror M 17 and a mirror M 28 obtaining the reference beam angle satisfying the design, and adjusting the reflector M 39 the angle of the object beam 4 is obtained which satisfies the design.
The prism system is composed of two identical triangular prisms 10, a fixed interval is reserved between the two triangular prisms 10 and used for placing photosensitive materials of the grating 11 to be recorded, and the prism system is installed and fixed through a support for 3D printing.
Cutting glass or PMMA material according to the size of the optical waveguide designed in the first step to form a flat optical waveguide; then, adhering the holographic grating photosensitive material according to the shape, size and position of the holographic grating in the step one, and placing the holographic grating photosensitive material in a fixed gap between the coupling prisms; opening laser, forming interference fringes on the photosensitive material by the object beam and the reference beam, recording the volume holographic grating, and recording the effect of a narrow-band and wide-band holographic grating optical waveguide condensing system; the diffraction efficiency of the volume reflection grating was measured and compared with the simulation results, which are shown in fig. 13.
The foregoing merely represents preferred embodiments of the invention, which are described in some detail and detail, and therefore should not be construed as limiting the scope of the invention. It should be noted that, for those skilled in the art, various changes, modifications and substitutions can be made without departing from the spirit of the present invention, and these are all within the scope of the present invention. Therefore, the protection scope of the present patent shall be subject to the appended claims.

Claims (8)

1. The design method of the holographic grating optical waveguide plane light gathering system facing LiFi communication is characterized in that: the planar light condensing system comprises a holographic grating, an optical waveguide and a visible light detector, wherein the holographic grating and the visible light detector are integrated on the optical waveguide; the holographic grating is formed by exposing two beams of structural beams on a photosensitive material film and then developing, wherein one beam of structural beam is called a reference beam, the incident direction of the reference beam is vertical to the photosensitive material, the other beam of structural beam is called an object beam, the included angle between the incident direction of the object beam and the photosensitive material is larger than the total reflection angle of the optical waveguide, and the incident beam has focal power; the holographic grating condenses and deflects the incident light; the optical waveguide folds the light path of the diffraction light of the holographic grating; when the diffraction light of the holographic grating is coupled into the optical waveguide, the coupling angle is larger than the total reflection angle of the optical waveguide;
the design method comprises the following steps:
s1, selecting different integration schemes between the holographic grating and the visible light detector according to the requirements of the plane light-gathering system;
s2, designing a holographic grating visible light convergence deflection initial system: calculating an initial system without optical waveguide folding light according to the spatial position and the light-gathering area of the holographic grating in the integration scheme of the step S1 and combining the spatial position of the visible light detector;
s3, designing a holographic grating optical waveguide condensing system: establishing a holographic grating optical waveguide simulation model in ZEMAX according to the initial system designed in the step S2;
s4, solving the propagation vector and grating period constant of the emergent light of the holographic grating through light tracing;
s5, calculating the diffraction efficiency of the holographic grating;
and S6, recording the holographic grating and testing.
2. The design method of the holographic grating optical waveguide planar light gathering system facing LiFi communication in claim 1, wherein: the optical waveguide is a flat optical waveguide made of glass and PMMA materials.
3. The design method of the holographic grating optical waveguide planar light gathering system facing LiFi communication in claim 2, wherein: the integration scheme between the holographic grating and the visible light detector in the step S1 includes three types: (1) the visible light detector is positioned on the side surface of the optical waveguide, and the planar holographic grating is integrated on the flat optical waveguide; (2) the planar holographic grating and the visible light detector are positioned on the same plane of the flat optical waveguide; (3) the annular holographic grating and the visible light detector are positioned on the same plane of the flat optical waveguide, and the visible light detector is positioned at the circle center of the annular holographic grating.
4. The design method of the holographic grating optical waveguide planar light gathering system facing LiFi communication in claim 3, wherein: the coordinates of the center position of the visible light detector of the initial system in step S2 are
Figure DEST_PATH_IMAGE001
Wherein, in the step (A),
Figure 298819DEST_PATH_IMAGE002
is the abscissa of the position where the visible light detector is placed,
Figure DEST_PATH_IMAGE003
is the thickness of the optical waveguide;
the holographic grating converges and deflects the incident light in a collimation state
Figure 357430DEST_PATH_IMAGE004
The point is that, among others,
Figure DEST_PATH_IMAGE005
the number of times of total reflection of the light on the optical waveguide interface is represented by a positive integer;
after incident light is reflected, converged and deflected by the holographic grating, the deflection angles of the edge and the center are respectively
Figure 908497DEST_PATH_IMAGE006
，
Figure DEST_PATH_IMAGE007
，
Figure 707825DEST_PATH_IMAGE008
The deflection angle is equal to the incident angle of the light on the optical waveguide interface, the minimum incident angle
Figure 559107DEST_PATH_IMAGE008
Greater than the full emission angle of the upper and lower surfaces of the optical waveguide
Figure DEST_PATH_IMAGE009
Wherein
Figure 51268DEST_PATH_IMAGE010
，
Figure DEST_PATH_IMAGE011
Is the refractive index of air and is,
Figure 824052DEST_PATH_IMAGE012
is the refractive index of the optical waveguide material.
5. The design method of the LiFi communication oriented holographic grating optical waveguide plane condenser system as claimed in claim 1, wherein the step S4 is characterized in that the formula of the propagation vector of the holographic grating emergent ray is as follows:
Figure DEST_PATH_IMAGE013
in the formula (I), the compound is shown in the specification,
Figure DEST_PATH_IMAGE015
is the incident and diffracted light propagation vectors of the structured beam,
Figure 489388DEST_PATH_IMAGE016
is the construction beam wavelength;
Figure 475404DEST_PATH_IMAGE018
is the incident and diffracted light propagation vectors of the reconstructed beam,
Figure DEST_PATH_IMAGE019
is the reconstruction beam wavelength;
Figure DEST_PATH_IMAGE021
is the grating vector perpendicular to the interference fringes of the holographic grating, and has a size of
Figure 76150DEST_PATH_IMAGE022
Wherein
Figure DEST_PATH_IMAGE023
Is the grating period constant;
having a wavelength of
Figure 726443DEST_PATH_IMAGE024
Constructed beam and wavelength of
Figure DEST_PATH_IMAGE025
Required to reconstruct the beam of lightAnd (3) deriving grating period constants of the grating in the x direction and the z direction according to the Bragg matching condition:
Figure 601995DEST_PATH_IMAGE026
，
Figure DEST_PATH_IMAGE027
in the formula (I), the compound is shown in the specification,
Figure 896710DEST_PATH_IMAGE028
and
Figure DEST_PATH_IMAGE029
is the angle of the reference beam and the object beam in the constructed beam,
Figure 730674DEST_PATH_IMAGE030
is the refractive index of the diffraction grating material,
Figure DEST_PATH_IMAGE031
are the wavelengths of the two constructed beams.
6. The design method of the LiFi communication oriented holographic grating optical waveguide planar light gathering system as claimed in claim 1, wherein the formula for calculating the diffraction efficiency of the holographic grating in the step S5 is as follows:
Figure 478050DEST_PATH_IMAGE032
in the formula (I), the compound is shown in the specification,
Figure DEST_PATH_IMAGE033
in order to be the diffraction efficiency of the holographic grating,
Figure 284856DEST_PATH_IMAGE034
is a refractive index modulation of a holographic gratingThe refractive index is made to be a constant value,
Figure DEST_PATH_IMAGE035
is the thickness of the holographic grating.
7. The design method of the LiFi communication-oriented holographic grating optical waveguide planar light gathering system as claimed in claim 3, wherein the recording of the holographic grating and the testing in the step S6 specifically comprises:
laser is divided into object beam and reference beam by beam splitter, and is adjusted by adjusting reflector M1Mirror M2Obtaining the reference beam angle satisfying the design, and adjusting the reflector M3Obtaining an object beam angle satisfying the design;
the prism system consists of two identical triangular prisms, a fixed interval is reserved between the two triangular prisms and used for placing photosensitive materials, and the prism system is fixedly installed through a support for 3D printing;
designing a flat optical waveguide according to the optical waveguide size designed in the step S1, then attaching a holographic grating photosensitive material according to the size and the position, and placing the holographic grating photosensitive material in a fixed gap between the coupling prisms; opening laser, forming interference fringes on the photosensitive material by the object beam and the reference beam, and recording a volume holographic grating; and testing the diffraction efficiency of the volume holographic grating, and comparing the diffraction efficiency with a simulation result.
8. The design method of the holographic grating optical waveguide planar light gathering system facing LiFi communication in claim 1, wherein: the photosensitive material adopts dichromated gelatin or silver halide.
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