CN110221447B - Structured light projection diffraction optical device based on super-structured surface - Google Patents
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Abstract
The invention provides a structured light projection diffraction optical device based on a super-structured surface, and belongs to the technical field of depth perception and optical imaging. The invention is composed of a substrate and a plurality of sub-wavelength optical antenna arrays which are arranged on the surface of the substrate in a periodic mode, wherein the sub-wavelength optical antenna arrays are one-dimensional arrays or two-dimensional arrays; when the array is a one-dimensional array, each optical antenna is a wire grid which is arranged along the direction of the row and has a rectangular or trapezoidal cross section; when the array is a two-dimensional array, each optical antenna is a cylinder with a circular or any regular polygon cross section along a plane formed by the directions of rows and columns; in each sub-wavelength optical antenna array, the diameter of each optical antenna is 1/20-1/2 of the working wavelength, the heights of the optical antennas are equal and are all in the sub-wavelength range, and the center distances of two adjacent optical antennas are equal and are not more than half of the working wavelength. The invention can realize structured light projection with large diffraction angle, high diffraction efficiency, high uniformity and insensitivity to incident light polarization.
Description
Technical Field
The invention can be applied to the fields of depth perception, optical imaging and the like, and particularly relates to a structured light projection diffraction optical device based on a super-structured surface.
Background
Structured light refers to a specifically encoded laser dot matrix or laser speckle. The structured light is projected on a measured object, a camera is used for capturing a pattern reflected by the measured object, and the depth information of each point of the measured object can be obtained through the comparison between the shape and the size of the received light spot and the projected light spot, so that the three-dimensional appearance of the object is restored. The three-dimensional stereoscopic vision that structured light can realize can be widely applied to a great deal of fields such as industrial detection, life security protection, unmanned driving, receive more and more extensive attention. At present, products based on structured light include a face recognition module of an apple mobile phone, a body sensing game machine of Microsoft corporation and the like.
The structured light projector is mainly composed of components such as a laser and a diffractive optical element, wherein the performance of the diffractive optical element largely determines key indexes such as a field angle, the number of light spots and uniformity of light spots of the structured light. The traditional diffractive optical element for implementing structured light lattice projection is Dammann grating, which was originally proposed by Dammann et al in the 70 s of the 20 th century, is a phase type grating with a special aperture function, has a periodic structure, and the phase within each period is binary (0 and pi). By designing the phase distribution in the period, the Dammann grating can realize the 64 × 64 lattice projection. However, the efficiency of the dammann grating cannot exceed 60% at large diffraction angles (> ± 45 °), since the phase of the dammann grating can only be obtained in two ways.
In recent years, the introduction of a metassurface (metassurface) has attracted much attention. Through the structural design of sub-wavelength scale on the surface, any value can be taken within the range of 0-2 pi phase, so that the limitation of the phase value of the Dammann grating is hopefully overcome, and the structured light projection with large diffraction angle and high efficiency is possible. The prior art works with a super-structured surface projecting structured light, mostly based on geometric phase (Pancharatnam-Berry phase), but has the disadvantage that the incident light must be circularly polarized.
Disclosure of Invention
Aiming at the problems of small diffraction angle and low efficiency of structured light in the prior art, the invention provides a device for realizing structured light projection with large diffraction angle, high efficiency and high uniformity by designing the geometric dimension and the arrangement period of a sub-wavelength nano cylindrical antenna.
In order to achieve the purpose, the invention adopts the following technical scheme:
the invention provides a structured light projection diffraction optical device based on a super-structured surface, which is characterized by comprising a substrate and a plurality of sub-wavelength optical antenna arrays arranged on the surface of the substrate in a periodic manner;
the sub-wavelength optical antenna array is a one-dimensional array, and the directions parallel to the rows and columns of the sub-wavelength optical antenna array are respectively used as an x axis and a y axis; the sub-wavelength optical antenna array at least comprises 3 optical antennas, each optical antenna is a wire grid which is arranged along the y axis and has a rectangular or trapezoidal cross section, the width of each optical antenna is 1/20-1/2 of the working wavelength, the heights of the optical antennas are equal and are within the sub-wavelength range, the center distances of every two adjacent optical antennas are equal and are not more than half of the working wavelength.
Further, the distribution of each optical antenna within each sub-wavelength optical antenna array is determined as follows:
1) calculating the modulation condition of the phase and the transmittance of the incident light of a single antenna under the conditions of different heights, center distances and widths by utilizing a finite difference time domain or strict coupled wave analysis method according to the working wavelength lambda; make the center distance d between two adjacent optical antennas0And the heights of all the optical antennas are kept unchanged, and the requirements are screened out: transmittance is connectedNearly 1, the phase modulation range of the incident light is [0,2 pi ]]A width range of the conditioned optical antenna;
2) determining the number N of antennas contained in a single sub-wavelength optical antenna array by using a grating equation according to the number of structured light points and the requirement of diffraction angle, wherein the expression of the grating equation is as follows:
dsinθm＝mλ
wherein,
d represents the period of a single sub-wavelength optical antenna array, d ═ Nd0；
M is an integer representing the diffraction order, M is 0, ± 1, ± 2, …, ± M; m is a positive integer greater than or equal to 1;
θmrepresents the diffraction angle of the m-th order;
3) and (3) optimizing the width and the distribution of each optical antenna in each period by using a machine learning algorithm, wherein the diffraction efficiency of the diffraction order selected in the step 2) and the root-mean-square error of the light intensity of each diffraction order are used as target functions of the machine learning algorithm.
The invention provides another structured light projection diffraction optical device based on a super-structured surface, which is characterized by comprising a substrate and a plurality of sub-wavelength optical antenna arrays arranged on the surface of the substrate in a periodic manner;
the sub-wavelength optical antenna array is a two-dimensional array, and the directions parallel to the rows and columns of the sub-wavelength optical antenna array are respectively used as an x axis and a y axis; each row and each column in the array at least comprise 3 optical antennas, the cross section of each optical antenna along an xy plane is circular or any regular polygon, the diameter of each optical antenna is 1/20-1/2 of the working wavelength, the heights of the optical antennas are equal and are in a sub-wavelength range, the center distances of every two adjacent optical antennas are equal and are not more than half of the working wavelength.
Further, the distribution of each optical antenna within each sub-wavelength optical antenna array is determined as follows:
1) according to the working wavelength lambda, calculating the position of a single antenna at different heights by utilizing a finite difference time domain or strict coupled wave analysis method,The modulation condition of the phase and transmittance of the incident light under the condition of the center distance and the width; make the center distance d between two adjacent optical antennas0And the heights of all the optical antennas are kept unchanged, and the requirements are screened out: transmittance is close to 1, and incident light phase modulation range is [0,2 pi ]]A width range of the conditioned optical antenna;
2) respectively determining the number N of antennas contained in the corresponding direction in a single sub-wavelength optical antenna array according to the requirements of the number of structured light points and the diffraction angle in the x direction and the y direction by using a grating equationx、NyThe expression of the grating equation is:
wherein,
dx，dyrespectively representing the periods in the x and y directions of a single sub-wavelength optical antenna array, dx＝Nxd0，dy＝Nyd0；
mx，myAre integers respectively representing the diffraction orders in the x and y directions, mx＝0，±1，±2，...，±Mx，my＝0，±1，±2，...，±My；Mx，MyAre all positive integers greater than or equal to 1;
3) and (3) optimizing the width and the distribution of each optical antenna in each period by using a machine learning algorithm, wherein the diffraction efficiency of the diffraction order selected in the step 2) and the root-mean-square error of the light intensity of each diffraction order are used as target functions of the machine learning algorithm.
The invention has the following characteristics and beneficial effects:
the device designs an optical antenna for phase modulation by using a dielectric material with high refractive index, selects the design range of geometric parameters of the antenna by using simulation algorithms such as a time domain finite difference algorithm and the like so as to ensure that the light in the ultraviolet to terahertz wave band realizes random phase modulation in the range of 0 to 2 pi and keeps higher transmittance or reflectivity. The antennas are arranged at a fixed interval and are periodically extended into a periodic array by taking a fixed number of antennas as a period. The size and arrangement of each antenna within the design cycle, i.e., the phase distribution within the design cycle, is designed and optimized using a machine learning algorithm. The structured light projection diffraction optical device obtained by the method can realize structured light projection with large diffraction angle, high diffraction efficiency, high uniformity and insensitivity to incident light polarization.
Drawings
FIG. 1 is a side view of a structured light projection diffraction optic based on a nanostructured surface of this example 1;
FIG. 2 is a top view of a structured light projection diffractive optic based on a nanostructured surface according to this example 1;
FIG. 3 is a side view of a structured light projection diffractive optic based on a nanostructured surface of this example 2;
FIG. 4 is a top view of a structured light projection diffractive optic based on a nanostructured surface according to this example 2;
FIG. 5 is the relationship between the modulation amount of the one-dimensional optical antenna for the incident light phase and the variation of the optical antenna width in embodiment 2;
FIG. 6 is a simulation result of one-dimensional structured light projection in example 2;
fig. 7 is a schematic view of a transmission type structured light projection apparatus constituted by embodiment 1 or 2 and its optical path;
fig. 8 is a schematic view of a reflection type structured light projection device formed in embodiment 1 or 2 and its optical path.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.
A front view and a top view of a structured light projection diffractive optic based on a nanostructured surface of the present embodiment. The structured light projection diffraction optical device consists of a substrate (102, 202) and a plurality of sub-wavelength optical antenna arrays (101, 201) which are arranged on the surface of the substrate (102, 202) in a periodic mode. The height of each optical antenna in the sub-wavelength optical antenna array is in the sub-wavelength range, and each optical antenna is made of a dielectric material with a high refractive index (refractive index >2), and comprises silicon, silicon nitride, titanium dioxide, gallium phosphide, gallium nitride, gallium arsenide and the like.
The sub-wavelength optical antenna array can be a two-dimensional array or a one-dimensional array, the directions parallel to the rows and the columns of the sub-wavelength optical antenna array are respectively used as an x axis and a y axis, and a coordinate system xyz is established according to the right-hand rule. Referring to fig. 1 and fig. 2, which are a front view and a top view of a two-dimensional array, respectively, in the diagram, a periodic sub-wavelength optical antenna array is located inside a dashed line frame 103, each row and each column in the array includes at least 3 optical antennas, a cross-sectional shape of each optical antenna along an xy plane may be a circle or any regular polygon (a circle is used in this embodiment), and a diameter of each optical antenna is 1/20 to 1/2 of an operating wavelength; due to its symmetry, the optical antenna is insensitive to the polarization state of the incident light. Referring to fig. 3 and 4, which are a front view and a top view of a one-dimensional array, respectively, a periodic array of sub-wavelength optical antennas is shown inside a dashed box 203, each optical antenna 201 in the array is a wire grid arranged along the y-axis and having a rectangular or trapezoidal cross section (along the xz plane), and the width of each optical antenna is 1/20 to 1/2 of the operating wavelength; the one-dimensional antenna array can realize structured light projection on light with the polarization direction in the incident plane.
The structured light projection diffraction optical device can be divided into a transmission type and a reflection type according to different materials of the substrates (102, 202). For transmissive structured light projection diffraction optics, the substrate (102, 202) may be selected from a transparent substrate of fused glass, quartz, or the like; for a reflective structured light projection diffraction optical device, the substrate (102, 202) can be made of materials with the reflectivity higher than 90% such as gold and silver, and can also be made of a distributed Bragg reflector. A distributed bragg mirror is composed of an alternating stack of two dielectric layers of different refractive index, the thickness of the dielectric layers typically taking the product of a quarter of the operating wavelength and the inverse of the refractive index of the dielectric. Dielectric materials commonly used to construct distributed bragg mirrors are silicon dioxide, titanium dioxide, silicon nitride, and the like.
Further, as shown in fig. 3 and 4, for the transmissive one-dimensional structured light projection diffraction optical device (composed of a one-dimensional sub-wavelength amorphous silicon antenna array 203 and a molten glass substrate 202, the sub-wavelength antennas 201 are all in a grid shape), the distribution of each optical antenna in each sub-wavelength optical antenna array is determined according to the following steps:
1) calculating the modulation conditions of the phase and the transmittance of the incident light of a single antenna under the conditions of different heights, center distances and widths by using methods such as time domain finite difference or strict coupled wave analysis and the like according to the working wavelength lambda; make the center distance d between two adjacent optical antennas0And the heights of all the optical antennas are kept unchanged, wherein the heights of all the optical antennas are equal, the center distances of two adjacent optical antennas are equal and not more than half of the working wavelength, (obtaining the modulation amount of a single antenna on the phase of incident light
The quantitative relationship with the width W of the antenna, and the effect of the width W of the antenna on the antenna transmittance) are selected to satisfy: transmittance is close to 1, and incident light phase modulation range is [0,2 pi ]]The width range of the optical antenna of the condition. In this embodiment, the working wavelength is 1550 nm, the height of the fixed silicon pillar antenna is 850 nm, the center distance is 750 nm, and the width of the antenna is 150-500 nm. As shown in FIG. 5, in this width range, the phase modulation range can include all values within 0-2 π, and the transmittance is close to 1.
2) Determining the number N of antennas contained in a single sub-wavelength optical antenna array by using a grating equation according to the number of structured light points and the requirement of the maximum diffraction angle, wherein the expression of the grating equation is as follows:
dsinθm＝mλ
wherein,
d represents the period of a single sub-wavelength optical antenna array, d ═ Nd0；
M is an integer representing the diffraction order, M is 0, ± 1, ± 2, …, ± M; m may be a positive integer from 1 to 1000;
θmrepresenting the diffraction angle of the m-th order.
At normal incidence, the grating period d varies with the number of antennas N within the period, thereby varying the number of diffraction orders. Therefore, when the number of structured light points is large, a larger period, that is, a larger number of antennas in a single period should be selected. And selecting N' optical antennas slightly larger than the number of the structured light points as a period by utilizing a grating equation according to the number N of the adopted structured light points, and screening N optical antennas meeting the requirement of the maximum diffraction angle as a period according to the grating equation. In this embodiment, the number n of structured spots is 9, and the diffraction angle is greater than ± 45 °. It was determined from the grating equation that 11 silicon columns were one period, and thus there were 11 diffraction orders (0, ± 1, ± 2, ± 3, ± 4, ± 5) in total, 9 of them (0, ± 1, ± 2, ± 3, ± 4) were selected, and the outermost diffraction angle (corresponding to ± 4 diffraction orders) was about ± 48.7 °, satisfying the diffraction angle requirements.
3) And optimizing the width and the distribution of each optical antenna in each period by using a machine learning algorithm. Optimization algorithms that may be used include simulated annealing algorithms, genetic algorithms, particle swarm algorithms, and the like.
The optimization algorithm used in this embodiment is a particle swarm algorithm. The particle swarm algorithm is an iteration-based optimization algorithm, each solution is called a particle in the algorithm, and the number of the particles in each iteration, the number of iterations and an objective function need to be specified initially. The method comprises the following specific steps:
31) in the embodiment, the diffraction efficiency η of the diffraction order selected in the step 2) and the root mean square error RMSE of the light intensity of each diffraction order are taken as the target functions, and the expressions are respectively as follows:
in the formula:
i is each diffraction order selected in the step 2), namely the diffraction order needing to be optimized; in the present embodiment, i is 0, ± 1, ± 2, ± 3, ± 4;
Iithe intensity corresponding to the ith diffraction secondary; i is0Is the incident light intensity;
32) calculating the objective function value of each particle in the current iteration times according to the objective function set in the step 31), and finding out local optimum and global optimum;
33) calculating the moving direction and speed of each particle in the next generation according to the local optimum and the global optimum in the current iteration times obtained in the step 32);
34) updating the position of the particle in the next iteration process according to the moving direction and the speed calculated in the step 33);
35) repeating steps 32) to 34) until all particles converge to the same position.
According to the optimization result, the widths of the silicon pillars are determined to be 242.5 nanometers, 293.0 nanometers, 226.9 nanometers, 214.4 nanometers, 456.1 nanometers, 282.6 nanometers, 456.1 nanometers, 214.4 nanometers, 226.9 nanometers, 293.0 nanometers and 242.5 nanometers in sequence. The simulation results for this design are shown in FIG. 6, where the diffraction efficiency is 90.12% and the root mean square error of the intensity of each diffraction order is 9.84%.
The determination mode of the distribution of each optical antenna in each sub-wavelength optical antenna array of the two-dimensional structured light projection optical device is similar to that of the one-dimensional structured light projection optical device. The specific differences are as follows:
in step 2), according to a grating equation, the number of the optical antennas in the x and y directions in the two-dimensional array is respectively determined by the number of structured light points and the diffraction angle in the corresponding direction.
In step 3), the target function adopted by the optimization algorithm, the expressions of the diffraction efficiency η of the diffraction order selected in step 2) and the root mean square error RMSE of the light intensity of each diffraction order are respectively as follows:
in the formula:
i, j are the diffraction orders in the x and y directions selected in step 2), respectively;
Ii，jthe light intensity corresponding to the ith diffraction order in the x direction and the jth diffraction order in the y direction; i is0Is the incident light intensity;
It should be noted that, according to the optimization algorithm, various distribution forms of each optical antenna in each sub-wavelength optical antenna array can be obtained, and the purpose of the present invention can be achieved, that is, the structured light projection with large diffraction angle, high diffraction efficiency, high uniformity and insensitivity to incident light polarization is realized.
The structured light projection device composed of the structured light projection diffraction optical device of the present invention is divided into two types, namely, a transmission type and a reflection type, and includes lasers (501, 601), collimating lenses (502, 602) and structured light projection diffraction optical devices (503, 603) based on a super-structured surface, which are arranged coaxially, as shown in fig. 7 and 8, respectively. The laser can be selected from semiconductor lasers, including vertical surface cavity lasers (VCSELs) and arrays thereof, Edge Emitting Lasers (EELs) and arrays thereof, and the like. The operating wavelength of the laser can be from an ultraviolet band to a terahertz band (200 nanometers to 300 micrometers).
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.
Claims (8)
1. A structured light projection diffraction optical device based on a super-structured surface is characterized by comprising a substrate and a plurality of sub-wavelength optical antenna arrays arranged on the surface of the substrate in a periodic mode;
the sub-wavelength optical antenna array is a one-dimensional array, and the directions parallel to the rows and columns of the sub-wavelength optical antenna array are respectively used as an x axis and a y axis; the sub-wavelength optical antenna array at least comprises 3 optical antennas, each optical antenna is a wire grid which is arranged along the y axis and has a rectangular or trapezoidal cross section, the width of each optical antenna is 1/20-1/2 of the working wavelength, the heights of the optical antennas are equal and are within the sub-wavelength range, the center distances of every two adjacent optical antennas are equal and are not more than half of the working wavelength;
the distribution of each optical antenna within each sub-wavelength optical antenna array is determined in the following manner:
1) calculating the modulation condition of the phase and the transmittance of the incident light of the single optical antenna under the conditions of different heights, center distances and widths by utilizing a finite difference time domain or strict coupled wave analysis method according to the working wavelength lambda; make the center distance d between two adjacent optical antennas0And the heights of all the optical antennas are kept unchanged, and the requirements are screened out: transmittance of lightClose to 1, the phase modulation range of the incident light is [0,2 pi ]]A width range of the conditioned optical antenna;
2) determining the number N of optical antennas contained in a single sub-wavelength optical antenna array by using a grating equation according to the number of structured light points and the requirement of diffraction angle, wherein the expression of the grating equation is as follows:
dsinθm＝mλ
wherein,
d represents the period of a single sub-wavelength optical antenna array, d ═ Nd0；
M is an integer representing the diffraction order, M is 0, ± 1, ± 2, …, ± M; m is a positive integer greater than or equal to 1;
θmrepresents the diffraction angle of the m-th order;
3) and (3) optimizing the width and the distribution of each optical antenna in each period by using a machine learning algorithm, wherein the diffraction efficiency of the diffraction order selected in the step 2) and the root-mean-square error of the light intensity of each diffraction order are used as target functions of the machine learning algorithm.
2. A structured light projection diffraction optical device based on a super-structured surface is characterized by comprising a substrate and a plurality of sub-wavelength optical antenna arrays arranged on the surface of the substrate in a periodic mode;
the sub-wavelength optical antenna array is a two-dimensional array, and the directions parallel to the rows and columns of the sub-wavelength optical antenna array are respectively used as an x axis and a y axis; each row and each column in the array at least comprise 3 optical antennas, the cross section of each optical antenna along an xy plane is circular or any regular polygon, the diameter of each optical antenna is 1/20-1/2 of the working wavelength, the heights of the optical antennas are equal and are within a sub-wavelength range, the center distances of every two adjacent optical antennas are equal and are not more than half of the working wavelength;
the distribution of each optical antenna within each sub-wavelength optical antenna array is determined in the following manner:
1) calculating the height of a single optical antenna at different heights by utilizing a finite difference time domain or strict coupled wave analysis method according to the working wavelength lambdaThe modulation condition of the phase and the transmittance of the incident light under the conditions of the degree, the center distance and the width; make the center distance d between two adjacent optical antennas0And the heights of all the optical antennas are kept unchanged, and the requirements are screened out: transmittance is close to 1, and incident light phase modulation range is [0,2 pi ]]A width range of the conditioned optical antenna;
2) respectively determining the number N of optical antennas contained in the corresponding direction in a single sub-wavelength optical antenna array according to the requirements of the number of structured light points and diffraction angles in the x direction and the y direction by using a grating equationx、NyThe expression of the grating equation is:
wherein,
dx,dyrespectively representing the periods in the x and y directions of a single sub-wavelength optical antenna array, dx＝Nxd0，dy＝Nyd0；
mx,myAre integers respectively representing the diffraction orders in the x and y directions, mx＝0,±1,±2,…,±Mx，my＝0,±1,±2,…,±My；Mx,MyAre all positive integers greater than or equal to 1;
3) and (3) optimizing the width and the distribution of each optical antenna in each period by using a machine learning algorithm, wherein the diffraction efficiency of the diffraction order selected in the step 2) and the root-mean-square error of the light intensity of each diffraction order are used as target functions of the machine learning algorithm.
3. The structured light projection diffraction optical device according to claim 1 or 2, wherein the machine learning algorithm in step 3) is selected from a particle swarm algorithm, a genetic algorithm or a simulated annealing algorithm.
4. The structured light projection diffraction optical device according to claim 1, wherein the machine learning algorithm in step 3) is a particle swarm algorithm, and the expressions of the diffraction efficiency η of the diffraction order selected in step 2) and the root mean square error RMSE of the light intensity of each diffraction order in the objective function are respectively as follows:
i is the selected diffraction order of step 2);
Iithe intensity corresponding to the ith diffraction secondary; i is0Is the incident light intensity;
5. The structured light projection diffraction optical device according to claim 2, wherein the machine learning algorithm in step 3) is a particle swarm algorithm, and the expressions of the diffraction efficiency η of the diffraction order selected in step 2) and the root mean square error RMSE of the light intensity of each diffraction order in the objective function are respectively as follows:
in the formula:
i, j are the diffraction orders in the x and y directions selected in step 2), respectively;
Ii,jthe light intensity corresponding to the ith diffraction order in the x direction and the jth diffraction order in the y direction; i is0Is the incident light intensity;
6. A structured light projection diffraction optical device according to claim 1 or 2, wherein the substrate is made of a transparent substrate or a high reflectivity material, or the substrate is a distributed bragg reflector.
7. The structured light projection diffraction optical device according to claim 1 or 2, wherein each of the optical antennas is made of a dielectric material having a refractive index greater than 2, and includes silicon, silicon nitride, titanium dioxide, gallium phosphide, gallium nitride, and gallium arsenide.
8. The structured light projection diffraction optical device of claim 1 or 2, wherein the structured light projection diffraction optical device operates at a wavelength from the ultraviolet band to the terahertz band.
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Application publication date: 20190910 Assignee: Weiwu Photon (Beijing) Technology Co.,Ltd. Assignor: TSINGHUA University Contract record no.: X2023980037583 Denomination of invention: A Structured light Projection Diffractive Optical Device Based on Hyperstructured Surface Granted publication date: 20200616 License type: Common License Record date: 20230704 |