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Xia R X, Zhao D, Li Z Q, et al. Metasurface beamsplitter with large field of view and equal diffraction angle interval[J]. Opto-Electron Eng, 2024, 51(8): 240141. doi: 10.12086/oee.2024.240141
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Metasurface beamsplitter with large field of view and equal diffraction angle interval
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Abstract
LiDAR currently mainly uses a Dammann grating as the laser beamsplitter. However, as a periodic diffraction optical device, the Dammann grating satisfies the grating equation requiring each diffraction angle's sine value to form an arithmetic progression, which cannot achieve uniform angular beam-splitting. The theoretical diffraction efficiency is also limited. This paper uses the angular spectrum and random search optimization algorithm to design a more flexible non-periodic beamsplitter. Simulations show that the metasurface beamsplitter can achieve a 70-degree field angle of 41 beams with an equal diffraction angle interval. The simulated diffraction efficiency reaches 84% which is higher than the diffraction limit of a binary phase device. In experiments, the metasurface beamsplitter has good beam-splitting uniformity and can promote the development of LiDAR.-
Keywords:
- metasurface /
- beamsplitter /
- Dammann grating /
- DOE
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References
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Overview
Autonomous driving has made significant progress in the past few years. LiDAR which can precisely measure the distance to reflecting surfaces is an important sensor for autonomous driving. Multi-line LiDAR is required to improve the accuracy and frame of LiDAR. Dammann gratings are commonly used as laser beamsplitters in multi-line LiDAR. As binary phase diffraction optical devices, they are relatively easy to process. Numerical solutions for Dammann gratings with common numbers of beam-splitting have been studied. However, as a periodic diffraction optical device, Dammann grating satisfies the grating equation requiring each diffraction angle's sine value to form an arithmetic progression.
In this paper, we use the angular spectrum and random search optimization algorithm to design a more flexible non-periodic beamsplitter. The angular spectrum, a discrete Fourier transform of the incident complex amplitude array, can calculate the angular response in the far field. We propose to expand the diffraction calculation window to reduce the frequency interval of the Fourier transform. Using the finer frequency grid, we build the far-field angular target function with equal angle interval beam-splitting. We set the evaluation function as a linear combination of root mean square error and efficiency. This evaluation function setting can take into account both error control and device diffraction efficiency. The random search optimization algorithm only adopts the optimization with a decreasing evaluation function according to the evaluation function. Simulations show that the metasurface beamsplitter can generate 41 beams with a 70° field angle. The normalized standard deviation of the simulated light intensity of each spot is 0.011. The simulated diffraction efficiency reaches 84% which is higher than the diffraction limit of the binary phase device.
To realize such phase modulation, we use full-wave simulation to design an efficient amorphous silicon nanopillar with geometric phase modulation at the 1550-nm wavelength. We use electron beam lithography to process a 1-mm diameter metasurface beamsplitter. Experimental measurements confirm the beam-splitting function of the metasurface. The experimentally measured standard deviation of beam power is 0.179, and the average half-maximum full width of the beam-splitting direction is 0.143°.
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Xia R X, Zhao D, Li Z Q, et al. Metasurface beamsplitter with large field of view and equal diffraction angle interval[J]. Opto-Electron Eng, 2024, 51(8): 240141. doi: 10.12086/oee.2024.240141
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Figure 1.
Schematic diagram of the designed metasurface beamsplitter and Dammann grating
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Figure 2.
(a) Schematic diagram of the metasurface cell, with a length of 480 nm, a width of 240 nm, a height of 760 nm, and a period of 650 nm. The substrate is 500-μm thick; (b) Efficiency of cross circularly polarized light at 650 nm period. The black dot in (b) is the selected metasurface cell; the length and width of the nanopillar are 480 nm and 240 nm, respectively; the conversion efficiency is 99.2%
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Figure 3.
(a, b) One-dimensional simulation of the beamsplitter intensity angle distribution. The blue line is the simulation result, and the orange dashed line is the optimization target; (c) The unwrapped phase of the beamsplitter; (d) The full width at half maximums (FWHMs) of spots in (a). The blue line is the simulation result, the yellow dashed line is the diffraction limit, and the red dotted line is the optimization target; (e) 41 rays on the 40 cm distancing plane obtained by the numerical Rayleigh-Somerphy diffraction integral. Each light spot is cut into 3 mm squares and spliced together
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Figure 4.
(a, b) Metasurface morphology imaged by 5x and 100x objective microscopes. Scalebars: 200 μm and 2 μm; (c) Design drawing of the same area in Fig. 4(b), with a scale of 2 μm; (d) Schematic diagram of the experimental optical path. P: polarizer; QWP: quarter-wave plate; MS: metasurface; (e) Captured 41 rays. Each beam is cut into 3 mm squares and spliced together; (f) Experimentally measured power of beams. The blue line represents the beams from the 1st to 20th orders and the orange dotted line is from the −1st to −20th orders; (g)Experimentally measured FWHMs. The solid blue line with asterisks and the orange dotted line with circles are the horizontal FWHMs from the 1st to the 20th orders and from the −1st to the −20th orders, respectively. The dashed yellow line with regular triangles and the dashed-dotted purple line with inverted triangles are the vertical FWHMs from the 1st to the 20th orders and from the −1st to the −20th orders, respectively. The solid green line is the horizontal diffraction limit.
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