Ultra-compact structured light projector with all-dielectric metalenses for 3D sensing

3D sensing is everywhere in the age of artificial intelligence (AI) and is widely used in the field of machine vision, for example, face recognition, 3D depth sensing, gesture motion sensing, object 3D scanning, etc.^1,2 3D sensing technology has been also used for facial recognition in iPhone X, but not just for this. Apple’s 3D sensing technology has driven the follow-up of major mobile phone manufacturers around the world. At the same time, it can be used in Advanced Driver Assistance System (ADAS), Augmented Reality and Virtual Reality (AR/VR), self-service store and so on.^3,4 It can be seen from the recent Consumer Electronics Show (CES 2019) that 3D sensing technology has been adopted more in practical applications.

In today’s consumer landscape, there are two primary 3D sensing technologies:^5,6 Time-of-Flight (TOF) and Structured-light (SL). A TOF 3D sensor measures the time that light has been in flight to estimate distance.⁷ A Structured-Light 3D sensor projects an active pattern and obtains depth by analyzing the deformation of the pattern.⁸ Two kinds of technology have its own special advantages and the user can select the corresponding technical product according to the actual application requirements. Structured-Light technology is more suitable for close-range, non-contact, high-precision three-dimensional measurement applications.⁹ The structured light projector currently used on smartphone produces a fixed speckle pattern through a diffractive optical element (DOE), and its measurement accuracy can achieve the order of millimeter.^10,11 If the three-dimensional data measurement of objects with sub-millimeter accuracy can be realized on a smartphone, it may lead to an innovation product, thus a high-speed, high-resolution and small-volume structured light projector is essential.

The smallest structured light projector, that can achieve sub-millimeter measurement accuracy, is the RealSense-SR300 module produced by Intel Corporation.¹² The optical system of the projector is shown in Figure 1(b). The infrared light emitted from a laser, is collimated after a collimating lens, passes through a cylindrical lens and then illuminates on the MEMS mirror. By controlling the switching of the laser and the scanning angle of the MEMS mirror,^13,14 the space projection of the high-speed and high-resolution coded structured light can be realized. Though RealSense-SR300 is the smallest in structured light projector, its volume is still too large for smartphone.

As shown in Figure 1, conventional refractive lenses are used in the structured light projector. If the size of the lens can be reduced and integrated with a laser chip, overall dimensions of the infrared projector will be greatly decreased. Capasso Group of Harvard University has done a lot of research and experimental verification on metalens technology.^15–19 Phase is a basic physical parameter of electromagnetic waves. Conventional refractive optical elements utilize a difference in the refractive index of a material or a change in a surface shape to achieve a specific phase distribution, thereby to construct a corresponding functional device. For natural optical materials, the range of choice is usually limited, so traditional optical functional components are mostly realized by constructing curved surface shapes, the volume and weight of systems is usually large. Subwavelength-sized plasmonic nano-antennas on a planar surface have been shown to create phase shifts covering the full range (from 0 to 2π) in cross-polarized scattered light due to their asymmetric plasmonic resonances.^20,21 These subwavelength-sized plasmonic nano-antennas can be used to build optical lenses with surprising properties. Metalens have been investigated as potential alternatives for integrated optical free space components.²² Metalens has the significant advantage of high integration, therefore metalens used in structured light projector will greatly reduce the size of an infrared projector.

In the following, the optical integration concept, meta-lenses design and characterization of the ultra-compact structural light projector are presented. The volume of the structured light projector is approximately 70 mm³, and it is expected to be highly integrated into smartphones for sub-millimeter 3D measurement applications.

In this paper, we proposes to use advanced all-dielectric metalens instead of traditional lenses in structured light projector, and accordingly, metalens is highly integrated with laser chips, which achieves ultra-small size. The structured light projector realized the ultra-compact concept and can be easily integrated into smartphones used as 3D sensing. The design of the structured light projector is shown in Figure 2, which working principle is that the laser chip emits light to the metalens 1 for collimation, and then through metalens 2 to diffuse light in one direction (the function of the cylindrical lens), and the diffused light is transmitted to the MEMS mirror and reflected onto the object being measured. It should be pointed out here that it is sufficient to select a MEMS mirror with the mirror size of 1 mm, metalens is much smaller than the MEMS mirror in Figure 2 because its optical path is compactness. The structured light projector is approximately 70mm³ in size and its overall volume is reduced by a factor of 4.3 compared to Figure 1, which is very easy to integrate into the smartphone.

The principle of the ultra-compact structured light projector is similar to that of the conventional technique in Figure 1. In order to reduce the size of the projector module, the most popular meta-lenses are used to replace the conventional refractive lenses. The schematic diagram is shown in Figure 2(a), when the laser is turned on, the light passes through the collimating metalens 1 and then through the metalens 2 to expand the beam in one direction, the expanded light is projected through the MEMS mirror, then the MEMS scanning angle and the laser switch can be precisely controlled to accurately project the desired structured light pattern. According to the system requirements, metalens needs to be designed in detail. The emission source is a key component of the system. The VECSEL (vertical-external-cavity surface-emitting-laser) is very popular in the face recognition field. Usually, to achieve optical power of 200mW, the array type VECSEL is required. The metalens with a matching VECSEL needs exactly the same array design, and the assembly requirements are relatively high. However, EEL (Edge Emitting Laser) does not require an array design. In this paper, OSRAM’s SPL_TR85 (Edge Emitting Laser Chip) is used as the projection light source, which major parameters are as follows: Center wavelength is 850nm, optical power is 200mW, light-emitting area is 170*1μm², maximum divergence is 19 degrees (a beam divergence of 19 degree corresponds to an NA of 0.165), and the chip size is about 0.4mm*1.5mm*0.2mm. The light emitted from SPL_TR85 is elliptically polarized light, which need to be considered in designing metalens. Because the laser diode is a heat-generating device, the temperature has a very important influence on the life of the laser diode. Therefore, the heat dissipation of the laser chip should be considered while designing the optical machine. In the design, we use copper as the mounting base material for a better heat dissipation. Considering the requirements, the following parameters of metalens are need to be designed: the focal length of metalens 1 is about 500μm and the aperture is at least 100μm (We choose diameter is 100um as the aperture size.), the focal length of metalens 2 is about 50um (FOV is about α=90°) and the aperture is equal to metalens 1, respectively. The parameters of the two meta-lenses are shown in Figure 2(b).

Metalens is a new ultrathin components, which is based on the generalized Snell’s Law to manipulate light beam focusing and diffusing.^23–25 It shows broad application prospects due to its novel mechanism and flexible design. In previous work, the choice of cell structure and materials for metalens was varied.²⁶ According to the requirements shown in Figure 3, we use nano-pillars of varying diameters arranged at the center of the square lattice unit as a cell structure that is polarization insensitive, (see Figure 3(c) below), wherein the material of the nano-pillar is amorphous (α-Si), and the material of the substrate is BF33 Glass, respectively.

To ensure high efficiency of meta-lenses, geometric dimensions of the unit cell structure need to be rationally designed. Such as, the lattice constant U of the unit cell, the height H and the radius of the nanopillar. In this design, the lattice constant U should be small enough to meet the Nyquist sampling criterion (U<λ/(2NA)).²⁷ The height H of the nanopillars should be tall enough to provide 2π phase coverage through a range of diameters. In addition, it is need to consider the current manufacture standards constraints and costs of semiconductors. We designed a metalens at the wavelength λ=850nm with the lattice constant U is 310nm, the diameter D of the nanopillars varies from 80nm to 250 nm, and the height H of the nanopillars varies from 250nm to 650 nm. The phase shift and intensity transmission of the unit cell structure were simulated by the Lumerical’s FDTD solutions.²⁸ Figure 4(a) shows the relationship between the height and diameter of the nanopillar and the phase shift. Figure 4(b) shows the relationship between the height and diameter of the nanopillars and the transmittance. From the Figure 4(b) (c), it appears that when the height of the nanopillar is 425nm, the phase can completely cover 2π and the transmittance is sufficiently high. At the same time, a diameter of 80nm is chose as the smallest nanopillars, and the depth-to-width ratio 0.19 of the nanopillars meets processing requirements, therefore, hence it can be easily implemented.²⁹ Figure 4(c) shows the phase modulation capability, transmittance of nanopillars with the height of 425 nm and the diameter range of 80-250nm with a phase modulation step of π/16. Its transmittance of the nanopillar is greater than 84%, an absorption peak appear will be appearing when the diameter of the nanopillar is 200 nm. Practically, the wavelength of the laser in this paper is ±10nm fluctuation instability. The achromatic design^30,31 of the metalens is a necessary consideration and a challenge. In this paper, the achromatic design has not been considered.

To accomplish metalens 1 with point-focusing at a certain focal length f1, the phase distribution φ(x, y) along the interface is given by equation (1),³² the metalens 2 with line-focusing at a focal length f2 can be realized that only considering the phase distribution in the x-axis direction. Thus, the new phase distribution along the interface is given by equation (2).³³ The phase profiles (black solid line) of the two metalenses are shown in Figure 4(a) (b), the blue solid dots in the figure represent nanopillars.

Where λ=850nm is the wavelength in vacuum. In addition, f1 and f2 here is the predefined focal length of metalens 1 and metalens 2. Each nanopillar at position (x, y) of the metalenses must impart the required phase by Eq. (1) and Eq. (2), respectively. Note that the transmission media after the lens is air. According to the relationship between the phase map of the nanopillars and the phase profile of the meta-lenses, the coordinate position of the nanopillars can be obtained accurately. The phase configuration distribution and coordinates of the nanopillars are as shown in the blue point in the Fig. 4(a),(b). Top-view of the metalens 1 and the metalens 2 are as shown in the Fig. 4(c),(d).

According to the meta-lenses structure designed in Fig. 4, FDTD simulations were performed on two meta-lenses respectively. Simulation results show that the metalens 1 has a good focusing function and can realize the beam collimation function of the laser diode. The electric field intensity of the metalens 1 is shown in Fig. 5a, and the electric field profile at z=500μm is shown in Fig. 5b, and its FWHM=0.323 λ. When the light passes through the metalens 1, the light becomes a collimated beam. So when performing FDTD simulation on metalens 2, it is postulated first that the incident source is ideal collimated light. It can be seen from Fig. 6 that this metalens has a good focusing effect at z=50um. When z is greater than 50um, the beam exhibits a divergent effect with a divergence angle of about 90 degrees. The metalens 2 implements a line lens function that replaces the cylindrical lens of Figure 1.

When the metalens 1 and the metalens 2 are bonded together, a projection of a line pattern can be realized. The optical path and the intensity distribution of the line pattern are shown in Fig. 7 (The MEMS scanner is closed in the figure). Compared with the traditional scheme, the scheme of this paper greatly realizes the volume reduction without performance sacrifice. By placing the MEMS mirror at the focus of the metalens 2, a structured light projector can be formed. A structured light pattern of any grating type can be realized by precisely controlling the switching of the laser and the scanning angle of the MEMS mirror. The MEMS mirror in Fig. 7(a) is provided by China Key System & Integrated Circuit Co., Ltd, which mirror size is 1mm, the scan frequency is about 2.8 kHz, scanning optical angle is 60 degrees and chip size is 3.9mm*2.2mm*0.5mm. Figure 7(b) is a simulated structured light projection pattern of the grating stripe. The projector of Fig. 7 can generate a Phase-shift pattern or a Binary pattern, and it can be used to perform high-precision measurement of 3D object combined with a reasonable algorithm.⁹ 

Two metalenses are assembled by bonding. At the beginning, the laser chip is mounted to a heat sink base and wiring bonding is done by connecting golden wires to pins. Then, bonded meta-lenses are attached to the light-emitting surface of the laser chip by micro-manipulation, and then the MEMS mirror is placed at the focus of the metalens 2 to form an assembly, as shown in Fig. 8. As can be seen from Figure 8, it has a high integration in small size with a volume of only 70 mm³. Compared with the model in Figure 1, it is reduced by 77% in volume.

This paper first proposed replacing traditional refractive lenses with some high-performance meta-lenses to achieve an ultra-compact structured light projector. The meta-lenses used 16 kinds of diameter nano-pillars to realize the function of focusing lens and cylindrical lens. Through the design and simulation, the feasibility of the idea was demonstrated. All-dielectric metalens is used in a structured light projector to achieve an ultra-compact concept, which is 77% smaller in size than conventional technology, and it is expected to be highly integrated in smartphones for 3D sensing applications with sub-millimeter measurement accuracy.

This work was sponsored by National Natural Science Foundation of China (61528401), National Defense Basic Scientific Research Project of China (JCKY2016208A002) and General Armament Department Advanced Manufacture of China (41423020111).