Researchers: Youmin Wang
Funding Agency: Texas Instruments, DARPA
Light detection and ranging (LIDAR) is a key enabling technology for self-driving cars and other autonomous vehicles. Most of current LIDARs employ mechanical scanning units such as motors, which are bulky and often intrusive in their deployment. Solid state LIDARs with non-mechanical scanning elements have received increasing interests. In particular, the optical phased array (OPA) provides non-mechanical scanning in a compact form factor. More importantly, it enables sophisticated beamforming such as simultaneous scanning, pointing, and tracking of multiple objects, or even direct line-of-sight communications. OPAs with various types of phase modulators have been demonstrated, most notably with liquid crystal, silicon photonics, and MEMS technologies. Large-scale OPA has been realized with liquid crystal spatial light modulators, albeit with slow response time. Large scan angle has been demonstrated in silicon photonics waveguides, however, they have high insertion loss and are limited to infrared wavelength transparent to Si. MEMS OPA offers faster response time than liquid crystal, but to date the mirror size is larger than 20µm, limiting their scan angles. In this paper we report on a fine-pitch MEMS OPA with fast response time. A 2.4µm pitch 1-D mirror array is designed to achieve 22° scanning angle at 905nm wavelength. Since micromirror is a broadband reflective device, the OPA can operate at other wavelengths. At 1550nm, a larger scan angle of 40° can be realized. The resonance frequency of the MEMS device is measured to be 500kHz and the actuation voltage is <10V.
The optical performance of an OPA is determined by its pitch, aperture, and the number of elements in the array. The ranging distance is proportional to the square of the aperture size, the field of view (FOV) is inversely proportional to the pitch, and the number of resolvable spots is approximately equal to the number of elements in the array. We have designed our OPA for 100m ranging distance, 22° steering angle at 905nm, and 256 resolvable spots in the far field.
The schematic of our OPA device is shown in Figure. 1. It consists of a one-dimensional (1D) array of micromirrors. Each mirror is 2.1µm wide and 35µm long, and the pitch is 2.4µm. To achieve high fill factor while minimizing the crosstalk between adjacent mirrors, we employ vertical combdrive actuators underneath the mirror. Each actuator consists of two lower and one upper comb fingers. Both comb fingers are 300nm wide, and the spacing between them is also 300nm. The mirrors are tethered to the anchors through a pair of springs. To operate the OPA, the mirrors and the top combs are grounded while the bottom combs are individually addressed to produce the desired beam profiles. The scanning electron micrographs (SEMs) of fabricated OPA device is shown in Figure 2.