University of Colorado Boulder researchers, with colleagues at Boston University, have developed a new optical phased array concept—the serpentine OPA (SOPA)—that could support medium- to long-range lidar. An open-access paper on their work appears in the journal Optica.

Optical phased arrays (OPAs) implemented in integrated photonic circuits could enable a variety of 3D sensing, imaging, illumination, and ranging applications, and their convergence in new lidar technology. However, current integrated OPA approaches do not scale—in control complexity, power consumption, or optical efficiency—to the large aperture sizes needed to support medium- to long-range lidar.

… The SOPA is based on a serially interconnected array of low-loss grating waveguides and supports fully passive, 2D wavelength-controlled beam steering. A fundamentally space-efficient design that folds the feed network into the aperture also enables scalable tiling of SOPAs into large apertures with a high fill-factor. We experimentally demonstrate, to the best of our knowledge, the first SOPA using a 1450–1650 nm wavelength sweep to produce 16,500 addressable spots in a 27×610 array. We also demonstrate, for the first time, far-field interference of beams from two separate OPAs on a single silicon photonic chip, as an initial step towards long-range computational imaging lidar based on novel active aperture synthesis schemes.

—Dostart et al.

Serpentine optical phased array 2D wavelength steering. (a) Schematic of SOPA tile topology. An array of 𝑀 rows of grating waveguides (red) are serially connected by flybacks (blue) in a serpentine configuration. Each row has 𝑁 grating periods. (b) Coarse (slow) wavelength steering. (c) Fine (fast) wavelength steering. (d) For coarse steering along 𝜃_𝑥, each grating waveguide diffracts light to an angle determined by the wavelength-dependent tooth-to-tooth phase delay. (e) For fine steering along 𝜃_𝑦, the array of gratings diffracts light to an angle determined by the wavelength-dependent row-to-row phase delay. Dostart et al.

Current commercial lidar systems use large, rotating mirrors to steer the laser beam and thereby create a 3-D image. For the past three years, Dostart and his colleagues have been working on a new way of steering laser beams called wavelength steering—where each wavelength, or “color,” of the laser is pointed to a unique angle.

They’ve not only developed a way to do a version of this along two dimensions simultaneously, instead of only one, they’ve done it with color, using a “rainbow” pattern to take 3-D images. Since the beams are easily controlled by simply changing colors, multiple phased arrays can be controlled simultaneously to create a bigger aperture and a higher resolution image.

We’ve figured out how to put this two-dimensional rainbow into a little teeny chip.

—Kelvin Wagner, co-author of the new study and professor of electrical and computer engineering

Autonomous vehicles are currently a $50-billion dollar industry, projected to be worth more than $500 billion by 2026. While many cars on the road today already have some elements of autonomous assistance, such as enhanced cruise control and automatic lane-centering, the long-term goal is to create a car that drives itself with no input or responsibility from a human driver—requiring sensors such as lidar systems.

Lidar is a remote sensing method that uses laser beams to measure distances; a sensor collects these reflections to create a precise, three-dimensional picture of the surrounding environment in real time. While great strides have been made in the size of lidar systems, they remain the most expensive part of self-driving cars by far.

In order to work broadly in the consumer market one day, lidar must become even cheaper, smaller and less complex. Some companies are trying to accomplish this feat using silicon photonics. The research team’s new finding is an important advancement in silicon chip technology for use in lidar systems.

Electrical communication is at its absolute limit. Optics has to come into play and that’s why all these big players are committed to making the silicon photonics technology industrially viable.

—Miloš A. Popović, co-author and associate professor of engineering at Boston University

We’re proposing a scalable approach to lidar using chip technology. And this is the first step, the first building block of that approach. There’s still a long way to go.

—lead author Nathan Dostart, who will continue his work at NASA Langley Research Center

Resources

• Nathan Dostart, Bohan Zhang, Anatol Khilo, Michael Brand, Kenaish Al Qubaisi, Deniz Onural, Daniel Feldkhun, Kelvin H. Wagner, and Miloš A. Popović (2020) “Serpentine optical phased arrays for scalable integrated photonic lidar beam steering,” Optica 7, 726-733 doi: 10.1364/OPTICA.389006