Smaller, cheaper lidar with new chip-based beam steering device

Researchers have developed a new chip-based beam steering technology that offers a promising path to small, inexpensive, and powerful lidar systems. Lidar, or Light Detection and Ranging, uses laser pulses to capture 3D information about a scene or object. It is used in a variety of applications such as autonomous driving, 3D holography, biomedical sensing, free-space optical communications, and virtual reality.

“Optical beam steering is a key technology for lidar systems, but traditional mechanical-based beam steering systems are bulky, expensive, sensitive to vibration and speed limited,” said research team leader Hao Hu from the Technical University of Denmark. “Although devices known as chip-based optical phased arrays (OPAs) can direct light quickly and precisely in a non-mechanical manner, until now these devices have had poor beam quality and a field of view typically under 100 degrees.”

Hao Hu and Yong Liu have developed a chip-based OPA that enables beam steering with a wide field of view without sacrificing beam quality. The device could enable small, inexpensive and powerful lidar systems. Photo credit: Hao Hu, Technical University of Denmark

Hu and co-author Yong Liu describe their new chip-based OPA, which solves many of the problems that have plagued OPAs optics, Optica Publishing Group’s journal of high-impact research. They show that the device can eliminate a key optical artifact known as aliasing and achieve beam steering over a large field of view while maintaining high beam quality. This combination could significantly improve lidar systems.

“We believe our results are groundbreaking in the field of optical beam steering,” Hu said. “This development lays the groundwork for a low-cost and compact OPA-based lidar that would enable widespread use of lidar for a variety of applications, such as security.”

A new OPA design

OPAs perform beam steering by electronically controlling the phase profile of light to form specific light patterns. Most OPAs use an array of waveguides to emit many rays of light, and then far-field (away from the emitter) interference is applied to form the pattern. However, the fact that these waveguide emitters are typically far apart and produce multiple beams in the far field creates an optical artifact known as aliasing. To avoid the aliasing error and achieve a 180° field of view, the emitters must be close together, but this causes a lot of crosstalk between adjacent emitters and degrades the beam quality. Therefore, until now there has been a trade-off between OPA field of view and beam quality.

To overcome this tradeoff, scientists designed a new type of OPA that replaces the multiple emitters of traditional OPAs with a plate grating to create a single emitter. This setup eliminates the aliasing error since the adjacent channels in the plate lattice can be very close to each other. Coupling between the adjacent channels is harmless in the slab grating as it allows interference and beamforming in the near field (near the individual radiator). The light can then be emitted at the desired angle into the far field. To reduce background noise and other optical artifacts such as sidelobes, the researchers also applied additional optical techniques.

High quality and wide field of view

To test their new device, the scientists built a special imaging system to measure the average optical power in the far field along the horizontal direction over a 180° field of view. They demonstrated aliasing-free beam steering in this direction, including steering beyond ±70°, although some beam degradation was observed.

They then characterized beam steering in the vertical direction by tuning the wavelength from 1480 nm to 1580 nm, achieving a tuning range of 13.5°. Finally, they demonstrated the versatility of the OPA by using it to create 2D images of the letters “D”, “T” and “U” centered at the angles of -60°, 0° and 60° by they tuned both the wavelength and the phase shifters. The experiments were conducted with a beamwidth of 2.1°, which the researchers are now working to narrow to achieve higher-resolution, longer-range beamsteering.

“Our new chip-based OPA shows unprecedented performance and overcomes the long-standing problems of OPAs by simultaneously achieving aliasing-free 2D beam steering over the entire 180° field of view and high beam quality with a low sidelobe level,” he said.

Reference: “Silicon optical phased array with 180 degree field of view for 2D optical beam steering” by Y Liu, H Hu, 4 Aug 2022, optics.
DOI: 10.1364/OPTICA.458642

This work is funded by VILLUM FONDEN and Innovationsfonden Denmark.