Improving Lidar with Single-Emitter Phased Arrays

Lidar systems, which have applications ranging from autonomous driving to free-space optical communications, depend on effective optical beam steering. Chip-based optical phased arrays (OPAs) electronically steer optical beams without moving parts, which mitigates the bulk, expense, sensitivity to vibration and speed limitations of conventional mechanical options, such as MEMS. However, electronic steering comes with its own issues: OPAs have typically had poor beam quality and a field of view under 100 degrees.

Now, researchers from the Technical University of Denmark have developed a new OPA that they say can maintain high beam quality over a large field of view—a combination that could lead to improved lidar systems (Optica, doi: 10.1364/OPTICA.458642).

Single emitter

Previous OPAs have generally consisted of a waveguide grating array used as multiple emitters to create specific light patterns. The desired radiation pattern is formed and steered by controlling the phase of each emitter to create constructive interference in the far field at certain angles. To achieve a 180-degree field of view without spatial aliasing—which manifests as spurious “grating lobes” due to strong constructive interference at more than one far-field angle—the emitters must be placed half a wavelength or less apart.

However, spacing the emitters close enough to avoid aliasing creates strong evanescent coupling or increased background noise between adjacent emitters, degrading the beam. So until now, there has been a necessary trade-off between OPAs’ field of view and beam quality.

The researchers behind the new work say their design overcomes this trade-off by replacing the multiple emitters of traditional OPAs with a half-wavelength-pitch waveguide array combined with a trapezoidal slab grating that acts as a single emitter. The use of a single emitter enables interference and beam formation in the near field, allowing the adjacent channels to be close enough together to eliminate aliasing errors without causing detrimental uncontrolled coupling. The light can then be emitted to the far field with the desired angle. The team also applied a Gaussian amplitude distribution for the emitter to suppress side lobes and reduce background noise.

Testing the device

The researchers tested their setup by measuring the average far-field optical power over a 180-degree field of view. They successfully demonstrated aliasing-free beam steering in the horizontal direction, sweeping a range from −70 degrees to 70 degrees at a fixed wavelength of 1550 nm. (The device also steered free of aliasing beyond ±70 degrees, to a full 180-degree range, but with some degradation in beam quality.) When the researchers steered the beam from −40 degrees to 40 degrees, they achieved a side-lobe level of −19 dB, which they say is the lowest side-lobe level demonstrated to date.

To test the vertical direction, they tuned the wavelength from 1480 nm to 1580 nm, achieving a 13.5-degree tuning range. They also used the OPA to form 2D images of the letters “D”, “T” and “U” centered at the angles of −60 degrees, 0 degrees and 60 degrees by tuning both the wavelength and phase shifters.

The beam width in the experiments was 2.1 degrees at the angle of 0 degrees—a relatively large beam width mainly limited by the effective aperture size. The researchers are now working to decrease this width to achieve beam steering with even higher resolution and longer range.

Toward better lidar

“This development lays the groundwork for OPA-based lidar that is low cost and compact, which would allow lidar to be widely used for a variety of applications such as high-level advanced driver-assistance systems that can assist in driving and parking and increase safety,” said research team leader Hao Hu in a press release.

The OPA chips are fabricated on a silicon-on-insulator wafer and can be produced at high volume in CMOS foundries, which the researchers say makes them a more cost-effective option than other OPAs.