Researchers have developed a new chip-based beam steering technique that offers a promising avenue for small, low-cost and high-performance lidar systems. Lidar, or light detection and ranging, uses laser pulses to acquire three-dimensional information about a scene or object. It is used in a wide range of applications such as autonomous driving, 3D holography, biomedical sensing, free-space optical communication and virtual reality.
Beam steering is a key technology for LIDAR systems, but conventional mechanical-based beam steering systems are bulky, expensive, vibration-sensitive, and limited in speed, and although devices called chip-based optical phased arrays (OPAs) are capable of guiding light quickly and precisely in a non-mechanical manner, the beam quality of these devices has so far been poor, with fields of view typically below 100 degrees”. Hu and co-author Yong Liu describe their new chip-based OPA in Optica, the high-impact research journal of the Optica Publishing Group, as solving many of the problems that plague OPAs. They show that the device can eliminate key optical artifacts known as aliasing and achieve beam steering with a large field of view while maintaining high beam quality. This combination can greatly improve LIDAR systems.
This development sets the stage for OPA-based lidars, which are low-cost and compact, which will allow lidars to be used in a wide variety of applications, such as high-level advanced driver assistance systems that can assist in driving and parking and improve safety. OPAs perform beam steering by electronically controlling the phase profile of the light to form a specific light pattern. Most OPAs use a waveguide array to emit many beams and then apply interference in the far field (away from the emitter) to form a pattern. However, the fact that these waveguide emitters are typically spaced far apart from each other and produce multiple beams in the far field creates an optical artifact known as aliasing. To avoid aliasing errors and achieve a 180° field of view, the emitters need to be close together, but this causes strong crosstalk between adjacent emitters and degrades the beam quality. Therefore, there has been a trade-off between field of view and beam quality for OPA so far.
To overcome this trade-off, the scientists designed a new type of OPA that replaces the multiple emitters of a traditional OPA with a slab grating to create a single emitter. This setup eliminates aliasing errors because adjacent channels in a plate grating can be very close to each other. In a plate grating, coupling between adjacent channels is not detrimental because it allows interference and beam formation in the near field (close to the single emitter). The light can then be emitted to the far field at the desired angle. To reduce background noise and reduce other optical artifacts, such as side lobes, the researchers also applied other optical techniques.
To test their new device, the scientists built a special imaging system to measure the average far-field optical power along the horizontal in a 180° field of view. They demonstrated blur-free beam steering in this direction, including steering beyond ±70°, although some beam degradation was seen. They then characterized the beam steering in the vertical direction by adjusting the wavelength from 1480 nm to 1580 nm, achieving a tuning range of 13.5°. Finally, they demonstrated the versatility of OPA by adjusting the wavelength and phase shifter to form 2D images of the letters “D”, “T” and “U” centered at -60°, 0° and 60°. The experiments were conducted with a beam width of 2.1°, and the researchers are now working to reduce the beam width to achieve higher resolution and longer distance beam steering.
