Researchers from the Technical University of Denmark have developed a chip-based beam-steering device to reduce the size and cost of high-performance lidar technology. The device could have applications in autonomous diving, free-space optical communications, 3D holography, biomedical sensing, and virtual reality. Conventional mechanical beam-steering in current lidar systems is bulky, expensive, sensitive to vibration, and limited in speed, said research team leader Hao Hu. Although chip-based optical phased arrays (OPAs) can quickly and accurately steer light nonmechanically, the devices tend to suffer from poor beam quality and limited fields of view, he said. An OPA demonstrated by a team from the Technical University of Denmark replaces the multiple emitters of traditional OPAs with a slab grating to create a single emitter. The design enabled a wide field of view without sacrificing beam quality. Courtesy of Hao Hu/Technical University of Denmark. OPAs approach beam steering by electronically controlling the phase profile of light to form specific patterns. Most use an array of waveguides to emit multiple beams. Interference is applied in far field to form the pattern. However, because the waveguides are usually spaced far apart from one another and the interference occurs in far field, aliasing is introduced. To achieve a 180º field of view, the emitters have to be positioned close together. However, that closeness can introduce crosstalk, which degrades beam quality. Rather than using multiple emitters, the researchers used a slab grating to create a single emitter. Aliasing is eliminated because adjacent channels can be placed in close proximity to one another. Additionally, coupling between the adjacent channels is not detrimental in the slab grating because it enables the interference and beam formation in the near field. The light can then be emitted in the far field with the desired angle. The team also incorporated additional optical techniques to reduce background and other optical artifacts, such as side lobes. Hao Hu, senior researcher at the Technical University of Denmark, and Ph.D. student Yong Liu developed a chip-based OPA that accomplished beam steering with a wide field of view without compromising beam quality. The device could enable small, cost-effective, and high-performance lidar systems. Courtesy of Hao Hu/Technical University of Denmark. To test the device, the team built an imaging system to measure the average far-field optical power along the horizontal direction over a 180º field of view. It demonstrated aliasing-free beam steering in this direction, including steering beyond ±70°, though the researchers observed some beam degradation. The researchers then characterized beam steering in the vertical direction by tuning the wavelength from 1480 to 1580 nm, achieving 13.5° tuning range. Finally, they showed the versatility of the OPA by using it to form 2D images of three letters of the alphabet centered at the angles of −60°, 0°, and 60° by tuning both the wavelength and the phase shifters. They performed the experiments with a beam width of 2.1°, which the researchers are currently working to decrease in order to achieve beam steering with a higher resolution and longer range. The research was published in Optica (www.doi.org/10.1364/OPTICA.458642).
