While beam steering systems have been used for years for applications such as imaging, display, and optical trapping, they require bulky mechanical mirrors and are overly sensitive to vibrations. Compact optical phased arrays (OPAs), which change the angle of an optical beam by changing the beam’s phase profile, are a promising new technology for many emerging applications. These include ultra-small solid-state LiDAR on autonomous vehicles, much smaller and lighter AR/VR displays, large-scale trapped-ion quantum computers to address ion qubits, and optogenetics, an emerging research field that uses light and genetic engineering to study the brain.
Long-range, high-performance OPAs require a large beam emission area densely packed with thousands of actively phase-controlled, power-hungry, light-emitting elements. To date, such large-scale phased arrays for LiDAR have been impractical since the technologies in current use would have to operate at untenable electrical power levels.
Researchers have developed a low-power beam steering platform that is a non-mechanical, robust, and scalable approach to beam steering. The team demonstrated low-power, large-scale optical phased array at near infrared and on-chip at blue wavelength for autonomous navigation and augmented reality, respectively. They also developed an implantable photonic chip based on an optical switch array at blue wavelengths for precise optogenetic neural stimulation.
The team designed a multi-pass platform that reduces the power consumption of an optical phase shifter while maintaining both its operation speed and broadband low loss for enabling scalable optical systems. The light signal recycles through the same phase shifter multiple times so that the total power consumption is reduced by the same factor it recycles. They demonstrated a silicon photonic phased array containing 512 actively controlled phase shifters and optical antenna, consuming very low power while performing 2D beam steering over a wide field of view. The results are a significant advance towards building scalable phased arrays containing thousands of active elements.
Phased array devices were initially developed at larger electromagnetic wavelengths. By applying different phases at each antenna, researchers can form a very directional beam by designing constructive interference in one direction and destructive in other directions. In order to steer or turn the beam’s direction, they can delay light in one emitter or shift a phase relative to another.
Current visible light applications for OPAs have been limited by bulky table-top devices that have a limited field of view due to their large pixel width. Previous OPA research done at the near-infrared wavelength faced fabrication and material challenges in doing similar work at the visible wavelength.
A major challenge was working in the blue range, which has the smallest wavelength in the visible spectrum and scatters more than other colors because it travels as shorter, smaller waves. Another challenge in demonstrating a phased array in blue was that in order to achieve a wide angle, the team had to overcome the challenge of placing emitters half a wavelength apart or at least smaller than a wavelength — 40-nm spacing, 2,500 times smaller than human hair — which was very difficult to achieve. In addition, in order to make optical phased array useful for practical applications, they needed many emitters. Scaling this up to a large system would be extremely difficult.
Solving these issues for blue meant that the team could easily do this for red and green, which have longer wavelengths. The team is now aiming to optimize the electrical power consumption because low-power operation is crucial for lightweight, head-mounted AR displays and optogenetics.