Almost all optical systems, including LiDAR for autonomous vehicles and robotic applications, medical imaging systems such as optical coherent tomography, and high-speed point-to-point optical communication links greatly benefit from a photonic beamforming and beam steering sub-system. This sub-system directs and concentrates the optical beam in the desired direction, improving the system's sensitivity and imaging resolution.
Most of the today's solutions for photonic beamforming are mechanical or MEMS with moving parts, which makes the beam steering performance susceptible to mechanical vibrations. Furthermore, most mechanical beam steering devices are slow (tens of kilohertz), limiting their usefulness for high-speed (megahertz rate) beam steering applications.
The emergence of integrated silicon photonic platforms enables the realization of optical phased arrays (OPAs), which address the limitations of MEMS and mechanical beam steering devices. Similar to phased array radars, these devices achieve beamforming and beam steering by adjusting the relative optical phase of the optical antennas in the aperture.
The relative phase can be adjusted using electro-optic phase shifters which can operate at gigahertz frequencies and hence are high-speed and fully solid-state with no moving part. Another advantage of OPAs is their ability to arbitrarily change the wavefront profile, which can be used to adaptively change the wavefront for different applications and scenarios, making them more versatile than their MEMS and mechanical counterparts.
Furthermore, silicon photonic platforms fabrication processes are becoming mature and can be used for making optical phased arrays, which significantly reduces the cost of integrated OPAs. This makes the OPAs more accessible for consumer electronics applications.
Current integrated photonic optical phased arrays have their drawbacks. Optical phased arrays suffer from various performance-limiting factors when scaled to larger array sizes. The two most common solutions either require a precision, widely-tunable laser source to provide steering in the second dimension that adds to system cost and complexity (1D-grid OPAs) or have a very limited field-of-view (FOV) (2D-grid OPAs). In addition, the interconnect complexity between the photonic components and the electrical components can further hinder the scalability of OPA systems.
Dr. Aroutin Khachaturian, Dr. Reza Fatemi, Artsroun Darbinian, and Professor Ali Hajimiri from California Institute of Technology's Holistic Integrated Circuits Laboratory (CHIC), inspired by the diffraction pattern of an annular ring, have proposed and demonstrated a multi-annular ring optical phased array architecture that achieves a radiator-limited FOV with reduced system complexity. The research results are published in Photonics Research, Volume 10, No. 5, 2022 (Aroutin Khachaturian, Reza Fatemi, Artsroun Darbinian, Ali Hajimiri. Discretization of annular-ring diffraction pattern for large-scale photonics beamforming[J]. Photonics Research, 2022, 10(5): 05001177).
Fig. Multi-annular-ring OPA concept and implementation. (left) An annular ring aperture is approximated using a discretized annular ring. (right) A multi-annular-ring aperture enables ultra-compact, high performance, and low-complexity optical phased-arrays for solid-state beamforming applications.
The diffraction pattern of an annular ring (a simple Bessel function) can concentrate the incident plane wave to a single point in the far field. This continuous structure can be approximated by placing optical antennas (radiators) along the circle's periphery. This discretized annular ring structure can be easily implemented using an OPA, which can steer the beam in both directions by adjusting the relative optical phase of the different optical antennas.
The proposed OPA architecture, incorporating discretized annular rings as apertures, provides a design methodology to systematically address the challenges with integrated phased array. It does not require a precision tunable laser and has a large FOV, limited by the FOV of the individual radiating elements.
Furthermore, the system complexity is significantly reduced. The implemented system can project the optical beam in over 9,000 different directions using only 255 radiating elements, which require only 100 electrical control circuitries. This simplifies beamforming by order of magnitude in the photonic implementation and almost two orders of magnitude in control electronic complexity.
Professor Hajimiri said: "As optical phased arrays transition from a curiosity to relevant active devices, it becomes more important to explore the design space of such arrays by looking at all variations of the system architecture. The polar multi-annular ring approach to OPA design can overcome some of the limitations of the more conventional approaches used to realize OPAs, by providing a systematic approach to design more optimum sparse arrays."
The group implemented the proposed multi-annular ring OPA in a standard foundry fabricated silicon photonic process demonstrating both active beamforming and beam steering. The entire structure was less than 5 mm2 in area. This beamforming and beam steering system can be used as an ultra-compact, low-cost, and low-power solution in compact LiDARs, medical imaging devices, and free-space data transmission links.