# Integrated Optical Phased Arrays for Beam Forming and Steering
## 1 Introduction  
In recent years, optical phased arrays (OPAs) have attracted a great deal of attention. At optical wavelengths, small diffraction angles for a given aperture size can be achieved, compared with microwave radar systems. Featured by precise and agile free-space beam forming and steering [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12], OPA techniques are highly desirable. However, current commercially available OPA components are mainly based on mechanical control, such as motor-driven rotating collimation stages/mirrors. Mechanical beam steering provides high efficiency and a relatively large scanning angle, through mechanically moving parts such as a gimbal, which can be susceptible to mechanical wear and tear, less stable, and limited in speed [13; 14] due to the masses involved. The bulky mechanical steering approach is less preferred as it typically degrades in performance due to acceleration, temperature, vibration, and inertial forces. Additionally, mechanical steering often consumes significant energy. Furthermore, they require a complex assembly and calibration process, causing a greatly high unit cost. More importantly, these mechanical approaches are only capable of steering a single light beam. Arbitrary pointing and rapid steering as needed for advanced communication and sensing applications then become difficult or even incapable [14; 15; 16; 17; 18; 19]. Optical beam forming and steering have been demonstrated with minimum mechanical movements or completely solid-state OPAs, and the seminal concept was demonstrated by a 46-channel lithium tantalate phase modulator for one-dimensional (1D) optical beam steering [20]. Existing techniques mainly include liquid crystal (LC) OPAs [21; 22; 23; 24; 25; 26; 27], microelectromechanical system (MEMS) OPAs [28; 29; 30; 31], optical fiber phased arrays [32], III-V laser arrays [33], and photonic crystal waveguides with diffraction gratings [34]. LC OPAs are the main OPA technology and take advantage of high birefringence. In these systems, the orientations of liquid crystals induce a change in the refractive index. They thus act as a programmable lens that can steer the light at will. Although the OPA based on cascaded liquid crystal has a large field of view (FOV) and low driving voltage [19], it has a limited operation speed due to the time needed for molecular reorientation [6; 14; 19; 35; 36] (typically a few milliseconds response time [6; 9; 18]). MEMS technology typically suffers from the same drawback [13; 36] with tuning speeds typically in milliseconds [18; 37]. Additionally, the limited FOV [29; 30; 37] is another drawback due to the multi-wavelength sizes of elements, which gives rise to grating lobes in the far-field patterns [37]. Additionally, tunable dielectric metasurfaces modulated by liquid crystals provide opportunities to develop the next generation light detection and ranging (LiDAR) and display technologies. A metasurface-based transmissive spatial light modulator generates active beam steering with >35% efficiency and a large beam deflection angle of 11\({}^{\circ}\)[38]. A CMOS-compatible all-dielectric silicon (Si)-based metasurface enabled beam steering with high transmission for wavelengths ranging from 2.96 to 3.215 \(\upmu\)m. Different steering angles can be achieved by changing the periodicity of the structure [39]. As one of the promising solutions for the next-generation dynamic beam steering, integrated OPAs simultaneously possess the advantages of being stable, rapid, and precise without relying on any mechanical motion, making them robust and insensitive to external constraints such as accelerations. Some of the integrated OPAs are presented in Figure 1. A wire-bonded device on a chip carrier plugged into a breadboard is presented in Figure 1a. Figure 1b shows a one-dimensional 64-channel chip-scale blue light phased array. A kind of III-V/Si phase-shifter-based OPA is shown in Figure 1c. Figure 1d is the photo of a fabricated OPA chip on a penny. Using an integrated approach, the OPAs can be made very small.