# Integrated Optical Phased Arrays for Beam Forming and Steering
## 4 Configurations
### WGA-Based OPAs  
The first OPA introduced in Si photonics was implemented based on WGA structures, as presented in Figure 8a. The 1D 16-element WGA-based OPA was fabricated on the SOI platform. An FOV of 2.3\({}^{\circ}\)\(\times\) 14.1\({}^{\circ}\) was demonstrated with both TO phase modulation and wavelength tuning [7]. A 4 \(\times\) 4 WGA-based OPA with an FOV of 1.5\({}^{\circ}\) was subsequently realized by the same group (Figure 8b). Wavelength tuning based on grating dispersion was exploited to decrease the number of phase controls [8]. The achieved FOV of 1.5\({}^{\circ}\) is relatively narrow due to the small fill factor of the OPA, which can be further improved by redesigning OPA with a proper fill factor. An 8 \(\times\) 8 WGA-based OPA with Si heaters presented in Figure 8c enabled an FOV of 6\({}^{\circ}\)\(\times\) 6\({}^{\circ}\) and a beamwidth of 1.6\({}^{\circ}\)\(\times\) 1.6\({}^{\circ}\). Lightly doped Si waveguides, which provided considerable resistances and low optical losses, and heavily doped Si leads were used as heaters to drop most of the heating power on waveguides. Zigzagged Si lead structures were used to increase the heating length and to provide thermal isolations between waveguides and metal contacts. The measured TO efficiency was about 8.5 mW per \(\pi\) phase shift [58]. \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline \(\uplambda\)**\(\upmu\)m** & **FOV** & **Spot** & \begin{tabular}{c} **Pitch,** \\ \(\upmu\)m \\ \end{tabular}  & **Array** & \begin{tabular}{c} **Size,** \\ **mm2** \\ \end{tabular}  & **Material** & **Shifter** & \begin{tabular}{c} **Power,** \\ **mW/\(\uppi\)** \\ \end{tabular}  & \ \begin{tabular}{c} **Speed,** \\ **\(\upmu\)s** \\ \end{tabular}  & \begin{tabular}{c} **SLL,** \\ **dB** \\ \end{tabular}  & **Ref. ** \\ \hline
0.488 & \textgreater{}50\({}^{\circ}\) & 0.17\({}^{\circ}\) & \textgreater{}1.95 & 64 & 0.97 & Si\({}_{3}\)N\({}_{4}\) & TO & 30 & 20 & 6.05 & [41] \\  
Based on the same physical principle and the reciprocity of electromagnetic wave propagation, a WGA-based transmitter can be theoretically used as its counterpart. Figure 8d shows the fabricated chip of a 64-element WGA-based OPA receiver [43]. Integrated OPAs suffer from a trade-off between the FOV and the beamwidth. A narrow beamwidth in the far field can be obtained by a large aperture aperiodic emitter array. In contrast, an aperiodic OPA with inherent sidelobe suppression enables a wide FOV, and millimeter-scale aperiodic WGAs can achieve almost uniform emission [108]. As shown in Figure 8e, a millimeter-long aperiodic Si/Si\({}_{3}\)N\({}_{4}\) WGA obtained a wide FOV and a diffraction-limited beamwidth of 0.089\({}^{\circ}\) simultaneously [109]. A 2D 125-element OPA based on aperiodic WGAs enabled an FOV of 18.5\({}^{\circ}\)\(\times\) 16\({}^{\circ}\) by leveraging wavelength tuning and phase control. A narrow beamwidth of 0.15\({}^{\circ}\)\(\times\) 0.25\({}^{\circ}\) was achieved by this OPA with a large aperture size of 0.5 \(\times\) 0.5 mm\({}^{2}\)[14]. A sparse aperiodic OPA based on custom WGAs with tapered slabs and multiple dielectric sections enabled a high FOV-to-beamwidth ratio of 16\({}^{\circ}\)/0.8\({}^{\circ}\). More than 400 resolvable spots were achieved by the 128-element OPA [45]. Some novel WGA-based OPAs have been implemented [110, 113]. A high-contrast WGA-based OPA with a flat interference zone presented in Figure 8f was used to achieve greatly enhanced emission efficiency [110]. The emission efficiency exceeded 50% in a broad wavelength range from 1.48 to 1.62 um, with its peak value of 93.9% at 1.55 um. The proposed structure worked as a whole device to achieve beam steering without crosstalk. The 16-channel OPA enabled an FOV of 45.6\({}^{\circ}\)\(\times\) 20.2\({}^{\circ}\) and a beamwidth of 2.4\({}^{\circ}\)\(\times\) 2.5\({}^{\circ}\)[110]. In another work, a WGA-based multi-beam steering OPA enabled a flat-top envelope near the wavelength of 905 nm, which is achieved by using five non-uniformly placed WGAs in one channel.