# Integrated Optical Phased Arrays for Beam Forming and Steering
## 3 Building Blocks
### Emitters  
As a part of integrated OPAs, antennas are used to transmit or receive light. There are two kinds of emitters, i.e., the EF structures with power radiated from the facets of waveguides and the WGAs with nearly vertical emission. Typically, the EF OPAs and the WGA-based OPAs enable 1D and 2D beam forming and steering, respectively. In EF OPAs, 1D beam forming and steering are demonstrated only by adjusting the phase difference between adjacent elements [41; 59; 61; 94; 107]. Using long WGAs as emitters instead, a 1D OPA allows for 2D dynamic beam forming and steering through wavelength-tuning in the polar direction, where the azimuthal dynamical control is still based on the refractive-index tuning [12; 42]. Fully reconfigurable 2D patterns can only be generated with 2D WGA-based OPAs by varying phase differences [10; 58; 86]. By changing the phase differences with control circuits, flexible manipulation and dynamic control can be achieved. The beamwidth in the far-field pattern can be compressed by large apertures [108].  
On-chip guided optical modes are scattered into free space at every periodically perturbed section of a WGA, where a local refractive index contrast is introduced commonly through etching, and the strength of light scattering in a WGA is determined by etching depths. Millimeter-scale WGAs enable uniform emission by varying duty cycles [14; 109]. This essentially builds a large aperture [108]. A millimeter-long Si/Si\({}_{3}\)N\({}_{4}\) WGA was used to achieve a diffraction-limited beamwidth of 0.089\({}^{\circ}\) and maximum FOV simultaneously [109]. The 500-um-long WGA enabled a large emitting aperture and a small beamwidth of 0.15\({}^{\circ}\)[14].  
Without breaking the up/down symmetry in WGAs, about half of light radiates to substrates [99]. Any downward radiation causes decreased efficiency. Additionally, the downward radiation experiences multiple reflections and leads to beam interference and fluctuation in the far-field pattern [71; 99]. Considering the power losses of sidelobes, the estimated efficiency can be even as low as \(\sim\)25% [99]. To significantly improve the efficiency, distributed Bragg reflectors [90] or metal layers [76] underneath the WGAs were fabricated to reflect the undesirable downward emission, which increased manufacturing costs.  
A dual-layer millimeter-scale Si\({}_{3}\)N\({}_{4}\) WGA with upward radiation efficiency of 93% at 1.55 um was proposed [108]. The perturbation strength was aperiodically modified along the millimeter-scale antenna such that uniform and efficient emission was achieved for the first time. This allows for the elimination of blind spots [108]. A new type of high-contrast WGAs with a flat interference zone greatly improved the emission efficiency and were used to achieve beam steering without crosstalk [110].  
Usually, a deposited overlayer is used to protect OPAs. The overlayer interface together with a BOX-substrate interface unavoidably forms a Fabry-Perot cavity. In this case, light emitted into free-space experiences a transmission response varying with angles [108]. Directionality of WGAs depends on the thicknesses of both the upper and lower claddings, which should be optimized to enhance transmission toward free space [71; 76; 90]. To match the optimal thickness, commercial-available customized Si-on-insulator (SOI) wafers [76] and fabrication processes with designed overlayer thickness should be applied. Optical lengths of the cavity (including BOX layer and waveguide) should be odd-integer multiplesof a quarter wavelength to maximize upward emission. Such a cavity suppresses reflection at the air interface [71].