# Integrated Optical Phased Arrays for Beam Forming and Steering
## 4 Configurations
### EF OPAs  
Typically, EF OPAs can only be tuned by the phase difference between adjacent elements. As presented in Figure 7a, the first experimentally demonstrated aperiodic OPA was implemented on SOI [9]. The 12-channel unequal spaced OPA with a silica (SiO\({}_{2}\)) overlay and individual TO phase shifters enabled an FOV of 31.9\({}^{\circ}\) at 1.55 \(\upmu\)m [9]. The highly desirable OPAs with wide FOVs are usually limited by grating lobes. An OPA with half-wavelength spacing eliminates the grating lobes and maximizes main lobe power. The first demonstrated OPA with half-wavelength spacing was presented by EF structures [61], as illustrated in Figure 7b. The OPA formed an FOV of 64\({}^{\circ}\) with a beamwidth of 17\({}^{\circ}\) at 1.55 \(\upmu\)m [61]. FOVs are limited by the strong crosstalk of waveguides that are placed in close proximity, compared with their mode sizes. Crosstalk should be suppressed to achieve high-performance OPAs. A dispersion-engineered half-wavelength EF OPA with low crosstalk is shown in Figure 7c [94]. Even for several millimeter lengths, light propagates still in the input waveguide. The coupling among the nearest, second nearest, and third-nearest neighbors of the OPA was below 17 dB. An FOV of 180\({}^{\circ}\) and >72% efficiency was obtained by this EF OPA [94]. The dispersion-engineered OPAs enable wide FOVs and high efficiencies with low crosstalk. However, it is important to note that the optical intensity at large angles are weak under a Gaussian envelope in the far-field pattern. It is difficult to obtain precise imaging with this OPA. A plateau envelope is meaningful for the precise imaging of OPAs, especially for the OPAs with a large steering angle. A 1-\(\upmu\)m-long SiO\({}_{2}\) cavity was used to tailor the envelope of the EF OPA presented in Figure 7d. The steered beam exhibited a peak intensity fluctuation of less than 0.45 dB from \(-\)30\({}^{\circ}\) to 30\({}^{\circ}\). The OPA incorporates a  
Figure 7: EF OPA structures. **(a)** A SOI-based EF OPA. Reproduced from [9] with the permission of AIP Publishing. **(b)** The first demonstrated half-wavelength spacing OPA. Reproduced from [61] with the permission of AIP Publishing. **(c)** A dispersion-engineered half-wavelength EF OPA with low crosstalk. Reprinted with permission from the author of [94] published on Arxiv, 2018. **(d)** A SOI-based EF OPA with a SiO\({}_{2}\) cavity for envelope tailoring. Reprinted with permission from [59] © The Optical Society. **(e)** A 1D 64-channel blue OPA with sparse aperiodic spacing. Reprinted with permission from [41] © The Optical Society. **(f)** Schematic of a 16-channel polymer EF OPA. Reprinted with permission from [98] © The Optical Society. coupling-suppressed curved waveguide array with a near-half-wavelength pitch and enables aliasing-free beam steering with an FOV of 64\({}^{\circ}\) and a beamwidth of 6.71\({}^{\circ}\)[59]. Due to fabrication limitations, the pitch of 0.8 \(\upmu\)m is slightly larger than half wavelength. Sparse OPAs can partially release the fabrication difficulties in forming a small pitch. A schematic of a 1D 64-channel sparse blue light OPA is presented in Figure 7e. The Si\({}_{3}\)N\({}_{4}\) EF aperiodic OPA enabled an FOV of 50\({}^{\circ}\) and a full width at half-maximum (FWHM) beamwidth of 0.17\({}^{\circ}\) in the visible spectral range (488 nm) [41]. Typically, most implemented EF OPAs are based on Si photonics. EF OPAs can also be fabricated based on polymers [97; 98]. Polymers have high TO effects and low thermal conductivities, which can be used as efficient phase modulators with a low drive power. As shown in Figure 7f, a 16-channel polymer EF OPA with TO modulators implemented a lower P\({}_{\pi}\) of 2.5 mW [98]. As mentioned above, EF OPAs typically enable 1D beam forming and steering. Complicated EF OPA structures enable 2D patterns. A mirror mounted on a rotating stage was used to achieve a 2D dynamical pattern [41].