# Integrated Optical Phased Arrays for Beam Forming and Steering
## 7 Performances
### Crosstalk and Suppression  
In large-scale OPAs, array performances can be degraded by unwanted coupling (optical or thermal) among tightly spaced components. Crosstalk is typically suppressed by enhanced light confinement and a mode mismatch between adjacent components. Increased thermal resistance can be applied among thermally sensitive components [92]. In theory, an OPA with a half-wavelength pitch enables a highly desirable FOV of 180\({}^{\circ}\)[59]. Compared with metal materials in radio-frequency phased arrays, the mode confinement of a dielectric waveguide is weak. A significant portion of the guided mode resides in cladding layers [59]. Crosstalk produced by mode superposition in OPAs with narrow pitches leads to scattered spots [117]. Standard 220 nm-high and 400 nm-wide Si waveguides should be placed at least \(\sim\)1 um apart to minimize evanescent crosstalk [130]. Some efforts made to suppress optical crosstalk are presented in Figure 12. Demonstrated approaches to suppress optical crosstalk mainly include waveguide pairs with inserted periodic strips [131; 132], waveguide superlattices with phase mismatches [133; 134], subarray superlattice structures [60; 116], half-wavelength-pitch waveguide arrays with minimized overlaps in phase space [94], curved waveguide arrays with different radii [59], sparse aperiodic structures [114], and waveguide arrays with photonic crystal slab insulators [135]. As shown in Figure 12a, a periodic Si strip array was introduced between Si waveguide pair for efficient crosstalk reduction in PICs [131]. Two asymmetrical Si strips [132] can be used as well. Waveguide superlattice structures in Figure 12b were specially designed waveguide array with submicrometer/subwavelength pitches to achieve a very low crosstalk [134]. The waveguide superlattice with phase mismatches achieved by periodically varying widths of waveguides enabled a half-wavelength-pitch OPA with a low crosstalk [133; 134]. SEM of an OPA with varying waveguide widths for minimizing the overlap between adjacent waveguides in phase space is shown in Figure 12c [94]. The half-wavelength-pitch EF OPA without crosstalk achieved an FOV of 180\({}^{\circ}\) and a \(>\)72% efficiency simultaneously [94]. Phase mismatches of a curved waveguide array can be introduced by waveguides with different radii. A 16-channel curved waveguide array with 0.8 \(\upmu\)m pitch achieved crosstalk of \(-\)27.5 dB at a wavelength of 1.55 \(\upmu\)m [59]. For WGA-based OPAs, crosstalk can also be suppressed by other approaches. The photonic bandgap of a 2D photonic crystal was used to optically isolate each element in a 16-channel WGA-based OPA with a small element spacing [135]. By changing the width of each element of a WGA-superlattice, a phase mismatch was introduced between adjacent antennas to prevent crosstalk. A WGA-superlattice-based OPA with a spacing of 0.9 \(\upmu\)m achieved an FOV of 106\({}^{\circ}\)[60]. Another WGA-superlattice-based OPA with a near-half-wavelength spacing (0.8 \(\upmu\)m) overcame conventional crosstalk problems and achieved an FOV of 130\({}^{\circ}\) and a beamwidth of 2.52\({}^{\circ}\)[116]. High-contrast WGAs of an OPA with a flat interference zone worked as a whole device to achieve beam steering without crosstalk. The emission efficiency of the OPA was 93.9% at 1.55 \(\upmu\)m and exceeded 50% in a range of wavelength from 1.48 to 1.62 \(\upmu\)m [110]. Both optical crosstalk and thermal crosstalk affect the applications of OPAs. As mentioned above, many effective efforts have been proposed to suppress optical crosstalk between adjacent waveguides. The tunability of a phase shifter is affected by heat leakage generated by other ones. Thermal crosstalk and phase mismatches disturb beam forming and steering. OPAs with cascaded phase shifters [77; 78; 99] are more susceptible to thermal crosstalk [82]. Compact thermally isolated phase shifters are highly desirable for large-scale OPAs. FOVs of OPAs can be widened by decreasing spacing.