# Integrated Optical Phased Arrays for Beam Forming and Steering
## 7 Performances
### Crosstalk and Suppression
In terms of heat leakage, OPAs with large pitches should be implemented to avoid thermal crosstalk among heaters [8; 74; 86; 98]. However, the FOV is narrow. Some typical thermal crosstalk suppression structures for OPAs are presented in Figure 13. The thermal crosstalk problem can be settled by fan-in/fan-out sections [41; 64; 95; 104], S-bends [81; 98], deep etched isolation trenches [12; 45; 59; 72; 74], and a combination of fan-in/fan-out sections and deep etched isolation trenches [75]. Additionally, dummy phase shifters can also be used to reduce temperature gradient and crosstalk [38]. Figure 12: Some optical crosstalk-suppressed structures. (**a**) The configuration of Si waveguide pair with an inserted periodic Si strip array. Reproduced from [131] published by Springer Nature, 2017. (**b**) Schematic of waveguide superlattice structures. Reprinted with permission from [134] © 2020 Chinese Laser Press. (**c**) Top-view SEM of an OPA with varying waveguide widths for minimizing overlaps in phase space. Reprinted with permission from the author of [94] published on Arxiv, 2018. Based on the fan-in/fan-out sections shown in Figure 13a, TO phase shifters of a 64-channel blue light OPA were 30 \(\upmu\)m apart in the lateral direction to ensure minimal thermal crosstalk. The outputs of the phase shifters were routed to a sparse aperiodic array of 970 \(\upmu\)m spanning [41]. The spacing of the phase shifters of a Si3N4 OPA for visible light communications was more than 20 \(\upmu\)m. After phase shifters, the distance between the waveguides was narrowed down to 1.52 \(\upmu\)m for beam steering [95]. TO phase shifters in a 512-element Si OPA were 20 \(\upmu\)m separated to minimize thermal crosstalk. The OPA with a decreased emitter spacing of 1.3 \(\upmu\)m shown an FOV of 70\({}^{\circ}\)\(\times\) 14\({}^{\circ}\)[104]. To reduce bend losses of waveguides, the bend radii of fan-in/fan-out sections should be large enough. The chip sizes are typically large. In a III-V/Si hybrid OPA, S-bends after splitters were 150 \(\upmu\)m separated to reduce thermal crosstalk. Additional S-bends reduced the spacing to 5.5 \(\upmu\)m for output [81]. S-bends after TO phase modulators were used in a 16-channel polymer OPA to decrease the output waveguide pitch to 12 \(\upmu\)m [98]. A cross section of the TO phase shifter of a MIR OPA is shown in Figure 13b. Deeply etched trenches reduce thermal crosstalk and decrease the required power for a 2\(\uppi\) phase shift by more than an order of magnitude [72]. Based on compact spiral TO phase shifters and deep trenches around, 20 \(\upmu\)m apart placed phase shifters in a sparse aperiodic OPA yielded low thermal crosstalk [45]. For aliasing-free OPAs with plateau envelopes, deep trenches among heaters were etched down to Si substrates to suppress thermal crosstalk and improve power efficiencies [59]. Deeply etched thermal isolating trenches on sides of TO phase shifters in an \(N\times N\) OPA with 2\(N\) phase shifters worked as well [74]. A combination of fan-in/fan-out sections and deeply etched isolation trenches shown in Figure 13c was used to suppress thermal crosstalk [75]. To reduce thermal crosstalk, waveguide pitches in conventional structure was on the order of 100 \(\upmu\)m, which can be reduced by one order using direct contact graphene heaters. Deeply etched trenches, which taking advantage of the poor thermal conductivity of air, improved heating efficiency by a factor of 2 [75]. A variety of effective implementations have been demonstrated for optical crosstalk suppression. Compared with fan-in/fan-out sections and S-bend structures, which occupy large areas, chip sizes of OPAs with deeply etched trenches are smaller. Well-designed compact spiral TO shifters with deeply etched trenches are attractive. Contact heaters with deeply etched trenches are useful to suppress thermal crosstalk.