# Si Photonics for Practical LiDAR Solutions
## 4 Si Photonics Optical Phased Array
### Key Components
#### 4.2.3 Phase Shifter  
An OPA generally needs a phase shifter for each antenna to steer output beam via optical inference. In a Si photonics OPA, thermo-optic effect is a widely used method to tune phase and it works for both Si and SiN thought the tuning power efficiency is much lower for the latter which has smaller thermo-optic coefficient (refer to \(\beta_{\text{TO}}\) in Table 3). The on-chip heater to induce temperature change in a waveguide-based phase shifter can be realized by TiN or other metal resistors [48; 50; 68], doped polysilicon resistors [62; 70], or a section of doped Si waveguide as resistors [54; 57; 64; 65]. The former two approaches generally consume more power than the latter which directly heats waveguides at the expense of possible optical loss introduced by the dopants via free-carrier absorption. The tuning efficiency can be improved by adding thermal isolation, e.g., deep-etched trench, around phase shifters whereas array density or tuning speed may be negatively impacted. An OPA generally requires 2\(\pi\) phase shifting for each phase shifter. The typical power efficiency of a thermo-optic phase shifter is about 10-20 mW/\(\pi\) (refer to Table 5) though higher efficiency is possible with resonance design at the expense of optical loss and wavelength sensitivity [68]. Thermo-optic tuning is not a fast mechanism, with the fastest bandwidth at around 100 kHz [52; 57] due to its passive cooling process. Thermal isolation may reduce the speed by at least several times. Low tuning speed limits the frame rate of a LiDAR demanding a large number of scanned points. The tuning efficiency and speed must be carefully balanced according to the application. The refractive index of Si can also be changed with free carriers through free carrier absorption induced index change governed by Kramers-Kronig relations [80; 81]. A Si diode is usually used in Si photonics to apply the free carrier effect to shift phase. A PIN diode is generally preferred for carrier injection scheme for more injected carrier volume while a PN diode is preferred for carrier depletion scheme for larger overlap between depletion width and optical mode. This method introduces inevitable free carrier-induced optical loss, and the design trade-offs are calculated and shown in Figure 7a,b. In the calculation it assumes that carrier-changing volume completely overlaps waveguide optical field which often inapplicable in practice due to limited optical mode confinement and/or limited depletion field volume. While telecom applications prefer a carrier depletion scheme capable of fast modulation, the carrier injection scheme is more often used for OPA designs as it has smaller footprint and less optical loss. However, even a carrier injection-based phase shifter is still an order of magnitude longer than its thermo-optic counterpart. On the other hand, it supports several orders of magnitude faster tuning speeds at the expense of often larger power consumption [56; 61]. The circuits to drive phase shifters are usually off-chip. As analyzed in earlier, a practical OPA may be not able to avoid a large number of antennae and phase shifters even for 1D design. When hundreds of phase shifters are used, the interconnection between an OPA chip and an external driver chip may not be easily accomplished with wire bonding due to the limited number of bond pads a chip can fit at its edges. Flip-chip bonding or 2.5D type of advance packaging [68] may be required which may pose challenges in fiber coupling and antenna emission. Monolithically integrating IC with photonics on a chip solves this interconnection issue but the availability of fabrication processes is currently limited. Standard RF-SOI CMOS processes have been used to demonstrate impressive IC-integrated OPAs [62; 70]. However, the optical performance, e.g., waveguide loss, grating coupler efficiency, etc. of such processes are much poorer than that of dedicated Si photonics processes and an OPA made of such processes may not be able to achieve performance goals a LiDAR product requires.