# Si Photonics for Practical LiDAR Solutions
## 4 Si Photonics Optical Phased Array
### Key Components
#### 4.2.3 Phase Shifter
As more foundries are entering the Si photonics space, and some have brought monolithic photonics and electronics processes to market in recent years, we expect to see more OPA designs with monolithic controlling IC on chip. Another way to tackle this interconnection issue is to group some phase shifters and use a single driver circuit to control each group simultaneously [64]. It works well in theory because the relative phase change between adjacent antennae during beam steering is the same for a uniformly spaced array. However, in practice, the accumulated phase of a waveguide is highly susceptible to unavoidable and random waveguide dimension errors from fabrication non-uniformity. The initial phases among all the antennae are generally indeterministic and may vary from chip to chip. Therefore, the capability of individually controlling each antenna guarantees correct phase condition for beam steering. Such random waveguide dimension error induced phase uncertainty grows with propagation length. Nevertheless, with careful designs, it is possible to group certain number of waveguides with equal and short length and control them by a single driver with minimal impact on beam steering [70]. The phase tuning power of a practical OPA can easily approach a watt or more. Consider a 1D antenna with a uniform spacing of 1.15\(\lambda\), corresponding to aliasing angle of 60\({}^{\circ}\), and 1.17 mm aperture, corresponding to 0.1\({}^{\circ}\) beam divergence, the total number of antennae or phase shifters is over 600. With an approximate 10 mW/\(\pi\) phase shifter efficiency, the average power (assuming an average of \(\pi\) shift out of all phase shifters) is about 6 W and peak power may be even higher. Grouping of phase shifter electrodes reduces the number of driving circuits but consumes the same power. Such higher power requires good heat dissipation management. As a comparison, a modern smartphone applications processor generally has a thermal design power around 3-5 W with a die size of 50-100 mm\({}^{2}\) and it can be passively cooled by a good thermal design. Such IC chips are often flip-chip bonded to a substrate where heat dissipates through its dense bonding bumps into a substrate  
Figure 7: (**a**) Calculated refractive index change and extinction ratio change in Si as a function of free electron or hole density, respectively; (**b**) Calculated \(\pi\)-shift length and free carrier absorption loss at such length in Si as a function of free electron or hole density, respectively. The calculation is based on formula and coefficients found in Ref. [81]. as well as through the entire backside to a heat sink. The OPA chip we consider above draws a similar level of power but may only be cooled from its backside due to light emission from its top surface and desired minimal thermal crosstalk between phase shifters. Therefore, a large chip area and a better thermal conduction design may be needed as active or air cooling is not desired in most cases. An OPA using SiN waveguides faces a more severe power management issue because of poorer thermo-optic phase tuning efficiency. The required power is about an order of magnitude higher [71], i.e., tens of watts, and it may demand active cooling which draws even more power from the system. It effectively makes the solution unattractive or even inapplicable given most other LiDAR products consume 10-20 W. The authors believe that if a SiN OPA is used for its benefit in transparency of 800-1000 nm wavelength range, certain performance may need to be traded off in order to simplify the power management. The monolithic integration of III-V materials on Si is slowly progressing toward practicality as it is called for both advanced CMOS and photonics applications. It allows the phase tuning in III-V materials via the same carrier injection mechanism therefore it doesn't solve the power and footprint challenges.