# Si Photonics for Practical LiDAR Solutions
## 3 Si Photonics for LiDAR
### LiDAR System Design Considerations
#### 3.2.3 Lasers and Photodetectors  
The availability of high-power laser sources and high sensitivity photodetectors at reasonable cost is also a key consideration when designing a LiDAR product. Table 4 summarizes commonly used laser sources and photodetectors for different wavelengths.  
Most mechanical or MEMS-based LiDAR systems adopt 800-1000 nm wavelength band mainly because of the exceptional signal-to-noise ratio of Si avalanche photodiode (APD) or single photon avalanche detector (SPAD) [40]. With the consideration of eye-safety, pulsed (direct) ToF is often used for long distance ranging. For a Si photonics implementation in this waveguide range, SiN waveguides may be used with careful designs for coping with phase nonlinearity at high peak optical power (e.g., tens of watts) and high phase tuning power which complicates thermal management and crosstalk. At least one well-funded Si photonics LiDAR startup [41] is targeting this wavelength range using an OPA approach [42].  
GaAs based semiconductor diode lasers, including both edge emitting lasers (EELs) and vertical cavity surface emitting lasers (VCSELs), are widely available at these wavelengths. Both EELs and VCSELs can achieve hundreds of watts peak power, by introducing lateral multi-mode and multi-junction stacking for the former and by arranging a 2D array of emitters for the latter, respectively. Both types of lasers present challenges for use in LiDAR to form a high-quality collimated beam due to highly asymmetric divergence and nonuniform wave front, respectively. This problem becomes more severe when they pair with a Si photonics chip because a waveguide generally requires single mode coupling. In practice, such laser sources are often packaged with a single mode fiber which feeds to a waveguide chip. A significant amount of loss is expected when coupling between a multi-mode laser and a single mode fiber.  
At 1064 nm diode-pumped solid-state lasers exist and help high efficiency coupling with a single mode fiber or a waveguide. One the other hand, the sensitivity of Si APD or SPAD sensors reduces by  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Wavelength (nm)** & **Lasers** & **Photodetectors** \\ \hline
850, 905, 940 & VCSEL, EEL & Si PD/APD/SPAD \\
1064 & DPSSL & Si SPAD \\
1310 & EEL & InGaAs PD/APD \\
1550 & EEL (+EDFA), fiber laser & InGaAs PD/APD \\ \hline \hline \end{tabular}
\end{table}
Table 4: Common laser sources and photodetectors for a LiDAR at different wavelengths.  
an order of magnitude compared to its peak value around 800 nm. The authors are not aware of any companies pairing Si photonics with this wavelength at the time of writing.  
At 1310 nm or 1550 nm, diode EELs and photodetectors are very mature thanks to the telecom industry though such lasers are single mode and usually emit much lower power than what direct ToF demands. High power multi-mode lasers also exist but the same coupling issue applies. At 1550 nm, mature erbium-doped fiber amplifier (EDFA) technology, which was commercialized in the 90s and dominate optical amplification in long-haul networks, enables high output power by either amplifying an EEL or enabling a fiber laser. These lasers, however, are bulky and expensive due to the use of long fiber coils and pump laser sources. Nevertheless, some LiDAR companies are using such high power 1550 nm lasers though combining it with Si photonics has yet to be known.  
When FMCW is used, the requirement on output optical power may be greatly reduced to tens of mW or less [43] thus a single diode EEL may provide sufficient optical power. Many LiDAR companies combining Si photonics with FMCW choose 1550 nm diode lasers [44; 45].