# Si Photonics for Practical LiDAR Solutions
## 2 An Overview of LiDAR Technologies
### Distance Calculation Methods  
Direct ToF calculates distance from the travel time of short light pulses reflected by targets. It is a straightforward approach and is by far the most widely used approach for automotive LiDAR. It can measure a long range from hundreds of meters for automotive applications to tens of kilometers for aerial or terrestrial mapping applications. It has a high ambient light rejection thanks to the short pulse duration, e.g. several to tens of nanoseconds, and high peak optical power. It is usually paired with a high sensitivity receiver therefore gains significant momentum as Si CMOS based single-photon avalanche detector (SPAD) matures in recent years. The light pulse travel time can be calculated by a  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & & & \\ \cline{3-5}  & & & **Distance Calculation** & \\ \cline{3-5}  & & & **Time-of-Flight** & \\ \cline{3-5}  & & **Direct** & **Indirect (Modulated CW)** & **FMCW** \\ \cline{3-5}  & & **Pulsed** & **AM** & **FM** & **(Optical Freq. Chirp)** \\ \hline \multirow{5}{*}{**Mapping**} & **Mech. Scan** & Production & / & / & R\&D \\  & **MEMS Scan** & Early Prod. & R\&D & / & R\&D \\ \cline{1-1}  & **Imaging** & Production & Production & R\&D & / \\ \cline{1-1}  & **OPA** & R\&D & / & / & R\&D \\ \cline{1-1}  & **SLM** & R\&D & / & / & / \\ \hline \hline \end{tabular}
\end{table}
Table 1: Status of popular LiDAR solutions for autonomous driving, ADAS and consumer 3D sensing with respective to the design choices in distance calculation and scene mapping.  
time-correlated Geier mode detection method [7] or by a coincidence detection method which better tackles a premature sampling issue [8].  
Indirect ToF refers to a group of approaches using continuous optical wave with certain modulation and sampling schemes. In an amplitude modulation (AM) scheme, the amplitude of an optical wave is modulated at a frequency so that the corresponding modulation period is longer than the round-trip travel time and the distance is calculated from the phase shift of reflected light. This approach is sometimes called AMCW, phase-shift ToF, or phase-comparison ToF. The phase shift of reflected light is determined by either directly sampling the received photocurrent or, more commonly, by integrating the photocurrent within gated period synchronized and shifted by every \(\pi\) (or \(\pi\)/2), from the source signal. The latter method is essentially a demodulation process therefore it is also called homodyne indirect ToF. In general, the AM scheme has limited unambiguous range corresponding to the modulation period though the range can be extended by using more than one modulation frequencies to unwrap ambiguous phases. The modulation frequency can also be chirped in more steps so that a beat frequency is generated with better signal-to-noise ratio (SNR). It is called stepped frequency (SF) CW [9] or, less often, frequency modulation (FM) CW which must not be confused with a different FMCW approach described below.  
FMCW is often referred to a scheme using optical wave frequency chirp, i.e., wavelength varying over time, to calculate the distance from the beat frequency generated from mixing the emitting and the reflected light. It essentially uses a local optical oscillator, often from the same light source, as a low noise optical amplifier to achieve a better SNR than what can be obtained in RF domain. Therefore, it can operate at much lower optical power for a given range than most other approaches. On the other hand, it requires a narrow linewidth laser for good temporal coherence within a targeted range, a good linear chirping and receiving system and demanding compute power for signal processing. These requirements currently make a low-cost solution challenging.