LiDAR: “dToF”, “iToF” vs. “FMCW”
There are many buzz words around Lidar nowadays. This article tries to compare a few key terms from academic perspective. Lidar can take on several different modalities, which can be classified by their dependence on incoherence or coherence of the laser source which is used. Each of these methods uses the optical path of the target reflector to effectively modulate the envelope intensity of the detected signal. Time-of-flight (pulsed) and amplitude-modulated continuous-wave (AMCW) sensors detect range by measuring temporal properties of the received light intensity. Frequency-modulated continuous-wave (FMCW) and optical coherence tomographic (OCT) sensors map properties of the received optical field (amplitude and phase) into intensity and attempt to leverage the knowledge of both the amplitude and phase in order to detect range.
- Pulsed Time-of-Flight (TOF), often also known as dToF
TOF lidar uses the known fact that light travels at a fixed speed through a medium with a constant refractive index (3x108 m/s in air). Examples of pulsed time-of-flight (TOF) systems can be found in [12,13]. The transmitted pulse must be reflected by the target object, and collected by an aperture at the receiver. Range is measured by determining the difference in time of arrival and the time of transmission of the pulse. Fig. 1–1 shows a simple schematic outlining the operating principles of pulsed TOF lidar.
dToF often uses APD, SiPM or SPAD as receiver array.
2. Amplitude Modulated Continuous Wave (AMCW) Lidar
Amplitude-modulated continuous-wave (AMCW) lidar uses similar principles to TOF lidar, in that a target delay is measured at the receiver. However, in the case of AMCW, an intensity pattern is encoded on the transmitted optical power, such as a linear radio frequency chirp. For AMCW, the free-space path encodes a phase shift on the RF chirp, which can be detected accurately by measuring the intermediate frequency after mixing the received intensity signal with a non-delayed electronic version of the chirp. Examples of AMCW lidar systems have been studied in [15,16]. Fig. 1–2 shows a simple schematic outlining the operating principles of AMCW lidar.
3 Frequency Modulated Continuous Wave (FMCW) Lidar
Frequency-modulated continuous-wave (FMCW) lidar can analytically be shown as a comparable method to RF-chirped AMCW lidar, except where the chirped field is the optical field of a tunable laser. Where chirped AM lidar uses the laser as a carrier for an RF signal, and the RF signal is applied to the intensity of the light source, chirped FM lidar modulates the phase of the light source (usually a single-mode laser) such that the optical frequency of the light source is modulated directly. A free-space path encodes a phase shift on the optical chirp, and the phase shift is detected by mixing the reflected chirp with a non-delayed version of the chirp. This mixing occurs at the photodiode upon detection, so no special design beyond good detector design is needed to achieve this mixing effect. A schematic for FMCW lidar is shown in Fig. 1–3.