Coherent LiDAR
Understanding Coherent LiDAR
In an FMCW coherent LiDAR, a narrow-linewidth laser (typically external-cavity or fiber laser with linewidth below 100 kHz) is linearly chirped across a bandwidth B over a sweep period T. A portion of this chirped signal serves as the LO, while the remainder is transmitted toward the target. The returned signal, delayed by the round-trip time τ = 2R/c, is mixed with the LO on a balanced photodetector. The resulting beat frequency contains two components: a range-dependent beat fR = Bτ/T proportional to target distance, and a Doppler shift fD = 2vr/λ proportional to radial velocity. By analyzing both the up-chirp and down-chirp beat frequencies, the system separates range and velocity unambiguously.
The heterodyne detection process provides intrinsic optical amplification: the LO power (typically 1 to 10 mW) amplifies the weak return signal above the detector's thermal noise floor, achieving shot-noise-limited sensitivity. This gives coherent LiDAR a 10 to 20 dB sensitivity advantage over direct-detection systems at the same transmitted power. The balanced detector configuration also rejects common-mode intensity noise from the laser and ambient sunlight, providing robust outdoor operation. Silicon photonics integration has reduced coherent LiDAR transceivers to single-chip implementations with on-chip lasers, modulators, and detector arrays.
FMCW Range and Velocity Equations
fR = 2RB / (cT)
Doppler Frequency (velocity component):
fD = 2vr / λ
Range Resolution:
ΔR = c / (2B)
Where R = target range, B = chirp bandwidth (Hz), T = sweep period (s), c = speed of light, vr = radial velocity, λ = wavelength. With B = 10 GHz chirp at 1550 nm: ΔR = 1.5 cm, and a target at 1 m/s produces fD = 1.29 MHz.
Coherent vs. Direct-Detection LiDAR
| Parameter | Coherent (FMCW) | Coherent (Pulsed) | Direct Detection (ToF) | Design Impact |
|---|---|---|---|---|
| Measurement | Range + velocity | Range + velocity | Range only | Object classification |
| Sensitivity | Shot-noise limited | Shot-noise limited | Thermal-noise limited | Detection range |
| Sunlight immunity | High (balanced det.) | High | Moderate (filtering) | Outdoor reliability |
| Range resolution | c/(2B), < 5 cm | cτ/2, 3 to 15 cm | cτ/2, 3 to 15 cm | Point cloud density |
| Complexity | High (laser coherence) | High | Low to moderate | Cost and integration |
Frequently Asked Questions
How does coherent LiDAR measure velocity?
Coherent LiDAR measures velocity through the Doppler shift of the returned optical signal. When the received light mixes with the local oscillator on a balanced photodetector, the beat frequency contains a Doppler component equal to 2vr/λ. At 1550 nm, a target moving at 1 m/s produces a Doppler shift of approximately 1.29 MHz. The system resolves velocity by measuring this frequency, typically achieving 0.1 to 0.5 m/s precision with 1 to 10 microsecond measurement windows.
What is the advantage of coherent LiDAR over direct-detection LiDAR?
Coherent LiDAR provides simultaneous range and velocity measurement in a single scan, near-quantum-limited sensitivity through heterodyne gain where the LO amplifies the signal above detector thermal noise, and inherent immunity to ambient sunlight through balanced detection. The trade-off is greater system complexity, narrower field of view, and sensitivity to speckle. Coherent systems typically achieve 10 to 20 dB better sensitivity than direct-detection at the same optical power.
What applications use coherent LiDAR?
Key applications include automotive ADAS and autonomous driving (FMCW at 1550 nm for simultaneous range and velocity of vehicles and pedestrians), wind energy (Doppler wind profiling at 50 to 300 m ahead of turbine rotors for predictive pitch control), aerospace (clear-air turbulence detection at ranges up to 30 km), atmospheric science (tropospheric wind profiling for weather models), and defense (target identification through micro-Doppler signatures of rotating or vibrating surfaces).