The role of laser coherence length in continuous-wave coherent laser radar

Abstract

The effect of laser coherence length on the performance of continuous-wave coherent laser radar (lidar) is examined. A laboratory lidar investigation using semiconductor lasers with linewidths of the order of 1 MHz has been carried out for ranges much shorter than, comparable with and much longer than the laser coherence length (about 50m). The signal strength, spectrum and fluctuation statistics are all shown to be sensitive to the various effects resulting from limited laser coherence. Different laser line-broadening mechanisms are considered, including phase diffusion and jitter, and the fundamental difference between single-and multiple-mode sources is examined. Limited laser coherence is also shown to give rise to an excess noise contribution, and this can be severely detrimental to the lidar system performance.

Reduction of phase-induced intensity noise in a fiber-based coherent Doppler lidar using polarization control

“…intermittent noise increase in the low-frequency region) occur in fractions of a second. These observations are reminiscent of those reported in previous studies [15,16]. The linewidth of the high-power MOPA-SL used in the current study was earlier measured to be ~100 kHz [6], which are comparable to that of the laser used in Refs.…”

“…The dependence of the excess noise to polarization state differences between LO and residual signals, although straightforward, has not been thoroughly discussed in the literature. Previously proposed models generally treated the level of PIIN to be dependent only on scalar quantities [15,16]. It was shown that for delay times d τ between LO and a stray signal much shorter than the laser coherence time c τ (in our CDL, d τ ≈ 11 ns and c τ ≈ 1.6 µs), the PIIN contribution to the spectra in the typical CDL frequency region may be approximated as a constant noise offset PIIN S to the shot-noise level [15,16], such as…”

An all-fiber image-reject homodyne coherent Doppler wind lidar

“…For instance, due to the presence of phase noise and cross-talk in optical circulators the estimated signal may suffer from interferometric noise [30,31]. Reflections from optical components such as telescope lenses can also be compounding.…”

Monostatic coaxial 15 μm laser Doppler velocimeter using a scanning Fabry-Perot interferometer

“…Because the required reference signal is much weaker (typically ~10 pW versus ~1 mW local oscillator in heterodyne detection), the sFPI-LDV is not strongly affected by local oscillator shot noise and has a more relaxed phase noise and/or coherence length requirements of the laser source unlike heterodyne detection [11]. Laser linewidth affects the speed resolution but not the maximum sensing range of our direct detection LDV whereas in heterodyne detection it is normally limited by the coherence length.…”