The design and performance of a simple, multifunction 1.55-mum continuous-wave (cw) and frequency-modulated cw coherent laser radar system with an output power of 1 W is presented. The system is based on a semiconductor laser source plus an erbium-doped fiber amplifier, a polarization-independent fiber-optic circulator used as the transmit-receive switch, and digital signal processing. The system is shown to be able to perform wind-speed measurements even in clear atmospheric conditions when the visibility exceeds 40 km. The aerosol measurements indicate the potential to use single-particle detection for wind measurements with enhanced sensitivity. The system can perform range and line-of-sight velocity measurements of hard targets at ranges of the order of several kilometers with a range accuracy of a few meters and a velocity accuracy of 0.1 m/s by use of triangular-wave frequency modulation with compensation of the frequency-modulation response of the semiconductor laser. The system also demonstrates a capability for vibration sensing.

The performance of a frequency-modulated continuous-wave (FMCW) semiconductor laser radar has been examined. Frequency modulation (linear chirp) has been studied experimentally in detail. To create a linear frequency sweep, we modified the modulating function according to the measured frequency response of the laser, using an arbitrary function generator. The measurements indicate the possibility of achieving a spectral width of the signal peak that is transform limited rather than limited by the frequency modulation response of the laser, which permits the use of a narrow detection bandwidth. The narrow width results in a relatively high signal-to-noise ratio for low output power and thus also in relatively long-range and high-range accuracy. We have performed measurements of a diffuse target to determine the performance of a test laser radar system. The maximum range, range accuracy, and speed accuracy for a semiconductor laser with an output power of 10 mW and a linewidth of 400 kHz are presented. The influence of the laser's output power and coherence length on the performance of a semiconductor-laser-based FMCW laser radar is discussed.

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.

We demonstrate the successful operation of a cw laser Doppler wind sensor at a wavelength of 1.55 mum. At longer ranges (>100 m) the signal conforms closely to complex Gaussian statistics, consistent with the incoherent addition of contributions from a large number of scattering aerosols. As the range is reduced, the probe volume rapidly diminishes and the signal statistics are dramatically modified. At the shortest ranges (<8 m) the signal becomes dominated by short bursts, each originating from a single particle within the measurement volume. These single-particle events can have a very high signal-to-noise ratio (SNR) because (1) the signal becomes concentrated within a small time window and (2) its bandwidth is much reduced compared with multiparticle detection. Examples of wind-signal statistics at different ranges and for a variety of atmospheric backscatter conditions are presented. Results show that single-particle-scattering events play a significant role even to ranges of ~50 m, leading to results inconsistent with complex Gaussian statistics. The potential is assessed for a low-power laser Doppler wind sensor that exploits the SNR enhancement obtained with single-particle detection.

Tel: +46 13 31 80 00, Fax +45 13 33 16 65 1 Abstract A C02-ladar system is used for measurements. The signal from the system is a sinusoidal FMmodulated multi-component signal. To extract the modulating frequencies time-frequency representations, e.g., the Wigner-Ville distribution (WVD) and the Choi-Williams distribution (CWD) are used. The estimation method is applied both to simulated and real data. Estimation of the vibration frequencies is shown to be feasible even for low SNR, e.g., -4 dB.