Four-Element Receiver for Pulsed 10-mum Heterodyne Doppler Lidar

Heterodyne detection with a weak local oscillator

Heterodyne detection in the limit of weak (a few photons) local oscillator and signal power levels has been largely neglected in the past, as authors almost always assumed that the noise was dominated by the shot noise from a strong local oscillator. We present the theory for heterodyne detection of diffuse and specular targets at arbitrary power levels, including the case where the local oscillator power is only a few photons per coherent integration period. The theory was tested with experimental results, and was found to show good agreement. We show how to interpret the power spectral density of the heterodyne signal and how to determine the optimal number of signal and local oscillator photons per coherent integration.

Effectiveness of simultaneous independent realizations at low carrier-to-noise ratio to improve heterodyne Doppler lidar performance. I. Experimental results

A 1.55-microm continuous-wave heterodyne Doppler lidar (HDL) with three receiver-detector units is used to validate experimentally the findings presented in the Guérit et al. companion paper [Appl. Opt. 41, 2232 (2002)] on the effectiveness of independent realizations to improve HDL performance (velocity or power estimates or both) at a low carrier-to-noise ratio (CNR). In fact, noise has a detrimental effect on the accumulation techniques, so in the Guérit et al. companion paper, the chances of getting "heavy" speckles in HDL signal from many receiver-detector units on a single- or several-shot basis are investigated theoretically and numerically with the Zrnic HDL model. The experimental results enable us to conclude there is a very good agreement (better than 95%) between the performance computed from actual HDL data and from the theoretical prediction.

Effectiveness of simultaneous independent realizations at low carrier-to-noise ratio to improve heterodyne Doppler lidar performance. I. Theory and numerical simulations

It is shown that the performance of heterodyne Doppler lidar (HDL) can be improved by (i) at least one good realization for every single shot or (ii) several simultaneous good realizations for accumulation. Until now, several simultaneous independent realizations at high carrier-to-noise ratio (CNR) have been considered. At low CNR, noise may have a detrimental effect on the accumulation techniques. We determine the chances of getting "heavy" speckles in HDL signals from many receiver-detector units on a single-shot basis and several good realizations on a single-shot basis, which is required for an effective accumulation. The use of multiple receiver-detector units at low CNR is worthwhile in contexts such as space lidar, where optimized treatment is at a premium. We conclude on the effectiveness of many receiver-detector units in parallel in order to achieve simultaneous independent realizations at low CNR to improve the performance of HDL on a statistical basis.

Analytical empirical expressions of the transverse coherence properties for monostatic and bistatic lidars in the presence of moderate atmospheric refractive-index turbulence

There have been many analyses of the reduction of lidar system efficiency in bistatic geometry caused by beam spreading and by fluctuations along the two paths generated by refractive-index turbulence. Although these studies have led to simple, approximate results that provide a reliable basis for preliminary assessment of lidar performance, they do not apply to monostatic lidars. For such systems, calculations and numerical simulations predict an enhanced coherence for the backscattered field. However, to the authors' knowledge, a simple analytical mathematical framework for diagnosing the effects of refractive-index turbulence on the performance of both bistatic and monostatic coherent lidars does not exist. Here analytical empirical expressions for the transverse coherence variables and the heterodyne intensity are derived for bistatic and monostatic lidars as a function of moderate atmospheric refractive-index turbulence within the framework of the Gaussian-beam approximation.