Dual laser heterodyne optical particle detection technique

Abstract

A gas-borne optical scattering particle counter uses intracavity optical scattering and heterodyne detection techniques to overcome the lower limit on particle size detection stemming from background light scattering by the gaseous carrier in which a particle is immersed. The particle counter uses a heterodyne technique to exploit a basic physical difference between target particle scattered light and the background light. The carrier gas molecules have a pronounced temperature-induced Maxwell-Boltzmann translational velocity distribution and an associated Doppler broadened spectral scattering characteristic that are dissimilar to those of the target particle. The Doppler broadened background Rayleigh light is orders of magnitude spectrally wider than that scattered by a particle in a particle detector view volume. This difference in bandwidth allows the local oscillator light to 'tune in' the target particle light in a beat frequency signal and 'tune out' the background radiation. In this way, most of the Rayleigh scattered light signal can be removed from the total signal, leaving a dominant target particle signal. To develop sufficient local oscillator power, an embodiment using intracavity optical scattering and heterodyne detection techniques is implemented in a dual laser configuration in which a first laser serves for intracavity light scattering and a second laser functions as the local oscillator. The first and second lasers are frequency locked to maintain a substantially constant frequency difference between them and thereby obtain a stable beat frequency signal. The beat frequency signal is proportional to the square root of the product of the target particle signal optical power and the local oscillator power and can be many orders of magnitude larger for coherent (i.e., heterodyne) detection than the scattered light signal for direct optical detection.