Pacific Scientific Instruments Company

Mingguang Li

Kenneth L Girvin

Richard K DeFreez

An optical scattering particle counter uses optical scattering and heterodyne detection techniques to overcome the lower limit on particle size detection stemming from background light scattering by the fluid 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. For gas-borne particulate monitoring, 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. For liquid-borne particulate monitoring, background optical noise generated by Brillouin scattering by the liquid carrier places a lower limit on particle size detection. With heterodyne detection techniques, the Brillouin broadening of the background light signal significantly reduces the background light signal seen by the photodetector. For gas-borne or liquid-borne particulate monitoring, the heterodyne beat frequency signal not only reduces the background light signal but also increases the signal representing the target particle light. With heterodyne detection, the beat frequency signal is proportional to the square root of the product of the target particle signal optical power and local oscillator beam optical power. Because the local oscillator beam optical power can be many orders of magnitude greater than the target particle signal optical power, the beat frequency signal can be many orders of magnitude larger for coherent (i.e., heterodyne) detection than the scattered light signal for direct optical detection.

High sensitivity optical fluid-borne particle detection

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.

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Grants Pass, OR, United States of America

Mingguang Li