Abstract: A particle counter for optically detecting an unconstrained particle of less than one micron in size suspended in a flowing liquid includes a sample chamber having a fluid inlet and a fluid outlet; a laser module producing a laser beam; a beam shaping optical system providing a multiple laser beam pattern in the sample chamber, and a CMOS optical detector located to detect light scattered by the particles in the sample chamber, the detector producing an electric signal characteristic of a parameter of the particle. The particle counter has a particle sensing area within the sample chamber in which the intensity of light is at least 10 Watts/mm2, the sensing area having an area of 0.5 square mm or more. The detector has thirty or more detector array elements. In the preferred embodiment, the laser optical system reflects and refocuses the laser beam to effect multiple passes of the same laser beam through the sensing area.

Abstract: A fluid particle counter comprising an intracavity diode pumped solid state laser having a solid state lasing material having a non-reflective coating and a concave mirror having a reflective coating, with the coatings isolated from the sample flow by Brewster windows. The laser beam is apertured by an aperture assembly including an inner aperture closest to the inlet nozzle assembly and an outer aperture farther from the inlet nozzle assembly, with the outer aperture significantly farther from the inner aperture than the inner aperture is from the inlet nozzle assembly.

Abstract: An optical particle counter has a gain-apertured laser cavity producing laser light, an inlet jet providing fluid flow into a particle detecting region within the laser cavity, the inlet jet having an inlet jet orifice; a detection optics assembly located to collect light scattered from particles with the detecting region for producing an output signal indicative of the particles; and an optical barrier complex located to reduce noise as compared to the gain-apertured system without the optical barrier complex for fluid flow rates greater than or equal to about 0.1 cubic feet per minute. The optical barrier complex inhibits laser light from illuminating turbulent eddy currents originating on the interior walls of the inlet jet. The optical barrier complex includes one or more physical apertures, one or more optical stops, or both which are located to prevent laser light from illuminating the eddy currents.

Abstract: An optical particle counter has a gain-apertured laser cavity producing laser light, an inlet jet providing fluid flow into a particle detecting region within the laser cavity, the inlet jet having an inlet jet orifice; a detection optics assembly located to collect light scattered from particles with the detecting region for producing an output signal indicative of the particles; and an optical barrier complex located to reduce noise as compared to the gain-apertured system without the optical barrier complex for fluid flow rates greater than or equal to about 0.1 cubic feet per minute. The optical barrier complex inhibits laser light from illuminating turbulent eddy currents originating on the interior walls of the inlet jet. The optical barrier complex includes one or more physical apertures, one or more optical stops, or both which are located to prevent laser light from illuminating the eddy currents.