a particle detector that determines the presence of particles in an enclosed volume includes a laser that directs an optical beam to a beam splitter that produces first and second beams . an optical system directs the first beam into the enclosed volume . a detector is positioned adjacent the volume in order to receive back scattered optical energy arising from a particle in the volume encountering the first beam . the back scattered optical energy and the second beam are optically combined so as to overlap in a region and in the overlap region the back scattered optical energy and second beam are in the same state of focus , of the same polarization and are substantially parallel . a detector located at the overlap region produces an electrical signal indicative of the intensity of the back scattered light . a signal processor analyzes the electrical signal to determine the presence of the particle .