A system and method is provided for an improved sensor for detecting particles in air such as smoke particles from a fire. The detector includes a laser diode that generates a light beam that is spread by a lens to fill a specific area of the interior of a sensing chamber before entering into a light trap for extinction. The detector consequently has a better signal to noise ratio and therefore may be more sensitive than prior art detectors. The detector may also use a shorter wavelength laser to allow the detector to sense smaller particles than prior detectors and consequently provide faster response to flaming fires.