Patent ID: 7002682

Claim:
In an optical particle detection system of a light-extinction type that includes a view volume through which a laser beam propagates and a stream of target particles immersed in a carrier fluid flows, the laser beam striking the target particles and thereby affecting an amount of the laser beam that is incident on a light-sensitive detector positioned in optical communication with the view volume, the light-sensitive detector producing in response to the incident laser beam a detection signal corresponding to a number and/or size of target particles in the stream, the laser beam propagating from a laser source in response to a drive current amplitude that reaches a lasing threshold and characterized by noise energy that is produced by spurious switching of laser operating modes, a method of controlling the laser source to emit a laser beam in which the noise energy is suppressed to a level that does not appreciably affect the fidelity of the signal produced by the light-sensitive detector, the method comprising: modulating the drive current amplitude to transition across the lasing threshold by a selected amount and at a selected frequency to purposefully cause multiple simultaneous laser operating modes, the selected amount and frequency of modulation of the drive current amplitude cooperating to cause the spurious mode switching to occur among a number of laser operating modes sufficient to spread the noise energy produced over a spectral range established by the number of laser operating modes, the spreading of the noise energy causing the noise energy to maintain a level to which the light-sensitive detector does not appreciably respond in the production of the detection signal.