Method and apparatus for operating a laser in an extinction-type optical particle detector

A light extinction-type optical particle detection system, comprising:

• a laser source that produces a laser beam, the laser beam propagating from the laser source in response to a drive current having an amplitude that reaches a lasing threshold, the laser beam characterized by mode-hopping noise energy that is produced by spurious switching of laser operating modes;
• a view volume through which the laser beam propagates and a stream of target particles immersed in a carrier fluid flows;
• a light-extinction type detector positioned on a side of the view volume opposite of the laser course, the laser beam striking the target particles and thereby affecting an amount of the laser beam that is incident on the detector, the detector producing in response to the incident laser beam a detection signal corresponding to a number and/or size of target particles in the view volume;
• and
• a signal generator that produces the drive current such that the drive current amplitude transitions across the lasing threshold by a selected amount and at a selected frequency to purposefully cause the laser beam to operate simultaneously in multiple lasing modes, the selected amount and frequency cooperating to spread the mode-hopping noise energy over a spectral range established by the number of simultaneously operating lasing modes, such that the detector does not appreciably respond to the mode-hopping noise energy when producing the detection signal.

An apparatus comprising:

• a view volume through which a laser beam propagates and a stream of target particles immersed in a carrier fluid flows;
• means for detecting light, positioned in optical communication with the view volume;
• means for generating a laser beam striking the target particles and thereby affecting an amount of the laser beam that is incident on the means for detecting light, the laser beam being characterized by noise energy that is produced by spurious switching of laser operating modes;
• means for producing, in response to the incident laser beam, a detection signal corresponding to a number of target particles in the stream;
• and
• means for spreading the noise energy over a spectral range, whereby the noise energy is maintained at a level to which the means for producing does not appreciably respond in the production of the detection signal.