In an optical communications system, it is necessary to control the extinction ratio of a laser device of an optical transmission unit. The system randomly selects an end time for the auction by picking an auction duration from a probability distribution, so that bidders have no preferences over the time of bidding. In an embodiment, the cytometer includes a field stop having an H-shaped aperture and light diverters over the ends of the aperture that divert a portion of the light to peripheral detectors.