425. How does alternating current affect the laser? Does it make the laser reverse?
A diode laser will only emit light (lase) when current flows through it in the proper direction. It is, after all, a diode and only conducts current in one direction. But small fluctuations in current do affect the light emission. If you run a modest current through a laser diode, so that it emits a steady stream of light, and then begin to modulate that current up and down slightly, the light emitted by the laser will modulate up and down slightly, too. In this manner, you can send sound or other information over a laser beam. This technique is useful as a private means of communicating over long distances. Only someone who can "see" the blinking laser beam can detect the information that it contains.