Optical devices based on internally seeded cascaded nonlinearities

Large nonlinear phase shifts of the fundamental wave can occur even for phase-matched second-harmonic generation if a weak seed wave at the harmonic frequency is injected into the /spl chi//sup (2)/ medium. This seed wave could be internally produced inside a device. We propose to utilize such internal seeding to build a single-beam nonlinear switch that is in the configuration of a Sagnac loop. Since the seed wave controls the amount of the second harmonic generated, this effect can also be used to make a modulator, which can electronically switch the output optical frequency of a pulse train from the fundamental to the second harmonic at rates that are well into the gigahertz range. Such a modulator would require much smaller phase shifts, and correspondingly smaller driving voltages, than a conventional electrooptic modulator based on the Mach-Zehnder geometry.