This invention relates to nonlinear optical apparatus of the type that provides limiting and clipping characteristics and more particularly, to such apparatus that uses Fabry-Perot resonators as the central element.
In the copending application entitled "Interferometer Apparatus Using Electrooptic Material with Feedback", by P. W. Smith, Ser. No. 902,733, filed May 3, 1978 an optical apparatus is disclosed that is capable of providing a large number of nonlinear functions to an input optical signal. The apparatus disclosed can not only function as a bistable memory element, but can also function to provide the well known limiter characteristic.
In this prior art apparatus a single frequency linearly polarized single mode light beam is coupled to one end of a Fabry-Perot resonator. The resonator is substantially filled with an electrooptic material such as lithium niobate having electrodes to which a voltage is applied. A photodetector is positioned so as to be responsive to the optical energy transmitted by the resonator, and this photodetector develops a voltage which is applied to the electrodes of the electrooptic material. By adjusting the resonator so as to provide a predetermined phase shift with zero voltage on the electrodes, the well known limiter charateristic can be provided to limit the amplitude variations that occur in the input optical energy.
Another characteristic that is useful in transmission systems is a clipper characteristic, that is, one which provides substantial loss to low level input signals and which provides little loss or essentially unity gain to signals that exceed a predetermined threshold. By using apparatus that provides a clipper characteristic, an information signal can be separated from a low level extraneous noise signal that may have been introduced in transmitting the information signal over a transmission medium. The apparatus disclosed in the above-identified Smith application cannot provide this highly desirable clipper characteristic.