Optical Amplifiers for Photonic Switches

Abstract

Optical switching elements, such as LiNbO3 or semiconductor waveguide devices are inherently lossy. Large arrays using such devices will inevitably accumulate losses that will not be tolerable. The utility of such arrays may be enhanced if optical amplifiers are employed to compensate for these losses. Although semiconductor optical amplifiers [1,2,3,4] hold the promise of allowing lossless switch design, current optical amplifiers have properties that render them unusable in many applications. Two problems currently limiting their use are rapid Fabry-Perot variations in the gain spectra and bidirectionality of the gain. Figure 1 illustrates schematically the gain spectrum of a semiconductor optical amplifier. It can be seen that the broad (≈50-nm) semiconductor gain spectrum is multiplied by a rapidly varying Fabry-Perot spectrum. The spacing between the Fabry-Perot peaks in the figure has been increased for illustrative purposes; the gain bandwidth of a 1.5-µm optical amplifier typically contains about 70 Fabry-Perot peaks spaced 1.3 nm apart. The depth of the rapid variation is determined by the product of the amplifier chip gain and facet reflectivity while the optical length of the amplifier waveguide determines the separation between the peaks.