Abstract

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. The figure below schematically illustrates the gain spectrum of a semiconductor optical amplifier. It can be seen that the broad (≈70-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). Semiconductor optical amplifiers are usually made by putting an anti-reflective coating on the facets of a laser diode. It is the residual refectivity of the amplifier facets that causes Fabry-Perot resonances in the gain spectrum. The amplitude of this 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. Since optical amplifiers typically have Fabry-Perot periods of 1.3 nm, it can be seen that there are actually around 50 Fabry-Perot periods within the material gain bandwidth rather than the 7 illustrated in the figure. While is possible to obtain high gain from an amplifier with strong Fabry-Perot resonances, the use of such an amplifier obviously presents wavelength control problems to the switch designer. Other less obvious problems in amplifiers with strong Fabry-Perot structure are: 1) the locations of the Fabry-Perot peaks are strongly temperature dependent, 2) the location and gain of the peaks are sensitive functions of the bias current of the amplifier, 3) they have more cross-talk and lower saturated output power than amplifiers with a smooth gain spectrum, and 4) the backward reflected gain is comparable to the forward gain. Given these problems, it is unlikely that amplifiers with large Fabry-Perot resonances will be of much use in photonic switching systems.

© 1987 Optical Society of America