Abstract

Optical fibers are currently being designed for 980- and 1480nm pumped-amplifier systems. Optimum fiber designs (choice of erbium radius, fiber-core radius, and refractive-index difference) have been suggested for single-mode operation,¹ and the effect of launching the higher-order LP₁₁ mode has been investigated.² For fibers with an LP₁₁ cutoff wavelength λ_c > 980 nm Desurvire et al.³ have demonstrated a variation of as much as 20 dB in amplifier gain when the pump power (λ_p = 980 nm) shifts from the LP₀₁ to the LP₁₁ mode in a fiber with E= 0.25 (ϵ = r_Er/e). This result signifies that amplifier performance is highly dependent on the manner in which the pump power is distributed among the fiber modes and that repeatability in the performance of several amplifier modules cannot be guaranteed. Similarly, for 1480-nm pumping, diameter fluctuations or departures in refractive indices from predesigned profiles result in changes in the fiber cutoff wavelength, thereby leading to unoptimized amplifier performance. In this paper we show that placing the erbium within a ringlike structure helps to resolve both these issues and minimizes performance variations.

© 1993 Optical Society of America