Abstract

We report continued progress in reducing the pump power required for transparency in a dispersion-shifted distributed erbium amplifier as well as measurements and theory to better describe the signal evolution along its length. Erbium-doped fiber for this study was fabricated by using a variation of the modified chemical-vapor deposition method, referred to as the “seed” fiber method, in which the rare earth is introduced into the preform as a doped fiber.¹ Several demonstrations of distributed erbium fiber using alternative fabrication methods, fiber designs, and pump wavelengths have been reported.^2–6 The dispersion-shifted fiber design chosen here uses a triangular refractive-index core profile, which has been shown to produce low-loss properties in erbium-free applications.⁷ An absorption loss spectrum indicates a low SiOH peak height of approximately 0.5 dB/km, at 138 μm (0.01 in 10⁶ by weight OH), a 1.48-μm pump band absorption of 1.29 dB/km, and a minimum loss of 0.32 dB/km at 1.63 μm (Fig. 1). A background loss of 0.5 dB/km at 1.48 μm was estimated by measuring the loss of a 6.3-km length with a 40-mW pump signal to bleach the loss from the erbium. The typical minimum dispersion wavelength for the chosen design was 1.51 μm with a dispersion slope of 0.055 ps nm⁻²km⁻².

© 1991 Optical Society of America