Fig. 1. Schematic diagram of fiber preform fabrication.

Fig. 2. (a) Radial refractive index profile. Inset: micrograph of MS-EDF cross section. (b) Concentrations of Al³⁺ and Er³⁺ in the MS-EDF and SG-EDF from EPMA measurement.

Fig. 3. (a) Background loss and (b) absorption of the MS-EDF.

Fig. 4. (a) Simplified energy level diagram of isolated Er³⁺ ions and Er³⁺-Er³⁺ pairs with an excitation at 980 nm. (b) Unsaturable absorptions at 976 nm of a 25 cm SG-EDF (blue) and a 32 cm MS-EDF (red). (c) Fluorescence decay curves obtained from the two fibers.

Fig. 5. (a) Schematic of EDFA experiment. (b) Dependence of the gain and NF of the MS-EDF on the wavelength with different fiber lengths.

Fig. 6. (a) Schematic of short-cavity laser experiment. (b) Dependence of laser output power on absorption pump power for different fiber length. (c) Laser spectra of 3.7 and 1.8-cm-long fibers. Inset: the enlarged and normalized spectrum (left) and the picture of the laser cavity (right) using the 3.7-cm-long MS-EDF.