Abstract

An improvement in scaling active fiber devices with single-transverse-mode output to higher power levels can be achieved through an increasing in mode-field area. A multimode core allows for a significant increase in mode volume. The gain of the given mode depends on the overlap between mode-field distribution and the spatial distribution of the gain within the multimode fiber core. Spatial distribution of the active ions was designed to provide maximum total gain for the fundamental mode while keeping the gain for the other modes below the level required for significant amplification. To study the effect of mode selection, three fibers were drawn from the same preform [1]. The fibers developed had a step-index design with Al as an index rising dopant, a fiber NA of 0.2 and core diameters of 4.5, 9.5 and 19 μm. The absorption for the fundamental mode in these fibers at 1.53 μm was measured to be 25, 37.5 and 47 dB/m, respectively. The overlap integral between the erbium distribution and mode fields was determined directly from absorption measurements [2] and then compared with the overlap calculated using measured Al and Er profiles. Figure shows the dependence of the overlap integral on the fiber V-number for an LP₀₁ signal mode and higher-order modes. The curves show the overlap computed numerically. The overlap integral and therefore the gain are higher for the fundamental mode. The higher-order modes discrimination is significantly gained from high doping level. As expected and observed experimentally, increasing the doping level increases the modal gain difference and thus improves the fundamental mode selection.

© 1998 IEEE