Fiber Lasers: Unclad Yb:YAG crystalline fiber acts as purely waveguided laser

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As is true with bulk solid-state lasers, transitioning from the use of dopedglass to the use of a doped single-crystal gain medium is an important step inscaling up the output power of individual fiber lasers.

For this reason, single-crystal fiber(SCF) is under intensive development. Various configurations have been created,including crystalline-clad crystalline fiber created using adhesive-freebonding (difficult to fabricate and thus not suitable for mass production),glass-clad crystalline fiber (not suitable for power scaling because of thethermal insulating properties of the glass cladding), and unclad SCF (which isvery suitable for research, but will need a suitable cladding depositiontechnique for wider use).

Previous tested unclad SCFconfigurations have consisted of short (30–60 mm long) flood-illuminatedcrystalline rods with a 1-mm-or-less diameter in which the pump light bouncesalong inside the rod because of total internal reflection, but the modestructure of the output laser light itself is defined solely by the cavitymirrors—in other words, the SCF has not acted as a waveguide for the laserlight and is more a bulk than a fiber laser.

In contrast, a longer and thinnerdiode-pumped unclad ytterbium YAG (Yb:YAG) crystalline fiber recently developedand tested by researchers at the U.S. Army Research Laboratory (ARL, Adelphi,MD) and Shasta Crystals (San Francisco, CA) has demonstrated pure waveguidedlaser operation (see figure).

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