  We present a high power single-frequency ytterbium fiber amplifier system
with an output power of 30 W at 1091 nm. The amplifier system consists of two
stages, a preamplifier stage in which amplified spontaneous emission is
efficiently suppressed (>40 dB) and a high power amplifier with an efficiency
of 52 %. Two different approaches of frequency doubling are compared. We
achieve 8.6 W at 545.5 nm by single-pass frequency doubling in a MgO-doped
periodically poled stoichiometric LiTaO3 and up to 19.3 W at 545.5 nm by
frequency doubling with a lithium-triborate (LBO) crystal in an external
enhancement cavity.
