  Large amounts of hyperpolarized 3He gas with high nuclear polarization rates
are required for use in neutron spin filters or nuclear magnetic resonance
imaging of human lung. Very high efficiency can be obtained by metastability
exchange optical pumping using multimode lasers to excite the 23S-23P
transition at 1083 nm. Broadband Ytterbium-doped tunable fiber lasers have been
designed for that particular application. Different options for the
architecture of the fiber oscillator are presented and compared. Emphasis is
given to a linear cavity configuration that includes a high reflectivity fiber
mirror and a low reflectivity tunable fiber Bragg grating. Optical measurements
are performed to finely characterize the spectral behavior of the lasers.
Atomic response is also quantitatively probed to assess the optimal design of
the oscillator for optical pumping. Multimode operation matching the 2 GHz
Doppler-broadened helium resonance line and tunability over more than 200 GHz
are demonstrated. Boosting the output of this fiber laser with a Yb-doped fiber
power amplifier, all-fiber devices are built to provide robust, high power
turn-key sources at 1083 nm for improved production of laser polarized 3He.
