  Here we investigate the use of passive intracavity optical filters for
controlling the laser output spectrum of a polarization-mode-locked, ultrafast
Ytterbium fiber laser. With strategic placement of the filter cutoff frequency,
the overall lasing bandwidth can be increased or extended. Overall laser
performance, including pulse compression and intensity noise, is investigated
for both shortpass and longpass filters with a range of cutoff frequencies,
tuned by rotating the filters. We demonstrate the use of an intracavity filter
not only shapes the output spectra, it provides a route for overall broader
bandwidths and shorter pulses in Yb:fiber lasers. These results demonstrate
that spectral shaping is a useful tool to routinely achieve sub-45 fs pulse
durations in Yb:fiber lasers.
