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