  Cylindrical vector beams (CVBs) have a wide range of applications owing to
their particular polarization characteristics and optical field distributions.
For the first time, an azimuthally polarized fiber laser without any
polarization controller is proposed and demonstrated experimentally. The scheme
is based on a self-designed ring-core fiber and transverse mode filter (TMF).
The ring-core fiber can break the degeneracy of LP11 modes and make the TE01
mode propagate stably in the fiber laser. The TMF, which made from the
ring-core fiber by depositing a layer of Aluminum on the cladding surface, can
effectively suppress the modes other than TE01 mode. The fiber laser can stably
operate at TE01 mode with a narrow 30dB linewidth of 0.18nm, which indicates
the laser is polarization-maintained. This study opens a new avenue toward the
true application of CVBs fiber lasers.
