  We demonstrate an all-fiber mode-locked laser at all-normal dispersion with a
saturable absorber, which is based on a section of grade-index multimode fiber
with inner microcavity embedded in the splice points. The absorption modulation
depth of this nonlinear saturable absorber is 2.2% in the suitable bent state.
The fiber laser operates at central wavelength of 1035nm with average power of
~1.26mW, pulse-duration of 17.05 ps, repetition rate of 37.098MHz. This kind of
saturable absorber has superiorities of low cost, high stability and high
damage threshold, which may facilitate applications in optical sensing,
material processing and light detection.
