  A novel mode-locked fiber laser based on offset splicing technology between
two kinds of graded index multimode fibers is proposed and experimentally
demonstrated. The offset splice spot structure acts as a saturable absorber via
nonlinear multimodal interference mechanism. The absorption modulation depth of
the device is 2.8%, with a saturation intensity of 2.5 MW/cm2. The fiber laser
operates at a stable mode-locked state when the pump power reaches 114 mW. The
laser oscillates at 1039 nm with a pulsewidth of 17.6 ps, a repetition rate of
32.619 MHz, and a high signal noise ratio of 65 dB. The fiber laser is superior
in terms of low cost, high damage threshold, and easy fabrication, thus
providing many potential applications in ultrafast photonics and industrial
processing.
