We　present　a　nonlinear　ytterbium-doped　fiber　amplifier　based　on　enhanced　nonlinear　effects　that　can　produce　a　flat　broadband　spectrum　ranging　from　1050-1225　nm　with　a　maximum　average　output　power　of　7.8　W　at　14　W　pump　power.　Its　repetition　rate　is　89　MHz.　Using　a　pair　of　gratings　and　two　knife　edges　as　a　filter,　wavelength　tunable　picosecond　pulses　of　tens　to　hundreds　of　milliwatts　can　be　obtained　in　the　broadband　spectrum　range.　The　output　power,　pulse　width,　and　spectrum　(center　wavelength　and　linewidth)　are　adjusted　by　tuning　the　distance　of　the　grating　pair　and/or　the　knife　edges.　Fixing　the　distance　between　the　two　gratings　at　15　mm　and　keeping　the　output　spectrum　linewidth　at　approximately　20　nm,　the　shortest　pulse　width　obtained　is　less　than　1　ps　centered　at　1080　nm.　The　longest　wavelength　of　the　short　pulses　is　around　1200　nm,　and　its　output　power　and　pulse　width　are　40　mW　and　5.79　ps,　respectively.　The　generation　of　a　flat　broadband　spectrum　is　also　discussed　in　this　paper.