Hi Ebraheem,
let me discuss two points with you:
first point:
the output of the Yb fiber is the green spectrum which centers around 1 um. the blue spectrum is the residual of the pump not the output of the Yb.
So you can either decrease the pump level or increase the Yb fiber length to decrease the residual pump spectrum from the Yb other side.
but the frequency conversion happened in the Yb fiber is represented by the output green spectrum.
Second point:
you want the output spectrum to be centered around 520 nm. you can NOT get this spectrum directly from Yb-doped fiber laser ONLY. I think after the Yb doped fiber laser you need a second harmonic generator (SHG) to double the frequency (half the wavelength).
So please check your resources for this required output. so you might need to do SHG or another nonlinear process to double the output frequency.
Hope my answer clarifies your issues.
Regards,
Mohamed Abouseif