The paper reports on the experimental demonstration of the photon number squeezing improvement by controlling the input spectral phase of ultrashort pulses to optical fibers. Schematic of experimental setup is described. Improvement of squeezing levels is observed even at a normal dispersion regime of a standard quartz fiber. However, spatio temporal coupling at the 4 f pulse shaper spatially shifts the spectral components that are shaped with a steep phase graduation at the spectral mask. Thus, the optical coupling at the fiber input is degraded for those spectral components. This spatio temporal coupling effect is analyzed with a numerical model and the influence on the scheme is discussed. Experimental results at an anomalous dispersion regime with 1.55 μm light source or with a photonic crystal fiber are also presented.