Various types of pre- and post-treatments to optical fibers are typically used to improve and/or change the properties of fiber Bragg grating (FBG). Here, we investigate experimentally the effects of NIR femtosecond (fs) laser pulses used for pre- and post-treatment on the refractive index of the fiber, and the resulting center wavelength shift of an fs inscribed FBG. We observe “red”-shift when applying a suitable pre-treatment, and both “blue”- and “red”-shifts when applying post-treatment. We characterize the photo-treatment parameters and compare to an FBG inscribed on a fresh fiber without any treatment. We also show that when the photo-treatment is saturated the result is a phase-shifted grating. Our results give further insight into the process of fs photo-treatments and quantify the effects in the case of fs FBG inscription.