The defects generate new absorption line called as color center.

Due to the color center, radiation induced absorption (RIA) increases.

so, can i imagine to put a maximum defects while the fabrication in order to maximise the sensibility of my finer ? if yes, how can i control during the drawing for this.

What will mostly influence the fiber RIA sensitivity is the level of precursor sites meaning defects that will be upon irradiation converted into other defects with stronger RIA at the wavelength of interest. This is the case for example for the GLPC in Ge-doped optical fibers that will be converted during irradiation in other Ge-related defects such as Ge(1) or Ge(2). Having already-absorbing strong defects may not help to enhance the sensitivity.

So, to have the most radiation sensitive fiber, you must try to enhance the number of precursor sites that will lead to absorption at the wavelength of interest. One example: if your system works at 633nm, you could try to create a fiber with a lot of strained Si-O-Si bonds by using specific drawing conditions. When the fiber will be irradiated, these weak bonds will be converted into NBOHC absorbing at 630nm...