Thermally regenerated fiber Bragg gratings written in photosensitive fibers with nanosecond laser pulses without hydrogen loading have been shown to become temperature stable up to 600 °C. In this paper, we give an experimental analysis of the refractive index development for fabricating the gratings and for the thermal regeneration process. Furthermore, we identify a thermal induced type IIa (type In) behavior of the refractive index to be responsible for the regeneration. The variation of refractive index modulation and the average index in the grating region is discussed. With additional annealing it is possible to improve the temperature stability beyond 1000 °C.