In the past decade, fused silica glass has been a special case of interest in the field of photoinscription for integrated optics. Positive refractive index (Type 1) changes have shown importance in the photoinscribed waveguide domain. Annealing fused silica glass under glass transition temperature shows significant enhancement in refractive index which intern is useful for integrated optics. Using post-mortem phase contrast microscopy and single mode light guiding measurements on photoinscribed waveguides, we determine the influence of the thermal history on the laser-induced refractive index changes. This is a positive development in developing efficient photoinscription techniques.