The femtoWrite™ process is used to locally increase the index of refraction of a glass. One can turn this local refractive index increase into three-dimensional optical waveguides written within the body of the fused silica block. These waveguides are unusual, as both the core and the cladding are made out of pure fused silica. The femtoWrite process is based on the use of femtosecond laser pulses, which are focused inside a fused silica substrate.  At the focal point, the optical field is so intense that, through multiphoton or tunneling ionization and subsequent impact ionization, an electron-plasma is formed.  Yet, since the pulse is extremely short and has little energy content, the glass re-solidifies on a short time scale compared to network vibrations and heat distribution.  This process has been studied extensively.  Raman spectroscopy data indicate that the laser-illuminated zone contains an excess of smaller member SiO2 rings when compared to the unexposed material.  A SiO2 network weighted towards smaller ring sizes is associated with an elevated index of refraction.