In the non-ablative regime, femtosecond laser pulse duration is known to affect the nature of the modification induced in the microstructure of fused silica. It has been demonstrated than below 200 fs, two different regimes are found, one at low energy, leading to bulk densification while the second one – for higher energy, leading to self-organized structure - nicknamed nanogratings - that induce a net and localized volume expansion of the material. The first regime is particularly interesting for waveguides fabrication, although, so far, the reported refractive index gain remains modest, typically within 10-3 relative net increase that limits the level of compactness for photonics circuits making use of it. Here, we investigate further how shorter pulses, i.e. in the sub-50 fs range, can increase the level of densification and in turn, the net refractive index gain, and possibly lead to an improve process for photonics device fabrication. First results show that indeed, higher level of densification can be obtained, level that we quantify, and that can be further correlated to a net increase of refractive index.