Mechanism of femtosecond-laser induced refractive index change in phosphate glass under a low repetition-rate regime

Raman microscopy and refractive near-field profilometry were used to analyze waveguides written in Yb-doped Kigre QX glass under the low repetition-rate (noncumulative-heating) regime. It was found that femtosecond-laser induced refractive index change was due to an increase in the proportion of Q¹ P-tetrahedra and the associated increase in the polarizability of the glass. The role of color center formation and removal in this process is clearly defined, phosphorous-oxygen hole centers (POHCs) and PO₃^- ions form as a result of P-O bonds being broken during the modification process, and the subsequent removal of POHCs give rise to the increased proportion of Q¹ P-tetrahedra. This result, when compared to other studies undertaken in the cumulative-heating regime, show conclusively that the mechanism of refractive index change in a particular type of glass can be very different, depending on the irradiation conditions.