Thermal regime of femtosecond writing for a phosphate glass

We report on peculiarities of femtosecond writing in phosphate glass at a thermal regime in the sense of two different types of written tracks demonstrated. The operation diagram is shown for a wide range of laser repetition rates (< 2 MHz) and pulse energies (< 400 nJ). In addition, we explain the mechanism of formation of periodic bubble-like structures and present some discussions for a thermal regime in other materials.