Laser writing in tellurite glasses

The market for devices operating in the mid-infrared wavelength range has been booming since the year 2010 aiming at several vital fields like medical, environmental sensing/monitoring, military surveillance, astronomy, and high-sensitivity gas detection. Tellurite glasses have received much attention due to their mid-infrared transparency up to 6 μm. This is one of the most significant spectroscopic windows due to the high absorbance level of several trace gases and the low-loss transmission in the atmosphere. Tellurites possess better mechanical robustness and thermal stability when compared to other low phonon energy glasses. Waveguide fabrication in tellurite glass had a challenging phase until the first successful demonstration by the ultrafast laser writing method, the only technique capable of realizing 2D/3D active and passive waveguides in the glass. Due to the high nonlinear third-order susceptibility of tellurite glasses, initial reports were mostly focused on controlling the filamentation effects which motivated the use of longer femtosecond or picosecond pulses. Modifying the tellurite base matrix using niobium or phosphorus was the most common practice to avoid nonlinear propagation effects during ultrafast laser writing. Picosecond pulses could produce only transient refractive index changes in niobium-modified tellurite glasses, whereas femtosecond pulses could make permanent changes. Low-loss active waveguides showing broadband amplification with net gain at telecom wavelengths were reported in phosphorus-modified tellurite glasses. Very recently, strong ion migrations were also reported in laser-written tellurite channel waveguides, widening the parameter space to increase the refractive index change accessible. This in fact will help to produce 2D/3D channel waveguides at mid-infrared wavelengths.