The absorption coefficient of the 1.0 mol%Er3+ -doped TZNT glass is shown in Fig. 4. Absorption bands and their peak positions are assigned based on the free-ion Hamiltonian model [27]. The peak absorption bands (in nanometers), their experimental fexp and calculated fcal oscillator strengths have been obtained using the well-known expressions [28–31] and are shown in Table2. The small value (70.13_10_6) of the root mean square (r.m.s.) deviation between the experimental and calculated oscillator