This study investigates the optical absorption and frequency upconversion properties of Erbium (Er3+) ion co-doped quaternary tellurite glasses (TZWE), which have garnered attention due to their promising applications in photonic devices. Tellurite glasses, known for their low phonon energy, high refractive index, and broad transmission window, serve as effective hosts for rare earth ions. The optical band gap energies of the synthesized glasses were determined using Tauc’s plot method and found to decrease from 3.14 eV to 3.01 eV with increasing Er3+ concentration. Correspondingly, the refractive index increased from 2.36 to 2.39, indicating enhanced polarizability of the glass matrix. UV-Vis-NIR absorption spectra confirmed multiple characteristic transitions of Er3+ ions, with intensity rising proportionally to dopant concentration. Upconversion photoluminescence under both 377 nm and 980 nm excitation revealed green and red emissions, primarily attributed to transitions such as 2H1 1/2  ⁴ I₁ 5 /2 and 4F9 /2  ⁴ I₁ 5/2 . A detailed energy transfer mechanism, involving ground-state absorption, excited-state absorption, energy transfer upconversion, and cross-relaxation processes, was proposed to explain the luminescent behavior. The results confirm the potential of Er3+ -doped tellurite glasses for advanced photonic applications such as lasers and optical amplifiers.