Er-doped phosphate glasses for near infrared laser sources

Eye-safe lasers operating at the wavelength of 1550 nm are of great interest for the development of coherent sources for remote sensing, such as LIDARs or range finding systems. The most efficient activator ion commonly employed for this emission wavelength is Er3+, while inorganic glasses are suitable host materials because of their low cost, high versatility and the possibility they offer of obtaining the active cavity in the form of an optical fibre. Among the possible host glasses, silica is the most popular choice thanks to its high thermal stability and good mechanical properties. However, its reduced solubility for rare earth ions does not allow the implementation of compact optical devices, while on the contrary phosphate glasses represent an interesting alternative. This work reviews the design, fabrication and characterization of a series of Er-doped phosphate glasses suitable for the realization of eye-safe laser sources. The glasses were doped with increasing Er3+ ion amounts, namely 0.02, 1, 2, 6 and 10 mol%. A series of phosphate glasses with increasing Er3+ ion concentration (Table 1) was prepared by mixing powdered chemicals with 99+% purity. The batched chemicals were melt quenched at temperature above 1300 °C under a dry air flux and finally casted onto pre-heated brass moulds. The physical, thermal and optical properties of the glasses were thoroughly evaluated. All the prepared glasses were homogeneous and highly stable: average values of 486 ± 3 °C and 982 ± 3 °C were measured for glass transition (Tg) and crystallization (Tx) temperatures, respectively. The term ΔT = Tx−Tg is commonly used as an estimate of the glass stability and the obtained value of ΔT = 496 ± 6 °C demonstrates the ability to draw crystal free optical fibres. Prism coupling technique was used to measure the refractive index of the manufactured glasses (Fig. 1) at five different wavelengths. Emission spectra were measured by exciting the glass samples with a laser diode operating at the wavelength of 976 nm, using a lock-in and a chopper to improve S/N ratio. Lifetime values up to 7 ms were measured at the wavelength of 1550 nm for the Er3+:4I13/2 excited state level. The results of the full characterization of the prepared samples will be presented, together with the selection of the best candidate for the fabrication of the active medium of the eye-safe optical fibre laser.