Optical properties of Tm3+ ions in alkali germanate glass

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Abstract

Tm-doped alkali germanate glass is investigated for use as a laser material. Spectroscopic investigations of bulk Tm-doped germanate glass are reported for the absorption, emission and luminescence decay. Tm:germanate shows promise as a fiber laser when pumped with 0.792 μm diodes because of low phonon energies. Spectroscopic analysis indicates low non-radiative quenching and pulsed laser performance studies confirm this prediction by showing a quantum efficiency of 1.69.

Introduction

One approach to Tm lasers uses materials co-doped with Yb and Er and ≈0.95 μm pump diodes. With this approach, Yb absorbs the pump photons, transfers the energy to Er, which subsequently transfers the energy to the Tm upper laser manifold. Even in the ideal case where every energy transfer process has unity efficiency, the ratio of photon energies limits the efficiency to ≈0.5. Another approach pumps Tm directly in the 3H4 manifold at 0.79 μm. If each pump photon produces only one Tm atom in the upper laser manifold, the 3F4, then the ratio of pump to laser photon energies limits the efficiency to ≈0.4. However, Tm pumped by a 0.79 μm diode laser undergoes self-quenching. This produces two Tm atoms in the upper laser manifold. In this situation, the quantum efficiency is ideally 2.0 and the efficiency limit increases to ≈0.8. In Tm doped laser materials non-radiative decay competes with self-quenching. In silica glass, non-radiative decay is very competitive because the phonon energies of silica are high, extending to 1100 cm−1[1]. Thus, the quantum efficiency is near 1.0 rather than 2.0. Germanate glass does not suffer as severely from this problem because the phonon energies are lower, extending to only 900 cm−1[2]. In lanthanide doped glasses and crystals the highest energy phonons exercise the most influence in non-radiative relaxations because multiphonon decay occurs with the fewest number of phonons required to bridge the energy gap between two manifolds. To combat the deleterious effects of non-radiative decay, Tm:germanate glass is investigated. There are very few studies of the optical properties of Tm in germanate glass [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], and those in recent years concentrate on lead germanates [8], [9], [10], [12], [13]. This study concentrates on alkali germinates.

One of the important reasons that we use alkali germanate glasses is that such a glass can be processed in a platinum crucible safely. Lead germanate glass could lead to damage of the platinum crucible, especially under inert atmosphere environment.

Section snippets

Experimental method

The alkali germanate glasses studied here are composed of, in mol%, GeO2 (50–70%), Na2O/Li2O (5–20%), BaO/CaO (5–20%), Al2O3 (2–15%), La2O3 (0.5–6%). The exact percentages for the sample compositions studied here are not known since the samples were obtained from a commercial company, NP Photonics, and the exact composition is proprietary. The samples were doped with 2 and 4 mol% Tm2O3, which replaces La2O3 substitutionally.

High purity chemicals with less than 5 ppm of iron and copper are used as

Spectroscopy

In observing the absorption of the 3F4 manifold in Tm:germanate glass it is noticed that it exhibits a double peak structure which is not seen in Tm:silica or Tm:ZBLAN glasses [14]. This is partially true for the 1G4 manifold as well, where it is somewhat evident for Tm:ZBLAN, but not for Tm:silica. This likely indicates a localized symmetry about the Tm ions in germanate glasses. The absorption cross sections measured look very similar to those measured by Wang et al. [8] for Tm-doped lead

Discussion

A substantial number of results have been presented here on the spectroscopy and pulsed laser action of Tm:germanate glass. The absorption cross section measurement shown in Fig. 1 is quite consistent with results in the literature [8], [13]. A Judd–Ofelt analysis relies on accurate absorption measurements. In particular, the integrated absorption cross section over the wavelength range of as many manifolds possible. This is especially true in the case of Tm ions since there are few manifolds

Summary

Pulsed laser experiments demonstrate that Tm:germanate fiber lasers can operate with a quantum efficiency of 1.7. Spectroscopic parameters needed to characterize a Tm:germanate fiber laser, including cross sections and lifetimes were measured and analyzed. A diode pumped, Tm:germanate fiber laser was constructed and evaluated. Both the spectroscopic and fiber laser performance data support the inference of quantum efficiencies near 1.7. This is probable due to low non-radiative quenching of Tm 3

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