Spectroscopy of chromium doped glass for amplifiers and lasers

Abstract

To date, there has been no demonstration of a transition metal laser that uses a glass host. Calculations show that if reasonable pump power could be confined to the small cross-sectional area of a fibre optic core, the threshold pump power could be brought down to a realistic level for laser action. A transition metal fibre laser, if practicable, would make a cheap, compact and highly tunable laser source in the near infrared. This project comprised two parts. Firstly, spectroscopy and developmental work on a host glass identified as being an excellent prospect for a Cr³⁺ fibre laser. Secondly, the spectroscopic investigation of a new family of glass found to host Cr⁴⁺.

Lithium lime silicate (LLS) glass was identified as an excellent prospect for a Cr³⁺ fibre laser. Not only possessing suitable thermal properties for fibre drawing, LLS had a radiative quantum efficiency (QE) measured as 15% - a very high figure for Cr³⁺ doped glass. It absorbed in two broad bands, 400-500nm and 600-750nm and the emission spectrum was also extremely broad, extending between 700nm and 1050µm. The upper state lifetime was about 35 µs and the peak cross-section of emission was found to be 6 x 10^-21cm². The LLS composition was optimised in terms of its spectroscopy and thermal properties and a fibre was fabricated. An excited state absorption (ESA) investigation was performed on the fibre - this is believed to be the first report of an ESA experiment on transition metal doped glass. Unfortunately, the fibre quality was not sufficient to make a realistic attempt at a demonstration of a fibre laser.

There are very few glasses known to host Cr⁴⁺, and none of these have thermal properties suitable for fibre drawing. This thesis reports the discovery that sulphide chalcogenide glasses, which are stable enough for fibre drawing, also host the chromium (IV) ion. A wide range of sulphide based glass was fabricated and the spectroscopic investigation revealed the QE to be low, ranging from 0 to 3%. The absorption spectra typically extended between 600nm and 1300nm and the emission between 0.8µm and 1.5µm. As the samples were warmed up from liquid nitrogen to room temperature, the peak of emission experienced a remarkable blue shift of up to 400nm owing to a site-dependent non-radiative decay mechanism. The upper state lifetime was found to be about 1 µs. Despite the low QE and short lifetime, calculations suggest that laser action in a low-loss narrow-cored fibre might still be feasible. However, the lifetime cross-section product for arsenic trisulphide, the best of the glasses tried, was about a factor of six times smaller than the Cr³⁺ doped LLS.

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Identifiers

ORCID for William S. Brocklesby: orcid.org/0000-0002-2123-6712

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