Improved blue laser results and photochromic effects in Tm:ZBLAN fibre

Barber, P.R., Paschotta, R., Tropper, A.C. and Hanna, D.C. (1995) Improved blue laser results and photochromic effects in Tm:ZBLAN fibre. 12th UK National Quantum Electronics Conference (QE12), , Southampton, United Kingdom. 03 - 07 Sep 1995.

Abstract

Upconversion lasing in Thulium-doped fluoride fibre has proved to be an efficient method of upconverting infrared light into blue laser emission and has been identified as a promising method of producing a compact short wavelength laser. There remains considerable scope for optimisation of performance, and we have been able to improve on previously reported results. However, attempts at optimisation by going to higher dopant concentrations have revealed interesting and unexpected photochromic effects. Optimisation of the output coupling has produced a slope efficiency for blue lasing output with respect to absorbed infrared power of 34% in 1000ppm Tm-doped fibre. Further, a diffraction grating has been used as an external tuning element and tuning of the blue output over a wavelength range of 475 to 486 nm has been achieved.

To date the best results have all been achieved with fibres of very similar parameters; 1000ppm Tm-doped, NA = 0.2, single mode cutoff at 800 nm. We have recently investigated the effect of increasing the Tm concentration and have encountered a number of unexpected results. In more heavily doped fibres, pumping the Tm absorption at 1.14 µm, the optimum wavelength for upconversion pumping, results in an induced fibre loss across the visible spectrum. While this loss appears stable at room temperature it can be reversed by pumping with a significantly lower power at the same wavelength. The spectrum of the induced loss has been measured from 0.4 to 1 µm and the peak loss is dependant on the inducing power but has been measured as high as 20 dB/m. Such a loss has serious implications for lasing performance. Continuing work is aimed at discovering the source of this photochromic loss.

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