Thulium-doped upconversion fibre-laser with 230mW of 480nm blue output

Blue laser sources are required for a number of applications such as colour displays, printing and data recording. Three main approaches are currently pursued. Blue emitting laser diodes have recently been demonstrated, albeit with a number of limitations at present regarding power lifetime and operating temperature. Another approach is frequency doubling of an infrared source; for example 49mW of 473nm light have recently been obtained by frequency doubling the output of a diode-pumped 946nm Nd:YAG laser in a single pass through a periodically poled LiNbO3 crystal. The third approach is upconversion lasing, for which the highest reported power to date, at blue wavelengths, was 106mW from a diode-pumped Tm:ZBLAN upconversion fibre laser. In this paper we report a blue output of up to 230mW, achieved by using a more powerful pump laser and a Tm:ZBLAN fibre with a modified composition, which has allowed higher power operation. Long term operation at the highest power is not yet possible however due to an optically induced loss in the fibre as observed in earlier work. The pump laser was a Nd:YAG laser operating at 1123nm and pumped by a 7W diode-bar. This laser produced 1.6W in a circular Gaussian-beam of M2 < 1.1. The high beam quality allowed overall launch efficiencies into the fibre of between 50 and 60%. The Tm-doped fibre used, produced by Le Verre Fluoré, had a Tm concentration of 1000ppm (by weight), a NA of 0.2, a core diameter of 3µm and length of 2.2m. To form the cavity, dielectric mirrors were dry butted against both ends of the fibre. The input mirror had high reflectivity for blue light, and a transmission of 90% for the 1123nm pump, which was launched through this mirror using an aspheric lens. The output coupler had a transmission of 37% for the blue. This laser had a threshold of 100mW of incident pump power, and a slope efficiency of 18.5%, again with respect to incident power. For high pump powers the slope efficiency rolled off, and the maximum output obtained was 230mW for 1.6W of incident power. It was noted that this output power could not be sustained. Instead, at a constant pump power, the output would gradually decrease to some sustainable level. Around 140mW was the highest sustainable power achieved. The underlying cause of this effect was an induced loss in the fibre at blue wavelengths, thus increasing the threshold and decreasing the slope efficiency. Further tests revealed that this loss is not permanent and it can be entirely removed by operation of the fibre laser at low powers for a time of the order of an hour. It is hoped that an understanding of the loss mechanism and its relation to material composition could lead to upconversion lasers with even higher sustainable powers