Two micron Tm doped ZBLAN double clad fibre laser.

Abstract

Tm:ZBLAN fibre lasers have potential applications in many research areas, such as eye safe range detection, medical science and materials processing. In this thesis we investigate improving the laser performance of a thulium doped fluoride fibre (specifically ZBLAN) to generate laser output with wavelengths near 2 μm. Two approaches are demonstrated to investigate laser performance at 2 μm. One is 790 nm pumping in which the cross-relaxation mechanism is used for efficient operation by populating the upper laser level. Another is in-band pumping, where 1570 nm light is directly pumps the emitting laser level, reducing the quantum defect between the pump and lasing wavelengths, reducing the threshold and increasing the efficiency of the fibre laser. Tm:ZBLAN fibres fabricated in-house, we also characterized with the aim of processing towards the development of new classes of fibre laser. A Tm:ZBLAN double clad fibre laser operating at 1.96 μm is demonstrated with a 1.53 W maximum output power and slope efficiency of 45% with respect to the launch pump power. Thereafter, the laser performance is analysed using cavity mirrors of various reflectivities. A laser with an operating wavelength of 1.57 μm is demonstrated with 1 watt output power using an Er:Yb co-doped double clad fibre. The slope efficiency of this laser is measured to be 28.3% with respect to the diode pump power, with a measured beam quality (M²) of 1.3. This 1.57 μm in-band pump source is then used to pump the 2 μm Tm:ZBLAN fibre laser. In addition to the laser development, this thesis also analyses the pump absorption for different shaped double clad fibres numerically. Understanding of pump absorption is required to develop an efficient double clad fibre laser. The pump absorption is measured using cut-back method and compared with the modeled results.