High power two-micron fibre-bulk hybrid lasers

Abstract

The design of high-power efficient solid-state lasers with good output beam quality has long been a general goal of many researchers within the solid-state laser community. Progress towards the realization of such solid-state lasers has mainly been limited by poor brightness and limited choice of operating wavelengths of the optical sources available to pump such lasers.

This thesis presents the results of an investigation into a new approach for power and brightness scaling of two-micron solid-state lasers. The strategy described in this thesis is based on reduction of heat generation and heat loading within the laser medium. This was achieved by employing a fibre-bulk hybrid scheme; where a high-power diode-pumped double-clad fibre laser is used to end pump a bulk solid-state laser. Thus, the work described here is split into two related research areas.

The first area of research was devoted to the construction of a high-power, tunable two-micron source based on thulium(Tm)-doped silica fibre. The result was a tunable Tm fibre laser that produced a maximum output power of 10.5W at a wavelength of 1921nm for 44W of launched diode power at 790nm. The output of the laser was tunable over a wavelength range of 215nm from 1855nm to 2070nm at multi-watt power levels, and over 150nm from 1860nm to 2010nm at output power levels in excess of 9W.

In this part of the research results for the first wavelength-combined Tm-doped silica fibre laser system with maximum output power of 14W on four lines at wavelengths, 1926nm, 1957nm, 1980nm and 2000nm, have also been presented. The aim of this work was to explore a new concept for power scaling in fibre lasers. The beam-combined system was tunable over a range of 200nm from 1900nm to 2100nm at multi-watt power levels, and over a range of 120nm, from 1900nm to 2020nm, at power levels in excess of 10W. Beam propagation factors, M2, of the beam-combined laser system were equal to; Mx2 ~ 4.9 and My2 ~ 4, in the x-z and y-z planes, respectively. Potential sources of degradation in beam quality of the system were also investigated.

In the second part of the research a high-power tunable Tm-doped silica fibre laser was used to end-pump a Ho:YAG crystal. Ho:YAG has been chosen due to its robust thermo-mechanical properties and favourable spectroscopic characteristics. Highly efficient room-temperature operation of a Ho:YAG laser has been demonstrated. 5.2W of output at 2097nm in a near diffraction-limited beam from a Ho:YAG laser was achieved using 9.1W of incident pump power from a Tm-doped silica fibre laser at 1905nm. The slope efficiency with respect to incident pump power (80%) was close to the theoretical maximum taking into account the pump absorption efficiency, resonator losses and output coupling loss. The optical-to-optical efficiency of the laser was 57%. Room-temperature laser operation of Ho:YAG lasers with different Ho3+ concentrations was also investigated. The design and performance of these lasers for highly efficient continuous-wave operation is briefly discussed.

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