High power diode-pumped Nd³⁺ fibre laser

In the medical field high power fibre-coupled AlGaAs diode lasers operating around 800nm are already making an impact in surgery but for more specialised applications high powers at other wavelengths are required. It is an attractive idea to develop active fibres as add-ons to such fibre coupled diode systems increasing the range of applications of such devices at little extra cost and minimal additional complexity. Rare-earth-doped optical fibres exhibit the traditional advantages of a glass laser host of broad emission and absorption spectra but without the usual associated thermal problems. With fibre lasers wavelength selection and temperature stabilisation of the pump diode is unnecessary, and the emission spectra allows for broad tuning. Double-clad fibres comprising a doped core, usually single-moded located within a large multimode waveguide enable efficient pumping of fibre lasers with diode ways or diode bars. Fibre lasers can thus be considered as simple wavelength convertors and brightness enhancers for the high power but poorly specified output of diode sources. In the this paper we describe a multiwatt Nd3+ fibre laser pumped via a second cladding by the DIOMED 25 laser diode unit. This multi diode array source is designed for coupling up to 25 Watts of diode power into a plastic-clad silica fibre of 400µm diameter. The laser fibre which is a double-clad structure fabricated from lead-silicate glasses is interchangeable with the normal PCS delivery fibre. The device operates at 1.058µm with a slope efficiency >50% and a 150 times brightness enhancement. This laser though useful in itself is also a key intermediate laser for generation of high powers at other wavelengths. Tandem pumping of Tm3+ and Er3+/Yb3+ fibre lasers at 1.058µm enables efficient generation of 2.0µm and 1.55µm radiation respectively. In addition the Nd3+ laser can be operated close to 1.3µm and there are prospects for in-fibre frequency doubling of the 1.06µm line to generate a high power source in the green