Multi-wavelength pumping of high power fibre Raman lasers

Fibre Raman lasers are an alternative to high power rare-earth-doped fibre lasers with benefits such as wavelength versatility and resilience to photodarkening. However, stimulated Raman scattering is a weak nonlinear process that requires high pump intensities, which are difficult to reach with high power multimode diode lasers, which are favoured for pumping. Nevertheless, research has shown that power-scaling of fibre Raman lasers pumped directly by such diode laser is possible. Whereas high-power diodelaser-pumped fibre Raman lasers have been investigated with only a single wavelength (narrow-bandwidth) pumping thus far, this study aims to further power-scale diode laserpumped fibre Raman laser by using wide-span multi-wavelength pumping. First, I examined two-wavelength (976 and 950 nm) pumped fibre Raman laser. Two high-power diode lasers are spectrally combined and launched into graded-index fibre. Although the separation between the two wavelengths was far from ideal (neither smaller than the Raman linewidth nor comparable to the Raman shift), the laser emitted 23 W at a single wavelength. This indicates that the separation among pump wavelengths is not so critical. Next, another three high power diode lasers at 969, 940, and 915 nm were spectrally added to the pump, expanding the total pump bandwidth to 683 cm-1 (915 – 976 nm). Three different fibres (silica-core step-index fibre, graded-index fibre with high germanium-doping, and step-index fibre with high germanium-doping) were tested. All three cases led to high-power single-wavelength output, reaching 32, 67, and 101 W, respectively. Also, this showed the advantages of germanium-doping. (e.g., higher Raman gain and pump launching efficiency) Lastly, numerical simulations showed that a pump bandwidth of up to ~1000 cm-1 can be used to pump a fibre Raman laser with an efficiency of more than 70%, for a clad-to-core area ratio up to ~7. Overall, the results show that multi-wavelength pumping of fibre Raman laser allow for efficient high power lasers. Thus, the basic concept of a multi-wavelength-pumped fibre Raman laser that may evolve into a new type of kW-laser is established in this thesis