Cladding-pumped Raman fibre laser sources

Abstract

In this thesis, I investigate cladding-pumped Raman fibre lasers and amplifiers. Such devices, offering a novel way to generate Raman gain, combine the advantages of the hugely successful cladding-pumped rare-earth doped fibre lasers with those of stimulated Raman scattering. They not only inherit most advantages of conventional fibre devices, such as flexibility, high efficiency, compactness, and robustness, but also provide their own advantages and distinct properties relative to conventional fibre sources, i.e., wavelength flexibility and nearly instantaneous gain without energy storage.

Cladding-pumped Raman fibre laser sources utilise double-clad Raman fibres as the gain medium. These are similar to a rare-earth doped double-clad fibre except that there is no laserion doping of the core. With double-clad fibres, the high-power output from low-cost multimode pump sources can be converted into diffraction-limited signal beams, e.g., through stimulated Raman scattering. Thus, cladding-pumped Raman fibre laser sources are a kind of brightness enhancers. In the beginning of this thesis, I theoretically analyse various factors that limit the brightness enhancement of such devices. One of the limits is unwanted 2nd-Stokes generation, which restricts the area ratio between the inner cladding and core. By designing a new DCRF with a W-type core, I successfully relax this restriction by nearly five times. Combined with other factors, i.e., core damage threshold, walk-off, numerical aperture, and background loss, a brightness enhancement of more than 3500 for the designed fibre could be achieved in such devices shown by a model with right pump sources and parameters.

Secondly, I focus on the conversion efficiency of such devices. A well-designed fibre with inner-cladding-to-core area ratio around six was used as a double-clad Raman fibre, pumped by a source with nearly rectangular pulse shapes. The nearly rectangular pulses were obtained from an erbium and ytterbium co-doped master optical power oscillator through prepulse shaping. A sufficiently short piece was chosen to reduce the background loss and walk-off. The highest peak power conversion into the 1st Stokes was 75% and the energy conversion efficiency was over 60% in a pulsed cladding-pumped Raman fibre amplifier.

Thirdly, I study the power scalability. Theoretically, I analyse the achievable power of such devices. The core size turns out to be a critical factor in most cases. The ultimately output power is limited to around 24 kW by thermal lensing if the core is large enough and enough pump power available. Experimentally, in collaboration with co-workers, a 100 W claddingpumped Raman fibre laser was demonstrated at 1116 nm. The output beam was nearly diffraction-limited. It shows the potential of power scalability of such devices and the ability of generating high power diffraction-limited sources at wavelengths outside the conventional range that rare-earth doped fibres offer.

Since a large core size is a critical factor for power scaling, new double-clad Raman fibres with large-mode areas were introduced. They were experimentally demonstrated to work as efficiently as the previous fibre. An Nd:YAG laser was used to pump one of these fibres, and a 1 mJ Raman fibre source with good beam quality was thus demonstrated. This shows that double-clad Raman fibres offer another approach to obtaining high-brightness high-energy sources. In addition, based on a cladding-pumped Raman fibre converter, a simple and efficient method was proposed to generate supercontinuum sources.

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Identifiers

ORCID for Johan Nilsson: orcid.org/0000-0003-1691-7959

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