Fibre lasers relying on optical fibres with a doped optical fibre as the gain media have shown tremendous progress in recent years. Ensuring the very high stability of the characteristics of the generated pulses is integral to many applications. The EU-funded training project HARMOFIRE (Harmonic mode-locked fibre lasers) was launched to train a promising young researcher in fibre laser development with a focus on cost-effective stabilisation. Activities combined both experimental and theoretical approaches. Harmonic mode locking is an advanced technique that enables delivery of very high pulse repetition rates. Multiple ultrashort pulses are circulated in the laser resonator with constant temporal spacing to overcome the limits of the resonator regarding the repetition rate of a single pulse. Novel harmonic MLFLs with a carbon nanotube (CNT) saturable absorber and highly erbium-doped gain fibre were designed and fabricated. CNT saturable absorbers were widely used in mode-locking applications, demonstrating fast responses and broadband operation. The polarisation insensitivity of CNT-based saturable absorbers makes them useful for the study of polarisation attractors in mode-locked lasers. The team went on to study in detail polarisation dynamics of vector soliton molecules in this paradigm. Solitons are pulses with a precise balance of non-linear and dispersive effects such that the temporal and spectral shape of the pulses was preserved even over long propagation distances. Further, the solitons can exhibit bound states with fixed and discrete phase separations. Vector solitons with various polarisation attractors with up to 11th harmonic mode locking were observed (harmonics are integer multiples of the original frequency of the light). Integration of a mode-coupling mechanism (tilted fibre Bragg gratings) into the laser cavity stabilised the repetition rate and improved the noise performance of the lasers when operating in the high harmonic (more than 5) range. It enabled stable harmonic operation for 16 hours at the 19th harmonic with pulse repetition at 460 MHz. Stabilised MLFLs and high flexibility in the generation of dynamic polarisation states will enable important innovation in communications, atoms and nanoparticles trapping, and control of magnetisation. HARMOFIRE's outcomes are therefore expected to find widespread application.
Optical lasers, mode-locked fibre lasers, ultrashort pulses, stabilisation technologies, harmonic mode locking