1 Department of Photonics Engineering, Technical University of Denmark2 Fiber Optics, Devices and Non-linear Effects, Department of Photonics Engineering, Technical University of Denmark3 Risø National Laboratory for Sustainable Energy, Technical University of Denmark
This Ph.D. thesis investigates how intramodal and intermodal nonlinear processes in few-moded fibres can be used to generate light sources at wavelengths outside the spectral gain-bands of rare-earth-doped opticalfibres. The design of two specialty few-moded fibres for use in a widely tunable femtosecond fibre laser is presented. The two fibres are used to facilitate the shifting of a soliton in a cascade configuration from the ytterbium gain-band and to a wavelength of 1280 nm. The temporal pulse duration is on a femtosecond scale with a pulse energy of 5 nJ. The experimentally observed soliton self-frequency shift and thereby the outcome of the experimental demonstration of the widely tunable femtosecond fibre laser is shown to depend highly on the chirped of the input pulse into the first few-moded fibre in the cascade setup. Furthermore, an alternative splicing process, with a combination of a fusion splicer and a gas-line burner, is applied to the few-moded fibres. An intermodal four-wave mixing process and a novel intermodal Cerenkov generation process are demonstrated experimentally in one of the two speciality few-moded fibres. The two intermodal processes are described theoretically and numerically. For the intermodal four-wave mixing experiment an alternative version of the Generalised Non-Linear Schrödinger Equation is derived, which includes the correct dispersion of the transverse field. It is observed that the alternative version of the Generalised Non-Linear Schrödinger Equation, as opposed to the commonly used version, is able to reproduce the intermodal four-wave mixing experiment. The relation between the intramodal self-phase modulation and the intramodal Raman effect is determined from experimental measurements on a number of step-index fibres. The Raman fraction is found to vary with the germanium concentration. For the considered step- index fibres the Raman fraction varies from 0.16 to 0.15 with increasing germanium concentration, which is lower than the often cited value of 0.18. Furthermore, an extensive work regarding modelling of mode-locked lasers was performed. The result of this is reported for an all-normal dispersive polarisation-maintaining laser.