Development of short pulse sources at 1.5μm for non-linear propagation studies in optical fibres
The development of sources of short mode-locked pulses at 1.5μm is described. Pulses of sufficient power in this spectral region are of great interest for the study of non-linear pulse propagation in optical fibres, where the interplay of negative group velocity dispersion and self-phase modulation can lead to soliton formation and pulse compression. Initial work concentrated on a Nd:YAG pumped methane Raman laser at 1.54μm. The use of a high pressure cell and a capillary waveguide reduces the stimulated Raman scattering threshold to just ~190kW. The use of synchronous pumping is shown to reduce this even further, to ~50kW, which is an order of magnitude less than the peak power available from a typical cw pumped, mode-locked and Q-switched Nd:YAG laser. These threshold values are shown to be in close agreement with theoretical predictions. Peak output powers of nearly 70kW are available in bandwidth-limited, 100ps full width half maximum duration pulses. We then describe a Nd:YAG pumped Yb:Er phosphate glass laser, showing it to be a versatile source in the 1.5μm spectral region. Pulsed, cw, mode-locked and Q-switched operation have been demonstrated in bulk and fibre forms. A simple rate equation model of this sensitised 3-level laser system. is shown to be in rough agreement with experimental results, with absorbed power thresholds as low as ~500mW and ~12mW being found in the bulk and fibre forms respectively. Typical mode-locked pulse durations of ~70ps are found and subsequent pulse compression via high order soliton propagation has given pulses of ~400fs. Finally the Yb:Er laser is assessed as a candidate for enhanced mode-locking via a non-linear external cavity, as in the soliton laser.