Novel optical fibres in mode-locked lasers and short pulse manipulation

Abstract

This thesis covers work developing novel optical fibres for use as a delay fibre to decrease the pulse repetition rate of a short pulse mode-locked fibre laser operating at 1064 nm. The limits on the length of these laser cavities, and in turn the pulse repetition rate, was found to be the nonlinear phase shift accumulated by a pulse in one round trip of the laser cavity. The addition of extra fibre increases the cumulative nonlinear phase shift to the detriment of fundamental mode-locked pulses. Here, different optical fibres were developed with negligible nonlinear response to minimise additional nonlinearity when extending the laser cavity length. The fibres included large mode area (LMA) solid core fibres, hollow core photonic crystal fibres (PCF) and the more recently developed negative curvature anti-resonant hollow core fibre (NCF).The LMA fibres had an increased modal area which decreases the intensity of theguided light and thus decreases the nonlinear response. These fibres were used to reduce the repetition rate of a 20 MHz mode-locked fibre laser to 7.6 MHz operating at 1064 nm with a pulse duration of approximately 4 ps.The use of a hollow core fibre reduces the nonlinearity experienced by a pulse by changing the medium the light is propagating through. The low nonlinear response of air allowed for the reduction of a 4 ps pulse operating at 1064 nm and 37 MHz to be reduced to 27 MHz in PCF hollow core fibre and to a record 5.4 MHz in NCF.The PCF was found to be limited by polarisation effects and high fibre attenuation. However, the results using NCF showed no detriment to the near transform limited pulse. The use of hollow core fibre as a method of reducing the pulse repetition rate of mode-locked fibre lasers was demonstrated.The NCF developed for this work was also used as a means to induce spectralbroadening of a high power, amplified pulse pulse by the means of self phase modulation in a single pass through the fibre. A pulse was broadened by self phase modulation alone producing a symmetrical and broad spectrum that was controlled by the pressurisation of argon within the hollow NCF core. Using a grating pair on the fibre output a pulse of 10 ps at 1064 nm was compressed to 420 fs with an output pulse energy of 8µJ.

Date of Award	26 Apr 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Jonathan Knight (Supervisor), William Wadsworth (Supervisor) & Paulo Almeida (Supervisor)