Spatio-temporal beam tailored fibre lasers

Abstract

This thesis reports the development of an important class of next generation fibre laser system offering unprecedented levels of simultaneous control of the spatial, temporal and polarisation properties of the output beam in the nanosecond and picosecond pulse regimes. This work is predominately assigned to the EPSRC funded Energy Resilient Manufacturing (ERM) project in collaboration with the Institute for Manufacturing (IfM) of the University of Cambridge and SPI Lasers (UK) Ltd.

In the nanosecond regime, an Yb-doped fibre master oscillator power amplifier (MOPA) system with the capability of selectively generating doughnut-shaped radially and azimuthally polarised beams with user-defined temporal pulse shapes is investigated. The system is seeded by a directly modulated super-luminescent diode (SLD) using a computer controlled arbitrary waveform generator (AWG) to generate nanosecond pulses with user-defined temporal pulse shapes. Using the SLD substantially increases the robustness of the system by raising the threshold for stimulated Brillouin scattering (SBS) induced damage. The spatial mode shaping is achieved by using a nanograting spatially variant half-waveplate (S-waveplate) to convert the linearly-polarised fundamental (LP01) mode from the pre-amplification stages into a doughnut-shaped radially-polarised beam prior to the power amplifier. A maximum output pulse energy of ∼860 µJ was achieved for ∼100 nanosecond pulses at 25 kHz repetition rate with user defined pulse shape for both radial and azimuthal polarisation states. The packed laser system was delivered to University of Cambridge for materials processing experiments to investigate its performance and capabilities.

Next, the generation of high average output power, high peak power, and high pulse energy radially polarised picosecond pulses from a compact gain-switched laser diode seeded Yb-doped fibre MOPA system was investigated. A 1030 nm Fabry-Pérot laser diode was gain-switched using a train of sinusoidal RF pulses at a repetition rate of 87.5 MHz and self-seeded to produce ∼150 ps, ∼4 pJ pulses at 1034.5 nm with a 3-dB spectral bandwidth of ∼0.03 nm. A fibre pigtailed electro-optic modulator (EOM) used as a pulse picker and a fibre pigtailed acoustic-optic modulator (AOM), synchronised to the EOM, was used to remove inter-pulse amplified spontaneous emission (ASE) prior to second pre-amplifier stage. A q-plate was employed as a mode converter prior to the final power amplifier to efficiently convert the linearly polarised Gaussian-shaped beam into a doughnut-shaped radially polarised beam. The desired vector beam was efficiently amplified yielding ∼110 ps pulses with a maximum output pulse energy of 30.7 µJ and a peak power of ∼280 kW at a repetition rate of 1.367 MHz. The average power was scaled up to 106 W by increasing the repetition rate to 5.468 MHz, which is being used in materials processing trails at the University of Cambridge.

Investigation of picosecond temporal pulse shaping in a femtosecond mode-locked laser oscillator based MOPA system is carried out for 10 W average output power. A ∼23 MHz repetition rate mode-locked laser source is built and followed by a length of fibre as stretcher to implement picosecond temporal pulse shaping principle via far-field frequency-to-time mapping (FF-FTM) using a WaveShaper and then amplified up to 10W average output power. Verification of temporal pulse profiles scrutinised using a house-built cross-correlator. Then, the laser performance extended and improved by replacing the laser source with a commercial laser has smoother input spectra and higher repetition rate of 100 MHz, resulting in lower peak power through the entire MOPA system. Final novel high average power spatial and temporal beam tailored data demonstrated radially polarised beam in any desired temporal pulse shapes such as square, single, double, triple pulse peaks, and Gaussian shapes were obtained for ∼70 ps at an average power level exceeding 50 W, after repetition rate doubling. The shapes were verified by using a cross-correlator. Further power scaling was limited by the peak power increase and introduction of higher nonlinear phase shifts, producing distorted temporal profiles. These results represent the highest optical power demonstrated from a fibre MOPA for spatio-temporal beam tailoring in tens of picosecond optical pulses.

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