Operating Regimes and Automated Control of Holmium and Thulium Non-linear Polarisation Rotation Mode-locked Fibre Optic Lasers

Abstract

Mode-locked fibre lasers have applications in research, defence, biological science, manufacturing and spectroscopy. The aim of this research was to investigate auto- matic control of a passive mode-locking technique known as non-linear polarisation rotation (NLPR) that exploits polarisation differences across a pulse’s profile that manifest due to different intensities. This research seeks to focus on how NLPR laser sources can be made stable, especially at longer wavelengths (such as 2 µm).

Three different fibre lasers were developed to investigate different aspects of NLPR. An erbium mode-locked source was used to explore measurement techniques and competing operating regimes. The operating regimes were controlled using a manual polarisation controller. Characterisation revealed that the source produced 570 fs pulses with a central wavelength of 1567 nm, repetition rate of 17 MHz and pulse energy of 3 nJ.

A thulium NLPR source was developed to further explore possible automation metrics and characterise a range of additional undesired operating regimes. Automation metrics tested to determine if the laser was continuously mode-locked included discontinuities in Stokes parameters, two-photon absorption signals and amplitude modulation across the radio-frequency spectrum of the laser. Characterisation revealed that the source produced 1.15 ps pulses with a central wavelength of 1990 nm, repetition rate of 21 MHz and pulse energy of 5 nJ.

Lastly, a holmium NLPR source with electronic control of the intra-cavity polarisation state was built. To the best of our knowledge, this is the first diode-pumped NLPR mode-locked holmium source in the 2µm waveband. An automation mechanism was devised and implemented. Characterisation revealed that the source produced 770 fs pulses with a central wavelength of 2061 nm, a repetition rate of 17 MHz and pulse energy of 0.41 nJ.