The development of a diode laser pumped modelocked Nd:YAG laser for the study of non-linear effects in optical fibre

Abstract

This thesis deals primarily with the development of a diode laser pumped modelocked Nd:YAG laser operating at 1.3 µm for the production of ultra-short pulses in optical fibre. The Nd:YAG laser was modelocked using both amplitude and frequency modulation (AM and FM) techniques. An acousto-optic modulator was used to AM modelock the laser, bandwidth limited pulses of a duration of 46 psec and a peak power of 0.46W were obtained from this system. FM modelocking was accomplished with a lithium niobate phase modulator and produced two times bandwidth limited pulses of a duration of 19 psec with a peak power of 4.6W. The Nd:YAG laser was simultaneously FM modelocked and Q-switched resulting in peak powers in excess of 1kW in the Q-switch envelope.

The high peak power from this laser was used to generate wavelengths in the range of 1.34 to 1.55 µm, with high conversion efficiencies, by making use of stimulated Raman scattering (SRS) in optical fibre. The radiation formed by SRS fell in the anomalous dispersion region of the fibre and so could support optical solitons. The generated Stokes radiation was found to contain soliton like pulses of a duration as short as 88 fsec.

The diode laser pumped Nd:YAG laser was used in a study of the processes which determine the formation of optical solitons via SRS. In this study the importance of modulational instability in the non-linear process was discovered. This process was found to act as a seed for SRS with the result that the threshold power was lowered for the generation of Stokes radiation and the output spectrum was modified. In this study modulational instability sidebands as high as 8.94 THz were observed.

The thesis concludes with possible methods for the development of a cw source of sub-picosecond pulses by the use of SRS in optical fibre.

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