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UBC Theses and Dissertations

Femtosecond laser pulse compression Chin, Roger S.


Once the Spectra-Physics Femtosecond Laser System had arrived, it had to be characterized. For further pulse compression, various techniques had to be considered. The best of these were chosen considering our needs and limitations. First, the Spectra-Physics Femtosecond Laser System is described and its 616 nm laser pulses are characterized. By using an autocorrelation technique based on the nonlinear optical characteristics of a potassium dihydrogen phosphate (KDP) crystal and assuming a particular intensity pulse shape (such as that described by a symmetric exponential decay), the pulse width (full width at half maximum) could be obtained. Assuming a pulse shape described by a symmetric exponential decay function, the "exponential" pulse width was measured to be 338 ± 6 fs. The nominal average power of the 82-MHz modelocked pulse train was 225 mW. The "exponential" pulse energy was 2.7 nJ with a peak pulse power of 2.8 kW. Theoretical calculations for fibre grating pulse compression are presented. Experimentally, I was able to produce 68 ± 1 fs (exponential) pulses at 616 nm. The average power was 55 mW. The "exponential" pulse energy was 0.67 nJ with a peak power of 3.4 kW. The pulse compressor consisted of a 30.8 ±0.5 cm fibre and a grating compressor with the effective grating pair distance of 103.8 ± 1 cm. Various techniques were considered for further pulse compression. Fibre-grating pulse compression and hybrid mode locking appeared to be the most convenient and least expensive options while yielding moderate results. The theory of hybrid mode locking is presented. Experimentally, it was determined that with the current laser system tuned to 616 nm, DODCI is better than DQOCI based on pulse shape, power, stability and expense. The recommended DODCI concentration is 2-3 mmol/l. The shortest "exponential" pulse width was 250 fs. The average power was 185 mW. The exponential pulse energy was 2.3 nJ with a peak pulse power of 2.6 kW. An attempt to increase the bandwidth of the laser pulse by replacing the one-plate birefringent plate with a pellicle severely limited the tunability of the dye laser and introduces copious noise. Attempts to reduce group velocity dispersion (responsible for pulse broadening) with a grating compressor was indeterminate, but did result in a slightly better pulse shape. Interferometric autocorrelation is recommended for such a study. An increase or decrease from the nominal power output of the pulse compressor showed a decrease in pulse compression.

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