Investigation into the generation of ultrashort pulses from Nd:YAG and Nd:glass lasers

Abstract

The purpose of the work described in this thesis was to investigate the generation of ultrashort mode-locked pulses from Nd-doped lasers, in particular the Nd:YAG and Nd:glass lasers. A flashlamp-pumped, pulsed, Nd:YAG laser was mode-locked by the addition of a nonlinear external coupled cavity. The nonlinear element used was a second harmonic generating crystal. Pulse durations of 30 psec (averaged over a 700 nsec long Q-switched pulse train) were obtained, at peak powers of up to 135 kW. This was the first report, to the author's knowledge, of self-starting coupled-cavity mode-locking of a Nd-doped laser, and certainly the first use of a second harmonic generating crystal in an external coupled cavity to achieve mode-locking.

The possibility of generating shorter pulses than those available from the Nd:YAG laser was investigated by the mode-locking of the continuous wave Nd:glass laser. This laser medium is capable of supporting sub-picosecond pulses. Initial work focused on the active mode-locking of a Nd:glass laser pumped by a 500 mW laser diode array. Both amplitude modulation and frequency modulation mode-locking techniques were investigated, with pulses as short as 9 psec being obtained in each case, at repetition rates of around 240 MHz. The peak powers obtained were, respectively, 12.1 W and 6.6 W. FM operation of the laser-diode-pumped Nd:glass laser was also observed, with a maximum FM bandwidth of 990 GHz being obtained.

As a necessary step towards obtaining sub-picosecond pulses from the laser-diode-pumped Nd:glass laser via coupled-cavity mode-locking, the cw performance of the laser was first significantly improved. This was accomplished using broad stripe (40 µm) high brightness 500 mW laser diodes as pump sources, and a lower doped Nd:glass than that used previously. When four 500 mW broad stripe devices were used, an output power as high as 415 mW was obtained for an absorbed pump power of around 1.4 W (for an output coupling of 5%). The optical slope efficiency was 32%. Using this source, with the output coupling increased to 15%, coupled-cavity mode-locking was achieved.

To obtain mode-locking, an external coupled cavity containing an 85 cm length of single mode optical fibre was appended to the main laser cavity. With the coupled cavity blocked, pulses of 20 psec duration were obtained from the active mode-locking of the Nd:glass laser. On unblocking the coupled cavity, this pulse duration was observed to decrease to around 700 fsec. Self-starting coupled-cavity mode-locking of this laser was also observed, although the laser power was only slightly above the mode-locking threshold of ~180 mW coupled into the fibre. This work constituted the first demonstration of sub-picosecond pulses from an all solid state laser system, and clearly indicated the viability of obtaining stable, all solid state sources of sub-picosecond mode-locked pulses.

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