REU REPORT draft

The creation of pulsed lasers is a field of study that developed 
immediately after the creation of the laser in 1960. Researchers 
were quickly able to produce pulses of nanosecond duration with 
various laser types, and today, researchers continue to apply 
pulsing techniques to modern lasers such as semiconductor lasers 
to ahcieve shorter and more powerful pulses. For this work we 
have have used extra-cavity modulation techniques to mode lock a 
laser and make pulses of ~3ns duration (FWHM) with peak 
intensities three to four times higher than the average 
intensity of the unmodulated laser.

The modes that lase within the cavity of a laser generally 
oscillate in random phase fashion and thus the output of the 
laser is an average of the mode intensities at any given time. 
If these modes can be made to oscillate in phase with one 
another, the profile of the output will consist of peaks of high 
amplitude when the modes add constructively and moments of very 
low intensity when they are simultaneously at nodal points in 
their oscillation.

Different schemes have been developed to try to mode lock 
lasers, often consisting of intracavity loss modulation. In 
essence these techniques periodically modify the quality of the 
cavity, alternating between low loss and high loss states. When 
the cavity is in a high loss (low quality) state, the gain can 
grow to a level higher than its threshold value as it does not 
experience a saturation limit from the stimulated emission 
field. If the quality of the cavity is increased, the large gain 
results in a quick buildup of stimulated emission in the laser 
cavity resulting in a pulse of energy. This rapid alternation of 
cavity qualities is called Q-switching and can be done with many 
different methods.