Volume 31, number 2 OPTICS COMMUNICATIONS November 1979 OBSERVATION OF THE INFLUENCE OF EXTERNAL FEED-BACK ON PASSIVE MODE-LOCKING Franz R. AUSSENEGG, Alfred LEITNER and Max E. LIPPITSCH Institi~t f~r Experimentalphysik, Universitiit Graz, A-8010 Graz, Austria Received 17 July 1979 Considerable detoriation of the build-up of a picosecond pulse train in a passively mode-locked Nd+-glass laser by feed- back of laser emission of about 10 -4 of the intra cavity energy is observed. A computer simulation is presented to explain this observation. Experimental observations show that the probabil- ity for build-up of a single picosecond pulse circulating in the resonator of a passively mode-locked solid-state laser is reduced considerably by feeding back even a very small fraction of the laser output into the reso- nator. This phenomenon has been investigated using a Nd÷-glass laser with the following specifications: laser rod 5" X 3/8", ED-2 glass, 2 linear flash-lamps in close-coupled configuration, resonator length L = 100 cm, rear mirror concave with 2 m radius of curvature, 100% reflectivity, output mirror plane with R = 60% refiectivity, dye cell contacted to the rear mirror, spacing 1 mm, Kodak 9860 dye in dichloroethane, initial transmission 70%. Feed-back was achieved by a 100% mirror in a dis- tance of 356.5 mm from the output mirror to avoid the distance being a rational fraction of the resonator length. Neutral density filters (Schott NG 4, NG 9) were inserted between output mirror and feed-back mirror under Brewster's angle to attenuate the feed- back to the intensity required. The laser output was monitored by a photodiode and an oscilloscope. The device had an overall time constant of 1 ns. Observation of the picosecond pulse train was possible down to 10 -5 of the maximum en- ergy. With the laser described above under usual mode- locking conditions (pumping close above mode- locking threshold) a picosecond pulse train with the following properties was observed: pulse duration ~6 ps (measured by two-photon fluorescence), pulse sepa- ration 6 ns, duration of the pulse train ~200 ns (full width at half maximum), total energy in the train ~15 mJ. The order of magnitude of the feed-back to reduce the mode-locking probability drastically was about R r = 10 -4. The feed-back is defined as the fraction of the intra-cavity energy reentering the resonator and is determined by the transmission of the output mirror and the filters. The relative number Z of mode-locked pulse trains observed is plotted versus the feed-back R r in fig. 1. Each point in the diagram is the result of Z~ 100 80 60 ,~0 20 0 0 I f I Q 0 0 o [ ~ I ° I0-,~ 2.10-,~ Rr Fig. 1. Relative number Z of mode-locked pulse trains ob- served in the experiment versus feed-back R r. 231