Pulse stacking, or synchronous pumping, is a novel approach that offers important
advantages in cavity ring-down spectroscopy. Using an ultrashort pulse, high repetition rate
laser source we have shown that it is possible to resonantly stack pulses in a high finesse
cavity, significantly enhancing the decay wave forms obtained when the laser source is
abruptly terminated. We have achieved signal-to-noise ratio improvements of several orders
of magnitude compared to single pulse injection systems, demonstrating a sensitivity of 2× …