An apparatus is described that includes an optical storage cavity comprising a planar mirror and a spherically curved mirror that form an optical path therebetween. A pump laser introduces optical pulses into the cavity. A folding mirror is disposed in the optical path between the planar mirror and the spherically curved mirror such that a beam waist is formed between the folding mirror and the second spherically curved mirror. The optical pulses preferably interact with selected particles disposed in the optical path at the waist. The apparatus further comprises a first transducer which adjusts the distance between the spherical mirror and the folding mirror to change a characteristic of the beam waist. A second transducer adjusts the distance between the planar mirror and the spherical mirror to change the optical path length of the cavity. The optical cavity is arranged such that the change in path length due to the second transducer does not significantly affect the characteristic of the beam waist.