Photon locking

Modulation of a cw laser by a traveling-wave acousto-optic modulator has recently been utilized to implement sequences of phase-coherent optical pulses.' By controlling the relative phases of the rf pulses applied to the crystal, one can control the phases of the light pulses that are created by the acoustic grating. That is, the phase of an rf pulse is effectively transferred to the corresponding light pulse. The execution of a sequence of phase-coherent light pulses is thus reduced to generating the corresponding sequence in the rf domain. We describe here the first reported use of this technique to effect the optical analog of spin locking. In both the spin-locking experiment and its optical analog, a r/2 pulse along the x axis of the rotating frame is followed by the application of an intense field along the y axis. In our experiment, the first pulse serves to prepare a narrowband coherence within the inhomogeneously broadened linewidth of gaseous iodine. The intense y-axis field (phase shifted by 900 from the preparation pulse) serves to prevent dephasing of the initially prepared coherence by frequency offsets (which may arise from inhomogeneous broadening or velocity-changing collisions). T, the decay time of the coherence under these conditions, is obtained by continuously varying the duration of the locking field pulse. To detect the locked coherence, we add a pulse to the usual twopulse locking sequence; this third pulse serves to convert the locked coherence to an excited-state population enhancement or depletion. 3 4 The coherent signal of interest is in this manner detected by observing only (incoherent) fluorescence, using the following sequence: