We present a new method for determining the long-term stability of UV-induced fiber Bragg gratings. We use a continuous temperature ramp method in which systematic variation of the ramp speed probes both the short-and long-term stability. Results are obtained both for gratings written in D 2 loaded and nonloaded fibers. The results for the nonloaded fibers are in good agreement with those previously obtained. Precise predictions of the grating decay were made. We find good agreement with a broad trap energy distribution where the defects with the lowest energy decay first. For the D 2 loaded fiber grating we resolve two separate energy distributions, suggesting that two different defects are involved. The experiments show that complicated decays originating from various energy distributions can be analyzed with this continuous isochronal anneal method. The results have both practical applications in determining the long-term stability of fiber gratings and fundamental importance since they can be used to determine the energy distribution when using different fiber types and writing techniques.

Compositional control in the preparation of rovibrational wave packets is demonstrated in the E(1Σg+) state of gas-phase Li2 molecules using ultrafast pump–probe laser spectroscopy combined with quantum-state-resolved intermediate state selection. The intermediate state, from which subsequent ultrafast excitation occurs, is a stationary rovibrational level in the A(1Σu+) state of Li2, produced by cw laser excitation from the ground X(1Σg+) state. The effect that the intermediate state has on the final composition of the wave packet is investigated by comparing the transients resulting from ultrafast pump–probe excitation of two different intermediate states (vA=14, JA=18 versus vA=13, JA=18). In these experiments the pump wavelength is compensated so that in each case the same E-state eigenstates (vE=13–18, JE=JA±1) make up the wave packet, but with different amplitudes. Theory predicts, and experiments confirm, that the relative amplitudes of the rovibrational eigenstates are strongly dependent upon the intermediate state and determine the spatial and temporal evolution of the wave packet. Evidence for this includes differences in the observed pump–probe transients and dramatically different amplitudes of the beat frequencies in the Fourier analysis of the time-domain transients. Theoretical three-dimensional wave packet simulations highlight how the composition of the wave packet is used to vary its spatial and temporal evolution.

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