Abstract

We measured the axial velocity distribution produced by frequency-chirp-cooling a sodium atomic beam, including heating effects produced by an optional frequency-fixed postcooling interaction. Frequency sweeps were produced via a voltage-tuned oscillator feeding a broadband, efficient electrooptic modulator to produce a frequency-agile sideband on the output of a dual-frequency dye laser(Δv = 1.712GHz). After the chirp-cooling, the frequency was held stationary for an adjustable time T before the intensity was reduced 30-fold and the frequency jumped to provide a single readout point on the number of atoms vs frequency curve. All frequency sweeps were derived with rf techniques from a single laser so the velocity scale is absolute and uncontaminated by drifts. The + 880-MHz sweep in 1.6 ms gave an effective absorption rate of 10.7 photons/μs (saturation = 1 gives 31 photons/μs, 1 photon gives 3 cm/s). The swept sideband power was 10 mW in Wo ~ 4 mm, decreasing to ~ 350μW for readout. The measured velocity-offset and –drift rate were 24 MHz and 8 photons/μs at T = 15μs, with the velocity offset increasing to 40 MHz and the rate decreasing to 4 photons/μs at T = 100 μs. Over the same time interval, the velocity width monotonically increased from 12.1 to 17 MHz. The presence of heating—rather than cooling—infixed frequency conditions implies the existence of an optimum chirp rate and is not anticipated by some theories of the laser atom cooling dynamics.

© 1986 Optical Society of America