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Doppler frequently shifts for a laser field interacting with a diverging supersonic atomic beam are canceled using spatially varying Zeeman shifts. Dense atomic beams with long interaction path lengths and narrow linewidths are obtained for spectroscopic and nonlinear-optics applications.

We report the observation of continuous spatial photon echoes in a magnetically compensated supersonic beam of atomic samarium. Atoms cross two spatially separated cw laser pump fields to produce an intense cw echo downstream. In contrast to previous studies of continuous coherent radiation in separated fields, the signals are readily observable with the unaided eye. The echo signal, which is generated in a magnetic-field gradient, differs from usual rephasing phenomena owing to a quadratic time dependence for the optical phase in the frame of the moving atoms.

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