Reducing the first-order Doppler shift in a Sagnac interferometer

Abstract

We demonstrate a technique to reduce first-order Doppler shifts in crossed atomic/molecular and laser beam setups by aligning two counterpropagating laser beams as part of a Sagnac interferometer. Interference fringes on the exit port of the interferometer reveal minute deviations from perfect antiparallelism. Residual Doppler shifts of this method scale with the ratio v / ͑4d͒ of the typical atomic/molecular velocity v and the laser beam diameter d. The method is implemented for precision frequency calibration studies at deep-UV wavelengths, both in one-and two-photon excitation schemes: the 6s 2 → 6s30p 3/2 J =1 line in Yb at 199 nm and the 4p 6 → 4p 5 p͓1/2͔ 0 transition in Kr at = 212 nm. The achieved precision of 6 ϫ 10 −10 is limited by the characteristics of the laser system.

Precision measurements and test of molecular theory in highly excited vibrational states of H2 (v = 11)

“…The probe or spectroscopy laser beam is split into two equidistant paths and subsequently steered in a counter-propagating orientation, making use of a Sagnac interferometer alignment for near-perfect cancelation of the residual first-order Doppler shifts [24]. Moreover, the probe laser beams pass through respective lenses, of f = 50-cm focal length, to focus and enhance the probe intensity at the interaction volume.…”

High-precision frequency measurement of the 423-nm Ca i line

“…The counter-propagating laser beams cross the Ca atomic beam at a (near) perpendicular orientation, where the effect of (residual) Doppler broadening is least. To ensure the best alignment of the counter-propagating laser beams, a Sagnac interferometric technique is used [9].…”

“…When using the Sagnac geometry, the Doppler shift of the transition frequency obtained for each beam will have the same magnitude but opposite signs, so the average of the transition-center frequency results in a cancellation of the first-order Doppler shifts [9]. Figure 2 shows separate recordings of the counter-propagating laser beams, illustrating the oppositely directed shifts of the peak positions for 40 Ca.…”

A narrow-band injection-seeded pulsed titanium:sapphire oscillator-amplifier system with on-line chirp analysis for high-resolution spectroscopy

“…30 In addition, the laser excitation of atoms and molecules in a crossed-beam configuration was performed with counterpropagating laser beams in a geometry of Sagnac interferometer to reduce Doppler shifts. 31 In one-photon laser-induced-fluorescence atomic beam studies, the transition frequency of the singlet 3s → 4p resonance in Mg at = 202 nm was investigated, yielding a transition frequency at an accuracy of Ϸ1 MHz. 32 Also the 3s → 3p resonance in Mg at = 285 nm was calibrated to the megahertz level of accuracy using the third harmonic ultraviolet output.…”