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arXiv:1205.0459 · Optics Express

Polarization spectroscopy and magnetically-induced dichroism of the potassium D_2 lines

K Pahwa, Lawrence Mudarikwa, J. Goldwin

Published Jul 17, 2012• 28 citations
arXivHTMLPDF

Abstract

We study modulation-free methods for producing sub-Doppler, dispersive line shapes for laser stabilization near the potassium D(2) transitions at 767 nm. Polarization spectroscopy is performed and a comparison is made between the use of a mirror or beam splitter for aligning the counter-propagating pump and probe beams. Conventional magnetically-induced dichroism is found to suffer from a small dispersion and large background offset. We therefore introduce a modified scheme, using two spatially separated pump-probe beam pairs. Finally we compare our results to methods using phase modulation and heterodyne detection.

DOI
10.1364/oe.20.017456

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We study modulation spectroscopy of the potassium D 2 transitions at 766.7 nm. The vapour pressure, controlled by heating a commercial reference cell, is first optimized using conventional saturated absorption spectroscopy. Heterodyne detection then yields sub-Doppler frequency discriminants suitable for stabilizing lasers in experiments with cold atoms. Comparisons are made between spectra obtained by direct modulation of the probe beam and those using modulation transfer from the pump via nonlinear mixing. Finally, suggestions are made for further optimization of the signals.
We discuss the physics of sub-Doppler DAVLL spectroscopy, which employs a pump beam with an axial magnetic field to induce dichroism in an atomic vapour. The dichroism is measured by a counterpropagating probe beam, while the pump generates sub-Doppler spectral features. The magnitude of the field is chosen to shift the frequency of the absorption features by an amount comparable to their linewidth. The reference signals obtained are ideal for laser frequency discriminant signals (laser 'locking' to the atomic transition) without frequency modulation. We discuss the sensitivity of the spectra to magnetic field, laser power and polarization purity, and suggest operating parameters for the 87 Rb F = 2 → F' = 3, 2 crossover transition which maximize the signal amplitude and gradient.
Doppler-free laser spectroscopy using polarization modulation has been carried out in Na D₁ and D₂ lines. The polarization of the pump beam is modulated between identically and orthogonally to a linear or circular polarization of the probe beam. The recorded signals of the hyperfine and crossover resonances are in agreement with a recent theoretical analysis of Nakayama which is based on velocity-selective optical pumping in a four-level system. Some exceptional discrepancy occuring in the D₂ is considered to be due to several relatively close lying resonances.
Doppler-free spectra of linear and circular dichroisms for the hyperfine structures of Na D1{D}_{1}D1​ and D2{D}_{2}D2​ lines have been obtained by means of polarization-modulated optical pumping spectroscopy. The experimental results are compared with recent theoretical calculations based on velocity-selective optical pumping in a four-level atomic system.
We demonstrate polarization spectroscopy of an excited state transition in room temperature cesium vapor. An anisotropy induced by a circularly polarized pump beam on the D2 transition is observed using a weak probe on the 6P3/2 to 7S1/2 transition. When the D2 transition is saturated, a sub-natural linewidth feature is observed which theoretical modeling shows is enhanced by Doppler averaging. Polarization spectroscopy provides a simple modulation-free signal suitable for laser frequency stabilization to excited state transitions.
The atomic magnetometer exhibits high sensitivity in weak magnetic field detection, with diverse applications in dark matter detection, biomagnetic sensing, wireless communication, etc. However, its performance, especially in environmental robustness and sensitivity, is largely limited by laser characteristics. Here, we realized two potassium Faraday lasers operating at potassium D1 (770 nm) and D2 (767 nm) lines, serving as pump and probe lasers for a potassium atomic magnetometer. Utilizing the Faraday anomalous dispersive effect for frequency selection, the lasers achieve automatic wavelength locking within the atomic Doppler broadening, enabling robust, self-stabilizing operation without tedious adjustments. Furthermore, both lasers exhibit strong immunity to environmental disturbances, such as vibration and temperature variations. By implementing the modulation transfer spectroscopy (MTS) technique, the frequency stability of the pump laser and probe laser reaches approximately 5.1 × 10 − 13 / τ and 6.2 × 10 − 13 / τ , respectively. Notably, the optimized frequency stability is accompanied by better power stability, showing threefold and fivefold improvements over their free-running counterparts, which reaches the 10 − 5 level at 1 s. The enhancements in laser performance directly improved the magnetometer’s performance, yielding a magnetic noise floor of approximately 100 fT / Hz at 10 Hz under a bias field of 1000 nT. Therefore, with hands-off operation, environmental robustness and competitive sensitivity, this achievement holds promising prospects for both laboratory and field-deployable applications.