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arXiv:2510.11859 · Optics Letters

Lithium Faraday filter: some like it hot

Maximilian Luka, Yijun Wang, Denis Uhland, Ilja Gerhardt

Published Dec 3, 2025• 0 citations
arXivHTMLPDF

Abstract

Magnetically induced rotation of linearly polarized light near an atomic resonance, combined with Doppler-broadened absorption windows, enables narrowband transmission of optical frequencies. A narrowband (2.2 GHz) lithium vapor Faraday filter at about 671 nm is investigated experimentally and theoretically. The resulting Faraday filter transmittance is demonstrated using a lithium heat pipe oven under longitudinal magnetic fields ranging from 0 to 300 G. The optimized lithium Faraday filter exhibits the peak transmittance of 82% with commonly chosen optimal parameters around the 7 Li transitions center. The lithium D 1 - and D 2 -transitions are only 10 GHz apart, and temperature broadening leads to an overlap of the isotopes’ D -lines. Thus, the applied theoretical model needs to consider both transitions simultaneously. For this purpose, we extended an existing Python library (ElecSus), which now allows for the calculation of the atomic susceptibilities of lithium.

DOI
10.1364/ol.581808

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Magnetically induced rotation of linearly polarized light near an atomic resonance, combined with Doppler-broadened absorption windows, enables narrowband transmission of optical frequencies. A narrowband (2.2 GHz) lithium vapor Faraday filter at about 671 nm is investigated experimentally and theoretically. The resulting Faraday filter transmittance is demonstrated using a lithium heat pipe oven under longitudinal magnetic fields ranging from 0 to 300 G. The optimized lithium Faraday filter exhibits the peak transmittance of 82% with commonly chosen optimal parameters around the 7 Li transitions center. The lithium D 1 - and D 2 -transitions are only 10 GHz apart, and temperature broadening leads to an overlap of the isotopes’ D -lines. Thus, the applied theoretical model needs to consider both transitions simultaneously. For this purpose, we extended an existing Python library (ElecSus), which now allows for the calculation of the atomic susceptibilities of lithium.
We demonstrate an atomic bandpass optical filter with an equivalent noise bandwidth less than 1 GHz using the D1 line in a cesium vapor. We use the ElecSus computer program to find optimal experimental parameters and find that, for important quantities, the cesium D1 line clearly outperforms other alkali metals on either D-lines. The filter simultaneously achieves a peak transmission of 77%, a passband of 310 MHz, and an equivalent noise bandwidth of 0.96 GHz, for a magnetic field of 45.3 G and a temperature of 68.0°C. Experimentally, the prediction from the model is verified. The experiment and theoretical predictions show excellent agreement.
This thesis presents both a computational and experimental investigation into light propagation in hot alkali-metal vapours, with a particular focus on utilizing the Faraday effect for practical applications. A model to calculate various spectra for a weak-probe laser beam in an atomic medium with an applied axial magnetic field is presented. A computer program (ElecSus) was developed which implements this model efficiently. Using ElecSus we design optical devices such as Faraday filters and laser frequency stabilizing references. The design of Faraday filters utilizing compact vapour cells is shown, along with excellent agreement with experiment. The importance of including the effect of self broadening in the model is shown for these short path length vapour cells. Also, a Faraday filter is presented that can potentially be used for quantum optics experiments on the caesium D1_11​ line (894~nm). The filter displays the highest ratio of transmission to equivalent noise bandwidth to date for a linear Faraday filter, demonstrating the power of computerized optimization for this application. Furthermore, a Faraday filter is presented for use as an intra-cavity element in an external-cavity diode laser. A proof-of-principle experiment is demonstrated which shows that using a short external cavity with the Faraday filter eliminates mode-hops. Experimentally and theoretically the Faraday effect is investigated in large magnetic fields where alkali-metal atoms enter the hyperfine Paschen-Back regime. This hyperfine Paschen-Back Faraday effect is shown to allow a direct measure of the refractive indices for left and right circular polarized light. Furthermore, fitting the weak-probe spectra using ElecSus is found to give measures of the magnetic field with a fractional precision of the order of 10−410^{-4}10−4. In addition we study slow-light pulse propagation in a high density rubidium vapour, showing that our theoretical model for the electric susceptibility is valid for short pulses as well as continuous-wave light. This shows that the model is accurate for predicting weak-probe pulse propagation.
The Faraday laser, with Faraday anomalous dispersion optical filter (FADOF) as frequency-selection element, is a natural narrow-bandwidth light source for laser physic experiments. In this work, a dual-frequency (DF) Faraday laser is demonstrated for the first time on Cs D2 line at 852 nm. The frequencies of the two modes of DF Faraday laser hinge on the peak transition frequencies of 852 nm FADOF transmittance spectrum corresponding to the ground state F=4 and F=3. The frequency difference between the two modes is tunable over a range of 1.4 GHz with the temperature of Cs vapor cell inside the FADOF. The linewidth of each laser mode is less than 33 kHz. Furthermore, the most probable linewidth of the beat-note spectra between the two modes is 902.95 Hz at different vapor-cell temperatures. Such a DF Faraday laser has many potential applications in atomic physics, such as sub-Doppler spectroscopy and coherent population trapping atomic clocks.
Abstract The nature of atomic vapors, their natural alignment with interatomic transitions, and their ease of use make them highly suited for spectrally narrow-banded optical filters. Atomic filters come in two flavors: a filter based on the absorption of light by the Doppler broadened atomic vapor, i.e. a notch filter, and a bandpass filter based on the transmission of resonant light caused by the Faraday effect. The notch filter uses the absorption of resonant photons to filter out a small spectral band around the atomic transition. The off-resonant part of the spectrum is fully transmitted. Atomic vapors based on the Faraday effect allow for suppression of the detuned spectral fraction. Transmission of light originates from the magnetically induced rotation of linear polarized light close to an atomic resonance. This filter constellation allows selective acceptance of specific light frequencies. In this manuscript, we discuss these two types of filters and elucidate the specialties of atomic line filters. We also present a practical guide on building such filter setups from scratch and discuss an approach to achieve an almost perfect atomic spectrum backed by theoretical calculations.
We demonstrate a Cs FADOF (Faraday anomalous dispersion optical filter) with a single transmission peak resonant with the 6S(1/2), F = 4 → 7P(3/2), F' = 3, 4, 5 transition at 455 nm. The filter achieves a single peak transmission of 86%. With the technique of saturated absorption spectra, we obtain the bandwidth of the single peak, which is 1.5 GHz. While most of other FADOFs operate at frequencies far from absorption, the filter we have realized can provide light exactly resonant with atomic transitions with a high transmission. We also find that, at a particular temperature, we can achieve a single transmission peak rather than many peaks far from absorption by changing the strength of magnetic field. This technique can also be applied to other alkali atoms.