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arXiv:2203.14767 · Optics Letters

Better magneto-optical filters with cascaded vapor cells

Fraser D. Logue, Jack D. Briscoe, Danielle Pizzey, Steven A. Wrathmall, Ifan G. Hughes

Published May 16, 2022• 14 citations
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Abstract

Single-cell magneto-optical Faraday filters find great utility and are realized with either “wing” or “line center” spectral profiles. We show that cascading a second cell with independent axial (Faraday) or transverse (Voigt) magnetic field leads to improved performance in terms of figure of merit (FOM) and spectral profile. The first cell optically rotates the plane of polarization of light creating the high transmission window; the second cell selectively absorbs the light eliminating unwanted transmission. Using naturally abundant Rb vapor cells, we realize a Faraday–Faraday wing filter and the first, to the best of our knowledge, recorded Faraday–Voigt line center filter which show excellent agreement with theory. The two filters have FOM values of 0.86 and 1.63 GHz−1, respectively.

DOI
10.1364/ol.459291

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arXiv:2305.00570•Jan 1, 2023•0 citations
Single-cell magneto-optical Faraday filters find great utility and are realized with either “wing” or “line center” spectral profiles. We show that cascading a second cell with independent axial (Faraday) or transverse (Voigt) magnetic field leads to improved performance in terms of figure of merit (FOM) and spectral profile. The first cell optically rotates the plane of polarization of light creating the high transmission window; the second cell selectively absorbs the light eliminating unwanted transmission. Using naturally abundant Rb vapor cells, we realize a Faraday–Faraday wing filter and the first, to the best of our knowledge, recorded Faraday–Voigt line center filter which show excellent agreement with theory. The two filters have FOM values of 0.86 and 1.63 GHz−1, respectively.
Abstract Cascading light through two thermal vapour cells has been shown to improve the performance of atomic filters that aim to maximise peak transmission over a minimised bandpass window. In this paper, we explore the atomic physics responsible for the operation of the second cell, which is situated in a transverse (Voigt) magnetic field and opens a narrow transmission window in an optically thick atomic vapour. By assuming transitions with Gaussian line shapes and magnetic fields sufficiently large to access the hyperfine Paschen–Back regime, the window is modelled by resolving the two transitions closest to line centre. We discuss the validity of this model and perform an experiment which demonstrates the evolution of a naturally abundant Rb transmission window as a function of magnetic field. The model results in a significant reduction in two-cell parameter space, which we use to find theoretical optimised cascaded line centre filters for Na, K, Rb and Cs across both D lines. With the exception of Cs, these all have a better figure of merit than comparable single cell filters in literature. Most noteworthy is a Rb-D2 filter which outputs > 92 % of light through a single peak at line centre, with maximum transmission 0.71 and a width of 330 MHz at half maximum.
Magneto-optical filters are ultra-narrow frequency selection devices that use atoms as the active medium. Frequency dependent dispersion, scattering and absorption make alkali atoms ideal for polarisation sensitive filtering, traditionally with equivalent noise bandwidths of several GHz and little control over the lineshape profile. Our investigations focus on two major improvements to magneto-optical filter performance: a) cascading two vapour cells with independent parameters and b) exerting magnetic fields at oblique angles to the light propagation direction. Optimised two-cell cascaded setups have a polarisation ‘transformation’ cell and a cell which ‘extinguishes’ unwanted features. Oblique magnetic fields result in frequency dependent non-orthogonal propagation eigenmodes which are directly responsible for narrower birefringent regions and better extinction of light outside these regions. We find very good agreement with theory and realise filters with equivalent noise bandwidths as low as ∼ 100 MHz with greater control of lineshape features. We consider theoretically further performance improvements which utilise exceptional points where the propagation eigenmodes coalesce completely.
We present an investigation of magneto-optic rotation in both the Faraday and Voigt geometries. We show that more physical insight can be gained in a comparison of the Faraday and Voigt effects by visualising optical rotation trajectories on the Poincare sphere. This insight is applied to design and experimentally demonstrate an improved ultra-narrow optical bandpass filter based on combining optical rotation from two cascaded cells - one in the Faraday geometry and one in the Voigt geometry. Our optical filter has an equivalent noise bandwidth of 0.56 GHz, and a figure-of-merit value of 1.22(2) GHz−1^{-1}−1 which is higher than any previously demonstrated filter on the Rb D2 line.
Atomic bandpass filters are used in a variety of applications due to their narrow bandwidths and high transmission at specific frequencies. Predominantly these filters in the Faraday (Voigt) geometry, using an applied axial(transverse) magnetic field with respect to the laser propagation direction. Recently, there has been interest in filters realized with arbitrary-angle magnetic fields, which have been made by rotating permanent magnets with respect to the kkk-vector of the interrogating laser beam. However, the magnetic-field angle achievable with this method is limited as field uniformity across the cell decreases as the rotation angle increases. In this work, we propose and demonstrate a new method of generating an arbitrary-angle magnetic field, using a solenoid to produce a small, and easily alterable, axial field, in conjunction with fixed permanent magnets to produce a large transverse field. We directly measure the fields produced by both methods, finding them to be very similar over the length of the vapor cell. We then compare the transmission profiles of filters produced using both methods, again finding excellent agreement. Finally, we demonstrate the sensitivity of filter profile to changing magnetic-field angle (solenoid current), which becomes easier to exploit with the much improved angle control and precision offered by our new design.
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.
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.