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arXiv:1806.08705 · Optics Letters

Optimized ultra-narrow atomic bandpass filters via magneto-optic rotation in an unconstrained geometry

James Keaveney, Steven A. Wrathmall, Charles S. Adams, Ifan G. Hughes

Published Aug 27, 2018• 33 citations
arXivHTMLPDF

Abstract

Atomic bandpass filters are widely used in a variety of applications, owing to their high peak transmission and narrow bandwidths. Much of the previous literature has used the Faraday effect to realize these filters, where an axial magnetic field is applied across the atomic medium. Here we show that by using a non-axial magnetic field, the performance of these filters can be improved in comparison to the Faraday geometry. We optimize the performance of these filters using a numerical model and verify their performance by direct quantitative comparison with experimental data. We find excellent agreement between experiment and theory. These optimized filters could find use in many of the areas where Faraday filters are currently used, with little modification to the optical setup, allowing for improved performance with relatively little change.

DOI
10.1364/ol.43.004272

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Atomic bandpass filters are widely used in a variety of applications, owing to their high peak transmission and narrow bandwidths. Much of the previous literature has used the Faraday effect to realize these filters, where an axial magnetic field is applied across the atomic medium. Here we show that by using a non-axial magnetic field, the performance of these filters can be improved in comparison to the Faraday geometry. We optimize the performance of these filters using a numerical model and verify their performance by direct quantitative comparison with experimental data. We find excellent agreement between experiment and theory. These optimized filters could find use in many of the areas where Faraday filters are currently used, with little modification to the optical setup, allowing for improved performance with relatively little change.
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.
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.
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.
Narrow-band optical filtering is required in many spectroscopy applications to suppress unwanted background light. One example is quantum communication where the fidelity is often limited by the performance of the optical filters. This limitation can be circumvented by utilizing the GHz-wide features of a Doppler broadened atomic gas. The anomalous dispersion of atomic vapours enables spectral filtering. These, so-called, Faraday anomalous dispersion optical filters (FADOFs) can be by far better than any commercial filter in terms of bandwidth, transition edge and peak transmission. We present a theoretical and experimental study on the transmission properties of a sodium vapour based FADOF with the aim to find the best combination of optical rotation and intrinsic loss. The relevant parameters, such as magnetic field, temperature, the related optical depth, and polarization state are discussed. The non-trivial interplay of these quantities defines the net performance of the filter. We determine analytically the optimal working conditions, such as transmission and the signal to background ratio and validate the results experimentally. We find a single global optimum for one specific optical path length of the filter. This can now be applied to spectroscopy, guide star applications, or sensing.
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.
We show that homogeneous line broadening drastically affects the performance of atomic Faraday filters. We study the effects of cell length and find that the behaviour of 'line-centre' filters are quite different from 'wing-type' filters, where the effect of self-broadening is found to be particularly important. We use a computer optimization algorithm to find the best magnetic field and temperature for Faraday filters with a range of cell lengths, and experimentally realize one particular example using a micro-fabricated 87 Rb vapour cell. We find excellent agreement between our theoretical model and experimental data.