Improving Magneto-Optical Filter Performance: Cascading and Oblique B-fields

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.