A device for magnetic-field angle control in magneto-optical filters using a solenoid-permanent magnet pair.

Atomic bandpass filters are used in a variety of applications due to their narrow bandwidths and high transmission at specific frequencies. Predominantly, these filters are 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 k-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 the 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.

Better magneto-optical filters with cascaded vapor cells.

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.