Abstract:Whether in art or for QR codes, images have proven to be both powerful and efficient carriers of information. Spatial light modulators allow an unprecedented level of control over the generation of optical fields by using digital holograms. There is no unique way of obtaining a desired light pattern however, leaving many competing methods for hologram generation. In this paper, we test six hologram generation techniques in the creation of a variety of modes as well as a photographic image: rating the methods according to obtained mode quality and power. All techniques compensate for a non-uniform mode profile of the input laser and incorporate amplitude scaling. We find that all methods perform well and stress the importance of appropriate spatial filtering. We expect these results to be of interest to those working in the contexts of microscopy, optical trapping or quantum image creation.

We investigate the conversion between transverse mode structures in four-wave mixing in a heated rubidium vapour. While angular momentum conservation in this nonlinear process dictates the selection rules for the angular quantum number, the role of the radial quantum number is more esoteric. We demonstrate experimentally that a clean Laguerre-Gauss mode LG p =LG 0 1 can be generated by converting LG 1 0 and LG −1 0 near-infrared pump beams -but only if the length of the atomic medium exceeds the Rayleigh range. In the same regime we also observe a direct transfer of the total radial quantum number from input to generated light modes.

Laguerre-Gauss beams, and more generally the orbital angular momentum of light (OAM) provide valuable research tools for optical manipulation, processing, imaging and communication. High-efficiency frequency conversion of OAM is possible via four-wave mixing in rubidium vapour. Conservation of the OAM in the two pump beams determines the total OAM shared by the generated light fields at 420 nm and 5.2 µm -but not its distribution between them. Here we experimentally investigate the spiral bandwidth of the generated light modes as a function of pump OAM. A small pump OAM is transferred almost completely to the 420 nm beam.Increasing the total pump OAM broadens the OAM spectrum of the generated light, indicating OAM entanglement between the generated light fields. This clears the path to high-efficiency OAM entanglement between widely disparate wavelengths.

We report the first use of a ring cavity to both enhance the output power and dramatically narrow the linewidth (<1  MHz) of blue light generated by four-wave mixing in a rubidium vapor cell. We find that the high output power available in our cavity-free system leads to power broadening of the generated blue light linewidth. Our ring cavity removes this limitation, allowing high output power and narrow linewidth to be achieved concurrently. As the cavity blue light is widely tunable over the Rb855S_1/2F=3→6P_3/2 transition, this narrow linewidth light would be suitable for near-resonant rubidium studies including, for example, second-stage laser cooling.

Phase only spatial light modulators (SLMs) have become the tool of choice for shaped light generation, allowing the creation of arbitrary amplitude and phase patterns. These patterns are generated using digital holograms and are useful for a wide range of applications as well as for fundamental research. There have been many proposed methods for optimal generation of the digital holograms, all of which perform well under ideal conditions. Here we test a range of these methods under specific experimental constraints, by varying grating period, filter size, hologram resolution, number of phase levels, phase throw and phase nonlinearity. We model beam generation accuracy and efficiency and show that our results are not limited to the specific beam shapes, but should hold for general beam shaping. Our aim is to demonstrate how to optimise and improve the performance of phase-only SLMs for experimentally relevant implementations.