Crossed optical cavities with large mode diameters

We report on an ultrahigh-vacuum compatible optical assembly to enhance the optical power of two separate laser beams with the aim of creating large-scale, two-dimensional optical lattices for use in experiments with ultracold atoms. The assembly consists of an octagon-shaped spacer made from ultra-low-expansion glass, to which we optically contact four fused-silica cavity mirrors, making it highly mechanically and thermally stable. The mirror surfaces are nearly plane-parallel which allows us to create two orthogonal cavity modes with diameters $\sim$1 mm. Such large mode diameters are desirable to increase the optical lattice homogeneity, but lead to strong angular sensitivities of the overlap between the two cavity modes. We demonstrate a procedure to precisely position each mirror substrate that achieves a mode overlap of $99.9\pm^{0.1}_{1.3}\%$. Creating large optical lattices at visible and near infrared wavelengths requires significant power enhancements to overcome limitations in the available laser power. The cavity mirrors have a customized low-loss mirror coating that enhances the power at a set of relevant wavelengths from the visible to the near infrared by up to three orders of magnitude.