Robust Optical Design of Angled Multilayer Dielectric Mirrors Optimized for Rubidium Vapor Cell Return Reflection

This paper reports on the design and implementation of thin film multilayer dielectric to form Bragg reflectors on the sidewalls of micromachined atomic cells. Due to deposition shadowing, significant variations in the thicknesses of the thin films are encountered when the layers are deposited using PECVD. These gradients in thickness may limit optical performance of the reflector in atomic cells. An optimized design procedure is described to maximize the performance of the reflector at a wavelength of 795 nm under the variations in material thickness. In addition, an optical design based on two shifted quarter wave Bragg reflectors in series is used to form a reflector with extended optical bandwidth for added robustness. The extended reflectance range maintains high reflection at the D1 absorption wavelength of Rb despite variations in uniformity greater than 70 %. The developed reflector technology is ideally suited for use in miniature rubidium vapor cells. We demonstrate less than 1.5 dB of return loss with circular polarization ellipticity maintained to ±2◦ is demonstrated, as required for many atomic MEMS applications. INTRODUCTION Miniature vapor cells for emerging atomic MEMS applications, such as chip scale atomic clocks [1], magnetometers [2], [3] and gyroscopes, depend on the efficient routing of laser light using micromachined reflectors. Cells containing rubidium alkali vapor need low reflection losses at the Rb D1 transition wavelength of 795 nm after multiple reflections inside the vapor cell cavity formed in bulk micromachined wet-etched silicon (Figure 1). However, uncoated silicon is not sufficiently reflective for use as a high performance mirror, losing 67 % of optical energy in bulk transmission. Previously, rubidium vapor cells with optical return performance superior to uncoated silicon have been demonstrated by use of multilayer dielectric reflectors integrated on the sidewalls of wet-etched silicon cavities fabricated using Plasma Enhanced Chemical Vapor Deposition (PECVD) [4]. Although these reflectors have the potential to reflect light with negligible loss, large variations in the thin film thicknesses were observed due to the deposition technology, which limited the reflector performance. PECVD has many practical advantages over other thin film fabrication methods, such as Physical Vapor Deposition (PVD), including higher deposition rates and the formation of films with better mechanically and environmental robustness [5]. However,