VCSELs for atomic sensors

A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies. These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells. The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium. The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam. This paper describes the required VCSEL characteristics for atomic clocks and magnetometers. The fundamental VCSEL requirement is single-frequency output with tunability to the particular spectroscopic line of interest. Single-polarization and single-transverse-mode operation are implicit requirements. VCSEL amplitude noise and frequency noise are also important because they contribute significantly to the sensor signal-to-noise ratio. Additional desired VCSEL attributes are low cost, low power consumption, and several years of continuous operating lifetime. This paper also describes the 894-nm VCSELs that we have developed for cesium-based atomic sensors. In particular, we discuss VCSEL noise measurements and accelerated lifetime testing. Finally, we report the performance of prototype atomic clocks employing VCSELs.