Physics of the sapphire whispering-gallery-mode solid-state MASER oscillator

Abstract

The Cryogenic Sapphire Oscillator (CSO) is currently the best available technology that can provide a relative frequency stability better than 10^-15 for integration times between 1 s and 10,000 s. But, the CSO remains a complex instrument requiring multiple loop controls to achieve the best frequency stability. The possibility to use of the sapphire resonator in a self-sustained MASER oscillator constitutes an elegant alternative to the CSO. Here, the sustaining amplification is achieved through the interaction between a high-Q factor whispering gallery mode and the paramagnetic Fe3+ ions, which are present in small concentration in the sapphire crystal. The Fe3+ ion exhibits three energy states enabling to realize a self-sustaining solid-state maser. Although, this principle has been already experimentally demonstrated few years ago, its development as a truly usable ultra-stable source has not yet been completed, mainly due to the lack of control of the
 complex physical phenomena involved. This paper completes the previous theoretical work based on the rate equations model. Here we derive the full quantum equations describing the evolution of the Fe3+ ions inside the sapphire lattice and submitted to a pump and a maser signal. The
 influence of the ions concentration and spin-spin relaxation time will be pointed out.