Progress in the development of cryogenic sapphire resonator oscillator at NMIJ/AIST

In this paper, we describe the progress in the development of a cryogenic sapphire oscillator (CSO) at NMIJ, AIST. The oscillator was based on a high Q-factor cryogenic-sapphire-resonator cooled with liquid helium which operates on a Whispering Gallery mode. Two CSOs (CSO1 and CSO2) have been built to evaluate their stability at short averaging times. We measured a fractional frequency stability of 1.1 × 10
^-15
at averaging times of 1 s and a minimum frequency instability of 5.5 × 10
^-16
at averaging times of 20 s.