High-Q whispering gallery travelling wave resonators for oscillator frequency stabilisation

Usually a frequency stabilised standing wave resonator-oscillator that incorporates a resonator as a frequency discriminator, requires a circulator to separate the input and reflected wave. A ferrite circulator is a noisy device and can limit the phase noise or frequency stability. Moreover, we show that the noise in a circulator varies, and detailed low noise measurements are necessary to choose an appropriate quite circulator. Thus, by realising a configuration that does not require a circulator an improvement in performance and reliability can be obtained. A solution to this problem is to design a high-Q Whispering Gallery Travelling Wave WGTW resonator. This device naturally separates the input and reflected wave in the same way as a ring cavity at optical frequencies without degrading the frequency discrimination. Q-factor measurements of a WGTW sapphire resonator are presented, along with a derivation of critical parameters to maximise the frequency discrimination. Measurements of noise in ferrite circulators and isolators have also been made which we follow with a discussion on oscillator design.