High-Q optical resonators for laser stabilization in microwave photonics oscillators

Abstract

New resonators are investigated in order to design compact and high performances microwave oscillators. Because of long wavelengths in microwave range, dimensions of resonators are intrinsically large. Secondly, the quality factor of these resonators is now limited to about 2.105 at 10 GHz with whispering gallery modes (WGM) sapphire resonators. Nevertheless, it is possible to solve these difficulties if we use optical waves as a carrier for the RF frequencies, for example a 1.5 µm-laser. Resonators become optical ones, with low dimensions due to short wavelengths. The optical resonator creates an optical frequency comb with microwave spacing. Each mode of this comb is characterized by an optical Q factor (Qopt) and an equivalent RF Q factor (QRF). QRF is the product of Qopt and the RF to optical frequencies ratio. At 1.55 µm, this frequency ratio is about 104 at 20GHz and Qopt must be at least equal to 108 to obtain a quality at 20 GHz better than the microwave resonators. We present different high-Q optical resonators and an application of one of them to microwave photonics oscillators.