Abstract

The utilisation of optical carriers is of strong interest for microwave applications such as microwave generation [1]. For example, optical resonators can replace microwave resonators and allow to overcome the performance of traditional microwave oscillators in terms of compactness and quality factor, which will strongly reduce the phase noise of the overall microwave oscillator. Whispering gallery modes resonators have been studied and used in this aim [2, 3]. Whereas they exhibit very interesting characteristics in terms of quality factor, the long-term stability of the resulting microwave oscillator is reduced because of the mechanical instability induced by the fibre-to-resonator coupling systems. An interesting alternative consists in using optical fibres based ring resonators [4]. This kind of set-up presents easily a high stability in time in terms of light coupling, while having great potential for exhibiting high-Q values. It is composed of a single-mode fibre with a length between 10 and 20 m, and two low losses fibered couplers. The choice of the fibre length and the couplers ratios are critical for the optical quality factor of the resonator. A first study of an optoelectronic oscillator (OEO) based on this fibre ring (Fig. 1) has shown that, despite a very high Q-factor of the resonator (about 2.10⁸ at 1.55 µm for a length of 10 m), the phase noise of the OEO can be high (Fig. 2) and has to be decreased [4]. We have thus based our optimisation of resonant fibre rings on a theoretical study of the OEO phase noise to understand the influence of the resonator on the noise of the oscillator. This study concerns mainly the phase noise due to the white frequency noise induced by our resonators and is comparable to the one proposed for purely microwave oscillators by J. Everard [5].

© 2011 Optical Society of America