Characteristics and control of fiber ring resonator
Authors: Song Li-Jun , Zhang Peng-Fei , Wang Xin , Wang Chen-Xi , Li Gang , Zhang Tian-Cai ,   State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China,   Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
Published: December 2020

Optical resonators play an active role in fundamental research and applications in atomic fine spectra, laser generation, precision measurements, and quantum information processing because of their high-resolution spectra and strong optical field enhancement. The fiber ring resonators, as a derivative of the resonant resonators, have the advantages of simple structure, small size, stable performance and easy integration. The fiber ring resonators are widely used in fiber lasers, optical communication devices, optical fiber sensing, etc. In this paper, we demonstrate the characteristics of a fiber ring resonator based on a tunable fiber beam splitter experimentally. Control of under-coupling, critical coupling and over-coupling state of the fiber ring resonator can be achieved by adjusting the splitting ratio of the tunable fiber beam splitter. The relationship between the minimum resonator reflectance and resonator loss is given. The intrinsic decay rate of the fiber ring resonator is <inline-formula><tex-math id="M92">\begin{document}${\kappa _0}{\rm{ = }}2{\text{π}} \times \left( {1.60 \pm 0.03} \right)\;{\rm{ MHz}}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M92.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M92.png"/></alternatives></inline-formula>, and the quality factor is <inline-formula><tex-math id="M93">\begin{document}$Q = \left( {1.10 \pm 0.02} \right) \times {10.8}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M93.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="7-20182296_M93.png"/></alternatives></inline-formula>. The resonance frequency of the fiber ring resonator is controlled by stretching the fiber. The fiber resonator is kept straight and fixed on a self-made U-shaped holder by gluing two points. A piezoelectric transducer is used to change the distance between the two glued points. The fiber ring resonator length is changed and controlled when the fiber is stretched. The Pound-Drever-Hall technique is used to lock the resonator to resonance with the laser. The phase of the laser beam is modulated by using an electro-optical modulator, and two sidebands of the laser frequency are generated. Due to the phase sensitivity of the fiber resonator, the reflected light of the fiber resonator with an intensity modulation is observed when the fiber ring resonator is locked. The intensity modulation is caused by the interference between the resonance frequency and the sidebands of the fiber ring resonator. The reflected spectrum of the fiber ring resonator carries the same-frequency modulation as the phase modulation. This is a disadvantage for the usage of the fiber ring resonator. Thus, we reduce the phase modulation power to reduce the intensity modulation of the resonator reflectance. The linear relationship between the phase modulation power and the intensity modulation of the resonator reflectance caused by the phase modulation is obtained. The fiber ring resonator can be locked when the phase modulation power decreases to –9 dBm. The fiber ring resonator has laid a solid experimental foundation for experimental research on the interaction between the fiber ring resonator and quantum emitters such as atoms and color centers in diamond.