Magnesium Fluoride Whispering Gallery Mode Disk-Resonators for Microwave Photonics Applications

Abstract

Abstract-We have manufactured a high-whispering gallery mode resonator with magnesium fluoride for microwave photonics applications in the gigahertz frequency range. This crystal is scarcely used for resonator fabrication despite its numerous advantages, that are mainly high mechanical hardness, low sensitivity to water vapor pollution, and low sensitivity to photo-and thermo-refractive fluctuations at optimal temperatures. Using a customized machining procedure, we have successfully fabricated and characterized a 5-mm resonator with a surface rugosity of the order of 1 nm (from 3 to 12 atoms). Cavity ring-down measurements enabled us to determine that the resonator has a quality factor = 3 4 10 8 at 1550 nm. is the ratio between the central frequency and the bandwidth of the resonance. It can, therefore, be deduced by inference that -factors superior to can in principle be obtained at 1550 nm. On the other hand, crystalline media generally enable to achieve superior performances. For example, quartz WGM resonators with a -factor of at 1550 nm have already been reported [11]. However, CaF seems to be the most interesting alternative as it yields the highest -factor reported so far at 1550 nm, a record value of [12]. In this letter, we consider an unusual bulk medium for the resonator, magnesium fluoride MgF . This tetragonal crystal has many specific advantages that may be of particular interest for various applications. For example, MgF is not sensitive to water vapor pollution [13], [14]. Its lifetime in free atmosphere is, therefore, significantly longer that the one of fused silica, which generally needs a purged nitrogen local atmosphere, or a special surface treatment. Another advantage of MgF is its mechanical hardness, which is in the range of 6 Mohs. This relatively high value makes this material resistant to mechanical shocks and to hazardous surface scratches, at the opposite of CaF which is mechanically softer. Therefore, MgF resonators are relatively easy to manipulate and their -factors are satisfyingly stable over time. Table I presents a comparison of some interesting properties of fused silica, quartz, CaF and MgF materials. The advantageous properties of MgF appear straightforwardly: it is the unique material having at the same time a good resistance to mechanical shocks and a high resistance to water pollution. Index Terms-MagnesiumMagnesium fluoride WGM resonators also have other positive characteristics. In [15], the authors have performed a detailed analysis of the influence of photo-and thermo-refractive fluctuations on the noise performance of WGM resonators, and they have found that in MgF , there are optimal temperatures for which these fluctuations are strongly inhibited. This is one of the properties that strongly motivate our interest for this material in view of microwave photonics applications, where noise is a critical parameter. To the best of our knowledge, the typical -factor of MgF resonators is not available in the literature, even though they have already been fabricated in ...

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