Ultrahigh-Q WGM microspheres from ZBLAN for the mid-IR band

The advantages of high-quality-factor (high-Q) whispering gallery mode (WGM) microresonators can be applied to develop novel photonic devices for the mid-infrared (mid-IR) range. ZBLAN (glass based on heavy metal fluorides) is one of the most promising materials to be used for this purpose due to low optical losses in the mid-IR. We developed an original, to the best of our knowledge, fabrication method based on melting of commercially available ZBLAN-based optical fiber to produce high-Q ZBLAN microspheres with the diameters of 250 to 350 μm. We effectively excited whispering gallery modes in these microspheres and demonstrated high quality factor both at 1.55 μm and 2.64 μm. Intrinsic quality factor at telecom wavelength was shown to be (5.4 ± 0.4) × 10⁸ which is defined by the material losses in ZBLAN. In the mid-IR at 2.64 μm we demonstrated record quality factor in ZBLAN exceeding 10⁸ which is comparable to the highest values of the Q-factor among all materials in the mid-IR.