A Neck-down Microsphere Resonator with an Improved Quality Factor of the Whispering-Gallery Mode

A new type of microsphere resonators is designed to improve the quality factor (Q factor) of the whispering-gallery mode (WGM). The neck-down microsphere was fabricated by tapering and discharging to a single-mode fiber using a commercial fusion splicer. The diameter of the microsphere ranges from 250 μm to 330 μm, and the stem is from 50 μm to 100 μm. In addition, the concept of the sphere-to-stem ratio (STSR, the diameter ratio of the sphere to the stem) to evaluate the quality of the resonator is proposed for the first time to our knowledge. The STSR could be adjusted by changing the diameter of the taper waist and the length of the tapered tip. The experimental results show a trend that the larger the sphere-to-stem ratio is, the higher the Q factor is. Using the same interrogating system, we demonstrate that the Q factor of the WGM with a neck-down sphere resonator could reach 1.07 × 10
⁷
, which is 1~2 orders of magnitude higher than that of a normal sphere with a similar size. This method represents a simple and low-cost approach to efficiently increase the Q factor.