Fs-Ablated Trenches on the Surface of Microsphere for Whispering Gallery Modes Cleaning

This study addresses the problem of whispering gallery mode (WGM) selection in spherical microresonators by means of their femtosecond micro-processing. The proposed method involves fabrication on the microsphere surface of defects playing the role of scattering elements for higher-order modes with low azimuthal mode indices. These two T-shaped trenches are created using femtosecond laser ablation, with a depth of 2 microns, gap of 30 microns between them, and each of length of 20 microns along the equatorial direction. A tapered fiber with a sub-micron waist diameter serves as the excitation element for WGMs. This method allows for spectral purification of the WGMs, reducing the number of resonances by 180 times, with a quality factor of Q>105 for the non-inverted spectrum in the form of resonance dips. Additionally, an inverted spectrum with narrow resonance peaks of about 35%, low background level and single mode regime with 3 dB side peak suppression has been simultaneously achieved in the taper transmission, for the first time to our knowledge. The latter was obtained by exciting the microsphere at the taper waist. These results hold promise for the development of narrowband filters, laser mode selectors, and optical sensors based on microresonators.