Investigation of thermal effects in a resonance condition of microfibre double-knot resonators as high-order filter

Theoretical and experimental investigations are conducted into the thermal effect on resonance conditions of microfibre double-knot resonators. A double-knot microfibre resonator which utilises heat for optical path correction is introduced to increase the finesse. The proposed resonator consists of two knots with radii of 357.66 and 357.73 μm which are placed in series. Each individual knot has a finesse of 5 and an extinction ratio of 6 dB. When placed in series, the extinction ratio of the comb spectrum varies from 3.5 to 15 dB by controlling the resonance wavelength of both knots through optical correction. Optical path correction is implemented by placing a hot metal bar with a temperature of 30°C close to the smallest knot. This path resulted in a flat comb spectrum with a high extinction ration of 15 dB. The produced resonance has an unchanged free spectral range of 715 pm, a quality factor of 20 000, FWHD of 85 pm and a finesse of 9.5.