Relative humidity sensor based on a few-mode microfiber knot resonator by mitigating group index difference

A few-mode microfiber knot resonator (FM-MKR) incorporating a polyvinyl alcohol (PVA) is investigated for measurement of relative humidity (RH). Two modes, such as HE
₁₁
and HE
₁₂
, are excited and interfered in a nonadiabatically tapered structure of the single-mode fiber (SMF). After making a tie with the few-mode microfiber, the FM-MKR is fabricated. In the FM-MKR, two modes, such as HE
₁₁
and HE
₁₂
, must be circulated within the optical knot and cross-coupled independently with a phase delay. By optimizing the diameter of the microfiber, the difference of group effective refractive indices between two modes is dramatically suppressed resulting in the improvement of RH sensitivity of the proposed FM-MKR.