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Optical fiber sensing techniques have been attractive in various applications to mechanical, chemical, biological, and environmental industries for measurement of temperature, strain, ambient index, liquid level, and so on. Fiber-optic interferometric sensing probes based on all-fiber Mach-Zehnder interferometers (MZIs), Fabry-Perot interferometers (FPIs), and Saganc interferometers have been investigated extensively. Various In-line MZIs using the combination of a single-mode fiber (SMF) with a multimode fiber (MMF) were proposed to realize refractometers or pressure sensors. To obtain high temperature sensing probe, multiple MZIs have been developed by using a piece of MMF sandwiched between two SMFs cascaded with a tapered SMF. Fiber-optic relative humidity (RH) sensors have been widely investigated in a variety of applications in meteorology, medicine, agriculture, and architectural engineering fields. To improve the RH sensitivity of the fiber-optic sensing probe, many structures of fiber-optic sensing probes have been proposed, such as a polyvinyl alcohol (PVA) coated photonic crystal optical fiber, a fiber FPI with a PVA thin film, a Michelson interferometer with chitosan coating. In this paper, a few-mode microfiber knot resonator (FM-MKR) is presented for measurement of RH. The proposed FM-MKR includes two optical phenomena, such as optical modal interference in the few mode microfiber and optical coupling in the FM-MKR. When the waist diameter of the microfiber is 4 m, two modes, such as HE11 and HE12, should be excited and interfered together in the nonadiabatically tapered region of the SMF. After making a tie with the few-mode microfiber with a diameter of 4 m, the FM-MKR can be fabricated. In the FM-MKR, two modes must be circulated within the optical knot and cross-coupled independently with a phase delay. To detect RH, the FM-MKR is coated by using the PVA which effectively absorbs humidity in the external environment. For the microfiber with a diameter of 4 m, the difference of group effective refractive indices between HE11 and HE12 modes becomes nearly zero and the sensitivity of the FM-MKR to RH can be successfully improved. Transmission characteristics of the proposed FM-MKR with variations in RH are measured.
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