Abstract
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.
