Mach-Zehnder interferometer based on fiber taper and fiber core mismatch for humidity sensing

Abstract

A simple and high sensitivity optical fiber relative humidity (RH) sensor based on Mach-Zehnder interferometer (MZI) is proposed and demonstrated in this paper. A single-mode fiber and a graded-index multimode fiber are connected by a fiber taper to form a section. Then an uncoated dispersion compensation fiber is sandwiched between two short sections of the graded-index multimode fiber.Therefore, a sensing structure is set up as a single-mode fiber-taper fiber-graded-index multimode fiber-dispersion compensation fiber-graded-index multimode fiber-taper laser-single-mode fiber. The taper fiber is used to augment the energy of the cladding mode. The two nodes of the graded-index multimode fiber can be looked as a mode coupler. Thus an MZI is constructed. Since the external RH change can make the transmission spectrum energy changed, we can obtain the RH by detecting the peak energy variation of the interference pattern induced by the evanescent-field interaction. The experimental results show that the peak energy changes linearly with surrounding relative humidity. Under the condition of 35%Rh-85%RH, the sensitivity of the sensor with a 20 mm dispersion compensation fiber is -0.0668 dB/%RH and the linearity is 0.995. Moreover, temperature response characteristics are investigated. Experimental results suggest that the transmission spectrum energy of the sensor is insensitive to temperature. With temperature increasing, the transmission spectrum presents obviously a red-shift, yet the peak energy of the monitoring point barely moves, which demonstrates its potential for measuring simultaneously RH and temperature. The proposed sensor has a small size and simple manufacturing process, which can make it widely used to measure RH.

Multiple interference components in a fiber Mach–Zehnder interferometer sensor and their influence on wavelength shift tracing

“…The irregularity in the transmission spectrum of a fiber MZI sensor is common in literature [13][14][15][16][23][24][25], and has also been observed in our experiments described in section 3. The regular curve rarely shows in the literature, to the best of our knowledge.…”

“…Shao et al fabricated an SMF-MMF-SMF fiber sensor through arc fusion splicing, for which the spatial frequency spectrum possesses a dominant peak at 0.0246 nm −1 [15]. Cheng et al fabricated two fiber MZI humidity sensors with different interference arm lengths [16]. For each of them, the spatial frequency spectrum contains several peaks with one peak outstanding.…”

“…As a result, three different IFFT spectra were obtained and related to the three MZIs. However, multiple interference components in the transmission spectrum of a single MZI, which are a common phenomenon as revealed by [13][14][15][16][17], are left out of consideration.…”