A Mach–Zehnder interferometric humidity sensor based on waist-enlarged tapers
Introduction
Relative humidity (RH) is an important parameter in chemical engineering, health, medical, industrial and agricultural fields. In a number of these areas, the detection devices are required to be small-size, immune to electromagnetic interference and work in severely poisonous environments. Optical fiber sensors could satisfy these requirements better than traditional electrical humidity sensors, which makes the fibers good candidates for humidity sensing applications. Many types of optical fiber humidity sensors have been reported, such as fibre gratings [1], [2], [3], U-bend fibre structures [4], [5], photonic crystal fibre (PCF) structures [6], fiber taper structures [7], [8] and interference structures [9], [10], [11]. Generally, these sensors are coated with thin poly-film to improve humidity sensitivity. The sensitivity of the sensors is thus determined and limited by film quality and coating technology.
As one of the excellent fiber-based optical interferometers, in-fiber Mach–Zehnder interferometers (MZI) are widely used to measure temperature, strain, and refractive index [12], [13], [14], [15]. The key principle for this device is the interference between the core mode and cladding modes of a fiber. To excite the cladding modes from the fundamental core mode, the coupling technology is focused on three methods. One method is making fiber tapers through arc fusion splicing or flame brushing techniques, the second is making a small lateral offset in two pieces of fibers by arc fusion splicing, and the third is fusion two pieces of fibers with different core diameters. In MZI structures with mismatched core diameters, multimode fiber (MMF) has attracted researchers' interest because of its large core diameter. Wang et al., [16] proposed a fiber refractometer based on an MMF taper sandwiched between two single-mode fibers (SMF) that achieved high sensitivity by combining the second and third methods listed above. The MMF based MZI has also been exploited for measuring humidity. Shohei et al., [17] constructed an MZI humidity sensor by sandwiching a hetero-core fiber into an MMF. However, the sensitivity of that sensor remained low despite being coated with multiple hygroscopic polymer layers. Li et al., [18] demonstrated a hybrid structure of an MMF taper and fiber Bragg Grating with a polyvinyl alcohol film coating on the taper. However, the cross-sensitivity to temperature and humidity was not mentioned in their paper.
In this paper a novel humidity sensor based on an SM–MM–SM in-fiber structure is proposed and experimentally demonstrated. The sensor consists of two waist-enlarged tapers created through cleaving and arc fusion splicing. The advantages of this sensor are simple fabrication, compact structure and insensitivity to temperature. More importantly, the humidity sensitivity is greatly improved without requiring polymer coating.
Section snippets
Sensor principle and fabrication
The schematic diagram of the proposed MZI is shown in Fig. 1. A section of MMF is sandwiched in an SMF, while the two coupling sections are formed as waist-enlarged fiber tapers by arc fusion splicing. The cladding modes are excited from the fundamental mode at the first waist-enlarged taper and then propagate in the MMF. The cladding modes could recouple with each other or with the fundamental mode at the second coupling point. The distance between the two coupling points is taken as the
Measurement and discussion
The MZI with 53 mm interferometer length is chosen for the humidity measurement due to its clearer interference fringes and large dynamic range. Fig. 5 shows the schematic diagram of the experiment. The humidity environment is provided by a saturated salt solution in a beaker, which is placed in a closed vessel. The performance of this type of MZI is affected by bending loss, so the sensor is straightened and fixed on plexiglass that has a distribution of holes for moisture to pass through.
Conclusions
In this paper, a novel type of MZI based on a piece of MMF sandwiched in an SMF for humidity sensing was demonstrated. The coupling points are fabricated as waist-enlarged tapers, which excite multiple modes and cause intermodal interference in the core of the MMF. The theoretical analysis indicates that interference primarily occurs between the LP01 and LP1n modes in such an MZI. The linear humidity response with an enhanced sensitivity of −0.119 dBm/%RH is achieved over the humidity range of
Acknowledgments
This work is supported by the National Science Foundation of China under Grant F050304 and the National “863” Project of China under Grant 2009AA06Z203. We Thank Jing Zhang for useful discussions.
References (22)
- et al.
Improving the sensitivity of a humidity sensor based on fiber bend coated with a hygroscopic coating
Opt Laser Technol
(2011) - et al.
Chitosan based fiber-optic Fabry–Perot humidity sensor
Sensors Actuat B
(2012) - et al.
Humidity insensitive TOPAS polymer fiber Bragg grating sensor
Opt Exp
(2011) - et al.
Simultaneous measurement of humidity and temperature based on an SiO2-Nanospheres film deposited on a long-period grating in-line with a fiber Bragg grating
IEEE Sensors J
(2011) - et al.
Relative humidity sensor based on cascaded long-period gratings with hydrogel coatings and fourier demodulation
IEEE Photon Technol Lett
(2009) - et al.
A fiber bend based humidity sensor with a wide linear range and fast measurement speed
Sensors Actuat A
(2012) - et al.
Relative humidity sensor based on an agarose-infiltrated photonic crystal fiber interferometer
IEEE J Sel Top Quantum Electron
(2012) - et al.
Fast detection of humidity with a subwavelength-diameter fiber taper coated with gelatin film
Opt Exp
(2008) - et al.
Humidity sensor based on a multimode-fiber taper coated with polyvinyl alcohol interacting with a fiber Bragg grating
IEEE Sensors J
(2012) - et al.
Humidity sensor based on a single-modehetero-core fiber structure
Opt Lett
(2011)
Optical fiber relative humidity sensor based on FBG incorporated thin-core fiber modal interferometer
Opt Exp
Cited by (77)
High-sensitive and temperature-immune curvature sensor based on bitaper sandwiching in SMS fiber structure
2023, Optics CommunicationsHumidity sensor based on fibre bragg grating for predicting microbial induced corrosion
2022, Sustainable Energy Technologies and AssessmentsRelative humidity sensor based on cascaded Fabry-Perot interferometers and Vernier effect
2022, OptikCitation Excerpt :As a new type of humidity sensor, optical fiber humidity sensor has the advantages of small volume, low cost, high sensitivity and anti-electromagnetic interference, which has attracted extensive research interest [2,3]. At present, many types of optical fiber humidity sensors with excellent performance have been reported, which includes made of special optical fibers [4–7], fiber gratings [8–10], special optical fiber structures [11–13] and optical fiber interferometers [14–17]. However, these sensors also have some disadvantages, such as high manufacturing cost or large light source power fluctuation or low sensitivity or fragile structure and so on.
Unobtrusive vital signs and activity monitoring based on dual mode fiber
2021, Optical Fiber TechnologyCitation Excerpt :Therefore, it is strongly desirable that a simple unobtrusive sensor could be developed, realizing vital signs and activity monitoring, which could also minimize the risk of cross-infection, especially during COVID-19 pandemic period. Fiber-optic sensors have intrinsic advantages of fast response, high sensitivity, compact size, immunity to EMI and remote sensing capabilities [3–5]. As for vital sign monitoring, technologies including fiber Bragg grating (FBG)-based [6,7] micro-bend-based [8,9] and fiber interferometers-based [10–13] have been already investigated.
Cascaded few-mode fiber down-taper modal interferometers and their application in curvature sensing
2020, Optics Communications