Relative Humidity Sensor Based on an Optical Microfiber Knot Resonator With a Polyvinyl Alcohol Overlay

Au nanorod–coupled microfiber optical humidity sensors

“…3(b)), from which we obtain a sensitivity of about 0.51 nm/% RH. While this sensitivity is comparable with other high-sensitivity microfiber RH sensors [10,13,16,[18][19][20][28][29][30][31], the hybrid nanorodmicrofiber sensor is much more compact in size (i.e., the 2-μm cavity size of this sensor versus > 40-μm cavity sizes of other sensors) [9][10][11][12][16][17][18][19][20][21]. The miniature cavity size is also beneficial to fast response, as has been shown in other types of microfiber optical sensors [10,25,32].…”

“…Typical microfiber resonators, in forms of assembled loops [8][9][10], tied knots [11][12][13], or stacked multicoils [14,15], are ring-type resonators. The ring size of this kind of resonators, from hundreds of micrometers to several millimeters, is limited by the bending loss of the microfiber, preventing the further miniaturization of the microresonator for higher compactness and spatial resolution (typically > 40 μm in cavity size) [9][10][11][12][16][17][18][19][20][21].…”

Mechanism and Characteristics of Humidity Sensing with Polyvinyl Alcohol-Coated Fiber Surface Plasmon Resonance Sensor

“…Compared to a traditional electric RH sensor, optical fiber-based sensors exhibit a great number of advantages, such as security to the human body, miniaturization for integration, and good resistance to electromagnetic interference. In recent years, a variety of fiber devices have been implemented in to the design of optical fiber RH sensors, configurations including interferometers [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ], long period fiber gratings [ 8 , 9 ], tapered fibers [ 10 , 11 , 12 , 13 ], fiber Bragg gratings (FBGs) [ 14 , 15 , 16 , 17 , 18 ], mode couplers [ 19 , 20 ], ZnO nanostructures [ 21 , 22 ], evanescent wave absorption structures [ 23 , 24 ], and so on. Among these configurations, light intensity demodulation is often used since it is simple and cost effective.…”

Relative Humidity Sensors Based on Microfiber Knot Resonators—A Review

“…The key component of the MKR is a microfiber fabricated by using a micro-tapering technique with various heating sources, including a flame, a laser-based heating tube, and electric strip heater [2,3,4,5,6,29,30]. Two critical parameters in the micro-tapering technique are the temperature of the heating process and elongation of the pulling process.…”

“…After fabricating a silica or polymer-based microfiber, its two freestanding ends are assembled to form a comparatively large knot and tightened into the desired knot diameter by progressively pulling the free ends of the microfiber, as shown in Figure 2a, so that an MKR with a certain knot diameter can be achieved as shown in Figure 2b [1,29,30]. Mode coupling and interference occur in the microfiber knot region (coupling region) where the microfiber is twisted.…”