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1. Introduction The optical fiber humidity sensor has the advantages of high measuring accuracy, high relative speed, simple fabrication, and compact size. Different types of humidity sensors based on fiber optical sensing have been proposed, such as polymer-coated Bragg grating fiber [1], long period fiber gratings (LPFG) [2], agarose-infiltrated photonic crystal fiber (PCF) interferometers [3], nonadiabatic tapered fiber coated with PDDA/Poly-R-478 film [4], U-shaped fiber coated by phenol red-doped PMMA [5], and interference structures [6]. Above all, the interference structures, particularly MZI, are more practical because of their compact structure, low cost, and easy fabrication and are widely used for temperature, strain, and refractive index measurement [7]. The humidity sensor based on MZI with waist enlarged structure is constructed by two waist enlarged fusion bitapers sandwiched between the SMFs. The core mode and the cladding mode have been coupled and recombined, the latter sensitive to outside environment refractive index (RI) with some humidity sensitive material coated on the SMF-MZI [8]. In the research of optical fiber humidity sensor, although optical fiber humidity sensor type and structure design are very important, humidity sensitive material also plays a key role in improving measurement performance of optical fiber humidity sensor [9]. In order to improve humidity sensor sensitivity, researchers tried to adopt various materials as humidity sensitive material, such as polyvinyl alcohol [10], metal oxide films [11], and chitosan [8]. However, all of these sensors have their own disadvantages. MC, as a special kind of high polymer chemical products, has good dispersion, thickening, film forming, and so on and is becoming a feasible material for biological sensing studies [12]. Recently, the layer-by-layer self-assembly technique has attracted significant attention because of its precise control of thickness at the nanometer level and construction on nonflat surfaces [8, 13]. Owing to its unique properties, it has been widely used for the construction of ultrathin multilayer films. In this article, a methylcellulose coated humidity sensor based on MZI with waist enlarged structure is demonstrated. It can be monitored by intensity, which makes this sensor production of low cost and easy to make. 2. Design and Principle A schematic diagram of a proposed humidity sensor structure is shown in Figure 1. It consists of two waist enlarged structures. The light which comes from the optical power source passes through the SMF to the first waist enlarged region; the cladding mode is excited and propagates inside the SMF cladding region [8]. When the two lights reach the second waist enlarged structure region, since two lights of optical path are different, interference will occur in the second waist enlarged structure region. The first waist enlarged structure is equivalent to the beam splitter, and the second waist enlarged structure plays the role of coupler. 
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