You are presented with multiple paper abstracts, alongside the associated literature review and chapter titles. Given the provided information, you are going to do a literature review. Please generate a coherent literature review that synthesizes the content, draws connections between different works, highlights major findings, and identifies any gaps in the existing research.

Literature Review Title: A Review of Multimode Interference in Tapered Optical Fibers and Related Applications 
Chapter Title:  Humidity Sensor 
1. Abstract of Cited Paper (BIB001):  Abstract We demonstrate the proof of principle for a novel fibre optic sensor for the detection of high humidity. The device uses single-mode fibre at telecommunications wavelengths, making it particularly suitable for application in passive optical networks. The sensing mechanism relies on a humidity-induced refractive index change in an organic film coated onto the cladding of a bent fibre. The sensor was shown to respond at relative humidities >62% and >80% using gelatine and polyethylene oxide films respectively, by the onset of a strong interaction between the LP 01 mode and a whispering gallery mode. 
2. Abstract of Cited Paper (BIB002):  Abstract This paper presents a detailed study of the characteristics of a polymer-coated fibre Bragg grating (FBG) sensor for relative humidity (RH) detection. The sensing scheme used in this work builds upon previous research and extends the application of FBGs in chemical sensing by employing a moisture sensitive polymer coating to induce a mechanical strain on the device through volume expansion. The swelling of the polymer coating as a result of the moisture absorption changes the Bragg wavelength of the FBG, thus giving a direct indication of the humidity level. Sensors with different coating thicknesses were evaluated through a series of experiments carried out over a range of values of RH and temperature to investigate various sensing characteristics which include the RH and temperature sensitivity, the time response and the hysteresis effect. All the sensors tested show a linear and reproducible response with a small degree of hysteresis. 
3. Abstract of Cited Paper (BIB003):  A long-period fiber grating (LPFG) humidity sensor is reported utilizing poly(ethylene oxide)/cobalt chloride (PEO/CoCl2) as a hybrid hygrosensitive cladding coating. A thin overlay of the material is deposited on the LPFG and with exposure to different ambient humidity levels, its spectral properties are modified. The material parameters associated with the sensing mechanism may include those of refractive index, absorption, and morphological alterations of the overlaid material. Relative humidity variations in the range from 50% to 95% have been detected with a resolution better than 0.2%. The response time constant of the fiber sensor is of the order of a few hundred milliseconds. 
4. Abstract of Cited Paper (BIB004):  A subwavelength-diameter tapered optical fiber coated with gelatin layer for fast relative humidity (RH) sensing is reported. The sensing element is composed of a 680-nm-diameter fiber taper coated with a 80-nm-thickness 8-mm-length gelatin layer, and is operated at a wavelength of 1550 nm. When exposed to moisture, the change in refractive index of the gelatin layer changes the mode field of the guided mode of the coated fiber, and converts a portion of power from guided mode to radiation mode, resulting in RH-dependent loss for optical sensing. The sensor is operated within a wide humidity range (9–94% RH) with high sensitivity and good reversibility. Measured response time is about 70 ms, which is one or two orders of magnitude faster than other types of RH sensors relying on conventional optical fibers or films. 
5. Abstract of Cited Paper (BIB005):  Humidity induced change in the refractive index and thickness of the polyethylene glycol (PEG) coatings are in situ investigated for a range from 10 to 95%, using an optical waveguide spectroscopic technique. It is experimentally demonstrated that, upon humidity change, the optical and swelling characteristics of the PEG coatings can be employed to build a plastic fibre optic humidity sensor. The sensing mechanism is based on the humidity induced change in the refractive index of the PEG film, which is directly coated onto a polished segment of a plastic optical fibre with dip-coating method. It is observed that PEG, which is a highly hydrophilic material, shows no monotonic linear response to humidity but gives different characteristics for various ranges of humidity levels both in index of refraction and in thickness. It undergoes a physical phase change from a semi-crystalline structure to a gel one at around 80% relative humidity. At this phase change point, a drastic decrease occurs in the index of refraction as well as a drastic increase in the swelling of the PEG film. In addition, PEG coatings are hydrogenated in a vacuum chamber. It is observed that the hydrogen has a preventing effect on the humidity induced phase change in PEG coatings. Finally, the possibility of using PEG coatings in construction of a real plastic fibre optic humidity sensor is discussed. 
6. Abstract of Cited Paper (BIB006):  A photonic crystal fiber (PCF)-model interferometer (MI)-based relative humidity (RH) sensor is proposed and experimentally demonstrated. The PCF-MI is formed by fusion splicing a short length of PCF between two single-mode fibers with the air holes of the PCF in the splicing regions being fully collapsed. It was then coated with a layer of polyvinyl alcohol by using a dip-coating process. Experimental results show that a high humidity sensitivity of 40.9 pm/%RH is achieved within a measurement range of 20–95%RH. The sensor also shows good repeatability and low temperature independence. 
7. Abstract of Cited Paper (BIB007):  Optical microfiber couplers (MFCs) with certain geometries are sensitive to local refractive changes. By incorporating a humidity-sensitive polyethylene oxide coating, a tapered optical MFC can work as a humidity sensor. We fabricated such a humidity sensor and demonstrated its sensing ability by monitoring spectral shifts in a wavelength domain. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:457–460, 2015

Literature Review:
Over the last few decades, accurate humidity sensing measurement has become increasingly important in numerous applications, such as industrial processing, air conditioning, civil engineering, environment controlling and many others. Optical fiber humidity sensors offer the same advantages previously mentioned elsewhere in this paper. Many types of optical fiber based humidity sensors have been reported, including a relative humidity sensor based on FBG structure coated with polymer BIB003 , a relative humidity sensor based on LPG structure BIB004 , a gelatin coated sub-wavelength-diameter fiber based humidity sensor with a fast response BIB005 , and a PCF coated with PVA interferometer humidity sensor BIB007 . Here, a humidity sensor based on MFC structure coated with a polyethylene oxide (PEO) coating BIB008 is presented. As described in an earlier section, MFC based sensors have shown a high sensitivity to the surrounding RI variation. PEO is known as a strongly hydrophilic material and its RI value varies with the water vapor concentration BIB004 BIB006 BIB002 with the additional advantage that PEO demonstrates good adhesion to the silica of optical fiber surfaces BIB001 . The fabricated MFC sample with total length of 25-mm composes of a uniform 3-mm-long waist region and two 11-mm-long transition regions. The approach of the PEO solution preparation is dissolving a small amount of PEO (Sigma Aldrich, St. Louis, MO, USA) in deionized water. When the MFC was totally immersed in the PEO solution for 20 s and is taken out from the solution, the surface of MFC is coated with a thin layer of PEO film. The two MFC ends were fixed on two PEO cubes using a UV curable adhesive, which ensured a good mechanical stability. Figure 25 presents a schematic diagram of the experimental setup for humidity sensing measurements.  Over the last few decades, accurate humidity sensing measurement has become increasingly important in numerous applications, such as industrial processing, air conditioning, civil engineering, environment controlling and many others. Optical fiber humidity sensors offer the same advantages previously mentioned elsewhere in this paper. Many types of optical fiber based humidity sensors have been reported, including a relative humidity sensor based on FBG structure coated with polymer BIB003 , a relative humidity sensor based on LPG structure BIB004 , a gelatin coated sub-wavelength-diameter fiber based humidity sensor with a fast response BIB005 , and a PCF coated with PVA interferometer humidity sensor BIB007 . Here, a humidity sensor based on MFC structure coated with a polyethylene oxide (PEO) coating BIB008 is presented. As described in an earlier section, MFC based sensors have shown a high sensitivity to the surrounding RI variation. PEO is known as a strongly hydrophilic material and its RI value varies with the water vapor concentration BIB004 BIB006 BIB002 with the additional advantage that PEO demonstrates good adhesion to the silica of optical fiber surfaces BIB001 . The fabricated MFC sample with total length of 25-mm composes of a uniform 3-mm-long waist region and two 11-mm-long transition regions. The approach of the PEO solution preparation is dissolving a small amount of PEO (Sigma Aldrich, St. Louis, MO, USA) in deionized water. When the MFC was totally immersed in the PEO solution for 20 s and is taken out from the solution, the surface of MFC is coated with a thin layer of PEO film. The two MFC ends were fixed on two PEO cubes using a UV curable adhesive, which ensured a good mechanical stability. Figure 25 presents a schematic diagram of the experimental setup for humidity sensing measurements.  Over the last few decades, accurate humidity sensing measurement has become increasingly important in numerous applications, such as industrial processing, air conditioning, civil engineering, environment controlling and many others. Optical fiber humidity sensors offer the same advantages previously mentioned elsewhere in this paper. Many types of optical fiber based humidity sensors have been reported, including a relative humidity sensor based on FBG structure coated with polymer BIB003 , a relative humidity sensor based on LPG structure BIB004 , a gelatin coated sub-wavelength-diameter fiber based humidity sensor with a fast response BIB005 , and a PCF coated with PVA interferometer humidity sensor BIB007 . Here, a humidity sensor based on MFC structure coated with a polyethylene oxide (PEO) coating BIB008 is presented. As described in an earlier section, MFC based sensors have shown a high sensitivity to the surrounding RI variation. PEO is known as a strongly hydrophilic material and its RI value varies with the water vapor concentration BIB004 BIB006 BIB002 with the additional advantage that PEO demonstrates good adhesion to the silica of optical fiber surfaces BIB001 . The fabricated MFC sample with total length of 25-mm composes of a uniform 3-mm-long waist region and two 11-mm-long transition regions. The approach of the PEO solution preparation is dissolving a small amount of PEO (Sigma Aldrich, St. Louis, MO, USA) in deionized water. When the MFC was totally immersed in the PEO solution for 20 s and is taken out from the solution, the surface of MFC is coated with a thin layer of PEO film. The two MFC ends were fixed on two PEO cubes using a UV curable adhesive, which ensured a good mechanical stability. Figure 25 presents a schematic diagram of the experimental setup for humidity sensing measurements. The fabricated MFC structure coated with PEO was placed inside an environment controllable chamber (5503-00 with Package F, Electro Tech Systems, Glenside, PA, USA), of which humidity could be adjusted within a humidity range of 25-85% RH. When broadband light from a BBS (SLD 6593, Covega, MD, USA) was launched into the input port of the MFC, an OSA (Q8384, Advantest, Tokyo, Japan) was employed to record the spectral variations. Noting that the temperature inside the chamber was kept constant at 24 • C during the experiments. The RI of PEO coating decreases as relative humidity (RH) increases, causing the spectral change. The experimental results in Figure 26 show that within the humidity range of 70% RH to 85% RH, this humidity sensor based on MFC coated with PEO possesses a high sensitivity of circa 2.23 nm/% RH, as well as a good linear response. Sensors 2018, 18, x FOR PEER REVIEW 20 of 28 The fabricated MFC structure coated with PEO was placed inside an environment controllable chamber (5503-00 with Package F, Electro Tech Systems, Glenside, PA, USA), of which humidity could be adjusted within a humidity range of 25-85% RH. When broadband light from a BBS (SLD 6593, Covega, MD, USA) was launched into the input port of the MFC, an OSA (Q8384, Advantest, Tokyo, Japan) was employed to record the spectral variations. Noting that the temperature inside the chamber was kept constant at 24 °C during the experiments. The RI of PEO coating decreases as relative humidity (RH) increases, causing the spectral change. The experimental results in Figure 26 show that within the humidity range of 70% RH to 85% RH, this humidity sensor based on MFC coated with PEO possesses a high sensitivity of circa 2.23 nm/% RH, as well as a good linear response.