A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> In this work, the numerical and experimental analysis on the use of thinned fiber Bragg gratings as refractive index sensors have been carried out. Wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Experimental characterization for an almost full etched cladding sensor is presented demonstrating good agreement with numerical results and resolutions of /spl ap/10/sup -5/ and /spl ap/10/sup -4/ for outer refractive index around 1.45 and 1.333, respectively. <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> We combine fiber Bragg grating (FBG) technology with a wet chemical etch-erosion procedure and demonstrate two types of refractive index sensors using single-mode optical fibers. The first index sensor device is an etch-eroded single FBG with a radius of 3 μm, which is used to measure the indices of four different liquids. The second index sensor device is an etch-eroded fiber Fabry-Perot interferometer (FFPI) with a radius of ~1.5 μm and is used to measure the refractive indices of isopropyl alcohol solutions of different concentrations. Due to its narrower resonance spectral feature, the FFPI sensor has a higher sensitivity than the FBG sensor and can detect an index variation of 1.4 X 10(-5). Since we can measure the reflection signal, these two types of sensors can be fabricated at the end of a fiber and used as point sensors. <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> Abstract When a thin metal layer is deposited on a single-mode tapered optical fiber the layer takes a semi-cylindrical shape. The fundamental fiber mode can excite the different hybrid surface plasmon modes supported by the semi-cylindrical metallic shell. As a consequence, the transmission spectrum of the device exhibits multiple resonance peaks with either TE- or TM-polarized light. The positions of those peaks are remarkably shifted when the refractive index of the medium adjacent to the metal layer changes. Experimentally, we have found that by monitoring the first three resonance peaks a refractive index resolution of about 7 × 10 −7 can be achieved. Our devices can be used for high-resolution biosensing. <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> An optical fiber sensor based on the change of optical confinement in a subwavelength tip is presented. The optical spot is substantially increased when the environmental refractive index (RI) increases from 1.3 to 1.4. By measuring the intensity of low angular spectral components, an intensity sensitivity up to 8000% per RI unit is achieved. The fiber tip sensors take advantage of the small detection volume and real-time responses. We demonstrate the application of the nanofiber sensors for measuring concentrations of acids and evaporation rates of aqueous mixtures. <s> BIB004 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> We design and fabricate a novel multicore fiber (MCF), with seven cores arranged in a hexagonal array. The fiber properties of MCF including low crosstalk, attenuation and splice loss are described. A new tapered MCF connector (TMC), showing ultra-low crosstalk and losses, is also designed and fabricated for coupling the individual signals in-and-out of the MCF. We further propose a novel network configuration using parallel transmissions with the MCF and TMC for passive optical network (PON). To the best of our knowledge, we demonstrate the first bi-directional parallel transmissions of 1310nm and 1490nm signals over 11.3-km of seven-core MCF with 64-way splitter for PON. <s> BIB005 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> We have investigated the influence of multimode fiber core (MMFC) diameters and lengths on the sensitivity of an SMS fiber based refractometer. We show that the MMFC diameter has significant influence on the refractive index (RI) sensitivity but the length does not. A refractometer with a lower MMFC diameter has a higher sensitivity. Experimental investigations achieved a maximum sensitivity of 1815 nm/ RIU (refractive index unit) for a refractive index range from 1.342 to 1.437 for a refractometer with a core diameter of 80 μm. The experimental results fit well with the numerical simulation results. <s> BIB006 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> All-fiber in-line single-mode—multimode—single-mode (SMS) and single-mode—tapered-multimode—single-mode (STMS) fiber structures are investigated. A wide-angle beam propagation method in cylindrical coordinates is developed and employed for numerical simulations of the light propagation performance of such fiber devices. The effect of strong mode interference on the performance of the devices is studied and verified numerically; results indicate that the proposed STMS structure can be exploited for measuring a broad refractive index range with reasonable high resolution, compared with the conventional SMS structure. <s> BIB007 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> A refractive index (RI) sensor based on a novel fiber structure that consists of a single-mode–multimode–single-mode (SMS) fiber structure followed by a fiber Bragg grating was demonstrated. The multimode fiber in the SMS structure excites cladding modes within output single-mode fiber (SMF) and recouple the reflected cladding Bragg wavelength to the input SMF core. By measuring the relative Bragg wavelength shift between core and cladding Bragg wavelengths, the RI can be determined. Experimentally we have achieved a maximum sensitivity of 7.33 nm/RIU (RI unit) at RI range from 1.324 to 1.439. <s> BIB008 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> We propose and experimentally demonstrate an enhanced evanescent field fiber refractometer based on a tapered multimode fiber sandwiched between two single-mode fibers. Experiments show that this fiber sensor offers ultrahigh sensitivity [better than 1900nm/RIU at a refractive index (RI) of 1.44] for RI measurements within the range of 1.33 to 1.44, in agreement with the theoretical predictions. This is the highest value reported to date (to our knowledge) in the literature <s> BIB009 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> We propose a novel way to enhance the sensitivity of single-mode-multimode-single-mode (SMS) fiber structure in the measurement of surrounding refractive index (RI), which is realized by depositing the multimode fiber section with a high RI overlay. First, as an essential basis of the analysis of the novel structure, characteristics of mode reorganizations in the deposited multimode fiber are discussed. Second, based on full-vector mode expansion, a theoretical analysis of transmission characteristics of the deposited SMS fiber structure is presented. Third, aimed at RI measurement ranges which are of common interest in liquid and gas sensing, the transmission spectra of the structure in response to the surrounding RI are investigated by simulations. To well utilize the collective effect of multiple interference modes in mode reorganizations, the monitoring based on wavelength shifts of valleys and the optimization of the overlay are analyzed for various application requirements. Lastly, largely enhanced sensitivities for RI measurements are demonstrated with properly designed overlay. For RI ranges from 1.31 to 1.35 and 1.0 to 1.03, average sensitivities of 900 and 206 nm/RIU are achieved with overlay thicknesses of 340 and 470 nm, respectively. <s> BIB010 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> An all-fiber refractive index (RI) sensor with a simple configuration of periodical tapers is proposed and investigated experimentally. The proposed fiber RI sensor consists of a small core fiber sandwiched between two standard singlemode fibers, with tapers periodically fabricated along the small core fiber using a focused CO2 laser beam. Such a structure can be used for RI sensing by measuring the dip wavelength shift of the multimode interference within the small core fiber cladding. An average sensitivity of 226.6 nm/RIU (RI Unit) has been experimentally achieved in the RI range from 1.33 to 1.38. The refractometer is sensitive to temperature and an experimental investigation of this sensitivity is presented. It is found that the peak shift response has a linear variation with temperature; therefore, temperature dependence can be mitigated by a suitable RI correction process. The proposed RI sensor benefits from simplicity and low cost and achieves a competitive sensitivity compared with other existing fiber-optic sensors. <s> BIB011 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> In this Letter, an all-fiber refractometer with a simple configuration of periodical tapers on a photonic crystal fiber (PCF) is proposed and investigated experimentally. The proposed fiber refractive index (RI) sensor consists of a PCF sandwiched between two standard single-mode fibers, with tapers periodically fabricated along the PCF using a CO2 laser beam focused by a ZnSe cylindrical lens. The proposed fiber sensor can be used for RI sensing by measuring the wavelength shift of the multimode interference dip over the transmission spectrum. An average sensitivity of 222 nm/RIU has been experimentally achieved over a RI range from 1.33 to 1.38. The proposed refractometer is also significantly less sensitive to temperature, and an experimental demonstration of this reduced sensitivity is presented. The proposed RI sensor benefits from simplicity and low-cost and achieves a competitive sensitivity compared with other existing fiber-optic sensors. <s> BIB012 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> An optical microfiber coupler-based refractive index sensor is proposed and experimentally demonstrated in this letter. A microfiber coupler with a minimum diameter of 2.5 μm and a uniform waist with a length of 2 mm is fabricated and used for sensing surrounding refractive index. Both theoretical and experimental results have shown that the spectral response of the proposed microfiber coupler is highly sensitive to the surrounding refractive index over the range from 1.3340 to 1.3800. An average sensitivity of 2723 nm/RIU over this entire refractive index range has been achieved experimentally. Furthermore, for the refractive index range from 1.3340 to 1.3515, this sensor has demonstrated high sensitivity of 4155 nm/RIU. <s> BIB013 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> RI Sensor <s> We demonstrate refractive index sensors based on single mode tapered fiber and its application as a biosensor. We utilize this tapered fiber optic biosensor, operating at 1550 nm, for the detection of protein (gelatin) concentration in water. The sensor is based on the spectroscopy of mode coupling based on core modes-fiber cladding modes excited by the fundamental core mode of an optical fiber when it transitions into tapered regions from untapered regions. The changes are determined from the wavelength shift of the transmission spectrum. The proposed fiber sensor has sensitivity of refractive index around 1500 nm/RIU and for protein concentration detection, its highest sensitivity is 2.42141 nm/%W/V. <s> BIB014
RI is an important physical parameter to measure in itself but importantly measuring RI is also a foundation for sensing a variety of other measurands, for example by utlising coatings on sensors where the coating RI is influenced by the level of the desired measurand. To date many types of fiber RI sensors have been reported and developed. Traditionally, the common implementation structures include FBG BIB001 BIB002 , LPG  , surface plasmons BIB003   , SMS fiber structures BIB006 BIB007 BIB010 , and TOF structures BIB004  BIB014 . Recently, various fiber structure based RI sensors have been investigated; for example, in reference BIB006 , the relationship between the diameter and length of MMF core within an SMS fiber structure and its RI sensing properties has been investigated; a novel RI sensor based on a hybrid structure of an SMS fiber and an FBG was demonstrated experimentally in BIB008 ; an SMS 28-SCSMF-SMF 28 structure was demonstrated experimentally to have a maximum sensitivity of 1808 nm/RIU BIB005 . TOF structures have also been utilized to implement RI sensors, such as STMS fiber structures BIB009 , SPTS fiber structures BIB011 , SPS fiber structures BIB012 , and MFC fiber structures BIB013 , just to cite a few. In this section, a comprehensive review is described for the RI sensors mentioned above. Generally, the approach to RI measurement for different TOF structures are similar, therefore, the STMS fiber structure is taken as an example to illustrate the measurement process. Figure 19 presents the experimental setup for the RI measurement, which consists of a broadband source (BBS), an OSA, and the fabricated STMS fiber sample. The RI sensing measurement was implemented at room temperature of about 25 °C, and a series of RI liquids (1.33-1.44 with an interval of 0.01, RI error ±0.0002) was used. Broadband light with wavelength range of 1450-1650 nm was launched from a Figure 18 . Schematic of (a) a MFC structure and (b) a MFCT structure. RI is an important physical parameter to measure in itself but importantly measuring RI is also a foundation for sensing a variety of other measurands, for example by utlising coatings on sensors where the coating RI is influenced by the level of the desired measurand. To date many types of fiber RI sensors have been reported and developed. Traditionally, the common implementation structures include FBG BIB001 BIB002 , LPG  , surface plasmons BIB003   , SMS fiber structures BIB006 BIB007 BIB010 , and TOF structures BIB004  BIB014 . Recently, various fiber structure based RI sensors have been investigated; for example, in reference BIB006 , the relationship between the diameter and length of MMF core within an SMS fiber structure and its RI sensing properties has been investigated; a novel RI sensor based on a hybrid structure of an SMS fiber and an FBG was demonstrated experimentally in BIB008 ; an SMS 28-SCSMF-SMF 28 structure was demonstrated experimentally to have a maximum sensitivity of 1808 nm/RIU BIB005 . TOF structures have also been utilized to implement RI sensors, such as STMS fiber structures BIB009 , SPTS fiber structures BIB011 , SPS fiber structures BIB012 , and MFC fiber structures BIB013 , just to cite a few. In this section, a comprehensive review is described for the RI sensors mentioned above. Generally, the approach to RI measurement for different TOF structures are similar, therefore, the STMS fiber structure is taken as an example to illustrate the measurement process. Figure 19 presents the experimental setup for the RI measurement, which consists of a broadband source (BBS), an OSA, and the fabricated STMS fiber sample. The RI sensing measurement was implemented at room temperature of about 25 • C, and a series of RI liquids (1.33-1.44 with an interval of 0.01, RI error ±0.0002) was used. Broadband light with wavelength range of 1450-1650 nm was launched from a BBS, transmitted through the STMS fiber structure, and recorded by an OSA. The prepared RI liquids were placed around the STMS fiber sample by using a dropper. The taper waist region of the STMS fiber sample was immersed totally. Before dropping and measuring the next RI liquid, the remained RI liquid on the fiber was washed off by deionized water. Finally, the experimental optical spectrum shift with respect to different RI values was obtained by processing the measured data of OSA. BBS, transmitted through the STMS fiber structure, and recorded by an OSA. The prepared RI liquids were placed around the STMS fiber sample by using a dropper. The taper waist region of the STMS fiber sample was immersed totally. Before dropping and measuring the next RI liquid, the remained RI liquid on the fiber was washed off by deionized water. Finally, the experimental optical spectrum shift with respect to different RI values was obtained by processing the measured data of OSA. The relationship between the resonance wavelength shifts and RI values was also presented in Figure 20 . Over the RI range of 1.33-1.44, the RI sensor based on an STMS fiber structure has an average sensitivity of 487 nm/RIU. In addition, when the RI value is 1.44, this proposed sensor can achieve a maximum sensitivity of 1913 nm/RIU, which means that there is an RI resolution of up to 5.23 × 10 −6 assuming the minimum wavelength shift of the OSA was 0.01 nm. Using a probe type sensor for RI measurement is useful for certain applications. Here, in case of trivial description, we presented the experiment process of the SPST fiber structure. As shown in Figure 21 , the input/output SMF of the fabricated SPST sample was divided into two ports by adopting a circulator, which was connected to a BBS source and an OSA, respectively. The FIB-milled gold coated SPST end was inserted into a series of RI liquids of which RI values is increased from 1.33 to 1.44 with an increment of 0.01 at room temperature of about 25 °C. The ends of the SPST fiber structure are respectively connected to the BBS source and an OSA through a circulator, so as to input a broadband light and receive the reflection spectrum. From the measured results in Figure 22 , the spectrum showed a redshift, and its average sensitivity is up to 39.1 nm/ RIU with RI values increment. This means the sensor could achieve a high RI resolution of 2.56 × 10 −4 for an OSA resolvable wavelength of 0.01 nm. The relationship between the resonance wavelength shifts and RI values was also presented in Figure 20 . Over the RI range of 1.33-1.44, the RI sensor based on an STMS fiber structure has an average sensitivity of 487 nm/RIU. In addition, when the RI value is 1.44, this proposed sensor can achieve a maximum sensitivity of 1913 nm/RIU, which means that there is an RI resolution of up to 5.23 × 10 −6 assuming the minimum wavelength shift of the OSA was 0.01 nm. Sensors 2018, 18, x FOR PEER REVIEW 16 of 28 BBS, transmitted through the STMS fiber structure, and recorded by an OSA. The prepared RI liquids were placed around the STMS fiber sample by using a dropper. The taper waist region of the STMS fiber sample was immersed totally. Before dropping and measuring the next RI liquid, the remained RI liquid on the fiber was washed off by deionized water. Finally, the experimental optical spectrum shift with respect to different RI values was obtained by processing the measured data of OSA. The relationship between the resonance wavelength shifts and RI values was also presented in Figure 20 . Over the RI range of 1.33-1.44, the RI sensor based on an STMS fiber structure has an average sensitivity of 487 nm/RIU. In addition, when the RI value is 1.44, this proposed sensor can achieve a maximum sensitivity of 1913 nm/RIU, which means that there is an RI resolution of up to 5.23 × 10 −6 assuming the minimum wavelength shift of the OSA was 0.01 nm. Using a probe type sensor for RI measurement is useful for certain applications. Here, in case of trivial description, we presented the experiment process of the SPST fiber structure. As shown in Figure 21 , the input/output SMF of the fabricated SPST sample was divided into two ports by adopting a circulator, which was connected to a BBS source and an OSA, respectively. The FIB-milled gold coated SPST end was inserted into a series of RI liquids of which RI values is increased from 1.33 to 1.44 with an increment of 0.01 at room temperature of about 25 °C. The ends of the SPST fiber structure are respectively connected to the BBS source and an OSA through a circulator, so as to input a broadband light and receive the reflection spectrum. From the measured results in Figure 22 , the spectrum showed a redshift, and its average sensitivity is up to 39.1 nm/ RIU with RI values increment. This means the sensor could achieve a high RI resolution of 2.56 × 10 −4 for an OSA resolvable wavelength of 0.01 nm. Using a probe type sensor for RI measurement is useful for certain applications. Here, in case of trivial description, we presented the experiment process of the SPST fiber structure. As shown in Figure 21 , the input/output SMF of the fabricated SPST sample was divided into two ports by adopting a circulator, which was connected to a BBS source and an OSA, respectively. The FIB-milled gold coated SPST end was inserted into a series of RI liquids of which RI values is increased from 1.33 to 1.44 with an increment of 0.01 at room temperature of about 25 • C. The ends of the SPST fiber structure are respectively connected to the BBS source and an OSA through a circulator, so as to input a broadband light and receive the reflection spectrum. From the measured results in Figure 22 , the spectrum showed a redshift, and its average sensitivity is up to 39.1 nm/ RIU with RI values increment. This means the sensor could achieve a high RI resolution of 2.56 × 10 −4 for an OSA resolvable wavelength of 0.01 nm. Additionally the temperature dependence of the SPST based RI sensor was also investigated. Figure 23 shows the experimental setup for studying the temperature dependence of the SPST structure based RI sensor. The SPST fiber sample was fixed on a thermoelectric cooler (TEC), the temperature of which was controlled by a TEC controller. The thermometer was used to monitor the current temperature value of the TEC surface. The entire experimental setup was placed inside a controlled small environment box to improve accurate temperature regulation. The temperature dependence of the SPST fiber sample could be determined from the measured optical spectra shift over the temperature range from 20 °C to 80 °C. These experimental results exhibit that the SPST fiber structure based RI sensor has a low temperature dependence of circa 7.67 pm/°C, which is in good agreement with the intrinsic characteristic of the fused silica material. Additionally the temperature dependence of the SPST based RI sensor was also investigated. Figure 23 shows the experimental setup for studying the temperature dependence of the SPST structure based RI sensor. The SPST fiber sample was fixed on a thermoelectric cooler (TEC), the temperature of which was controlled by a TEC controller. The thermometer was used to monitor the current temperature value of the TEC surface. The entire experimental setup was placed inside a controlled small environment box to improve accurate temperature regulation. The temperature dependence of the SPST fiber sample could be determined from the measured optical spectra shift over the temperature range from 20 °C to 80 °C. These experimental results exhibit that the SPST fiber structure based RI sensor has a low temperature dependence of circa 7.67 pm/°C, which is in good agreement with the intrinsic characteristic of the fused silica material. Additionally the temperature dependence of the SPST based RI sensor was also investigated. Figure 23 shows the experimental setup for studying the temperature dependence of the SPST structure based RI sensor. The SPST fiber sample was fixed on a thermoelectric cooler (TEC), the temperature of which was controlled by a TEC controller. The thermometer was used to monitor the current temperature value of the TEC surface. The entire experimental setup was placed inside a controlled small environment box to improve accurate temperature regulation. The temperature dependence of the SPST fiber sample could be determined from the measured optical spectra shift over the temperature range from 20 • C to 80 • C. These experimental results exhibit that the SPST fiber structure based RI sensor has a low temperature dependence of circa 7.67 pm/ • C, which is in good agreement with the intrinsic characteristic of the fused silica material. Additionally the temperature dependence of the SPST based RI sensor was also investigated. Figure 23 shows the experimental setup for studying the temperature dependence of the SPST structure based RI sensor. The SPST fiber sample was fixed on a thermoelectric cooler (TEC), the temperature of which was controlled by a TEC controller. The thermometer was used to monitor the current temperature value of the TEC surface. The entire experimental setup was placed inside a controlled small environment box to improve accurate temperature regulation. The temperature dependence of the SPST fiber sample could be determined from the measured optical spectra shift over the temperature range from 20 °C to 80 °C. These experimental results exhibit that the SPST fiber structure based RI sensor has a low temperature dependence of circa 7.67 pm/°C, which is in good agreement with the intrinsic characteristic of the fused silica material. It is also useful to systematically summarize the TOF based RI sensors proposed in our previous papers. In Table 1 , a comprehensive comparison of RI range, average sensitivity, maximum sensitivity, resolution and temperature dependence are presented. It is noteworthy that MFC structure based RI sensors achieve the highest sensitivity of about 4155 nm/RIU, and it is the first time that an MFC structure with a diameter of 2.5 µm is used as a high-sensitive refractometer sensor. The most significant advantage of the SPST based RI sensor is its low temperature dependence and high spatial resolution, which offers the potential to be useful for a very broad range of applications. 