A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Biosensor <s> It is demonstrated that through control of the refractive index surrounding a fused biconical-taper single-mode coupler the coupling coefficient can be changed from 0% to 96%. Data are also presented that show the coupling mechanism cannot be attributed to power transfer between the modal fields guided by the fiber cores in the taper-waist region. The results support the model of the coupling mechanism as a beat phenomenon between modes guided by a glass core with a dumbbell cross section and an external refractive index. <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Biosensor <s> An automated optical biosensor system based on fluorescence excitation and detection in the evanescent field of a quartz fiber was used to detect 16-mer oligonucleotides in DNA hybridization assays. A biotinylated capture probe was immobilized on the fiber surface via avidin or streptavidin. The hybridization with fluorescein-labeled complementary strands was monitored in real time by fluorescence detection. The double strands formed by hybridization could be dissociated by chemical or thermal regeneration, allowing one to perform hundreds of assay cycles with the same fiber. The signal loss during long-time measurements, i.e., consecutive hybridization assays, can be described by a single-exponential function. Over more than 200 cycles, the net signal decreased by 50% with a signal variation of 2.4% after correction for this signal loss. By binding the capture probe with the 5‘-end to the optical fiber surface, and by using a 50% (w/w) aqueous urea solution for chemical regeneration, the duration of an a... <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Biosensor <s> *A special report on the International Union of Pure and Applied Chemistry, Physical Chemistry Division, Commission I.7 (Biophysical Chemistry), Analytical Chemistry Division, Commission V.5 (Electroanalytical Chemistry). <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Biosensor <s> Remarkable developments can be seen in the field of optical fibre biosensors in the last decade. More sensors for specific analytes have been reported, novel sensing chemistries or transduction principles have been introduced, and applications in various analytical fields have been realised. This review consists of papers mainly reported in the last decade and presents about applications of optical fiber biosensors. Discussions on the trends in optical fiber biosensor applications in real samples are enumerated. <s> BIB004 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Biosensor <s> This article reviews the recent progress in optical biosensors that use the label-free detection protocol, in which biomolecules are unlabeled or unmodified, and are detected in their natural forms. In particular, it will focus on the optical biosensors that utilize the refractive index change as the sensing transduction signal. Various optical label-free biosensing platforms will be introduced, including, but not limited to, surface plasmon resonance, interferometers, waveguides, fiber gratings, ring resonators, and photonic crystals. Emphasis will be given to the description of optical structures and their respective sensing mechanisms. Examples of detecting various types of biomolecules will be presented. Wherever possible, the sensing performance of each optical structure will be evaluated and compared in terms of sensitivity and detection limit. <s> BIB005 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Biosensor <s> This paper presents a fast, highly sensitive and low-cost tapered optical fiber biosensor that enables the label-free detection of biomolecules. The sensor takes advantage of the interference effect between the fiber’s first two propagation modes along the taper waist region. The biomolecules bonded on the taper surface were determined by demodulating the transmission spectrum phase shift. Because of the sharp spectrum fringe signals, as well as a relatively long biomolecule testing region, the sensor displayed a fast response and was highly sensitive. To better understand the influence of various biomolecules on the sensor, a numerical simulation that varied biolayer parameters such as thickness and refractive index was performed. The results showed that the spectrum fringe shift was obvious to be measured even when the biolayer was only nanometers thick. A microchannel chip was designed and fabricated for the protection of the sensor and biotesting. Microelectromechanical systems (MEMS) fabrication techniques were used to precisely control the profile and depth of the microchannel on the silicon chip with an accuracy of 2 μm. A tapered optical fiber biosensor was fabricated and evaluated with an Immune globulin G (IgG) antibody-antigen pair. <s> BIB006
Biosensors have important applications in biomolecule measurements such as DNA sequences and antibody and protein detection since such sensors can utilize some specific biological recognition element to investigate analytical information both quantitatively and semi quantitatively BIB003 . Several biosensors based on optical fiber structure have been proposed previously BIB005 BIB002 BIB004 . Recently, the development of TOF provides a potential for high-sensitivity and fast-response biosensors BIB006  . In this sub-section an MFC-structure-based label-free biosensor is presented. Firstly, a common microheater brushing technique ( Figure 14 ) was employed to fabricate an MFC structure. The fabricated MFC structure comprised of two weakly fused microfibers with its minimum tapered waist diameter of circa 2 µm, the lengths of the uniform waist region and transition region of this MFC structure were 3-mm and 13-mm long, respectively. Based on previous work BIB001 , this MFC is known to have an effective response to external RI variations. In order to enhance its stability, the fabricated MFC structure was covered by UV curable polymer (Efiron UVF PC363, Luvantix, Daejeon, Korea) with a low RI value. Two blocks of poltdimethylsiloxane (PDMS) were fixed in parallel on a microscope slide coated with a thin layer of the UV curable polymer by mean of which an open-top channel was constructed. The fabricated MFC was placed into the channel. Finally, the two ends of the channel is sealed with the help of s the viscous UV curable Efiron polymer, and thus the MFC structure can be immobilized. The whole experimental setup is depicted in Figure 27 . Experimentally, a broadband light was launched from a BBS (Fiber Coupled SLD, Thorlabs, Newton, NJ, USA) to the input port of the MFC, then the output signal was detected by an OSA. Next, the fibrinogen immobilization was carried out by adopting human fibrinogen contained 95% clottable and plasminogen depleted, from Calbiochem (Merck KGaA, Darmstadt, Germany). In order to ensure that the fibrinogen molecules were totally adsorbed on the MFC surface, the MFC sensing area was covered by a fibrinogen solution in PBS (0.2 mL, 100 µg/mL) for 20 min. The inset in  Biosensors have important applications in biomolecule measurements such as DNA sequences and antibody and protein detection since such sensors can utilize some specific biological recognition element to investigate analytical information both quantitatively and semi quantitatively BIB003 . Several biosensors based on optical fiber structure have been proposed previously BIB005 BIB002 BIB004 . Recently, the development of TOF provides a potential for high-sensitivity and fast-response biosensors BIB006  . In this sub-section an MFC-structure-based label-free biosensor is presented. Firstly, a common microheater brushing technique ( Figure 14 ) was employed to fabricate an MFC structure. The fabricated MFC structure comprised of two weakly fused microfibers with its minimum tapered waist diameter of circa 2 µm, the lengths of the uniform waist region and transition region of this MFC structure were 3-mm and 13-mm long, respectively. Based on previous work BIB001 , this MFC is known to have an effective response to external RI variations. In order to enhance its stability, the fabricated MFC structure was covered by UV curable polymer (Efiron UVF PC363, Luvantix, Daejeon, Korea) with a low RI value. Two blocks of poltdimethylsiloxane (PDMS) were fixed in parallel on a microscope slide coated with a thin layer of the UV curable polymer by mean of which an open-top channel was constructed. The fabricated MFC was placed into the channel. Finally, the two ends of the channel is sealed with the help of s the viscous UV curable Efiron polymer, and thus the MFC structure can be immobilized. The whole experimental setup is depicted in Figure 27 . Experimentally, a broadband light was launched from a BBS (Fiber Coupled SLD, Thorlabs, Newton, NJ, USA) to the input port of the MFC, then the output signal was detected by an OSA. Next, the fibrinogen immobilization was carried out by adopting human fibrinogen contained 95% clottable and plasminogen depleted, from Calbiochem (Merck KGaA, Darmstadt, Germany). In order to ensure that the fibrinogen molecules were totally adsorbed on the MFC surface, the MFC sensing area was covered by a fibrinogen solution in PBS (0.2 mL, 100 µg/mL) for 20 min. The inset in Figure 24 illustrates schematically the surface of the MFC with fibrinogen immobilized on it. During the entire immobilization process, the optical spectra was recorded and are shown in Figure 28 . It was found that in the first five minutes, a rapid wavelength shift occurred, then the wavelength shift rate reduced and finally the wavelength shift reached a plateau within 20 min. Figure 24 illustrates schematically the surface of the MFC with fibrinogen immobilized on it. During the entire immobilization process, the optical spectra was recorded and are shown in Figure 28 . It was found that in the first five minutes, a rapid wavelength shift occurred, then the wavelength shift rate reduced and finally the wavelength shift reached a plateau within 20 min. When the biosensor based MFC was built up, rabbit anti-fibrinogen solution (Calbiochem) in PBS buffer (0.2 mL, 100 µg/mL) and anti-IgG solution in PBS (0.2 mL, 100 µg/mL) were applied to the sensor. The experimental results exhibit that the anti-fibrinogen molecules caused a wavelength blueshift of circa 10.35 nm while the anti-IgG caused a redshift of only 0.45 nm. Therefore, it is demonstrated that this sensor can distinguish the difference between the specific anti-fibrinogen solution and the non-specific anti-IgG solution. In addition, an experiment for various concentrations of fibrinogen solution was performed. The used fibrinogen solution has a constant concentration of 100 µg/mL, while the concentration of anti-fibrinogen solution was increased from 25 µg/mL to 100 µg/mL. Based on experimental results in Figure 28 , we can find that the sensor output was proportional to the concentration of the anti-fibrinogen. It is noteworthy that this is the first time a label-free MFC based biosensor using a fibrinogen and anti-fibrinogen pair has been demonstrated experimentally.