A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> Since the discovery of photosensitivity in optical fibers there has been great interest in the fabrication of Bragg gratings within the core of a fiber. The ability to inscribe intracore Bragg gratings in these photosensitive fibers has revolutionized the field of telecommunications and optical fiber based sensor technology. Over the last few years, the number of researchers investigating fundamental, as well as application aspects of these gratings has increased dramatically. This article reviews the technology of Bragg gratings in optical fibers. It introduces the phenomenon of photosensitivity in optical fibers, examines the properties of Bragg gratings, and presents some of the important developments in devices and applications. The most common fabrication techniques (interferometric, phase mask, and point by point) are examined in detail with reference to the advantages and the disadvantages in utilizing them for inscribing Bragg gratings. Reflectivity, bandwidth, temperature, and strain sensitivity ... <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors. <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> This work presents an overview of progress and developments in the field of fiber optic sensor technology, highlighting the major issues underpinning recent research and illustrating a number of important applications and key areas of effective fiber optic sensor development. <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> We report the observation of whispering-gallery-mode lasers based on Er(3+) -doped ZBLAN microspheres coupled with half-tapered and tapered optical fibers. A multimode lasing effect is observed at 1.56mu when the taper diameters are properly adjusted so that their fundamental propagation constants match those of the resonant modes of interest. Overlap between the pump and the laser geometrical modes is optimized by pumping at 1.48mu . <s> BIB004 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> We describe the fundamentals of two new types of fiber, photonic crystal fiber (PCF) and photonic bandgap fiber (PBF). PCF is fabricated by forming small air holes in the cladding of the fiber rather than doping the fiber core with high-index materials. This fiber has very interesting new features including endlessly single-mode operation, zero GVD in the visible region, and an easily controllable spot-size due to the strongly guiding characteristics that result from the very high relative refractive index between the core and air-holed cladding. By contrast, PBF has air holes not only in its cladding but also in its core, and this enables us to confine an optical beam by using the principle of two-dimensional Bragg diffraction. The air core structure greatly reduces fiber nonlinearity and suggests the feasibility of high power transmission. <s> BIB005 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> Silica waveguides with diameters larger than the wavelength of transmitted light are widely used in optical communications, sensors and other applications. Minimizing the width of the waveguides is desirable for photonic device applications, but the fabrication of low-loss optical waveguides with subwavelength diameters remains challenging because of strict requirements on surface roughness and diameter uniformity. Here we report the fabrication of subwavelength-diameter silica 'wires' for use as low-loss optical waveguides within the visible to near-infrared spectral range. We use a two-step drawing process to fabricate long free-standing silica wires with diameters down to 50 nm that show surface smoothness at the atomic level together with uniformity of diameter. Light can be launched into these wires by optical evanescent coupling. The wires allow single-mode operation, and have an optical loss of less than 0.1 dB mm(-1). We believe that these wires provide promising building blocks for future microphotonic devices with subwavelength-width structures. <s> BIB006 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> A tunable photonic crystal fiber (PCF) coupler, which couples part of the optical power in one PCF with that in another PCF, has been made by side polishing. We fabricated the PCF coupler by mating two side-polished PCFs. We achieved evanescent field coupling between the core modes of the two PCFs by using side polishing to bring the cores close to each other. By adjusting the mating angle between the two side-polished PCFs we obtained as much as 90% tunability in the coupling ratio. The spectrum of the coupling ratio was almost flat, with small ripples, over a 400-nm wavelength range. <s> BIB007 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> We report on research towards application of birefringent photonic crystal fibers as active and passive elements of fiber optic sensors for measurements of different physical parameters. Using experimental and theoretical methods, the sensing characteristics of different photonic structures are studied, including spectral behavior of phase and group modal birefringence, polarization dependent losses, sensitivity to temperature and hydrostatic pressure. <s> BIB008 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> Here we demonstrate a supercontiuum spectrum generated in a tapered fiber pumped by a Ti sapphire laser with 130fs pulse width at 800nm centre wavelength. The tapered fiber with the diameter of 1.25µm and the length of 16cm was made by using a tapered machine. The supercontiuum range is from 500nm to 900nm. We simulated the supercontinuum with the nonlinear Schrodinger Equation and discussed the affecting factors. In some conditions, the experimental results are agreed very well with the theoretical analyses. <s> BIB009 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <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> BIB010 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> In this paper, spectrally tailored tapered chirped fiber Bragg gratings (TCFBGs) are considered for use as strain sensors. Both gratings were written in fused tapered optical fiber using linearly chirped fiber Bragg gratings in co-directional and counter-directional chirp configurations. Theoretical and numerical analysis as well as experimental verification of the influence of applied strain on spectral width of both TCFBG structures were carried out. The results show that TCFBGs exhibit monotonic strain response over the wide operating range of the applied force. Compared with standard tapered FBG written using uniform phase mask, in the case of co-directionally written TCFBG with substantially larger grating chirp the monotonic operating range can be easily broadened toward the higher strain values (even above the applied force at which the optical fiber breaks). In turn, the intrinsic chirp of the counter-directionally written TCFBG can be tailored in such a way as to ensure that the monotonicity of its strain response is always satisfied when tensile force is applied. This is due to the spectral broadening of the reflected spectrum when strain increases. <s> BIB011 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> A novel concept of a Fabry-Perot (F-P) cavity composed of two linearly chirped fiber Bragg gratings written in a thermally fused fiber taper is presented. Both chirped gratings are written in counter-directional chirp configuration, where chirps resulting from the optical fiber taper profile and linearly increasing grating periods cancel each other out, forming a high-quality F-P resonator. A new strain-sensing mechanism is proposed in the presented structure, which is based on strain-induced detuning of the F-P resonator. Due to the different strain and temperature responses of the cavity, the resonator can be used for the simultaneous measurement of these physical quantities, or it can be used as a temperature-independent strain sensor. <s> BIB012 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Introduction <s> Abstract The development of highly-sensitive miniaturized sensors that allow real-time quantification of analytes is highly desirable in medical diagnostics, veterinary testing, food safety, and environmental monitoring. Photonic Crystal Fiber Surface Plasmon Resonance (PCF SPR) has emerged as a highly-sensitive portable sensing technology for testing chemical and biological analytes. PCF SPR sensing combines the advantages of PCF technology and plasmonics to accurately control the evanescent field and light propagation properties in single or multimode configurations. This review discusses fundamentals and fabrication of fiber optic technologies incorporating plasmonic coatings to rationally design, optimize and construct PCF SPR sensors as compared to conventional SPR sensing. PCF SPR sensors with selective metal coatings of fibers, silver nanowires, slotted patterns, and D-shaped structures for internal and external microfluidic flows are reviewed. This review also includes potential applications of PCF SPR sensors, identifies perceived limitations, challenges to scaling up, and provides future directions for their commercial realization. <s> BIB013
Optical fiber devices have been a significant research focus since low loss optical fibers were first proposed in the 1960s. Due to their excellent properties, which include low loss, compact structure, wide bandwidth and immunity to electromagnetic interference, optical fiber devices have been intensively exploited in many fields ranging from optical sensing to optical communication BIB003 . With the development of sophisticated optical fiber fabrication techniques, many types of fiber structures have been reported in the past several decades, including common fiber structures such as fiber Bragg grating (FBG), long period grating (LPG) and photonic crystal fiber (PCF), as well as more targeted structures such as tapered optical fiber (TOF), side polished fibers, interference devices and rare earth doped fibers. Fiber gratings are usually manufactured by modulating the refractive index (RI) along the fiber axis and various approaches have been developed to fabricate them, including interferometric techniques, the phase mask technique and the point-by-point technique BIB001  BIB002 . Although fiber gratings exhibit many outstanding properties such as compact size, wavelength selectivity, and mature fabrication technique, it has inherent limitations, especially for high-precision sensing measurement. Moreover, fiber grating based sensors struggle to work at high temperatures since inscribed gratings are erased. In order to increase sensing performance, research based on a combination of fiber grating structures and fiber taper techniques has been reported BIB012 BIB011 . PCF, as an emerging technology, has been intensively investigated and used in a wide range of applications, such as lasers, couplers, and sensors BIB005 BIB008 BIB007 BIB013 . However, given its high-cost and poor compatibility to existing fiber types, PCF does present challenges for sensing applications. Compared with the conventional fiber structures above, tapered optical fibers (TOF) can provide a number of useful features including a large evanescent field, strong mode confinement capability and small-scale diameter. Thus, TOF exhibits great potential for applications in communication systems and optical sensing, since it was reported for first time in 2003 BIB006 . Over the past few decades, TOFs have been investigated and widely used in many fields ranging from telecommunications to sensing. For instance, supercontinuum (SC) generation in TOFs was reported in BIB009 ; whispering gallery mode (WGM) lasers based on Er 3+ doped ZBLAN microspheres coupled with half-TOFs and TOFs were observed in BIB004 and a sensitive liquid refractive index sensor using TOF tips was demonstrated in BIB010 . These TOF based devices have exhibited many superior properties over conventional optical fiber structures and for this reason our group has investigated the TOF structure. In this paper, we systematically summarize the various TOF technologies that exploit multimodal interference and also the range of sensing applications for TOF technology. In Section 2 "TOF Structures and Their Fabrication" the fundamental principles of light propagation within TOF structures and associated fabrication techniques are introduced. Section 3 "Applications of TOFs as Optical Fiber Sensors" provides an overview of sensing applications for structures in sensing for measurands such as refractive index (RI), temperature, humidity and biochemical. Finally Section 4 provides a conclusion and some comments on the future directions of TOF technology.