A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We report the fabrication of a new type of optical waveguide: the photonic crystal fiber. It consists of a pure silica core surrounded by a silica–air photonic crystal material with a hexagonal symmetry. The fiber supports a single robust low-loss guided mode over a very broad spectral range of at least 458–1550 nm. <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> The confinement of light within a hollow core (a large air hole) in a silica-air photonic crystal fiber is demonstrated. Only certain wavelength bands are confined and guided down the fiber, each band corresponding to the presence of a full two-dimensional band gap in the photonic crystal cladding. Single-mode vacuum waveguides have a multitude of potential applications from ultrahigh-power transmission to the guiding of cold atoms. <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We report the fabrication and properties of soft glass photonic crystal fibers (PCF's) for supercontinuum generation. The fibers have zero or anomalous group velocity dispersion at wavelengths around 1550 nm, and approximately an order of magnitude higher nonlinearity than attainable in comparable silica fibers. We demonstrate the generation of an ultrabroad supercontinuum spanning at least 350 nm to 2200 nm using a 1550 nm ultrafast pump source. <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We report what is believed to be the first example of structural long-period gratings written in pure silica photonic crystal fibers (PCFs). The gratings are realized by periodic collapse of the holes of the PCF by heat treatment with a CO2 laser. The resulting periodic hole-size perturbation produces core-to-cladding-mode conversion. These results can lead to a new family of structural all-fiber devices that use the unique properties of PCFs. <s> BIB004 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> In this paper, we present the results of a high power photonic crystal fiber laser, where the well-known concepts of double-clad and large-mode-area design are transfered to an actively doped microstructured fiber. The double-clad design is achieved by surrounding the inner cladding (pump core) with a web of silica bridges (as thin as 390 nm and approximately 50 /spl mu/m long) which are substantially narrower than the wavelength of the guided radiation. It is concluded that air-clad microstructure ytterbium-doped large-mode-area fibers should be scalable to kW-level output powers, where nonlinearity will limit the performance. <s> BIB005 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We demonstrate highly efficient evanescent-wave detection of fluorophore-labeled biomolecules in aqueous solutions positioned in the air holes of the microstructured part of a photonic crystal fiber. The air-suspended silica structures located between three neighboring air holes in the cladding crystal guide light with a large fraction of the optical field penetrating into the sample even at wavelengths in the visible range. An effective interaction length of several centimeters is obtained when a sample volume of less than 1 microL is used. <s> BIB006 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <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> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We demonstrate implementation of an all-fiber Mach-Zehnder interferometer formed in a photonic crystal fiber (PCF). We formed the all-PCF Mach-Zehnder interferometer by mechanically inducing two identical long-period fiber gratings (LPGs) in the PCF. The spectral properties of a LPG and a LPG pair were investigated. The interference fringe formed within the stop band of the LPG pair varied with the period and the strength of the gratings, and the fringe spacing was decreased with increasing grating separation. From the fringe spacing measurement the differential effective group index of the PCF was calculated to be deltam approximately equal to 2.8 x 10(-3). <s> BIB008 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We demonstrate, for the first time to our knowledge, three-dimensional (3D) trapping and manipulation of microscopic objects by use of supercontinuum white light generated from photonic crystal fibers. Furthermore, we show that the supercontinuum white-light optical tweezers used have the unique capability to perform optical scattering spectroscopy of a single 3D trapped object over a broad wavelength range. These novel tweezers can potentially open a promising avenue toward simultaneous manipulation and characterization of microscopic objects. <s> BIB009 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> Tunable bandgap guidance is obtained by filling the holes of a solid core photonic crystal fiber with a nematic liquid crystal and applying an electric field. The response times are measured and found to be in the millisecond range. <s> BIB010 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed. <s> BIB011 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We study the sensitivity of fiber grating sensors in the applications of strain, temperature, internal label-free biosensing, and internal refractive index sensing. New analytical expressions for the sensitivities, valid for photonic crystal fibers are rigorously derived. These are generally valid, and we identify a previously unaccounted term for temperature and strain sensing. It is shown that dispersion plays a central role in determining the sensitivity, and that dispersion may enhance or suppress sensitivity as well as change the sign of the resonant wavelength shifts. We propose a quality factor, Q, for characterizing long period gratings sensors. <s> BIB012 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> A whole temperature sensor in one package utilizing a fiber Bragg grating (FBG) made in a conventional single-mode fiber and which uses a long-period grating (LPG) made in a photonic crystal fiber is proposed and experimentally demonstrated. The function of the interrogation is that the wavelength change of the FBG with environmental temperature is transferred to the intensity of the output via the LPG. Utilizing the temperature-insensitivity of the LPG in the PCF, the interrogation is stable and enables a cheap and practical temperature measurement system with a wide dynamic range. <s> BIB013 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We demonstrate a novel method for low-loss splicing small-core photonic crystal fibers (PCFs) and single-mode fibers (SMFs) by repeated arc discharges using a conventional fusion splicer. An optimum mode field match at the interface of PCF-SMF and an adiabatic mode field variation in the longitudinal direction of the small-core PCF can be achieved by repeated arc discharges applied over the splicing joint to gradually collapse the air holes of the small-core PCF. This method is simple and offers a practical solution for light coupling between small-core PCFs and SMFs. <s> BIB014 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> Abstract We report that the proposed long-period gratings (LPGs), which were fabricated in an endlessly single-mode photonic crystal fiber (ESM-PCF) via point-by-point residual stress relaxation utilizing focused CO 2 laser irradiation, exhibit a high sensitivity to variations in the external refractive index, with an identified shifting of attenuation band as large as 4.4 pm for 1 × 10 −5 change in the surrounding refractive index over the range from 1.42 to 1.45. Contrary to LPGs inscribed in conventional all-solid optical fiber under similar condition, the mode-coupling resonance band of the ESM-PCF-LPG moves towards longer wavelength with increasing refractive index in the surrounding medium of the fiber cladding. The resonance wavelength has a low-temperature coefficient of 2.2 pm/°C over 30–200 °C. <s> BIB015 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> The year 2009 marks the tenth anniversary of the first report of white-light supercontinuum generation in photonic crystal fibre. This result had a tremendous impact on the field of nonlinear fibre optics and continues to open up new horizons in photonic science. Here we provide a concise and critical summary of the current state of nonlinear optics in photonic crystal fibre, identifying some of the most important and interesting recent developments in the field. We also discuss several emerging research directions and point out links with other areas of physics that are now becoming apparent. <s> BIB016 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> We introduce a refractive index sensing geometry exploiting modes beyond cutoff in a selectively infiltrated PCF. We demonstrate a detection limit of 4.6×10−7 RIU and sensitivity of 30,100nm/RIU, a one-order-of-magnitude improvement over previous PCF sensors. <s> BIB017 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> Abstract A polyvinyl alcohol (PVA) coated photonic crystal optical fiber (PCF) sensor has been proposed as a relative humidity (RH) sensor. It was fabricated by collapsing the holes of PCF at both ends to form a Michelson interferometer with cladding mode excitation. PVA was dip coated onto the sensor and the interference shift was measured when the sensor was exposed to varying RH. The sensor with 9% (w/w) coating showed a high sensitivity of 0.60 nm/%RH, displayed little hysteresis, high repeatability, low cross-sensitivity to temperature and ammonia gas and stability over 7 days of testing. A rise/fall time of 300/500 ms was achieved respectively. <s> BIB018 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <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> BIB019 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Tapered Fiber Structures Incorporating Photonic Crystal Fiber <s> A compact singlemode - photonic crystal fiber - singlemode fiber tip (SPST) refractive index sensor is demonstrated in this paper. A CO2 laser cleaving technique is utilised to provide a clean-cut fiber tip which is then coated by a layer of gold to increase reflection. An average sensitivity of 39.1 nm/RIU and a resolvable index change of 2.56 x 10-4 are obtained experimentally with a ~3.2 µm diameter SPST. The temperature dependence of this fiber optic sensor probe is presented. The proposed SPST refractometer is also significantly less sensitive to temperature and an experimental demonstration of this reduced sensitivity is presented in the paper. Because of its compactness, ease of fabrication, linear response, low temperature dependency, easy connectivity to other fiberized optical components and low cost, this refractometer could find various applications in chemical and biological sensing. <s> BIB020
In the last two decades, photonic crystal fiber (PCF) as a novel fiber type has attracted much attention from researchers. The cross section of a PCF consists of a solid or hollow core surrounded by a periodic arrangement of air holes BIB001 BIB002 BIB003 . These fibers provide a new approach for many applications, such as nonlinear optics, SC generation, fiber lasers, and fiber sensors BIB016 BIB005 BIB011 BIB009 BIB017 BIB006 . Various  In the last two decades, photonic crystal fiber (PCF) as a novel fiber type has attracted much attention from researchers. The cross section of a PCF consists of a solid or hollow core surrounded by a periodic arrangement of air holes BIB001 BIB002 BIB003 . These fibers provide a new approach for many applications, such as nonlinear optics, SC generation, fiber lasers, and fiber sensors BIB016 BIB005 BIB011 BIB009 BIB017 BIB006 . Various optical devices based on PCF structure have been proposed, such as a liquid-crystal-filled PCF BIB010 , LPG written PCF BIB004 , functional material coated PCF BIB018 , and PCF based on a side-polishing technique BIB007 . Among them, inscribing an LPG in PCF has been widely investigated as a basis for sensing elements since it can provide a strong wavelength dependency and a strong evanescent field, for sensor types such as a refractometer, biosensor, strain sensor and temperature sensor BIB012 BIB008 BIB013 BIB015 . An effective approach of writing LPGs in PCF is to utilise a series of uniform microtapers, is reported in BIB019 , is adopted to develop an SMF-periodically tapered PCF-SMF structure, as shown schematically in Figure 13 . It comprises a periodically tapered PCF inserted between two SMFs. The transmission spectrum of this SMF-PCF-SMF (SPS) structure strongly depends on the spatial filtering because of the multimode interference present in the periodically tapered PCF section, as shown in Figure 14 . In practice, as an initial step an SMF-PCF-SMF structure was fabricated by cutting and splicing a short length of PCF (LMA-8, NKT Photonics) with two SMFs utilizing a precise cleaver and a commercial splicer. The fusion process between the SMF and PCF is carefully executed to avoid air hole collapse BIB014 . The experimental setup for fabricating the SMF-periodically tapered PCF-SMF structure was similar to that used for fabricating the SPTS structure in BIB019 , which is shown in Figure 6 . In this fabrication process, a CO 2 laser and a ZnSe cylindrical lens are employed to taper the PCF section of this fiber structure. A periodically tapered PCF section can be obtained by repeating this tapering process 40 times over. As observed previously, the actual period of the tapers was larger than the movement distance of the laser beam, due to the tapering induced elongation of the fiber. The spliced fiber joint between the singlemode fiber and the actual periodic PCF taper section is shown in Figure 15 . optical devices based on PCF structure have been proposed, such as a liquid-crystal-filled PCF BIB010 , LPG written PCF BIB004 , functional material coated PCF BIB018 , and PCF based on a side-polishing technique BIB007 . Among them, inscribing an LPG in PCF has been widely investigated as a basis for sensing elements since it can provide a strong wavelength dependency and a strong evanescent field, for sensor types such as a refractometer, biosensor, strain sensor and temperature sensor BIB012 BIB008 BIB013 BIB015 . An effective approach of writing LPGs in PCF is to utilise a series of uniform microtapers, is reported in BIB019 , is adopted to develop an SMF-periodically tapered PCF-SMF structure, as shown schematically in Figure 13 . It comprises a periodically tapered PCF inserted between two SMFs. The transmission spectrum of this SMF-PCF-SMF (SPS) structure strongly depends on the spatial filtering because of the multimode interference present in the periodically tapered PCF section, as shown in Figure 14 . In practice, as an initial step an SMF-PCF-SMF structure was fabricated by cutting and splicing a short length of PCF (LMA-8, NKT Photonics) with two SMFs utilizing a precise cleaver and a commercial splicer. The fusion process between the SMF and PCF is carefully executed to avoid air hole collapse BIB014 . The experimental setup for fabricating the SMF-periodically tapered PCF-SMF structure was similar to that used for fabricating the SPTS structure in BIB019 , which is shown in Figure 6 . In this fabrication process, a CO2 laser and a ZnSe cylindrical lens are employed to taper the PCF section of this fiber structure. A periodically tapered PCF section can be obtained by repeating this tapering process 40 times over. As observed previously, the actual period of the tapers was larger than the movement distance of the laser beam, due to the tapering induced elongation of the fiber. The spliced fiber joint between the singlemode fiber and the actual periodic PCF taper section is shown in Figure 15 . Transmission (dB) Figure 13 . Schematic of singlemode-periodically tapered PCF-singlemode fiber structure. Figure 15 . the Figure 15 . During the fabrication process, the transmitted spectrum of the SPS fiber structure for taper periods ranging from 0 to 40 was monitored in real time. The attenuation increased gradually for an increase in the number of tapers due to the taper transition regions that induced radiation loss. For example there is an attenuation of 10 dB associated with 10 taper periods, while the total attenuation was circa 35 dB for 40 taper periods over the wavelength range from 1450 to 1650 nm. When the number of taper periods increases, the intensity of multiple mode interferences increases correspondingly. Also the optical performance of the multiply tapered structure is determined by the physical size and the geometrical shape of the taper region. The tapering process not only induces the fiber elongation and a diameter decrease, but also causes collapse of air holes in the PCF, which in turn produces periodical effective RI modulation. Furthermore, we also investigated the transmission spectra of an SMF-periodically tapered PCF-SMF structure with different period length of 410, 430, 450, 470 µm. From these experimental results, we found no resonant interference for period lengths of 410 µm; there was a recognizable but tiny contrast interference fringe for period lengths of 430 µm, which is due to the higher loss of higher-order cladding modes. As the period lengths were increased to 450 and 470 µm, a discernable interference pattern was observed. Considering the intensity of resonant fringes, an SMF-periodically tapered PCF-SMF structure with a period length of 450 µm was thus selected for a further investigation. Although the periodically tapered fiber devices can measure the surrounding refractive index with a high resolution, the relatively large size of this fiber structure for RI sensing (usually 125 µm in diameter but several centimeters in length) restrict their sensing application in a confined space, or sensing measurements that demand a high spatial resolution. To address this issue we have developed a compact singlemode photonic crystal fiber singlemode fiber tip (SPST) structure, the schematic is shown in Figure 16 . Although the periodically tapered fiber devices can measure the surrounding refractive index with a high resolution, the relatively large size of this fiber structure for RI sensing (usually 125 µm in diameter but several centimeters in length) restrict their sensing application in a confined space, or sensing measurements that demand a high spatial resolution. To address this issue we have developed a compact singlemode photonic crystal fiber singlemode fiber tip (SPST) structure, the schematic is shown in Figure 16 . Figure 16 presents the schematic of the SPST structure as reported in our previous work BIB020 , and it is composed of an input/output SMF and a PCF half-taper section coated with gold film. The gold film is employed to increase the reflectivity of the tip end. Here, the SPST structure was fabricated by CO2 laser irradiation and is presented in Figure 7 . The prefabricated singlemodephotonic crystal fiber-singlemode fiber structure is vertically fixed on a support, and a 50 g weight is hung at the other end of this fiber structure to apply a constant predictable tension. The combined effect of heating by the CO2 laser and the constant tension applied was to cause the glass to become molten and stretch to a point where breakage occurred, resulting in half-tapered PCF tip with a diameter of circa 3.2 µm. Furthermore, the half-tapered PCF tip was coated with a gold film of circa 40 nm thickness by utilizing a thermal evaporator to improve reflectivity. The gold film on the PCF and the conical transitional region was then partially removed using a gold-etching solvent. Finally, a thin gold film was left on the half-tapered PCF tip end. In the next stage, we expect to fabricate a SPS tip with an increased reflectivity by employing a FIB milling and gold coating process, therefore its sensing performance can be also enhanced.