A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> This report summarizes the activites at the 4th International Workshop on Optical Waveguide Theory, held at Leuwenhorst Holland on 13–16 September 1979. <s> BIB001 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> General propagation properties and universal curves are given for doubly clad single-mode fibers with inner cladding index higher or lower than outer cladding index, using the two parameters: inner cladding/core radii ratio and inner cladding/core index differences ratio. LPO1, LP11, and LPO2 cutoff conditions are examined. It is shown that dispersion properties largely differ from the singly clad single-mode fiber case, leading to large new possibilities for low-loss dispersion-free fibers at any wavelength between 1.3 and 1.7 μm. <s> BIB002 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Ions of kiloelectron volt energies incident on a solid surface produce a number of effects: several atoms are sputtered off, several electrons are emitted, chemical reactions may be induced, atoms are displaced from their equilibrium positions, and ions implant themselves in the solid, altering its properties. Some of these effects, such as sputtering and implantation are widely used in semiconductor device fabrication and in other fields. Thus the capability to focus a beam of ions to submicrometer dimensions, i.e., dimensions compatible with the most demanding fabrication procedures, is an important development. The focused ion beam field has been spurred by the invention of the liquid metal ion source and by the utilization of focusing columns with mass separation capability. This has led to the use of alloy ion sources making available a large menu of ion species, in particular the dopants of Si and GaAs. The ability to sputter and to also induce deposition by causing breakdown of an adsorbed film has produced an immediate application of focused ion beams to photomask repair. The total number of focused ion beamfabrication systems in use worldwide is about 35, about 25 of them in Japan. In addition, there are many more simpler focused ion beam columns for specialized uses. The interest is growing rapidly. The following range of specifications of these systems has been reported: accelerating potential 3 to 200 kV, ion current density in focal spot up to 10 A/cm2, beam diameters from 0.05 to 1 μm, deflection accuracy of the beam over the surface ±0.1 μm, and ion species available Ga, Au, Si, Be, B, As, P, etc. Some of the applications which have been demonstrated or suggested include: mask repair, lithography (to replace electron beamlithography), direct, patterned, implantationdoping of semiconductors, ion induced deposition for circuit repair or rewiring, scanning ion microscopy, and scanning ion mass spectroscopy. <s> BIB003 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Biconically tapered single-mode fibers were fabricated, and their characteristics were studied experimentally. The optical throughput was measured as the fiber was being pulled to produce the required radial profile. The tapered single-mode fiber was bent in a simple fixture, and the optical throughput was measured as a function of the bending angle. It is seen that the bending resulted in very strong oscillations of the optical power as the bend angle was varied. At the point when the light in the core was minimal, the cladding region was bright, indicating that the light has moved to the cladding. The propagation characteristics of the tapered single-mode fiber subjected to bending are analyzed using a stepwise approximation. A simple sensor based on this principle is discussed. > <s> BIB004 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Two processes have been developed to enable both focused ion beam (FIB) repair of advanced masks and FIB device modification. Silicon dioxide- based films can be deposited by rastering a focused ion beam across a surface onto which a combination of siloxane and oxygen gases have been adsorbed. The deposited material exhibits sufficient dielectric strength to be used for FIB modification of devices. Applications of FIB dielectric deposition include: (1) Local passivation. (2) Backfilling vias to allow for probing buried metal layers without contacting exposed metal layers. (3) Electrical isolation between crossed metal lines. (4) Optically transparent films for phase shift mask repair. In the first half of this paper we discuss the gas delivery system, and the material and electrical characteristics of the films, as well as describing typical device modifications using FIB dielectric films. In the second half of the paper we describe a process for deposition of tantalum- containing films using a tantalum-based organometallic precursor for repair of clear defect on X-ray masks. Although FIB gold films are adequate for repair of gold-absorber, silicon-membrane X-ray masks, gold films are not acceptable in the fab line, and tantalum is preferred for repair of either tungsten or tantalum absorber X-ray masks. <s> BIB005 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> We present a new class of long-period fiber gratings that can be used as in-fiber, low-loss, band-rejection filters. Photoinduced periodic structures written in the core of standard communication-grade fibers couple light from the fundamental guided mode to forward propagating cladding modes and act as spectrally selective loss elements with insertion losses act as backreflections <-80 dB, polarization-mode-dispersions <0.01 ps and polarization-dependent-losses <0.02 dB. <s> BIB006 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Long-period fiber gratings are used to f latten the gain spectrum of erbium-doped fiber amplifiers. A broadband amplifier with <0.2-dB gain variation over 30 nm is presented. We also show that a chain of amplifiers can be equalized, leading to a bandwidth enhancement by a factor of 3. <s> BIB007 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> A new, simple method of fabricating long-period fibre gratings by direct exposure of the fibre to focused 10.6 /spl mu/m wavelength CO/sub 2/ laser pulses is presented. No ultraviolet exposure is used. Hydrogen loading is found to enhance the writing sensitivity. <s> BIB008 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Narrow band, large dynamic range, polarisation independent spectral resonances of long-period fibre gratings fabricated period-by-period with focused CO2 laser pulses are demonstrated at 1490 and 1538 nm. The observed cladding mode near field patterns in this wavelength range are presented. <s> BIB009 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Long-period fibre gratings fabricated period-by-period with focused CO2 laser pulses are shown to be highly stable even when subjected to temperatures up to 1200°C. <s> BIB010 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> In this paper the possibilities of focused ion beam (FIB) applications in microsystem technology are reviewed. After an introduction to the technology and the operating principles of FIB, two classes of applications are described. First the subject of FIB for microsystem technology inspection, metrology and failure analysis is outlined. A procedure for cross sectioning on samples is presented, as well as some examples of how this technique can be applied to study processing results. The second part of the paper is on the use of FIB as a tool for maskless micromachining. Both subtractive (etching) and additive (deposition) techniques are discussed, as well as the combination of FIB implantation of silicon with subsequent wet etching. We will show the possibility to fabricate three-dimensional structures on a micrometre scale, and give examples of recent realizations thereof. <s> BIB011 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> Abstract A long-period grating (LPG) coated with gelatin was developed as a high relative humidity (RH) sensor. The resonance dip or coupling strength of the LPG spectrum varies with RH. Experimental investigations on the sensor yield a sensitivity of 0.833%RH/dB with an accuracy of ±0.25%RH, and a resolution of ±0.00833%RH. The LPG RH sensor also offers repeatability, hysteresis and stability errors of less than ±0.877, ±0.203 and ±0.04%RH, respectively. In addition to the characterization of the LPG RH sensor, further studies were conducted to determine the effect of grating periodicities on the sensitivity of the sensor. Results show that higher order cladding modes from smaller grating periods enable the sensor to achieve higher sensitivity to humidity. This method is proposed to be more cost effective as compared to more complex spectroscopic methods based on wavelength detection. This sensor can also help to solve problems in measuring high humidity with existing relative humidity measurement systems. <s> BIB012 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> A macrobending-based all-fiber refractometer sensor with a simple optical configuration is proposed and investigated both theoretically and experimentally. The proposed fiber refractometer sensor consists of a single-loop structure of bare macrobending standard single-mode fiber (SMF28) with a selected bending radius and reduced cladding diameter. The well-known scalar approximation theory is employed to theoretically predict the characteristics of the proposed fiber refractometer sensor. An approach to improve the resolution of the refractometer is presented, which shows that the refractometer with a reduced cladding diameter of 81 μm has an experimentally verified resolution of 5.75×10−5 for a refractive index range from 1.4586 to 1.5396 at the wavelength of 1550 nm. <s> BIB013 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <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> BIB014 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> We present a new fiber-optic refractive-index sensor based on a fiber modal interferometer constituted by a thin-core optical fiber, whose cut-off wavelength is around three times shorter than normal single-mode fiber. In such a core diameter mismatching structure, the high-order cladding modes are efficiently excited and interfere with the core mode to form a high extinction-ratio filter (>30 dB). Both transmissive and reflective thin-core fiber modal interferometers are experimentally demonstrated, and show a high sensitivity to a small change of external refractive-index (>100 nm/R.I.U.), but a low sensitivity to the change of temperature (<0.015 nm/°C). Such a fiber device possesses an extremely simple structure, but excellent refractive-index sensing properties, and thus is an ideal candidate for fiber-optic biochemical sensing applications. <s> BIB015 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> A comprehensive theoretical model for an SMF28–small-core SMF (SCSMF)–SMF28 structure based refractometer is developed based on the modal propagation analysis (MPA) method. The simulation result shows that the wavelength shift of this refractometer changes exponentially as the surrounding refractive index (SRI) varies. The core diameter of SCSMF does not have a significant influence on the sensitivity of the refractometer but cladding diameter does have. The simulation results are verified experimentally and it is also experimentally demonstrated that there is a maximum sensitivity of 1808 nm/RIU (refractive index unit) for an SRI range from 1.324 to 1.431 and that, as expected, the wavelength shift response is an exponential function of SRI. <s> BIB016 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <s> A new fiber-optic relative humidity (RH) sensor based on a thin-core fiber modal interferometer (TCFMI) with a fiber Bragg grating (FBG) in between is presented. Poly (N-ethyl-4-vinylpyridinium chloride) (P4VP·HCl) and poly (vinylsulfonic acid, sodium salt) (PVS) are layer-by-layer deposited on the side surface of the sensor for RH sensing. The fabrication of the sensing nanocoating is characterized by using UV-vis absorption spectroscopy, quartz crystal microbalance (QCM) and scanning electron microscopy (SEM). The incorporation of FBG in the middle of TCFMI can compensate the cross sensitivity of the sensor to temperature. The proposed sensor can detect the RH with resolution of 0.78% in a large RH range at different temperatures. A linear, fast and reversible response has been experimentally demonstrated. <s> BIB017 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <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> BIB018 </s> A Review of Multimode Interference in Tapered Optical Fibers and Related Applications <s> Periodically Tapered SMS Fiber Structures <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> Periodically Tapered SMS Fiber Structures <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> BIB020
In addition to using multimode fiber as the center section fiber, there are many other kinds of While the CO 2 laser technique described above can be used to fabricate a tip, it does not allow the tip parameters to be determined very precisely or repeatably. Focused ion beam (FIB) technology BIB003 BIB005 BIB011 can also be used to fabricate the fiber tip. FIB technology can support the micro-operations required to form the tip with high precision, such as localized milling and metal coating deposition. In order to achieve a good manufacturing accuracy in nanoscale on fiber tips, therefore, an FIB system (Helios 600, FEI Inc., Hillsboro, OR, USA) was employed to flat cut the tapered MMF tip, which can greatly increase the reflectivity of the fiber tip. Firstly, to avoid charging during FIB milling, a gold layer with thickness of circa 50 nm was deposited on the tapered MMF tip surface by employing an electron beam evaporator. A FIB beam with a size smaller than 30 nm can be obtained through adjusting the accelerating voltage and current into optimal values (30.0 kV and 93 pA. Thus, the tapered MMF tip end shape can be controlled with precision. After FIB milling, the remained gold layer on the tapered MMF tip surface was removed totally by gold-etching solvent, which contained iodine and potassium iodine. Then, another 40 nm thickness of gold, to be used as mirror, was coated on the tapered MMF tip end to increase the reflectivity. From a comparison of tapered MMF tip structures with and without FIB milling, we found that the FIB milling process can provide an improvement of a circa 40 dB for the reflectivity. Thus, it is demonstrated that the FIB milling technology is an appropriate approach to improve the capability of TOF-based MMF devices. In addition to using multimode fiber as the center section fiber, there are many other kinds of fibers used in the center section to fabricate a fiber structure similar to the SMS, such as the coreless fiber  , the small core fiber BIB016 , and the multi core fiber BIB014 . In our previous work, an SMF 28-small core singlemode fiber (SCSMF)-SMF 28 fiber structure was investigated both theoretically and experimentally BIB014 . Periodic tapering has also been proposed and investigated, for example a singlemode-periodically tapered small core fiber-single mode fiber structure using fiber tapering technology was investigated in BIB019 . In BIB015 BIB017 , a limited theoretical analysis of the SCSMF is presented, which assumes that there exists a interference between the core and cladding modes. However, according to the theoretical analysis in BIB001 BIB004 , there is no guided core mode within a SCSMF structure, which is consistent with our experimental results. Initially in  , the SCSMF structure before tapering was investigated by a comprehensive theoretical analysis. To the best of our knowledge, in this a comprehensive theoretical analysis for the SCSMF structure based on an MPA was described for the first time. Figure 10a shows the schematic configuration of the SMF-SCSMF-SMF structure, and Figure 10b illustrates a cross-sectional view of it. It is clear that the SCSMF has a step index distribution and can be equivalently seen as three layers, respectively denoted as the fiber core, cladding and the surrounding environment. When considering the circular symmetry characteristic of the input field and the SCSMF structure used, there are only zero-order azimuthal modes excited in the SCSMF of the SMF-SCSMF-SMF structure. It is assumed that the mode propagation constant and the field distribution of the SCSMF are respectively denoted as and ( ) and the input field distribution can be expressed as BIB002  : and: where , , , denote zero-order Bessel and modified Bessel function, and are the radii of fiber core and cladding, , and respectively denote the RI values of the fiber core, cladding and surrounding environment. In addition, , , , ′, A , A , A , A′ , A′ , A′ in the above equations can be respectively defined as: When considering the circular symmetry characteristic of the input field and the SCSMF structure used, there are only zero-order azimuthal modes excited in the SCSMF of the SMF-SCSMF-SMF structure. It is assumed that the mode propagation constant and the field distribution of the SCSMF are respectively denoted as β and ψ(r) and the input field distribution can be expressed as BIB002  : and: where J 0 , Y 0 , I 0 , K 0 denote zero-order Bessel and modified Bessel function, a and b are the radii of fiber core and cladding, n 1 , n 2 and n 3 respectively denote the RI values of the fiber core, cladding and surrounding environment. In addition, µ, µ , ϑ, ϑ , A 1 , A 2 , A 3 , A 1 , A 2 , A 3 in the above equations can be respectively defined as: where k 0 = 2π/λ, c = a/b, and λ is the free space wavelength. When the field distribution of the input SMF and the field distribution within the SCSMF are assumed to be E(r, 0) and ψ(r) respectively, we have: where E(r, 0) is the eigenmode of the SMF, ψ m (r) is m-th eigenmode of the SCSMF, M is the mode number and b m is the excitation coefficient, which can be calculated by: When the light propagates along the SCSMF section, the field intensity distribution at distance z can be obtained from: where β m denotes the propagation constant of the m-th eigenmode. Based on the above equations, the transmission power of the output SMF can be calculated by:3 From the above equations, it is clear that as the surrounding environment changes, the propagation constant and the excited eigenmodes in the cladding have a corresponding variation, resulting in changes of the output to the SMF. Additionally in BIB014 , we also investigated the influence of the core and cladding size of the SCSMF on operation and characteristics of the SMF-SCSMF-SMF structure theoretically and experimentally. We found that the cladding diameter has a much more significant influence than core diameter. The main reason for this is that since the SCSMF has a very small core, the normalized constant V (V = 2πa λ n 2 1 − n 2 2 < 1) is less than 1, which demonstrates that no core mode guides within the SCSMF. Based on the above theoretical analysis and experimental results, it can be concluded that the SMF-SCSMF-SMF fiber structure presents an improved sensing behavior when the diameter of the SCSMF cladding becomes smaller. Traditionally, hydrofluoric acid etching cladding is an effective approach; but its induced fabrication difficulty and surface roughness are inevitable, as well as using an etching liquid is harmful to the human body and the environment BIB013 . Thus, a novel fiber tapering technique is adopted to replace the chemical etching method, specifically a CO 2 laser irradiation method, which has been previously widely used to inscribe grating in conventional silica fibers BIB008 . Compared with the UV-laser exposure technique  BIB012 BIB006 BIB007 , the CO 2 laser irradiation method can provide many merits of low cost, easy fabrication and flexible since there is no need for photosensitive fiber or any other pretreatment process BIB008  BIB009 BIB010 . When combining the fiber tapering technique and the CO 2 laser irradiation method, a series of microtapers were fabricated to produce a periodically tapered (SPTS) fiber structure BIB019 . The SPTS fiber structure is composed of a periodically tapered SCSMF spliced axially with two standard SMFs, as shown schematically in Figure 11 . By comparison to the SMF-SCSMF-SMF fiber structure BIB016 , the SPTS fiber structure shows a better response to a number of physical parameters within an external environment, such as RI, because of the multiple foci effects and its induced cladding mode interference in the periodically tapered SCSMF section BIB018 . As illustrated in BIB016 , the SCSMF fiber can be effectively seen as the surrounding environment as a coated cladding, because the SCSMF core has no guided mode. A WA-BPM with cylindrical coordinates using a Padé (3, 3) approximation operator and PML boundary conditions was used to simulate light transmission in the SMF-SCSMF-SMF fiber structure and SPTS fiber structure. From the calculated amplitudes of the optical fields presented in BIB019 , it is clear that the optical power propagating through the SPTS fiber core is much lower than that in the SCSMF fiber core, because of the enhanced MMI and weak confinement of the fiber core mode. The SPTS fiber structure is composed of a periodically tapered SCSMF spliced axially with two standard SMFs, as shown schematically in Figure 11 . By comparison to the SMF-SCSMF-SMF fiber structure BIB016 , the SPTS fiber structure shows a better response to a number of physical parameters within an external environment, such as RI, because of the multiple foci effects and its induced cladding mode interference in the periodically tapered SCSMF section BIB018 . As illustrated in BIB016 , the SCSMF fiber can be effectively seen as the surrounding environment as a coated cladding, because the SCSMF core has no guided mode. A WA-BPM with cylindrical coordinates using a Padé (3, 3) approximation operator and PML boundary conditions was used to simulate light transmission in the SMF-SCSMF-SMF fiber structure and SPTS fiber structure. From the calculated amplitudes of the optical fields presented in BIB019 , it is clear that the optical power propagating through the SPTS fiber core is much lower than that in the SCSMF fiber core, because of the enhanced MMI and weak confinement of the fiber core mode. To fabricate the SPTS structure we adopted a CO2 laser irradiation technique, which was introduced in Section 2.2. It is noteworthy that this technique is frequently used to inscribe grating in fibers. The fabrication process for an SPTS fiber structure is similar to that for an STMS fiber structure, as depicted in Figure 7 . A prefabricated SMF-SCSMF-SMF fiber structure was fixed directly on two 3D translation stages. The SCSMF section was exposed to the CO2 laser beam, which was moved by using gold-coated mirrors on a motorized translation stage. A LabView program was designed to control the shutter opening and closing timing. Tapering occurs when the strained SCSMF section was exposed to the CO2 laser beam with an output power of 10.5 W for 120 s. After that, the laser beam was moved at a distance of 400 µm along the fiber axis to heat and taper another segment of the fiber. When this taper process is repeated 20 times, a periodically tapered SCSMF structure is fabricated. It is worth noting that while the movement distance of laser beam is 400 µm, the actual distance between consecutive taper waists is circa 500 µm due to a tapering induced elongation effect. The microscope images of the obtained periodically tapered SCSMF structure is shown in Figure 12 . To fabricate the SPTS structure we adopted a CO 2 laser irradiation technique, which was introduced in Section 2.2. It is noteworthy that this technique is frequently used to inscribe grating in fibers. The fabrication process for an SPTS fiber structure is similar to that for an STMS fiber structure, as depicted in Figure 7 . A prefabricated SMF-SCSMF-SMF fiber structure was fixed directly on two 3D translation stages. The SCSMF section was exposed to the CO 2 laser beam, which was moved by using gold-coated mirrors on a motorized translation stage. A LabView program was designed to control the shutter opening and closing timing. Tapering occurs when the strained SCSMF section was exposed to the CO 2 laser beam with an output power of 10.5 W for 120 s. After that, the laser beam was moved at a distance of 400 µm along the fiber axis to heat and taper another segment of the fiber. When this taper process is repeated 20 times, a periodically tapered SCSMF structure is fabricated. It is worth noting that while the movement distance of laser beam is 400 µm, the actual distance between consecutive taper waists is circa 500 µm due to a tapering induced elongation effect. The microscope images of the obtained periodically tapered SCSMF structure is shown in Figure 12 . was exposed to the CO2 laser beam with an output power of BIB020 Figure 12 . 