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We investigate the waist diameter of Tapered Optical Fiber (TOF) using the mode evolution during the tapering process of single mode optical fiber (SMF28) through Short Time Fourier Transform (STFT) analysis. During the fiber pulling process, some of modes exited due to waveguide modification of TOF, hence, interfere with each other, resulting an up-chirp signal. Analyzing with STFT, reveals spatial beat frequencies between different modes. Determining the cut-off value for spatial frequencies and the knowledge of theoretical cut-off diameter for different modes, the final diameter of waist region with precision of 4 nm can be estimated. Finally, the cut-off diameter was calibrated with conventional scanning electron microscopy (SEM) and then different spectra for different TOF structure are analyzed. The calculated and imaging data is in a good agreement with each other. We propose this method for characterization and optimizing of TOF fabrication process such as control the waist radius, the number of modes and their propagation constants.

Measurement of small force in biological applications could be helpful especially in the field of diagnostic and prognostic procedure. For this purpose, a Hybrid Fabry Perot fiber optic Micro Cavity is proposed based on Micro Silica Sphere Cavity integrated on the capillary tube, and is bound to the single mode fiber with PDMS layer. Since PDMS acts as an elastic material, under small loads the cavity length was affected. To study this mechanical behavior, the sensor structure was simulated with Finite element method. The force measurement was studied experimentally with analyzing wavelength shifts of sensor. Consequently, the force sensitivity was equal to -3pm/mN. The force resolution of our sensor was equal to 340 μN in the range of 0 to 950 mN.

A fiber optic interrogation sensor scheme based on a “figure-of-eight” configuration created from a single directional 3 $\,\times\,$ 3 fiber optic coupler is proposed. Two loops are formed in each arm and one of them contains the sensing head and the other is used as reference signal. A theoretical study based on Jones matrix analysis of this fiber loop mirror combination is reported. The optical configuration is tested as an interrogation scheme for a fiber strain sensor where the spectral response arises from the combination of the reference signal modulated by the sensor signal. The strain sensor configuration shows a phase sensitivity of $6.7\pm 4.38\times 10^{-2}~{\rm mrad/}\mu{\rm strain}$ by linear regression.

usc Refereed/Peer-reviewed Focused ion beam technology is combined with dynamic chemical etching to create microcavities in tapered optical fiber tips, resulting in fiber probes for temperature and refractive index sensing. Dynamic chemical etching uses hydrofluoric acid and a syringe pump to etch standard optical fibers into cone structures called tapered fiber tips where the length, shape, and cone angle can be precisely controlled. On these tips, focused ion beam is used to mill several different types of Fabry-Perot microcavities. Two main cavity types are initially compared and then combined to form a third, complex cavity structure. In the first case, a gap is milled on the tapered fiber tip which allows the external medium to penetrate the light guiding region and thus presents sensitivity to external refractive index changes. In the second, two slots that function as mirrors are milled on the tip creating a silica cavity that is only sensitive to temperature changes. Finally, both cavities are combined on a single tapered fiber tip, resulting in a multi-cavity structure capable of discriminating between temperature and refractive index variations. This dual characterization is performed with the aid of a fast Fourier transform method to separate the contributions of each cavity and thus of temperature and refractive index. Ultimately, a tapered optical fiber tip probe with sub-standard dimensions containing a multi-cavity structure is projected, fabricated, characterized and applied as a sensing element for simultaneous temperature and refractive index discrimination.

Refereed/Peer-reviewed Fiber probe structures composed of two physical microcavities were created using focused ion beam technology. These structures have a tip-like shape as they were milled in preciously etched tapered fiber tips. The microprobes are then characterized for temperature and refractive index sensing using a signal filtering technique to discriminate signals from distinct microcavities. Using fast Fourier transforms combined with band-pass filters, it is possible to reconstruct the spectra of each cavity independently and thus measure their individual spectral shifts.

In this article, a novel Micro Silica Sphere Cavity Hybrid Fabry Perot optical fiber sensor is reported where refractive index (RI) and temperature can be simultaneously measured. The sensor is based on Micro Silica Sphere that was fabricated using a capillary tube. The micro silica sphere and optical fiber form a Hybrid Fabry Perot cavity. The temperature cross sensitivity of this sensor is small enough to be used for accurate RI measurement. The temperature sensitivity and RI sensitivity are -0.0028 dBm/oC, -0.0044 dBm/oC , -24.09 dBm/RIU and -20.6 dBm/RIU respectively, using two selected resonances.

In this paper the potential of real time monitoring of AS1411 Guanine-quadruplex folding by using long period fiber grating (LPFG) biosensor was described. AS1411 ss-DNA folds into parallel bimolecular quadruplex in solution containing metal ions. While immobilized AS1411 folds, molecular crowding of sensing area will increase and cause change the refractive index (RI) around the sensor. In addition RI change is absolutely dependent on G-quadruplex factor concentration.

Focused ion beam milling is used on chemically etched tapered fiber tips to create fiber Bragg gratings. These fiber Bragg gratings are based on a modulation of silica and external medium. This leads to a wide and structured spectrum obtained due to imperfections and the inherent structure of the tip. The fiber Bragg gratings presented are very short and have a length of 27 μm and 43 μm and are milled on the tapered fiber tip. They are characterized in the high temperature range 350-850oC and a sensitivity of 14.4 pm/K is determined.

In this work, fiber in-line Mach-Zehnder Interferometer (MZI) based on triangular-shape suspended core fibers (SCFs) is investigated. The sensitivity of the sensing head was determined for pressure and temperature, respectively. The sensitivities are 0.4 pm/psi and 13 pm/psi for longitudinal and radial pressure, respectively. The sensing head was also subjected to temperature and presented very low sensitivity.

Ahigh pressure sensor based on an optical fiber micro sphere cavity Fizeau interfer ometer (MSC-FI) is presented in this paper.The sensorwas fabricated by use of a fusion splicer. Th e achieved pressure sensitivity of MSC-FI sensor was in the range of 500 to 7000pi to be 4×10 -4 nm/psi. With a 10 pm resolution for the wavelength shift detection, the resolution of the device at room temperature for pressure measurement is to be 25 psi. This sensor can be used as a high pressure sensor in downhole application. Keywords: optical fiber sensor, Mi cro sphere cavity, Fizeau interferometer, pressure. 1. INTRODUCTION Efficient and complete recovery of petroleum reserves is difficult in part due to lack of robust instrumentation for characterization of physical conditions in the downhole environment. Accurate evaluation of pressure, temperature and volumetric flow would permit optimization of oil recovery processes.This parameters were traditionally measured by electrical gauges based on electrical gauge [1,2].They are al so sensitive to electromagnetic noise and have low potential for multiplexing application. The use of optical fiber sensors for measurement of physical parameters such as temperature, pressure and flow plays an important role in in dustry and engineering sciences [2]. Optical fibersensorsdue to thecompact size, potential low cost,immunity to electromagnetic interference and the ab ilityof multiplexing, arevery important.Many different optical fiber pressure sensors have been reported, including those based on fiber bragg gratings, highly birefringent (Hi-Bi) microstructure and photonic crystal fiber fibers [3-4], Fabry-Perot interferometers (FPIs) [2], FPI based on capillary micro cavity[5], hollow core fiber [6]. For many practical application of interfe rometry small sensor heads is required. Fizeau interfero meters with short optical cavity are specially attractive as they have the advantages of being simple, compact, low cost with minimum cross sensitivity to temperature [7]. Enbang Li et al reported a strain sensor based on Fizeau interferometer in-fiber spherical micro cavity, in their work an air bubble was cr eated in splice mechanism by fusion splicing.[8] In this paper, we report a novel pressure sensor based on a spherical micro cavity extrin sic Fizaue interferometerc. The sensor was made by melting the capillary at the tip to form a micro sphere with fusing, and then putting a single mode fiber into the capillary tube. The sensor has a compact size and good mechanic strength and it can be used for high pressure applications.

In this paper, we report a design and fabrication of MEMS electrostatically tunable Fabry-Perot (METFP) cavity sensor. The mirror of METFP sensor is fabricated by SU-8 photoresist for the first time. The sensor is the combination of optical fiber and microelectromechanical systems (MEMS) technologies. The voltage sensitivity of METFP is 0.036 nm/V and the resolution of voltage measurement is 0.277 V.

In this work a fiber optic interferometric system for differential refractive index measurement is described. The system is based on a white light Mach-Zehnder configuration, with serrodyne phase modulation, to interrogate two similar nonadiabatic tapered optical fiber sensors in a differential scheme. In this situation it is possible to measure the refractive index independent of temperature. Signal processing with low cost digital instrumentation developed in Labview environment allows a detectable change in refractive index of Δn≈2x10-6, which is, from the best of our knowledge the highest resolution achieved using a fiber taper device. The results demonstrate the potential of the proposed scheme to operate as chemical and biological sensing platform. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

A single-mode nanoadiabatic tapered optical fiber biosensor was utilized for real-time monitoring of the antimicrobial activity of silver nanoparticles (Ag-NPs) against E. coli. For measuring growth-inhibitory effects, the bacteria were immobilized on the tapered surface. Then, fiber sensor surrounded in glucose growth medium conation different concentration Ag-NP. At the same condition, the growth inhibition of E. coli was measured by colony counting method as a controlling experiment. Interacting Ag-NPs with cells, by anchoring to and penetrating the bacterial cell, the refractive index of the tapered region can be changed that leads to changes in the optical characteristics of the taper. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

A high-birefringent fiber (HBF) was tapered as adiabatic in sequence steps by utilizing a CO 2 laser and its birefringence was measured in fiber loop mirror (FLM) setup. The birefringence of tapered section and total sensor was obtained to be -8.02×10 -2 , and 2.46×10 -4 , respectively. Then, refractive index (RI) sensitivity increased and temperature sensitivity of the tapered Hi-Bi fiber (THBF) decreased. The sensitivity of the proposed FLM interferometer for RI changes in the range from 1.3380 to 1.3470 was measured to be 389.85 nm /RIU. The temperature sensitivity in the range from 50°C to 90 °C was measured to be -1.19nm/°C. Keywords: high-birefringent fiber, taper, birefringence, refractive index, temperature. 1. INTRODUCTION Optical biochemical fiber sensors have been studied based on a range of different concepts. The main physical phenomena exploited for the realization of biochemical fiber sensors are absorption and fluorescence. Another well-known concept for detection of biochemical parameters is base d on its interaction with the evanescent field (EVF) of the light that travels in an optical fiber. The amplitude of the EVF is typically very low in a normal single mode fiber (SMF), but it can be enhanced significantly by tapering, and thereby th e interaction of the transmitted light and the analyte lying on the taper is enhanced [1]. The measurement of changes in refractive index (RI) enables to continuously monitor unlabeled biomolecular interactions occurring at their interface which has many applications such as the study of a molecular structure and the identification of organic compounds in the medical, pharmaceutical, fluid, petrochemical and food industries [2]. Fiber loop mirrors (FLMs) are attractive devices and very useful components for the implementation of optical devices and systems. The FLM is formed by a splice between the output ports of a directional optical coupler. When a section of highly birefringent fiber (HBF) is spliced inside of the FLM, a path unbalance is introduced between the light that propagates along different polarization eigenaxis and an interferometric spectrum is observed at the output [3].The FLM interferometer has many advantages, such as high sensitivity to external perturbation changes, polarization independence to input light, low insertion loss, electromagnetic immunity, and high measuring precision [3]. In optical sensing, besides the gyroscope application, it has been used in temperature and strain measurements [4-5]. In this paper sensing parameters of an adiabatic tapered HBF (THBF) in FLM setup was characterized. 2. EXPERIMENTS AND RESULTS 2.1 Experimental setup and tapering of Hi-Bi fiber For characterizing of THBF and study of changing birefringence in during process of tapering, experimental setup figure 1 was used. The proposed setup consists of a broadband light source, an optical spectrum analyzer (OSA) with a maximum resolution of 10 pm, a 50:50 power splitting directional coupler, a polarization controller (PC) and a THBF inserted into the loop. The THBF has three parts, a tapered section between two HBF sections. The transmission spectrum of the fiber loop is approximately a periodic function of the wavelength, namely sin 2 2 cos 2 (

In this work, a novel approach to application of tapered long period fiber grating (TLPFG) sensor is demonstrated, which are based on the modification of gold nanoparticles (GNPs). The sensor we propose here is a TLPFG fabricated with CO2 laser by heat-pulling method. TLPFG was immobilized with GNP and its refractive index sensitivity was measured. Experimental results show that modification of the TLPFG with GNPs enhances the evanescent wave and penetration depth around the sensor. Comparing different resonance dips of gold modified and bare TLPFG, improvement of 10.3 and 6.6 times can be estimated for wavelength and intensity sensitivities, respectively.

A theoretical and experimental study of a new fiber loop mirror based on a “figure-of-eight”configuration, is reported. For the theoretical model, the Jones matrix analysis is analyzed. The configuration is tested as an interrogation system where the spectral response arises from the combination of the reference signal modulated by the sensor signal. The configuration is characterized in mechanical strain and presents a phase sensitivity of 8.2 mrad/∝e.

We report fabrication of a high pressure optical fiber sensor by using a fat long period fiber grating (FLPFG) for downhole applications. The pressure sensitivity of the bare optical fiber is low so we have designed a mechanical transducer for increasing the pressure sensitivity and possibility of installation the sensor in downhole. The pressure along the longitudinal direction changes the physical characteristic of the FLPFG and result in shifting the resonance wavelength. The FLPFG sensor has been installed on transducer and the pressure sensitivity of the fiber sensor has been measured. Since the temperature changes can affect the FLPFG output, the high pressure vessel was isolated and the temperature was kept constant during the experiment. Pressure sensitivity of the FLPFG sensors has been measured by increasing the pressure from 1500 psi to 10000 psi in steps of 700 psi which is equal to -1.04 pm/psi. With a 10 pm resolution for the wavelength shift detection our OSA, detection limit of our device at room temperature for pressure measurement is calculated to be 10 psi.

A single-mode non-adiabatic tapered optical fiber (NATOF) sensor was inserted into a Sagnac loop interferometer allowing tuning its sensitivity to refractive index (RI) by use of polarization control. By adjusting any polarization controllers inserted in the Sagnac loop interferometer, various cladding modes are selectively excited in each arm of the interferometric taper resulting into different phase changes for the clockwise and counterclockwise beams. By this method, sensitivity of the sensor for RI in the range from 1.3380 to 1.3510 was tuned between 876.24 RIU/nm to 1233.07 RIU/nm.

A nonadiabatic tapered optical fiber sensor was utilized for studying of bimolecular interactions including DNA-DNA and DNA-Drug interaction. This work presents a simple evanescent wave sensing system based on an interferometric approach, suitable to meet the requirements of lable-free sensor systems for detecting biomolecular interactions. We have demonstrated the measuring refractive index and the real time detection of interactions between biomolecules. Furthermore basic experiments were carried out, for detecting the hybridization of 25-mer DNA with an immobilized counterpart on the surface. The overall shift after the successful DNA hybridization was 9.5 nm. In this work, a new approach for studying DNA-drug interactions was successfully tested. Nicotine as a carcinogenic compound in cigarette smoke plays an important role in interaction with DNA. Different concentrations of nicotine were applied to observe the Longmuir interaction with DNA.

We report fabrication of a high pressure nonadiabatic tapered optical fiber (NATOF) for downhole applications by using a mechanical transducer. The mechanical transducer has been used for increasing the pressure sensitivity and possibility of installation the sensor in downhole. The NATOF is fabricated by heat pulling method, utilizing a CO 2 laser. The limit of detection of the NATOF was 15 psi.

In this work, two all-fibre interferometric configurations based on suspended core fibres (SCF) are investigated. A Fabry-Perot cavity made of SCF spliced in-between segments of single-mode and hollow-core fiber is proposed. An alternative sensing head configuration formed by the insertion of a length of SCF as a birefringence element in a Sagnac interferometer is also demonstrated. The sensitivity to pressure and temperature was determined for both configurations.

A single-mode tapered fiber optic biosensor was utilized for real-time monitoring of the Escherichia coli ( E. coli K-12) growth in an aqueous medium. The applied fiber tapers were fabricated using heat-pulling method with waist diameter and length of 6-7μm and 3mm, respectively. The bacteria were immobilized on the tapered surface using Poly-L-Lysine. By providing the proper condition, bacterial population growth on the tapered surface increases the average surface density of the cells and consequently the refractive index (RI) of the tapered region would increase. The adsorption of the cells on the tapered fiber leads to changes in the optical characteristics of the taper. This affects the evanescent field leading to changes in optical throughput. The bacterial growth rate was monitored at room temperature by transmission of a 1558.17nm distributed feedback (DFB) laser through the tapered fiber. At the same condition, after determining the growth rate of E. coli by means of colony counting method, we compared the results with that obtained from the fiber sensor measurements. This novel sensing method, promises new application such as rapid analysis of the presence of bacteria.

A single-mode tapered fiber optics biosensor was utilized for the real-time attachment of model protein bovine serum albumin (BSA) to the antibody-immobilized surface of the taper. The applied fiber tapers were fabricated with waist diameter of 6-7μm and of 3mm waist length using heat-pulling method. The surfaces of the tapers were modified with an amine group to allow for the formation of a covalent bond between the amine and one of the carboxylic groups of the antibody. The attachment of BSA to the antibody-immobilized surface of the taper at (20-25°C) temperature was monitored by transmission of a 1558.17nm distributed feedback (DFB) laser through the tapered fiber. While cuvette measurements established that BSA was non-absorbing at 1558 nm showing no significant changes in optical throughput through the cuvette, tapered fibers with antibody-immobilized surfaces showed changes in optical throughput at bulk concentrations down to100 fg/mL of BSA. We postulate that the adsorption of the protein to the tapered fiber leads to changes in the optical characteristics of the taper. This affects the evanescent field leading to changes in optical throughput.