Post chemical etching of tapered seven-core fiber sensor for enhanced figure of merit.

A post chemical etching process to a tapered seven-core fiber (TSCF) is proposed and experimentally demonstrated to effectively adjust the mode profiles of high-order supermodes, aimed to improve the figure of merit (FOM). The experimental results show that the FOM of an etched TSCF is as high as 1431.36 1/RIU, a 7.32-times enhancement compared with that of TSCF without etching, provided the TSCF has the same taper waist diameter of 19.20 µm. The proposed method opens a new, to the best of our knowledge, method for optimizing optical fiber sensor performance.

Multicore Fiber Bending Sensors with High Sensitivity Based on Asymmetric Excitation Scheme.

Bending sensing was realized by constructing a tapered four-core optical fiber (TFCF) sensor. The four-core fiber (FCF) between the fan-in and fan-out couplers was tapered and the diameter became smaller, so that the distance between the four cores arranged in a square became gradually smaller to produce supermodes. The two ends of the TFCF were respectively connected to the fan-in and fan-out couplers so that the individual cores in the FCF could link to the separate single-mode fibers. A broadband light source (superluminescent diodes (SLD)) spanning 1250-1650 nm was injected into any one of the four cores, and the orientation was thus determined. In the tapering process, the remaining three cores gradually approached the excitation core in space to excite several supermodes based on the tri-core structure first, and then transited to the quadruple-core structure. The field distributions of the excited supermodes were asymmetric due to the corner-core excitation scheme, and the interference thus resulted in a higher measurement sensitivity. When the diameter of the TFCF was 7.5 µm and the tapered length was 2.21 mm, the sensitivity of the bending sensor could reach 16.12 nm/m-1.

High Sensitivity Fiber Interferometric Strain Sensors Based on Elongated Fiber Abrupt Tapers.

We demonstrate high-sensitivity fiber strain sensors based on an elongated abrupt taper. The fiber abrupt taper, with a tapered diameter ranging from 40-60 µm, was made by using a hydrogen microflame to break the waveguide adiabaticity so as to convert the fundamental mode into cladding modes. The abrupt taper was further uniformly tapered by using a normal moving flame with a torch diameter of 7 mm to elongate the tapered region until the tapered diameter was down to 2.5-5 µm. The excited high-order modes were confined to propagate along the cladding and then recombined at the rear edge of the fiber taper to produce interferences with extinction ratios of up to 16 dB. The tapered region was pulled outwardly to change the optical path difference (OPD) between modes to measure the tensile strain with all the interfering wavelengths blue-shifted. The measured best strain sensitivity was 116.21 pm/µÎµ and the coefficient of determination R2 of linear fitting exhibits high linearity. This strain sensor based on elongated abrupt taper is several times higher than that of most of the fiber strain sensors ever reported.

High Sensitivity Strain Sensors Using Four-Core Fibers through a Corner-Core Excitation.

A weakly-coupled multicore fiber can generate supermodes when the multi-cores are closer to enter the evanescent power coupling region. The high sensitivity strain sensors using tapered four-core fibers (FCFs) were demonstrated. The fan-in and fan-out couplers were used to carry out light coupling between singlemode fibers and the individual core of the FCFs. A broadband lightsource from superlumminescent diodes (SLDs) was launched into one of the four cores arranged in a rectangular configuration. When the FCF was substantially tapered, the asymmetric supermodes were produced to generate interferences through this corner-core excitation scheme. During tapering, the supermodes were excited based on a tri-core structure initially and then transited to a rectangular quadruple-core structure gradually to reach the sensitivity of 185.18 pm/µÔ‘ under a tapered diameter of 3 µm. The asymmetric evanescent wave distribution due to the corner-core excitation scheme is helpful to increase the optical path difference (OPD) between supermodes for improving the strain sensitivity.

Observation of split evanescent field distributions in tapered multicore fibers for multiline nanoparticle trapping and microsensing.

The optical attractive force in tapered single-mode fibers (SMFs) is usually uniformly distributed around the tapered section and has been found to be important for trapping and manipulating targeted atoms and nanoparticles. In contrast, a peculiar phenomenon of the evanescent field splitting along the azimuth axis can be experimentally observed by tapering a weakly-coupled MCF into a strongly-coupled MCF to generate supermode interference. Moreover, the supermode interference produces a hexagonally distributed evanescent field and its six vertices give rise to the multiline optical attractive force. For such spectral resonances, the optimum extinction ratio for the transmission dips is given by 47.4 dB, this being determined using an index liquid to cover the tapered MCF. The resonant dips move to a greater extent at longer wavelengths, with the optimum tuning efficiency of 392 nm/RIU for index sensing. The split evanescent fields respectively attract the excited upconversion nanoparticles in the liquid to be linearly aligned and running down the tapered region over the fiber surface, emitting green light with 60° symmetry. The charged nanoparticles were periodically self-organized, with a period of around 1.53 µm. The parallel lines, with 60° rotational symmetry, can be useful for (1) indicating the exact locations of the side-cores or orientations of the tapered MCF; (2) as precision alignment keys for micro-optical manipulation; and (3) enhancing the upconversion light, or for use in lasers, coupling back to the MCF. The split evanescent fields can be promising for developing new evanescent field-based active and passive fiber components with nano-structures.

High sensitivity micro-fiber Mach-Zehnder interferometric temperature sensors with a high index ring layer.

The influence of the high index ring layer (HIRL) in a tapered fiber Mach-Zehnder interferometer (MZI) on the interference observed, and thus on its potential applications in temperature sensing, has been investigated. The MZI was comprised of a tapered Ring Core Fiber (RCF), spliced between two single mode fibers (SMF). Since part of core mode from the SMF was converted into cladding modes in the RCF, due to the mismatch in the cores between the RCF and SMF, the residual power enters and then propagates along the center of the RCF (silica). The difference in phase between the radiation travelling along these different paths is separated by the HIRL to generate an interference effect. Compared with fiber interferometers based on core and cladding mode interference, the thin fiber HIRL is capable of separating the high order cladding modes and the silica core mode, under grazing incident conditions. Therefore, the optical path difference (OPD) and the sensitivity are both substantially improved over what is seen in conventional devices, showing their potential for interferometric temperature sensor applications. The optimum temperature sensitivity obtained was 186.6 pm/°C, which is ∼ 11.7 times higher than has been reported previously.

Micro-fiber Mach-Zehnder interferometer based on ring-core fiber.

A micro-fiber Mach-Zehnder interferometer (MZI), with a thousands-µm-long ring-core fiber (RCF), is demonstrated, and its performance investigation is also implemented. In this paper, the proposed MZI is manufactured by ends-splicing the short RCF segment with single-mode fiber (SMF-28), respectively. The scheme of the MZI is a typically core-mismatch structure, which has the advantages of miniaturization and simplification. Due to the core mismatch between RCF and SMF, the light from the SMF can be well separated into ring core (RC) and silica center (SC) of the RCF at the first splicing point. After transmitting through the RC and SC, the two separated light beams encounter each other and interfere at the second splicing point. Different from conventional micro-fiber MZIs using SMFs or few-mode fibers, the RCF has a higher numerical aperture, which can generate a larger optical path-length difference with a short length fiber, accumulates a higher extinction ratio and suppresses the crosstalk between the core and cladding modes. Therefore, our proposed MZI is more stable and the best extinction ratios can reach up to 18.2 dB. Meanwhile, owing to the core structure of RCF (where SC is surrounded by high-index ring core), the power propagating through low-index area of RCF is mostly confined into SC (termed the silica-center modes). These characteristics would lead to the lower sensitivity to external disturbances.

LSPR-based cholesterol biosensor using a tapered optical fiber structure.

Accurate cholesterol level measurement plays an important role in the diagnosis of severe diseases such as cardiovascular diseases, hypertension, anemia, myxedemia, hyperthyroidism, coronary artery illness. Traditionally, electrochemical sensors have been employed to detect the cholesterol level. However, these sensors have limitations in terms of sensitivity and selectivity. In this paper, a localized surface plasmon resonance (LSPR) -based biosensor is demonstrated that accurately detects and measures the concentration of cholesterol. In the present study, a tapered optical fiber-based sensor probe is developed using gold nanoparticles (AuNPs) and cholesterol oxidase (ChOx) to increase the sensitivity and selectivity of the sensor. Synthesized AuNPs were characterized by UV-visible spectrophotometer, transmission electron microscope (TEM), and energy dispersive X-ray spectroscopy (EDS). Further, coating of AuNPs over fiber was confirmed by scanning electron microscope (SEM). The developed sensor demonstrates for a clinically important cholesterol range of 0 to 10 mM, and the limit of detection is found to be 53.1 nM.

Broadband Ce/Cr-doped crystal fibers for high axial resolution OCT light source.

The fabrication and characteristics of Ce/Cr-doped crystal fibers employing drawing tower technique are reported. The fluorescence spectrum of the Ce/Cr fibers at the core diameter ranging from 10 to 21 µm exhibited a 200-nm near-Gaussian broadband emission which enabled to provide an axial resolution of 1.8-µm and a power density of 79.1 nW/nm. The proposed broadband Ce/Cr-doped crystal fibers may be provided as a high-resolution light source for the use in optical coherence tomography system as well as industrial inspection and biomedical imaging applications.

In-situ detection of density alteration in non-physiological cells with polarimetric tilted fiber grating sensors.

Tilted fiber Bragg grating (TFBG) biosensors can be used as a cost-effective and relatively simple-to-implement alternative to well established biosensor platforms for high sensitivity biological sample measurements in situ or possibly in vivo. The fiber biosensor presented in this study utilizes an in-fiber 12° tilted Bragg grating to excite a strong evanescent field on the surface of the sensor over a large range of external medium refractive indices. The devices have minimal cross-sensitivity to temperature and their fabrication does not impact the structural integrity of the fiber and its surface functionalization. Human acute leukemia cells with different intracellular densities and refractive index (RI) ranging from 1.3342 to 1.3344 were clearly discriminated in-situ by using the differential transmission spectrum between two orthogonal polarizations for the last guided mode resonance before "cut-off", with an amplitude variation sensitivity of 1.8 × 10(4) dB/RIU, a wavelength shift sensitivity of 180 nm/RIU, and a limit of detection of 2 × 10(-5)RIU. The detection process was precisely controlled with a micro-fluidic chip which allows the measurement of nL-volumes of bio-samples. The proposed in-fiber polarimetric biosensor is an appealing solution for rapid, sub-microliter dose and highly sensitive detection of analytes at low concentrations in medicine, chemical and environmental monitoring.

Tapered fiber Mach-Zehnder interferometers for vibration and elasticity sensing applications.

We demonstrate the optical measurements of heart-beat pulse rate and also elasticity of a polymeric tube, using a tapered fiber Mach-Zehnder interferometer. This device has two abrupt tapers in the Er/Yb codoped fiber and thus fractional amount of core mode is converted into cladding modes at the first abrupt taper. The core and cladding modes propagate through different optical paths and meet again at the second abrupt taper to produce interferences. The mechanical vibration signals generated by the blood vessels and by an inflated polymeric tube can perturb the optical paths of resonant modes to move around the resonant wavelengths. Thus, the cw laser signal is modulated to become pulses to reflect the heart-beat pulse rate and the elasticity of a polymeric tube, respectively.

Tapered optical fiber sensor based on localized surface plasmon resonance.

A tapered fiber localized surface plasmon resonance (LSPR) sensor is demonstrated for refractive index sensing and label-free biochemical detection. The sensing strategy relies on the interrogation of the transmission intensity change due to the evanescent field absorption of immobilized gold nanoparticles on the tapered fiber surface. The refractive index resolution based on the interrogation of transmission intensity change is calculated to be 3.2×10⁻5 RIU. The feasibility of DNP-functionalized tapered fiber LSPR sensor in monitoring anti-DNP antibody with different concentrations spiked in buffer is examined. Results suggest that the compact sensor can perform qualitative and quantitative biochemical detection in real-time and thus has potential to be used in biomolecular sensing applications.

Core-pumped gain-guided index-antiguided continuous wave lasing in dispersion-engineered erbium-doped fiber.

We demonstrate core-pumped gain-guided index-antiguided fiber lasers using a 22 cm long dispersion-engineered erbium-doped fiber. The erbium-doped fiber is dispersion-engineered using optical liquid to replace the most part of silica cladding so that the index of core turns out to be higher (lower) than that of new cladding at pump (lasing) wavelength, respectively. The pump light is confined to propagate in core based on an index-guiding mechanism to efficiently excite the gain medium running through the fiber whereas the cw lasing is constructed in a long, small-core waveguide under a gain-guided, index-antiguided situation.

Pr3+-sensitized Er3+-doped bismuthate glass for generating high inversion rates at 2.7 µm wavelength.

With a 980 nm laser diode pumping, the 2.7 µm emission and energy transfer processes of Er3+/Pr3+ codoped germanium-gallium-bismuthate glasses have been investigated. For Er3+ (1 mol. %) and Pr3+ (1 mol. %) molar concentrations, an intense 2.7 µm emission was obtained based on the high excited-state absorption of Er3+ ions and energy transfer (ET) between Er3+ and Pr3+ ions codopant (ET). The intrinsic lifetime of Er3+:4I(13/2) level is quenched effectively (from 6.85 ms down to 0.24 ms) and the population inversions between Er3+:4I(11/2) and 4I(13/2) levels are enhanced to achieve a four-level energy system at 2.7 µm.

Highly efficient femtosecond pulse stretching by tailoring cavity dispersion in erbium fiber lasers with an intracavity short-pass edge filter.

We demonstrate highly efficient pulse stretching in Er(3+)-doped femtosecond mode-locked fiber lasers by tailoring cavity dispersion using an intracavity short-pass edge filter. The cavity dispersion is preset at around zero to obtain the shortest pulsewidth. When the cutoff wavelength of the short-pass edge filter is thermo-optically tuned to overlap the constituting spectral components of mode-locked pulses, large negative waveguide dispersion is introduced by the steep cutoff slope and the total cavity dispersion is moved to normal dispersion regime to broaden the pulsewidth. The time-bandwidth product of the mode-locked pulse increases with the decreasing temperature at the optical liquid surrounding the short-pass edge filter. Pulse stretch ratio of 3.53 (623.8 fs/176.8 fs) can be efficiently achieved under a temperature variation of 4 °C.

Broadband micro-Michelson interferometer with multi-optical-path beating using a sphered-end hollow fiber.

We demonstrate a high-sensitivity broadband (1250-1650 nm) fiber micro-Michelson interferometer using a single-mode fiber end-spliced with a sphered-end hollow-core fiber. The hollow core is slightly smaller than the solid core of a single-mode fiber, so the fractional power of the core mode is converted into cladding modes. The excited cladding modes propagate at distinct optical paths along the hollow-core fiber and have individual foci outside the spherical lens. The reflected core mode, generated at the solid core-air interface, and the reflected cladding modes, generated at external material, interfere with each other to produce beating in the interference signals.

Multicolor upconversion emissions in Tm 3+/Er3+ codoped tellurite photonic microwire between silica fiber tapers.

We report multicolor upconversion emissions including the blue-violet, green, and red lights in a Tm 3+/Er3+codoped tellurite glass photonic microwire between two silica fiber tapers. A silica fiber is tapered until its evanescent field is exposed and then angled-cleaved at the tapered center to divide the tapered fibers into two parts. A tellurite glass is melted by a gas flame to cluster into a sphere at the tip of one tapered fiber. The other angled-cleaved tapered fiber is blended into the melted tellurite glass. When the tellurite glass is melted, the two silica fiber tapers are simultaneously moving outwards to draw the tellurite glass into a microwire in between. The advantage of angled-cleaving on fiber tapers is to avoid cavity resonances in high index photonic microwire. Thus, the broadband white light can be transmitted between silica fibers and a special optical property like high intensity upconversion emission can be achieved. A cw 1064 nm Nd:YAG laser light is launched into the Tm 3+/Er3+ codoped tellurite microwire through a silica fiber taper to generate the multicolor upconversion emissions, including the blue-violet, green, and red lights, simultaneously.

Effect of gain-dependent phase shift for tunable abrupt-tapered Mach-Zehnder interferometers.

A differential gain-dependent phase shift between the core mode and the cladding mode can be all-optically tuned by a cw 975 nm pump light for Mach-Zehnder interferometers using two abrupt fiber tapers in a 1.2-cm-long highly Er/Yb codoped fiber. The highly doped fiber has large absorption and emission cross sections to efficiently introduce the phase shift when the inversion rate is changed by varying the pump power. The first abrupt taper converts part of core mode into the cladding mode, while the second abrupt taper recombines both modes to interfere after propagating for a length of a few millimeters.

Influence of depressed-index outer ring on evanescent tunneling loss in tapered double-cladding fibers.

A tapered fiber with a depressed-index outer ring is fabricated and dispersion engineered to generate a widely tunable (1250-1650 nm) fundamental-mode leakage loss with a high cutoff slope (-1.2 dB/nm) and a high attenuation for stop band (>50 dB) by modification of both waveguide and material dispersions. The higher cutoff slope is achieved with a larger cross angle between the two refractive index dispersion curves of the tapered fiber and surrounding optical liquids through the use of depressed-index outer ring structures in double-cladding fibers.

Widely tunable asymmetric long-period fiber grating with high sensitivity using optical polymer on laser-ablated cladding.

We demonstrate high-efficiency, wideband-tunable, laser-ablated long-period fiber gratings that use an optical polymer overlay. Portions of the fiber cladding are periodically removed by CO(2) laser pulses to induce periodic index changes for coupling the core mode into cladding modes. An optical polymer with a high thermo-optic coefficient with a dispersion distinct from that of silica is used on a deep-ablated cladding structure so that the effective indices of cladding modes become dispersive and the resonant wavelengths can be efficiently tuned. The tuning efficiency can be as high as 15.8 nm/ degrees C, and the tuning range can be wider than 105 nm (1545-1650 nm).