Strong Evanescent Field Research Articles

All-fiber-optic temperature sensor based on reduced graphene oxide

We demonstrate a novel all-fiber-optic temperature sensor based on a reduced graphene oxide (rGO) film coated onto a side-polished fiber (SPF). Significantly enhanced interaction between the propagating light and the rGO film can be obtained via strong evanescent field of the SPF. The strong light–graphene interaction results in temperature sensing with a maximum optical power variation of 11.3 dB in the SPF experimentally. The novel temperature fiber sensor has a linear correlation coefficient of 99.4%, a sensitivity of 0.134 dB °C−1, a precision of better than 0.03 °C, and a response speed of better than 0.0228 °C s−1. Such an rGO-based all-fiber-optic temperature sensor is easy to fabricate, is compatible with fiber-optic systems, and possesses high potentiality in photonics applications such as all-fiber-optic temperature sensing networks.

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 1.8×104dB/RIU, a wavelength shift sensitivity of 180nm/RIU, and a limit of detection of 2×10−5RIU. 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. @ 2013 Elsevier B.V. All rights reserved.

Optical microfibers and nanofibers

AbstractAs a combination of fiber optics and nanotechnology, optical microfibers and nanofibers (MNFs) have been emerging as a novel platform for exploring fiber-optic technology on the micro/nanoscale. Typically, MNFs taper drawn from glass optical fibers or bulk glasses show excellent surface smoothness, high homogeneity in diameter and integrity, which bestows these tiny optical fibers with low waveguiding losses and outstanding mechanical properties. Benefitting from their wavelength- or sub-wavelength-scale transverse dimensions, waveguiding MNFs exhibit a number of interesting properties, including tight optical confinement, strong evanescent fields, evident surface field enhancement and large and abnormal waveguide dispersion, which makes them ideal nanowaveguides for coherently manipulating light, and connecting fiber optics with near-field optics, nonlinear optics, plasmonics, quantum optics and optomechanics on the wavelength- or sub-wavelength scale. Based on optical MNFs, a variety of technological applications, ranging from passive micro-couplers and resonators, to active devices such as lasers and optical sensors, have been reported in recent years. This review is intended to provide an up-to-date introduction to the fabrication, characterization and applications of optical MNFs, with emphasis on recent progress in our research group. Starting from a brief introduction of fabrication techniques for physical drawing glass MNFs in Section 2, we summarize MNF optics including waveguiding modes, evanescent coupling, and bending loss of MNFs in Section 3. In Section 4, starting from a “MNF tree” that summarizes the applications of MNFs into 5 categories (waveguide & near field optics, nonlinear optics, plasmonics, quantum & atom optics, optomechanics), we go to details of typical technological applications of MNFs, including optical couplers, interferometers, gratings, resonators, lasers and sensors. Finally in Section 5 we present a brief summary of optical MNFs regarding their current challenges and future opportunities.

Guided-mode resonance enhanced excitation and extraction of two-photon photoluminescence in a resonant waveguide grating

Guided-mode resonances enhanced excitation and extraction of two-photon photoluminescence (TPP) is demonstrated with a one-dimensional resonant waveguide grating (RWG) with a layer of fluorescent polymer (polyfluorene, PFO) on top. In this work, we design and fabricate a PFO RWG, in which two dispersive resonant modes in TE-polarization were measured. By aligning the red-shifting resonant mode with excitation wavelength in the infrared range, and the blue-shifting resonant mode with TPP spectrum in the visible range, the intensity of TPP can be enhanced up to 300-fold compared with that from a flat film with the same thickness coated on a glass slide. Such high enhancement results from firstly the strong evanescent local field in the waveguide layer due to the resonance between the incident light and the waveguide structure according to the results of rigorous coupled-wave analysis calculation, and secondly the enhanced extraction of the emission light which also resonates with the waveguide structure.

All-Fiber Modal Interferometer Based on a Joint-Taper-Joint Fiber Structure for Refractive Index Sensing With High Sensitivity

We propose and demonstrate an all-fiber modal interferometer (MI) formed by sandwiching a thinned fiber taper (TFT) between two core-offset joints (COJs) for refractive index sensing. The COJs are fabricated using the splicing method on a standard single-mode fiber and their core-offset distances are over 10 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> . The TFT is made by the flame-heated drawing technology, and has a very thin waist with only several microns in diameter for which satisfies the core-mode cutoff condition. The all-fiber MI device with a TFT waist diameter of 6.8 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm m}$</tex></formula> has extremely high sensitivities because of the strong evanescent field interaction, for example, 5339 and 10377 nm/refractive index unit at the surrounding refractive indexes of 1.334 and 1.35, respectively. In addition, the device has the advantages of compact size, simple fabrication, and low cost.

Broadband all-optical modulation using a graphene-covered-microfiber

We demonstrate all-optical modulation based on ultrafast saturable absorption in graphene-covered-microfiber. By covering the microfiber surface with polydimethylsiloxane supported graphene film along the fiber length, a greatly enhanced interaction between the propagating light and the graphene can be obtained via the strong evanescent field of the microfiber. The strong light–graphene interaction results in high-speed, broadband all-optical modulation with maximum modulation depths of 5 dB and 13 dB for single-layer and bi-layer graphene, respectively. Such a graphene all-optical modulator is easy to fabricate, is compatible with optical fiber systems and has high potential in photonics applications such as all-optical switching and all-optical communications.

Design of nanofibres for efficient stimulated Raman scattering in the evanescent field

Nanofibres can be produced with diameters smaller than the wavelength of the light they guide. In this regime, the guided mode presents a strong evanescent field well adapted to the excitation of “evanescent nonlinearities”. We theoretically investigate such an evanescent nonlinearity: the Raman interaction between the evanescent field and a liquid surrounding the nanofibre. Our calculations demonstrate that the Raman conversion is obtained with nanofibre lengths an order of magnitude lower than those required for liquid core photonic crystal fibres.

Surface WGM sensor based on a cylindrical dielectric waveguide

The dispersion equation of a layered cylindrical dielectric waveguide operating in the whispering gallery mode (WGM) is derived. We demonstrate that when the waveguide is coated with a negative permittivity layer, a surface WGM can be generated. It exposes a strong evanescent field to the ambient substance, rendering the negative permittivity material assisted microring sensor much more sensitive to a slight change in surrounding medium. This opens up an avenue towards designing optical WGM sensors with superior performance.

Evanescent-Light Deposition of Graphene Onto Tapered Fibers for Passive Q-Switch and Mode-Locker

We demonstrate the fabrication of graphene-deposited tapered fibers (GDTFs), which can be used as saturable absorbers (SAs) for pulsed lasers. The advantages of GDTF SAs include flexibility, all-fiber configuration, and high optical damage threshold. The fabrication process is based on the interaction of the evanescent field of a tapered fiber with graphene. By in situ monitoring the transmitted power, the deposition process can be controlled, and the GDTF with a desirable level of nonsaturable absorption loss can be fabricated. We also study the dynamic deposition process by employing different waist diameters of tapered fibers and the different deposition powers. The results show that the deposition time can be significantly shortened with stronger evanescent field by decreasing the taper diameter or increasing the deposition power. Furthermore, by exploiting the GDTF as an intracavity passive power modulating element, we demonstrate efficient Q-switched and mode-locked erbium-doped fiber lasers, respectively.

Highly Sensitive In-Fiber Refractive Index Sensor Based on Down-Bitaper Seeded Up-Bitaper Pair

We report a novel highly sensitive in-fiber refractive index (RI) sensor based on the Mach-Zehnder interferometer (MZI). This sensor is composed of an up-fusion-bitaper (UFBT) pair and an embedded down-stretching-bitaper (DSBT), where the UFBT excited high-order cladding modes and the DSBT enhanced the evanescent field. By employing the interaction between the strong evanescent field of high-order cladding mode and ambient environment, an RI sensitivity of 86.565 nm/cm/RIU is achieved in the RI range from 1.3332 to 1.4140. This sensitivity is about an order of magnitude higher than that of the conventional and taper-based improved in-fiber MZIs.

Multiwatt octave-spanning supercontinuum generation in multicore photonic-crystal fiber

High-power supercontinuum spanning over more than an octave was generated using a high power femtosecond fiber laser amplifier and a multicore nonlinear photonic crystal fiber (PCF). Long multicore PCFs (as long as 20 m in our experiments) are shown to enable supercontinuum generation in an isolated fundamental supermode, with the manifold of other PCF modes suppressed due to the strong evanescent fields coupling between the cores, providing a robust 5.4 W coherent supercontinuum output with a high spatial and spectral quality within the range of wavelengths from 500 to 1700 nm.

Passively Mode-Locked Fiber Laser Based on Reduced Graphene Oxide on Microfiber for Ultra-Wide-Band Doublet Pulse Generation

Graphene with zero-band gap can be easily saturated under strong excitation due to Pauli blocking. In this work, the graphene oxide membrane is reduced on the surface of microfiber by use of high temperature heating. Such reduced graphene oxide can interact with the strong evanescent field of the microfiber and enable saturable absorption in passively mode-locked fiber laser. The system can be effectively utilized to directly generate ultra-wide-band doublet pulses for high-capacity wireless communication applications.

Experimental confirmation of strong fluorescence enhancement using one-dimensional GaP/SiO_2 photonic band gap structure

In this paper we report the experimental confirmation of the fluorescence enhancement effect using a one-dimensional photonic band gap (1D PBG) structure. This 1D PBG structure consists of periodic multilayer thin films with gallium phosphide (GaP) and silicon dioxide (SiO2) as the alternating high and low index materials. Strong evanescent field enhancement can be generated at the last interface due to the combination of total internal reflection and photonic crystal resonance for the excitation wavelength. In addition, the 1D PBG structure is designed as an omnidirectional reflector for the red-shifted fluorescent signal emitted from the surface bounded molecules. This omnidirectional reflection function helps to improve the collection efficiency of the objective lens and further increase the detected fluorescent signal. Compared with the commonly used bare glass substrate, an average enhancement factor of 69 times has been experimentally verified with quantum dots as the fluorescent markers. This fluorescence enhancer may find broad applications in single molecular optical sensing and imaging.

Characteristics of Near-Surface-Core Optical Fibers

A near-surface-core optical fiber is proposed that consists of a cladding and a circular or an elliptical core close to the surface of the cladding. The finite element method (FEM) is utilized to analyze characteristics of the near-surface-core optical fiber, including the mode characteristic, birefringence, and evanescent field. The results reveal that the near-surface-core optical fiber has polarization- preserving properties and a non-zero cut-off frequency for the fundamental mode. The near-surface-core optical fiber has a strong evanescent field due to a small distance between the core and the ambient medium and is a promising candidate for evanescent wave sensors. The near-surface-core optical fiber has an interesting phenomenon of surface-enhanced optical effect, which is similar to that of the nanofiber.

Characteristics of embedded-core hollow optical fiber

We propose a novel embedded-core hollow optical fiber composed of a central air hole, a semi-elliptical core, and an annular cladding. The fiber characteristics are investigated based on the finite element method (FEM), including mode properties, birefringence, confinement loss, evanescent field and bending loss. The results reveal that the embedded-core hollow optical fiber has a non-zero cut-off frequency for the fundamental mode. The birefringence of the hollow optical fiber is insensitive to the size of the central air hole and ultra-sensitive to the thickness of the cladding between the core and the air hole. Both thin cladding between the core and the air hole and small core ellipticity lead to high birefringence. An ultra-low birefringence fiber can be achieved by selecting a proper ellipticity of the core. The embedded-core hollow optical fiber holds a strong evanescent field due to special structure of thin cladding and therefore it is of importance for potential applications such as gas and biochemical sensors. The bending losses are measured experimentally. The bending loss strongly depends on bending orientations of the fiber. The proposed fiber can be used as polarization interference devices if the orientation angle of the fiber core is neither 0° nor 90°.

Plasmonic enhanced fluorescence spectroscopy using side-polished microstructured optical fiber

We report excitation of surface plasmon in a gold-coated side-polished D-shape microstructure optical fiber (MOF). As the leaky evanescent field from the fiber core becomes highly localized by the plasmon wave, its intensity also gets amplified significantly. Here we demonstrate an efficient use of this intensified field as excitation in fluorescence spectroscopy. The so-called plasmonic enhanced fluorescence emission from Rhodamine B has been investigated experimentally. First, plasmonic effect alone was found to provide an immediate fluorescence enhancement factor of two. Second, experimental results show a good agreement with theoretical modeling. Strong evanescent field generation and surface enhancement with simple metallic coating makes this fiber based device a good candidate for compact fluorescence spectroscopy.

Modeling and analysis of localized biosensing and index sensing by introducing effective phase shift in microfiber Bragg grating (µFBG)

We report a novel micro-fiber Bragg grating (µFBG) sensor that takes advantage of the degeneracy of stop-band and rapid emergence of spectral modes when an effective phase shift occurs. The phase shift can be enabled by a range of perturbations in a central segment of the grating, including monolayer immobilization of bio-molecules or change in refractive index in the surrounding, thereby constituting the possibility of a highly sensitive sensor with the merit of scalable performance. The use of µFBG ensures strong evanescent field coupling to the surrounding in order to maximize signal transduction. Simulation results indicate very favorable sensor signal characteristics such as large wavelength shift and sharp reflection dips. A general relation between the peak position within the stop-band and the amount of effective phase shift is also provided, and may generally serve as helpful guideline for FBG sensor design. A typical µFBG sensor device may detect surface protein/DNA adsorption with limit-of-detection (LOD) as low as 3.3 pg.mm(-2) for surface mass density and 51.8 fg for total mass. For refractive index (RI) sensing, the LOD is 2.5×10(-6) refractive index unit (RIU).

A half wave retarder made of bilayer subwavelength metallic apertures

We demonstrate a half wave plate whose principle of operation is based on the strong evanescent field coupling between two metal layers with arrays of subwavelength slits. The device is divided into two kinds of pixels in which the slits are oriented in orthogonal directions. By tuning the phase delay of the transmitted light through the lateral displacement between the top and bottom layers, the polarization of linearly polarized light at 1.55 μm can be rotated by up to 90°. The polarization extinction ratio of the transmitted light exceeds 22 dB.

Fluorescence-Based Aluminum Ion Sensing Using a Surface-Functionalized Microstructured Optical Fiber

The first microstructured optical fiber-based sensor platform for aluminum ions using a surface-attached derivative of lumogallion (3), a known fluorescence-based indicator, has been fabricated. These fibers allow for strong evanescent field interactions with the surrounding media because of the small core size while also providing the potential for real-time and distributed measurements. The fluorescence response to aluminum ions was first demonstrated by applying the procedure to glass slides. This was achieved through the covalent attachment of the fluorophore to a polyelectrolyte-coated glass surface and then to the internal holes of a suspended-core microstructured optical fiber to give an effective aluminum sensor. Whereas the sensor platform reported is fabricated for aluminum, the approach is versatile, with applicability to the detection of other ions.

Stable low-loss optical nanofibres embedded in hydrophobic aerogel

Nanofibres, optical fibres narrower than the wavelength of light, degrade in hours on exposure to air. We show that encapsulation in hydrophobic silica aerogel (refractive index 1.05) provides protection and stability (over 2 months) without sacrificing low attenuation, strong confinement and accessible evanescent field. The measured attenuation was <0.03 dB/mm, over 10 × lower than reported with other encapsulants. This enables many nanofibre applications based on their extreme small size and strong external evanescent field, such as optical sensors, nonlinear optics, nanofibre circuits and high-Q resonators. The aerogel is more than a waterproof box, it is a completely-compatible gas-permeable material in intimate contact with the nanofibre and hydrophobic on both the macroscopic and molecular scales. Its benefits are illustrated by experiments on gas sensing (exploiting the aerogel's porosity) and supercontinuum generation (exploiting its ultra-low index).