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Long-range surface plasmon resonance (LRSPR), generated from a coupled plasmon polariton in a thin metal slab sandwiched by two dielectrics, has attracted more and more attention due to its merits, such as longer propagation and deeper penetration than conventional single-interface surface plasmon resonance. Many useful applications related to light–medium interaction have been demonstrated based on the LRSPR effect, especially in the sensing area. Here, we propose and demonstrate an LRSPR-based refractive index sensor by using a SiO2-Au-TiO2 heterostructure, in which a D-shaped honeycomb-microstructure optical fiber (MOF) is designed as the silica substrate and then deposited with a gold film and thin-layer titanium dioxide (TiO2). By using the full-vector finite-element method (FEM), this heterostructure is numerically investigated and demonstrated to excite LRSPR without a buffer layer, which is usually necessary in previous LRSPR devices. Through comprehensive discussion about the influence of structural parameters on the resonant wavelength, the excitation of the LRSPR in the proposed heterostructure is revealed to be highly related to the effective refractive index of MOF’s fundamental core mode, which is mainly determined by the MOF’s pitch, the thicknesses of the silica web and the planar-layer silica. Moreover, the thin-layer TiO2 plays an important role in significantly enhancing the resonance and the sensitivity to analyte’s refractive index as well, when it is coated on the top of the Au film rather than between the metal and waveguide. Finally, the proposed LRSPR sensor based on SiO2-Au-TiO2 heterostructure shows an ultra-high wavelength sensitivity of 20,100 nm/RIU and the corresponding minimum resolution is as low as 4.98×10−7 RIU. Thus, the proposed LRSPR device offers considerable potential for sensing applications in biomedical and biochemical areas.

In this paper, we propose a metal-semiconductor-metal slot waveguide (MSMSW) for producing mid-infrared (MIR) light from 10.2 μm by means of third harmonic generation (THG). In this waveguide, the required interaction length is limited by the large Ohmic loss, and therefore, perfect phase-matching condition (PMC) which ensures long coherent length is not necessary. Without the limitation of perfect PMC, the THG interaction mainly occurs between two fundamental modes at pump and harmonic wavelength, respectively leading to high nonlinear coefficient. According to our simulation, the conversion up to ~ 9.1 × 10−4 can be achieved within micron-scale waveguide under 1 W pump power. Furthermore, the waveguide demonstrates both large fabrication error and wavelength shift tolerance. The proposed waveguide can be a promising candidate for MIR light generation with large integration capability.

Long-range surface plasmon resonances (LRSPRs) are featured with longer propagation and deeper penetration, compared with conventional surface plasmon resonances (SPRs). Thus, LRSPR-based fiber sensors are considered to have great potential for highly sensitive detection in chemistry or biomedicine areas. Here, we propose and demonstrate a near-infrared LRSPR sensor based on a D-shaped honeycomb microstructured optical fiber (MOF) directly coated with gold film. Although there is no additional heterogeneous buffer layer, the optical field of the long-range surface plasmon polariton (LRSPP) mode penetrates strongly into the analyte region. Thus the effective refractive index of the LRSPP mode depends highly on the analyte’s material refractive index and an abnormal dispersion relationship between the LRSPP mode and MOF’s y-polarized core mode is observed. The mechanism of the LRSPR excitation in the coupling zone is attributed to an avoided crossing effect between these two modes. It also results in the generation of a narrow-bandwidth peak in the loss spectrum of the core mode. Further discussion shows that the resonance wavelength is mainly determined by the core size that is contributed by the MOF’s cladding pitch, silica-web thickness and planar-layer-silica thickness together. It indicates that the operation wavelength of the proposed LRSPR device can be flexibly tuned in a broadband wavelength range, even longer than 2 µm, through appropriately designing the MOF’s structural parameters. Finally, the proposed LRSPR sensor shows the highest wavelength sensitivity of 14700 nm/RIU and highest figure of merit of 475 RIU−1 for the analyte refractive index range from 1.33 to 1.39.

We propose an internal asymmetric plasmonic slot waveguide (IAPSW) containing two different materials in the slot region for third harmonic generation (THG) from 2.25 μm. In the IAPSW, the required phase matching condition is satisfied between the 0th-order mode at the fundamental frequency and 1st-order mode at the third harmonic frequency. By choosing an appropriate slot geometry and materials, the third harmonic electric field distribution can be engineered to significantly enhance the nonlinear overlap coefficient for THG. According to our simulation, a conversion efficiency up to 0.67 % with 1 W pump power is predicted within a ∼10 μm IAPSW. Additionally, the waveguide shows large fabrication tolerance in terms of geometry parameters. The proposed waveguide can find potential applications for high-speed all-optical signal processing.

We studied the measurement and data analysis of modes effective refractive indexes for few-mode optical fiber and found that measuring refractive index difference from the modes interference pattern was affected by dispersion in the optical fiber. A comprehensive method of accurate measurement of modes effective refractive index differences in few-mode optical fiber was developed. It consists of the measurements of the FBG reflection spectrum and the modes interference spectrum, the simulation of interference with dispersion effect in the interferometer configuration, and the data optimization to match with the measured modes interference. The results show much improvement in the few-mode optical fiber characterization. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Phase-matching between fundamental plasmonic mode and photonic mode for third harmonic and triplet photon generation is achieved in a dielectric-loaded waveguide. A THG efficiency of 10-4 and a TPG rate of 28 Hz are predicted.

On-chip third harmonic generation (THG) and its inverse process, namely the generation of entangled triplet photons with Greenberger-Horne-Zeilinger (GHZ) state, have attracted much interest in the last decade. To enhance the conversion efficiency or photon creation rate in optical waveguides, phase matching is a crucial condition which must be satisfied. However, material dispersion usually prevents the phase matching condition (PMC) between input pump and generated radiation when they are both guided in the fundamental modes. In this paper, a dielectric-loaded waveguide based on fundamental mode photon-plasmon coupling is proposed for efficient THG and triplet photon generation (TPG). Leveraging on the unique dispersion properties of transparent conductive oxide, the third harmonic radiation can be guided by the conventional photonic mode in the near-infrared, while the pump frequency is confined by surface plasmon polariton (SPP) mode in the mid-infrared. According to our simulations, the THG efficiency and TPG rate can achieve ~10-4 and 32 Hz, respectively. The proposed waveguide can be a promising platform for all-optical and quantum signal processing.

Helical-structured fiber devices has attracted increasing attention due to their very promising applications in both communication and sensing. In this paper, we will present our recent works on sensors based on helical-structured seven-core fiber, including the principle, fabrication and as well as their performance of ultra-sensitive strain and directional torsion measurements.

We propose and experimentally demonstrate a directional torsion sensor based on a Mach-Zehnder interferometer formed in a multicore fiber (MCF) with a ~570-μm-long helical structure (HS). The HS was fabricated into the MCF by simply pre-twisting and then heating with a CO2 laser splicing system. This device shows the capability of directional torsion measurement from −17.094 rad/m to 15.669 rad/m with the sensitivity of ~0.118 nm/(rad/m). Moreover, since the multiple interferences respond differently to torsion and temperature simultaneously, the temperature cross-sensitivity of the proposed sensor can be eliminated effectively. Besides, the sensor owns other merits such as easy fabrication and good mechanical robustness. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

Spatially arrayed long period gratings (LPGs) are inscribed in a multicore fiber by CO2 laser. The LPGs in the center core and one outer core exhibit different responses to bending and temperature. The directional bending sensor based on the LPGs has large sensitivity even for ultra-low curvature values ranging from 0 to 0.359 m−1.

Using laser ablation method, we taper photonic crystal fiber (PCF) with keeping its inner structure well and then we achieved the TPG where the phase matching point PW is 512nm and THG is 1536nm with effective refractive index is 1.1314. Finally we calculate the evolution of the conversion efficiency as a function of the fiber length.

We fabricate a helical long period grating (HLPG) in a multicore fiber. A fiber torsion sensor with directional discrimination based on the HLPG is experimentally demonstrated. The resonant dips offer the ability of simultaneously measurement of torsion and temperature.

Optical fiber sensors for strain measurement have been playing important roles in structural health monitoring for buildings, tunnels, pipelines, aircrafts, and so on. A highly sensitive strain sensor based on helical structures (HSs) assisted Mach-Zehnder interference in an all-solid heterogeneous multicore fiber (MCF) is proposed and experimentally demonstrated. Due to the HSs, a maximum strain sensitivity as high as −61.8 pm/με was experimentally achieved. This is the highest sensitivity among interferometer-based strain sensors reported so far, to the best of our knowledge. Moreover, the proposed sensor has the ability to discriminate axial strain and temperature, and offers several advantages such as repeatability of fabrication, robust structure and compact size, which further benefits its practical sensing applications. MOE (Min. of Education, S’pore) Published version

A compact refractive index (RI) sensing probe with controllable sensitivities based on a θ-shaped microfiber resonator and Vernier effect is reported. By cascading the θ-shaped microfiber resonator with a fiber Fabry-Perot interferometer, Vernier effect is generated to enhance the RI sensitivity. Both theoretical analyses and experimental results demonstrate that the RI sensitivity can be tuned by changing the cavity length of the θ-shaped microfiber resonator. The RI sensitivity is widely tuned from 311.77nm/RIU to 2460.07nm/RIU in the experiment. The θ-shaped microfiber resonator and the proposed method of generating Vernier effect could find important applications in optical fiber sensing field. MOE (Min. of Education, S’pore) Published version

We theoretically propose an asymmetric plasmonic slot waveguide (APSW) with a thin silicon nanocrystal (Si-nc) layer and a thinner silicon layer that fills the bottom and the top of the metallic slot region to increase third-harmonic generation (THG) efficiency. The needed modal phase-matching condition is satisfied with properly mode dispersion engineering by exploiting the waveguide geometrical parameters. Combining the asymmetric waveguide structure and the high third-order susceptibility of the nonlinear materials, efficient phase-matched THG from mid-infrared (IR) to near-IR regions is realized. Then, the THG performance was further improved by increasing the silicon slot width wherein the THG nonlinear coefficient as large as 21452 $\hbox{m}^{-1} \cdot \hbox{W}^{-1}$ is achieved. The corresponding THG efficiency comes up to 0.5% at a waveguide length of 9.3 $\mu\hbox{m}$ with a pump power of 1 W.

An all-fiber high-sensitivity temperature sensor based on highly Germania-doped few-mode fiber (HGFMF) is presented. The sensor employs Mach–Zenhder interferometer configuration and fiber mode LP01 and LP11 are excited in HGFMF. Temperature sensitivity higher than 97 pm/°C is demonstrated in experiment.

We propose the asymmetrical plasmonic slot waveguide (APSW) design for third-harmonic generation (THG) from 2.25 $\mu\hbox{m}$ . In this configuration, the phase-matching condition is fulfilled between the zeroth-order mode at fundamental frequency (FF) and the first-order mode at third-harmonic frequency (THF). Due to the asymmetrical geometry, the mode overlap between the two involved modes is significantly enhanced, leading to an efficient THG process. According to the numerical calculation, the conversion efficiency is predicted up to 1.4% with 1-W pump power. The proposed APSW has the potential to realize an integrated efficient THG device in nanometer scale.

We experimentally demonstrate a highly sensitive strain sensor based on a helical structure in a multicore fiber. The maximum sensitivity is about −61.13 pm/μ∊. The sensor has the capability of discriminating strain and temperature.

An all-fiber torsion sensor based on a helical structure in a multicore fiber is demonstrated. The sensor exhibits the ability of discriminating torsion direction, torsion angle and temperature. MOE (Min. of Education, S’pore) Published version

International audience; We experimentally demonstrate a Type I grating effect in a lanthanum-aluminum silicate fiber for the first time, to the best of our knowledge. This grating shows a temperature sensitivity ~8.71 pm/oC, which is lower than that of FBG in standard single-mode fiber.

We provide a comprehensive study on the efficient third harmonic generation (THG) in a lossy metal-hybrid-metal asymmetric plasmonic slot waveguide (MHM) to develop a method for efficient THG by focusing on the modal phase-matching condition (PMC), the third-order nonlinear susceptibility of the nonlinear interactive material, and the pump-harmonic modal overlap in conjunction with reasonable linear propagation loss. In addition to the PMC and the nonlinear material, the stimulated THG process can be greatly enhanced by the large pump-harmonic modal overlap. With 1 W pump power, simulation results present that THG conversion efficiency up to 2.79 × 10−4 within 4.5 𝜇m MHM can be achieved.

Intermodally phase matched up- and down-conversion processes based on the third order nonlinearity have been proposed to efficiently generate light in the UV and mid-IR wavelength regions in solid core silica optical fibers and optical microfibers. We study waveguide parameters and practical considerations required for optimum conversion.

We study third-harmonic generation (THG) in parallel-coupled waveguides where the spatial modulation of the mode intensity provides quasi-phase matching, called coupling-length phase matching (CLPM), for efficient nonlinear frequency conversion. Different types of CLPM are investigated for THG, and it is found that two sets of CLPM conditions can be practically implemented with traditional waveguides. These two CLPM conditions are further investigated by considering nonlinear phase modulations, which can degrade the CLPM-based THG conversion. However, up to 45% efficiency is still possible in this scheme. The greatest significance of this approach is that the requirement of perfect phase matching in a single waveguide is no longer necessary, leading to an alternative waveguide design for THG.

An all-fiber multiwavelength Tm-doped laser based on four-wave mixing in highly-germania-doped-highly-nonlinear-fiber (HG-HNLF) has been demonstrated. By using 50-m HG-HNLF to provide intensity-depended gain, 36 lasing lines with wavelength spacing of 0.86 nm within 30-dB bandwidth is obtained.

Recently NTU built up a new fibre fabrication system, benefit from which we will have the capability to fabricate most of our designed specialty fibresIn this talk, we will introduce the recent progresses in the area of specialty fibre design and application achieved in our group.

We report on a curvature sensor based on a long-period grating in a dual-concentric-core fiber. By measuring the relative shifting of two adjacent resonant dips, the curvature sensitivity reaches 7.635 nm/m−1.

We theoretically investigate three photon generation with our fabricated highly Germania-doped fiber with center index dip wherein the efficiency can reach up to 9% which shows an improvement compared with the step-index fiber.

We theoretically study one-third harmonic generation (OTHG) in silica-germanate fibers wherein the efficiency, enhanced by pump attenuation, can reach up to 31% conversion in 34-meter-long fiber, which is 5 times that of the lossless OTHG.

Intermodally phase matched up- and down-conversion processes based on the third order nonlinearity can be used to efficiently generate UV and mid-IR wavelength regions in solid core silica optical fibers and optical fiber tapers. We theoretically study the waveguide parameters and practical considerations required for optimum conversion. Published version

We report on a theoretical and experimental investigation to demonstrate that the birefringence of a commercial polarization-maintaining photonic crystal fiber (PMPCF) can be enhanced almost one order of magnitude by scaling its diameter down to ~50um.

Synaptotagmin I is a Ca2+-binding protein of synaptic vesicles that serves as a Ca2+ sensor for neurotransmitter release and was the first member found of a large family of trafficking proteins. We have now identified a novel synaptotagmin, synaptotagmin XI, that is highly expressed in brain and at lower levels in other tissues. Like other synaptotagmins, synaptotagmin XI has a single transmembrane region and two cytoplasmic C2-domains but is most closely related to synaptotagmin IV with which it forms a new subclass of synaptotagmins. The first C2-domain of synaptotagmin I (the C2A-domain) binds phospholipids as a function of Ca2+ and contains a Ca2+-binding site, the C2-motif, that binds at least two Ca2+ ions via five aspartate residues and is conserved in most C2-domains (Shao, X., Davletov, B., Sutton, B., Sudhof, T. C., Rizo, J. R. (1996) Science 273, 248–253). In the C2A-domains of synaptotagmins IV and XI, however, one of the five Ca2+-binding aspartates in the C2-motif is substituted for a serine, suggesting that these C2-domains do not bind Ca2+. To test this, we produced recombinant C2A-domains from synaptotagmins IV and XI with either wild type serine or mutant aspartate in the C2-motif. Circular dichroism showed that Ca2+stabilizes both mutant but not wild type C2-domains against temperature-induced denaturation, indicating that the mutations restore Ca2+-binding to the wild type C2-domains. Furthermore, wild type C2A-domains of synaptotagmins IV and XI exhibited no Ca2+-dependent phospholipid binding, whereas mutant C2A-domains bound phospholipids as a function of Ca2+ similarly to wild type synaptotagmin I. These experiments suggest that a class of synaptotagmins was selected during evolution in which the Ca2+-binding site of the C2A-domain was inactivated by a single point mutation. Thus, synaptotagmins must have Ca2+-independent functions as well as Ca2+-dependent functions that are selectively maintained in distinct members of this gene family.

Synaptotagmin I is a synaptic vesicle protein that is thought to act as a Ca2+ sensor in neurotransmitter release. The first C2 domain of synaptotagmin I (C2A domain) contains a bipartite Ca2+-binding motif and interacts in a Ca2+-dependent manner with syntaxin, a central component of the membrane fusion complex. Analysis by nuclear magnetic resonance spectroscopy and site-directed mutagenesis shows that this interaction is mediated by the cooperative action of basic residues surrounding the Ca2+-binding sites of the C2A domain and is driven by a change in the electrostatic potential of the C2A domain induced by Ca2+ binding. A model is proposed whereby synaptotagmin acts as an electrostatic switch in Ca2+-triggered synaptic vesicle exocytosis, promoting a structural rearrangement in the fusion machinery that is effected by its interaction with syntaxin.

We report on the fabrication of a fiber Bragg grating (FBG) with multiple resonances in a two-dimensional waveguide array microstructured optical fiber containing 91 cores. Theoretical investigation reveals that these resonances originate from the identical and nonidentical mode couplings between forward-propagating and backward-propagating LP0m-like (m=1, 2, 3; LP refers to linearly polarized) supermodes. Since both the central wavelength and minimum transmission of these resonant dips respond differently to curvature and axial strain, this FBG can be applied in the simultaneous measurement of curvature and axial strain. Published version

We provide a comprehensive study on one-third harmonic generation (OTHG) in highly Germania-doped fiber (HGDF) by analyzing the phase matching conditions for the step index-profile and optimizing the design parameters. For stimulated OTHG in HGDF, the process can be enhanced by fiber attenuation at the pump wavelength which dynamically compensates the accumulated phase-mismatch along the fiber. With 500 W pump and 35 W seed power, simulation results show that a 31% conversion efficiency, which is 4 times higher than the lossless OTHG process, can be achieved in 34 m of HGDF with 90 mol. % GeO2 doping in the core. Published version

We present a method for generating photon triplets in mid-infrared in a length of micro-fiber. By intermodal phase-matching, we show that the photon creation rate can achieve 1.6 triplets/s in 1-m fiber with 1-W pump power.

The crystal structure of the V alpha domain of a T cell antigen receptor (TCR) was determined at a resolution of 2.2 angstroms. This structure represents an immunoglobulin topology set different from those previously described. A switch in a polypeptide strand from one beta sheet to the other enables a pair of V alpha homodimers to pack together to form a tetramer, such that the homodimers are parallel to each other and all hypervariable loops face in one direction. On the basis of the observed mode of V alpha association, a model of an (alpha beta)2 TCR tetramer can be positioned relative to the major histocompatibility complex class II (alpha beta)2 tetramer with the third hypervariable loop of V alpha over the amino-terminal portion of the antigenic peptide and the corresponding loop of V beta over its carboxyl-terminal residues. TCR dimerization that is mediated by the alpha chain may contribute to the coupling of antigen recognition to signal transduction during T cell activation.

International audience; We report on a high-performance curvature sensor based on a long-period grating (LPG) in a dual-concentric-core fiber (DCCF). The LPG is inscribed to couple light from the fundamental mode of the central core to the ring-core modes, resulting in the generation of a series of resonant dips. Two adjacent dips shift toward each other when the LPG is bent. By monitoring the variation of the wavelength interval between these two dips, this LPG can be applied in curvature measurement with a sensitivity as high as −9.046 nm/m−1. More importantly, such a wavelength interval is almost immune to the cross impacts of temperature and axial strain, since the sensitivities to temperature and axial strain are only 2.6 pm/°C and 0.083 pm/με, respectively.

A maximum likelihood sequence detection (MLSD) receiver is used to detect data sequences in single-carrier coherent optical systems in the presence of laser phase noise. It requires no explicit phase estimation and involves only linear operations. It consistently shows improvement in the OSNR penalty (e.g., 1.1 dB at BER = 10−4 with memory length L =3) and the laser linewidth tolerance (e.g., around 4 times that of DAML at 1dB OSNR penalty at BER = 10−4 with memory length L =3) over the well-known DAML and Mth power approaches in laser phase noise (LPN)-impaired coherent optical systems.

We propose an output 2-queue optical buffer system to provide multi-class QoS in terms of throughput, loss and delay, that can be realized by implementing the state-of-art optical buffer technology in the near future. The proposed optical buffer system can be used at the output port of an optical router in an optical switching network (OPS). It resembles its electronic equivalent in terms of operational functionality. Analytical modeling derives the performance metrics and the numerical results are demonstrated.

We propose a realizable output optical buffer structure that resembles its electronic equivalent in terms of operational functionality. Analytical modeling is done to investigate the switch with this buffer structure, which can provide differentiate services.