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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.

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

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.

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.

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.

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.

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.

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 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 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.

An all-fiber multiwavelength Tm-doped laser assisted by four-wave mixing (FWM) in highly Germania-doped highly nonlinear fiber (HG-HNLF) has been experimentally demonstrated. Benefiting from the high nonlinearity of the HG-HNLF, intensity-dependent gain caused by FWM is introduced into the laser cavity to mitigate the gain competition in Tm-doped fiber. Thanks to a 50-m HG-HNLF, 9, 22, and 36 lasing lines with considering 10-dB, 20-dB, and 30-dB bandwidth, respectively is obtained at room temperature with wavelength spacing of 0.86 nm. More than 30-nm broad-band lasing can be obtained. The stability of the proposed fiber laser has also been studied. Repeat measurements show the power fluctuations and wavelength drifts of the lasing lines are less than 1.6 dB and 0.05 nm, respectively. The laser performances without the assistance of HG-HNLF have fewer center wavelengths lasing, which indicates that FWM in HG-HNLF plays an important role for the multiwavelength laser operation.

The conversion efficiency of third harmonic generation (THG) from mid-IR (3600 nm) to near-IR (1200 nm) regions in a silicon-silicon-nanocrystal hybrid plasmonic waveguide (SSHPW) was calculated. The required modal phase-matching condition (PMC) between the 0-th mode at fundamental wave (FW) and the 2-nd mode at third harmonic (TH) is achieved by carefully designing the waveguide geometry. Benefiting from the hybridized surface plasmon polariton (SPP) nature of the two guided modes, the SSHPW is capable of achieving both high THG nonlinear coefficient I6 and reasonable linear propagation loss, thereby resulting in large figure-of-merits (FOMs) for both FW and TH. According to our simulation, THG conversion efficiency up to 0.823% is achieved at 62.9 𝜇????m SSHPW with pump power of 1 W. Accepted version