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Most of the existing schemes for extracting the Brillouin frequency shift (BFS) are based on the line shape of the Brillouin gain spectrum (BGS) curve. However, in some circumstances, such as in this paper, there is a cyclic shift in the BGS curve, causing difficulty in obtaining the BFS accurately with traditional methods. To solve this problem, we propose a method for extracting Brillouin optical time domain analyzer sensing information in the transform domain—the fast Fourier Lorentz curve fitting method. It shows better performance especially when the cyclic start frequency is near the BGS central frequency position or when the full width at half maximum is large. The results show that our method can obtain BGS parameters more accurately in most cases than the Lorenz curve fitting method.

This paper presents a design and simulation study of a tapered few-mode fiber based broadband mode coupler for multimode optical coherence tomography in the O-band for better signal collection efficiency and improved penetration depth.

International audience; Specialty fibers have introduced new functionalities and opportunities in distributed fiber sensing applications. Particularly, Brillouin optical time domain analysis (BOTDA) systems have leveraged the unique features of specialty fibers to achieve performance enhancement in various sensing applications. This paper provides an overview of recent developments of the specialty fibers based BOTDA technologies and their sensing applications. The specialty fibers based BOTDA systems are categorized and reviewed based on the new features or performance enhancements. The prospects of using specialty fibers for BOTDA systems are discussed.

Surface plasmon waves are coupled electron–photon modes at the metal–dielectric interface. They can significantly enhance light–matter interactions that are favorable in many applications including nanophotonics, data storage, microscopy, solar cells, and sensing. Compared with the prism-based coupling configuration, optical fiber-based plasmonic devices offer more compact and robust configuration for exciting the plasmon modes. Photonic crystal fibers (PCFs) are a special class of optical fibers in which the presence of holey structures or periodic microstructures of refractive index modulations can provide a wide scope of flexibility in the control and engineering of the optical properties, thus opening up the potential for many new applications and scientific explorations. Notably, PCFs are a desirable platform to incorporate plasmonic structures for the excitation of surface plasmon resonance (SPR) and localized surface plasmon resonance (LSPR). Three main types of plasmonic PCF structures have been developed and reported in the literature, including metal nano-/microwire-filled plasmonic PCF, metal-coated plasmonic PCF, and nanoparticle-deposited/filled plasmonic PCF. This chapter provides a comprehensive review on the recent progress of these reported plasmonic PCF structures in terms of design and applications. Firstly, the operating principles based on surface plasmon polaritons and localized surface plasmon polaritons are presented. Secondly, the experimental studies of plasmonic PCF structures for various application areas are reviewed, including refractive index sensing, biosensing, temperature sensing, polarization, and birefringent devices. Lastly, design considerations and challenges are discussed.

We have designed a CNN model to detect Bragg wavelengths in overlapping spectra. The mean RMS error of 0.123pm and mean testing time of 12.4ms are achieved, which outperforms most of the existing techniques.

We proposed a temperature sensor by selectively infiltrating refractive index liquid (RIL) in the central airhole of a dual core photonic crystal fiber (PCF). The temperature dependent propagation characteristics of modes in the dual core PCF are analyzed numerically.

We numerically investigate the optical properties of three-dimensional (3D) microfiber/nanofiber (MNF) couplers and resonators, by using the finite-difference time domain method. It is the first time that the polarization-dependent transmission properties of 3D MNF couplers and resonators are investigated numerically. For the knot coupler, the twisted structure contributes to the inter-polarization coupling, and the envelope of intra- and inter-polarization coupling coefficients, respectively, has the trend as a $(\rm cosc)^{2}$ or $(\rm sinc)^{2}$ function. For the loop resonator, the inter-polarization coupling coefficients are relatively small. However, the coupling coefficients for both polarization states are discriminated obviously. The high $Q$ -factor resonance spectrum in one polarized state may be overwhelmed by that of the other. We also investigate the mechanism of resonance spectrum splitting, and find out that the inter-polarization coupling at the coupler region degenerates the resonance spectrum and gives rise to the splitting. The results provide guidance to design 3-D MNF devices, for example, high $Q$ -factor resonators.

We propose and demonstrate a novel plasmonic nanopore platform based on a bowtie-nanopore structure, for single-molecule sensing. In this nano-structure, nano-bowties are integrated with solid-state nanopores to provide localized surface plasmon resonances for signal enhancement. We design and optimize the nano-structure by tuning both the bowtie gap and the bowtie angle, and investigate their influences on field enhancement, thereby achieving single-molecule sensitivity. In addition, we study the field enhancement by introducing an engineered photonic nano-cavity. This further strengthens the electric enhancement. An overall Raman enhancement factor of 2×108 is achieved in our simulation. This is believed to be sufficient for single-molecule sensing. The proposed bowtie-nanopore structure can be multiplexed on a single substrate for simultaneous multi-channel detection, paving the way for demanding applications such as DNA sequencing.

A two-core photonic crystal fiber (TC-PCF) based highly-sensitive temperature sensor was proposed and demonstrated. By selectively infiltrating the central airhole with refractive index liquid (RIL), a three-parallel-waveguide structure was formed. A dual-component interference pattern, consisting of a large spectrum envelope and fine interference fringes, was observed in the transmission spectrum. The simulation results confirmed that the interference was arising from a few hybrid supermodes in the fiber coupler structure. They were in good agreement with the experimental observation on the discrete temperature windows with different temperature sensitivities due to couplings between different hybrid supermodes in respective temperature windows. By tracing the wavelength shifts of the large spectrum envelope, high sensitivities were achieved at 42.621 nm/°C in the temperature range from 54.2 °C to 55 °C and 32.159 nm/°C from 51.8 °C to 52.6 °C.

We report the investigation of a hybrid photonic crystal fiber (PCF) realized by filling liquid crystal to a twin-core PCF. Both the index-guiding and photonic bandgap guiding mechanism co-exists in the fiber, leading to interesting properties for sensing applications. This paper presents both simulation results of the hybrid PCF structure and shows preliminary experiment results of the temperature response of the hybrid PCF.

A simulation design was presented to optimise the model of long period grating based fibre devices. The simulation used a modified phase matching condition to better estimate the resonance wavelengths in the transmission profile. Experiments were conducted to verify the results obtained in the simulation. The experimental results matched closely with the simulations.

In this paper, for the first time Python was used to model the long period fibre grating devices and simulate the light propagating in the waveguide structures in order to design and optimize LPFG devices for sensing applications. It begin with resolving the effective refractive index of the LP01 core mode and different cladding modes, followed by calculating the mode electric and magnetic fields to determine the strength of coupling for core mode as well as between core mode and different cladding modes. After then, the transmission power spectrum within typical telecommunication wavelengths range was plotted. The spectrums and the wavelength shift under different surrounding refractive index was then plotted to analyze the SRI sensitivity of LPFG devices. Eventually, the results of Python and MATLAB will be compared for verification.

International audience; A side-channel photonic crystal fiber (SC-PCF) is proposed and fabricated to serve as a high sensitivity surface enhanced Raman scattering (SERS) sensing platform. The SC-PCF has a solid core with triangular lattice cladding structure, and one third of the cladding is removed intentionally to enable fast liquid infiltration and enlarge effective interaction area between liquid samples and the fiber core guided lightwave. The geometry of the SC-PCF is designed to obtain high evanescent field power in cladding side-channel, high free space coupling efficiency and low transmission loss at the excitation wavelength of 632.8 nm. Rhodamine 6G (R6G) solutions mixed with gold nanoparticles (AuNPs) are used to test the SERS performance of the proposed platform. A low detection limit of 50 fM R6G solution is achieved. Moreover, an accumulative effect of SERS signal along the SERS-active fiber length is demonstrated experimentally, and a numerical–analytical model for backward scattering scheme is developed to investigate the dependence of Raman intensity on fiber length.

We design a high-birefringent microstructured optical fiber for surface plasmon resonance sensor. Our proposed sensor provides birefringence as large as 4.2×10−4, which effectively reduces the measurement offset induced by polarization crosstalk.

Optical fiber based surface plasmon resonance (SPR) sensors are favored by their high sensitivity, compactness, remote and in situ sensing capabilities. Microstructured optical fibers (MOFs) possess microfluidic channels extended along the entire length right next to the fiber core, thereby enabling the infiltrated biochemical analyte to access the evanescent field of guided light. Since SPR can only be excited by the polarization vertical to metal surface, external perturbation could induce the polarization crosstalk in fiber core, thus leading to the instability of sensor output. Therefore for the first time we analyze how the large birefringence suppresses the impact of polarization crosstalk. We propose a high-birefringent MOF based SPR sensor with birefringence larger than 4 × 10−4 as well as easy infiltration of microfluidic analyte, while maintaining sensitivity as high as 3100 nm/RIU. MOE (Min. of Education, S’pore) Accepted version

A directional coupler structure formed by a nematic liquid crystal (NLC)-filled photonic crystal fiber (PCF) represents a promising configuration in sensing applications. Because of large refractive index difference between the NLC and silica material, the mode coupling between the NLC waveguide and the silica core is more complicated than the situation of coupling between two fundamental modes of the waveguides. Therefore, it is necessary to perform a theoretical investigation of the mode properties associated with the experimental studies of the coupling characteristics. In this paper, we present a thorough analysis, both theoretically and experimentally, of the directional coupler structure, including the mode properties, coupling characteristics, and thermal sensing properties. The temperature response of the device is experimentally measured, showing a polynomial curve in nematic phase and a linear curve in isotropic phase. The nonlinearity of the temperature response of the device in nematic phase and the linearity in isotropic phase are attributed to the temperature dependence of the refractive index of the NLC. Specifically, the sensitivity is -3.86 nm/°C in isotropic phase of the 6CHBT with good linearity and shows good agreement with simulation results.

We report on the fabrication and characterization of a highly sensitive temperature sensor by selectively filling the nematic liquid crystal (NLC) 6CHBT into a single void within the photonic crystal fiber (PCF) structure. The temperature response of the device is experimentally characterized, showing good linearity, repeatability, and sensitivity at around -3.90 nm/°C within the temperature range from 44°C to 53°C. The mode properties of the device are theoretically investigated, confirming the mode coupling principle and the temperature sensitivity of the device.

We report the fabrication of a highly sensitive refractometer based on photonic crystal fiber (PCF) interferometry. Using fusion arc splicing technique, a short length of PCF (∼3 mm) was spliced to conventional single-mode fibers on both sides. The voids of the PCF were completely collapsed in the splice region, which acted as mode splitter and combiner on both sides. Microbubbles were formed in the splice region and acted as diverging lens for exciting cladding modes efficiently. The transmission spectrum exhibited detectable “dips” for refractive index (RI) sensing. The bulk liquid sensitivity can reach up to 320 nm/RIU (RI unit) based on the linear curve fit for RI ranging from 1.33 to 1.34. Biosensing experiment was carried out using streptavidin and biotin as respective analyte and receptor. To the best of authors’ knowledge, this is the first time of reporting bioaffinity test on the external surface of PCF using modal interferometry.

In this paper, we report an optical fiber Sagnac interferometer (OFSI)-based temperature sensor constructed by a selectively filled polarization-maintaining photonic crystal fiber (PM-PCF). The transmission spectrum of the OFSI is in sinusoidal form and is sensitive to temperature. Simulation results predicted a higher sensitivity by selective filling than nonselective filling. In experiments, we used an extremely low-cost process to realize the selective filling. A sensitivity of 2.58 $\hbox{nm}/^{\circ}\hbox{C}$ was achieved with an 11.7-cm-long PM-PCF. The sensitivity dependence on the infiltration length ratio was also investigated.

We investigate the dispersion property of space filling mode of photonic crystal structures and find a new type of dispersion—structure induced dispersion. By incorporating this new source of dispersion we designed PCF with large normal dispersion ~ 350 ps 2 /km. Our simulation indicates the dispersion of such fiber changes less than 3% in 1.4–1.7 μm wavelength range and we also show that our design is insensitive to the structure changes.

A novel fiber design of large mode area (LMA) all-glass leakage channel fibers (LCF) with triangular core is proposed. The effective single mode behavior and bend characteristics of the fiber are numerically investigated. The designed LCFs can achieve LMA larger than 1400 μm 2 , effective single mode operation and low bend loss by carefully choosing the diameters of the fluorine-doped silica rods.

Theoretical analysis of propagation properties in a hollow-core Bragg fiber for bio-sensing applications has been demonstrated. Based on the Bragg fiber we propose a resonant sensor that operates on changes in refractive index of aqueous solution placed inside the hollow core. By using the transfer matrix method we analyzed the confinement loss of the TE01 mode in the hollow-core Bragg fiber. We have shown the influence of the fiber geometry on the changes in the confinement loss. Spectral sensitivity and resolution of the sensor are also presented.

An approximate analytical approach for revealing bandgap information for photonic band gap fibers with depressed index cladding rods is presented. The method considers isolated rods in cladding, with three layers structure: higher index in the core, lower-index depressed layer and silica cladding. Good consistency is found between the results of the approximate analytical method and those from accurate numerical simulations.

A hybrid photonic crystal fiber (PCF) is proposed and demonstrated for highly sensitive temperature measurement. The hybrid PCF is formed by selectively infiltrating liquid crystal 5CB into the first ring of air holes around one silica core of a twin-core PCF (TCPCF). In the modified TCPCF, one silica core guides light with a total internal reflection (TIR) mechanism, while the other silica core guides light through a photonic bandgap (PBG) mechanism, due to the infiltration of high refractive index 5CB. The co-existing of TIR and PBG waveguiding mechanisms in the TCPCF leads to a hybrid light guiding mechanism PCF, i.e. hybrid PCF. Theoretical analysis reveals that the TIR-guided core mode, PBG-guided core modes and 5CB channel modes in the hybrid PCF can concurrently propagate and interfere with each other, which is further verified by experimental results. Besides, experimental investigation on the temperature response of the hybrid PCF shows that the temperature sensitivities can be up to 4.91 nm °C−1 for nematic phase of 5CB, and −3.68 nm °C−1 for isotropic phase of 5CB, respectively. The proposed hybrid PCF is promising for applications of temperature-tunable optical filtering in optical fiber communication, sensing and laser systems. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore)

In this paper, we propose a novel photonic crystal fiber design to achieve high birefringence, high nonlinearity with a flattened dispersion profile. The air holes which are embedded in silica are arranged in triangular lattice configuration. In the core region, an air hole is introduced as the central defect to flatten the dispersion curve. The shape of the defect air hole is deliberately designed elliptical so that high birefringence in the PCF is obtained for polarization-maintaining purpose. The structural parameters are carefully selected to optimize the optical properties.

We propose a novel tunable photonic crystal fiber (PCF) coupler scheme, that the light is coupled between a total internal reflection (TIR) guiding core and a photonic bandgap (PBG) guiding core. By introducing the thermo-optic responsive nematic liquid crystals (NLC) into the air holes around the PBG core, the hybrid PCF based coupler exhibits enhanced thermal tunability. The thermo-optical characteristics of the proposed coupler are numerically studied using finite element method (FEM) and beam propagation method (BPM). The proposed design can find promising applications in filtering and sensing devices.

We propose a three-dimensional (3D) finite-difference time-domain (FDTD) method to analyze the pulse propagation characteristics in microstructured optical fibers (MOFs). The computation domain size is greatly reduced by adopting the technique of moving problem space. The propagating pulse is virtually held in the buffer cell of the problem space as simulation continues. This method is capable to investigate the temporal evolution of the propagating pulse. Spectral information can be obtained by Fourier analysis. As an example, the influence of the kerr nonlinearity on the optical pulse propagation in a Lorentz dispersive MOF is demonstrated. The model is also used to simulate the nonlinear interactions between the pump spectral broadening and third harmonic generations in a highly nonlinear fused silica nanowire with good agreement with the generalized nonlinear envelop equation (GNEE) model.