A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation, photonic crystal fiber

The development of highly-sensitive miniaturized sensors that allow real-time quantification of analytes is highly desirable in medical diagnostics, veterinary testing, food safety, and environmental monitoring. Photonic Crystal Fiber Surface Plasmon Resonance (PCF SPR) has emerged as a highly-sensitive portable sensing technology for testing chemical and biological analytes. PCF SPR sensing combines the advantages of PCF technology and plasmonics to accurately control the evanescent field and light propagation properties in single or multimode configurations. This review discusses fundamentals and fabrication of fiber optic technologies incorporating plasmonic coatings to rationally design, optimize and construct PCF SPR sensors as compared to conventional SPR sensing. PCF SPR sensors with selective metal coatings of fibers, silver nanowires, slotted patterns, and D-shaped structures for internal and external microfluidic flows are reviewed. This review also includes potential applications of PCF SPR sensors, identifies perceived limitations, challenges to scaling up, and provides future directions for their commercial realization.

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Photonic crystal fiber based plasmonic sensors

R EIEW A R TCLE Abstract As an emerging topic, photonic-assisted microwave measurements with distinct features such as wide frequency coverage, large instantaneous bandwidth, low frequencydependent loss, and immunity to electromagnetic interference, have been extensively studied recently. In this article, we provide a comprehensive overview of the latest advances in photonic microwave measurements, including microwave spectrum analysis, instantaneous frequency measurement, microwave channelization, Doppler frequency-shift measurement, angle-of-arrival detection, time–frequency analysis, compressive sensing, and phase-noise measurement. A photonic microwave radar, as a functional measurement system, is also reviewed. The performance of the photonic measurement solutions is evaluated and compared with the electronic solutions. Future prospects using photonic integrated circuits and software-defined architectures to further improve the measurement performance are also discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/lpor.201600019

Photonics for microwave measurements

We numerically demonstrate a surface plasmon resonance biosensor-based on dual-polarized spiral photonic crystal fiber (PCF). Chemically stable gold material is used as the active plasmonic material, which is placed on the outer layer of the PCF to facilitate practical fabrication. Finite-element method-based numerical investigations show that the proposed biosensor shows maximum wavelength sensitivity of 4600 and 4300 nm/RIU in ${x}$ - and ${y}$ -polarized modes at an analyte refractive index of 1.37. Moreover, for analyte refractive index ranging from 1.33 to 1.38, maximum amplitude sensitivities of 371.5 RIU−1 and 420.4 RIU−1 are obtained in ${x}$ - and ${y}$ -polarized modes, respectively. In addition, the effects of changing pitch, different air hole diameter of the PCF and thickness of the gold layer on the sensing performance are also investigated. Owing to high sensitivity, improved sensing resolution and appropriate linearity characteristics, the proposed dual-polarized spiral PCF can be implemented for the detection of biological analytes, organic chemicals, biomolecules, and other unknown analytes.

Spiral Photonic Crystal Fiber-Based Dual-Polarized Surface Plasmon Resonance Biosensor

High-quality optical fibers can be produced now at a low cost and large quantity, and this has further promoted the development of fiber optic (chemical) sensors. After over 30 years of innovation, fiber optic sensing technology has become mature because of acceptable costs, compact instrumentation, high accuracy and the capability of performing measurements at inaccessible sites, over large distances, in strong (electro)magnetic fields and in harsh environment. The technology is still proceeding quickly in terms of innovation, and respective applications have been found in highly diversified fields. This review covers work published in the time period between October 2015 and October 2019. It is written in continuation of previous reviews.

Fiber-Optic Chemical Sensors and Biosensors (2015-2019).

Two kinds of novel plasmonic high sensitivity of refractive index (RI) sensors based on analyte-filled photonic crystal fiber (AF-PCF) are proposed in this paper. The metallic gold and silver is used as the surface plasmon resonance activity metal. A full-vector finite-element method is applied to analyze and investigate the sensing and coupling characteristics of this designed AF-PCF with the gold or silver layer. Phase matching between the second surface plasmon polariton and fundamental modes can be met at different wavelengths as the analyte of the RI is increased from 1.40 to 1.42. The phase-matching wavelength of the designed AF-PCF with the gold layer is shifted to the longer wavelength direction compared with that with the silver layer, and the resonance strength is much stronger. The average sensitivities of 7040 and 7017 nm/RIU in the sensing are arranged from 1.40 to 1.42 with high linearity are achieved for the designed sensors with the gold and silver layers, respectively, which are almost the same. However, the figure of merit with the silver layer is much better than that with the gold layer.

High Sensitivity of Refractive Index Sensor Based on Analyte-Filled Photonic Crystal Fiber With Surface Plasmon Resonance

We demonstrate a photonic crystal fiber temperature sensor based on surface plasmon resonance and evaluate it using the finite element method. A temperature-sensitive material is injected into the central air hole of the photonic crystal fiber. The air hole is coated with nanoscale gold film. Six cores are formed by removing air holes in the second layer, which supports the core mode. The coupling between the core mode and the surface plasmon polariton mode occurs as the phase matching condition is satisfied. The average sensitivity and linearity become −2.15 nm/°C and 0.99991, respectively. The length of this fiber is only 1 mm. Our temperature sensor is competitive within the temperature sensor field.

https://iopscience.iop.org/article/10.7567/APEX.8.046701/pdf

High-sensitivity plasmonic temperature sensor based on photonic crystal fiber coated with nanoscale gold film

A high-sensitivity temperature sensor of compact photonic crystal fiber (PCF) based on the coupling between liquid-core mode and defect mode has been analyzed by the finite element method. The temperature sensitive materials with high refractive index ( $n=1.65$ at 25 $^{\circ}\hbox{C} $ ) , which support liquid-core modes, are filled into the central air hole of PCF. Six cores are formed by removing air holes in the second layer. The six cores work as defect modes and show high confinement losses. The liquid-core mode couples to defect mode as the phase matching condition is satisfied. The sensitivity and figure of merit reach $-1.85\ \hbox{nm}/^{\circ}\hbox{C}$ , $-0.072/^{\circ}\hbox{C}$ and $-1.95\ \hbox{nm}/^{\circ}\hbox{C}$ , $-0.035/^{\circ}\hbox{C} $ . The temperature sensor is competitive in the reported temperature sensors. The simple structure is easy to fabricate, and the structure can be further improved.

Photonic Crystal Fiber Temperature Sensor Based on Coupling Between Liquid-Core Mode and Defect Mode

A highly sensitive magnetic field sensor based on Sagnac interferometer consisted of a polarization maintaining photonic crystal fiber (PCF) filled with magnetic fluid (MF) is proposed and demonstrated. The refractive index of MF is sensitive to magnetic field H and the fiber Sagnac interferometer is sensitive to refractive index, so the sensing structure of our magnetic field sensor is designed. The water-based nanoparticles Fe3O4 is used which possesses low refractive index. The refractive index of MF follows the Langevin function and changes linearly with magnetic field H in our considerable magnetic field region. The parameters of λ, B(λ, H) on the influences of sensitivity are analyzed. We find a new phenomenon the two dip wavelengths shift in the opposite direction as magnetic field increases. The phenomenon is explained by group birefringence Bg which decreases and its sign changes as magnetic field increases. The sensitivity increases infinitely near the special wavelength related with zero-point Bg. The average sensitivities are 384 pm/Oe (51,569 nm/RIU), −430 pm/Oe (−57,747 nm/RIU) which turn out pretty well and the R-squares are 0.98113, 0.98415 respectively for the detected range 410–600 Oe.

Sensing characteristics of a MF-filled photonic crystal fiber Sagnac interferometer for magnetic field detecting

A tunable fiber polarization filter by filling different index liquids into the central hole of photonic crystal fiber (PCF) is proposed and demonstrated. The dispersion characteristics and loss spectra of the polarization filter are evaluated by finite element method (FEM). The gold wires are selectively filled into the cladding air holes of the PCF. When the phase matching condition is satisfied, the liquid-core mode couples to surface plasmon polaritons (SPP) mode intensely. The resonance wavelength varies with the change of the structural parameters and liquids. By adjusting the refractive index of the liquid, we realize the polarization filter at the wavelength of 1.31, 1.49, and 1.55 $\mu$ m, respectively, under the optimized structural parameters. This is the first time to propose the narrowband polarization filter at the communication wavelength of 1.31 $\mu$ m to our best knowledge based on the coupling between liquid-core mode and SPP mode, and the full width half maximum (FWHM) is only 16 nm. The loss of X-polarized mode is 44336 dB/m at $\lambda$ = 1.31 $\mu$ m, and the corresponding loss of Y-polarization mode is 224 dB/m. By comparison, we find the birefringence in our structure is further better than that in conventional structure. High birefringence is helpful to separate the resonance wavelength positions of the two orthogonal polarized modes. The result also reveals that resonance loss becomes small with increasing the distance between liquid core and gold wire.

Qiang Liu

Papers

High Sensitivity of Refractive Index Sensor Based on Analyte-Filled Photonic Crystal Fiber With Surface Plasmon Resonance

High-sensitivity plasmonic temperature sensor based on photonic crystal fiber coated with nanoscale gold film

Photonic Crystal Fiber Temperature Sensor Based on Coupling Between Liquid-Core Mode and Defect Mode

Sensing characteristics of a MF-filled photonic crystal fiber Sagnac interferometer for magnetic field detecting

Tunable Fiber Polarization Filter by Filling Different Index Liquids and Gold Wire Into Photonic Crystal Fiber