The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of hos...

Lanthanide-Activated Phosphors Based on 4f-5d Optical Transitions: Theoretical and Experimental Aspects.

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

Photonic crystal fibers are a kind of fiber optics that present a diversity of new and improved features beyond what conventional optical fibers can offer. Due to their unique geometric structure, photonic crystal fibers present special properties and capabilities that lead to an outstanding potential for sensing applications. A review of photonic crystal fiber sensors is presented. Two different groups of sensors are detailed separately: physical and biochemical sensors, based on the sensor measured parameter. Several sensors have been reported until the date, and more are expected to be developed due to the remarkable characteristics such fibers can offer.

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Photonic crystal fibers for sensing applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

We describe the history, guiding mechanism, recent advances, applications, and future prospects for hollow-core negative curvature fibers. We first review one-dimensional slab waveguides, two-dimensional annular core fibers, and negative curvature tube lattice fibers to illustrate the inhibited coupling guiding mechanism. Antiresonance in the glass at the core boundary and a wavenumber mismatch between the core and cladding modes inhibit coupling between the modes and have led to remarkably low loss in negative curvature fibers. We also summarize recent advances in negative curvature fibers that improve the performance of the fibers, including negative curvature that increases confinement, gaps between tubes that increase confinement and bandwidth, additional tubes that decrease mode coupling, tube structures that suppress higher-order modes, nested tubes that increase guidance, and tube parameters that decrease bend loss. Recent applications of negative curvature fibers are also presented, including mid-infrared fiber lasers, micromachining, and surgical procedures. At the end, we discuss the future prospects for negative curvature fibers.

Negative curvature fibers

A highly sensitive torsion sensor, which can measure the torsion angle and determine the torsion direction simultaneously, is proposed and demonstrated based on Sagnac interferometer by incorporating a segment of novel side-leakage photonic crystal fiber (SLPCF). The SLPCF was designed by introducing an elliptical Ge-doped core and a symmetrical linear side-leakage defect. The experimental results show that the proposed sensor has different torsion sensitivities at different valley wavelengths and in different twist directions. The achieved maximum torsion sensitivity is about 0.9354 nm/°, which is much higher than that of the previously reported torsion sensors. In order to eliminate the temperature effect and obtain an accurate value of the torsion angle, the matrix method is adopted.

Highly Sensitive Torsion Sensor Based on Sagnac Interferometer Using Side-Leakage Photonic Crystal Fiber

Switchable multi-wavelength fiber ring laser with an in-fiber Mach-Zehnder interferometer incorporated into the ring cavity serving as wavelength-selective filter at room temperature is demonstrated. The filter is formed by splicing a section of few-mode photonic crystal fiber (PCF) and two segments of single mode fiber (SMF) with the air-holes on the both sides of PCF intentionally collapsed in the vicinity of the splices. By adjusting the states of the polarization controller (PC) appropriately, the laser can be switched among the stable single-, dual- and triple-wavelength lasing operations by exploiting polarization hole burning (PHB) effect.

Switchable multi-wavelength fiber ring laser based on a compact in-fiber Mach-Zehnder interferometer with photonic crystal fiber

A novel refractive index (RI) sensor based on an asymmetrical fiber Mach–Zehnder interferometer (AFMZI) was realized by concatenating single-mode abrupt taper and core-off section. Compared with the normal two-taper MZI, the proposed sensor enlarges the evanescent field in the surrounding medium through the core-offset section. As a result, the RI sensitivity to the surrounding medium can be improved. Experimental results show that the RI sensitivity of the proposed sensor is 28.2 nm/RIU (refractive index unit) and 59.2 nm/RIU for interferometer length of 30 mm and 50 mm, respectively. It is much higher than the RI sensitivity of the normal two-taper MZI which is 14.7 nm/RIU and 19.1 nm/RIU for the corresponding interferometer length. Such kinds of low cost and highly sensitive RI sensor would find application in chemical and biological sensing fields.

Refractive index sensor with asymmetrical fiber Mach–Zehnder interferometer based on concatenating single-mode abrupt taper and core-offset section

Luminescent properties and energy transfer of Sr3La(PO4)3:Sm3+, Eu3+ for white LEDs

A rectangle lattice silica-host photonic crystal fiber with large mode area (LMA) is successfully fabricated for the first time. The introduction of rectangle lattice contributes to distinct leakage of second order modes and two large air holes provide an effective solution to suppress bend distortion. Numerical results demonstrate that a large mode field area (MFA) of 2500 μm2 is achieved at the wavelength of 2 μm when the fiber is straight. While the fiber is bent to a radius of 30 cm, it confirms to single-mode (SM) operation conditions and the decrements of MFA at x polarization and y polarization are just 4.3% and 5%. Within a bend orientation angle of ± 10° and a wide wavelength range from 1.8 μm to 2.4 μm, the fiber can operate under single-mode (SM) conditions with low bend loss. LMA, SM operation, and small bend distortion make this fiber of great potential in developing 2 μm compact high-power fiber lasers.

Shuqin Lou

Papers

Highly Sensitive Torsion Sensor Based on Sagnac Interferometer Using Side-Leakage Photonic Crystal Fiber

Switchable multi-wavelength fiber ring laser based on a compact in-fiber Mach-Zehnder interferometer with photonic crystal fiber

Refractive index sensor with asymmetrical fiber Mach–Zehnder interferometer based on concatenating single-mode abrupt taper and core-offset section

Luminescent properties and energy transfer of Sr3La(PO4)3:Sm3+, Eu3+ for white LEDs

Rectangle Lattice Large Mode Area Photonic Crystal Fiber for 2 $\mu$ m Compact High-power Fiber Lasers