Temperature- and strain-insensitive curvature sensor based on ring-core modes in dual-concentric-core fiber.

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/µÎµ, respectively.

Invited article: Dielectric material characterization techniques and designs of high-Q resonators for applications from micro to millimeter-waves frequencies applicable at room and cryogenic temperatures.

Dielectric resonators are key elements in many applications in micro to millimeter wave circuits, including ultra-narrow band filters and frequency-determining components for precision frequency synthesis. Distributed-layered and bulk low-loss crystalline and polycrystalline dielectric structures have become very important for building these devices. Proper design requires careful electromagnetic characterization of low-loss material properties. This includes exact simulation with precision numerical software and precise measurements of resonant modes. For example, we have developed the Whispering Gallery mode technique for microwave applications, which has now become the standard for characterizing low-loss structures. This paper will give some of the most common characterization techniques used in the micro to millimeter wave regime at room and cryogenic temperatures for designing high-Q dielectric loaded cavities.

Broadband terahertz transmission within the air channel of thin-wall pipe.

We report broadband transmissions of terahertz radiations through the air channel of thin-wall pipe. The impacts of the wall thickness and of the refractive index of the material on the transmission window bandwidth are investigated. An extension of the bandwidth by at least 5.5 times is reported with a commercial drinking straw. The salient properties of the antiresonant reflecting guiding mechanism are studied with the terahertz time domain spectroscopy method, including the reduction of the attenuation coefficient of the propagated field by 60 times the material absorption coefficient.

Semiconductor-doped liquid-core optical fiber.

A semiconductor liquid-core optical fiber has been made by simply filling the hollow core of a capillary waveguide with nanoparticles suspended in toluene media. Under a low continuous optical power excitation at 532 nm, the emission of PbSe particles was clearly demonstrated in the infrared region and then partially maintained in the core of the fiber by the total internal reflection mechanism. Finally, due to the guided propagation, which results in multiple absorption effects, a linear shift of the emission peak toward longer wavelengths was observed (~0.32 nm/cm). As a proof of concept, this original demonstration of visible-to-infrared conversion could lead to the development of active fibered devices at wavelengths not covered by the conventional rare-earth ion doping.

Simplified hollow-core photonic crystal fiber.

An original design of hollow-core photonic crystal fiber composed of a thin silica ring suspended in air by six silica struts is proposed. This structure can be viewed as a simplified Kagomé-lattice fiber reduced to one layer of air holes. By working on the core surround parameters, an efficient antiresonant air guiding was successfully demonstrated. Two large low-loss windows (visible/IR) were measured with a minimum attenuation less than 0.2 dB radicalm at yellow wavelengths, comparable with state-of-the-art designs. The curvature behavior was also studied, showing low bending loss sensitivity for the fundamental transmission band. These relevant features might open a new route to propose original hollow-core fiber designs while making their production simpler and faster than previously.

Highly dispersive large mode area photonic bandgap fiber.

An all-silica photonic bandgap fiber composed of a low-index core surrounded by alternating high- and low-index rings allows us to achieve a large mode area (500 microm(2)) and large chromatic dispersion. Sharp resonances from the even Bragg mode to odd ring modes theoretically lead to 20,000 ps/(nm km) chromatic dispersion when large bends are applied. By nature, sharp resonances are sensitive to inhomogeneities along the fiber length. Under experimental conditions, the resonances are broadened and the dispersion coefficient is decreased to 1000 ps/(nm km). However, to the best of our knowledge, this is the largest dispersion coefficient reported using a large mode area fiber.

Stimulated Raman scattering in an ethanol core microstructured optical fiber.

We show that high efficiency stimulated Raman scattering can be obtained using hollow core photonic crystal fiber with the core filled with a low refractive index nonlinear liquid. This new architecture opens new perspectives in the development of nonlinear functions as any kind of nonlinear liquid media can now be used to implement them, with original properties not accessible with silica core fibers.

Ultra wide band supercontinuum generation in air-silica holey fibers by SHG-induced modulation instabilities.

Second harmonic generation in an air-silica microstructured optical fiber pumped by subnanosecond pulses is used in order to initiate modulation instability processes in normal and anomalous dispersion regimes. This allows us to generate an ultra wide and flat supercontinuum (350-1750 nm), covering the entire transparency window of silica and exhibiting a singlemode transverse profile in visible range.