Ben McMillen

Sensitivity enhancement of fiber Bragg gratings to transverse stress by using microstructural fibers

We present simulation and experimental results of fiber Bragg grating responses to transverse stress in microstructure fibers. The grating wavelength shifts and peak splits are studied as a function of external load and fiber orientation. Both simulation and measurement results indicate that the sensitivity of grating sensors to the transverse stress can be enhanced by a factor of eight in a two-hole fiber over that in a standard fiber.

Self-heated fiber Bragg grating sensors

This letter demonstrates an approach for tuning fiber Bragg grating sensors with optical energy carried in the same optical fiber. Optical energy carried in the optical fiber was used to heat in-fiber Bragg gratings in order to alter the grating’s optical response to surrounding media. The functional enhancement of optically heated Bragg gratings as sensor devices is demonstrated by a dual-function Bragg grating temperature and level sensing array for liquid at room and cryogenic temperatures.

Ultrafast laser fabrication of low-loss waveguides in chalcogenide glass with 0.65 dB/cm loss

We present 0.65-dB/cm loss symmetric waveguides fabricated in Gallium Lanthanum Sulfide glasses using spatial and temporal shaped ultrafast laser pulses. Micro-Raman analysis was used to correlate optical loss versus laser-induced lattice damages.

Waveguiding and nonlinear optical properties of three-dimensional waveguides in LiTaO3 written by high-repetition rate ultrafast laser

We report the fabrication of waveguides in lithium tantalate using a 250kHz high-repetition rate ultrafast laser at 771nm and the characterization of the resulting laser induced structure with second harmonic microscopy. Waveguides operating at the 1.5μm telecommunication wavelength were formed above and below the focal volume using pulse energies ranging from 100to1.6J and translation speeds from 100μm∕sto5mm∕s. The second harmonic microscopy reveals no degradation of the electro-optic coefficient in the guiding region above the focal volume.

High repetition rate UV ultrafast laser inscription of buried channel waveguides in Sapphire: Fabrication and fluorescence imaging via ruby R lines

We report on the fabrication of buried cannel waveguides in Sapphire crystals by 250-kHz high repetition rate ultrafast laser inscription with 385 nm pulses. The propagation properties of the waveguides were studied as a function of the writing conditions. The micro-fluorescence analysis of the R lines generated by trace Cr(3+) dopant in Sapphire is used to elucidate the micro-structural modifications induced in the crystal network. It is revealed that waveguide has been formed due to local dilatation of the Sapphire network generated in the surroundings of the focal volume. The refractive index increment due to the dilatation induced electronic polarizability enhancement has been estimated to be of the order of Deltan approximately 10(-4).

Fiber Bragg grating vacuum sensors

This letter demonstrates functional enhancements of fiber Bragg grating sensors powered by in-fiber light. High-power laser light transmitted in double-clad optical fiber was extracted from the fiber core to heat an on-fiber metal coating. When the power-laser is turned off, the fiber Bragg grating is used as a passive component for temperature sensing. When the laser is turned on, the thermal response of the optically heated grating was used to monitor ambient air pressure. The sensitivity and dynamic range of optically powered fiber sensors can be actively adjusted by in-fiber light to measure vacuum pressures over four orders of magnitude.

Thermal stability of microstructural and optical modifications induced in sapphire by ultrafast laser filamentation

We report on the thermal stability of both structural and optical micromodifications created by ultrafast laser written filaments in sapphire crystals. By using the Cr3+ traces as optical probes, we have concluded that the filaments are constituted by both reversible and nonreversible defects with very different spatial locations. The strain field measured from the analysis of R lines has been found to be erased at the same time when the reversible centers are recombined (∼1100 °C). This fact seems to indicate that these defects act as pinning centers for the induced stress. Furthermore, we have found that the waveguide generated in the proximity of the filament disappear for annealing temperatures above 1100 °C. This clearly supports the assumption that waveguiding is produced by the strain stress induced refractive index increment based on the dominant electronic polarizability enhancement.

Microstructural imaging of high repetition rate ultrafast laser written LiTaO3 waveguides

The microstructural changes associated with the formation of lithium tantalate waveguides after high repetition rate ultrafast laser inscription has been investigated by confocal micro-Raman experiments. While the laser beam focal volume is characterized by significant lattice damage, no reduction of Raman mode strength has been observed at the guiding region, suggesting the preservation of the nonlinear optical coefficient in the waveguide. A general blueshift of the Raman modes has been observed at the guiding region, suggesting lattice compression as the dominant mechanism of waveguide formation.

Waveguide saturable absorbers in chalcognide glass fabricated by ultrafast lasers

We report a waveguide gap structure for nonlinear coupling, written in chalcognide glass with an ultrafast laser. The coupling efficiency between two waveguides is enhanced by the self focusing of shorter and stronger input pulse.

Self-powered multi-functional fiber sensors

Fiber optical components such as fiber gratings, fiber interferometers, and in-fiber Fabry-Perot filters are key components for optical sensing. Fiber optical sensors offer a number of advantages over other optical and electronic sensors including low manufacturing cost, immunity to electromagnetic fields, long lifetimes, multiplexing, and environmental ruggedness. Despite the advantages of purely passive optical components described above, fiber sensor performance and applications have been limited by their total passivity and solid-core/solid cladding structure configurations. Passive sensors can only gather limited information. Once deployed; set point, sensitivity, trigging time, responsivity, and dynamic range for each individual fiber sensor cannot be adjusted or reset to adapt to the changing environment for active sensing. Further, the fiber sensor sensitivity is also limited by the traditional solid core/solid cladding configuration. In this paper, we present a concept of active fiber sensor that can directly powered by in-fiber light. In contrast to a passive sensor, optical power delivered with sensing signal through the same fiber is used to power in-fiber fiber Bragg grating sensors. The optical characteristics of grating sensors can then be adjusted using the optical energy. When optical power is turned off, in-fiber components can serve as traditional passive sensor arrays for temperature and strain measurements. When optical power is turned on, the fiber sensor networks are capable of measuring a wide array of stimuli such as gas flow, wall shear stress, vacuum, chemical, and liquid levels in cryogenic, micro-gravity, and other hostile environments. In this paper, we demonstrate in-fiber light powered dual-function active FBG sensor for simultaneous vacuum, hydrogen fuel gas, and temperature measurement in a cryogenic environment.