Charles G. Askins

Fiber grating sensors

We review the recent developments in the area of optical fiber grating sensors, including quasi-distributed strain sensing using Bragg gratings, systems based on chirped gratings, intragrating sensing concepts, long period-based grating sensors, fiber grating laser-based systems, and interferometric sensor systems based on grating reflectors.

STEPPED-WAVELENGTH OPTICAL-FIBER BRAGG GRATING ARRAYS FABRICATED IN LINE ON A DRAW TOWER

More than 450 fiber Bragg gratings have been fabricated in line during the draw process in a period of 1 h with single pulses of a KrF excimer laser. Arrays of gratings have been written at different Bragg wavelengths by using an automated, computer-controlled interferometer.

Optical fiber waveguides in radiation environments, II

Abstract This paper will review recent progress in understanding the behavior of optical fiber waveguides when they are exposed to ionizing radiation. Not only have the growth and recovery of the radiation-induced attenuations been thoroughly characterized, in some cases the defect centers which cause these absorptions have been identified, and means for reducing the radiation sensitivity of the fibers have become apparent. The behavior of the radiation-induced loss is described in terms of parameters such as fiber composition and dopants, fiber structure, wavelength and intensity of the light source, temperature, total dose, time after irradiation, dose rate, and radiation history.

Fiber Bragg reflectors prepared by a single excimer pulse

Narrow-line, permanent Bragg reflection gratings have been created in Ge-doped silica-core optical fibers by interfering beams of a single 20-ns pulse of KrF excimer laser light. Of the fibers studied, the highest reflectance value of ~2% was observed with a linewidth (FWHM) of 0.1 nm, which corresponds to a 2-mm grating length with an index modulation of ~3 x 10(-5).

Instrumentation for interrogating many-element fiber Bragg grating arrays

Fiber Bragg grating (FBG) arrays have provided the Smart Structures community with a powerful means for real-time and absolute point measurements of strain throughout extended structures. The mechanical properties of these optical fiber sensors uniquely suit them for unobtrusive and reliable incorporation into composite materials, while wavelength encoding of the measurand enables independent interrogation of many devices distributed along a single fiber. In view of recent progress towards practical and economical FBG array fabrication via the computer-controlled in-line writing process during fiber draw, there is a complementary need for economical instrumentation which makes efficient use of the reflected signals. This is an important issue, especially since the reflectivity of narrow-line, type I gratings produced during fiber draw is typically less than 5%.

BROADBAND SQUARE-PULSE OPERATION OF A PASSIVELY MODE-LOCKED FIBER LASER FOR FIBER BRAGG GRATING INTERROGATION

A long-cavity, passively mode-locked erbium fiber laser operated in the square-pulse regime is demonstrated as a useful light source for interrogating fiber Bragg grating arrays. Output pulses with 4-W peak-power, 10-ns pulse widths, and bandwidths greater than 60nm were used successfully to interrogate 2% fiber Bragg gratings.

Interferometric method for concurrent measurement of thermo-optic and thermal expansion coefficients

A Fabry-Perot-type interferometer is described that permits the concurrent measurement of the coefficient of thermal expansion (alpha) and the thermooptic coefficient (dn/dT) of transparent materials. Measurements of the a and the dn/dT of vitreous silica and a heavy-metal fluoride glass show that the technique is accurate, reproducible, and easily performed. In addition, the technique is used to show that the dn/dT of heavy-metal fluoride glasses is negative and temperature dependent. The magnitude of dn/dT is found to increase with increasing temperature.

Photoinduced grating and intensity dependence of defect generation in Ge‐doped silica optical fiber

Ge E’ centers photoinduced in Ge‐doped silica by 5 eV photons of various intensities and fluences were found by electron spin resonance to be induced and bleached by one‐ and two‐photon absorption processes, respectively. The observation that Ge E’‐type centers are the only paramagnetic centers induced by very low intensity 5 eV photons in Ge‐doped silica supports the proposal that Ge E’‐type centers are responsible for the photoinduced gratings observed in both Bragg grating and second‐harmonic generation fibers.

Effect of low dose rate irradiation on doped silica core optical fibers.

The optical attenuation induced in multimode doped silica core optical fiber waveguides by a years exposure to low dose rate (1 rad/day) ionizing radiation was studied, allowing a characterization of fibers deployed in these environments and a determination of the permanent induced loss in the waveguides. Variations in the induced attenuation at 0.85 microm have been observed with changes in the dose rate between 1 rad/day and 9000 rads/min. These dose rate dependences have been found to derive directly from the recovery that occurs during the exposure; the recovery data predict little or no dose rate dependence of the damage at 1.3 microm. The low dose rate exposure has been found to induce significant permanent attenuation in the 0.7-1.7-microm spectral region in all fibers containing P in the core, whether doped uniformly across the diameter or constrained to a narrow spike on the centerline. Whereas permanent loss was induced at 0.85 microm in a P-free binary Ge-doped silica core fiber by the years exposure, virtually no damage was observed at 1.3 microm.

Correlation of single-mode fiber radiation response and fabrication parameters

Statistically significant correlations have been established between certain fabrication parameters of matched clad, single-mode optical fiber waveguides and their response to an ionizing radiation dose of 2000 rad. The reCOVE:ry data measured at -35 degrees C following exposure have been fit to nth-order kinetic behavior where the adjustable parameters are the initial and permanent incremental losses (A(o) and A(f), respectively), the half-life of attenuation tau, and the order of kinetics n. The set of fibers chosen for analysis had Ge-doped silica cores. In fibers with Ge-F-doped silica clads, A(o) correlates with the concentration of Ge-doped into the fiber core; A(f) correlates with the ratio of oxygen to reagents used during core deposition; and tau and n correlate with a two-way interaction of core oxygen and fiber draw speed. In P-F-doped clad fibers, the P concentration has been found to correlate with the order of the kinetics of recovery.