Barbara M. Wright

Pulse Compression using a Tapered Microstructure Optical Fiber

We calculate the pulse compression in a tapered microstructure optical fiber with four layers of holes. We show that the primary limitation on pulse compression is the loss due to mode leakage. As a fibers diameter decreases due to the tapering, so does the air-hole diameter, and at a sufficiently small diameter the guided mode loss becomes unacceptably high. For the four-layer geometry we considered, a compression factor of 10 can be achieved by a pulse with an initial FWHM duration of 3 ps in a tapered fiber that is 28 m long. We find that there is little difference in the pulse compression between a linear taper profile and a Gaussian taper profile. More layers of air-holes allows the pitch to decrease considerably before losses become unacceptable, but only a moderate increase in the degree of pulse compression is obtained.

Projecting the performance of erbium-doped fiber devices in a space radiation environment

The optical attenuation induced in two erbium-doped fibers by 60Co (gamma) -ray exposures to 200 krad has been measured at 3 different dose rates, approximately, 35, 1000, and 20,500 rad/min. The growth of the induced loss was measured at 980, 1300, and 1500 nm. In some instances recovery of the loss after exposure was also measured. The effect of temperature on radiation-induced loss was examined by irradiating one of the erbium-doped fibers at three different temperatures, -54, 30, and 80 C. A simple physical model describing the thermal annealing process of the induced loss in the fibers is presented. The model yields expressions, with only two adjustable parameters, which describe the dose rate and temperature dependence of both the radiation-induced loss growth and recovery kinetics. The simple model accurately describes the form of the growth and recovery kinetics but overestimates the dependence of induced loss on dose rate and temperature.

Inscription of Fiber Bragg Gratings in Multicore Fiber

We present solutions to unique problems associated with simultaneously writing FBGs in 4 cores of a multicore fiber used for bend and twist sensing, including exposure conditions, photosensitivity balance, and draw-induced refractive index changes.

Ho-doped fiber for high energy laser applications

Ho-doped fiber lasers are of interest for high energy laser applications because they operate in the eye safer wavelength range and in a window of high atmospheric transmission. Because they can be resonantly pumped for low quantum defect operation, thermal management issues are anticipated to be tractable. A key issue that must be addressed in order to achieve high efficiency and minimize thermal issues is parasitic absorption in the fiber itself. Hydroxyl contamination arising from the process for making the Ho-doped fiber core is the principal offender due to a combination band of Si-O and O-H vibrations that absorbs at 2.2 μm in the Ho3+ emission wavelength region. We report significant progress in lowering the OH content to 0.16 ppm, which we believe is a record level. Fiber experiments using a 1.94 μm thulium fiber laser to resonantly clad pump a triple clad Ho-doped core fiber have shown a slope efficiency of 62%, which we also believe is a record for a cladding-pumped laser. Although pump-power limited, the results of these studies demonstrate the feasibility of power scaling Ho-doped fiber lasers well above the currently-reported 400-W level.1

Optical limiting properties of nonlinear multimode waveguide arrays

An experimental investigation of the transmission of multimode capillary waveguide arrays containing a liquid nonlinear absorber shows an enhanced nonlinear response relative to that found in a single waveguide and to the same length of bulk material. Comparison of the nonlinear response of arrays with different pitch to diameter (d/Lambda) ratios confirm that both the intensity distribution within an individual waveguide and coupling between the elements of the array influence the overall nonlinear response.

Optical properties of solid-core photonic crystal fibers filled with nonlinear absorbers

A theoretical and experimental investigation of the transmission of solid-core photonic crystal fibers (PCFs) filled with nonlinear absorbers shows a sharp change in the threshold for optical limiting and in leakage loss as the refractive index of the material in the holes approaches that of the glass matrix. Theoretical calculations of the mode profiles and leakage loss of the PCF are in agreement with experimental results and indicate that the change in limiting response is due to the interaction of the evanescent field of the guided mode with the nonlinear absorbers in the holes.

Optical limiting in solid-core photonic crystal fibers

Optical limiting in solid-core photonic crystal fibers filled with reverse-saturable absorbers has been observed. A sharp change in limiting threshold was found for materials in the fiber holes with refractive indices near n = 1.44.

Reliability of ultrahigh sensitivity optical fiber sensors embedded in graphite composites

Fiber cavity etalon (FCE) sensors have demonstrated ultrahigh static strain sensitivity (~1 nε) when they are either surface-mounted to, or embedded, in graphite reinforced resin composites. Although a significant amount of data has been acquired at very low strain, little is known about their performance and durability in typical installations. Graphite/epoxy composite test specimens were fabricated to address practical concerns and to evaluate the reliability of embedded FCE sensors. Two different specimen configurations using two different composite fabrication methods were selected for sample installations: thin flat laminates and cylindrical struts. After fabrication, the FCE sensors were interrogated to ensure that they were still intact, to record a baseline response, and to determine any changes in response that might have occurred during manufacturing. Next, to determine the survivable strain limits of the embedded sensor, the specimens were loaded in tension to a predetermined strain level, unloaded, and then interrogated. Once these limits were found, the specimens were subjected to cyclic loading and periodically interrogated until sensor failure. The results from these tests provide practical strain limits for the embedded FCE sensor and show that the response does not change as a result of tensile cyclic loading.

Embedding and testing of ultrahigh-sensitivity optical fiber sensors in prototype graphite composite spacecraft strut tubes

Ultrahigh sensitivity fiber cavity etalon (FCE) sensors have been embedded in graphite-reinforced polymer tubes fabricated by two different methods: resin transfer molding (RTM) and standard autoclave curing, and FCEs have been embedded in autoclave-cured unidirectional flat laminates. Significant issues encountered in embedding the sensors include protecting the fiber egress during layup, curing, and breakout, survival of the butt-coupled splice between the cavity and lead fiber during composite cure, maintaining sensor location, and sensor reliability and response. Methods were successfully devised to overcome these obstacles.