Glen M. Williams

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.

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).

Index structure of fiber Bragg gratings in Ge-SiO(2) fibers.

The behavior of the concentration of photoinduced color centers in Ge-SiO(2) optical fibers was compared with that of the index modulation associated with fiber Bragg gratings (FBGs) written in the same fibers. We find that the fluence dependence of the photoinduced Ge E? center, its thermal annealing behavior, and its reaction with H(2) are similar to that of the index modulation generated in both H(2)-loaded and unloaded Ge-SiO(2) fibers. The much higher photosensitivity of H(2)-loaded Ge-SiO(2) fibers is attributed to the much higher formation efficiency of Ge E? centers, with an additional contribution from GeH. A diamagnetic structure, possibly densification, is also found to contribute to the index modulation of FBGs.

Separation of Intrinsic and extrinsic Optical-Absorption in a Fluoride Glass

The contribution of impurity ions to the total optical absorption of a heavy metal fluoride glass has been determined at 532 and 1064 nm. Four ZrF4‐BaF2‐LaF3‐AlF3‐NaF glasses were prepared from various purity raw materials. The absorption coefficients of these glasses range from 0.92 to 45.4×10−4 cm−1 at 1064 nm and from 7.43 to 11.1×10−4 cm−1 at 532 nm as determined by laser calorimetry. The concentrations of Fe, Ni,Cu, and Co ions in each glass were determined by graphite furnace atomic absorption spectroscopy. These two measurements enable the absorption, due to transition metal ions to be differentiated from the intrinsic absorption of the glass. At 1064 nm, the absorption coefficient of these glasses is controlled entirely by the transition metal ion content. However, at 532 nm, the absorption by the transition metal ions accounts for 4–42% of the total absorption depending on impurity concentration. The intrinsic absorption of this fluoride glass calculated from these data at 532 nm is (7.69±0.99)×10−4 cm−1.

Space radiation effects on erbium-doped fibers

The effect of (gamma) and proton radiation on the optical attenuations at 980, 1300, and 1550 nm has been measured in two Lucent Technologies erbium-doped fibers (EDFs) designed for use in amplifiers pumped at 980 nm. Growth of the induced loss at the 3 wavelengths followed a power law of dose with an exponent of approximately 0.8 independent of fiber, radiation type or dose rate. The incremental attenuation was found to have only a weak dependence on dose rate. The recovery could be described by the sum of 3 exponential decay terms, but simple color center models overestimated the effect of dose rate.

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.

Radiation-induced coloring of erbium-doped optical fibers

The radiation-induced coloring of erbium-doped optical fibers (EDFs) is reported. The radiation hardness of the EDFs is observed to be strongly dependent on composition. The implications for erbium-doped fiber amplifier (EDFA) performance is modelled.

Permanent photowritten optical gratings in irradiated silicate glasses.

We show that permanent optical gratings can be photowritten into simple silicate glasses by exposure to interfering beams of an Ar-ion laser after the glass has been treated by x rays. Gratings with index modulations as large as Deltan = 10(-5) can be formed in less than a minute by exposure to write beams with intensities of the order of 50 W/cm(2).

Uniform component of index structure induced in Ge-SiO2 fibers by spatially modulated ultraviolet light

Experimental data are presented to show that Ge(1) and Ge(2) centers are induced by trapping photoinduced electrons from the conduction band, in agreement with our previous proposal that both are trapped electron centers. The spacing (Λ) dependence of ultraviolet (UV) light bleaching of the pre-existing Ge E′ centers illustrates that the electron diffusion length is greater than Λ of the spatially modulated UV light used in the fabrication of fiber Bragg gratings (FBGs) with Bragg wavelengths ⩽1.5 μm (short period grating) for laser powers as low as 25 mJ/cm2. The Ge(1) and Ge(2) centers are uniformly induced by the spatially modulated UV light and therefore contribute to the uniform component of the index structure of FBGs.

Permanent Photoinduced Refractive Index Changes in Rare Earth Doped Glasses

Permanent refractive index gratings were holographically photowritten into rare earth doped glasses using the 488 rn line of a Ar Ion laser. The rare earths Pr 3+ , Tb 3+ , and Eu 2+ were examined. Experiments were performed to elucidate the mechanism responsible for the photoinduced index change. A model is presented in which the index change is accomplished through photoionization of the rare earth dopant and subsequent trapping of the free carrier. The viability of these materials for use in optical fiber grating devices is discussed.