Fabrication of Bragg gratings in microstructured and step index Bi-SiO2 optical fibers using an ArF laser.

An ArF excimer laser was used to fabricate Bragg gratings in fibers with Bi-SiO(2) core and microstructured or F-doped claddings without fiber presensitization. Average and modulated refractive index changes of 2.7 × 10(-4) and 1.0 × 10(-4) were induced in pristine microstructured fiber while 1.0 × 10(-4) and 0.7 × 10(-4) were observed in the F-doped-cladding fiber. Fiber luminescence was also measured under 1064 nm pumping for both fibers. Photosensitivity and luminescence were compared to a Bi-Al(2)O(3)-SiO(2) core optical fiber.

Fabrication and thermal decay of fiber Bragg gratings in pristine and H2-loaded Bi-Al co-doped optical fibers.

A cw-244-nm-Ar(+) laser was used to fabricate Bragg gratings in pristine and H(2)-loaded Bi-Al-SiO(2) optical fibers with index changes as high as 3.6 × 10(-4) and 19.3 × 10(-4), respectively. For comparison, fiber Bragg gratings in pristine and H(2)-loaded SMF-28e showed index changes of 13.6 × 10(-4) and 63.3 × 10(-4). Continuous isochronal thermal annealing revealed higher thermal stability for the H(2)-loaded Bi-Al-SiO(2) fiber compared to the pristine one. The SMF-28e fibers, with and without hydrogen, were more stable than the Bi-Al-SiO(2) fibers.

Photosensitivity and stress changes of Ge-free Bi-Al doped silica optical fibers under ArF excimer laser irradiation.

The photosensitivity of germanium free Bi-Al-doped silica fibers with different bismuth concentrations was investigated using ArF excimer laser radiation at 193 nm and fiber grating formation. For the fiber with the highest bismuth concentration maximum refractive index changes of 2.2 × 10(-3) and 2.0 × 10(-4) were obtained for hydrogen loaded and unloaded fibers, respectively. Irradiation induced tensile stress changes were observed in the fiber core of H(2)-loaded and unloaded fibers. The results indicate a contribution of compaction to the total refractive index change in both cases.

Distributed strain measurements using fiber Bragg gratings in small-diameter optical fiber and low-coherence reflectometry.

Optical low coherence reflectometry and fiber Bragg gratings written in small diameter (50 micrometer) optical fibers were used for measurements of non-homogenous internal strain fields inside an epoxy specimen with sub-grating length resolution. The results were compared with measurements using Fiber Bragg gratings in standard size (125 micrometer) single mode fibers and show that smaller fibers are less intrusive at stress heterogeneities.

Absence of UV-induced stress in Bragg gratings recorded by high-intensity 264 nm laser pulses in a hydrogenated standard telecom fiber.

We report on photochemical two-photon Bragg grating preparation in hydrogenated fiber without any UV-induced stress in the core or cladding, leaving only the color-center model responsible for refractive index changes for UV femtosecond irradiation. Without hydrogen loading strong stress changes are observed in the core and in the cladding indicating glass compaction. The irradiation does not change the inelastic strains, in contrast to H(2)-loading.

Distributed measurements of fiber birefringence and diametric load using optical low-coherence reflectometry and fiber gratings.

Polarization sensitive optical low-coherence reflectometry (OLCR) is used for measuring the complex fiber Bragg gratings (FBG) reflection coefficient. We determine the beat length directly from oscillations in the OLCR amplitude with a resolution of 10(-6) and a spatial resolution only limited by the minimum beat length or the coherence length of the light source. Using the OLCR amplitude and phase in combination with an inverse scattering algorithm the birefringence is retrieved with a resolution of 2x10(-5) while the spatial resolution is 25 mum. The two developed techniques are applied for measuring position, magnitude and footprint of induced birefringence of an FBG under uniform and non-uniform diametric loading.

Polarization microscopy by use of digital holography: application to optical-fiber birefringence measurements.

We present a digital holographic microscope that permits one to image polarization state. This technique results from the coupling of digital holographic microscopy and polarization digital holography. The interference between two orthogonally polarized reference waves and the wave transmitted by a microscopic sample, magnified by a microscope objective, is recorded on a CCD camera. The off-axis geometry permits one to reconstruct separately from this single hologram two wavefronts that are used to image the object-wave Jones vector. We applied this technique to image the birefringence of a bent fiber. To evaluate the precision of the phase-difference measurement, the birefringence induced by internal stress in an optical fiber is measured and compared to the birefringence profile captured by a standard method, which had been developed to obtain high-resolution birefringence profiles of optical fibers.

Relative humidity sensor with optical fiber Bragg gratings.

A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.