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Photodegradation has been observed in Tm(3+)-doped ZBLAN fiber lasers pumped with laser diodes at 1135 nm. After upconversion lasing at 482 nm, the fiber develops color centers that absorb strongly at wavelengths below ~650 nm, affecting further upconversion lasing. The rate of damage formation is strongly dependent on the pump power level and on the thulium concentration. The color centers are bleached by intense blue light but recover with thermal excitation and can be removed by thermal annealing at temperature near 100 degrees C.

A new technique has enabled us to resolve the strong UV absorption spectrum in UV-exposed germanosilicate preforms. Our results show, for the first time to our knowledge, that the large photoinduced absorption changes between 165 and 300 nm (~700 dB/mm at 195 nm has been observed) can account for, ~3 ×10(-4) of index change at 1.5 µm. The thermal-annealing dynamics of the photoinduced absorption changes also resemble those of fiber gratings, evidence that the color-center model of photosensitivity plays an important role in the photoinduced index changes in UV-written fiber gratings. We also find that the photoinduced absorption changes depend linearly on germania (i.e., germanium dioxide) concentrations in the preforms, and their exposure-time dependence can be fitted with a stretched-exponential function.

We propose a new method for the simultaneous interrogation of conventional two-beam interferometers and Bragg grating sensors. The technique employs an unbalanced Mach-Zehnder interferometer illuminated by a single low-coherence source, which acts as a wavelength-tunable source for the grating and as a path-matched f ilter for the Fizeau interferometer, thus providing a high phase resolution output for each sensor. The grating sensor demonstrates a dynamic strain resolution of ~0.05 microepsilon/ radicalHz at 20 Hz, while the interferometric phase resolution is better than 1 mrad/ radicalHz at 20 Hz, corresponding to an rms mirror displacement of 0.08 nm.

The dispersion of picosecond pulses on reflection from efficient photorefractive fiber gratings is explored experimentally. Unlike simple measurements of reflectivity, this approach allows both the amplitude and the phase of the grating response to be probed as a function of frequency.

We report what is to our knowledge the first fiber distributed-feedback laser using a single Bragg grating at 1.5 microm written directly into a 2-cm-long Er(3+)-doped fiber codoped with Yb(3+). We obtained robust single-frequency operation by either using one end reflector or locally heating the center of the grating to create the necessary phase shift.

A new type of internally written grating filter is proposed and demonstrated in single-mode optical fibers. A photoinduced rocking birefringence is created by polarization interference between two counterpropagating and orthogonal linear-polarization modes at 514.5 nm. This birefringent grating has the correct submicrometer pitch for efficient phase-matched reflection of a linearly polarized mode into the orthogonally polarized counterpropagating mode. The phenomenon represents a new form of holography in which the image (the orthogonal counterpropagating mode) is reconstructed by scattering off a hologram consisting of a periodic birefringence.