Measurement of UV-induced absorption and scattering losses in photosensitive fibers.

UV-assisted fabrication of gratings using photosensitive fibers is a well-established technique, based on the UV-induced permanent modification of the refractive index of the fiber material. As a result, the absorption coefficient is also changed. Here, we exploit the thermal sensitivity of whispering gallery mode resonances of the fiber itself to measure the profile of the heating along an irradiated fiber versus the input power and for different UV radiation intensities. Our technique allows discriminating between the absorption and scattering contributions to the overall losses, by comparing the results obtained with our technique with direct transmission measurements. Different photosensitive fibers are characterized by means of this technique. Nonuniform UV irradiation of photosensitive fibers can be characterized with this technique.

Fabrication of long period fiber gratings of subnanometric bandwidth.

This Letter reports on the fabrication of long period fiber gratings having subnanometric bandwidths in the 1500 nm spectral region. Large gratings have been photo inscribed in a high NA fiber; the grating pitch and the order of the HE cladding mode are optimized to produce gratings with a large number of periods and prevent the coupling to TE, TM, or EH modes. Resonances with a FWHM of 0.83 and 0.68 nm have been achieved for gratings 15 and 20 cm long, respectively; the free spectral range between the transmission notches is 125 nm. The polarization effects and the sensitivity of the gratings to temperature and to strain variations are presented as well.

Measurement of Pockels' coefficients and demonstration of the anisotropy of the elasto-optic effect in optical fibers under axial strain.

The elasto-optic effect in optical fibers under axial strain can be characterized by means of the whispering gallery mode (WGM) resonances of the fiber itself. This technique enables a direct measurement of the anisotropy, the determination of the individual Pockels' coefficients, and the study of the wavelength dependence. The method is based on a rigorous theoretical study of WGM resonances in cylindrical microresonators. The shift of the WGM resonances as a function of strain was measured for the TE and TM modes, showing a strong modal anisotropy. In particular, the shift rate for TE modes was 1.84 times the one for TM modes. From these measurements, experimental values for the Pockels' coefficients were obtained: p11=0.116 and p12=0.255 at 1531 nm, and p11=0.131 and p12=0.267 at 1064 nm. The dispersion of p44 with wavelength was shown to be 5% µm-1.