Increasing fiber photosensitivity to near-UV radiation by rare earth doping

The writing of Bragg gratings with near-UV light is desirable because there exist fiber coatings transparent to this wavelength. This will enable gratings to be written without first removing the protective coating. However, the writing time is typically long when near-UV wavelengths are used. We demonstrate that the writing time can be substantially reduced by using fibers doped with holmium and thulium rare earth ions. These ions are absorbing at these wavelengths but transparent at telecommunication wavelengths. We used the 355-nm output from an efficient all solid-state laser to write the gratings and observed a 30% reduction in writing time.