Using longer wavelengths to write fiber Bragg gratings could ease the process-if researchers can speed up the writing at these wavelengths. By doping the fiber with holmium and thulium ions, researchers at the University of Sydney (Eveleigh, NSW, Australia) shorten the length of time required to write gratings with near-UV by a third, which makes this a more manufacturable process.1
Fiber Bragg gratings form the basis of many essential WDM components. Gratings are typically written with 244-nm radiation because germanium-doped silica fiber is highly photosensitive at this wavelength, and because argon ion lasers can provide this wavelength via frequency doubling. To write at 244-nm, however, the fiber coating (which is highly absorbing at this wavelength) must be stripped before writing, then replaced afterwards.
New coatings that are transparent around 355 nm make possible writing without stripping the coating, but the fiber core is roughly three orders of magnitude less absorbing of these wavelengths. Researchers have written Bragg and long-period gratings with greater than 30 dB of rejection using 1.3 W of near-UV light at 355-nm, but it took 30 min.2
Doping fibers with rare-earth ions can increase their photosensitivity. Justin L. Blows, Philip Hambley, and Leon Poladian at the University of Sydney, decreased the writing time by 30%, using 355-nm light on a fiber doped with rare-earth ions.
Fabricators at the university's Optical Fibre Technology Centre doped germanosilicate fiber with triply ionized holmium and thulium, which resonantly absorb at wavelengths near 355 nm. Their absorption spectra show that Ho3+ does not strongly absorb 1250- to 1650-nm radiation, so the fiber can be used in this extended telecommunication window (see figure). The Tm3+-doped fiber, while more ab sorbing in this range, may be sufficiently transparent for lightwave communications between 1300 and 1400 nm.
The fibers were hydrogenated at 200 atm at 100°C for roughly two days, then the coating was stripped (because it was not one of the near-UV coatings mentioned above). A 355-nm beam from a frequency-tripled Nd:YAG laser providing 15 kHz with an output power of 1 W, was focused onto the fiber through a phase-mask. The irradiance at the fiber was 200 W/cm2. A 1-mm-long slit placed in the beam reduced the power reaching the fiber to significantly less than 1 W. The fibers were exposed for 20 min.
Several days later, the gratings were annealed for 10 min. The gratings appear to have the same decay rates as more conventional fiber gratings, suggesting that neither the doping nor the change in wavelength exacts lifetime penalties.
Twenty minutes is still a long time for making Bragg gratings, although the benefits of being able to write through the coating may make this process worthwhile. The writing speed must only be increased by a factor of 10 to become comparable in speed to the writing of gratings using 244-nm radiation. The researchers have optimized neither the process nor the fiber composition, yet.
For more information contact Justin Blows at email@example.com.
- J. L. Blows et al., IEEE Photon. Tech. Lett. 14(7), 938 (July 2002).
- J. L. Guttman et al., Proc. 6th Intl. Workshop on Laser Beam and Optics Characterization, Munich, Germany (June 18-20, 2001).