Abstract

The possibility of using high-dopant concentrations in phosphorus-doped silica fibers makes it very attractive for making compact fiber amplifiers and lasers, particularly for single-frequency DFB lasers. Phosphorus doping, however, reduces the 240-nm germanium oxygen deficient centers (GODC) in germanosilicate fibers and makes grating formation very difficult in these fibers when writing at this wavelength.¹ Grating formation in phosphorus-doped silica fibers has been observed only after low-temperature H₂ loading and writing at 193 nm² or at 248 nm after the H₂-loaded fibers were heated to react the H₂ with the glass to form a strong absorption at ~240 nm.³ It has also been found that Type Ha gratings formed in germanosilicate fibers are much more stable than Type I gratings.⁴ Such stable gratings are very desirable, especially for applications at higher temperatures because they lengthen the device lifetime. Type IIa gratings, however, take a long time to form when writing at around 240 nm (>10 kj/cm²). We report, for the first time, index changes as high as 5.4 × 10⁻⁴ in phosphorus-doped germanosilicate fibers without H₂ loading and observation of Type IIa formation after short exposure to a 193 nm laser (~3 kj/cm²). The dynamics of the process was also studied.

© 1996 Optical Society of America