Overtone absorption and Raman spectra of H2 and D2 in silica optical fibers

We have recorded quantitative optical absorption and Raman spectra of H
2
and D
2
diffused into silica optical fibers at a pressure of 500 atm. The absorption spectra recorded were fundamental through the third overtone for H
2
(0.6 through 2.5 μm) and the first through fourth overtone for D
2
(0.7 through 1.7 μm). The observed spectra show fine structure similar to what has been seen in gas-phase Raman spectra. By considering all our absorption and Raman spectra for both H
2
and D
2
, including temperature dependence and anharmonicities, and comparing the spectra to gas-phase Raman spectra, we have made unambiguous assignments for all observed spectral lines. As a result of these assignments, we conclude that H
2
and D
2
are trapped singly in interstitial sites in silica and are free to rotate. The absorption spectra provide information on the added loss due to H
2
saturation of silica optical fibers. For example, extrapolating the experimental results to the case of a fiber saturated under 1 atm of H
2
, the added loss due to the intense first overtone Q-branch line is 14 dB/km at 1.2445 /im and 0.14 dB/km at 1.30 μm. High loss due to the fundamental absorption of H
2
occurs beyond 1.5 μm; at 1.55 μm it is 0.8 dB/km for 1 atm of H
2
. There is, however, a transmission window in H
2
-saturated fibers between 1.32 and 1.45 μm.