Abstract

Absorption and luminescence spectroscopy can be useful in probing the defect state properties of glass optical fibers. Defects play an important role in photoinduced effects in Germanium-doped fibers such as second harmonic generation (SHG) and refractive index gratings. Using a cutback method, we present absorption measurements in communication single-mode fibers from 280 to 700 nm and find absorption to be generally the same in fibers as in preforms.^[1] Figure 1 shows the absorption spectra of two Ge-doped fibers and one pure silica fiber. The peak at 325 nm and the edge at 290 nm are associated with the Germanium Oxygen deficiency center (GODC) which, in preforms, has an absorption peak at 325 nm and a 1000 times larger peak at 242 nm. The 242 nm peak can not be resolved further because of the difficulty in measuring absorption levels higher than 3 dB/cm (.69 cm⁻¹) in fibers. These Germanium related details are not seen in the pure silica fiber. The effects of preparation for SHG on the absorption spectrum of a fiber are shown in Fig. 2. The 325 nm absorption band is increased from .25 to .6 dB/cm (.06 to .13 cm⁻¹) and a wide ultraviolet absorption band is introduced extending beyond 530 nm. A Gaussian curve fit reveals that the tail arises from a band centered at 280 nm with a width of 1.8 eV. This corresponds well with the Ge(1) defect observed in preforms which has a width of 1.97 eV and is centered at 280 nm. These changes are not seen in Ge-doped fibers which are exposed to the same intense infra-red preparation conditions but which do not prepare for SHG. Thus the Ge(1) defect is believed to play an important role in SHG in fibers.

© 1992 IQEC