Defect production in silica fibers doped with Tm3+.

Defects in undoped germanosilicate fibers have a profound effect on optical properties in the visible region of the spectrum. Photoexcitation of these defects can produce a refractive-index change, which in turn can be used for many useful purposes, including the formation of gratings and second-harmonic generation in fibers. The defects responsible for this behavior have been associated with the germanium doping present in the core.`14 Glasses doped with rare-earth ions also show photochromic effects on excitation with blue light.5 Ceriumand europium-doped fibers have both shown photochromic effects, which have been used for photoinduced second-harmonic generation6 and to produce distributed-feedback gratings.7 Photochromic effects in thulium-doped germanosilicate fibers have been observed by Millar et al.,8 who observed changes in the optical absorption of the Tm3+doped fibers on irradiation with light at 476 nm. These effects were monitored by measurement of induced absorption at 514.5 and 633 nm before and after irradiation with light at 476 nm. 476-nm light is resonant with the 'G4 level of thulium, and it is also close to the wavelength observed to produce photochromic effects in undoped germanosilicate fibers through two-photon absorption, 488 nm. The absorption bands were observed to appear on irradiation at 476 nm and disappear on irradiation with 514.5-nm light. In this Letter we describe further experiments on thulium-doped silicate fibers. We have measured the spectrum of the induced absorption on irradiation at 476 nm for Tm3+-doped silica-germania and silica-alumina fibers. We have also studied the vibrational spectrum of these glasses both before and after photochromic effects occur and find that the Raman spectra show significant changes on irradiation. Peaks appear at 495 and 606 cm-'. These peaks are associated with defects in the silica network structure itself. These photochromic effects occur in both silica-alumina and silica-germania fibers, showing that the defects are not associated with the presence of germanium in the glass structure. We prepared the fibers used in these experiments, using modified chemical vapor deposition, and doped them with thulium using solution doping. 9 The concentrations of rare earth were 1000 parts in 106 (ppm) in the aluminosilicate fiber and 3000 ppm in the germanosilicate fiber, determined from optical absorption measurements based on cross sections determined by Smith and Cohen. 10 Raman spectra of the core regions of the fibers were taken with a Jobin-Yvon S3000 micro-Raman spectrometer. The pump wavelength for Raman measurements was 488 nm, and all spectra were taken in a backscattering geometry. The pump light used for Raman scattering slowly erased the changes in the glass, so care was taken to irradiate the fibers for the shortest possible time when spectra were taken. The micro-Raman geometry permits accurate coupling into the fiber, and in all cases the coupling was not changed between spectra taken before and after 476-nm irradiation. Absorption spectra were taken with a standard spectrometer/photomultiplier combination, with a Hamamatsu S1 photocathode photomultiplier and photon-counting electronics. The 476-nm pump light and white light were counterpropagated, which allowed us to measure the absorption spectra without ever changing the coupling of the white-light source, guaranteeing that the changes measured were real effects and not just artifacts of changes in the fiber coupling. Figure 1 shows the Raman spectrum of a germanosilicate fiber doped with 3000 ppm of thulium before and after irradiation with -50 mW of light at 476 nm for 1 min. As can be seen from the figure, peaks at 495 and 606 cm-' were enhanced by the irradiation. On further irradiation with -100 mW of 514.5-nm light, these peaks reduced in magnitude, although they did not return to their former size. A similar effect was seen in the aluminosilicate fiber, although in this fiber the effect was less pronounced because peaks at these frequencies are already present in the vibrational spectrum of aluminosilicate glass before irradiation. Trace (a) of Fig. 2 shows the change in the absorption spectrum of the germanosilicate fiber before and after irradiation with 476-nm light. A broad absorption band across most of the visible spectrum