Defect production in silica fibers doped with Tm^3+

Abstract

Irradiation of Tm 3+ fibers with blue light at 476 nrm induces a broad-bandwidth loss in these fibers. We have measured the spectral dependence of the loss for both silica-germania and silica-alumina fibers and show through micro-Raman studies of the core regions of the fibers that this induced loss is correlated with the production of structural defects in the glass host.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 Cerium-and europium-doped fibers have both shown photochromic effects, which have been used for photoinduced second-harmonic generation 6 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 Tm 3 +-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 'G 4 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 Tm 3 +-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. 1 0 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 ba...

Progress and Summary of Photodarkening in Rare Earth Doped Fiber

“…When the TDF is exposed to strong blue light (475 nm), the visible and near-infrared transmission spectrum of the fiber is significantly reduced, and it is easy for the Ge 4+ ions in the fiber to produce two-photon absorption close to the pump light. In 1993, Brocklesby et al [9] compared Tm-doped silica fiber and Tm-doped germanosilicate fiber; it was found that thulium ion itself affects the generation of PD. In the same year, Broer et al [10] found that Tm-doped silica fiber appeared PD when excited by a near-infrared laser (1064 nm).…”

“…It is known that Tm ions with 1 G 4 level show a strong absorption band at about 488 nm and Tm ions at 1 G 4 level can be excited to 1 I 6 -3 P 0 level by ESA, so as to up-convert 488 nm radiation to its ultraviolet [61]. PD was observed in Tm-doped silica optical fibers pumped at 475 nm wavelength [8,9].…”

Transverse mode instability and thermal effects in thulium-doped fiber amplifiers under high thermal loads

“…It is generally accepted that the additional losses are related to the formation of color centers (CC) [46]. Photodarkening has also been reported in thulium-doped aluminosilicate fibers [47][48][49] and the degradation is commonly believed to originate from upconversion-induced, short-wavelength emission of the thulium ion. Another process that might be correlated to photodarkening in TDFA is a reduction of the cross-relaxation rate, which significantly contributes to the overall efficiency and heat-load upon amplification.…”

Manufacture and spectroscopic analysis of Tm3+-doped silica glass and microstructure optical fiber through the laser sintering technique

“…20) The 490 and 604 cm −1 are defect bands connected with the vibrations of four-membered and three-membered siloxane rings, respectively. 21) The broad peak at 798 cm −1 is due to symmetric Si-O-Si stretching mode. 22) The weak and broad peaks at 1077 and 1209 cm −1 originate from antisymmetric Si-O-Si stretching vibrations.…”