Combined excitation-emission spectroscopy of bismuth active centers in optical fibers S. V.Firstov,1 V. F. Khopin,2 I. A. Bufetov,1,* E. G. Firstova,1 A. N. Guryanov,2 and E. M. Dianov1 1 Fiber Optics Research Center of the Russian Academy of Science, 38 Vavilov St., 119333, Moscow, Russia Institute of High-Purity Substances of the Russian Academy of Sciences, 49 Tropinin St.,603950, Nizhnii Novgorod, Russia *
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Abstract: For the first time, 3-dimensional luminescence spectra (luminescence intensity as a function of the excitation and emission wavelengths) have been obtained for bismuth-doped optical fibers of various compositions in a wide spectral range (450-1700 nm). The bismuthdoped fibers investigated have the following core compositions: SiO2, GeO2, Al-doped SiO2, and P-doped SiO2. The measurements are performed at room and liquid nitrogen temperatures. Based on the experimental results, the positions of the low-lying energy-levels of the IR bismuth active centers in SiO2- and GeO2-core fibers have been determined. Similarity of the energy-level schemes for the two core compositions has been revealed. ©2011 Optical Society of America OCIS codes: (160.2290) Fiber materials; (160.2540) Fluorescent and luminescent materials; (060.2320) Fiber optics amplifiers and oscillators.
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Received 21 Jul 2011; revised 31 Aug 2011; accepted 6 Sep 2011; published 22 Sep 2011
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1. Introduction Bismuth-doped glasses and optical fibers are new active optical materials featuring a broad luminescence spectrum in the spectral range 1000-1700 nm, the luminescence lifetime in a number of such glasses being as large as 0.1-1 ms [1–4]. Interest in these new materials is due to the possibility to harness them in lasers and for the amplification of optical signals in the range 1200-1500 nm in the next-generation optical communication. After the first demonstration of Bi-doped fiber laser in 2005 [5], laser generation in bismuth-doped fibers have been obtained in the range 1150-1550 nm (see, for example, the review paper [6] and latest results [7, 8]). Recently Bi-doped fiber amplifiers with a gain of 20-25 dB under LD pump power of ~100 mW have been demonstrated for a spectral region of 1300-1340 nm and 1409-1445 nm [9, 10]. The absence of an adequate model of the bismuth active centers (BAC) is a serious obstacle on the way of advancing bismuth lasers and amplifiers. A number of hypotheses on BAC nature has been already formulated ([11–13]); but none of the models discussed has been confirmed by direct experimental data. BACs best manifest their gain properties at a low bismuth concentration (typically below 0.02 at.%), consequently, at a low BAC concentration. This fact necessitates increasing the sensitivity of the investigation methods applied. We use a widespread luminescence analysis, which has been also used in the numerous preceding works in this field (see review [6] and refs. therein). A peculiarity of our paper is that we have carried out detailed measurements of the luminescence intensity (Ilum) depending on both emission (λem) and excitation (λex) wavelengths, which were varied in a wide spectral range, from 450 to 1700 nm. The data measured allowed us to construct contour graphs of dependence Ilum(λem, λex). In this way the BAC luminescence properties were investigated in bismuth fibers with simplest host glasses: ν-SiO2 and ν-GeO2. In such fibers, one may expect to obtain easy-to-interpret results. Thereafter, Ilum(λem, λex) spectra were measured in bismuth fibers with more complex host glasses: aluminum- and phosphorus-doped silicas. These results are of much practical interest, because optical amplification and laser generation in most previous papers have been observed in bismuth fibers with an aluminum-, germanium, or phosphorus-doped silica host. Preliminary reports on the investigation of Bi-doped silica fibers free of other dopants were published in [14, 15]. 2. Experimental samples and methods The luminescence spectra were measured in four fibers with different core compositions (Table 1). The bismuth concentration did not exceed the sensitivity threshold of our analytical device (0.02 at.%) and, therefore, it is not indicated in Table 1. The relative BAC concentration can be estimated from the absorption spectra (Figs. 1-4). All the fibers had an outer diameter of 125µm. They were either single-mode (at the wavelength of 1.2 µm), or multimode, which was found to be of no significance.
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Received 21 Jul 2011; revised 31 Aug 2011; accepted 6 Sep 2011; published 22 Sep 2011
26 September 2011 / Vol. 19, No. 20 / OPTICS EXPRESS 19552
Table 1. Designation, core composition, and fabrication method of the fibers investigated No. 1 2 3 4
Designation SBi GBi ASBi PSBi
Core composition 100SiO2+Bi 100GeO2+Bi 3Al2O3+97SiO2+Bi 10P2O5+90SiO2+Bi
Fabrication method powder-in-tube MCVD MCVD MCVD
Fiber SBi was fabricated by means of the powder-in-tube technology [14]; its core was surrounded by a silica cladding with a reduced refractive index due to fluorine doping. The rest fibers were MCVD-produced, with all the dopants being doped from vapor phase. In order to reduce the fiber optical loss, at the very beginning of the MCVD-process a layer of high-purity F- and P-codoped silica was deposited onto the inner wall of a Heraeus F300 subsrtate tube (the P2O5 concentration was СP2O5≤1 mol%, and the fluorine concentration was chosen so as to have the resultant refractive index equal to that of silica). After that, the core layers were deposited. It is worth noting that during fiber drawing, phosphorus, fluorine, and SiO2 could diffuse into the core periphery from the cladding. All the fibers were drawn in the same conditions, which included heating to ~2000 °С followed by fast cooling in the standard drawing process to the below glass transition temperature. Formation of BACs is determined by presence of the bismuth atoms, theirs concentration, by the core composition, as well as by the thermal treatment of the fiber. In the general case, all these factors can define the redox processes in glass, the final bismuth valence state, and, consequently, presence or absence of BACs and their structure. Because in our fibers the heat treatment in the drawing process (melting with subsequent shock cooling) was the same, the distinctions in the BACs luminescence properties were mainly due to the distinctions in the core composition. The optical loss was measured by the common cut-back technique. Luminescence was received through the fiber lateral surface in order to exclude the re-absorption phenomena. To vary the luminescence excitation wavelength, we used an SC450 supercontinuum light source of Fianium. Narrow-band radiation (Δλ=3 nm) was isolated from a broad spectrum with the help of an acousto-optic filter and launched into the fiber core. The emission spectra were registered by an HP 71450B spectrum analyzer in the range 875 nm
