Rare Earth Doped Photonic Glass Materials for the Miniaturization and Integration of Optoelectronic Devices Ju H. Choi1, Alfred Margaryan2, Ashot Margaryan2, Wytze van der Veer3 and Frank G. Shi1 1University of California Irvine, Dept. of Chemical and Materials Science Irvine, CA 92697 2AFO Research Inc., P.O. Box 1934, Glendale, CA 91209 3University of California Irvine, Dept of Chemistry, Irvine, CA 92697 1Phone: 949.824.7385, Fax: 949.824.2541 1Email: [email protected] Abstract 3 Er doped alkaline-free glass systems based on ,MgF2-BaF2-Ba(PO3)2-Al(PO3)3 (MBBA system), was investigated with the aim of using as high gain media The absorption spectra were -20 2 recorded to obtain the intensity parameters (Ωt) which are found to be Ω2= 4.47×10 cm , -20 2 -20 2 Ω4=1.31×10 cm , Ω6=0.81× 10 cm for the MBBA system. The emission cross section for the 4 4 I13/2 → I15/2 transition is determined by the Fuchtbauer-Ladenburg method and found to be 2.35 ×10-20 cm2 for the MBBA systems. Comparison of the measured spectroscopic values to those of Er3+ transitions in other glass hosts suggests that new MBBA systems are good candidates for broadband compact optical fiber and waveguide amplifier applications. Keywords: Rare earth ion, Fluorophosphate glass 1.0 Introduction size. In order to evaluate the potential for The use of compact lasers has attracted compact laser media, we conducted a increasing interest operating in the infrared systematic investigation of the spectroscopic region for optical communications, medical properties of Er3+ doped MBBA system. The and eye-safe light detecting and ranging previous results on Nd3+, Yb3+ doped applications in the visible region for data fluorophosphate glass systems already storage, undersea communications [1-3]. showed the strong potentials for each With the development of 980nm laser diodes, characteristics transitions of rare earth ions the diode pumped solid state lasers can [5-9] to develop new host materials with a provide a compact and efficient device with high emission cross section for compact gain the advantage of easy coupling with fiber media. We obtained intensity parameters, integrated optical systems. Specifically, the the radiative lifetime and gain coefficient for 4 4 3+ optically excited luminescence originating the I13/2 → I15/2 transition of Er . Finally, 4 4 from the dipole-forbidden I13/2 Æ I15/2 the potential of these systems for short fiber transition of Er3+ has a wavelength of 1.54 amplifiers or planar waveguides is evaluated µm that matches one of the minimum loss by comparison to other reported glass hosts. windows of commercial silica-based optic fibers. Typical Er3+ doped fiber amplifiers 2.0 Experimental procedures utilize approximately several meters of silica 2.1 Glass formation: fiber doped with a few hundred ppm weight The batch materials of 40MgF2-40BaF2- 3+ Er ions [4] 10Ba(PO3)2-10Al(PO3)3 system (the MBBA In the construction of integrated light system) was purchased from reagent grade amplifiers it is desirable to obtain the materials (City Chemicals, except for Er2O3, maximum gain with minimum component Spectrum Materials), all have better than 99.99% purity. The ingredients of the doped MBBA system was used in this work. glasses were weighed with 0.1% accuracy The refractive index nD, and Abbe number and mixed thoroughly for 3 hours. Next, the are 1.5885 and 68.1, respectively. raw mixed materials were melted in a 3.2. Absorption spectrum properties vitreous carbon crucible in Ar-atmosphere at of Er3+ in MBBA system. 1200 - 1250 °C. The melt was quenched by pouring it in a room temperature stainless 3.00E-024 MBBA system steel mold. Next, the samples were annealed below the glass transition temperature, 2.00E-024 around 400 - 430 °C, to remove internal stress, which was verified by examination 1.00E-024 with a polariscope (Rudolph Instruments). 400 600 800 1000 1200 1400 1600 The samples for optical and spectroscopic Wavelength (nm) measurements were cut and polished to a 3 size of 15×10×2mm . Figure 1: The absorption cross section of 2.2 Spectroscopic measurements: Er3+ in the range of 300nm to 1700nm. The refractive index nd of the samples was measured at 588 nm, using an Abbe The absorption spectrum of Er3+ ion refractometer (ATAGO). The absorption consists of 11 absorption bands centered at spectra were obtained at room temperature 1532, 804, 650, 544, 520, 488, 451, 406, 377, in the range of 400-1700 nm with a Perkin- 365, and 356 nm, corresponding to the Elmer photo spectrometer (Lambda 900). 4 absorptions from the ground state I15/2 to The lifetime and fluorescence spectrum of the excited states in the 4f11 electronic both samples was recorded using a chopped configuration respectively. The radiative Ti-Sapphire laser (Coherent 890) tuned to nature of trivalent rare earth ions in a variety 800 nm, pumped by the 514 nm line of an of laser host materials is usually investigated Ar laser (Innova 300), see Figure 3. The using the Judd-Ofelt model [10, 11]. The fluorescence signal was recorded with a 0.25 observed oscillator strengths fmed at each m monochromator (Oriel 77200), using a absorption peak is calculated by integration InGaAs PIN detector (Thorlabs DET 410), a the optical absorption spectra over each peak, trans-impedance amplifier and a Lock-In as given by following expression Eq. (2): amplifier (Oriel Merlin 70100). The lifetimes of both samples, was recorded with 2 the same system, recording the temporal mc α(λ) fmed = 2 ∫ 2 dλ behavior of the fluorescence signal with a πe N λ . (2) 100 MHz digital Oscilloscope. Here c is the velocity of light, N is the Er3+ 3.0 Results and discussion ion concentration (ion/cm3). α(λ) 3.1 Optical properties (=2.303Do(λ)/d) is the optical absorption The value of (nF-nC) and Abbe number coefficient at a particular absorption (νd) normally describes dispersion in glasses wavelength λ, which is calculated from the as below sample thickness d and the measured absorption density D0(λ). The oscillator ()n D − 1 (1) ν d = strengths as predicted by Judd-Ofelt model ()n F − n C fcal were also calculated. The oscillator strengths of the observed electronic where nD, nF and nC are the refractive indices transition are due to three interactions, at the D, C and F spectral lines. The variation electrical dipole (ed), magnetic dipole (md) of refractive index as a function of rare earth and electric quadrupole (eq). In most dopant concentration was systematically instances in the Er3+ system, the oscillator investigated in previous work. 2wt% of Er3+ -6 strength of the eq component is of the order deviation δrms of the fits was 8.52 x 10 , of 10−10, and the md component is of the which indicates that these fits are reliable. In order of 10−8. These contributions are thus Table I [12-15] these values are compared to unimportant compared with the ed those for other reported laser glasses. The contribution to the oscillator strength, which value of Ω2 indicates the strength of the −6 is in the order of 10 [1]. However, a covalent binding between the tri-valent rare significant contribution of the md earth ion and the host material [16, 17]. The 4 4 component is involved for the I15/2 → I13/2 value of Ω2 of the MBBA systems is smaller 3+ absorption transition for the Er . Therefore, than those of BK20 oxide glass and theoretical oscillator strengths f(aJ, bJ’) of phosphate glass, which show strong co- the J → J’ transition at the mean frequency valent bonds. The measured value is higher ν is given for both the electric and the than that of ZBLAN, which has a very high magnetic dipole transition by below Eq. (3) fluoride content causing strong ionic bonds and thus weaker co-valent bonds. The values 8π2mν (3) of Ω2 of the MBBA, system are comparable fcal()aJ,bJ' = 2 2 []χedSed()aJ,bJ' + χmdSmd ()aJ,bJ' 3()2J +1 he n to those of other fluorophosphate glass with similar fluorine content. where me is the mass of the electron. e and h The effect of co-valent bonding between are the charge of the electron and plank’s the Er3+ ions and the host material can be 2 2 constant, respectively. χed =n(n +2) /9 and understood in terms of the Judd-Ofelt 3 3+ χmd =n are local field corrections and are parameters. In case of a Er doped system functions of the refractive index n of the the t=2 transition matrix elements [U(2)]2 of 4 4 medium. Sed and Smd are the electrical dipole the transitions between the I11/2, I13/2 and 4 and the magnetic dipole line strength I15/2 states are very small. The quality of respectively these transitions for laser operation is thus 3.2. Intensity parameters & quality factor characterized by Ω4 and Ω6 via the The Judd-Ofelt intensity parameters spectroscopic quality factor Q (=Ω4/Ω6), as were determined by a least squares fit of the introduced by Kaminskii [18]. The Q values theoretical (free ion) oscillator strengths to are found to be 1.62 for the MBBA system. the measured (glass matrix) values obtained These values are larger than those found in from optical absorption spectra. By fitting most laser glasses as well as in FP20, see the measured oscillator strengths fmed to the Table I. The MBBA glass system is thus calculated values fcal we obtained the better suitable for laser applications than following values for three Judd-Ofelt other published glass systems -20 2 parameters Ω2= 4.47×10 cm , Ω4= .