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= . -20 2 -20 2 1.31×10 cm and Ω6= 0.81× 10 cm are obtained for the MBBA system. The . -20 2 -20 2 -20 2 Glasses Ω2 (10 cm ) Ω4 (10 cm ) Ω6 (10 cm ) Ω4 /Ω6 Ref BK20 5.66 1.84 1.18 1.56 12 ZBLAN 2.20 1.40 0.91 1.54 13 Phosphate 6.65 1.52 1.11 1.34 14 FP20 4.71 1.61 1.62 0.99 15 MBBA 4.47 1.31 0.81 1.62 Current work Table I: Comparison of Judd-Ofelt parameters of Er3+ doped MBBA system and other reported laser glasses

3.3 The emission cross section and gain The efficiency of a laser transition is 4 4 coefficient of the I13/2 → I15/2 transition. evaluated by considering stimulated emission cross-section (σem(λ)). In this work, σem(λ) was determined from the emission 3 as a function of the population inversion γ, spectrum using Fuchtbauer-Ladenburg using the relation below method (FL) [19]

G(λ) = γσem (λ)− (1− γ)()σabs λ (5) β λ 4 A σ = J − > J ' p ra em 8πcn λ 2 ∆ λ ()p eff (4) Using this equation, we calculated the gain spectra as shown in Fig. 3. where λp is the peak wavelength of the emission, λeff is the width of the emission 4 line, βJ->J’ is the branching ration, which is in γ=1 4 4 3 case of the I13/2 → I15/2 transition equal to 1, γ=0.8 γ=0.6 ) c is the speed of light in vacuums, and n(λp) 2 2 γ=0.4 γ=0.2 is the refractive index at emission peak , cm -20 1 γ=0 wavelength. In our case an effective line 0 width is used instead of the full width at half maximum to compensate for the a- -1 symmetric profile of the emission line. Fig. -2 Gain coefficient (x10 2 shows the absorption cross section, σabs(λ), -3 and the emission cross section, σem(λ), -4 determined by FL method. 1400 1450 1500 1550 1600 1650 1700 Wavelength (nm)

Figure 3: Gain coefficient in the eye-safe 0.7 range of Er3+ in the MBBA system 6 MBBA system 5 0.6 Note that the gain will be positive at 1536 4 0.5 nm, when the population inversion is larger 3 than 0.5. The maximum value for the gain is 0.4 achieved in the case of complete population 2 inversion (γ = 1), in this case cross section Absorbdance (a.u.) 1 0.3 −20 2 Emission Intensity (a.u.) Intensity Emission for stimulated emission is 2.35 × 10 cm 0 for the MBBA system. 0.2 1350 1400 1450 1500 1550 1600 1650 1700 Wavelength (nm) 4.0 Conclusions The novel MBBA system was successfully Figure 2: Absorption cross section and developed and the absorption and emission measured emission cross section of Er3+ in spectra of Er3+ were measured and analyzed. the MBBA system. Three intensity parameters are found to be -20 2 -20 2 Ω2= 4.47×10 cm Ω4=1.31×10 cm For the MBBA system, the peak -20 2 Ω6=0.81× 10 cm for the MBBA system. absorption cross sections of σabs(λ) turned out to be 1.58 × 10-20 cm2 and the peak The strong emission bands were observed at 1536 nm and the effective bandwidths were emission cross sections of σ (λ) are 1.86 × em found to be 91 nm. Emission cross section 10-20 cm2. Comparing the Er3+ ion to a determined by FL method for the 4I simplified two level system, we assume the 13/2 →4I transition are found to be 2.35 × population is either in the 4I ground state 15/2 15/2 10−20 cm2 and population inversion of above or the 4I excited state. In this case the 13/2 50 % were obtained. These spectroscopic optical gain properties are directly results show that these novel materials are associated with the absorption and emission strong candidates for developing broadband cross sections. Gain spectra is shown in Fig. optical amplifiers and compact fiber lasers. References 11 G.S. Ofelt: “Intensities of crystal spectra 1. S. Taccheo, P. Laporta, S. Longhi, O. of rare-earth ions”. J. of Chem. Phys., Svelto, and C. Svelto, ”Diode-pumped 37, 1962, p. 511. bulk erbium-ytterbium lasers” Appl. 12. S. Tanabe, T. Ohyagi, N. Soga, and T. Phys. B: Lasers Optics. 63,1996,.p. 425. Hanada: “Compositional dependence of 2 K. Seneschal, F. Smektala, S. Jiang, T Luo, Judd-Ofelt parameters of Er3+ ions in B. Bureau, J. Lucas, N. Peyghambarian, alkali-metal glasses”. Phys. Rev. B. 46, “Alkaline-free phosphate glasses for 1992, p. 3305. ultra compact optical fiber amplifiers at 13. J. McDougall, D.B. Hollios, and M.J. 1.5µm” J of Non-Cryst. Solids 324 2003, Payne: “Spectroscopic properties of p.197. Er3+ in ZBLAN fluoride glass”. Phys. 3. N. P. Barnes, W. J. Rodriguez, and B. M. Chem. Glasses 37 1996, p. 256. Walsh, “Ho:Tm:YLF laser amplifiers” J 14. X.L. Zou, and T. Izumitani: Opt. Soc. Am. B. 13 1996, p. 2872. “Fluorescence mechanisms and 4. M. Shimizu, M. Yamada, M. Horigucho, dynamics of Tm3+ singly doped and Yb3+, E. Sugita, “Gain characteristics of an Tm3+ doubly doped glasses”. J. Non- Er3+-doped multicomponent glass Cryst. Solid. 162, 1993, p. 68. single-mode optical fiber” IEEE Photon. 15. A. Lira, I. Camarillo, E. Camarillo, F. Tech. Letter. 2 1990, p. 43. Ramos, M. Flores, and U. Caldino: 5. J. H. Choi, F. G. Shi, A. Margaryan, A. “Spectroscopic characterization of Er3+ Margaryan “Spectroscopic properties of transitions in Bi4Si3O12” J. Phys.: Yb3+ in heavy metal contained Condensed. Matter 16, 2004, p. 5925. fluorophosphate glasses” Mat. Res. 16. W.T. Carnall, J.P. Hessler, and F. Bull., 40, 2005, p. 2189. Wagner: “Transition-probabilities in 6. J. H. Choi, F. G. Shi, A. Margaryan, A. absorption and fluorescence-spectra of Margaryan “Judd-Ofelt analysis of lanthanides in molten lithium nitrate- spectroscopic properties of Nd3+ doped potassium nitrate eutectic”. J. Phys. novel fluorophosphate glass” J. of Chem., 82, 1978, p. 2152. Lum.114, 2005, p. 167. 17. S. Tanabe, T. Ohyagi, N. Soga, and T. 7. J. H. Choi, F. G. Shi, A. Margaryan, A. Hanada: “Compositional dependence of Margaryan “Optical transition Judd-Ofelt parameters of Er3+ ions in properties of Yb3+ in new alkali-metal glasses”. Phys. Rev. B. 46, fluorophosphate glasses with high gain 1992, p. 3305. coefficient” Journal of Alloy and 18. J. McDougall, D.B. Hollios, and M.J. Compound 396, 2005, p. 79. Payne: “Spectroscopic properties of 8. J. H. Choi, F. G. Shi, A. Margaryan, A. Er3+ in ZBLAN fluoride glass” Phys. Margaryan “Refractive index and low Chem. Glasses 37, 1996, p. 256. dispersion properties of new 19. M. P. Hehlen, N.J. Cockroft, T.R. fluorophosphate glasses highly doped Gosnell, and A.J. Bruce: ”Spectroscopic with rare earth ions” J. Mat. Res., 20, properties of Er3+ and Yb3+doped soda- 2005, p. 264. lime silicate and aluminosilicate 9. Margaryan, A. Margaryan, J. H. Choi, F. glasses” Phys. Rev. B. 56, 1997, p. 9302. G. Shi “Spectroscopic properties of Mn2+ in new bismuth and lead contained fluorophosphates glasses” Appl. Phys. B 78, 2004, p. 409. 10. B.R. Judd: “Optical absorption intensities of rare earth ions” Phys. Rev., 127, 1962, p. 750.