Spectroscopic properties of fluorophosphate glass (779683)
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Appl. Phys. B 86, 83–89 (2007)Applied Physics BDOI: 10.1007/s00340-006-2410-zLasers and OpticsSpectroscopic propertiesof fluorophosphate glasswith high Er3+ concentrationm. liao1,2,ul. hu1z. duan1l. zhang3l. wen11Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences,Shanghai 201800, P.R.
China2 Graduate School of Chinese Academy of Sciences, Shanghai 201800, P.R. China3 College of Information Engineering, China Jiliang University, Hangzhou, P.R. ChinaReceived: 30 April 2006/Revised version: 25 June 2006Published online: 8 August 2006 • © Springer-Verlag 2006Fluorophosphate glass with 4 mol. % ErF3 content was prepared. The different scanning calorimetry wasconducted.
Raman spectrum, infrared transmission spectrum,absorption spectrum were measured. Fluorescence spectrumand lifetime of emission around 1.53 µm were measured under970 nm laser diode excitation. The metaphosphate content inthe composition is limited, but the maximum phonon energy ofglass amounts to 1290 cm−1, and is comparatively high. Thefull width at half maximum is about 56 nm, and is wider thanfor most of the materials investigated. The measured lifetime of4I413/2 → I15/2 transition, contributed by the high phonon energy, inefficient interaction of Er3+ ions, and low water content,amounts to no less than 7.36 ms though the Er3+ concentration is high. This work might provide useful information for thedevelopment of compact optical devices.ABSTRACTPACS 78.20.-e;142.70.Ce; 42.70.Hj; 32.70.CsIntroductionIn recent years researchers have paid a great dealof attention to miniature rare earth-doped glass devices.
Compact devices such as microchip lasers, amplifiers, losslesssplitters, and waveguide lasers have already been demonstrated [1–5]. Among them erbium-doped glass waveguideshave been of particular interest because of their potential foruse as active elements in integrated optical devices for opticalcommunication system.Miniature devices, which are only a few centimeters inlength, require the glass to provide a gain per unit lengthas large as possible.
The gain per unit length is linear proportional to NEr × τ × σemi , where NEr is the concentrationof Er3+ ions, τ is the fluorescence lifetime of Er3+ : 4I13/2 →4I15/2 transition and σemi is the stimulated-emission cross section, so in order to obtain high gain per unit length, the concentration of Er3+ ions should be as high as possible. However,the concentration is limited by the concentration quenching.Generally the concentration of Er3+ ions for the onset of concentration quenching is no more than 1 – 2 × 1020 ions/cm3u Fax: +86-21-39910393, E-mail: liaomeisong2005@yahoo.com.cnfor tellurite glasses or phosphate glasses [6, 7]. Above theconcentration, the lifetime decreases sharply with the increase of Er3+ concentration. The fluorescence lifetime ofEr3+ : 4I13/2 → 4I15/2 transition is a critical factor not onlybecause it contributes to high gain properties, but also because a long lifetime permits the required high populationinversion to be obtained under steady-state conditions by exertion of modest pump power [8].
In this paper, a glass withconsiderably high Er3+ concentration as well as long fluorescence lifetime was obtained. The Er3+ concentration is7.45 × 1020 ions/cm3, and the measured lifetime amounts tono less than 7.36 ms.2ExperimentalGlasses were prepared by the conventional melting and quenching method. The composition is as following: 19NaF-38RF2 -30AlF3 -5YF3 -4Al(PO3 )3 -4ErF3 .
RF2 represents the alkaline earth fluoride including MgF2 , CaF2 ,SrF2 and BaF2 . All starting materials are of analytical grade.A batch of 40 g was weighed and mixed, then was placed intoplatinum crucible and melt at 950 – 1000◦ . After completelymelting and fining, the glass liquid was cast into graphitemould. Then the glass was transformed to oven at transitiontemperature and annealed for 2 h. After that it was cooleddown to room temperature at a rate of 10 ◦ C/h.Glass was cut and polished carefully to meet the requirements for optical measurements.
To avoid the re-absorptionof Er3+ ions in the spectral measurements as possible aswe can, glass was as thin as 0.5 mm. Transition temperature Tg and onset of the crystallization temperature Txwere determined by different scanning calorimetry (DSC)using NETZSCH STA 409PC. Density was measured byArchimedes method in distilled water. The Er3+ concentration was calculated from the initial compositions and thedensity. The refractive index was measured on prism minimum deviation method.
Raman spectrum was measured byusing a Micro Raman Spectra Lab Ram-1B. Infrared transmittance was measured by a bio-rad fourier transform infraredspectrometer (FTIR) within the wavelengths 2.5 – 7.0 µm.The absorption spectrum was recorded with a Perkin-ElmerLambda A 900UV/VIS/NIR spectrophotometer over a spectral range of 300 – 1700 nm. The lifetime of 1.53 µm fluo-84Applied Physics B – Lasers and Opticsrescence was measured with pulse signal of 970 nm laserdiode (LD) and a HP546800B 100-MHz oscilloscope. Thepulse used for the decay time measurement is a rectangular pulse with pulse duration of 5 ms. The decay time of therectangular pulse is less than 0.1 ms, and has very little influence on the decay of the fluorescence.
The fluorescencespectrum was measured with a Triax 550 spectrofluorimeter on excitation at 970 nm. The upconversion luminescencespectra were obtained with a TRIAX550 spectrofluorimeterunder excitation of 970 nm LD. The position of the pumping beam and the width of the slit of the spectroscopy tocollect the luminescence signal were adjusted in order to obtain the upconversion luminescence as intense as we can. Theresolution is 1 nm for the absorption, emission and upconversion spectra.
All the measurements were taken at roomtemperature.3Results and discussionDSC graph is shown in Fig. 1. The transition temperature Tg and the onset of crystallization peak temperatureTx , listed in Table 1, are 398◦ and 479◦ respectively. The temperature gap between Tx and Tg , suggesting the resistanceagainst crystallization of glasses, is 81◦ . The concentration ofEr3+ , density and refractive index n d are indicated in Table 1as well.Raman spectrum of undoped glass is shown in Fig.
2.Four absorption peaks are prominent. They centre around400 cm−1 , 580 cm−1 , 1000 cm−1 and 1290 cm−1 , respectively, and are named a, b, c, d in turn. Peak a does not exist inprevious research about fluorophosphate glasses which contain no YF3 [9–12]. Further experiment suggests that peaka becomes intense with the increment of YF3 content, so peakFIGURE 1 Differential scanning calorimetry graph of thefluorophosphate glassFIGURE 2 Raman spectrum of the undoped fluorophosphate glass in the range of 150–1800 cm−1LIAO et al.Spectroscopic properties of fluorophosphate glass with high Er3+ concentrationTg(◦ C)Tx(◦ C)Tx –Tg(◦ C)NEr(×1020 ions/cm3 )(g/cm3 )nd397478817.453.3511.459Transition temperature Tg , onset of the crystallization temperature Tx , temperature gap Tx –Tg , concentration of Er3+ ions NEr , density and refractive index n d of fluorophosphate glassTABLE 1a is ascribed to vibration of Y–F bond. Peak b is ascribed tothe overlap vibration of Al–F and M–F [9–11], where M represents alkaline earth ions and Na+ .
Peak c is ascribed to thevibration of O–P–F [12]. Peak d, by which the phonon energy of glass is determined, is ascribed to the vibration of theO–P–O [13]. Generally peak d is characteristic for metaphosphate glasses and a decrease in its intensity can be ascribedto the shortening of linear metaphosphate chains [14]. It is interesting that the phonon energy is comparatively high thoughthe metaphosphate content in the composition is limited.Transmission spectrum is indicated in Fig. 3.
The absorption peak centered around 4.7 µm is ascribed to the secondharmonics of stretching vibration of P–O bonds [15, 16]. Theintensity of absorption peak centered 3.175 µm indicates thewater content in the glass. The absorption coefficient of OHgroup in the glass can be calculated according to formula (1):ln(θ0 /θ),(1)lwhere α is the absorption coefficient of OH group, l is thethickness of the sample, θ0 is the transmission of matrix glass,θ is the transmission at the absorption peak of OH group.
Hereα is 0.12 cm−1 . It is lower than most of other Er3+ dopedglasses in which water was cleaned by special measures suchα=85as using oxygen atmosphere [7, 17]. This is because the fluoride content in the composition is very high. OH groups weredecomposed by F− ion. HF gas was formed and then wasgiven off.The absorption spectrum is shown in Fig. 4. The cutoffband of ultraviolet region is 340 nm. The energy levels, determined according to the wavelengths at which the absorptionis the maximum, are listed in Table 2. Based on the absorption spectrum, the stimulated-emission cross section σemi ofEr3+ : 4I13/2 → 4I15/2 transition is calculated according to theMcCumber theory:σemi (λ) = σabs (λ) exp(ε − hν)/KT ,(2)where K is the Boltzmann constant, T is the temperature ofthe sample, ε is the net free energy required to excite one Er3+from the 4I15/2 state to 4I13/2 state at temperature T .
ε wasdetermined using the procedure provided in [18]. σabs is theabsorption cross section at given wavelength, and can be calculated by (3)σabs (λ) =2.303OD(λ) ,NEr l(3)in (3), OD(λ) is optical density. The dependences of absorption and stimulated-emission cross sections on the wavelength are indicated in Fig. 5. The maximum σabs and σemi ,listed in Table 3, are 5.92 × 10−21 cm2 and 6.77 × 10−21 cm2respectively.
The absorption and stimulated-emission crosssections decrease with the increment of Er3+ concentration.As is shown in Table 3, the maximum stimulated-emissioncross section of the fluorophosphate glass is higher than thatof germinate and silicate glasses in spite of the high Er3+concentration.FIGURE 3 Infrared transmittance spectrum of the fluorophosphate glass in the range of 2.5–7.0 µmExcited states4I13/24I11/24I9/24F9/22H11/24F7/24F5/22H9/2Energy levels (cm−1 )653210 25612 48415 36119 19420 53422 22224 6314G11/226 455Energy levels of Er3+ ion in thefluorophosphate glassTABLE 286Applied Physics B – Lasers and OpticsFIGURE 4 Absorption spectrum of the fluorophosphateglass in the range of 300–1700 nmFIGURE 5 Absorption cross section and stimulatedemission cross section calculated by the McCumber theoryof the fluorophosphate glassBased on the absorption spectrum, the Judd–Ofelt theory has been applied in evaluating the Judd–Ofelt parameterΩt for characterizing the spectral intensities.
Four absorption peaks 4I15/2 → 4F7/2 , 4I15/2 → 4F9/2 , 4I15/2 → 4I9/2 and4I15/2 → 4I11/2 were selected to fit the Judd–Ofelt model. TheGlassesThis glassPhosphate [21]Germanante [22]Silicate [23][U (t) ]2 for this work were collected from the literature [19].Judd–Ofelt parameters Ω2 , Ω4 , Ω6 are 7.36 × 10−20 cm2 ,2.01 × 10−20 cm2 and 1.11 × 10−20 cm2 respectively. The rootmean square δrms of the measured and calculated oscillatorstrength is 0.27 × 10−6.τmea (ms)σabs (×10−21 cm2 )σemi (×10−21 cm2 )FWHM (nm)σemi × τmeaσemi × FWHM≥ 7.367.95.926.778.005.685.5056555340≥ 49.863.2379.1440.0301.0220.0The measured lifetime τmea , the absorption cross section σabs , the stimulated-emission cross section σemi , the full width at half maximumFWHM, σemi × τmea and σemi × FWHM of Er3+ 4 I13/2 → 4 I15/2 in various glassesTABLE 3LIAO et al.Spectroscopic properties of fluorophosphate glass with high Er3+ concentrationFor the 4I13/2 → 4I15/2 transition, calculated lifetime τcalwas resolved based on the Judd–Ofelt parameters Ωt .
It is9.12 ms. Between the calculated lifetime τcal and measuredlifetime τmea , the following relationship exists:1τmea=1+ WNR ,τcal(4)where WNR is the non-radiative decay rate. However, hereτcal is a little lower than τmea which is 9.20 ms based on theexponential decay curve shown in Fig. 6. We ascribe it tothe radiative trapping of Er3+ ions in the glass.
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