Golding, Paul S.

Jackson, Stuart D.

King, Terence A.

Pollnau, Markus

English

A detailed characterization of energy level lifetimes and energy-transfer processes in Er3+-doped and Er3+, Pr3+-codoped ZBLAN bulk glasses is presented. Energy transfer upconversion parameters from the Er3+ 4I13/2 and 4I11/2 levels have been measured and are compared to energy transfer from Er3+ to a Pr3+ codopant. To assess the practicality of energy transfer a high power, diode pumped, 3 μm, Er3+, Pr3+- codoped ZBLAN fiber laser is presented

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Determination of energy transfer parameters in Er3+-doped and Er3+, Pr3+-codopedZBLAN glassesPaul S. Golding, Stuart D. Jackson*, Terence A. KingDepartment of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK.paul@fs4.ph.man.ac.ukMarkus PollnauOn leave from: Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland.Abstract:  A detailed characterization of energy level lifetimes and energy-transferprocesses in Er3+-doped and Er3+, Pr3+-codoped ZBLAN bulk glasses is presented.Energy transfer upconversion parameters from the Er3+ 4I13/2 and 4I11/2 levels have beenmeasured and are compared to energy transfer from Er3+ to a Pr3+ codopant.  To assessthe practicality of energy transfer a high power, diode pumped, 3 µm, Er3+, Pr3+- codopedZBLAN fiber laser is presented.OCIS codes: (160.5690) Rare earth doped materials, (300.6280) Spectroscopy, fluorescence andluminescence.IntroductionErbium 3 µm fiber lasers have long been suggested as devices with great potential for surgical use,owing to the strong absorption of radiation by water at this wavelength, and their efficient, compact,design.  High output powers from Er3+-doped ZBLAN fiber lasers is often limited, however, by ground-state bleaching arising from undesired excited-state absorption (ESA) from the 4I13/2 and 4I11/2 laserlevels.  This can lead to saturation of the 2.7 µm emission by competitive co-lasing at 850 nm [1].  Asolution to this problem is the cascade laser system which has been shown to provide unsaturated outputbut requires high pump intensities and careful mirror selection [2].  Under low-brightness diode laserexcitation, however, the cascade laser mechanism is no longer attainable and a different approach isrequired.  Two different operational regimes have been suggested: quenching of the 4I13/2 lifetime byenergy transfer (ET) to a Pr3+ codopant or energy recycling from 4I13/2 level by energy transferupconversion (ETU) (Fig. 1).  As part of a study to determine ultimately which of these provides thebest approach, we have measured the intrinsic lifetimes of the Er3+ 4I13/2, 4I11/2, and 4S3/2 / 2H11/2 levels,and investigated the major energy-transfer processes such as ETU from the Er3+ 4I13/2 and 4I11/2 laserlevels, ET from these levels to the corresponding levels of a Pr3+ codopant, and cross relaxation (CR)from the thermally coupled Er3+ 4S3/2 / 2H11/2 levels in Er3+-doped and Er3+, Pr3+- codoped ZBLAN glass.The macroscopic energy-transfer parameters of all these processes are derived and their dependence onEr3+ and Pr3+ concentrations is investigated.  In addition, the validity of energy transfer for high powerlaser operation is assessed with the design and construction of a 3 µm, Er3+, Pr3+ codoped fiber laserMethodLifetimes as well as the parameters of ETU, CR, and ET for Er3+-doped and Er3+,Pr3+-codoped inZBLAN bulk glasses (Le Verre Fluoré, France) were determined by measuring luminescence decay© 2000 OSA/ASSL 2000from the 4I13/2, 4I11/2 levels, and the thermally coupled 4S3/2 / 2H11/2 levels to the 4I15/2 ground statefollowing direct excitation of these levels at 532, 979, and 1510 nm, respectively.The excitation source for the spectroscopic measurements was an in-house KTP opticalparametric oscillator (OPO) pumped by a frequency-doubled, 10-Hz, Q-switched Nd:YAG laser(Spectron SL802G) at 532 nm.  To excite the 4I13/2 energy level a center pump wavelength of 1510 nmwas selected.  For the 4I11/2 energy level the center pump wavelength was tuned away from theground-state absorption (GSA) peak at 973 nm to 979 nm to ensure that GSA was significantly greaterthat ESA [3].  The lifetime of the thermally coupled 4S3/2 / 2H11/2 energy levels was measured after directexcitation by the second harmonic of the Q-switched Nd:YAG laser at 532 nm.  The luminescence fromthe samples was imaged onto the entrance slit of a 30-cm monochromator, and the resulting signal wasdetected using an InGaAs photodiode (Hamamatsu G5746-01) to detect the 4I13/2 luminescence at1560 nm and a silicon photodiode (IPL 10530DAL) to detect the 1010-nm (4I11/2) and 550-nm (4S3/2)luminescences.  The spectra were analyzed and ET, ETU and CR parameters were calculated accordingto a previously published method [4].4I13/24I15/24I11/24I9/24F9/22H11/24S3/2Pr3+Er3+3F23H43H53H63F41G43F31D24F7/2ETETETUETUCRCR3 µm LASERTRANSITIONFig 1. Partial energy-level diagram of Er3+ and Pr3+ in ZBLAN glass.ResultsErbium singly doped ZBLAN.The fluorescence lifetimes of the 4I13/2, 4I11/2, and 4S3/2 erbium energy levels are shown in Fig 2.  For thelowest doped samples the measured luminescent lifetimes of the 4I13/2 and 4I11/2 levels were 9.0 ± 0.2 msand 6.9 ± 0.1 ms respectively.  The effect of the Er3+ concentration upon the luminescent lifetime of the4S3/2/2H11/2 levels is also shown in Fig. 2.  The 4S3/2 level is in thermal equilibrium with the 2H11/2 level,therefore, after excitation, the population relaxes to the Boltzmann distribution within the two levels.  Athigh Er3+ concentrations, the luminescent lifetime of these levels is shortened as a result of the CRprocesses (2H11/2, 4I15/2) → (4I9/2, 4I13/2) and (2H11/2, 4I15/2) → (4I13/2, 4I9/2).  The observed luminescencefrom the 4S3/2 / 2H11/2 levels shows a dramatic reduction in lifetime with a value of 518 ± 6 µs for an Er3+concentration of 4×1019 cm-3 falling to 21 ± 1 µs for an Er3+ concentration of 1.4×1021 cm-3.© 2000 OSA/ASSL 20000.0 5.0x10201.0x1021 1.5x10210.010.11104S3/24I11/24I13/2Lifetime (ms)Er3+ concentration (cm-3)Fig. 2. Erbium energy level lifetimes.An example luminescence decay curve is shown in Fig. 3.  It can be seen that the ln(I/I0) versustime curve begins with a nonlinear section which is dependent upon excitation density, N0, as a result ofion-ion interactions.  This is followed by an exponential decay which is the intrinsic decay of the erbiumion. This decay was used to calculate the fluorescence lifetimes shown in Fig. 2.  ETU parameters werecalculated according to a previously published method [4], as follows.  If N represents the time-dependent population density of the excited level, then the rate equation for N is given by:2ETU N2WNdtdN −τ−= . (1)where τ is its intrinsic lifetime, and WETU is the macroscopic ETU parameter. It has been shown [5] thatwith N(t=0) = N0, integration yields)]t/exp([1N2W1)t//N(t)]exp([N 0ETU0 τ−−τ=−τ− . (2)Replotting Fig. 3. with respect to equation (2) gives Fig. 4.  From the initial gradient of this curve, whichhas a value of 2WN0τ, the upconversion parameter can be obtained.-2.5-2-1.5-1-0.500 1 2 3 4 5 6 7 8Time (ms)ln(I/I0)N0 = 2.2 x 1019 cm-3UpconversionExponentialdecayN0 = 8.6 x 1018 cm-30123450 0.1 0.2 0.3 0.4 0.5 0.61-exp(-t/τ)N0/N(t)exp(-t/ τ)-1 N0 = 2.2 x 1019 cm-3N0 = 8.6 x 1018 cm-32WN0τFig. 3. Luminescence decay curve from the 4I13/2 energylevel. (Er3+ concentration = 1.4 x 1021 cm-3). N0 is the initialexcitation density.Fig 4.  Linearization of Fig. 3 with respect to W. N0 isthe initial excitation density.© 2000 OSA/ASSL 2000The degree of upconversion is dependent upon both the excited-state population density for agiven concentration and the Er3+ concentration within a sample.  Energy transfer upconversionparameters have been calculated for the 4I11/2 and 4I13/2 and are displayed on Fig. 5. along with the 4S3/2 /2H11/2 cross relaxation parameters.  The findings from this investigation show strong upconversionparameters for the 4I13/2 level with the 4I11/2 level parameter being approximately three times lower.These values are comparable to that of crystal hosts, e.g. LiYF4:Er3+ [6], and as such should provide aneffective mechanism for energy recycling in ZBLAN, provided that optimum concentrations areavailable.4I13/24I11/24S3/2W (cm3 /s)Er3+ concentration (cm-3)Fig. 5. Energy transfer upconversion and cross-relaxation parameters as a function of Er3+ concentrationPraseodymium codoping.Pr3+ has been suggested as a possible codopant for erbium in the crystal host BaY2F8 and LiYF4 [6], andin ZBLAN glasses [7], to reduce the lifetime of the 4I13/2 lower laser level.  The introduction of a smallamount of Pr3+ to the samples rapidly depletes the 4I13/2 level whilst having a much lesser effect on the4I11/2 level (Fig. 6.).  Increasing the Pr3+ concentration leads to enhanced quenching of both levels whilstretaining the large lifetime difference between them.  The overall change in intrinsic lifetime of theenergy levels, however, decreases with increasing Pr3+ concentration to the maximum level used.Therefore, we suggest that only small amount of Pr3+ is necessary in an efficient codoped fiber lasersystem.The luminescence decay from the codoped samples can be described by the following equation:1/τeff(Er*) = 1/τ(Er*) + WET N(Pr) (3)where τeff(Er*) is the effective lifetime of the excited level of Er3+ in which ET is observed, τ(Er*) is theintrinsic lifetime of the same level, N(Pr) is the Pr3+ concentration in the sample and WET is the energytransfer parameter.  The ET parameters have been calculated for each of the codoped samples (Fig. 7),where it can be seen that for each energy level the parameters are reasonably insensitive to Pr3+concentration, but increases with erbium concentration in a similar manner to the ETU parameters.  Thissuggests that energy migration within the Er3+ 4I13/2 and 4I11/2 levels is fast.© 2000 OSA/ASSL 20000.010.11100Pr3+ concentration (cm-3)Lifetime (ms)4I11/24I13/2[Er 3+] = 8 x 1020  cm-3[Er 3+] = 1.4 x 10211x10202x10203x1020 cm-3Pr3+ concentration (cm-3)W (cm3 /s)4I13/24I11/2[Er  cm[Er3+] = 1.4 x 1021 cm-33+] = 8 x 1020 -310-1510-1610-1710-1810 1910 2010 21Fig. 6. Effect of Pr3+ codoping on Er3+ lifetime. Fig. 7. Energy transfer parameters.High power Er, Pr – codoped fiber laser.In order to show the usefulness of the Pr3+ as a suitable codopant for Er3+ doped fiber lasers, we havedemonstrated 1.7W of unsaturated output from a diode pumped Er3+, Pr3+-doped ZBLAN double cladfiber laser [8].  The fiber laser was pumped at a wavelength of 790 nm, which excites the 4I9/2 level ofEr3+, and the core was doped with 6.3 × 1020 cm-3 Er3+ and 5.4 × 1019 cm-3 Pr3+.  The experimental setupis shown in Fig. 8.  A mirror which was highly (∼ 99%) reflecting at 790 nm placed at 45o to the pump-light direction steered the pump light onto the input end of the fiber while also effectively transmitting(>99.5%) the fiber-laser output.  Two different cavity configurations were used.  The first used just theFresnel reflection at each end of the fiber to provide feedback (denoted resonator A) whilst the secondcavity arrangement incorporated a mirror which was highly reflecting at both the pump (>99.9%reflecting) and laser (>99.7% reflecting) wavelengths butted against the distal end of the fiber (denotedresonator B).  The performance of this laser is shown in Fig. 9.  In the case of resonator A, the counter-propagating output was determined to have a slope efficiency of 12% with respect to the launched pumppower and for the co-propagating output this value was 5.2%. The overall threshold launched pumppower for this fiber-laser configuration was measured to be 0.52 W.  For resonator B, the slopeefficiency was determined to be 17.3% with a slightly reduced threshold pump power of 0.47 W.Optional HighReflectorDichroic MirrorEr,Pr-ZBLANDiodePumpSourceOutputL2 L10 1 2 3 4 5 6 7 8 9 10 110.00.20.40.60.81.01.21.41.61.8Resonator BCounter-PropagatingResonator ACo-PropagatingResonator ACounter-PropagatingLaser Output Power (W)Launched Pump Power (W)Fig. 8. Experimental setup for fiber laser. Fig. 9. Laser performance.DiscussionThe 3µm laser transition of erbium is based on a simple 4 level laser scheme.  This is complicated,however, by the metastable lifetime of the 4I13/2 lower laser level which results in the accumulation of© 2000 OSA/ASSL 2000excited ions.  To overcome this problem, erbium crystal lasers are highly doped to exploit upconversionprocesses.  This provides a mechanism that both depletes the 4I13/2 lower laser level and recycles some ofthe energy to the 4I11/2 upper laser level.  Based on this process, the 3-µm crystal laser is operated CWeven for unfavorable lifetime ratios of the laser levels [9] and output powers exceeding 1 W have beenobtained.  To obtain the highest slope efficiency from such a laser it is important to use optimum Er3+doping levels, which causes the ratio of the probabilities of the 4I11/2 and 4I13/2  ETU processes to belargest.  If this condition is met the slope efficiency may be twice the Stokes efficiency [10,11].  Untilrecently, ZBLAN glass has been unavailable with high Er3+ concentrations and detrimental ESA, ratherthan ETU, was the dominant factor that affected laser performance.  The aim of this research was toinvestigate whether ETU in Er3+ could be exploited to allow for efficient 3µm radiation production, orwhether a second operational regime, ET to a Pr3+ codopant, would be preferable.The findings from this investigation show strong upconversion parameters for the 4I13/2 level,with the 4I11/2 level parameter being approximately three times lower. At the Er3+ concentrationsinvestigated here, the ratio of the probabilities of ETU from the 4I13/2 lower laser level, which recyclesenergy to the 4I11/2 upper laser level, versus ETU from the 4I11/2 upper laser level, which depletesinversion, is more favorable for ZBLAN compared to LiYF4. It is therefore predicted that the optimumEr3+ concentration is probably smaller in a fiber laser than the optimum concentration of 2×1021 cm-3(= 15 at.%) in LiYF4 [12, 4].For fibers with lower dopant concentrations, co-doping Er3+ with Pr3+ allows the system tooperate as a four-level laser with short lower and longer upper-state lifetimes.  This may be a practicalalternative if sufficiently high doping levels are unavailable in optical fibers. This technique allows adoped optical fiber to act as a conventional 4-level laser but limits the slope efficiency [6] to valuesbelow the Stokes limit.  To demonstrate the effectiveness of praseodymium codoping to enhance theperformance of erbium doped ZBLAN fiber lasers, a codoped double clad fiber laser was constructed.This, 790nm diode pumped, device provided up to 1.7 W of unsaturated 2.7 µm output power at a slopeefficiency of ~17% with respect to launch pump power (for a 10.5 m long fiber) and a totaloptical-to-optical efficiency of ~7.6%.For the future, the possible development of highly Er3+-doped fibers may provide a route to moreefficient fiber lasers.  Optimized energy transfer upconversion from the lower laser level can recycleions back to the upper laser level.  This energy-recycling regime with its inherent enhancement of theslope efficiency by a factor of two will, thus, be an attractive alternative to Pr3+ codoping at higherdopant concentrations.ConclusionEnergy transfer processes in Er3+-doped and Er3+,Pr3+-codoped ZBLAN bulk glasses have beeninvestigated.  Energy transfer upconversion parameters for the 4I13/2 and 4I11/2 energy levels have beenfound to be comparable to those of LiYF4:Er3+.  This should provide an effective mechanism for energyrecycling in ZBLAN and possibly enhanced slope efficiencies.  Optimum concentrations for this modeof operation are most likely higher than those studied here, but maybe less than that required forLiYF4:Er3+ by virtue of a more favorable ratio of the ETU parameters in ZBLAN.  Energy transfer fromthe Er3+ 4I13/2 and 4I11/2 levels to the Pr3+ codopant has been shown to effectively reduce the lifetime ofthe 4I13/2, lower, laser level by an order of magnitude more that the 4I11/2, upper, level – resulting in aconventional 4-level laser scheme. The validity of co-doping has been demonstrated practical by theproduction of 1.7W of unsaturated output power from a diode pumped, Er3+, Pr3+- codoped ZBLANfiber laser.© 2000 OSA/ASSL 2000AcknowledgementsThis work was financially supported by Engineering and Physical Sciences Research Council.M. Pollnau is indebted to Hans-Ulrich Güdel from the Department of Chemistry and Biochemistry,University of Bern, Switzerland, for his support.References1. S. Bedö, M. Pollnau, W. Lüthy, and H. P. Weber, "Saturation of the 2.71 µm laser output in erbium dopedZBLAN fibers", Opt. Commun. 116, 81-86 (1995).2. M. Pollnau, Ch. Ghisler, G. Bunea, M. Bunea, W. Lüthy, and H. P. Weber, "150 mW unsaturated outputpower at 3 µm from a single-mode-fiber erbium cascade laser", Appl. Phys. Lett. 66, 3564-3566 (1995).3. R. S. Quimby, W. J. Miniscalco, and B. Thompson, "Excited state absorption at 980 nm in erbium dopedglass", SPIE Vol. 1581 Fiber Laser Sources and Amplifiers III, 72-79 (1991).4. T. Jensen, "Upconversion-Prozesse und Wirkungsquerschnitte in Er3+-dotierten 3 µm Fluorid- und Granat-Lasern, gepumpt mit cw und quasi-cw Dioden-Arrays", Ph. D. thesis, Institute of Laser-Physics, Universityof Hamburg, Germany, 1996.5. B. A. Wilson, J. Hegarty and W. M. Yen, “Biexciton decay in MnF2” Phys. Rev. Lett. 41, 268-270, (1978).6. D. S. Knowles and H. P. Jenssen, "Upconversion versus Pr-deactivation for efficient 3 µm laser operation inEr", IEEE J. Quantum Electron. 28, 1197-1208 (1992).7. L. Wetenkamp, G. F. West, and H. Többen, "Co-doping effects in erbium3+- and holmium3+-dopedZBLAN glass", J. Non-Crystalline Sol. 140, 25-30 (1992).8. S. D. Jackson, T. A. King, and M. Pollnau, "Diode-pumped 1.7-W erbium 3-µm fiber laser", Opt. Lett. 24,1133-1135 (1999).9. K. S. Bagdasarov, V. I. Zhekov, V. A. Lobachev, T. M. Murina, and A. M. Prokhorov, “Steady-statestimulated-emission in lasers on Y3Al5O12:Er3+ (λ = 2.94 µm, T=300K)”, Kvant. Elektr. (Moscow) 10,452-454 (1983) [Sov. J. Quantum Electron. 13, 262-263 (1983)].10. M. Pollnau, R. Spring, Ch. Ghisler, S. Wittwer, W. Lüthy, and H. P. Weber, “Efficiency of erbium 3µmcrystal and fiber lasers.” IEEE J. Quantum Electron. 32, 657-663 (1996).11. Ch. Wyss, W. Lüthy, H. P. Weber, P. Rogin, and J. Hulliger, “Emission properties of an optimized 2.8 µmEr3+:YLF laser  Opt. Commun. 139, 215-218 (1997).12. T. Jensen, A. Diening, G. Huber, and B. H. T. Chai, “Investigation of diode-pumped 2.8 µm Er:LiYF4lasers with various doping levels.” Opt. Lett. 21, 585-587 (1996).* Present address: Optical Fibre Technology Centre, Australian Photonics CRC, 101 NationalInnovation Centre, Australian Technology Park, Eveleigh NSW 1430, Australia.© 2000 OSA/ASSL 2000

Determination of energy transfer parameters in Er3+-doped and Er3+, Pr3+-codoped ZBLAN glasses

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Determination of energy transfer parameters in Er<sup>3+</sup>-doped and Er<sup>3+</sup>, Pr<sup>3+</sup>-codoped ZBLAN glasses

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