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Fluorescence lifetime and 980nm pump energy transfer dynamics in erbium and ytterbium co-doped phosphate laser

glasses Ruikun Wu, J.D. Myers, M.J. Myers, Charles Rapp

Kigre, Inc.

100 Marshland Road, Hilton Head Island, SC 29926 Email: kigreinc@cs.com WEB PAGE: http://www.kigre.com

Abstract

Phosphate glasses are attractive laser oscillator/amplifier materials because unlike fluoride, silicate, and other laser glass materials they combine such useful properties as good chemical durability, ion-exchangeability, high gain, low concentration quenching, and low upconversion losses. Phosphate glasses also exhibit a very high solubility for rare earth ions. This feature permits the introduction of large concentrations of active ions into relatively small volumes resulting in smaller laser devices with high-energy storage capabilities. These high dopant concentrations also result in very rapid and efficient energy transfer between rare earth ions. This allows for the effective use of Yb3+ as a sensitizer for the Er3+ laser ion. Effective Er:Yb:Glass pumping, energy storage, and energy extraction involves the population of the 2F5/2 level of Yb3+ (~2ms fluorescence lifetime); a moderately rapid and efficient nonradiative transfer of energy to the 4I11/2 level of Er3+ (about 500 µsec transfer time); and a very rapid (< 1µsec) nonradiative decay of the Er3+ from the 4I11/2 to the metastable 4I13/2 state (with an 8ms fluorescence lifetime). In this study we measured the fluorescence lifetime for the 4I13/2 level of Er+3 on different glass samples with various concentrations of erbium. The data indicate that for doping levels up to 7 wt.% Er2O3 the lifetime remains above 7.0 ms. Theoretically, this highly doped glass could produce over 18 dB gain in a 1cm path length. In additional fluorescence lifetime testing, ytterbium doped and erbium/ytterbium co-doped glasses were evaluated for concentration quenching and energy transfer rate as a function of the Er3+ concentration. The effect on the energy transfer efficiency and laser efficiency was analyzed. Key Words: Fluorescence, Erbium Glass, Diode pump

1. Introduction Erbium doped glass lasers operating at about 1540 nm are commonly used in “eye safe” applications, optical amplifiers in telecommunications, and other applications(1,2,3,4). Fused silica is a very useful host for some applications where very long amplifiers can be used. However, it is generally found to be less suitable for small laser devices because the rare earth concentration is limited by ion “clustering’, low solubility, and other undesirable effects(5,6). More complex silicate glasses can overcome some of these problems. However, the Er3+ emission cross section and the laser efficiency is generally found to be low for these glasses. Phosphate glasses have been found to overcome most of these problems(7, 8, 9). Unlike silicate, fluoride, and other laser glass materials, phosphate glasses combine such useful properties as good chemical durability, ion-exchangeability, high gain, low concentration quenching, and low upconversion losses. Phosphate glasses also exhibit a very high solubility for rare earth ions. This feature permits the

SPIE Paper# 4968-1 Photonics West 2003

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introduction of large concentrations of active ions into the glass and allows for the design of very small but efficient laser devices. The high concentration of rare earth ions in phosphate glasses has several advantages. These include:

- efficient pumping in short lengths; - high energy storage per unit volume; - high gain per unit length; and, - high efficiency of energy transfer between rare earth ions.

2. Yb3+ Sensitized Er3+ Laser

The Yb3+ - Er3+ laser was first reported by Snitzer and Woodcock in 1965(10). In this system the pump energy is absorbed by the Yb3+ and transferred to the Er3+. The advantage of this co-doped system is that the Er3+ concentration can be chosen to give the optimum laser threshold, energy storage, and gain for the size of the device being designed, while the Yb3+ concentration can be chosen to give the optimum absorption of the pump light. In practice the concentration of the Yb3+ may be as much as 10 to 100 times that of the Er3+. The energy absorption and transfer process for the sensitization of Er3+ by Yb3+ is shown in Figure 1. Absorption of light in the 900 to 1000 nm region results in the excitation of the Yb3+ from the 2F7/2 to the 2F5/2 state. The lifetime of this excited state is about 2 msec in a singly doped (Yb3+ only) phosphate glass. The energy transfer time between the Yb3+ 2F3/2 level and the Er3+ 4I11/2 is about 500 µsec for a glass doped with 19 weight % Yb2O3 and 0.22 weight % Er2O3. Therefore, the energy transfer efficiency between these ions is about 80% for these dopant concentrations. This rate and efficiency is dependent on both the Er3+ and Yb3+ concentrations and approaches 90% at about 0.5 weight % Er2O3. A determination and discussion of this energy transfer rate as a function of the concentration in the QX phosphate glass is given in a later section. After the energy is transferred to the Er3+, a very rapid nonradiative transition takes place between the 4I11/2 and 4I13/2 states. This transition has been reported to take place in less than 1 µsec in phosphate glass(11). This is an additional advantage of phosphate glass. The relatively high energy multiphonon absorption in the phosphate glass rapidly quenches the Er3+ from the 4I11/2 state. The multiphonon absorption edge in silicate glass is at a lower energy than phosphate glass and, therefore, the lifetime of the 4I11/2 state is about 10 to 100 times longer(11). Borate glasses have even higher mulitphonon energies and a shorter lifetime for the 4I11/2 state than phosphate glasses. However, the multiphonon energies are so high that a significant quenching of the 4I13/2 state is also seen in borate glasses. This greatly reduces the lifetime, quantum efficiency, and laser efficiency of the Er-borate glasses. Phosphate glasses appear to have the optimum balance between these two effects.

The lifetime of the 4I13/2 state is typically about 8 msec in a phosphate glass. This long lifetime is ideal for high energy storage, high gain, and high efficiency in small laser systems.

3. Yb3+ Absorption Spectra The Yb3+ 2F7/2 – 2F5/2 absorption covers the spectral region from about 880 to 1020 nm, with the most intense absorption falling between about 915 nm and 980 nm. The absorption spectrum of a Yb3+ doped phosphate glass is shown in Figure 2. This glass contains 6.80 weight % Yb2O3 and has an absorption path length of 1.0 cm. The spectrum has been corrected for reflection loss. As can be seen from the figure, the glass absorbs more than 80 % of the light between about 930 and 985 nm. Yb2O3 concentrations can be increased to well over 20 weight %. In these glasses, the absorption over this same spectral region should

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be greater than 80 % in < 4mm. Double pass pumping could further increase the absorption efficiency or reduce the required sample thickness. Since up to 90 % of the absorbed energy can then be transferred to Er3+ (when present at an effective 0.5 weight % Er2O3 in the ground state), very efficient small laser devices can be designed.

4. Er3+ Fluorescence Lifetime The design of microlasers and short fiber lasers may require doping with relatively high levels of Er2O3 in order to obtain the high energy storage and high gain per unit length required for these devices. Therefore, the Er3+ 4I13/2 lifetime was measured as a function of the erbium concentration up to 7.0 weight %. The results of these measurements are shown in Figure 3. As can be seen, the lifetime at low concentrations approaches 9 msec. As the concentration is increased, the lifetime slowly decreases to about 8.5 msec at 5 weight % Er2O3, and about 7.5 msec at 7 weight % Er2O3. These lifetimes would imply quite high quantum efficiencies for the Er3+ fluorescence in the highly doped samples. It is estimated that the 7 weight % sample would have a Q. E. about 85 % of that of the low doped samples. The mechanism for concentration quenching is not known. However, one possible mechanism is an increase in the energy migration along the Er3+ ions to energy sinks such as Fe2+ or OH groups that can quench or accept the energy from the excited Er3+ ions. This would suggest that very pure glasses are needed when working at high rare earth dopant levels. The 7 weight % sample contains about 6 x 1020 Er3+ ions/cm3. The cross section for stimulated emission is about 7 x 10-21 cm2. Therefore, if most of the Er3+ ions were in the excited state, the gain for this glass would be > 4.1 cm-1, or > 18 dB cm-1.

5. Yb3+ - Er3+ Energy Transfer Rate In order to model laser performance and calculate the optimum Er3+ concentration for various laser configurations, it is necessary to know the energy transfer rate and energy transfer efficiency as a function of the Er3+ concentration. Therefore, the Yb3+ fluorescence decay time was measured on a series of glass samples containing about 19 weight % Yb2O3 and from 0.010 to 0.075 weight % Er2O3. From this data, both the energy transfer rate and energy transfer efficiency can be calculated. The equation describing the decay of the excited Yb* can be expressed as: Eq.1 k = kYb + kET[Er3+]x

where: k = 1/τ or the inverse of the measured Yb fluorescence decay time; kYb = the decay rate constant for the Yb; kET = the energy transfer rate constant for Yb to Er; [Er3+] = the erbium concentration; x = a constant that is dependent on the energy transfer mechanism. kYb is actually a complex term that includes the spontaneous fluorescence decay rate of the Yb, as well as nonradiative processes including quenching by impurities. However, as long as the impurities and host are constant, then kYb can be treated as a constant.

In this equation, x would have a value of 2 (for a 1/r6 dependence) for a dipole-dipole energy transfer mechanism (without energy migration); greater than 2 for a dipole-quadrupole and higher order interactions; and less than 2 if rapid energy migration between donor ions is important(12). If the energy

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exchange between donor ions is very rapid compared to the energy transfer between donor and acceptor, then the value of x will approach 1.

The value of x can be determined by several graphical means. One simple way is to assign various values to x in Eq. 1. A linear plot of 1/τ vs [Er3+] should be obtained for the best value of x. At very high concentrations of Yb3+, such as those used here, we would expect a very rapid migration of energy among the Yb3+ ions. Therefore, we might expect the value of x to be near 1. Snitzer and Young found this linear dependence of the energy transfer rate on the Er concentration in a silicate glass for Er concentrations up to about 5 weight % Er2O3

(13).

If we assume that the correct value of x is 1, then a linear plot of 1/τ vs. [Er] can be made. This plot is shown in Figure 4. The R2 value for the plot is 0.998, which confirms that a value of 1.0 for x is the correct choice. The intercept of the plot gives the value of kYb and the slope the value of kET. The value of kYb is 499.04 sec-1, which would give a value of 0.00200 seconds for the Yb fluorescence decay time with no Er. The slope of the plot is 9247 sec-1 wt%-1.

6. Energy Transfer Efficiency The energy transfer efficiency for a 19 wt% Yb2O3 phosphate glass can be calculated from the equation: Eq. 2 Energy Transfer Eff. = kET[Er]/k Where: k = kYb + kET[Er] kET = 9247 sec-1wt%-1 [Er] = Wt% Er2O3 kYb = 499.04 sec-1 It is estimated that the energy transfer efficiency for a phosphate glass containing 19 weight % Yb2O3 + 0.5 weight % Er2O3 would be about 90%. However, under cw pumping and lasing conditions, the effective Er2O3 concentration would be about half this amount so the expected energy transfer efficiency would be about 80 %.

7. Conclusions

1. Heavily doped Yb-Er phosphate glasses containing over 20 wt% Yb2O3 can efficiently absorb pump light in very thin sections (< 5 mm).

2. The fluorescence lifetime of the Yb3+ in a phosphate glass at “zero” Er2O3 is about 2.0 msec.

3. The energy transfer from Yb3+ to Er3+ has a linear dependence on the Er3+ concentration. This

would suggest the energy migration among Yb3+ ions is very rapid.

4. The energy transfer rate between Yb3+ and Er3+ can be expressed as: (9247 sec-1 wt%-1) [wt% Er2O3] (for 19 wt% Yb2O3).

5. The energy transfer efficiency between Yb3+ and Er3+ would be about 90% for a glass containing

19 wt% Yb2O3 and 0.5 wt % Er2O3. This efficiency would drop to about 80 % or less under cw lasing conditions.

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8. References

1. A. M. Johnson, Eyesafe lasers for military use, Lasers Optron, June 1988. 2. S. J. Hamlin, J. D. Myers, and M. J. Myers, High repetition rate Q-switched Erbium glass lasers,

Proc. SPIE Vol. 1419, 100, 1991. 3. B. J. Ainslie, A review of the fabrication and properties of erbium doped fibers for optical

waveguides, IEEE J. Lightwave Technol. 9, 220, 1991. 4. P. Laporta, S. Taccheo, S. Longhi, O. Svelto, and C. Svelto, Erbium-ytterbium microlasers: optical

properties and lasing characteristics, Optical Materials 11, 269, 1999. 5. S. T. Davey, B. J. Ainslie, and R. Wyatt, Waveguide Glasses, In Handbook of Laser Science and

Technology, Supplement 2: Optical Materials, M. J. Weber, Ed., CRC Press, Boca Raton, 635, 1995.

6. P. C. Becker, N. A. Olsson, and J. R. Simpson, Erbium-Doped Fiber Amplifiers Fundamentals and Technology, Academic Press, San Diego, 1999.

7. R. Wu, J. D. Myers, G. M. Bishop, S. J. Hamlin, Characteristics of diode-pumped Erbium-Ytterbium doped glass laser, SPIE Nol. 2986, Photonics West ’97, High power lasers and application, solid state laser VI, 1997.

8. S. Jiang, J. D. Myers, D. L. Rhonehouse, Laser and thermal performance of a new Erbium doped phosphate laser glass, SPIE, Vol 2138, Longer-Wavelength and Applications, 1994.

9. L. Jiang, J. D. Myers, D. L. Rhonehouse, Further investigation of 1.5 µm Er3+ doped novel phosphate glass and lasers, CLEO’95, CWH#, 1995.

10. E. Snitzer and R. Woodcock, Yb3+-Er3+ glass laser, Appl. Phys. Lett. 6, 45, 1965. 11. N. E. Alekseev, V. P. Gapontsev, M. E. Zhabotinskii, V. B. Kravchenko, and Yu. P. Rudnitskii,

Laser Phosphate Glasses, Moscow, Nauka Publishing House, 1983. 12. D. L. Dexter, A theory of sensitized luminescence in solids, J. Chem. Phus., 21 (5), 836, 1953. 13. E. Snitzer and C. G. Young, Glass Lasers, In Lasers, Vol. 2, A. K. Levine, Ed., Marcel Dekker,

New York, 1968.

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Figure 1. Energy absorption and transfer process for Er3+ & Yb3+. (Transfer rates and lifetimes are for approximately 19 wt. % Yb2O3 and

0.22 wt. % Er2O3 in a phosphate glass.)

Figure 2. Absorption spectrum of a phosphate glass containing 6.80 wt. % Yb2O3 (t = 1.0 cm; spectrum corrected for reflection loss)

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Figure 3. Plot of the Er3+ 4I13/2 fluorescence lifetime vs. Er2O3 concentration for a phosphate glass.

Figure 4. Plot of 1/ττττ for Yb3+ fluorescence vs. Er2O3 in a phosphate glass. (Yb2O3 = 19 weight %)