Absorption and emission properties of Tm 2 + ions in germanosilicate glass fibers

Absorption and emission properties between 350nm and 1600nm of the Tm ions in optical fibers were investigated using the Tm-Tm codoped germanosilicate glass fibers and its fiber preform. Strong broad absorption band due to Tm ions was found to appear from 350nm to ~900nm together with the absorption bands due to Tm ions. Broad emission from ~600nm to ~1050nm and the other emission from ~1050nm to ~1300nm, which were not shown in the Tm ions, were found upon the Ar ion laser pumping at 515nm. Both absorption and emission results confirm that the Tm ions in the germanosilicate glass have the 4f-5d energy band from 350nm to ~900nm and the 4f-4f energy level at ~1115nm. 2003 Optical Society of America OCIS codes: (060.2290) Fiber materials; (160.5690) Rare earth doped materials; (300.1030) Absorption; (300.2140) Emission. References and links 1. M. J. F. Digonnet, R. W. Sadowski, H. J. Shaw, and R. H. Pantell, “Resonantly enhanced nonlinearity in doped fibers for low-power all-optical switching: a review,” Opt. Fiber Technol. 3, 44-64 (1997). 2. M. Janos, J. Canning and M.G. Sceats, “Transient transmission notches induced in Er doped optical fibre bragg gratings,” Electron. Lett. 32, 245-246 (1996). 3. R.H Pantell and M.J.F. Digonnet, “A model of nonlinear all-optical switching in doped fibers,” J. Lightwave Technol. 12, 149-156 (1993). 4. J. W. Arkwright, P. Elango, G. R. Atkins, T. Whitbread, and M. J. F. Digonnet, “Experimental and theoretical analysis of the resonant nonlinearity in ytterbium-doped fiber,” J. Lightwave Technol. 16, 798806 (1998). 5. D.-H. Cho, K. Hirao, N. Soga, and M. Nogami, “Photochemical hole burning in Sm-doped aluminosilicate and borosilicate glasses,” J. Non-Cryst. Solids 215, 192-200 (1997). 6. J. Qiu, K. Miura, H. Inouye, S. Fujiwara, T. Mitsuyu, and K. Hirao, “Blue emission induced in Eu-doped glasses by an infrared femtosecond laser,” J. Non-Cryst. Solids 244, 185-188 (1999). 7. K. Oh, U.-C. Paek, T. F. Morse, and L. Reinhart, “Photoinduced refractive-index change in Sm/Sm codoped aluminosilicate fiber by irradiation of an Ar-ion laser,” Opt. Lett. 22, 1192-1194 (1997). 8. K. Oh, T. F. Morse, L. Reinhart, A. Kilian, and W. M. Risen Jr., “Spectroscopic analysis of a Eu-doped aluminosilicate optical fiber preform,” J. Non-Cryst. Solids 149, 229-242 (1992). 9. Z. J. Kiss, “Energy levels of divalent thulium in CaF2,” Phys. Rev. 127, 718-724 (1962). 10. F. A. Cotton and G. Wilkinson, Advanced inorganic chemistry (John Wiley & Sons, New York, 1988), Chap. 20. 11. Y. Kim, Y. Chung, U. Paek, W. Han, “Fabrication of Tm/Tm co-doped silica fiber and its fluorescence characteristics,” in Optical Fiber Communication Conference, Tech. Dig., Postconference ed., Vol. 86 of OSA Trends in Optics and Photonics (TOPS) (Optical Society of America, Washington, D.C., 2003), pp. 301-302. 12. G.H. Dieke and H.M. Crosswhite, “The spectra of the doubly and triply ionized rare earths,” Appl. Opt. 2, 675-686 (1963). 13. E. Loh, “4f → 4f5d spectra of rare-earth ions in crystals,” Phys. Rev. 175, 533-536 (1968). (C) 2003 OSA 20 October 2003 / Vol. 11, No. 21 / OPTICS EXPRESS 2672 #3065 $15.00 US Received September 23, 2003; Revised October 04, 2003 14. J. E. Townsend, S. B. Poole, and D. N. Payne, “Solution-doping technique for fabrication of rare-earthdoped optical fibres,” Electron. Lett. 23, 329-331 (1987). 15. P.C Becker, N.A. Olsson, and J.R. Simpson, Erbium-doped fiber amplifiers: fundamentals and technology (Academic, New York, 1999), Chap. 2. 16. A. Paul, Chemistry of glasses (Chapman & Hall, New York, 1990), Chap. 9. 17. M.J. Weber, “Lanthanide and actinide lasers,” in Lanthanide and actinide chemistry and spectroscopy, N.M. Edelstein, ed. (American Chemical Society, Washington, D.C., 1980).


Introduction
Development of high nonlinear optical (NLO) glass fibers is one of issues in making various all-optical devices, such as optical switches and optical tunable filters, in optical communications applications [1,2].Since rare-earth doped fibers are expected to have large nonlinear optical properties, there have been much research activities on development of fibers doped with different rare earth ions [3,4].Divalent thulium (Tm 2+ ) ions are expected to bring about high optical nonlinearity among rare-earth ions [4].However, it is not easy to dope the Tm 2+ ions into the glass fiber core because rare-earth ions are usually stable in the trivalent state.Even though the divalent state of some rare-earth ions, such as samarium and europium, in bulk glasses or in glass fibers using aerosol delivery technique in modified chemical vapor deposition (MCVD) process has been demonstrated [5][6][7][8], and there have been a few reports to obtain the divalent thulium ions in alkaline-earth fluoride crystals by irradiating trivalent thulium (Tm 3+ ) ions with γ-ray or in tetrahydrofuran (THF) solution by reducing TmCl 3 with Na naphthalide [9,10], no experimental results have been reported for the Tm 2+ ions even in any bulk glasses to the best knowledge.
Recently, we have fabricated Tm 2+ -Tm 3+ co-doped germanosilicate glass optical fibers for the first time [11].Based on the spectroscopic study of rare-earth ions [12], the divalent rareearth ions have unique 4f-5d electronic transitions together with 4f-4f electronic transitions at the visible or infrared region, whereas the trivalent rare-earth ions have only 4f-4f transitions.In general, the 4f-5d transitions are affected by a strong crystal field, which depends on the host materials of the ions, but the 4f-4f transitions are not [13].In this paper, the absorption and emission properties of the Tm 2+ ions in the germanosilicate glass fibers were investigated and the energy transition levels of the Tm 2+ ions in the glass were confirmed.

Preparation of Tm 3+ -Al 3+ co-doped germanosilicate glass optical fibers
Silica glass based fibers doped with the Tm 3+ ions were fabricated as a reference by the conventional solution doping technique [14].Doping solution (Solution I) was prepared by dissolving TmCl 3 (0.04M) and AlCl 3 (0.2M) into ethanol, where aluminum ions were used to increase the solubility of the Tm 3+ ions in the glass fiber [15].Porous core layers with a germanosilicate glass composition were deposited onto the inside of a silica tube by the MCVD process.Then, the porous layers were soaked with Solution I for an hour.After draining the doping solution, the soaked porous layers were dried below 300 o C, oxidized at ~1100 o C and then sintered at ~2000 o C by flowing helium, oxygen and chlorine gases.Then the silica glass tube with the sintered core layers was collapsed over 2200 o C to prepare a fiber preform.Finally, the preform was drawn into the fiber, the Tm 3+ -Al 3+ co-doped fiber, by using a draw tower.

Preparation of Tm 2+ -Tm 3+ co-doped germanosilicate glass optical fibers
To fabricate silica glass based optical fibers containing Tm 2+ ions, the same solution doping technique together with the MCVD process was basically used.But sucrose (C 12 H 22 O 11 , 2.2M) was added to the solution containing TmCl 3 (0.04M) in de-ionized (DI) water to reduce the valence state of thulium ions from trivalent to divalent (Solution II).A preform doped with the Solution II was prepared as the Tm 2+ -Tm 3+ co-doped preform except that only helium gas was used during sintering and collapsing processes in order to keep a strong reduction environment.

Measurement of absorption and emission properties
Absorption spectrum between 350nm and 1600nm of both thulium doped fibers with and without sucrose addition in the doping solution was measured by the cut-back method using white light source (Ando, Japan) and optical spectrum analyzer (OSA; Ando, Japan).
Emission spectrum of the 57cm long Tm 3+ -Al 3+ co-doped fiber and the 46cm long Tm 2+ -Tm 3+ co-doped fiber was measured by pumping at 515nm using Ar-ion laser from 0 to 200mW, respectively.Ar ion laser was launched into a thulium doped fiber passing through the fiber collimator, and the output signals were monitored from 600nm to 1600nm by the OSA.The launching efficiency of the Ar-laser beam at 515nm was ~25%.
In the case of the fiber preforms, on the other hand, emission spectrum between 800nm and 1300nm of both thulium doped fiber preforms was measured at room temperature by pumping with 7W Ar-ion laser covering wavelengths of 458nm, 476nm, 488nm, 497nm and 515nm.The Ar-ion laser beam was focused on the core of the preform samples through the convex lens after passing the optical chopper.Then, light emitted from the preform samples was measured by the PIN diode detector after filtering out only Ar-ion laser source beam with an optical filter.

Absorption properties of the fibers
Fiber with Tm and sucrose (Tm 2+ -Tm 3+ co-doped fiber) Fiber with Tm and Al (Tm 3+ -Al 3+ co-doped fiber) Fiber with only sucrose Fig. 1.The optical absorption spectra of the Tm 2+ -Tm 3+ co-doped germanosilicate glass optical fibers soaked with TmCl 3 and sucrose solution, the Tm 3+ -Al 3+ co-doped germanosilicate glass optical fibers soaked with TmCl 3 and AlCl 3 solution, and the germanosilicate glass fiber soaked with only sucrose solution.
Fig. 1 compares the absorption spectrum of the Tm 3+ -Al 3+ co-doped germanosilicate glass fiber and the Tm 2+ -Tm 3+ co-doped germanosilicate glass fiber.The absorption spectrum of the Tm 2+ -Tm 3+ co-doped fiber showed additional broad absorption band from 350nm to ~900nm as well as the same absorption bands at 465nm, 680nm, 785nm, 1210nm and 1600nm of the Tm 3+ ions shown in the Tm 3+ -Al 3+ co-doped fibers.Based on the absorption spectrum of the Tm 2+ ions in alkaline-earth fluoride crystals [9,13], the Tm 2+ ions have a broad absorption band from ~200nm to ~700nm due to the 4f-5d transitions and an absorption band at ~1115nm due to the 4f-4f transitions at a room temperature.Therefore, the additional broad absorption band from 350nm to ~900nm in the Tm 2+ -Tm 3+ co-doped fibers is attributed to the 4f-5d transitions of the doped Tm 2+ ions.The different position of the 4f-5d transition absorption band of the Tm 2+ ions from that in the alkaline-earth fluoride crystals can be explained by the ligand field (or the crystal field) theory such that the host materials affect the spectroscopic properties of the doping ions [16].Particularly, the crystal field effect on the 5d electron is strong and the 4f-5d configurations are very different for the different host materials [13].On the other hand, the crystal field effect on the 4f electron is weak and the 4f-4f configurations are not so different in the different host materials.In the absorption spectrum of the Tm 2+ -Tm 3+ co-doped fibers, however, the 4f-4f transitions absorption band near 1115nm was not clearly shown.This is because the doping concentration of the Tm 2+ ions in the fibers was low.The oscillator strength of the 4f-5d transitions is usually over ten thousands times larger than that of the 4f-4f transitions [17], and the 4f-5d transitions absorption is more easily detected than the 4f-4f transitions absorption.
To confirm the effect of the sucrose as a reducing agent that was added in the doping solution (Solution II), germanosilicate glass fibers without adding thulium ions were also made using a doping solution containing only sucrose (2.2M) in DI water (Solution III).Absorption spectrum of the fiber doped with Solution III is also shown in Fig. 1.No strong absorption band between 350nm and ~900nm appeared and thus the sucrose in the Tm 2+ -Tm 3+ co-doped fibers is believed to reduce the valence state of the thulium ions from trivalent to divalent without forming an additional absorption spectrum.Emission spectrum between 600nm and 1600nm of the 57cm long Tm 3+ -Al 3+ co-doped germanosilicate glass fibers and the 46cm long Tm 2+ -Tm 3+ co-doped germanosilicate glass fibers upon the pumping at 515nm is shown in Figs.2(a) and (b), respectively.In order to excite divalent thulium ions exclusively, pumping wavelength of 515nm was chosen from the absorption results shown in Fig. 1, where the absorption coefficient at 515nm of the Tm 3+ -Al 3+ co-doped fibers and the Tm 2+ -Tm 3+ co-doped fibers was ~0.001cm -1 (0.4dB/m) and ~0.153cm -1 (66.36dB/m), respectively.Even though the launched pump power increased from 7.8mW to 52mW, no significant change in the emission spectrum of the Tm 3+ -Al 3+ co-doped fibers was found as shown in Fig. 2(a).However, the emission spectrum of the Tm 2+ -Tm 3+ codoped fibers showed a pump power dependence as shown in Fig. 2(b).As the pump power increased, overall transmitted power increased between 600nm and 1150nm except the regions corresponding to the absorption bands at ~670nm and ~780nm of the Tm 3+ ions.Since the Tm 2+ and the Tm 3+ ions are co-existed in the fibers, the emission due to the Tm 2+ ions would be absorbed by the Tm 3+ ions.The emission at ~1150nm also seems to have been affected by the re-absorption due to the absorption band at ~1210nm of the Tm 3+ ions by the abrupt decrease of the emission with the pump powers.Probably, the re-absorption can occur by other Tm 2+ ions as well as the Tm 3+ ions as the emission lights propagate through the fiber core.

Emission properties of the fibers
To examine the re-absorption effect more clearly, the emission spectrum of the Tm 2+ -Tm 3+ co-doped fibers with different fiber length, 187cm and 258cm, was measured upon pumping at 515nm and shown in Fig. 3.As the fiber length became longer, the emission between 600nm and 800nm and that at ~1100nm decreased largely with the launched pump powers from 4.6mW to 53.6mW.Moreover, even though the emission intensity for the fiber lengths of 46cm and 187cm at ~830nm was larger than that at ~905nm for all pump powers, the emission intensity for the fiber length of 258cm at ~830nm was smaller than that at ~905nm with the pump powers.This fact indicates that the re-absorption effect of the fibers should be considered to understand the emission properties of the Tm 2+ ions in the Tm 2+ -Tm 3+ co-doped fibers.If the re-absorption effect is neglected, the emission upon the pumping at 515nm of the Tm 2+ ions should be shown from ~600nm to ~1150nm (or larger).Also, the emission spectra due to the Tm 2+ ions must be separated by considering the electronic transition configurations of the Tm 2+ ions into two parts such as the 4f-5d transitions emission from ~600nm to ~1050nm and the 4f-4f transitions emission from ~1050nm to ~1150nm (or larger).Fig. 3. Emission spectra with the launched pump powers at 515nm (a) 187cm long Tm 2+ -Tm 3+ co-doped germanosilicate glass fiber and (b) 258cm long Tm 2+ -Tm 3+ co-doped germanosilicate glass fiber.

Emission properties of the fiber preforms
Emission spectrum of the Tm 2+ ions in the core region of the two different fiber preforms, where the re-absorption effect shown in the fibers would be negligible, is shown in Fig. 4(a).Since the pumping light excited both ions of the Tm 2+ and the Tm 3+ during the emission experiments of the preforms, there should be the emissions due to the Tm 3+ ions as well as the Tm 2+ ions in the Tm 2+ -Tm 3+ co-doped fiber preform and the emissions due to only Tm 3+ ions in the Tm 3+ -Al 3+ co-doped fiber preform.
To examine the difference of the emission spectra explicitly, the emission spectrum was normalized by the peak intensity in the spectrum and the results are shown in Fig. 4(b).The Tm 2+ -Tm 3+ co-doped preform showed additional emission bands between ~800nm and ~1300nm together with the emission bands at ~800nm and ~1200nm due to the Tm 3+ ions which were also shown in the Tm 3+ -Al 3+ co-doped preform.Similarly to the emission results in the fibers, the additional emission bands are separated into the 4f-5d transitions emission from ~800nm to ~1050nm, in which the peak was at ~930nm, and the 4f-4f transitions emission from ~1050nm to ~1300nm, in which the peak was at ~1160nm.Actually, it is hard to compare the emission spectra of the fiber preforms with those of the fibers directly because the emission spectra of the fibers and the preforms were measured using different Ar-ions laser source and different spectrometer, and there was a waveguide effect, such as reabsorption of the doped ions, in the fibers.However, it should be pointed out that the 4f-4f transitions of the Tm 2+ ions are clearly shown in the emission spectrum of the Tm 2+ -Tm 3+ codoped preform but not so in the absorption and emission spectrum of the Tm 2+ -Tm 3+ co-doped fibers.This difference is due to low concentration of Tm 2+ ions in the fibers or the reabsorption effect of the doped ions in the fibers.Also, the 4f-5d transitions emission peak near 900nm in the Tm 2+ -Tm 3+ co-doped fibers can be found in the emission spectrum of the Tm 2+ -Tm 3+ co-doped preform as shown in the inset of Fig. 4(b).Therefore, the emission properties of the Tm 2+ ions in the germanosilicate glass can be understood by those of the Tm 2+ ions in the fibers and the preform.For the Ar-ion laser pumping at 515nm, the Tm 2+ ions in the germanosilicate glass would have a broad emission band from 600nm to ~1050nm due to 4f-5d electronic transitions and an emission band from ~1050nm to ~1300nm due to 4f-4f electronic transitions in the spectral region between 600nm and 1600nm.
From the absorption and emission properties of the Tm 2+ -Tm 3+ co-doped germanosilicate glass fibers and the preform we have found out that the 4f-5d transitions energy band of the Tm 2+ ions in the germanosilicate glass is very different from that in the alkaline-earth fluoride, such as CaF 2 , crystals.Thus, the energy level diagram of the Tm 2+ ions doped in the germanosilicate glass is proposed as shown in Fig. 5.The lowest 4f-5d transition energy level of the Tm 2+ ions would be at ~11,110cm -1 (~900nm) in the germanosilicate glass, which is comparable to ~14290cm -1 (~700nm) in the CaF 2 crystals even though the 4f-4f transition energy level would be the same at ~8970cm -1 (~1115nm).
The theoretical model to explain the high NLO properties of rare-earth doped fibers by the optical pumping shows that the NLO properties at optical communication windows (1310nm and 1550nm) of the fibers will be higher as 4f-5d transitions absorption band is closer to the windows [1,4].Since the 4f-5d transitions absorption band of the Tm 2+ ions in the germanosilicate glass fibers is distributed up to ~900nm, the contribution of the 4f-5d transitions to the pump-induced refractive index change at the optical communication windows is expected to increase.Thus, the glass fibers containing the Tm 2+ ions can be used to the optical devices requiring high NLO properties at the windows.

Conclusion
The absorption and emission properties between 350nm and 1600nm of the Tm 2+ ions in the germanosilicate glass were investigated using the Tm 2+ -Tm 3+ co-doped germanosilicate glass optical fiber and its fiber preform.The Tm 2+ ions showed a strong broad absorption band from 350nm to ~900nm corresponding to the 4f-5d transitions and a weak absorption band at ~1115nm corresponding to the 4f-4f transitions.Also, there were a broad emission band from ~600nm to ~1050nm due to the 4f-5d transitions and an emission band from ~1050nm to ~1300nm due to the 4f-4f transitions upon the Ar ion laser pumping at 515nm.The 4f-5d configurations in the germanosilicate glass were much different from those in the CaF 2 crystals.Therefore, the energy level diagram of the Tm 2+ ions in the germanosilicate glass was proposed such that the lowest 4f-5d transition energy level of the Tm 2+ ions would be at ~11,110cm -1 (~900nm) and the 4f-4f transition energy level would be at ~8970cm -1 (~1115nm), which is the same as that of the CaF 2 .

Fig. 4 .
Fig. 4. (a) Emission spectra between 800nm and 1300nm of the thulium doped fiber preforms upon the 7W Ar ion laser pumping together at 458nm, 476nm, 488nm, 497nm and 515nm (b) Normalized emission spectra of the spectra shown in (a).