The infrared emission spectra and decay lifetimes of Tm -doped and Tm3+-Ho3+, Tm3+-Yb3+ co-doped tellurite fibres were measured using 808 nm and 978 nm diode laser pump sources in the range 1.35 μm to 2.2 μm. The spectra were compared with varying fibre lengths and core diameters. Tm3+-doped fibre shows strong emission at ~1.8 μm and when co-doped with Ho3+, energy transfer results in strong Ho3+ fluorescence at ~2.0 μm. These fibres show promise for compact mid-IR fibre laser sources.
©2007 Optical Society of America
Tellurite glass (TeO2) has several properties which makes it an interesting material to investigate the fluorescence properties of rare earth ions for compact fibre laser applications. The main advantages of TeO2 glasses over other laser glasses, such as the silicates, are large solubility of rare earth oxides and extended infrared cut-off which are both comparable with fluoride hosts, and moderate phonon energy (780 cm-1) which is lower than germanate glass (880 cm-1) and silica glass (1100 cm-1) . Nearly 15000 ppm of Er2O3 was dissolved without concentration quenching and up to 70000 ppm without devitrification . It is also a chemically and environmentally stable glass unlike fluorides making it more desirable for device manufacture . The glass exhibits high-resistance against crystallization over the fibre drawing temperature range (570 K to 730 K) and thereby enables fabrication of single and multimode fibres . Its large refractive index (>1.95) and broad fluorescence bandwidth, in comparison with silicates and fluoride glasses [2,4], are additional attractive features for considering design for a tunable laser host in mid-IR.
Mid-IR fibre lasers in the range 1.5 – 2.1 μm have applications such as eye-safe lasers for lidar, remote chemical sensing and atmospheric monitoring. Er3+, Ho3+ Dy3+ and Pr3+-doped fibre lasers operating at ~3 μm have potential for medical applications due to the strong absorption of water at this wavelength . Lasing has been demonstrated at 1.9 μm in a short length (465 mm) of multi mode Tm3+-doped silica fibre when pumped with an Yb3+-doped fibre laser at 1.09 μm , and at 2.05 μm in a 4.2 m double-clad Tm3+-Ho3+ co-doped fluoride fibre when pumped with a high power 800 nm laser diode . To the author’s knowledge there are no reports of tellurite fibre lasers operating at these wavelengths. Higher absorption and emission cross-sections and solubility of rare earths in tellurite glass make them very promising for short near and mid-IR fibre laser sources.
This paper examines the emission spectra of Tm3+ and Ho3+ ions doped into tellurium oxide fibres in the infrared region of 1.35 – 2.10 μm when excited with an 808 nm laser diode and a 978 nm laser diode through the use of Yb3+ as a sensitizer. The visible upconversion emission has also been measured and the energy transfer processes discussed.
The tellurite fibres were manufactured from glass with the composition 78TeO2-12ZnO-10Na2O (mol%) for the core and 75TeO2-15ZnO-10Na2O (mol%) for the cladding. Tm2O3 (0.5 wt%) doped, Tm2O3-Ho2O3 (0.2-1.0 wt%) co-doped and Tm2O3-Yb2O3 (0.5-1.0 wt%) co-doped fibres were investigated. The starting chemicals for the core and cladding glasses were weighed, ground, mixed and then placed into gold crucibles and melted in separate electric tube furnaces in a dry O2 atmosphere. The powders are initially dried overnight and then melted at 820°C. The mixtures were stirred after 1 hour with a gold rod and then homogenized at 720°C for 2 hours. The fibre preform is cast using the suction method into a preheated brass mould and then annealed at 300°C for 3 hours, after which it is allowed to cool slowly in the furnace to room temperature. The preforms are extruded into structured rods and then the “rod-in-tube” method is used to achieve a smaller core to cladding diameter ratio.
Fluorescence spectra of fibre and bulk glass were measured using an Edinburgh Instruments FLS920 Steady State and Time Resolved Fluorescence Spectrometer fitted with an InGaAs detector and photomultiplier tube, (PMT), for infrared and visible measurements, respectively. Bulk samples were excited either with a collimated 808 nm or 978 nm laser diode source. The laser light was launched so that was incident on the edge of the sample and the fluorescence detected perpendicularly from the incident beam. This ensures that the fluorescence travels a minimum distance through the sample, (<1 mm), thus reducing the effect of radiation reabsorption on the line width . Lifetime measurements were made by modulating the laser diode current supply with an external function generator and capturing the fluorescence decay using a digital oscilloscope (Tektronics, TDS3012). All experiments were carried out at room temperature.
3. Results and discussion
Figure 1 shows the near infrared emission spectra of a range of lengths of 0.5 wt% Tm3+ -doped tellurite fibres with a core diameter of 12 μm when excited with the 808 nm laser source. The spectra have been normalized with respect to the peak at ~1.86 μm which is attributed to 3F4→3H6 transition of Tm3+. As the length of the fibre increases, the peak shifts to longer wavelengths due to radiation trapping where light is absorbed from the ground state to the 3F4 level and then re-emitted. The figure also compares the fibre emission spectra to that of the bulk glass sample with the same doping concentration. The 3F4→3H6 peaks measured in the fibre samples are narrower than the bulk glass sample and also shifted to longer wavelengths. In fibre the peak at 1.46 μm due to the 3H4→3F4 transition is lower in intensity relative to the ~1.86 μm peak than in the bulk sample showing that the increased pump density in the fibre geometry is increasing the energy transfer efficiency between Tm3+ ions.
The measured fluorescence lifetime from the 3F4 Tm3+ level was 1633 μs in bulk glass and increased from 1941 μs to 2290 μs in increasing lengths of singly doped multi mode fibre. This lifetime increase is due to the radiation trapping process re-populating the 3F4 level. Radiation trapping causing an increase in the upper level lifetime has also been reported for Er3+ ions in tellurite glasses .
Figure 2 shows the emission spectra of varying lengths of 0.2 wt% Tm3+ and 1.0 wt% Ho3+ co-doped tellurite fibre which are compared to the bulk glass sample of the same dopant concentration when excited with the same 808 nm laser source. The fibres are multi mode with a core diameter of 12 μm. The spectra have been normalized with respect to the peak at ~2.0 μm which is attributed to the Ho3+: 5I7→5I8 transition. In fibre the peak narrows and shifts to longer wavelengths and also appears as a single peak instead of a double peak as seen in the bulk glass. Radiation trapping in the Ho3+ ions due to the increased path length and energy confinement in the fibre causes the narrowing and shifting of the peak. This is due to emission from the same Stark levels within the 5I7 manifold to higher energy Stark levels in the 5I8 manifold, leading to longer wavelength transitions. The peak at 1.46 μm and shoulder at ~1.86 μm are due to the 3H4→3F4 and 3F4→3H6 transitions, respectively in Tm3+. The intensities of these peaks relative to the ~2.0 μm peak are much weaker in fibre than in bulk glass. Ho3+ ions cannot be directly excited with an 808 nm source which means that energy must be transferred to Ho3+ from the Tm3+ ions. The reduction in intensity of the Tm3+ peaks in Fig. 3 shows that the energy transfer processes from Tm3+ to Ho3+ are becoming more efficient in fibre over bulk glass. The intensity of the peak at 1.46 μm increases with fibre length, suggesting a back energy transfer process from Ho3+ to Tm3+ ions due to the upconversion in Ho3+ which increases in longer lengths of fibre. The evidence for back energy transfer also suggests that for operation at ~2.0 μm in tellurite fibre, shorter lengths are preferable. The lifetime of the Ho3+: 5I7 level in bulk glass is 3148 μs and in multi mode fibre increases to 4200-4500 μs, however the lifetime does not appear to vary with fibre length, as observed in the case of Tm3+-doped fibres. The increase in lifetime in fibre over bulk glass further confirms the increased energy transfer to the 5I7 level in the fibre geometry.
Figure 3 shows the emission spectra of 0.5 wt% Tm3+ and 1.0 wt% Yb3+ co-doped fibre of varying length with a core diameter of 12 μm. The use of Yb3+ as a sensitizer in this fibre allowed the use of a 978 nm pump source to populate the 3H5 level of Tm3+ through energy transfer from Yb3+. Yb3+ has an absorption peak centred at 977 nm due to the 2F7/2→2F5/2 transition which is more intense than the Tm3+ absorption peaks making this an efficient pumping scheme . The emission spectra have been normalized with respect to the peak from the Tm3+: 3F4→3H6 transition. The results again show, as in Fig. 2, a narrowing and shifting to longer wavelengths with increasing fibre length. The peak at ~1.5 μm is from the 3H4→3F4 transition in Tm3+ which must be caused by upconversion in Tm3+ due to the fact that energy transfer from Yb3+ populates the lower lying 3H5 level of Tm3+. The lifetime of the 3F4 level in this fibre was generally slightly shorter than in the pure Tm3+-doped fibre due to the increased upconversion, and did not seem to vary with length.
Strong visible emission due to upconversion was observed and in the Tm3+-Ho3+ and Tm3+-Yb3+ co-doped fibres, and so was measured to further help in the understanding of the fluorescence and energy transfer mechanisms. The visible emission spectra were measured at room temperature using the same pump sources as for the infrared measurements. These results can be found in Figs. 4(a) and 4(b). Visible emission could not be observed from the Tm3+-doped fibre with the naked eye, but measurements showed that there was very weak blue emission at 480 nm due to the 1G4→3H6 transition arising from a two pump photon excited state absorption (ESA) process. The Tm3+-Ho3+-doped fibre shows additional emission at 550 nm which is due to the Ho3+: (5S2, 5F4)→5I8 transition [Fig. 4(a)]. The Tm3+-Yb3+ co-doped fibre shows strong emission at 480 and 650 and 800 nm due to the 1G4→3H6, 1G4→3F4 and Tm3+: 3H4→3H6 transitions, respectively [Fig. 4(b)].
Figure 5 shows the energy level diagrams for the three systems studied showing the pumping, energy transfer and upconversion mechanisms. In Tm3+-doped fibre the 808 nm pump excites the Tm3+: 3H4 level, which decays to the 3F4 level emitting a 1.46 μm photon and then to the 3H6 level emitting the 1.86 μm photon. At high Tm3+ concentration and in fibre, a quenching mechanism takes place between Tm3+ ions and energy is directly transferred non-radiatively from one ion to another. One ion decays from the 3H4 to the 3F4 while the other ion is excited from the 3H6 ground state to the 3F4 level. This process results in two Tm3+ ions being excited to the 3F4 level for one 808 nm pump photon. Stokes energy transfer also occurs from an ion decaying from the 3F4 level exciting another ion to the 3F4 level. There is upconversion from 3H5 to 1G4, which then decays to the ground state giving rise to weak emission at 480 nm. This peak is very low in intensity because the upconversion to 1G4 is originating from the 3H5, level which will undergo fast non-radiative decay to 3F4. Upconversion emission has been observed  at ~376 nm in Tm3+-doped fluoride glass due to the 1D2→3H6 transition which is not seen in this tellurite glass because the corresponding energy of this transition falls in the UV absorption edge and is therefore quenched.
In the Tm3+-Ho3+ fibre, the Tm3+ ions are pumped in the same way as above and the energy is transferred non-radiatively to the Ho3+ ions. The Tm3+ ion decays from the 3F4 level to the ground state and the Ho3+ ion is excited from the 5I8 ground state to the 5I7 level from which it decays emitting a 2.05 μm photon. Upconversion excites the Ho3+ ions to the 5I5 level, from which the states decays non-radiatively to the 5I6 level. A back-energy transfer repopulates the Tm3+: 3H5 level which decays non-radiatively to the 3F4 level. Energy is then transferred back to the Ho3+: 5I7 level emitting a 2.05 μm photon . Upconversion also populates the 5S2 and 5F4 levels in Ho3+ which de-excite to the ground state giving rise to the green emission at 550 nm. The addition of the Ho3+ ions enhances the upconversion to the Tm3+: 1G4 level and consequently the blue emission intensity at 480 nm.
In the Tm3+-Yb3+ co-doped fibre, the 976 nm pump excites the Yb3+: 2F5/2 level which exhibits fluorescence at 1020 nm exciting the Tm3+ ions into the 3H5 level through a non-resonant phonon-assisted energy transfer process . This level decays non-radiatively to 3F4 and then radiatively to the ground state emitting the 1.86 μm photon. The two photon upconversion populates the Tm3+: 3H4 level, which decays to the ground state emitting an 800 nm photon. By comparison the three photon upconversion populates the 1G4 which decays to the 3F4 and 3H6 levels giving strong emission at 650 nm (red) and 480 nm (blue) respectively[3,13,14].
The lifetimes of the 1G4 and 3H4 levels in Tm3+ were measured in a 0.5 wt% Tm3+-doped bulk glass samples to be 204 μs and 343 μs, and are in good agreement with predictions made using the Judd-Ofelt theory of 210 μs and 345 μs, respectively . Despite the calculated radiative lifetime of the 3H5 level being relatively long (~2 ms) and the branching ratio for the 3H5→3H6 transition large (99%) , no emission at around 1.2 μm due to this transition was observed in either Tm3+ or Tm3+-Yb3+-doped bulk glass or fibre. This is due to multi-phonon absorption depopulating the 3H5 level into the 3F4 level, as only 4 phonons are required to bridge the ~2600 cm-1 energy gap. The very low intensity of blue emission in Tm3+-doped samples further confirms this process, because in order to populate the 1G4 level an 808 nm pump photon must be absorbed by an electron in the 3H5 level.
Lasing has been demonstrated for the Tm3+ -Yb3+ -doped fibre pumped by a Yb3+ -doped silica fibre laser operating at 1088 nm. This Yb-fibre laser was chosen for its high power and good beam quality and, even though the absorption coefficients of Tm3+ and Yb3+ are small at 1088 nm, the long fibre length enables significant pump absorption. Initial results show lasing from the Tm3+: 3F4→3H6 transition at 1.9 μm with output powers up to 67 mW, slope efficiency of 10% with respect to absorbed pump power and threshold of ~400 mW for a 16 cm long tellurite fiber. A full description of these experiments is presently being prepared for publication.
Mid-IR emission in the 1.8 – 2.1 μm range has been measured in Tm3+-doped and Tm3+-Ho3+ co-doped tellurite fibres when excited with an 808 nm pump. Tm3+-doped fibres show strong emission at ~1.86 μm while emission at 2.05 μm is observed in the Tm3+-Ho3+ co-doped fibres via energy transfer to the neighbouring Ho3+ ions. Emission has also been observed at ~1.86 μm in Tm3+-Yb3+ co-doped fibres when pumped with a 976 nm source. These measurements show narrowing and red-shifting of emission peaks and increased energy transfer in the smaller geometries associated with these fibres. The visible upconversion emission has been measured and used to help explain the energy transfer processes involved. Laser emission has been observed at 1.9 μm in Tm3+-Yb3+-doped fiber.
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