1. Introduction

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) [1]. Nearly 15000 ppm of Er₂O₃ was dissolved without concentration quenching and up to 70000 ppm without devitrification [2]. It is also a chemically and environmentally stable glass unlike fluorides making it more desirable for device manufacture [3]. 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 [2]. 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 [5]. 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 [6], 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 [7]. 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.

2. Experimental

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 [8]. 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 [9].

 

Fig. 1. The emission spectra of 0.5 wt% Tm³⁺-doped tellurite fibre (core diameter = 12μm) and bulk glass when excited with an 808 nm laser source. The spectra have been normalized with respect to the peak at ~1.86 μm.

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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.

 

Fig. 2. The emission spectra of 0.2 wt% Tm³⁺-1.0 wt% Ho³⁺ co-doped tellurite fibre (core diameter = 12μm) and bulk glass when excited with an 808 nm laser source. The spectra have been normalized with respect to the peak at ~2.0 μm.

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Fig. 3. The emission spectra of 0.5 wt% Tm³⁺-1.0 wt% Yb³⁺ co-doped tellurite fibre (core diameter = 12μm) when excited with an 978 nm laser source. The spectra have been normalized with respect to the peak at ~1.86 μm.

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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 ²F_7/2→²F_5/2 transition which is more intense than the Tm³⁺ absorption peaks making this an efficient pumping scheme [1]. 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)].

 

Fig. 4. The visible upconversion emission spectra for (a) 0.2 wt% Tm³⁺ - 1.0 wt% Ho^{3 +} co-doped tellurite fibre with 808 nm pump and (b) 0.5 wt% Tm³⁺ - 1.0 wt% Yb³⁺ co-doped tellurite fibre with 978 nm pump.

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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 [10] 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 [11]. 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⁺: ²F_5/2 level which exhibits fluorescence at 1020 nm exciting the Tm³⁺ ions into the ³H₅ level through a non-resonant phonon-assisted energy transfer process [12]. 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 [13]. 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%) [13], 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.

 

Fig. 5. Energy level diagrams for the Tm³⁺-Tm³⁺, Tm³⁺-Ho³⁺ and Tm³⁺-Yb³⁺ systems showing the pumping schemes, multiple photon upconversion and energy transfer.

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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.

4. Conclusions

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.