1. Introduction

For multi-channel WDM transmission, it is essential to have a large bandwidth with a flat-gain spectrum for minimising the channel-to-channel gain excursion and crosstalk in a high-speed network. Although Er³⁺-doped fibre and waveguide amplifiers are available, the bandwidth is limited in silicate hosts to a maximum of 40 nm at C-band (1530–1565 nm). By comparison, a tellurium oxide host with Er³⁺ ion as dopant exhibits 80 nm of bandwidth across which the small-signal gain is 20 dB [1]. It was recently shown that the inclusion of Tm-doped fluoride fibres could extend the bandwidth into the S-band (1460–1530 nm). However, the gain spectrum is not likely to be continuous [2]. A Tm³⁺-doped and Er³⁺-doped fibre-based device can potentially supersede the cascaded configuration of Tm³⁺-doped fluoride fibre and Er³⁺-doped tellurite fibre amplifiers [3]. The available bandwidth in a tellurite fibre amplifier with Tm³⁺/Er³⁺ dopants is over 200 nm which can be potentially pumped with both 800 nm and 980 nm lasers. Recently, Jeong et al. [4] reported spectral characteristics of amplified spontaneous emission (ASE) from an Er³⁺-Tm³⁺ co-doped silica fiber. When pumped at 980 nm, the ASE yielded a 3 dB bandwidth over 90 nm, from 1460 to 1550 nm.

In this paper, a broad seamless emission extending from 1.35 µm to 1.6 µm in Er³⁺-Tm³⁺ codoped tellurite glasses and fibres were observed using the pump excitation at 800 nm. The possibility of amplifying signals beyond 1600 nm is also discussed. The visible upconversion emission spectra were also recorded to understand the luminescence mechanisms. The energy transfer processes between Er³⁺ and Tm³⁺ ions are discussed.

2. Experimental

The composition of tellurite glass chosen was 89.91TeO₂-5.73Na₂O-4.36ZnO (wt%). The doping concentration of Er³⁺ in the doped glass was 0.2 wt%, while the Er³⁺-Tm³⁺-codoped tellurite glasses had a fixed concentration of 0.2 wt% Er₂O₃ and different concentrations of Tm₂O₃: 0.2, 0.4, 0.6, 0.8, and 1.0 wt%, respectively. The fibre produced for the characterisation of emission was doped with 0.2 wt% Er₂O₃ and 1.0 wt% Tm₂O₃. The oxide ingredients were calculated, weighed and mixed thoroughly inside a dry glove box. The weighed material was transferred into a gold crucible inside the glove box and then melted at 800 °C in an atmosphere of dry O₂. The melted mixture was homogenized for 30 minutes, stirred, and allowed to equilibrate by releasing the gas bubbles formed during melting. The homogenized melt was quenched either into a preheated brass mould to cast a bulk glass, or into a preheated brass cylindrical mould for making a cylindrical shape rod of 10 mm in diameter with 20 mm in length. After casting, the glass was annealed at 285 °C for 3 hours in a muffle furnace, after which it was allowed to cool slowly inside the furnace. The glass samples were polished carefully for the optical measurements, whereas the glass rod was drawn into a 125 µm diameter unclad fibre in a fibre drawing tower.

The absorption spectrum of the sample was performed by a Perkin-Elmer Lambda 19 model spectrometer at room temperature. The 800 nm excitation line produced by a Ti-sapphire (Schwartz Electro Optics, Titan CWBB) laser pumped with two argon-ion lasers (Coherent, Innova 90) was used. The near infrared emission spectra and visible emission spectra were recorded using a scanning spectrometer equipped with an InGaAs detector (Macam Photometrics). The cut-off wavelength in the infrared range of the scanning spectrometer was 1700 nm. Lifetimes were measured by using a mechanical chopper and a digital oscilloscope (Tektronics, TDS3012). All optical experiments were carried out at room temperature.

3. Results and discussion

The absorption spectra of 0.2 wt% Er₂O₃ and 0.2 wt% Tm₂O₃ codoped tellurite glass sample at room temperature is shown in Fig. 1. The symbols E and T represent lines due to Er³⁺ and Tm³⁺, respectively. Absorption bands of Er³⁺ and Tm³⁺ ions are all from their ground states ⁴I_15/2 and ³H₆ to the level specified, respectively. The band positions for Er³⁺ and Tm³⁺ are similar to fluoride glasses [5]. The absorption spectra are similar for all the samples used in this study and the absorption coefficients are proportional to the concentrations of Er³⁺ and Tm³⁺ ions in the samples.

Figure 2 shows the normalized near infrared emission spectra of Er³⁺ doped and Er³⁺- codoped with Tm³⁺ tellurite glasses with a pump excitation at 800 nm and a pump power of 200 mW. For the Er³⁺ doped glass, the emission peak is at 1532 nm with a full width at half-maximum (FWHM) of ~50 nm, which is attributed to the ⁴I_13/2→⁴I_15/2 transition of Er³⁺. With the increasing addition of Tm₂O₃, the emission spectra broadened significantly, starting from 1350 nm to 1600 nm. It is clear from Fig. 2 that the emission at 1465 nm, due to Tm³⁺: ³H₄ → ³F₄ transition, becomes stronger with the increasing concentration of Tm³⁺ ions in the glass. The ³H₄ → ³F₄ transition of Tm³⁺ at 1465 nm overlaps significantly with the 1532 nm emission of Er³⁺. The FWHM is ~134 nm for 0.2 wt% Er₂O₃ and 1.0 wt% Tm₂O₃ codoped tellurite glass. In addition, we also observe the short-wavelength tail of the Tm³⁺ emission at 1650 nm due to the ³F₄ → ³H₆ transition. The spectroscopic analysis was limited due to the upper limit of spectrometer at 1700 nm.

 

Fig. 1. The absorption spectra of 0.2 wt% Er₂O₃ and 0.2 wt% Tm₂O₃ co-doped tellurite glass (thickness=4.38 mm) at room temperature. A is the absorption (¹⁰log(I/I₀).

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Fig. 2. The normalized near infrared emission spectra of Er³⁺ singly doped and codoped with Tm³⁺ tellurite glasses with the excitation of an 800 nm laser.

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Figure 3 compares the near infrared emission spectra of 0.2 wt% Er₂O₃ and 1.0 wt% Tm₂O₃ co-doped tellurite fibres as a function of fibre length, using pump excitation at 800 nm with a pump power of 200 mW. The 24 cm long Er³⁺-Tm³⁺ codoped fibre exhibits a broad emission spectrum with 160 nm bandwidth. The longer wavelength part of 1.46 µm emission of Tm³⁺ disappeared in a longer fibre due to the absorption via the Er³⁺:⁴I_15/2→⁴I_13/2 transition. With increasing fibre length, both intensities of 1.46 µm emission of Tm³⁺ and 1.53 µm emission of Er³⁺ decrease due to the relative high fibre background loss in the unclad geometry, Er³⁺: ⁴I_15/2→⁴I_13/2 absorption, short wavelength tail of Tm³⁺: ³H₆→³F₄ absorption, and the energy transfers from Tm³⁺: ³H₄ to Er³⁺: ⁴I_9/2 and from Er³⁺: ⁴I_13/2 to Tm³⁺: ³F₄, see Fig. 5. Moreover, the intensity of 1.53 µm emission of Er³⁺ decreases faster than that of the 1.46 µm emission in Tm³⁺ since an energy transfer from Er³⁺: ⁴I_13/2 to Tm³⁺: ³F₄ becomes more efficient in a longer fibre. The optimised Er³⁺-Tm³⁺ codoped tellurite fibres, thus is a potential short fibre device for WDM system in the S+C+L bands (1460–1625). If the length is optimised, it is possible to access L+U bands (1565–1675 nm) in the 1600 to 1700 nm regions by sacrificing the amplification below 1500 nm wavelength.

 

Fig. 3. The normalized near infrared emission spectra of 0.2 wt% Er₂O₃ and 1.0 wt% Tm₂O₃ co-doped tellurite unclad fibres with different fibre length under the excitation of an 800 nm laser.

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In order to further understand the luminescence mechanisms, the visible emission spectra of Er³⁺-doped and Er³⁺-Tm³⁺-codoped tellurite glasses were also measured at room temperature under 800 nm excitation within the ⁴I_9/2 (Er³⁺) and ³H₄ (Tm³⁺), as shown in Fig. 4. It is well known that the excitation from the ⁴I_15/2 to the ⁴I_9/2 levels of Er³⁺ generates the green (²H_11/2→⁴I_15/2 and ⁴S_3/2→⁴I_15/2) and red (⁴F_9/2→⁴I_15/2) upconversion fluorescences in Er³⁺- doped glass due to the excited state absorption and the upconversion by energy transfer. On the other hand, no upconversion fluorescence was recorded in a Tm³⁺-single-doped tellurite glass with the excitation of 800 nm laser. Therefore, the green emissions at ~525 nm and ~545 nm and the red emission at ~660 nm are identified as relevant Er³⁺ transitions in Figure 5. From Fig. 4, it should be noted that with the increasing concentrations of Tm³⁺, the intensities of green emissions at ~525 nm and ~545 nm decrease drastically, while the intensity of red emission at ~660 nm increases moderately.

The near infrared and visible luminescence mechanisms for Er³⁺ and Tm³⁺ codoped tellurite glasses and fibres are explained on the basis of Fig. 5. First, the 800 nm laser excitation of Tm³⁺ and Er³⁺ populates ³H₄ and ⁴I_9/2 levels from the ground states Tm³⁺: ³H₆ and Er³⁺: ⁴I_15/2, respectively. The relaxation in Tm³⁺ from ³H₄→³F₄ level yields 1465 nm emission, whereas the Er³⁺ de-excites non-radiatively to ⁴I_11/2 then to ⁴I_13/2. Finally, Tm³⁺ and Er³⁺ relax to the respective ground states ³H₆ and ⁴I_15/2 generating the ~1800 nm and 1532 nm emissions. On the other hand, the Er³⁺ at ⁴I_13/2 level may be excited to the ²H_11/2, ⁴S_3/2 level by absorbing an 800 nm photon or via energy transfer from another Er³⁺ ion. The radiative relaxation from ²H_11/2 and ⁴S_3/2 levels to the ground state yields green emissions. Alternatively, non-radiative depopulation of the ²H_11/2, ⁴S_3/2 to ⁴F_9/2 level can also take place, from where the emission at 660 nm takes place. Due to the high concentrations of Tm³⁺ and Er³⁺ ions, the energy transfer processes between Tm³⁺ and Er³⁺ can potentially occur. The dominant energy transfers are described as follows [5–8]:

 

Fig. 4. The visible upconversion emission spectra of Er³⁺-doped and Er³⁺-Tm³⁺-codoped tellurite glasses excited by 800 laser.

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Fig. 5. Energy level diagram of Er³⁺ and Tm³⁺ ions and the near infrared and visible upconversion luminescence mechanisms of Er³⁺ and Tm³⁺ codoped tellurite glasses and fibres with an 800 nm laser excitation at room temperature. The solid lines stand for the absorption and emission transitions for Er³⁺ and Tm³⁺ ions. The dashed lines represent the nonradiative relaxations. The curves stand for energy transfers (cross relaxations) between Tm³⁺ and Er³⁺.

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The energy transfer process (1), which is a resonant energy transfer because of a very small gap between the Er³⁺: ⁴I_9/2 and Tm³⁺: ³H₄ levels, depopulates Tm³⁺: ³H₄ level resulting in the reduction of the intensities of the emissions at 1465 nm and ~1800 nm from Tm³⁺, and the increase of the intensity of 1532 nm emission from Er³⁺, whereas the process (2) is un-resonant and causes the reduction of the intensity of 1532 nm emission from Er³⁺ and consequently enhances the emission at ~1800 nm from Tm³⁺. The process (3) depopulates Er³⁺: ⁴I_13/2 level and consequently decreases the intensity of 1532 nm emission from Er³⁺ and also results in the decrease of the green and red upconversion emissions due to the second upconversion process commencing from the ⁴I_13/2 level and the increase of the 1800 nm emission from Tm³⁺. The process (4) decreases the emission at ~1800 nm, but preferably promotes a large number of Er³⁺ions to the ⁴F_9/2 level favouring the red upconversion emission. In the co-doped tellurite fibres, the energy transfers between Tm³⁺ and Er³⁺ would be more efficient due to long path length of the interaction between Tm³⁺ and Er³⁺ with the increasing fibre length. The energy transfer rate W_ET can be obtained experimentally using the following equation:

where τ_f and τ₀ are the lifetimes in the presence and absence of acceptors, respectively [5]. In addition, the energy transfer efficiency η_ET can be calculated with

The calculated energy transfer rates are 140 s^-1 and 985 s^-1 for the energy transfer (1) and (3) in the 0.2 wt% Er₂O₃ and 1.0 wt% Tm₂O₃ co-doped tellurite fibre, respectively, using the measured lifetimes, whereas the corresponding energy transfer efficiencies are 3.3 % and 79.2 % for the energy transfer steps (1) and (3). Clearly, a flat and broad emission is achievable in Er³⁺ and Tm³⁺ codoped tellurite fibre by modifying Er³⁺ and Tm³⁺ ions concentrations, their concentration ratios, and the fibre length. The optimised Er³⁺-Tm³⁺ codoped tellurite fibres, thus is a potential broadband fibre light source and a short fibre device for WDM systems.

4. Conclusions

Broad emissions were observed when the Er³⁺-Tm³⁺ codoped tellurite glasses and fibres were excited with an 800 nm pump. Especially, a broad emission extending from 1.35 µm to 1.6 µm with the full width at half-maximum (FWHM) of ~160 nm was recorded at room temperature in a 24 cm long 0.2 wt% Er₂O₃ and 1.0 wt% Tm₂O₃ codoped tellurite fibre using an 800 nm laser pump. The energy transfer processes between Er³⁺ and Tm³⁺ play important roles in the luminescence mechanism. The results indicate Er³⁺-Tm³⁺ codoped tellurite fibre can be optimised for an ASE source and broadband amplifier applications. By optimising the fibre length, it is also possible to access amplification wavelengths beyond 1600 nm using the ³F₄ to ³H₆ emission in Tm³⁺.