The visible and near infrared emission spectra of Er3+-Tm3+- codoped tellurite glasses and fibres were measured with the excitation of an 800 nm laser. A broad emission extending from 1.35 µm to 1.6 µm with a full width at half-maximum (FWHM) of ~160 nm was recorded in a 24 cm long 0.2 wt% Er2O3 and 1.0 wt% Tm2O3 codoped tellurite fibre. Energy transfer between Er3+ and Tm3+ play important roles in the luminescence mechanism. These results indicate that Er3+-Tm3+ codoped tellurite fibre could be a promising material for broadband light source and broadband amplifier for the wavelength-division-multiplexing (WDM) transmission systems.
©2004 Optical Society of America
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 . 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 . 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 . 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.  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.
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 4I15/2 and 3H6 to the level specified, respectively. The band positions for Er3+ and Tm3+ are similar to fluoride glasses . 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 4I13/2→4I15/2 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.
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+:4I15/2→4I13/2 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+: 4I15/2→4I13/2 absorption, short wavelength tail of Tm3+: 3H6→3F4 absorption, and the energy transfers from Tm3+: 3H4 to Er3+: 4I9/2 and from Er3+: 4I13/2 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+: 4I13/2 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.
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 4I9/2 (Er3+) and 3H4 (Tm3+), as shown in Fig. 4. It is well known that the excitation from the 4I15/2 to the 4I9/2 levels of Er3+ generates the green (2H11/2→4I15/2 and 4S3/2→4I15/2) and red (4F9/2→4I15/2) 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 4I9/2 levels from the ground states Tm3+: 3H6 and Er3+: 4I15/2, respectively. The relaxation in Tm3+ from 3H4→3F4 level yields 1465 nm emission, whereas the Er3+ de-excites non-radiatively to 4I11/2 then to 4I13/2. Finally, Tm3+ and Er3+ relax to the respective ground states 3H6 and 4I15/2 generating the ~1800 nm and 1532 nm emissions. On the other hand, the Er3+ at 4I13/2 level may be excited to the 2H11/2, 4S3/2 level by absorbing an 800 nm photon or via energy transfer from another Er3+ ion. The radiative relaxation from 2H11/2 and 4S3/2 levels to the ground state yields green emissions. Alternatively, non-radiative depopulation of the 2H11/2, 4S3/2 to 4F9/2 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]:
The energy transfer process (1), which is a resonant energy transfer because of a very small gap between the Er3+: 4I9/2 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+: 4I13/2 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 4I13/2 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 4F9/2 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 WET can be obtained experimentally using the following equation:
where τf and τ0 are the lifetimes in the presence and absence of acceptors, respectively . In addition, the energy transfer efficiency ηET 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.
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+.
The authors acknowledge this work was supported by EPSRC of UK.
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