Amplification properties have been compared in Er3+, Er3+/Eu3+ and Er3+/Ce3+ doped tellurite glass fibers using a 980 nm pumping scheme. The pump efficiency has been compared in the 3 types of fibers. Er3+ ion upconversion in bulk glasses and fibers in visible range has been measured and the Er3+ ion IR fluorescence intensity and lifetimes have been recorded to understand the amplification characteristics. Codoping with Ce3+ is more efficient in Er3+ doped tellurite fibre.
©2006 Optical Society of America
Tellurium oxide glasses are known to have a larger absorption and emission cross-section, lower phonon energy than the commercial silica glass used for the fabrication of EDFA. The glass also exhibits 10 to 50 times larger solubility of Er-ions than in silica [1, 2]. These spectroscopic features make Er3+ ion doped TeO2 glass fiber a much better host for designing broadband amplifiers with much shorter device lengths. In Er3+-doped TeO2 glass fiber small signal gains exceeding 20 dB were obtained over a bandwidth as wide as 80 nm from 1530 to 1610 nm . Commercial EDFAs utilize either 980 nm or 1480 nm pumping scheme, but the latter is a resonant pumping scheme and limits the full inversion, therefore, the noise figures are adversely affected . Another issue is about pump to signal cross-talk at operating wavelength in a broadband amplifier, which is why the 1480 nm pumping is not useful as the 980 nm.
In tellurite glass based EDFA, the 980 nm pump is not efficient. This is due to the relatively low phonon energy of tellurite glass (780 cm-1) compared with the values for silicate (1100 cm-1) and phosphate (1400 cm-1) hosts, resulting into a slow feeding rate from the 4I11/2 to 4I13/2 levels and a large transitional probability for the excited state absorption (ESA). From the energy level diagram of Er3+, shown in Fig.1, for an efficient use of 980 nm pump, it is critical to increase the transition rate from 4I11/2 to 4I13/2 level, which can increase the population of upper level of 4I13/2 for 1.5 µm fluorescence and reduce the pump ESA. Codoping with a suitable rare-earth ion is a method to alleviate the problems of large ESA at 980 nm and slow multiphonon relaxation to 4I13/2 level. The Ce3+ ion is an ideal candidate for this purpose since the energy gap between the 2F7/2 and 2F5/2 levels in Ce3+ ion is resonant with the gap between 4I11/2 and 4I13/2 in Er3+ ion. The use of Ce3+ ion as a co-dopant with Er3+ was studied by Choi et al [5, 6] in a tellurite bulk glass, in which the authors demonstrated the enhancement in the transition rate from 4I11/2 to 4I13/2 without shortening the lifetime of 4I13/2 level. The gain enhancement in Er3+/Ce3+ doped tellurite fiber has been reported . Eu3+ ion is another candidate which was used to increase the decay rate of Er3+ 4I11/2 energy level by cross-relaxation from Eu3+: 7F0→7F4 . The energy transfer process from Er3+ to Ce3+ and Eu3+ are shown in Fig. 1.
In this paper we have compared the gain characteristics and pump inversion efficiency in three different types of fibers: Er3+, Er3+/Eu3+ and Er3+/Ce3 tellurite glass fibers, which were excited with the 980 nm diode laser. The difference in the amplification characteristics between these fibers is understood by comparing the Er3+ fluorescence in IR and visible ranges in the rare earth doped bulk glass and fiber samples.
Single mode tellurite glass fibers were produced using the suction casting method. The host glass was based on a ternary TeO2-ZnO-Na2O system, reported elsewhere . Using this technique, three types of fibers were produced: a) Er fiber: core glass doped with 0.5 wt% Er2O3; b) ErEu fiber: core glass doped with 0.5 wt% Er2O3 and 1 wt% Eu2O3, and c) ErCe fiber: core glass doped with 0.5 wt% Er2O3 and 1 wt% CeO2. The measured fiber length of each single mode fiber was 9 cm with NA=0.2, and the core diameter was 6 µm. As the 3 fibers are fabricated in the same process, the fiber single mode profile is near identical in near IR region. The input signals were generated from a tunable laser or broadband light sources, which were WDM coupled into the fiber. The signal and pump were focused into the tellurite fiber using an objective lens. The output signal was butt-coupled with an OSA. The gain measured is the internal gain, which was calculated by subtracting absorption from the relative gain in the fiber. Bulk glass samples of three types of core glasses were made for fluorescence and lifetime measurements. The fluorescence spectra were recorded using spectrometer (Model FS 920, Edinburgh Instruments, UK) pumped either at 808 nm or 980 nm laser diodes for which the near-IR photomultiplier tube (PMT) and visible PMT detectors were used. The fluorescence decay curves were obtained by using an ultra fast pulsed flash lamp as an excitation source.
3. Results and discussions
In Fig. 2, the internal gain for 3 different fibers are compared with different pump power at the peak wavelength of 1534 nm and input power of -15 dBm. It is obvious that the internal gain increased with the addition of Eu3+ and Ce3+ ions. The highest gain was observed in ErCe fiber, followed by that in the ErEu fiber. Especially the internal gain nearly saturated with launched pump power around 50 mW in ErCe fiber, which is much lower than that in other two types of fibers, which is around 80 mW. In these measurement, we have kept the pump coupling efficiency as the same as possible for different fibers. The pump inversion efficiency (PIE) of each fiber, which is defined as the internal gain in the fiber divided by Er3+ ion absorption, is compared in Fig. 3. The input signals are from broadband light source. It is also clear that the PIE in ErCe fiber is much larger across the whole spectral range than that in other two fibers, whereas the PIE in ErEu fiber is only marginally higher than that in Er-doped fiber. We also observed that the PIE increased with the increase in the input signal wavelength. This may be due to the lower absorption cross section in the longer wavelengths and the energy migration or re-absorption/re-emission of Er3+ ions in this 3-level energy system, which confirms that the reduction in ESA enhances the bandwidth of amplifier operation.
In Er3+, Er3+/Eu3+ and Er3+/Ce3+ doped tellurite glasses, the fluorescence spectra in near IR range and lifetimes of Er3+ ions at 1534 nm have been measured with 808 nm pump, which are shown the Fig. 4 and 5, respectively. It is clear that the 980 nm fluorescence intensity (4I11/2→4I13/2) decreased with Eu3+ and Ce3+ ions in the glass since Ce3+ ions are more efficient than the Eu3+ ions. From the fluorescence decay curve in Fig. 5, the 1534 nm fluorescence decay lifetime is 4.4 ms, 3.9 ms and 3.1 ms in Er, ErCe and ErEu doped glasses, respectively. Both the Eu3+ and Ce3+ ions reduce the metastable lifetimes, however the codoping with Eu3+ ions is much shorter, reducing it by 30%, than in the ErCe glass, which reduces by 11%. This can be understood from the energy level diagrams in Fig. 1 as the Eu3+ ion has more closely packed energy levels and the stokes energy difference between 7F6 :Eu3+ and 4I13/2:Er3+ is smaller than the energy gap between 4I13/2:Er and 2F7/2:Ce3+. Shortening lifetime at 4I13/2 level with co-doping ions will reduce the internal gain in the fiber.
The upconversion spectra in Er3+ doped bulk glass samples and 9 cm glass fibers were measured with 980 nm pump, which are shown in Fig. 6 (a) and (b). The fibers for the upconversion measurement are the same with which were used for gain measurement. The launched pump power is kept at the same in the 3 bulk glass samples and fibres. In both glass and fiber samples, the intensities of pump ESA have reduced significantly in the ErCe doped samples, since the Ce3+ ions are more efficient in reducing pump ESA at 980 nm than the Eu3+ ions. This also corresponds to the IR fluorescence spectra in Fig.4. Especially in the 9 cm fiber, the decrease in ESA is much more than in the bulk glass due to the long interaction length of photons. In ErCe fiber, the pump ESA at 550 nm and 670 nm are rarely recognizable. That means there is much less pump wasted in the visible upconversion in the ErCe fiber. The results in Fig. 4 to 6 provided unambiguous evidence why the internal gain and PIE are the highest in ErCe fiber. In ErEu fiber, it is understandable that the internal gain and PIE are slightly better than pure Er3+ doped fiber by combining both effects: the decrease in pump ESA and shortening of the lifetime of 4I13/2 level. The reduction in ESA has therefore been compensated at the expense of shortening of 4I13/2 level lifetime.
In conclusion, for Er3+ doped tellurite glass fiber amplifier with 980 nm pump scheme, codoping with Ce3+ and Eu3+ can improve internal gain and pump inversion efficiency though both ions shorten the 1534 nm fluorescence lifetimes. The Ce3+ ion is more efficient in reducing ESA and has less quenching effect on the 1.5 µm fluorescence than Eu3+ ions. Therefore, the Ce3+ ion is a suitable codopant for Er3+ doped tellurite glass fiber amplifier with 980 nm pump scheme.
The authors acknowledge the support from the EPSRC, UK.
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