We present the first in-band diode-pumped TDFAs operating in the 2 µm wavelength region and test their suitability as high performance amplifiers in potential future telecommunication networks. We demonstrate amplification over a 240 nm wide window in the range 1810 – 2050 nm with up to 36 dB gain and noise figure as low as 4.5 dB.
© 2013 Optical Society of America
As a result of the exponentially increasing volume of internet traffic, today’s telecom networks are rapidly being driven towards their capacity limits [1–3]. The quest for increasing transmission capacity has stimulated interest in radical approaches [4,5], e.g., space division multiplexing (SDM) employing multi-mode [6,7] or multi-core [8,9] fibers. These novel techniques currently under study have shown impressive data transmission performance and certainly offer interesting prospects. It is worth noticing, however, that most of the research work directed towards enhancing network capacity has been focusing on the 1.55 µm optical waveband, which falls in the well-known low-loss transmission window of silica fiber and the amplification band of erbium doped fiber amplifier (EDFA).
Thulium-doped fiber amplifiers (TDFAs) operating around 2 µm offer the broadest gain spectrum of all rare-earth doped fiber amplifiers  and represent an attractive route towards significantly enhanced transmission bandwidths, which may eventually justify a shift away from the traditional operating wavelengths around 1.55 µm. Additionally, their operating region overlaps with the predicted minimum loss window of hollow-core photonic bandgap fibers (HC-PBGFs) [11,12], which hold great promise as a transmission medium due to their ultra-low nonlinearity and faster transmission speed as compared to conventional solid core fibers . Recently, the first amplified data transmission system at 2 µm using TDFAs and HC-PBGFs has been demonstrated, confirming the viability/practicability of this approach .
TDFAs have recently been characterized extensively in an optical communications context, demonstrating high gain, low noise amplification over more than a 100 nm bandwidth around 2 µm . The current implementations are typically in-band fiber laser pumped, which would limit their applicability in real-life transmission systems. Ultimately, TDFAs will have to reach the same level of compactness, reliability and efficiency as current Erbium-based systems in order to be considered as a true alternative solution, i.e. laser diode pumping is indispensable. Diode-pumped TDFAs have been developed for emission in the first communication window around 800 nm  and in the S-band at 1480 – 1510 nm [17,18], but to date no such pumping scheme has been demonstrated suitable for optical communications in the new waveband of interest at 2 µm.
In this contribution we present the first implementations of TDFAs operating in the 2 µm wavelength region, in-band pumped by laser diodes at 1550 nm. We demonstrate amplification over a 240 nm wide window with up to 36 dB gain, noise figures as low as 4.5 dB and up to 100 mW saturated output power by combining three TDFA designs optimized for short, central and long wavelength operation, respectively. These results represent a major stepping stone in the assessment of such a radically new solution for next generation transmission systems.
2. Experimental setup
The experimental setup of the diode-pumped TDFA is shown in Fig. 1. We employed a commercially available single-mode Tm3+-doped fiber (OFS TmDF200) with ~6.2 µm mode-field diameter at 2000 nm and core absorption of ~20 dB/m at 1565 nm. Three TDFA implementations were investigated using 2 m, 4 m, and 8 m of gain fiber, denoted TDFA-S/C/L, which optimize the amplifier performance at short, central, and long wavelength bands, respectively. The fiber was core pumped in a bidirectional configuration by two Fabry–Pérot (FP) laser diodes (Princeton Lightwave) operating at 1550 nm with 3 dB bandwidth of ~4 nm, each delivering up to 210 mW (23.2 dBm) of pump power. Pump and signal wavelengths were combined using two 1570 / 2000 nm filter based WDM couplers. Isolators were placed both at the input and output ends to prevent parasitic lasing. For characterization, the TDFAs were seeded by an in-house built tunable laser source (TLS) providing narrow linewidth operation in the range 1790 – 1990 nm. Additionally, we used three discrete-mode (DM) laser diodes (Eblana Photonics) emitting at 2008 nm, 2025 nm, and 2045 nm to evaluate the TDFA performance at the long wavelength edge of the amplification band. A power meter (Ophir 3A-FS) and an optical spectrum analyzer (AQ6375) were used to measure gain and noise figure (NF) of the amplifiers.
3. Results and discussion
Figure 2(a) shows the detailed characterization of the TDFA-C implementation, which uses 4 m of gain fiber. The figure presents the wavelength dependence of the small-signal gain (measured with an input signal power of −20 dBm), the saturated gain (measured with an input signal power of 0 dBm), as well as the external NF for both gain curves. The total pump power delivered by both pump diodes was 26.2 dBm in all cases. The amplifier achieves a small-signal peak gain of 36 dB at 1900 nm and provides gain over a more than 215 nm wide window in the range 1830 – 2045 nm. Note that we could not perform measurements at longer wavelengths due to the lack of a suitable seed source. The saturated gain curve is flat and only varies between 18 – 20 dB in the 1840 – 2010 nm waveband. The relatively low output powers of the DM diodes (<0 dBm) at 2025 nm and 2045 nm do not allow us to perform the saturated gain measurements beyond 2010 nm. There is no significant difference in NF between small and saturating input signal powers, varying between 5 and 7 dB over the entire spectral range tested.
A single, compact and diode-pumped TDFA is therefore able to deliver more than 20 dB small-signal gain and less than 7 dB NF over a nearly 200 nm wide transmission window in the 2 µm wavelength region. The saturated amplified spectra have more than 30 dB optical signal to noise ratio (OSNR), as shown in Fig. 2(b).
The amplifier performance at the short and long wavelength edge of the amplification band can be improved by varying the length of employed gain fiber. Figure 3(a) compares the external small-signal gain and NF performance of all three different amplifier configurations. As the gain fiber length is increased, the gain maximum shifts from 1880 nm for TDFA-S (2 m of TDF) to 1950 nm for TDFA-L (8 m of TDF). This behavior is due to the fact that at room temperature Tm3+ is effectively a three level laser system resulting in the reabsorption of short wavelength ASE components with an increase in fiber length.
In comparison to the performance of TDFA-C discussed above, TDFA-S provides up to 10 dB gain enhancement and up to 3 dB improvement in NF for wavelengths below 1860 nm. Similarly, TDFA-L provides up to 4 dB higher gain and up to 1 dB lower NF than TDFA-C for wavelengths beyond 1970 nm. TDFA-L also exhibits the lowest overall measured NF of 4.5 dB at 2025 nm. However, in this amplifier configuration the NF rises sharply for wavelengths below 1900 nm. We found that increasing the length of gain fiber above 8 m does not provide any further performance advantage at long wavelengths.
The above discussion highlights the possibility that high gain, low noise amplification over the entire 240 nm band investigated can be achieved by combining all three amplifier configurations in a transmission system, where short wavelengths up to 1860 nm are amplified by TDFA-S, the central waveband 1860 – 1960 nm by TDFA-C, and TDFA-L provides amplification for longer wavelengths. The combined amplifier system provides more than 20 dB small-signal gain in the waveband 1810 – 2025 nm, and more than 16 dB at up to 2050 nm. These gain figures can be improved in the future using higher pump power. Figure 3(b) shows the combined amplified spectra and the corresponding NF for −20 dBm input signal power. The in-band OSNR is more than 25 dB across almost the whole band under test, with a slight degradation below 1820 nm. Over 30 dB in-band OSNR is observed from 1840 – 2045 nm. The NF for the combined system is flat at ~5 dB over almost the entire waveband tested, but rises rapidly below 1840 nm. It is worth noting that the isolator and the WDM coupler used at the input end of the TDFAs have a total insertion loss of ~1.5 dB in the 1840 – 2050 nm region. Taking this into account, we estimate the internal NF values of the TDFAs to be of order of 1 dB lower than the external NF values shown in Fig. 3(a).
The degradation of gain and NF below 1840 nm in both TDFA-S and TDFA-C is caused by the exponentially increasing insertion loss of the WDM couplers, as shown in the inset of Fig. 3(a). We therefore stress that the reported performance, especially the NF, is mainly limited by the insertion loss and operating bandwidth of the first generation fiber components at 2 µm. There is no fundamental reason why TDFAs should be noisier than erbium based systems, and we expect the NF to approach the values known for EDFAs once a new generation of low-loss components becomes available for the 2 µm region. Therefore, we expect the true operating window of TDFAs to be even broader than physically demonstrated here, especially on the short wavelength side as indicated by the patterned shaded area in Fig. 3(b).
Note that in , it was demonstrated that the TDFA gain at long wavelength could be enhanced by introducing a strand of TDF pumped by the backward travelling amplified spontaneous emission (ASE). However, this method could not be adopted here due to the ASE power being low, limited by the available pump power. A more powerful pump would allow a considerable amount of ASE to be recycled and enhance the TDFA gain above 1960 nm.
Seeding of TDFA-C at 1900 nm was chosen to demonstrate the power scaling capability of the amplifier. Figure 4 shows the variation of output power with increasing pump power of the backward-pumping LD-2, while the forward-pumping LD-1 was operating at maximum power of 210 mW. We obtained more than 100 mW saturated output power at a total pump power of 420 mW with a slope efficiency of 40%. If the insertion losses of the employed passive components (WDM couplers, isolators) are taken into account then the estimated (internal) slope efficiency of TDFA-C is ~55%. For telecom grade amplifiers, another important factor for consideration is the wall-plug efficiency, which in our case is limited by the electrical-to-optical (E-O) efficiency of the 1550 nm pump diodes used in our experiments. The E-O efficiency of our pump diodes is ~10% (ignoring the TEC power consumption), which is slightly lower than that of 1480 nm pump diodes (~15%) commonly used for EDFAs. We believe that it will ultimately be possible to increase the efficiency of 1550 nm pump modules for TDFAs by exploiting the continuing technological advances in material science, chip design, and diode packaging and that the wall-plug efficiency of the TDFA will ultimately match that of the EDFA.
We presented the first in-band diode-pumped TDFAs operating in the 2 µm region and assessed their suitability as high performance amplifiers in potential future telecommunication networks. The TDFAs are analogous in implementation and function to current EDFAs, but offer a far more extended bandwidth in this new waveband of interest. By combining three different designs optimized for short, central and long wavelength operation, respectively, we demonstrated amplification over a 240 nm wide window in the range 1810 – 2050 nm with up to 36 dB gain, NF as low as 4.5 dB, and up to 100 mW saturated output power. Bandwidth and performance were limited by the insertion loss of the employed components and further bandwidth extension (particularly to short wavelengths) should be anticipated in due course.
Our results confirm the practicality of 2 µm optical fiber communications from an amplifier perspective, and represent a significant advancement in terms of compactness, robustness, controllability and power consumption of high performance TDFAs compared to earlier fiber-laser-pumped systems.
The authors acknowledge OFS Denmark for providing the thulium doped fiber and Eblana Photonics for providing the 2 µm laser diodes. We thank S. Butler and J. Cook for their help with the mechanics needed for these experiments. Z. Li thank M. Ibsen, S. Ganapathy and R Slavík for valuable discussions, P. Teh and E. Lim for their help with the experiment, as well as the China Scholarship Council for its financial support. A. M. Heidt acknowledges funding from the EU People Programme (Marie Curie Actions) under grant agreement 300859 (ADMIRATION). This work was supported by the EU 7th Framework Program under grant agreement 258033 (MODE-GAP) and by the UK EPSRC through grant EP/I01196X/1 (HYPERHIGHWAY).
References and links
1. P. Winzer, “Beyond 100G Ethernet,” IEEE Commun. Mag. 48(7), 26–30 (2010). [CrossRef]
2. A. D. Ellis, J. Zhao, and D. Cotter, “Approaching the non-linear Shannon limit,” J. Lightwave Technol. 28(4), 423–433 (2010). [CrossRef]
4. T. Morioka, Y. Awaji, R. Ryf, P. Winzer, D. Richardson, and F. Poletti, “Enhancing optical communications with brand new fibers,” IEEE Commun. Mag. 50(2), s31–s42 (2012). [CrossRef]
5. D. J. Richardson, J. M. Fini, and L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photonics 7(5), 354–362 (2013). [CrossRef]
6. S. Randel, R. Ryf, A. Sierra, P. J. Winzer, A. H. Gnauck, C. A. Bolle, R.-J. Essiambre, D. W. Peckham, A. McCurdy, and R. Lingle Jr., “6×56-Gb/s mode-division multiplexed transmission over 33-km few-mode fiber enabled by 6×6 MIMO equalization,” Opt. Express 19(17), 16697–16707 (2011). [CrossRef] [PubMed]
7. V. A. J. M. Sleiffer, Y. Jung, V. Veljanovski, R. G. H. van Uden, M. Kuschnerov, H. Chen, B. Inan, L. G. Nielsen, Y. Sun, D. J. Richardson, S. U. Alam, F. Poletti, J. K. Sahu, A. Dhar, A. M. Koonen, B. Corbett, R. Winfield, A. D. Ellis, and H. de Waardt, “73.7 Tb/s (96 x 3 x 256-Gb/s) mode-division-multiplexed DP-16QAM transmission with inline MM-EDFA,” Opt. Express 20(26), B428–B438 (2012). [CrossRef] [PubMed]
8. B. Zhu, T. F. Taunay, M. Fishteyn, X. Liu, S. Chandrasekhar, M. F. Yan, J. M. Fini, E. M. Monberg, and F. V. Dimarcello, “112-Tb/s Space-division multiplexed DWDM transmission with 14-b/s/Hz aggregate spectral efficiency over a 76.8-km seven-core fiber,” Opt. Express 19(17), 16665–16671 (2011). [CrossRef] [PubMed]
9. S. Matsuo, Y. Sasaki, T. Akamatsu, I. Ishida, K. Takenaga, K. Okuyama, K. Saitoh, and M. Kosihba, “12-core fiber with one ring structure for extremely large capacity transmission,” Opt. Express 20(27), 28398–28408 (2012). [CrossRef] [PubMed]
10. S. D. Jackson, “Towards high-power mid-infrared emission from a fibre laser,” Nat. Photonics 6(7), 423–431 (2012). [CrossRef]
11. P. Roberts, F. Couny, H. Sabert, B. Mangan, D. Williams, L. Farr, M. Mason, A. Tomlinson, T. Birks, J. Knight, and P. St J Russell, “Ultimate low loss of hollow-core photonic crystal fibres,” Opt. Express 13(1), 236–244 (2005). [CrossRef] [PubMed]
12. J. K. Lyngsø, B. J. Mangan, C. Jakobsen, and P. J. Roberts, “7-cell core hollow-core photonic crystal fibers with low loss in the spectral region around 2 microm,” Opt. Express 17(26), 23468–23473 (2009). [CrossRef] [PubMed]
13. F. Poletti, N. V. Wheeler, M. N. Petrovich, N. Baddela, E. Numkam Fokoua, J. R. Hayes, D. R. Gray, Z. Li, R. Slavík, and D. J. Richardson, “Towards high-capacity fibre-optic communications at the speed of light in vacuum,” Nat. Photonics 7(4), 279–284 (2013). [CrossRef]
14. M. N. Petrovich, F. Poletti, J. P. Wooler, A. M. Heidt, N. K. Baddela, Z. Li, D. R. Gray, R. Slavík, F. Parmigiani, N. V. Wheeler, J. R. Hayes, E. Numkam, L. Grüner-Nielsen, B. Pálsdóttir, R. Phelan, B. Kelly, M. Becker, N. MacSuibhne, J. Zhao, F. C. Garcia Gunning, A. D. Ellis, P. Petropoulos, S. U. Alam, and D. J. Richardson, “First demonstration of 2µm data transmission in a low-loss hollow core photonic bandgap fiber,” in ECOC (2012), paper Th.3.A.5.
15. Z. Li, A. M. Heidt, J. M. O. Daniel, Y. Jung, S. U. Alam, and D. J. Richardson, “Thulium-doped fiber amplifier for optical communications at 2 µm,” Opt. Express 21(8), 9289–9297 (2013). [CrossRef] [PubMed]
16. R. M. Percival, D. Szebesta, J. R. Williams, R. D. T. Lauder, A. C. Tropper, and D. C. Hanna, “Diode pumped operation of thulium doped fluoride fibre amplifier suitable for first window systems,” Electron. Lett. 30(19), 1598–1599 (1994). [CrossRef]
17. T. Kasamatsu, Y. Yano, and T. Ono, “Laser-diode-pumped highly efficient gain-shifted thulium-doped fiber amplifier operating in the 1480-1510-nm band,” IEEE Photon. Technol. Lett. 13(5), 433–435 (2001). [CrossRef]
18. T. Kasamatsu, Y. Yano, and T. Ono, “1.49 µm band gain-shifted thulium-doped fiber amplifier for WDM transmission systems,” J. Lightwave Technol. 20(10), 1826–1838 (2002). [CrossRef]