First demonstration of a 2μm few-mode TDFA for mode division multiplexing Y. Jung,1,* P. C. Shardlow,1 M. Belal,1 Z. Li,1 A. M. Heidt,1 J. M. O. Daniel,1 D. Jain,1 J. K. Sahu,1 W. A. Clarkson,1 B. Corbett,2 J. O’Callaghan,2 S. U. Alam,1 and D. J. Richardson1 1
Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK 2 Tyndall National Institute, University College Cork, Cork, Ireland *[email protected]
Abstract: We report the first demonstration of an inline few-mode thulium doped fiber amplifier (TDFA) operating at 2μm for mode division multiplexed transmission. Similar gain and noise figure performance for both the LP01 and LP11 modes are obtained in a cladding pumped 2-mode group TDFA. A maximum gain of 18.3dB was measured at 1970nm with a 3dB gain bandwidth of 75nm while the average noise figure was measured to be between 7 and 8dB for wavelengths longer than 1970nm. ©2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2320) Fiber optics amplifiers and oscillators.
References and links 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13.
14.
D. J. Richardson, J. M. Fini, and L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photonics 7(5), 354–362 (2013). T. Morioka, Y. Awaji, R. Ryf, P. J. Winzer, D. Richardson, and F. Poletti, “Enhancing optical communications with brand new fibers,” IEEE Commun. Mag. 50(2), s31–s42 (2012). I. P. Kaminow, T. Li, and A. E. Willner, Optical Fiber Telecommunications VIB Systems and Networks, Sixth Edition (Elsevier, 2013). R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. C. Burrows, R. Essiambre, P. J. Winzer, D. W. Peckham, A. H. McCurdy, and R. Lingle, “Mode-division multiplexing over 96 km of fewmode fiber using coherent 6 × 6 MIMO processing,” J. Lightwave Technol. 30(4), 521–531 (2012). V. A. J. M. Sleiffer, Y. Jung, V. Veljanovski, R. G. H. van Uden, M. Kuschnerov, H. Chen, B. Inan, L. GrünerNielsen, Y. Sun, D. J. Richardson, S. U. Alam, F. Poletti, J. K. Sahu, A. Dhar, A. M. J. Koonen, B. Corbett, R. Winfield, A. D. Ellis, and H. de Waardt, “73.7Tb/s (96x3x256-Gb/s) mode-division-multiplexed DP-16QAM transmission with inline MM-EDFA,” Opt. Express 20(26), B428–B438 (2012). 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). E. Ip, M. Li, Y. Huang, A. Tanaka, E. Mateo, W. Wood, J. Hu, Y. Yano, and K. Koreshkov, “146λx6x19-Gbaud wavelength and mode-division multiplexed transmission over 10x50km spans of few-mode fiber with a gainequalized few-mode EDFA,” in Optical Fiber Communication Conference 2013, paper PDP5A.2. S. Mumtaz, R.-J. Essiambre, and G. P. Agrawal, “Nonlinear propagation in multimode and multicore fibers: generalisation of the Manakov equations,” J. Lightwave Technol. 31(3), 398–406 (2013). 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 μm,” Opt. Express 17(26), 23468–23473 (2009). F. Poletti, N. V. Wheeler, M. N. Petrovich, N. K. Baddela, E. Numkam, 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). Z. Li, A. M. Heidt, N. Simakov, Y. Jung, J. M. O. Daniel, S. U. Alam, and D. J. Richardson, “Diode-pumped wideband thulium-doped fiber amplifiers for optical communications in the 1800 - 2050 nm window,” Opt. Express 21(22), 26450–26455 (2013). 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). 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. Grűner-Nielsen, B. Pálsdóttir, R. Phelan, B. Kelly, J. O’Carroll, M. Becker, N. MacSuibhne, J. Zhao, F. C. Garcia Gunning, A. D. Ellis, P. Petropoulos, S. U. Alam, and D. J. Richardson, “Demonstration of amplified data transmission at 2 µm in a low-loss wide bandwidth hollow core photonic bandgap fiber,” Opt. Express 21(23), 28559–28569 (2013). N. Mac Suibhne, Z. Li, B. Baeuerle, J. Zhao, J. Wooler, S. Alam, F. Poletti, M. Petrovich, A. Heidt, N. Wheeler, N. Baddela, E. R. Numkam Fokoua, I. Giles, D. Giles, R. Phelan, J. O'Carroll, B. Kelly, B. Corbett, D. Murphy,
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10544
15.
16. 17. 18.
A. D. Ellis, D. J. Richardson, and F. Garcia Gunning, “WDM transmission at 2μm over low-loss hollow core photonic bandgap fiber,” in Optical Fiber Communication Conference 2013, paper OW1I.6. V. Sleiffer, Y. Jung, N. Baddela, J. Surof, M. Kuschnerov, V. Veljanovski, J. Hayes, N. Wheeler, E. Fokoua, J. Wooler, D. Gray, N. Wong, F. Parmigiani, S. Alam, M. Petrovich, F. Poletti, D. J. Richardson, and H. de Waardt, “High capacity mode-division multiplexed optical transmission in a novel 37-cell hollow-core photonic bandgap fiber,” J. Lightwave Technol. 32(4), 854–863 (2014). D. M. Baney and J. Stimple, “WDM EDFA gain characterization with a reduced set of saturating channels,” IEEE Photon. Technol. Lett. 8(12), 1615–1617 (1996). Y. Jung, Q. Kang, V. A. J. M. Sleiffer, B. Inan, M. Kuschnerov, V. Veljanovski, B. Corbett, R. Winfield, Z. Li, P. S. Teh, A. Dhar, J. Sahu, F. Poletti, S.-U. Alam, and D. J. Richardson, “Three mode Er3+ ring-doped fiber amplifier for mode-division multiplexed transmission,” Opt. Express 21(8), 10383–10392 (2013). K. S. Abedin, T. F. Taunay, M. Fishteyn, D. J. DiGiovanni, V. R. Supradeepa, J. M. Fini, M. F. Yan, B. Zhu, E. M. Monberg, and F. V. Dimarcello, “Cladding-pumped erbium-doped multicore fiber amplifier,” Opt. Express 20(18), 20191–20200 (2012).
1. Introduction Space division multiplexing (SDM) [1–4] has attracted considerable attention amongst the high-capacity fiber-optic communications community as a radical approach to increase the data capacity by employing multiple distinguishable spatial information channels through the same fiber. Indeed space is the only remaining physical dimensions yet to be explored in the struggle to accommodate the ever increasing growth in demand for data capacity at an economically viable cost-per-bit: all other signaling dimensions (i.e. wavelength, polarization, phase and amplitude) are already close to optimally exploited in current single mode fiber (SMF) based transmission systems. There are two basic strategies for achieving multiple spatial channels within a single fiber: 1) multicore fiber whereby multiple cores are incorporated in a single fiber cladding, and 2) multimode fiber that utilizes multiple spatial modes in a large core fiber. Both of these approaches are being intensively investigated around the globe and a 10-fold increase in overall fiber capacity has already been achieved in little more than 2 years [5–7]. However the nonlinearity of these solid-core silica fibers [3, 8] still remains at a similar level to that of SMF and this will ultimately limit the maximum power that can be used. Moreover, the gain bandwidth of the erbium doped fiber amplifier (EDFA) will further constrain the overall maximum achievable per-fiber capacity. In recent years, there has been emerging research activity looking at overcoming the aforementioned limitations by moving to hollow core photonic bandgap transmission fiber (HC-PBGF) which offer the potential for ultralow nonlinearity (three orders of magnitude less), lower loss (~0.1dB/km theoretical limit) and lower latency (1.45 times faster) than conventional silica glass fibers [9, 10]. Furthermore, the anticipated low loss spectral window of HC-PBGFs is around 2μm, which overlaps with the gain bandwidth of the thulium doped fiber amplifier (TDFA) [11, 12], which offers the broadest gain bandwidth (extending potentially from 1700 to 2100nm) amongst all rare earth doped fiber amplifiers. To this end, in [13, 14], the first 2μm data transmission over a HC-PBGF was accomplished for both single and wavelength division multiplexed (WDM) data channels (4 channels) using an inline single-mode TDFA. This transmission experiment demonstrated the technical viability of using a new broadband optical communication window around 2μm. Moreover, low loss HCPBGFs are inherently multimode, raising the intriguing prospect of exploiting SDM to further enhance fiber capacity. Indeed the first experiments on three mode transmission at 1550nm in such fibers have now been reported [15]. To maximize the overall capacity of HC-PBGF then one needs to be able to perform SDM at 2μm resulting in a need for both passive and active multimode fiber devices operating at this wavelength. In this paper we present the first demonstration of a multimode (two mode-group, namely LP01 and LP11) TDFA for SDM transmission around 2μm. Employing a 2μm-phase plate based mode multiplexer/demultiplexer, amplifier gain was measured under constant saturation conditions by saturating the amplifier with one strong signal and measuring the gain experienced by a weaker probe. Our cladding pumped TM-TDFA exhibits similar gain/noise figure performance for both the LP01 and LP11 modes. The maximum gain was measured to be ~18.3dB at 1970nm with a 3dB gain bandwidth of 75nm and the average noise figure was measured to be around 7-8dB for wavelengths longer than 1970nm. There is considerable #206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10545
scope to improve both the gain and noise performance of the device as new and better amplifier components become available. 2. Experimental setup for the TM-TDFA MM-AMP BS
LD(2000nm) + SMF AMP+NBPF
TMF
f1
f1=4.5mm f2=125mm f3=8mm
MMUX
1
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2
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+ LD (2045nm)
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(λp=790nm)
f3
π
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Probe signal, λps
TLS
f1
f2
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SMF
Saturating signal, λss
DEMUX
π
π 0
OSA
Fig. 1. Schematic diagram of the TM-TDFA using backward cladding pumping at 790nm. BS: non-polarizing beam splitter, PP: phase plate, DM: dichroic mirror, SMF: single mode fiber, MMF: multimode fiber, TMF: passive 2-mode group fiber, TM-TDF: 2-mode group thulium doped fiber, AMP: amplifier (TDFA), NBPF: narrow bandpass filter.
Figure 1 shows a schematic diagram of our TM-TDFA and associated setup for gain characterization of the two lowest order spatial mode groups (namely LP01 and LP11). To emulate the amplifier performance under a wavelength division multiplexing (WDM) configuration, the gain of the amplifier was measured under constant saturation conditions by saturating the amplifier with one strong signal and measuring the gain with a much weaker probe [16]. To provide a saturating signal, the output from a discrete-mode (DM) laser diode (Eblana Photonics) operating at 2000nm was pre-amplified using an in-house built singlemode TDFA and spectrally filtered by a narrow optical bandpass filter (full-width halfmaximum, FWHM = 3nm) to filter out amplified spontaneous emission (ASE) noise. The resulting saturating signal input power to the amplifier was 0dBm. As a probe signal, an inhouse built, narrow-linewidth tunable laser source (TLS) operating over the range of 19302020nm was used. Additionally, a second DM laser diode at 2045nm was used to evaluate the TM-TDFA performance at the long wavelength edge of the amplifier. The optical power of the probe signal in the 1930-2045nm range was −15dBm. A phase-plate based mode multiplexer (MMUX) [4, 17] was used to selectively excite the pure LP01 and LP11 signal modes in a 10m long passive 2-moded fiber (TMF) via a telecentric lens arrangement. The passive TMF (a graded index fiber with 20μm core diameter and an NA of 0.19) was then spliced directly to a 1.1m-long double-clad TM-TDF with a circular core to guide the signal and a D-shaped silica inner cladding with a low-index polymer outer cladding to guide the heavily multimode 790nm pump beam. The refractive index profile and fiber geometry are shown in Fig. 2(a). The D-shaped inner cladding promotes pump mode mixing for efficient pump absorption. The in-house fabricated active TM-TDF has a high thulium concentration of approximately 3.2 ± 0.3% by weight. The core diameter of the fiber is 12.6μm with a numerical aperture (NA) of 0.22 and the inner cladding diameter is 125μm with an NA of 0.45. The large mode field diameter mismatch between active and passive fibers results in a ~3dB splice loss for both LP01 and LP11 modes. The measured cladding pump absorption was found to be ~12.1dB/m at 790nm. The 790nm multimode semiconductor pump laser had a 3dB spectral bandwidth of 3.9nm and generated up to 3.2W of pump power (emitted from a 105/125μm multimode pigtail fiber). The pump light was free-space coupled into the inner cladding of the TM-TDF in a backward pumping configuration [18] (i.e. the pump counter propagates with respect to the signal light) and the pump coupling efficiency into the inner
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10546
cladding was estimated to be ~80% (estimated through transmission measurements on a short length of gain fiber < 5cm). To suppress any Fresnel reflections of the signal at the fiber facets into the core, coreless end caps (diameter = 200μm, length~500μm) were spliced at both ends of the spliced active/passive fiber assembly. The amplified output was separated from the incoming pump light by a dichroic mirror (high reflection at 2000nm and high transmission at 790nm) and de-multiplexed by a simplified phase plate based de-multiplexer (DEMUX). By placing the amplified signal beam at different spatial positions on the binary LP21 phase plate, e.g. uniform sector for LP01 excitation (region 1 in Fig. 1), vertical halfsector for LP11a (region 2) and horizontal half-sector for the LP11b (region 3), the desired modes can be converted into LP01 and coupled efficiently into a single mode fiber. Note that the two LP11 modes experience continuous mode-mixing along the length of the fiber and the spatial orientation of the lobes can also be altered by external perturbations of the fiber. Therefore in order to minimize the uncertainty, the gain of the LP11 mode should be measured in two directions, perpendicular to each other, and the average of the two measurements then used to estimate the LP11 modal gain. (b) from passive TMF (10m)
(a) 0.020
RIP LP01 LP11
Δn
0.015
from TMF+active TDF (15cm)
0.010 0.005
After amplification
0.000 -10
0
10
20
Radius [μm]
Fig. 2. (a) Refractive index and mode profiles of the (in-house fabricated) TM-TDF, and (b) signal mode profiles after passive TMF (top row), across the splice (middle row) and after amplification (bottom row).
To confirm clean amplification of the input signals, mode images of the various input and output signals from the amplifier were taken by a CCD for a signal wavelength of 2045nm (which is located at the edge of the thulium absorption band). The core absorption was less than 2dB/m at 2045nm allowing us to measure clean spatial mode images of the input signals after propagating through 15cm of TM-TDF. As shown in Fig. 2(b), the two spatial modes are well defined after propagating through 10m of passive TMF (top row) and can be preserved across the splice point (middle row). However due to the large splice loss between the passive and active fibers, a certain amount of signal light is coupled out of the core and is guided by the inner cladding of the double clad TM-TDF, which complicated making accurate measurements. To reduce measurement error a 4cm-long section of the low-index polymer coating was removed from the TM-TDF and a high refractive index liquid was applied over the stripped section in order to remove any signal light coupled into the cladding. The beam quality of the individual input signals was largely preserved during amplification although some low level degradation is noticeable by careful inspection of the CCD images in the top and bottom rows of Fig. 2(b). We attribute this to the presence of unpolarized amplified spontaneous emission (ASE) light and modal crosstalk originating at the mode-mismatched splice points. This could be further minimized by optimizing the mode field diameters between the passive and active fibers used. Nevertheless the mode quality is sufficient for our needs.
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10547
3. Gain and noise figure performance of the TM-TDFA (a)
(b)
20 LP01 LP11
75nm
5
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2050
Wavelength [nm]
1940
10
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1950
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λss
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10
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15
20
1960
1980
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Wavelength [nm]
2020
2040
0 1920
1940
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1980
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Wavelength [nm]
Fig. 3. Measured modal gain (a) and noise figure (b) as a function of wavelength at a fixed input probe signal power of −15dBm and pump power of 3.2W. Inset: output spectra of the fiber amplifier.
As depicted in the inset of Fig. 3(a), the total output power of the amplifier is determined by the saturating signal (λss) at 2000nm and the small signal gain of the TM-TDFA can be measured by sweeping the wavelength of the probe signal (λps). Firstly, we assigned the saturating signal to the LP01 spatial mode while the probe signal was assigned to either the LP01 or LP11 mode. Secondly the saturating signal was assigned to the LP11 mode while the probe signal was assigned to either the LP01 or LP11 mode. In both cases similar performance in terms of gain and noise figure were observed. Figure 3 shows the amplifier performance for the first case (i.e. λss = LP01 and λps = LP01 or LP11). A total output signal power of 15.6dBm was achieved for a cladding pump power of 3.2W which is limited by our pump diode. The saturating and small probe input signal powers in the gain measurement were 0dBm and −15dBm, respectively. The maximum signal gain was found to be about 18.3dB for the LP11 and 17dB for the LP01 modes at 1970nm for the maximum available pump power of 3.2W. The 3dB gain bandwidth was measured to be ~75nm for both modes. Note that nearly equal modal gain was achieved using the cladding pumping configuration with slightly higher gain for the high-order mode which is beneficial as the high-order modes generally experience higher losses in the transmission medium [7]. The achievable maximum gain is limited by the available pump power. Compared with the core-pumped TDFA in [11, 12], our cladding pumped TDFA operates at substantially longer wavelengths. The red-shift of the gain peak is mainly due to the increased Tm3+ re-absorption in the highly-doped double clad fiber. With cladding pumping, only a small average excitation density (relatively low fraction of excited ions) can be achieved due to the reduced pump brightness which in essence promotes longer wavelength gain at the expense of gain at shorter wavelengths. Figure 3(b) shows that the NF rises sharply for wavelengths shorter than 1960nm while it tends to decrease at longer wavelengths. The high NF at short wavelengths is mainly due to insufficient population inversion (and hence effective signal loss) at the amplifier input which is clearly evident from the sharp drop in signal gain. The NF could be improved either by increasing the pump power or decreasing the core/inner cladding diameter ratio so as to increase the pump intensity and hence the average population inversion. The average NF was about 7-8dB for wavelengths longer than 1970nm for both modes. The NF values are relatively high compared to those of typical core pumped EDFAs but they are still reasonable when compared to cladding pumped EDFAs [18]. Moreover, our current lack of availability of a suitable optical isolator at 2μm likely also contributes to the degradation of NF, since any undesirable signal reflection from the cleaved input and output ends leads to ASE build-up. The relatively low pump power and backward cladding pumping implementation also contribute to the increase in overall amplifier NF. We envisage that an improvement in noise figure will be possible with the
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10548
availability of appropriate 2μm optical isolators, increased pump power and more optimal amplifier configurations. (a)
(b)
20
15
Gain [dB]
Gain [dB]
15
20
10
5
10
5 LP01 LP11
0
-30
-25
-20
-15
Input probe signal power [dBm]
-10
LP01 LP11
0
1
2
3
Pump power [W]
Fig. 4. Measured modal gain as a function of (a) input probe signal power at a fixed pump power (3.2W) and (b) cladding pump power at a fixed probe signal power (−15dBm).
Figure 4(a) plots the mode dependent signal gain as a function of probe signal power for a fixed saturating signal power (0dBm) and pump power (3.2W). Both spatial modes experienced gain reduction with an increase in input signal power due to the amplifier saturation but the slope was relatively small because of the degree of amplifier saturation induced by the saturating signal. Figure 4(b) shows the mode dependent gain as a function of cladding pump power. The gain of the TDFA increases monotonically as the pump power is increased to 3.2W with a maximum achievable gain of about 17-18dB. As can be seen from Fig. 4(b), there is no sign of signal gain saturation and the maximum signal gain is limited only by the available pump power. 4. Conclusion A two-mode group thulium-doped fiber amplifier has been successfully demonstrated for the first time and exhibits relatively good performance. The amplifier should allow for mode division multiplexed data transmission in a potential new transmission window around 2μm. Similar gain and noise figure performance were obtained for both spatial modes (LP01 and LP11) under cladding pumped operation. A maximum signal gain of about 18.3dB was measured at 1970nm for the LP11 mode with a 3dB gain bandwidth of 75nm while the average noise figure was measured to be between 7 and 8dB for wavelengths longer than 1970nm. The amplifier performance can be improved by increasing the pump power, improving the pumping configuration, reducing the core to inner cladding area ratio and by improvements in a variety of passive 2μm optical components such as optical isolators and WDM couplers. We consider this to be an important step in extending SDM transmission to the new 2μm transmission window providing new options for fiber capacity scaling. Acknowledgments The authors would like to thank Eblana Photonics for providing the 2μm discrete-mode laser diodes. This work was supported in part by the European Communities 7th Framework Programme under grant agreement 258033 (MODE-GAP) and 287732 (ISLA).
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10549
Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK 2 Tyndall National Institute, University College Cork, Cork, Ireland *[email protected]
Abstract: We report the first demonstration of an inline few-mode thulium doped fiber amplifier (TDFA) operating at 2μm for mode division multiplexed transmission. Similar gain and noise figure performance for both the LP01 and LP11 modes are obtained in a cladding pumped 2-mode group TDFA. A maximum gain of 18.3dB was measured at 1970nm with a 3dB gain bandwidth of 75nm while the average noise figure was measured to be between 7 and 8dB for wavelengths longer than 1970nm. ©2014 Optical Society of America OCIS codes: (060.0060) Fiber optics and optical communications; (060.2320) Fiber optics amplifiers and oscillators.
References and links 1. 2. 3. 4. 5.
6. 7. 8. 9. 10. 11. 12. 13.
14.
D. J. Richardson, J. M. Fini, and L. E. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photonics 7(5), 354–362 (2013). T. Morioka, Y. Awaji, R. Ryf, P. J. Winzer, D. Richardson, and F. Poletti, “Enhancing optical communications with brand new fibers,” IEEE Commun. Mag. 50(2), s31–s42 (2012). I. P. Kaminow, T. Li, and A. E. Willner, Optical Fiber Telecommunications VIB Systems and Networks, Sixth Edition (Elsevier, 2013). R. Ryf, S. Randel, A. H. Gnauck, C. Bolle, A. Sierra, S. Mumtaz, M. Esmaeelpour, E. C. Burrows, R. Essiambre, P. J. Winzer, D. W. Peckham, A. H. McCurdy, and R. Lingle, “Mode-division multiplexing over 96 km of fewmode fiber using coherent 6 × 6 MIMO processing,” J. Lightwave Technol. 30(4), 521–531 (2012). V. A. J. M. Sleiffer, Y. Jung, V. Veljanovski, R. G. H. van Uden, M. Kuschnerov, H. Chen, B. Inan, L. GrünerNielsen, Y. Sun, D. J. Richardson, S. U. Alam, F. Poletti, J. K. Sahu, A. Dhar, A. M. J. Koonen, B. Corbett, R. Winfield, A. D. Ellis, and H. de Waardt, “73.7Tb/s (96x3x256-Gb/s) mode-division-multiplexed DP-16QAM transmission with inline MM-EDFA,” Opt. Express 20(26), B428–B438 (2012). 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). E. Ip, M. Li, Y. Huang, A. Tanaka, E. Mateo, W. Wood, J. Hu, Y. Yano, and K. Koreshkov, “146λx6x19-Gbaud wavelength and mode-division multiplexed transmission over 10x50km spans of few-mode fiber with a gainequalized few-mode EDFA,” in Optical Fiber Communication Conference 2013, paper PDP5A.2. S. Mumtaz, R.-J. Essiambre, and G. P. Agrawal, “Nonlinear propagation in multimode and multicore fibers: generalisation of the Manakov equations,” J. Lightwave Technol. 31(3), 398–406 (2013). 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 μm,” Opt. Express 17(26), 23468–23473 (2009). F. Poletti, N. V. Wheeler, M. N. Petrovich, N. K. Baddela, E. Numkam, 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). Z. Li, A. M. Heidt, N. Simakov, Y. Jung, J. M. O. Daniel, S. U. Alam, and D. J. Richardson, “Diode-pumped wideband thulium-doped fiber amplifiers for optical communications in the 1800 - 2050 nm window,” Opt. Express 21(22), 26450–26455 (2013). 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). 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. Grűner-Nielsen, B. Pálsdóttir, R. Phelan, B. Kelly, J. O’Carroll, M. Becker, N. MacSuibhne, J. Zhao, F. C. Garcia Gunning, A. D. Ellis, P. Petropoulos, S. U. Alam, and D. J. Richardson, “Demonstration of amplified data transmission at 2 µm in a low-loss wide bandwidth hollow core photonic bandgap fiber,” Opt. Express 21(23), 28559–28569 (2013). N. Mac Suibhne, Z. Li, B. Baeuerle, J. Zhao, J. Wooler, S. Alam, F. Poletti, M. Petrovich, A. Heidt, N. Wheeler, N. Baddela, E. R. Numkam Fokoua, I. Giles, D. Giles, R. Phelan, J. O'Carroll, B. Kelly, B. Corbett, D. Murphy,
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10544
15.
16. 17. 18.
A. D. Ellis, D. J. Richardson, and F. Garcia Gunning, “WDM transmission at 2μm over low-loss hollow core photonic bandgap fiber,” in Optical Fiber Communication Conference 2013, paper OW1I.6. V. Sleiffer, Y. Jung, N. Baddela, J. Surof, M. Kuschnerov, V. Veljanovski, J. Hayes, N. Wheeler, E. Fokoua, J. Wooler, D. Gray, N. Wong, F. Parmigiani, S. Alam, M. Petrovich, F. Poletti, D. J. Richardson, and H. de Waardt, “High capacity mode-division multiplexed optical transmission in a novel 37-cell hollow-core photonic bandgap fiber,” J. Lightwave Technol. 32(4), 854–863 (2014). D. M. Baney and J. Stimple, “WDM EDFA gain characterization with a reduced set of saturating channels,” IEEE Photon. Technol. Lett. 8(12), 1615–1617 (1996). Y. Jung, Q. Kang, V. A. J. M. Sleiffer, B. Inan, M. Kuschnerov, V. Veljanovski, B. Corbett, R. Winfield, Z. Li, P. S. Teh, A. Dhar, J. Sahu, F. Poletti, S.-U. Alam, and D. J. Richardson, “Three mode Er3+ ring-doped fiber amplifier for mode-division multiplexed transmission,” Opt. Express 21(8), 10383–10392 (2013). K. S. Abedin, T. F. Taunay, M. Fishteyn, D. J. DiGiovanni, V. R. Supradeepa, J. M. Fini, M. F. Yan, B. Zhu, E. M. Monberg, and F. V. Dimarcello, “Cladding-pumped erbium-doped multicore fiber amplifier,” Opt. Express 20(18), 20191–20200 (2012).
1. Introduction Space division multiplexing (SDM) [1–4] has attracted considerable attention amongst the high-capacity fiber-optic communications community as a radical approach to increase the data capacity by employing multiple distinguishable spatial information channels through the same fiber. Indeed space is the only remaining physical dimensions yet to be explored in the struggle to accommodate the ever increasing growth in demand for data capacity at an economically viable cost-per-bit: all other signaling dimensions (i.e. wavelength, polarization, phase and amplitude) are already close to optimally exploited in current single mode fiber (SMF) based transmission systems. There are two basic strategies for achieving multiple spatial channels within a single fiber: 1) multicore fiber whereby multiple cores are incorporated in a single fiber cladding, and 2) multimode fiber that utilizes multiple spatial modes in a large core fiber. Both of these approaches are being intensively investigated around the globe and a 10-fold increase in overall fiber capacity has already been achieved in little more than 2 years [5–7]. However the nonlinearity of these solid-core silica fibers [3, 8] still remains at a similar level to that of SMF and this will ultimately limit the maximum power that can be used. Moreover, the gain bandwidth of the erbium doped fiber amplifier (EDFA) will further constrain the overall maximum achievable per-fiber capacity. In recent years, there has been emerging research activity looking at overcoming the aforementioned limitations by moving to hollow core photonic bandgap transmission fiber (HC-PBGF) which offer the potential for ultralow nonlinearity (three orders of magnitude less), lower loss (~0.1dB/km theoretical limit) and lower latency (1.45 times faster) than conventional silica glass fibers [9, 10]. Furthermore, the anticipated low loss spectral window of HC-PBGFs is around 2μm, which overlaps with the gain bandwidth of the thulium doped fiber amplifier (TDFA) [11, 12], which offers the broadest gain bandwidth (extending potentially from 1700 to 2100nm) amongst all rare earth doped fiber amplifiers. To this end, in [13, 14], the first 2μm data transmission over a HC-PBGF was accomplished for both single and wavelength division multiplexed (WDM) data channels (4 channels) using an inline single-mode TDFA. This transmission experiment demonstrated the technical viability of using a new broadband optical communication window around 2μm. Moreover, low loss HCPBGFs are inherently multimode, raising the intriguing prospect of exploiting SDM to further enhance fiber capacity. Indeed the first experiments on three mode transmission at 1550nm in such fibers have now been reported [15]. To maximize the overall capacity of HC-PBGF then one needs to be able to perform SDM at 2μm resulting in a need for both passive and active multimode fiber devices operating at this wavelength. In this paper we present the first demonstration of a multimode (two mode-group, namely LP01 and LP11) TDFA for SDM transmission around 2μm. Employing a 2μm-phase plate based mode multiplexer/demultiplexer, amplifier gain was measured under constant saturation conditions by saturating the amplifier with one strong signal and measuring the gain experienced by a weaker probe. Our cladding pumped TM-TDFA exhibits similar gain/noise figure performance for both the LP01 and LP11 modes. The maximum gain was measured to be ~18.3dB at 1970nm with a 3dB gain bandwidth of 75nm and the average noise figure was measured to be around 7-8dB for wavelengths longer than 1970nm. There is considerable #206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10545
scope to improve both the gain and noise performance of the device as new and better amplifier components become available. 2. Experimental setup for the TM-TDFA MM-AMP BS
LD(2000nm) + SMF AMP+NBPF
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Fig. 1. Schematic diagram of the TM-TDFA using backward cladding pumping at 790nm. BS: non-polarizing beam splitter, PP: phase plate, DM: dichroic mirror, SMF: single mode fiber, MMF: multimode fiber, TMF: passive 2-mode group fiber, TM-TDF: 2-mode group thulium doped fiber, AMP: amplifier (TDFA), NBPF: narrow bandpass filter.
Figure 1 shows a schematic diagram of our TM-TDFA and associated setup for gain characterization of the two lowest order spatial mode groups (namely LP01 and LP11). To emulate the amplifier performance under a wavelength division multiplexing (WDM) configuration, the gain of the amplifier was measured under constant saturation conditions by saturating the amplifier with one strong signal and measuring the gain with a much weaker probe [16]. To provide a saturating signal, the output from a discrete-mode (DM) laser diode (Eblana Photonics) operating at 2000nm was pre-amplified using an in-house built singlemode TDFA and spectrally filtered by a narrow optical bandpass filter (full-width halfmaximum, FWHM = 3nm) to filter out amplified spontaneous emission (ASE) noise. The resulting saturating signal input power to the amplifier was 0dBm. As a probe signal, an inhouse built, narrow-linewidth tunable laser source (TLS) operating over the range of 19302020nm was used. Additionally, a second DM laser diode at 2045nm was used to evaluate the TM-TDFA performance at the long wavelength edge of the amplifier. The optical power of the probe signal in the 1930-2045nm range was −15dBm. A phase-plate based mode multiplexer (MMUX) [4, 17] was used to selectively excite the pure LP01 and LP11 signal modes in a 10m long passive 2-moded fiber (TMF) via a telecentric lens arrangement. The passive TMF (a graded index fiber with 20μm core diameter and an NA of 0.19) was then spliced directly to a 1.1m-long double-clad TM-TDF with a circular core to guide the signal and a D-shaped silica inner cladding with a low-index polymer outer cladding to guide the heavily multimode 790nm pump beam. The refractive index profile and fiber geometry are shown in Fig. 2(a). The D-shaped inner cladding promotes pump mode mixing for efficient pump absorption. The in-house fabricated active TM-TDF has a high thulium concentration of approximately 3.2 ± 0.3% by weight. The core diameter of the fiber is 12.6μm with a numerical aperture (NA) of 0.22 and the inner cladding diameter is 125μm with an NA of 0.45. The large mode field diameter mismatch between active and passive fibers results in a ~3dB splice loss for both LP01 and LP11 modes. The measured cladding pump absorption was found to be ~12.1dB/m at 790nm. The 790nm multimode semiconductor pump laser had a 3dB spectral bandwidth of 3.9nm and generated up to 3.2W of pump power (emitted from a 105/125μm multimode pigtail fiber). The pump light was free-space coupled into the inner cladding of the TM-TDF in a backward pumping configuration [18] (i.e. the pump counter propagates with respect to the signal light) and the pump coupling efficiency into the inner
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10546
cladding was estimated to be ~80% (estimated through transmission measurements on a short length of gain fiber < 5cm). To suppress any Fresnel reflections of the signal at the fiber facets into the core, coreless end caps (diameter = 200μm, length~500μm) were spliced at both ends of the spliced active/passive fiber assembly. The amplified output was separated from the incoming pump light by a dichroic mirror (high reflection at 2000nm and high transmission at 790nm) and de-multiplexed by a simplified phase plate based de-multiplexer (DEMUX). By placing the amplified signal beam at different spatial positions on the binary LP21 phase plate, e.g. uniform sector for LP01 excitation (region 1 in Fig. 1), vertical halfsector for LP11a (region 2) and horizontal half-sector for the LP11b (region 3), the desired modes can be converted into LP01 and coupled efficiently into a single mode fiber. Note that the two LP11 modes experience continuous mode-mixing along the length of the fiber and the spatial orientation of the lobes can also be altered by external perturbations of the fiber. Therefore in order to minimize the uncertainty, the gain of the LP11 mode should be measured in two directions, perpendicular to each other, and the average of the two measurements then used to estimate the LP11 modal gain. (b) from passive TMF (10m)
(a) 0.020
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Fig. 2. (a) Refractive index and mode profiles of the (in-house fabricated) TM-TDF, and (b) signal mode profiles after passive TMF (top row), across the splice (middle row) and after amplification (bottom row).
To confirm clean amplification of the input signals, mode images of the various input and output signals from the amplifier were taken by a CCD for a signal wavelength of 2045nm (which is located at the edge of the thulium absorption band). The core absorption was less than 2dB/m at 2045nm allowing us to measure clean spatial mode images of the input signals after propagating through 15cm of TM-TDF. As shown in Fig. 2(b), the two spatial modes are well defined after propagating through 10m of passive TMF (top row) and can be preserved across the splice point (middle row). However due to the large splice loss between the passive and active fibers, a certain amount of signal light is coupled out of the core and is guided by the inner cladding of the double clad TM-TDF, which complicated making accurate measurements. To reduce measurement error a 4cm-long section of the low-index polymer coating was removed from the TM-TDF and a high refractive index liquid was applied over the stripped section in order to remove any signal light coupled into the cladding. The beam quality of the individual input signals was largely preserved during amplification although some low level degradation is noticeable by careful inspection of the CCD images in the top and bottom rows of Fig. 2(b). We attribute this to the presence of unpolarized amplified spontaneous emission (ASE) light and modal crosstalk originating at the mode-mismatched splice points. This could be further minimized by optimizing the mode field diameters between the passive and active fibers used. Nevertheless the mode quality is sufficient for our needs.
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10547
3. Gain and noise figure performance of the TM-TDFA (a)
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Fig. 3. Measured modal gain (a) and noise figure (b) as a function of wavelength at a fixed input probe signal power of −15dBm and pump power of 3.2W. Inset: output spectra of the fiber amplifier.
As depicted in the inset of Fig. 3(a), the total output power of the amplifier is determined by the saturating signal (λss) at 2000nm and the small signal gain of the TM-TDFA can be measured by sweeping the wavelength of the probe signal (λps). Firstly, we assigned the saturating signal to the LP01 spatial mode while the probe signal was assigned to either the LP01 or LP11 mode. Secondly the saturating signal was assigned to the LP11 mode while the probe signal was assigned to either the LP01 or LP11 mode. In both cases similar performance in terms of gain and noise figure were observed. Figure 3 shows the amplifier performance for the first case (i.e. λss = LP01 and λps = LP01 or LP11). A total output signal power of 15.6dBm was achieved for a cladding pump power of 3.2W which is limited by our pump diode. The saturating and small probe input signal powers in the gain measurement were 0dBm and −15dBm, respectively. The maximum signal gain was found to be about 18.3dB for the LP11 and 17dB for the LP01 modes at 1970nm for the maximum available pump power of 3.2W. The 3dB gain bandwidth was measured to be ~75nm for both modes. Note that nearly equal modal gain was achieved using the cladding pumping configuration with slightly higher gain for the high-order mode which is beneficial as the high-order modes generally experience higher losses in the transmission medium [7]. The achievable maximum gain is limited by the available pump power. Compared with the core-pumped TDFA in [11, 12], our cladding pumped TDFA operates at substantially longer wavelengths. The red-shift of the gain peak is mainly due to the increased Tm3+ re-absorption in the highly-doped double clad fiber. With cladding pumping, only a small average excitation density (relatively low fraction of excited ions) can be achieved due to the reduced pump brightness which in essence promotes longer wavelength gain at the expense of gain at shorter wavelengths. Figure 3(b) shows that the NF rises sharply for wavelengths shorter than 1960nm while it tends to decrease at longer wavelengths. The high NF at short wavelengths is mainly due to insufficient population inversion (and hence effective signal loss) at the amplifier input which is clearly evident from the sharp drop in signal gain. The NF could be improved either by increasing the pump power or decreasing the core/inner cladding diameter ratio so as to increase the pump intensity and hence the average population inversion. The average NF was about 7-8dB for wavelengths longer than 1970nm for both modes. The NF values are relatively high compared to those of typical core pumped EDFAs but they are still reasonable when compared to cladding pumped EDFAs [18]. Moreover, our current lack of availability of a suitable optical isolator at 2μm likely also contributes to the degradation of NF, since any undesirable signal reflection from the cleaved input and output ends leads to ASE build-up. The relatively low pump power and backward cladding pumping implementation also contribute to the increase in overall amplifier NF. We envisage that an improvement in noise figure will be possible with the
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10548
availability of appropriate 2μm optical isolators, increased pump power and more optimal amplifier configurations. (a)
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Fig. 4. Measured modal gain as a function of (a) input probe signal power at a fixed pump power (3.2W) and (b) cladding pump power at a fixed probe signal power (−15dBm).
Figure 4(a) plots the mode dependent signal gain as a function of probe signal power for a fixed saturating signal power (0dBm) and pump power (3.2W). Both spatial modes experienced gain reduction with an increase in input signal power due to the amplifier saturation but the slope was relatively small because of the degree of amplifier saturation induced by the saturating signal. Figure 4(b) shows the mode dependent gain as a function of cladding pump power. The gain of the TDFA increases monotonically as the pump power is increased to 3.2W with a maximum achievable gain of about 17-18dB. As can be seen from Fig. 4(b), there is no sign of signal gain saturation and the maximum signal gain is limited only by the available pump power. 4. Conclusion A two-mode group thulium-doped fiber amplifier has been successfully demonstrated for the first time and exhibits relatively good performance. The amplifier should allow for mode division multiplexed data transmission in a potential new transmission window around 2μm. Similar gain and noise figure performance were obtained for both spatial modes (LP01 and LP11) under cladding pumped operation. A maximum signal gain of about 18.3dB was measured at 1970nm for the LP11 mode with a 3dB gain bandwidth of 75nm while the average noise figure was measured to be between 7 and 8dB for wavelengths longer than 1970nm. The amplifier performance can be improved by increasing the pump power, improving the pumping configuration, reducing the core to inner cladding area ratio and by improvements in a variety of passive 2μm optical components such as optical isolators and WDM couplers. We consider this to be an important step in extending SDM transmission to the new 2μm transmission window providing new options for fiber capacity scaling. Acknowledgments The authors would like to thank Eblana Photonics for providing the 2μm discrete-mode laser diodes. This work was supported in part by the European Communities 7th Framework Programme under grant agreement 258033 (MODE-GAP) and 287732 (ISLA).
#206334 - $15.00 USD (C) 2014 OSA
Received 12 Feb 2014; revised 11 Apr 2014; accepted 17 Apr 2014; published 24 Apr 2014 5 May 2014 | Vol. 22, No. 9 | DOI:10.1364/OE.22.010544 | OPTICS EXPRESS 10549
