OW1I.2.pdf OFC/NFOEC Technical Digest © 2013 OSA Combined SDM and WDM transmission over 700-km Few-Mode Fiber R. Ryf(1) , M. A. Mestre(1) , S. Randel(1) , X. Palou(1) , A. H. Gnauck(1) , R. Delbue(2) , P. Pupalaikis(2) , A. Sureka(2) , Y. Sun(3) , X. Jiang(3) , and R. Lingle, Jr.(3) 1 Bell Laboratories, Alcatel-Lucent, 791 Holmdel-Keyport Rd, Holmdel, NJ, 07733, USA. Corporation, 700 Chestnut Ridge Road, Chestnut Ridge, NY 10977, USA 3 OFS, 2000 Northeast Expressway, Norcross, GA 30071, USA. Roland.Ryf@alcatel-lucent.com 2 LeCroy Abstract: We experimentally demonstrate multiple-input multiple-output transmission over a 700-km few-mode fiber of a combined 3-space-, 2-polarization-, and 34-wavelength-division multiplex, using low-loss 3-spot mode couplers. OCIS codes: 060.4510, 060.1660, 060.2280, 040.1880, 060.4230. 1. Introduction Space-division multiplexing (SDM) over a single fiber is currently under investigation for its potential to overcome the capacity limit of single-mode fibers (SMFs). Few-mode fibers (FMFs) [1–4] have recently been proposed to overcome the SMF shortcomings. In this work we experimentally demonstrate SDM transmission of 34 WDM channels spaced at 50 GHz over 700-km FMF based on a 70-km span. To our knowledge this is the longest combined WDM/SDM transmission distance reported yet in FMFs, more than an order of magnitude longer than the result reported in . The record distance is achieved by using low-loss spot-based mode multiplexers (SMUX)  in combination with a DGD compensated span. In our experiment, 20-Gbaud/s QPSK signals are multiplexed over 3 spatial modes, 2 polarizations, and 34 wavelengths, resulting in an aggregate capacity of 6.5 Tbit/s over a bandwidth of 1.7 THz. The signals are recovered after 700 km transmission by off-line multiple-input multiple-output (MIMO) digital signal processing (DSP) and we obtain a BER < 10−2 for all SDM and WDM channels. Further, the impulse response and the mode dependent loss (MDL) of the 6 × 6 MIMO channel is analysed and compared to previous single channel results  obtained with phase-plate-based mode multiplexer (PPB-MMUX), showing that the system wide MDL is < 2.1 dB even after 700-km transmission. 2. Three-spot based mode coupler The spot based mode-coupler was first demonstrated for a FMF supporting 6 spatial and polarization modes (FMF6) in . The coupler consists in an equilateral triangular spot arrangement centered around the FMF core. This simple spot arrangement can be used to sample the light coupled out from the FMF, and each spot will contain linear combination of the fields (phase and amplitude) and polarization of the fiber modes. If the distance between the spots is chosen correctly, it can be shown , that the linear transformation between the fields and polarizations in the spots and the fiber modes is unitary, up to a common loss term. Unitary transformation can be undone completely by using MIMO DSP and will therefore not cause any penalty for MIMO transmission. By choosing optimal spot diameter, also the insertion loss of the coupler can be reduced and minimal insertion loss < 2dB is theoretically possible. Even smaller insertion losses can be achieved if the spots are brought together by tapered fiber bundles so called “photonic lanterns” [5,6]. In this experiment the SMUXs were implemented using 3 collimators, which are brought close together by using three clipping mirrors (M1, M2, and M3), and the resulting spot pattern is then demagnified and imaged on the FMF6 facet using a lens pair ( f1 = 100 mm and f2 = 3 mm) in double telecentric configuration (see Fig. 1 b). Using individual collimators offers the advantage that spot coupler can be optimized to produce low loss and have a transfer function that is very close to a unitary function. We used a phase-plate based coupler on the second end of the FMF to analyze the modal content of the light coupled by each spot. For a correctly aligned coupler for FMF6, all spots will have the same coupling efficiency and will all couple the same amount of power in all fiber modes (LP01 , LP11a , and LP11b ). The resulting experimental insertion loss of the spot couplers was 4.1 dB and 3.9 dB for the multiplexing SMUX and the demultiplexing SMUX, respectively. 3. DGD compensated FMF span The FMF span with a total length of 70 km was realized by using a graded-index (GI)-FMF that supports exactly 6 spatial and polarization modes. The fiber characteristics are the following: The effective area was 64 µm2 and 67 µm2 for LP01 and LP11 modes, respectively, the absorption was 0.226 dB/km and 0.32 dB/km at a wavelength of 1.55 µm and 1.45 µm, respectively, and the chromatic dispersion is 18.5 ps/(nm km) at 1550 nm. The span was built by splicing 978-1-55752-962-6/13/$31.00 ©2013 Optical Society of America 6 × 6 coherent MIMO experiment OW1I.2.pdf 97 ns … DFB ECL DFB DN-MZM Q I … 2ch – PPG 20 GBaud Q I DN-MZM DFB 49 ns 25 ns O Interleaver E Delay generator PD-CRX 0 ECL LO SMUX LN-SW 1 BPF PD-CRX 2 LeCroy 12 ch, 20 GHz, 40 GS/s modular oscilloscope BPF 0 SMUX 1 1 2 2 3x Loop loading signal BPF PD-CRX 1 70 km FMF 0 LN-SW 2 PBS Pattern sync a) LN-SW 0 Blocker 3 x 10:90 DFB OFC/NFOEC Technical Digest © 2013 OSA b) SMUX SMF M2 port 1 SMF port 2 SMF Periscope port 0 M4 M3 M1 FMF Side view f2 f1 Lenses Fig. 1. a) Experimental setup for coherent MIMO transmission. PBS: Polarization beam splitter. BPF: Band-pass filter. Erbium-doped fiber amplifiers are denoted by triangles. b) Experimental setup for a 3-spot mode multiplexer. fiber spools with the following lengths (DGDs): 12.5 km (-2 ns), 25 km (1.64 ns), 12.5 km (-1.17 ns), and 20 km (1.52 ns). The order of the spools was chosen to continuously compensate most of the DGD after each spool pair. The maximal excursion in DGD occurring within theAll whole span is therefore reduced to 2 ns, and the resulting total Rights Reserved © Alcatel-Lucent 201 DGD for the entire span is less than 200 ps based on the accuracy of our DGD measurement method. The DGD was measured using a phase-plate-based mode coupler and a 100 ps test pulse that was selectively launched and detected into different modes. The fiber spools were spliced together using a commercial fusion splicer and resulting span loss including the SMUXs was 24 dB. 4. Coherent MIMO Transmission experiment The MIMO transmission measurement was performed using the setup shown in Fig. 1 a). The wavelength channels on a 50-GHz grid were generated by independently modulating odd and even wavelength from 34 distributed feedback lasers (DFBs). Two double-nested LiNbO3 modulators (DN-MZM), driven by a two-channel programmable pattern generator (PPG) were used to generated a 20-GBaud QPSK signal, and two independent De Bruijn bit sequences (DBBS) of length 212 were used for the in-phase (I) and quadrature (Q) components of the QPSK signal, respectively. We used an external cavity laser (ECL) with a line width of 100 KHz as the light source for the channel under test, and a second ECL was used as a local oscillator (LO) in intradyne configuration. After traversing a polarization multiplexing stage, that adds a orthogonally polarized copy of the signal delayed by 25-ns, the polarization multiplex QPSK signal is divided into 3 paths with a relative delay of 49 and 97 ns, respectively. The three delayed copies are then fed into a 3-fold recirculating loop. Three LiNbO3 switches (LN-SWs) are used to control the loading and the closing of the loop. The loop consists of a pair of mode multiplexers (SMUXs) connecting the 70 km compensated FMF span, and 3 two-stage Erbium-doped fiber amplifiers (EDFAs), where a multichannel blocker is inserted between amplification stages in order to spectrally equalize the power in the loop. Note that the three paths of the 3-fold loop have to be accurately aligned in order to avoid introduction of delays between loop paths. The relative length of the loop was accurately tuned to within < 200 ps using optical delay lines. Finally, three 10:90 couplers are used to extract the signals from the loop which are detected, after traversing three EDFAs and three 0.6 nm bandpass filters, by three polarization-diverse coherent receivers (PD-CRX). The 12 electrical signals from the PD-CRXs were captured by a modular digital storage oscilloscope (DSO) (LeCroy LabMaster 9zi) operating at 40 GS/s with a bandwidth of 20 GHz. The waveforms captured by the DSO were processed off-line using the MIMO DSP algorithm described in , which consists of a network of 6 × 6 feed-forward equalizers (FFEs) with 400 taps each. The FFE coefficients are determined according to the least-mean-square algorithm (LMS), and initial convergence is obtained by data aided operation. The resulting BER as function of the distance are shown for all SDM and WDM channels for an input power of -3 dBm per mode-, wavelength-, and polarization in Fig. 2 c). After 700 km transmission the maximum observed BER is < 10−2 , which can be compensated with a state-of-the-art forward error correction (FEC) with a 20% overhead. | h | 2 (dB) -20 70 km | h | 2 (dB) -20 140 km | h | 2 (dB) -20 350 km -20 700 km OFC/NFOEC Technical Digest © 2013 OSA 4 b) MDL (dB) -40 -60 -40 -40 -60 -4 SMUX 1 0 100 200 300 400 L (km) 500 600 700 -3 -4 10 -60 -5 2 -2 c) -40 PPB-MMUX 3 0 -60 log (BER) a) | h | 2 (dB) OW1I.2.pdf -3 -2 -1 0 1 2 3 4 5 t (ns) -5 -6 140 km 210 km 350 km 560 km 700 km Fig. 2. a) Impulse response |h|2 of a DGD compensated FMF span at a transmission distance of 70, 140, 350, and 700 km. b) Mode dependent loss as function of the distance for spot-based and phaseplate based MMUX transmission experiments. c) BER as function of the transmission distance for all 6 SDM and 34 WDM channels. The impulse response as function of the distance was calculated according to a least-square error (LSE) channel estimator. For a 3-spot SMUX, all of the 6 × 6 impulse responses look qualitatively similar and only one representative All Rights Reserved © Alcatel-Lucent 201 impulse response is reported in Fig. 2 a). After 70 km a strong central double peak produced by the LP01 and LP11 modes is clearly visible at the center of the impulse response, surrounded by a 5-ns wide plateau generated by mode coupling occurring in the fiber and at various splice locations along propagation in the DGD compensated FMF span. For longer transmission distances, the impulse response becomes wider and becomes more Gaussian-like. In particular, after 700 km, the impulse response starts to fill-up the whole 10 ns windows of Fig. 2 a), which also corresponds to the maximum delay that the 400 tap equalizer is capable of compensating. This explains the reduced transmission performance after 700 km. In particular, at that distance, the spread of the impulse response, is clearly wider than for the single channel experiment  performed with the same kind of fiber and using phase-plate based mode couplers. Possible explanation for the faster broadening of the impulse response in this experiment are the fact that the 70 km span is not as well compensated as the 30 km span measured in . In fact, after the first loop the central peak has already a spread of ≈ 300 ps. Also it is more difficult to precisely align the relative loop delays using the SMUX, because the impulse response offers fewer sharp feature that can be used to derive a precise delay measurement. The width of the impulse response is one criterion to anticipate the performance of the MIMO channel. A second relevant quantity is the MDL, which is obtained performing a singular value decomposition of the estimated channel matrix. The MDL is then defined as the ratio between the largest and the smallest squared singular values. We have evaluated the MDL for the SMUX and for the PPB-MMUX based transmission experiments, the results are reported in Fig. 2 b). The experiment based on the SMUX has initially a low MDL of 0.7 dB confirming the excellent alignment of both SMUXs. As a function of the distance, both experiments accumulate approximately the same amount of MDL, of about 0.2 dB/100km, which could indicate that the MDL is caused by the fiber, note however that in the PPB-MMUX experiments the couplers are traversed more than twice more frequently because of the shorter 30 km span used. In conclusion we have demonstrated a combined WDM/SDM transmission in few-mode fiber over a record distance of 700 km. We transmitted 34 wavelength and 6 spatial and polarization modes resulting in an aggregate capacity of 6.5 Tbit/s over a bandwidth of 1.7 THz. References 1. 2. 3. 4. 5. 6. 7. R. Ryf et al., J. Lightwave Technol., 30 (4), 521 (2012). N. Bai et al., Opt. Express, 20 (3), 2668 (2012). R. Ryf et al., Proc. OFC 2012, PDP5B.5 (2012). S. Randel et al., Proc. OFC 2012, PDP5C.5 (2012). N. K. Fontaine et. al., Proc. ECOC 2012, Th.2.D.6 (2012). S. G. Leon-Saval et. al., Opt. Express, 18 (8), 8430 (2010). S. Randel et al., Optics Express, 19 (17), 16 697 (2011).