1 COPYRIGHT 2011 ALCATEL-LUCENT. ALL RIGHTS RESERVED.

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2 ECOC 2011 WORKSHOP Space-Division Multiplexed Transmission in Strongly Coupled Few-Mode and Multi-Core Fibers Roland Ryf September 18 th 2011

3 CONTENTS 1. THE CAPACITY CRUNCH 2. SPACE DIVISION MULTIPLEXING AND CROSSTALK 3. COHERENT MIMO 4. FEW-MODE FIBER 5. COUPLED-CORE FIBER 6. CONCLUSION 3

4 CAPACITY CRUNCH IN OPTICAL COMMUNICATION AND SPACE DIVISION MULTIPLEXING (SDM) Capacity of a single mode fiber is approaching the theoretical limit SDM by system duplication has an unfavorable cost structure SDM over a single fiber is the solution System capacity Gbits/s Tbits/s Spectral efficiency (bits/s/hz) db/year (12%/year) db/year (78%/year) Multi-channel (WDM) Single channel (ETDM) Spectral efficiency 0.01 Essiambre et. al., JLT, v. 28, WDM channels 4

5 SDM OVER A SINGLE FIBER CROSSTALK FRIEND OR FOE? When multiple spatial path are introduced in an optical fiber crosstalk between the different paths is unavoidable. At the system level additional crosstalk is introduced by optical elements like couplers and switches Fundamentally two solutions are possible: LOW CROSSTALK FIBER Advantages Simple system design Simple couplers Drawbacks Limited number of spatial paths ELECTRONIC CROSSTALK SUPRESSION Advantages Large number of modes Shared pump for optical amplification Drawbacks Added complexity of the electronic crosstalk suppression Low modal differential group delay required Complex mode couplers required 5

6 COHERENT MIMO BASED TRANSMISSION AND ELECTRONIC CROSSTALK SUPRESSION Ch 1 Multimode fiber Coh-Rx 1 n 1 Out 1 Ch 2 Ch 3... Ch n SDM MUX Orthogonal mode coupling Mode Mixing SDM DEMUX Orthogonal mode coupling Coh-Rx 2 Coh-Rx 3... Coh-Rx n n 2 n 3... n 6 MIMO DSP Out 2 Out 3... Out 6 All guided modes of the fiber are selectively launched Modes are strongly coupled during propagation in the fiber PD: Polarization Diversity All guided modes are simultaneously detected with coherent receivers Digital signal processing decouples the received signals to recover the transmitted signal 6

7 COHERENT MIMO DIGITAL SIGNAL PROCESSING SPACE DIVISION MULTIPLEXING IS THE SOLUTION The multiple-input multiple output (MIMO) digital signal processing (DSP) can be implemented by a Network of n x n feed-forward equalizers (FFEs) Number of taps L required depend on modal differential group delay (DGD) Numerous algorithm are available to determine the equalizer coefficients h ij Randel et. al., Opt. Exp., V. 19, N. 17,

8 MODES IN A 6-MODE FEW-MODE FIBER (FMF) THE LINEAR POLARIZED (LP) MODES Design: Depressed cladding index profile with V 5 at 1550 nm DGD (between LP 01 and LP 11 across C-band): Loss: db/km, dispersion 20 ps/nm/km <60 ps/km Effective Areas: 155 µm 2 for LP 01 and 159 µm 2 for LP 11 LP 01 X-pol LP 01 Y-pol LP 11a X-pol LP 11a Y-pol LP 11b X-pol LP 11b Y-pol Phase Intensity 8

9 MODE MULTIPLEXER FOR FEW MODE FIBER BASED ON PHASE PLATES Coupler loss: 8.3 db / 9.0 db / 10.6 db for LP 01 / LP 11a / LP 11b resp. Crosstalk rejection for MMUX pair > 28 db MMUX port 1 Phase Plates port 0 port 2 Beam Splitters f 1 f 2 Lenses Mirror Few-Mode Fiber 33 km MMUX Mirror Lenses f 2 f 1 Beam Splitters port 2 port 0 Phase Plates port 1 9

10 MODE PROFILES OF THE FEW MODE FIBER MEASURED WITH IR CAMERA AT ONE END OF FIBER LP 01 LP 11a LP 11b LP 11a LP 11b After 96 km After 33 km Theory (amplitude) Theory (phase) Ryf et. al., submitted to JLT,

11 EXPERIMENTAL SETUP FOR MODE-MULTIPLEXING OVER FEW-MODE FIBER TX: 3 PD-QPSK@28Gbaud RX: 3 PD-coherent receiver Pattern Sync Pattern generator 28 GBaud De Bruijn 11 Delay 16 Symb Q QPSK Mod I Splitter Laser Delay 12ns Splitter PBS OVA LO Splitter Splitter 4ch 80Gs/s Oscilloscope Coherent RX Delay 27ns 4ch 80Gs/s Oscilloscope Coherent RX MMUX 4ch 50Gs/s Oscilloscope FMF 33 km Coherent RX MMUX Delay 52ns Port 0: LP 01 mode Port 1: LP 11a mode Port 2: LP 11b mode 11

12 BER FOR 6 x 6 MIMO TRANSMISSION OVER FMF WITH 33 km LENGTH AND 6 x 28-Gbaud QPSK SIGNALS Data aided least-mean square estimator (LMS) for first 500,000 symbols Switching to decision directed LMS Bit-error ratio evaluated for last 1 Million bits Low penalty < 2 db observed for all 6 transmitted channels Best performance obtained for 120 taps Randel et. al., Opt. Exp., V. 19, N. 17,

13 IMPULSE RESPONSE MATRIX FOR 96-km 6-MODE FEW-MODE FIBER Transmitted ports The impulse response was characterized for all 6 outputs as function of all 6 inputs Strong coupling is observed within the LP 01 and the LP 11 mode Weaker coupling is observed between the LP 01 and LP 11 mode Received ports LP 11bx LP 11ay LP 11ax LP 01y LP 01x h 2 (db) h 2 (db) h 2 (db) h 2 (db) h 2 (db) LP 01x LP 01y LP 11ax LP 11ay LP 11bx LP 11by -10 h 11 h h 21 h h 31 h h 41 h h 51 h h 13 h 23 h 33 h 43 h 53 h 14 h 24 h 34 h 44 h 54 h 15 h 25 h 35 h 45 h 55 h 16 h 26 h 36 h 46 h h 61 h 62 h 63 h 64 h 65 h 66 LP 11by h 2 (db) t (ns) t (ns) t (ns) t (ns) Ryf et. al., submitted to JLT, t (ns) t (ns) 13

14 MULTI-MODE TRANSMISSION IN COUPLED CORE FIBERS CHARACTERISTICS OF A 24 km 3-CORE COUPLED-CORE FIBER Number of cores 3 Core diameter is 11.2 µm Refractive Index step = 0.32% Distance between cores 38 µm Effective core area 104±1 µm 2 Attenuation db/km Dispersion 20 ps/nm/km Dispersion slope 0.06 ps/nm 2 /km m P nm n Normalized output power or Crosstalk [db] #1 #2 # #1 Input #2 core #3 Crosstalk #1 #2 #3 Output core Ryf et. al., PTL, n. 99, 2011 (early access) 14

15 SPATIAL CORE MULTIPLEXER FOR COUPLE-CORE FIBER Insertion loss < 2 db Crosstalk suppression > 40 db Spatial core MMUX port 1 port 0 Lenses f 1 Coupled-Core Fiber 24 km Lenses port 2 Mirror Mirror f 2 f 2 f 1 port 1 port 2 port 0 Spatial core MMUX 15

16 EXPERIMENTAL SETUP FOR MODE-MULTIPLEXING OVER COUPLED-CORE FIBER PPG 8 symbols delay Laser Tx I/Q Modulator 12 ns PBS VOA 53 ns 27 ns Spatial Core Mux MCF (24 km) Spatial Core Mux LO Fiber delay () Rx 3 Rx 2 Rx 1 90 o Hybrid 90 o Hybrid X 90 o Hybrid 90 o Hybrid 90 o Hybrid Y 90 o Hybrid Clock recovery MIMO equalization Carrier Phase Estimation Symbol detection Error counting Coherent Rx Offline DSP 16

17 BER FOR 6 x 6 MIMO TRANSMISSION OVER CCF WITH 24 km LENGTH AND 6 x 14-Gbaud QPSK SIGNALS Data aided least-mean square estimator (LMS) for first 500,000 symbols Switching to decision directed LMS Bit-error ratio evaluated for last 1 Million bits Low penalty < 2 db observed for all 6 transmitted channels Best performance obtained for 100 taps BER Theorerical Limit B2B RX 1 X-Pol B2B RX 1 Y-Pol B2B RX 2 X-Pol B2B RX 2 Y-Pol B2B RX 3 X-Pol B2B RX 3 Y-Pol MCF RX 1 X-Pol MCF RX 1 Y-Pol MCF RX 2 X-Pol MCF RX 2 Y-Pol MCF RX 3 X-Pol MCF RX 3 Y-Pol OSNR Pol (db) Ryf et. al., PTL, n. 99, 2011 (early access) 17

18 CONCLUSION We have experimentally confirmed that MIMO based crosstalk suppression is possible even in the presence of large crosstalk We have demonstrated mode-multiplexed transmission of 6 channels in 33 km few-mode fiber and 24 km coupled-core fiber We experimentally determined the impulse response matrix of the multimode fiber, which gives a complete instantaneous characterization of the fiber This clearly indicate the coherent MIMO applied over a full set of modes allow to reach maximum capacity gains Further technological development are required to improve the fiber characteristics and to perform MIMO DSP in real time 18

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