Optimization of DSP-based Nyquist-WDM PM-16QAM Transmitter V. Curri(1), A. Carena(1), G. Bosco(1), P. Poggiolini(1), F. Forghieri(2) (1) DET, Politecnico di Torino, Torino, Italy. curri@polito.it (2) Cisco Photonics Italy srl, Monza, Italy. fforghie@cisco.com ECOC 2012 Paper Tu.4.A.5

Motivation and outline Assuming to use a DSP, what is the best Tx setup for PM-16QAM in Nyquist WDM links? Details of Tx structure based on DAC Evaluation of the optimal CP and BWDAC for each DAC resolution (Nbit) with and without compensation of electro-optical nonlinearity of MZM Optimal bandwidth (BWDAC) and clipping (CP) Sensitivity vs. Df, ER and MD Analysis vs. Tx loss Comments and conclusions ECOC 2012 Paper Tu.4.A.5

Transmitter layout +1 Vp Vin -1 DSP Nested MZM b MZM DAC Nbit RDAC LSBIx MSBIx DSP DAC LSBQx MSBQx I/Q Nested MZM LSBIy MSBIy LSBQy MSBQy Laser PBC Pol Rotator Etx(t) Ptx VIx VQx VIy VQy Ideal 4-PAM Nyquist filter ArcSin ON/OFF Pre-emphasis DSP G2 G1 b Vin Vp -1 +1 MZM Vin Vout DVDAC 5-Bessel LPF Vin,DAC DAC Vout,DAC G3 RDAC Nbit BWDAC ECOC 2012 Paper Tu.4.A.5

ArcSin operation vout -1 +1 vin VDAC,in k·Vnyquist Vnyquist is the max swing of the signal at the output of the Nyquist filter k properly scales the signal amplitude, depending on MD and CP, in order to achieve the exact compensation of the MZ nonlinearity k=f(MD,CP) Exact MZ nonlinearity compensation is possibile only if MD>(100-CP) ECOC 2012 Paper Tu.4.A.5 4 4 4

Analysis: general parameters PM-16QAM Symbol rate: Rs=32 Gsymbol/s (Rb=256 Gb/s) Simulation with error counting on 128 Ksymbols (1 Mbit) 5 channels BER target: 10-3 Back-to-back setup with noise loading Rol-off: b = 0.05 DAC rate: RDAC = 2 SpS ECOC 2012 Paper Tu.4.A.5

Optimization process Ranges for parameters Optimization process Tx ASE 51-taps LMS Rx ASE noise + Prx OSNR Df Nbit CP MD ER Tx spectrum – Df=33 GHz Ranges for parameters Df = 32 -> 50 GHz MD = 30 -> 70% CP = 0 -> 40% BWDAC = 8 -> 32 GHz Nbit = 4 -> 8 Optimization process Evaluation of the optimal CP and BWDAC for each Nbit with and without ArcSin operation and ER = 20 dB Sensitivity vs. Df, ER and MD with CP and BWDAC fixed Analysis vs. transmitter loss with respect to ideal PM-16QAM ECOC 2012 Paper Tu.4.A.5

Results vs. BWDAC For a given OSNR, for each Df, Nbit we obtained a grid of BER vs.(BWDAC, MD,CP) with ArcSin ON/OFF An example Optimal BWDAC resulted to be 0.5·RS, i.e. 16 GHz, independently of channel spacing (Df) and DAC resolution (Nbit) Therefore BWDAC = 16 GHz is used in the following analyses log10(BERmin) ECOC 2012 Paper Tu.4.A.5

Results vs. CP An example For a given OSNR and the optimal BWDAC, for each Df, Nbit we obtained a grid of BER vs. (CP, MD) with ArcSin ON/OFF An example log10(BERmin) ECOC 2012 Paper Tu.4.A.5

Optimal clipping percentage Nbit=8 Nbit=6 Nbit=5 Nbit=4 ArcSin OFF 100-CP [%] ArcSin ON 100-CP [%] Nbit 8 6 5 4 CP 12% 14% 20% 29% Nbit 8 6 5 4 CP 20% 26% 36% ECOC 2012 Paper Tu.4.A.5

Analysis vs. modulator ER For optimal BWDAC and CP, for each Df, Nbit we carried on an analysis of OSNR penalty varying the modulator ER with ArcSin ON/OFF OSNR vs. MD: Df = 33.00 GHz, Nbit=5 ArcSin OFF ArcSin ON ECOC 2012 Paper Tu.4.A.5 10 10

From previous results we can evaluate the Optimal MD vs. ER From previous results we can evaluate the optimal MD vs. ER Optimal MD vs. ER, Df = 33.00 GHz ArcSin OFF ArcSin ON ECOC 2012 Paper Tu.4.A.5 11 11

Summary: OSNR@BER=10-3 vs. ER 20 30 40 22 24 26 28 ER [dB] OSNR [dB] N bit =8 - DSP no arcsin =8 - DSP arcsin =6 - DSP no arcsin =6 - DSP arcsin =5 - DSP no arcsin =5 - DSP arcsin =4 - DSP no arcsin =4 - DSP arcsin Quantum Limit Df = 33.00 GHz ER = 20 dB limits penalty to 0.5 dB ECOC 2012 Paper Tu.4.A.5 12 12

Summary: OSNR@BER=10-3 vs. Df ER = 20 dB 35 40 45 50 20 22 24 26 28 30 D f [GHz] OSNR [dB] Df=1.1·Rs limits penalty to 0.5 dB N bit =8 - DSP no arcsin =8 - DSP arcsin =6 - DSP no arcsin =6 - DSP arcsin =5 - DSP no arcsin =5 - DSP arcsin =4 - DSP no arcsin =4 - DSP arcsin Quantum Limit Optical Nyquist ECOC 2012 Paper Tu.4.A.5 13 13

Power Tx evaluator Analysis vs. Loss Loss (Nbit,CP), MD, ER ArcSin ON/OFF Power evaluator Loss Reference: CW (Loss = 0 dB) Ideal rectangular PM-16QAM: Loss =-10·log10{1/2·[(1)2+(1/3)2]} = 2.55 dB We evaluate Loss varying MD and ER for each Nbit, given the optimal CP previously evaluated Given ER, we have a MD  Loss correspondence Results vs. MD can be re-plotted vs. Loss ECOC 2012 Paper Tu.4.A.5 14 14

OSNR@BERtarget vs. Loss ER = 20 dB, Df = 33 GHz Solid lines: ArcSin ON Dashed lines: ArcSin OFF Zoom: Nbit = 5 ~3 dB ~0.2 dB IDEAL PM-16QAM Similar results for all scenarios ECOC 2012 Paper Tu.4.A.5 15

Comments and conclusions With b=0.05 in the shaping filter and ER=20 dB we can pack channels down to 1.1·Rs with penalty below 0.5 dB. The requirement in the DAC resolution is Nbit≥5 with a clipping percentage of 20%. The use of ArcSin gives very limited sensitivity advantages that we can roughly estimate below 0.5 dB permits to reduce the transmission Loss of 2 to 3 dB, i.e., using the same laser source we can get 2 to 3 dB more power at the modulator output with respect to the absence of ArcSin ECOC 2012 Paper Tu.4.A.5

This work was supported by CISCO Systems within a SRA contract Acknoledgemets This work was supported by CISCO Systems within a SRA contract The simulator OptSimTM was supplied by RSoft Design Group (now part of Synopsys) ECOC 2012 Paper Tu.4.A.5

OSNRpenalty vs. BWDAC/Rs given RDAC Df = 33.00 GHz – b = 0.05 OSNRpenalty vs. BWDAC/Rs given RDAC 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.5 2 2.5 3 3.5 4 BW DAC /R S OSNR penalty [dB] 1.27 SpS 1.40 SpS 1.56 SpS 1.75 SpS 2.00 SpS 2.33 SpS 2.80 SpS ECOC 2012 Paper Tu.4.A.5