We demonstrate all-optical 1-to-5 differential phase-shift keyed (DPSK) wavelength multicasting at 40 Gbit/s using a degenerate four-wave mixing (FWM) based phase sensitive amplifier (PSA). Phase regenerative properties are reported with a sensitivity improvement of more that 10 dB

Grüner-Nielsen, L.

Herstrøm, S.

Jakobsen, D.

Kakande, J.

Parmigiani, F.

Petropoulos, P.

Richardson, D.J.

Slavík, R.

English

Southampton (e-Prints Soton)

All-Optical Phase-Regenerative Multicasting of 40 Gbit/s DPSK Signal in a Degenerate Phase Sensitive Amplifier  Radan Slavík(1), Joseph Kakande(1), Francesca Parmigiani(1), Lars Grüner-Nielsen(2),  Dan Jakobsen(2), Søren Herstrøm(2), Periklis Petropoulos(1), and David J. Richardson(1)  (1) Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK Tel.:+44-2380594524,    Fax +44-2380593142, e-mail:rxs@orc.soton.ac.uk (2) OFS, Priorparken 680, 2605 Brøndby, Denmark Abstract  We  demonstrate  all-optical  1-to-5  differential  phase-shift  keyed  (DPSK)  wavelength multicasting at 40 Gbit/s using a degenerate four-wave mixing (FWM) based phase sensitive amplifier (PSA). Phase regenerative properties are reported with a sensitivity improvement of more that 10 dB.  Introduction All-optical  wavelength  conversion  and multicasting, where a single data wavelength is simultaneously  replicated  onto  multiple  output wavelengths,  are  essential  functions  in applications where the information is broadcast to  a  large  number  of  destinations  and  users (high-definition  television,  video-conferencing, etc.).  Although  at  the  optical  level  multicasting can  be  performed  passively,  e.g.,  by  splitters that deliver the information to multiple users at a single  wavelength￿ 1,  this  passive  approach  is inherently lossy and does not offer regenerative properties.  Thus,  it  is  limited  in  scalability  and functionality.  A  preferred  approach  is  thus wavelength-division-multiplexed  (WDM)  multi- casting  which  allows  for  simultaneous wavelength  conversion,  offers  network functionalities inherent to WDM networks (such as routing and monitoring), and  which has the potential for signal regeneration￿ 2. Most  of  the  existing  multicasting approaches presented up to date are suited for amplitude  modulated  signals,  such  as  on-off keyed (OOK) and the optical phase (which does not  carry  any  information  in  OOK)  is  not necessarily  preserved  during  the  multicasting process3,4. Thus, many published schemes that perform  simultaneous  regeneration  and multicasting are limited to the OOK modulation format5,6.  However,  DPSK  modulation  formats are  recently  emerging  as  preferred  solutions over OOK, especially for long-haul transmission, thanks to the improved receiver sensitivity and nonlinear tolerance they offer. As DPSK may be widely employed in the near future, it is highly desirable  to  explore  wavelength  multicasting schemes for such signals7. Here, we demonstrate a scheme based on  a  FWM-based  phase  sensitive  amplifier (PSA) in a dual pump configuration operating on an initial  DPSK signal at  40 Gbit/s and that is capable  of  simultaneously  multicasting  and regenerating  the  output  signals  when  a significant amount of phase noise is present at the input to the system. Five 40 Gbit/s multicast channels are obtained and properly regenerated at 400 GHz spacing.   Principle of Operation and Set-up The operating principle  of the proposed DPSK wavelength multicasting and phase regenerator scheme is shown in Fig.1. It is based on a dual pump degenerate FWM process in a fibre-based PSA. As sketched in Fig.1, two processes occur: (i) the PSA regenerates the phase of the data signal  through  properly  optimizing  the  energy exchange  with the two phase-locked pumps8,9; (ii)  new  (secondary)  pumps  as  well  as  new copies  of  the  data  signals  are  generated  via phase  insensitive  amplifier  (PIA)  interactions3.   The corresponding experimental set-up is shown in Fig. 2. The data signal is a 40 Gbit/s non-return-to-zero  (NRZ)-DPSK,  231-1  pseudo-random bit sequence (PRBS). In order to study the  robustness  of  the  multicasting  scheme  to phase noise, the signal phase was modulated in DataPump 1 Pump 2Regeneration:Multicasting:DataPump 1 Pump 2Copy-1Copy1Pump 4 Pump 3 Fig. 1: Operation principle of the multicasting and regeneration scheme. ECOC 2010, 19-23 September, 2010, Torino, Italy978-1-4244-8535-2/10/$26.00 ©2010 IEEEMo.1.A.2a  deterministic  fashion  using  an  additional phase  modulator,  driven  at  a  single-frequency close  to  20 GHz.  The  core  of  the  multicasting and regenerating scheme consists of three main blocks: (i) a phase insensitive amplifier (PIA) to generate  an  idler  that  is  automatically  phase- locked to the data signal and a locally generated pump  (Pump  1);  (ii)  a  continuous  wave  (CW) laser  that  is  injection-locked10  by  the  PIA generated  idler  to  produce  the  second  phase locked  pump  (Pump  2);  and  (iii)  a  dual  pump degenerate FWM-based PSA that performs the multicasting as well as the regeneration8,9.  As  the  injection  locking  is  a  much  slower process  than  FWM,  any  high  frequency fluctuations  (e.g.,  bit-to-bit  phase  variations) present  on  the  original  data  signal  are  not transferred  onto  the  output  of  the  injection locked laser, allowing phase regeneration in the subsequent PSA8,9. More  details  of  the  customised components  used  are  as  follows.  The  highly non-linear  fibre  (HNLF)  used  in  the  PIA  was 500 m long with a dispersion of 0.09 ps/nm/km, nonlinear  coefficient  of  11.5 W-1km-1,  and  an attenuation of 0.8 dB/km. The HNLF used in the PSA  was  an  alumino-silicate  fibre  with  an applied  strain  gradient  to  increase  the Stimulated  Brillouin  Scattering  (SBS)  threshold to > 28 dBm thereby avoiding the need for any active  SBS  suppression  scheme.  This  HNLF was  177 m  long  with  a  dispersion  of 0.13 ps/nm/km,  nonlinear  coefficient  of  7.1 W-1km-1, and an attenuation of 15 dB/km. The very low  dispersion  and  dispersion  slope  values facilitate the generation of multiple copies of the signal  and  pumps.  The  injection  locked  laser was a discrete-mode semiconductor laser from Eblana  Inc.,  Ireland  with  a  natural  linewidth  of 300 kHz11.The laser used to generate the input DPSK  signal  had  a  natural  linewidth  below 10 kHz. As may be appreciated from Fig. 2, the data and two phase locked pumps propagate in different paths before being combined together in the PSA. Thus, their relative phase difference is subject to phase noise pick up due to acoustic and  thermal  effects.  This  relatively  slow  drift (kHz-range) is compensated for by an electrical phase-locked loop that controls a piezoelectric-based  fibre  stretcher  in  the  pump  path  (to simplify the schematic, this feedback system is not shown in Fig.2). Fig.3  shows  the  PSA  output  spectrum  for  the optimal  total  input  power  of  34  dBm.  We  see that it consists of many pumps (generated from the input Pumps 1 and 2 via FWM) and many copies  of  the  signal.  Here,  we  consider  five copies  only,  which  have  similar  power  levels: Copy 0 (at the original wavelength), Copies ±1 with  about  the  same  powers  as  Copy  0,  and Copies ±2 with powers about 2 dB below that of Copy 0.  1545 1550 1555 1560 1565-50-40-30-20-10 Pump 2 Pump 1Copy 2Copy 1Copy -2Copy -1Power spectral density, dB/nmWavelength, nmCopy 0Fig. 3: Output PSA spectral characteristics. Signal at the input signal wavelength (Copy 0) and four copies with  power  difference  of  less  than  2  dB  are considered.   Results For the sake of simplicity and due to the lack of a  suitable  filter  at  the  corresponding wavelengths, we characterized only Copies 0, -1, and -2, respectively. However, results similar to that for Copy -1 and -2 are also expected for Copies 1 and 2, respectively.. To evaluate the performance of the system, we measured the bit error ratios (BERs) and the eye diagrams.  First,  we  assessed  the  performance  of  our multicasting  scheme  using  a  clean  (no  noise) input data signal. The eye diagrams are shown in Table 1 (left part) and the BER curves in Fig. 4a. As also observed previously in Ref. 9, Copy 0  has  a  slightly  negative  power  penalty  as compared  to  the  back-to-back  (about  1  dB). Analysis of the corresponding eye diagrams in Transmitter231-1  231-1 40 Gbit/sPhasenoiseemulation~20 GHz CWpump 1EDFAHNLF 1CWpump 2(IL)Pump 1DataDataPhase Sensitive MulticasterINPIA Pump 2seed Pump 2MUXHPEDFAHNLF 2PSADeM UXCopy 0Copy 1Copy 2Copy -1Copy -2PRBSNRZ DPSKPhasemod.  Fig. 2: Experimental set-up of the 40 Gbit/s multicasting and phase regenerator scheme.  Mo.1.A.2Table  1  suggest  that  the  shape  of  the demodulated  pulse  is  different  at  the  PSA input/output,  which  may  explain  this  slight  improvement  in  the  receiver  power  sensitivity. BER  measurements  of  the  other  copies  show error free operation with power penalties of less than 1 dB as compared to the back-to-back (and less than 3 dB as compared to the Copy 0).   Table 1: Eye diagrams of Copy 0,-1 and -2 for noise off (left) and on (right), respectively. Noise  OFF  ON Input    Copy 0    Copy -1    Copy -2     Subsequently,  we  introduced  phase  distortion on  the  data  at  the  input  of  the  multicasting scheme, severely degrading the corresponding BER curve and eye diagram at the input of the multicasting scheme (Fig. 4b and Table 1) – a power  penalty  of  13  dB  was  obtained  at  the  BER~10-7  level  (Fig.  4a  and  4b).  The  BER curves for all the characterized copies (Fig. 4b) have better BER sensitivity as compared to the input  signal  clearly  demonstrating  the regenerative property of the presented scheme. This  regenerative  property  is  also  visible  from the  corresponding  eye  diagrams  shown  in  the second column of the Tab. 1.   Conclusions We  have  proposed  and  experimentally demonstrated  phase  regenerative  multicasting of 40 Gbit/s DPSK signal using cascaded PIA-PSA  processes  in  a  dual  pump  configuration. The use of a high SBS-threshold HNLF with low dispersion and dispersion slope has enabled the generation of several copies of the input signal (as well as of the pumps).  We note that the use of  smaller  wavelength  spacing  between  the signal  and  the  pumps  should  allow  the generation  of  a  larger  number  of  high  fidelity signal copies.  -35 -33 -31 -29 -27 -25 -23 -21 -19 -17 -1510987654 PSA input (b2b) Copy 0 (original) Copy -1  Copy -2 -log(BER)Received power, dBm(a) Noise OFF-35 -33 -31 -29 -27 -25 -23 -21 -19 -17 -1510987654  PSA input Copy 0         (original) Copy -1  Copy -2 -log(BER)Received power, dBm(b) Noise ONFig. 4: BER curves of Copy 0,-1 and -2 for noise off (a) and on (b), respectively. Acknowledgments We would  like to thank Eblana Photonics Inc., Dublin, Ireland, for providing the injection locked laser used in the experiment and the group of Dr. Ellis from Tyndall National Institute for useful discussions. This research has received funding from  the  European  Communities  Seventh Framework  Programme  FP/2007-2013  under grant agreement 224547 (PHASORS). References 1.  B.M.N.K.  Singhal,  Proc.  OFC’01,  TuG8-1-TuG8-3 (2001). 2.  K.K.  Chow  and  C.  Shu,  Opt.  Express  12, 3050 (2004). 3.  C.-S. Bres et al., J. Lightw. Technol. 27(3), 356-363, (2009). 4.  W.  Wang  et  al.,  J.  Lightw.  Technol.  23(1), 211-218, (2005). 5.  M. Matsuura et al., IEEE Phot. Technol. Lett., 18 (24), 2659-2661, (2006). 6.  K. K. Chow et al., J. Selec. Topic Quantum Electronics. 12(4), 838-842, (2006). 7.  M. P. Fok et al., J. Lightw. Technol. 27(15), 2953-2957, (2009). 8.  F.  Parmigiani  et  al.  Proc.  OFC’10,  PDPC3 (2010). 9.  K.  Croussore  et  al.,  IEEE  Phot.  Technol. Lett., 19 (11), 864-866, (2007). 10.  R. Weerasuriya, et al., Proc. OFC’10, OWT6 (2010). 11.  R. Phelan et al, Electron. Lett.  45, 43 (2009). Mo.1.A.2

Topic Quantum Electronics.

All-optical phase-regenerative multicasting of 40 Gbit/s DPSK signal in a degenerate phase sensitive amplifier

All-optical phase-regenerative multicasting of 40 Gbit/s DPSK signal in a degenerate phase sensitive amplifier

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