We demonstrate a four-wave mixing based wavelength conversion scheme at 1.55µm in a 1.5m long highly nonlinear, dispersion tailored one-dimensional (1D) soft glass microstructured optical fibre

Camerlingo, Angela

Feng, Xian

Horak, Peter

Loh, Wei H.

Parmigiani, Francesca

Petropoulos, Periklis

Poletti, Francesco

Richardson, David J.

English

Southampton (e-Prints Soton)

Four-wave mixing-based wavelength conversion in a Short-Length  of a Solid 1D Microstructured Fibre  Angela Camerlingo, Francesca Parmigiani, Xian Feng, Francesco Poletti, Peter Horak,  Wei H. Loh, David J. Richardson and Periklis Petropoulos Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK Email: anc@orc.soton.ac.uk Abstract We demonstrate a four-wave mixing based wavelength conversion scheme at 1.55mm in a 1.5m long highly nonlinear, dispersion tailored one-dimensional (1D) soft glass microstructured optical fibre. Introduction The  use  of  soft  glasses  for  the  fabrication  of microstructured optical fibres (MOFs) and the ability to tailor their dispersion profile have opened up new opportunities  for  the  realization  of  compact  highly nonlinear  devices.  Among  the  nonlinear  processes, optical parametric effects based on four-wave-mixing (FWM)  play  an  essential  role  in  the  realization  of many  devices,  including  all-optical  wavelength converters  [1].  In  general,  the  most  important  fibre parameters that contribute towards achieving efficient and broadband FWM are a high nonlinear coefficient, a  low  dispersion,  low  dispersion  slope  and  a  short fibre length [2]. Soft-glass holey fibres have already been  demonstrated  as  good  candidates  for  FWM processes  (see  e.g.  [3]).  However,  to  achieve  the accurate dispersion control required places onerous demands on the level of structural control needed for the complex ‘holey’ microstructure (both transversally and  longitudinally  down  the  fibre).  One  way  to improve the situation is to move to all-solid 1D MOFs since the microstructure geometry is defined in this case  during  manufacture  of  the  macroscopic  solid preform  rather  than  during  the  delicate  fibre  draw process [3]. Recently,  we  have  reported  the  fabrication  of  a highly nonlinear 1D MOF based on two commercial glasses  (Schott  SF6  and  LLF1)  which  exhibits  the lowest reported value of losses, while maintaining a high nonlinear coefficient and low dispersion values at telecommunications  wavelengths  [3].  In  this  paper, we  demonstrate  the  use  of  this  fibre  to  realise  a compact  (1.5 m  long)  FWM-based  wavelength converter  operating  at  1.55  mm.  Eye-diagrams  and bit-error  rate  (BER)  curves  confirm  the  high  signal quality achieved in the conversion process.  Experimental set-up and results   The  experimental  setup  for  the  FWM-based wavelength  converter  is  shown  in  Fig.  1.  The converter was based on a 1D MOF that exhibited a high refractive index core surrounded by alternating high and low index rings. The high index glass was Schott SF6 (n=1.76 @ 1550 nm) while Schott LLF1 glass (n=1.53 @ 1550 nm) was used for the low index rings.  The  core  diameter  was  ~3.7 mm,  while  the thickness  of  the  surrounding  layers  varied  in  the range 0.3 to 1.1 mm [4]. This fibre showed effective single mode guidance at 1.55 mm and its propagation loss  was  measured  to  be  ~0.8±0.2 dB/m  using  the cut-back method. This is the lowest reported value for a  non-silica  MOF  at  telecoms  wavelengths  so  far. Using  the  Boskovic  method  [5],  the  nonlinear coefficient  of  the  fibre  was  measured  to  be  ~120 W-1km-1.  Numerical  simulations  on  the dispersion  profile  of  the  fabricated  fibre  show  a dispersion slope of 0.15 ps/nm2/km at 1.55 mm; this value  is  largely  insensitive  to  any  small  structural variations. Using the FWM method, the dispersion of the  fibre  was  measured  to  be  12.5 ps/nm/km  at 1.55 mm. The zero dispersion wavelength (ZDW) of this fibre is therefore estimated to be ~1.47 mm.  The  pump  signal  to  the  FWM-based  wavelength converter was generated by a 10GHz mode locked laser,  which  produced  ~7ps full-width-half-maximum (FWHM)  pulses  at  1545nm.  The  pulses  were amplitude  modulated  by  a  231-1  pseudorandom  bit sequence  (PRBS)  using  a  lithium-niobate  Mach-Zehnder modulator (MOD) and amplified using a high power  erbium-doped  fibre  amplifier  (HP-EDFA).  A band  pass  filter  (BPF)  was  used  to  reject  any undesired  amplified  spontaneous  emission  (ASE) noise  arising  from  the  HP-EDFA.  The  probe  signal was generated by a continuous wave (CW) tuneable laser, amplified and filtered to reject any out-of-band ASE  noise.  Note  that  two  different  amplifiers  were used to independently control the powers of the two   CW  ~ PC 231-1 PRBS Generator  @ 10Gbit/s HP-EDFA MOD PC EDFA  PC  Dl=2nm  10%  Dl=2nm  EDFA 90%  1.5m fibre Pulsed pump ~7ps ls=1545.5nm 10GHz  Dl=2nm  Dl=3nm BER and diagnostic     Fig. 1: Experimental setup. PC: polarization controller signals.  Polarization  controllers (PCs)  were  used  to align the state of polarization of the two beams to the polarization axis of the 1D MOF. The pump pulses and the CW signal were then combined in a 90/10 coupler and free-space launched into 1.5 m of fibre with  a  coupling  efficiency  of  ~  35%.  The  average powers of the pump and the signal at the very input of the fibre were 24.5 dBm and 3 dBm respectively.  Under conditions of phase matching, the gain and the pulse shape of the signal and the idler depend on the pump pulse shape and peak power as well as the fibre  parameters.  Owing  to  the  low  loss,  low dispersion and the high nonlinearity per unit length of the  MOF,  we  have  observed  optical  parametric generation of pulses over a range of wavelengths that cover  the  upper  part  of  the  C-band  in  our experiments,  even  if  the  pump  wavelength  sits  far away from the ZDW. This is illustrated in Fig. 2, which shows the spectral traces at the output of the system when the CW wavelength was tuned from 1542nm to 1530nm. The Figure shows that an optical signal to noise ratio (OSNR) of ~ 12-18 dB has been achieved at  the  output  of  the  system.  It  is  worth  noting however,  that this is mainly limited  by the  dynamic range  of  the  spectrum  analyser,  and  in  reality  the OSNR  was  much  higher  than  this  (the  BER measurements presented below are in support of this argument). The conversion efficiency (which depends on  the  power  of  both the  pump  and  signal  waves) varies  by  ~4.5 dB  across  this  12-nm  tuning  range. The  Figure  also  shows  newly  generated  frequency components around the CW wavelengths, which are due  to  the  combined  effects  of  cross-phase modulation and parametric amplification. We  then  filtered  and  further  characterised  the parametrically  generated  pulses  at  one  particular wavelength (lidler = 1552 nm) in terms of their noise properties and pulse width. The insets in Fig.3 show eye diagrams obtained at the input and the output of the system, respectively. A very clean eye diagram with very low intensity noise is obtained at the output of the FWM wavelength converter, confirming a good OSNR for the generated FWM component. The high quality  of  the  eye  diagram  is  reflected  in  the corresponding  BER  measurement  shown  in  Fig.3. The wavelength converted signal has a power penalty of  ~0.5 dB  as  compared  to  the  input  signal  at BER = 10-9  (error-free  operation).  Finally,  we  have measured  autocorrelation  traces  of  the  input/output signals  (the  results  are  not  reported  here)  and observed an output signal pulse reduction from 7ps to 4.8ps (e.g. as discussed in [7]). These values have also  been  predicted  from  simulations  on  our wavelength conversion system.  Conclusions We  have  successfully  demonstrated  the  use  of  a recently  fabricated  soft  glass  1D  MOF  in  a  FWM-based wavelength converter. The fibre shows a high nonlinear  coefficient,  low  dispersion  and  dispersion slope and the lowest value of losses reported so far for  a  non-silica  fibre  at  telecommunication wavelengths. Using a sample of only 1.5m length we have  demonstrated  FWM-based  wavelength conversion over a wide range of wavelengths even when the pump signal sits at wavelengths relatively far  from  the  fibre  ZDW.  We  envisage  that  further improvements in the fabrication of 1D MOFs will yield significant benefits in both the conversion efficiency and  the  bandwidth  of parametric devices  based  on this technology.  This  work  was  partially  funded  by  the  European Commission  STREP  project  PHASORS  (FP7-ICT-2007-2  22457)  and  NoE  BONE  (FP7-ICT-2007-1 216863).  References 1 M.N. Islam et al., IEEE JST-QE, 8, (2002), 527. 2  J. Hansryd et al., IEEE JST-QE, 8, (2002), 506. 3. Asimakis et al., Opt. Exp., 15,(2007), 596-601.  4  X. Feng et al., CLEO/Europe 2009, CE3.4. 5  A.Boskovic et al., Opt. Lett., 21, (1996) 1966. 6 T. Hasegawa et al. Opt. Comm. (2008), 281, 782. 7 T. Torounidis, et al., IEEE J. Lightwave Technol.       (23), (2005), 4067-4073.  Average Optical Power[dBm]-26 -25 -24 -23 -22 -21 -20-log(BER)3456789101112Back-to-backConverted  signal OutputInputAverage Optical Power[dBm]-26 -25 -24 -23 -22 -21 -20-log(BER)3456789101112Back-to-backConverted  signalBack-to-backConverted  signal OutputInput Fig. 3: BER measurements at the input (back-to-back) and output of the system. Inset: Corresponding eye –diagrams.  Wavelength [nm]1530 1540 1550 1560Spectral density [10dB/div] lcw=1542nm lcw=1540nm lcw=1538nm lcw=1536nm lcw=1534nm lcw=1532nm lcw=1530nm   Fig. 2: Spectral traces at the output of the 1D MOF for various input CW signal wavelengths (Resolution =0.5nm).  

Feng et al., CLEO/Europe 2009, CE3.4. 5 A.Boskovic et al.,

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Four-wave mixing-based wavelength conversion in a short-length of a solid 1D microstructured fibre

Four-wave mixing-based wavelength conversion in a short-length of a solid 1D microstructured fibre

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