We experimentally study saturation effects in degenerate phase sensitive amplifiers, revealing and explaining a gain regime suitable for all-optical signal processing functions such as phase regeneration

Kakande, J.

Parmigiani, F.

Slavík, R.

Grüner-Nielsen, L.

Jakobsen, D.

Herstrøm, S.

Petropoulos, P.

Richardson, D.J.

English

Southampton (e-Prints Soton)

Experimental setup and results The experimental setup is shown in Fig. 1. Two narrow  linewidth  signals  200  GHz  apart  were mixed  in  a  Ge-doped  HNLF  (OFS,  Denmark) generating  an  idler  wave.  The  HNLF  length, dispersion, nonlinear coefficient and attenuation were  500m,  0.09ps/nm/km,  11.5W-1km-1  and  Fig. 2: Saturation characteristics as signal-to-pumps ratio is varied for PS-FOPA (total power of 29 dBm). 0.83dB/km  respectively.  The  idler  was  filtered out, in so doing generating a third wave locked in frequency and phase to the two  signals at the HNLF input. The three waves spaced 200 GHz apart could then be used as the two pumps and signal  for  the  PS-FOPA.  A  variable  attenuator allowed  control  of  the  signal  power  relative  to the pumps. The degenerate PS-FOPA utilised a novel  high  SBS  threshold  strained  alumino-silicate  HNLF  (OFS,  Denmark).  The  length, dispersion,  polarization  mode  dispersion, nonlinear coefficient and attenuation of the fiber were 177m, -0.13ps/nm/km, 0.11pskm-0.5, 7.1W-1km-1 and 15dB/km, respectively. The high SBS threshold meant that in excess of 1W per pump could be coupled into the fibre.  The  phase  of  the  signal  wave  was  modulated using a 50 kHz ramp function applied to a piezo-electric fibre stretcher. The choice of the ramp frequency  was  chosen  to  correspond  to  the resonant  frequency  of  the  piezo-electric stretcher used, allowing the phase to be swept by  many  multiples  of  180°   for  a  peak-to-peak drive  voltage of under  100V. The  three  waves were then combined, amplified in a high power polarization  maintaining  EDFA  to  a  fixed  total power, and then  launched into the degenerate PS-FOPA. Following the parametric interaction, the signal at the PS-FOPA output was filtered, attenuated  and  coupled  into  a  low  power photodiode.  The  electrical  signal  from  the photodiode  was  then  observed  using  a  high speed digital oscilloscope. Post-processing was then  carried  out  to  eliminate  high-frequency noise and normalise the gain curves. The  normalised  signal  gain  curves  for  a  total power of 29 dBm (pumps and signal combined) coupled into the fibre are shown in Fig. 2. At this moderate  pump  power,  for  low  input  signal  Fig. 3: Saturation characteristics as signal-to-pumps ratio is varied for PS-FOPA (total power of 32 dBm). levels  (solid  curve  in  Fig.  2)  the  gain  curve  is sinusoidal  with  relative  signal  input  phase  as expected  from  theory9.  As  the  signal  power increases, the curves accumulate a skew, with the gain peak seemingly detuned in phase from the central phase position. Because each curve is captured for a fixed signal power at the PS-FOPA  input,  this  differs  from  PI  saturation because a nonlinear dependence on input signal phase  exists.  In  this  case,  every  point  on  the curve  experiences  a  gain  shift  due  to  the alteration of the phase matching condition by the variation  in  the  signal/pump  power  proportions during  propagation.  As  the  signal  and  pumps propagate  down  the  fibre  in  which  the  PS interaction  occurs,  at  every  point  a  nonlinear contribution to the phase matching exists as a result of self- and cross-phase modulation (SPM and  XPM  respectively).  Because  the  gain  is dependent  on  the  signal  phase,  the  absolute powers  and  therefore  the  nonlinear  phase matching varies with the input phase. For strong signal  levels,  this  phase  dependent  behaviour leads to a modification of the gain characteristic, and ultimately to gain saturation.  For  applications  such  as  the  regeneration  of binary  phase  shift  keyed  (BPSK)  signals,  a crucial requirement would  be that  the PS  gain curves have a flat-top, minimising the transfer of phase  to  amplitude  noise.  To  obtain  such flattening, it was necessary to increase the total power coupled into the PS-FOPA to 32 dBm. A selection of the resulting gain curves are shown in Fig. 3. At the lowest signal level assessed a sinusoidal gain curve is obtained indicating the absence of significant gain saturation (thick solid line in Fig. 3). As the signal power is increased, the  gain  curve  is  seen  to  acquire  a  flat-top (dashed-dotted  line  in  Fig.  3).  Increasing  the signal power further reveals that the flattening is in fact due to the growth of a second peak in the gain  curve  that  detunes  in  phase  from  the central position as the signal level is increased.    Fig. 4: Saturation characteristics as signal-to-pumps ratio is varied for PS-FOPA (total power of 33 dBm). The  total  signal  power  was  then  increased further to 33 dBm and the gain curves obtained are shown in Fig. 4. The gain curve goes from sinusoidal  at  low  signal  levels  to  skewed  at moderate power levels, as previously discussed. At  high  signal  levels,  though,  it  is  possible  to have two separate peaks in the gain curve for a 180°   change  in  the  input  signal  phase, contrasting  with a single peak for lower power levels. This can be seen in the dotted line in Fig. 4.  This  points  further  towards  the  saturation effect  stemming  from  a  modification  of  the nonlinear phase matching condition and not just from pump power depletion. For 0°  input phase as an example, the gain goes from maximum for the  signal  23.2  dB  lower  than  the  pumps,  to minimum at -3.1 dB, and then starts to increase again  for  stronger  signal  levels.  The  ability  to tune  the  periodicity  of  the  phase  response  as well as the peak gains of a PSA by saturation opens up a whole new host of applications, an example being the phase regeneration of QPSK and M-PSK signals.  In  the  dotted  line  in  Fig.  4,  the  peaks  have  a 14%  difference  in  power.  To  obtain  two  equal peaks, the signal level and the total power into the PSA were varied and the results are shown in  Fig.  5.  Generating  dual  peaks  can  be achieved by increasing the signal level beyond -8 dB in proportion to the total power coupled into the PSA. It is also worth pointing out that for a total power of 32 dBm (Fig. 3 thin solid line) the spacing between the peaks was approximately 90° , despite a  peak to peak variation of 50%. At 33 dBm (dotted line Fig. 5) the spacing between the  peaks  was  70°   without  significantly compromising the phase sensitive swing (97%). Finally moving to 34 dBm (solid line Fig. 5) this reduced  to  52°   with  equal  gain  peaks.  These studies suggest that by optimising the signal and pump  powers  further  it  should  be  possible  to achieve the desired gain shape, such as the 90° spacing  required  for  regeneration  of  QPSK signals in a single PS-FOPA.   Fig. 5: PS-FOPA saturation optimised to achieve two gain peaks within a 180°  signal phase period.  Conclusions We  have  experimentally  studied  saturation effects in PS-FOPAs showing the impact of the nonlinear  phase  matching  term  on  the  gain characteristics.  The  ability  to  experimentally access  this  high  saturation  regime  stems from the very high SBS threshold levels for the HNLF used,  allowing  access  to  a  flat  top  regime suitable  for  BPSK  phase  regeneration,  as  well as a further saturation regime characterised by multiple gain peaks within a 180°  signal phase periodicity. Acknowledgements We  would  like  to  thank  OFS  Denmark  and Eblana  Photonics  Inc.,  Dublin,  Ireland,  for respectively providing HNLFs and lasers used in the  experiment.  This  research  has  received funding  from  the  European  Communities Seventh Framework Programme FP/2007-2013 under grant agreement 224547 (PHASORS).  References  [1] A.  Durecu-Legrand  et  al.,  Photonics Technology Letters, IEEE 17, 1178 (2005). [2] J. M. C. Boggio et al., Photonics Technology Letters, IEEE 21, 612 (2009). [3] C. J. McKinstrie, S. Radic, and A. H. Gnauck, Opt. Photon. News 18, 34 (2007). [4] K. Inoue, Electronics Letters 36, 1016 (2000). [5] P.  Kylemark  et  al.,  Lightwave  Technology, Journal of 24, 3471 (2006). [6] A.  Bogris,  and  D.  Syvridis,  Ieee  Photonics Technology Letters 18, 2144 (2006). [7] K. Croussore, and G. F. Li, IEEE Photonics Technology Letters 19, 864 (2007). [8]   Parmigiani  F.  et  al.  Proc.  OFC’10, PDPC3 (2010). [9] R.  Tang  et  al.,  Optics  Express  13,  10483 (2005).

Lightwave Technology,

Saturation effects in degenerate phase sensitive fiber optic parametric amplifiers

Saturation effects in degenerate phase sensitive fiber optic parametric amplifiers

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Southampton (e-Prints Soton)