More compact, stable, and efficient configuration of a recently-developed regenerator is presented. The regenerator is assessed at data rates up to 56 Gbit/s using white phase noise for the first time

Grüner-Nielsen, Lars

Kakande, Joseph

Parmigiani, Francesca

Petropoulos, Periklis

Phelan, Richard

Richardson, David J.

Sköld, Mats

Slavík, Radan

Westlund, Mathias

English

Southampton (e-Prints Soton)

Robust design of all-optical PSK regenerator based on  phase sensitive amplification  Radan Slavík1*, Francesca Parmigiani1, Joseph Kakande1, Mathias Westlund2, Mats Sköld2, Lars Grüner-Nielsen3, Richard Phelan4, Periklis Petropoulos1, and David J. Richardson1 1. Optoelectronics Research Centre, University of Southampton, Southampton, SO17 1BJ, UK 2. EXFO Sweden AB, Arvid Hedvalls Backe 4, SE-41133 Göteborg, Sweden 3. OFS, Priorparken 680, 2605 Brøndby, Denmark 4. Eblana Photonics, Dublin, Ireland *r.slavik@soton.ac.uk  Abstract: More compact, stable, and efficient configuration of a recently-developed regenerator is presented. The regenerator is assessed at data rates up to 56 Gbit/s using white phase noise for the first time.    ©2011 Optical Society of America  OCIS codes: (060.2320) Fiber optics amplifiers and oscillators, (190.4410) Nonlinear optics, parametric processes   1. Introduction Phase noise introduced during transmission both from optical amplifiers and the nonlinear interactions between channels represents a significant limiting factor to data transmission using (differential) phase-shift keyed (D)PSK, modulation formats [1]. Consequently, there is great interest in developing all-optical regeneration schemes capable of  eliminating  phase  (and  ideally  additionally  amplitude)  noise  for  PSK  signals.  Phase  regeneration  can  be implemented directly by exploiting the phase-squeezing capability of phase sensitive amplifiers (PSAs) [2], and amplitude regeneration can be simultaneously performed by operating in the saturated regime. In order to achieve PSA, a phase relationship between the PSA pump(s), the signal and any idlers present needs to be maintained, which is challenging in practice. Recently, we demonstrated a scheme in which the carrier from a carrier-less PSK signal is recovered and then used to phase-lock the locally generated pumps with the incoming data prior to a degenerate PSA. In this way, a “black-box” operation was obtained [3]. In this scheme, a fraction of the incoming data stream was tapped off and this tapped signal was used for phase locking of the locally-generated pumps. The pump phase locking building block, however, included a relatively long (300 m) highly nonlinear fiber (HNLF), which made the unit susceptible to acoustic/thermal pick up capable of causing strong variations of the phase (more than 2p) of the locked pumps with respect to the original data. Although it proved possible to compensate for this using a relatively slow  (kHz  range)  feedback  loop  driving  a  piezo-transducer  (PZT)  in  the  relatively  ‘quiet’  conditions  in  our laboratory this would not be practical for real-world deployment. Here, we propose and experimentally demonstrate a new scheme that strongly reduces the effective fiber length over which acoustic/thermal pickup can influence the performance of the regenerator (potentially reducing it to hundreds  of  millimeters  rather  than  hundreds  of  meters).  Further,  we  assess  the  performance  of  the  device  at repetition rates up to 56 Gbit/s with broadband phase noise applied to the data signal rather than the deterministic phase perturbations at discrete frequencies as we did in our earlier work [3].  2. Principle of operation and experimental results Our setup is shown in Fig. 1. The non-return-to-zero (NRZ)-DPSK, 231-1 pseudo-random bit sequence (PRBS) was sent through a phase modulator to add the noise. The modulator was driven by a fast photodiode that was fed by ASE noise.  The distorted signal was then launched into the black-box regenerator that was made of polarisation maintaining (PM) components other than the part operating in reflection (where the polarization was inherently preserved as a result of the bidirectional propagation) and the HNLF used for PSA (HNLF 2). Rather than tapping off  the data signal  for the synchronization  of the pumps as  previously,  the whole data  signal  was used  in the synchronization process. First, the signal was combined with a CW local pump (‘Pump 1’, its frequency was 200 GHz apart from the data carrier frequency) using an add-multiplexer (data insertion loss ~0.5 dB) and then sent to a PM-HNLF (HNLF 1) to generate an idler wave that was inherently phase-locked to the data and Pump 1. The length, dispersion, nonlinear coefficient and insertion loss of HNLF 1 were 300 m, 1.3 ps/nm/km, 10.5 W-1km-1 and 0.9 dB, respectively. Note that due to the phase erasure process, the binary data modulation was not transferred to the idler [4]. Then, the three signals (Data, Pump 1, Idler) were demultiplexed in a 4-channel 200-GHz demux (insertion loss ~1dB) placed behind a circulator. A mirror provided retro-reflection in the data path as well in the path for Pump 1 which also included a PZT stretcher (60 mm total length, maximum phase change of 5p) and a variable attenuator. A semiconductor laser [5] which was injection locked to the idler, was used to generate Pump 2        OMT2.pdf   OSA/OFC/NFOEC 2011       OMT2.pdf©Optical Society of Americain the idler path. Pump 2 was thus phase-locked to the idler (and thus also to the data and Pump 1). Note that since the injection locking is a much slower process than four wave mixing (having a (sub)-GHz bandwidth) then 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 slave laser [4].  The operation of the demux in the double-pass (reflection) mode has several advantages:  (i)  it  allows  for  direct  injection  locking  of  the  semiconductor  laser,  (ii)  it  improves  the  filtering characteristics of the demux which is essential in the following PSA stage, and (iii) all the components operating in reflection  (WDM  demux,  PZT  transducer,  attenuator,  fiber-coupled  mirrors)  do  not  need  to  be  polarization maintaining  in  order  to  obtain  stable  polarization  operation.  Note  that  all  the  optical  waves  involved  in  the regeneration process share a common path through the regenerator other than on the output side of the demux. The lengths of fiber at the demux output ports can however be kept short (<2 m in this case, although this could be reduced to  tens of  cm if needed) and  all can be kept in  close proximity during packaging  to ensure that  they experience similar acoustic/thermal pickup further reducing the environmental sensitivity of the system.  DataPump 1(cw)HNLF 1AddPump 1DataPump 1DataIdler(DE)MUXPZTAttenuatorMirrorPump 1Slave laser(Injection locked)IdlerPump 2DataPump 1 Pump 2DataEDFAHNLF 2PSAOUTDataPump 2 Pump 1DEMUXDataPump 1Pump 2Detector231-1  3140 or 56 Gbit/sBroadbandnoiseWhite noise40 GHzPhasemodulatorRegenerator(black-box)INFeedback controlDPSKTransmitterDifferentpathsboxMirrorPol. controller Figure 1: Experimental set-up.   The data stream with the two properly locked pumps was then launched into the PSA which consisted of a high-power EDFA (total power of 29 dBm) and a 180-m sample of a high-SBS-threshold HNLF [5]. HNLF2 had an alumino-silicate core and had a linear strain gradient along its length (ranging from 400-20 g). The dispersion, polarization mode dispersion, nonlinear coefficient and attenuation of the fiber were -0.17 ps/nm/km, 0.35 pskm-0.5, 7.5 W-1km-1 and 15 dB/km, respectively. The relative powers of the pumps and signals were adjusted for optimal regeneration  performance  (using  the  attenuator  and  the  bias current  of  the  slave  laser  in  pump paths  1  and 2 respectively). For optimum performance the PSA was operated in deep saturation resulting in a strong variation of pump power at the PSA output as a function of the relative phase between the pump and signal beams. We used this feature to control the PZT fiber stretcher (i.e. the minimum of the pump power corresponds to the maximum PSA gain of the data, which is the correct operation point of the regeneration). No sign of acoustic pick-up was observed in the output signal and only low bandwidth control (Hz-range) was needed to compensate for thermal drift. Just <5p of continuous phase adjustment was required to keep the system locked for extended periods of time.  The performance of the regenerator was first studied using an EXFO constellation analyzer (PSO-200) based on all-optical sampling and capable of operation up to 100 Gbaud, Fig.2. The results at 40 Gbit/s and 56 Gbit/s (the maximum data rate of our transmitter) show that the phase noise can be significantly squeezed by the regenerator with negligible induced amplitude noise and also illustrate the bit rate transparency of our regenerator. We measured the bit error ratio (BER) at 40 Gbit/s (the only modulation speed for which a differential receiver was available) at the input and output of the regenerator for various levels of phase noise, see corresponding measurements in Fig. 3. The receiver consisted of a bandpass filter (0.5 nm bandwidth measured at -0.5 dB), attenuator (the received power was  measured  at  its  output),  low-noise  EDFA,  a  bandpass  filter,  a  1-bit  delay  interferometer  (DLI),  and  a photodetector receiving average power of 7 dBm. In Fig. 3a the level of added noise is such that the regenerator is fully capable of restoring the data fidelity despite the fact that the noisy data exhibited an error floor close to 10-7. In Fig. 3b we show the same data for the noise level used in Fig. 2. In this case, the regenerator fully restores the signal fidelity up to BER 10-4 (well within the FEC limit), whilst the non-regenerated data suffers from a power penalty of 9 dB. Analyzing Fig. 3 it can be concluded that the regenerator actually improves BER, which is counter-intuitive, as regenerators typically do not improve BER when placed in front of a receiver. This phenomenon is due to a correctable class of errors specific to a ‘differential’ receiver – e.g., given two consecutive bits with phases +60o and -60o the signal after a DLI would represent a (differential) phase error of 120o  clearly resulting in an erroneous detection, while the regenerator can restore them both back to 0o  leading to a correctly detected 0 o phase difference.         OMT2.pdf   OSA/OFC/NFOEC 2011       OMT2.pdfGbit/s Back-to-back   Regenerated  Not regenerated     40         56       Figure  2:  Constellation  diagrams  for  back-to-back  and  for  added  noise  level:  2  for  two  data  rates  and  the  corresponding demodulated eyes for 40 Gbit/s.   -40 -38 -36 -34 -32 -30 -28 -26 -241098765432Noise OFF: Regen. output Regen. input (B2B)Noise ON : Regen. output Regen. input-log(BER)Received power, dBm(a) Noise level: 1 -40 -38 -36 -34 -32 -30 -28 -26 -241098765432B2BNoise ON: Regen. output Regen. input-log(BER)Received power, dBm(b) Noise level: 2 Figure 3: BER curves at the input (open) and output (full) of the regenerator for no added phase noise (black) and two levels of phase noise added (red): (a) noise level 1, (b) noise level: 2 (corresponding to data shown in Fig. 2).  4. Conclusions We have proposed and experimentally demonstrated an improved, compact and more environmentally stable black-box  PSK  regenerator  using  degenerate  FWM  in  a  high  SBS-threshold  highly  nonlinear  fiber.  We  tested  the regenerator at data rates up to 56 Gbit/s using – for the first time - white phase noise as would be present in a real fiber network. For moderate levels of phase noise, the regenerator was able to fully restore the signal fidelity (down to a BER of 10-9), while the non-regenerated data showed a floor at a BER of 6×10-6.  For a high level of phase noise, the regenerator fully restored the fidelity of the data (with no power penalty) down to the 10-4 BER level, where it gave sensitivity improvement of 9 dB as compared to the detection of the non-regenerated data.   This research has received funding from the European Communities Seventh Framework Programme FP/2007-2013 under grant agreement 224547 (PHASORS).  6. References [1]  J. P. Gordon, and L. F. Mollenauer, “Phase noise in photonic communications systems using linear amplifiers,” Opt. Lett. 15(23), 1351–1353 (1990).  [2]  K. Croussore, et al., “Demonstration of phase-regeneration of DPSK signals based on phase-sensitive amplification,” Opt. Express, 13, 3945-3950 (2005). [3]  R. Slavík et al, “All-optical phase and amplitude regenerator for next-generation telecommunications systems,” Nature Photonics 4 , 690–695 (2010). [4]  R. Weerasuriya, et al., “Generation of frequency symmetric signals from a BPSK input for Phase Sensitive Amplification”, OWT6, paper 1928, OFC 2010.  [5]  R. Phelan, B. Kelly, J. O’Carroll, C. Herbert, A. Duke and J. O’Gorman, “- 40 oC < T < 95 oC mode-hop free operation of an uncooled [5]   AlGaInAs-MQW discrete-mode laser diode with emission at l = 1.3 mm.” Electron. Lett. 45, 43-45 (2009). [6]  L. Grüner-Nielsen  et  al., “A Silica Based Highly  Nonlinear  Fibre with Improved  Threshold  for Stimulated Brillouin  Scattering,”  paper Tu.4.D.3, ECOC 2010.        OMT2.pdf   OSA/OFC/NFOEC 2011       OMT2.pdf

A Silica Based Highly Nonlinear Fibre with Improved Threshold for Stimulated Brillouin Scattering,” paper

All-optical phase and amplitude regenerator for next-generation telecommunications systems,”

Demonstration of phase-regeneration of DPSK signals based on phase-sensitive amplification,”

Generation of frequency symmetric signals from a BPSK input for Phase Sensitive Amplification”, OWT6, paper

oC mode-hop free operation of an uncooled [5] AlGaInAs-MQW discrete-mode laser diode with emission at l

Phase noise in photonic communications systems using linear amplifiers,”

Robust design of all-optical PSK regenerator based on phase sensitive amplification

Robust design of all-optical PSK regenerator based on phase sensitive amplification

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