Improved WDM performance of a fibre optical parametric amplifier using Raman-assisted pumping M.F.C. Stephens,1,* I.D. Phillips,1 P. Rosa,1 P. Harper,1 and N.J. Doran1 1
Aston Institute of Photonic Technologies, Aston University, Aston Triangle, Birmingham B4 7ET, UK * [email protected]
Abstract: We experimentally demonstrate a Raman-Assisted Fibre Optical Parametric Amplifier (RA-FOPA) with 20dB net gain using wavelength division multiplexed signals. We report amplification of 10x58Gb/s 100GHz-spaced QPSK signals and show that by appropriate tuning of the parametric pump power and frequency, gain improvement of up to 5dB can be achieved for the RA-FOPA compared with combined individual contributions from the parametric and Raman pumps. We compare the RAFOPA with an equivalent-gain conventional FOPA and find that four-wave mixing crosstalk is substantially reduced by up to 5.8 ± 0.4dB using the RA-FOPA. Worst-case performance penalty of the RA-FOPA is found to be only 1.0 ± 0.2dB over all measured OSNRs, frequencies and input powers, making it an attractive proposal for future communications systems. ©2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (190.4970) Parametric oscillators and amplifiers; (190.4380) Nonlinear optics, four-wave mixing.
References and links 1. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
R. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity limits of optical fiber networks,” J. Lightwave Technol. 28(4), 662–701 (2010). Z. Li, S. U. Alam, J. M. O. Daniel, P. C. Shardlow, D. Jain, N. Simakov, A. M. Heidt, Y. Jung, J. K. Sahu, W. A. Clarkson, and D. J. Richardson, “90 nm gain extension towards 1.7 μm for diode-pumped silica-based thulium-doped fiber amplifiers,“ in European Conference and Exhibition on Optical Communication (ECOC 2014), paper Tu.3.4.2. R. H. Stolen and E. P. Ippen, “Raman gain in glass optical waveguides,” Appl. Phys. Lett. 22(6), 276–278 (1973). V. E. Perlin and H. G. Winful, “Optimal design of flat-gain wide-band fiber Raman amplifiers,” J. Lightwave Technol. 20(2), 250–254 (2002). M. N. Islam, Raman amplifiers for telecommunications, 1: physical principles (Springer, 2004). P. B. Hansen, L. Eskildsen, A. J. Stentz, T. A. Strasser, J. Judkins, J. J. DeMarco, R. Pedrazzani, and D. J. DiGiovanni, “Rayleigh scattering limitations in distributed Raman pre-amplifiers,” IEEE Photon. Technol. Lett. 10(1), 159–161 (1998). M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge, 2008). T. Torounidis, P. A. Andrekson, and B. E. Olsson, “Fibre-optical parametric amplifier with 70-dB gain,” IEEE Photon. Technol. Lett. 18(10), 1194–1196 (2006). M. E. Marhic, K. Y. K. Wong, and L. G. Kazovsky, “Wideband tuning of the gain spectra of one-pump fiber optical parametric amplifiers,” IEEE J. Sel. Top. Quantum Electron. 10(5), 1133–1141 (2004). I. Sackey, F. Da Ros, T. Richter, R. Elschner, M. Jazayerifar, C. Meuer, C. Peucheret, K. Petermann, and C. Schubert, “Design and performance evaluation of an OPC device using a dual-pump polarization-independent FOPA,” in European Conference and Exhibition on Optical Communication (ECOC 2014), paper Tu.1.4.4. J. M. Chavez Boggio, A. Guimarães, F. A. Callegari, J. D. Marconi, and H. L. Fragnito, “Q penalties due to pump phase modulation and pump RIN in fiber optic parametric amplifiers with non-uniform dispersion,” Opt. Commun. 249(4), 451–472 (2005). Á. Szabó, B. J. Puttnam, D. Mazroa, A. Albuquerque, S. Shinada, and N. Wada, “Numerical comparison of WDM interchannel crosstalk in FOPA and PPLN-based PSAs,” IEEE Photon. Technol. Lett. 26(15), 1503–1506 (2014). C. J. S. de Matos, D. A. Chestnut, P. C. Reeves-Hall, and J. R. Taylor, “Continuous-wave-pumped Ramanassisted fiber optical parametric amplifier and wavelength converter in conventional dispersion-shifted fiber,” Opt. Lett. 26(20), 1583–1585 (2001).
#229416 - $15.00 USD © 2015 OSA
Received 9 Dec 2014; revised 8 Jan 2015; accepted 9 Jan 2015; published 14 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.000902 | OPTICS EXPRESS 902
14. S. H. Wang, L. Xu, P. K. A. Wai, and H. Y. Tam, “6.4-dB enhancement of the gain of a Raman-assisted fiber optical parametric amplifier over the sum of the gains of individual amplifiers,” in Optical Fiber Communication Conference (OFC 2008), paper JThA13. 15. X. Guo, X. Fu, and C. Shu, “Gain saturation in a Raman-assisted fiber optical parametric amplifier,” Opt. Lett. 38(21), 4405–4408 (2013). 16. M. Jamshidifar, A. Vedadi, and M. E. Marhic, “Reduction of Four-Wave-Mixing Crosstalk in a Short FiberOptical Parametric Amplifier,” IEEE Photon. Technol. Lett. 21(17), 1244–1246 (2009). 17. S. K. Korotky, P. B. Hansen, L. Eskildsen, and J. J. Veselka, “Efficient phase modulation scheme for suppressing stimulated Brillouin scattering,”in Tech. Dig. Int. Conf. Integrated Optics and Optical Fiber Communications 2, 110–111, Paper WD2–1 (1995). 18. A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory 29(4), 543–551 (1983). 19. R. Schmogrow, B. Nebendahl, M. Winter, A. Josten, D. Hillerkuss, S. Koenig, J. Meyer, M. Dreschmann, M. Huebner, C. Koos, J. Becker, W. Freude, and J. Leuthold, “Error vector magnitude as a performance measure for advanced modulation formats,” IEEE Photon. Technol. Lett. 24(1), 61–63 (2012). 20. F. Yaman, Q. Lin, S. Radic, and G. P. Agrawal, “Impact of pump-phase modulation on dual-pump fiber-optic parametric amplifiers and wavelength converters,” IEEE Photon. Technol. Lett. 17(10), 2053–2055 (2005). 21. A. Mussot, A. Durécu-Legrand, E. Lantz, C. Simonneau, D. Bayart, H. Maillotte, and T. Sylvestre, “Impact of pump phase modulation on the gain of fiber optical parametric amplifier,” IEEE Photon. Technol. Lett. 16(5), 1289–1291 (2004). 22. D. Derickson, Fiber Optic Test and Measurement (Prentice Hall, 1998).
1. Introduction The continuing increase of fibre optical signal data rates and spectral density is resulting in systems rapidly approaching capacity limits for those windows of the spectrum where low transmission loss and easily-accessible optical gain co-exist (i.e. the ‘C’ and ‘L’ bands) [1]. Commercial optical amplification technology in these bands has been dominated for >30 years by the erbium doped fibre amplifier (EDFA) which limits gain in wavelength division multiplexed (WDM) systems to a combined C/L bandwidth of ~10THz (80nm). In order to help further increase the information-carrying capacity of a single mode optical fibre, research is ongoing into alternative fibre-based optical amplifiers which can provide useable gain in other spectral regions as well as traditional C/L bands. One research approach is the use of fibre dopants other than erbium to provide the gain – for example thulium has recently been demonstrated within a silica-based host fibre to provide >20dB gain between 1750 and 1950nm [2]. These amplifiers show very promising performance, in particular a significant improvement in operating bandwidth, but lack some potentially useful features such as the ability to tune the operating wavelength region. Another approach is the (discrete or distributed) Raman amplifier which can provide polarisation-independent gain across a bandwidth of ~3THz using a single depolarised pump, and which provides a gain spectrum position defined purely by the pump frequency [3]. The Raman gain spectrum can easily be extended in range by adding additional frequency-offset pump lasers, but at the expense of pump-pump interactions requiring a complex pump control algorithm to obtain flat gain [4]. Other limitations of the Raman amplifier include the comparatively low Raman gain coefficient which restricts the magnitude of the gain to 1214dB in standard single mode fibre (assuming distributed amplification) [5], and the effect of double Rayleigh backscatter which causes a multi-path interference degradation of signal quality [6]. The Fibre Optical Parametric Amplifier (FOPA) is another actively-researched amplification scheme which employs a phase-matched four-wave mixing process between pump(s) and signal(s) in a highly nonlinear fibre (HNLF) to provide gain more efficiently than via the Raman effect [7]. By careful combination of the HNLF properties (dispersion profile, nonlinear parameter and length) together with pump power and pump frequency tuning, gains as high as 70dB and gain bandwidths of ~200nm have been demonstrated [8, 9]. The FOPA has also been shown operating in polarisation diverse arrangements which has allowed amplification of polarisation-multiplexed signals [10]. However, FOPA performance is also highly dependent on pump quality and can be compromised via instantaneous transfer of pump fluctuations such as relative intensity noise (RIN) to the amplified signals [11], as well as by the generation of detrimental four-wave mixing crosstalk terms when amplifying multiple WDM channels [12].
#229416 - $15.00 USD © 2015 OSA
Received 9 Dec 2014; revised 8 Jan 2015; accepted 9 Jan 2015; published 14 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.000902 | OPTICS EXPRESS 903
Recently, the Raman-Assisted (RA)-FOPA has been proposed and studied experimentally, analytically and numerically by a number of groups in order to potentially employ the best properties of both schemes. The RA-FOPA consists of a forward travelling parametric pump within an HNLF which is ‘augmented’ by a backward travelling Raman pump, usually in the same fibre [13]. The Raman pump provides gain not only to the signals but also to the parametric pump and, through careful selection of their respective frequencies, can widen/flatten the gain spectrum as well as provide enhanced gain over and above the linear sum of the individual contributions in certain regions of the spectrum [13,14]. To date, RA-FOPAs have typically been experimentally characterised using a single continuous wave (CW) probe to measure the gain and noise figure in long lengths of HNLF (~1km). A notable exception, where the performance was examined using two 10Gb/s nonreturn-to-zero (NRZ) data signals, showed a reduced susceptibility to signal gain saturation [15]. It is recognized that the long HNLF length helps maximize the Raman gain, but at the expense of an increased interaction length, resulting in buildup of unwanted four-wave mixing crosstalk products [16]. In this paper we experimentally investigate for the first time the performance of a RAFOPA when amplifying 10x58Gb/s 100GHz-spaced QPSK signals for an average net gain of ~20dB over the band, split approximately equally between Raman and parametric contributions. We use a short (200m) length of HNLF and compare the RA-FOPA performance directly with a standard FOPA at the same level of gain and find that the RAFOPA offers significantly reduced inter-channel crosstalk levels and lower performance penalty for the same received optical signal to noise ratio (OSNR). 2. Experimental set-up The experimental set-up is shown in Fig. 1. The transmitter consisted of ten 100GHz-spaced standard distributed feedback (DFB) lasers ranging from 193.5 to 194.4THz and multiplexed together in a polarisation-maintaining arrayed waveguide grating (PM-AWG). The DFB frequencies were combined with a 100kHz-linewidth laser which was used as the signal under test whereby the relevant DFB laser could be turned off and the 100kHz laser tuned to fill the spectral gap. The ten continuous wave (CW) frequencies were IQ-modulated in a nested Mach-Zehnder modulator using two 29Gb/s decorrelated 231-1 bit streams, producing 10x58Gb/s single polarisation QPSK signals. The signals were transmitted through 5.4km of standard single-mode fibre (SSMF) to provide data decorrelation. The per-signal input power was adjusted using a variable optical attenuator (VOA) to either −20dBm or −12dBm, measured at the output of the VOA (indicated as point A). This was considered to be the input to the black-box RA-FOPA subsystem. The RA-FOPA subsystem consisted of a parametric pump (PP) from a 100kHz CW laser emitting at 191.57THz (fP) which was spectrally broadened using a lithium niobate phase modulator (Vπ = 5.2V) to provide mitigation against stimulated Brillouin scattering (SBS) [17]. The electrical drive to the phase modulator was a comb of three electrical tones at 100MHZ, 320MHz and 980MHz. The PP was amplified using a high-power EDFA and optically filtered to remove amplified spontaneous emission (ASE) using a circulator and reflective apodised fibre Bragg grating (FBG) centred at fP and with a 3dB bandwidth of 112GHz. The QPSK signals were combined with the PP by transmitting the signals through the FBG, which had
Aston Institute of Photonic Technologies, Aston University, Aston Triangle, Birmingham B4 7ET, UK * [email protected]
Abstract: We experimentally demonstrate a Raman-Assisted Fibre Optical Parametric Amplifier (RA-FOPA) with 20dB net gain using wavelength division multiplexed signals. We report amplification of 10x58Gb/s 100GHz-spaced QPSK signals and show that by appropriate tuning of the parametric pump power and frequency, gain improvement of up to 5dB can be achieved for the RA-FOPA compared with combined individual contributions from the parametric and Raman pumps. We compare the RAFOPA with an equivalent-gain conventional FOPA and find that four-wave mixing crosstalk is substantially reduced by up to 5.8 ± 0.4dB using the RA-FOPA. Worst-case performance penalty of the RA-FOPA is found to be only 1.0 ± 0.2dB over all measured OSNRs, frequencies and input powers, making it an attractive proposal for future communications systems. ©2015 Optical Society of America OCIS codes: (060.2320) Fiber optics amplifiers and oscillators; (190.4970) Parametric oscillators and amplifiers; (190.4380) Nonlinear optics, four-wave mixing.
References and links 1. 2.
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
R. Essiambre, G. Kramer, P. J. Winzer, G. J. Foschini, and B. Goebel, “Capacity limits of optical fiber networks,” J. Lightwave Technol. 28(4), 662–701 (2010). Z. Li, S. U. Alam, J. M. O. Daniel, P. C. Shardlow, D. Jain, N. Simakov, A. M. Heidt, Y. Jung, J. K. Sahu, W. A. Clarkson, and D. J. Richardson, “90 nm gain extension towards 1.7 μm for diode-pumped silica-based thulium-doped fiber amplifiers,“ in European Conference and Exhibition on Optical Communication (ECOC 2014), paper Tu.3.4.2. R. H. Stolen and E. P. Ippen, “Raman gain in glass optical waveguides,” Appl. Phys. Lett. 22(6), 276–278 (1973). V. E. Perlin and H. G. Winful, “Optimal design of flat-gain wide-band fiber Raman amplifiers,” J. Lightwave Technol. 20(2), 250–254 (2002). M. N. Islam, Raman amplifiers for telecommunications, 1: physical principles (Springer, 2004). P. B. Hansen, L. Eskildsen, A. J. Stentz, T. A. Strasser, J. Judkins, J. J. DeMarco, R. Pedrazzani, and D. J. DiGiovanni, “Rayleigh scattering limitations in distributed Raman pre-amplifiers,” IEEE Photon. Technol. Lett. 10(1), 159–161 (1998). M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge, 2008). T. Torounidis, P. A. Andrekson, and B. E. Olsson, “Fibre-optical parametric amplifier with 70-dB gain,” IEEE Photon. Technol. Lett. 18(10), 1194–1196 (2006). M. E. Marhic, K. Y. K. Wong, and L. G. Kazovsky, “Wideband tuning of the gain spectra of one-pump fiber optical parametric amplifiers,” IEEE J. Sel. Top. Quantum Electron. 10(5), 1133–1141 (2004). I. Sackey, F. Da Ros, T. Richter, R. Elschner, M. Jazayerifar, C. Meuer, C. Peucheret, K. Petermann, and C. Schubert, “Design and performance evaluation of an OPC device using a dual-pump polarization-independent FOPA,” in European Conference and Exhibition on Optical Communication (ECOC 2014), paper Tu.1.4.4. J. M. Chavez Boggio, A. Guimarães, F. A. Callegari, J. D. Marconi, and H. L. Fragnito, “Q penalties due to pump phase modulation and pump RIN in fiber optic parametric amplifiers with non-uniform dispersion,” Opt. Commun. 249(4), 451–472 (2005). Á. Szabó, B. J. Puttnam, D. Mazroa, A. Albuquerque, S. Shinada, and N. Wada, “Numerical comparison of WDM interchannel crosstalk in FOPA and PPLN-based PSAs,” IEEE Photon. Technol. Lett. 26(15), 1503–1506 (2014). C. J. S. de Matos, D. A. Chestnut, P. C. Reeves-Hall, and J. R. Taylor, “Continuous-wave-pumped Ramanassisted fiber optical parametric amplifier and wavelength converter in conventional dispersion-shifted fiber,” Opt. Lett. 26(20), 1583–1585 (2001).
#229416 - $15.00 USD © 2015 OSA
Received 9 Dec 2014; revised 8 Jan 2015; accepted 9 Jan 2015; published 14 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.000902 | OPTICS EXPRESS 902
14. S. H. Wang, L. Xu, P. K. A. Wai, and H. Y. Tam, “6.4-dB enhancement of the gain of a Raman-assisted fiber optical parametric amplifier over the sum of the gains of individual amplifiers,” in Optical Fiber Communication Conference (OFC 2008), paper JThA13. 15. X. Guo, X. Fu, and C. Shu, “Gain saturation in a Raman-assisted fiber optical parametric amplifier,” Opt. Lett. 38(21), 4405–4408 (2013). 16. M. Jamshidifar, A. Vedadi, and M. E. Marhic, “Reduction of Four-Wave-Mixing Crosstalk in a Short FiberOptical Parametric Amplifier,” IEEE Photon. Technol. Lett. 21(17), 1244–1246 (2009). 17. S. K. Korotky, P. B. Hansen, L. Eskildsen, and J. J. Veselka, “Efficient phase modulation scheme for suppressing stimulated Brillouin scattering,”in Tech. Dig. Int. Conf. Integrated Optics and Optical Fiber Communications 2, 110–111, Paper WD2–1 (1995). 18. A. J. Viterbi and A. M. Viterbi, “Nonlinear estimation of PSK-modulated carrier phase with application to burst digital transmission,” IEEE Trans. Inf. Theory 29(4), 543–551 (1983). 19. R. Schmogrow, B. Nebendahl, M. Winter, A. Josten, D. Hillerkuss, S. Koenig, J. Meyer, M. Dreschmann, M. Huebner, C. Koos, J. Becker, W. Freude, and J. Leuthold, “Error vector magnitude as a performance measure for advanced modulation formats,” IEEE Photon. Technol. Lett. 24(1), 61–63 (2012). 20. F. Yaman, Q. Lin, S. Radic, and G. P. Agrawal, “Impact of pump-phase modulation on dual-pump fiber-optic parametric amplifiers and wavelength converters,” IEEE Photon. Technol. Lett. 17(10), 2053–2055 (2005). 21. A. Mussot, A. Durécu-Legrand, E. Lantz, C. Simonneau, D. Bayart, H. Maillotte, and T. Sylvestre, “Impact of pump phase modulation on the gain of fiber optical parametric amplifier,” IEEE Photon. Technol. Lett. 16(5), 1289–1291 (2004). 22. D. Derickson, Fiber Optic Test and Measurement (Prentice Hall, 1998).
1. Introduction The continuing increase of fibre optical signal data rates and spectral density is resulting in systems rapidly approaching capacity limits for those windows of the spectrum where low transmission loss and easily-accessible optical gain co-exist (i.e. the ‘C’ and ‘L’ bands) [1]. Commercial optical amplification technology in these bands has been dominated for >30 years by the erbium doped fibre amplifier (EDFA) which limits gain in wavelength division multiplexed (WDM) systems to a combined C/L bandwidth of ~10THz (80nm). In order to help further increase the information-carrying capacity of a single mode optical fibre, research is ongoing into alternative fibre-based optical amplifiers which can provide useable gain in other spectral regions as well as traditional C/L bands. One research approach is the use of fibre dopants other than erbium to provide the gain – for example thulium has recently been demonstrated within a silica-based host fibre to provide >20dB gain between 1750 and 1950nm [2]. These amplifiers show very promising performance, in particular a significant improvement in operating bandwidth, but lack some potentially useful features such as the ability to tune the operating wavelength region. Another approach is the (discrete or distributed) Raman amplifier which can provide polarisation-independent gain across a bandwidth of ~3THz using a single depolarised pump, and which provides a gain spectrum position defined purely by the pump frequency [3]. The Raman gain spectrum can easily be extended in range by adding additional frequency-offset pump lasers, but at the expense of pump-pump interactions requiring a complex pump control algorithm to obtain flat gain [4]. Other limitations of the Raman amplifier include the comparatively low Raman gain coefficient which restricts the magnitude of the gain to 1214dB in standard single mode fibre (assuming distributed amplification) [5], and the effect of double Rayleigh backscatter which causes a multi-path interference degradation of signal quality [6]. The Fibre Optical Parametric Amplifier (FOPA) is another actively-researched amplification scheme which employs a phase-matched four-wave mixing process between pump(s) and signal(s) in a highly nonlinear fibre (HNLF) to provide gain more efficiently than via the Raman effect [7]. By careful combination of the HNLF properties (dispersion profile, nonlinear parameter and length) together with pump power and pump frequency tuning, gains as high as 70dB and gain bandwidths of ~200nm have been demonstrated [8, 9]. The FOPA has also been shown operating in polarisation diverse arrangements which has allowed amplification of polarisation-multiplexed signals [10]. However, FOPA performance is also highly dependent on pump quality and can be compromised via instantaneous transfer of pump fluctuations such as relative intensity noise (RIN) to the amplified signals [11], as well as by the generation of detrimental four-wave mixing crosstalk terms when amplifying multiple WDM channels [12].
#229416 - $15.00 USD © 2015 OSA
Received 9 Dec 2014; revised 8 Jan 2015; accepted 9 Jan 2015; published 14 Jan 2015 26 Jan 2015 | Vol. 23, No. 2 | DOI:10.1364/OE.23.000902 | OPTICS EXPRESS 903
Recently, the Raman-Assisted (RA)-FOPA has been proposed and studied experimentally, analytically and numerically by a number of groups in order to potentially employ the best properties of both schemes. The RA-FOPA consists of a forward travelling parametric pump within an HNLF which is ‘augmented’ by a backward travelling Raman pump, usually in the same fibre [13]. The Raman pump provides gain not only to the signals but also to the parametric pump and, through careful selection of their respective frequencies, can widen/flatten the gain spectrum as well as provide enhanced gain over and above the linear sum of the individual contributions in certain regions of the spectrum [13,14]. To date, RA-FOPAs have typically been experimentally characterised using a single continuous wave (CW) probe to measure the gain and noise figure in long lengths of HNLF (~1km). A notable exception, where the performance was examined using two 10Gb/s nonreturn-to-zero (NRZ) data signals, showed a reduced susceptibility to signal gain saturation [15]. It is recognized that the long HNLF length helps maximize the Raman gain, but at the expense of an increased interaction length, resulting in buildup of unwanted four-wave mixing crosstalk products [16]. In this paper we experimentally investigate for the first time the performance of a RAFOPA when amplifying 10x58Gb/s 100GHz-spaced QPSK signals for an average net gain of ~20dB over the band, split approximately equally between Raman and parametric contributions. We use a short (200m) length of HNLF and compare the RA-FOPA performance directly with a standard FOPA at the same level of gain and find that the RAFOPA offers significantly reduced inter-channel crosstalk levels and lower performance penalty for the same received optical signal to noise ratio (OSNR). 2. Experimental set-up The experimental set-up is shown in Fig. 1. The transmitter consisted of ten 100GHz-spaced standard distributed feedback (DFB) lasers ranging from 193.5 to 194.4THz and multiplexed together in a polarisation-maintaining arrayed waveguide grating (PM-AWG). The DFB frequencies were combined with a 100kHz-linewidth laser which was used as the signal under test whereby the relevant DFB laser could be turned off and the 100kHz laser tuned to fill the spectral gap. The ten continuous wave (CW) frequencies were IQ-modulated in a nested Mach-Zehnder modulator using two 29Gb/s decorrelated 231-1 bit streams, producing 10x58Gb/s single polarisation QPSK signals. The signals were transmitted through 5.4km of standard single-mode fibre (SSMF) to provide data decorrelation. The per-signal input power was adjusted using a variable optical attenuator (VOA) to either −20dBm or −12dBm, measured at the output of the VOA (indicated as point A). This was considered to be the input to the black-box RA-FOPA subsystem. The RA-FOPA subsystem consisted of a parametric pump (PP) from a 100kHz CW laser emitting at 191.57THz (fP) which was spectrally broadened using a lithium niobate phase modulator (Vπ = 5.2V) to provide mitigation against stimulated Brillouin scattering (SBS) [17]. The electrical drive to the phase modulator was a comb of three electrical tones at 100MHZ, 320MHz and 980MHz. The PP was amplified using a high-power EDFA and optically filtered to remove amplified spontaneous emission (ASE) using a circulator and reflective apodised fibre Bragg grating (FBG) centred at fP and with a 3dB bandwidth of 112GHz. The QPSK signals were combined with the PP by transmitting the signals through the FBG, which had
