Amplification of DWDM channels at 1.28 Tb/s in a bidirectional fiber optical parametric amplifier Gordon K. P. Lei* and Michel E. Marhic College of Engineering, Swansea University, Singleton Park, Swansea, SA2 8PP, Wales, UK * [email protected]
Abstract: We experimentally demonstrate amplification of 1.28 Tb/s DWDM channels using a bidirectional fiber optical parametric amplifier. The amplifier can provide more than 13 dB on-off gain on all 32 DWDM channels. Error-free operation has been achieved for all data streams, with an average power penalty of 2.5 dB compared with conventional unidirectional configuration. ©2014 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (060.2320) Fiber optics amplifiers and oscillators.
References and links 1. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
16.
M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge University, 2007). N. El Dahdah, D. S. Govan, M. Jamshidifar, N. J. Doran, and M. E. Marhic, “Fiber optical parametric amplifier performance in a 1-Tb/s DWDM communication system,” IEEE J. Sel. Top. Quantum Electron. 18(2), 950–957 (2012). M. Jazayerifar, S. Warm, R. Elschner, D. Kroushkov, I. Sackey, C. Meuer, C. Schubert, and K. Petermann, “Performance evaluation of DWDM communication systems with fiber optical parametric ampliers,” J. Lightwave Technol. 31(9), 1454–1461 (2013). W. Lee, M. Y. Park, S. H. Cho, J. Lee, C. Kim, G. Jeong, and B. W. Kim, “Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers,” IEEE Photon. Technol. Lett. 17(11), 2460–2462 (2005). J. Prat, C. Arellano, V. Polo, and C. Bock, “Optical network unit based on a bidirectional reflective semiconductor optical amplifier for fiber-to-the-home networks,” IEEE Photon. Technol. Lett. 17(1), 250–252 (2005). V. Mizrahi, “Bidirectional WDM optical communication systems with bidirectional optical amplifiers,” Ciena Corp., US Patent 5,742,416 (1998). S. Radic, S. Chandrasekhar, P. Bernasconi, J. Centanni, C. Abraham, N. Copner, and K. Tan, “Feasibility of hybrid Raman/EDFA amplification in bidirectional optical transmission,” IEEE Photon. Technol. Lett. 14(2), 221–223 (2002). I. Tafur Monroy, R. Kjær, F. Öhman, K. Yvind, and P. Jeppesen, “Distributed fiber Raman amplification in long reach PON bidirectional access links,” Opt. Fiber Technol. 14(1), 41–44 (2008). K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent one-pump fiberoptical parametric amplifier,” IEEE Photon. Technol. Lett. 14(11), 1506–1508 (2002). T. Torounidis, B.-E. Olsson, H. Sunnerud, M. Karlsson, and P. A. Andrekson, “Fiber-optic parametric amplifier in a loop mirror configuration,” IEEE Photon. Technol. Lett. 17(2), 321–323 (2005). G. Kalogerakis, M. E. Marhic, K. Uesaka, K. Shimizu, K. K.-Y. Wong, and L. G. Kazovsky, “Methods for full utilization of the bandwidth of fiber optical parametric amplifiers and wavelength converters,” J. Lightwave Technol. 24(10), 3683–3690 (2006). K. S. Yeo, F. R. M. Adikan, M. Mokhtar, S. Hitam, and M. A. Mahdi, “Fiber optical parametric amplifier with double-pass pump configuration,” Opt. Express 21(25), 31623–31631 (2013). E. Myslivets, N. Alic, S. Moro, B. P. P. Kuo, R. M. Jopson, C. J. McKinstrie, M. Karlsson, and S. Radic, “1.56micros continuously tunable parametric delay line for a 40-Gb/s signal,” Opt. Express 17(14), 11958–11964 (2009). B. P.-P. Kuo, E. Myslivets, A. O. J. Wiberg, S. Zlatanovic, C.-S. Brès, S. Moro, F. Gholami, A. Peric, N. Alic, and S. Radic, “Transmission of 640-Gb/s RZ-OOK channel over 100-km SSMF by wavelength-transparent conjugation,” J. Lightwave Technol. 29(4), 516–523 (2011). J. D. Marconi, J. M. Chavez Boggio, H. L. Fragnito, and S. R. Bickham, “Nearly 100 nm bandwidth of flat gain with a double-pumped fiber optic parametric amplifier,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper OWB1. K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent two-pump fiber optical parametric amplifier,” IEEE Photon. Technol. Lett. 14(7), 911–913 (2002).
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8726
1. Introduction Fiber optical parametric amplifiers (OPAs) have some properties which can potentially be exploited for improving the performance of optical communication systems [1], including: low noise figure (NF), which can be as low as 0 dB in phase-sensitive amplification; ability to perform wavelength conversion with high efficiency; broad gain bandwidth and unidirectional gain. With these interesting properties, it would be valuable to investigate the performance of OPAs in dense wavelength division multiplexing (DWDM) systems. Experimental study [2] has demonstrated amplification of >1 Tb/s DWDM channels in an OPA, while theoretical study [3] has shown that with optimized design and configuration, >10 Tb/s transmission can be achieved. For these demonstrations, only a single pump was used for amplification in one direction. It would also be interesting to investigate the performance of OPA when bidirectional pumps are used. Bidirectional amplifiers have found applications mainly in passive optical networks [4, 5] and bidirectional transmission link. Different schemes have been proposed to achieve bidirectional amplification, including bidirectional erbium-doped fiber amplifier (EDFA) [6], hybrid EDFA/Raman amplifier [7], bidirectional Raman amplifier [8] and reflective semiconductor optical amplifier [4]. OPAs can also operate in a bidirectional manner by pumping it in both directions, and this has been previously demonstrated in polarizationindependent OPA [9], OPA in loop mirror configuration for reduced loss [10] and full utilization of gain bandwidth by including filters in OPA configuration [11]. A recent study [12] has shown that with a Faraday mirror, double-pass OPA could be achieved with enhanced on-off gain. However, the additional signal gain bandwidth provided by the bidirectional pumping has not been fully explored yet. Here we present amplification of DWDM channels using a bidirectional OPA. We investigate in detail the penalties associated with bidirectional versus unidirectional propagation in a DWDM system, for the first time to our knowledge. First we demonstrate 20 × 40 Gb/s data channels (10 channels per direction) amplified by the bidirectional OPA with >10 dB on-off gain. Error-free operation has been achieved for all channels in the proposed bidirectional OPA. Compared with a unidirectional OPA, a maximum power penalty of 2 dB was achieved. Furthermore we demonstrated amplification of 32 × 40 Gb/s DWDM channels (16 channels per direction) with > 13 dB on-off gain, which is the largest channel count in a single OPA to date. An average power penalty of 2.5 dB has been achieved compared with conventional OPA. With access to the idler, the bidirectional OPA can act as both an amplifier and a wavelength converter in two directions, which could potentially be a costeffective device for optical communication systems. 2. Experimental setup First we describe the experiment for 20-channel amplification. The experimental setup is shown in Fig. 1. 10 continuous wave (CW) lasers are obtained from a distributed feedback (DFB) laser bank and combined by an arrayed waveguide grating (AWG). All channels are modulated in a Mach-Zehnder modulator with a 40 Gb/s 231-1 pseudorandom binary sequence (PRBS) to generate non-return-to-zero on-off keying (NRZ-OOK) signals. The channels are then de-correlated by a 1.5-km single-mode fiber and amplified by an erbium-doped fiber amplifier (EDFA). A variable optical attenuator (VOA) is used to control the input power to the OPA, while a 50/50 coupler is used to monitor the signal input power of the OPA. An optical isolator is used to block the backscattered stimulated Brillouin scattering (SBS) light.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8727
Fig. 1. Experimental setup for bidirectional OPA. AWG: arrayed waveguide grating; PRBS: pseudorandom binary sequence; SMF: single-mode fiber; EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; ISO: isolator; TLS: tunable laser source; PM: phase modulator; TBPF: tunable bandpass filter; HNLF: highly nonlinear fiber; OSA: optical spectrum analyser; PD: photodetector.
The OPA pump is generated by a tunable laser source at 1562 nm. It is then phasemodulated by 4 RF tones (100 MHz, 320 MHz, 980 MHz, and 2200 MHz) to increase the SBS threshold. The modulated pump is further amplified by another EDFA and the amplified spontaneous noise (ASE) is removed by a 1-nm tunable bandpass filter (TBPF). The pump and the 10 signal channels are then split by two separate 50/50 couplers. For each direction, the signals and the pump are combined using a 90/10 coupler. An optical circulator is used to direct the signals and pump to a 99/1 coupler. The 1% ports are used for monitoring the SBS power and total input power to the highly nonlinear fiber (HNLF) in backward and forward direction, respectively. Parametric amplification occurs inside the HNLF. The HNLF has a length of 340 m, zero dispersion at 1560 nm, nonlinearity coefficient of 15 W−1km−1, and dispersion slope of 0.023 ps nm−2 km−1. After parametric amplification, the signals and the pump are obtained through the port 3 of the circulator in the other direction (output1 and output 2 in Fig. 1). Each direction is investigated individually. For each output, 1% of the power is used for monitoring the signals on the optical spectrum analyzer (OSA). Individual data channels are filtered using a 0.6-nm flat-top TBPF. The signal is further amplified and filtered before detection by the photodetector. Signals are analyzed on oscilloscope and error analyzer. 3. Bidirectional OPA with 20 channels First we examine the gain profile for the bidirectional OPA. As a comparison, a unidirectional OPA is also included by removing the signals and pump in one of the directions. Table 1 shows the parameters of the two OPAs in both directions and their gain spectra are shown in Fig. 2. Table 1. Parameters of unidirectional and bidirectional OPAs in both directions. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA. Pump @ 1562 nm
D1_uni
D2_uni
D1_bi
D2_bi
Gain peak (nm)
1540.5
1540.2
1541.5
1542
3-dB bandwidth (nm)
21
21
26.5
24
Maximum gain (dB)
11.5
12.4
11.3
12.0
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8728
Fig. 2. Gain spectra for bidirectional and unidirectional OPAs in both directions. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA.
Although the bidirectional OPA has a larger 3-dB gain bandwidth, it exhibits a slightly smaller maximum gain for both directions compared with the unidirectional OPA; it also has an overall flatter gain spectrum as shown in Fig. 2. Furthermore it is observed that direction 2 has a higher gain for both configurations, which is caused by the slightly unbalanced pump powers in the two directions. The difference in gain profiles for the two configurations could be caused by self-phase modulation (SPM) of the counter-propagating pumps in the bidirectional case, by which the phase matching condition for the OPA could be altered. Figure 3 shows the 1% output optical spectrum from the bidirectional OPA in one of the directions. The signals are generated from 1537.4 nm to 1544.6 nm with 0.8 nm spacing, situating at the gain peak region of the bidirectional OPA. The intensity variation is caused by the non-flat gain spectrum and the difference in transmitter power. The intensity variation is < 2 dB among the channels, while the on-off gain ranges from 10.9 dB to 12.2 dB. Moreover, the idlers are generated in the L-band with conversion efficiency over 90%. The idler wavelength could be used for different applications including parametric tunable delay [13] and dispersion compensation with mid-span spectral inversion [14].
Fig. 3. Output optical spectrum from bidirectional OPA in one of the directions.
Fig. 4. Eye diagrams of selected channels amplified by either unidirectional or bidirectional OPA in direction 1.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8729
Figure 4 shows the eye diagrams of three selected channels (Ch.1: 1537.4 nm; Ch.5: 1540.6 nm; Ch.10: 1544.6 nm) amplified by either unidirectional or bidirectional OPA in the same direction as Fig. 3. Clear eye diagrams have been achieved for both configurations. However for bidirectional OPA, the eye diagrams exhibit slightly more noise at bit “1” and also reduced extinction ratios. This may be caused by additional noise from the SPM between the counter-propagating pumps, the co-propagating SBS light and also the Rayleigh scattering in the system. To further evaluate the performance, bit error rate (BER) measurements were performed on the selected channels amplified by the two OPAs in both directions. The measurements were performed by varying the total input power to the OPA, so the individual channel power should be obtained by dividing by the number of signal channels. Figure 5 (a) and (b) show the experimental results. For all channels in two directions, error-free operation (BER < 10−9) could be achieved with similar BER performance in each OPA. As expected, the bidirectional OPA has inferior performance compared with the unidirectional OPA. For direction 1, the power penalties range from 1 dB to 1.8 dB at BER = 10−9, while for direction 2 the penalties range from 0.8 dB to 2 dB. The difference in receiver sensitivities is caused by the different gain profiles of the two amplifiers and also the additional nonlinear noise in bidirectional OPA as discussed previously.
Fig. 5. Bit error rate measurements on selected channels in unidirectional and bidirectional OPAs in (a) direction 1 and (b) direction 2. Ch1/5/10_bi: bidirectional OPA; Ch1/5/10_uni: unidirectional OPA.
We also measured the receiver sensitivities (input optical power @ BER = 10−9) for all 20 channels in the bidirectional OPA. Figure 6 shows the experimental results. It is observed that the two directions exhibit similar performance. The receiver sensitivities vary from −13.6 dBm to −15.5 dBm, resulting from the non-flat gain spectrum and also variation in the transmitter power. To further optimize the flatness across the gain spectrum, a two-pump approach could be utilized as demonstrated in [15], with a ripple of 2 dB across 100 nm.
Fig. 6. Receiver sensitivities for all channels in both directions in bidirectional OPA.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8730
4. Bidirectional OPA with 32 channels To further explore the potential of the bidirectional OPA, we increased the channel count to 16 and the maximum gain level to ~15 dB. To implement the experiment, 16 CW lasers were used instead of 10 before the AWG. The pump wavelength was moved to 1561.5 nm to ensure that the 3-dB gain bandwidth covered the signal wavelengths. The pump power was increased to 28.1 dBm for each direction. To reduce the influence of the counter-propagating pump, the pump polarizations were set to be orthogonal as this can reduce the nonlinear interaction by 1/3 [1], while the signal polarizations were aligned to their respective pump. The pumps were set to orthogonal polarization by maximizing the total output power from the HNLF, and with this configuration BER could be improved by an order of magnitude. Finally three modulation tones were used (100 MHz, 320 MHz, and 980 MHz) instead of four, as we observed that for bidirectional OPA, the additional modulation tone would cause excessive noise on the eye diagrams. The reason behind is still under investigation. Table 2 shows the parameters of the two OPAs in both directions with enhanced gain and the gain spectra are shown in Fig. 7. Table 2. Parameters of unidirectional and bidirectional OPAs in both directions with enhanced gain. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA. Pump @ 1562 nm
D1_uni
D2_uni
D1_bi
D2_bi
Gain peak (nm)
1531
1529
1533.5
1531
3-dB bandwidth (nm)
24.2
24.2
25.5
24.9
Maximum gain (dB)
16.6
17.6
16.6
17.7
Fig. 7. Gain spectra for bidirectional and unidirectional OPAs in both directions with increased gain. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA.
We can see that the gain peak was shifted by ~10 nm by just a 0.5-nm shift in pump wavelength. Compared with the previous amplifiers for the 20-channel experiment, the current ones can provide more than 10-dB gain from 1520 nm to 1560 nm, showing that there is potential to accommodate more data channels. For both bidirectional and unidirectional OPAs, similar gain profiles were obtained. Also direction 2 has a 1-dB higher gain compared with direction 1 for both configurations. Figure 8 shows the 1% output optical spectrum from the bidirectional OPA in one of the directions. 16 DWDM channels were generated from 1534.2 nm to 1546.2 nm. As the EDFAs for amplifying signals had degraded performance at wavelength shorter than 1535 nm, we could not match the signal wavelengths with the gain peak region (~1530 nm) as shown in Fig. 7. The intensity variation was reduced to < 1.5 dB among the channels, while the on-off gain ranged from 13.2 dB to 15.3 dB. Moreover, the idlers were generated in the L-band with conversion efficiency over 95%.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8731
Fig. 8. Output optical spectrum from bidirectional OPA in one of the directions with 16 channels.
Figure 9 (a) and (b) show the BER measurements on five selected channels (Ch.1: 1534.2 nm; Ch.4: 1536.6 nm; Ch.8: 1539.8 nm; Ch.12: 1543 nm; Ch.16: 1546.2 nm) across the signal bandwidth in both directions. Similar to the 20-channel experiment, the BER performances for different channels in each amplifier are similar. Still the bidirectional OPA exhibits power penalties compared with the unidirectional OPA. Also it is observed that for unidirectional OPAs channels at shorter wavelengths have better BER performance while the case is reversed for bidirectional OPA. The phenomenon is caused by the different gain profiles of the two OPAs, and it leads to a higher power penalty variation. As a result, the power penalties range from 1.5 to 3.7 dB in direction 1, while for direction 2 they range from 0.8 to 3.6 dB.
Fig. 9. Bit error rate measurements on selected channels in 15-dB gain unidirectional and bidirectional OPAs in (a) direction 1 and (b) direction 2. Ch1/4/8/12/16_bi: bidirectional OPA; Ch1/4/8/12/16_uni: unidirectional OPA.
Fig. 10. Receiver sensitivities for all channels in both directions in bidirectional OPA.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8732
Finally we measured the receiver sensitivities (input optical power @ BER = 10−9) for all 32 channels in the bidirectional OPA. Figure 10 shows the experimental results. It is observed that the two directions exhibit similar performance, while more than 1-dB difference is observed for Ch. 10 and Ch. 11. The receiver sensitivities vary from −12.4 dBm to −14.8 dBm, which is similar to the 20-channel experiment. It is worth noting that the sensitivity is measured by the total input optical power to the OPA, meaning that it should be divided by the channel count to obtain the individual channel power. 5. Discussion For OPAs, polarization dependency is still a barrier for practical application, and it is even more important for DWDM amplification. In our experiment, as we have only a single polarization controller for all data channels, it is critical to align all of them to the pump polarization. Polarization misalignment could cause degradation in optical signal-to-noise ratio, and that could also be one of the reasons for the BER variation in Fig. 6 and Fig. 10. A dual-pump approach [16] could reduce the polarization dependency of the OPA and also provide a flat gain spectrum, although the usable bandwidth could be reduced. Nevertheless, the bidirectional OPA could be a useful tool when amplification and wavelength conversion are needed in both directions. A bidirectional OPA could provide four functions (two per direction) in a single device, so the equipment cost could be reduced. With the unused bandwidth in the proposed bidirectional OPA, more channels could be supported with higher data rate to achieve multi-Tb/s amplification. 6. Conclusion We have experimentally demonstrated a bidirectional OPA for over 1 Tb/s DWDM amplification and analyzed its performance. 32 DWDM channels have been amplified with more than 13-dB on-off gain and error-free operation has been achieved for all data streams. An average power penalty of 2.5 dB has been measured compared with conventional unidirectional OPA. This characterization could be helpful for the design of high-performance bidirectional OPAs for a variety of applications. To further optimize the performance, different gain flattening techniques could be used to equalize the channel powers, while higher gain levels could be attained by using higher pump power. Acknowledgment This work was supported part by the UK’s EPSRC grant EP/J009709/2. The HNLF was provided by Sumitomo Electric Industries, Japan.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8733
Abstract: We experimentally demonstrate amplification of 1.28 Tb/s DWDM channels using a bidirectional fiber optical parametric amplifier. The amplifier can provide more than 13 dB on-off gain on all 32 DWDM channels. Error-free operation has been achieved for all data streams, with an average power penalty of 2.5 dB compared with conventional unidirectional configuration. ©2014 Optical Society of America OCIS codes: (190.4970) Parametric oscillators and amplifiers; (060.2320) Fiber optics amplifiers and oscillators.
References and links 1. 2. 3.
4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
16.
M. E. Marhic, Fiber Optical Parametric Amplifiers, Oscillators and Related Devices (Cambridge University, 2007). N. El Dahdah, D. S. Govan, M. Jamshidifar, N. J. Doran, and M. E. Marhic, “Fiber optical parametric amplifier performance in a 1-Tb/s DWDM communication system,” IEEE J. Sel. Top. Quantum Electron. 18(2), 950–957 (2012). M. Jazayerifar, S. Warm, R. Elschner, D. Kroushkov, I. Sackey, C. Meuer, C. Schubert, and K. Petermann, “Performance evaluation of DWDM communication systems with fiber optical parametric ampliers,” J. Lightwave Technol. 31(9), 1454–1461 (2013). W. Lee, M. Y. Park, S. H. Cho, J. Lee, C. Kim, G. Jeong, and B. W. Kim, “Bidirectional WDM-PON based on gain-saturated reflective semiconductor optical amplifiers,” IEEE Photon. Technol. Lett. 17(11), 2460–2462 (2005). J. Prat, C. Arellano, V. Polo, and C. Bock, “Optical network unit based on a bidirectional reflective semiconductor optical amplifier for fiber-to-the-home networks,” IEEE Photon. Technol. Lett. 17(1), 250–252 (2005). V. Mizrahi, “Bidirectional WDM optical communication systems with bidirectional optical amplifiers,” Ciena Corp., US Patent 5,742,416 (1998). S. Radic, S. Chandrasekhar, P. Bernasconi, J. Centanni, C. Abraham, N. Copner, and K. Tan, “Feasibility of hybrid Raman/EDFA amplification in bidirectional optical transmission,” IEEE Photon. Technol. Lett. 14(2), 221–223 (2002). I. Tafur Monroy, R. Kjær, F. Öhman, K. Yvind, and P. Jeppesen, “Distributed fiber Raman amplification in long reach PON bidirectional access links,” Opt. Fiber Technol. 14(1), 41–44 (2008). K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent one-pump fiberoptical parametric amplifier,” IEEE Photon. Technol. Lett. 14(11), 1506–1508 (2002). T. Torounidis, B.-E. Olsson, H. Sunnerud, M. Karlsson, and P. A. Andrekson, “Fiber-optic parametric amplifier in a loop mirror configuration,” IEEE Photon. Technol. Lett. 17(2), 321–323 (2005). G. Kalogerakis, M. E. Marhic, K. Uesaka, K. Shimizu, K. K.-Y. Wong, and L. G. Kazovsky, “Methods for full utilization of the bandwidth of fiber optical parametric amplifiers and wavelength converters,” J. Lightwave Technol. 24(10), 3683–3690 (2006). K. S. Yeo, F. R. M. Adikan, M. Mokhtar, S. Hitam, and M. A. Mahdi, “Fiber optical parametric amplifier with double-pass pump configuration,” Opt. Express 21(25), 31623–31631 (2013). E. Myslivets, N. Alic, S. Moro, B. P. P. Kuo, R. M. Jopson, C. J. McKinstrie, M. Karlsson, and S. Radic, “1.56micros continuously tunable parametric delay line for a 40-Gb/s signal,” Opt. Express 17(14), 11958–11964 (2009). B. P.-P. Kuo, E. Myslivets, A. O. J. Wiberg, S. Zlatanovic, C.-S. Brès, S. Moro, F. Gholami, A. Peric, N. Alic, and S. Radic, “Transmission of 640-Gb/s RZ-OOK channel over 100-km SSMF by wavelength-transparent conjugation,” J. Lightwave Technol. 29(4), 516–523 (2011). J. D. Marconi, J. M. Chavez Boggio, H. L. Fragnito, and S. R. Bickham, “Nearly 100 nm bandwidth of flat gain with a double-pumped fiber optic parametric amplifier,” in Optical Fiber Communication Conference and Exposition and The National Fiber Optic Engineers Conference, OSA Technical Digest Series (CD) (Optical Society of America, 2007), paper OWB1. K. K. Y. Wong, M. E. Marhic, K. Uesaka, and L. G. Kazovsky, “Polarization-independent two-pump fiber optical parametric amplifier,” IEEE Photon. Technol. Lett. 14(7), 911–913 (2002).
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8726
1. Introduction Fiber optical parametric amplifiers (OPAs) have some properties which can potentially be exploited for improving the performance of optical communication systems [1], including: low noise figure (NF), which can be as low as 0 dB in phase-sensitive amplification; ability to perform wavelength conversion with high efficiency; broad gain bandwidth and unidirectional gain. With these interesting properties, it would be valuable to investigate the performance of OPAs in dense wavelength division multiplexing (DWDM) systems. Experimental study [2] has demonstrated amplification of >1 Tb/s DWDM channels in an OPA, while theoretical study [3] has shown that with optimized design and configuration, >10 Tb/s transmission can be achieved. For these demonstrations, only a single pump was used for amplification in one direction. It would also be interesting to investigate the performance of OPA when bidirectional pumps are used. Bidirectional amplifiers have found applications mainly in passive optical networks [4, 5] and bidirectional transmission link. Different schemes have been proposed to achieve bidirectional amplification, including bidirectional erbium-doped fiber amplifier (EDFA) [6], hybrid EDFA/Raman amplifier [7], bidirectional Raman amplifier [8] and reflective semiconductor optical amplifier [4]. OPAs can also operate in a bidirectional manner by pumping it in both directions, and this has been previously demonstrated in polarizationindependent OPA [9], OPA in loop mirror configuration for reduced loss [10] and full utilization of gain bandwidth by including filters in OPA configuration [11]. A recent study [12] has shown that with a Faraday mirror, double-pass OPA could be achieved with enhanced on-off gain. However, the additional signal gain bandwidth provided by the bidirectional pumping has not been fully explored yet. Here we present amplification of DWDM channels using a bidirectional OPA. We investigate in detail the penalties associated with bidirectional versus unidirectional propagation in a DWDM system, for the first time to our knowledge. First we demonstrate 20 × 40 Gb/s data channels (10 channels per direction) amplified by the bidirectional OPA with >10 dB on-off gain. Error-free operation has been achieved for all channels in the proposed bidirectional OPA. Compared with a unidirectional OPA, a maximum power penalty of 2 dB was achieved. Furthermore we demonstrated amplification of 32 × 40 Gb/s DWDM channels (16 channels per direction) with > 13 dB on-off gain, which is the largest channel count in a single OPA to date. An average power penalty of 2.5 dB has been achieved compared with conventional OPA. With access to the idler, the bidirectional OPA can act as both an amplifier and a wavelength converter in two directions, which could potentially be a costeffective device for optical communication systems. 2. Experimental setup First we describe the experiment for 20-channel amplification. The experimental setup is shown in Fig. 1. 10 continuous wave (CW) lasers are obtained from a distributed feedback (DFB) laser bank and combined by an arrayed waveguide grating (AWG). All channels are modulated in a Mach-Zehnder modulator with a 40 Gb/s 231-1 pseudorandom binary sequence (PRBS) to generate non-return-to-zero on-off keying (NRZ-OOK) signals. The channels are then de-correlated by a 1.5-km single-mode fiber and amplified by an erbium-doped fiber amplifier (EDFA). A variable optical attenuator (VOA) is used to control the input power to the OPA, while a 50/50 coupler is used to monitor the signal input power of the OPA. An optical isolator is used to block the backscattered stimulated Brillouin scattering (SBS) light.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8727
Fig. 1. Experimental setup for bidirectional OPA. AWG: arrayed waveguide grating; PRBS: pseudorandom binary sequence; SMF: single-mode fiber; EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; ISO: isolator; TLS: tunable laser source; PM: phase modulator; TBPF: tunable bandpass filter; HNLF: highly nonlinear fiber; OSA: optical spectrum analyser; PD: photodetector.
The OPA pump is generated by a tunable laser source at 1562 nm. It is then phasemodulated by 4 RF tones (100 MHz, 320 MHz, 980 MHz, and 2200 MHz) to increase the SBS threshold. The modulated pump is further amplified by another EDFA and the amplified spontaneous noise (ASE) is removed by a 1-nm tunable bandpass filter (TBPF). The pump and the 10 signal channels are then split by two separate 50/50 couplers. For each direction, the signals and the pump are combined using a 90/10 coupler. An optical circulator is used to direct the signals and pump to a 99/1 coupler. The 1% ports are used for monitoring the SBS power and total input power to the highly nonlinear fiber (HNLF) in backward and forward direction, respectively. Parametric amplification occurs inside the HNLF. The HNLF has a length of 340 m, zero dispersion at 1560 nm, nonlinearity coefficient of 15 W−1km−1, and dispersion slope of 0.023 ps nm−2 km−1. After parametric amplification, the signals and the pump are obtained through the port 3 of the circulator in the other direction (output1 and output 2 in Fig. 1). Each direction is investigated individually. For each output, 1% of the power is used for monitoring the signals on the optical spectrum analyzer (OSA). Individual data channels are filtered using a 0.6-nm flat-top TBPF. The signal is further amplified and filtered before detection by the photodetector. Signals are analyzed on oscilloscope and error analyzer. 3. Bidirectional OPA with 20 channels First we examine the gain profile for the bidirectional OPA. As a comparison, a unidirectional OPA is also included by removing the signals and pump in one of the directions. Table 1 shows the parameters of the two OPAs in both directions and their gain spectra are shown in Fig. 2. Table 1. Parameters of unidirectional and bidirectional OPAs in both directions. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA. Pump @ 1562 nm
D1_uni
D2_uni
D1_bi
D2_bi
Gain peak (nm)
1540.5
1540.2
1541.5
1542
3-dB bandwidth (nm)
21
21
26.5
24
Maximum gain (dB)
11.5
12.4
11.3
12.0
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8728
Fig. 2. Gain spectra for bidirectional and unidirectional OPAs in both directions. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA.
Although the bidirectional OPA has a larger 3-dB gain bandwidth, it exhibits a slightly smaller maximum gain for both directions compared with the unidirectional OPA; it also has an overall flatter gain spectrum as shown in Fig. 2. Furthermore it is observed that direction 2 has a higher gain for both configurations, which is caused by the slightly unbalanced pump powers in the two directions. The difference in gain profiles for the two configurations could be caused by self-phase modulation (SPM) of the counter-propagating pumps in the bidirectional case, by which the phase matching condition for the OPA could be altered. Figure 3 shows the 1% output optical spectrum from the bidirectional OPA in one of the directions. The signals are generated from 1537.4 nm to 1544.6 nm with 0.8 nm spacing, situating at the gain peak region of the bidirectional OPA. The intensity variation is caused by the non-flat gain spectrum and the difference in transmitter power. The intensity variation is < 2 dB among the channels, while the on-off gain ranges from 10.9 dB to 12.2 dB. Moreover, the idlers are generated in the L-band with conversion efficiency over 90%. The idler wavelength could be used for different applications including parametric tunable delay [13] and dispersion compensation with mid-span spectral inversion [14].
Fig. 3. Output optical spectrum from bidirectional OPA in one of the directions.
Fig. 4. Eye diagrams of selected channels amplified by either unidirectional or bidirectional OPA in direction 1.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8729
Figure 4 shows the eye diagrams of three selected channels (Ch.1: 1537.4 nm; Ch.5: 1540.6 nm; Ch.10: 1544.6 nm) amplified by either unidirectional or bidirectional OPA in the same direction as Fig. 3. Clear eye diagrams have been achieved for both configurations. However for bidirectional OPA, the eye diagrams exhibit slightly more noise at bit “1” and also reduced extinction ratios. This may be caused by additional noise from the SPM between the counter-propagating pumps, the co-propagating SBS light and also the Rayleigh scattering in the system. To further evaluate the performance, bit error rate (BER) measurements were performed on the selected channels amplified by the two OPAs in both directions. The measurements were performed by varying the total input power to the OPA, so the individual channel power should be obtained by dividing by the number of signal channels. Figure 5 (a) and (b) show the experimental results. For all channels in two directions, error-free operation (BER < 10−9) could be achieved with similar BER performance in each OPA. As expected, the bidirectional OPA has inferior performance compared with the unidirectional OPA. For direction 1, the power penalties range from 1 dB to 1.8 dB at BER = 10−9, while for direction 2 the penalties range from 0.8 dB to 2 dB. The difference in receiver sensitivities is caused by the different gain profiles of the two amplifiers and also the additional nonlinear noise in bidirectional OPA as discussed previously.
Fig. 5. Bit error rate measurements on selected channels in unidirectional and bidirectional OPAs in (a) direction 1 and (b) direction 2. Ch1/5/10_bi: bidirectional OPA; Ch1/5/10_uni: unidirectional OPA.
We also measured the receiver sensitivities (input optical power @ BER = 10−9) for all 20 channels in the bidirectional OPA. Figure 6 shows the experimental results. It is observed that the two directions exhibit similar performance. The receiver sensitivities vary from −13.6 dBm to −15.5 dBm, resulting from the non-flat gain spectrum and also variation in the transmitter power. To further optimize the flatness across the gain spectrum, a two-pump approach could be utilized as demonstrated in [15], with a ripple of 2 dB across 100 nm.
Fig. 6. Receiver sensitivities for all channels in both directions in bidirectional OPA.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8730
4. Bidirectional OPA with 32 channels To further explore the potential of the bidirectional OPA, we increased the channel count to 16 and the maximum gain level to ~15 dB. To implement the experiment, 16 CW lasers were used instead of 10 before the AWG. The pump wavelength was moved to 1561.5 nm to ensure that the 3-dB gain bandwidth covered the signal wavelengths. The pump power was increased to 28.1 dBm for each direction. To reduce the influence of the counter-propagating pump, the pump polarizations were set to be orthogonal as this can reduce the nonlinear interaction by 1/3 [1], while the signal polarizations were aligned to their respective pump. The pumps were set to orthogonal polarization by maximizing the total output power from the HNLF, and with this configuration BER could be improved by an order of magnitude. Finally three modulation tones were used (100 MHz, 320 MHz, and 980 MHz) instead of four, as we observed that for bidirectional OPA, the additional modulation tone would cause excessive noise on the eye diagrams. The reason behind is still under investigation. Table 2 shows the parameters of the two OPAs in both directions with enhanced gain and the gain spectra are shown in Fig. 7. Table 2. Parameters of unidirectional and bidirectional OPAs in both directions with enhanced gain. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA. Pump @ 1562 nm
D1_uni
D2_uni
D1_bi
D2_bi
Gain peak (nm)
1531
1529
1533.5
1531
3-dB bandwidth (nm)
24.2
24.2
25.5
24.9
Maximum gain (dB)
16.6
17.6
16.6
17.7
Fig. 7. Gain spectra for bidirectional and unidirectional OPAs in both directions with increased gain. D1/D2: direction 1/2; uni: unidirectional OPA; bi: bidirectional OPA.
We can see that the gain peak was shifted by ~10 nm by just a 0.5-nm shift in pump wavelength. Compared with the previous amplifiers for the 20-channel experiment, the current ones can provide more than 10-dB gain from 1520 nm to 1560 nm, showing that there is potential to accommodate more data channels. For both bidirectional and unidirectional OPAs, similar gain profiles were obtained. Also direction 2 has a 1-dB higher gain compared with direction 1 for both configurations. Figure 8 shows the 1% output optical spectrum from the bidirectional OPA in one of the directions. 16 DWDM channels were generated from 1534.2 nm to 1546.2 nm. As the EDFAs for amplifying signals had degraded performance at wavelength shorter than 1535 nm, we could not match the signal wavelengths with the gain peak region (~1530 nm) as shown in Fig. 7. The intensity variation was reduced to < 1.5 dB among the channels, while the on-off gain ranged from 13.2 dB to 15.3 dB. Moreover, the idlers were generated in the L-band with conversion efficiency over 95%.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8731
Fig. 8. Output optical spectrum from bidirectional OPA in one of the directions with 16 channels.
Figure 9 (a) and (b) show the BER measurements on five selected channels (Ch.1: 1534.2 nm; Ch.4: 1536.6 nm; Ch.8: 1539.8 nm; Ch.12: 1543 nm; Ch.16: 1546.2 nm) across the signal bandwidth in both directions. Similar to the 20-channel experiment, the BER performances for different channels in each amplifier are similar. Still the bidirectional OPA exhibits power penalties compared with the unidirectional OPA. Also it is observed that for unidirectional OPAs channels at shorter wavelengths have better BER performance while the case is reversed for bidirectional OPA. The phenomenon is caused by the different gain profiles of the two OPAs, and it leads to a higher power penalty variation. As a result, the power penalties range from 1.5 to 3.7 dB in direction 1, while for direction 2 they range from 0.8 to 3.6 dB.
Fig. 9. Bit error rate measurements on selected channels in 15-dB gain unidirectional and bidirectional OPAs in (a) direction 1 and (b) direction 2. Ch1/4/8/12/16_bi: bidirectional OPA; Ch1/4/8/12/16_uni: unidirectional OPA.
Fig. 10. Receiver sensitivities for all channels in both directions in bidirectional OPA.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8732
Finally we measured the receiver sensitivities (input optical power @ BER = 10−9) for all 32 channels in the bidirectional OPA. Figure 10 shows the experimental results. It is observed that the two directions exhibit similar performance, while more than 1-dB difference is observed for Ch. 10 and Ch. 11. The receiver sensitivities vary from −12.4 dBm to −14.8 dBm, which is similar to the 20-channel experiment. It is worth noting that the sensitivity is measured by the total input optical power to the OPA, meaning that it should be divided by the channel count to obtain the individual channel power. 5. Discussion For OPAs, polarization dependency is still a barrier for practical application, and it is even more important for DWDM amplification. In our experiment, as we have only a single polarization controller for all data channels, it is critical to align all of them to the pump polarization. Polarization misalignment could cause degradation in optical signal-to-noise ratio, and that could also be one of the reasons for the BER variation in Fig. 6 and Fig. 10. A dual-pump approach [16] could reduce the polarization dependency of the OPA and also provide a flat gain spectrum, although the usable bandwidth could be reduced. Nevertheless, the bidirectional OPA could be a useful tool when amplification and wavelength conversion are needed in both directions. A bidirectional OPA could provide four functions (two per direction) in a single device, so the equipment cost could be reduced. With the unused bandwidth in the proposed bidirectional OPA, more channels could be supported with higher data rate to achieve multi-Tb/s amplification. 6. Conclusion We have experimentally demonstrated a bidirectional OPA for over 1 Tb/s DWDM amplification and analyzed its performance. 32 DWDM channels have been amplified with more than 13-dB on-off gain and error-free operation has been achieved for all data streams. An average power penalty of 2.5 dB has been measured compared with conventional unidirectional OPA. This characterization could be helpful for the design of high-performance bidirectional OPAs for a variety of applications. To further optimize the performance, different gain flattening techniques could be used to equalize the channel powers, while higher gain levels could be attained by using higher pump power. Acknowledgment This work was supported part by the UK’s EPSRC grant EP/J009709/2. The HNLF was provided by Sumitomo Electric Industries, Japan.
#207180 - $15.00 USD (C) 2014 OSA
Received 25 Feb 2014; revised 26 Mar 2014; accepted 26 Mar 2014; published 3 Apr 2014 7 April 2014 | Vol. 22, No. 7 | DOI:10.1364/OE.22.008726 | OPTICS EXPRESS 8733
