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s differential quadrature phase-shift keying transmission over 2000 km in a 64-channel WDM system
Optics Communications 237 (2004) 319–323 www.elsevier.com/locate/optcom
20 Gb/s differential quadrature phase-shift keying transmission over 2000 km in a 64-channel WDM system Pierpaolo Boffi a,*, Lucia Marazzi a, Livio Paradiso a, Paola Parolari a, Aldo Righetti a, Dario Setti a, Rocco Siano a, Roberto Cigliutti b, Diego Mottarella b, Pierluigi Franco c, Mario Martinelli a,d a
CoreCom, via G. Colombo, 81 20133 Milano, Italy Pirelli Submarine Telecom Systems, Milano, Italy c Pirelli Labs, Milano, Italy Department of Electronics and Information-Politecnico di Milano, Milano, Italy b
d
Received 30 January 2004; accepted 2 April 2004
Abstract We present a 20 Gb/s return-to-zero differential quadrature phase-shift keying transmission over a 2000 km in a 33 GHz spaced 64-channel dense-WDM system. Successful transmission is demonstrated in a laboratory straight line, designed for standard intensity-modulated submarine systems. 2004 Elsevier B.V. All rights reserved. PACS: 42.81; 42.79.s Keywords: Optical fiber communication; Modulation formats; Wavelength division multiplexing
1. Introduction Nowadays there is a growing effort to upgrade the transmission capacity of optical systems by increasing the data transmission spectral efficiency. For this purpose modulation formats alternative to usual Intensity Modulation – Direct Detection (IMDD) are explored [1,2]. Differential phase shift
*
Corresponding author. Tel.: +39-02-239-98900; fax: +39-02239-98922. E-mail address: boffi@corecom.it (P. Boffi).
keying (DPSK) format has demonstrated to extend transmission distance, especially in high bit rate wavelength division multiplexing (WDM) systems (40 Gb/s), thanks to its tolerance towards nonlinear effect impairments [3,4]. To increase spectral efficiency, while maintaining low bit rate (10 Gb/s), optical differential quadrature phase shift keying (DQPSK) format appears very promising, combining multi-level phase modulation with direct detection. Recently a few papers have presented first transmission experiments demonstrating also for DQPSK robustness toward fiber nonlinear effects [5–7].
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.04.008
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P. Boffi et al. / Optics Communications 237 (2004) 319–323
In this letter, we investigate the exploitation of the DQPSK modulation format over already installed links. The robustness of DQPSK modulation format in systems not specifically designed on that purpose in terms of repeater spacing, dispersion and non-linearity management, appears very attractive in order to avoid new link installations while enhancing existing plant performance. We demonstrate the transmission of return-to-zero (RZ) DQPSK signal at 10 Gsymbol/s (20 Gb/s equivalent) over 2000 km in a Dense WDM (DWDM) environment. The experimented fiber link is a 2000-km straight line with a wet plant, designed for a standard submarine IMDD transmission. DWDM channel spacing is 33 GHz. RZ-DQPSK performance in long-haul DWDM propagation is tested by BER measurements as a function of received optical signal-to-noise ratio (OSNR). RZ-DQPSK phase modulation impact on propagation and on performance flatness for all the DWDM channels is also addressed and discussed.
2. Experimental setup and results The experimental setup is presented in Fig. 1. The 64 DWDM channels, coming from standard external cavity laser (ECL) diodes, are 33 GHz spaced from 1543.03 to 1559.83 nm and are combined together as shown in figure. The DQPSK channel under test is generated by a tunable laser -
linewidth full width half maximum (FWHM) 100 kHz – and inserted after the multiplexers. RZDQPSK modulation is obtained by cascading three modulators. The RZ-shaping is performed by the first Mach–Zehnder modulator (MZM) driven by the 10 GHz clock signal, obtaining a duty cycle of 50% and an extinction ratio of 13 dB. The second MZM operates in push–pull mode and is driven by a 10 Gb/s pseudo random bit sequence (PRBS) of length 27 )1 achieving a p-modulation depth. At last, a phase modulator (PM), driven by the complementary PRBS signal suitably delayed (about 1 ns) in order to obtain uncorrelation, performs an additional p=2-phase modulation. An RF phase shifter is used in order to synchronize data at the two-phase modulators. Owing to setup complexity, the other 63 channels are only binary phase shift keying (BPSK) modulated by a single PM, while RZ format is performed as for the channel under test (duty cycle 50%, extinction ratio 13 dB). Due to the same RZ format and to the BPSK modulation with p-depth, the spectral widths of the 63 channels are very similar to the RZ-DQPSK channel one, as evidenced in the inset of Fig. 2, where both spectra are compared. At the receiving unit, an interleaver (22 GHz FWHM) demultiplexes to 66 GHz the DWDM grid, and then a tunable band pass filter (FWHM of 33 GHz) selects the DQPSK channel under test for bit error rate (BER) measurements. The in-phase and in-quadrature phases are directly detected by a
Fig. 1. Experimental setup.
P. Boffi et al. / Optics Communications 237 (2004) 319–323
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Fig. 2. BER vs. system OSNR for back-to-back (grey diamonds); for back-to-back with the interleaver (grey triangles); after 2000 km with the channel under test RZ-DQSPK modulated and the 63 channels RZ-modulated (black circles); after 2000 km with the channel under test RZ-DQSPK modulated and the 63 channels RZ-BPSK modulated (black squares). In the inset the spectral width of the RZDQPSK channel compared with the RZ-BPSK one.
pair of Mach–Zehnder delay interferometers (MZDI) biased at þp=4 and p=4, respectively [8]. We employ a pair of originally designed MZDIs, realized by SiON technology with a high index contrast between the guide core and upper cladding/substrate, allowing minimum bending radius with negligible losses in the 100 ps delay arm. Due to MZDI sensitivity toward the state of polarization, Polarization Controllers (PC) are used to couple the signal in the DQPSK receiver. In order to set the differential optical phase between the interferometer arms to þp=4 and p=4, respectively, the integrated SiON chips are thermally controlled by a heater inside a metallic case. The two outputs of each MZDI are detected by a commercially available balanced receiver (14 GHz bandwidth). The optical path lengths of the balanced receiver are mechanically controlled. The two received DQPSK signal components are measured separately at the data rate of 10 Gb/s. Being the differential quaternary precoder not
available, the BER-tester is programmed with the expected data sequence of 27 )1 bits. The 64 DWDM channels co propagate over a 1994-km straight line. The link consists of 33 nonzero dispersion (NZD) spans: 25 spans 55 km long and 8 spans of overall 319.5 km; they are compensated, approximately every 6 spans, by 6 single mode (SM) fibers 50 km long. NZD fiber has a negative dispersion of )2.82 ps/nm km and SM fiber dispersion is +16.28 ps/nm km at 1548 nm, thus total residual dispersion is 97.37 ps/nm and average link dispersion at 1548 nm is 0.0488 ps/ nm km. In the straight line 39 EDFAs with 17 nm flat optical bandwidth, 12 dBm saturation output power and 4.2 dB noise figure are employed. This laboratory link was originally designed for an IMDD submarine transmission by Pirelli Submarine Telecom Systems. We investigate BER performance of the RZDQPSK system at 10 Gsymbol/s as a function of the received OSNR. In the next figures we show
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P. Boffi et al. / Optics Communications 237 (2004) 319–323
measurements relative to þp=4 biased MZDI receiver; similar results are obtained for p=4 biased MZDI. In Fig. 2 we show the filtering impairments due to the receiving interleaver and phase modulation effects during propagation over 2000 km; the presented results are obtained for 1548 nm channel which is also polarization-scrambled. Comparison between grey diamond curve (backto-back) and gray triangle one (back-to-back in presence of the interleaver at the receiver) shows RZ-DQPSK robustness towards narrow-band filtering induced by the interleaver. The black circle curve represents BER measurements after 2000
km propagation when the channel under test is RZ-DQPSK modulated, while the other 63 DWDM channels are only RZ-modulated by the 10 GHz clock signal (no phase-modulation is applied). The black square curve is instead obtained when also the 63 neighbor channels are RZ-BPSK modulated. By comparing these last two curves with the back-to-back one, penalties due to propagation are evidenced. In particular, the phase modulation on the neighbor channels further impairs propagation, owing to linear inter-channel cross talk due to PM spectral broadening and owing to non-linear interactions.
Fig. 3. Eye diagrams at the þp=4-biased MZDI receiver (OSNR 17.5 dB): (a) back to back, (b) after 2000 km propagation.
Fig. 4. BER after 2000 km of 10 (over 64) DWDM channels RZ-DQPSK modulated with OSNR equalized within 1 dB. Residual dispersion is completely post-compensated for each channel. In the inset the spectrum of all the 64 DWDM channels having 33 GHz spacing. Dashed line indicates the BER values corrigible by a 1st generation FEC.
P. Boffi et al. / Optics Communications 237 (2004) 319–323
In Fig. 3, the eye diagrams before and after propagation are also shown, clear eye opening is still present after 2000 km. Fig. 4 shows BER measurement for 11 different DQPSK DWDM channels covering the entire transmission bandwidth; channel OSNR is equalized within 1 dB. Each measurement is performed when the residual dispersion of the channel under test is completely post-compensated by a suitable fiber Bragg grating. In the inset of Fig. 4 the spectrum of all 64 DWDM channels is also shown. As can be seen, all channels show an uncorrected BER after 2000 km propagation around 106 , ranging within one decade. The experimented system, designed for standard IMDD propagation, is intrinsically limited in OSNR by the channel power. After 2000 km the maximum OSNR for the 64 DWDM channels is 18 dB, at this OSNR value the experimented DQPSK propagation is not error-free. Although noise statistics in phase-modulated systems is different from that of on-off keying (OOK) format, it is expected that standard Forward Error Correction performs on DQPSK signals similar coding gain to OOK modulation [9,10]. The obtained experimental BER results are considerably below the BER values correctable by a 1st generation FEC (1.5 · 104 ), indicated in Fig. 4 by the dashed line. Thus, also with the needed overhead, by exploiting a conventional FEC code a corrected BER below 1012 is achievable for 2000 km propagation with 0.6 b/s/Hz spectral efficiency in the considered link, not optimized for DQPSK transmission.
3. Conclusion We have experimentally demonstrated the transmission of a RZ-DQPSK channel at 20 Gb/s equivalent bit rate over 2000 km in a 33 GHz spaced 64-channel DWDM system. Employed equipment used in our experimentation includes an originally designed integrated MZDI realized in SiON technology for direct detection of the DQPSK signal. Propagation over 2000 km is experimented in a link not optimised for this type of modulation format. Even if the other 63 DWDM channels are not DQPSK modulated, the em-
323
ployed RZ-BPSK modulation, due to spectra similarities, should allow to extend transmission results to a 64-channel RZ-DQPSK DWDM system. Moreover thanks to a conventional FEC, error-free propagation can be guaranteed, increasing previously reported [3–5] transmission distances and channel numbers in presence of a reduced spectral efficiency. These preliminary results demonstrate that DQPSK modulation can be usefully employed in order to double the transmission capacity of inservice optical communication links.
Acknowledgements The authors thank CoreCom Optical Technologies and Integrated Optics Labs for the project and the realization of the MZDI in collaboration with Pirelli Labs – Milan, Italy. References [1] T. Ono, Y. Iano, IEEE J. Quantum Electron. 34 (1998) 2080. [2] T. Hoshida, O. Vassilieva, K. Yamada, S. Choudhary, R. Piecqueur, H. Kuwahara, J. Lightwave Technol. 20 (2002) 1989. [3] A. Agarwal, S. Banerjee, D.F. Grosz, A.P. Kung, D.N. Maywar, T.H. Wood, IEEE Photon. Technol. Lett. 15 (2003) 1779. [4] B. Zhu, L.E. Nelson, S. Stulz, A.H. Gnauck, C. Doerr, J. Leuthold, L. Gruner-Nielsen, M.O. Pedersen, J. Kim, R.L. Lingle, J. Lightwave Technol. 22 (2004) 208. [5] C. Wree, N. Hecker-Denschlag, E. Gottwald, P. Krummrich, J. Leibrich, E-D. Schmidt, B. Lankl, W. Rosenkranz, IEEE Photon. Technol. Lett. 15 (2003) 1303. [6] P.S. Cho, V.S. Grigoryan, Y.A. Godin, A. Salamon, Y. Achiam, IEEE Photon. Technol. Lett. 15 (2003) 473. [7] H. Kim, R.J. Essiambre, IEEE Photon. Technol. Lett. 15 (2003) 769. [8] P.R.A. Griffin, A.C. Carter, Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission, in: Proceedings of Optical Fiber Conference 2002, WX6, p. 367–368, 2002. [9] A.H. Gnauck et al., 2.5 Tb/s (64 · 42.7 Gb/s) transmission over 40 · 100 km NZDSF using RZ-DPSK format and all-Raman-amplified spans, in: Proceedings of the Optical Fiber Conference 2002, Postdeadline Paper FC2, 2002. [10] G. Kramer, A. Ashikhmin, A.J. van Wijngaarden, X. Wei, J. Lightwave Technol. 21 (2003) 2438.
20 Gb/s differential quadrature phase-shift keying transmission over 2000 km in a 64-channel WDM system Pierpaolo Boffi a,*, Lucia Marazzi a, Livio Paradiso a, Paola Parolari a, Aldo Righetti a, Dario Setti a, Rocco Siano a, Roberto Cigliutti b, Diego Mottarella b, Pierluigi Franco c, Mario Martinelli a,d a
CoreCom, via G. Colombo, 81 20133 Milano, Italy Pirelli Submarine Telecom Systems, Milano, Italy c Pirelli Labs, Milano, Italy Department of Electronics and Information-Politecnico di Milano, Milano, Italy b
d
Received 30 January 2004; accepted 2 April 2004
Abstract We present a 20 Gb/s return-to-zero differential quadrature phase-shift keying transmission over a 2000 km in a 33 GHz spaced 64-channel dense-WDM system. Successful transmission is demonstrated in a laboratory straight line, designed for standard intensity-modulated submarine systems. 2004 Elsevier B.V. All rights reserved. PACS: 42.81; 42.79.s Keywords: Optical fiber communication; Modulation formats; Wavelength division multiplexing
1. Introduction Nowadays there is a growing effort to upgrade the transmission capacity of optical systems by increasing the data transmission spectral efficiency. For this purpose modulation formats alternative to usual Intensity Modulation – Direct Detection (IMDD) are explored [1,2]. Differential phase shift
*
Corresponding author. Tel.: +39-02-239-98900; fax: +39-02239-98922. E-mail address: boffi@corecom.it (P. Boffi).
keying (DPSK) format has demonstrated to extend transmission distance, especially in high bit rate wavelength division multiplexing (WDM) systems (40 Gb/s), thanks to its tolerance towards nonlinear effect impairments [3,4]. To increase spectral efficiency, while maintaining low bit rate (10 Gb/s), optical differential quadrature phase shift keying (DQPSK) format appears very promising, combining multi-level phase modulation with direct detection. Recently a few papers have presented first transmission experiments demonstrating also for DQPSK robustness toward fiber nonlinear effects [5–7].
0030-4018/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2004.04.008
320
P. Boffi et al. / Optics Communications 237 (2004) 319–323
In this letter, we investigate the exploitation of the DQPSK modulation format over already installed links. The robustness of DQPSK modulation format in systems not specifically designed on that purpose in terms of repeater spacing, dispersion and non-linearity management, appears very attractive in order to avoid new link installations while enhancing existing plant performance. We demonstrate the transmission of return-to-zero (RZ) DQPSK signal at 10 Gsymbol/s (20 Gb/s equivalent) over 2000 km in a Dense WDM (DWDM) environment. The experimented fiber link is a 2000-km straight line with a wet plant, designed for a standard submarine IMDD transmission. DWDM channel spacing is 33 GHz. RZ-DQPSK performance in long-haul DWDM propagation is tested by BER measurements as a function of received optical signal-to-noise ratio (OSNR). RZ-DQPSK phase modulation impact on propagation and on performance flatness for all the DWDM channels is also addressed and discussed.
2. Experimental setup and results The experimental setup is presented in Fig. 1. The 64 DWDM channels, coming from standard external cavity laser (ECL) diodes, are 33 GHz spaced from 1543.03 to 1559.83 nm and are combined together as shown in figure. The DQPSK channel under test is generated by a tunable laser -
linewidth full width half maximum (FWHM) 100 kHz – and inserted after the multiplexers. RZDQPSK modulation is obtained by cascading three modulators. The RZ-shaping is performed by the first Mach–Zehnder modulator (MZM) driven by the 10 GHz clock signal, obtaining a duty cycle of 50% and an extinction ratio of 13 dB. The second MZM operates in push–pull mode and is driven by a 10 Gb/s pseudo random bit sequence (PRBS) of length 27 )1 achieving a p-modulation depth. At last, a phase modulator (PM), driven by the complementary PRBS signal suitably delayed (about 1 ns) in order to obtain uncorrelation, performs an additional p=2-phase modulation. An RF phase shifter is used in order to synchronize data at the two-phase modulators. Owing to setup complexity, the other 63 channels are only binary phase shift keying (BPSK) modulated by a single PM, while RZ format is performed as for the channel under test (duty cycle 50%, extinction ratio 13 dB). Due to the same RZ format and to the BPSK modulation with p-depth, the spectral widths of the 63 channels are very similar to the RZ-DQPSK channel one, as evidenced in the inset of Fig. 2, where both spectra are compared. At the receiving unit, an interleaver (22 GHz FWHM) demultiplexes to 66 GHz the DWDM grid, and then a tunable band pass filter (FWHM of 33 GHz) selects the DQPSK channel under test for bit error rate (BER) measurements. The in-phase and in-quadrature phases are directly detected by a
Fig. 1. Experimental setup.
P. Boffi et al. / Optics Communications 237 (2004) 319–323
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Fig. 2. BER vs. system OSNR for back-to-back (grey diamonds); for back-to-back with the interleaver (grey triangles); after 2000 km with the channel under test RZ-DQSPK modulated and the 63 channels RZ-modulated (black circles); after 2000 km with the channel under test RZ-DQSPK modulated and the 63 channels RZ-BPSK modulated (black squares). In the inset the spectral width of the RZDQPSK channel compared with the RZ-BPSK one.
pair of Mach–Zehnder delay interferometers (MZDI) biased at þp=4 and p=4, respectively [8]. We employ a pair of originally designed MZDIs, realized by SiON technology with a high index contrast between the guide core and upper cladding/substrate, allowing minimum bending radius with negligible losses in the 100 ps delay arm. Due to MZDI sensitivity toward the state of polarization, Polarization Controllers (PC) are used to couple the signal in the DQPSK receiver. In order to set the differential optical phase between the interferometer arms to þp=4 and p=4, respectively, the integrated SiON chips are thermally controlled by a heater inside a metallic case. The two outputs of each MZDI are detected by a commercially available balanced receiver (14 GHz bandwidth). The optical path lengths of the balanced receiver are mechanically controlled. The two received DQPSK signal components are measured separately at the data rate of 10 Gb/s. Being the differential quaternary precoder not
available, the BER-tester is programmed with the expected data sequence of 27 )1 bits. The 64 DWDM channels co propagate over a 1994-km straight line. The link consists of 33 nonzero dispersion (NZD) spans: 25 spans 55 km long and 8 spans of overall 319.5 km; they are compensated, approximately every 6 spans, by 6 single mode (SM) fibers 50 km long. NZD fiber has a negative dispersion of )2.82 ps/nm km and SM fiber dispersion is +16.28 ps/nm km at 1548 nm, thus total residual dispersion is 97.37 ps/nm and average link dispersion at 1548 nm is 0.0488 ps/ nm km. In the straight line 39 EDFAs with 17 nm flat optical bandwidth, 12 dBm saturation output power and 4.2 dB noise figure are employed. This laboratory link was originally designed for an IMDD submarine transmission by Pirelli Submarine Telecom Systems. We investigate BER performance of the RZDQPSK system at 10 Gsymbol/s as a function of the received OSNR. In the next figures we show
322
P. Boffi et al. / Optics Communications 237 (2004) 319–323
measurements relative to þp=4 biased MZDI receiver; similar results are obtained for p=4 biased MZDI. In Fig. 2 we show the filtering impairments due to the receiving interleaver and phase modulation effects during propagation over 2000 km; the presented results are obtained for 1548 nm channel which is also polarization-scrambled. Comparison between grey diamond curve (backto-back) and gray triangle one (back-to-back in presence of the interleaver at the receiver) shows RZ-DQPSK robustness towards narrow-band filtering induced by the interleaver. The black circle curve represents BER measurements after 2000
km propagation when the channel under test is RZ-DQPSK modulated, while the other 63 DWDM channels are only RZ-modulated by the 10 GHz clock signal (no phase-modulation is applied). The black square curve is instead obtained when also the 63 neighbor channels are RZ-BPSK modulated. By comparing these last two curves with the back-to-back one, penalties due to propagation are evidenced. In particular, the phase modulation on the neighbor channels further impairs propagation, owing to linear inter-channel cross talk due to PM spectral broadening and owing to non-linear interactions.
Fig. 3. Eye diagrams at the þp=4-biased MZDI receiver (OSNR 17.5 dB): (a) back to back, (b) after 2000 km propagation.
Fig. 4. BER after 2000 km of 10 (over 64) DWDM channels RZ-DQPSK modulated with OSNR equalized within 1 dB. Residual dispersion is completely post-compensated for each channel. In the inset the spectrum of all the 64 DWDM channels having 33 GHz spacing. Dashed line indicates the BER values corrigible by a 1st generation FEC.
P. Boffi et al. / Optics Communications 237 (2004) 319–323
In Fig. 3, the eye diagrams before and after propagation are also shown, clear eye opening is still present after 2000 km. Fig. 4 shows BER measurement for 11 different DQPSK DWDM channels covering the entire transmission bandwidth; channel OSNR is equalized within 1 dB. Each measurement is performed when the residual dispersion of the channel under test is completely post-compensated by a suitable fiber Bragg grating. In the inset of Fig. 4 the spectrum of all 64 DWDM channels is also shown. As can be seen, all channels show an uncorrected BER after 2000 km propagation around 106 , ranging within one decade. The experimented system, designed for standard IMDD propagation, is intrinsically limited in OSNR by the channel power. After 2000 km the maximum OSNR for the 64 DWDM channels is 18 dB, at this OSNR value the experimented DQPSK propagation is not error-free. Although noise statistics in phase-modulated systems is different from that of on-off keying (OOK) format, it is expected that standard Forward Error Correction performs on DQPSK signals similar coding gain to OOK modulation [9,10]. The obtained experimental BER results are considerably below the BER values correctable by a 1st generation FEC (1.5 · 104 ), indicated in Fig. 4 by the dashed line. Thus, also with the needed overhead, by exploiting a conventional FEC code a corrected BER below 1012 is achievable for 2000 km propagation with 0.6 b/s/Hz spectral efficiency in the considered link, not optimized for DQPSK transmission.
3. Conclusion We have experimentally demonstrated the transmission of a RZ-DQPSK channel at 20 Gb/s equivalent bit rate over 2000 km in a 33 GHz spaced 64-channel DWDM system. Employed equipment used in our experimentation includes an originally designed integrated MZDI realized in SiON technology for direct detection of the DQPSK signal. Propagation over 2000 km is experimented in a link not optimised for this type of modulation format. Even if the other 63 DWDM channels are not DQPSK modulated, the em-
323
ployed RZ-BPSK modulation, due to spectra similarities, should allow to extend transmission results to a 64-channel RZ-DQPSK DWDM system. Moreover thanks to a conventional FEC, error-free propagation can be guaranteed, increasing previously reported [3–5] transmission distances and channel numbers in presence of a reduced spectral efficiency. These preliminary results demonstrate that DQPSK modulation can be usefully employed in order to double the transmission capacity of inservice optical communication links.
Acknowledgements The authors thank CoreCom Optical Technologies and Integrated Optics Labs for the project and the realization of the MZDI in collaboration with Pirelli Labs – Milan, Italy. References [1] T. Ono, Y. Iano, IEEE J. Quantum Electron. 34 (1998) 2080. [2] T. Hoshida, O. Vassilieva, K. Yamada, S. Choudhary, R. Piecqueur, H. Kuwahara, J. Lightwave Technol. 20 (2002) 1989. [3] A. Agarwal, S. Banerjee, D.F. Grosz, A.P. Kung, D.N. Maywar, T.H. Wood, IEEE Photon. Technol. Lett. 15 (2003) 1779. [4] B. Zhu, L.E. Nelson, S. Stulz, A.H. Gnauck, C. Doerr, J. Leuthold, L. Gruner-Nielsen, M.O. Pedersen, J. Kim, R.L. Lingle, J. Lightwave Technol. 22 (2004) 208. [5] C. Wree, N. Hecker-Denschlag, E. Gottwald, P. Krummrich, J. Leibrich, E-D. Schmidt, B. Lankl, W. Rosenkranz, IEEE Photon. Technol. Lett. 15 (2003) 1303. [6] P.S. Cho, V.S. Grigoryan, Y.A. Godin, A. Salamon, Y. Achiam, IEEE Photon. Technol. Lett. 15 (2003) 473. [7] H. Kim, R.J. Essiambre, IEEE Photon. Technol. Lett. 15 (2003) 769. [8] P.R.A. Griffin, A.C. Carter, Optical differential quadrature phase-shift key (oDQPSK) for high capacity optical transmission, in: Proceedings of Optical Fiber Conference 2002, WX6, p. 367–368, 2002. [9] A.H. Gnauck et al., 2.5 Tb/s (64 · 42.7 Gb/s) transmission over 40 · 100 km NZDSF using RZ-DPSK format and all-Raman-amplified spans, in: Proceedings of the Optical Fiber Conference 2002, Postdeadline Paper FC2, 2002. [10] G. Kramer, A. Ashikhmin, A.J. van Wijngaarden, X. Wei, J. Lightwave Technol. 21 (2003) 2438.