Flexible Terabit/s Nyquist-WDM Superchannels with net SE > 7bit/s/Hz using a Gain-Switched Comb Source V. Vujicic1*, J. Pfeifle2, R. T. Watts1, P. C. Schindler2, C. Weimann2, R. Zhou1, W. Freude2,3, C. Koos2,3, L. P. Barry1 1
The Rince Institute, School of Electronic Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland 2 Institute of Photonics and Quantum Electronics (IPQ), Karlsruhe Institute of Technology (KIT), Germany 3 Institute of Microstructure Technology (IMT), Karlsruhe Institute of Technology (KIT), Germany *Author e-mail address:
[email protected]
Abstract: We demonstrate two 1Tbit/s superchannel architectures using a compact, FSR-tunable gain-switched comb source. SSMF transmission of 18GBaud Nyquist-WDM shaped PDM-QPSK and PDM-16QAM modulation is reported, with a capacity up to 1.296Tbit/s and SE of 7.2bit/s/Hz. OCIS codes: (060.0060) Fiber (140.3520) Lasers, injection-locked;
optics
and
optical
communications;
(060.1660) Coherent
communications;
1. Introduction With the progress of flexible provisioning of spectral bandwidth and mixed modulation formats within wavelength division multiplexed (WDM) networks, the deployment of optical superchannels with capacity of 1 Tbit/s is foreseeable [1]. Several concepts have been proposed to scale down cost and footprint of multi-carrier sources enabling 1 Tbit/s superchannel architectures, including ensembles of independent oscillators [2], concatenated modulators [3], and four-wave mixing in high-Q microresonators [4]. Each of these options provide a trade-off in achievable bandwidth and required driving complexity. The first method is the most complex to control – with temperature and current control required for each spectral line, whereas the second is more stable but is synonymous with high insertion loss of the original carrier. The last requires sophisticated pumping schemes, exhibits large lineto-line power variations and has only very limited means for tuning the line spacing. In this work we demonstrate two distinct Tbit/s superchannel architectures which utilize a Gain-Switched Comb Source (GSCS) laser as the optical multi-carrier source. The free spectral range (FSR) of the GSCS can be electrically tuned between 5-35 GHz [5]. The GSCS exhibits high optical carrier-to-noise ratio (OCNR) on each tone, good spectral flatness, and low optical linewidth for each comb line. Here we develop different Tbit/s superchannel systems by choosing between polarization division multiplexed (PDM) QPSK or 16QAM modulation at 18 GBaud with a comb source FSR of 18.5 GHz. System performance is investigated for propagation distances up to 300 km of SSMF, demonstrating that it is possible to achieve very good performance using the GSCS. The highest capacity superchannel architecture presented here consists of a 9-channel GSCS with an FSR of 18.5 GHz, and 18 GBaud Nyquist-WDM-shaped PDM-16QAM modulation. This corresponds to 1.296 Tbit/s before FEC overhead, with a net spectral efficiency (SE) of 7.2 bit/s/Hz. This work represents a considerable advancement in both system capacity and SE for GSCS-based transmitter systems, as the best previously reported capacity is 74.9 Gbit/s and SE = 0.64 bit/s/Hz [6]. 2. Gain Switched Comb Source Fig. 1(a) depicts the experimental setup employed to realize the externally-injected GSCS. More details about the GSCS can be found in [5]. The multi-carrier signal consisting of 9-10 tones within the 3 dB bandwidth of the spectral envelope, shown in Fig. 1(b), each with an OCNR in excess of 50 dB within a 20 MHz resolution bandwidth (RBW).
Fig.1 (a) Setup schematic of the GSCS. (b) Comb spectrum for an FSR of 18.5 GHz (RBW = 20 MHz). (c) Schematic of the GSCS Terabit/s superchannel transmitter and coherent receiver.
FSR tunability is achieved by changing the frequency of the RF synthesizer signal. Moreover, with the master laser (ECL; Agilent N7711A) injecting the slave laser (DFB laser; NEL NLK5C), the low optical linewidth (80 kHz) and low RIN (-145 dBc/Hz) of the master laser is transferred to each individual comb lines of the GSCS, which highlights its merit for use in systems employing multi-level modulation formats. Further cost and complexity reduction, and stability improvement can be achieved by monolithic integration of the master and slave lasers [7]. 3. Terabit/s transmission using PDM-QPSK and PDM-16QAM The experimental setup for the 1 Tbit/s superchannel transmitter is shown in Fig 1(c). It comprises a wavelength selective switch (WSS) which equalized the power in the comb spectrum and dis-interleaved adjacent spectral lines into ‘odd’ and ‘even’ channels. The two sets of channels were modulated using an IQ modulator driven by an 18 GBaud Nyquist-WDM-shaped data source. The data sources are programmable to give either QPSK or 16QAM modulation [8, 9]. The odd and even channels were recombined and passed through a polarization multiplexer. The polarization-multiplexed data was transmitted back-to-back (B2B) to an Agilent N4391A Optical Modulation Analyzer (OMA) for analysis, with DSP performed offline, or transmitted through up to four amplified spans of 75 km SSMF before analysis. Channel performance was determined using error-vector magnitude (EVM) calculation of the measured constellation diagrams, and also error counting to establish a bit error rate (BER). The optical spectra of the flattened GSCS with 18.5 GHz FSR that were employed in the 18 GBaud PDM-QPSK and PDM-16QAM experiments are shown in Fig. 2(a) and (d), respectively. Fig. 2(b) and (e) show the polarizationaveraged EVM for the data channels. All 15 channels for the PDM-QPSK superchannel had performance far better than the 7% FEC limit, whilst for PDM-16QAM case some channels had performance slightly above the 7% FEC limit after transmission over 150km. Single-channel B2B measurements employing a low-linewidth ECL, shown in Fig. 2(b) and (e), was used as a benchmark for system performance. Constellation diagrams for both polarizations, shown in Fig. 2(c), depict performance of a middle and outer channel for PDM-QPSK, and the central channel for PDM-16QAM is shown in Fig. 2(f). The superchannel based on the PDM-QPSK had an aggregate capacity of 1.08 Tbit/s (15 18 GBaud PDM-QPSK), net SE of 3.6 bit/s/Hz and occupied a bandwidth of 278 GHz, whilst the superchannel based on PDM-16QAM had an aggregate capacity of 1.296 Tbit/s (9 18 GBaud PDM-16QAM), net SE of 7.2 bit/s/Hz if 7% FEC is assumed, or 6.2 bit/s/Hz for 20% FEC and requires a bandwidth of only 167 GHz.
Fig. 2 (a) Spectrum of flattened GSCS with 18.5 GHz channel spacing. (b) Measured EVM of PDM-QPSK data for each channel number, over different propagation distances. (c) Measured constellations of channel 1 and 8 (B2B). Fig. 2 (d,e) show the same results as (a,b), but for PDM-16QAM, and Fig. 2 (f) shows the measured constellations of channel 4 (B2B) for PDM-16QAM.
4. Conclusion In this work we demonstrate Tbit/s data transmission using an externally-injected gain-switched comb source operating at 18.5 GHz FSR, in a Nyquist-WDM system employing 18 GBaud PDM-QPSK or PDM-16QAM. A net spectral efficiency up to 7.2 bit/s/Hz was demonstrated, as well as transmission over SSMF up to 300 km. 5. References [1] P. J. Winzer, IEEE Communication Magazine, pp. 26-30, (July 2010). [2] Y. Huang, et. al., in ACP Conference, 2012, paper PAF4C.2. [3] X. Liu, et. al., Optics Express, vol. 19, B958-B964 (2011). [4] J. Pfeifle, et. al., Preprint - arXiv:1307.1037v2. [5] P. Anandarajah, et. al., in OFC/NFOEC, 2013, paper OTh3I.8. [6] R. Maher, et. al., ECOC, 2010, paper P3.07.
[7] R. Zhou, et. al., accepted for publication OFC/NFOEC, 2014, paper Th3A.3. [8] R. Schmogrow, et. al., Photonics Technology Letters, vol. 22, pp.1601-1603 (2010). [9] R. Schmogrow, et. al., Optics Express, vol. 20, pp. 317-337 (2012).
