We experimentally demonstrate a highly filtering-tolerant multi-modulus equalization (MMEQ) process for very aggressively spectrum-shaped 9-ary quadrature-amplitude-modulation (9-QAM)-like polarization division multiplexing quadrature phase shift keying (PDM-QPSK) signal to achieve 400-Gb/s wavelength-division-multiplexing (WDM) channels on the 100-GHz grid for ultra-long-haul reach and high tolerance of the filter narrowing effect caused by reconfigurable optical add-drop multiplexers (ROADMs). We successfully transmitted 8 channels 480-Gb/s super-Nyquist (channel occupancy much less than signal baud rate) WDM signals at 100-GHz grid over 25 × 200 km conventional single-mode fiber-28 (SMF-28) with post Raman amplification and 25 ROADMs at a net spectral efficiency (SE) of 4b/s/Hz, after excluding the 20% soft-decision forward-error-correction (FEC) overhead. The system performance is significantly enhanced by the MMEQ based on 9-QAM-like constellations compared to the conventional 4 point QPSK constellation. A record transmission distance over conventional SMF-28 with a large number of ROADMs is firstly reported on the 400-Gb/s channels at 100-GHz grid.
© 2013 OSA
With the fast growth of Internet bandwidth demand year by year, 400-Gb/s wavelength-division-multiplexing (WDM) transmissions on ITU-T grid have attracted increasing research interest as the next-generation transport standard [1–8]. Several experimental demonstrations of 400-Gb/s per channel long haul transmission have been reported most recently by using higher level modulation formats with orthogonal frequency division multiplexing (OFDM) or Nyquist multiplexing technologies, most of which utilizes ultra-large-effective-area and ultra-low-loss newly designed fiber [1–3, 6, 7]. There are very few experimental demonstrations, however, for transmissions over existing standard fiber link employing multiple reconfigurable optical add-drop multiplexers (ROADMs), which can seriously degrade the system performance due to the filter narrowing effect [8–11]. On the other hand, going for higher modulation levels, i.e. 16-ary quadrature amplitude modulation (16QAM) and beyond, will in turn come with the expense of higher optical signal-to-noise ratio (OSNR) requirement, limited transmission distance and also increased complexity and area/power of digital signal processing (DSP) in coherent receivers. Alternatively, quadrature-phase-shift-keying (QPSK) modulation format demonstrates the best tradeoff between spectral efficiency (SE) and transmission distance so far. Thus, increasing SE of established 100G PDM-QPSK channels on 50/100GHz ITU-T grid to achieve the 400-Gb/s WDM channels is a promising and cost effective solution for future large-capacity optical transmission networks. Recently, we have achieved 50GHz-grid Nyquist-WDM transmission consisting of 8 × 216.4-Gb/s polarization-division-multiplexing carrier-suppressed return-to-zero QPSK (PDM-CSRZ-QPSK) channels with the SE of 4b/s/Hz over 1750-km G.652 fiber with Erbium-doped fiber amplifier (EDFA)-only amplification . However, the system performance in multiple ROADM link with transoceanic transmission distance was not achieved due to limited equalization capability for narrow filtering compensation when using conventional QPSK algorithms. Different from the regular constant modulus equalization method in [8–11], in our previous work [12, 13], we have proposed and demonstrated a novel DSP scheme for this optical Nyquist filtering 9-QAM like signals based on multi-modulus equalization (MMEQ) without post filter, which directly recovers the Nyquist filtered QPSK to a 9-QAM like signal based on the multi-modulus algorithm with better tolerance for super-Nyquist filtered channels.
In this paper, we experimentally demonstrate a highly filtering-tolerant MMEQ process for very aggressively spectrum-shaped 9-QAM-like PDM-QPSK signal to achieve 400-Gb/s WDM channels on the 100-GHz grid for ultra-long-haul reach and high tolerance of the filter narrowing effect caused by ROADMs. We successfully transmitted 8 channels 480-Gb/s super-Nyquist (channel occupancy much less than signal baud rate) WDM signals at 100-GHz grid over 25 × 200 km conventional single-mode fiber-28 (SMF-28) with post Raman amplification and 25 ROADMs at a net SE of 4b/s/Hz, after excluding the 20% soft-decision forward-error-correction (FEC) overhead. In each 480-Gb/s, 100GHz-grid super-Nyquist channel, two sub-channels with carrier spacing of 50 GHz are used, each carrying 240 Gb/s. The system performance is significantly enhanced by the MMEQ based on 9-QAM-like constellations -compared to the conventional 4-point QPSK constellation. A record transmission distance over conventional SMF-28 with a large number of ROADMs is firstly reported on the 400-Gb/s channels at 100-GHz grid.
2. Principle of 9-QAM-like signal generation, novel DSP algorithms, and experimental setup
The principle and experimental setup of 9-QAM-like signals generation by spectrum shaping on QPSK signals are shown in Fig. 1, where 8 × 480-Gb/s spectrum shaped PDM-9-QAM signals are generated and transmitted over 5000-km SMF-28 and 25 ROADMs at 100-GHz grid. In each 480-Gb/s, 100GHz-grid super-Nyquist channel, two sub-channels with carrier spacing of 50 GHz are used, each carrying 240 Gb/s. At the transmitter, 16 external cavity lasers (ECLs) with a linewidth less than 100 kHz, spacing of 50 GHz and output power of 14.5 dBm are divided into two groups as the odd and even sub-carriers. The two pairs of 60-Gbaud binary electrical signals are generated from an electrical 4:1 multiplexer after multiplexing four-channel 15-Gb/s binary signals. Each I/Q modulator (I/Q MOD) driven by two 60-Gb/s pseudo-random binary sequence (PRBS) electrical signals with a word length of 215-1, is used to modulate odd/even sub-carriers. The polarization multiplexing of each path is realized via the polarization-multiplexer, which consists of a polarization-maintaining optical coupler (PM-OC) to split the signal, an optical delay line (DL1 and DL2) to provide over 100 symbols delay, and a polarization beam combiner (PBC) to recombine the signal. The odd and even channels are spectrally filtered to achieve the 9-QAM-like constellation signals and combined by using a programmable wavelength selective switch (WSS) with 50-GHz fixed grid and 44-GHz 3-dB bandwidth. The measured pass-band transfer function of the 50GHz-grid WSS is shown in Fig. 1(a). The optical spectra of 240-Gb/s single sub-channel signals before and after the 50GHz-grid WSS are shown in Fig. 1(b), where the spectrum-shaped signal occupies a narrower bandwidth. The constellation evolutions of PDM-QPSK signal to the 9-QAM-like signal before and after optical spectrum shaping are shown as Figs. 1(c) and 1(d).
The generated 8 × 480-Gb/s, 100GHz-grid channel signals are then launched into a re-circulating transmission loop, which consists of four 50-km spans of conventional SMF-28 fiber with average loss of 10.5 dB. Each span of the 50-km SMF-28 is followed by a post Raman amplifier to compensate for the fiber loss. The on-off Raman gain is 10 dB per span from ~1450nm pumps. After 4 spans of 50-km SMF-28 transmission, the eight 480-Gb/s channels pass through a 100-GHz spaced WSS to emulate the filtering effect from a 100GHz-grid ROADM. Odd and even channels are then sent to separate WSS output ports, for maximum filtering, and a relative delay of 175 symbols de-correlates the odd and even channels before they are combined in a 3-dB optical coupler. Filtering from the WSS pass-band is measured, as the 3-dB bandwidth of 94 GHz, shown in Fig. 1(e). One EDFA is used to compensate for the switch loss in the loop after the 100-GHz ROADM. Thus, the 8 channels pass the ROADM after one round trip with 200-km distance and totally 25 ROADMs after 5000-km transmission.
At the receiver, one tunable optical filter with 3-dB bandwidth of 0.9 nm is employed to select the desired sub-channel. An ECL with a linewidth less than 100 kHz is utilized as local oscillator (LO). A polarization-diversity 90° hybrid is used for polarization- and phase-diversity coherent detection. The analog-to-digital conversion is realized in the digital scope with 80-GSa/s sample rate and 30-GHz electrical bandwidth.
Figure 2 shows the major DSP functional blocks. A novel cascaded, 9-QAM based, highly filtering-tolerant MMEQ is used for polarization demultiplexing, robust filtering compensation, and other channel distortion mitigation [12, 13]. First, a 17-tap T/2-spaced constant-modulus-algorithm (CMA) equalizer is used to perform the pre-equalization. The output of this CMA equalizer is used for the initial frequency-domain frequency offset estimation and compensation. Then, a 17-tap, T/2-spaced 2 × 2 equalizer as the second stage equalization based on decision-directed least radius distance (DD-LRD) algorithm are used for polarization demultiplexing. The carrier frequency and phase recovery are performed within the DD-LRD loop. The frequency offset is also estimated and compensated using a frequency-domain method. The phase recovery is realized by novel decision-directed blind phase search (BPS) method within a small phase-varying range: the initial phase is recovered by the last symbol but then refined using the BPS over a nonlinear distributed phase range. Such a two-stage algorithm can effectively mitigate cyclic phase slipping. Before the calculation of the bit error ratio (BER), the 1-bit maximum likelihood sequence estimation (MLSE) based on Viterbi algorithm is utilized for symbol decoding and detection to eliminate the inter-symbol interference (ISI) impact. The total errors are counted over 12 × 106 bits.
3. Experimental results and discussions
Figure 3(a) shows the optical spectrum (0.02-nm resolution) of the eight channels of 480-Gb/s signal before fiber transmission. The back-to-back (BTB) BER results of 240-Gb/s subchannel 2 of channel 4 as a function of OSNR (0.1-nm resolution) under different spectrum shaping filter bandwidth are shown in Fig. 3(b). The 3-dB bandwidth of the WSS is changed from 44 GHz to 36.6 GHz. Negligible OSNR penalty for bandwidth larger than 38.8 GHz and less than 0.5-dB penalty are observed by our proposed robust MMEQ process for even 36.6-GHz filtering at the BER of 1 × 10−2. The required OSNR for that Nyquist WDM 240 Gb/s channel at the BER of 1 × 10−2 is 19 dB/0.1 nm, while 22 dB/0.1 nm for 480-Gb/s channels. We can see that, the super-Nyquist 9-QAM signals based on proposed MMEQ have a high tolerance of the filter narrowing effect. In addition, we also verified that all other channels exhibit similar performance except that the side channel has 0.5-dB better OSNR tolerance.
Figure 4 shows the measured BER of channel 4 with and without ROADMs versus transmission distance ranging from 1000 km to 6000 km. The measured BERs after 5000-km SMF-28 transmission without and with 25 ROADMs are 1.4 × 10−2 and 2.0 × 10−2, respectively. It shows that the proposed MMEQ for this super-Nyquist-spectrum-shaped 9-QAM-like signal has a high tolerance of the filter narrowing effect caused by ROADMs. The optical spectrum after transmission over 25 × 200-km SMF-28 with ROADMs is inserted in Fig. 4, where clear filter narrowing effect is observed on the 100GHz-grid channels. The results are obtained under the optimal input power for each distance. The optimal input power of total 8 channels are 0 dBm for 5000-km transmission, which is quite small due to the strong nonlinearity impairments in the low-noise and high power Raman post amplified fiber link. However, the OSNR after 5000-km transmission is still larger than 20 dB for each sub-channel. For all channels, the measured BER after transmission with and without ROADMs are shown in Fig. 5 (average of the two sub-channels). After 5000-km transmission, the BER for all super-Nyquist-WDM channels are below the 2.7 × 10−2 BER threshold for 20% soft-decision FEC using low-density parity-check (LDPC) encoding and layered decoding algorithm . The constellations of the received signal in X and Y polarization of channel 4 with ROADMs after transmission processed by the filtering-tolerant 9-QAM based MMEQ are also inserted in Fig. 5.
A highly filtering-tolerant MMEQ for the super-Nyquist PDM-QPSK signal based on a 9-QAM-like constellation is experimentally investigated to achieve 400-Gb/s channels on the 100-GHz grid for ultra-long-haul optical reach. Negligible OSNR penalty for the 240-Gb/s sub-channel with bandwidth larger than 38.8 GHz and less than 0.5-dB penalty are observed by our proposed robust MMEQ process for even 36.6-GHz filtering at the BER of 1 × 10−2. Finally, 8 channels 480-Gb/s WDM signals at 100-GHz grid over 25 × 200 km conventional SMF-28 with post Raman amplification and 25 ROADMs at a net SE of 4b/s/Hz (after excluding the 20% soft-decision FEC overhead) are experimentally demonstrated with BER below the 2.7 × 10−2 BER threshold.
This work is partly supported by “863” projects with No. 2012AA011303 and 2013AA010501.
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