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ECL 2 … ECL 12 ECL 1 … ECL 11 7 Gb/s PM-OC PM-OC DSB: frequency doubler EA: electrical amplifier ECL: external cavity laser EDFA: erbium-doped optical fiber amplifier S pecial Topic 1 Tb/s Nyquist-WDM PM-RZ-QPSK Superchannel Transmission over 1000 km SMF-28 with MAP Equalization Ze Dong, Jianjun Yu, and Hung-Chang Chien Mux I/Q MOD 7 Gb/s Mux I/Q MOD 28 Gb/s PM EDFA 28 Gb/s PM EDFA I/Q MOD: I/Q modulator IL: optical interleaver IM: intensity modulator OSC: real-time scope ▲Figure 1. Testbed setup for 1Tb/s Nyquist-WDM superchannel transmission. the ISI. The aggregated terabit Nyquist-WDM superchannel is then launched into a recirculating loop comprising five 80 km SMF-28 spans with an average loss of 16.7 dB and chromatic dispersion of 17 ps/km/nm at 1550 nm. The loop has no optical dispersion compensation modules. For each span, an EDFA with midstage adjustable-tilt filter is used to provide flat gain. A tunable optical bandpass filter is also used to remove RZ IM IM RZ EA ATT DSB ATT EA amplified spontaneous emission (ASE) noise. The total launch power into the transmission fiber is 10.7 ± 4 dBm (-4 to approximately 4 dBm per subchannel) at 112 Gb/s. After that, a subcarrier is selected using a tunable optical filter (TOF) for coherent detection. At the receiver, an ECL with a linewidth less than 100 kHz is used as the fiber laser local oscillator (LO). A polarization-diverse 90 degree hybrid is used for polarization and phase-diverse coherent detection of the LO and received optical signal before balance detection is performed. Analog to digital sampling and digitization occurs in the digital scope, which has a 40 GSa/s sample rate and 16 GHz electrical bandwidth. The captured data is processed through an offline DSP. First, the clock is extracted using a square and filter method, and the digital signal is resampled at twice the baud rate of the recovery clock. Second, a T/2-spaced time-domain finite impulse response (FIR) filter is used to compensate for chromatic dispersion. Third, two complex-valued, 13-tap, T/2-spaced adaptive PM PM 25 GHz IL 14 GHz RF 25 GHz IL Signal LO Fiber 25 GHz IL Optical Power (dBm) Optical Power (dBm) TOF 90° Hybrid Offline DSP PD PD PD PD Coherent Detection Receiver O S C PM: polarization multiplexer PM-OC: polarization maintenance optical coupler TOF: tunable optical filter 0 -20 -40 -60 -80 1541 0 -20 -40 -60 -80 -100 1541 (a) (c) FIR filters, based on classic constant modulus algorithm (CMA), are used to retrieve the modulus of the QPSK signal. Carrier recovery is then performed. The feed-forward fourth power is used to estimate the frequency offset between the LO and received optical signal. Then, the Viterbi-Viterbi algorithm is used to estimate the carrier phase. To improve the transmission performance of a Nyquist-WDM PM-RZ-QPSK signal subject to tight optical filtering and crosstalk, we propose MAP equalization with high data-pattern dependence. First, a sequence of data symbols with BER less than 3 × 10 -4 is decided before averaging so that the symbols can be arranged into a data dependence pattern (64 kinds in the case of QPSK) to be a decision reference. Then, the data received after DSP is calculated by correlating with the decision reference to the maximum extent and mapping the QPSK data dependence decisions. 3 Experiment Results The CW light waves (ECL1-12) range from 1541.5 nm to 1543.7 nm with wavelength spacing of 25 GHz. An IM with appropriate DC bias and electrical amplifier power control are used to create an RZ pulse with 44% duty cycle. The Vpp of the 28 GHz RF signal is 17 V. Fig. 2(a) shows a single-carrier 28 Gbaud QPSK signal before the RZ carver, and Fig. 2(b) shows a single-carrier 28 Gbaud QPSK signal after the RZ 1542 1543 1544 -100 1541 1542 1543 1544 Wavelength (nm) Wavelength (nm) 0 (d) 1 3 5 7 9 11 1542 1543 Wavelength (nm) 18 dB 1544 ▲Figure 2. Optical spectra (0.1 nm) (a) before RZ, (b) after RZ, (c) after two ILs for ECL7, and (d) after two ILs for ECL1-11. Optical Power (dBm) Optical Power (dBm) 0 -20 -40 -60 -80 -20 -40 -60 -80 -100 (b) 1541 1542 1543 Wavelength (nm) 1544 March 2012 Vol.10 No.1 <strong>ZTE</strong> COMMUNICATIONS 51
S pecial Topic 1 Tb/s Nyquist-WDM PM-RZ-QPSK Superchannel Transmission over 1000 km SMF-28 with MAP Equalization Ze Dong, Jianjun Yu, and Hung-Chang Chien Optical Power (dBm) Optical Power (dBm) ▲Figure 3. Optical spectra (0.1 nm) of 1Tb/s signal in (a) back-to-back transmission and after (b) 1000 km, (c) 2000 km, and (d) 2400 km SMF-28 transmission. carver. Fig. 2(c) shows the optical spectrum of the subchannel (ECL7) after two stages of 25/50 ILs. The ratio of in-band signal power to out-of-band signal leakage is about 18 dB. Such out-of-band leakage can cause ICI to spread to adjacent even channels that are 50 GHz apart. However, this is not an issue in a practical system where each subchannel is independently modulated and then combined through an arrayed waveguide grating (AWG) with the subchannel bandwidth highly confined. Fig. 2(d) shows the optical spectrum of subchannels ECL1-11 after two stages of ILs. To simulate a practical system, we turn the fifth and ninth subchannels off to mitigate crosstalk and assess the performance of the seventh subchannel after transmission. Fig. 3 shows the optical spectra of ten subcarriers with and without transmission. -log (BER) 52 0 -20 -40 -60 -80 -20 -40 -60 1 2 3 4 5 (a) 1 2 3 4 6 7 8 10 11 12 (5) (9) 1541 1542 1543 1544 Wavelength (nm) (c) 1541 1542 1543 1544 Wavelength (nm) 12 16 20 24 <strong>ZTE</strong> COMMUNICATIONS 28 32 36 Optical Power (dBm) Optical Power (dBm) -20 -40 -60 -20 -40 -60 ■ : Single Subchannel Without IL ▲: Single Subchannel With 2ILs (a) ●: Even Subchannel after 2ILs ★: WDM Subchannels without MAP ❋: WDM Subchannels ● ■ ★ with 3 Tap MAP ▲ ❋★ ▲ ★ ● ❋ ★ ■ ▲ ❋ ★ ● ❋❋★ ■ ▲● ★ ❋ ★ ❋❋★ ★ ■ ▲ ● ❋❋ ▲ ● ■ ▲● ▲● ■ ▲ ▲● ■ ■ OSNR (dB) 40 March 2012 Vol.10 No.1 (b) 1541 1542 1543 1544 Wavelength (nm) (d) 1541 1542 1543 1544 Wavelength (nm) -log (BER) 1 2 3 -5 -4 ● We then characterize the back-to-back performance of the seventh subchannel by aligning its subwavelength to that of the LO and turn all the other subchannels off (Fig. 4a). The required OSNR for a BER of 3.8 × 10 -3 is 14.5 dB with aggressive filtering and 19.5 dB without aggressive filtering for 0.1 nm resolution. The filtering is performed by two stages of interleavers (two ILs), and 650, 000 symbols are counted as BER. When all the odd subchannels are turned on, the OSNR penalty caused by ICI between 50 GHz spaced subcarriers is 0.9 dB. For a 1 Tb/s Nyquist-WDM signal, the OSNR penalty caused by 25 GHz adjacent subcarriers and tight filtering is about 13 dB. For 1000 km SMF-28 Nyquist-WDM transmission (Fig. 4b), the BER of the seventh subcarrier can barely reach the hard-decision (HD) pre-FEC limit of 3.8 × 10 -3 at any launch power level unless MAP equalization is used. With MAP equalization, the BER can be brought well below the FEC limit at subchannel launch powers of -1 and -2 dBm. Similarly, for 2000 km SMF-28 Nyquist-WDM transmission (Fig. 4c), BER below soft-decision (SD) pre-FEC limit of approximately 1 × 10 -2 can be obtained with MAP equalization at input power of -1 dBm. (Fig. 4c). The bit rate should be 120 Gb/s or higher if SD FEC overhead is considered. 4 Conclusion We have described the generation, transmission, and coherent detection of a 1 Tb/s Nyquist-WDM superchannel where each subchannel carries a 112 Gb/s PM-RZ-QPSK signal on a 25 GHz grid. The robustness of MAP nonlinear equalization against tight Nyquist-WDM filtering and crosstalk has also been shown in a widely deployed SMF-28 fiber with an 80 km-span EDFA system. With subchannel launch power (b) ■ : 1 Tb/s Signal Over 1000 km (c) ● ■ : 1 Tb/s Signal Over 1000 km SMF-28 Transmission without MAP ● ■ -3 SMF-28 Transmission with 3 Tap MAP ● ■ -2 Launch Power (dBm) ▲Figure 4. (a) BER curve of seventh subchannel (back-to-back), (b) BER curve for 1000 km SMF-28 transmission, and (c) BER curve for 2000 km SMF-28 transmission. ● ■ -1 ● ■ 0 ● ■ 1 ● ■ 2 ● ■ FEC 3 4 -log (BER) 1 2 3 ■ -2 ● ■ ● ■ : 1 Tb/s Signal Over 2000 km SMF-28 Transmission : 1 Tb/s Signal Over 2000 km SMF-28 Transmission with 3 Tap MAP ● -1 ■ ● 0 ■ ● Launch Power (dBm) 1 ■ ● SD-FEC 2
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