Coherent Nyquist pulses have been used for optical time division multiplexed (OTDM) digital coherent transmission, and a single-channel 1.92 Tbit/s, Pol-Mux-64 QAM coherent Nyquist pulse transmission over 150 km is demonstrated. The ability to considerably reduce the spectral bandwidth of the data signal enabled us to increase the spectral efficiency from 3.2 bit/s/Hz to 7.5 bit/s/Hz when using a Gaussian pulse train.
© 2014 Optical Society of America
Recently, high-speed optical communication research has been focusing on channel capacities of 1 Tbit/s and beyond. Several groups have already demonstrated a single-channel transmission capacity of over 1 Tbit/s by combining multi-level modulation formats and optical time division multiplexing (OTDM) for coherent return-to-zero (RZ) pulses. For example, a single-channel 10.2 Tbit/s, 16 quadrature-amplitude modulation (QAM) coherent transmission at 10 Gbaud x 128 OTDM over 29 km has been demonstrated using self-homodyne detection . We also demonstrated a single-channel 1.92 Tbit/s, 64 QAM coherent optical pulse transmission over 150 km at 10 Gbaud x 16 OTDM by employing optical phase-locked loop (OPLL), RZ-CW conversion and frequency domain equalization (FDE) schemes . However, coherent OTDM transmissions employing ultra-short Gaussian or sech pulses generally occupy large bandwidths, making it difficult to realize higher spectral efficiencies. For example, the spectral efficiency of the 1.92 Tbit/s, 64 QAM transmission was 3.2 bit/s/Hz, since the spectrum occupies a bandwidth of 562.5 GHz even at the −20 dB level.
We recently proposed a novel high-speed TDM transmission scheme that employs an optical Nyquist pulse train, in which the signal bandwidth can be reduced to as low as the symbol rate . Nyquist pulse generation does not simply consist of installing a Nyquist filter as originally reported in 2008 , which is sometimes referred to as “Nyquist pulse shaping”  but does not involve actual optical pulses and their TDM. The envelopes of optical Nyquist pulses possess a sinc-like waveform that has a periodic oscillating tail with zero crossing at every symbol period. Using the optical Nyquist pulse train, we can achieve a high-speed OTDM transmission without inter-symbol interference (ISI), by setting the OTDM period equal to the zero-crossing interval . This is in contrast to Gaussian and sech pulses that require adequate inter-symbol separation to avoid ISI. The Nyquist pulse allows high data rate OTDM at a reduced signal bandwidth, which leads to increased tolerance to group velocity dispersion (GVD) and polarization mode dispersion (PMD), as well as improved spectral efficiency. 1.28~2.56 Tbit/s DPSK transmissions have already been demonstrated with non-coherent Nyquist pulses [6, 7]. In addition, an efficient add-drop operation has been carried out in a Nyquist OTDM-WDM system . As a different approach for high-speed Nyquist signal generation, optical Nyquist filtering to OTDM RZ pulses has also been demonstrated to generate 107 Gbaud, 16 QAM signals .
Thus far, the Nyquist pulse has been non-coherent because we did not use coherent Nyquist pulses in the transmission system. If we can generate a coherent Nyquist pulse, we can greatly improve the spectral efficiency using, for example, QAM. We recently applied Nyquist pulses to coherent OTDM transmission and demonstrated a preliminary transmission experiment with a polarization-multiplexed (Pol-Mux) 160 Gbaud (10 Gsymbol/s x 16 OTDM) 64 QAM signal . In this paper, we describe the generation of coherent Nyquist pulses and their application to Pol-Mux-64 QAM transmission in detail. A single-channel 1.92 Tbit/s (10 Gbaud x 16 OTDM x 2 pol. x 6 bits) signal was transmitted over 150 km within a bandwidth of 240 GHz, corresponding to a spectral efficiency of as high as 7.5 bit/s/Hz.
2. Principle of coherent OTDM-Nyquist pulse transmission
The principle of coherent Nyquist pulse transmission is shown in Fig. 1.First, a broad optical comb is generated from a CW laser. This comb is subsequently passed through a pulse shaper,thus generating a Nyquist pulse train. Here, the waveform and spectral profile of Nyquist pulses are given by11]. The pulse waveform is a sinc-like waveform with an oscillating tail that has zero-crossing points at periodic intervals. After Nyquist pulse generation, the optical signal is QAM modulated at a baud rate of fs. Thereafter, the modulated signal can be OTDM multiplexed to a baud rate of Nfs with bit interleaving by aligning the interval to the zero-crossing periods, which can be achieved by setting the symbol period T of the optical Nyquist pulses at T = 1/Nfs. In this way, high symbol rates can be achieved at a reduced signal bandwidth without ISI as shown in Fig. 1. After transmission, the signal is OTDM demultiplexed and converted to a data signal loaded on a CW carrier by employing an RZ-CW conversion process . This is done with the aim of increasing the signal optical signal-to-noise ratio (OSNR) within the demodulation bandwidth. The RZ-CW converted data signal is thereafter coherently detected using a CW-local oscillator (LO) phase locked to the data signal through an OPLL circuit. Finally, demodulation is carried out using digital signal processing (DSP) in an offline condition.
3. Experimental setup
The experimental setup for a single-channel 1.92 Tbit/s, Pol-Mux-64 QAM coherent optical Nyquist pulse transmission is shown in Fig. 2.At the transmitter, we used an acetylene (C2H2) frequency-stabilized fiber laser with a linewidth of 4 kHz  as a coherent optical source. The optical output from this laser was fed into a comb generator consisting of a dual-drive LiNbO3 (LN) Mach-Zehnder modulator . Figure 3 shows the optical comb spectrum generated at the comb generator measured using an optical spectrum analyzer with a 0.01 nm resolution bandwidth. The 10 dB bandwidth was 490 GHz. Using a pulse shaper  followed by single-mode fiber (SMF) for chirp compensation, we obtained a 10 GHz optical Nyquist pulse train with a roll-off factor α of 0.5. Figures 4(a) and 4(b) show the coherent optical Nyquist pulse spectrum and waveform, respectively. From Fig. 4, it can be seen that the generated Nyquist pulse spectrum and waveform accurately fit the Nyquist profile. The coherent Nyquist pulse train was then modulated at an IQ modulator driven by a 64 QAM data signal from an arbitrary waveform generator (AWG). At the AWG, we carried out pre-compensation for the nonlinear phase rotation induced by self-phase modulation (SPM) during transmission. The generated 10 Gbaud Nyquist pulse signal was then multiplexed to160 Gbaud using a planar light-wave circuit (PLC)-type OTDM multiplexer. Pol-Mux was then performed by using a polarization beam combiner (PBC), which doubled the bit rate to 1.92 Tbit/s. At the same time, we extracted the 11th harmonics of the optical comb output spectrum using an optical filter with a bandwidth of 6.5 GHz. This signal was used as a pilot tone signal for the optical phase-locked loop (OPLL) process employed at the receiver . The 1.92 Tbit/s signal was combined with the pilot signal before being fed into the transmission link. Figure 5 shows the optical spectrum of the 1.92 Tbit/s, Pol-Mux-64 QAM data combined with a pilot signal before transmission. Here the signal bandwidth, including the pilot tone signal, was 240 GHz (160 Gbaud x (1 + α)).
The transmission link was a 150 km-long dispersion-managed fiber (DMF) consisting of two 75 km spans with an average loss of 17.5 dB/span. Each span consisted of a 50 km super large area (SLA) fiber with a dispersion of 19.5 ps/nm/km and a 25 km inverse dispersion fiber (IDF) with a dispersion of −40 ps/nm/km. Here, an SLA fiber with a large effective area Aeff of 106 μm2 was employed to reduce phase rotation arising from fiber nonlinearities. We adopted in-line dispersion compensation using DMF, not during the DSP, due to the optical demultiplexing required prior to coherent detection.
At the receiver, the 1.92 Tbit/s, Pol-Mux-64 QAM data signal was first polarization-demultiplexed using a polarization beam splitter (PBS) followed by OTDM demultiplexing with a nonlinear optical loop mirror (NOLM) to the baseline 10 Gbaud signal. The NOLM operation, illustrated in Fig. 6, involved ultrashort optical sampling, and extracting data from the overlapped multiplexed Nyquist sequence only at the ISI-free point . The loop in the NOLM consisted of a 100 m-long highly nonlinear fiber (HNLF) with a nonlinear coefficient γ of 20.4 W−1km−1, a dispersion slope of 0.029 ps/nm2/km, and its zero dispersion wavelength at 1522 nm. The control pulse source for the NOLM consisted of a CW-distributed feedback (DFB) laser diode (LD) operating at 1564.74 nm, a comb generator, a pulse shaper, an SMF, and 2 km of HNLF with a nonlinear coefficient γ of 5 W−1km−1, a dispersion slope of 0.003 ps/nm2/km and its zero dispersion wavelength at 1550 nm. Here, the comb generator was driven by a clock signal extracted from the transmitted 1.92 Tbit/s, Pol-Mux-64 QAM Nyquist pulse data signal . The output beam from the DFB LD was fed into the comb generator followed by a pulse shaper that carved out a 2.4 ps 10 GHz Gaussian pulse train after chirp compensation with the SMF. The pulse train was then amplified and compressed through the HNLF to 800 fs.
After OTDM demultiplexing, the 10 Gbaud, 64 QAM data signal was passed through an RZ-CW conversion circuit. RZ-CW conversion is a process that leads to the narrowing of the spectral width, which results in an increase in the optical peak power at the center frequency . This enables the received signal to be modulated at a higher signal-to-noise ratio (SNR) within a narrow demodulation bandwidth. This circuit consisted of a dispersion compensation fiber (DCF) with a dispersion of −69 ps/nm and an LN phase modulator driven by a 10 GHz clock signal extracted from the transmitted data signal at a modulation depth of 2.5π. Following the RZ-CW conversion, the data signal was homodyne detected with a CW-local oscillator (CW-LO), which is a frequency-tunable fiber laser with a linewidth of 4 kHz , using a 90-degree optical hybrid. The CW-LO was phase-locked to the data signal through an OPLL process using the pilot tone signal. After homodyne detection, the data signal was A/D-converted at a sampling rate of 40 Gsample/s and demodulated in an offline condition using digital signal processing (DSP). Here, waveform distortion compensation was carried out in the frequency domain using FDE .
4. Experimental results
We first investigated the optimum roll-off factor for the Nyquist pulse used in the transmission experiment. Figure 7(a) shows the bit error rate (BER) of the demodulated 10 Gbaud, 64 QAM data signal under a back-to-back condition for different values of α. For lower α values, the slow decay of the oscillating tail results in a smaller margin against timing jitter. The BER performance converged to a minimum level at around α = 0.5, which we chose as our optimum value. The optimization of the launch power into the transmission fiber is shown in Fig. 7(b). Here, launch power is defined as the optical power of the 1.92 Tbit/s data and pilot tone signals combined at the transmitter. This figure shows the BER for thedemodulated 10 Gbaud, 64 QAM data signal after a 150 km transmission as a function of the transmission power. The best BER was obtained when the transmission power was 4 dBm. At lower transmission powers the demodulation results were degraded due to insufficient OSNR. On the other hand, at higher powers, the BER deteriorated as a result of cross-phase modulation (XPM) between the two orthogonal polarizations. The optical spectra of the 1.92 Tbit/s, Pol-Mux-64 QAM Nyquist pulse data signal before and after transmission at the optimum power are shown in Fig. 8.The spectra were measured with a resolution of 2 nm. This figure shows that there was an OSNR degradation of 9 dB, from 33 dB to 24 dB, during transmission.
Figure 9(a) shows the relationship between the control pulse width used in the OTDM demultiplexing operation and the error vector magnitude (EVM) of the demodulated 64 QAM signal. There was a decrease in the EVM as the control pulse width was reduced, and it eventually converged to 3.2% at around 800 fs. With a wider control pulse, there was an increase in the EVM due to the increased leakage of ISI from adjacent tributaries. Figure 9(b) shows the EVM of the demodulated 64 QAM data signal with respect to the input power to the NOLM. As the input power was reduced to below 17.5 dBm, the EVM values increased, owing to insufficient OSNR. For input powers higher than 17.5 dBm, there was a waveform distortion in the NOLM associated with the nonlinear propagation in the HNLF. We therefore set the launch power at 17.5 dBm.
Figure 10 shows the received signal spectra before and after the signal passed through the RZ-CW conversion circuit. The black and red curves correspond to the optical spectrum of the received 10 Gbaud, 64 QAM signal before and after RZ-CW conversion, respectively. The figure reveals that there was a considerable reduction in the spectral bandwidth, and the optical peak power was increased by 9 dB at the center frequency. The shoulder peaks at 1536.6 nm and 1540.6 nm result from interference caused by the overlapping of adjacent pulses after they had passed through the DCF in the RZ-CW conversion circuit.
Figures 11(a) and 11(b), respectively, show the constellation maps of the demodulated 10 Gbaud, 64 QAM signals before and after 150 km transmission. Their respective EVMs were 3.2% and 4.9%. From the figure, it can be seen that there was broadening of the constellation points caused by OSNR degradation during transmission.
Figure 12(a) shows the BER characteristics of the demodulated signals with respect to the received power for one tributary. The red line represents the back-to-back BER characteristics while the blue curve shows the characteristics after transmission. The BER for all the tributaries at a received power of −12 dBm is shown in Fig. 12(b). BERs below the forward error correction (FEC) limit (2x10−3) were obtained for all the tributaries. Furthermore, at a BER of 2x10−3, there was a power penalty of 4.5 dB resulting from the transmission. We attributed this mainly to the OSNR degradation in the transmission link. There was also an error floor observed in the BER after transmission, which was a consequence of the XPM between the two polarizations in the fiber link. In this experiment, a 1.92 Tbit/s signal transmission was achieved within a 240 GHz bandwidth, which corresponds to a potential spectral efficiency of 7.5 bit/s/Hz in a multi-level transmission, taking the 7% FEC overhead into consideration. This is a considerable improvement from the spectral efficiency of 3.2 bit/s/Hz that we obtained when we used a Gaussian pulse train.
We demonstrated a single-channel, 1.92 Tbit/s, Pol-Mux-64 QAM coherent Nyquist pulse transmission over 150 km using RZ-CW conversion and FDE schemes. By employing Nyquist pulses for the coherent OTDM transmission, we were able to increase the spectral efficiency to 7.5 bit/s/Hz. This is, to the best of our knowledge, the highest spectral efficiency yet achieved in a single-channel transmission beyond Tbit/s.
We thank T. Hara and S. Oikawa of Sumitomo Osaka Cement Co., Ltd for providing the low Vπ dual-drive LN Mach-Zehnder modulator used in the optical comb generator.
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