1. Introduction

In recent years, with the increasing bandwidth requirement of future optical network, coherent detection and digital signal processing (DSP) have been introduced into high-speed optical transmission systems to combat the transmission impairments efficiently [1, 2]. Among various DSP methods, the orthogonal frequency division multiplexing (OFDM) is promising for its intrinsic advantages such as flexibility of spectral division, high spectral efficiency (SE) and the outstanding tolerance against chromatic dispersion (CD) and polarization mode dispersion (PMD) [3, 4]. Moreover, the OFDM enables orthogonal band multiplexing (OBM) [5], in which multiple bands can be multiplexed or demultiplexed with small or zero frequency guard band while avoiding inter-band interference due to inter-band orthogonality. So the OBM-OFDM can improve spectral efficiency and enable continuous waveband, which is sometimes named as a superchannel. The OFDM superchannel is different from conventional WDM scheme with large guard bands between adjacent channels. High-speed optical superchannel transmissions beyond 1 Tb/s have been experimentally demonstrated [3–7] and the superchannel is usually generated by individually modulating multiple optical carriers from an optical comb with equidistant frequency spacing [3–5, 8]. If the frequency spacing between the optical carriers is an integral multiple of OFDM subcarrier spacing, the modulated bands can be seamlessly aggregated to form a superchannel signal. Thus a broader Tb/s signal can be generated in parallel to reduce the bandwidth requirement of commercial electrical and optical components.

Although OFDM technique has some impressive advantages, the main drawback of OFDM is the high peak-to-average-power ratio (PAPR), which may induce severe nonlinear impairments and inefficient power consumption [9–11]. Recently, the coherent optical single-carrier frequency-division-multiplexing (CO-SCFDM) utilizing discrete Fourier transform (DFT)-Spread-OFDM has been proposed [12, 13], which is a modified form of OFDM and has been employed in uplink single-carrier frequency-division-multiple-access (SC-FDMA) scheme for the Long Term Evolution (LTE) of cellular systems by the Third Generation Partnership Project (3GPP) [14]. Simulation results have shown that the SCFDM has similar CD tolerance as OFDM while achieves better nonlinear impairment tolerance with lower PAPR [12, 13].

In this paper, we demonstrate the generation of 1.08 Tb/s CO-SCFDM superchannel signal and transmission over 3170 km standard single mode fiber (SSMF). The paper is structured as follows. In Section 2 the experimental setup for 1.08 Tb/s CO-SCFDM re-circulating loop transmission is described. Subsequently, the DSP method for polarization-division multiplexing (PDM) CO-SCFDM is discussed. In Section 3 the experimental results including back-to-back transmission performance, optical spectra for CO-SCFDM superchannel, nonlinear transmission performance and the maximum transmission reach are presented. In Section 4 we draw the conclusions.

2. Experiment setup

Figure 1 shows the block diagram of the experiment setup. At the transmitter, an optical comb with equidistant frequency spacing is required to construct the Tb/s SCFDM superchannel. Some approaches have been proposed for the generation of the optical comb. In [5], a recirculating frequency shifter is utilized, which recirculates the optical carriers and shift the frequency each time to generate a new tone. The number of optical tones can be adjusted by the optical band-pass filter (OBPF) in the loop. We adopt similar approach with [3], in which the optical comb is generated by modulating a group of lasers with a 5-tone generator. In our experiment, 4 lasers with the frequency spacing of 93.75 GHz (10 times of tone spacing f) are combined by cascaded 2 × 1 polarization maintain couplers (PMC). A Tektronix arbitrary waveform generator (AWG) is used to generate the baseband SCFDM or OFDM signals operating at 10 GS/s. 4 original odd subbands are obtained after the IQ modulator1. Then we adopt the same scheme in [8] to generate the even subbands. The odd subbands are splitted into two streams by PMC4. The lower stream passes directly without any change while the upper one passes a frequency shifter, which is realized by driving the IQ modulator2 with 9.375 GHz radio frequency (RF) signal. Two phase shifters (PS) are used for adjusting the phase difference between the two arms. Before being combined by PMC5, the even and odd subbands are delayed by optical patchcords with different length for decorrelation. Then all the 8 subbands pass the 5-tone generator, which is realized by driving the intensity modulator with the combination of 18.75 and 37.5 GHz (2 and 4 times of f, respectively) RFs. Thus the 40-subband superchannel with the spacing f of 9.375 GHz is generated. The PDM is emulated with a polarization controller (PC), a polarization beam splitter (PBS), a tunable optical delay line and a polarization beam combiner (PBC). The delay of the optical delay line is exactly one SCFDM or OFDM block. The transmitter lasers have the linewidth of about 5 kHz.

 

Fig. 1 Block diagram of the PDM CO-SCFDM/CO-OFDM experiment setup.

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The transmission loop consists of 4 spans, each including an Erbium-doped fiber amplifier (EDFA) and a SSMF. No inline chromatic dispersion compensation is used. At the end of the loop, we applied an OBPF1 with a bandwidth of 3.4 nm to mitigate the noise accumulation. A loop controller drives the two optical switches and triggers the acquisition of oscilloscope.

At the receiver, the OBPF2 with a bandwidth of 0.4 nm is used to remove out-of-band ASE noise and there will be multiple subbands sent into the coherent receiver at a time. The filtered signal is sent into a 90°hybrid afterwards to interfere with a local oscillator (LO). The LO is a tuneable laser with the linewidth of 100 kHz. 4 balanced detectors (BD) are used to detect the polarization diverse signals. The electrical signals after the BDs pass electrical low-pass filters with a bandwidth of 7.2 GHz to select only one subband. Then the signals are sampled by a real-time digital storage oscilloscope (Tektronix DPO72004B) operating at 50 GS/s and then processed offline.

Figure 2 shows the DSP method of the baseband PDM SCFDM signals, which is very similar to that of PDM OFDM signals [13]. But a better understanding can be achieved by comparing the SCFDM with the single-carrier frequency-domain equalization (SCFDE) [15]. At the SCFDE transmitter, the single-carrier quadrature amplitude modulation (QAM) signals are grouped into blocks and the insertion of cyclic prefix (CP) and cyclic suffix (CS) ensure that each block is not ‘polluted’ by adjacent blocks. At the receiver, each block should be regarded as a whole for frequency-domain processing. A DFT is utilized to transform the block into frequency-domain for equalization. After the equalization, the block should be transformed into time-domain for decision. The only difference between SCFDM and SCFDE is the precoder in the SCFDM transmitter, which is realized by a pair of Fourier transforms and subcarrier mapping. During the subcarrier mapping, image subcarriers can be introduced to reshape the frequency spectrum and reduce the out-of-band energy leakage, which is similar with the subcarrier arrangement at an OFDM transmitter. Moreover, if the guard band between adjacent SCFDM subband is the multiple of subcarrier spacing, the DFT and IDFT processing will guarantee that orthogonality condition is not only satisfied within a specific SCFDM subband, but also satisfied for any two subcarriers in different subbands. So multiple SCFDM subbands can construct an optical superchannel with small or even zero guard band, just like the OFDM. The PAPR of the SCFDM is lower than that of the OFDM, but is higher than pulse-shaped SCFDE. The SCFDM has two pairs of Fourier transforms and the computation complexity is higher than that of the OFDM.

 

Fig. 2 Block diagrams for the baseband SCFDM DSP method (a) at the transmitter (b) at the receiver.

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In our experiment, the mapped quadrature phase shift keying (QPSK) signals are first grouped into blocks of 98 symbols each, and 4 pilots are time-multiplexed into each block. Then the signal is transformed into frequency domain by 102-point DFT. In the frequency domain, 26 guard subcarriers are added and equally allocated at both sides of the band. After subcarrier mapping, the signal is converted back to time domain by 128-point IDFT. The CP and CS with 8-symbol length each are inserted in each data block before transmitted [16]. The length of SCFDM block is 14.4 ns. The preamble is inserted after each 200 SCFDM blocks, which consists of two synchronization symbols for synchronization and 4 time-multiplexed training symbols for polarization division demultiplexing and channel equalization. The training symbol design is the same as that of PDM CO-OFDM frame [17]. Considering the overhead of image subcarriers, pilot subcarriers, CP and CS, the net bit-rate of total 40 subbands is 1.08 Tb/s with the SE of about 2.9 b/s/Hz. At the receiver, after removing CP and CS, the signals for both polarizations are transformed to frequency domain to remove guard subcarriers and channel equalization is jointly performed. Then the equalized signals are transformed back to time domain, in which the phase of each block is firstly corrected utilizing pilots and then averaged with the Viterbi-Viterbi method. At last, bit-error rate (BER) is calculated. In this experiment, we also generated OFDM signal which has the same frame structure and bit rate with SCFDM signal for comparison.

3. Results and discussion

Figure 3 shows the BER performances for back-to-back transmission. We measure the 30th subband of the SCFDM/OFDM superchannel. Here the small band index corresponds to the short wavelength. For comparison, we also measure the performances of single channel SCFDM/OFDM signals when the multi-carrier generation unit in Fig. 1 is bypassed and only one laser is used. The OSNR penalty after multi-band multiplexing at the BER of 1 × 10−3 is about 1.5 dB. We can see that the CO-SCFDM performs better than the CO-OFDM for both single channel and 40-subband scenarios. This is because that the OFDM has higher PAPR than SCFDM and suffers more nonlinear distortion induced by the IQ-modulator.

 

Fig. 3 BER curves of the 30th subband for back-to-back transmission. The BER curves of single channel signals are also shown for comparison.

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Figure 4(a) shows the spectrum of all the 4 optical carriers after PMC3. Because the gain spectra of EDFA1 and EDFA2 that we used are not flat, the output powers of 4 lasers are adjusted to compensate for the slope. Figure 4(b) shows the spectrum of the combination of even and odd subbands after PMC5. Figure 4(c) shows the spectra of 1.08 Tb/s PDM CO-SCFDM/CO-OFDM signal. The black line shows the spectrum of the signal generated from the transmitter before optical switch1. The optical signal-to-noise ratio (OSNR) is always larger than 30 dB for all the subbands. The red line shows the spectrum of the 1.08 Tb/s signal after 317 km SSMF transmission and the blue line shows the spectrum after 317 km SSMF and OBPF1.

 

Fig. 4 Optical spectra for (a) all the 4 optical carriers after PMC3 (b) the combination of even and odd subbands after PMC5 (c) 1.08 Tb/s CO-SCFDM/CO-OFDM signal of back-to-back and after 317 km SSMF transmission.

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Figure 5 shows the BER of 30th SCFDM/OFDM subband as a function of launched power per subband over 2536 km SSMF. We can see that the optimum launched power of SCFDM is about 1.2 dB higher than that of OFDM, which means it suffers less transmission nonlinearity.

 

Fig. 5 Measured BER of 30th subband vs. launched power per subband over 2536 km SSMF transmission.

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We have measured the maximum reach for CO-SCFDM and CO-OFDM, which are 3170 km and 2536 km with our experimental setup, respectively. The launched powers per subband for them are −7 and −8 dBm, respectively, which are optimized by experiments. Figure 6(a) gives the measured BERs for the CO-SCFDM superchannel after 3170 km SSMF transmission. Figure 6(b) gives the measured BERs for the CO-OFDM superchannel after 2536 km SSMF transmission. The BERs for all subbands are smaller than the FEC threshold of 3.8 × 10⁻³. The inset in Fig. 6(a) gives the constellations of subband 35, which has the worst performance and the inset in Fig. 6(b) gives the constellations of subband 29.

 

Fig. 6 Measured BERs for (a) the CO-SCFDM superchannel after 3170 km SSMF transmission and (b) the CO-OFDM superchannel after 2536 km SSMF transmission.

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4. Conclusions

We experimentally demonstrate the generation and transmission experiment of 1.08 Tb/s PDM CO-SCFDM signal. We compare the CO-SCFDM with the CO-OFDM with the same frame structure and bit rate. The back-to-back transmission performances show that the CO-SCFDM suffers less nonlinear distortion by optical modulator because of lower PAPR. The nonlinear transmission performances show that the CO-SCFDM suffers less nonlinear transmission impairments. Moreover, the CO-SCFDM can achieve a larger maximum transmission reach than the CO-OFDM.