1. Introduction

The wavelength division multiplexing passive optical network (WDM-PON), taking advantages of high capacity, strong security, and high flexibility, is a future-proof access technology in which a wavelength splitter is used to realize a virtual point-to-point connection with dedicated bandwidth as well as to save on the loss budget for possible network reach extension [1–4]. With the rapid growth of emerging data-centric services, 400 Gb/s or even 1 Tb/s per channel tends to be the next targeted bit rate for long-haul optical transmission. At the same time network carriers are actively seeking a 100 GbE capable, highly convergent metro and access network solution with unprecedented broadband connectivity from end to end [5–7]. Delivering 100 GbE services over WDM-PON is technically feasible but remains challenging in many aspects especially when the channel spacing has to be narrower to accommodate more users sharing the available bandwidth in the near future. Orthogonal frequency division multiplexing (OFDM) modulation technique has recently been introduced in high-speed optical transmission systems [8–13], which is potential to be leveraged in aforementioned WDM-PON to enhance the spectral efficiency. However, implementing FFT-based OFDM at such high bit rate is restricted by the operation speed and expense of electrical circuitries such as DAC/ADC [8]. In principle, coherent optical OFDM (CO-OFDM) can also be implemented all optically with sub-carriers demodulated by either direct detection [9] or digital coherent receivers [11–13]. The latter with significant improvement on the receiver sensitivity has been widely recognized, and the crosstalk between carriers can be further reduced if they are optically interleaved beforehand [13]. CO-OFDM is an attractive candidate for next generation access network because of its high spectral efficiency and superior tolerance to ISI. However, the performance of a CO-OFDM uplink may be restricted by the symbol transition misalignment and relative power mismatch between subcarriers, which can’t be easily controlled in a lightwave-centralized PON system where the CO-OFDM modulation is performed at remote optical network units (ONUs) with all the uplink optical subcarriers launched from the central office. Therefore, as part of the research effort to investigate on the next-generation access technologies with feasible high bit-rate and high spectral efficiency, in this paper, we propose an Ultra-dense (UD) WDM-PON system delivering carrier-centralized Nyqusit-WDM uplink signals with coherent detection, and demonstrate that Nyquist-WDM optical spectral shaping technique is promising for terabit WDM-PON applications [14,15]. In this paper, 13 coherent, 25-GHz spaced optical carriers generated from a single centralized laser source are individually carrying 25 Gbaud/s dual-polarization (DP) QPSK signals and narrow optical filtered, 80-km SSMF uplink transmission is experimentally demonstrated. We introduce an ‘ultra-dense’ concept into next-generation WDM-PON systems and experimentally investigate the performance of a Terabit ultra-density super-channel using 13 × (25GBaud) DP-QPSK subcarriers (1.3 Tb/s), with coherent detection. The spacing was set at 25 GHz, which exactly equals to the Baud rate, and subcarrier narrow optical filtering after I/Q modulation to achieve low crosstalk. We also experimentally investigate the performance of a 13-subcarrier CO-OFDM uplink at 25 Gbaud/s and 25 GHz spacing, which has a much stringent requirement on the symbol transition-aligning and relative power match between subcarriers as compared to the proposed Nyquist-WDM case.

2. Operating principles of ultra-dense WDM-PONs

Figure 1(a) shows the operating principle of the proposed UD-WDM-PON system delivering centralized, bandwidth efficient Nyquist-WDM uplink signals with coherent demodulation. In the central office (CO), a coherent multi-wavelength light source with a frequency spacing of f Hz is generated and will be delivered to i-th optical network unit (ONUi) for uplink Nyquist-WDM modulation passing through the i th channel (CHi) of AWG at the remote note (RN), while a portion of them can be preserved by using an optical coupler (OC) and directly serve as an individual LO signal selected by a wavelength switch for homodyne detection of each uplink Nyquist-WDM subcarriers. Each ONU is equipped with both x- and y-polarization Nyquist-WDM functional blocks comprising five signaling stages: (1) channel interleaving, (2) wavelength de-multiplexing, (3) I/Q modulation, (4) Nyquist pulse shaping, and (5) wavelength multiplexing shown as illustrated in Fig. 1(b). The centralized uplink optical subcarriers are first interleaved into even and odd channels, and after wavelength de-multiplexing, each subcarrier will be I/Q modulated independently at a symbol rate identical to the channel spacing. After that a Nyquist pulse shaping is applied to each of the subcarriers so that the spectral overlapping between the odd and even channels is minimized after the channel combination by wavelength multiplexing. All the modulated Nyquist-WDM uplink subcarriers will be polarization-combined, and then be transported to the CO through RN and another feeder fiber. Note that, dual feeder fibers are considered to avoid any signal back scattering and the RN may simply contain two identical 1 × N AWGs. In CO, upstream Nyquist-WDM signals are retrieved via coherent detection, which will simplify the uplink management as all the wavelength has been kept correspondingly.

 

Fig. 1 Conceptual diagram of the proposed UD-WDM-PON with carrier-centralized Nyquist-WDM uplink and coherent demodulation (PM: phase modulator, WSS: wavelength selective switch; AWG: arrayed waveguide grating).

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3. System setup

3.1 Experimental setup

Figure 2 shows the proof-of-concept experimental setup for the proposed UD-WDM-PON system delivering Nyquist-WDM uplink signals demodulated by using coherent receiver with centralized LO. The coherent uplink carrier generator consists of an optical phase modulator (PM), a Mach-Zehnder modulator (MZM1) and a WSS. The PM is driven by a 25-GHz clock signal to create 25-GHz frequency-locked optical carriers. The MZM1 is then utilized to flat the optical spectral peaks, and the WSS is used to remove high order carrier. The obtained thirteen coherent uplink optical carriers with a uniform optical power level of −10 dBm and an OSNR of more than 50 dB are shown as inset Fig. 3(i) . After transmission over 80-km SMF-28, odd and even carriers are separated by a 25/50 GHz optical interleaver. Two I/Q modulators (I/Q MOD) driven by two sets of 25-Gb/s random binary sequence with a word length of (2¹³-1) × 4, are used to modulate the two sets of carriers with independent QPSK signals, respectively. Each I/Q MOD contains two parallel Mach-Zehnder modulators, which are both biased at the null point and driven at full swing to achieve zero-chirp 0/π phase modulation. The phase difference between the upper and the lower branch of I/Q modulator is controlled at π/2. After I/Q modulation, both even and odd carriers are then combined by using a polarization maintenance optical coupler (PM-OC), and an optical delay line (DL1) is utilized along the optical path of odd carriers for symbol synchronization between two tributaries before polarization multiplexing of each path via the polarization-multiplexer, comprising a PM-OC to halve signal, an optical delay line (DL2 and DL3) to provide 150 symbols delay, and a polarization beam combiner (PBC) to recombine the signal. We utilize a reconfigurable optical wavelength selective switch (WSS) to shape and combine two paths of sub-carriers with individually wavelength narrow shape before 80-km SMF transmission. The 3-dB bandwidth of each subcarrier after wavelength shape is 0.15 nm. The odd and even carriers after WSS are shown in Fig. 3 as inset (ii) and (iii), and Fig. 3(iv) illustrates the measured optical spectra of Nyquist-WDM uplink carriers after WSS. At the receiver, an optical band-pass filter with tunable bandwidth (set as 0.4 nm) and central wavelength is utilized to de-multiplex the channels. An external caver laser (ECL) with a line-width less than 100 kHz is utilized as the fiber laser local oscillator (LO). A polarization-diverse 90 degree hybrid is used to realize the polarization and phase-diverse coherent detection of the LO and received optical signal before balance detection. The sample and digitization (A/D) is realized in the digital scope with 80 Gs/s sample rate and 30 GHz electrical bandwidth.

 

Fig. 2 Proof-of-concept experimental setup (ECL: external cavity laser; IL: optical interleaver; TOF: tunable optical filter; PPG: pulse pattern generator; PM-OC: polarization maintenance optical coupler; ATT: attenuator; WSS: wavelength select switch).

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Fig. 3 Optical spectra (0.01 nm) (i): after WSS; (ii): odd carriers before modulation; (ii): even carriers before modulation; (iv): all subcarriers after modulation.

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3.2 Digital signal processing

The captured data is processed through offline DSP. Firstly, the clock is extracted by using “square and filter” method, the digital signal is re-sampled at twice of the baud rate based on the recovery clock. Secondly, a T/2-spaced time-domain finite impulse response (FIR) filter is utilized for the compensation of chromatic dispersion, where the filter coefficients is calculated from known fiber CD transfer function using frequency-domain truncation method [16]. Thirdly, two complex-valued, 13-tap, T/2-spaced adaptive FIR filters, found on classic constant modulus algorithm (CMA), is used to retrieve the modulus of the QPSK signal [16]; The carrier recovery is performed in the subsequent step where the 4-th power is used to estimate the frequency offset between the LO and the received optical signal, the frequency offset is obtained from the speed of the phase rotation of Mth-power of the signal after CMA process, and then the MLSE algorithm is utilized to estimate the carrier phase [6,17].

4. Experimental results

We measured the performance of the 6th subcarrier by aligning its sub-wavelength to that of the LO. Figure 4(a) illustrates the optical spectra and corresponding retrieved constellations in error free condition. The distortion of constellations is generated by certain intra-channel ISI generated from aggressive pre-filtering used to suppress WDM crosstalk. In Fig. 4(b), we measured and compared the back to back (BTB) BER performance of the proposed Nyquist-WDM and traditional CO-OFDM (without the Nyquist pulse shaping). With respect to the single-wavelength dual polarization QPSK uplink case, the OSNR penalty for the 13-subcarrier Nyquist-WDM one is about 1.5 dB while it is about 0.5 dB for the 13-subcarrier CO-OFDM case because of the ISI and intra-channel ISI. The required OSNR of Nyquist-WDM technique for the BER of 2 × 10⁻³ is 18 dB with sets consisting of 10 × 65000 symbols. In addition, we also measured and confirmed that all the other sub-channels exhibited similar performance. We experimentally investigated the performance with relative synchronization of one sub-carrier with respect to the others by introducing optical delay via DL1. Fig. 5(a) shows the required OSNR at constrained BER = 1 × 10⁻³ versus the relative symbol delay between even and odd carriers. Note that one symbol (25 GBaud/s) period is 40 ps. The performance of the traditional CO-OFDM varies periodically while the proposed Nyquist-WDM can tolerance more on the relative symbol delay since the intra-channel crosstalk coming from dis-synchronized sub-carriers has been removed by the Nyquist optical filtering.

 

Fig. 4 (a) Optical spectra of 6th carrier before detection, and retrieved constellations at x- and y- polarizations, (b) Uplink BER curves for single-channel, Nyquist-WDM and CO-OFDM cases, respectively. (A tunable TOF with bandwidth of 0.4 nm is utilized before coherent detection)

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Fig. 5 (a) Required OSNR of the 6th sub-carrier constrained BER of 1 × 10⁻³ with respect to relative symbol delay between odd and even carriers before and after 80km SMF transmission, (b) BER curves of 6th sub-carrier in CO-OFDM.

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We also experimentally confirmed that traditional CO-OFDM is restricted by symbol transition-aligning and relative power match with all subcarrier. Figure 5(b) shows the BER performance of CO-OFDM versus power level for even and odd subcarriers. It can be found that BER performance will be worse off shapely when the orthogonal relationship is just broken down by 2.5-dB power difference.

5. Conclusion and discussion

We proposed and demonstrated an Ultra-dense WDM-PON uplink transmission method employing Nyquist-WDM technique cooperated with digital coherent detection to meet stringent requirement of future PON systems delivering high-capacity data and bandwidth-demanding video services. The use of centralized optical carriers for Nyquist-WDM uplink takes advantages of both high spectral efficiency and source-free operation for remote ONUs. On the other hand, the coherent detection is utilized to demodulate each optical carrier. Delivering 13 × 100-Gb/s dual polarization QPSK uplink signal in forms of Nyquist-WDM with 25-GHz subcarrier spacing has been successfully demonstrated over 80-km SMF with negligible power penalty and 1-dB crosstalk penalty of OSNR at BER of 2 × 10⁻³. Rather than using a single feeder fiber, in our experiment dual feeder fibers were utilized for both carrier distribution and upstream, which cannot only avoid the signal back scattering but also help to reduce the hardware complexity of ONUs. We believe the proposed technique is promising for future access network, and might requires lightwave integrated all-optical OFDM modulators as well as DSP-intensive CMOS coherent receivers to lower the capital expenditure. As sub-carrier aggression can be gained from either Nyquist-WDM or CO-OFDM technique, both of them have to solve the issue. Referred to the restriction of symbols synchronization and power matching, it seems to be more convenient to employ Nyquist-WDM technique that is realized from intra-subcarrier crosstalk suppression by individually subcarrier spectrally shaping.