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We propose and demonstrate a novel wavelength-division-multiplexing orthogonal-frequency-division-multiplexing passive-optical-network (WDM-OFDM-PON) architecture with centralized lightwave sources and polarization shift keying (PolSK) multicast overlay. The 10-Gb/s 16QAM-OFDM point to point (P2P) signal, 2.5-Gb/s multicast PolSK signal and 2.5-Gb/s on-off keying (OOK) upstream signal are experimentally demonstrated. After transmission over 25km standard single mode fiber (SMF), 1.5dB crosstalk between the downstream signals is eliminated by employing a low pass electrical filter at the PolSK receiver. The power penalty of the upstream OOK signal at BER of 10−9 is less than 0.1dB.
©2010 Optical Society of America
1. Introduction
The wavelength division multiplexed passive optical network (WDM-PON) has been considered as a promising approach to meet the requirement of future access network due to its high data bandwidth, protocol transparency, enhanced security and flexible scalability [1–15]. In order to realize more flexible network, there are many schemes to provide both point-to-point data service and broadcast or multicast video/data service [1,3–7], such as time-division multiplexing (TDM), multi-light sources, sub-carrier multiplexing (SCM), and orthogonal modulation. Usually, the TDM is complicated because of the exact time control and communal downstream bandwidth. Using additional light sources has the simple configuration, but it causes significant increase in cost and complexity [3,5]. Orthogonal modulation scheme based on superimposing the multicast data onto point-to-point (P2P) data has many advantages and it is believed to be one of the most cost-effective schemes. Several orthogonal modulation schemes has been proposed, including NRZ multicast signal orthogonally modulated on the differential phase shift keying(DPSK) P2P signal, DPSK multicast signal orthogonally modulated on the inverse RZ P2P signal, and so on [4,5]. Optical orthogonal frequency division multiplexing (OFDM) has recently gained much attention due to its high spectral efficiency and the resistance to various dispersions including chromatic dispersion [9–13]. Polarization shift keying (PolSK) is considered as one of the promising modulation formats in future network, which possesses constant energy per bit to eliminate the cross-talk between the P2P signal and multicast signal [14–16]. In practice, the random birefringence of buried optical fibers would typical cause 2°-10° fluctuations in the polarization angles of the signal [17], but dynamic polarization control can be adopted to compensate for this fluctuation [18].
In this paper, we propose a novel WDM-OFDM-PON architecture which simultaneously supports both P2P and multicast data transmission. We superimpose the 2.5-Gb/s PolSK modulated broadcast overlay on the 10-Gb/s 16QAM-OFDM modulated downstream P2P signal. At the optical network unit (ONU), the downstream signal is re-modulated by the 2.5-Gb/s OOK upstream signal. We also studied the cross-talk between the P2P and multicast signal, which cause 1.5dB power penalty in this architecture, and can be removed by using a low-pass electrical filter.
2. The architecture
The principle of proposed architecture is illustrated in Fig. 1 . The optical line terminal (OLT) consists of N channels, and there is a Mach-Zehnder modulator (MZM) in each channel to generate optical 16QAM-OFDM signals for downstream transmission. We adopt off-line program and a Tektronix arbitrary waveform generator (AWG) to generate the 16QAM-OFDM signal. As the figure shows, the P2P OFDM signal is mixed with a local oscillation (LO) source to produce an electrical signal that drives the MZM. Since the spectrum bandwidth of 16QAM-OFDM is much narrower compared with the on-off keying (OOK) modulation, a 10GHz LO source is enough to carry the high-speed OFDM signal. The output optical P2P signal is double side band (DSB) form, so we adopt an optical filter (OF) after the MZM to generate single side band (SSB) 16QAM-OFDM signal for increasing the spectral efficiency and reducing the fading effect. After a multiplexer (MUX), the N channel combined 16QAM-OFDM signals are sent for the multicast modulation. The combined signals are fed into a Polsk modulator, which is consisted of a polarization controller (PC) and a phase modulator (PM). With properly adjusting the PC to fit the input state of polarization (SOP) [16], the PolSK multicast data can be added onto the combined signals.
Fig. 1 Principle of proposed WDM-OFDM-PON architecture with centralized and multicast overlay (LO: local oscillation; OF: optical filter; PC: polarization control)
After fiber transmission, a demultiplexer (DeMUX) is used to separate the WDM channels and then deliver them to different ONUs. In the ONU, the downstream signals are firstly passed through a 3dB coupler. One coupler output is used for upstream link, which is re-modulated by an OOK signal via an intensity modulator (IM) with large extinction ratio. The other output is divided again by another 3dB coupler. One part is fed into the OFDM receiver. The OFDM receiver employs direct-detection optical OFDM scheme, which is insensitive to polarization dependent effect. The other part is fed into the PolSK receiver, which consists of a polarization controller and polarization beam splitter (PBS) to convert the PolSK signal to OOK signal for detection and performance analyses.
3. Experimental transmission and results
The experiment setup is shown in Fig. 2 . In the OLT, a 1550.68nm distributed feed back (DFB) laser is employed as the optical source. The 10-Gb/s baseband 16QAM-OFDM signal is generated off-line by MATLAB program. The original bit stream is packed into 256 sub-carriers, and each symbol is 16QAM mapping format. In a 16QAM-OFDM symbol, 200 channels are filled for data, 7 pilot sub-carriers are used for phase estimation, and the other are for over-sampling, which can eliminate signal aliasing at the AWG. The cyclic prefix is 1/16 symbol time. The training sequence is added to every 100 16QAM-OFDM symbol signal for time synchronization. The 16QAM-OFDM waveform is generated through the AWG at 10-Gsample/s and 8 bit digital-analog conversion (DAC) and then mixes with a 10Gz LO source. The electrical spectrum of the up-converted signal is shown in Fig. 2 inset, and the bandwidth of the signal is 2.5-Gb. This RF signal is applied to a MZM with proper bias via a bias-tee. The MZM is driven by the signal with 1.7V corresponding to half-wave voltage of 3.5V. Then the output DSB P2P signal is converted into SSB format through a tunable optical filter with bandwidth of 25GHz before entering the PM, where the Polsk multicast at 2.5-Gb/s is encoded. A revolving PC is inserted before the PM to obtain the exactly required input SOP. Then the 16QAM-OFDM/Polsk downstream signal is amplified by an erbium-doped optical fiber amplifier (EDFA) and then launched into the 25-km single mode fiber (SMF) link.
Fig. 2 Experimental setup of the proposed architecture (SMF: single mode fiber, TOF: tunable optical filter; EL: electrical low-pass filter; P-to-P: point to point; PBS: polarization beam splitter)
After transmission, the downstream signal is firstly filtered by a tunable optical filter (TOF) with the bandwidth of 0.6nm to suppress the ASE noise, and then split into two parts by a 3-dB optical coupler. One part is for downstream signal receivers, and the other is reused for 2.5-Gb/s OOK upstream. Figures 3(a) -3(d) show the optical spectra at the corresponding points (a)-(d) in Fig. 2, respectively.
Fig. 3 Optical spectra at the corresponding measured points in Fig. 2. (a) After the MZM; (b) After the OF; (c) Before the PD; (d) After the OOK re-modulation (optical spectrum analyzer resolution: 0.02nm)
For the downstream signal receive, another 3-dB optical coupler is employed to split the downstream signal before receive. One part is fed into a 40GHz photodiode (PD) for O/E conversion. The RF OFDM signal is sampled by a 20-GSa/s real time digital sampling scope (TDS) and then down-converted in the offline processing. The measured bit error rate (BER) curve and 16QAM constellations with and without transmission are shown in Fig. 4(a) . For the other part, we obtain intensity detection of the PolSK multicast by a PBS that performs the conversion from polarization modulation to intensity modulation. Then we use a PD for the O/E conversion. Because the detected signal includes both Polsk signal and part of P2P OFDM signal, we adopt a 3.9GHz electrical low pass filter (EL) to suppress the frequency components higher than the Polsk bit rate, which can eliminate the crosstalk between the P2P OFDM signal and the Polsk multicast signal. The BER curve and eye diagrams are presented in Fig. 4(b). We can see in Fig. 4(b) that the crosstalk between the downstream signals leads to 1.5dB power penalty for the Polsk signal in our proposed scheme, and the eye diagrams with and without EL also indicate their different performance.
Fig. 4 Measured BER curves and corresponding constellations and eye diagrams of downstream signals with and without transmission: (a) 16QAM-OFDM signal; (b) Polsk signal (EL: electrical low-pass filter).
For the upstream link, the OFDM/Polsk signal is re-modulated by an IM at 2.5-Gb/s with a PRBS of length 231-1. Because the central carrier of the OFDM signal is blank, it can be re-modulated with an IM with large extinction ratio even if the original signal is intensity modulated [10]. After 25-km transmission, a 5GHz commercial APD is used for directly detection. The eye diagrams and BER curve is illustrated in Fig. 5 . The power penalty is less than 0.1dB after 25-km SMF transmission at the BER of 10−9.
4. Discussion and outlook
In our experiment, the cost of the ONU is an important issue we care about. We have made many efforts to reduce the cost of ONU. First, we have employed software down conversion for the OFDM signal instead of conventional down conversion which adopts RF source and mixer. In engineering application, this down conversion operation can also be achieved by the FPGA chip. Second, we have used the low cost mechanical polarization controller which is 90 U.S. dollars worth. As the access network is short, the Polsk receiver consisting of mechanical polarization controller and polarization beam splitter has got a good signal demodulation. Third, the bandwidth of the PD at OUN can be reduced to 10-GHz when the carrier frequency of the OFDM signal became 5GHz. The performance of the upstream and downstream signal will remain unchanged. Due to our lab conditions, we have to adopt the higher speed PD (only 10GHz RF source and 40GHz PD).
Duo to the one-tap equalization algorithm of OFDM, the performance after 25-km transmission is almost the same as back-to-back transmission. When we transmitted longer length, the performance of OFDM signal is almost unchanged, but the Polsk signal deteriorate severely duo to the polarization rotation effect and polarization mode dispersion. This will be our next research point.
5. Conclusion
We have proposed and demonstrated a novel WDM-OFDM-PON architecture with centralized lightwave sources and PolSK multicast overlay. In this scheme, the downstream 10-Gb/s 16QAM-OFDM modulated P2P signal and 2.5-Gb/s PolSK modulated multicast signal have been transmitted over 25-km SMF successfully. The fading effect is removed when the OFDM modulated DSB P2P signal is converted to SSB form after the OF. There is 1.5dB crosstalk between P2P and multicast signal observed in our experiment, which can be eliminated by a 3.9GHz EL at the PolSK receiver. The downstream signal is re-modulated by 2.5-Gb/s OOK signal with a large extinction ratio IM. The power penalty for the downstream multicast signal is 0.2dB at 10−9, while the power penalty is less than 0.1dB for the upstream signal after transmission over 25-km SMF.
Acknowledgments
The financial support from National Basic Research Program of China with No. 2010CB328300, National Natural Science Foundation of China with No. 60932004, 60677004, 60977046, National High Technology 863 Research and Development Program of China with No. 2009AA01Z220 are gratefully acknowledged. The project is also supported by the Teaching and Scientific Research Foundation for the Returned Overseas Chinese Scholars (State Education Ministry), and Program for New Century Excellent Talents in University of China with No. NECT-07-0111.
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