MDM-TDM PON Utilizing Self-Coherent Detection-Based OLT and ...

MDM-TDM PON Utilizing Self-Coherent Detection-Based OLT and RSOA-Based ONU for High Power Budget Volume 8, Number 3, June 2016 Yuanxiang Chen Juhao Li, Member, IEEE Peng Zhou Paikun Zhu Yu Tian Zhongying Wu Jinglong Zhu Ke Liu Dawei Ge Jingbiao Chen Yongqi He Zhangyuan Chen

DOI: 10.1109/JPHOT.2016.2557623 1943-0655 Ó 2016 IEEE

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

MDM-TDM PON Utilizing Self-Coherent Detection-Based OLT and RSOA-Based ONU for High Power Budget Yuanxiang Chen, Juhao Li, Member, IEEE, Peng Zhou, Paikun Zhu, Yu Tian, Zhongying Wu, Jinglong Zhu, Ke Liu, Dawei Ge, Jingbiao Chen, Yongqi He, and Zhangyuan Chen State Key Laboratory of Advanced Optical Communication Systems and Networks, Peking University, Beijing 100871, China DOI: 10.1109/JPHOT.2016.2557623 1943-0655 Ó 2016 IEEE. Translations and content mining are permitted for academic research only. Personal use is also permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.

Manuscript received March 19, 2016; revised April 19, 2016; accepted April 19, 2016. Date of publication April 22, 2016; date of current version May 16, 2016. This work was supported in part by the National Basic Research Program of China (973 Program) under Grant 2014CB340101 and Grant 2014CB340105; by the National Natural Science Foundation of China under Grant 61505002, Grant 61377072, and Grant 61275071; and by the China Postdoctoral Science Foundation under Grant 2015M580926. Corresponding author: J. Li (e-mail: [email protected]).

Abstract: A cost-effective mode-division-multiplexing and time-division-multiplexing passive optical network (MDM-TDM PON) utilizing a self-coherent detection for upstream signal at optical line terminal (OLT) and a high-gain reflective semiconductor optical amplifier (RSOA) as an optical amplifier for a downstream signal at optical network units (ONUs) to enhance bidirectional power budget is proposed. Meanwhile, to further reduce the cost of ONU, RSOA is also used as upstream on–off keying (OOK) signal modulator by time compression multiplexing. A novel dual-modulus algorithm is proposed for OOK signal detection at the OLT. We experimentally verify the proposed MDM-TDM PON architecture with an upstream 1-Gb/s OOK signal and a downstream 10-Gb/s orthogonal frequency division multiplexing (OFDM) signal. Due to the high gain of RSOA and high receiver sensitivity of self-coherent detection, a 30-dB bidirectional power budget is achieved after 10-km few-mode fiber and a 20-km standard single-mode fiber at the bit error rate (BER) of 10−3. Optimal seed power and signal power that input to the RSOA are investigated in this paper. Index Terms: Mode-division-multiplexing (MDM), time-division-multiplexing (TDM), passive optical network (PON), dual-modulus algorithm (DMA), power budget.

1. Introduction The passive optical network (PON) has been considered as a dominant solution for optical access due to the advantages of cost effectiveness, energy savings, and service transparency [1], [2]. Besides the current commercial PON based on time-division-multiplexing (TDM), several alternative approaches such as PON architecture based on wavelength-division-multiplexing (WDM), orthogonal frequency-division-multiplexing (OFDM), and optical code division multiplexing (OCDM) were also proposed [3]–[5]. For next-generation PON technologies supporting more subscribers, multidimensional PON architectures, including WDM-TDM PON, TDM-OFDM PON, and TDM-OCDM PON, were proposed [6]–[9]. For example, in WDM-TDM PON, a number of wavelengths are deployed and each wavelength is shared through TDM among several optical

Vol. 8, No. 3, June 2016

7904207

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

Fig. 1. Schematic of MDM-TDM PON architecture.

network units (ONUs) rather than being dedicated to a single ONU in the WDM PON case. The multidimensional multiplexing technology can effectively expand PON scale and offer both high bandwidth and high resource utilization efficiency. With the explosive demand of broadband Internet, mode has been proposed as a new independent multiplexing dimension for PON to support more subscribers [10], [11]. In our previous works [12] and [13], we have experimentally verified the feasibility of mode-division-multiplexing (MDM) optical distribution network (ODN) cascaded with multiple conventional TDM ODNs or WDM ODNs to extend the scale of MDM-PON. In both works, costly external modulators are used in the ONU. When constructing hybrid PON of larger scale, how to improve power budget while reduce the cost and complexity of ONU is a critical concern. In this paper, we focus on enhancing bidirectional power budget of MDM-TDM-PON while keeping ONU cost-effective. We propose a novel MDM-TDM PON architecture with self-coherent detection for upstream signal at optical line terminal (OLT) and cost-effective reflective semiconductor optical amplifier (RSOA) as optical amplifier for downstream signal at ONUs. Meanwhile, to further reduce the cost at each ONU, RSOA is also used as upstream OOK signal modulator by reusing the wavelength from OLT utilizing time compression multiplexing [14]. A novel dual-modulus algorithm (DMA) is proposed for OOK signal detection at the OLT. With the high gain RSOA and selfcoherent detection combined with robust DMA equalization scheme, the double ODN power budget demand owing to the signal round trip through the transmission line is effectively compensated. Meanwhile, the effective area of few-mode fiber (FMF) can be much larger than that of single-mode fibers (SMF) thus it can support higher input power to enhance power budget [15]. We experimentally verify the proposed MDM-TDM PON architecture with 1 Gb/s upstream on-off keying (OOK) signal and 10 Gb/s downstream OFDM signal with transmission distance of 10-km FMF and 20-km SSMF. 30 dB bidirectional power budget is achieved. What is more, we sweep the seed power and signal power that input to the RSOA to validate the feasibility of the proposed scheme with dynamic input power range.

2. Technique Principle The schematic of MDM-TDM PON architecture is shown in Fig. 1. MDM-ODN is cascaded with multiple conventional TDM-ODNs to extend the scale of PON. By utilizing low modal-crosstalk FMF and all-fiber mode multiplexer/demultiplexer (MUX/DEMUX), mode is operated as independent dimension and the signal at each ONU/OLT can be individually detected without multipleinput multiple-output (MIMO) digital signal processing (DSP) that joints mode processing. The proposed bidirectional MDM-TDM PON architecture can be operated in burst mode, as shown in the inset of Fig. 1. The transmitter and receiver are synchronized. The first burst section is downstream data to all the ONUs and then, after a guard band, unmodulated optical carrier is sent for upstream modulation purpose. The downstream and upstream are time compression multiplexed and only unidirectional signal is transmitted and received at each time slot, so the back-propagation contaminations from opposite stream signal can be omitted. The time scheduling is operated in

Vol. 8, No. 3, June 2016

7904207

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

Fig. 2. DSP method for coherent detection of OOK signal.

one TDM ODN with single mode. Different modes can be operated at they own time scheduling. The RSOA is biased at a proper DC current to operate as a pre-amplifier for downstream transmission and a re-modulator for upstream transmission simultaneously. We adopt OOK modulation for upstream and OFDM for downstream. The OOK modulation simply the transmitter structure for ONU and it can be compatible with polarization division multiplexing (PDM). The downstream OFDM modulation scheme can provide advantages such as great resistance to fiber dispersion, high spectral efficiency, and extreme flexibility for both multiple services access and dynamic bandwidth allocation [16]–[18]. Limited by the bandwidth of the RSOA, the scheme applies to the asymmetric traffic scenario where the downstream bandwidth is large while the upstream bandwidth is relative low [19]. Fig. 1 shows the detailed architecture, at the OLT, a transmitted laser is power split to two 1:N splitters and one part is utilized as downstream modulation carrier while the other part is as the local oscillator (LO) of upstream. For downstream, the split signals are respectively input to the MZM1 to MZMN for OFDM modulation. Then the OFDM signals from all the transmitters are combined and converted to specific modes by a mode MUX. After the FMF transmission, OFDM signals are mode demultiplexed and converted to the LP01 mode at each TDM-ODN. Then the OFDM signals with the same mode are split to all the ONUs of TDM-ODN and then amplified and reflected to the receiver by the RSOA. Each ONU receives and demodulates its corresponding OFDM subcarriers. Dynamic bandwidth allocation can be achieved by configuring the subcarrier number and its modulation order for each ONU. When for the upstream transmission, the seed light from the laser will not be modulated at OLT and directly input to the RSOA for upstream OOK modulation at ONU. Similar to the downstream, the signal from each ONUs are combined and converted to the specific modes of the FMF. At the OLT, the upstream signals are mode demultiplexed after few-mode circulator (FMC). Self-coherent detection with robust DMA equalization scheme is adopted for upstream OOK signal. Due to the high gain of RSOA and higher receiver sensitivity of self-coherent detection, the power budget can be effectively enhanced. What is more, thanks to the large effective core area of FMF compared with the SSMF, the power input FMF can be enhanced without additional nonlinear impairments to support larger splitting ratio. The proposed scheme can be expanded to multi-band and be compatible with WDM system attributed to the colorless characteristic of the ONU. Fig. 2 shows the proposed dual-modulus algorithm (DMA) for OOK signal detection. After coherent detection, blind equalization is performed to combat the linear channel impairments such as chromatic dispersion (CD) and polarization-mode-dispersion (PMD). Constant Modulus Algorithm (CMA) and Cascaded Multi-modulus Algorithm (CMMA) [20] are two commonly used blind equalization algorithms. However, the CMA cannot equalize OOK signal due to its inherent inconstant modulus while CMMA is specifically used to equalize QAM signal, which replaces a single radius of convergence with multiple new radiuses of convergence by a cascaded error function. To perform blind equalization for OOK signals, we can take the origin of coordinates as the second convergence radius, which is the basic idea behind the proposed DMA. The origin of coordinates can be taken as one convergence radius (i.e., R1 ¼ 0) together with the other modulus R2 ¼ 1. Thus, two new convergence radiuses A1 and A2 are defined to make the convergence error of OOK turn out to be zero A1 ¼ 0:5ðR1 þ R2 Þ A2 ¼ 0:5ðR2  R1 Þ:

Vol. 8, No. 3, June 2016

(1) (2)

7904207

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

Fig. 3. Experimental setup for MDM-TDM PON transmission.

Accordingly, the error function is exy ¼ jZxy j  A1  A2 :

(3)

In the above equation, Zxy denotes the equalized symbols. Then the filter tap weight updating criteria are similar to CMMA [20]. After blind equalization, OOK hard-decision is performed. Compared with conventional coherent de-modulation, the frequency offset estimation and compensation are removed due to the self-coherent scheme. The laser phase noise will also cause constellation rotation in the constellation diagram, but evidently for OOK demodulation phase recovery can be omitted without causing penalty, which further simplifies receiver-side DSP compared with other complex modulation schemes.

3. Experimental Setup To verify the feasibility of our proposed scheme, we have experimentally demonstrated a proof of concept the MDM-TDM PON scheme as shown in Fig. 3. We utilize low modal-crosstalk twomode FMF, MUX/DEMUX and FMC to implement the few mode structure. When the number of the mode increases, low modal crosstalk between the mode groups and the degenerate modes in one mode groups are essential to support more ONUs. For the downstream transmission, we adopt similar scheme that was shown in our previous work [21] to generate the 10 Gb/s 16QAM-OFDM signals. An arbitrary waveform generator (AWG 7122B) with sampling rate of 10 GS/s generates baseband OFDM signal. The baseband OFDM signal is up-converted to 5-GHz by I-Q modulation. The DFT size is 1024, from which 968 subcarriers are used for data transmission, and the cyclic prefix (CP) size is 16. 16-QAM is used as modulation format and the bit rate of single channel is 10-Gb/s. A Mach–Zehnder modulator (MZM) is utilized to convert the OFDM signal to double-sideband optical signal. Then the two signals at the output of the MZMs are power amplified by EDFA and then combined by mode MUX. The upper signal is converted from LP01 mode to LP11 mode and the below signal is LP01 mode for FMF transmission. The signal power and seed light power launched into FMF is fixed at 16 dBm. After 10-km FMF transmission, the MDM signals are firstly demultiplexed by a mode DEMUX and sent to two TDM-ODNs. The TDM-ODN consists of 20 km SSMF and a variable optical attenuator (VOA) to emulate optical splitter. The fabrication parameters of this FMF for transmission are as follows: the core/cladding diameters of the fiber are 13.5-m and 125-m, respectively. The refractive index of the core and cladding are 1.446 and 1.440, respectively. The relative index difference is 0.42%. The normalized frequency V is 3.24. Thus, the FMF only supports LP01 and LP11 modes transmission. The attenuation is 0.21 dB/km. The mode MUX/DEMUX are realized in the form of fused-type coupler and fabricated with an SMF and an FMF by heating and tapering according to phase-matching condition [22], [23]. The modal-crosstalk from LP01 to LP11 is about −16 dB, while from LP11 to LP01 is −26 dB. At the ONU, the OFDM signals are amplified and reflected by RSOA and then detected by a linear photo-diode with 20 GHz bandwidth. RSOA is biased at 45 mA DC current. The received signal is then amplified by an electrical amplifier before being sampled by a real-time digital storage oscilloscope (Tektronix DPO72004B) operating at 50 GS/s. The sampled OFDM signal is decoded offline. For the upstream transmission, the seed light generated by the laser at the OLT is reflected and modulated by the RSOA to

Vol. 8, No. 3, June 2016

7904207

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

Fig. 4. Output mode intensity profiles for (a) LP01 before transmission, (b) LP11 before transmission, (c) LP01 after transmission, and (d) LP11 after transmission.

Fig. 5. Eye diagram for (a) OOK at output of RSOA, (b) LP01 at CoRx1, and (c) LP11 at CoRx2.

generate 27  1 PRBS upstream OOK signal. Limited by our available RSOA, the data rate is limited to 1 Gb/s. A new type of RSOA with higher modulation bandwidth and optimized drive circuit board can improve the modulation data rate. After upstream transmission, the OOK signals are self-coherent detected at OLT with DMA algorithm to improve receiver sensitivity.

4. Experimental Results To investigate the performance of mode MUX/DEMUX and FMF, we measure far-field mode patterns of the downstream transmission at the points A and B in MDM-PON system as shown in Fig. 4. These results show that LP01 mode is successfully converted to LP11 mode and then converted back using mode MUX/DEMUX after 10 km FMF transmission. Fig. 5(a)–(c) shows the eye diagram of upstream OOK signal at the output of RSOA, at the input of CoRx1 and CoRx2, respectively. We can see after upstream transmission, the signal in the two modes are deteriorated due to chromatic dispersion and mode crosstalk. Compared with LP11, LP01 has a clearer eye opening due to smaller mode crosstalk. Fig. 6 shows the constellation diagrams before and after blind equalization for OOK signal under 10-km FMF and 20-km SSMF transmission. All data points finally converge to two radiuses (0 and 1). Thus, the decision can be performed by calculating the modulus of each bit and optimizing threshold detection. Fig. 7(a) and (b) show the BER performances versus the received optical power for upstream and downstream, respectively. For upstream transmission, self-coherent detection can greatly improve the receiver sensitivity. Combined with advanced DMA algorithm for signal impairment compensation, only 1 dB power penalty is observed at BER of 10−3 after link transmission. The output power of upstream signal after modulation and amplification is 0 dBm, so the power budget of the upstream is 30 dB. For downstream transmission, due to the high gain of RSOA, the receiver sensitivity is increased from −8 dBm to −17 dBm for both modes. The downstream power budget is 33 dB. From the results, we can see the system power budget bottleneck is in the upstream and the bidirectional power budget is 30 dB. Even though different mode coupling

Vol. 8, No. 3, June 2016

7904207

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

Fig. 6. Constellation diagram before and after DMA.

Fig. 7. (a) BER performances of upstream OOK transmission. (b) BER performances of downstream OFDM transmission with and without RSOA.

Fig. 8. (a) Downstream receiver sensitivity versus the signal power. (b) Upstream receiver sensitivity versus the seed power.

crosstalk between the two modes after FMF transmission, however, the crosstalk power is relative low compared with the original signal power. Combined with powerful DSP equalization, thus no obvious performance difference is observed for the two modes. We also investigate the optimal seed power and signal power that input to the ONU. Fig. 8(a) shows the sensitivity of the receiver that embed in the ONU. We can see when the input power is below −20 dBm, the amplified spontaneous emission (ASE) consume the gain of the RSOA and the downstream receiver has a low receiver sensitivity. When the signal power exceeds −17 dBm, the receiver sensitivity is about −9 dBm and starts to saturate. Higher input power will not improve the receiver sensitivity. Fig. 8(b) shows the upstream receiver sensitivity versus the seed power that input to the ONU. We can see when the seed power is low, the ASE of the RSOA will worsen the performance and degrade the receiver sensitivity. When the seed power

Vol. 8, No. 3, June 2016

7904207

IEEE Photonics Journal

MDM-TDM PON Utilizing OLT and ONU

beyond −13 dBm, the increasing power enhances the bias level optically inside the RSOA, and it degrades the extinction ratio of the OOK signal.

5. Conclusion In this paper, we propose and experimentally demonstrate MDM-TDM PON utilizing selfcoherent detection for upstream signal at OLT and high gain RSOA as optical amplifier for downstream signal at ONUs to enhance bidirectional power budget. A novel dual-modulus algorithm (DMA) is proposed for OOK signal detection at the OLT. With the proposed scheme, 30 dB bidirectional power budget is achieved after 10-km FMF and 20-km SSMF at the BER of 10−3.

References [1] T. Koonen, “Fiber to the home/fiber to the premises: What, where, and when?” Proc. IEEE, vol. 94, no. 5, pp. 911–934, May 2006. [2] R. Davey, J. Kani, F. Bourgart, and K. McCammon, “Options for future optical access networks,” IEEE Commun. Mag., vol. 44, no. 10, pp. 50–56, Oct. 2006. [3] L. Kazovsky, W. Shaw, D. Gutierrez, N. Cheng, and S. Wong, “Next-generation optical access networks,” J. Lightw. Technol., vol. 25, no. 11, pp. 3428–3442, 2007. [4] Q. Dayou, N. Cvijetic, H. Junqiang, and W. Ting, “A novel OFDMA PON architecture with source-free ONUs for next-generation optical access networks,” IEEE Photon. Technol. Lett., vol. 21, no. 17, pp. 1265–1267, Sep. 2009. [5] T. Kodama, N. Wada, G. Cincotti, and K. Kitayama, “Asynchronous OCDM-based 10 G-PON using cascaded multiport E/Ds to suppress MAI noise,” J. Lightw. Technol., vol. 31, no. 20, pp. 3258–3266, 2013. [6] H. Lee, P. Iannone, K. Richmann, and B. Kim, “A bidirectional SOA-Raman hybrid amplifier shared by 2.5 Gb/s, 60 km long-reach WDM-TDM PON,” in Proc. IEEE ECOC, 2008, pp. 1–2. [7] Y. Luo et al., “Time- and wavelength-division multiplexed passive optical network (TWDM-PON) for next-generation PON stage 2 (NG-PON2),” J. Lightw. Technol., vol. 31, no. 4, pp. 587–593, 2013. [8] H. Yang et al., “DSP-based evolution from conventional TDM-PON to TDM-OFDM-PON,” J. Lightw. Technol., vol. 31, no. 16, pp. 2735–2741, 2013. [9] T. Kodama et al., “Scaling the system capacity and reach of a 10G-TDM-OCDM-PON system without an en/decoder at an ONU,” J. Opt. Commun. Netw., vol. 5, no. 2, pp. 134–143, 2013. [10] D. Richardson, J. Fin, and L. Nelson, “Space-division multiplexing in optical fibres,” Nat. Photon., vol. 7, no. 5, pp. 354–362, 2013. [11] C. Xia et al., “Time-division-multiplexed few-mode passive optical network,” Opt. Exp., vol. 23, no. 2, pp. 1151–1158, 2015. [12] F. Ren et al., “Cascaded mode-division-multiplexing and time-division-multiplexing passive optical network based on low modal-crosstalk FMF and mode MUX/DEMUX,” IEEE Photon. J., vol. 7, no. 5, Oct. 2015, Art. no. 7903509. [13] Y. Chen et al., “Novel MDM-PON scheme utilizing selfhomodyne detection for high-speed/capacity access networks,” Opt. Exp., vol. 23, no. 25, pp. 32054–32062, 2015. [14] D. Lee, W. Kwon, C. Chae, and C. Park, “Cost-effective TCM-based WDM-PON for highly asymmetric traffic conditions,” Opt. Exp., vol. 23, no. 23, pp. 29802–29807, 2015. [15] H. Wen, H. Zheng, B. Zhu, and G. Li, “Experimental demonstration of long-distance analog transmission over fewmode fibers,” in Proc. IEEE OFC, 2015, pp. 1–3. [16] C. Chow et al., “WDM extended reach passive optical networks using OFDM-QAM,” Opt. Exp., vol. 16, no. 16, pp. 12096–12101, 2008. [17] D. Qian, N. Cvijetic, J. Hu, and T. Wang, “108 Gb/s OFDMA-PON with polarization multiplexing and direct detection,” J. Lightw. Technol., vol. 28, no. 4, pp. 484–493, 2010. [18] T. Duong et al., “Very high bit rate transmission for NGPON using AMOOFDM direct modulation of linear laser,” in Proc. IEEE OFC, 2010, pp. 1–3. [19] K. Tanaka, A. Agata, and Y. Horiuchi, “IEEE 802.3av 10G-EPON standardization and its research and development status,” J. Lightw. Technol., vol. 28, no. 4, pp. 651–661, 2010. [20] X. Zhou, J. Yu, and P. Magill, “Cascaded two-modulus algorithm for blind polarization de-multiplexing of 114-Gb/s PDM-8QAM optical signals,” in Proc. IEEE OFC, 2009, pp. 1–3. [21] B. Lin et al., “Comparison of DSB and SSB transmission for OFDM-PON,” J. Opt. Commun. Netw., vol. 4, no. 11, pp. B94–B100, Nov. 2012. [22] R. Ismaeel, T. Lee, B. Oduro, Y. Jung, and G. Brambilla, “All-fiber fused directional coupler for highly efficient spatial mode conversion,” Opt. Exp., vol. 22, no. 10, pp. 11610–11619, 2014. [23] J. Love and N. Riesen, “Mode-selective couplers for few-mode optical fiber networks,” Opt. Lett., vol. 37, no. 19, pp. 3990–3992, 2012.

Vol. 8, No. 3, June 2016

7904207