Demonstration of hybrid orbital angular momentum ... - OSA Publishing

Demonstration of hybrid orbital angular momentum multiplexing and time-division multiplexing passive optical network Andong Wang,1,3 Long Zhu,1,3 Jun Liu,1,3 Cheng Du,2 Qi Mo,2 and Jian Wang1,* 1

Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China 2 Fiberhome Telecommunication Technologies Co. Ltd, Wuhan 430074, Hubei, China 3 These authors contributed equally to this work. *[email protected]

Abstract: Mode-division multiplexing passive optical network (MDMPON) is a promising scheme for next-generation access networks to further increase fiber transmission capacity. In this paper, we demonstrate the proof-of-concept experiment of hybrid mode-division multiplexing (MDM) and time-division multiplexing (TDM) PON architecture by exploiting orbital angular momentum (OAM) modes. Bidirectional transmissions with 2.5-Gbaud 4-level pulse amplitude modulation (PAM-4) downstream and 2Gbaud on-off keying (OOK) upstream are demonstrated in the experiment. The observed optical signal-to-noise ratio (OSNR) penalties for downstream and upstream transmissions at a bit-error rate (BER) of 2 × 10−3 are less than 2.0 dB and 3.0 dB, respectively. ©2015 Optical Society of America OCIS codes: (060.2330) Fiber optics communications; (060.4230) Multiplexing; (060.4250) Networks.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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Received 4 Aug 2015; revised 27 Oct 2015; accepted 28 Oct 2015; published 3 Nov 2015 16 Nov 2015 | Vol. 23, No. 23 | DOI:10.1364/OE.23.029457 | OPTICS EXPRESS 29457

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1. Introduction Passive optical network (PON) is widely considered as the most promising optical access network solution in terms of its cost effectiveness. Time-division multiplexing (TDM) based gigabit-capable PON (GPON) and gigabit Ethernet PON (GE-PON) widely used in fiber-tothe-home (FTTH) deployment nowadays have already been standardized in the ITU-T G.984 and IEEE 802.3ah respectively [1–3]. Besides, the 10 GE-PON [4] and ten gigabit passive optical network (XG-PON) [5] known as the next-generation passive optical networks (NGPON1) are standardized in 2009. In 2011, the next-generation PON stage 2 (NG-PON2) requiring an aggregated capacity of at least 40 Gbit/s was initiated by the Full Service Access

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Network (FSAN) community [6]. In order to satisfy the requirements of providing high capacity, high split-ratio, and long transmission distance in next-generation access networks, different PON techniques have been extensively studied, such as wavelength-division multiplexing PON (WDM-PON) [7, 8], orthogonal-frequency-division multiplexing PON (OFDM-PON) [9], time-wavelength-division multiplexing PON (TWDM-PON) [10–12] and the 40 Gigabit TDM-PON (XLG-PON) [13]. Among these techniques, TWDM-PON has attracted the majority support from global vendors and was selected by FSAN community as a primary solution to next-generation NG-PON2 due to its cost effectiveness, lower power consumption and backward compatibility [10, 14]. On the other hand, deploying expenditure is also a critical concern in various PON schemes. Therefore, intensity modulation-direct detection (IM-DD) and binary on-off-keying (OOK) modulation are desired for current commercial PON systems. In addition, symmetric data rates might be preferred as the peer-topeer traffic proliferates in the future access networks [11]. Achieving 10 Gbit/s data rate per wavelength in both downstream and upstream is necessary for future TWDM PONs. Driven by the increasing demand of supporting higher data rate to end-customers, more multiplexing dimensions are desired to be combined to provide higher capacity and support more users. Recently, mode-division multiplexing (MDM) [15] has been receiving increasing attention as a potential scheme of space-division multiplexing (SDM) for high capacity optical communications without adding extra spectral band. In MDM, multiple independent data channels can be located on different spatial modes. Spatial modes can be represented by different modal bases, such as linearly polarized (LP) modes [16, 17], orbital angular momentum (OAM) modes [18], and vector modes [19]. One typical example of MDM is the use of few-mode fiber (FMF) to simultaneously transmit data on different LP modes [16, 17]. Besides using LP modes in FMF as one modal basis, OAM modes as another modal basis to represent spatial modes, featuring a helical phase front of exp(ilφ), where l is topological charge number and φ is azimuthal angle, have also shown great potential of MDM both in free-space [18] and fiber optical communications [20]. In the respect of cost and energy consumption, MDM could be a promising approach in PON application for its potential in cost, space, and energy savings. Actually, MDM over long-distance transmission using computation-complex coherent detection and multiple-input-multiple-output (MIMO) digital signal processing (DSP) has been widely reported [15–17]. Meanwhile, there are also some recent works on MDM in short haul transmission system (e.g. data centers) without MIMO DSP [21, 22]. Due to the increasing demand for high-splitting-ratio PONs, some recent experiments introduce MDM into PONs. By combining MDM with traditional PONs employing specially designed mode couplers and splitters, the combining losses of different modes can be reduced and the splitting ratio in PONs would be increased. In 2014, C. Xia et al. demonstrated a few-mode PON network which can eliminate upstream combining loss. An average 3.5 dB improvement in the combining loss compared to conventional single-mode splitters was achieved in the experiments [23, 24]. In a more recent experiment, a low loss of 4.3 dB was achieved for upstream transmission by using a novel large-scale (1x16) mode splitter, which is 7.7 dB less than the theoretical loss (12 dB) of a 1x16 conventional splitter [25]. Besides, by employing MDM in conventional PONs, the transmission capacity could be also increased. The recent PON access architecture based on OAM multiplexing over 0.4 m free-space optics link shows the potential for increasing the transmission capacity [26]. To the best of our knowledge, fiber-based OAM multiplexing PON has not yet been reported so far. In this scenario, a laudable goal would be to combine fiber-based OAM multiplexing and other multiplexing technique such as TDM in PON network for providing higher capacity and supporting more users [27]. In this paper, we propose and demonstrate a MDM-TDM-PON architecture, i.e. hybrid OAM multiplexing and TDM PON using two OAM modes (OAM+1 and OAM-1). We cascade MDM optical distribution network (ODN) and conventional TDM ODN to achieve the expansion of existing TDM-PON systems. A 1.1-km FMF is used for short-distance OAM

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multiplexing transmission. IM-DD based optical network unit (ONU) is adopted in the experiment. 2.5-Gbaud 4-level pulse amplitude modulation (PAM-4) and 2-Gbaud OOK are employed as downstream data and upstream data, respectively. The obtained results show that OAM multiplexing has potential to be combined with other dimension multiplexing techniques to construct hybrid multiplexing PONs. 2. Principle of OAM-based MDM-TDM-PON architecture Figure 1 depicts the schematic of OAM-based MDM-TDM-PON architecture of downstream transmission for simplicity, where the hybrid ODN is achieved by cascading an OAM-based MDM ODN and conventional TDM ODNs. The configuration of the proposed scheme can be expanded to bidirectional transmission similar to bidirectional TWDM-PON. For the downstream link (OLT to ONU), TDM signals from several transmitters (Tx 1… Tx N) are converted to specific OAM modes and combined by mode multiplexer at the optical line terminal (OLT) side. After propagating through FMF, the OAM channels are demultiplexed by mode demultiplexer. Then these mode demultiplexed channels are launched into singlemode fiber (SMF) and signals within specific time slots are received by each ONU. The ONUs in one group share a downstream OAM mode by selecting the same OAM mode, while different ONU groups own different downstream OAM modes in a MDM way. For the upstream link (ONU to OLT), multi-user access to the transmission media is achieved by assigning different time slots to users. At the ONU side, different ONUs in the same group are encoded with specific time slots in a TDM way. To reduce multi-channel interference between different ONUs, time synchronization is necessary in upstream transmission. Then different groups of ONUs carrying different OAM modes are combined through mode multiplexer in a MDM way. After passing through the FMF, upstream signals from different ONU groups are divided into multiple OAM channels and detected at OLT side in specific time slots.

Fig. 1. Schematic of the proposed MDM-TDM-PON architecture (hybrid OAM multiplexing and TDM PON). OLT: optical line terminal; ODN: optical distribution network; ONU: optical network unit; Mux/Demux: multiplexing/demultiplexing; SMF: single-mode fiber; OC: optical coupler; MDM: mode-division multiplexing; TDM: time-division multiplexing; OAM: orbital angular momentum.

3. Experimental setup Figure 2(a) shows the experimental setup of downstream transmission in OAM-based MDMTDM-PON. At the OLT side, the signal at 1550 nm is modulated by an optical intensity modulator (IM) with 2.5-Gbaud PAM-4 from an arbitrary waveform generator (Tektronix AWG 70002). The 2.5-Gbaud PAM-4 signal is pre-amplified by an erbium-doped optical fiber amplifier (EDFA). The amplified signal light is converted into two OAM modes using two spatial light modulators (SLM1 and SLM2) and combined by a polarization beam splitter (PBS). The SLMs employed in the experiments are Holoeye PLUTO phase-only SLMs based on reflective liquid crystal on silicon (LCOS) microdisplays enabling 0-2π phase modulation at 1550 nm. These SLMs have a spatial resolution of 1920 x 1080 pixels and a small pixel pitch size of 8 μm. After that, the light is expanded by two lenses L1 (f = 100 mm) and L2 (f = 200 mm). A quarter-wave plate (QWP) is used to convert the two linearly polarized OAM #247254 (C) 2015 OSA


modes to two circularly polarized OAM modes before coupling into the fiber, i.e. one right circularly polarized OAM+1 mode and the other left circularly polarized OAM-1 mode. Then the expanded light is focused by a 10X objective lens with a focal length f = 20 mm and coupled into the FMF. As shown in Fig. 3, the employed 1.1-km FMF in the experiment is a conventional circular core optical fiber with a step index profile. The radii of the fiber core and cladding are rcore = 6.35 μm and rcladding = 62.5 μm , respectively. The relative refractive index difference ( Δ = (n1 − n2 ) / n2 ) between the fiber core ( n1 ) and cladding ( n2 ) is Δ = 0.377%. The normalized frequency V of the fiber is 3.23. The designed and fabricated FMF supports six eigenmodes in total ( HE11even , HE11odd , TE01 , TM 01 , HE21even , HE21odd ). Those six eigenmodes are divided into two mode groups with relatively large effective refractive index difference (>2.3X10−3) between mode group 1 ( HE11even , HE11odd ) and mode group 2

( TE01 ,TM 01 , HE21even , HE21odd ). One can get right circularly polarized OAM+1 ( OAM +R1 ) and OAM-1 ( OAM −R1 ) modes and left circularly polarized OAM+1 ( OAM +L1 ) and OAM-1 ( OAM −L1 ) modes through proper linear combinations of eigenmodes in the mode group 2, i.e. even odd OAM +R1 = HE21 + i * HE21 , OAM −R1 = TM 01 − i * TE01 , OAM +L1 = TM 01 + i * TE01 , and odd OAM −L1 = HE21even − i * HE21 [28]. In the OAM-based MDM-TDM-PON experiments, we

employ OAM +R1 and OAM −L1 modes. At the ODN side, after 1.1-km FMF transmission, the output light is collimated by another 10X objective lens. The coupling and transmission loss is about 2.75 dB. Another QWP is used to convert the light into linear polarization. Then the light splits into two parts and coupled into SMF1 (20 km) and SMF2 (23.5 km). At the ONU side, a photodetector (PD) is used to detect the optical signal, then the electrical signal is sampled using a real-time oscilloscope (Tektronix DPO72004B) operating at 50 GS/s for offline processing.

Fig. 2. Experimental setup for hybrid OAM multiplexing and TDM PON. (a) Downstream transmission. (b) Upstream transmission. PBS: polarization beam splitter; HWP: half-wave plate; QWP: quarter-wave plate; PC: polarization controller; EDFA: erbium-doped fiber amplifier; AWG: arbitrary waveform generator; IM: intensity modulator; SMF: single-mode fiber; OC: optical coupler; FMF: few-mode fiber; FMF-PC: few-mode fiber polarization controller; Pol.: polarizer; SLM: spatial light modulator; Col.: collimator; VOA: variable optical attenuator; PD: photodetector.

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Fig. 3. (a) Cross-section view of the FMF. (b) Relative refractive index profile of the FMF. (c) Supported six eigenmodes in two mode groups of the FMF.

Figure 2(b) shows the experimental setup of upstream transmission in OAM-based MDMTDM-PON. Each ONU transmits 2-Gbaud optical OOK signal at pre-defined time slot controlled by a tunable optical delay line (ODL). Different ONUs in the same group are coupled together by optical coupler (OC). To distinguish waveforms of ONU1 and ONU2 on the real-time oscilloscope, we inset groups of zero-sequence to make 25-symbol blank space between ONU1 and ONU2. In each frame of ONU1, 500 data symbols are included with 500symbol zero-sequence, while there are 525-symbol zero-sequence, 450 data symbols and 25symbol zero-sequence in each frame of ONU2. In order to emulate another ONU, we split the coupled light into two parts and send them to SLM3 and SLM4 to generate two OAM modes. With the assistance by SLMs, ONU1 and ONU2 share the same OAM+1 mode, while ONU3 is converted to another OAM-1 mode. After passing through the MDM-ODN and TDM-ODN similar to the downstream, each OLT port receives the corresponding signal at the correct time slot after been divided into two different OAM channels. In the experiment, we introduce OAM multiplexing technique in the traditional TDM PON to construct a hybrid OAM multiplexing and TDM PON architecture, which enables increased transmission capacity by employing OAM multiplexing. 4. Experimental results

We first characterize the transmission performance of OAM modes in the FMF. The complex phase patterns employed in the experiment for generating OAM+1 and OAM-1 modes are plotted in Fig. 4(a), which are fork patterns for pure generation of OAM modes. Figure 4(b) shows the intensity profiles of the generated input OAM modes. By using a reference Gaussian beam with the same polarization to interfere with the OAM modes, one can tell the topological charge of the OAM modes. The interferograms of the input OAM modes are shown in Fig. 4(b). After 1.1-km FMF propagation, the output modes are recorded by a camera (HAMAMATSU InGaAs Camera C10633). By appropriately adjusting the few-mode fiber polarization controller (FMF-PC) to minimize the mode crosstalk, one can get the corresponding output OAM modes with high quality, as shown in Fig. 4(c). Such phenomenon might be briefly explained as follows. In the experiments, we are able to control mode coupling in the FMF by using FMF-PC. The use of FMF-PC is somehow equivalent to mitigating mode crosstalk using MIMO DSP. One can change the “channel matrix” of the FMF until it is approximately diagonal by adjusting the FMF-PC. A diagonal “channel matrix” means little mode coupling. Actually, in a basis of circularly polarized OAM modes, a complete characterization of the mode coupling requires the measurement of crosstalk between right circularly polarized OAM+1 and OAM-1 modes and left circularly polarized OAM+1 and OAM-1 modes. We measure the full “channel matrix” in the basis of four circularly polarized OAM modes ( OAM +R1 , OAM −R1 , OAM +L1 , OAM −L1 ) in the mode group 2 of the employed FMF. We adjust the FMF-PC to get an optimized approximately diagonal “channel matrix”. For each input circularly polarized OAM mode, we observe its minimized

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mode coupling to the other three circular polarized OAM modes. In the OAM-based MDMTDM-PON experiments, we only employ OAM +R1 and OAM −L1 modes at the input of the FMF, and the relative mode crosstalk to OAM +R1 ( OAM −L1 ) induced from OAM −L1 ( OAM +R1 ) at the output is measured to be about −18.2 dB (−18.4 dB). We analyze the polarization of the light at the output of the FMF. By sending the output light directly through a polarizer, slight output power fluctuation is observed, indicating that the output light is circular polarization. By sending the output light through a QWP followed by a polarizer, it is possible to determine the right and left circular polarization. Together with the OAM topological charge value determination by SLM, we are able to confirm the right circularly polarized OAM+1 ( OAM +R1 ) mode and left circularly polarized OAM-1 ( OAM −L1 ) mode at the output of the FMF. The interferograms of the output OAM modes are observed after converting the circularly polarized OAM modes to linearly polarized OAM modes. After the light coming out from the FMF, we use a QWP to convert the circularly polarized OAM modes to linearly polarized OAM modes. We use a reference Gaussian beam with the same linear polarization to interfere with the linearly polarized OAM modes for the generation of interferograms, as shown in Fig. 4(c). One can clearly see from Fig. 4(b) and (c) that OAM multiplexing transmission in a 1.1km FMF is successfully demonstrated with favorable transmission performance.

Fig. 4. Experimental results for the OAM generation and OAM transmission in a 1.1-km FMF. (a) Complex phase patterns for generating OAM+1 and OAM-1 modes. (b) Measured intensity profiles and interferograms for input OAM modes. (c) Measured intensity profiles and interferograms for output OAM modes.

We further measure the downstream and upstream link performance of the OAM-based MDM-TDM-PON. The measured bit-error rate (BER) curves for back-to-back (B2B) and OAM-based MDM-TDM-PON are shown in Fig. 5. Figure 5(a) plots BER performance for the 2.5-Gbaud PAM-4 downstream transmission link. Compared to the back-to-back case bypassing the MDM-TDM-PON, the measured optical signal-to-noise ratio (OSNR) penalties at a BER of 2 × 10−3 (enhanced forward-error correction (EFEC) threshold) for OAM+1 mode and OAM-1 mode are about 2.0 dB and 1.7 dB, respectively. The OAM+1 mode and OAM-1 mode shown in Fig. 5(a) correspond to the ONU1/ONU2 and ONU3 shown in Fig. 2(a), respectively. Figure 5(b) plots BER performance for the 2-Gbaud OOK upstream transmission link. In the upstream transmission link, BER curves are measured for three cases: 1) Single OAM mode transmission case: only one OAM mode is used for data transmission without crosstalk from the other OAM mode, i.e. “OAM+1 (ONU1 only, w/o ONU3)” and “OAM-1 (ONU3 only, w/o ONU1)” as shown in Fig. 5(b).

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2) MDM transmission case: two multiplexed OAM modes are simultaneously used with distinct (decorrelated) data streams for transmission and OAM mode crosstalk from one to another is considered, i.e. “OAM+1 (ONU1, w/ ONU3)” and “OAM-1 (ONU3, w/ ONU1)” as shown in Fig. 5(b). 3) MDM-TDM transmission case: OAM+1 mode with time slot multiplexed ONU1 and ONU2 and OAM-1 mode with ONU3 are simultaneously used for transmission and the crosstalk from OAM-1 mode to OAM+1 mode is considered, i.e. “OAM+1 (TDM: ONU1+ONU2, w/ ONU3)” as shown in Fig. 5(b). In the single OAM mode transmission case, the measured OSNR penalty at a BER of 2 × 10−3 is 0.81 dB for OAM+1 (ONU1 only, w/o ONU3) and 1.16 dB for OAM-1 (ONU3 only, w/o ONU1), respectively. In the MDM transmission case, the measured OSNR penalty increases to 1.71 dB for OAM+1 (ONU1, w/ ONU3) and 1.82 dB for OAM-1 (ONU3, w/ ONU1), respectively. One can observe slight BER performance degradation when two multiplexed OAM modes are simultaneously used for transmission. Such performance degradation is mainly due to the OAM mode crosstalk from one to another. In the MDMTDM transmission case, the measured OSNR penalty at a BER of 2 × 10−3 is about 3.0 dB for OAM+1 (TDM: ONU1 + ONU2, w/ ONU3), which might be ascribed to both the mode crosstalk from OAM-1 (ONU3) and the imperfect time synchronization and different optical link loss between ONU1 and ONU2. Moreover, we plot temporal waveforms of signals from ONU1, ONU2 and time slot multiplexed ONU1 and ONU2 recorded by the real-time sampling oscilloscope, as shown in Fig. 6, from which one can clearly visualize the process of TDM.

Fig. 5. Measured BER performance for hybrid OAM multiplexing and TDM PON (OAMbased MDM-TDM-PON). (a) BER performance for downstream transmission link. (b) BER performance for upstream transmission link.

Fig. 6. Recorded temporal waveforms of TDM signals. (a) Waveform of ONU1. (b) Waveform of ONU2. (c) Waveform of time slot multiplexed ONU1 and ONU2.

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5. Discussions

We further study the longer term stability of the OAM multiplexing and transmission through the 1.1-km FMF. In the experiments, we measure the received optical powers of the demodulated OAM+1 (OAM-1) mode and the crosstalk from OAM-1 (OAM+1) mode over a 30 minutes time period. As shown in Fig. 7, relatively stable powers of demodulated OAM modes are observed with slight power fluctuations less than ± 1 dB. Moreover, we record the captured intensity profiles of the demodulated OAM+1 (OAM-1) mode and the crosstalk from OAM-1 (OAM+1) mode over a 30 minutes time period in the experiments. As shown in Fig. 8, we do not observe significant changes of the intensity profiles. As a consequence, the obtained results indicate the relatively longer term stability of the system. In the proof-of-concept experiment, we employ two OAM modes (right circularly polarized OAM+1 mode and left circularly polarized OAM-1 mode). With future improvement, the proposed fiber OAM and TDM PON might be also applicable to multiple OAM modes. For the extended work of fiber OAM PON with large number of OAM modes, the challenge is the OAM fiber supporting more than two OAM modes. Actually, different kinds of OAM fibers supporting multiple OAM modes have already been designed or fabricated for multiple OAM modes fiber transmission, such as 4 OAM modes in a graded-index few-mode optical fiber [29, 30], 12 high-order OAM modes in an air core fiber [31], 22 modes with 18 OAM ones in a trench-assisted multi-OAM multi-ring fiber [32], and multiple OAM modes in a supermode OAM fiber [33]. In this scenario, one could use these types of multi-OAM fiber to further extend the OAM PON work to a larger number of OAM modes.

Fig. 7. Longer term stability measurement of the OAM multiplexing and transmission through the 1.1-km FMF. Received optical powers of the demodulated OAM+1 (OAM-1) mode and the crosstalk from OAM-1 (OAM+1) mode over a 30 minutes time period.

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Fig. 8. Longer term stability measurement of the OAM multiplexing and transmission through the 1.1-km FMF. Captured intensity profiles of the demodulated OAM+1 (OAM-1) mode and the crosstalk from OAM-1 (OAM+1) mode over a 30 minutes time period.

6. Conclusions

In summary, we have proposed a hybrid MDM-TDM-PON scheme based on traditional TDM-PON structure assisted by OAM multiplexing. In the hybrid OAM multiplexing and TDM PON, different ONUs in the same group are encoded at specific time slots in a TDM way, while different groups of ONUs are converted to different OAM modes in a MDM way. A proof-of-concept experiment is demonstrated for the hybrid OAM multiplexing and TDM PON with 2.5-Gbaud PAM-4 downstream transmission link and 2-Gbaud OOK upstream transmission link with favorable operation performance. 1.1-km FMF, 20-km SMF and 23.5km SMF are employed in the experiment. The longer term stability of the OAM multiplexing and transmission through the 1.1-km FMF is also measured in the experiment. The obtained results imply that a hybrid combination of OAM multiplexing and TDM technique might provide a promising approach to facilitating future high-capacity optical access networks. Acknowledgments

This work was supported by the National Basic Research Program of China (973 Program) under grants 2014CB340004 and 2014CB340003, the National Natural Science Foundation of China (NSFC) under grants 11274131, 11574001 and 61222502, the Program for New Century Excellent Talents in University (NCET-11-0182), the Wuhan Science and Technology Plan Project under grant 2014070404010201, and the seed project of Wuhan National Laboratory for Optoelectronics (WNLO).

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