Full-duplex bidirectional data transmission link using twisted lights multiplexing over 1.1-km orbital angular momentum fiber

We present a full-duplex bidirectional data transmission link using twisted lights multiplexing over 1.1-km orbital angular momentum (OAM) fiber. OAM+1 and OAM−1 modes carrying 20-Gbit/s quadrature phase-shift keying (QPSK) signals are employed in the downlink and uplink transmission experiments. The observed mode crosstalks are less than −15.2 dB, and the full-duplex crosstalks are less than −12.7 dB. The measured full-duplex optical signal-to-noise ratio (OSNR) penalties at a bit-error rate (BER) of 2 × 10−3 are ~2.4 dB in the downlink transmission and ~2.3 dB in the uplink transmission. The obtained results show favorable full-duplex twisted lights multiplexing data transmission performance in a km-scale OAM fiber link.

Scientific RepoRts | 6:38181 | DOI: 10.1038/srep38181 cost of the equipment to be shared between the two directions of traffic in a full-duplex communication link [22][23][24][25][26] . Previously, single-mode fiber (SMF) or free space is considered for uplink and downlink transmission between both ends of the architecture. So far there have been very few research efforts devoted to bidirectional transmission systems using twisted lights multiplexing in fiber. In this scenario, a laudable goal would be to develop a full-duplex bidirectional data transmission link by exploiting twisted lights multiplexing using an OAM fiber.
In this paper, we propose and experimentally demonstrate a full-duplex data transmission link using twisted lights multiplexing over 1.1-km OAM fiber. The downlink and uplink transmit OAM +1 and OAM −1 modes carrying 20-Gbit/s quadrature phase-shift keying (QPSK) signals, respectively. The obtained results show that the mode crosstalks are less than − 15.2 dB for both downlink and uplink transmission, while the full-duplex crosstalks are less than − 12.7 dB. The measured full-duplex optical signal-to-noise ratio (OSNR) penalties at a bit-error rate (BER) of 2 × 10 −3 (enhanced forward-error correction (EFEC) threshold) are about 2.4 dB for downlink and 2.3 dB for uplink. The demonstrated full-duplex OAM multiplexing transmission in 1.1-km OAM fiber shows favorable operation performance.

Results
Concept of OAM-fiber based full-duplex architecture. Figure 1 shows the concept of full-duplex bidirectional data transmission link using twisted lights multiplexing over 1.1-km OAM fiber. In the downlink direction, two multiplexed twisted lights from channel ① and channel ② propagate through an OAM fiber, while the other two twisted lights from channel ③ and channel ④ in the uplink direction share the same transmission path. Thus these four bidirectional modes from four channels transmit in the same OAM fiber simultaneously. Note that all the four channels are orthogonal to each other by employing twisted lights with different OAM values and polarizations. One can also employ multiple OAM modes in the downlink and uplink directions to further increase the transmission capacity by employing OAM fiber supporting multiple OAM modes. After full-duplex bidirectional data transmission, the downlink and uplink twisted lights are separated from each other at the demodulation side and then sent to the receiver for followed offline processing. Experimental setup. The experimental setup of full-duplex bidirectional data transmission link using twisted lights multiplexing over 1.1-km OAM fiber is shown in Fig. 2. At the transmitter side, an arbitrary waveform generator (Tektronix AWG 70002) is used to drive the IQ modulator generating 20 Gbit/s QPSK signal at a wavelength of 1550 nm. The signal is pre-amplified and split into four channels (channel ① and channel ② for downlink, channel ③ and channel ④ for uplink), and then relatively delayed by SMFs with different lengths for decorrelation. These four channels are launched onto four Holoeye PLUTO phase-only liquid crystal spatial light modulators (SLM1 and SLM2 for downlink, SLM3 and SLM4 for uplink) which are loaded with four hologram phase masks to create four OAM beams with topological charge of l = − 1. The employed SLMs are polarization sensitive, i.e. working only for the x-polarization while having no response to the y-polarization. Thus the four generated OAM beams are initially at x-polarization (xOAM −1 ). Then we use a beam splitter (BS) to combine the two OAM beams together and expand the beams by lens pairs both for downlink and uplink. Note that each reflection can flip the topological charge sign of the OAM beam. So the combined OAM beams are OAM +1 and OAM −1 , respectively. Before coupling into the 1.1-km OAM fiber by a 10X objective lens, the OAM beams for downlink are converted to y-polarization from x-polarization by a half-wave plate (HWP) while the uplink beams stay x-polarization. As a consequence, the OAM beams in the downlink direction are yOAM −1 for channel ① and yOAM +1 for channel ② , while in the uplink direction are xOAM −1 for channel ③ and xOAM +1 for channel ④ , respectively. After transmission through the 1.1-km OAM fiber, the OAM beams are collimated by another 10X objective lens. Then the uplink x-polarization beams are converted to y-polarization by the HWP and reflected by the polarization beam splitter (PBS), while the downlink beams stay y-polarization and are reflected by another PBS. Here the PBS works like an optical polarization circulator as the x-polarization OAM beams transmit through the PBS (uplink before PBS2 and downlink before PBS1), while the y-polarization OAM beams are reflected by the PBS (uplink before PBS1 and downlink before PBS2) to the demodulation side. The four output OAM beams after full-duplex bidirectional transmission are shrunk by lens pairs and projected to another SLMs (SLM6 for downlink, SLM5 for uplink) for demultiplexing/demodulation. The demodulated Gaussian-like beam is followed by coherent detection at the receiver. In the OAM-fiber based full-duplex experiment, y-polarization for downlink and x-polarization for uplink through the OAM fiber are adopted to minimize the bidirectional crosstalk. Meanwhile, we adjust the polarization controllers on OAM fiber (PC-OAMF) to minimize the mode crosstalk and achieve desired output OAM modes with high quality.

Experimental Results
We first study the performance over the 1.1-km OAM fiber transmission for both downlink and uplink. The hologram phase masks 10,12 loaded to SLM1, SLM2, SLM3 and SLM4 for generating four OAM modes with topological charge l = − 1 are showed in Fig. 3(a1)-(a4). The followed reflections by BS and HWP enable the generation of four OAM modes, i.e. xOAM +1 , xOAM −1 , yOAM +1 and yOAM −1. For the downlink transmission, Fig. 3(b1) and (b2) show the observed intensity profiles of the generated input OAM modes when only channel ① or channel ② is on, respectively. The interferograms of the input OAM modes are obtained by interfering OAM mode with a reference Gaussian beam with the same polarization, as shown in Fig. 3(c1) and (c2). According to the number of twist and the twist direction, one can determine the topological charge number of OAM mode to be − 1 or + 1. At the demodulation side, hologram phase mask with corresponding inverse OAM charge number is loaded to SLM6. Thus the OAM beam is converted to Gaussian-like beam with a bright spot at the beam center. The obtained intensity profiles of the output OAM modes after demultiplexing with single channel on (only channel ① or channel ② on) are shown in Fig. 3(d1) and (d2). Moreover, Fig. 3(e1) and (e2) show the demultiplexing intensity profiles with both channel ① and channel ② on, while the demultiplexing intensity profiles with all four channels ① -④ on are shown in Fig. 3(f1) and (f2). Similarly, for the uplink transmission, the intensity profiles and interferograms of the input OAM modes in channel ③ and channel ④ are shown in Fig. 3(b3), (b4), (c3) and (c4). The observed demultiplexing intensity profiles of output OAM modes with single channel on, double channels on and all four channels on are displayed in Fig. 3(d3), (d4), (e3), (e4), (f3) and (f4), respectively. Figure 4 records the crosstalks with double channels on and all four channels on for downlink and uplink transmission. Taking channel ① as an example, the crosstalk for channel ① with double channels on is exactly the mode crosstalk between channel ① and channel ② when channel ③ and channel ④ are off. Similarly, in the case of full-duplex when all four channels are on, the crosstalk for channel ① include both mode crosstalk and bidirectional crosstalk. One can see that the mode crosstalks are less than − 15.2 dB, and the bidirectional crosstalks are less than − 12.7 dB.
We further measure the BER performance of full-duplex 20-Gbit/s QPSK transmission link using twisted lights multiplexing. Figure 5  20-Gbit/s QPSK signal constellations with single channel on, double channels on, and all four channels on for downlink and uplink transmission at a BER of ~1 × 10 −4 are shown in Fig. 5(c). The back-to-back QPSK constellation is also shown for reference. According to the obtained results shown in Figs 3, 4 and 5 one can clearly see that the full-duplex data transmission link using twisted lights multiplexing over 1.1-km OAM fiber is successfully demonstrated in the experiment with a favourable transmission performance.

Discussions
In summary, we report a full-duplex data transmission link using twisted lights multiplexing over 1.1-km OAM fiber. We employ OAM +1 and OAM −1 modes carrying 20-Gbit/s QPSK signals in the uplink and downlink transmission. The measured mode crosstalks between OAM +1 and OAM −1 modes are less than − 15.2 dB, and the bidirectional crosstalks in the full-duplex link are less than − 12.7 dB. At a BER of 2 × 10 −3 , the measured OSNR penalties with single channel on, double channels on and all four channels on for OAM +1 and OAM −1 modes are about 1 dB, 1. Crosstalk. Remarkably, the mechanisms for the generation of crosstalk include two parts. One is the mode crosstalk between the two multiplexing modes along the same direction data transmission, while the other is the bidirectional crosstalk between the downlink and uplink directions. In the experiments, the measured mode crosstalk less than − 15.2 dB and bidirectional crosstalk less than − 12.7 dB are relatively high. With future improvement, the mode crosstalk could be improved by employing other specialty fiber structures such as high-index ring fiber designed to increase the effective refractive difference among different OAM modes (> 10 −4 ) 15 . The bidirectional crosstalk might come from the reflection on the fiber facet, which could be improved by making an angled fiber facet.    Scalability. In the experiments, the employed 1.1-km OAM fiber can only support OAM +1 and OAM −1 modes. The main limitations in expanding the system for multi-OAM modes or higher-order OAM modes are specialty fiber structures supporting multi-OAM modes. Fortunately, different kinds of specialty fiber designs have been proposed and even fabricated to support multi-OAM modes with low-level mode crosstalk such as 12 high-order OAM modes in an air core fiber 27 , 22 modes with 18 OAM ones in a trench-assisted multi-OAM multi-ring fiber 17 , and 36 OAM modes spanning 9 OAM orders supported in an annular fiber 28 . In this scenario, one could use these multi-OAM fibers with low-level mode crosstalk to further expand the system for multi-OAM modes or higher-order OAM modes.

Methods
The employed 1.1-km OAM fiber in the experiment supports six eigenmodes in total (HE even 11 , HE odd 11 , T E 01 , T M 01 , HE even 21 , HE odd 21 ). Figure 6(a) shows the refractive index profile of the OAM fiber. The diameters of the fiber core and cladding are 2r core = 12.7 μ m and 2r cladding = 125 μ m, respectively. The refractive index of the pure-SiO 2 cladding and GeO 2 -doped core are 1.444 and 1.449 at 1550 nm, respectively. The cross-section view is shown in Fig. 6(b). We further evaluate the mode properties of the OAM fiber, including effective modal index (n eff ), chromatic dispersion coefficient (D λ ), differential mode delay (DMD) and bandwidth, as shown in Fig. 6(c). It is noted that xOAM +1 , xOAM −1 , yOAM +1 and yOAM −1 can be obtained by proper linear combinations of TE 01 , TM 01 , HE even 21 and HE odd 21 .