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Experimental Demonstration of 260-meter Security FreeSpace Optical Data Transmission Using 16-QAM Carrying Orbital Angular Momentum (OAM) Beams Multiplexing Yifan Zhao, Jun Liu, Jing Du, Shuhui Li, Yan Luo, Andong Wang, Long Zhu, Jian Wang* Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan 430074, Hubei, China. *Corresponding author:
[email protected]
Abstract: We experimentally demonstrate a 260-meter security free-space optical data transmission link using orbital angular momentum (OAM) beams multiplexing and 16-ary quadrature amplitude modulation (16-QAM) signals. We study the beam wandering, power fluctuation, channel crosstalk, bit-error rate (BER) performance, and link security. OCIS codes: (060.2605) Free-space optical communication; (050.4865) Optical vortices; (060.4510) Multiplexing.
1. Introduction As an alternative dimension for optical communications, orbital angular momentum (OAM), in principle, provides great potential for spatial-division multiplexing (SDM) due to the unlimited achievable OAM states and orthogonality between each two different OAM states. Therefore, it has attracted increasing interest in utilizing OAM beams multiplexing to improve the system transmission capacity and spectral efficiency [1-5]. Recently, Pbit/s free-space data transmission with 112.6-bit/s/Hz spectral efficiency was demonstrated[5]. Additionally, there is another distinct advantage that OAM can lead improved security to optical data transmission [6]. The eavesdropper cannot measure the accurate OAM information while wiretapping an angular of less than 2π. However, these previous works with impressive performance were demonstrated in the range of few meters, which ignore the influence of OAM resulting from the real atmospheric turbulence. Inhomogeneity in the pressure and temperature or the dust in the atmosphere results in variations of the refractive index along the transmission path, which can degrade the performance of free-space optical link, especially for OAM multiplexing communication link. Some reports evaluated the performance of long-distance free-space OAM transmission link and demonstrated the information transfer [7-9]. Very recently, a laudable experiment was reported that multiplexing of 4 collocated OAM beams achieved 120-meter 400-Gbit/s free-space optical communications link [10]. To the best of our knowledge, security free-space optical data transmission over hundreds of meters using OAM multiplexing has not yet been studied.In this paper, we experimentally demonstrate a 260-meter security free-space optical data transmission using spatial multiplexing of 2 OAM beams, where each channel is modulated with 10-Gbaud (40-Gbit/s) 16-ary quadrature amplitude modulation (16-QAM) data signal. We study the OAM link performance after 260-meter propagation, including beam wandering, received power fluctuation, channel crosstalk, bit-error rate (BER), and link security.. The obtained results show that the average mode crosstalk is over 20 dB when demultiplexing by a full pattern and it degrades to ~10 dB when demultiplexing by a 1/4-block pattern, which indicates the 260-meter security OAM transmission link. 2. Concept, principle and experimental setup
Fig. 1. Layout of a 260-meter security OAM multiplexing free-space optical transmission link between WNLO-E building and WNLO-H building. WNLO: Wuhan National Laboratory for Optoelectronics.
Figure 1 illustrates the layout of a 260-meter security OAM multiplexing free-space optical transmission link between the corridors from WNLO-E building to WNLO-H building, which is exposed to the atmospheric conditions. The transmitter and the receiver are located in the front of the gate of WNLO-E building and the reflection mirror (M) is located at the end of the corridors. The single way distance is 130 mand thus the total distance of double-pass transmission is 260 m after reflection. The experimental setup is shown in Fig. 2. A narrow linewidth laser at 1550 nm is sent to an IQ modulator to produce a 10-Gbaud (40-Gbit/s) 16-QAM signal. The 16QAM signal is split into two copies for two OAM channels. In one copy, the signal is delayed with a 2-km singlemode fiber (SMF) to decorrelate the data sequence and thus the two channels are decorrelated.. Before connecting to collimators, the two channels are sent to erbium-doped fiber amplifier (EDFA), variable optical attenuator (VOA),
Th1H.3.pdf
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Fig. 2. Experimental setup for 260-meter security OAM-multiplexed link. SLM-1: spatial light modulation, Pol.: polarizer, Col.: collimator, BS: beam splitter, PC: polarization controller, VOA: variable optical attenuator. OC: optical coupler, EDFA: erbium-doped fiber amplifier, M: mirror, NDF: neutral density filter, TX: transmitter, RX: receiver.
and polarization controller (PC) for proper power and polarization control. The two spatial light modulators (SLMs) in two paths modulate the light beams to OAM state 𝑙 = +3. After combination using a beam splitter (BS-1) with the OAM state reversed in the reflective path (mirror image effect), two OAM beams with opposite states are multiplexed together (𝑙 = ±3). Meanwhile, a He-Ne laser at 632.8 nm produces a clear Gaussian beam (size: 0.8 mm), which is combined with the two OAM channels using another beam splitter (BS-2). This red Gaussian beam is mainly used for easy system alignment. The two OAM channels and the red beam pass through a 1:20 expander with the beam size magnified to ~4 cm. Note that we can produce a converged beam by sliding the lens adjustment and adjust the beam waist position at the reflection site in the end of corridor. As a result, we can receive a ~4 cm beam in the OAM RX. The received OAM beam size is reduced by two lens (f=300 mm and 40 mm) but still converged. At the proper position, OAM channel can be demodulated by loading an inverse fork hologram pattern on SLM-3. After a telescope system (f=50mm and 400mm), the converged demodulated beam is magnified and collimated and then coupled into an SMF for coherent detection assisted by off-line digital signal processing. 3. Experimental results and discussions Fig. 3 shows the intensity profiles of generated OAM beams and their superposition at the transmitter ((a1)-(a3)), the received OAM beams after 260-meter propagation ((b1)-(b3), and demodulated beams for different patterns loaded
Fig. 3. Intensity profiles of (a1)-(a3) generated OAM beams (𝑙 = +3, 𝑙 = −3 and superposition of 𝑙 = ±3) at TX; (b1)-(b3) received OAM beams (𝑙 = +3, 𝑙 = −3 and superpositions of 𝒍 = ±𝟑) at RX; (c1)-(c4) demodulated beams for different loading patterns (𝑙 = −3, 𝑙 = −1, 𝑙 = +1, 𝑙 = +3) when transmitting 𝑙 = −3. TX: transmitter, RX: receiver.
onto SLM-3 when transmitting 𝑙 = −3 ((c1)-(c4)). It is found that only when the demodulated pattern is inverse to the transmit OAM state the OAM beam can be converted into a Gaussian-like beam with a bright spot at the beam center ((c4)). Owing to the atmospheric turbulence, it is valuable to investigate the fluctuation of demodulated position and received power. Fig. 4(a1) and 4(a2) depict the center displacement of the received demodulation beam. It is observed that the maximum displacement is around ~0.45mm for 𝑙 = +3 and ~0.5mm for 𝑙 = −3. Fig. 4(b1) and 4(b2) present the space light power fluctuation after the beam reduction and the power is stable with slight difference for two channels due to the atmospheric loss. The fluctuations of the received power of signal channel and crosstalk channel after SMF are shown in Fig. 4(c1)(c2) and (d1)(d2). The received power fluctuations up to ~8 dB for signal channel demultiplexed by 𝑙 = +3, ~10 dB for crosstalk channel demultiplexed by 𝑙 = +3, ~4 dB for signal channel demultiplexed by 𝑙 = −3, and ~6 dB for crosstalk channel demultiplexed by 𝑙 = −3 are observed within a 50% probability distribution range. The crosstalk between two channels is ~20 dB (𝑙 = +3) and ~24 dB (𝑙 = −3). All the received data are recorded in 200 seconds at the interval of 1 second.
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Fig. 4. Statistic results of fluctuations after 260-meter OAM multiplexing transmission (recorded in 200 seconds at the interval of 1 second). (a1)(a2) center displacement of the received demodulation beam (𝑙 = +3, 𝑙 = −3); (b1)-(b2) space light power fluctuation after the beam reduction (𝑙 = +3, 𝑙 = −3), (c1)-(2) received power of signal channel after SMF; (d1)-(d2) received power of crosstalk channel after SMF.
Furthermore, we also study the security of the OAM multiplexing transmission link. As shown in Fig. 5, we respectively load the full pattern, 1/16-block, 1/8-block and 1/4-block pattern onto SLM-3 and present their average BER performance demultiplexed by 𝑙 = +3 and 𝑙 = −3. When the eavesdropper wiretaps a majority of the OAM beams with 1/16-block, the BER curves can be still below the enhanced forward error correction (EFEC) limit of 2e3 but with significantly increased optical signal-to-noise ratio (OSNR) penalty compared to the case with full pattern. Such phenomenon can be ascribed to the increased OAM channel crosstalk when blocking a part of OAM beams, which enhances the security of OAM multiplexing transmission link As the blocked part of OAM beams increases, the BER curves degrade rapidly. When 1/4 part of OAM beams is blocked (1/4-block pattern), the BER performance of the data transmission link cannot be below the EFEC limit, i.e. the eavesdropper loses lots of correct data information. As a consequence, the eavesdropper wiretapping a part of OAM beams fails to get correct data information. The more OAM beams blocked, the more errors received by the eavesdropper. Only complete reception of OAM beams corresponds to the best BER performance. The obtained results shown in Fig. 5 indicate successful demonstration of 260-meter security free-space optical data transmission using 16-QAM carrying OAM multiplexing.
Fig. 5. Measured average BER performance for 260-meter security 10-Gbaud 16-QAM OAM multiplexing free-space optical data transmission link.
4. Acknowledgement This work was supported by the National Basic Research Program of China (973 Program) under grant 2014CB340004, 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). The authors thank Yongxiong Ren at University of Southern California for helpful discussions. 5. Reference [1] J. Wang et. al., Nat. Photonics 6, 488 (2012). [2] I. B. Djordjevic et. al., Al., Opt. Express 19, 6845 (2011). [3] T. Su et. al., Opt. Express 20, 9396 (2012). [4] H. Huang et. al, Opt. Lett. 39, 197 (2014) [5] Jian Wang, et. al., ECOC Mo.4.5.1(2014).
[6] G. Gibson et. al., Opt. Express, 12, 5448 (2004). [7] M. Krenn et. al arXiv:1402.2602 (2014). [8] G. Vallone et. al, PRL 113, 060503 (2014). [9] J. A. Anguita et. al, FIO paper F5h3b.5 (2014). [10] Y. Ren, et. al., OFC M2F. 1 (2015)
