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Abstract
The availability of the underwater wireless optical communication (UWOC) based on red (R), green (G) and blue (B) lights makes the realization of the RGB wavelength division multiplexing (WDM) UWOC system possible. By properly mixing RGB lights to form white light, the WDM UWOC system has prominent potentiality for simultaneous underwater illumination and high-speed communication. In this work, for the first time, we experimentally demonstrate a 9.51-Gb/s WDM UWOC system using a red-emitting laser diode (LD), a single-mode pigtailed green-emitting LD and a multi-mode pigtailed blue-emitting LD. By employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) modulation in the demonstration, the red-light, the green-light and the blue-light LDs successfully transmit signals with the data rates of 4.17 Gb/s, 4.17 Gb/s and 1.17 Gb/s, respectively, over a 10-m underwater channel. The corresponding bit error rates (BERs) are 2.2 × 10−3, 2.0 × 10−3 and 2.3 × 10−3, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10−3.
© 2017 Optical Society of America
1. Introduction
The vast ocean is yet an unsolved mystery, which mankind has never ceased to explore. In the past, underwater acoustic communication technology carried considerable weight in ocean exploration and marine environmental monitoring. However, with the dramatically increased demands for broadband transmission in underwater communication, underwater acoustic communication alone can no longer meet the needs of bandwidth-intensive applications due to its drawbacks of low bandwidth, bulky antennas and low propagation speed. In contrast, underwater wireless optical communication (UWOC) technology, featuring high bandwidth, low power consumption and low latency, has gradually emerged and spread as a new trend to meet the increasing bandwidth requirement. In recent years, UWOC systems using light emitting diodes (LEDs) as light sources have attained great achievement and progress [1–3]. However, LEDs with limited modulation bandwidth are attractive only for applications requiring medium data rate. Alternately, laser diodes (LDs), which have much higher modulation bandwidth than LEDs, are more applicable for high-speed UWOC [4–14]. With the current progress in the manufacturing of LDs operating in the visible range, LDs at different wavelengths have been investigated in UWOC systems to cope with various water types. So far, a great deal of effort has been made in UWOC utilizing blue (B)-light LDs, as blue light has the smallest absorption coefficient in pure water [4–10]. Lately, a 10-meter underwater transmission at a data rate of 10 Gbps has been achieved, using a 405-nm LD with light injection and optoelectronic feedback techniques [10]. Meanwhile, researchers have begun to shift their focus to longer wavelengths, which suffer smaller attenuation than blue light in certain realistic water types. In [13], the authors demonstrated a 4-Gbit/s underwater transmission of multiplexed four green orbital angular momentum (OAM) beams using a directly-modulated 520-nm LD, over a 1.2-meter-long underwater channel. However, the UWOC system based on green-light LDs is still in the initial stage [11–13], due to the immature manufacturing technologies of green-light LDs. Compared with the infancy green-light LDs, mature red (R)-light LDs have the advantages of larger modulation bandwidth, higher power, smaller multipath effect and lower price. Not long ago, we started studying the transmission performance of red light in water, with an interesting finding that the red light with smaller scattering coefficient could outperform the blue/green light in turbid water [14]. Using a TO56 red-light LD, a data rate of 4.883 Gb/s was achieved over a 6-m underwater channel [14]. The realization of UWOC based on non-intuitive red light is a very important step, which makes it possible for the implementation of RGB wavelength division multiplexing (WDM) in UWOC. By properly mixing the RGB lights into a common transmission path, not only system capacity can be efficiently increased, but also white light can be generated, which is essential for simultaneous underwater illumination and communication, similar to the indoor visible light communication (VLC). Besides, the RGB WDM UWOC system can alleviate the electronic bandwidth requirements for a given aggregate data rate, which is especially important for oceanic engineering.
In this paper, we propose and experimentally demonstrate a WDM UWOC system based on a red-light LD, a single-mode pigtailed green-light LD and a multi-mode pigtailed blue-light LD. To improve the spectral efficiency of the UWOC system, 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) technology is employed. An aggregate data rate of 9.51 Gb/s (R: 4.17 Gb/s, G: 4.17 Gb/s, B: 1.17 Gb/s) is successfully achieved over a 10-m underwater channel in the proposed WDM UWOC system. The bit error rates (BERs) of the three wavelength channels (R, G and B) are 2.2 × 10−3, 2.0 × 10−3 and 2.3 × 10−3, respectively, which are below the forward error correction (FEC) limit of 3.8 × 10−3. To our knowledge, this is the first demonstration of using RGB WDM to implement high-speed UWOC. At present, WDM technology is mostly investigated in the areas of optical fiber communication [15–17] and VLC [18–25]. As reported in [24], using 16-QAM OFDM modulation, in conjunction with red, blue, and green LDs, the authors achieved data rates of 4.4 Gb/s, 4 Gb/s and 4 Gb/s over a 0.2-m omnidirectional air channel. Note that we employ a multi-mode pigtailed blue-light LD with a relatively small 3-dB modulation bandwidth and a collimation lens with poor converging effect in our experiment due to limited component availability, which significantly limit the underwater transmission rate and distance. In addition, a spatial beam combiner with relatively large insertion loss is used to combine the three laser beams. Significant performance enhancement can be expected via using an improved blue-light LD and a fiber-based beam combiner.
2. Experimental setup
Figure 1 shows the experimental setup of the proposed RGB LDs-based WDM UWOC system. The photographs of the transmitter, the receiver and the channel are shown in the insets of Fig. 1. At the transmitter, two different 32-QAM OFDM signals, OFDM 1 and OFDM 2, were first generated with off-line MATLAB programs and then loaded into two channels (CH 1 and CH 2) of an arbitrary waveform generator (AWG, Tektronix AWG70002A). The sampling rate and output peak-to-peak voltage (Vpp) of the two OFDM signals were set at 5 GSamples/s and 450 mV, respectively, with their specific parameters listed in Table. 1.
Fig. 1 The experimental setup of the proposed RGB LDs-based WDM UWOC system. Inset: (a) the transmitter module, (b) the receiver module, and (c) the water tank.
Table 1. Parameter values of OFDM 1 and OFDM 2
We used two output ports of CH 1, namely CH 1 ( + ) and CH 1 (-), which are 180 degrees out of phase, to transmit OFDM 1. The outputs from CH 1 ( + ) and CH 1 (-) were used to drive a 35-mW red-light LD (HL6501MG) and a 15-mW single-mode pigtailed green-light LD (Thorlabs LP520-SF15), respectively. OFDM 2 was output from CH 2 ( + ) before being superimposed on a 1-W multi-mode pigtailed blue-light LD (FJ450A11), which has a smaller 3-dB modulation bandwidth than the red-light and green-light LDs. After adjusting the voltage via three 25-dB Mini-Circuits ZHL-6A-S + amplifiers (AMPs) and three key-press variable electrical attenuators (ATTs), the three baseband OFDM signals were fed into the three LDs, respectively. The process of signal superposition was realized by three bias-tees. The bias currents of the red-light, green-light and blue-light LDs were 77 mA, 80.89 mA and 250 mA, respectively. The green-light and blue-light LDs were followed by air-spaced doublet collimators (Thorlabs FB10FC-543) to achieve parallel emitting light, while the red-light LD was installed in a cupreous heat diffuser with an ordinary collimation lens, which has poor converging effect. The RGB laser beams from three different directions were then combined by a beam combiner and transmitted through a 10-m underwater channel. A pair of mirrors were put in the water tank, which is full of fresh tap water, to realize underwater transmission by mirror reflection. At the receiver, a plano-convex lens was employed to focus the mixed light onto a 1-GHz avalanche photo diode (APD, Menlo Systems, APD210) for signal detection. Red, green and blue dichroic filters (Thorlabs FD1R, FD1G, FD1B), used for wavelength separation, were in turn placed between the lens and the APD. The electrical signals were captured by an oscilloscope (OSC, Rohde&Schwarz RTO1044) with a sampling rate of 20 G Samples/s and finally transmitted to a computer for demodulation.
3. Experimental results
Firstly, we measured the optical spectra of the red-light, green-light and blue-light LDs. The peak wavelengths of the red-light, green-light and blue-light LDs are around 660 nm, 520 nm and 440 nm, respectively, as shown in Fig. 2. Figure 3 presents the power-to-current (P-I) characteristics of the red-emitting, green-emitting and blue-emitting LDs, all of which show good linearity.
Fig. 2 The optical spectra of (a) the red-light LD, (b) the green-light LD and (c) the blue-light LD.
Fig. 3 The P-I characteristics of (a) the red-light LD, (b) the green-light LD and (c) the blue-light LD.
Over the 10-m underwater channel, the achieved gross data rate of OFDM 1 transmitted by the green-light and red-light LDs was 4.17 Gb/s and the net data rate was 3.42 Gb/s after removing the overheads of training symbols, CP and FEC (7%). The BERs of the OFDM signals transmitted by the green-light and red-light LDs are 2.0 × 10−3 and 2.2 × 10−3, respectively. For OFDM 2 transmitted by the blue-light LD, we achieved the gross data rate of 1.17 Gb/s at a BER of 2.3 × 10−3, whereas the net data rate was 959.40 Mb/s, after 10-m underwater transmission. The limited transmission rate of the blue-light LD is ascribed to its small 3-dB modulation bandwidth.
The electrical spectra of the 32-QAM OFDM signals transmitted by the red-emitting, green-emitting and blue-emitting LDs over the 10-m underwater channel are presented in Fig. 4. The corresponding constellation maps are shown in Fig. 5, which were well converged. Figures 6(a) and 6(b) display the BERs and EVMs of the OFDM signals transmitted by the red-light, green-light and blue-light LDs for the different subcarriers over the 10-m underwater channel, respectively. Note that most subcarriers have a BER of zero that is not suitable to be plotted in log scale. Higher BERs in the low-frequency region were caused by larger beating noise among subcarriers.
Fig. 4 The spectra of the OFDM signals over the 10-m underwater channel using (a) the red-emitting LD at 4.17 Gb/s, (b) the green-emitting LD at 4.17 Gb/s and (c) the blue-emitting LD at 1.17 Gb/s.
Fig. 5 The constellation maps of the 32-QAM OFDM signals over the 10-m underwater channel using (a) the red-emitting LD at 4.17 Gb/s, (b) the green-emitting LD at 4.17 Gb/s and (c) the blue-emitting LD at 1.17 Gb/s.
Fig. 6 BERs (a) and EVMs (b) of the OFDM signals transmitted by the red-light, green-light and blue-light LDs for the different subcarriers after 10-m underwater transmission.
4. Discussion
At first glance, the application of mature WDM technology in UWOC is straightforward. However, the superiority of WDM technology in increasing system capacity and reducing requirement of electronic bandwidth is particularly important for oceanic engineering. In addition, it is the first step toward developing simultaneous underwater illumination and communication for underwater platforms, which have been widely applied for monitoring underwater environment, seafloor activities and submarine life in recent years. It also paves the way to design a future-proof UWOC system that can flexibly assign varying data rates and/or modulation formats to each wavelength according to different water types. Nevertheless, the complex transmitter and receiver structure in the current form has to be further simplified in the future (e.g., via photonic integration).
5. Conclusion
In this paper, for the first time, we propose and experimentally demonstrate a WDM UWOC system based on a red-light LD, a single-mode pigtailed green-light LD and a multi-mode pigtailed blue-light LD. In our experiment, using spectrally efficient OFDM modulation technique, we have achieved an aggregate data rate of 9.51 Gb/s (R: 4.17 Gb/s, G: 4.17 Gb/s, B: 1.17 Gb/s) over a 10-m underwater channel. The BERs of the 32-QAM OFDM signals transmitted by the red-light, green-light and blue-light LDs are 2.2 × 10−3, 2.0 × 10−3 and 2.3 × 10−3 respectively, which are under the FEC limit of 3.8 × 10−3. To further improve the system performance, a blue-light LD with a higher 3-dB modulation bandwidth can be employed in the future. The transmission distance can also be increased by using a fiber-based beam combiner and better collimators.
Funding
National Natural Science Foundation of China (NSFC) (61671409, 61301141).
References and links
1. P. Wang, C. Li, B. Wang, and Z. Xu, “Real-Time 25Mb/s Data Transmission for Underwater Optical Wireless Communication Using a Commercial Blue LED and APD Detection,” in Asia Communications and Photonics Conference 2016, OSA Technical Digest (online) (Optical Society of America, 2016), paper AS2C. [CrossRef]
2. J. Xu, M. W. Kong, A. B. Lin, Y. H. Song, X. Y. Yu, F. Z. Qu, J. Han, and N. Deng, “OFDM-based broadband underwater wireless optical communication system using a compact blue LED,” Opt. Commun. 369, 100–105 (2016). [CrossRef]
3. B. Zhuang, C. Li, N. Wu, and Z. Xu, “First Demonstration of 400Mb/s PAM4 Signal Transmission Over 10-meter Underwater Channel Using a Blue LED and a Digital Linear Pre-Equalizer,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (online) (Optical Society of America, 2017), paper STh3O.3. [CrossRef]
4. K. Nakamura, I. Mizukoshi, and M. Hanawa, “Optical wireless transmission of 405 nm, 1.45 Gbit/s optical IM/DD-OFDM signals through a 4.8 m underwater channel,” Opt. Express 23(2), 1558–1566 (2015). [CrossRef] [PubMed]
5. H. M. Oubei, J. R. Duran, B. Janjua, H. Y. Wang, C. T. Tsai, Y. C. Chi, T. K. Ng, H. C. Kuo, J. H. He, M. S. Alouini, G. R. Lin, and B. S. Ooi, “4.8 Gbit/s 16-QAM-OFDM transmission based on compact 450-nm laser for underwater wireless optical communication,” Opt. Express 23(18), 23302–23309 (2015). [CrossRef] [PubMed]
6. S. P. Najda, P. Perlin, T. Suski, L. Marona, M. Leszczyński, P. Wisniewski, R. Czernecki, R. Kucharski, G. Targowski, M. A. Watson, H. White, S. Watson, and A. E. Kelly, “AlGaInN laser diode technology for GHz high-speed visible light communication through plastic optical fiber and water,” Opt. Eng. 55(2), 026112 (2016). [CrossRef]
7. J. Baghdady, K. Miller, K. Morgan, M. Byrd, S. Osler, R. Ragusa, W. Li, B. M. Cochenour, and E. G. Johnson, “Multi-gigabit/s underwater optical communication link using orbital angular momentum multiplexing,” Opt. Express 24(9), 9794–9805 (2016). [CrossRef] [PubMed]
8. C. Shen, Y. Guo, H. M. Oubei, T. K. Ng, G. Liu, K. H. Park, K. T. Ho, M. S. Alouini, and B. S. Ooi, “20-meter underwater wireless optical communication link with 1.5 Gbps data rate,” Opt. Express 24(22), 25502–25509 (2016). [CrossRef] [PubMed]
9. T. C. Wu, Y. C. Chi, H. Y. Wang, C. T. Tsai, and G. R. Lin, “Blue laser diode enables underwater communication at 12.4 Gbps,” Sci. Rep. 7, 40480 (2017). [CrossRef] [PubMed]
10. C. Ho, C. Lu, H. Lu, S. Huang, M. Cheng, Z. Yang, and X. Lin, “A 10m/10Gbps Underwater Wireless Laser Transmission System,” in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2017), paper Th3C.3. [CrossRef]
11. H. M. Oubei, C. Li, K. H. Park, T. K. Ng, M. S. Alouini, and B. S. Ooi, “2.3 Gbit/s underwater wireless optical communications using directly modulated 520 nm laser diode,” Opt. Express 23(16), 20743–20748 (2015). [CrossRef] [PubMed]
12. J. Xu, A. B. Lin, X. Y. Yu, M. W. Kong, Y. H. Song, F. Z. Qu, J. Han, W. Jia, and N. Deng, “High-speed underwater wireless optical communication using a compact OFDM-modulated green laser diode,” IEEE Photonics Technol. Lett. 28(20), 2133–2136 (2016). [CrossRef]
13. Y. Ren, L. Li, Z. Zhao, G. Xie, Z. Wang, N. Ahmed, Y. Yan, A. Willner, Y. Cao, C. Liu, N. Ashrafi, S. Ashrafi, M. Tur, and A. Willner, “4 Gbit/s Underwater Transmission Using OAM Multiplexing and Directly Modulated Green Laser,” in Conference on Lasers and Electro-Optics, OSA Technical Digest (2016) (Optical Society of America, 2016), paper SW1F. [CrossRef]
14. J. Xu, Y. Song, X. Yu, A. Lin, M. Kong, J. Han, and N. Deng, “Underwater wireless transmission of high-speed QAM-OFDM signals using a compact red-light laser,” Opt. Express 24(8), 8097–8109 (2016). [CrossRef] [PubMed]
15. R. Kruglov, J. Vinogradov, O. Ziemann, S. Loquai, and C. A. Bunge, “10.7-Gb/s discrete multitone transmission over 50-m SI-POF based on WDM technology,” IEEE Photonics Technol. Lett. 24(18), 1632–1634 (2012). [CrossRef]
16. M. Joncic, R. Kruglov, M. Haupt, R. Caspary, J. Vinogradov, and U. H. P. Fischer, “Four-channel WDM transmission over 50-m SI-POF at 14.77 Gb/s using DMT modulation,” IEEE Photonics Technol. Lett. 26(13), 1328–1331 (2014). [CrossRef]
17. R. Caspary, M. Joncic, M. Haupt, U. F. Hirchert, R. Kruglov, J. Vinogradov, H. H. Johannes, and W. Kowalsky, “High speed WDM transmission on standard polymer optical fibers,” in Proceedings of IEEE Conference on Transparent Optical Networks (IEEE, 2015), pp. 1–4. [CrossRef]
18. A. Neumann, J. J. Wierer Jr, W. Davis, Y. Ohno, S. R. J. Brueck, and J. Y. Tsao, “Four-color laser white illuminant demonstrating high color-rendering quality,” Opt. Express 19(S4), A982–A990 (2011). [CrossRef] [PubMed]
19. W. Y. Lin, C. Y. Chen, H. H. Lu, C. H. Chang, Y. P. Lin, H. C. Lin, and H. W. Wu, “10m/500 Mbps WDM visible light communication systems,” Opt. Express 20(9), 9919–9924 (2012). [CrossRef] [PubMed]
20. F. M. Wu, C. T. Lin, C. C. Wei, C. W. Chen, Z. Y. Chen, H. T. Huang, and S. Chi, “Performance comparison of OFDM signal and CAP signal over high capacity RGB-LED-based WDM visible light communication,” IEEE Photonics J. 5(4), 7901507 (2013). [CrossRef]
21. C. Y. Lin, Y. P. Lin, H. H. Lu, C. Y. Chen, T. W. Jhang, and M. C. Chen, “Optical free-space wavelength-division-multiplexing transport system,” Opt. Lett. 39(2), 315–318 (2014). [CrossRef] [PubMed]
22. N. Chi, Y. Q. Wang, Y. G. Wang, X. X. Huang, and X. Y. Lu, “Ultra-high-speed single red-green-blue light-emitting diode-based visible light communication system utilizing advanced modulation formats,” Chin. Opt. Lett. 12(1), 22–25 (2014).
23. Y. Wang, X. Huang, L. Tao, J. Shi, and N. Chi, “4.5-Gb/s RGB-LED based WDM visible light communication system employing CAP modulation and RLS based adaptive equalization,” Opt. Express 23(10), 13626–13633 (2015). [CrossRef] [PubMed]
24. B. Janjua, H. M. Oubei, J. R. Durán Retamal, T. K. Ng, C. T. Tsai, H. Y. Wang, Y. C. Chi, H. C. Kuo, G. R. Lin, J. H. He, and B. S. Ooi, “Going beyond 4 Gbps data rate by employing RGB laser diodes for visible light communication,” Opt. Express 23(14), 18746–18753 (2015). [CrossRef] [PubMed]
25. D. Tsonev, S. Videv, and H. Haas, “Towards a 100 Gb/s visible light wireless access network,” Opt. Express 23(2), 1627–1637 (2015). [CrossRef] [PubMed]
