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Abstract
With the increasing demands for underwater monitoring and military applications, underwater wireless optical communication (UWOC) is desired to be an alternative approach to provide higher data rate than acoustic communication. Twisted light carrying orbital angular momentum (OAM) has recently gained increasing interest in diverse areas, especially in free-space and fiber-based optical communications. OAM-based UWOC between underwater and aerial users, a promising technique to enable a variety of applications, which however, has not yet been reported so far. Here we experimentally demonstrate an adaptive water-air-water data information transfer using OAM. According to the feedback information of the received intensity distribution, the reflection element is adjusted for mitigating the misalignment-induced degradation effect due to water level change. The experimental results show favorable performance of the feedback-assisted water-air-water twisted light data information transfer.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, the increasing demands of oceanography monitoring, air/underwater rescue, military warship-to-submarine communication have stimulated great development of underwater communication [1–3]. While fiber and copper cabling can be directly employed for stationary devices, a wireless scheme is preferable for diverse situations, e.g. underwater communication between unmanned vehicles and submarines. Acoustic communication is a well-established method but suffers from narrow bandwidth, high latency and bulky devices [4–8], which make it increasingly difficult for short-range, mobile, multi-user environments in future high-data-rate underwater communication systems. Radio frequency communication is also not applicable to underwater communication because of heavily attenuation in water [9].
Very recently, underwater wireless optical communication (UWOC) that can provide larger data rates than acoustic communication by orders of magnitude has attracted increasing interest in underwater communication links over short and moderate distances (<100 m) [10,11]. By taking advantage of low attenuation of seawater in blue-green (400-550 nm) wavelengths of electromagnetic spectrum [12], the UWOC system is capable of providing high data rates, low-cost, compact and low latency communication systems [13–18].
Additionally, in order to keep up with the demands in capacity growth of underwater communication, the spatial domain of lightwaves is explored and desired to be combined with the present UWOC system. As an alternative mode base set, twisted light carrying orbital angular momentum (OAM), which featuring helical phase front, has shown great potential both in free-space and fiber-based optical communications in terms of OAM modulation (encoding/decoding) and OAM multiplexing [19–30]. Very recently, OAM-based UWOC systems have also been reported [31–35]. In 2016, Baghdady et al. demonstrated a 3-Gbit/s UWOC link by multiplexing 2 OAM modes and investigated the degrading effects in turbid water [31]. Ren et al. reported an impressive demonstration of 40-Gbit/s aggregate capacity in UWOC by multiplexing 4 green OAM modes at 1064 nm [32]. Apart from those OAM-based UWOC systems with high capacity, some degrading effects such as oceanic turbulence and bubbles have also been investigated. Wang et al. presented a theoretical investigation of a OAM-based UWOC system over weak turbulent ocean [33]. Zhao et al. experimentally evaluated the transmission performance of UWOC employing different spatial modes subjected to dynamic bubbles and static obstructions [34].
Besides UWOC connecting different underwater users, UWOC between underwater and aerial users is also a promising technique to enable a variety of applications such as air/water search and rescue, real-time video transmission during an inspection of oil platforms or ships. Remarkably, most of the aforementioned UWOC systems are based on the communication between underwater users, and the OAM-based underwater wireless optical link across the air-to-water interface has not yet been reported so far. Moreover, it is still a great challenge to ensure accurate detection under the environment of ebb and flow of the tide, which would be a common circumstance for communication link across the water-to-air interface. In this scenario, a laudable goal would be to exploit a robust OAM-based UWOC system across the water-to-air interface.
In this paper, we propose and experimentally demonstrate an adaptive water-air-water data information transfer using OAM. We investigate the degrading effects of the change of water surface height on the received OAM beam quality and system performance under three different relative surface heights of the water tank (0 mm, 25 mm and −10 mm), finding that the change of water level induced misalignment can influence the successful reception of the information carried by the OAM modes. To address this problem, a feedback is introduced to enable the alignment. A reflection element is controlled by the feedback information of the received intensity distribution for feedback-assisted UWOC across the air-to-water interface. Moreover, we employ discrete multi-tone (DMT) modulation in underwater to air link for flexible communications with proposed feedback scheme, and the gross bit rate of 1.08 Gbit/s is achieved. By comparing the system performance with and without feedback under two relative surface heights (25 mm/-10 mm), the measured power penalty at hard-decision forward error correction (HD-FEC) limit with 7% overhead (3.8 × 10−3) shows an improvement of 2.5 dB and 1 dB at relative surface height of 25 mm and −10 mm with feedback, respectively.
2. Concept and principle
In general, unmanned vehicles, robots and devices deployed underwater or in the air are distributed sparsely over the area of interest. There is much interest to build collaborative communication networks between underwater and air users featuring high mobility, low time latency and deployment flexibility. This technology has many potential applications, such as air/underwater rescue and resource exploration. In the applications of exploration and rescue, there are great demands to transmit high-speed data information in real time. To improve the exploration efficiency, collaborative communication configuration between underwater users (e.g. multi autonomous underwater vehicles (AUVs), robots) and air users (e.g. multi unmanned aerial vehicles (UAVs)) can be employed. OAM-based water-air-water wireless optical communication system has potential to satisfy the requirement of providing high-capacity underwater to air short distance data transmission. As shown in Fig. 1, there are three envisioned collaborative communication scenarios for a group of users with OAM-based wireless optical communication across the water-to-air interface. Figure 1(a) illustrates the OAM-based communication between an air user and an underwater user for high-speed data transmission. Collaborative communication between air user and underwater users is depicted in Fig. 1(b). In this scenario, the air user can receive data information from one underwater user, and sends instructions to another underwater user. The most common link between users is a line-of-sight (LOS) link, as illustrated in Figs. 1(a) and 1(b). However, in some underwater scenarios the line of sight is not available because of obstructions such as half-submerged rocks, which would be common circumstance for the mobile underwater users. The reflection communication link between an air user and underwater users is illustrated in Fig. 1(c). The light from one underwater user reaching the air user is reflected back to another underwater user, which is capable of bypassing obstacles.
Fig. 1 Concept and principle of collaborative communication scenarios with underwater twisted green light across the air-to-water interface. (a) Communication between an air user and an underwater user. (b) Collaborative communication between an air user and two underwater users. (c) Reflection communication between an air user and two underwater users.
3. Experimental setup
Figure 2 illustrates experimental configuration for feedback-assisted water-air-water twisted green light communication. At the transmitter, an arbitrary waveform generator (Tektronix AWG 70002A) is used to generate DMT signal, and the sampling rate of the AWG is set at 800 MSamples/s. The analog bandwidth and vertical resolution of the AWG is 14 GHz and 10 bit, respectively. The gross transmission bit rate is 1.08 Gb/s. Note that before the actual transmission, signal-to-noise ratios (SNRs) for different subcarriers are estimated by transmitting a pilot DMT signal. With the measured SNR, the Chow’s bit loading algorithm is employed to obtain the bit allocation for each subcarrier. Figure 3 shows measured SNR per subcarrier with orthogonal frequency division multiplexing 16-ary quadrature amplitude modulation (OFDM-16QAM) and bit allocation per subcarrier. After amplified by an electrical amplifier (EA), the baseband DMT signal is directly modulated on a 520-nm single mode pigtailed laser diode (LD). Then, the pigtailed fiber of LD is connected to a collimator to generate a data-carrying Gaussian beam in free space. The output green light is converted into an OAM mode with a phase-only spatial light modulator (SLM). 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 19201080 pixels and a small pixel pitch size of 8 μm. A half-wave plate (HWP) and a polarizer (Pol.) are used to adjust the polarization of the light to be aligned to the optimal working polarization of SLM. A pinhole is used as a spatial filter to choose the desired diffraction order. We emulate the ebb and flow of the tide by using a 2-meter-long rectangular tank (with 40cm in width and 40 cm in height) filled with tap water and changing the water surface height in the tank. Two reflection mirrors are fixed on the bottom of the tank to make the light across the water-to-air interface. Note that a reflection element set above the tank is employed in the feedback scheme for beam steering. At the receiver side, the light passes through two lenses for reducing the beam size for avalanche photo detector (APD). A neutral density filter (NDF) is used to adjust the power for camera. Then, another SLM (SLM-2) is used to convert the output OAM mode back to Gaussian-like mode for detection. With the information of the received intensity distribution captured by the camera, the reflection element is adjusted to ensure perfect alignment. The demodulated beam is detected by a 1-GHz APD (Menlo Systems, APD210, active area diameter: 0.5 mm). After amplified by another EA, the DMT signal is sampled using a real-time oscilloscope operating at 20 GS/s for offline processing.
Fig. 2 Experimental configuration for feedback-assisted underwater twisted green light communications across the air-to-water interface. Inset is the schematic of the link across the air-to-water interface. Col.: collimator; HWP: half-wave plate; Pol.: polarizer; NDF: neutral density filter; BS: beam splitter; SLM: spatial light modulator.
Fig. 3 Bit-loading parameters for DMT transmission. (a) Measured SNR per subcarrier. (b) Bit allocation per subcarrier (insets are received 16-QAM, 8-QAM and 4-QAM constellations at different subcarriers).
Inset of Fig. 2 shows a schematic of the water-air-water link. The green light pass through the air-to-water interface and the relationship between the angles of incidence and refraction is governed by Snell’s law. To emulate the ebb and flow of the tide, we change the relative surface height of the water tank (0 mm, 25 mm and −10 mm) by adding and reducing the tap water in the water tank. It can be clearly seen that the angles of incidence stays constant when the relative water surface height changes. By adjusting the height of the feedback-assisted reflection element, one can keep the light path same with the orignal water level to ensure successful data transmission when the water surface height changes.
4. Experimental results
We then characterize the influence of the ebb and flow of the tide on the received OAM beam quality and system performance. In general, a light beam propagating across the water-to-air interface may suffer from misalignment caused by the change of the water level. Before changing the relative surface height of the water tank, the intensity profiles of generated Gaussian, OAM+1, OAM+3 and OAM+5 modes at the transmitter side and the receiver side are shown in Fig. 4(a). Then we compare the intensity profiles of OAM+3 beam without and with feedback. Note that the centers of the measured output and demodulated OAM intensity profiles before introducing surface height change (0 mm) are used as the baseline, as shown in Figs. 4(b) and 4(c). Then, we add or reduce water in the water tank to raise or lower the surface height. In Fig. 4(b), we show the measured output and demodulated OAM intensity profiles under three relative surface heights (0 mm, 25 mm and −10 mm) without feedback.
Fig. 4 Measured output and demodulated OAM intensity profiles under three relative surface heights (0 mm, 25 mm and −10 mm) in underwater wireless optical link across the water-to-air interface. (a) Measured input intensity, output intensity and demodulated intensity profiles of Gaussian, OAM+1, OAM+3, OAM+5 modes. (b), (c) Measured output and demodulated OAM+3 intensity profiles under three relative surface heights (0 mm, 25 mm and −10 mm). (b) w/o feedback. (c) w/ feedback.
Compared to the intensity profiles at 0 mm relative surface height, the center of the intensity profiles for relative surface height of 25 mm moves to the left, while the center of the intensity profiles for −10 mm moves slightly to the right. The larger the change of the water level is, the larger the deviation from the baseline is, which can be clearly seen from Fig. 4(b). In order to correct such deviation, feedback-assisted correction is employed to enable the alignment in the center of the beam at the receiver side. By appropriately adjusting the reflection element according to the received information of the demodulated OAM intensity distribution detected by the camera at the receiver side, one could correct the deviation of the output and demodulated OAM intensity profiles at the receiver side. After feedback-assisted correction, the measured output and demodulated OAM intensity profiles under two relative surface heights (25 mm and −10 mm) are depicted in Fig. 4(c). Compared to the results in Fig. 4(b) without feedback, both the center of the measured output and demodulated OAM intensity profiles are aligned to the baseline. One can clearly see the bright spots at the center of the demodulated intensity profiles, which show favorable performance with feedback.
We further demonstrate the data-carrying performance of DMT signal across water-air-water link. Note that the APD is only sensitive to optical power and for different modes the bit-error rate (BER) value will be equal when receiving the same power, so we adopt the attenuation of NDF for BER measurement in the experiment. Figure 5(a) plots the measured BER performance with and without feedback when the water level is raised by 25 mm. Here, we take Gaussian mode as the reference channel. It can be seen that the observed penalties at HD-FEC threshold with 7% overhead (BER = 3.8 × 10−3) without feedback is about 2.5 dB, as shown in Fig. 5(a). For comparison, we also plot the measured BER performance with and without feedback when the water level is lowered by 10 mm in Fig. 5(b). The insets give the obtained constellations for 16-QAM with and without feedback at the attenuation level 0 dB under two relative surface heights, respectively. The observed penalties at 7% HD-EFEC threshold (BER = 3.8 × 10−3) without feedback is about 1 dB. The measured penalty decreases with the decrease of relative surface height. It means that OAM mode suffers more misalignment-induced performance degradation when the change of the water surface height becomes larger. Note that the BER curves with feedback are almost coincident with the BER curve at 0 mm relative surface height, which shows favorable performance of the feedback-assisted water-air-water twisted light data information transfer.
Fig. 5 Measured BER performance with and without feedback under three relative surface heights. (a) Measured BER performance with and without feedback when the water level is raised by 25 mm. (b) Measured BER performance with and without feedback when the water level is lowered by 10 mm.
5. Discussions
The obtained results indicate the successful demonstration of adaptive water-air-water data information transfer communication link using OAM. There are several distinct features of water-air-water OAM communication link: i) OAM, exploiting the spatial domain of lightwaves, can potentially increase the transmission capacity; ii) water-air-water link may find actual applications, such as air/water search and rescue, real-time video transmission during an inspection of oil platforms or ships; iii) OAM is fully compatible with advanced modulation formats (e.g. DMT). Remarkably, the water-air-water link may suffer from performance degradation due to light path fluctuation (change of water surface height). Accordingly, we provide an effective feedback-enabled adaptive solution.
In the proof-of-concept experiment, we study the degrading effects of the change of water surface height on the received OAM beam quality under three different relative surface heights of the water tank (0 mm, 25 mm and −10 mm), and the system performance (BER) with and without feedback under different relative surface heights. In our feedback assisted water-air-water link, one can ensure successful data transmission when the water surface height changes by adjusting the height of the feedback-assisted reflection element. We believe that the proposed scheme could also work in deeper propagation depths. Limited by the low height of the rectangular tank used in the experiment, the demonstrated propagation depths are relatively small.
Moreover, the experimental results including the mode intensity patterns and BER curves in our work are obtained when the water is calm. However, capillary waves are common in real applications. The wavelength of capillary waves in water is typically less than a few centimeters, with a phase speed in excess of 0.2-0.3 meter/second. When a light beam passes through the capillary wave, the refraction angle of the output light will change rapidly with the wave. The feedback system is hard to track the output light when the dynamic response of the system is not fast enough. Thus, capillary waves will cause greater degradation in the received signal. In the proof-of-concept experiment, the influence of capillary waves on the received signal is not considered. To deal with the capillary waves in practical applications, the feedback system needs to be faster and more precise. With the help of fast steering mirror and high-speed camera, a faster adaptive system might be realized for handling the capillary waves. Actually, fast steering mirrors can provide response time of microseconds [36–39], so they will be suitable for fast dynamic process in adaptive water-air-water link. In this scenario, future improvement of fast adaptive underwater OAM communcation link across the air-to-water interface assisted by fast steering mirror and high-speed camera could be considered towards practical OAM-based water-air-water data information transfer applications.
6. Conclusions
In summary, we experimentally demonstrate an adaptive water-air-water data information transfer using OAM. In general, OAM-based underwater wireless optical link across the air-to-water interface requires precise alignment between the transmitter and receiver, and the misalignment induced by the change of the water level will result in poor receiving quality of OAM beam. A feedback-assisted correction process is employed in the experiment to enable the alignment through an adjustable reflection element. The measured output and demodulated OAM intensity distributions show successful alignment when employing feedback-assisted correction scheme in the presence of the change of water surface height. Moreover, we measure the communication link performance with DMT signals. The experimental results show favorable transmission performance of the feedback-assisted water-air-water data information transfer using OAM. The demonstrated feedback-assisted underwater wireless optical link exploiting OAM-carrying twisted light may open a door to facilitate wide applications in future grooming OAM communications and networks between underwater and aerial users.
Funding
The National Natural Science Foundation of China (NSFC) (11574001, 61761130082, 11774116, 11274131, 61222502); National Basic Research Program of China (973 Program) (2014CB340004); Royal Society-Newton Advanced Fellowship; National Program for Support of Top-notch Young Professionals; Natural Science Foundation of Hubei Province of China (ZRMS2017000403); Program for HUST Academic Frontier Youth Team; Shenzhen Strategic Emerging Industry Development Special Fund (JCYJ20160531194518142, JCYJ20170307172132582).
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