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Abstract
The orbital angular momentum (OAM) multiplexing technology is an essential method to boost underwater wireless optical communication (UWOC) capacity. However, state-of-art UWOC systems are often demonstrated in the laboratory using bulky and high power-consumption instruments, which can be impractical in a realistic environment. In this work, we propose, design and demonstrate a compact and energy-efficient OAM multiplexing UWOC prototype with complete packaging. Indeed, we improve the signal generation, modulation, receiving and processing components by employing the integrated programmable chips. We also employ two geometric phase Q-plate chips as an OAM multiplexer and de-multiplexer, respectively. Owing to the improvement of these components and the optical design, we package the complete UWOC system in two 65cm×35cm×40cm boxes with the power consumption of 20W. Our experiment demonstrates such a completely packaged prototype can support two 625Mbit/s channels (OAM+3, OAM-3) multiplexing in a 6-meter underwater environment with fidelity.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Recent decades witnessed a fast-rising need for high-speed reliable underwater wireless communication (UWC) systems due to increasing human activities in the rivers, lakes, seas and oceans [1–3]. Conventionally, owing to low transmission loss, acoustic waves are commonly used for UWC net-working ships, oceanography studies, offshore oil exploration, seafloor surveys and monitoring, etc. [4]. However, underwater acoustic communications intrinsically suffer from low transmission capacity, low spectral efficiency, high power consumption and expensive price [5]. As a result, in short-range communication applications where loss is typically not significant, acoustic communications may not be preferable. On the other hand, an alternative approach to UWC is using radio frequency (RF) signaling which has a relatively high communication speed and low cost. However, the RF-based UWC is seriously limited by strong attenuation in water [6]. As a favorable trade-off between acoustic and RF communication schemes, underwater wireless optical communication (UWOC) system is developed to gain considerable attention from the scientific, commercial and military communities [7–14]. Firstly, UWOC uses light in blue-green wavelength regions that can propagate through clean water with relatively low attenuation. Secondly, compared with the acoustic waves, UWOC uses optical waves with enormous bandwidth resources to enable high communication capacity. Lastly, compared with underwater acoustic communication and networks, UWOC demands lower power consumption and provides higher levels of security [13].
Recently, demanded by the ever-increasing need for higher communication capacities, breaking the limitation of the UWOC capacity is a critical task for the UWOC system. For this purpose, people turn their attention to the space domain of lightwaves. Space division multiplexing (SDM), especially mode division multiplexing (MDM), has been widely used both in fiber-based and free-space optical communications [15–20]. In particular, the orbital angular momentum (OAM) multiplexing, as one important form of MDM, is developed to enable OAM as possible mode bases, with theoretically infinite orthogonal OAM channels (l) and unlimited communication capacity [21–25]. Up to now, the OAM multiplexing technique has been introduced to UWOC to improve the transmission speed and capacity [26–29]. Important development includes achieving degradation compensation under different water conditions [26,30,31] and establishing optical links across the water-to-air interface [32].
However, previous OAM multiplexing UWOC systems are mostly demonstrated principally in labs, while it is challenging to adapt to realistic field environments [26–29,31]. The most important reason is that they require large size, high energy consumption, high price, and a friendly lab environment due to three major problems. For the first problem, conventional OAM (de-)multiplexing methods use multiple spatial light modulators (SLMs), spiral phase phase-plates, and gratings to generate multiple OAM beams and then use a beam combiner to overlap the beams into one path [22,33–35]. These devices typically demand large spaces and require independent wiring. For the second problem, conventional signal generation and re-processing methods use arbitral wave generators (AWG) and signal modulators to generate the desired modulation signal, and use oscilloscopes, coherent optical receivers and workstations to receive and process the signal [36]. All these devices are independent, large, heavy, expensive, and consume excessive power. For the third problem, conventional systems are not packaged and waterproof so it’s hard to work in the practical underwater environment. As a result, previous OAM multiplexing UWOC systems are typically expensive, delicate and unportable, not ideal for practical applications.
To overcome these limitations, here we propose and demonstrate a packaged integrated OAM multiplexing UWOC prototype with high-speed, low-cost, desirable compactness and convenience. Indeed, we use two geometric phase Q-plate chips to multiplex/de-multiplex OAM beams, respectively. Also, we integrate the signals generation, reception and processing components into the algorithm carrying field-programmable gate arrays (FPGA). With the optical design and delicate installation, we package all components into two 65cm×35cm×40cm waterproof boxes, as transmitter and receiver respectively. This OAM multiplexing prototype system realizes a 6-meter OAM UWOC with a 1.25Gbit/s data transmission rate (in l=+3 and l=-3 channels, each has the rate of 625Mbit/s), and the bit error rate (BER) is lower than the 1.5e-2 threshold.
2. Principle and experiment setup
Figure 1(a) shows the principle and concept of the prototype for OAM multiplexing UWOC. In this prototype, we use multiplexing l=+3 and l=-3 OAM beams carrying signals. After propagating through the underwater channel, the OAM beams are de-multiplexed and received by the receiver. The different OAM beams carry different signals enabling an OAM multiplexing UWOC link.
Fig. 1. (a) Concept of UWOC employing OAM mode (b) Concept of geometric Phase Q-plate chip multiplexer.
Figure 2 shows the experimental setup of an OAM multiplexing UWOC prototype. In the transmitter, the first component is a signal generation and modulation assembly. To improve the integration of the system, we use FPGAs to generate two electrical 625 Mbit/s Pseudo-Random Binary Sequence (PRBS) signals, then signals are reshaped and amplified by Small Form-factor Pluggable (SFPs) and applied to two 520-nm single-mode pigtailed laser diodes (LDs) by direct modulation. Owing to these integrated devices, the assembly has a compact size and low energy consumption. Then, we use two beams shaping system which consists of two lenses and an aperture to reshape two beams respectively. We also use two polarizers and half wave-plates to modulate the two beams as x-/y-linearly polarized (LP) respectively. After combination by a polarization beam splitter (PBS), we use a half-wave plate to change the transmission light as orthogonal circularly polarized (CP) light. Then, the light beams are converted to multiplexing OAM beams by a geometric phase Q-plate (q=1.5).
Fig. 2. (a) Experimental setup for underwater wireless optical multiplexing employing OAM mode. LD: laser diode; POL: polarizer; HWP: half-wave plate; PBS: polarization beam splitter SFP: Small Form-factor Pluggable; FPGA: field-programmable gate arrays; APD: avalanche photodiode detectors; A/D: analog-to-digital. (b) The pictures for the transmitter prototype models. (c) The pictures for the receiver prototype models. (d) The concept of FPGA in signals generation and processing. (e) The concept of OAM generation and multiplexing by Q-plate.
In the experiment, we emulate the underwater condition by using a 6-meter-long rectangular tank (40 cm width × 40 cm height) filled with tap water. Since the tap water has been filtered by the waterworks, we consider it as clean water without extra attenuation. At the receiver side, we firstly use an optical collimator system which is an inverse telescope to collect and collimate the transmission light. A neutral density filter (NDF) is used to adjust the received optical power. Then, we use another geometric phase Q-plate (q=1.5) chip to de-multiplex the OAM multiplexing beam as below:
After de-multiplexing, OAM multiplexing beams are converted back to orthogonal CP Gaussian beams. Using a quarter-wave plate and a half-wave plate, the orthogonal CP light beams are changed into orthogonal LP beams, then they are split by PBS into two beams. Next, we employ two high sensitivity silicon avalanche photodiode detectors (APDs) to receive the optical signals, respectively. Finally, we use FPGAs to process the signals with pre-burn programming.
After design the transmitter and receiver, we present the package of the prototype. We firstly carefully design the optical path and align the devices on a prefabricated optical plane to make the system compact and efficient. All devices above are placed into two 65cm×35cm×40cm customized acrylic boxes respectively, illustrated in Figs. 2(b) and 2(c). The boxes are sealing and waterproof, so they can conveniently work in the underwater environment. Since the prototype has low energy consumption (<20W) so the power system consisted of commercial charge batteries are packaged into the boxes. The prototype model can work up to 12 hours without any external supplements.
3. Results
Firstly, we observe the intensity distribution of the beams at the corresponding position. Figure 3(a) and Fig. 3(b) plot the intensity distribution of the Q-plate chip generated beams, which exhibit ‘donut’ shapes with a dark center due to the phase singularity. The multiplexing beam is shown in Fig. 3(c). Moreover, Figs. 3(d) and 3(e) exhibits the measured intensity profiles of the de-multiplexed beam at the receiver side after 6-meter underwater transmission which shows a bright spot in the beam center because the phase singularity is compensated by the de-multiplexer.
Fig. 3. (a, b) OAM+3 and OAM-3 intensity distribution generated by Q-plate. (c) multiplexing of OAM+3 and OAM-3 beam. (d, e) demodulation beam intensity distribution. Demod: demodulation. Measurement of polarization of transmitted light. (e) The power distribution of each polarization angle. (f) Measured the polarization after the light pass through a quarter-wave plate.
Secondly, we also study the polarization performance of the generated orthogonally CP OAM beam in the UWOC prototype system. We use a linear polarizer to detect the power of the transmission beam at each angle. As is plotted in Fig. 3(f), the power is almost the same at different angles for both channel 1 and channel 2. Moreover, we use a quarter-wave plate to convert the orthogonal CP light to LP light and measure the power again. Figure 3(g) depicts the received power when rotating the optical axis of the polarizer. The results indicate that the light pass through the liquid crystal Q-plate is two orthogonal CP beams.
Thirdly, we measure the crosstalk which is defined by the ratio of the lowest power of the multiplexing OAM channels to the highest power of other unwanted channels in the OAM spectrum in this OAM multiplexing UWOC prototype. Figure 4(a) illustrates the crosstalk performance of the underwater optical OAM multiplexing communication prototype system, which is estimated to be about −14 dB.
Fig. 4. (a) Measured crosstalk matrix of two channels. (b) Measured BER performance for UWOC employing OAM multiplexing after 6-meter underwater propagation.
Finally, we measure the BER of the OAM multiplexing UWOC prototype. The BER curves of channel 1 (l=+3) and 2 (l=-3) are plotted in Fig. 4(b), which is lower than the 1.5e-2 threshold. For comparison, the BER curves of free space back-to-back communication and the multiplexing UWOC without crosstalk are also depicted in Fig. 4(b).
4. Discussion
Here, we discuss some important points in this work. First and most important, performance degradation of OAM-multiplexing UWOC is the key factor. There are several sources of degradation for our prototype. The first one is the underwater environment such as the scattering, bubbles, and turbulences. These unideal environmental factors break the symmetry of the amplitude and phase profile of the OAM beams and results in signal degradation and the crosstalk between channels [26,27]. This kind of degradation is unpredictable and erratic [26,30]. Here, we choose “l=+3, -3” channels due to such crosstalk will decrease while the increase of the topological charge difference between channels
The second source is incomplete modulation of the geometric phase multiplexer and the de-multiplexer. In principle, the transmission light of the geometric phase contains both converted polarized components which get a phase modulation and the original polarized component. The remaining original polarized component will induce crosstalk. Fortunately, we can avoid it by designing the π phase difference of anisotropic unit axes [37,38].
The third source in this prototype is the varying polarization. When the light propagates through the free space and the water, the ever-changing environment will slightly change the polarization states of the receiving light. However, the geometric phase chip only recognizes the CP basis, so it will influence the de-multiplexing causing the crosstalk. We elaborate its quantification as follows:
The optical field of channel 1 $\overrightarrow {{\boldsymbol{E}_{Q1}}}$ can be expressed:
Similarly, the optical field of channel 2$\overrightarrow {{\boldsymbol{E}_{Q2}}}$:
So, the crosstalk in de-modulation and de-multiplexing $Cros{s_{de1 - 2}}$ and $Cros{s_{de1 - 2}}$ are
Hence, by using the real-time polarization compensation device, we can rectify the received polarization, and improve the overall system performance.
Finally, we discuss the capacity extensibility of our prototype. In this prototype, we employ the two OAM modes to do the multiplexing communication. While, it is well known that OAM has infinite topological charges, hence or prototype has the potential to realize more OAM channels multiplexing information transmission. However, two challenges need to address first to achieve more OAM channels. The first one is the high performance multiplexer and de-multiplexer. The conventional scheme which needs multiple SLMs and spiral phase plates is not suitable for the desired compact and energy-efficient device. A better method to multiplex/de-multiplex the several OAM modes can be using the OAM mode-sorter, Dammann grating and multi-tasking geometric phase elements [39–44]. The second challenge is that the signal degradation and the crosstalk will typically increase with the increase of the multiplexing channels. To solve this problem, on one hand, we can establish an adaptive system to feed back the environment changes, on the other hand, we can use the FPGAs based algorithms to compensate for the degradation.
5. Conclusion
In summary, we propose and experimentally demonstrate a prototype for UWOC system employing OAM multiplexing with high-speed, low-cost, compactness and convenience. To reduce the energy consumption and volume, we propose a single compact geometric phase Q-plate chip to generate, multiplex and de-multiplex the OAM mode with high efficiency and accuracy. Then, we apply the FPGAs to generate, preprocess and reprocess the communication signals with high speed, small volume, and low energy consumption. With the assistance of these compact and integration devices and the system package design, we package the complete system into two 65cm×35cm×40 cm boxes powered by two batteries. Using this compact system, we demonstrate a UWOC link using the OAM mode carrying 1.25Gbit/s on-off keying (OOK) signals (two channels each with 625Mbit/s). The BER of the multiplexing system is lower than 1.5e-2. Such a system is flexible to upgrade and ready to be used in various realistic underwater environments.
Funding
Science and Technology Innovation Commission of Shenzhen (JCYJ20200109114018750); National Key Research and Development Program of China (2020BAB001, 2019YFB2203604); Key R&D Program of Guangdong Province (2018B030325002); National Natural Science Foundation of China (11774116).
Acknowledgments
The authors gratefully acknowledge many helpful discussions with Dr. Xiaoping Cao, Yifan Zhao from Huazhong University of Science and Technology.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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