1. Introduction

In recent years, there is a growing interest in monitoring the ocean environment for scientific, commercial, and military applications [1–3], and wireless communications between various underwater vehicles and sensors are essential. The traditional approach for underwater wireless communications is dominated by acoustic modulation schemes due to the low attenuation, enabling long-range underwater communications with a reach up to tens of kilometers [4]. However, underwater acoustic communications also suffer major drawbacks of relatively low transmission capacity and spectral efficiency because of the very limited bandwidth, which make them less preferable in short-range applications [5–7]. On the other hand, radio-frequency (RF) signaling can also hardly serve as a viable underwater wireless communication technique because of the strong attenuation.

Fortunately, as an alternative approach, underwater wireless optical communication (UWOC) provides a trade-off between acoustic and RF communication schemes. It is proved that light with wavelength in the blue-green window has a relatively low attenuation and provides more sufficient bandwidth, lower time latency and higher security under the water [8]. Directly modulated light-emitting diode (LED) is a low-cost and easy-to-operate optical source. An impressive demonstration of 1-Gbit/s underwater data transmission at 532 nm was reported by F. Hanson et al. [9]. Orthogonal frequency-division multiplexing (OFDM) based broadband UWOC employing different advanced modulation formats (161.3 Mbps with 16-QAM, 156.31 Mbps with 32-QAM, and 127.07 Mbps with 64-QAM, QAM: quadrature amplitude modulation) and a 450-nm LED was demonstrated by J. Xu et al. [10]. Besides, laser diode (LD) is also employed as another optical source featuring highly collimated beam, narrow linewidth and high mode purity. 2.3-Gbit/s over 7-m tap water UWOC was demonstrated using a 520-nm LD [11]. Moreover, 4.88-Gbit/s data transmission utilizing 32-QAM-OFDM over 6-m underwater channel was also demonstrated [12].

Additionally, in order to further improve the transmission capacity, the space domain of lightwaves has gained increasing interest [13]. Different spatial modes including orbital angular momentum (OAM) modes have been extensively studied both in fiber-based and free-space optical communications [13–20]. Typically, OAM communications include OAM modulation, OAM multiplexing and OAM multicasting [20]. Very recently, OAM multiplexing technique has been introduced to UWOC. J. Baghdady et al. reported 3-Gbit/s UWOC employing 2 OAM modes multiplexing [21]. Y. Ren et al. further increased the UWOC transmission capacity to 4 Gbit/s (directly modulated LD) and 40 Gbit/s (external modulation & frequency doubling) by multiplexing 4 green OAM modes [22]. Beyond OAM multiplexing optical communications, C. Shi et al. also reported an interesting work by introducing OAM into acoustic communications [23]. It provides a potential solution to increase the transmission capability for long-range underwater acoustic communications. Remarkably, in an OAM multiplexing system, different OAM channels usually carry different data information and the aggregate transmission capacity is increased. Actually, there are also some situations in today’s networks that require the multi-copy replication of a seed optical signal, which is a kind of one-to-many communication known as optical multicasting. OAM-based optical multicasting has been reported in free-space optical communications [24–29]. In underwater communications, multicasting is also widely desired such as the application in delivering the data information between submarine formations or other multiple underwater vehicles. For instance, the center station can send the same instruction to multiple unmanned vehicles and sensors for ocean environment monitoring via multicasting. In this scenario, a laudable goal would be to implement underwater optical multicasting, which however, has not yet been demonstrated so far to the best of our knowledge.

In this paper, we propose and demonstrate an OAM-based underwater wireless optical multicasting link. 2-meter underwater 4-fold OAM multicasting link with different topological charges of −6, −3, + 3 and + 6 is demonstrated. The crosstalk between multicasting OAM channels and other unwanted channels are less than −6 dB after underwater propagation. Data-carrying OAM multicasting with 8-QAM-OFDM, 16-QAM-OFDM and 32-QAM-OFDM signals are demonstrated in the experiments.

2. Concept and principle

The concept and principle of OAM-based underwater wireless optical multicasting is illustrated in Fig. 1. In the scenario of underwater communications, sometimes there is a need to send multi-copy replication of signal to multiple users (e.g. multiple submarines). Considering that OAM modes, having helically phased structures, can take multiple orthogonal values, one can assign different OAM modes as unique signatures to different users. As a result, N-fold OAM multicasting enables the signal distribution to N users. As shown in Fig. 1, the seed signal carried by a Gaussian mode ($\mathcal{l}$ = 0) is multi-copy replicated to multiple OAM modes ($k, m, n$≠0) through a complex phase mask which shapes the spatial structure of the Gaussian mode. Different multicasting OAM modes are respectively assigned to different users, enabling an OAM-based underwater wireless optical multicasting link.

 

Fig. 1 Concept and principle of an orbital angular momentum (OAM)-based underwater wireless optical multicasting link.

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3. Experimental setup

Figure 2 shows the experimental setup of an OAM-based underwater wireless optical multicasting link. An electrical 1.5-Gbaud signal is generated by an arbitrary waveform generator (AWG) and m-QAM-OFDM advanced modulation format is adopted. The signal is amplified by an electrical amplifier (EA) and then applied to a 520-nm single mode pigtailed laser diode (LD) for direct modulation. The LD should stably operate at the linear region through appropriate adjustment of the current and temperature by the controller. The pigtailed fiber of LD is connected to a collimator (Col.) so that a fundamental Gaussian mode carrying advanced modulation signal is coupled out from fiber to free space. The output green light is launched onto a phase-only spatial light modulator (SLM-1) loaded with a complex phase mask (forked multicasting phase pattern). Considering that SLM is a polarization-sensitive device, a half-wave plate (HWP) and a polarizer (Pol.) are used to adjust the polarization state to be aligned to the optimal working direction of SLM for efficient phase modulation. A pinhole is used as a spatial filter to select the desired output and remove other unwanted diffraction orders. In the experiment, we emulate the underwater condition by using a 2-meter-long rectangular tank (40 cm width x 40 cm height) filled with tap water. At the receiver side, two lenses work together as an inverse telescope to reduce the beam size. A neutral density filter (NDF) is used to adjust the received optical power. After 2-meter underwater wireless optical multicasting link propagation, the received multicasting light beam is demodulated by another spatial light modulator (SLM-2) loaded with a switchable particular forked phase pattern for different multicasting OAM channels. The demodulated beam, having a bright spot at the beam center of the intensity profile, is sent to a high-sensitivity silicon avalanche photodiode detector (APD). The signal after detection is amplified by another EA and then sent to an oscilloscope (OSC) for bit-error rate (BER) performance measurement. A camera is used to record the intensity profile assisted by a flip mirror (FM).

 

Fig. 2 Experimental setup of an OAM-based underwater wireless optical multicasting link. AWG: arbitrary waveform generator; EA: electrical amplifier; Col.: collimator; HWP: half-wave plate; Pol.: polarizer; SLM: spatial light modulator; NDF: neutral density filter; FM: flip mirror; APD: avalanche photo detector; OSC: oscilloscope.

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4. Experimental results

The complex phase mask (forked multicasting phase pattern) used for OAM multicasting consists of an OAM multicasting phase pattern and a linear phase ramp pattern, as shown in Fig. 3(a). The OAM multicasting phase pattern is designed based on the pattern search assisted iterative (PSI) algorithm [28] to simultaneously generate multiple OAM modes. The linear phase ramp pattern is used to obtain clean multicasting OAM modes. The designed complex phase mask enables 4-fold multicasting OAM modes (OAM_-6, OAM_-3, OAM₊₃, OAM₊₆) at the first-order diffraction. Figure 3(b) shows the measured intensity profiles of generated 4-fold multicasting OAM modes at the transmitter (Tx) side and the receiver (Rx) side after 2-meter underwater transmission. It is shown that the 4-fold multicasting OAM modes have slight distortion after 2-meter underwater propagation (~5 dB power attenuation). Also, one can clearly see the null intensity at the beam center of the multicasting OAM modes due to the phase singularity property of OAM modes.

 

Fig. 3 (a) Design of forked multicasting phase pattern loaded onto SLM-1 for generating 4-fold multicasting OAM modes (OAM_-6, OAM_-3, OAM₊₃, OAM₊₆). (b) Measured intensity profiles of generated 4-fold multicasting OAM modes at the transmitter side (Tx) and the receiver side (Rx).

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We also study the OAM spectrum performance after 2-meter underwater propagation in the experiment. For the 4-fold OAM multicasting (OAM_-6, OAM_-3, OAM₊₃, OAM₊₆), Fig. 4 displays simulated and measured demodulation intensity distributions of different OAM channels (OAM spectrum) using different order forked phase pattern loaded onto SLM-2. Insets show measured demodulation intensity profiles of typical multicasting and unwanted channels. It can be seen that for multicasting OAM_-6, OAM_-3, OAM₊₃ and OAM₊₆ channels, the demodulated intensity profiles appear a bright spot at the beam center, while for other unwanted channels (e.g. OAM_-9, OAM₀, OAM₊₈), the beam center remains dark. At the transmitter, the crosstalk, defined by the ratio of the lowest power of the multicasting OAM channels to the highest power of other unwanted channels in the OAM spectrum, is estimated to be about −13 dB. After 2-meter underwater propagation, the OAM spectrum suffers some degradation and the crosstalk is measured to be about −6 dB. Such phenomenon might be ascribed to the scattering and turbulence during underwater propagation. Remarkably, for OAM multiplexing communications, −6 dB crosstalk is relatively large which will significantly degrade the transmission performance in an OAM multiplexing system with each OAM carrying an independent channel data information. The multiplexing with large crosstalk will mess up the data information. In contrast, for OAM multicasting communications, i.e. one-to-many communications, multiple OAM modes share the same data information which is delivered to different users with different OAM signatures. As a result, the OAM multicasting still works well even with −6 dB crosstalk. However, low-level crosstalk is always expected. To further reduce the crosstalk, adaptive optics method can be applied to compensate the spatial structure (e.g. wavefront) distortion of lightwaves during underwater propagation, which can benefit the crosstalk mitigation of OAM multicasting.

 

Fig. 4 Normalized power distribution of different OAM channels (OAM spectrum) for 4-fold OAM multicasting (OAM_-6, OAM_-3, OAM₊₃ and OAM₊₆). Insets show measured demodulation intensity profiles of typical multicasting and unwanted channels. Sim.: simulated OAM spectrum at the transmitter. Exp.: measured OAM spectrum at the receiver after underwater propagation.

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We further demonstrate the data-carrying 4-fold OAM multicasting link under 2-meter underwater propagation condition. 8-QAM-OFDM advanced modulation signal is employed in the experiment. At the transmitter, a random binary sequence is converted into parallel binary data by serial-to-parallel (S/P) conversion. After being mapped into 8-QAM symbols, these binary data are finally assigned to different subcarriers. The 8-QAM symbols are then assigned to 100 OFDM subcarriers, employing inverse fast Fourier transform (IFFT) with the size of 256, to generate baseband OFDM signals. 20 training symbols (TS) are used to train the adaptive frequency domain equalizer for every 200 symbols and another 20 cyclic prefixes (CP) are also added to each OFDM symbol. Finally, the parallel OFDM signals are converted into serial signals by parallel-to-serial (P/S) conversion, before being loaded into an AWG. The digital to analog converter (DAC) speed (or sampling rate) is 1.5 GS/s (or 1.5 Gbaud signal). For BER measurements, since the APD is only sensitive to optical power, for different modes the BER values will be equal when receiving the same power. Hence, we adopt the attenuation of NDF as the horizontal axis of the BER curve. Figure 5 displays the measured BER performance for the data-carrying OAM multicasting link after 2-meter underwater propagation. Here, we take the Gaussian mode as the reference channel. For different multicasting OAM channels (OAM_-6, OAM_-3, OAM₊₃, OAM₊₆), the BER curves are almost the same, having ~2 dB penalty compared to the reference channel at the forward error correction (FEC) threshold (BER = 1x10⁻³). The measured typical constellation diagrams above, at and below the FEC threshold are also depicted in Fig. 5.

 

Fig. 5 Measured BER performance for data-carrying (8-QAM-OFDM) 4-fold OAM multicasting (OAM_-6, OAM_-3, OAM₊₃ and OAM₊₆) after 2-meter underwater propagation. Insets show typical constellation diagrams of 8-QAM-OFDM. Mul.: multicasting. Ref.: reference.

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For comparison, we also demonstrate the data-carrying 4-fold OAM-based multicasting link over 2-meter free space. As shown in Fig. 6, intensity profiles and BER performance are measured for OAM₊₆ channel as one typical example. The multicasting OAM modes at the transmitter and receiver have null intensity at the beam center, while the demodulated OAM_-6 channel has bright spot at the beam center. 8-QAM-OFDM, 16-QAM-OFDM and 32-QAM-OFDM advanced modulation signals are employed in the experiment. Despite increased penalty of 16-QAM-OFDM and 32-QAM-OFDM, it holds the potential to further implement data-carrying OAM-based underwater wireless optical multicasting link exploiting higher-order advanced modulation signals.

 

Fig. 6 Measured intensity profiles and BER performance for data-carrying 4-fold OAM-based multicasting link over 2-meter free space. Insets show typical constellation diagrams of 8-QAM-OFDM, 16-QAM-OFDM and 32-QAM-OFDM.

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5. Discussion

In general, there are two kinds of multicasting approaches. One is the traditional spatial path multicasting with multiple users located along different directions. This can be realized by use of a power splitter. The other is like OAM multicasting with multiple users along the same propagation direction. The former uses different spatial directions for multicasting user allocation, while the latter employs different OAM modes for multicasting user allocation. In the experiment, we generate multiple OAM modes using a complex phase mask, and the multicasted OAM modes are spatially overlapping. In order to also incorporate the spatial direction allocation ability in OAM multicasting, one can simply use an OAM mode sorter based on transform optics to separate them along different directions with high efficiency [30–32]. In addition, one can also use a spatially designed complex phase mask to enable spatial direction steering of different multicasted OAM modes [33]. Remarkably, OAM multicasting provides added flexibility compared to traditional spatial path multicasting. OAM multicasting can realize the multicasting either at the transmitter via a complex phase mask with beam steering function [33] or at the receiver after transmission via an OAM mode sorter [30–32]. OAM multicasting with collinear transmission of multiple OAM modes functions as a bus line. In branch lines that share the same data information from the bus line, different users are linked to different OAM modes, i.e. using OAM as unique signature for each user.

For underwater wireless communications, OAM can be both introduced into acoustic communications and optical communications. OAM-based acoustic communications provide a promising tool to increase the transmission capability for long-range underwater wireless communications [23]. Compared to acoustic communications, optical communications have shorter transmission distance due to large underwater propagation loss. However, optical communications provide a distinct advantage of large transmission capacity (~Gbit/s) as lightwaves have much larger bandwidth than acoustic waves. That is, both acoustic communication and optical communication have their pros and cons. For long-range propagation with less attention to the transmission capacity, acoustic communication is the best choice. In contract, for high-speed (~Gbit/s) short-range propagation, optical communication is preferred.

As lightwaves propagate in water, absorption, scattering and turbulence cause loss and distortion [34]. So far underwater optical communication up to 30 meters has been demonstrated [35]. It is believed that the transmission distance may reach up to hundreds of meters in the clean water. For underwater optical OAM multicasting link, tens of meters transmission distance would be possible.

6. Conclusion

In summary, we present an underwater wireless optical multicasting link by accessing the space domain of lightwaves and exploiting OAM modes with helically phased structures. Using direct modulation of 1.5-Gbaud 8-QAM-OFDM signal, green light (520 nm) 4-fold OAM multicasting (OAM_-6, OAM_-3, OAM₊₃, OAM₊₆) underwater transmission over 2-meter is demonstrated in the experiment. The crosstalk between multicasting OAM channels and other unwanted channels is measured to be less than −6 dB after 2-meter underwater propagation. The BER performance for 8-QAM-OFDM-carrying OAM-based underwater wireless optical multicasting link is evaluated with ~2 dB penalty at a BER of 1x10⁻³. In addition, we also demonstrate the data-carrying OAM multicasting link over 2-meter free space for comparison by adopting higher-order advanced modulation signals (16-QAM-OFDM, 32-QAM-OFDM). The demonstrated underwater OAM multicasting link can be further extended to more OAM channels and applied to one-to-many underwater communication applications.