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Abstract
Underwater wireless communication plays an increasingly important role in more and more emerging ocean activities. Underwater wireless optical communication is a potential underwater communication technology with the advantages of high communication rate, large information capacity, and high bandwidth, but its ability to resist environmental turbulence is challenged. Different from free space optical communication, in underwater optical communication, turbulence not only directly affects the beam transmission, but also influences the transceiver, causing severe mechanical vibration of the transceivers, thus interfering with the optical communication link. At present, the research on mechanical vibration underwater has not received much attention. Hence, we propose and demonstrate fast auto-alignment underwater wireless optical communications employing orbital angular momentum (OAM) modes. The fast auto-alignment system is used to against the mechanical vibration. Two OAM modes multiplexing transmission link with 4-Gbit/s aggregate capacity is demonstrated in the experiment under four different vibration conditions assisted by the fast auto-alignment system with a response time of 244 Hz. After the fast auto-alignment system, the vibrations under for conditions are all greatly reduced to maintain a stable link. The demonstrations may open up new perspective to robust stable underwater wireless optical communications exploiting spatial modes in practical environment.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the last few years, terrestrial, space and underwater wireless optical communications have been widely explored for ever-increasing data capacity demands. [1,2] Compared with terrestrial and space links, underwater wireless optical links are relatively less explored due to their complexity. [3] Various physical processes in underwater environments make stable underwater wireless optical links hard to realize. [4] However, owing to great commercial, military and scientific value, the interest towards underwater wireless optical communication has increased. [5,6] The performance of underwater acoustic communication, which is the mainstream technology for underwater communications, is limited by the shortcomings of low bandwidth, high delay, low energy efficiency, time varying multipath propagation and Doppler spread. [7–9] Underwater wireless optical communication (UWOC) without these drawbacks is a better option, especially for high data rate transmission in short distances. Apropos of the distance limitation due to the severe water absorption at optical frequency, it is fortunate that there is a relatively low attenuation optical window of blue and green light, which is capable of proving high bandwidth underwater communication over adequate distance. [10–14]
However, underwater wireless optical communication will still encounter many challenges. Due to the influence of absorption, scattering and turbulence, the underwater wireless optical communication suffers from signal attenuation, delay spread and fading, resulting in limited communication distance and reduced reliability. [15,16] Underwater mechanical vibration, ocean undercurrent or other turbulent sources are inevitable, so maintaining a stable link in this environment is also a great challenge. In order to solve problems caused by absorption, scattering and turbulence, scientists have done a lot of research, and have made a lot of progress. [17] However, less attention has been paid to the signal loss caused by the vibration of the transceiver. Earlier related research and practical application dealt with this problem by increasing the divergence angle, but the problem of low security and low confidentiality has also been put forward recently. [18,19] Hence, the fast auto-alignment system is used to quickly resist beam vibration and maintain beam transmission stability. In this way, we can not only solve the problem of optical path stability, but also ensure the security and confidentiality of optical transmission path.
The space-division multiplexing (SDM) is a promising technique for increasing the capacity scalability in free space and optical fiber communications. [20] To improve the transmission capacity, the mode-division multiplexing (MDM) with orbital angular momentum (OAM) modes, one subset of SDM, has gained increasing interest. [21–26] Recently, OAM has been introduced into underwater wireless optical communications [27–31] with impressive performance. OAM mode set is an option for MDM because of the potentiality to accommodate infinite states with orthogonality. With different OAM modes carrying different data information, the OAM multiplexing technique can improve the transmission capacity of the communication system. [10,32] Adaptive turbulence compensation and fast auto-alignment technology have been used in free-space optical vortex beams communication to overcome the atmospheric turbulence. [33] For OAM-based underwater wireless optical communication, the underwater turbulence channel has been modelled and a unified statistical distribution of OAM propagation in UWOC systems has been researched. [34] Adaptive optics (AO) technique has also been proposed to mitigate the turbulence effects in OAM-based UWOC system. [35] In addition, the longitudinal orbital angular momentum multiplexing (LOAMM) system has been proved to have potential advantages for improving the security performance in UWOC. [36]
In this paper, we demonstrate a fast auto-alignment OAM-based 2-meter underwater wireless optical link. Two OAM modes with topological charges of +3 and -3 are multiplexed. The intermodal crosstalk after underwater propagation is less than -9 dB. Data-carrying OAM multiplexing with quadrature phase-shift keying (QPSK) signals is demonstrated under different vibration conditions. After the fast auto-alignment system, the vibrations under for conditions are all greatly reduced to maintain a stable link.
2. Concept and principle
The concept and principle of fast auto-alignment underwater wireless optical link employing OAM modes are illustrated in Fig. 1. The transmitter side sends optical signals to the receiver side. The MDM technology with OAM modes is employed to increase the transmission capacity. The fluctuation of light beam originating from machine vibration is a major element for the loss of signal in the underwater transmission link. To overcome the beam fluctuation and get a stable output, the fast auto-alignment system is built. The schematic diagram of the fast auto-alignment system is shown in Fig. 2(a). The system consists of two alignment stages. Each alignment stage consists of one quadrant detector, one position sensing detector (PSD) auto aligner, two piezo controllers, one beamsplitter, and one piezo mirror mount. The flow chart of the auto-alignment process is shown in Fig. 2(b). (a). The light is incident on the first tuning mirror mount. The tuning mirror directs light to a beamsplitter which divides the light into two parts. The reflected light is directed towards the quadrant detector. (b). The quadrant detector can accurately measure the displacement of an incident beam relative to the calibrated center. If a symmetrical beam is centered on the sensor, four equal photocurrents will be detected, resulting in null difference signals. The photocurrents will change if the beam moves off center, thereby giving rise to difference signals that are related to the beam displacement from the center of the sensor. The misalignment of the link caused the displacement of the beam point on the quadrant detector which outputs difference signals in horizontal and vertical direction. And the displacements are proportional to the intensity of signals. Quadrant detector is connected with the PSD auto aligner. (c). The PSD auto aligner can read the difference signals (X DIFF and Y DIFF) of the quadrant. The digital signal processor (DSP), located inside the PSD auto aligner, is a key component of the PSD auto aligner. The DSP deals with the difference signals and outputs the corresponding position demand signals which will be used as the inputs to the piezo drivers. (d). When piezo driver is operated together with the PSD auto aligner, high-precision closed-loop operation can be achieved due to the complete feedback. The piezo drivers receive the position demand signals and output corresponding drive voltages which drive the piezo mirror mount to center the beam on the detector. The detector is sensitive to the beam’s power density. By this way, the laser beam can be fixed on the center of the detector array. (e). The beamsplitter sends part of the beam to the quadrant position sensor. The quadrant position sensor can acquire the difference in quantity of the beam’s position relative to the detector center. The computer is used to control and monitor the whole feedback system. (f). The transmitted light from the beamsplitter would go through the second alignment stage which is the same as the first alignment stage. So light beams would be fixed on the center of quadrant detectors. Two fixed points determine a fixed line. When the two alignment response time, is an essential feature for a piezo controller in high-speed applications. The response time mainly results from the charging and discharging process of the piezo driver and can be calculated through the process. Figure 2(c) shows a large sample of beam’s displacements in the experiment. According to the flow chart, the beam’s displacements, the difference signals, the position demand signals and the drive voltages all are proportional to each other. Each point in Fig. 2(c) also represents two drive voltages. The auto-alignment system would center every non-central point. The response time of the system is also the time when the voltage value becomes zero. The movements of the beam are random, which means the average time of the voltage variations is needed. The average drive voltage can be calculated according to the scatter plot, which is about 10V. The bandwidth would be calculated by the given parameters. The piezo controller can achieve up to 7.5mA current. And the load capacitance of the pizeo is 1uF. The response time would be larger if the capacitance is higher. The signal voltage amplitude is determined to be 37.5V, which represents the length that the piezo extends. The function of a sinusoidal signal with peak amplitude A, average drive voltage Vpp, and frequency f is shown below.
Fig. 1. Concept and principle of fast auto-alignment underwater wireless optical link employing OAM modes.
Fig. 2. (a) Schematic diagram of the fast auto-alignment system. (b) Flow chart of auto-alignment process. (c) Scatter plot of beam’s displacements. (d) Diagram of drive voltage as a function of the time.
The Fig. 2(d) shows the diagram of the voltage fluctuation. The max response frequency at the steepest slew rate point is reached at t = 2npi (n = 0, 1, 2, …).
Thus
By calculation the response frequency would be about 244 Hz. Compared with the feedback system composed of SLM, the response speed of this system is two orders of magnitude faster. Hence the fast auto-alignment system can deal with the beam vibration more effectively. [30]
3. Experimental setup
Figure 3 shows the experiment setup of underwater wireless optical communication with fast auto-alignment system. The arbitrary waveform generator (AWG) generates two electrical 1-Gbaud signals with QPSK modulation format. Two electrical amplifiers (EAs) amplifiers the signals respectively. And two 520-nm single mode pigtailed laser diodes (LD) modulate the signals directly. The collimators (Col.) are able to couple the Gaussian mode signals from fiber to free space. Two phase-only spatial light modulators (SLMs) convert the Gaussian mode beams into OAM beams by loading complex phase masks. For efficient phase modulation, the polarization states of signals are aligned to the optimal working direction by using the two half-wave plates (HWP). A beam splitter (BS) combines the two spatially orthogonal OAM beams together. To select the desired output and remove other unwanted diffraction orders, a pinhole is used as a spatial filter. To reduce the beam size, two lenses work together as an inverse telescope system.
Fig. 3. Experimental setup of underwater wireless optical link employing OAM modes with fast auto-alignment system. (a) Quadrant detector (sensor). (b) PSD auto aligner. (c) Piezo driver. (d) Spatial light modulator. (e) Piezo mirror mount. (f) Water tank, (g) Auto-alignment system.
To generate the vibration, the mirror is connected with a motor. A 2-meter-long rectangular tank (40cm width x 40 cm height) filled with tap water is used to emulate the underwater condition.
We connect mirror with motor to simulate the vibration conditions. The experiments are processed under four different conditions: (a) vibration from the transmitter side; (b) vibration from receiver side; (c) vibration from both transmitter side and receiver side; (d) non-line-of-sight link, vibration from both transmitter side and receiver side. The OAM modes multiplexing light propagates through the 2-meter underwater condition. A neutral density filter (NDF) is placed before the fast auto-alignment system to adjust the received optical power.
The received light beams after the fast auto-alignment system are demodulated by the SLM on the received side with a switchable specific fork phase pattern for different OAM channels. After the demodulation, the center of the beam, the intensity profile of which is a bright spot, is sent to a high sensitivity silicon avalanche photodiode detector (APD). The signal detected by the APD are amplified by another electrical amplifier (EA). The amplified signal is received by the oscilloscope (OSC) for bit-error rate (BER) performance measurement. To record the intensity profile of the light beam, a camera assisted by a flip mirror (FM) is used.
4. Results and discussion
To study the multiplexing process of the system, the OAM mode states before and after the transmission are recorded. Two signals with ${{\boldsymbol{OAM}}_{ + 3}}$ and ${\boldsymbol{OA}}{{\boldsymbol M}_{ - 3}}$ modes are generated by the SLMs. The left part of Fig. 4 shows the transmitted side beam intensity profiles with corresponding patterns on the SLMs. At the transmitted side, two different modulated OAM modes with topological charges of +3 and -3 are perfect with doughnut profiles.
Fig. 4. Measured intensity profiles for ${\boldsymbol OA}{{\boldsymbol M}_{ + 3}}$ and ${\boldsymbol{OA}}{{\boldsymbol M}_{ - 3}}$ at the transmitter side (Tx), receiver side (Rx) and the side after demodulation by patterns.
After the 2-meter underwater propagation, the intensity of the received light beams becomes weaker due to the propagation loss. However, the intensity profiles still maintain complete with negligible deformation. Then the received OAM beams would be demodulated by the SLM at the received side. When the pattern on the received side SLM is opposite to the pattern on the transmitted side SLM, the OAM mode would be demodulated as Gaussian beam. The intensity profiles of the demodulated light beam have a Gaussian-like bright center spot. When the pattern on the received side SLM is the same as the pattern on the transmitted side SLM, the topological charge of the OAM beam is doubled up to +6/ -6, leading to a larger doughnut profile, as shown in Fig. 4.
To evaluate the performance of the fast auto-alignment system, we simulate the vibration of underwater working machine by using a mirror with a vibrating motor. When reflected by the oscillatory mirror, the affected light would propagate in a random orientation. 1000 pictures with a time interval of 1/30 second are taken at the Camera-1 to record the beam trajectory. We make scatter plots of the displacements of the spot which represent the vibration degree of the beam center. The larger the scatter distribution, the more serious the vibration degree is. Figures 5(a)–5(d) show the beam’s trajectory of 1000 point without the fast auto-alignment system under the four conditions: (1) vibration from the transmitter side; (2) vibration from the receiver side; (3) vibration from both transmitter side and receiver side; (4) non-line-of-sight link, vibration from both transmitter side and receiver side. Figures 5(e)–5(h) show the beam’s trajectory of 1000 point with the fast auto-alignment system under the four conditions. Under the condition (1) in Figs. 5(a) and 5(e), the maximum variation of scatter distribution shrinks from 0.7 mm to about 0.2 mm after the fast auto-alignment process. Under the condition (2) in Figs. 5(b) and 5(f), the maximum variation of scatter distribution shrinks from 0.7 mm to about 0.15 mm after the fast auto-alignment process. Under the condition (3) in Figs. 5(c) and 5(g), the maximum variation of scatter distribution shrinks from 0.6 mm to about 0.2 mm after the fast auto-alignment process. Under the condition (4) in Figs. 5(d) and 5(h), the maximum variation of scatter distribution shrinks from 0.7 mm to about 0.2 mm after the fast auto-alignment process. When the variation of scatter distribution is too large (0.6∼0.75mm), the demodulation would be Insufficient, leading a huge power loss. Therefore, it is necessary to compress the variation of scatter distribution below 0.2mm, which does not affect the accuracy of the demodulation. [37] One can clearly see that after the auto-alignment process, the distribution range of the beam is obviously reduced, which represents the excellent performance to stabilize the light path. These results also prove that the system is fast enough.
Fig. 5. Beam’s trajectory under different vibration condition. (a)-(d) Without fast auto-alignment. (e)-(h) With fast auto-alignment. (a)(e) Vibration from the transmitter side. (b)(f) Vibration from the receiver side. (c)(g) Vibration from both transmitter side and receiver side. (d)(h) Non-line-of-sight link, vibration from both transmitter side and receiver side.
The data-carrying performance of the 2-meter underwater OAM modes multiplexing communication is further measured. Each OAM mode carries a 1-Gbaud QPSK signal, thus the aggregate capacity is 4 Gbit/s. When the received powers are the same, the BER values of different OAM modes will be equal. For the power sensitivity of the APD, the attenuation of NDF is a good approach for the BER measurement. Figures 6(a)–6(d) plot the measured BER performance with and without fast auto-alignment under different vibration conditions: (a) Vibration from the transmitter side. (b) Vibration from the receiver side. (c) Vibration from both transmitter side and receiver side. (d) Non-line-of-sight link, vibration from both transmitter side and receiver side. Each plot contains 5 channels of BER performance, including the reference Gaussian beam, the $OA{M_{ + 3}}$ beam with/without auto-alignment system and the $OA{M_{ - 3}}$ beam with/without auto-alignment system. The forward error correction (FEC) threshold is $3.8 \times {10^{ - 3}}$. Without auto-alignment system, the variation of beam distribution is too large to ensure the coupling stability with the receiver. The BER cannot be measured because of the loss of information. With auto-alignment system, the reduced variation of beam distribution makes sure that the coupling with the receiver is uninterrupted. In this situation, the BER can be measured accurately. Under different vibration conditions: vibration from the transmitter side, vibration from the receiver side and vibration from both transmitter side and receiver side, the variation of scatter distribution after auto-alignment system are commensurate, resulting in the similar 2dB power penalty to reference curve at the forward error correction (FEC) threshold of $3.8 \times {10^{ - 3}}$. While in non-line-of-sight link, the power penalty increases to 4dB due to the excess loss of 2dB caused by the water surface reflection. The constellation diagrams shown in Fig. 6 corresponding to the first point of four curves. When the points of the constellation diagram are divided into four separate parts, this represents that communication quality is favorable when the receiving power is sufficient. When the points of the constellation diagram converge together, this means that the communication signals cannot be distinguish. From the obtained results, we can see that the optical link is completely disconnected without using the fast auto-alignment system. After using the fast auto-alignment system, the UWOC employing OAM mode multiplexing is re-established and vibration-resistant. With future improvement, more OAM modes and faster auto-alignment system could be considered to further facilitate the capacity scaling and enhance the robustness. The demonstrations pave a way to implement practical underwater wireless optical communications with increased transmission capacity by OAM multiplexing and with robustness against environmental disturbance by fast auto-alignment system.
Fig. 6. Measured BER performance with (w/) and without (w/o) fast auto-alignment under different vibration conditions. (a) Vibration from the transmitter side. (b) Vibration from the receiver side. (c) Vibration from both transmitter side and receiver side. (d) Non-line-of-sight link, vibration from both transmitter side and receiver side. The bottom shows the constellation diagrams. Left: w/ fast auto-alignment (re-established link). Right: w/o fast auto-alignment (disconnected link).
Although the demonstration is only a transmission of 2m, the function of auto-alignment system has been fully verified in the experiment with good performance obtained. The conclusions obtained from experiments still have great significance for the practical UWOC link of tens to hundreds of meters or even kilometers. In order to prevent the beam from widening seriously in optical transmission, the transmission distance is generally designed within the Rayleigh distance. The formula of Rayleigh distance ${Z_R}$ is as follows:
At the transmitting side, if the beam is widened to the waist radius of 13mm through the optical 4f expanding system, the Rayleigh distance of 520nm laser can reach 1km, which means that the transmission process of 1km can be regarded as collimated transmission. At the receiving side, the beam can be reduced to the waist radius of 3mm by the optical 4f reduction system to fit to the fast auto-alignment system. In this way, the vibration range is effectively further reduced. Therefore, in a practical UWOC link of tens to hundreds of meters or even kilometers, the auto-alignment system in the paper can still be applied with good performance.
5. Conclusions
In summary, we have proposed and demonstrated a fast auto-alignment underwater wireless optical system. From the obtained results, we can see that the optical link is completely disconnected without using the fast auto-alignment system. When using the fast auto-alignment system, the UWOC employing OAM mode multiplexing is re-established and vibration-resistant. With future improvement, more OAM modes and faster auto-alignment system could be considered to further facilitate the capacity scaling and enhance the robustness. The demonstrations pave a way to implement practical underwater wireless optical communications with increased transmission capacity by OAM multiplexing and with robustness against environmental disturbance by fast auto-alignment system.
Funding
National Natural Science Foundation of China (62125503); Key R&D Program of Guangdong Province (2018B030325002); Key R&D Program of Hubei Province of China (2020BAB001, 2021BAA024); Science and Technology Innovation Commission of Shenzhen Municipality (JCYJ20200109114018750).
Disclosures
The authors declare that they have no competing interests.
Data availability
The data that support the findings of this study are available from the corresponding author on request.
Ethics approval and consent to participate
Not applicable.
Consent for publication
Not applicable.
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