A B S TRACTA normalized expression for the frequency character of the reinforce direction of the phased array transducers is established and its character is discussed in much detail. Through this discussion, we find that good beam steering results can be obtained only if the frequency character of the effective acoustic direction, which makes the momentum matching condition fulfilled, has negative slope and positive curvature. For many cases, this can be obtained only by taking the fast mode as the incident light. Moreover, for the cases where the curvature of the frequency character of the effective acoustic direction nearly equals twice the absolute value of its slope, the beam steering results will be excellent. New designs about the phased array Te02 and lithium niobate devices have been carried out to illustrate these new ideas. These new designs of equals twice the absolute value of the slope. For the Te02 device discussed in Section 3, through solving Dixon equation, we find that the frequency character of the effective acoustic wave direction also has negative slope and positive curvature if the fast mode is taken as the incident light. So the beam steering result will be better than that of phased array isotropic device (its frequency character always has slope -1 and zero curvature). Moreover, for the lithium niobate (LN) device discussed in Section 4 , the magnitude of the curvature does nearly equal twice the absolute value of the slope for many specific k-vector configurations. In this case, the beam steering result will be excellent. Thus, these new designs of phased array birefringent devices best solve the contradictory requirements imposed by a high diffraction efficiency and a large Bragg bandwidth. THEORETICAL ANALYSISphased array birefringent devices best solve the conAs is well-known, the reinforce direction protradictory requirements imposed by a high diffracduced by the interference of multiple acoustic tion efficiency and a large Bragg bandwidth. beams is determined by the angle 7 s a t i~f y i n g [~~~] (1) 1. INTRODUCTION 7 s i n 7 = PA -= @! . L 2s 2s f' T h e birefringent acousto-optic (AO) devices have the unique advantage of high dynamic range. Usually, the tangential condition is used to increase the bandwidth while still keeping high diffraction efficiency11t2]. However, the working frequency range of the tangential condition devices mi!st be around the tangential frequency and cannot be chosen arbitrarily as in isotropic devices. Recently, the design and use of phased array birefringent A 0 devantages of such devices are far from well exploration. In this paper, a normalized expression for the frequency character of the reinforce direction of the phased array transducers is established. It has been found that the slope of this frequency character is always negative and its curvature is always pA2 1 a positive. Moreover, the magnitude of the curvature where 7 is the angle between the reinforce direction and the normal direction of the transducer array plane. Usually, a fixed ac...

We experimentally demonstrate a high-speed air-water optical wireless communication system with both downlink and uplink transmission employing 32-quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM) and a single-mode pigtailed green-light laser diode (LD). This work is an important step towards the future study on optical wireless communications between underwater platforms and airborne terminals. Over a 5-m air channel and a 21-m water channel, we achieve a 5.3-Gbps transmission without power loading (PL) and a 5.5-Gbps transmission with PL in the downlink. The corresponding bit error rates (BERs) are 2.64×10 and 2.47×10, respectively, which are below the forward error correction (FEC) criterion. A data rate of 5.5 Gbps with PL at a BER of 2.92×10 is also achieved in the uplink.

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We first study the transmission property of red light in water in terms of extinction coefficient and channel bandwidth via Monte Carlo simulation, with an interesting finding that red light outperforms blue-green light in highly turbid water. We further propose and experimentally demonstrate a broadband underwater wireless optical communication system based on a simple and cost-effective TO56 red-light laser diode. We demonstrate a 1.324-Gb/s transmission at a bit error rate (BER) of 2.02 × 10^-3 over a 6-m underwater channel, by using 128-QAM OFDM signals and a low-cost 150-MHz positive-intrinsic-negative photodetector, with a record spectral efficiency higher than 7.32 bits/Hz. By using an avalanche photodetector and 32-QAM OFDM signals, we have achieved a record bit rate of 4.883 Gb/s at a BER of 3.20 × 10^-3 over a 6-m underwater channel.

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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, 2.0 × 10 and 2.3 × 10, respectively, which are below the forward error correction (FEC) threshold of 3.8 × 10.

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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.