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An Efficient Pulse Position Modulation Scheme to Improve the Bit Rate of Photoacoustic CommunicationWireless communication from air-to-underwater is quite challenging because of the lack of proper physical signal that propagates well in both air and water medium. Photoacoustic energy transfer mechanism is the most promising method for such cross-medium communication, where a high energy pulsed light is focused on the water surface, causing the generation of an acoustic signal inside the water. Since acoustic signals can travel a long distance inside the water, this method enables an airborne unit to reach nodes at increased underwater depth. Yet the achievable bit rate for this process is very low. When a pulsed laser light with a higher repetition rate is focused inside the water, a vapor cloud is generated around the focus point, which blocks subsequent generation of acoustic signal and consequently limits the achievable bit rate. This paper opts to overcome such a limitation by proposing a novel pulse position modulation technique which can avoid such generation of vapor cloud and increases the bit rate significantly.more » « less
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We design a three-way silicon optical waveguide with the Bloch dispersion relation supporting a stationary inflection point (SIP). The SIP is a third order exceptional point of degeneracy (EPD) where three Bloch modes coalesce forming the frozen mode with greatly enhanced amplitude. The proposed design consists of a coupled resonators optical waveguide (CROW) coupled to a parallel straight waveguide. At any given frequency, this structure supports three pairs of reciprocal Bloch eigenmodes, propagating and/or evanescent. In addition to full-wave simulations, we also employ a so-called “hybrid model” that uses transfer matrices obtained from full-wave simulations of sub-blocks of the unit cell. This allows us to account for radiation losses and enables a design procedure based on minimizing the eigenmodes’ coalescence parameter. The proposed finite-length CROW displays almost unitary transfer function at the SIP resonance, implying a nearly perfect conversion of the input light into the frozen mode. The group delay and the effective quality factor at the SIP resonance show an $N^3$ scaling, where N is the number of unit cells in the cavity. The frozen mode in the CROW can be utilized in various applications like sensors, lasers and optical delay lines.more » « less
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The current era is notably characterized by the major advances in communication technologies. The increased connectivity has been transformative in terrestrial, space, and undersea applications. Nonetheless, the water medium imposes unique constraints on the signals that can be pursued for establishing wireless links. While numerous studies have been dedicated to tackling the challenges for underwater communication, little attention has been paid to effectively interfacing the underwater networks to remote entities. Particularly it has been conventionally assumed that a surface node will be deployed to act as a relay using acoustic links for underwater nodes and radio links for air-based communication. Yet, such an assumption could be, in fact, a hindrance in practice. The paper discusses alternative means by allowing communication across the air–water interface. Specifically, the optoacoustic effect, also referred to as photoacoustic effect, is being exploited as a means for achieving connectivity between underwater and airborne nodes. The paper provides background, discusses technical challenges, and summarizes progress. Open research problems are also highlighted.more » « less
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An Adaptive DPPM for Efficient and Robust Visible Light Communication Across the Air-Water InterfaceThe scarcity of the optical power is the main challenge for underwater visible light communication. It becomes worst for communication across the air-water interface because of the reflection of light from the air-water interface. Differential pulse position modulation (DPPM) is one of the power efficient modulation techniques. In L-DPPM a block of M = log 2 L input data is mapped into one of the L distinct waveforms containing only one 'on' chip. The size of the DPPM packet is variable and depends on the value of input data and L, which makes error detection quite challenging. In this paper, we propose a frame structure that efficiently enables error detection within a packet for various symbol length, L, of DPPM. We also propose an algorithm using such a frame structure to enable effective detection of packet errors and for adaptively changing the value of L for optimal power efficiency while meeting a certain bound on the packet error rate (PER). We have named our proposed protocol as adaptive differential pulse position modulation (ADPPM). The Bit rate and PER have been studied for different signal-to-noise ratio (SNR) through simulation. A comparison between ADPPM and OOK, DPPM with fixed L is provided.more » « less