Optoacoustic communication enables an airborne unit to directly reach nodes deep underwater. To achieve high data rates in optoacoustic communications, implementing a multilevel modulation scheme is necessary where distinct acoustic signals can convey multiple symbols. However, demodulating these signals proves challenging amidst the complexities of underwater environments characterized by multipath propagation and resultant inter-symbol interferences. To overcome these challenges, this paper presents a novel demodulation scheme using a Spiking Neural Network (SNN). Our SNN model has undergone training using a laboratory-constructed dataset, comprising eight levels of optoacoustic signals recorded from three different underwater positions. Validation is conducted with a dataset deliberately designed to include severe interference from multipath-generated echoes and reverberations. The results indicate that our SNN-based demodulation scheme achieves an impressive accuracy of 90.16%, surpassing the 65.30% accuracy obtained through conventional peak detection-based techniques.

In many applications of underwater networks, surface nodes cannot be employed for security and logistical reasons. For these scenarios an airborne unit ought to reach the underwater nodes directly from the air in order to provide commands to and configure the network. In particular, the airborne unit should facilitate localizing the underwater nodes and enable the establishment of a global coordinate system. Visible Light Communication (VLC) stands out as a prime choice for communication across the air-water interface. Using VLC, an underwater node uses the light intensity to infer proximity of the beam incident point. When receiving a sufficient number of VLC transmissions, prior work could localize the underwater nodes. However, such an approach requires very fine-grained area coverage, especially in shallow water environments, which is not practical in many application scenarios. This paper tackles such a limitation by introducing a novel Hybrid Airborne-Enabled Underwater Localization (HAUL) method. HAUL leverages knowledge of the underwater network topology in the localization process. Specifically, both acoustic ranging and light intensity measurements are used to estimate the node's global coordinates. HAUL is validated through extensive MATLAB simulations, demonstrating its effectiveness in calculating accurate positions.

The optoacoustic process can solve the longstanding challenge of wireless information transmission from an airborne unit to an underwater node (UWN). The nonlinear optoacoustic signal generated by proper laser parameters can propagate long distances in water. However, forming such a signal requires a high-power laser, and the buildup of a vapor cloud precludes the subsequent acoustic signal generation. Therefore, pursuing the traditional on-off keying (OOK) modulation technique will limit the data rate and power efficiency. In this paper, we analyze different modulation techniques and propose a vapor cloud delayed-differential pulse position modulation (VCD-DPPM) technique to improve the data rate and achieve high power efficiency for a single stationary laser transmitter. The symbol rate of VCD-DPPM is approximately 6.9 times and 1.69 times higher than OOK in our text communication simulation using a laser repetition rate of 10 kHz and 40 Hz, respectively. Furthermore, VCD-DPPM is 137% more power efficient than the OOK technique for both cases. We have generated different acoustic signal levels in laboratory conditions and simulated the bit error rate (BER) for different depths and positions of the UWN, while considering ambient underwater noises. Our results indicate that VCD-DPPM enables efficient data transmission.

Localization of underwater networks is important in many military and civil applications. Because GPS receivers do not work below the water surface, traditional localization methods form a relative topology of underwater nodes (UWNs) and utilize either anchor nodes or floating gateways with dual transceivers in order to determine global coordinates. However, these methods introduce logistical complications and security risks in deploying the anchor and/or surface gateways. This paper tackles such an issue by proposing new localization techniques which can remotely localize UWNs using optoacoustic signals. In our approach, GPS coordinates are transmitted from air to the UWN via creating an underwater temporary isotropic acoustic transmitter with the optoacoustic process. We analyze the process of controlling the shape and size of the plasma to create the isotropic acoustic transmitter and experimentally validate the generation of isotropic acoustic signals. Then two methods of localization are proposed for static and dynamic UWNs. Finally, the simulation results with experimental values show the effectiveness of our approach. Comparing to the traditional techniques, our approach achieves the same accuracy without using any surface or underwater anchor nodes.

In underwater wireless networks, optoacoustic energy conversion using high energy laser pulse is the only known viable option for communication from an airborne unit to a node at large depth, e.g., a submarine or an unmanned underwater vehicle. However, controlling the generated acoustic signal through this process is very complex. Specifically, if the repetition rate of laser pulses is low, the corresponding acoustic signal is very broadband. The higher frequency components of this broadband signal attenuate more if the underwater node is very far from the surface. Hence, a relatively narrowband signal with lower frequency components is desirable for long distance communication. The frequency component of the broadband acoustic signal depends on the incident angle of the laser light and observation angle of the receiver, i.e., the position of the underwater hydrophone. Both of these angles also change continuously for a wavy water surface, which makes it more complex to determine the frequency components of this kind of signal. In this paper, we show that by carefully choosing the relative position of the airborne unit and underwater node, we can generate a narrowband acoustic signal with lower frequency components for both flat and wavy water surfaces. We provide theoretical analysis and simulation results to capture the effect of these angles on the generated acoustic signals. We further provide guidelines for optimum angle setting for improving the quality of the optoacoustic communication link.

We report an adaptive capacitive transimpedance amplifier circuit for room temperature sensing of short wave infrared (SWIR) radiation. The photocircuit reduces junction leakage current by measuring the current across a photodiode held at zero bias. This is critical to enable room temperature SWIR detection using materials with smaller bandgaps (and higher leakage current) such as InGaAs, which are typically operated at cooled temperatures. The transimpedance amplifier incorporates a floating gate current mirror in order to precisely cancel offset using nonvolatile analog storage. We experimentally verify that we are able to precisely tune the input offset and demonstrate a reduction in dark current of an InGaAs photodiode by two orders of magnitude, from 700pA to 2.25pA, when comparing similar adaptive and non-adaptive circuits. These circuits have been fabricated in a standard 0.6μm CMOS process.