A maritime environment presents a complex problem for propagating a laser through its turbulent nature. The environment includes standard aerosols, sea surface spray, and atmospheric turbulence when propagating along horizontal, vertical, or slant angled paths. Atmospheric measurements of the turbulence structure (Cn 2) and Fried’s coherence length (r0) were taken in a maritime environment using an IR Laser beacon at 1.06 microns mounted on an unmanned aerial system (UAS) at varying distances and slant angles using a Differential Image Motion Monitor (DIMM) and Wide-Angle Teleradiometric Transmissometer (WATT). The DIMM measured the coherence length, r0 and atmospheric turbulence strength, Cn 2 from the IR Laser beacon source as it propagated through the atmosphere. The WATT measured the transmissivity of the IR Laser beacon through the atmosphere.
This paper describes a new concept of mitigating signal distortions caused by random air-water interface using an adaptive optics (AO) system. This is the first time the concept of using an AO for mitigating the effects of distortions caused mainly by a random air-water interface is presented. We have demonstrated the feasibility of correcting the distortions using AO in a laboratory water tank for investigating the propagation effects of a laser beam through an airwater interface. The AO system consisting of a fast steering mirror, deformable mirror, and a Shack-Hartmann Wavefront Sensor for mitigating surface water distortions has a unique way of stabilizing and aiming a laser onto an object underneath the water. Essentially the AO system mathematically takes the complex conjugate of the random phase caused by air-water interface allowing the laser beam to penetrate through the water by cancelling with the complex conjugates. The results show the improvement of a number of metrics including Strehl ratio, a measure of the quality of optical image formation for diffraction limited optical system. These are the first results demonstrating the feasibility of developing a new sensor system such as Laser Doppler Vibrometer (LDV) utilizing AO for mitigating surface water distortions.
A new technique has been developed for improving the Signal-to-Noise Ratio (SNR) of underwater acoustic signals measured above the water’s surface. This technique uses a Laser Doppler Vibrometer (LDV) and an Adaptive Optics (AO) system (consisting of a fast steering mirror, deformable mirror, and Shack-Hartmann Wavefront Sensor) for mitigating the effect of surface water distortions encountered while remotely recording underwater acoustic signals. The LDV is used to perform non-contact vibration measurements of a surface via a two beam laser interferometer. We have demonstrated the feasibility of this technique to overcome water distortions artificially generated on the surface of the water in a laboratory tank. In this setup, the LDV beam penetrates the surface of the water and travels down to be reflected off a submerged acoustic transducer. The reflected or returned beam is then recorded by the LDV as a vibration wave measurement. The LDV extracts the acoustic wave information while the AO mitigates the water surface distortions, increasing the overall SNR. The AO system records the Strehl ratio, which is a measure of the quality of optical image formation. In a perfect optical system the Strehl ratio is unity, however realistic systems with imperfections have Strehl ratios below one. The operation of the AO control system in open-loop and closed-loop configurations demonstrates the utility of the AO-based LDV for many applications.
There is much interest in detecting a target and optical communications from an airborne platform to a platform submerged under water. Accurate detection and communications between underwater and aerial platforms would increase the capabilities of surface, subsurface, and air, manned and unmanned vehicles engaged in oversea and undersea activities. The technique introduced in this paper involves a Laser Doppler Vibrometer (LDV) for acousto-optic sensing for detecting acoustic information propagated towards the water surface from a submerged platform inside a 12 gallon water tank. The LDV probes and penetrates the water surface from an aerial platform to detect air-water surface interface vibrations caused by an amplifier to a speaker generating a signal generated from underneath the water surface (varied water depth from 1” to 8”), ranging between 50Hz to 5kHz. As a comparison tool, a hydrophone was used simultaneously inside the water tank for recording the acoustic signature of the signal generated between 50Hz to 5kHz. For a signal generated by a submerged platform, the LDV can detect the signal. The LDV detects the signal via surface perturbations caused by the impinging acoustic pressure field; proving a technique of transmitting/sending information/messages from a submerged platform acoustically to the surface of the water and optically receiving the information/message using the LDV, via the Doppler Effect, allowing the LDV to become a high sensitivity optical-acoustic device. The technique developed has much potential usage in commercial oceanography applications. The present work is focused on the reception of acoustic information from an object located underwater.
New results for characterizing laser intensity fluctuation statistics of a laser beam transmitted through a random air-water interface relevant to underwater communications are presented. A laboratory watertank experiment is described to investigate the beam wandering effects of the transmitted beam. Preliminary results from the experiment provide information about histograms of the probability density functions of intensity fluctuations for different wind speeds measured by a CMOS camera for the transmitted beam. Angular displacements of the centroids of the fluctuating laser beam generates the beam wander effects. This research develops a probabilistic model for optical propagation at the random air-water interface for a transmission case under different wind speed conditions. Preliminary results for bit-error-rate (BER) estimates as a function of fade margin for an on-off keying (OOK) optical communication through the air-water interface are presented for a communication system where a random air-water interface is a part of the communication channel.
New results for investigating optical propagation through the random wavy air-water interface relevant to underwater optical communications are presented. A laser beam propagating through the air-water interface reaching a receiver below the water surface, as well as propagated through the water towards an airborne receiver, is significantly distorted due to the high geometric phase aberrations introduced by the random motion of the water surface waves. This causes a significant reduction in the received communications signal resulting in limiting the data transfer capability and the transmitting and receiving data rates. This research develops probabilistic models for optical propagation at the random air-water interface for both reflection and transmission cases under various wind speed conditions. Preliminary results from a laboratory water tank experiment provide information about histograms or the probability density functions of intensity fluctuations measured by a CCD camera for both reflection and transmission cases. Angular displacements of the centroid of the fluctuating laser beam generates the beam wander effects. Finally preliminary results for BER estimates for an on-off keying (OOK) for air-water interface only are presented for a communication system where random air-water interface is a part of communication channel.
Double-functional (optical and electrical) interferometer was realized using holographic recording of dynamic gratings in the semiconductor crystals of CdTe: V, CdTe:Ti and ferroelectric-pyroelectric crystal Sn2P2S6 (SPS).
Also we introduce novel holographic single-beam wave-front division interferometer that is compact, do not need stabilization and are well suited for real-world applications.
We will describe optical and electrical effects in photorefractive materials- in semiconductors and semiconductor-ferroelectric crystals.
Double-functional (optical and electrical) interferometer was realized using holographic recording of dynamic
gratings in the semiconductor crystal of CdTe: V. Two mechanisms of holographic phase grating recording is
considered: electro-optic effect (relatively slow, in microsecond) and free-carrier gratings {Drude-Lorentz
nonlinearity, fast response in nanoseconds). Fast optical response, based on Drude-Lorentz nonlinearity (also
called plasma nonlinearity) play essential role in the surface- plasmon resonance phenomena.
Optical and electrical effects in semiconductors and ferroelectric crystals will be modeled. Standard photorefractive equations are supplemented by the equation of state for the polarization density
following Devonshire-Ginsburg-Landau (DGL) approach. We have derived equations for pyroelectric and photogalvanic contribution to the holographic grating recording in ferroelectric materials. We will consider double-functional holographic interferometer, based on holographic pyroelectric current and optical beam coupling. Crystal electrostatic accelerators, based on charging of ferroelectric crystals by pyroelectric and photogalvanic effects are discussed in
relation to generation of self-focused electron beam, X-rays and neutrons.
We have developed model that describe transformation of time modulated optical signals by the double optical and electrical interferometer: beam coupling and holographic current in the semiconductor crystals. We consider non-local response, when dominant mechanism of the space charge formation is diffusion or drift in a high external electric field. Both phase and amplitude modulation is considered (linear ramp phase modulation, sinusoidal modulation, step-like modulation). For the sinusoidal amplitude modulation slow-down of optical signal is described. Experimental results on CdTe crystal with IR CW laser (P=100mW, wavelength 1064 nm) are in agreement with theoretical predictions. Effective group velocity was slow-downed to 555 cm/s for the modulated signal with period of 8ms.
Double-functional (optical and electrical) interferometer was realized using holographic recording of dynamic gratings in the semiconductor crystal of CdTe:V. Two mechanisms of holographic phase grating recording is considered: electro-optic effect (relatively slow, in microsecond) and free-carrier gratings {Drude-Lorentz nonlinearity, fast response in nanoseconds).
Both, electrical and optical signals from electro-optic gratings were strong enough to make direct registration on oscilloscope (without preamplifiers) using infrared (IR) CW (λ = 1064 nm) diode pumped 100 mW laser. Step-like phase modulation mimics the pulsed ultrasound modulation, used in biomedical acoustophotonic imaging.
Theoretical approach is in agreement with experimental results.
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