ABSTRACT
The creation of underwater optical turbulence is driven by density variations that lead to small changes in the water's refractive index, which induce optical path length differences that affect light propagation. Measuring a laser beam's optical phase after traversing these turbulent variations can provide insight into how the water's turbulence behaves. The sensing technique to measure turbulent fluctuations is a self-heterodyne beatnote enhanced by light's orbital angular momentum (OAM) to obtain simultaneous optical phase and amplitude information. Experimental results of this method are obtained in a water tank that creates a thermally driven flow called Rayleigh-Bénard (RB) convection. The results show time-varying statistics of the beatnote that depend on the incident OAM mode order and the strength of the temperature gradient. Beatnote amplitude and phase power spectral densities are compared to analytic theory to obtain estimates of the turbulent length scales using the Taylor hypothesis that include mean flow speed, turbulent strength, and length scales, and flow dynamics due to intermittency in the RB process.
ABSTRACT
Laser based optical applications such as imaging, ranging, and wireless communications are susceptible to environmental distortions. Inferring the strength of these optical distortions is crucial to obtaining information about the environment in which the system is operating. Our technique of inferring environmental distortion strength leverages the spreading of light's orbital angular momentum (OAM) spectrum combined with heterodyne detection. A laser encoded with OAM can be decomposed into a basis set of helical modes that spreads upon interaction with optical distortions. This mode spreading is quantified using the OAM spectrum that can be measured using mode projection or mode sorting techniques. This new technique, to the best of our knowledge, provides benefits compared to the latter two OAM detection methods such as: low-frequency noise rejection, a simpler optical receiver, lower noise floor, and an inherent optical phase component. Central to the method is the heterodyne detection of the zeroth-order OAM coefficient of a superimposed two-beam, two-frequency, probe. The measured heterodyne signal power is seen to be proportional to the coupling power of each beam's OAM spectra. To test the idea, wave-optic simulations and experiments using spatial light modulators are implemented using a simplified optical turbulence model to represent the environment. The experimental implementation agrees well with simulated and theoretical results.
ABSTRACT
An optical vortex is used to enhance the ranging accuracy of an underwater pulsed laser ranging system. An experiment is conducted whereby an underwater object is illuminated by a pulsed Gaussian beam, and both the object-reflected and scattered light are passed through a diffractive spiral phase plate prior to being imaged at the receiver. An optical vortex is formed from the spatially coherent non-scattered component of the return, providing an effective way to discriminate the desired objected reflected light from the spatially incoherent scatter. Experimental results show that the optical vortex permits a spatially coherent ballistic target return to be more easily discriminated from spatially incoherent forward scattered light up to eight attenuation lengths. The results suggest new optical sensing techniques for underwater imaging or lidar.
ABSTRACT
The propagation behavior of high-frequency intensity-modulated signals through turbid water is of significant interest for underwater laser ranging, imaging, and communications. Prior experimental measurements have focused only on the magnitude response of the underwater optical channel to forward-scattered and unscattered modulated light. In this study we include, for the first time to our knowledge, both the magnitude and phase of the underwater optical channel to forward-scattered light. The magnitude and phase response is measured out to 1 GHz, using three different artificial scattering agents in scattering environments in excess of 25 attenuation lengths. The phase response provides additional insight into the behavior of forward-scattered light carrying high-frequency intensity modulation.
ABSTRACT
Until recently, little has been done to study the effect of higher modulation frequencies (>100 MHz) or short (<2 ns) pulse durations on forward-scattered light in ocean water. This forward-scattered light limits image resolution and may ultimately limit the bandwidth of a point-to-point optical communications link. The purpose of this work is to study the propagation of modulated light fields at frequencies up to 1 GHz. Results from laboratory tank experiments and their impact on future underwater optical imaging and communications systems are discussed.
ABSTRACT
A new technique has been found that uses in-phase and quadrature phase (I/Q) demodulation to optimize the images produced with an amplitude-modulated laser imaging system. An I/Q demodulator was used to collect the I/Q components of the received modulation envelope. It was discovered that by adjusting the local oscillator phase and the modulation frequency, the backscatter and target signals can be analyzed separately via the I/Q components. This new approach enhances image contrast beyond what was achieved with a previous design that processed only the composite magnitude information.
Subject(s)
Lasers , Optics and Photonics , Data Interpretation, Statistical , Equipment Design , Image Processing, Computer-Assisted/methods , Models, StatisticalABSTRACT
Laser systems have been developed to image underwater objects. However, the performance of these systems can be severely degraded in turbid water. We have developed a technique using modulated light to improve underwater detection and imaging. A program, Modulated Vision System (MVS), which is based on a new theoretical approach, has been developed to predict modulated laser imaging performance. Experiments have been conducted in a controlled laboratory environment to test the accuracy of the theory as a function of system and environmental parameters. Results show a strong correlation between experiment and theory and indicate that the MVS program can be used to predict future system performance.
