High data-rate communication link supported through the exploitation of optical channels in a characterized turbulent underwater environment.

Underwater turbulence presents a myriad of challenges for underwater optical systems through wavefront distortion and beam deflection. In this work, an underwater turbulence emulator is developed and thoroughly characterized to experimentally test the proposed underwater turbulence mitigation technique. This technique applies a modified HOBBIT system introduced in atmospheric turbulence to the relatively unknown underwater turbulence domain. By varying a beam's spatial position and relative phase gradient, a volume of turbulence is rapidly probed to determine the beam state for optimal propagation. This probe and control method is applied in multiple facets, including improved optical power transmission as well as supporting a 25-Gbps communication link through a dynamic environment.

Sensing and coupling of optical channels in dynamic atmospheric turbulence using OAM beamlets for improved power and data transmission.

Propagation of laser light is distorted in the presence of atmospheric turbulence. This poses an issue for sensing, free-space optical communications, and transmission of power. The presented system offers a novel solution to mitigate the effects of turbulence. By rapidly probing a turbulent volume by varying a beam's spatial and phase characteristics, the best transmission mode can be determined and updated in real time. Unlike a traditional tip-tilt system, this scheme is fully electronic, and has a scalable architecture to leverage multiple optical transmission paths simultaneously. This optical control system greatly improves power efficiency and successful recovery of data through environments with strong turbulence.

OAM-based optical wavelet using a single pixel detection system for probing dynamic environments with application to real-time measurements of strong atmospheric turbulence.

This paper presents a novel method for optical probing by generating optical fields with characteristics of wavelets. The optical wavelets form a basis of rotated asymmetric beams with scaled orbital angular momentum (OAM) and beam sizes. The probing method was used experimentally to measure the continuous wavelet transform of a turbulent propagation path, giving insight into the angular properties about a fixed radius. The wavelet transform of a three-dimensional turbulence distribution was measured; the measurements are much faster than the turbulence changes, allowing characterization of an instantaneous realization of turbulence over time. Results show highly localized regions of OAM in space through the turbulence and characteristics of the turbulence can be extracted from the wavelet transforms.

Fractional orbital angular momentum conversion in second-harmonic generation with an asymmetric perfect vortex beam.

This Letter demonstrates the nonlinear conversion of asymmetric perfect vortex (APV) beams with fractional orbital angular momentum (OAM). By controlling the amplitude and phase of the fundamental light field, we create APVs whose global OAM demonstrates a one-to-one correspondence of the charge numbers for fractional OAM values. The results show that the OAM of the second-harmonic generation fields follow the OAM conservation law. The nonlinear interactions of multiple OAM beams with the APVs are also investigated as they relate to the nonlinear frequency conversion and are shown to exhibit unique frequencies as a result of the Doppler frequency tagged OAM values.

Experimental probing of turbulence using a continuous spectrum of asymmetric OAM beams.

Propagation of a continuous spectrum of orbital angular momentum (OAM) states through a realistic and controlled 3-dimensional turbulent condition has not been studied to date to the authors' knowledge. Using the Higher Order Bessel-gauss Beams Integrated in Time (HOBBIT) system and a 60 meter optical path Variable Turbulence Generator (VTG), we demonstrate that by changing the OAM in a continuous scan, a spectrum of OAMs provide an opportunity to take advantage of additional propagation channels within the aperture of the transmitter and optical path to the receiver. Experimental results are provided illustrating the HOBBIT system's ability to position the beam in space and time to exploit eigenchannels in the turbulent medium. This technique can be used to probe the turbulence at time scales much faster than the Greenwood frequency.

Second-harmonic generation of asymmetric Bessel-Gaussian beams carrying orbital angular momentum.

Nonlinear processes of laser beams carrying orbital angular momentum (OAM) offer a means to generate new wavelengths and to manipulate OAM charge numbers. We demonstrate the second-harmonic generation (SHG) of asymmetric Bessel-Gaussian (BG) beams carrying OAM of both integer and fractional charge numbers. Experimental results show a good one-to-one correspondence of the charge numbers and compliance with the OAM conservation law. The SHG conversion process and efficiency with different combined charge numbers are also discussed.

Rapidly tunable orbital angular momentum (OAM) system for higher order Bessel beams integrated in time (HOBBIT).

Beams with fast and continuously-tunable orbital angular momentum (OAM) have potential applications in classical and quantum optical communications, sensing, and in the study of beam propagation through turbulence. An acousto-optical deflector (AOD) is a sophisticated, well-studied device that continuously and rapidly tunes the deflection angle of an output beam. The log-polar HOBBIT setup can generate beams with OAM by wrapping elliptically shaped Gaussian beams with linear phase tilt to a ring. By combining the linear tilted output from the AOD with the OAM generation capabilities of the HOBBIT system, the generated OAM modes become continuously tunable at high speeds measured on the order of 400 kHz.

Multilevel quadrature amplitude multiplexing using coherently coupled orbital angular momentum modes.

Information can be encoded onto transverse spatial light modes, providing a platform that can improve the spectral efficiency of a communications link. Switching speed between these spatial modes and detection methods limit the information capacity of such links. This work demonstrates high speed capabilities of such a link. Transverse modes are created and switched at high rates by coherently coupling twisted light modes using passive optical elements in line with electro-optical modulators. Here we demonstrate the encoding and detection of two coherently coupled modes encoded with 0.5 Gbaud quadrature amplitude modulated (16- and 32-QAM) signals, for a 4X and 5X increase in spectral efficiency by exploiting both phase and amplitude of the coherently coupled modes. The receiver is able to successfully recover the signal with error rates below the forward error correction limit using passive optical techniques. The data rate of the system used is only limited by hardware, but similar devices are available that are capable of multi-Gigahertz operation.

Probability density function estimation of laser light scintillation via Bayesian mixtures.

A method for probability density function (PDF) estimation using Bayesian mixtures of weighted gamma distributions, called the Dirichlet process gamma mixture model (DP-GaMM), is presented and applied to the analysis of a laser beam in turbulence. The problem is cast in a Bayesian setting, with the mixture model itself treated as random process. A stick-breaking interpretation of the Dirichlet process is employed as the prior distribution over the random mixture model. The number and underlying parameters of the gamma distribution mixture components as well as the associated mixture weights are learned directly from the data during model inference. A hybrid Metropolis-Hastings and Gibbs sampling parameter inference algorithm is developed and presented in its entirety. Results on several sets of controlled data are shown, and comparisons of PDF estimation fidelity are conducted with favorable results.