Optical Control System for Atmospheric Turbulence Mitigation

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. With an ever-increasing demand for high-speed data communications, particularly between satellites, unmanned vehicles, and other systems that benefit from a point-to-point link, this issue is critical for the field. A variety of methods have been proposed to circumvent this issue. Some major categories include the manipulation of the light’s structure, an adaptive scheme at the optical receiver, scanning mirror systems, or a transmission of simultaneous signals with a goal to improve robustness. There is an inherent advantage to a turbulence mitigation scheme that can be performed on the transmission side of an optical link. By rapidly probing a turbulent volume by varying a beam’s spatial and phase characteristics, the best transmission mode for an optical beam can be determined and controlled in real time. With the fast mode-switching capabilities of the HOBBIT (Higher-Order Bessel-Beams Integrated in Time) system, the dynamics of turbulence can be probed incredibly quickly. This work presents an optical control system that takes advantage of such a probing method, and greatly improves power efficiency and successful recovery of data through environments with strong turbulence

Quality Metrics and Reliability Analysis of Laser Communication System

Beam wandering is the main cause for major power loss in laser communication. To analyse this prerequisite at our environment, a 155 Mbps data transmission experimental setup is built with necessary optoelectronic components for the link range of 0.5 km at an altitude of 15.25 m. A neuro-controller is developed inside the FPGA and used to stabilise the received beam at the centre of detector plane. The Q-factor and bit error rate variation profiles are calculated using the signal statistics obtained from the eye-diagram. The performance improvements on the laser communication system due to the incorporation of beam wandering mitigation control are investigated and discussed in terms of various communication quality assessment key parameters.

Integrated Approach to Free Space Optical Communications in Strong Turbulence

The propagation of a free space optical communication signal through atmospheric turbulence experiences random fluctuations in intensity, including signal fades which negatively impact the communications link performance. This research develops an analytical probability density function (PDF) to model the best case scenario of using multiple independent beams to reduce the intensity fluctuations. The PDF was further developed to account for partially correlated beams, such as would be experienced by beams having finite separation. The PDF was validated with results obtained from digital simulations as well as lab experiments. The research showed that as the number of transmitted beams increases the probability of fade decreases. While fade probability is reduced by adding more beams, using more than four transmitters does little to improve the overall performance. Additionally, the use of pulse position modulation (PPM) provided significant improvement over traditional fixed threshold on/off keying with the impact of signal fading reduced. Combining PPM with multiple transmitters produced the best overall bit error rate results

Investigation of Multiple Transceiver System for Free Space Optical Communication

In free space optical (FSO) communication, atmospheric turbulence has major impact on the stability of a free space optical transmission system. Laser beam with high coherency and intensity is commonly used to increase the transmission distance. However, with an increased distance, the stability of the system becomes more sensitive to the turbulence. In this work, a multiple transceiver system with signal summing approach is implemented to improve signal transmission stability. The performance of the proposed system is investigated with the intensity turbulence modeled. In comparison with the single transceiver system, the proposed system showed higher signal-to noise ratio and lower bit-error-ratio with doubled stability in terms of the improvement factor