Light Beams Lose Power Predictably through Turbulence, Model Confirms

Scientists at Technology Innovation Institute, led by Konstantin Kravtsov, have developed a new analytical model to directly measure the depletion of optical power and its redistribution between spatial light modes as light propagates through turbulent atmospheric conditions. The model provides a simplified, matrix-based solution for predicting power transfer at any distance within a turbulent channel, demonstrating a linear scaling relationship with propagation distance. Predictions generated by the model align with simulation results and validate previously established empirical relationships linking turbulence characteristics to mode order.

Analytical model reveals linear scaling of power transfer in turbulent free-space optical propagation

Turbulence-induced power transfer between spatial light modes now exhibits a linear scaling with propagation distance, representing a substantial improvement over prior methodologies that relied on approximations accurate only for single modes. Previously, modelling such propagation necessitated computationally intensive numerical simulations or restricted the analysis to specific, simplified scenarios. This new breakthrough enables exact calculations across a broad range of turbulence strengths and propagation distances, offering a significant advantage for both theoretical studies and practical applications. The model’s accuracy is rooted in a ‘split-step approach’, which dissects the light’s journey into infinitesimally small segments, allowing for incremental accounting of the cumulative effects of turbulence on the optical field. This method avoids the complexities of solving the full wave propagation equation directly, offering a computationally efficient alternative.

A linear scaling of power transfer with distance in turbulent conditions has been confirmed through extensive simulations, even in medium-to-strong turbulence regimes. The model directly assesses how optical power depletes from an initial mode and redistributes into neighbouring spatial modes, a crucial element for optimising free-space optical communication and imaging systems. Accurate prediction of the average power distribution amongst these spatial modes is essential for both optical communication, where it impacts bit error rates, and imaging applications, where it affects image resolution and contrast. The model considers the interplay between the initial spatial mode profile and the turbulence-induced phase fluctuations, providing insights into the mechanisms driving power transfer. This is particularly important as turbulence introduces random phase screens that distort the wavefront of the propagating light.

The rate of power transfer is primarily determined by the spatial spectral overlap between the turbulence spectrum and the acceptance spectrum for a pair of interacting spatial modes. The turbulence spectrum describes the distribution of energy across different spatial frequencies in the turbulent flow, while the acceptance spectrum characterises the spatial bandwidth of the optical mode. A greater overlap implies a stronger coupling and, consequently, a faster rate of power transfer. The Fried parameter, defining the best achievable resolution through turbulence, typically around 0.1 metres for moderate turbulence, is implicitly accounted for within the framework, linking theoretical predictions to observable atmospheric conditions. This provides a baseline for evaluating various scenarios and allows for the development of improved techniques to counteract the effects of atmospheric distortion, such as adaptive optics. The model’s ability to connect theoretical parameters to measurable atmospheric properties enhances its practical utility.

Predicting single-mode light beam distortion through turbulent atmospheres

Free-space optical communication promises exceptionally high bandwidth and enhanced security compared to traditional radio frequency communication, but atmospheric turbulence remains a persistent obstacle to reliable data transmission. This new analytical model represents a strong advance by precisely charting how light beams distort as they travel through swirling air, a key step towards mitigating these signal losses and improving link performance. The model allows for the quantification of the impact of turbulence on the spatial structure of the light beam, providing valuable information for designing robust communication systems. A limitation exists, however, as the model currently relies on a single, dominant initial light mode; real-world signals often comprise multiple modes simultaneously, potentially reducing its predictive power in complex scenarios involving multimode excitation.

Atmospheric turbulence is rarely uniform across the propagation path, and light signals typically consist of multiple interwoven modes, creating a more chaotic propagation environment. Calculating power transfer between these modes moves beyond previous approximations and provides a solution scaling linearly with distance travelled. This advancement is significant for free-space optical communication, where turbulence degrades signal quality by causing beam spreading and scintillation, and understanding these shifts in light distribution is vital for optimising transmission parameters such as beam divergence and power levels. The linear scaling simplifies the prediction of power transfer over long distances, reducing the computational burden associated with modelling atmospheric propagation. Furthermore, the model’s insights can be applied to imaging systems operating through turbulence, such as telescopes and remote sensing platforms, to improve image quality and resolution. Future work will focus on extending the model to encompass multimode signals, enhancing its applicability to real-world communication systems and imaging technologies, and potentially incorporating non-Kolmogorov turbulence models to account for more complex atmospheric conditions. Investigating the impact of atmospheric absorption and scattering on the power transfer process is also a potential avenue for future research.

The ability to accurately predict and compensate for turbulence-induced effects is crucial for realising the full potential of free-space optical communication and imaging. This model provides a valuable tool for researchers and engineers working in these fields, offering a simplified yet accurate method for analysing and optimising optical systems operating in turbulent environments. The linear scaling property, in particular, offers a significant advantage for long-range propagation scenarios, where computational efficiency is paramount.

The research developed an analytical framework to understand how light signals carrying spatial information are affected when travelling through turbulent air. This is important because turbulence scatters light, degrading the quality of free-space optical communication and imaging systems. The model demonstrates that power transfer between different light modes increases linearly with distance, simplifying predictions of signal behaviour. Researchers validated the model against simulations, achieving good agreement up to medium-to-strong turbulence, and plan to extend it to more complex scenarios involving multiple signals.