Atmospheric optical turbulence is the single most important channel impairment in terrestrial and ground-to-space free-space optical (FSO) links. Random fluctuations of the air's refractive index — driven mainly by small temperature variations mixed by wind and convection — distort a propagating laser beam, causing the received signal to fade, wander and lose spatial coherence. For an engineer designing an optical link, predicting and mitigating turbulence is what determines whether the link meets its availability target.
What causes it: refractive-index fluctuations
Turbulent eddies of slightly different temperature have slightly different refractive index. As the beam crosses many such eddies, its wavefront is progressively corrupted. The strength of these fluctuations is captured by the refractive-index structure parameter Cn2, which varies with altitude, time of day, and weather — strongest near a hot surface at midday, weaker at night. The classical statistical description uses the Kolmogorov power spectrum of turbulence.
How it degrades an optical link
- Scintillation — random intensity fluctuations (the "twinkling" of the received spot), which translate directly into signal fades and a higher bit-error rate.
- Beam wander — the beam centroid moves randomly, causing pointing loss and dropouts at the receive aperture.
- Beam spreading — turbulence broadens the beam beyond pure diffraction, lowering received power density.
- Angle-of-arrival (AOA) fluctuation — the wavefront tilt varies, degrading fibre coupling and focal-plane spot stability.
- Loss of spatial coherence — important for coherent and single-mode-fibre receivers.
Weak vs. strong turbulence
Link behaviour depends on the turbulence regime, often indexed by the Rytov variance. In weak turbulence, scintillation follows a log-normal distribution; as the path lengthens or Cn2 grows, the link enters moderate-to-strong turbulence where the gamma-gamma (or related) distribution better fits the measured fading statistics. Choosing the right fading model is essential for accurate BER and outage prediction.
Modelling turbulence
Engineering analysis typically combines a turbulence spectrum (Kolmogorov and variants), a propagation method (analytical scintillation-index expressions or split-step phase-screen simulation), and a statistical fading model to produce BER, fade-depth and outage-probability curves for a given Cn2, wavelength, range and aperture. SCOPT's Optical Turbulence Prediction & Compensation Tool (AOTA) predicts turbulence-induced distortion and evaluates aperture averaging as a passive compensation technique for exactly these scenarios.
Mitigation techniques
- Aperture averaging — a receive aperture larger than the spatial scale of the fluctuations averages out scintillation, reducing fade variance.
- Adaptive optics — actively corrects wavefront distortion for coherent/high-performance links.
- Spatial / aperture diversity — multiple transmit beams or receive apertures decorrelate fades.
- Error-control coding, interleaving and adaptive modulation — handle residual fading at the link layer.
Frequently asked questions
What is the Cn2 structure parameter?
It quantifies the strength of refractive-index fluctuations in the atmosphere. Higher Cn2 means stronger turbulence and more scintillation, beam wander and spreading. It is the key input for predicting optical-link performance.
Why does turbulence get worse during the day?
Solar heating of the ground drives convection and larger temperature gradients, raising Cn2. This is why FSO links often see the worst scintillation around midday and calmer conditions at night.
What is aperture averaging?
Using a receive aperture larger than the correlation scale of the intensity fluctuations so the receiver integrates over several speckles, which reduces the variance of the fading — a simple, passive way to improve link reliability.
SCOPT models, analyses and helps mitigate atmospheric turbulence in optical links. Try our AOTA turbulence tool, explore analysis & optimization and simulation services, or talk to our consultants. See also our companion guide on underwater optical turbulence.