Overview
Self-defocusing in free-space optics (FSO) describes the divergence of a laser beam beyond that predicted by diffraction. It commonly results from thermal blooming—absorption of beam energy by the medium leads to local heating, reduced refractive index, and a negative-lens effect. [1] In plain English ,as light going through an atmosphere is absorbed, it heats the air, reducing it’s density relative to the air just outside of the beam’s path. This works as a weak lens which defocuses the beam.
If the beam is moving to track a target or wind is blowing cooler air across the beam, the unevenness of this defocusing will bend the beam towards the cooler air. [2]
Mechanism of Thermal Blooming
There are a few mechanisms which cause thermal blooming.
- Laser energy absorption: Atmospheric constituents such as water vapor, CO₂, and airborne aerosols absorb a portion of the beam’s energy, even in wavelengths with low nominal absorption.
- Localized heating and expansion: Absorbed energy heats the air within the beam path, causing it to expand and become less dense. [3]
- Refractive index gradient: The heated, lower-density core has a lower refractive index compared to surrounding cooler air, forming a negative thermal lens. [3]
- Beam distortion and wander: Wavefront shape changes cause beam distortion, while wander refers to lateral displacement caused by thermal blooming.
These combined effects manifest as self-defocusing, directly impacting system performance. While these issues are greater in higher power systems, they are also a consideration for FSO systems.
Impact in FSO systems
-
Reduced receiver intensity: Beam spreading lowers signal intensity at the receiver, degrading signal-to-noise ratio and limiting communication distance.
-
Asymmetric distortions: Crosswinds can induce asymmetric heating, causing beam wander and distortions that degrade pointing accuracy and coherence.
-
Adaptive optics interactions: While adaptive optics correct turbulence by refocusing the beam, increased concentration of intensity may exacerbate local heating and thermal blooming.
Contributing Factors
Additional factors can worsen the self-defocusing effect. [4] These include:
-
High laser power increases heating, strengthening the negative lensing effect.
-
Wavelength-dependent absorption: Wavelengths with higher atmospheric absorption—especially in IR—worsen blooming.
-
Low wind conditions Low wind conditions allow heat to remain in the beam path, which intensifies refractive gradients.
-
Long propagation distances allow thermal effects to accumulate, magnifying beam degradation.
Mitigation Strategies
Decreasing power levels and wavelength selection can partially mitigate self-defocusing, though these methods have tradeoffs with other considerations. Because the heating of air takes time, short pulses of energy and wider beams are also advantageous. Beam shaping — for example, choosing a top-hat/flat-top beam versus a Gaussian beam — can also influence susceptibility to thermal blooming.
Beam pre-compensation and adaptive optics with thermal lensing compensation, where the beam is deliberately defocused at the transmitter to counteract expected thermal blooming along the path is an area of research.[5,6,7]
References
[1]Chemomechanics. “‘Thermal Blooming’ – Where Does the Term Come From?” Physics Stack Exchange, 14 July 2023. https://physics.stackexchange.com/questions/771990
[2] Luke Campbell. “Thermal Blooming.” PanoptesV.com. http://panoptesv.com/SciFi/LaserDeathRay/ThermalBlooming.html
[3] Rüdiger Paschotta. “Thermal Blooming.” RP Photonics Encyclopedia. Retrieved 3 September 2025. https://www.rp-photonics.com/thermal_blooming.html
[5] Vaseva, Irina A., et al. “Light self-focusing in the atmosphere: thin window model.” Scientific Reports 6, 30697 (2016). doi:10.1038/srep30697