Frequency response of a thermally driven atmosphere

Area of Science:

• Atmospheric optics
• Laser physics
• Fluid dynamics

Background:

• Thermal blooming is a phenomenon where a laser beam heats the atmosphere, causing refractive index changes that distort the beam.
• Understanding atmospheric response to thermal blooming is crucial for laser propagation.
• Previous studies have explored thermal blooming effects, but analytical descriptions of dithered atmospheric responses are less common.

Purpose of the Study:

• To investigate the atmospheric response to a thermally dithered beam in a laboratory setting.
• To analytically describe the dithered atmosphere's behavior using fluid dynamics principles.
• To validate experimental measurements with theoretical predictions.

Main Methods:

• Utilized a focused, dithered carbon dioxide (CO2) laser beam to induce thermal dithering in a laboratory atmosphere.
• Measured the amplitude and phase response of the dithered atmosphere using both a helium-neon (He-Ne) laser and a CO2 laser.
• Employed analytical methods based on acoustic wave propagation theory in absorptive and dispersive fluids.

Main Results:

• Successfully induced and measured thermal dithering in the laboratory atmosphere.
• Observed and quantified the amplitude and phase response of the atmosphere to the dithered beam.
• Demonstrated that the atmospheric response can be analytically described by introducing a relaxation time dependent on beam diameter and crosswind velocity.

Conclusions:

• The study provides an analytical model for describing atmospheric response to a dithered beam, incorporating a relaxation time.
• Experimental measurements using He-Ne and CO2 lasers validate the theoretical model.
• This research contributes to a better understanding of laser-atmosphere interactions and thermal blooming effects.