The atmosphere introduces two effects: absorption (or ‘reduction of transmission from unity’), and loss of beam quality (or ‘blurring of the beam spot’). The transmission of the atmosphere at a wavelength of 1 micron is extremely good, exceeding 90% at high altitude ground-based sites. Going to a high altitude site also significantly reduces atmospheric blurring, which would allow an adaptive optics system to achieve performance near to the diffraction limit.
The effects of atmospheric turbulence on the beam include a broadening of the beam footprint (equivalent to image blurring for telescope observations), random jitter of the beam spot, and intensity fluctuations (or ‘scintillations’). The blurring depends on turbulence and wind profile in the atmosphere. The turbulence amplitude is reduced by a factor of approximately 4 between sea level and an altitude of 5km.
The quality of an image is measured by the Strehl ratio, which reflects the ratio of the peak image intensity from a point source to the diffraction limit of an ideal optical system. The ratio measures phase deviations caused by lens aberrations and atmospheric turbulence. 10m-class telescopes, such as the Large Binocular Telescope (LBT), comprising two 8.4m telescopes, have demonstrated image resolution of 40 milliarcseconds and Strehl ratio of 80% at a wavelength of 1.6 microns.
Breakthrough Starshot aims to achieve the diffraction limit for an optical system of laser beams across 0.2-1km, which is 1-2 orders of magnitude beyond existing demonstrations. There are no fundamental physics limitations to achieving this improvement. A beacon on the nanocraft or near its launch point (for instance on the mothership) could be used to correct for phase variations in real time. The effect of the light beam on the atmosphere could be studied, and corrected for, by adaptive optics, again in real time. Additional beam focusing may also be explored to reduce the beam spot size using pulsed laser filamentation techniques.
- Giovanelli et al., “The Optical/Infrared Astronomical Quality of High Atacama Sites. II. Infrared Characteristics”, PASP, Vol. 113, p. 789 (2001)
- Giovanelli et al., The Optical/Infrared Astronomical Quality of High Atacama Sites. I. Preliminary Results of Optical Seeing, PASP, Vol. 113, p. 803 (2001)
- Couairon, A. & Mysyrowicz, A. Femtosecond filamentation in transparent media. Phys. Rep., Vol. 441, pp. 47–189 (2007).
- Mitrofanov et al., “Mid-infrared laser filaments in the atmosphere”, Scien. Rep., 8368 (2015)
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