Light Beamer | Atmosphere

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.

Jul 05, 2020 08:50 Posted on: Breakthrough Initiatives

Would it be possible to build it on the far side of the moon? I realize that if we were to do this today it would cost an enormous amount of money, but say 30 years, or so, from now we have a lunar base set up and we’ve begun to make use of the moon’s resources, could that be a viable option?

Jul 29, 2020 18:37 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Thanks for your comment, Daniel. The Breakthrough Starshot effort is designed to send a probe to Alpha Centauri at the lowest cost and highest reliability. The primary effort (and funding) is focused on the ability to coherently combine a near infinite number of lasers. This effort will benefit the program no matter where the installation is placed. Currently, our estimates of the economics of space flight suggest that the cost to place an installation on the far side of the moon is many trillions of dollars. However, if great advanced are made in lift and the price of lift drops to some very small fraction (1% ) of current costs we would consider the cost and risk dealing with the atmosphere versus of placing the installation beyond the atmosphere. But we have many years before this choice needs to be made.

Pete Klupar
Engineering Director
Breakthrough Initiatives

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