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.

Comments (44)

  1. danielwebber801@gmail.com:

    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?

  2. 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

  3. Sahil choudhary:

    We should set the sources of beam out in space just like we have hubble in space due atmospheric effects.......
    Or we can set the beamers on moon in near future

  4. Philip Lubin:

    Sahil

    Great question.

    In the long term space would be preferable from the point of view of avoiding atmospheric perturbation issues. The Moon is a possibility we have discussed for the distant future but space is vastly more expensive to deploy that is an Earth based system. For example, currently the cost to place 1 kg (~2.2 pounds) on the lunar surface is about 1 million USD). This cost will come down as increased heavy lift capability arises but the costs are still daunting for large structures in space. At the moment we do not feel that space deployment is required. More data is needed (see below). Since we are based on an exponential technology (photonics), time is on our side as the system costs will drop dramatically with time and we still need many years to develop all the technology required.

    If it turns out that we cannot mitigate the Earth's atmosphere well enough then space remains an option albeit a very expensive option. Before going to space it is important to work out the system issues and to find the limitations as much as possible on the Earth. As you no doubt know, ground based telescopes work remarkably well with modern adaptive optics in mitigating the Earth's atmosphere and though we do have space based observatories, including the HST and soon the JWST among many others, the largest telescopes are Earth based. For some astronomy applications space is essential (X-Ray, UV, far IR) but for visible and near IR the Earth's atmosphere is extremely transparent, though turbulent. Much of our effort on the laser array (photon engine) will focus on adaptive optics through the use of exo-atmospheric beacons to measure the phase on the Earth and correcting for it. Finding the limits of this are part of our future testing program.

    If you are interested in a technical discussion of this point see one of our recent papers:
    "Beam propagation simulation of large phased laser arrays"
    https://www.spiedigitallibrary.org/conference-proceedings-of-spie/11107/2528931/Beam-propagation-simulation-of-large-phased-laser-arrays/10.1117/12.2528931.full?SSO=1

    Philip Lubin
    UC Santa Barbara/ Starshot

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