Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: galaxysim on 06/07/2013 17:08:43
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Interstellar communication with lasers, watts per light year anyone ?
Q what is the effective range ? Stars shine everywhere but what about directional lasers ?
Q what kind of light spread do we get from Laser light at one light year or 1000 light years ?
any pointers ? (scuse the laser'ish pun )
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I can easily imagine lasers being used for communication 'within' a solar system, light minutes and light seconds i am less sure of their practical effectiveness out to 1 light year and beyond 1 light year
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Depends how big and sensitive your detector is, what frequency you work at, how narrow you can keep your laser beam etc... Lower frequency will get better energy per photon, but you have to consider signal to noise issues with background spectrum of skies.
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The principal limiting factors are beam divergence, required data transmission rate, and signal/noise ratio at the receptor. Starting with your transmitted beam power per unit area, divide by (divergence multiplied by distance) to get the received power per unit receptor area. Multiply by the receptor area to get the received power. Now consider the background noise spectrum at your chosen frequency (is your target near a bright star?) and calculate how many pulses you need to integrate in order to distinguish signal from noise.
I don't have the figures to hand but whilst you can bounce a few kilowatts of visible light off the moon with enough integrity to measure the distance, I don't think Mars represents a feasible target. But I'd like to be surprised.
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I understand that the laser used for Moon distance ranging has a beam width of about 4 km when it reaches the Moon, as the reflector is quite small very little energy is returned and the operation is quite marginal.
Radio communication is reckoned to be possible with Arecibo size antenna at each end out to 1000 ly but would need a lot of patience !!