ST401385@BROWNVM.BITNET (01/17/86)
What phase conjugation is particularly good for is correcting
errors due to atmospheric turbulance, eg., a laser propelling
a rocket being launched from the earth. Typically this would
use a low power pilot beam from the spaceship to the launch
laser. The pilot beam would be distorted by the atmosphere,
then phase conjugated and amplified into a reverse beam at high
power. This power beam is then already be pre-distorted for the
effects of the atmosphere, and atmospheric turbulance exactly
cancels this distortion, resulting in a perfect beam.
This works because the travel time through the atmosphere (at
the speed of light) is small compared to the time scale of atmospheric
turbulance.
This might also be the mechanism used for a ground-to-space
laser weapon, such as has been proposed for SDI, where a ground
based laser is bounced off a mirror in space, off another mirror
near the target, and then destroys ICBM's.
For a space-to-space laser, either weapon or spacecraft propulsion,
phase conjugation is much less useful. There is little or no
distortion due to interplanetary gas, and the distances are large
enough that sending a power signal down the reverse path of a pilot
beam would be pointless, since the ship would have moved in the
interim.
In fact, there was an article in Analog sometime last year, in the
Alternate View column, discussing phase conjugation.
The author suggested that this process violates the second law
of thermodynamics. Unfortunately, that is not the case: while the
entropy of the reversed pilot beam is, in fact, decreased (since the
random components are removed and it is restored to the original,
low entropy state), the entropy of the two pump beams is increased by
the same amount.
"You can't win, you can only break even at absolute zero, and
you can never reach absolute zero."
--GLLynn.es@XEROX.ARPA (01/21/86)
Why won't the phase conjugation technique work in reverse to build a large earth based telescope that removes the effects of atmospheric turbulence? One would need a laser in the field of view (on a satellite, say) to determine the distortion during the trip down through the atmosphere, then apply the correction for that distortion to all incoming light to produce an undistorted image. If it works, it sounds like it could make the Space Telescope obsolete. /Don Lynn
perkins@bnrmtv.UUCP (Henry Perkins) (01/22/86)
> Why won't the phase conjugation technique work in reverse to build a > large earth based telescope that removes the effects of atmospheric > turbulence? One would need a laser in the field of view (on a > satellite, say) to determine the distortion during the trip down through > the atmosphere, then apply the correction for that distortion to all > incoming light to produce an undistorted image. If it works, it sounds > like it could make the Space Telescope obsolete. > /Don Lynn There are two reasons this won't work. The first is that you'd have to move the "satellite" into a position on a line between the earth-based telescope and the object to be photographed. That's not a satellite anymore, but rather a precisely controlled station-keeping spacecraft. The second reason is that all you'd be taking would be a terrific photograph of a laser on a spacecraft. You'd only be correcting for the wavelength of the laser beam (different wavelengths are distorted differently), and it would overwhelm anything else you're attempting to have in the same field of view. It wouldn't even work to turn off the laser before taking the picture because atmospheric changes take place on the order of seconds, and typical exposures are on the order of minutes. -- {hplabs,amdahl,3comvax}!bnrmtv!perkins --Henry Perkins
dietz@SLB-DOLL.CSNET (Paul Dietz) (01/24/86)
n >Why won't the phase conjugation technique work in reverse to build a >large earth based telescope that removes the effects of atmospheric >turbulence? One would need a laser in the field of view (on a >satellite, say) to determine the distortion during the trip down through >the atmosphere, then apply the correction for that distortion to all >incoming light to produce an undistorted image. If it works, it sounds >like it could make the Space Telescope obsolete. I'm not sure if phase conjugation is what you want, but it doesn't seem impossible to use a laser beam to determine what the atmosphere is doing, then correct the image by computer. You might have some problems if the atmosphere causes different wavelength light to travel along different paths. There's already a technique, called speckle interferometry, that stacks short snapshots of a star into one image with resolution better than normally allowed by atmospheric distortion. I don't think it would make the space telescope obsolete; the ST can also be used to see infrared and ultraviolet light in wavelengths where atmospheric absorption and interference is important.
will@anasazi.UUCP (Will Fuller) (01/24/86)
In article <860120-145927-1076@Xerox> Lynn.es@XEROX.ARPA writes: >One would need a laser in the field of view (on a satellite, say) to >determine the distortion during the trip down through the atmosphere... One big problem with the atmosphere that can't be gotten rid of is the attenuation at certain wavelengths. If you had a tunable dye laser, you might be able to account for the degree of attenuation, but if you can't see it... Real time extinction information is of comparative little value (unless one is being astronometric). Instead the air mass can be made to work in your favor - as is the case of speckle interferometry. What advantage would a space borne laser in the field of view of a telescope have over any star in correcting the abberations caused by the air mass? The star might be "calibrated" by a space based detector. Further, the "spot" from such a remote laser (geosynchronous) would be damned big... -- William H. Fuller {decvax|ihnp4|hao}!noao!terak!anasazi!will