Date: 31 Jan 1994 22:30:03 GMT
From: Jordin Kare <jtk@s1.gov>
Subject: more on lasers for comm links
Newsgroups: sci.space,sci.astro

Bruce Macintosh writes:

>My intuition (anyone who was actually involved in laser communication studies
>want to back me up?) is that laser links really do require a reciever in
>space, or a 10-micron laser that's really out into the infrared, where daylight
>wouldn't be a factor.

This is not correct.  The main requirement for a daytime laser comm
system is that the receiver filter bandwidth be extremely narrow, and that
the receiving optics be well designed.  The sky is
not all that bright, especially in the red and near-IR -- so with a
very narrow field of view and a properly-designed telescope, there's
very little received power from the sky.  Actually, a larger problem with
planetary probes is sunlight reflected from the planet (or moon)
swamping the laser signal.

BTW, "narrow bandwidth" in this case means <<1 nm; possibly as little
as a few hundred MHz.  One way of getting such narrow bands is with 
a class of atomic absorption filters that can be tuned by magnetic
fields.  Don't assume that you're limited by the performance of
conventional interference filters.

	Jordin Kare	jtk@s1.gov	

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: DSN questions
Date: Fri, 7 Nov 1997 03:09:00 GMT

In article <63rfv5$miv@lace.colorado.edu>,
Frank Crary <fcrary@rintintin.Colorado.EDU> wrote:
>A minor but important detail: The detector would also be different.
>If you just used a conventional, astronomical detector (a CCD with
>the geometry set up for good resolution), you would be spreading
>the photons out among 640,000 or more pixels. Dark current and read-out
>noise would seriously reduce the signal to noise...

Very true.  In any case, precision mirror or not, you'd use an avalanche
photodiode as the data receiver -- there just is no other sensible choice
for this sort of application (photomultiplier tubes are the only thing
that even comes close).  With a precision mirror, you might also use a CCD
for acquisition and tracking; with a light bucket, you'd probably use a
whole small auxiliary telescope for that, again possibly with a CCD as the
sensor (although there are other possibilities) -- for acquisition and
tracking, you *do* want to form an image.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

From: madler@alumni.caltech.edu (Mark Adler)
Newsgroups: sci.space.tech
Subject: Re: DSN questions
Date: 6 Nov 1997 18:18:28 GMT

In article <itsd1-0411971949230001@ip-pdx27-11.teleport.com>,
Thomas L. Billings <itsd1@teleport.com> wrote:
>Would something as simple as one of Dr Angel's mirrors right out of his
>spinning furnace in Arizona be good enough?  I saw a photo of an image

I've heard 2 micron rms surface accuracy is good enough for about 1 micron
wavelength laser communication.

mark

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: DSN questions
Date: Tue, 4 Nov 1997 04:04:33 GMT

In article <345B4541.BC5@erols.com>,
Brian Pickrell  <emphyrio@erols.com> wrote:
>What exactly is a 10m light bucket?  I assume it's easier to build than
>a 10m telescope.

It's a 10m telescope with a mirror that's of poor quality by astronomical
standards.  Because a system like this is not doing imaging -- the laser
beam is much brighter than the background, so there is no need to focus
sharply enough to exclude all the background -- all you're concerned about
is whether the photons make it onto the detector, not exactly where on
the detector they hit.  So the initial shape of the mirror doesn't need
to be as good, and the shape doesn't need to be held as precisely either.

>I wonder how often all three WOULD be obscured.  Offhand, the odds of
>three desert sites having clouds at the same time seem vanishingly
>small...

Unfortunately, they aren't.  Small, yes, but not small enough to eliminate
the problem.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: DSN questions
Date: Thu, 6 Nov 1997 14:16:15 GMT

In article <itsd1-0411971949230001@ip-pdx27-11.teleport.com>,
Thomas L. Billings <itsd1@teleport.com> wrote:
>> >What exactly is a 10m light bucket? ...
>> It's a 10m telescope with a mirror that's of poor quality by astronomical
>> standards...
>
>Would something as simple as one of Dr Angel's mirrors right out of his
>spinning furnace in Arizona be good enough?

My guess would be that it would still require some polishing, but I don't
know for sure -- I don't know how good the initial surfaces of Angel's
mirrors are.

More to the point, though, this is a process that's aimed at making full
diffraction-limited mirrors, and its initial stages are not necessarily
optimum for making light buckets.  In particular, the thickness of a
conventional mirror is necessary for rigidity, but rigidity is less
crucial for a light bucket, especially one for use in space (where gravity
loads are absent).

For light buckets you might want to look at ideas like casting mirror
segments in molds, and at materials other than glass.  Optimizing the
process for the desired results will probably make things easier.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

From: Henry Spencer <henry@zoo.toronto.edu>
Newsgroups: sci.space.tech
Subject: Re: DSN questions
Date: Tue, 4 Nov 1997 04:12:00 GMT

In article <0000345E110E.NO_UCE@mauve.demon.co.uk>,
Ian Stirling  <0000345E110E.NO_UCE@mauve.demon.co.uk> wrote:
>: Right.  It doesn't have to image, so you don't have to worry about the
>: surface to fractions of a wavelength.
>
>However, I think the more accurate, the better, especially if it's near
>other light sources, for example planets, ....
>Filters can help quite a lot, but the planet is a LOT brighter than
>most laser sources.

No, sorry, not right -- nothing is, not with really narrowband filters.
With the best narrowband filter technology, it is actually possible to
continue laser reception with the *Sun* directly behind the satellite.  A
mere sunlit planet is no big deal; current experimental systems intended
for near-Earth data relay have to function with their whole field of view
full of sunlit Earth.
--
If NT is the answer, you didn't                 |     Henry Spencer
understand the question.  -- Peter Blake        | henry@zoo.toronto.edu

Newsgroups: sci.space.policy
From: Henry Spencer <henry@zoo.toronto.edu>
Subject: Re: Cassini -- small?
Date: Thu, 15 Aug 1996 13:04:51 GMT

In article <Em4G5ne00ay30S4IVb@andrew.cmu.edu> Jacob McGuire <mcguire+@andrew.cmu.edu> writes:
>>Who said anything about big mirrors?  We're talking about entire 
>>spacecraft that can fit in the palm of your hand...
>
>  I did some rough figures, and it looks like with a 20W lightbulb
>(since you mentioned optics), you'll get around one 10 quintillionth
>of a watt per square meter at earth.  You're gonna need some mighty
>big telescopes to even get one bit per second out of that sucker.

Let's talk serious optics, not silly strawman examples.  Using a 5cm
diffraction-limited transmitting telescope at 500nm, the spot size at a
distance of 1.4Tm is about 14000km (very rough calculations, disregarding
the occasional factor of 2).  With 4W of optical power and a 4m receiving
mirror, that gives about 0.2 picowatts at the receiver.  That may not
sound like much, but at 500nm it is about 650,000 photons per second... 
and the optical-detector people routinely detect single photons.  This
should give data rates comparable to what Galileo's high-gain antenna
would have delivered. 
-- 
 ...the truly fundamental discoveries seldom       |       Henry Spencer
occur where we have decided to look.  --B. Forman  |   henry@zoo.toronto.edu