Lasers, fiber optics, and ewes. (need answer fast!)

Factual Questions
1

How efficiently can electrical energy be converted into a laser beam? I’m assuming a bunch of it is lost as heat, is that correct?

Can a laser be routed through a fiber optic line? How about a 750 megawatt class IV laser?

How efficiently will the light be transmitted?

What’s the most efficient way to convert the laser energy back to electricity?

If I fired a 750 megawatt industrial class IV laser from an geosynchronous orbiting platform at an adult ewe on the moon 1) could I reasonably expect to hit it with current known targeting technology? 2) in the absence of oxygen, how would the ewe’s flesh respond to the beam?

Just curious.

2

On3e of the dirty little secrets about lasers is that they’re inefficient. a LOT of energy goes into waste heat. They’ve gotten better, but in most cases by far most of the energy does NOT come out in the laser beam.

Some of this loss is inevitable – lasers only work when you have a population inversion, and all the pump schemes I’m familiar with require that some energy be lost to heat in the gas, liquid, or (increasingly) the laser crystal. (The only exception I know of are chemical lasers, which have other inefficiency factors) You can’t do anything about this – if you want a 3- or 4-level system with a population inversion, you have inevitable loss. 25 years ago the best you could do in light-to-light conversion was 80%. Now there are systems with well over 90% efficiency. Unfortunately, these systems need to be pumped by other lasers or light systems (often diodes) which themselves are far less than 100% efficient.

I don’t know what the current wall plug - to - laser efficiency is. It used to be down on the order of 5-10%. I’ll bet it’s still below 50%.

You can certainly funnel energy into fiber optic carriers. it’s done all the time. Again, there is loss by various methods – reflection loss going into and coming out of the fiber (even with anti-reflection coatings), loss in focusing it down, losses due to bending, scattering, and absorption in the fiber.

Light is a pretty rotten way to transmit power – look at the poor efficiencies of solar cells, because you’d be using similar technology. You’d do a lot better microwaving the power, I think.
A Ewe on the Moon? A Ewe on the Moon??
What weird scenario to you have in mind? A Ewe on the moon, of course, is going to be seriously dead. And, if you don’t get to it pretty soon, seriously desiccated.

In any event, over a distance of a quarter of a million miles (vector added or subtracted to the 21,000 miles or so a geosynch orbit is) your laser beam is going to spread out pretty damned wide, and won’t be very effective for drilling holes in dead ovines (When Auric Goldfinger said you could “Project a spot of light on the moon”, he was talking about a pretty big spot). My guess is that the dessicated corpse would glow weakly in the color of the laser you shot at it. You might need instruments to be sure, though.
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3

Diode lasers are in excess of 65% efficient, and may be past 80% at this point (cite: SHEDS drives laser diode efficiencies towards 80% ), but I’ve never heard of one in the megawatt range.

Pulsed lasers can hit instantaneous powers in the megawatt range, but they aren’t necessarily very efficient.

Lasers can be and are routed through optical fibers all the time. If you’re going to move 750 MW, it’ll take a lot of fiber area to avoid frying the fibers.

Converting laser light back to electricity? The best solar panels are ~40% efficient; it may be possible to do better via a solar furnace and steam-cycle turbine/alternator.

Could you hit your moon-ewe with a laser? Depends on the size of your beam by the time it reaches the surface of the moon. The bigger the spread, the better chance the ewe has of being contacted by the beam. Lunar distance measurements are made with lasers all the time; they bounce them off of retroreflectors placed there by the Apollo lunar landing hoax program. The laser is fired from the earth and has a width of about four miles when it reaches the moon. Firing the laser from orbit would get you above the atmosphere, but your platform would be subjected to solar winds that might influence aiming accuracy. If you want your moon-ewe to feel any kind of warmth from the arriving beam, you’d have to keep it pretty tight. Dispersing it to a circle 800 meters across would put the power flux on par with solar , but I think aiming would be pretty difficult.

For the sake of argument, suppose you were able to focus your super-laser entirely upon the moon-ewe. Upon receiving an influx of 750 MW, the moon-ewe would rapidly pyrolize. and sublimate.

4

The lasers that they fire at the Moon for the purpose of reflecting them back and determining the exact distance to the Moon end up spread out into a circle about 6.5 km wide by the time they get to the Moon. The reflected light is too dim to be seen with the human eye (though it can be detected with instruments). I think your ewe is going to reflect light even more faintly than the reflectors, since the reflectors are designed to reflect light back to Earth and ewes, well, aren’t.

5

You might be able to achieve greater efficiency with a solid-state device that’s designed specifically to receive that exact wavelength, rather than one designed for white light. In fact, the receiving device would probably bear a striking similarity to the original diode laser.

6

Would “buckeyes” explode, carmelize, or keep staring creepily at you when hit by such a laser ? :slight_smile:

7

Why does the OP need the answer fast?

Presumably that’s humor or hyperbole, because otherwise, we’re all doomed.

I’m envisioning a scenario like Black Sheep or God Beast where monster Sheep – from the moon, in this case – are invading earth. I’m thinking it would be easier to kill them by dropping the laser on them than by trying to shoot them with it.

8

Forgot some details. The ewe would:

  1. pyrolize to a carbonaceous mass.

  2. incandesce rather brightly. Some portion of the laser light would initially be reflected, but as the charred remains of the ewe heated up, they would begin to radiate heat as a blackbody, glowing red at first, then white-hot.

  3. sublimate. In the absence of atmosphere, the charcoal ewe-briquet would not oxidize; it would continue heating until it began to sublimate, giving off carbon vapor.

9

An ewe is not an “it”, it’s a “she”. Even on the moon.

10

I feel truly sheepish. I will start my interplanetary pyrotechnics endeavors an ewe.

11

Suppose the moon-ewe has lice, who have laid eggs. When you use a laser to zap a moon ewe nit, frankly it gives me a sense of deja vu. :wink:

12

Frankly, this reminds me of rock and roll…

13

You’d better get to work quickly to stop the Space Ewe invasion. You’ll need to build a mirror or lens several kilometers in diameter if you want your diffraction-limited laser beam to be Ewe sized on the moon.

If you uniformly illuminate the lens, the spot on the moon should be roughly wavelength*(distance to moon)/(diameter of lens). It is a long way to the moon.

The good news is that I can confirm an earlier comment. Photovoltaic devices can be very efficient if you design the semiconductor bandgap to be very close to, but slightly less than, the photon energy of your laser. The inefficiency of silicon photovoltaics has mostly to do with the broad spectrum of incandescent light from the sun. Diode lasers can also be quite efficient, but count on losing half the light if you are trying to couple it to a single mode fiber.

14

We now know why the cow jumped OVER the moon!

15

What if “its” name is Pat and “it” dresses and acts oddly ?

16

Nothing like having somebody come along and explain your own joke to you, eh, AskNott?

17

Hint, whats a slang/none technical name for a planetary body, particulary one without an atmosphere ?

In space, no one can hear you whoosh, eh? , Seventh Deadly Finn.

18

I have nothing interesting to add to that exchange.

19

Let me spell it out for you in words of one syllable:

  1. AskNott had already made the Zappa joke you thought you were making.

  2. Everybody got it but you, and you looked dumb for making it again.

  3. Hi, Opal.

20

I didn’t see it as billfish making the same joke again; I saw it as him extending the joke.