Moon in them spotlights.

[Oxymorphone]

Hi, just thought you laserfreaks would find this info interesting.

This is what they use to light the moon up (and measure distance to it).

The laser is a Leopard solid-state picosecond product from Continuum Lasers. Pumped by
flashlamps at 20 Hz, 1.064 [ch956]m laser emission from the Nd:YAG rod in the oscillator is shaped into

120 ps pulses (FWHM) via the combined efforts of an acousto-optical mode-locker, a solid state
saturable absorber, and a GaAs wafer to clamp the pulse energy. A double-pass amplifier boosts
the cavity-dumped 1 mJ pulse to
250 mJ, after which a second-harmonic-generator crystal
frequency-doubles the light at approximately 50% efficiency to produce 115 mJ pulses of 532 nm
light with pulse widths of about 90 ps (FWHM).

Heat from the laser rods is carried away by de-ionized water flowing at a rate of approximately
6 liters per minute, taking away
1300 Watts of thermal power. An auxiliary pump maintains
flow in this loop when the laser is not powered on and the ambient temperature is near or below
freezing.

Remote setting of the laser output power is provided by digitally controlled potentiometers
placed in the electronics units that control the amplifier flashlamp delay and the voltage applied
to the oscillator flashlamp. The latter allow us to monitor the oscillator laser threshold and adjust
for optimal laser operation on a routine basis.

 

[Switch]

Yea , but they didn't actually light it up.Even with that monster hitting the retroreflector they barely got a reflection back.

 

[rocketparrotlet]

50% crystal efficiency? Whoa! I wish green pointers had that kind of efficiency! How powerful is this laser?

-Mark

 

[Montana64]

This LASER was on Mythbusters. The episode where they were busting the Moon landing conspiracy theories.

Ted

 

[Montana64]

50% crystal efficiency?  Whoa!  I wish green pointers had that kind of efficiency!  How powerful is this laser?

-Mark

"A double-pass amplifier boosts
the cavity-dumped 1 mJ pulse to
250 mJ, after which a second-harmonic-generator crystal
frequency-doubles the light at approximately 50% efficiency to produce 115 mJ pulses of 532 nm
light with pulse widths of about 90 ps (FWHM). "

Ted

 

[Montana64]

ok......so how does a mJ compare to a mW? :

Ted

 

[Hemlock_Mike]

That is a good homework problem, M64 !!!  Do the math and report to us tomorrow.

Mike

 

[rocketparrotlet]

ok......so how does a mJ compare to a mW?   :

Ted

I'm studying this in Physics right now. Joules and watts measure different things, so they cannot be compared. A joule is a measure of energy, and a watt is a measure of power.

1 watt = 1 joule/sec.

-Mark

 

[Montana64]

[quote author=Montana64 link=1226000880/0#5 date=1226016976]ok......so how does a mJ compare to a mW?   :

Ted

I'm studying this in Physics right now.  Joules and watts measure different things, so they cannot be compared.  A joule is a measure of energy, and a watt is a measure of power.

1 watt = 1 joule/sec.

-Mark  [/quote]

So, 1 mJ = 1 apple ...... and 1mW = 1 orange? can't compare apples to oranges!! ;D

BTW, thanks Mark

Ted

 

[randomlugia]

On mythbusters they said one gigawatt.

 

[Switch]

You can calculate the power within the duration of the pulse. 1mW = 1mJ/1s
The laser can deliver 115mJ in 90 picoseconds, so thats 115mJ / 0.00000000009s = 1.277.777.777.777 mW ~ 1.28GW  
But that just one tiny pulse.We can't calculate the CW equivalent or average power without knowing the frequency.

 

[Things]

there is no CW with that laser, its only pulsed. try and run it CW and you'll end up with a pool of molten metal on the floor

 

[Switch]

I said CW equivalent, meaning the constant power that a CW laser with the same average power would have.

 

[Cyparagon]

I got the same result Switch did.

 

[Oxymorphone]

So, what do you think will be the diameter of 200 mW red laser pointer beam, when and if it reaches the moon.

I guess some photons will reach the moon, right? I dont remember school physics, but I thought energy loss was smth like twice on every meter or smth like that, nevermind beam divergence...

 

[pullbangdead]

^I don't think a single photon would make it to the moon from your handheld, but not sure, I could be wrong. If it did make it, the beam would be massively wide. But you intensity cut in half every meter isn't quite right. Is your laser pointer 1/4 or 1/8 as bright by the time it reaches the oppostie wall of your bedroom? Probably not.

But let's do a rough calculation. For instance, if we say your thinking is correct and you lose 1/2 of the power every meter, and you start with 200mW of output. that's .2W, or .2 J/s. A single photon of wavelength 650nm has an energy of just over 1.9eV, or, just over 3(10)[sup]-19[/sup] J. So if you're dividing your .2 J /s by 2 for every meter traveled, we find that after 60 meters traveled, only 1.8(10)[sup]-19[/sup] J/s are still going. But oops! Photon energy is quantized, you can't have less than the energy of a single photon, it's either zero photons, or it's one photon. So if the energy still going is less than the energy of 1 photon, then it is actually zero energy and zero photons. Zero photons left after only 60 meters. Again, cutting in half each meter can't be right, or your 200mW laser wouldn't reach the other end of a football field. The math holds, so the inputs must be wrong.

But the same calculation can be done for whatever percentage of light is not transmitted by the atmosphere, say 1% maybe? If 1% of light's energy is lost per meter, there will be no light left at around 4084 meters, or 13400 feet. Now, that's not even as high as high as commercial airliners fly, so this number probably isn't quite right either. But is shows the idea that since the decay is exponential, and the energy of light is quantized (and therefore can't get below a certain value or can't get infinitesimally small), that all the light will get absorbed in relatively short distances. That's why they have such a huge laser for the moon measurements, you need MASSIVE amounts of light to make it through the atmosphere, especially twice.

If someone has the exact number for the transmissivity of the atmosphere for 650nm light, I can do the numbers, or you can, it's just: (power of the laser)/((1/fraction transmitted per meter)[sup]d[/sup]) = energy of a single photon, in this case 3(10)[sup]-19[/sup] J. When you get a high enough number for d, which is the nuymber of meters that will result in no light being transmitted anymore, that d is more than the thickness of the atmosphere, then you can hit the moon.