Lasers - Page 1
Forum section: electronics / projects

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 09:32:58 am
content: Some basic questions.We fire a laser to the moon. What is the width of the beam at the surface of the moon?Basic on that width of the beam, what kind of equipment do you have to aim the beam towards a particular spot on the surface? Or I presume you can sweep th esurface hoping to hit your mirror whichis placed on the surface by men in yellow hats, sorry , by Apollo austronauts.OK so you have trained your beam to the surface of the moon, with a good accuracy so that you are spot on the mirror . Can someone even estimate the level of accuracy required?OK so you have now hit the mirror, not a random rock, but your mirror. Now you get a reflection back. How do you adjust the mirror on the surface of the moon so that it reflects precisely back not only to earth, but to the point of origin and not say to the top of the Himalayas? Imean what would the degree of error would be on the mirror's plne to miss earth completely let alone return the beam to a specific reception point?Aren't there lots of rocks on the moon? Maybe some random rocks smooth enough to reflect light? Aren't you likely to get something back if you do fire radio waves at the moon with all those random rocks?

author: matseng
author_rank: Frequent Contributor
timestamp: October 19, 2015, 09:40:33 am
content: Google and wikipedia is your friend....https://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experimentAt the Moon's surface, the beam is about 6.5 kilometers wide and scientists liken the task of aiming the beam to using a rifle to hit a moving dime 3 kilometers away. The reflected light is too weak to be seen with the human eye: out of 1017photons aimed at the reflector, only one will be received back on Earth every few seconds, even under good conditions.

author: German_EE
author_rank: Super Contributor
timestamp: October 19, 2015, 10:09:20 am
content: The reflector placed on the moon has optics that direct a beam of light back in the direction it came no matter what the angle:https://en.wikipedia.org/wiki/RetroreflectorThis makes aiming from Earth a lot easier.

author: dmills
author_rank: Super Contributor
timestamp: October 19, 2015, 10:19:32 am
content: One of the things to understand is that you are diffraction limited at that distance, so using telescope optics to expand the bean to a meter plus in diameter at the source is necessary if you are to stand any chance (Makes the spot on the moon smaller).Lots of laser power, probably Q Switched and detected with some sort of lock in arrangement I would suspect.Note that pointing Q Switched lasers into the sky involves paperwork or the aviation crowd get very sniffy with you.This is very much not trivial.Radio is actually easier, and HAMs routinely communicate by using the moon as a reflector for VHF, UHF & Microwave links, where it is known as 'moonbounce', but it suffers from the fact that the moon is not a plane, so exact ranging is problematic.73, Dan.

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 12:01:22 pm
content: How bright would a light/LED on the surface of the Moon need to be in order to be detected with ordinary telescopes from Earth? Would such a device using solar panels and bright LEDs and some sort of Morse code or other simple encoding fit in a reasonably small enclosure (say carried by one man)?

author: hayatepilot
author_rank: Frequent Contributor
timestamp: October 19, 2015, 12:32:57 pm
content: Quote from: akis on October 19, 2015, 12:01:22 pmNo.

author: matseng
author_rank: Frequent Contributor
timestamp: October 19, 2015, 01:46:27 pm
content: No. You can't make a handheld unit that can turn the moon into a batman signal... :-)

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2015, 02:07:33 pm
content: See:https://what-if.xkcd.com/13/Steve

author: MikeW
author_rank: Regular Contributor
timestamp: October 19, 2015, 02:20:11 pm
content: Quote from: LaserSteve on October 19, 2015, 02:07:33 pmInteresting. But it has to be said that making a light that can be picked up by a telescope and illuminating the entire surface of a planet to a greater level than the star it orbits are two fairly different tasks though.

author: Marco
author_rank: Super Contributor
timestamp: October 19, 2015, 02:42:06 pm
content: It gives us some useful numbers, a good AO telescope will have <100m resolution to the moon.Lets assume the moon reflects 100% of the sunlight diffusely then you'd need about 10 MegaWatt of non directional light to double the light intensity of a pixel. This is not a problem, at low duty cycle, a slightly directional small'ish Xenon strobe should be enough to get a signal through without any significant signal processing.Just need a couple 100 Million bucks telescope first.

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 03:04:30 pm
content: How much light intensity is needed to be shone from the moon, to be visible from earth, when the moon is dark, using a commercial scope?Can someone do the calculations?A reasonable man can carry 12.5kg of weight over moderate distances. A bag of salt/sack of cement/concete block (before the health and safety crap) weighs 25kg and is a chore to move around but certainly doable, as construction workers used to carry them up 5 stories on the scaffolds. So say half that so as to be able to move it from my lunar module to a reasonable distance. On the moon this is 6 times that allowance, therefore say 80kg on earth.The Russian lunar rover had batteries and a solar panel, as well as scientific equipment and a motor to carry it over miles of distance. It weighed 840 kg. We only need a battery, a solar panel, and a bright light of some description. What can we fit inside 80kg?

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2015, 03:06:58 pm
content: OK, You want some numbers:www.iers.org/SharedDocs/Publikationen/EN/IERS/Publications/tn/TechnNote34/tn34_097.pdf?__blob=publicationFile&v=1[url=http://physics.ucsd.edu/~tmurphy/apollo/basics.html]http://physics.ucsd.edu/~tmurphy/apollo/basics.htmlI used to install lab lasers, so I can comment on the source they use with a 1 meter telescope.A 300 mJ Frequency Doubled Yag laser with pulse compression option would be 50,000 to 70,000 USD.It needs about 7 Kilowatts of power to run at a repetition rate that would appear quasi- continuous to the eye.That would be a 700 to 1000 picosecond, 532 nm, green, pulse with 350 mJ of energy, max rep rate would be 10, maybe 20 Hz.Not cheap... And no where near enough brightness to be seen without a 1 meter class telescope st each end, and photomultipliers.What the OP is also missing is there is no convection cooling on the moon, so not only do you need a power source, you need cooling.So lets get crazy, find something that would fit in the sack, be powered by batteries, and be visible if you knew where and when to lookIf you want it to fit in a sack of cement, your best choice would be a plutonium based nuclear bomb, surrounded by something dense to form a plasma, as there is no air to heat to incandescence on the moon. You get one flash pair per blast, plus associated afterglow from the moon's surface.Otherwise your looking at something like a big stack of 300KW Vortek water cooled arc lamps, and the associated telescopes, and small nuclear reactor to power them. Along with a not so small heat exchange system.You would be better off placing a few square kilometers of mirror on the moon.This guy did the math, which suggests that perhaps a hundred or so 300 Kw Vortek arcs might might make you a mag six star.http://www.science20.com/robert_inventor/could_you_see_moon_city_lights_or_a_greenhouse_from_earth_just_for_fun-157480[/urlAnd he's neglecting skyglow on Earth.Steve

author: MikeW
author_rank: Regular Contributor
timestamp: October 19, 2015, 04:00:44 pm
content: Just for a laugh I did some fag packet numbers too.What if we were to try to do it with LEDs?A magnitude 6 star has illuminance of about 0.000000008 lux. This is the (rough) naked eye limit so let's add a zero and now we are in the big telescope range.The moon averages 381000000 meters from earth.Let's be magically generous and pretend we can get our beam angle down to 10 degrees with zero losses.You will need a light source of 2776571.91 lumens.At a very efficient 150 lumens per watt, your led would need 18510 watts of power.At a more 'realistic' 30 degree beam angle we need 24862519.68 lumens and 165750 watts of power

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 04:21:36 pm
content: Quote from: MikeW on October 19, 2015, 04:00:44 pmGood stuff.So in very very short, sending VHF (or whatever) from the moon can be done with portable batteries, and can be received with a portable device, but shining a light, even for a few seconds requires hundreds of watts of power ?What size telescope have you calculated in? "big telescope range" ?

author: MikeW
author_rank: Regular Contributor
timestamp: October 19, 2015, 04:26:06 pm
content: That's just a bit of a fudge really.I figure 10% of the light level the human eye can see is a pretty good approximation for most telescopes.It's just to show how ridiculous the numbers get very quickly. Let's say it was 1% or even 0.1%, the numbers are still ridiculous to achieve.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 19, 2015, 04:55:37 pm
content: Nobody would call four 11 Element 144 MHz Yagi antennas in a Quad Array portable.However a sat phone to low earth orbit only needs a few watts and a folding dish antenna.Its when your signal source has 2.5 seconds round trip flight time to diverge, inverse square law starts to take quite a toll.Steve

author: dmills
author_rank: Super Contributor
timestamp: October 19, 2015, 06:50:55 pm
content: I don't know Steve, we set up a bigger array then that (But on 70cms) in a field for the chaos computer clubs summer camping trip this summer (And used it for EME), now granted the CCC can be a little bit hardcore upon occasion, but still..I am not convinced that moon to earth with handhelds is really a practical proposition, unless maybe using one of the totally down in the noise modes.One way from the moon is obviously easier, doubly so if running one of the very low bandwidth weak signal modes.By symmetry a 1M telescope with a laser behind it on the moon will throw a ~ 6.5Km spot on the top of the earths atmosphere, rather better.If going for the nuclear option, surely one could increase the bandwidth by placing some carefully doped rods between the nuke and the earth so you could get several bits per bomb by looking for the xray pumped lasing lines in the few tens of nanoseconds before the first optical flash from the bomb?Regards, Dan.

author: mark03
author_rank: Frequent Contributor
timestamp: October 19, 2015, 07:08:21 pm
content: Quote from: MikeW on October 19, 2015, 04:26:06 pmThe  human eye has an aperture of approx. 7 mm so your hypothetical telescope would only need a one-inch aperture to pass the 10% threshold. At perihelion, Pluto is easily visible in my 12.5" Newtonian, at roughly 14th magnitude. That is about 1600 times fainter than your 6th-magnitude star, so there are at least several orders of magnitude we can throw at the problem, before it starts getting expensive/difficult at the Earth end of things. One-meter class instruments aren't so exotic for amateurs anymore, so figure a gain of ~20,000 over the naked eye. Although, observing something this faint so close to the illuminated portion of the lunar disk would be quite difficult.Also, what is so hard about 10-degree beamwidth? Pretty sure amateurs go narrower than that for long-distance optical communications, using high-power LEDs with fresnel lenses.Admittedly, these numbers may not get you there. But on the surface of it, it doesn't sound that ridiculous to me.

author: akis
author_rank: Frequent Contributor
timestamp: October 19, 2015, 07:28:11 pm
content: Like the Russian modules, it can have a solar panel and can charge for days/weeks, if only to pulse an LED for say 10 seconds in the minute, for only a few minutes, so that amateur astronomers know it is it they are looking at.It can do that when the moon is completely dark, it does not need to compete with sunlight reflecting off the moon as well. Or maybe when it is in shadow with a margin of X miles, so it still can be seen even if it is a quarter moon (say).So what is the smallest light intensity we need to make it visible from the earth, using the best commercial telescopes?

author: MikeW
author_rank: Regular Contributor
timestamp: October 19, 2015, 08:10:40 pm
content: Quote from: mark03 on October 19, 2015, 07:08:21 pmSalient points.I was under the impression though that the narrower the beam the more losses you have through the lens?Or am I misguided there?

author: mark03
author_rank: Frequent Contributor
timestamp: October 19, 2015, 08:26:01 pm
content: Quote from: akis on October 19, 2015, 07:28:11 pmWhen the moon is completely dark, it is in conjunction with the sun as seen from earth. The moon is never in a dark sky unless it is somewhat illuminated, e.g. it needs to be > 18 degrees from the sun to still be above the horizon at the end of astronomical twilight. So, practically speaking we're looking at a quarter moon here.Sorry, I don't have the optics background to do the requisite calculations.

author: LabSpokane
author_rank: Super Contributor
timestamp: October 19, 2015, 09:38:54 pm
content: Before building a moon-illuminating laser, one really needs to establish a laboratory inside a dormant Pacific Island volcano.First things first.

author: Someone
author_rank: Super Contributor
timestamp: October 19, 2015, 10:35:59 pm
content: Quote from: akis on October 19, 2015, 04:21:36 pmRF has the advantage in s/n ratio as filtering the return signal to reject other frequencies is relatively cheap and easy (compared to the optical case).

author: LaserSteve
author_rank: Super Contributor
timestamp: October 20, 2015, 05:04:20 pm
content: "If going for the nuclear option, surely one could increase the bandwidth by placing some carefully doped rods between the nuke and the earth so you could get several bits per bomb by looking for the xray pumped lasing lines in the few tens of nanoseconds before the first optical flash from the bomb?"LOL, This monthsIdeas for DesignSubmission for EDN..."Nuclear Pumped Lunar Laser Communicator has Simple Frequency Division Modulation"I'd use green laser diodes or ablative flash lamps long before I'd use LEDs, for the simple reason that getting modern LEDs into less then a 7 degree FHWM beam Is difficult. Before I worked for the University I worked for an early LED flashlight company (Contributed to one Patent on LED Collimating Optics, sold 60,000 units)Here is some more math...http://browncubesat.org/wp-content/uploads/2012/08/XFT-Documentation.pdfLEO orbit is 250-375 miles for a Cubesat, the moon averages 238,900 miles give or take.So how did we drift from Naked Eye viewing to state of the art amateur telescope viewing? That 20K Gain or 38K gain from adding an image intensifier really takes away from the OP's nearly impossible goal of naked eye viewing of a man made light on the moon.The odds of some one funding this are comparable to my odds of meeting with, dancing with, and then marrying the lovely Peta Murgatroyd tomorrow morning before 10 AM.Steve

author: akis
author_rank: Frequent Contributor
timestamp: October 20, 2015, 05:29:10 pm
content: "So how did we drift from Naked Eye viewing to state of the art amateur telescope viewing?"****************It was always suggested thus from the beginning. Amateur telescope as in affordable and commercially available, rather than multi-million dollar reserved for rich universities and the military.

author: Gall
author_rank: Frequent Contributor
timestamp: October 20, 2015, 06:16:40 pm
content: Quote from: akis on October 19, 2015, 09:32:58 amDepending upon the beam quality and wavelength, could be between 2-3 km and 100s of km or more. Google for "Gaussian beams".QuoteMoving mirrors with precision piezo actuator, like the one used in scanning tunneling microscopes. Pretty useless, however.QuoteThe required accuracy is most likely impossible as it is smaller than the size of an atom if you want to do reasonably fine movements.QuoteThe returning beam will be distorted due to atmosphere refraction and diverges. What you'll get is no more a "beam", so you can't return it at all.A retrorflector is something that is at least usable.QuoteYou'll do. Provided you have a good TEM00 laser beam (not a laser pointer beam) and a good telescope, you may be able to see it.It all depends on the quality of the beam. Something like a good He-Ne TEM00 laser is needed. And oyu have to expand its beam with an telescope to one feet in diameter or so too keep the beam from diverging. The beam has to be VERY thick if oyu want to hit the Moon.

author: dmills
author_rank: Super Contributor
timestamp: October 20, 2015, 07:37:12 pm
content: Quote from: Gall on October 20, 2015, 06:16:40 pmHe-Ne has the fatal flaw in that it is pretty much inherently limited to <100mW or so, due to the triplet state quenching being effectively rate limited.Good beam quality, and real power? Tricky, tricky, maybe some sort of mode locked ion laser, but the efficiency sucks. Enormous power in ns pulses with sort of ok beam quality is actually very much easier.I do actually have access to a 1M class telescope, and I have an old greenlight HPS in the shed (And I am pretty sure I know how to hack around the famously annoying control system), but the beam is more then somewhat multimode (And air traffic control would not be pleased this close to Heathrow).Regards, Dan.

author: Gall
author_rank: Frequent Contributor
timestamp: October 20, 2015, 07:56:14 pm
content: I've seen a pulsed 10ns 1J 532nm laser with a very good beam quality. It was a Nd:YAG MOPA system with ring cavity and passive Q-switch at the master oscillator (50 mJ) and a two-pass power amplifier with Brillouin scattering mirror. The laser was capable of more than 1J (4J?) but never ran above 1J to prolong its life. It was used in the portrait holography.A He-Ne laser wasactuallyused to make a spot on the Moon in one of the early experiments. The power does not matter as long as you have reasonable way to see the light. Due to the very narrow spectrum, one may use a good filter to reject most of the white light.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 20, 2015, 08:12:55 pm
content: We had a laser show fellow who used the HPS's older predecessor who claimed with only 6 inch optics he could hit the moon and see the reflection by eye by binoculars. This mythical posting of his perpetuates on the internet to this day..I Hated to tell him about what he was seeing was trapped dust in the upper atmosphere doing Mie scattering and water droplets doing Rayleigh and Tyndall scattering. Down close to earth, say 2-3 miles, you get dust scattering of a laser beam, then there is a reasonably clear region, then a few more miles up is a concentrated layer of dust and water... Well above that is a layer of trapped dust and ions, including sodium, which is how the laser guide stars work, basically by resonant scattering and emission off the sodium ions.The answer is no, folks, the required power is extra-ordinary... LEO can see a more reasonable light source, a few watts if up-collimated by a small telescope.What most people don't also know, is while laser beams do stay relatively bundled together as they pass through the atmosphere, due to their coherence, once they enter localized areas of turbulence, they shed communications bandwidth like crazy. This is why laser sky scatter communications offer data rates at the low audio, or more or less slow speed Morse code bandwidths. Most optical comms distance records are set with LEDs, in order to break the coherence of the light source.Then you hit the LED's huge loss of optical power during collimation, which basically requires huge F1 lenses.So why do we not have free space laser communications:Yes, you can get a screaming open air data rate over a kilometer or so, till the fog sets in, it gets windy, or snows..Bell Labs was going so far as to try to make very straight buried vacuum pipes with re-focusing lenses ever few hundred meters, to get around this limitation, but that proved impractical. Then fiber optics was invented and hope appeared.The US Military and our Soviet/Russian friends spent tens of millions on space based laser to submarine communications research, only to achieve very low data rates, limited by noise... From issues with turbulence causing localized differences in the index of refraction in the air.So many myths perpetuated by science fiction.... Nothing I work with, and I've worked with MANY laser sources, goes "pew pew pew" either. A best a loud click or soft hum... Usually silent.. And I've been in the clean room of Terawatt and Pentawatt systems..Steve

author: akis
author_rank: Frequent Contributor
timestamp: October 20, 2015, 08:14:07 pm
content: we are talking about two distinct experimets, shine a laser to the moon and use a telescope to see it, or place a light on the moon and use a telescope to see it. I was interested in the latter.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 20, 2015, 08:21:10 pm
content: Akis,Its the exact same math, BOTH ways, up and down... No matter what the light source is...We've given you the numbers, yet you cannot see how impractical this is.Dream On, but if this was any where near practical, either the Apollo Manned or Soviet Probe missions would have done it.Steve

author: MikeW
author_rank: Regular Contributor
timestamp: October 20, 2015, 08:35:54 pm
content: Quote from: LaserSteve on October 20, 2015, 08:21:10 pmI'm not saying it's practical for one second, but surely one involves a two-way trip with reflection (and losses) and the other is a one way trip.How can that be the exact same maths?

author: LaserSteve
author_rank: Super Contributor
timestamp: October 20, 2015, 08:46:03 pm
content: The optical equivalent of the Friss Transmission Equation treats the scatter off the moon as path loss. When I say its the same math, its the same set of equations either way. The relative magnitude of the total light needed is similar. The divergence needed in the beam is about the same.I'll post some equations for the naysayers day after tomorrow if I can get to it... I have more serious work to do tonight...Steve

author: dmills
author_rank: Super Contributor
timestamp: October 20, 2015, 08:48:59 pm
content: Quote from: LaserSteve on October 20, 2015, 08:12:55 pmSeems somewhat unlikely, a quick orders of magnitude estimate confirms not even close.Is it just me or is the HPS optical bed one of the scariest things to work on, the 800 at least has those brass tubes so you know roughly where the many, many KW of IR intracavity power is (And the 800 has a sane lamp supply).QuoteWhat sort of glasses are appropriate up at the NIF sort of power levels?I actually remember a pulsed gas system back in the day that sort of did go "pew".... It used a self oscillating flyback as the charging supply and the whine from the flyback transformer would change in tone as the caps charged for each pulse, wish I could remember who made the thing.Regards, Dan

author: Gall
author_rank: Frequent Contributor
timestamp: October 20, 2015, 09:08:10 pm
content: Quote from: dmills on October 20, 2015, 08:48:59 pmThat was an error for sure. Seems not to be a fake, just misinterpretation. If one sends a beam in the direction of the Moon, chances are big that the observer sees some light "on the Moon" which is in fact just atmosphere scattering.There was a real scientific experiment about 40 years ago utilizing a large HeNe laser and a HUGE telescope. The reflection has been observed and was from the Moon for sure but I guess it was just above the detection threshold.

author: LaserSteve
author_rank: Super Contributor
timestamp: October 20, 2015, 09:33:59 pm
content: Dr Mills,I just helped a friend rebuild a 800 a few months ago.. The discoloration and burning of the brass tube where the intra-cavity beam was grazing the tube was amazing. My friend halted work until we upgraded his safety goggles after seeing that.As for the ultra high power systems,The required OD for any given amplifier sub-module is "The best you can get, and it had better have ANSI or Euro Certified Damage Testing", because you align the sub-modules at very low power. If your in the room, the lasing path is blocked to prevent lasing. Even then you worry about amplified spontaneous emission or the flash from the pump lamp.In our case Glasses are tested and certified for OD7- OD8 and shielding precautions are taken. The beam tubes have 6 mm or thicker walls. You are then out of the room for an actual full scale shot with all the modules combined, , so your OD answer is the cumulative OD of a double layer of concrete block.. plus 6.25 mm or more metal. Above 50 Joules we left the room....Hoya Corp loved us, because the first glass attenuator after the sampling pick-off was always crazing or cracking.On site safety briefings could take two hours and come with a quiz... Your in a clean room, and you are the major source of the dirt and dust, so actual time spent near the beam line is minimal. No one enters alone, either... Everything is briefed. This is not "fun", its very serious.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: October 20, 2015, 09:38:15 pm
content: There was a real scientific experiment about 40 years ago utilizing a large HeNe laser and a HUGE telescope. The reflection has been observed and was from the Moon for sure but I guess it was just above the detection threshold.With synchronous detection required, I would be wondering if it was scatter from the air or something. With a 2.5 second round trip, the patience required to align such a critical system would be amazing.Steve