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Thread: How to achieve the finest laser line possible with the least spread in the opposite..

  1. #1
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    Default How to achieve the finest laser line possible with the least spread in the opposite..

    Hi,

    I want to build a laser which has optics which can produce the finest line possible with the least spread in its opposite plane. How would one go about doing so? Just have as low a divergence as you can achieve without beam expansion and shoot that into a cylinder lens? The only way I can get a low divergence beam I know of with a laser diode (without using beam expansion) is to use a single mode diode with the smallest aperture you can get and then shoot that into a cylinder lens, hoping the lens doesn't destroy the divergence in the opposite plane, does it?

    Anyone who can think outside of my box willing to comment? Maybe I should just swing the beam across a mirror and produce the line that way with a low divergence laser? Anything better? What is the most amount of swing, in degrees, I can get with a vibrating mirror to produce a nice line? Should I use a flat mirror, or something else that rotates? I want to use the line as a long distance laser signal, something you can see flashing from a long distance away without having to be spot on target, not airplanes though, forbade the thought. Anyone know the fastest flash speed the eye can detect, as far as how short the flash can be? I suppose this really depends upon the intensity too.

    Thanks in advance! I need some help, maybe someone with projector knowledge can clue me in
    Last edited by Laser57; 04-03-2015 at 19:10.

  2. #2
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    There is no way to cheat the physics and you seem to suggest that you know that, but are hoping anyway. When you say you want to build a laser, you mean you want to project a laser. Right? If you are planning to stay with diode lasers then you are correct that a single mode laser may be the right choice. If the reason you are avoiding expansion is only because of the limited size of the available cylinder lens then you should expand to the largest size that that cylinder will accommodate.

    Another possibility is to use a multimode 445nm diode. In one plane the divergence is pretty high, but you could choose to use that plane for the expansion into a line. In the other plane the divergence is very low and if expanded to fill your cylinder the line would become finer still. These are very powerful diodes and you could reduce the line even more by masking the edges of the beam prior to expansion allowing you to expand it further and reducing the divergence further (at the expense of some power).

  3. #3
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    If you want the distant observer to see a flashing "beacon" then I would scan the beam at a frequency you want the flash to be. This would give the highest brightness to the observer (think lighthouse). I would use a collimated LED not laser (safety). If its a solid line you need, I would take a high power MM diode and shine it on a cylinder lens with no collimator. Use a cylinder with a Fl of around 50mm focusing the SA. This will give you about .25mrad X 35-40deg beam with MM cazio 445 diode.
    Last edited by logsquared; 04-04-2015 at 07:05.

  4. #4
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    Thank you for giving me some things to consider. I'm going to stick with a single mode laser diode for this project due to the lower divergence I can obtain from the get go using them.

    The use of a LED would work for shorter distances, but the divergence is horrible compared to what I can do with the output of a laser, especially at the distances I'm working on to flash the ISS, if they will cooperate and look. Here's the first beacon I built using some 100 watt R,G,B LED's: http://imageevent.com/qdf_files/tech...ndlaserproject - You can see some laser diodes in the unit too, they are all M140 445nm multimode laser diodes which spread too much due to their relatively high divergence, but much better than the LED's could do.

    To get to the low divergence I want, I won't be able to find a cylinder lens big enough because it appears the only way to get uber low divergence is if the beam is expanded to several inches in diameter. I was hoping there was a way of using a line lens to produce something close to .01 mRad of a divergence without the need to first expand the beam. I like the idea of using a M140 diode with one of its axis being so thin, but even then the mRad is much higher than I want. I had no idea that low of a divergence could be obtained by shooting the output of a M140 multimode diode straight into a cylinder lens, because of that I might use your suggestion for another project in a hand held unit, but for this project, I want much lower divergence.

    I've been playing around with some numbers to get extremely low beam divergence, if I start out with a single mode laser diode with a 5um aperture and let it expand to 150mm diameter and then collimate it, the divergence should be very low, but not sure if I should believe the number I'm seeing, can someone tell me what they get? If I expand a regular 2mm wide collimated laser beam with a known divergence of 1.5 mRad to 150mm diameter I'm seeing a number of close to .01 mRad of divergence, if I'm doing the math right.

    To help me get my bearings right I have a couple of questions, if someone can confirm, deny or straighten me out on these:

    Question 1: If shooting an un-collimated beam into a cylinder lens, through spreading of the beam to produce a line, does this action of the lens also reduce the divergence or thickness of the line as it travels forward through space, or is this property solely dependent upon the divergence of the beam before it goes through the cylinder lens?

    Question 2: I've been told the smaller the laser diode chip aperture, the lower the divergence. i.e., an aperture of 5um can produce a beam with 10 times less divergence than a diode with a 50um aperture, for a given sized beam width. True?

    Thanks!

    I sure appreciate the responses I've received already, if anyone can help me understand more of these things, thank you. I just started with lasers about three years ago, taking awhile to get my bearings on some of these things. I do a lot of googling and searching YouTube for answers, but some of my questions are outside of common discussion, it seems. Safety is number 1, I will have to use the device in the far out reaches away from anyone and use goggles too, as I live in Alaska that isn't too difficult to accomplish.

    Chris
    Last edited by Laser57; 04-04-2015 at 10:07.

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    I do not know your application, but with these dimensions, you will not be able to achieve this with scanning a point source to a line. The mirrors would be too large and the scanner too imprecise. So, its going to be a static system.

    The problem you will face is that the optical issues will not be limited to simple algebra as in a beam that is twice as large will have 1/2 the divergence WITHOUT LIMIT. I think that with a beam that is 150mm in diameter from a single mode diode, you might be able to get down into the 0.01mrad range, but "far less"? I doubt that. You will be up against spherical aberration and small misalignment's will cause coma. Lens imperfections in optics that large will introduce aberrations and with really big optics and long beam paths the rigidity of the optical bench becomes a real issue as well.

    Then there is the atmosphere to deal with. In astronomy, several arc second resolution of celestial objects is typical when most of the path length is high above the turbulence generating ground. Traveling long distances parallel to the earth will generate many, many times more optical distortion. One arc second equals approximately 0.005mrad. All of these complicating factors will tend to contribute to the divergence inherent in the design.

    What are you trying to do?

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    I'm wanting to see if I can get the cooperation of the international space station to look and see if they can see flashes from my laser if using a low divergence line, or moving the beam in a line across them in a fast oscillation. Before I can even ask through a university project, I need to know if the power will be enough if using a line beam, then I need to see if I can design it. If I can't build it, no reason to go further but I'm hoping a low divergence line beam will allow me to sweep across them without loosing too much power to be detected. I know this has been done before with a 1 watt 445nm laser, they saw it and a photo was taken, but not sure a line beam has enough power to be visible. I know that the lower the divergence, the more power is delivered so working off of that idea. The idea of using a line instead of a spot is targeting, I want to prove a line beam can be detected from space, if together the power is high enough with a low enough divergence. I have limits in what I can afford, a single mode diode might not have enough power less the divergence is extremely low.
    Last edited by Laser57; 04-04-2015 at 10:32.

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    Can you link to anything about the successful 445nm laser sighting, or give a lot specifics for the set up. Knowing how they detected it would help a lot. Furthermore, a line will spread the beam over hundreds of miles at their altitude. What about setting up a big Mead or Celestron to track them and allowing enough divergence to fill the tracking error circle. Consider using some power such as a several watt diode or DPSS green. Regarding safety. You will have to have a spotter and you may need to convince both the FAA and NASA that your intentions are "peaceful"

  8. #8
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    I agree, nothing will move forward until after I have found a way to do so with the equipment I can afford, certainly not without permission. I am also a pilot, have flown many hours and familiar with the laws regarding the use of lasers in open space which if held to the letter, we can't really do much in the USA outside, even below 5mw if within 10 miles of any airport.

    All I have been able to gather regarding the 445nm flashing of the ISS is this video:

    https://www.youtube.com/watch?v=2UoY15WDuHQ

    Edit: More info such as this can be found on the net:

    http://www.universetoday.com/93987/a...space-station/


    From an old ISS blog posting before the attempt shown in the video, above:

    Ironically, when earthlings can see us, we cannot see them. The glare from the full sun effectively turns our windows into mirrors that return our own ghostly reflection. This often plays out when friends want to flash space station from the ground as it travels overhead. They shine green lasers, xenon strobes, and halogen spotlights at us as we sprint across the sky. These well-wishers don’t know that we cannot see a thing during this time. The best time to try this is during a dark pass when orbital calculations show that we are passing overhead. This becomes complicated when highly collimated light from lasers are used, since the beam diameter at our orbital distance is about one kilometer, and this spot has to be tracking us while in the dark. And of course we have to be looking. As often happens, technical details complicate what seems like a simple observation. So far, all attempts at flashing the space station have failed.
    Last edited by Laser57; 04-04-2015 at 11:14.

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    " 15 characters"
    Last edited by Laser Wizardry; 11-13-2015 at 12:33.

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    I have a 4 watt output DPSS 532nm laser, perhaps I should consider using it if the beam is expanded enough, but targeting is my concern and thus the idea of using a line laser output instead. Also, I recently purchased a Special Optics 488nm 160X 1/2 wave accuracy beam expander I was hoping to use which can take as much as 15 watts input when at a 2mm beam width.

    Back with an edit:

    Maybe I can consider using several small servo mirrors all tied together with individual motors so most of my 150mm diameter beam falls on them? Those can be moved fast. Anyone have a suggestion on the largest mirror and servo I can buy at a reasonable cost to achieve that, since I would have to buy several of them? I suppose it would be cheaper just to hook some smaller mirrors all together with some kind of cogged rubber belts and spin them, maybe simpler to get a bunch of big mirrors and mount them all on a wheel and spin it, should do the same thing, then steer it in the direction I want a line painted in the sky. Not at all sure how the mechanics need to be worked it for this, still mulling the idea around.
    Last edited by Laser57; 04-04-2015 at 12:10.

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