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Thread: Thinking outside the box

  1. #1
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    Laser Warning Thinking outside the box

    Hi all,
    I'm new here & have absolutely no knowledge or history of lasers, however, I do use them for work.

    This is more of a feasibility question that I wish to evolve before putting it to our engineering dept.

    I would like to narrow down the design options from my ignorant perspective to a more realistic one.

    I would like to connect up 50+ green laser pens to a 50m power cable, the laser pens need to have as much power as possible with a broad beam.

    Then it needs to be cheap, since initially this is a prototype.

    What I'm looking for is a few markers to guide my uninformed imagination in the absence of laser knowledge.

    I know its vague, but bare with me.

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    Z-Bolt for in the future, they do volume discounts on quality green modules.

    Steve
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    Lightbulb Details please...

    Quote Originally Posted by Gimball View Post
    This is more of a feasibility question that I wish to evolve before putting it to our engineering dept.
    We need a lot more information before we can determine if something is feasible or not. (Let alone practical, or affordable, or legal, or SAFE.)

    1) What are you trying to accomplish? Be specific. First, is it a visual effect? Or does it have a utilitarian purpose. (This matters! If a visual effect ends up looking lame due to local conditions / budget constraints, that sucks, but there's no real harm done. If you are relying on this idea to perform a specific function, however, and it fails to do so, then the consequences are more serious.) And then, what *exactly* do you expect it to look like and *how* will that look accomplish what you want. (Because lasers don't always look/perform/behave like people think they do, so even if you can describe how you want it to look, you may not recognize that such a look may not satisfy the need you have in mind.) That's why we need specifics. Don't worry - we're not going to steal your idea.

    2) What is your budget? Both for the prototype and the finished product. (Assuming that it performs as desired and solves whatever problem or accomplishes whatever effect you described in step 1 above.)

    3) What is the location, and what are the local conditions, where the laser(s) will be used? (Indoor, outdoor, control of light level?, temperature, scattering medium present, etc...)

    I would like to connect up 50+ green laser pens to a 50m power cable
    Again, why? What is the purpose? Are you trying to use the lasers as markers for the cable? Or are you trying to create an effect where you have multiple, parallel beams in a line? Why Green?

    The better you describe the desired outcome, the better we can advise you on how to proceed. Right now we're just guessing, and that doesn't help anyone.

    the laser pens need to have as much power as possible with a broad beam.
    This is meaningless without context. "As much power as possible?" Well, it's possible to put a 9 watt blue diode in a small brass mount and splice the diode leads onto a cable... It would be grossly unsafe, however, and it would also likely overheat in short order, but it *is* possible.

    Why "broad beam"? Are you worried about eye exposure? And how broad? At what distance? Note that a simple maladjustment of the primary collimating lens on a laser diode module will give you a beam like a flashlight...

    Then it needs to be cheap, since initially this is a prototype.
    How cheap? Are you talking $100 or $10,000? Can the prototype be scaled down to save cost while you prove the concept? And what about personnel safety? Will there be any chance that human eyeballs could be exposed to this? (Mitigating safety hazards will add to the price.)

    What I'm looking for is a few markers to guide my uninformed imagination in the absence of laser knowledge.
    We're happy to help, really. And we can not only guide you in the correct direction but also explain pricing, difficulty, and safety issues along the way. But we have to understand what you want to do first, and understand it well.

    I know its vague, but bare with me.
    Sorry, but vague doesn't cut it. Too many unknowns, too many variables, and far to many safety risks involved. No LSO worth his salt is going to tell someone to just string up a bunch of high power laser pointers on a cable without a lot more information first.

    Adam

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    Buffo

    Many thanks,
    The idea is to create a laser fence, Certain species of fish respond to Green laser fencing, its a method of sorting & separating fish by age/type using laser gates.

    Lasers will hang down from a power cable pointing vertically into the water.

    Its a great way of holding some fish species in rivers between certain points.

    Cost wise, the budget is what it costs to make the best & cheapest version of the idea since its so effective.
    if its not possible with $10 lasers because they over heat after 30 seconds then $100 lasers it is & so on.

    In reference to the broad beam, I'd like the beam of light to be as thick as possible, if I have the choice between a 3mm beam & 30mm without an exceptional jump in price, i'd go for the latter.

    Is there a chart which explains the numbers & displays power/range/brightness.
    Its my early understanding that a 1w Laser can be quite variable

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    why not use a green only projector and scan a fence of green beams into the water. Might be a simpler and easier approach. You could build it pretty cheap considering you would not need a very fast scan set for what you are doing. You could probably get a whole projector for under a grand that would work just fine for what you are planning. If it has an sd player on it you could just put the ilda file with the fence pattern on it turn it on and viola LASER FENCE !!!! Of course this is all dependent on how wide a fence you need?

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    Quote Originally Posted by coil1002 View Post
    why not use a green only projector and scan a fence of green beams into the water. Might be a simpler and easier approach. You could build it pretty cheap considering you would not need a very fast scan set for what you are doing. You could probably get a whole projector for under a grand that would work just fine for what you are planning. If it has an sd player on it you could just put the ilda file with the fence pattern on it turn it on and viola LASER FENCE !!!! Of course this is all dependent on how wide a fence you need?
    ~
    If you use a scanner system, it would pretty much have to be submerged to get past MALUS' Law on dealing with variable reflection losses with regards to entry angle into the water. It would need an Antireflection coated window on the air side of the sealed projector box, with the window submerged and wet on the water side. Straight down will have a low loss from Fresnel reflection, and a 53 degree angle entry entry into the water will have a low loss for one polarization of the light.
    BREWSTER's Angle for water at index of refraction at 1.33 is about 53 degrees, this will have the other lowest Fresnel reflection.
    ~
    One way or another green DPSS lasers and laser diode based green lasers will need to be sealed very tight, prolonged exposure to water vapor will kill both laser diodes and the KTP crystals in the DPSS system. Water vapor may condense inside certain diode laser packages behind the protective window (Ask me how I know, cleaning the optics on a 445 Nichia diode package often kills them, no hermetic seal...) So long term you need a good physical package design.
    ~
    A submerged laser may allow higher powers to be used, avoiding the 5 or more percent reflection from the air-water interface. That reflection can be massive at certain angles, but the math is easy enough if you look up Fresnel Reflection and Snell's law. generally 5-15 mW could be perceived as eye safe, depending on beam diameter.
    The eye safety aspect is readily calculable, once a few quick measurements are made, and most nations allow for a hotter beam for industrial use.
    ~
    Unless the manufacturer tests, these days trusting the power level on the laser warning sticker is a crap shoot. Most will be much higher in power unless bought from a credible seller.
    I sent you to Z-Bolt for a reason, until you have a production line set up, you need good quality, tested units.
    ~
    Blinking the laser or having a color changing beam might be interesting, recent multispectral imaging studies on coral have found "hidden" markings using fluorescence on fish and coral in the visible spectrum. Recent data suggests many fish while not having perfect RGB color vision, can separate certain specific shades of red or green. Our eyes cant see them without filters,, but a highly adapted fish eye often can. Blue- Green light penetrates water well and causes some types of red-orange fluorescence that act as warnings or are attractive to mates. Its species selective, and just now becoming known in the bio community. (PBS Nova I believe had a special on that, that is why I remember it. The scientists were harvesting species looking for new efficient fluorescent molecules, and since I run a laser excited confocal microscopy lab which uses such dyes, it caught my attention!
    ~
    An aside for PLers, my work "projector" uses 404, 435, 488, 514, 561, and 635 or 670 nanometers and a dual pair of 6215s, for imaging tissue stained with multiple dyes, and each dye targets a specific biological function in cells. Two variable bandwidth tunable PCAOM/AOTFs look for the fluorescence excited from the lasers to up to 20 different dyes in tissue at one time. the second set of scanners lets us burn or bleach the dyes in the tissue at the same time we image it. It does this in 3D at the microscopic level, neat, huh?
    ~
    Yes Buffo, take some red-orange narrow band dichro filters the next time you guys have that Laser Experimenters Meeting with scuba on the reef in Honduras :-0


    Steve
    Last edited by mixedgas; 08-02-2017 at 07:24.
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    Can you please provide an example of the "scanner system"
    And what this might be projected onto?
    Last edited by Gimball; 08-02-2017 at 07:48.

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    Quote Originally Posted by Gimball View Post
    The idea is to create a laser fence
    Got it. How wide is the span of the fence, and how far apart do the individual beams need to be?

    Certain species of fish respond to Green laser fencing, its a method of sorting & separating fish by age/type using laser gates.
    Does this need to be set up/taken down several times, or is it something you set up once and then switch on and off as needed?

    Lasers will hang down from a power cable pointing vertically into the water.
    Will there be any people in the river (boaters, swimmers, etc)? Also, how deep is the water? (Beams may not be visible beyond a certain depth.)

    If I have the choice between a 3mm beam & 30mm without an exceptional jump in price, i'd go for the latter.
    You can make the beams wider, and even 30 mm is not out of the question. Making the beams wider also spreads the power out, though. That's good for eye safety if we're worried about human exposure, but we'd need to do some testing to see if the underwater scattering is adversely affected.

    Is there a chart which explains the numbers & displays power/range/brightness.
    Unfortunately, no. There are some rules of thumb, but there are many variables that will affect the apparent brightness of the beam. Given that we're also talking about the beam in water, that adds more uncertainty. Additional factors include the ambient light level (daytime vs night), the relative turbidity of the water, and the presence of substances in the water that might fluoresce (making the beams more visible).

    This project will likely require a good bit of field testing with one or two lasers to determine average propagation distance and "aversion response" for a given brightness level first. It would be great if you could do these tests in the area where you plan to use the fence. Then you could use that information to build your first prototype.

    Quote Originally Posted by mixedgas View Post
    If you use a scanner system, it would pretty much have to be submerged to get past MALUS' Law on dealing with variable reflection losses with regards to entry angle into the water.
    So long as the scanned angle wasn't too wide, he might be able to live with the losses that will be reflected at the water's surface. Consider the cost savings of one or two high-power projectors creating a "fence" of 250 beams verses the cost of 250 individual dpss modules.!. That alone would make me think very hard about trying to make a projector work, even if it had to be above the water. (I agree that a submerged unit would be ideal, if not for all the problems involved in water-proofing it.)

    Buffo, take some red-orange narrow band dichro filters the next time you guys have that Laser Experimenters Meeting with scuba on the reef in Honduras :-0
    Yeah, for a multitude of reasons! Brad and I did bring UV underwater flashlights though, which were quite handy. Hell, we go back in 2019 dude - why not come with us?

    Quote Originally Posted by Gimball View Post
    Can you please provide an example of the "scanner system"
    Essentially he's talking about a laser projector. So you'd have a high-power green laser and a set of scanners. (moving mirrors driven by galvometers) The galvos would position the beam at each location where you want a "picket" for your laser fence, and the laser would then turn on for a short period. Then the laser turns off, the mirrors move to the next position, and the laser turns on again. All this happens several thousand times per second, and you end up with a "fan" of beams (or a ring, or square, or whatever shape you want) that make up your laser fence.

    Unfortunately, since all the beams will originate from a single point (where the scanners are located), the pickets won't ever be parallel. They'll instead be splayed out in a fan, and as you get further away from the source the distance between individual beams will increase. This may not be appropriate for a "fish fence" however - you'll have to think about that some. If the depth isn't great (say, 5 to 10 ft) and you can mount the projector very high in the air (say, above 20 ft), then the difference in beam spacing will be reduced.

    The other problem with this approach is that you will lose some of your power to reflection. The closer the beam is to being perpendicular to the water's surface, the more power will penetrate into the water. But as the beam angle moves away from perpendicular (in either direction), the amount of light that is reflected will increase. Eventually you will reach a point of near total reflection, which is why you would need to keep the scan angle as narrow as possible. Mounting the projector higher will help.

    And what this might be projected onto?
    Essentially the "projection surface" would be the bottom of the river. But in practice, all you care about is the light that is scattered by the river water as the beam propagates from the surface of the water to the bottom of the river. (The resulting line of dots on the river bottom isn't what we're concerned about.)

    So to recap: we need to address any safety concerns first. If you want to go with a projector mounted in the air, you must keep people out of the area, or else you will need to ensure the projector's output is eye-safe. (And if it is eye-safe, it is unlikely to be bright enough to create the "fence" effect underwater that you want.) Will you have the ability to enforce an exclusion zone on the river when this fence is being used? If not, one other way to address the safety issue is to go with individual laser modules mounted right at, or just below, the water's surface, although for large numbers of beams this may not be cost-effective.

    We also need to know how long the fence will need to be, and how many pickets (beams) the fence will have. That will also give us the picket-spacing distance, which will figure in to the calculation of how high the projector would need to be mounted (assuming a projector is the ideal solution - we don't know yet).

    Another key issue is that we need to know how deep you need the fence pickets (the beams) to be. And when we have that distance, it's time to do some underwater tests to verify that a beam of a given power level can actually make a beam that is visible underwater for that distance.

    Suggest that you test with a normal, tightly-collimated beam (say, 3 or 4 mm) and also with an expanded beam (think reverse-telescope) so it's still collimated, but perhaps 20 or 30 mm. See which ones are more visible. (Better still if you can test with actual fish, but that's probably a longshot...)

    Then we can look at what it would cost to build an array of X number of beams that can penetrate to X depth. (Which will quickly let you know if it's better to think about multiple, single-beam lasers or a few scanning projectors.)

    One technical question: how important is the wavelength? DPSS green lasers will give you 532 nm. Direct-injection green diode lasers will give you 520 nm in a more compact and durable package. Also, what about other colors altogether? There are blue diode lasers (direct-injection) that are *very* affordable and will deliver several watts of 445 nm light. So even though it might not be the most optimal color, if it will still work - even if you need more power to make it work - it might still be cheaper to use blue.

    Adam

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    Why not take advantage of the property of diodes to have a fast and slow axis. Use a cylinder lens to make a line. This will give a wide but thin beam to maximize brightness. Also pulse the laser to keep it cooler. Now you need less lasers. Seal the whole thing is heat conducting epoxy is an aluminum bar. Then seal the front optics with a clear waterproof epoxy. Submerge and instant water cooling. Other option is fiber optics with cylinder lens on endo fibers. You might be able to use high power leds and lens again to do the same. Try dropping a few electrodes in the light fence so the fish sense a wall and see the wall. Few millivolts. Nothing crazy.

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    ... maybe super-bright green LED's could be used instead of the lasers? - would be much safer in handling ...

    Viktor

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