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Low-divergence laser pointer recommendation?

grw110

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Hi, I'm a complete and utter newb at laser pointers - but, scientifically inclined and an engineer by training. I'm hoping for a recommendation for an inexpensive consumer laser pointer with the lowest divergence that's reasonably possible.

The application is for a science project for my teens. Basically, it involves sending a laser beam over a long distance (1-2 miles), having it bounce back via a corner-cube retroreflector, and detecting the arriving laser light (hopefully via the naked eye / protected eye). My quick and possibly flawed back of the envelope calculations show that laser pointers with divergence rates in the 1mRad range will have a beam spread up to about 1-2 meters across upon arrival. Anything I can do to reduce that would be ideal.

I'm also hoping not to break the bank. Used gear is fine - as is using a lens to focus the beam.

thoughts?
 





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Hi, I'm a complete and utter newb at laser pointers - but, scientifically inclined and an engineer by training. I'm hoping for a recommendation for an inexpensive consumer laser pointer with the lowest divergence that's reasonably possible.

The application is for a science project for my teens. Basically, it involves sending a laser beam over a long distance (1-2 miles), having it bounce back via a corner-cube retroreflector, and detecting the arriving laser light (hopefully via the naked eye / protected eye). My quick and possibly flawed back of the envelope calculations show that laser pointers with divergence rates in the 1mRad range will have a beam spread up to about 1-2 meters across upon arrival. Anything I can do to reduce that would be ideal.

I'm also hoping not to break the bank. Used gear is fine - as is using a lens to focus the beam.

thoughts?

Hello grw110, I suggest you add your location to your profile so it shows under your name or you will get fewer answers especially if you ask what and where to buy something. This is a very global forum, people need to at least know what country your in. I am guessing your in the U.S. since you said miles instead of kilometers. If you decide to stick around I also suggest you post in the welcome area telling a little about yourself so people can get to know you better.

Putting a dot on something up to 2 miles away that's visible to the naked eye is easy and you don't even need a reflector. Some of us have lasers that can put a dot on the clouds at night (don't do that anymore here in the U.S. Unless in a very remote area) I have seen video of a laser taken from 8 miles distance, there was someone here who had his friends go to the side of a hill I forget how many miles while he pointed the laser at them and it was about 10 feet accross and very bright. A 1W green laser (very expensive) would be visible at 80 miles or more, depending on atmospheric conditions such a laser could even be seen from the ISS (don't point a laser at the ISS).

You didn't say what you mean by inexpensive or without breaking the bank. In USD your talking about $150 and up, a few hundred would be better, and you may want a beam expander, I think they are about $100 - $350, so $150 - $1000, am not sure a $150 445nm/450nm blue would do it but I think so. This would be at night, for daytime you would need a 532nm/520nm green and the distance of visibility would be much less in daytime. A beam expander gives you a larger beam to begin with but divergence is improved, at that distance it will make a smaller dot, there is some power loss from the additional optics, not sure you need one anyway at only up to 2 miles.

Alan
 

grw110

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Pi-r2: Wonderful info. Many many thanks!

Yes, good guesswork - am in the US; Seattle to be specific.

And right, by 'inexpensive' I am hoping for something less than $250-ish, total.
By the way, the inspiration for the project comes from the story of finding Lunokhod, the '60's-era Soviet lunar module that was thought lost but found with some neat laser work. Story is here if you haven't read it - a fun read: UCSD Physicists Signal Long Lost Soviet Reflector on Moon | Solar System Exploration Research Virtual Institute

I'm more inclined based on your info to go with the less-expensive blue... are there any particular models / retailers that come to mind? As mentioned, I'm fine buying 'used' - unless that's a bad idea i.e. there's undue risk in a used purchase of getting something banged up or truly less-divergent than needed.

Thanks!
 
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Only problem with 445nm+ blues (unless low power single mode) is due to being multimode their divergence is relatively high compared to a DPSS 532nm green. Also, a relatively inexpensive 100mw 532nm green will be 10 times brighter to the eye than a blue or red laser due to the sensitivity of the eye to green which is much higher. Last but not least, 532nm DPSS green lasers are typically 1.2 to 1.5 mRad of divergence, high power red or blue multimode diode based lasers much higher, 1.8 mRad and up, depending upon the design, 2.5 mRad is not unusual for blue, up to 5 mRad for red.

If you want to use a high power blue, I recommend getting a 10X beam expander for it. Some manufacturers such as Jetlasers offer beam expanders for their lasers. This will reduce the divergence to be better than a 532nm DPSS green laser, but you will have a fainter beam, milliwatt to milliwatt compared to green. A way around that is to use a high power blue such as 2 watts output with a 10X expander. Of course, the blue 445nm high power lasers, as well as a 100mw laser, need to be handled with great care, both power levels are dangerous and can blind, the blue extremely efficiently.

In my opinion, best of all is pairing a 532nm DPSS laser with a 10X beam expander which are available from just a few milliwatts of power all the way up to about a watt. With a 532nm DPSS green lasers you are already starting out with a laser beam which is brighter to the eye than red or blue, lower divergence too which with a 10X expander, reduces it far below what you would get with a blue multimode diode laser even if it were using a 10X expander. The reason for this is if you start out with low divergence and use an expander, i.e. 10X, the divergence is reduced by about 10 times less, so starting out with a high amount of divergence, you get 1/10th with a 10X expander, sure, but it won't compete with a 532nm green laser which had a relatively low divergence to start out with.

If you are considering a 520nm green multimode diode based laser, they also have poor divergence without a beam expander, compared to DPSS 532nm green lasers, but due to green being far more visible to our eyes than blue, a good trade-off, but in my biased view, only if using a 3X or higher beam expander to put them close to what you could get with a 532nm DPSS laser. Put a 10X beam expander on a 1 watt 520nm green multimode diode laser and now you have something impressive, the beam might be 25mm thick, but it will go a long distance without much spread and at that much power, will light up the night sky with an impressive beam. Even a 100-150mw 520nm multimode diode laser with a 10X expander does a good job lighting up the darkness, nothing wrong with that.

One last word, your project won't work if using a multimode diode based laser at any color, unless using a good beam expander, the end of the beam from one of those just fans out and dilutes in a huge spread at two miles. I've tried to put a spot on a cloud at 6000 feet using a 2 watt blue 445nm multimode laser and couldn't do it, but I could with a 100mw 520nm green single mode laser (50mw laser diode being pushed to 100mw out) which did not require a beam expander due to single mode diodes having such low divergence, even better than some of the 532nm DPSS lasers. You might try a single mode 100mw output 520nm laser with a beam expander on it to make an even tighter spot, it will deliver FAR more power to a spot two miles away than a 2 watt blue multimode diode based laser.
 
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grw110

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Hi,

Alaskan and Pi-R2 - just a quick note to say _thanks_ so much for the outstanding advice. Alaskan, it was great to get the detail in your post - invaluable, as I've mentioned previously, for a newb like myself.

A quick question - do you have a suggestion for either where to buy, or what to look for, in an expander that is most likely to work for my application?

As mentioned previously - am just hoping to send a beam about 3 miles, round-trip, and have it be visible to the naked eye (protected eyes, actually) on its return. As mentioned in the original - this is all for a science project with kids, akin to what they did with Lunokhod a while back, but on a much smaller scale. I've settled on a 532nm green laser, and am going to see how much progress i can get from something very low-power, before turning to higher-power (since the project involved kids, I don't want to jump into higher-powered lasers unless I have to - there are more logistics with juggling eye protection for everyone, etc.)

I don't have sufficient volume of posts to the forums in order to post a "WTB" / wanted to buy message. I see things on eBay that don't break the bank, such as this - but don't know if there are any considerations (applicability for 532nm? would it be a good idea to buy 2, so that the returning beam can be 'reduced' to a smaller diameter and hence made more visibile?)

Thanks. A short version of my question is if the eBay link above seems like a safe bet - if so I'll pull the trigger. All the other gear, including a nice corner-cube retroreflector, has arrived.

thank you!
 
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You won't need eye protection outside bouncing a beam off something a mile and a half away. You probably wouldn't even see it. Only inside when viewing the dot up close such as pointing it across the room or when burning something or measuring with an LPM is the dot too bright to look at. The beam is safe to view. Be sure not to do your experiment within 10 miles of an airport, especially not Sea-Tac.

Alan
 
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...A quick question - do you have a suggestion for either where to buy, or what to look for, in an expander that is most likely to work for my application?

If buying a beam expander, a 10X or higher is what I'd buy, perhaps as low as 8X. Which one depends upon your pointer, once you decide on a pointer, we can find an expander and a way to couple to it, but getting them to line up correctly is a pain, unless designed for your specific pointer. The best thing to do is just buy a pointer which offers a beam expander as an option, like JetLasers, I suppose there are others who do at a good price, but that's the company I like. Maybe Lasterbtb has a 532nm 100mw pointer with an expander designed for it? I'd stick with either DPSS 532nm or a single mode 520nm laser pointer, but the DPSS 532nm units come at higher power than single mode diodes can produce.

One advantage to 520nm single mode diodes is they are not temperature sensitive like the DPSS which can't get too cold, or they don't want to put out much, if any power. The 532nm DPSS lasers are best used near room temperature.

The airport warning is a good one, you have busy skies in your part of the country. For what you want to do, just keep it well above people but below the sky if you want to spot something on the ground.
 
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I have seen video of a laser taken from 8 miles distance, there was someone here who had his friends go to the side of a hill I forget how many miles while he pointed the laser at them and it was about 10 feet accross and very bright. A 1W green laser (very expensive) would be visible at 80 miles or more, depending on atmospheric conditions such a laser could even be seen from the ISS (don't point a laser at the ISS).

I think the video you're referring to was called "The Dangers of a Spartan 1W even At a 9km distance" (so 5.6 miles)

I'm pretty sure you're familiar with the experiment conducted using the Arctic Spyder 3 (1W) where they pointed it at the ISS (called "ISS FLASH PROJECT"). Here's a link with the picture taken by an astronaut from the ISS
http://www.wickedlasers.com/catalog/view/theme/techno/image/space-illustration.jpg
Here's a link with the youtube video if anyone hasn't watched it yet:
https://www.youtube.com/watch?v=2UoY15WDuHQ

There's also a thread about it somewhere in the forum.

Let's not forget that (if I remember correctly) the ISS is orbiting the earth at 400 kilometers above sea level! (248,5 miles!)
 




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