Beam Width Question

[andreasjva]

I am curious about the spread of a laser beam over x distance, assuming the laser is as perfect as humanely possible.

For example, if my beam was 1 millimeter at the aperture, how big would it be 1 meter out?

Hope I'm asking that right.

I'm trying to figure out what the natural spread would be over this distance void of any focusing to that point.

 

[chazz]

Hi, and welcome to the forums! :)

Since no one had responded.. I will give it a shot... Disclaimer: I AM NOT AN EXPERT, so if you really want proper info, you probably should do more research.. wikipedia should have some good info or at least be a gateway to more scientific articles/papers.

To answer your question.. I would have to say, it depends.

I would guess a theoretically perfect laser, in a monochromatic/single wavelength, in a perfect vacuum, (i.e. no atmosphere, dust particles, etc.) would have little to no spread especially at 1 meter out. Even current, good quality lasers in Earths atmosphere, you would have to have some pretty specialized equipment to measure spread at 1 meter.

The beam of a laser is inherently 'focused' as it is emitted coherently or highly polarized, with gas lasers being even more polarized than others.

With real world lasers I am pretty sure there will be some beam spread, just not sure on how much. (again, it would depend...)

It is a very interesting subject (at least to me.. LOL, I am a geek) and one I wish I knew more about.. If you want to see some really cool stuff being done with lasers, check out the Lawrence Livermore National Laboratory: National Ignition Facility & Photon Science Division

https://lasers.llnl.gov/

They have some pretty cool info and videos. I actually stopped by there last time I was in CA, but it was the weekend so was not able to get the tour.

You might also google: 'measuring the distance to the moon with a laser'

They talk some about beam spread being up to four miles wide at the moon. But that is on average 238,897 miles away. :)

I am guessing if they were to try to design a laser for specifically narrow beam over huge distances, they probably could do a lot better than that. Although, there would likely be limited practical use for such a device?

 

[andreasjva]

Thanks chazz!

Yes, I know it's a very difficult question to answer on many levels due to the variables involved. Focus is everything in lasers. That's the quandary I suppose. There is definitely a natural spread of some sort that occurs in any laser beam, so there must be some answer out there. I suppose the answer I was looking for is more like all the photons are parallel at the source, not necessarily forced to focus to a particular point or target down range.

I was working on a theory and needed that specific answer. I know a meter is an extremely short distance, but I could also use a 1 meter beam with a target 299792458 meters away.

 

[FRITZHID]

Diode lasers are usually not culminated very well right out of the package, hence the lens sets they come with.
Gas lasers on the other hand are typically very clean right out of the tube.
Depending on the beam size you desire (and the effect you require) is more dependent on the wattage of the laser. A low power laser will look fine at a small dia, but just like a photo, when you "blow it up", graininess and imperfections become exponentially more apparent..... especially with diode lasers.
Gas lasers inherently produce a MUCH cleaner beam, even when expanded.

 

[FRITZHID]

You planning a moon shot?? Hope you have the WATTAGE not mW available for that.
Not to mention you need a way to detect the very few photons that are returned.

 

[Sean168]

Diode lasers are usually not culminated very well right out of the package, hence the lens sets they come with.
Gas lasers on the other hand are typically very clean right out of the tube.
Depending on the beam size you desire (and the effect you require) is more dependent on the wattage of the laser. A low power laser will look fine at a small dia, but just like a photo, when you "blow it up", graininess and imperfections become exponentially more apparent..... especially with diode lasers.
Gas lasers inherently produce a MUCH cleaner beam, even when expanded.

most of diode lasers are multimode, and will expand and become hard to focus pretty within a short distance.

to answer the original question, in an ideal world when you have a well collimated laser beam, the beam will still expand due to diffraction. the angle of expansion is the wavelength of the laser beam divided by the radius of the beam.

so if you have a laser beam at about 1mm, and the wavelength is 1 microm, the angle of expansion is about 1/1000, which will expand about 1 mm per 1 meter of distance traveled, making the laser spot into 2 mm in size.

obviously in real world, you have multimode laser, so it's going to expand more; and when it's not collimated well, it's going to expand by the effective focal length.