Beam expander to focus laser beam

[Singlemode Laser]

Re: Beam expander to focus laser bearm

I think you misunderstood Alaska's question. His question was: What focal length lens would be needed to focus a beam at a mile distance? Not about beam waist.

The focus of a laser beam is the waist. So what do you mean exactly with:
focus a beam at a mile distance?

Singlemode

 

[steve001]

Re: Beam expander to focus laser bearm

The focus of a laser beam is the waist. So what do you mean exactly with:
focus a beam at a mile distance?

Singlemode

A finite conjugate. See the green video link. Do that at a mile.
The beams waist occurs when in the case of a beam expander the beam is at the smallest diameter. That happens within the Rayleigh Range.

 

[Alaskan]

I guess I essentially meant that, at it's smallest possible diameter at a terminated focal point. e: when the term waist was used, I was thinking of you were referring to an unterminated beam, that's what threw me off, but it doesn't really matter, does it.

I had seen that video, it was one of the first I saw when becoming interested in lasers. My curiosity is how far can a beam be focused to a point at great distances, where is the cut off distance for a given lens size, i.e. 3 inches or about 75 mm diameter at different FL's? I need to go ask my Google buddy, now that I know more about how to word the question.

 

[RedCowboy]

---EDITED---

Let me see if I have this right.

Is the beam waist the beam diameter at the laser aperture, and also at the point where the beam crosses over it's self when focused through a lens as those are the narrow points ?

Or does waist simply mean the laser beams diameter ?

I have been calling the expanded beams diameter where it strikes the output lens of a 2 lens beam expander the " print " but that in fact should be called the waist ?

Focal Spot Size Calculator for Gaussian Laser Beams | Ophir Photonics

 

[WizardG]

"Let me see if I have this right."

Nope. The beam waist is the part of the beam with the smallest diameter. A collimated beam (infinity focus) essentially has no waist. Only a converging beam will have a waist, after which the beam will be diverging.

I think I got that right....

 

[RedCowboy]

OK then it's like this diagram, for this calculator. *Note: This calculator is for spherical Gaussian beams.

Focal Spot Size Calculator for Gaussian Laser Beams | Ophir Photonics

Here's another image that's better labeled.

**Note: NEVER POINT A LASER AT AN AIRCRAFT !

 

[Singlemode Laser]

A collimated beam doesn't have a focus at infinity. Such a thing does not exist. A collimated beam just has the smallest far field divergence for a given lens.
For the maximum distance see here:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=4353

Singlemode

 

[RedCowboy]

I understand my impossible statement, what you are saying is a collimated beam is diverging by some amount, ( Mrad ) but with the use of a beam expander would be converging if focused to a near point.

But if that point of focus is perceived infinity the beam may appear to be converging but is actually diverging because of how we see things over a distance.

 

[steve001]

I guess I essentially meant that, at it's smallest possible diameter at a terminated focal point. e: when the term waist was used, I was thinking of you were referring to an unterminated beam, that's what threw me off, but it doesn't really matter, does it.

I had seen that video, it was one of the first I saw when becoming interested in lasers. My curiosity is how far can a beam be focused to a point at great distances, where is the cut off distance for a given lens size, i.e. 3 inches or about 75 mm diameter at different FL's? I need to go ask my Google buddy, now that I know more about how to word the question.

This will be an interesting experiment. Surplus Shed I think carry lenses with long focal lengths.

 

[paul1598419]

I believe much of the confusion comes from our use of the term "infinity focus" to mean focused to the smallest point at distance with a lens. Since no laser can actually be focused at infinity, that is the cause for confusion. Of course the waist is the point of focus, as it is the place where the beam is of the smallest diameter.

Trying to get a 1 watt laser focused to a 2.5mm diameter at a mile is likely not possible especially with a direct MM diode. You can decrease the divergence of your beam by using a BE in the near field, but all light diverges after a given distance and trying to use this as a way to get a MM diode focused to 2.5mm at a mile's distance is not possible using a BE.

 

[steve001]

Hi

"Let me see if I have this right."

Nope. The beam waist is the part of the beam with the smallest diameter. A collimated beam (infinity focus) essentially has no waist. Only a converging beam will have a waist, after which the beam will be diverging.

I think I got that right....

OK then it's like this diagram, for this calculator. *Note: This calculator is for spherical Gaussian beams.

Focal Spot Size Calculator for Gaussian Laser Beams | Ophir Photonics

Here's another image that's better labeled.

**Note: NEVER POINT A LASER AT AN AIRCRAFT !

A collimated beam doesn't have a focus at infinity. Such a thing does not exist. A collimated beam just has the smallest far field divergence for a given lens.
For the maximum distance see here:

https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=4353

Singlemode

I understand my impossible statement, what you are saying is a collimated beam is diverging by some amount, ( Mrad ) but with the use of a beam expander would be converging if focused to a near point.

But if that point of focus is perceived infinity the beam may appear to be converging but is actually diverging because of how we see things over a distance.

I believe much of the confusion comes from our use of the term "infinity focus" to mean focused to the smallest point at distance with a lens. Since no laser can actually be focused at infinity, that is the cause for confusion. Of course the waist is the point of focus, as it is the place where the beam is of the smallest diameter.

Trying to get a 1 watt laser focused to a 2.5mm diameter at a mile is likely not possible especially with a direct MM diode. You can decrease the divergence of your beam by using a BE in the near field, but all light diverges after a given distance and trying to use this as a way to get a MM diode focused to 2.5mm at a mile's distance is not possible using a BE.

Forget about multimode diode lasers and just consider a nice gaussian TEM00 beam. A beam expander is a refractor telescope. It is capable of doing three things. (1) It can focus a beam just like a magnifying lens can focus the Sun's rays to a small spot. Finite conjugate.
(2) It can be used to diverge the beam. This should not be confused with infinite focus.
(3) It is used normally to focus to infinity. Infinite conjugate.
When focused to infinite conjugate a new wider beam waist is created within the Rayleigh Length (RL). The interesting stuff happens within the (RL). Within the RL the beam expands 1.4 times it's initial diameter at the expander. What's actually happening is the positive lens is making the beam converge while the wave nature of light is expanding at a slightly
faster rate. Once passed the RL the beam will expand at a linear rate like a flashlight beam.

 

[RedCowboy]

Looks like styro has built a big beam expander, you can catch a look at it in the end of his latest video, he said we will see it soon.

 

[Alaskan]

Cool, he likes big lenses too

 

[RedCowboy]

I have some big optics, I just need to get off my last azz and make something.

I can already burn across my yard so I'm pretty much out of testing space, what I really want is a lot more horsepower, something in the hundreds of watts would be fun.

 

[Rintec]

Re: Beam expander to focus laser bearm

Steve001, Alaskan, Singlemode, and others.
Thank you for commenting in this thread.

I tried to summarize what I can understand from the discussion and from the source out there.
Please evaluate attached files. Whether theoretically I am right or completely misleading.

I think focusing laser at a distance using galilean expander is possible. Not only collimating, but also focusing at a point at where the beam spot is the smallest.
This can be done depend on the fineness and size of the lenses.

One of my reference is here:
https://lightmachinery.com/optical-...tical-design-tools/gaussian-beam-propagation/

Give me your comments. Thanks.

 

[WizardG]

Re: Beam expander to focus laser bearm

Try this: http://www.holoor.co.il/Diffractive_Optics_Products/Calculators.php?PN=DL

With a perfect TEMoo beam focusing down to a few mm spot size at a mile out should be doable with reasonably sized optics (neglecting for the moment any distortion of the beam by air currents). Unfortunately (or maybe it's a good thing?) our high power multimode diodes don't have nice clean Gaussian beams even when corrective optics are used.

It's a pretty cool website with some handy optical calculators.