Collimation - Down the rabbit hole

[paul1598419]

Singlemode Laser is a laser expert that I met while he was working in Berlin on single-mode laser diodes for diffraction tuning to narrow line widths at atomic transitions. He is now in California and you won't find a more knowledgeable member here on this subject.

 

[Junkers]

Yes, my objective is to focus a laser beam but my question is more specific than that. It relates to the optical mechanics of a lens and how NA influences curvature for a fixed focal length and diameter. I believe I've figured out the root of my misunderstanding and that is that lenses with a higher NA for a fixed focal length and diameter use a spherical convex interface as opposed to a standard plano interface and hence offset the greater incidence angle of light by bending it inwards at the initial interface. I hadn't noticed this and thought somehow they offset the higher incidence angle by manipulating the exit path e.g. using steeper curvature. That led me to think that the NA rating would determine a lenses ability to bend light. Total confusion.

I've only started to delve into lenses a little over a week ago. If my use of nomenclature is off, please forgive me.

Anyway... I've purchased a 50mm condenser lens with a 40mm focal length. Hopefully this does the trick. I setup a small 650nm diode with the expectant beam divergence as a test and the focal lens did a much better job of bringing the beam to a point. Now I'm just trying to figure out whether a positive meniscus lens is worthwhile. I'll post my findings once these lenses arrive.

 

[paul1598419]

I would look into aspheres. These are lenses that aren't spherical and do manage to focus coherent light better that spherical lenses can.

 

[Junkers]

Yep. An aspheric condenser is what I bought.

Does anyone know the maximum percentage of light you can project onto a lens before aberration etc become apparent?

A spherical lens has an aplanatic point (i.e., no spherical aberration) only at a radius that equals the radius of the sphere divided by the index of refraction of the lens material. A typical value of refractive index for crown glass is 1.5 (see list), which indicates that only about 43% of the area (67% of diameter) of a spherical lens is useful.

I gleaned this from Wikipedia but don't know how this translates to the plano side of a PCX lens. I imagine the percentage would be smaller?

 

[paul1598419]

It depends on the focal length of your aspheric lens. The G2s we use have a focal length of ~2.39 mm. That means at a distance of 2.39 mm from the diode's emitter you should get the laser focused to infinity focus which is as good as the divergence will be with that lens. But, the shorter the focal length the greater the divergence will be. A longer one like the G8 will help, but will also clip part of the beam with some of our most divergent diodes. So, there is "no one answer fits all" to your question. I have some with a focal length of 12 mm in the same standard size we use the most. It really depends on exactly what the end result is that you are aiming for and which diode you plan on using.

 

[steve001]

It depends on the focal length of your aspheric lens. The G2s we use have a focal length of ~2.39 mm. That means at a distance of 2.39 mm from the diode's emitter you should get the laser focused to infinity focus which is as good as the divergence will be with that lens. But, the shorter the focal length the greater the divergence will be. A longer one like the G8 will help, but will also clip part of the beam with some of our most divergent diodes. So, there is "no one answer fits all" to your question. I have some with a focal length of 12 mm in the same standard size we use the most. It really depends on exactly what the end result is that you are aiming for and which diode you plan on using.

His objective is to focus a laser beam to a very small spot as I recall.

 

[paul1598419]

His objective is to focus a laser beam to a very small spot as I recall.

If that is the case, any of these lenses will focus your beam's profile to a smaller size than infinity focus at the aperture, but only at very close distances. Maybe 2 inches or so.

 

[Junkers]

Well I have purchased a new focal lens (on top of a new collimator) with a focal length of 150mm. Hopefully the divergence I achieve with the collimator is enough to keep the spot size under 0.2mm. I have also read about using a positive meniscus lens combined with a PCX lens to achieve a smaller spot size but from what I gather this is technique is much more effective with lenses of a higher index of refraction. This will be plan B if things don't work out.

 

[paul1598419]

There are multi-element lenses available to us already. You can get 2 and 3 element lenses in the standard M9X0.5 size. A lens with a FL of 150 mm is very long and if used as a collimater it would need to be several inches in diameter depending on the diode you are using.

 

[Junkers]

Well the lenses arrived and it appears that I can't focus the beam into a small enough point for it to be useful. I guess the beam is still too divergent. I've yet to put together a decent testbench, I have some linear rail here so I will have another go soon and see if I can improve on it.

Failing that I suspect my only other option is to focus the laser by setting it two focal lengths away from a single focal lens. The problem is, if I go down this route I still require a focal length of at least 80mm without an overly large beam diameter. Is it possible to pair a convex lens with a concave lens to achieve a long but narrow beam which focuses to a point? If so, how would I determine the focal length?

Illustration:

 

[steve001]

Well the lenses arrived and it appears that I can't focus the beam into a small enough point for it to be useful. I guess the beam is still too divergent. I've yet to put together a decent testbench, I have some linear rail here so I will have another go soon and see if I can improve on it.

Failing that I suspect my only other option is to focus the laser by setting it two focal lengths away from a single focal lens. The problem is, if I go down this route I still require a focal length of at least 80mm without an overly large beam diameter. Is it possible to pair a convex lens with a concave lens to achieve a long but narrow beam which focuses to a point? If so, how would I determine the focal length?

Illustration:

View attachment 68255

What does a negative focal lens do to light? Think it through.

 

[Junkers]

It causes light to diverge. I realize it wouldn't have the same integrity but what else can I do..?

 

[steve001]

It causes light to diverge. I realize it wouldn't have the same integrity but what else can I do..?

As I recall all that you need to do is use a long focal length plano-convex lens. Check Surplus Shed for lenses that fit that description.

 

[lazeristasUVISIR]

Aspherical lenses usually are design to collimate beams. You can read about them at http://www.lightpath.com/wp-content/uploads/2015/11/Using-Precision-Molded-Aspheres.pdf

There are special ones for fiber coupling (or imaging) when a single one does a job. Usually the distances are very short.

You can sacrifice the quality and a spot size by using a single collimating lens to focus a beam at a farther distance.


In principle you have a typical problem how to deliver a radiation from one place to other. You put restrictions (sizes, lengths) you'll get less various solutions (optics between two points). You are somewhere in bewteen "how to couple into the SM fiber" and "how to shoot down drones"