Large Aspheric lense = reduced divergence?

What do you guys think of this Aspheric lense from Thorlabs:


Thorlabs.com - C240TME-A f=8.0 mm, 0.5 NA, Mounted Geltech Aspheric Lens, AR: 400-600 nm

It has an 8mm focal length and 8mm clear aperture, compared to the 4mm focal length and 5mm clear aperture in lenses like the 405-G1.

The only downside is the cost, and the fact that it has a lower Numeric Aperture (.5 vs .6), which means it won't capture quite as much of the beam.

This should improve divergence, right? (the beam will be a little larger as well)

Am I making sense here? I am pretty new to this...

Jubathoph said:

What do you guys think of this Aspheric lense from Thorlabs:


Thorlabs.com - C240TME-A f=8.0 mm, 0.5 NA, Mounted Geltech Aspheric Lens, AR: 400-600 nm

It has an 8mm focal length and 8mm clear aperture, compared to the 4mm focal length and 5mm clear aperture in lenses like the 405-G1.

The only downside is the cost, and the fact that it has a lower Numeric Aperture (.5 vs .6), which means it won't capture quite as much of the beam.

This should improve divergence, right? (the beam will be a little larger as well)

Am I making sense here? I am pretty new to this...

Click to expand...

I can't address what this type of lens will do but if it behaves like a plan-convex lens then it should reduce divergence. But if you really want to reduce divergence then a plano-convex (PCX) with a longer still effective focal length will work. Generally speaking PCX lenses are used.

This is for collimation; at a high numerical aperture the spherical lenses such as the PCX will have "spherical aberation", which is why all the laser collimation lenses I have seen are aspheric. It would definatly be cheaper to use the PCX lense so it might be worth a shot to try first, thks for the reply.

Jubathoph said:

This is for collimation; at a high numerical aperture the spherical lenses such as the PCX will have "spherical aberation", which is why all the laser collimation lenses I have seen are aspheric. It would definatly be cheaper to use the PCX lense so it might be worth a shot to try first, thks for the reply.

Click to expand...

You bring up a question I've not been able to find the answer to. If I were to compare two positive EFL lenses both of the same diameter and focal length say 30mm, but one is aspherical the other is a PCX lens. What would be the affect on beam profile ?

I believe the Aspheric will be able to better collimate. All spherical lenses such as the Plano convex lense are really "approximations" of a perfect lense. A perfect lense would focus collimated light to a point, or in the other direction, perfectly collimate light from a point source. The spherical approximation is a very close one, but it starts to break down when the focal point is very close to the lense, I.e. the high numeric aperture or "wide angle capture" that is needed to collimate an LD. Granted, the asperic is still an approximation of a perfect lense, but much improved by a 4th order polynomial equation (and harder to manufacture)

http:// http://www.google.com/m/url?client=ms-android-sprint-us&ei=WybsTcjJM8eHiAKthfO8AQ&gl=us&hl=en&q=http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/basics/g11/&source=android-browser-type&ved=0CDQQFjAJ&usg=AFQjCNHB1O5Ko-mbNdkf-6bLs3G2d9hmbw
The real question is how much is it improved, and is it worth the added cost. I went ahead and ordered the lense so I will post results when I finish the build.

This link explaines it better than I can

You seem to know your stuff about lenses. I am nowhere near that. I just want to share something I saw on TV, because everything you see on TV is absolutely true ;-) Well, this sounded credible enough. I heard that the reason we use spherical lenses isn't because they're the best, but because they're much much easier to manufacture than a good aspherical.

Let us know your results, I'd be interested to see what qualitative effects these lenses will have on the beam, and to understand what specific qualities of a lens contribute to the various qualities of the beam profile.

Here's Jubathoph's link: http://amazing-space.stsci.edu/resources/explorations/groundup/lesson/basics/g11/
pretty good beam path drawing.

Thanks,

I have done some research but everything I have learned is from the internet so not sure what to believe. That's why I'm checking with you guys; get some perspective from people who have actually built this stuff.