numerical aperture

[Ed Wesly]

Not only is a prespreading lens a good idea to zoom the size of the spread spatially filter beam, but translating one in the X-Y plane before the microscope objective can move the Gaussian beam for optimal results, like controlling the beam balance ratio in a single beam hologram set up. To see its practical application, check out: http://edweslystudio.com/Research/DE615/DE.html

[Brian]

An x y translator is very useful for that lens. Without one, fine tuning the lens position relative to the incident beam is a real test of patience.

To be complete, the optimal concave lens for this arrangement would have the same magnitude focal length, but of course with negative sign. Then the separation distance equation is the same, and the negative focal length results in a much shorter distance. I don't have any concave lenses, so I never thought about using one for this purpose (until now).

[Brian]

Okay in my other thread, I promised to ask only one question per week. I asked a question last Sunday and its only Friday. But its Friday after 5 pm, so I'm declaring it another week.

I am planning to purchase pinholes next week. I have a couple 40x objectives and a couple 16x objectives. What pinholes would you generally use with each of these to spatially filter an unmodified (say 2 mm diameter) beam?

[Ed Wesly]

10 - 15 microns for the 40X's, 20 - 25 on the 16X.

Optimation seemed to be always be the least expensive, but I am leaning toward Lenox Laser after having visited their plant with our old forum friend, John Pecora. Plus they sponsor what looks like fun annual Light Seminars.

Wouldn't it be nice if someone made a micro-iris?

[Din]

Remember that the pinhole diameter must be about, or slightly larger than the Airy diameter. However, in order to calculate the Airy diameter, you need to know the beam profile of the beam coming into the laser. Remember also, the objective of an objective is to create a real image for the eyepiece. Holographers are using the objective "backwards", as it were. However, the laser does not propagate from a point source, but the beam waist. So, effectively, you need to extend the beam waist to a point source, take the magnification into account and calculate the Airy diameter.

From a practical point of view, it depends a lot on the laser, the power of the laser, and the beam divergence at the objective. Back in the day when I was using a 30 mW HeNe, I used a 5 micron for the 40X and a 25 micron for the 10X. There's an inverse relationship, the higher the 'X', the lower the 'micron' for obvious reasons, I guess, since r ~ 1.22{(lambda)(f)}/d.

I am leaning toward Lenox Laser after having visited their plant with our old forum friend, John Pecora

So, he's a pro now is he? I assume that's the reason he's visiting a laser manufacturer. I seem to remember he swore blind that he was just a hobbyist and was never going to become a pro.
[edit] Please do give him my regards. Hope he's doing well.

[Ed Wesly]

Yes, pinhole selection depends on a lot, which is why I hedged and mentioned a range rather than a number. So I would advise ordering a range of sizes.

Lenox Laser doesn't make lasers to sell, but uses them to make pinholes and other micro apertures, especially arrays. John Pecora worked there as a technician.

[Brian]

I am familiar with using objectives to focus laser beams (in order to create optical traps. So I know about Airy disks, Rayleigh criterion, and so on. When I do those calculations, a 16x objective focuses a typical 2 mm diameter beam to an Airy disk smaller than 8 microns, and a 40x can squeeze it to under 2 microns.

But of course a pinhole is going to have some thickness to it (on the scale of the spot size, the hole starts to look more like a tunnel than a doorway) and the beam's convergence and divergence angles are going to be pretty steep. So I figured one needs an oversized hole to keep from losing the edges of the beam at the tunnel entrance.

So Dinesh and Ed, thanks much, your numbers give me a better feel for what I need. And thanks Ed for mentioning suppliers; I'll take a look.

Also, I'm glad to have read old posts. Saved me from making the mistake of using really high NA objectives I accumulated during the optical trapping days.

*sigh* now I have to wait a whole week to ask another question.

[Ed Wesly]

"*sigh* now I have to wait a whole week to ask another question"

We won't hold you to that!

[Din]

*sigh* now I have to wait a whole week to ask another question.

Well, you can always redefine 'week'. After all, the religious folk have been redefining 'day' for quite a while now.

[BobH]

I'd recommend using the largest diameter pinhole you can get away with. Matching it to the Airy disk "by the numbers" will only result in keeping the alignment of the beam going through it in your system a nightmare. At 40X, I'd use 10 micron pinhole. Anything lower, I'd be at 25 microns. the only reason to use a spatial filter is to remove objectionable patterns. Use the largest pinhole that achieves that goal, not the one that provides textbook perfection of the Airy disk. Pictorial holography is not a physics experiment, unless you want it to be.