Spatial filter for lasers (3-axis)

This is a spatial filter for laser beams. 

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EDIT: The V2 versions have been added because I noticed that inserting the spring for the XY stage was significantly too hard than needed. The first time I assembled it I did it in one shot but it was clearly fortuitous.

EDIT 2: It turns out that I messed up one of the dimensions for the spring for the XY stage. The spring I used there is actually 4.5x15mm. 5mm diameter will also work, but you should really not use too much longer than 15mm if you print the first version of the XY stage. If you choose V2, it is also good for up to 18mm length for this spring.
I also added a new pinhole holder which requires no pin, but it is slighly more complex to print because you print the pin yourself embedded into the holder (pinhole holder V3).
 

Why?

I needed one to expand a beam homogeneously for holography, but typical products of this type are very costly (can be above 1000$). This item can be printed for just about 2$ worth of plastic (75g of PETG in my case). You just need to add 3 micrometer heads (available on AliExpress for less than 10$ each), a few screws and inserts.
I have designed this component to be able to accept Thorlabs 1" pinholes, which are some of the most widely used ones, and common RMS thread microscope objectives. 

These are obviously the most costly parts of this project. A precision drilled pinhole from Thorlabs costs about 80$ and a microscope objective can be acquired for just about 30$ (the cheapest I could find on AliExpress, link below). Note that it is better if the objective is infinity corrected, but it may possibly work even without this feature (I am not sure as I only tried with infinity corrected microscope objectives).

 

What is this for?

When working with lasers, it is very common to send your beam through many optical components such as mirrors, lenses and gratings. These components will always have some amount of imperfections or dust on them, which disturbs the beam's transverse profile and makes it look messy and inhomogeneous. To solve this problem, one possibility is to use a spatial filter. The principle is simple: when you take a laser and focus it (using a microscope objective for example), you form a small dot on the focal plane. The focal plane is the 2D optical Fourier transform of the beam, and the high spatial frequencies (which have high angles) are focused off-center, while the TEM00 mode, which is the typical and perfect Gaussian mode, is focused at the center. If one now places a pinhole at the focal plane precisely such that only the central peak passes through, the high frequencies which correspond to diffraction with dust specks or fine imperfections on the optics are filtered out, and out of the pinhole emerges a perfect Gaussian profile. It basically is the same principle of a low-pass filter.

Often, a second objective (or even just a simple lens) is placed after the pinhole to recollimate the beam (not included in this design).
 

How big should my pinhole be? What magnification?

The size of the pinhole is very important. If the pinhole is too large, the filtering may be insufficient. If it is too small however, you may risk to crop the main beam, and you will get diffraction from a circular hole (Airy pattern) after the pinhole.

To know what size you need, you should find out the focal length of your objective. This is often somewhere in the ballpark of 16.5mm. Once you know the focal length, the formula for the optimal size of the pinhole is:

where lambda is the wavelength of the laser, f is the focal length of the microscope objective and r is the radius of the beam at the input of the objective (which should be collimated).

For example for a HeNe laser operating at 633nm, with a focal length of 16.5mm and r=0.6mm you get D = 17.4um (micromemeters). You will then approximate by excess to 20 microns to be safe (even 25 microns will most likely work). Remember that the beam size of your beam can be also given by the manufacturer, along with the divergence, so you can even calculate the beam size if you do not have a beam profile camera available.

Anyway, a general guideline is:
 

• 10X objective : 30 um pinhole
• 20X objective : 15 um pinhole
• 40X objective: 10 um pinhole

Please calculate what you need for your own situation though (bigger is better than smaller).

What do I need to make one?

To make this work, you will need the following (I had to shorten the links with url shortener websites as they were super long):
 

1. 4x M6 screws (small, I used 14mm ones but a little longer or shorter should work).
2. 5x M3 brass inserts, 5.7mm  long. These are easily available for example on Amazon: https://tinyurl.com/yn35y2z4 or on AliExpress for cheaper: https://shorturl.at/nlcrm. You will press them into the print with a soldering iron. 
3. 2x 80mm alumininum rods, 8mm diameter. I bought mine longer and cut them with a simple saw.
4. 2x M6 brass inserts, 12.7mm long. You can buy this exact type on Amazon: https://shorturl.at/3YWDy 
5. M3 grub screws (any length is good as long as not too long), just get a bunch of them here and see what is best, they are so cheap… Link for Amazon: https://shorturl.at/PS26T . They are used to fix the micrometer heads firmly in place. There are also 2 of them which are optional and can be used if you want to fix the rods in place. However I found that tightening them too much can bend the rods to the point that the tension springs for the microscope slider may stop functioning correctly, so you may even choose to not use them (the tolerance is such that you do not really need them anyway). You still need to use them to keep the micrometer heads however (very important).
6. 2x tension springs, 6mm x 22mm length + 1 tension spring, 5mm x 15mm length. I just bought an assortment on Amazon, but cannot find it on Amazon.com, so here is the one I bought (from Amazon.se): https://shorturl.at/HkfC6
7. 6x small (9mm long) pins to keep the springs in position. For this, I just took some PCB headers I had laying around, took out the small brass pins and clipped them to size using a nails clipper. You can use anything similar, but this is the kind I used: https://shorturl.at/onn00
8. 3x micrometer heads, 13mm long, 9.5mm shaft diameter. I had mine from Mitutoyo, but you can find equivalent ones for super cheap on AliExpress: https://shorturl.at/TfIDP 
9. A 1 inch housing pinhole of appropriate size (see guidelines above). I bought mine from Thorlabs, but I guess other manufacturers may work also as long as the size of the housing matches).  Here is the link: https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=14350
10. A microscope objective, better if infinity corrected. I used a Newport M-10X, but you may find much cheaper alternatives on AliExpress, for instance this: https://shorturl.at/7v7Nv

Printing instructions

Orient the models as you see in the pictures below. For all objects I used black PETG (Verbatim brand). Most items can be printed at 0.2mm layer height, but for the microscope slider I used 0.1mm to print the fine RMS threads better (you may wanna add grease and slow down the printing speed though). I used 25% infill and tree supports for the holes which are for the aluminium rods and the micrometer heads. Also, the XY stage needs support as well (check the pictures).

I printed all objects using a Bambulab P1S, and it took me just about 4h for the entire thing (you do not have to print each one individually, I just show the pictures for the orientations of each one and to show where the supports should go (green areas)). I printed the baseplate and xt stage together using “print by object” option in OrcaSlicer.

Assembly 

 Assembly is pretty straightforward, please observe the photos carefully if you do not understand. The only difficulty is in placing the springs. You take a pin and slide into the loop of one extremity of the spring. Then, you place the spring within its hole. Finally, use a small screwdriver (or something like that) to pull the loop of the spring through the hole, and carefully slide another pin in the loop with a pair of precision pliers. It may not work at first and the spring may bounce on your eyeball and permanently blind you, but it should eventually work.

 

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