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## Step 1: Why Use Counting ICs?

My work in the atom optics lab at my university has taken me through some real twists and turns. I've learned more in my two years as an intern there than I have in most of college, and it's way more rewarding than turning in homework. If you ever have the chance to work in a university laboratory, take it, no matter what. You'll never regret it. One of the things I've had to learn pretty thoroughly is electronics, since one of my duties is troubleshooting electronics and occasionally building additions and hacks to them. My most recent project brought me face to face with digital counting ICs.

The project involved using a Michelson Interferometer to compare the relative wavelengths of two lasers by counting the number of fringes of each beam when a retro-reflector cart was translated across the beam paths, thus changing the path lengths of both beams by identical amounts. Light has such a short wavelength that moving the cart even as slowly as a few centimeters per second produces fringe patterns at around 150kHz. The interferometer operation simplifies down to the relation:

λ

_{2}=λ

_{1}(N

_{2}/N

_{1})

where λ is the wavelength of a laser, and N is the number of fringes produced by translating the cart a certain distance. So if we know one laser pretty well, we can find the wavelength of the second laser by counting their fringes. For light being measured by translating the cart over 1 meter, N is going to be around 40 million. Try counting that by hand. No, we need high speed event counting methods.

Another use would be in something like a Geiger counter. If you need to know how much radiation you've been exposed to, you can hook up a traditional Geiger counter to a counter IC and count the number of radiation events, then use the appropriate math to convert the radiation count to rads.

Any suggestions about interferometry and fringe patterns? I am trying to use an interferometer to find the density of a protein sample. I was planning on using a Mach-Zehner Interferometer and taking an image of the fringes. It is my understanding that I would need to record the space between the fringes. Is this correct? The idea behind it is that the density relates to index of refraction, and when one beam goes through the sample it will be phase shifted from the original beam, and when that beam and the original beam interfere it will create horizontal fringes (which it does).

That sounds about right. Interferometry typically boils down to either a change in path length or a change in index of refraction. I haven't worked with Mach-Zehner (Zehnder?) setups, but from the schematic it looks useful in terms of being able to use a reference on one path to look for deviations from that reference in the other path. I'm not sure why there would be horizontal lines in the interferogram unless some angle is out of perfect alignment. If your sample is perfectly aligned and occupies a container identical to the reference, and the container is a perfect rectangular prism, I would think that no horizontal fringes would occur. As you added more protein concentration to the sample container, the entire interferogram would oscillate in brightness.

A closer look at the wiki seems to indicate that the interferometer is usually deliberately misaligned to produce fringes. If that's the case it will take some geometry to figure out what the expected path length difference should be. What is the shape of the container, what kind of index of refractions are you working with, and what is your light source?

That is a neat idea. There are a few conversions you'll need. Getting the index of refraction gradient shouldn't be too hard. That's geometric calculations and counting fringes produced by the interferometer, as you said earlier. More difficult will be converting the index gradient to a concentration gradient. Do you already have the data that would allow that conversion?

I just did a rough calculation and it looks like your index of refraction gradient will be roughly dn/dx = L/(t*a), where L is the wavelength of light in vacuum, t is the thickness of the sample container, and a is the distance between each horizontal fringe in your interferometer. I assumed the index gradient is constant, the incoming light is perpendicular to the container, and the container thickness doesn't vary much. Definitely do check that figure.

According to the LS7183 datasheet, the 40193 counter can be used as an up/down counter, but it's 4-bit parallel.

love counters though great how to on them!