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Comment by amelius | Hacker News Reader

Slightly offtopic, why is it so difficult to find a cheap and compact laser interferometer that can do sub-micron measurements?

This guy gets close:

https://www.youtube.com/watch?v=MUdro-6u2Zg&t=770s

But why isn't something cheap and small like this commercially available as an integrated system?

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It's mainly the laser itself that is the expensive part. If you only care about resolution it's easy, you just need a single-mode laser. But if you care about accuracy it's very difficult, because then the wavelength needs to be stable, and that requires a much more expensive laser. Most people looking for an interferometer are interested in accuracy, unless they're just measuring vibrations.

You can get pretty far with cheap diodes + current and temperature control. Unless you need coherence lengths in the meters range you can make do with cheaper lasers.

Can't you solve the stable wavelength issue by using a beamsplitter and a separate reference arm?

While not an integrated system itself, looking for convenient information on the matter I stumbled upon https://arxiv.org/abs/2011.05313 . As for integrated electro-optical hardware that does coherent optics, I'd like to point at SFP optics; the ones that are sufficiently non-fancy with their electronics (up to 10km single-mode optics; no high-end-multimode optics; no exotic coherent optics) should not have a retimer on the RX side. Once you go to 1-Gbit/s transceivers you'll find ones with a linear RX stage; those should be suitable for rigging simple homodyne interferometry by just feeding a little TX back to the RX taking care to let the RX AGC run the RX path at high gain.

They are available cheaply. You can purchase a blu ray reader and hack it. The reason a ready packaged sensor is not available cheaply is simply because of economics and market size.

I'm not convinced by the market/economics argument. Cheap and small ToF sensors exist, even though the market initially was also small.

ToF sensors have huge applications. Initially ToF sensors also cost thousands of dollars. What huge applications do you see that should have driven the prices of over the counter integrable interferometers down? As I stated blu ray (and DVD, CD and laser disc before it) readers are tiny purpose build inferometers. And they are cheap.

Since when are CD reader optics interferometers? The linked video shows a CW homodyne LIDAR used for measuring vibration frequency and counting vibration amplitude in fringes.

Last I looked CD readers used a 4-detector sensor's differential low-pass signals for closed-loop track-following so the rotation need not be optically centered. I also see no reason why optical disc readouts would need homodyne let alone heterodyne readout.

The pits and valleys are spaced λ/4 apart. The reason there is such a stark difference in intensity is because this λ/4 spacing causes interference at pit->valley and valley->pit transition points. Of course this not a standard interferometer but rather a purpose build one. Wikipedia says:

> Interferometry is a technique which uses the interference of superimposed waves to extract information.

And a blu ray player directly uses interference of superimposed waves to extract information. It squarely fits in the homodyne category.

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