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Comment by riedel | Hacker News Reader

Have you explored fiber bragg grating. 10 years ago a student of mine semi successfully explored sensing the shape of firefighter hose using that technique. Seems to gain new traction again lately for optical shape sensing.

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I explored FBG sensors early on and they are very cool -- I was aiming for a less expensive and more robotics-oriented application. Something that can be seamlessly integrated into a design at a lower level without the complexity of FBG technology.

I think you made the right choice. You've come up with a great sensor topology. FBGs are good, but there's so much to them that is just infuriating. I ended up late in my optics work spending most of my time finding ways to avoid them.

The perpetual issue with FBGs is cost, in my experience. For quality sensors, you can use cheap fibre with expensive detectors, or cheap detectors with expensive fibre. There's been a perpetual promise of the tech getting cheaper, but never seems to really drop significantly. We always struggled to find buyers once they saw the price tag.

Reading about them, the main reason is because you have to manufacture regularly repeating slices of glass in a column over large distances whose thicknesses are usually on the order of the wavelength you're dealing with. So visible light requires some really small slices.

I could probably make fiber bragg gratings in my ceramics studio, yet it would require buying some Very specific thickness glass sheet, setting up a bunch of hole punch jigs to get 1 m worth out of 1 um slices, figuring out some way to actually layer 1,000,000 slices correctly, and then not just melting them to slag accidentally.

The ones we purchased were made in two different ways. Production ones were made by using a diffraction pattern of UV light to modify the refractive index of the core slightly, the other was using femptosecond lasers to alter it more dramatically.

The UV light method worked fine, came out to about $600US per fibre with 32 gratings each, not exactly cheap. The measurement unit was on the order of $20k US though.

That kind of surprises me, since then I'd figure that somebody would be making the equivalent of a reprap that puts out glass filament like this 3D glass printer [1] or various glass resin methods ([2] and [3]) Just with some kind of inline laser processing, or post deposition modification. Most of those use UV or lasers anyways. The methods mentioned don't seem like those should cost $20,000/unit unless the unit's 1km+.

[1] https://www.nobula3d.com/

[2] https://ethz.ch/en/news-and-events/eth-news/news/2019/11/gla...

[3] https://www.glassomer.com/products/glass-3d-printing.html

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