Fiber Optics, But… Wetter?

Fiber optics are a great way to transfer huge quantity of data at lightning speed. Thanks to the property of total internal reflection, which allows light to flow through a glass fiber like fluid through a pipe, they can be used for communications at long distances and form the backbone of modern communication networks. However, water is also able to pull off the total internal reflection party trick, and [Mike Kohn] decided to see if it could be used as a communication medium, too.

The experimental setup consists of an ATTiny85 that receives signals over its serial port, and outputs the received bits by flashing an LED. This LED is attached to a plastic tube filled with water. On the receiving end, another ATTiny85 reads the voltage level of a photodiode placed in the other end of the tube. When the ADC detects voltage over a certain level, it toggles a pin connected to the serial RX pin.

Hooking the setup to a pair of terminals, [Mike] was able to successfully transmit 9600 baud serial data through a tube full of water with just an LED and a small microcontroller. To verify the success, he ran the test again with an air-filled tube instead, which failed. In doing so, he proved that the water was doing the work.

We’ve seen other optical data hacks, too – like this awesome laser ethernet build. Video after the break.

27 thoughts on “Fiber Optics, But… Wetter?

    1. I was going to say, we bought a fancy new research microscope a year or so ago and the fluorescence illumination box has a “liquid light guide” to pass the illumination to the microscope. I’ve no idea what it is filled with but I wouldn’t be surprised to hear it is something mundane like water.

  1. Um, no, the water was not “doing the work.” TIR happens when going from a higher-index material to a lower-index material, as (say) water and plastic. So you can’t get TIR from air to plastic; but anything with a higher index than the plastic tube will give you TIR at some critical angle.

    I don’t know if you’ve heard of them, but there are these things called “physics” and “equations” that can be used to understand and predict these effects. (sorry. I just can’t help myself).

    I can’t tell if the experimenter didn’t bother to do any research or if that is the author’s problem. I suspect the latter.

    1. Actually, the index of the plastic is likely higher than that of the water, so I was snarky for no good reason. Apologies. Interestingly, the TIR is probably happening at the plastic-air interface rather than the water-plastic interface. TIR does not occur when the tube is filled with air because the refractive difference between the air and plastic causes the light to bend past the critical angle for the plastic-air interface on the other side of the tubing wall.

      So it’s at least a _little_ non-obvious.

      1. There are glass optical fibres with hollow cores that use glass-air for their TIR interface. Air has a lower absorption than glass, so the fibres are more efficient over long distances.

  2. I could swear that in the 1990’s there was someone offering a fiber coupled laser for industrial metal cutting where the fiber coupling was a stream of laminar flow water, with the intent being that it’d clear debris and prevent beam obstruction by vaporized debris, but I can’t find any mention of it.

      1. liquid jet guided laser gets me a ton of results: I wish I’d thought of that. I was trying fluid coupled and not getting anything. There’s been a huge amount of progress in the field!

  3. So, you are saying that water cooling loops in modern desktop computers can now have meaningful lighting effects inside? PCMasterRace approves of this!
    Imagine having a RGB diodes on one end and color sensing photo diodes on other end and you have the desired visual effects as well as increased bandwidth for the crucial cooling elements to communicate between each other :)

    1. I don’t really know if high bandwidth comm is needed for such application, or if it is reachable with these circumstances, or if it’s even worth it, since the already present hardware and control firm/software is already coordinating these things, but I know that we’ve managed without it so far, and to great results. That’d be just more bloat for the already bloated pc parts market, and another excuse for manufacturers to charge stupid figures for something as simple as a cooling loop.

      There are some complications to this idea, cause usually you have additives on the water, or even a specialized substance meant for this app, that may have less than average performance on optical comm. And for the interface to cause Total Internal Reflection on the desired band, it would impose some serious restrictions on the combinations of tube material and coolant.

      But nevertheless, I would love to see more meaningful hardware and parts out there. not only your usual normal part with heatsinks and rgb for a ludicrous price and no reason

      1. Now, I commented as a semi joke. However there is a possibility of having communication trough water loops only thing is I have no idea to what end. Hoses rarely run longer than 50cm from joint to joint. Maybe having separate elements in the loop communicating status messages like coolant temperature and flow speed, on-die temp, radiator entry and exit temps and pressure; for all this you need low speed single channel comms with collision detection. And yes, you can bet your gluteus that they will double the product price for this.

  4. Water could be used to communicate but would not be much better than an empty tube with a reflective coating. One of the characteristics of fiber optic in communications is variable gradient meaning that light striking at a steep angle gets refracted at a different angle than shallow incidence angles. This keeps the beam aligned within the center of the fiber. If you don’t have this you have dispersion. This means different modes arrive at different times due to differing path length. This causes blurring of the pulses which limits the shortest duration you can send. So, bottom line is that anything that allows a light pulse to get from one place to another COULD be used to communicate however without a gradient index, it will be speed and distance (more distance, more dispersion) limited. Also, remember that as the medium gets denser the velocity of propagation gets slower. Looking only at Vp, vacuum would be best, air good, water, glass, and plastics OK. The last issue is ensuring continued purity of the water. Any contamination of the water will cause power loss and increase heating of the water. Plastic and glass used in fiber optics stay transparent over time much better than water does.

    1. I mean, I still try to keep a handful of floppies in stock, in case I need to work with older robots and machinery, where I may not have the bespoke multiple thousand dollar serial adapter for it that they stopped making 20 years ago. They’re obsolete, not out of circulation.

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