Optical fibers

I discussed total internal reflection briefly yesterday, and aside from allowing us to see quantum tunneling on a macroscopic scale, I also mentioned that a neat application is optical fibers. They turn out to be both interesting and practical. In essence, the way they work is by having a thin core surrounded by a layer of cladding material, which has a lower index of refraction than the core. As long as the light is mostly directed along the fiber, when it encounters the core-cladding boundary, it undergoes total internal reflection and stays inside the core. It bounces back and forth along the inside all the way from one end to the other, with pretty high efficiency. An interesting issue in the most easily-manufactured type of optical fiber is that different photons come into the fiber at slightly different angles, and thus end up with different path lengths as they travel through the fiber. This produces diffusion of the transmitted signal. But there's another form of fiber optic cable, called single-mode optical fibers, in which the core is small even compared to the wavelength of the propagating light. In this case, quantum mechanics comes into play, and only certain modes of light are permitted to propagate, which reduces or eliminates the diffusion.

And look, physics is useful! Optical fibers can be used for a variety of purposes. One you hear about a lot is the use of optical fibers for long-distance communication. That's because they transmit light, which tends to travel fast (though from a quick Google search it looks like the speed of light is around 35% lower in optical fibers than in vacuum), with very high efficiency and can operate over long distances. This is used in telephone communication, internet connection, etc. There's also a whole field based on the technology, with textbooks and conferences and papers up the wazoo. Outside of that, those cool light toys that people play with are based on optical fibers, and that's why it seems like the light is only present at the very end. But of course any damage to the fiber can produce a point at which the total internal reflection doesn't entirely work, so you'll see a spot of light there. They're also really useful as detectors, especially because they require no power and aren't affected by strong electric or magnetic fields. It's often a convenient way to get a measurement and propagate it away from the experiment site before data processing.