Stretching it a bit – nonlinear fiber optics

Imagine that all materials are made of springs, and networks of springs. For example, if you give a blob of jelly a nudge, it wobbles around a bit and then it relaxes. Remarkably, light nudges all materials too, but at the atomic scale. We don’t perceive it as motion of the object, but we perceive it as the color of the object. The question now is what happens if light, instead of a gentle nudge, gives the material a vigorous shake-up? Well, interesting things happen.

Imagine you are playing with a paddle pong. If you hit the pong softly, all is hunky-dory. But if you hit the pong a bit too hard, you wouldn’t be able to exactly predict how the pong would behave – the rubber band starts to respond nonlinearly to the applied forces. Essentially the same happens with atoms and light. When light interacts with atoms, it sets its electrons into oscillation. In the study of nonlinear optics, we hit the atoms a bit harder than usual and see how the atomic springs interact with light. Our paddle though, is a cool laser.

But even if we have a laser, to hit the atoms hard enough we need a lot of energy focussed in one place. We could use a magnifying glass (or a system of lenses) in principle to obtain a tight focus, but the rays would diverge beyond it, and the energy density will fall.

Optical fibers help us to walk around this hurdle. These fibers confine light within themselves by the principle of total internal reflection, but with two added advantages. One – the light can travel inside the fiber for kilometres without losing much of its energy. Two - and more importantly – it is confined to dimensions of the order of 7 to 8 microns – ten times smaller than the diameter of the human hair. Thus we have a medium in which we can confine a lot of light energy in a tiny space, and then make it travel for kilometres. This increases the interaction of light with the medium. So we ‘pump in’ light from a high power laser through one end of a very long optical fiber, and study what happens to it after it travels a substantial distance within it.

This essentially is the study of nonlinear fiber optics. It started with the question ‘what if…?’, yet it has resulted in many real world applications. For example, it is possible to amplify a weak signal of one color in a fiber, by making it interact with a stronger light signal of a different color – a technique that is used in fiber communications. By pumping in a bit more energy than usual, one can produce light of different colors (called higher harmonic generation), or even a super-continuum of colors, spanning tens of nanometers. Research into nonlinear fiber optics has also spawned fast, pulsed output fiber lasers, which are routinely used in surgery and industrial applications. At Aston University, we study the nonlinear phenomena in optical fibers in depth, with state of the art equipment. Quite literally, by pushing the limits we hope to tap into Nature’s hidden secrets, and inch closer to understanding why things are the way they are.

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