Amazing Science – Photons and Optical Fiber Cable

by Yang Yang on July 25, 2012

Today, most of our online data transmissions pass through fiber-optic cables. And as most of us know, fiber-optic cables are simply long, thin strands of glass which allow light to bounce and refract from one-end to the other without leaving the cable. It’s a bit like shining a flashlight down a long, mirrored tube.

One of the great benefits of fiber-optic cabling is that it can transmit multiple signals at the same time by combining different colors. (Photons with different wavelengths) Because of this, the number of simultaneous signals which can pass through a fiber optic cable is theoretically limited only by the accuracy and precision of the hardware. Compare this with a copper wire, which can only send one signal at a time.

But few people really appreciate how much incredible science goes into fiber optic networking, or the fascinating principles which govern this technology that has such a profound impact on our daily lives.

One of these principles is what’s often called the “Lifeguard Principle”, also referred to as the “principle of minimal action” or “principle of least action”.

Now I’m not a scientist, and I may slightly over-generalize in certain areas. But I’ll attempt to present these principles in the most accurate way that I can in laymen’s terms.

We all know that the shortest path from point A to point B is a straight line. But what about the FASTEST distance?

How to get from A to B?

For example, let’s assume that you’re a lifeguard that sees a drowning swimmer off in the distance. The shortest route to that swimmer would be to follow a straight line towards the swimmer through land and water.

From A to B in shortest path

But in a life-or-death situation, every second counts. The running portion of this trajectory may be very fast, but the swimming portion will be very slow. If you waste too much time swimming, the person you’re trying to save might drown.

From A to B in potentially shortest time (fasted path)

Another approach might be to run along the beach until you reach the point which offers the shortest swimming distance. Although this may take care of the swimming problem, now you’ve spent too much time running. Yes, this route may be faster than the “straight line” approach, but it’s still possible to shave off a few more precious life-saving seconds.

Somewhere in between these 2 strategies, there is a sweet spot where the combination of swimming and running time is reduced to the absolute minimum.

From A to B in shortest time (fasted path)

It’s possible to calculate this optimal trajectory using complex calculus, but I won’t bore you with that for this article.

Now let’s take this principle and apply it to a photon of light instead of a lifeguard.

If you’re standing in a boat and you point a laser at a fish in the water, what will happen?

Photons automatically travel in fasted path

Of course, we all know that the light beam will bend when it hits the surface. But how does the beam “decide” which angle to bend at?

Scientists have been able to calculate the speed at which light travels through different substances such as air and water. And when you combine the speed of light through air and the speed of light through water, you can make an interesting observation.

If you add up the time it takes for the light to go from the laser to the water and the time it takes for the light to travel from the water’s surface to the fish, it turns out that the trajectory which is taken by those photons is the shortest possible route for a photon of that wavelength.

In other words:

If the photon was a lifeguard and the fish was a drowning swimmer, the light would always pick the route which requires the least amount of time to reach the swimmer. And it’s this principle which dictates how the light decides to refract when it reaches the water’s surface.

To an untrained layman, this seems almost miraculous. It’s almost as if the photon could see into the future and plot a path to its final destination before it ever left the laser. And it’s almost as if the photon was able to perform the difficult calculus required to find the shortest possible path.

Of course, it’s not quite that simple. But it’s a clear demonstration of how beautiful science can be.

This least-action principle is also critical to optical networking technology. When you send a message through a fiber optic cable, it truly does take the fastest possible route to its final destination. And if it wasn’t for the principle described above, Internet connectivity would not be possible.

There are many other implications which result from this phenomenon, but they are simply beyond my understanding so I couldn’t speak to those issues.

About The Author: Patrick Jobin is a technical writer with Storagepipe Solutions, a leader in serious server online backup services for datacenters and corporate networks.

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