This is my last post for a while, and then I’m off on a sort of spring sabbatical. I’ll be back this summer though, so don’t miss me too much. You can still email me at firstname.lastname@example.org!
After last week’s post on LEDs, one of my favorite readers inquired about how fiber optic lightning works. Well, I’m a sucker for special requests.
As many of you may have observed, optical fiber technology is used not only to make those glowing, flexible sea-urchins that spice up your party-lighting, it has also become extremely prevalent in the field of information transmission. Cable television, phone, and internet companies have spent the last decade replacing their underground lines with fiber optic cables, and this transition alone is responsible for significant developments in high-speed, high-definition technology. It’s one thing to make a clear, bright picture, it’s another to transmit that gorgeous image to your house over several miles without a loss in clarity. It’s a rough world out there for digital data, and safe, undistorted passage through an underground network is essential. With fiber optics now nearly ubiquitous, phone calls sound more clear, internet speeds are up, and we can get hundreds of high-definition television channels into our home with only the slightest hesitation as we rapidly flip through channels. It’s a pretty hefty amount of information traveling over an impressive distance.
Yet despite our reliance on innovations such as these that make possible our vast, high-speed world of accessible information, I think it’s safe to say and lighting geeks and ravers everywhere probably appreciate this technology far more than the rest of us do. After all, they have some pretty wild stuff.
What they’re made of
Optical fibers are long thin strands of glass or plastic that can range in size from a few microns to a millimeter in diameter. A number of analogies can be made between these optical fibers and wires as methods for long-distance transmission. Metal wires are the railways for electrons, while optical fibers provide a pathway for electromagnetic waves, or light. The longer the distance that something has to travel, the greater the risk of loss. Electricity is wasted in the form of heat because the wire has resistance. Similarly, light can be lost if it “escapes” the tubing structure.
A fiber optic cable is usually a collection of many individual strands, and if you look very closely at a single fiber, you will see that it is essentially made of three parts: a core glass center, a material that surrounds the glass tube called the cladding, and some plastic coating that acts as a buffer from damage, moisture, and other effects of nature.
Starting with illumination
Optical fibers do not generate light, they just direct it. Generally, light travels in the direction that you point its source – a flashlight shines down a hallway, spreading outward in a generally forward direction. When it strikes a surface, it can do one of three things: transmit, reflect, or refract.
Objects that are transparent, like plastic wrap, allow light to travel through them unhindered. That is why we can see everything undistorted when we look through clear, thin materials – this is transmission. Those materials that we can’t see through, like metal, do not allow any light through at all. However, if a metal surface is smooth, light will strike it and bounce off. These types of objects are popularly known as mirrors, and they reflect light.
The last type of behavior is the most interesting. If you’ve ever looked down into a pool, you might have noticed that while you can see into it, things look a little off. This is due to the third behavior of light at a surface: refraction
Understanding refraction to understand reflection
Light travels at different speeds in different media. In empty space, where there isn’t any matter to slow it down, light travels at its very fastest – about 300,000 kilometers (186,000 miles) per second. It slows down a bit when it has air molecules to bump into, and even more so when it travels through something denser like water or glass. We quantify this difference with a property called the refractive index. This number is calculated by dividing the speed of light in a pure vacuum by the speed of light in a particular material. By definition, the index of refraction of empty space is exactly one. For air, it is about 1.0003, water 1.33, glass 1.5, and diamond 2.4. This makes intuitive sense – the denser the material, the slower light travels within it, and therefore the larger this ratio will be.
Because of this change in speed, when light strikes a boundary at an angle, it will change direction, or bend slightly. I was given the following analogy as a college freshman to explain why this happens:
Imagine that you and all your friends go to the beach, and when you get there, you all decide to stand in a line holding hands (because that’s obviously what everyone does at the beach). All at once, you and your hand-locked friends sprint forward towards the ocean, but the speed of the runners in your line is stacked. All your fast friends are on one end while all the slow ones lag behind on the other. You meet the ocean at a steep angle, fast runners first. As each person hits the water, naturally they slow down – you can’t run as fast in the water as you can on land. Because the very fast runners are slowed down first, it gives the slower ones a chance to catch up. Once you have all reached the water, still attempting to run (and still looking very silly), your group will still be moving forward at an angle, but that angle wont be as steep as it was when you hit the water.
While this analogy isn’t perfect, it helped me understand why light bends when it moves from one index of refraction to another at an angle. We define a line perpendicular to the boundary between two materials that we call the normal to the surface. When light moves from a material with a low index to a higher one (faster speed of light to slower), and it strikes at an angle other than 90 degrees, the transmitted light is bent towards the normal. The greater the difference in the indices, the stronger the bend. The physical law that allows you to calculate the exact angles is called Snell’s Law, and it’s a pretty simple one.
Total internal reflection
When light moves from a medium of a higher index of refraction to a lower one (such as from glass to air) it will bend away from the normal. As the angle of incidence increases, this bend will get sharper, and the refracted ray will move closer to the interface. Eventually, we reach what is called the critical angle – the incoming angle at which the refracted ray is bent perfectly along the surface. At all incoming angles above that, light can no longer escape the denser medium without being bent back inwards. At this point, we experience total internal reflection.
In fiber optics, light travels through a glass tube that has been coated with an optical material that I mentioned above, called cladding. This cladding has an index of refraction that is lower than that of glass, and so when light strikes it as it travels forward, it refracts back into the glass. And because the cladding is molded around the glass and flexes with it, this total internal reflection is maintained regardless of how the fiber itself travels or bends.
Remember that light striking the surface at a smaller angle than the critical angle can escape. But when we place a source of light at one end of the fiber, the only light that can get into such a narrow tube will be the light that is traveling along a forward direction within it, and all those rays would be primarily traveling at or above the critical angle.
Waves of information
So far, I have discussed the applications of fiber optic cables in the transmission of visible light, but what many people forget is that most of our modern digital information is transmitted via electromagnetic waves. For example, we receive music and cell phone signals from radio waves. Other types of signals, such as phone and internet communications, can also be transmitted through fiber-optic cables, if the cable is constructed to maximize the transmission of a particular wavelength. These cables are so physically small that many of them can be grouped together in a bundle, one which could support a range of different frequencies, increasing bandwidth capabilities.
The greatest advantage of fiber optic cables is their flexibility. They can be distorted and moved along any number of paths without compromising their efficiency or durability. Copper wires will hold their bent, rigid form, and they tend to wear out after too much shaping. For this reason, fiber-optic cables are often used for medical imaging. Cameras are placed on one end and the cable is pushed relatively safely through narrow passageways in the human body, such as through the esophagus or through very small incisions in the skin. The flexibility of the cable minimizes the risk of damage. This property also makes fiber optic cables useful as simple flashlights for diagnosing electrical problems or plumbing blockages.
Fiber optics’ second most important advantage is their efficiency. The signal loss compared to electrical transmission is very low, though exactly how much depends on the wavelength being transmitted. Most signal loss occurs because of impurities in the glass, allowing beams to bounce off at angles that cannot be refracted back inwards. In premium cable lines, the degradation can be less than 10% per kilometer, whereas that number can be closer to 50-60% in electrical transmission through copper wire.
Other advantages include their cost (glass is much cheaper and more abundant than copper), their small size, and their light weight. But additionally, light as an information carrier has several advantages over electricity. It is safer (it can’t start fires), emits less waste-heat and is therefore less energy consuming, and it doesn’t interfere with signals being carried in other nearby cables. Moving electrical signals generate magnetic fields, and these can interfere with other electric signals close by, but light waves contained in these casings do not suffer this weakness. Your phone conversations and the picture on your television come in much more clearly now than they used to.
In summation, optical fibers have been extremely useful in increasing our capacity to transmit digital signal. They operate on a relatively basic principle of physics, one that has been well understood for hundreds of years. But our advancement development of materials and our unquenchable thirst for speed has hurtled them into the front lines, literally. They’re below us, everywhere, but luckily, it’s hard to be afraid of something that makes for some pretty sweet party lighting.
Thanks for reading! Until next time!