Last time when we talked about CCDs, we were concerned with how to take an optical signal, like an image, and convert it to an electronic signal. Then it can be processed, moved, and stored using electronics. But there is an obvious question this idea raises: why is the conversion to electronic signal needed? Why can’t we process the optical signal directly? Is there a way to manipulate a stream of photons that’s analogous to the way that electronic circuits manipulate streams of electrons?
The answer is yes, and the field dealing with optical signal processing is called photonics. In the same way that we can generate electronic signals and manipulate them, signals made up of light can be generated, shuffled around, and detected. While the underlying physical mechanisms are different from those in electronics, much of the same processing can take place! There are a lot of cool topics in photonics, but let’s go over some of the most basic technology just to get a sense for how it all works.
The most common way to generate optical signals in photonics is by using a laser diode, which is actually another application of the p-n junction. Applying a voltage across the junction itself causes electrons to drift into the junction from one side, while holes (which are oppositely charged) drift in from the other side. This “charge injection” results in a net current flow, but it also means that some electrons and holes will meet in the junction. When this happens, they can recombine if the electron falls into the empty electron state that the hole represents. But there is generally an energy difference between the free electron and free hole state, and this energy can then be emitted as a photon. This is how light with a specific energy is generated in the semiconductor laser diode, and when the junction is attached to an enclosed area to amplify that light, you get a very reliable light source that is easy to modulate in order to encode a signal.
But how do you send that signal anywhere else? Whereas electronic signals pass easily through metal wires, photonic signals are commercially transmitted through transparent optical fibers (hence the term “fiber optic”). Optical fibers take advantage of total internal reflection, a really cool phenomenon where for certain angles at an interface, all incident light is reflected off the interface. Since light is a quantized electromagnetic wave, how it moves through its surroundings depends on how easy it is to make the surrounding medium oscillate. Total internal reflection is a direct consequence of Snell’s Law, which describes how light changes when it goes between media that are not the same difficulty for light to pass through (the technical term for this is refractive index). So optical fibers consist of a fiber with high refractive index which is clad in a sheath with lower refractive index, tuned so that the inner fiber will exhibit total internal reflection for a specific wavelength of light. You can see an example of total internal reflection below, for light travelling through a plastic surrounded by air. When optical fibers exhibit total internal reflection, they can transmit photonic signals over long distances, with less loss than an electronic signal moving through a long wire would experience, as well as less susceptibility to stray electromagnetic fields.
Photonic signals can then be turned back into electronic signals using semiconducting photodetectors, which take advantage of the photoelectric effect. This technology is the basis of most modern wired telecommunications, including the Internet!
But if you are remembering all the electronic components, like resistors and capacitors and transistors, which we use to manipulate electronic signals, you may be wondering what the corresponding parts are for photonics. There are photonic crystals, which have microstructure that affects the passage of light, of which opal is a naturally occurring example! And photonic signals can be recorded and later read out on optical media like CDs and DVDs. But in general, the commercial possibilities of optical data transmission have outweighed those of complex photonic signal analysis. That’s why our network infrastructure is photonic but our computers, for now, are electronic. However, there are lots of researchers working in this area, so that could change, and that also means that if you find photonics interesting there is much more to read!