LASERS AND PHOTONICS
Electronic circuits can be created to transmit, amplify, and filter signals. These signals can be digital bits or analog signals such as music or voices. The desire to push electronics to higher frequencies is driven by two main applications: computers and communication links. For computers, higher frequencies translate to faster performance. For communication links, higher frequencies translate to higher bandwidth. Oscillator circuits serve as timing for both applications. Computers are in general synchronous and require a clock signal. Communications links need a carrier signal to modulate the information for transmission. Therefore, a basic need to progress electronics is the ability to create oscillators.
In the past few decades, photonics has emerged as an alternative to electronics, mostly in communication systems. Lasers and fiber optic cables are used to create and transmit pulses of a single wavelength (frequency) of light. In the parlance of optics, single-frequency sources are known as coherent sources. Lasers produce synchronized or coherent photons; hence, the name photonics. The light that we encounter every day from the sun and lamps is noncoherent light. If we could look at this light on an oscilloscope, it would look like noise. In fact, the visible light that we utilize for our vision is noise—the thermal noise of hot objects such as the sun or the filament in a light bulb. The electrical term “white noise” comes from the fact that optical noise contains all the visible colors (frequencies) and appears white. The white noise of a light bulb extends down to electronic frequencies and is the same white noise produced by resistors and inherent in all circuits. Most imaging devices, like our eyes and cameras, only use the average squared-field amplitude of the light received. (Examination at the quantum level reveals imaging devices to be photon detectors/counters.) Averaging allows us to use “noisy” signals for vision, but because of averaging all phase information is lost. To create sophisticated communication devices, such light is not suitable. Instead the coherent, single-frequency light of lasers is used. Lasers make high-bandwidth fiber optic communication possible.
Until recently, the major limitation of photonics was that the laser pulsed signals eventually had to be converted to electronic signals for any sort of processing. For instance, in data communications equipment, major functions include the switching, multiplexing, and routing of data between cables. In the past, only electronic signals could perform these functions. This requirement limited the bandwidth of a fiber optic cable to the maximum available electronic bandwidth. However, with recent advances in optical multiplexing and switching, many tasks can now be performed completely using photonics. The upshot has been an exponential increase in the data rates that can be achieved with fiber optic technology. The ultimate goal for fiber optics communication is to create equipment that can route Internet protocol (IP) datapackets using only photonics. Such technology would also lead the way for optical computing, which could provide tremendous processing speeds as compared with electronic computers of today.