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.