One important use of fiber optics is in antenna remoting applications. In such an application wide bandwidth (multi-channel) radio frequency (RF) information is remotely collected and converted into an analog signal for transmission over the ground. Systems based on antenna remoting technology are often deployed as listening stations to gather information for intelligence purposes. Antenna remoting is also used where geographic barriers prohibit the use of high power or the housing of processing electronics at the receiver. The remotely located antenna can receive standard radio and television signals as well as military (RF) transmissions, over a very wide range of frequencies (virtually the entire RF spectrum). Very large amounts of data must be transmitted at high speed and often the system must be easily transportable. Consequently, conventional transmission via copper coaxial cable or RF waveguides (i.e., metal pipes or tubes) is not practical.
Converting the RF signals into an optical analog output for transmission through a fiber-optic cable is necessary in order to avoid the bandwidth and loss limitations of coaxial cables or waveguides. Externally modulated, fiber-optic links are one means of antenna remoting for ground-based systems (e.g., See U.S. Pat. No. 4,070,621). Elementary antenna remoting systems have used two polarized laser sources and single mode optical fiber between the sources and the modulator. Direct modulation detection is used. This approach is relatively inexpensive, although there is a 3 dB power budget penalty.
One difficulty of conventional antenna remoting systems is that such systems are sensitive to environmental effects. A "standard" single mode fiber carries two polarization modes. In a perfect waveguide without any external environmental effects, those two polarization modes will be degenerate (i.e. they will be in phase). As you introduce variations, either through an external effect, such as small temperature changes or just because it is difficult to make a perfect, totally unstressed waveguide, the two polarization modes will lose their degeneracy, introducing a phase difference between them. Thus, a polarized input light signal will tend to transfer power between those two polarization modes, thereby scrambling the polarization signal. So, in the real world, singlemode fibers do not maintain a stable state of polarization. That has an impact on polarization-sensitive devices, such as many external modulators, and explains why the fiberoptic community has developed an interest in polarization-maintaining fibers.
Polarization maintaining (PM) optical fibers are better. Typical designs of polarization-maintaining fibers today create a propagation difference between those two modes, favoring one at the expense of the other. A polarized light signal launched into that favored polarization mode will tend to have its polarization state maintained down the length of the fiber and the output signal's polarization will be identical to, or at least similar to, the input signal's. Unfortunately, such optical fibers are more expensive.