ation. This is why there are no applications at frequencies higher than the extra high frequency (EHF) band (Figure 4.10). 4.2.5.3 Optical Communications At the infrared light frequencies just below visible light (wavelength 400-700 nm) a controlled transmission medium, optical fiber, provides very low attenuation. Optical fiber is the most important media for high-capacity long-distance transmission. It is used in national long-distance networks as well as in international and intercontinental submarine systems. The commercial optical communication systems of today use binary light pulses for transmission. The transmitted information is usually in binary form, which means that the receiver either detects light or does not. The present optical systems are not able to use transmitted light as a carrier Transmission wave in the same way that radio systems do. Radio systems are able to change phase and frequency of the carrier wave, not just intensity. Traditionally, one optical signal occupies the whole fiber although a small portion of its very wide frequency would be feasible. Characteristics of optical fibers are intro- duced in Section 4.7. However, development of narrowband optical transmitters and optical filters has made it possible to increase the data transmission capacity by inserting multiple optical channels into the same fiber with the help of the dense wavelength division multiplexing (DWDM) system, which is introduced later in this chapter. As technology is developed, we will be able use light at a certain fre- quency as a carrier wave. Then we can increase fiber capacity further by util- izing the CW modulation methods discussed previously. The utilization of this so-called coherent optical technology will increase the transmission capacities of optical fibers dramatically in the future. 4.2.6 Free-Space Loss of Radio Waves Most radio systems of today operate well above 100 MHz where the radio wave travels a direct path from the transmitting antenna to the receiving antenna. This propagation mode is