Optical Windows and Spectrum
Wavelength remains a significant factor in fiber-optic developments. Figure 3 illustrates the wavelength “windows.” Table 1 shows the wavelength of each optical window and the typical application for multimode (MM) or single-mode (SM) operation.
The earliest fiber-optic systems were developed at an operating wavelength of about 850 nm. This wavelength corresponded to the so called “first window” in a silica-based optical fiber, as shown in Figure 3. This window refers to the wavelength region that will offer a low optical loss that sits between several large absorption peaks. The absorption peaks are caused primarily by moisture in the fiber and Rayleigh scat- tering, which is the scattering of light due to random variations in the index of refraction caused by irregu- larities in the structure of the glass.
The attraction to the 850 nm region came from its ability to use low-cost infrared LEDs and low-cost sili- con detectors. As technology progressed, the first win- dow lost its appeal due to its relatively high 3 dB/km losses. Most companies began to exploit the “second window” at 1310 nm with a lower attenuation of about 0.5 dB/km. In late 1977, Nippon Telegraph and Telephone developed the “third window” at 1550 nm. The third window offers an optical loss of about 0.2 dB/km.
The three optical windows—850 nm, 1310 nm, and 1550 nm—are used in many fiber-optic installations today. The visible wavelength near 660 nm is used in low-end, short-distance systems. Each wavelength has its advantages. Longer wavelengths offer higher performance, but always come with higher cost.
Table 2 provides the typical optic attenuation for each of the common wavelengths versus the fiber- optic cable diameter. A narrower core fiber has less optical attenuation.
The International Telecommunication Union (ITU), an international organization that promotes world- wide telecommunications standards, has specified six transmission bands for fiber-optic transmission. The first is the O band (“original band”), which is from 1260–1310 nm. The second band is the E band (“extended band”), which is 1360–1460 nm. The third band is the S band (“short band”), which is 1460–1530nm. The fourth band in the spectrum is the C band (“conventional band”), which is 1530–1565 nm. The fifth band is the L band (“longer band”), which is 1560–1625 nm. The sixth band is the U band (“ultra band”), which is 1625–1675 nm. There is a seventh band that has not been defined by the ITU that is in the 850 nm region. It is mostly used in private networks. The seventh band is widely used in high-speed computer networking, video distribution, and corporate applications.
Researchers have attempted to develop new fiber optics that could reduce costs or improve performance. Some alternative fiber materials have found specialized usage. Plastic fiber is ideal for short transmission distances that are ideal for home theater installations. Lower cost glass fiber reduces the need to develop longer distance plastic fiber and the higher cost of copper wire has expanded glass fiber-optic cable applications.