Temporal distortion due to chromatic dispersion. Fiber Dispersion Time Axis Photo- detector to 1.3 microns, the opposite beings to happen-longer wavelengths begin to travel slower. There is also a temperature dependence in chromatic disper- sion, as seen in Figure 2-13. Although this dependence can generally be ignored for speeds up to 10 Gbps, it becomes an issue for higher speeds: an example would be OC768/40 Gbps (its impact is 16 times more severe at 40 Gbps than at 10 Gbps). The effect of chromatic dispersion can be reduced in three ways: 1. Use a light source with only one wavelength. This idea, however, is somewhat theoretical because every source has some spectral spread. The simplest lasers to manufacture are multilongitudinal-mode lasers, which produce light at a number of different wavelengths spread over 5 to 10 nm or more. More sophisticated designs (distributed feedback, external cavity, and so on) reduce the spectrum to a single line, which is narrow enough SO that chromatic dispersion can be ignored in most applications; these devices, however, are expensive. 2. A more common way is to choose a transmitter center wavelength and fiber design so that the effect is minimized. Silica-based optical fiber has low loss at 1,300 nm and even lower loss at 1,550 nm, making these good operating wavelength regions. Close to 1,300 nm, two signals at 10 nm apart in wavelengths (a typical laser line width) will have little difference in velocity. In contrast, a laser pulse with a 1-nm Optics Primer Figure 2-13 Temperature- dependent chromatic dispersion. NRZ 40 Gb/s Limit L = 200 km NRZ 40 Gb/s Limit L = 500 km L = 750 km L = 1000 km Temperature Change (C) spectral width centered at 1,550 nm will be broadened by 16 picoseconds after traveling 1 km. If the spectral width is 10 nm and the fiber length is 10 km, the pulse will be broadened by 1,600 picoseconds (1.6 nanoseconds). Negative values of chromatic dispersion have the same effect as positive values (the positive or negative sign indicates if longer or shor