rated by the mode spacing of the cavity. Efforts to narrow the spectrum to permit greater bandwidth and repeater spacing have resulted in other structural designs. As an example, the structure of an internal DFB laser to reduce the spectrum width to a few megahertz is also shown in Figure 2-28. Multimode laser operation pro- duces mode-partition noise. This means that the relative intensity of the various modes fluctuates, while the total optical power remains constant. This fluctuation degrades system performance. Single-frequency lasers eliminate this noise. They reduce the dispersion bandwidth impairment to only that associated with the spectrum produced by the modulation. Fur- ther bandwidth impairments are caused by laser chirp, which is a fre- quency change in the laser output caused by on and completely off signaling. The change increases the undesired spectral lines. This effect can be suppressed by keeping the laser in the light-emitting state and changing the light intensity for signaling. 2.6.3 Line Signal Optical line signals are usually in the form of return to zero (RZ) or nonre- turn to zero (NRZ) pulses. The RZ pulses typically have an average 50 per- cent duty cycle. (Duty cycle is the fraction of time that the light source is on compared with the assigned time slot.) For off, the light from an LED is turned off; on the other hand, lasers are only dimmed (NRZ) to about 10 dB below the on light. Line signals are also used for clock recovery. The clock Chapter 2 Figure 2-28 p-InP LD structures. InGaAsP (Active) n-InP n-InP Light beam (a) Cleaved crystal facets at each end of the optical cavity p-InP p-InGaAsP p-InGaAsP n-InP InGaAsP Grating p-InGaAsP n-InP Light beam InGaAsP (Active) (b) Grating for internal distributed feedback for narrowing of spectrum recovery circuit requires a minimum number of transitions between Os and 1s and only short runs of contiguously successive 1s or 0s. To meet these conditions, line signals are typically scrambled to provide a balance between Os and 1s. 2.6.4 Tunabl