taken by the optical radiation to rise as a result of an applied pulse is the turn on delay t as shown in Figure 53.16, and is given by Equation 53.37, where T is the recombination time constant and Figure 53.14 The light/current characteristic of LEDs I the applied signal current. Optical sources 53/13 than the width of the pulse otherwise there will be again a pattern effect. It was mentioned above that the spectral density of a laser depends on the modulation. This leads to two related phenomena, spectrum broadening and partition noise. Spectral broadening manifest itself with the appearance of new spectral lines which where not there under CW driving conditions. The cause of partition noise is the variability of the energy of each laser mode as the device is modu- Stimulated lated. Now, assuming that the pulse propagates over a dispersive medium, each mode suffers different delay and at the receiver the emission pulse amplitude fluctuates, suggesting the existence of noise pro- regime cess in operation. The information bandwidth of lasers range for a few megahertz to gigahertz with current experimental devices reaching 20GHz. The theoretical bandwidth relation is as in Equa- tion 53.38. (53.38) LED regime Current (mA) In this equation W and B w are given by Equations 53.39 and 53.40, and t _ and ph are the spontaneous recombination time and Figure 53.15 The light-current characteristic of a laser mean photon lifetime respectively, as in Figure 53.17. Again as in LEDs the bandwidth depends on bias current. (53.37) increasing 0 with the maximum of t d obtained for lb=0. the device at or above threshold, the turn on delay is zero. This implies that for high bit rate systems pre-bias is mandatory, otherwise there will be a pattern effect. In addition to the turn on delay a laser driven by a digital signal exhibits relaxation oscilla- tions. Clearly the period of the relaxation oscillations should be less Figure 53.17 Small signal frequency response of semiconductor lasers (53.39) (53.40) At w = w the device exhib