Up to this point we have discussed each of the primary measures of link performance – gain, bandwidth, noise figure and dynamic range – in as complete isolation from the other parameters as possible. While such an approach permitted us to focus on the various aspects of each parameter, it did miss the effects of interactions among the parameters to a large extent. Clearly when designing a link, one needs to take into account such interactions; in fact one might argue that maturity in link design is gauged by the link designer's ability to balance often conflicting requirements to meet a given combination of link parameters.
As one might expect, there are myriad potential interactions among link parameters. Therefore in this chapter we can only offer a sampling of these interactions. We begin by exploring interactions among the primary parameters of the intrinsic link. In general the best link designs usually result from attaining the required performance via optimization of the intrinsic link.
However, there are situations where despite a link designer's best efforts, the intrinsic link performance falls below the requirements. In some of these cases electronic pre- and/or post-amplification may be used to improve performance. Consequently we expand our interaction space to include a sampling of tradeoffs between amplifier and link parameters.
Tradeoffs among intrinsic link parameters
Diode laser bias current
In Fig. 2.2 we saw that the slope efficiency of a directly modulated link is highest just above threshold and decreases as the bias current is increased above threshold – slowly at first and then more rapidly as the bias current is increased further.