In this chapter we review a few important aspects of externally optically modulated ROF links. The external RF-optical modulation arrangement was introduced in Chapter 2.
External modulation applies to modulation schemes where the light source (laser) is driven by a constant-bias current that is higher than the threshold current. This bias current is usually optimized to get the best performance from the laser. Under this condition, the laser diode is said to be operating in CW mode, meaning that it emits constant light output. The constant bias eliminates issues such as laser chirp, saturation, clipping, and instability problems.
This constant light is then modulated with an external optical modulator. The EOM approach is essentially more advanced than the direct modulation approach, with better performance at higher complexity. First, it alleviates the laser instability issues and clipping. External modulators are capable of providing very high bandwidths (over 100 GHz) and significantly better power budget (low loss or even gain) compared with directly modulated links. The widely used Mach–Zehnder interferometer (MZI) (also known as the Mach–Zehnder modulator) does not have the chirp phenomenon, although the EAM has small chirp behavior. Most external modulators can handle much higher RF input power compared with laser diodes.
Another important difference is the dependence of the RF link gain on the optical power. The RF gain of a directly ...