Optimum source concepts for satellite-airborne communication link with RZ coding

We discuss several potential methods of generating optical RZ data, signals, distinguishing between direct RZ modulation and modulation of a primary pulse train which is either generated by using a mode locked laser, by sinusoidally driving of an external modulator, or by gain witching of a laser diode. We analyze the properties of each method cycle, extinction ratio, timing jitter, frequency chirp, robustness. Most mode locked lasers are highly sensitive to ambient perturbations, necessitating accurate temperature control and mechanical stabilization. Directly modulated semiconductor lasers are compact and robust but suffer from large frequency chirp, which deteriorates the receive sensitivity. One reliable RZ source is a conventional DFB semiconductor laser with two intensity modulators, one for pulse generation and one for data modulation. Using the RZ modulation format, the quantum limit can be approached closer.