Analysis of Total Harmonic Modulation Distortion of a Directly Modulated and Optically Injected Fabry-Perot Laser Diode

This paper presents calculations on total harmonic modulation distortion level of an injection-locked Fabry-Perot laser diode directly modulated by a sinusoidal modulating signal accompanied by the harmonics of the fundamental modulation. The theory predicts that the total harmonic distortion level shows a resonance like effect followed by a fall-off with modulation frequency. The novel prediction of the theory is that optical injection favors total harmonic modulation distortion suppression and the lower the optical injection level, the better is the suppression. This is due to the fact that when relative optical injection power is small, the lockband is also narrow. So, the higher order modulation sidebands fall outside the lockband and get suppressed. This makes the output THD power level lower