Analog direct-modulation behavior of semiconductor laser transmitters using optical FM demodulation

In this paper, we report a theoretical investigation of the analog modulation performance of a semiconductor laser transmitter which employs the direct optical FM demodulation. This analysis is based on the rate equations in which Langevin noise functions are included. The optical FM response has firstly been derived using a chirp-power relation deduced from the rate equations. The chirp-to-power ratio (CPR) has secondly been computed and illustrated showing the effect of the novel damping parameter phenomenologically introduced in the rate equation for the optical phase. The curves show that the conventional CPR can be notably reduced using FM demodulation. Beside this, the comparison of the FM responses with prior studies show that our model may be more appropriate to describe optical FM behavior of many types of single-mode lasers. The relative-output noise (RON) at the transmitter has also been determined and subsequently used to calculate the carrier-to-noise ratio (CNR) available. It is shown that the transmitter inherent noise would significantly degrade as a result of the conversion of the optical frequency noise into intensity noise. Consequently, intrinsic noise is expected practically to contribute the most to CNR at high powers. This feature can be more easily appreciated through curves illustrating CNR and is in contrast to previous studies in which the receiver shot noise was assumed to be the main noise term.