On the injection laser contribution to mode partition noise in fiber telecommunication systems

The injection laser can initially turn on in a side mode rather than in the main mode when the risetime of the driving current is short compared to the electron lifetime, causing mode partition noise in digital transmission systems which have wavelength dispersion between the laser and the receiver. It is shown that increased differential loss between the main mode and side modes reduces both the incidence and duration of side-mode turn-ons and is the most desirable approach to reducing mode partition noise. The needed differential loss is a strong inverse function of the signal pulse width. Gain compression has a lesser influence on the rate of occurrence of side-mode turn-ons. Quantitative results from a computer model indicate that significant numbers of such turn-ons occur even when the side-mode power during quiescent bias conditions never exceeds the main-mode power. Biasing slightly above threshold will always increase the pulse rate at which a given laser can be operated and provides an attractive goal for designers of laser transmitter packages