Output intensity noise of lightwave transmitters employing chirp demodulation

Direct modulation of semiconductor lasers results in simultaneous variations of the lightwave intensity and frequency. Thus, information can be transmitted over fiber using either of both characteristics. The use of direct optical frequency modulation (FM) instead of intensity modulation (IM) requires an FM-to-IM conversion scheme which additionally converts the phase (or frequency) fluctuations into intensity, causing extra noise. In this letter, we theoretically analyze the relative-output noise of a single-mode laser followed by a Mach-Zehnder interferometer.