Chirp Stabilization in 1550-nm InGaAsP/InP Quantum Well Distributed Feedback Lasers with Controlled Optical Feedback

The frequency chirp of semiconductor lasers of directed modulated laser is investigated. It is shown that both thermal and adiabatic chirps can be controlled through the use of a proper controlled optical feedback. I. Introduction It is known that the current modulation of a diode laser's active region results generates both amplitude (AM) and optical frequency (FM) modulation. This effect is related to the modulation of both the photon density and the carrier density. Thus, the modulation of the carrier density also modulates the gain and so the optical index causing the resonant mode to shift. For high-speed applications, this frequency chirping is known to broaden the modulated spectrum, which is a serious limitation in optical fiber communications. Determining the FM contribution allows to extract the laser's frequency chirp1. This paper aims to show that the frequency chirp can be stabilized using a controlled optical feedback. These preliminary results obtained on quantum well distributed feedback lasers (QW DFB) demonstrate that under proper feedback conditions, the chirp-to-power (CPR) ratio remains constant and equal to 33 MHz/mW for modulation frequencies up to 130 MHz as compared to 0.54 GHz/mW in the free-running case. Stabilizing the frequency chirp can then be realized via a controlled optical feedback, which is of first importance for enhancing the transmission capabilities in optical telecommunication systems. II. Device Description and Experimental Setup The DFB laser is a buried ridge stripe (BRS) with a high reflection (HR) coating on the rear facet and an antireflection (AR) coating on the front facet in order to allow for high efficiency.