Proceedings - Viện Vật lý

_________________________________________________________________________________Những tiến bộ trong Quang học, Quang tử, Quang Phổ và Ứng dụng. 8/2006, Cần Thơ, Việt Namwere restricted to a power range below 10 mW. In this paper, we perform for thefirst time a detailed study on the linewidth of 780 nm RW DFB lasers up to anoutput power of 150mW. With the help of a heterodyne detection system detailinvestigations of the linewidth in dependence of the optical power were performed.The structure of theDFB is shown in Figure1. The DFB lasers weregrown by low-pressuremetal organic vaporphase epitaxy (MOVPE)in two steps /6/. First theepitaxy layers weregrown on GaAs substrateas follows: n-GaAsbuffer, n-AlGaAscladding, 250 nm n-AlGaAs waveguide,14 nm tensile-strainedGaAsP active quantumwell (QW), 250 nm p-AlGaAs waveguide. Thefirst part of the p-AlGaAscladding and an2. Laser structure and fabricationFigure 1. Transverse cross section showing epitaxiallayer sequence of the 780 nm DFB lasers investigatedInGaP/GaAsP/InGaP layer sequence in which the grating was formed byholographic photolithography and wet-chemical etching. Secondly, the remaining p-AlGaAs cladding and a p-GaAs contact layer were grown. Due to the tensilestrainedGaAsP QW, the laser emission is in TM mode.The grating period determines the lasing wavelength of the 780 nm DFB lasers.The periode of the applied second order gratings is nearly 235 nm (see Fig. 2). Thecoupling coefficient of the grating depends mainly on the thickness and thecomposition of the GaAsP grating layer and its distance to the active zone. Due to aprecise adjustment of these parameters a value of 1 cm -1 was realized.Figure 2. SEM picture showing the buriedsecond order Bragg grating L = 235 nm.Figure 3. SEM picture of the epitaxial layerstructure showing the ridge waveguide.268