Effect of index tapering on characteristics of DFB and DBR lasers

Abstract

ABSTRACT Some critical parameters of uniform and tapered DFB and DBR lasers are calculated and compared to each other. We haveshown that tapering improves some of them including internal quantum efficiency, modal selectivity and yield. Howeverspectral purity of tapered strictures would be worsen. We show that tapering is an ideal choice when the designer desires tomake the device asymmetric.Keywords: Quasi-periodic gratings, DFB and DBR lasers 1. INTRODUCTION Although, DFB laser now is accepted as a standard light source in long haul optical communication networks, there is atendency to improve its characteristic through manipulating its basic structure. In conventional DFB and DBR lasers,usually uniform gratings are used. These periodic structures cause coupling between the incident and reflected wavesprovided that the propagation constant is close to the Bragg's conditions. Tapering of periodic structure is a commonpractice in microwave region of spectrum. Matching is the main aim of using tapering there. However in DFB and DBRlasers, there is not an obvious reason for using tapered structures. Index tapering is implemented by reducing the gratingdepth gradually. It causes mainly a variation of coupling coefficient through the device length. Also it causes the variationof local propagation constant. By linear tapering we intend that the depth of grating reduces linearly with distance alongthe device length. Similarly exponential tapering indicates exponential grating depth variation. In this paper we considerlinear tapering exclusively.Tapering is a tailoring between grating depth and length. It is a known fact that coupling will increase if gratingdepth increases. Furthermore, coupling will be more strong for longer gratings. Shallow grating with long length can bereplaced virtually with a deep short grating. In this regard, tapering does nothing; we can replaced it with an equivalentuniform one. Even this equivalence is a drawback of tapering, since the uniform grating with equivalent coupling usuallyis smaller in length. However, as we will show the superiority of tapering for DFB and DBR lasers is on the other aspects.