Effect of internal reflections on wavelength access in widely tunable laser diodes

A key issue in widely tunable laser diodes employing codirectional mode coupling is the number of accessible wavelength channels. Depending on the device structure, only a fraction of about 25% of the available cavity modes can typically be accessed. In this paper, we identify the suppression of individual longitudinal modes by internal reflections at the grating and other waveguide inhomogenities as the relevant physical mechanism limiting the number of accessible modes. A vertical coupler filter (VCF) is analyzed in detail by the transfer matrix method, yielding analytical expressions for the internal reflections. Owing to fabrication tolerances the location of the index perturbation varies randomly, leading to a random phase of each reflection contribution. The internal reflections influence the filter transfer function and thus may randomly suppress or enhance individual modes. The fraction of accessible wavelength channels of a VCF-based laser structure has been calculated as a function of the relevant technological parameters, showing good agreement with experimental data. It is shown that by tapering the index discontinuities, a marked reduction of internal reflections can he obtained and a complete wavelength access to all longitudinal modes appears feasible.