Spectral linewidth of tunable twin-guide laser diodes

The excess broadening of the spectral linewidth in the tuning mode is analyzed for the case of the tunable twin-guide (TTG) distributed feedback (DFB) laser, taking into account the injection-recombination shot noise (IRSN) caused by the tuning current and the interaction between active and tuning region. It is shown, that linewidth broadening Δν_IRSN and tuning efficiency are mutually related by a square law dependence, if the tuning is based on carrier injection. This relationship is independent of the material parameters and the transverse device structure. At equal tuning efficiency, the smallest broadening occurs for the as-cleaved devices with a small × L-product, whereas the structures with a π/2 phase shifted grating exhibit a distinct enhancement of Δν_IRSN at × L ≥ 2, independently of the facet reflectivity. The influence of the impedance of the bias circuit for the tuning region is also investigated, revealing that Δν_IRSN can be reduced up to about an order of magnitude by applying a low-resistive bias source (voltage controlled tuning mode).