Multibranch frequency-selective reflectors and application to tunable single-mode semiconductor lasers

Multibranch reflectors (MBRs) are proposed as a flexible wavelength-selective element for optical integrated circuits generally and for tunable single-mode lasers specifically. The undesired recurring peaks of reflectivity obtained with two-branch MBRs are spread out in the spectrum by using three or more branches. Design relations are given for the lengths of an arbitrary number of branches, for the selectivity near the peak response, for the wavelength interval to the nearest undesired peak, and for tuning changes in the various branches. It is shown that an MBR is not sensitive to amplitude differences of the order of 20% between the branches, but requires phase control in order to achieve tuning to a specific wavelength. The effects of semiconductor gain variation with wavelength are obtained from a rate-equation solution including MBR. By changing the semiconductor temperature (gain peak) at the same time as the MBR is tuned, a broad range of tuning without jumps is achievable