Simulation analysis of high-order high-duty-cycle surface gratings

High-order surface grating distributed feedback lasers are known to operate with a fundamental mode, narrow linewidth, high power, and high slope efficiency. The adoption of high-order surface gratings can avoid epitaxial re-growth necessary for the fabrication of conventional buried gratings, which simplifies the fabrication process and reduces device cost. It is essential for the design and optimization of device structure to clarify the influence of the change of grating structure parameters on grating characteristics (coupling and loss). Based on Lumerical's Mode Solutions and multiple grating samples, we evaluated the coupling and loss coefficients of surface gratings as a function of duty cycle, order, and V-groove topography. As the order increases, the duty cycle corresponding to the peak value of the grating coupling coefficient increases gradually and approaches one. The grating coupling coefficient decreases with increasing order but increases at some specific orders. At high duty cycles, the width of the grating groove corresponding to the peak of the coupling coefficient remains substantially in the range of 100-150 nm, which is close to the length of a quarter-wavelength in the grating groove filling material. Regarding the grating groove morphology, the fabrication difficulty of the V-shaped groove grating is obviously less than that of the rectangular groove grating, but its coupling coefficient is slightly smaller than that of the rectangular shaped groove grating of the same depth. The larger the V-shaped groove width, the smaller the peak coupling coefficient and the corresponding sidewall inclination will be. Losses decrease with increasing duty cycle and decreasing sidewall inclination of the V-groove.