Efficiency-enhanced third-order grating coupler

A third-order grating coupler with a grating period 3 times that of a first -order grating is discussed in terms of efficiency enhancement. A reflectio n film is integrated between the optical buffer layer and the substrate ben eath the grating-guiding core. The power-distribution ratio for each diffra cted wave depends on the optical buffer layer's thickness because of interf erence effects. The thickness is determined to suppress first- and second-o rder diffraction and to enhance the power distribution to the output air-ra diation wave generated by third-order diffraction. A demonstrator was desig ned and examined at a wavelength of 0.82 mu m. The measured output coupling efficiency was 40%, whereas the theoretical prediction was 60%. (C) 1999 O ptical Society of America.