Numerical Simulation of Grating Input/Output Elements into Optical Waveguides Based on Si3N4/SiO2 Thin Films

This paper presents the results of modeling a grating element for input/output of optical radiation into Si
3
N
4
/SiO
2
waveguide structures in the wavelength range from 1.5 to
$1.6\ \mu \mathrm{m}$
. The results of the study of the geometric parameters of a diffraction grating on the optical transmission coefficient, in particular: the grating period, undercutting period, etching depth, filling factor, are presented. For a wavelength of
$1.55\ \mu \mathrm{m}$
the maximum optical transmission coefficient is observed at: grating period
$0.875\ \mu \mathrm{m}$
; etching depth
$0.181\ \mu \mathrm{m}$
; fill factor of
$0.595\ \mu \mathrm{m}$
; undercutting
$\pm 0.015\ \mu \mathrm{m}$
. The greatest contribution to the change in the transmission coefficient is made by the change in the grating period and is
$\pm 0.0022\ \mu \mathrm{m}/{\%}$
and the etching depth is
$\pm 0.00762\ \mu \mathrm{m}/{\%}$
. Based on the results of simulations, taking into account the operating wavelengths and refractive indices of Si
3
N
4
/SiO
2
films, the following permissible deviations of diffraction lattice parameters were put forward: grating period
$\pm 0.022\ \mu \mathrm{m}$
; etching depth
$\pm 0.0762\ \mu \mathrm{m}$
; filling factor ±20%; undercutting period
$\pm 0.14\ \mu \mathrm{m}$
. Permissible insertion angles were 10–12 degrees. The reflection loss at the input of light into the waveguide was −17 dB, and at the output from −20 to −35 dB. Transmission loss at −3dB was
$0.09\ \mu \mathrm{m}$
.