Grating Coupler Model version 2.0
A more formal
Program
Specification is also available.
for design of input and output grating couplers
arbitrary grating profile and grating order
arbitrary waveguide profile
TE and TM polarisation, any guided mode order
re-radiation losses - locates all radiation modes
coupling coefficients and coupling efficiencies for a length of grating
plots versus wavelength, grating depth and length or any waveguide dimension
multiple grating devices (stacked vertically)
based on coupled mode theory
This model was initially developed to design grating coupled waveguide structures for inter-sub band mid mid-infra red photodetectors. The coupled mode theory of grating coupling has been extended to include TM as well as TE modes.
Above: Input coupling coefficients of a grating coupler as a function of grating thickness, and for different grating shapes as shown to the right. Resonant coupling occurs at 18º to the normal; this causes the asymmetric profile A to have much higher efficiency. Its reverse, B, has accordingly a much lower efficiency
The model will calculate input and output coupling coefficients, radiation loss coefficients and efficiencies. It will deal with sinusoidal or trapezoidal profiles or user defined (tabulated) gratings, and with waveguide structures of arbitrary profile, including graded index regions and lossy materials and up to 40 distinct slabs or regions.
Above: a schematic of a waveguide-structure photodetector for 10 µm wavelength, incorporating a grating to couple light into the wave-guide. The grating/waveguide structure increases the interaction length and thus the probability of photon absorption.
Any number of gratings stacked above each other may be included in the structure. Any waveguide mode order may be modelled. The program includes an extensive user interface, allowing interactive control and inspection of various parameters and results. It will calculate all the data outlined below.
Input/output coupling coefficients for radiative to TE and radiative to TM modes, for the nth grating order.
Radiation loss coefficient for coupling from the waveguide mode to all radiation modes. The program will locate and list all valid radiation modes automatically, sum-ming their coupling coefficients to determine the total radiation loss coefficient of the waveguide.
Efficiencies, defined as the ratio of either guided power absorbed (detected?) or emerging at the end of the waveguide, to incident light power.
Plots of all coefficients and efficiencies versus wavelength, device length or any waveguide dimension.
The calculation takes into account other losses in the waveguide and light coupled back out to radiation modes.