Method to optimize a light coupling waveguide

A method for constructing a light coupling system wherein a grating is manufactured on an incident light surface of a multimode waveguide, said grating defining the entrance of the multimode waveguide for an incident light beam, said grating comprising a repetition of patterns, said method comprising a step of choosing a first set of parameters comprising

• wavelength distribution of the incident light to be transmitted by the multimode waveguide, said wavelength distribution being at least 40 nm,
• polarized or unpolarized nature of said incident light,
• incident angle standard deviation of the incident light with respect to the multimode waveguide,
• shape of the patterns, which is designed to have its local surface normal (Ns), in any location of said shape, making an angle (α) with respect to the average normal (N) of the surface of the multimode waveguide, said angle (α) being comprised between α=−90° and α=90°, refractive index (n1) of the medium surrounding the multimode waveguide,
• multimode waveguide refractive index (n3),
• said grating being defined by a second set of parameters comprising
• grating period (P), separating two adjacent patterns,
• grating depth (d) between the highest and the lowest point of the pattern,
• incident angle mean value (0) of the incident light with respect to the normal to said entrance of the multimode waveguide,
• said method comprises a step of optimization of the second set of parameters to obtain an optimized second set of parameters,
• said step of optimization being performed to obtain a transmission efficiency (Ce) of the incident light crossing through the grating into said multimode waveguide for the first or the second diffractive order exceeding 35% for unpolarized light, or exceeding 50% for polarized light, over at least the wavelength distribution of the incident light beam as defined in the chosen first set of parameters, said grating having a length (D) measured on the incident light surface of the multimode waveguide in the direction of the diffracted beam, said length (D) being related to the thickness (WT) of the multimode waveguide and to the angle β between the multimode waveguide surface normal and the light beam inside the multimode waveguide, said length D being defined by the following inequality:
• D≦2·WT·tan β
• said method comprising a step of manufacturing the grating on the surface of the multimode waveguide according to the chosen first set of parameters and to said optimized second set of parameters.