Improved design of optical directional full couplers based on new function modified by hamming function

Improved design of adiabatic directional full couplers based on new function modified by hamming function is investigated. The modification-hamming function weighting of new function is described. The validity of approximation theory of adiabatic directional full couplers becomes questionable when the coupling length is not enough long so the beam propagation method is used. We propose a new design and numerically show that some optical properties of the optimized one are better than the optimized coupler with hamming function weighting. The objective of the work including in the new design of adiabatic directional full couplers can obtain respectively low noise, large frequency and short couplers. When the crosstalk is demanded to be smaller than -40 dB, the coupler length for hamming function weighting for new weighting is 5.8 mm at 1.57 mum wavelength and the one for modification-hamming function weighting is only 4 mm at 1.57 mum wavelength. When the length of coupler is 12 mm and the crosstalk is smaller than -40 dB, the corresponding wavelength ranges of convention and modified cases are in the 1.38~1.67 mum and 1.3~1.7 mum, respectively. It is surely that the proposed modification-hamming function weighting is more effective than conventional weighting for adiabatic directional full couplers