edx Phot1x report template 3


MZI or Mach-Zender Interferometer (dotaapso@gmail.com)

Lets look at our design below:
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In our scheme we use adiabatic couplers. They work by adiabatically converting the mode of a single waveguide into either the even or odd mode of two identical waveguides separated by a small gap. Although adiabatic couplers look similar to directional couplers they work very differently in that adiabatic couplers ensure only one fundamental mode is excited at all times within the coupler. In a conventional directional coupler both the fundamental even and odd mode of the structure are excited initially which then interfere with each other and results in the characteristic power oscillation between the two waveguides. Therefore, key to designing with adiabatic couplers is that only one mode of the system is excited at all time and the transition of the mode from the that of a single waveguide to the even/odd mode of the two waveguides is done slowly (adiabatically) enough. Once this adiabatic transition occurs the light naturally stays in the even/odd mode, which automatically has 50% of the power in each waveguide. Consequently the adiabatic coupler is inherently broadband, insensitive to fabrication imperfections and   length (as long as it is long enough).