Temperature insensitive delay line interferometer

A delay line interferometer with temperature insensitive characteristics comprising:

• a first waveguide arm comprising a first plurality of segments connected in series to transfer a first light wave of a first peak frequency at an ambient temperature, each of the first plurality of segments being characterized by one of the first plurality of lengths and a material-specific group index;
• a heater disposed near at least one of the first plurality of segments to set a second temperature locally;
• a second waveguide arm comprising a second plurality of segments connected in series to transfer a second light wave of a second channel frequency with a time-delay difference relative to the first light wave at the same ambient temperature, each of the second plurality of segments being characterized by one of the second plurality of lengths and a material-specific group index;
• wherein a first change of phase-delay of the first waveguide arm for transferring the first light wave is configured to be equal to a second change of phase-delay of the second waveguide arm for transferring the second light wave upon any change of the ambient temperature, and the time-delay difference between the first light wave and the second light wave is equal to an inversed value of a free spectral range (FSR) associated with a designated frequency grid, and the second temperature is set above the ambient temperature to induce a frequency shift to align at least the first peak frequency to a channel frequency of the designated frequency grid.

A method for forming a temperature insensitive delay line interferometer, the method comprising:

• forming a first waveguide arm comprising a first length of a first material and a second length of a second material characterized by a second group index corresponding to a second phase delay;
• forming a second waveguide arm comprising the first length of the first material and a third length of a third material characterized by a third group index corresponding to a third phase delay, the third phase delay and the second phase delay being set to change a same amount upon temperature change;
• transferring a first light wave received from an input coupler through the first waveguide arm with a first peak frequency at a first temperature;
• transferring a second light wave received from the same input coupler through the second waveguide arm with a second peak frequency having a time-delay difference relative to the first light wave at the first temperature;
• setting the time-delay difference to be equal to an inversed value of free spectral range (FSR) associated with a designated frequency grid;
• disposing a heater on the first length of the first material to set a second temperature locally above the first temperature to induce a frequency shift to align at least the first peak frequency to one of channel frequencies of the designated frequency grid.