Polarization-independent, lithium-niobate-on-insulator directional coupler based on a combined coupling-sections design
Lei Zhang, Xin Fu, Lin Yang
Abstract
We have proposed a polarization-independent directional coupler (DC) by using sections with optimal coupling strength designs on the lithium-niobate-on-insulator platform. With this design, arbitrary polarization-independent coupling ratio ranging from 0% to 100% can be achieved by tuning the length of the identical coupling region. The DC exhibits ultralow excess losses ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo><</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.1</mml:mn> </mml:mrow> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">d</mml:mi> <mml:mi mathvariant="normal">B</mml:mi> </mml:mrow> </mml:mrow> </mml:math> ) and polarization-dependent losses ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo><</mml:mo> </mml:mrow> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>0.05</mml:mn> </mml:mrow> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">d</mml:mi> <mml:mi mathvariant="normal">B</mml:mi> </mml:mrow> </mml:mrow> </mml:math> for the complete coupling) over a bandwidth of 100 nm. Moreover, the proposed DC is compact in size, simple in structure, and easy to fabricate.