Million-Q integrated Fabry-Perot cavity using ultralow-loss multimode retroreflectors
Hongnan Xu, Yue Qin, Gaolei Hu, Hon Ki Tsang
Abstract
The monolithic integration of Fabry-Perot cavities has many applications, such as label-free sensing, high-finesse filters, semiconductor lasers, and frequency comb generation. However, the excess loss of integrated reflectors makes it challenging to realize integrated Fabry-Perot cavities working in the ultrahigh- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m1"> <mml:mrow> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> regime ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m2"> <mml:mrow> <mml:mo form="prefix">></mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> ). Here, we propose and experimentally demonstrate what we believe is the first silicon integrated million- <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m3"> <mml:mrow> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> Fabry-Perot cavity. Inspired by free-space optics, a novel monolithically integrated retroreflector is utilized to obtain near-unity reflectance and negligible reflection losses. The corner scattering in the retroreflector is prevented by the use of the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m4"> <mml:mrow> <mml:msub> <mml:mrow> <mml:mi>TE</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>1</mml:mn> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> mode, taking advantage of its zero central field intensity. Losses incurred by other mechanisms are also meticulously engineered. The measurement results show resonances with an ultrahigh intrinsic <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m5"> <mml:mrow> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> factor of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m6"> <mml:mrow> <mml:mo form="prefix">≈</mml:mo> <mml:mn>3.4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> spanning an 80-nm bandwidth. The measured loaded <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m7"> <mml:mrow> <mml:mi>Q</mml:mi> </mml:mrow> </mml:math> factor is <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m8"> <mml:mrow> <mml:mo form="prefix">≈</mml:mo> <mml:mn>2.1</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>6</mml:mn> </mml:mrow> </mml:msup> </mml:mrow> </mml:math> . Ultralow reflection losses ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m9"> <mml:mrow> <mml:mo form="prefix">≈</mml:mo> <mml:mn>0.05</mml:mn> <mml:mtext> </mml:mtext> <mml:mi mathvariant="normal">dB</mml:mi> </mml:mrow> </mml:math> ) and propagation losses ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" id="m10"> <mml:mrow> <mml:mo form="prefix">≈</mml:mo> <mml:mn>0.18</mml:mn> <mml:mtext> </mml:mtext> <mml:mtext>dB/</mml:mtext> <mml:mi mathvariant="normal">cm</mml:mi> </mml:mrow> </mml:math> ) are experimentally realized.