Ultralow voltage, high-speed, and energy-efficient cryogenic electro-optic modulator
Paolo Pintus, Anshuman Singh, Weiqiang Xie, Leonardo Ranzani, M. Gustafsson, Minh A. Tran, Chao Xiang, Jonathan Peters, John E. Bowers, Moe Soltani
Abstract
Photonic integrated circuits (PICs) at cryogenic temperatures enable a wide range of applications in scalable classical and quantum systems for computing and sensing. A promising application of cryogenic PICs is to provide optical interconnects by upconverting signals from the electrical to the optical domain, allowing a massive data transfer from 4 K superconducting (SC) electronics to the room temperature environment. Such a solution can overcome a major bottleneck in the scalability of cryogenic systems that currently rely on bulky coaxial cables that suffer from limited bandwidth, a large heat load, and poor scalability. A key element to realize a cryogenic-to-room temperature optical interconnect is a high-speed, electro-optic (EO) modulator operating at 4 K with a modulation voltage at the mV scale, compatible with SC electronics. Although several cryogenic EO modulators have been demonstrated, their driving voltages are substantially large (several hundred mV to a few V) compared to the mV scale voltage provided by SC circuits. Here, we demonstrate a cryogenic modulator with <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>10</mml:mn> </mml:mrow> <mml:mspace width="thickmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi mathvariant="normal">m</mml:mi> <mml:mi mathvariant="normal">V</mml:mi> </mml:mrow> </mml:math> peak-to-peak driving voltage and Gb/s data rate, with an ultralow electric energy consumption of <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>10.4</mml:mn> </mml:mrow> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mi mathvariant="normal">a</mml:mi> <mml:mi mathvariant="normal">J</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">b</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">t</mml:mi> </mml:math> and an optical energy consumption of <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>213</mml:mn> </mml:mrow> <mml:mspace width="thinmathspace"/> <mml:mspace width="thinmathspace"/> <mml:mi mathvariant="normal">f</mml:mi> <mml:mi mathvariant="normal">J</mml:mi> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mo>/</mml:mo> </mml:mrow> <mml:mi mathvariant="normal">b</mml:mi> <mml:mi mathvariant="normal">i</mml:mi> <mml:mi mathvariant="normal">t</mml:mi> </mml:math> . We achieve this record performance by designing and fabricating a compact optical ring resonator modulator in a heterogeneous InP-on-Si platform, where we optimize a multi-quantum-well layer of InAIGaAs to achieve a strong EO effect at 4 K. Unlike other semiconductors such as silicon, our platform benefits from the high-carrier mobility and minimal free-carrier freezing of III-V compounds at low temperatures, with a moderate doping level and a correspondingly low loss (intrinsic resonator <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mi>Q</mml:mi> </mml:mrow> <mml:mo>∼</mml:mo> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>272</mml:mn> </mml:mrow> <mml:mo>,</mml:mo> <mml:mspace width="negativethinmathspace"/> <mml:mrow class="MJX-TeXAtom-ORD"> <mml:mn>000</mml:mn> </mml:mrow> </mml:math> ). These modulators can pave the path for complex cryogenic photonic functionalities and massive data transmission between cryogenic and room-temperature electronics.