Dual comb spectroscopy using silicon electro-optical modulators

摘要

Optical frequency combs (OFCs) have played an important role over the past years for optical frequency metrology and synthesis, for astronomy, telecommunications, and spectroscopy. Among the different methods that have been studied for OFC generation, electro-optic frequency combs (EOFCs) using electro-optical modulators show a large flexibility in comb repetition rate, making it a solution of choice for absorption spectroscopy where a fine sampling in the frequency domain is usually required. Silicon photonics is a promising platform for EOFC generation, thanks to its high volume production and strong light confinement allowing to achieve small footprint photonic integrated circuits (PICs). Additionally, silicon PICs benefit from a direct compatibility with complementary metal-oxide semiconductor (CMOS) fabrication process. Carrier depletion-based modulators have already proved to be efficient and to achieve high bandwidth operation, which makes them suitable for EOFC generation. In this work we will show the first dual comb spectroscopy experiment using silicon optical modulators. As a proof of concept for spectroscopy applications, beating of two silicon EOFCs with slightly different repetition rates is observed in the RF domain using a multi-heterodyne detection technique. Each EOFC is generated from a silicon push-pull Mach-Zehnder modulator and shows typically 12 equally separated lines. The comb repetition rate is swept from 500 MHz to 12.5 GHz, thanks to the inherent flexibility of EOFCs, while their relative offset is kept steady (4 MHz). This technique is used to recover the transfer function of an optical band-pass filter without any tunable laser.