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.
Optical frequency combs (OFCs) are involved in a large diversity of applications such as metrology, telecommunication or spectroscopy. Different techniques have been explored during the last years for their generation. Using an electrooptical modulator (EOM), it is possible to generate a fully tunable OFC for which the optical repetition rate is set by the frequency of the applied electrical radio frequency (RF) signal. In order to realize on-chip OFC generators, silicon photonics is a well-suited technology, benefiting from large scale fabrication facilities and the possibility to integrate the electronics with the EOM. However, observing OFCs with a repetition rate lower than 10 GHz can be challenging since such spacings are smaller than the typical resolution of grating-based optical spectrum analyzers. To overcome this issue, two alternative solutions based on heterodyne detection techniques are used to image the OFC on the electrical RF domain. The first technique consists in applying two frequencies close to each other simultaneously on the modulator, and observing the beating between the resulting two combs. Another method consists in observing the beating between the OFC and the input laser, once the frequency of this input laser has been shifted from the center of the OFC by means of an acousto-optic modulator. Based on both measurement techniques, OFCs containing more than 10 lines spaced with repetition rates from 100 MHz to 15 GHz have been observed. They are generated using a 4-mm long silicon depletionbased traveling-wave Mach-Zehnder modulator (MZM) operating at a wavelength of 1550 nm.