Contents I. Introduction
The continued exponential growth in internet traffic has driven research on optical signaling at a baud rate of over 100 Gbaud [1]–[2]. Recent intensity modulation and direct detection (IM-DD) and coherent demonstrations of 100 Gbaud+ capable modulators have included the use of Indium Phosphide (InP) based modulators, Lithium Niobate (LiNbO3) modulators and Germanium-Silicon electro-absorption modulators (GeSi-EAM) [3]–[8]. In datacenter interconnects, the low-cost potential of silicon photonics (SiP) has attracted a widespread interest and product developments benefiting from the mature high-yield and scalable manufacturing of CMOS microelectronics, including the development on integration with electronic integrated circuits (EIC) [9], [10]. The intrinsic high index contrast of silicon and silicon oxide in the SiP platform allows for the integration of different passive and active optical components with a small footprint. The active devices include high-speed modulators, based on principles such as free carrier dispersion effect and Franz-Keldysh effect. These modulators include Mach-Zehnder modulators (MZM), microring modulators (MRM) and electro-absorption modulators (EAM) [11]–[16]. Among these modulators, GeSi-EAM based on the Franz-Keldysh effect enjoy the benefits of high bandwidth, very small footprint, low driving voltage and a good optical bandwidth of 30 nm. The low driving voltage and device capacitance gives it a high energy efficiency, reported to be 13.8 fJ/bit (NRZ) [8]. The GeSi-EAM can be more energy efficient than conventional carrier-depletion MZMs, which has a typical energy efficiency of ∼200 fJ/bit [17]. The GeSi-EAM is based on the Franz-Keldysh effect, which describes a change in the bandtail absorption with applied electric field. GeSi-EAM is compatible with advanced modulation formats such as 4-level pulse-amplitude modulation (PAM-4), up to 80 Gbaud (160 Gb/s) has been demonstrated with a single modulator [18]–[23], and 104 Gbaud (204 Gb/s) by interleaving 4 modulators operating at 26 Gbaud (56 Gb/s) [19]. Its small footprint allows it to be configured into transmitter structures for coherent modulation formats and other complex multiplexing structures using multiple EAMs, such as wavelength-division multiplexing (WDM) transmitters and space-division multiplexing (SDM) transmitters, without the requirement of a large chip area [18], [19], [21]–[23]. The GeSi-EAM is also a reliable device, the lifetime is predicted to be more than 10 years based on accelerated aging tests [24]. Modulation at over 100 Gbaud has been a challenging topic in SiP, because of the difficulty in optimizing for high bandwidth modulators beyond 50 GHz 3-dB bandwidth in the SiP platform [1]. Here we show the first operation of a single GeSi-EAM at 100 Gbaud+ in C-band. We demonstrate PAM-4 modulation with a GeSi-EAM and direct detection using an integrated GeSi waveguide photodiode. Back-to-back (B2B) and standard single mode fiber (SSMF) transmission are performed at different data rates (168 Gb/s-240 Gb/s). The modulator and photodiode have a measured 3-dB bandwidth of over 65 GHz and the photodiode has a responsivity of 0.8 A/W at 1550 nm. The devices are fabricated at a commercial foundry using CMOS compatible processes.
