Optical modulators provide the information encoding engines for most long-haul terrestrial and undersea fiber-optic transmission systems. The Internet that connects the people and businesses around the world can be possible because of the widespread of reliable, low-loss and high-speed optical modulators. As the trend toward ever-increasing bandwidths in fiber-optic communications continues, the optical modulator will remain an indispensable component for transmitting information. Today, hundreds of millions of modulators have been deployed worldwide, and many of them are lithium niobate (LiNbO3) modulators.
For more than two decades, LiNbO3 modulators have been the "bread and butter" for high speed and high fidelity electrical-to-optical conversion. LiNbO3 is a widely exploited crystal material that does not exist in nature. It has a trigonal crystal structure and has excellent physical properties, including large electro-optic coefficients and low optical absorption loss at a wide wavelength span from around 0.4 to 4 μm.
But in many ways, it has proven to be difficult to keep up with the trend toward ever-increasing bandwidths in fiber-optic communications. For example, the short reach links, such as metro and data-center interconnects, is a rapidly-growing field where the optical modulators must be operated in a compact space and featuring high performance in terms of power-consumption and speed. To date, the commercial LiNbO3 modulators are still bulky and power-consuming, with a moderate half-wave voltage (Vπ) of 3.5 V requiring device length of more than 5 cm, and with no clear route for further improving the modulation bandwidth (typically around 35 GHz), which limits their practical applications in the future optical links.
Many efforts have been made to realize compact and high-performance optical modulators in various material platforms, including silicon, indium phosphide, polymers and plasmonics. Among them, silicon photonics is apparently the leading platform due to the compatibility to the CMOS processing technology, the feasibility of dense integration, and the high-bandwidth and high-density I/O capability enabled by the compact waveguide dimensions. Because Pockels effect is absent in unstrained pure crystalline silicon, it is not possible to build a pure phase modulator in silicon in a similar way as the case in LiNbO3. Therefore, optical modulation in silicon mainly relies on free-carrier dispersion effect. Unfortunately, free-carrier dispersion is intrinsically absorptive and nonlinear. As a result, the silicon modulators normally suffer from the tradeoff among the bandwidth, optical losses and modulation efficiency. This is the reason why tremendous research efforts have been made to heterogeneously integrate silicon photonics with Pockels effect materials, like LiNbO3.
Prof. Xinlun Cai's group from the Sun Yat-sen University demonstrated a high-performance optical modulator by co-integrating LiNbO3 with silicon circuitry, in Photonics Research, Vol. 8, No. 12, 2020 (Shihao Sun, Mingbo He, Mengyue Xu, Shengqian Gao, Ziyan Chen, Xian Zhang, Ziliang Ruan, Xiong Wu, Lidan Zhou, Lin Liu, Chao Lu, Changjian Guo, Liu Liu, Siyuan Yu, Xinlun Cai. Bias-drift-free Mach–Zehnder modulators based on a heterogeneous silicon and lithium niobate platform [J]. Photonics Research, 2020, 8(12): 12001958). The present device shows a large modulation bandwidth (> 70 GHz), a low half-wave voltage ( 3 V) and low on-chip insertion loss ( < 1.8 dB). On–off keying (OOK) modulation up to 100 Gbit/s and PAM-4 up to 128 Gbit/s are successfully demonstrated.
Schematics of the bias-drift-free hybrid Si/LN modulator with silicon TOPS
Although the same approach has been demonstrated last year by our group, the performance here is boosted. Compared with the previous work, the present device shows much lower half-wave voltage while maintaining modulation bandwidth. Moreover, the present device exhibits a very stable DC bias point, which is very attractive for practical applications.
It's well known that the bias point in most LiNbO3 modulators change over time: this phenomenon is the so-called DC bias drift, which related to the flow and redistribution of electrical charges in LiNbO3 region under the application of the DC voltage. This requires a real time bias control loop to stabilize the operation point of the modulator. The present device utilize a thermo-optic phase shifter consisting of Ti heating film resistor on top of the silicon waveguide to control the DC bias point. The thermos-optic effect in silicon is very stable, leading to a much simpler control loop.
Prof. Cai believes that the demonstrated heterogeneous LN/SOI MZM achieve excellent optical modulation characteristics, featuring stable and substantially free of DC bias drift phenomena. LN phase modulation waveguides and the TOPS can be fabricated with lithographic precision and alignment accuracy in a back-end process after the SOI fabrication. This manufacturing procedure is highly scalable. As pure phase modulation capability in silicon photonic platform can only be achieved by utilizing slow thermal-optic effect, while carrier effects allow high-speed phase modulation accompanied by phase-dependent loss; the approach demonstrated here allows for the combination of both slow and fast pure phase modulation capabilities in silicon photonic platform. Therefore, the demonstrated approach potentially provides a new generation of compact, high-performance, and very stable optical modulators for telecommunications and data-interconnects, as well as opening up new avenues for many new applications, such as quantum photonics and microwave photonics, where pure phase modulation is crucial.
