Monolithic thin film lithium niobate electro-optic modulator with over 110 GHz bandwidth

Driven by the information technology, such as 5G, IoT, VR and AI, the global dataflow is dramatically growing over the past decades. Optical communication is an important technology in the information transmission system, which is facing the challenges of information capability, power consumption and cost. Electro-optic modulators (EOMs) are key components serving as the information encoding engines from the electrical domain to the optical domain. Modulators featuring large bandwidth, low power consumption, compact footprint, low cost and large-scale manufacturing are highly desired and tremendous efforts have been made towards realizing the high performance.

 

Several integrated thin film lithium niobate (TFLN) Mach–Zehnder modulators (MZMs) demonstrated high bandwidth and low half-wave voltage-length product (VπL), which met the requirements for future photonic systems. In those high-performance devices' fabrication, electron-beam lithography (EBL) with high-exposure resolution served a crucial function of defining high-quality waveguides or electrodes. However, the fabrication is still on the chip scale, spends more exposure time, and is expensive for large-scale manufacturing. Photolithography was previously utilized cooperating with dry etching, but the micro-masks and redeposition in dry etching limited the performance of the devices. There is a serious challenge ---- How to improve the performance while maintain low-cost manufacturing?

 

The group led by associate professor Yanping Li in Peking University presented in Chinese Optics Letters, Volume. 20, No. 2, 2022(Fan Yang, et al., Monolithic thin film lithium niobate electro-optic modulator with over 110 GHz bandwidth)a thin film lithium niobate Mach–Zehnder modulator with a 3 dB bandwidth exceeding 110 GHz, based on wafer-scale ultraviolet (UV) photolithography and wet etching. Wet etching eliminates the influence of micro-masks and redeposition in dry etching, thereby obtaining high-quality waveguides and electrodes, as shown in Figure 1.

 

Meanwhile, the half-wave voltage-length product VπL is 2.37 V cm, the extinction ratio is >23 dB, and the propagation loss of optical waveguides is ∼0.2 dB/cm, which indicate the overall superior performance. Based on our device, we also achieve modulation rates up to 250 Gb/s of pulse amplitude modulation (PAM)- 6 signals and 200 Gb/s of PAM-4 signals.

 

Fig. 1 (a) Microscope image of the TFLN Mach-Zehnder EOM. (b) SEM image of the etched TFLN waveguides and electrodes before SiO2 cladding deposition. (c) AFM measurement of the waveguide.

 

The researchers believed this work demonstrates the feasibility of large-scale and low-cost manufacturing of TFLN EOMs and serves as an important step for making TFLN EOMs widely used in future cost-sensitive scenarios such as telecommunications, microwave photonic links, and so on. Furthermore, the device will be optimized to get the better performance and applied in the integrated modules.