Thin-film lithium niobate (TFLN) shows strong potential as a platform for integrated photonics, due to its robust electro-optic coefficient, large optical nonlinearity, and wide transparency window. TFLN is used in the development of various optoelectronic components, but most TFLN devices must rely on external lasers and photodetectors because lithium niobate (LN) does not natively provide a light source and photodetection. An on-chip, integrated, high-performance photodetector is essential to exploit TFLN’s potential as a photonics integrated circuit (PIC) platform. (a) TFLN wafer with pre-defined waveguide and passive components. (b) Bare InP/InGaAs wafer. (c) InP/InGaAs wafer and TFLN wafer bonding. (d) InP/InGaAs wafer substrate removal. (e) N mesa dry etch. (f) P mesa dry etch. (g) SU-8 base for CPW pad. (h) Metal electroplating and lift-off. Courtesy of C. Wei et al. To address this need, researchers at Southwest Jiaotong University heterogeneously integrated a modified uni-traveling carrier (MUTC) photodiode wafer onto a TFLN wafer with pre-defined waveguides and passive components. The MUTC photodiode simultaneously boosts the bandwidth and the responsivity of the TFLN platform. The researchers initiated the fabrication process by dry-etching the LN waveguides and passive devices. They used a hybrid etching approach to form the device mesa. After metal plating and lift-off were completed, the chips were diced and polished. The team optimized the epitaxial layer structure, LN waveguide geometry, and coplanar waveguide pad geometry to achieve both a large bandwidth and high responsivity. To assess the performance of the TFLN device, the team applied the device to a data transmission system. It detected four-level pulse amplitude modulation (PAM4) signals at 32 Gbaud with high quality. These results demonstrate the potential of the photodiodes on the TFLN platform to enable next-generation, high-speed transmission systems. The device demonstrated a record-high 3-dB bandwidth of 110 GHz, which is comparable to the state-of-the-art for TFLN modulators. The waveguide-coupled photodiodes based on the wafer-level, TFLN-indium phosphide (InP), heterogeneous integration technique exhibited a dark current of approximately 1 nA (nanoampere) and a responsivity of 0.4 A/W (amperes per watt) at a 1550-nm wavelength. (a) Measured (blue circle) and simulated (black dashed line) responsivities of the devices with different lengths. (b) Transit-time-limited bandwidth (blue solid line), RC-limited bandwidth (red solid line), total bandwidth (black dashed line), and measured bandwidth of the devices with various active areas (black circle). (c) Measured bit error rates (BERs) versus the received optical power for 32-Gbaud PAM4 signal. (d) Eye diagrams and measured waveforms of the PAM4 signal with 10, 20, and 32 Gbaud. Courtesy of C. Wei et al. TFLN technology has enabled tight mode confinement and high nonlinear efficiency, leading to its wide adoption in optical communications. It has been used to build various compact, integrated photonics devices, including high-performance modulators, polarization management devices, and broadband frequency comb sources. However, the inherent difficulty of LN in realizing light sources and photodetectors has stood in the way of a TFLN-based, integrated photonics platform. The team’s wafer-level integration of ultrawideband photodiodes on a TFLN platform is a significant step toward addressing this issue. The researchers demonstrated that heterogeneously integrated photodiodes on a TFLN platform have the potential to be applied in the next generation of high-speed transmission systems. The work paves the way to achieving massive-scale, multifunctional, high-performance TFLN photonic integrated circuits. Moreover, it holds promise for ultrahigh-speed optical communications; high-performance integrated microwave photonics; and multifunctional, integrated quantum photonics. The research was published in Light: Advanced Manufacturing (www.light-am.com/article/doi/10.37188/lam.2023.030).
