High-performance thin film lithium niobate (LN) electro-optic modulators with low cost are in demand. Based on photolithography and wet etching, we experimentally demonstrate a thin film LN Mach–Zehnder modulator with a 3 dB bandwidth exceeding 110 GHz, which shows the potential of boosting the throughput and reducing cost. The fabricated modulator also exhibits a comparable low half-wave voltage-length product of ∼2.37V·cm, a high extinction ratio of >23dB, and the propagation loss of optical waveguides of ∼0.2dB/cm. Besides, six-level pulse amplitude modulation up to 250Gb/s is successfully achieved.

In this work, based on a conventional silicon photonic travelling-wave Mach-Zehnder modulator (MZM) with a 3-dB bandwidth of 22.5GHz, we experimentally demonstrate ultra-high speed optical interconnects with Nyquist shaped pulse amplitude modulation (PAM) signals. For bandwidth-limited systems, a two-tap digital post filter is employed to suppress the equalization-enhanced high frequency noise. The post filter induced inter-symbol interference (ISI) is subsequently eliminated by maximum likelihood sequence detection (MLSD). Enabled by the post filter and MLSD, we achieve 200Gb/s (80Gbaud) PAM-6 signal direct detection (DD) transmission over 1km standard single-mode fiber (SSMF) with a bit-error rate (BER) below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5×10 $^{-2}$ . For PAM-4 format, we successfully transmit 192Gb/s (96Gbaud) and 176Gb/s (88Gbaud) signals over 1km and 2km SSMF, respectively, with BERs lower than the 20% HD-FEC threshold. For PAM-8 format, 192Gb/s (64Gbaud) and 168Gb/s (56Gbaud) signals are generated at back-to-back (BTB) and transmitted over 1km SSMF, respectively. To our best knowledge, we achieve the highest single lane bitrates ever reported for single polarization PAM-4/6/8 signal generation and DD transmission with all-silicon MZM. The flexible receiver-side digital signal processing (DSP) can significantly enhance the performance of silicon MZM, which provides a promising solution for future single lane 200G data-center interconnects.

We propose an ultra-compact optical 90° hybrid with the smallest length of 107μm, consisting of a wedge-shaped 2 × 4 MMI coupler connected with a 2 × 2 MMI coupler using silicon nanowaveguide technology. Neither cascaded phase shifters nor waveguide crossings are attached to the proposed 90° hybrid in coherent receiving system. The proposed device is demonstrated on silicon-on-insulator (SOI) with 220nm thick top-silicon layer and 2μm thick buried oxide layer. A high performance of the proposed 90° hybrid is exhibited experimentally with a high extinction ratio larger than 20dB, an excess loss mostly less than 0.5dB, a common mode rejection ratio better than −20dB and phase deviation within the range of 5° over C-band spectral range.

A compact and broadband mode (de)multiplexer utilizing the asymmetrical directional coupling between a hybrid plasmonic waveguide and a silicon nanowire waveguide is proposed. The coupling length is 13.6 μm, which is the shortest in ${\mathrm{TE}}_0$ and ${\mathrm{TE}}_1$ mode multiplexers reported, to the best of our knowledge. It shows a high mode conversion efficiency of 99.2% with excess loss of only 0.35 dB at 1550 nm. The mode multiplexing circuit exhibits a low crosstalk of less than $-17\mathrm{dB}$ over a large bandwidth of 100 nm.

We review high baud rate operation of silicon photonic modulators with emphasis on short-reach and metro-haul applications. The current status of both devices and digital signal processing are discussed. The basic design of a typical carrier-depletion-based dual-drive traveling-wave Mach-Zehnder modulator (TW-MZM) is studied by simulation. Based on an all-silicon TW-MZM, we report the transmission experiments of high baud rate single sideband 4-ary pulse amplitude modulation (PAM-4) signals with linear equalization and artificial neural network based equalization, respectively. Our work indicates all-silicon modulators can address the requirements of future optical communication systems operating at 100 Gbaud.

The exponential growth in data-driven applications like artificial intelligence and metaverse poses an increasing demand for integrated optical interconnects with efficient and cost-effective high-capacity solutions. For short-reach optical interconnects dominated by intensity-modulated direct detection (IM-DD), high-speed and large-capacity transmission enabled by high-bandwidth electro-optic devices is always highly desired. In this article, based on a thin film lithium niobate (TFLN) modulator with a 3-dB bandwidth larger than 110 GHz fabricated by ourselves, we experimentally demonstrate ultrahigh-speed optical interconnects with probabilistic shaping pulse amplitude modulation (PS-PAM) signals. With linear pre-equalization and Volterra nonlinear equalizer (VNLE), a 120 Gbaud PS-PAM16 signal with a bit rate up to 408 Gb/s is generated. Compared to the regular-PAM8 signal with the same bit rate of 360Gb/s, the PS-PAM signal demonstrates a receiver sensitivity gain of 2.2 dB. With the help of VNLE, 384.5 Gb/s PS-PAM16 signal can be transmitted over 500 m standard single-mode fiber (SSMF) at the C band. The ultra-broadband TFLN enabled transmission provides a promising solution for future 400G per wavelength data-center interconnects.