Lithium niobate-on-insulator (LNOI) is a promising integration platform for various applications, such as optical communication, microwave photonics, and nonlinear optics. To make Lithium niobate (LN) photonic integrated circuits (PICs) more practical, low-loss fiber–chip coupling is essential. In this Letter, we propose and experimentally demonstrate a silicon nitride (SiN) assisted tri-layer edge coupler on LNOI platform. The edge coupler consists of a bilayer LN taper and an interlayer coupling structure composed of an 80 nm-thick SiN waveguide and an LN strip waveguide. The measured fiber–chip coupling loss for the TE mode is 0.75 dB/facet at 1550 nm. Transition loss between the SiN waveguide and LN strip waveguide is ∼0.15 dB. In addition, the fabrication tolerance of the SiN waveguide in the tri-layer edge coupler is high.

An on-chip 64-channel hybrid (de)multiplexer for wavelength-division multiplexing (WDM) and mode-division multiplexing (MDM) is designed and demonstrated on a 220 nm SOI platform for the demands of large capacity optical interconnections. The designed hybrid (de)multiplexer includes a 4-channel mode (de)multiplexer and 16-channel wavelength-division (de)multiplexers. The mode (de)multiplexer is comprised of cascaded asymmetric directional couplers supporting coupling between fundamental TE mode and higher-order modes with low crosstalks in a wide wavelength range. The wavelength-division (de)multiplexers consist of two bi-directional micro-ring resonator arrays for four 16-channel WDM signals. Micro-heaters are placed on the micro-resonators for thermal tuning. According to the experimental results, the excess loss is <3.9 dB in one free spectral range from 1522 nm to 1552 nm and <5.6 dB in three free spectral ranges from 1493 nm to 1583 nm. The intermode crosstalks are −23.2 dB to −33.2 dB, and the isolations between adjacent and nonadjacent wavelength channels are about −17.1 dB and −22.3 dB, respectively. The thermal tuning efficiency is ∼2.22 mW/nm over one free spectral range.

A 32-channel hybrid (de)multiplexer on silicon is designed and experimentally demonstrated to enable polarization division multiplexing (PDM) and wavelength division multiplexing (WDM) for improving the capacity of optical communication links. The hybrid (de)multiplexer is realized by monolithically integrating a polarization rotator and splitter (PRS) and two 16-channel microring resonators (MRRs) arrays. The through and cross ports of the PRS are connected to the bus waveguide of the MRR arrays, respectively. In order to ensure a larger free spectrum range (FSR) to cover all wavelength channels, the radius of the microring is about 3 $\mu$ m and the wavelength-channel spacing is 1.6 nm. For the hybrid (de)multiplexer, the crosstalk between the adjacent channels and the non-adjacent channels are −20 dB and −25 dB, respectively. When TE $_{0}$ mode input, the insertion loss is 1.5 dB–3.8 dB, the crosstalk between the through and cross port is less than −24 dB. When TM $_{0}$ mode inputs, the insertion loss is 1.9 dB–4.5 dB and the polarization crosstalk is less than −22 dB. In order to eliminate the influence of the fabrication errors and temperature on resonance wavelength, a TiN thermal tuning electrode is added to calibrate the deviation of the wavelength.

We demonstrate an integrated transmitter with a robust silicon nitride coarse wavelength division multiplexing 4-lane (CWDM4) filter and 4 high-speed Mach-Zehnder modulators (MZM). The fabrication-tolerant CWDM4 filter comprises two-stage cascaded asymmetric Mach-Zehnder interferometers (AMZI) utilizing multi-sectional phase shifters. This design eliminates the spectral shifts induced by manufacturing linewidth variabilities. Experimental results show a good match of the operating wavelengths with predefined values. The insertion loss is $<$ −0.72 dB, and crosstalk is $<$ −13.17 dB over four operating wavelengths. The temperature dependence is 16.3 pm/ $^{\circ }$ C. The 3-mm long silicon photonic MZM exhibits electro-optical bandwidths of 32.88 GHz at −3 V and 36.68 GHz at −7 V. The half wave voltage is measured to be 4.97 V. Transmissions of 4 × 100 Gbps pulse amplitude modulation-4 (PAM4) signals, and 4 × 70 Gbps non-return to zero-on-off keying (NRZ-OOK) signals were successfully carried out. The proposed transmitter with the robust CWDM4 design offers a practical solution for Ethernet networks in data centers and has the potential for future optical communications in high-performance computing systems.

In this paper, we proposed a 128-channel hybrid mode/polarization/wavelength (de)multiplexer by monolithically integrating four 16-wavelength-channel (de)multiplexers based on bi-directional MRRs arrays and an 8-channel hybrid mode/polarization (de)multiplexer. The hybrid mode/polarization (de)multiplexer consists of a polarization beam splitter (PBS) and cascaded six asymmetric directional couplers (ADCs). The present 128-channel hybrid (de)multiplexer utilizes four modes, dual polarizations, and sixteen wavelengths to improve the data transmission capacity of optical communication systems. For the fabricated hybrid (de)multiplexer, the channel spacing is 1.4 nm, and we used thermal tuning electrodes with a tuning efficiency of 0.45 nm/mW to calibrate resonance wavelengths. The measurement results show the insertion loss is 3∼8.5 dB, the inter-mode crosstalk is −7∼−23 dB, and the inter-wavelength crosstalk is−8∼−20 dB. The proposed (de)multiplexer is a promising approach to enhance the transmission capacity and has great potential in high-speed data transmission.

In this work, we propose a mode-conversion-based chirped Bragg grating on thin-film lithium niobate (TFLN). The device is mainly composed of a 4.7-mm long chirped asymmetric Bragg grating and an adiabatic directional coupler (ADC). The mode conversion introduced by the ADC allows the chirped Bragg grating operates in reflection without using an off-chip circulator. The proposed device has experimentally achieved a total time delay of 73.4 ps over an operating bandwidth of 15 nm. This mode-conversion-based chirped Bragg grating shows excellent compatibility with other devices on TFLN, making it suitable in monolithically integrated microwave photonics, sensing, and optical communication systems.