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.

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.

In this paper, we demonstrate a silicon forward-biased positive intrinsic negative (PIN) Mach–Zehnder modulator (MZM), which has two operating states of high efficiency and high speed. The two operating states are switched by changing the position where the electric signal is loaded. The modulator incorporates a PIN phase shifter integrated with the passive resistance and capacitance (RC) equalizer (PIN-RC), which expands the electro-optic (E-O) bandwidth by equalizing it with modulation efficiency. The fabricated modulator exhibits a low insertion loss of 1.29 dB in two operating states and a compact design with a phase shifter length of 500 $\mathrm{\mu }$m. The modulation efficiencies are 0.0088 V·cm and 1.43 V·cm, and the corresponding 3 dB E-O bandwidths are 200 MHz and 7 GHz, respectively. The high-speed modulation performance of the modulator is confirmed by non-return-to-zero (NRZ) modulation with a data rate of 15 Gbps without any pre-emphasis or post-processing. The presented modulator shows functional flexibility, low insertion loss, and a compact footprint, and it can be suitable for applications like optical switch arrays and analog signal processing.

We propose and demonstrate a novel on-chip optical sampling pulse interleaver based on time mode interleaving. The designed pulse interleaver was fabricated on a 220 nm silicon-on-insulator (SOI) platform, utilizing only one S-shaped delay waveguide. Interleaving is achieved by the relative time delay between different optical modes in the waveguide, eliminating the need for any active tuning. The total length of the delay waveguide is 5620.5 µm, which is reduced by a factor of 46.3% compared with previously reported time-wavelength interleaver schemes. The experimental results indicate that the device can convert an optical pulse into a 40 GHz pulse sequence composed of four pulses with a root mean square (RMS) timing error of 0.9 ps, making it well suited for generating high-frequency sampling pulses for optical analog-to-digital converters.