Arbitrary-ratio 1 × 2 optical power splitter based on thin-film lithium niobate.

Optical power splitters (OPSs) have been widely used in photonic integrated circuits, but an OPS with a large fabrication tolerance and free choice of power splitting ratio (PSR) is still highly desired for thin-film lithium niobate (TFLN) platform. Here, we propose and experimentally demonstrate several 1 × 2 OPSs with PSRs from 50:50 to 5:95 using TFLN platform. The proposed devices are built by multimode interference structure to achieve a broad bandwidth and large fabrication tolerance. Various PSRs can be obtained by adjusting the geometry structure of the multimode interference region. All of our fabricated devices feature an insertion loss lower than 0.3 dB at the wavelength of 1550 nm, and a PSR variation less than 3% in the range of 1520 nm to 1590 nm.

C-Band optical 90-degree hybrid using thin film lithium niobate.

The integrated optical 90-degree hybrid is a crucial component for coherent receivers. Here, we simulate and fabricate a 4 × 4 multimode interference coupler as a 90-degree hybrid using thin film lithium niobate (TFLN). The device features low loss (0.37 dB), high common mode rejection ratio (over 22 dB), compact footprint, and small phase error (below 2°) within the whole C-band experimentally, which is promising for integration with coherent modulators and photodetectors for TFLN-based high-bandwidth optical coherent transceivers.

Photonic integrated circuit-based fiber-optic temperature and strain sensing system.

A low-cost, multi-function fiber-optic sensing system is highly desirable for physical security monitoring. Using the silicon photonic integrated circuit technology, we propose and demonstrate a compact fiber-optic sensing system which can simultaneously measure the temperature and strain information. A key enabler of the proposed system is an on-chip optical interrogator consisting of a two-dimensional grating coupler, four microring resonators, and four on-chip photodetectors. The interrogator conveys the temperature and strain information via measuring the center wavelength of a fiber Bragg grating and the polarization state of back-reflected light through a single-mode fiber.

Compact substrate-removed thin-film lithium niobate electro-optic modulator featuring polarization-insensitive operation.

A compact polarization-insensitive electro-optic (EO) modulator, which allows the laser and modulator to be located at different locations while using a standard single-mode fiber to interconnect them, is highly desirable for 5G or future 6G wireless networks. Herein, we propose a modulator based on substrate-removed thin-film lithium niobate. The proposed device exhibits a polarization-dependent loss of 0.35 dB and on-chip loss of approximately 2 dB. The polarization insensitivity of the proposed device was experimentally demonstrated using a four-level pulse-amplitude modulation format at 50 Gbaud (100 Gb/ s).

High-performance polarization management devices based on thin-film lithium niobate.

High-speed polarization management is highly desirable for many applications, such as remote sensing, telecommunication, and medical diagnosis. However, most of the approaches for polarization management rely on bulky optical components that are slow to respond, cumbersome to use, and sometimes with high drive voltages. Here, we overcome these limitations by harnessing photonic integrated circuits based on thin-film lithium niobate platform. We successfully realize a portfolio of thin-film lithium niobate devices for essential polarization management functionalities, including arbitrary polarization generation, fast polarization measurement, polarization scrambling, and automatic polarization control. The present devices feature ultra-fast control speeds, low drive voltages, low optical losses and compact footprints. Using these devices, we achieve high fidelity polarization generation with a polarization extinction ratio up to 41.9 dB and fast polarization scrambling with a scrambling rate up to 65 Mrad s-1, both of which are best results in integrated optics. We also demonstrate the endless polarization state tracking operation in our devices. The demonstrated devices unlock a drastically new level of performance and scales in polarization management devices, leading to a paradigm shift in polarization management.

Electrically pumped widely tunable O-band hybrid lithium niobite/III-V laser.

Significant improvements in the lithium niobate on insulator (LNOI) platform are pushing LNOI-based laser sources to the forefront of integrated photonics. Here, we report the first, to the best of our knowledge, electrically pumped hybrid lithium niobate/III-V laser by butt coupling an InP-based optical gain chip with a LNOI photonic integrated circuit (PIC). In the PIC, a Vernier filter consisting of two LNOI microring resonators is employed to select the lasing wavelength. A wavelength tuning range of more than 36 nm is achieved in the O band. The hybrid laser has a maximum on-chip optical power of 2.5 mW and threshold current density of 2.5kA/cm2. A side mode suppression ratio better than 60 dB is achieved.

Broadband, low-loss silicon photonic Y-junction with an arbitrary power splitting ratio.

Photonic integrated circuits (PICs) often require broadband power splitters such as Y-junctions for signal monitoring, signal feedback, power distribution, etc., with various splitting ratios. Therefore, a parameterized Y-junction with an arbitrary power splitting ratio that can be selected in layout design is desired in a PIC library. Here, we propose an ultra-compact (1.4 µm × 2.3 µm) Y-junction on the 220-nm-thick silicon-on-insulator (SOI) platform for an arbitrary splitting ratio with a programmable design. It applies smooth curvatures for a good tolerance to fabrication errors. Rigorous 3D-FDTD simulations predict an excess loss below 0.36 dB and a splitting-ratio variation of less than 1 dB over 100 nm. Experimental results achieved using a CMOS-compatible silicon photonics process show an excess loss of lower than 0.5 dB for a splitting ratio varied from 0 to -18 dB across the entire C band. Both numerical and experimental results show that the power splitting ratio of the proposed device is weakly wavelength dependent.

Chip-scale, full-Stokes polarimeter.

The polarization of light conveys unique information that can be exploited by crucial applications. The bulky and costly discrete optical components used in conventional polarimeters limit their broad adoption. A compact, low-cost polarimeter would bring this functionality into a myriad of new scenarios and revolutionize its exploitation. Here we present a high-performance, full-Stokes polarimeter on a silicon chip. A surface polarization splitter and on-chip optical interferometer circuit produce the complete analysis matrix of an optimally conditioned polarimeter. A matrix analysis on measurement errors is also performed. This solid-state polarimeter is a system-on-a-chip with exceptional compactness, stability, and speed that could be used singly or in integrated arrays. Large arrays can increase the speed and resolution of full-Stokes imaging; therefore, our design provides a scalable polarimeter solution.

On the role of localized surface plasmon resonance in UV-Vis light irradiated Au/TiO2 photocatalysis systems: pros and cons.

The role of localized surface plasmon resonance (LSPR) in UV-Vis light irradiated Au/TiO2 photocatalysis systems has been investigated, and it is demonstrated experimentally for the first time that both pros and cons of LSPR exist simultaneously for this photocatalytic reaction. We have proved that when operating under mixed UV and green light irradiation, the LSPR injected hot electrons (from the Au nanoparticles to TiO2 under green light irradiation) may surmount the Schottky barrier (SB) formed between the Au nanoparticles and TiO2, and flow back into the TiO2. As a result, these electrons may compensate for and even surpass those transferred from TiO2 to the Au nanoparticles, thus accelerating the recombination of UV excited electron-hole pairs in TiO2. This is the negative effect of LSPR. On the other hand, more hot electrons existing on the surface of the Au nanoparticles due to LSPR would favor the photocatalytic reaction, which accompanied by the negative effect dominates the overall photocatalytic performance. The presented results reveal the multi-faceted essence of LSPR in Au/TiO2 structures, and is instructive for the application of metal-semiconductor composites in photocatalysis. Moreover, it is confirmed that the extent to which the above pros and cons of LSPR dominate the overall photocatalytic reaction depends on the intensity ratio of visible to UV light.