Mode-division multiplexing (MDM) technology can expand transimission capacity of a communication system and multimode waveguide bend (MWB) is a vital component. However, the reported MWBs either occupy large footprints or require complex fabrication steps. In this letter, the levelset method and adjoint method are combined to achieve an ultracompact three-mode MWB with an effective radius of 4.4 $\mu {\mathrm{ m}}$ . The MWB structure can be defined by levelset method which projects a three dimensional (3D) surface into a two dimensional (2D) curve. Moreover, the efficient evolution of the MWB structure are guaranteed by the adjoint method, this method only requires two simulations to derive the derivative of the figure of merit (FOM) with respect to design parameters. The simulation results of our MWB reveals that the average insertion losses for TE0, TE1 and TE2 modes are 0.04 dB,0.06 dB and 0.07 dB, respectively. In addition, the intermode crosstalks are all below −21dB in a wide spectral range from 1500 nm to 1600 nm. The MWB is also fabricated on a commerical silicon-on-insulator (SOI) wafer, the experimental spectra show that the insertion losses for three modes are 0.28 dB, 0.25 dB and 0.18 dB, the crosstalks between those modes are less than −13.4 dB across the wavelength range from 1525 nm to 1600 nm. The design methods can be applied not only to various nanophotonic components but also to other material platforms.

Silicon-based optical switches are indispensable for on-chip optical interconnects and have become essential components of optical neural networks and optical computing. Although broadband Mach–Zehnder interferometer (MZI) switches have been widely studied, they are large in size and difficult to implement wavelength or polarization selection functions at the switching element (SE) level. Here, we introduce a novel design for a polarization- and wavelength-selective switch on the standard silicon-on-insulator (SOI) platform, composed of mode-coupled micro-ring (MR) switches, polarization rotators (PRs), and mode exchangers (MEs). Parallel-cascaded MR switches facilitate the expansion of wavelength channels, while PRs and MEs enable two polarizations to route separately through two sets of MR switches without the need for polarization splitting. As a proof of concept, we demonstrate an ultra-compact 2 × 2 × 3λ × 2P (TE & TM) SE with full spatial, spectral, and polarization switching capabilities. Experimental results show that the insertion losses are less than 1.1 dB, and the crosstalk is better than −17 dB around 1550 nm. Additionally, all channels’ 3 dB optical bandwidth is over 60 GHz, and characterization with 50 Gbps on-off keying (OOK) modulated signals demonstrates aggregate throughput up to 600 Gbps.

Mode-division multiplexing (MDM) technology significantly increases transmission capacity, with mode (de)multiplexers serving as key components. However, designing lithium niobate-on-insulator (LNOI) mode (de)multiplexers is challenging due to potential mode hybridization. In this work, we propose an ultra-compact and high-performance mode (de)multiplexer based on inverse-design shape optimization for LNOI platform. By accounting for crystal anisotropy, sidewall angles, rib geometry, and minimum feature sizes, we optimize the device structure to achieve a compact device with low insertion loss (IL) and crosstalk (CT). The experimental results for the fabricated four-channel mode (de)multiplexer show IL of approximately 0.37 dB and maintains CT below −15.7 dB over a wide wavelength range of 1500–1600 nm. These results highlight the potential of LNOI in multimode photonics and reveal its suitability for high-density, low-loss photonic devices.

Lithium niobate on insulator (LNOI) has received much attention in recent years due to its excellent material properties, such as low optical loss, strong nonlinearities and high index contrast, possessing great potential in future nanophotonic devices and systems. However, most of current LNOI devices are designed by traditional physical models and inefficient parameter sweeps, which limit their performance and functionality. In this paper, we propose an inverse design methodology that introduces the levelset method to achieve ultra-compact, high-performance, and readily manufacturable LNOI devices. The levelset method offers a flexible and natural representation of device contours. In addition, it also yields simply-connected device structures (no holes, islands and gaps) that are robust to fabrication errors. We also factor in crystal anisotropy, ridge feature and sidewall angle of LN waveguide to substantially improve simulation accuracy. Using this methodology, we design and fabricate a waveguide crossing and a mode multiplexer, both of which exhibit ultra-compact footprints, simply-connected device contours and consistency between simulation and experimental results. Our design method can be applied to a wide range of LNOI devices and nanophotonic platforms with similar material properties and fabrication constraints, paving the way toward future integrated optical chips and nonlinear technologies.

Polarization management is a key technique in integrated photonic circuits. In this paper, a low loss and compact TE-pass polarizer based on lithium niobate on insulator (LNOI) platform is presented. By utilizing subwavelength grating (SWG) metamaterials and inverse design algorithm, the TE₀ mode propagates through the SWG region with minimal loss, while the TM₀ mode is efficiently coupled out and suppressed through shape-optimized algorithm, thereby achieving an expanded bandwidth of the polarization extinction ratio (PER). With a footprint of 66 μm, the polarizer exhibits low insertion loss (IL) < 0.174 dB and a PER > 10 dB over 176 nm (1465–1641 nm), reaching 33.2 dB at 1550 nm. Furthermore, the proposed polarizer demonstrates superior overall performance, along with promising potential for polarization management and mode conversion in high-performance LNOI-based integrated photonic systems.

Micro-racetrack resonators have become one of the key components for realizing signal processing, generation, and integration in microwave photonics, owing to their high Q factor, compact footprint, and tunability. However, most of the reported micro-racetrack resonators are confined to the single-mode regime. In this paper, we designed an ultra-compact multimode micro-racetrack resonator (MMRR) based on shape-optimized multimode waveguide bends (MWBs). Cubic spline curves were used to represent the MWB boundary and adjoint methods were utilized for inverse optimization, achieving an effective radius of 8 μm. Asymmetric directional couplers (ADCs) were designed to independently couple three modes into a multimode micro-racetrack, according to phase-matching conditions and transmission analysis. The MMRR was successfully fabricated on a commercial platform using a 193 nm dry lithography process. The device exhibited high loaded Q factors of 2.3 × 10⁵, 4.1 × 10⁴, and 2.9 × 10⁴, and large free spectral ranges (FSRs) of 5.4, 4.7, and 4.2 nm for ${\mathrm{TE}}_0$, ${\mathrm{TE}}_1$, and ${\mathrm{TE}}_2$ modes, with about a 19 × 55 μm² footprint.