Light detection and ranging (LiDAR), combining traditional radar technology with modern laser technology, has much potential for applications in navigation, mapping, and so on. Benefiting from the superior performance, an all-solid-state beam steering realized by integrated optical phased array (OPA) is one of the key components in the LiDAR system. In this review, we first introduce the basic principle of OPA for beam steering. Then, we briefly review the detailed advances of different solutions such as micro-electromechanical system OPA, liquid crystal OPA, and metasurface OPA, where our main focus was on the recent progress of OPA in photonic integrated chips. Finally, we summarize the different solutions and discuss the challenges and perspectives of all-solid-state beam steering for LiDAR.

/pdf/all-solid-state-beam-steering-via-integrated-optical-phased-tmgwd6c8.pdf

All-Solid-State Beam Steering via Integrated Optical Phased Array Technology

Thin‐film lithium‐niobate photonics has attracted intensive attention due to its superior material properties. In particular, multimode lithium‐niobate photonics is becoming attractive for the applications of high‐capacity optical interconnects and high‐efficiency nonlinear optics. However, multimode manipulation in lithium‐niobate‐on‐insulator (LNOI) waveguides is not easy because there might occur some mode hybridness due to the vertical asymmetry as well as the material anisotropy. In this paper, high‐performance mode‐multiplexing devices with anisotropic LNOI waveguides are proposed, consisting of a multi‐channel mode (de)multiplexer and a sharp multimode waveguide bend (MWB). The mode (de)multiplexer is designed with Z‐propagation LNOI waveguides to avoid the mode hybridness. Meanwhile, the sharp MWB is designed to connect with a Y‐propagation waveguide, enabling access to the highest electro‐optic coefficient and second‐order nonlinear coefficient. The fabricated mode (de)multiplexer has a low excess loss less than 0.2 dB as well as low crosstalk less than ‐19 dB in the wavelength range of 1520–1600 nm. These high‐performance multimode photonic devices are useful for developing multimode LN photonics.

High‐Performance Mode‐Multiplexing Device with Anisotropic Lithium‐Niobate‐on‐Insulator Waveguides

As a fundamental building block for photonic integrated circuits, waveguide crossing is playing an essential role for routing the increasingly complex network on chip. Driven by the demands for carrying a large capacity of data per waveguide, wavelengths and guiding modes are emerging as important degrees of freedom for optical multiplexing to embrace the advantages of signal parallelism. Although various waveguide crossing structures are proposed to either support broad spectra of wavelengths or a large number of modes, none of these are simultaneously achieved. Here, an ultra‐broadband multimode waveguide crossing based on subwavelength transmitarray (SWTA) is proposed. The SWTA is well designed for high transmittance in the propagation direction and excellent confinement in its orthogonal direction by creation of a bound state. A three‐mode crossing is designed and fabricated on silicon on insulator with a compact footprint of only 5.05 µm × 5.05 µm. The crossing is able to operate over a large bandwidth from 1.4 to 2.1 µm with an excess loss of less than 0.77 dB. For a proof‐of‐concept demonstration of the structure scalability, the crossing is predicted to support up to ten modes. The ultra‐broadbandwidth and compactness of this multimode crossing make it possible to densely cross connect the waveguides with large data capacity via wavelength and mode division multiplexing.

Ultra‐Broadband Multimode Waveguide Crossing via Subwavelength Transmitarray with Bound State

We propose and demonstrate two types of 1 × 2 power splitters based on multimode interference (MMI), which are ultra-compact, fabrication friendly, and low loss. The contours of MMI and output tapers are optimized with Bezier curves, which can implement arbitrary ratio power splitters (ARPSs) and ultra-broadband dual-polarization power splitters (UDPSs). For ARPSs, the experimental results show that arbitrary power splitting ratios can be obtained with an average excess loss (EL) of 0.17 dB at 1550 nm for fundamental TE polarization. For UDPSs, the experimental results show that the ELs for fundamental TE and TM polarization are less than 0.63 dB and 0.44 dB over a large bandwidth of 415 nm (1260-1675 nm). The footprints of the proposed devices are less than 10 µm × 2.5 µm (without input straight waveguide) with large fabrication tolerance.

Ultra-broadband dual-polarization and arbitrary ratio power splitters based on Bezier curve optimized multimode interference.

The asynchronous double deep Q-learning (A-DDQN) method is proposed to design the multi-mode interference (MMI) power splitters for low insertion loss and wide bandwidth from 1200 to 1650 nm wavelength range. By using A-DDQN to guide hole etchings in the interference region of MMI, the target splitting ratio (SR) can be obtained with much less CPU time (about 10 hours for one design) and more effective utilization of the computational resources in asynchronous/parallel manner. Also, this method can simplify the design by using relatively few holes to obtain the same SR with small return loss.

Inverse design of the MMI power splitter by asynchronous double deep Q-learning

Abstract The multi-dimensional multiplexing technology is very promising for further increasing the link capacity of optical interconnects. A 96-channel silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is proposed and demonstrated for the first time to satisfy the demands in hybrid mode/polarization/wavelengthdivision-multiplexing systems. The present ROADM consists of a six-channel mode/polarization de-multiplexer, a 6 × 16 array of microring-resonator (MRR)-based wavelength-selective switches, and a six-channel mode/polarization multiplexer. With such a ROADM, one can add/drop optical signals to/from any channels of the multimode bus waveguide arbitrarily. For the designed and fabricated ROADM chip, there are more than 1000 elements integrated monolithically, including 96 MRRs, 576 waveguide crossings, 192 grating couplers, 96 micro-heaters, 112 pads, six polarization-splitter-rotators (PSRs), four asymmetric adiabatic couplers and four asymmetric directional couplers. For any channel added/dropped with the fabricated ROADM, the on-chip excess loss is about 5–20 dB, the inter-mode crosstalk is <−12 dB, and the inter-wavelength crosstalk is <−24 dB. The system experiments are demonstrated by using 10-GBaud quadrature phase shift keying (QPSK) signals, showing that the observed optical signal noise ratio (OSNR) power penalties induced by the ROADM are less than 2 dB at a BER of 3.8 × 10−3.

/pdf/96-channel-on-chip-reconfigurable-optical-add-drop-2rq6pwag.pdf

96-Channel on-chip reconfigurable optical add-drop multiplexer for multidimensional multiplexing systems

We propose and demonstrate a $1\times2$ power splitter enabling arbitrary power splitting ratios. The device is based on a directional coupler with subwavelength structure in the coupling region and a trapezoid-structure in one arm. The measurement shows that arbitrary power splitting ratios, equal to 50:50, 60:40, 70:30, 80:20 and 90:10, can be obtained over an >80 nm bandwidth with an excess loss about 0.65 dB at 1550 nm, while the footprint is only $11.5\,\,\mu \text{m}\,\,\times 1.315\,\,\mu \text{m}$ . The proposed arbitrary ratio power splitter shows promising application prospect in photonic integrated circuits.

Broadband Arbitrary Ratio Power Splitters Based on Directional Couplers With Subwavelength Structure

We propose a large scanning range silicon optical phased array (OPA), which is polarization multiplexed and bi-directional with only one waveguide grating antenna array. Mach-Zehnder interferometers (MZIs) and polarization splitter-rotator (PSR) are utilized to select the polarization states and propagation directions of the input light to the waveguide grating emitter array. By optimizing the parameters of the waveguide grating, the longitudinal far-field steering range is increased by four times compared to that of the conventional ones among the same wavelength range. The simulation results show that the 54.5° longitudinal scanning range could be realized with the tuning wavelength ranging from 1500 nm to 1600 nm. The high wavelength tuning efficiency is up to 0.545 °/nm. The beam steering range in the lateral direction is 77.8° via phase tuning.

/pdf/dual-polarization-and-bi-directional-silicon-photonic-24fg3r59.pdf

Dual Polarization and Bi-Directional Silicon-Photonic Optical Phased Array With Large Scanning Range

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