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

Polarization-insensitive multimode interference coupler on an x-cut thin-film lithium niobate platform

Thin-film lithium niobate (TFLN) is a promising integrated photonics platform but currently lacks a polarization-insensitive multimode interference (MMI) coupler, a crucial component for polarization-related optical communication applications such as polarization management, polarization-division multiplexing, and polarization-insensitive modulation systems. This paper presents a novel, to the best of our knowledge, approach by rotating the MMI structure on an anisotropic x-cut TFLN at specific angles to compensate for the difference in the beat length between the two polarizations. A polarization-insensitive 1 × 2 MMI coupler is experimentally achieved with measured transmittances of -2.5 to -4 dB for both output ports and polarization modes in the wavelength range of 1520-1580 nm.

Spatial Information Lasing Enabled by Full-k-Space Bound States in the Continuum

Optical amplification and massive information transfer in modern physics depend on stimulated radiation. However, regardless of traditional macroscopic lasers or emerging micro- and nanolasers, the information modulations are generally outside the lasing cavities. On the other hand, bound states in the continuum (BICs) with inherently enormous Q factors are limited to zero-dimensional singularities in momentum space. Here, we propose the concept of spatial information lasing, whose lasing information entropy can be correspondingly controlled by near-field Bragg coupling of guided modes. This concept is verified in gain-loss metamaterials supporting full-k-space BICs with both flexible manipulations and strong confinement of light fields. The counterintuitive high-dimensional BICs exist in a continuous energy band, which provide a versatile platform to precisely control each lasing Fourier component and, thus, can directly convey rich spatial information on the compact size. Single-mode operation achieved in our scheme ensures consistent and stable lasing information. Our findings can be expanded to different wave systems and open new scenarios in informational coherent amplification and high-Q physical frameworks for both classical and quantum applications.

Impact of Carbon-Based Nanoparticles on Polyvinyl Alcohol Polarizer Features: Photonics Applications

Among different inorganic and organic polarizer elements, thin-film light polarizers occupy a special place because of their flexibility, ease of integration into any optoelectronic circuit, and good functioning in the visible and near-infrared spectral range and can compete with Glan and Nicolas volumetric prisms. This paper presents the results of a study on how carbon-based nanoparticles influence on the basic properties of a well-known PVA-based polymer matrix, using which it is possible to obtain good transparency for parallel light components. An accent is made on graphene oxide nanoparticles, which are used as PVA sensitizers. It was shown for the first time that the structuring of PVA with graphene oxides allows an increased transmittance of the parallel light component to be obtained, saving the transmittance of the orthogonal one. Moreover, the graphene network can increase the mechanical strength of such thin-film PVA-based polarizers and provoke a change in the wetting angle. These advantages make it possible to use graphene oxide-structured thin-film light polarizers based on a PVA matrix as an independent optoelectronic element. Some comparative results for polarizers based on PVA-C70 structures are shown as well.

Ultrafast polarization characterization with Mueller matrix based on optical time-stretch and spectral encoding

High-speed optical polarization characterization is highly desirable for a wide range of applications, including remote sensing, telecommunication, and medical diagnosis. The utilization of the Mueller matrix provides a superior systematic and comprehensive approach to represent polarization attributes when matter interacts with optical beams. However, the current measurement speed of Mueller matrix is limited to only seconds or milliseconds. In this study, we present an ultrafast Mueller matrix polarimetry (MMP) technique based on optical time-stretch and spectral encoding that enables us to achieve an impressive temporal resolution of 4.83 nanoseconds for accurate Mueller matrix measurements. The unique feature of optical time-stretch technology enables continuous, ultrafast single-shot spectroscopy, resulting in a remarkable speed of up to 207 MHz for spectral encoding Mueller matrix measurement. We have employed an effective Mueller linear reconstruction algorithm based on the measured modulation matrix, accounting for all potential non-ideal effects of polarization components like retardance error and azimuth error. To ensure high precision, prior to the actual measurement, high-order dispersion induced by time-stretch requires adjustment through proper modulation matrix design. Upon such correction, both the results of static and rapid dynamic samples measurements exhibit exceptional accuracy with root-mean-square error (RMSE) approximately equal to 0.04 and 0.07 respectively. This presented ultrafast MMP provides a significant advance over preceding endeavors, enabling superior accuracy and increased speed concurrently.

Ultra-compact electro-optic phase modulator based on a lithium niobate topological slow light waveguide

Electro-optic modulators (EOMs) are essential devices of optical communications and quantum computing systems. In particular, ultra-compact EOMs are necessary for highly integrated photonic chips. Thin film lithium niobate materials are a promising platform for designing highly efficient EOMs. However, EOMs based on conventional waveguide structures are at a millimeter scale and challenging to scale down further, greatly hindering the capability of on-chip integration. Here, we design an EOM based on lithium niobate valley photonic crystal (VPC) structures for the first time. Due to the high effective refractive index introduced by the strong slow light effect, the EOM can achieve an ultra-compact size of 4 μm×14 μm with a half-wave voltage of 1.4 V. The EOM has a high transmittance of 0.87 in the 1068 nm because of the unique spin-valley locking effect in VPC structures. The design is fully compatible with current nanofabrication technology and immune to fabrication defects. Therefore, it opens a new possibility in designing lithium niobate electro-optic modulators and will find broad applications in optical communication and quantum photonic devices.

High-speed polarization tracking using thin film lithium niobate integrated dynamic polarization controller

Dynamic polarization controllers (DPCs) are essential devices in various optical applications. We develop a thin film lithium niobate (TFLN) integrated DPC driven by the real-time implemented Jacobian control algorithm for fast polarization tracking. Experimental results demonstrate a high polarization tracking speed of 100 krad/s when targeting a specific linear state of polarization, with a low control loop delay of 420 ns, half-wave control voltages of 2.75 V, and a fast polarization restoring time of 1.6 us. Compared to previously reported integrated DPCs, the TFLN-based DPC achieves significantly higher tracking speed and lower loop delay. The results highlight the effectiveness of the Jacobian method and the outstanding performance of TFLN-based DPCs. The study opens up possibilities for further advancements in DPC solutions using TFLN technology.

TE/TM mode electro-optic conversion based on a titanium diffusion lithium niobate waveguide with a polarization-maintained fiber structure

For the development of photonic integrated circuits and lithium niobate (L i N b O 3, LN) optical waveguide technology, the implementation and application of polarization devices based on LN are also becoming more widespread, where titanium (Ti)-diffused LN waveguides form the basis of many important electro-optic (EO) integrated optical devices. Moreover, utilizing polarization conversion has the potential to enhance both the effectiveness and capacity of optical transmission. Thus, we have presented an EO polarization mode converter packaging with PANDA polarization-maintaining optical fibers (PMFs) in the broadband wavelength range (1440-1620 nm) to obtain the multiwavelength modulation, featuring the wavelength tunability. Additionally, the fabricated device is able to achieve transverse electric (TE) to transverse magnetic (TM) mode conversion efficiently with the applied voltage of  ±, which provides high conversion efficiency. Importantly, our device also features a high-frequency response of about 600 MHz with overall insertion loss below 5 dB. The rapid development of LN-based polarization devices holds great promise for chip-integrated systems in the field of polarization telecommunication.

Design of a broadband polarization controller based on silicon nitride-loaded thin-film lithium niobate

A novel design of a polarization controller based on "etch-less" Si3N4-loaded thin film LiNbO3 is described. Broadband operation in the spectral range between 1.45 and 1.65 µm is achieved by using a mode evolution TM/TE splitter/converter, two mode evolution 3-dB couplers, and two electro-optic phase shifters. Numerical simulations show that the on-chip insertion loss should not exceed 1 dB. A single TE-mode output can be adjusted by applying control voltages lower than 10 V for an arbitrary input polarization state.

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.

Low-loss and broadband polarization-diversity edge coupler on a thin-film lithium niobate platform

Fiber-to-chip coupling is an essential issue for taking high-performance integrated photonic devices into practical applications. On a thin-film lithium niobate platform, such a high-performance coupler featuring low loss, large bandwidth, and polarization independence is highly desired. However, the mode hybridization induced by the birefringence of lithium niobate seriously restricts a polarization-independent coupling. Here, we propose and experimentally demonstrate a high-performance and polarization-diversity cantilever edge coupler (EC) with the assistance of a two-stage polarization splitter and rotator (PSR). The fabricated cantilever EC shows a minimal coupling loss of 1.06 dB/facet, and the fully etched PSR structure shows a low insertion loss (IL) of -0.62 dB. The whole polarization-diversity cantilever EC exhibits a low IL of -2.17 dB and -1.68 dB for TE₀ and TM₀ mode, respectively, as well as a small cross talk of <-15 dB covering the wavelength band from 1.5 µm to 1.6 µm. A polarization-dependent loss <0.5 dB over the same wavelength band is also obtained. The proposed fiber-to-waveguide coupler, compatible with the fabrication process of popular thin-film lithium niobate photonic devices, can work as a coupling scheme for on-chip polarization-diversity applications.

Online polarization error suppressed optical vector analyzer based on Bayesian optimization

An optical vector analyzer (OVA) based on orthogonal polarization interrogation and polarization diversity detection is widely used to measure an optical device's loss, delay, or polarization-dependent features. Polarization misalignment is the OVA's primary error source. Conventional offline polarization alignment using a calibrator greatly reduces the measurement reliability and efficiency. In this Letter, we propose an online polarization error suppression method using Bayesian optimization. Our measurement results are verified by a commercial OVA instrument that uses the offline alignment method. The OVA featuring online error suppression will be widely used in the production of optical devices, not just in the laboratory.

Metasurface-Enabled On-Chip Manipulation of Higher-Order Poincaré Sphere Beams

An integrated way to generate and manipulate higher-order Poincaré sphere beams (HOPBs) is a sought-after goal in photonic integrated circuits for high-capacity communication systems. Here, we demonstrate a novel method for on-chip generation and manipulation of HOPBs through combining metasurface with optical waveguides on lithium niobate on insulator platform. With phase modulation by a diatomic geometric metasurface, guided waves are extracted into free space with a high signal-to-noise ratio in the form of two orthogonal circularly polarized optical vortices which are linearly superposed into HOPBs. Meanwhile, a dual-port waveguide crossing is established to reconfigure the output states into an arbitrary point on a higher-order Poincaré sphere based on in-plane interference of two guided waves. Our approach provides a promising solution to generate and manipulate the HOPBs in a compact manner, which would be further enhanced by employing the electro-optical modulation on a lithium niobate waveguide to access a fully tunable scheme.

Jacobian methods for dynamic polarization control in optical applications

Dynamic polarization control (DPC) is beneficial for many optical applications. It is often realized via tunable waveplates to perform automatic polarization tracking and manipulation. Efficient algorithms are essential to realize an endless polarization control process at high speed. However, the standard gradient-based algorithm is not well analyzed. Here, we model the DPC with a Jacobian-based control theory framework that finds a lot in common with robot kinematics. We then give a detailed analysis of the condition of the Stokes vector gradient as a Jacobian matrix. We identify the multi-stage DPC as a redundant system enabling control algorithms with null-space operations. An efficient, reset-free algorithm can be found. We anticipate more customized DPC algorithms to follow the same framework in various optical systems.

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.

Highly efficient and tunable polarization rotator based on lithium niobate on an insulator

The lithium niobate on an insulator (LNOI) platform has greatly advanced the development of integrated photonics recently, where efficient polarization management components are indispensable. In this work, we propose a highly efficient and tunable polarization rotator based on the LNOI platform and the low-loss optical phase change material antimony triselenide (S b ₂ S e ₃). The key polarization rotation region is formed by a LNOI waveguide with a cross section of the double trapezoidal shape and a S b ₂ S e ₃ layer deposited atop the LNOI waveguide in an asymmetrical way, where an isolating layer of silicon dioxide is sandwiched between them to reduce the material absorption loss. Based on such a structure, we have achieved the efficient polarization rotation in a length of only 17.7 µm, where the polarization conversion efficiency and insertion loss are 99.6% (99.2%) and 0.38 dB (0.4 dB) for the trans-electric (TE)-to-trans-magnetic (TM) rotation. If we further change the phase state of the S b ₂ S e ₃ layer, other polarization rotation angles besides 90° can also be obtained for the same device, revealing a tunable function. We believe that the proposed device and design scheme could offer an efficient method for realizing the polarization management on the LNOI platform.

One-dimensional grating coupler on lithium-niobate-on-insulator for high-efficiency and polarization-independent coupling

A metal-based one-dimensional grating coupler on an x-cut lithium-niobate-on-insulator wafer structure for a polarization-independent fiber interface is designed and demonstrated. By using a metal-based plasmonic mode, the diffractive angle for the two polarized modes in the lithium niobate ridge waveguide can be tuned to be the same. The polarization dependence of the grating coupler therefore can be effectively reduced. The fabricated device exhibits -3.56-dB and -4.08-dB peak coupling losses per coupler at 1573 nm for the TE and TM modes, respectively. The polarization-dependent losses are less than 0.69 dB in a 44-nm wavelength range. The demonstrated grating coupler can serve as a polarization-independent optical fiber interface on lithium-niobate-on-insulator and facilitate on-chip polarization diversity applications.

Three-dimensional on-chip mode converter

The implementation of transverse mode, polarization, frequency, and other degrees of freedom (d.o.f.s) of photons is an important way to improve the capability of photonic circuits. Here, a three-dimensional (3D) linear polarized (LP) LP₁₁ mode converter was designed and fabricated using a femtosecond laser direct writing (FsLDW) technique. The converter included multi-mode waveguides, symmetric Y splitters, and phase delaying waveguides, which were constructed as different numbers and arrangements of circular cross section waveguides. Finally, the modes (LP_11a and LP_11b) were generated on-chip with a relatively low insertion loss (IL). The mode converter lays a foundation for on-chip high-order mode generation and conversion between different modes, and will play a significant role in mode coding and decoding of 3D photonic circuits.

Self-Assembly of 2D Hybrid Double Perovskites on 3D Cs2 AgBiBr6 Crystals towards Ultrasensitive Detection of Weak Polarized Light

We report the self-assembly of 2D double perovskite (BLA)₂ CsAgBiBr₇ (BLA=benzylammonium) on 3D Cs₂ AgBiBr₆ crystals, providing the first demonstration of polarization-sensitive photodetection using lead-free double perovskite heterocrystals (HCs). The (BLA)₂ CsAgBiBr₇ /Cs₂ AgBiBr₆ HC successfully combines the anisotropy of 2D double perovskites with the well-defined interface provided by heterogeneous integration. Driven by the built-in electric field in junction, photodetectors of HCs exhibit unique polarization dependence of zero-bias photocurrent with a large anisotropy ratio up to 9, which is 6 times amplified as compared to the pristine 2D (BLA)₂ CsAgBiBr₇ . More importantly, the present devices can remain polarization-sensitive with incident light intensity down to the nW cm^-2 level. Our study on lead-free hybrid perovskite HCs marks a step toward establishing robust material foundations for fundamental scientific investigations and the development of optoelectronic devices.