Broadband Polarization Research Articles

Design of a Broadband Polarization Splitter Based on Anisotropy-Engineered Tilted Subwavelength Gratings

Polarization management is of paramount importance in integrated optics, particularly for highly birefringent photonic platforms such as silicon-on-insulator. In this paper, we present a polarization beam splitter based on a multimode interference coupler incorporating tilted subwavelength gratings. The tilt provides accurate control of the structural anisotropy and enables independent selection of the beat length for two orthogonal polarization states. As a result, device length is reduced to less than 100 μm while simultaneously achieving broadband operation through subwavelength grating dispersion engineering. Insertion losses below 1 dB and an extinction ratio higher than 20 dB are demonstrated through three-dimensional FDTD simulation in a 131-nm bandwidth.

Flexible broadband polarization converter based on metasurface at microwave band**Project supported by the Fundamental Research Funds for the Central Universities, China (Grant No. kfjj20180401), the National Natural Science Foundation of China (Grant No. 61471368), the Aeronautical Science Foundation of China (Grant No. 20161852016), the China Postdoctoral Science Foundation (Grant No. 2016M601802), and Jiangsu Planned Projects for Postdoctoral Research Funds, China

A flexible broadband linear polarization converter is proposed based on the metasurface operating at microwave band. To achieve bandwidth extension property, long and short metallic arc wires, as well as the metallic disks placed over a ground plane, are combined into the polarizer, which can generate three neighboring resonances. Due to the combination of the first two resonances and the optimized size and thickness of the unit cell, the polarization converter can have a weak incident angle dependence. Both simulated and measured results confirm that the average polarization conversion ratio is over 85% from 11.3 GHz to 20.2 GHz within a broad range of incident angle from 0° to 45°. Moreover, the proposed polarization converter based on flexible substrates can be applied for conformal design. The simulation and experiment results demonstrate that our designed polarizer still keeps high polarization conversion efficiency, even when it adheres to convex cylindrical surfaces. The periodic metallic structure of the designed polarizer has great potential application values in the microwave, terahertz, and optic regimes.

A broadband, low-crosstalk and low polarization dependent silicon nitride waveguide crossing based on the multimode-interference

A silicon nitride waveguide crossing based on multimode-interference is designed and fabricated. By optimizing the structure parameters with finite difference time domain method, 0.3 dB insertion loss and −75 dB crosstalk at 1550 nm are obtained. Then the designed waveguide crossing is fabricated based on the double strip silicon nitride waveguide. The measured results show the insertion loss and crosstalk are about 0.6 dB and below −42 dB, respectively, in the wavelength range of 1520 nm∼1640 nm with low polarization dependence. The waveguide crossing could be a key building block for the silicon nitride waveguide platform.

Frequency dependent multi-functional polarization convertor based on metasurface

Controlling the polarization state of light is an important research region in optics due to its enormous potential. Conventional polarization converters inevitably express huge size and low efficiency, which run counter to the requirement of modern optics. Herein, a frequency-dependent multi-functional metasurface is proposed and researched to design a broad-band and high-efficiency reflective-type polarization converter at near-infrared band. Based on the simulation results, 90° polarization convertor and quarter wave plate can be realized at different wavelengths. The broad-band 90° polarization convertor will transform gradually to a broad-band quarter wave plate by increasing the ”coupling gap” at the long wavelength region, and the functionalities of 90° polarization convertor and quarter wave plate can always be kept at the short wavelength region. Our results can provide a guide to design the ultrathin optical polarization converters.

A low scattering slot antenna based on broadband polarization conversion metasurface

An ultrawide band polarization conversion metasurface (PCM) is proposed to reduce the reflection of a slot antenna. The proposed PCM can convert a linearly polarized incident wave to the reflected wave with a 90° rotation. By arranging the PCM around the antenna symmetrically, an ideal phase cancelation is achieved. To verify the design, a metasurface-slot antenna is simulated and measured to confirm the proposed PCM for a significant reduction of the reflection. Results show that the normal reflection of the proposed antenna at boresight is reduced more than 10 dB over an ultrawide band. In addition, 2 dB enhancement of the gain is achieved compared with a reference antenna which has the same dimensions. Meanwhile, the small influence is observed for the antenna radiation by introducing the PCM.

Ultra-compact broadband polarization diversity orbital angular momentum generator with 3.6 × 3.6 μm2 footprint.

Orbital angular momentum (OAM), one fundamental property of light, has been of great interest over the past decades. An ideal OAM generator, fully compatible with existing physical dimensions (wavelength and polarization) of light, would offer the distinct features of broadband, polarization diversity, and ultra-compact footprint. Here, we propose, design, fabricate, and demonstrate an ultra-compact chip-scale broadband polarization diversity OAM generator on a silicon platform with a 3.6 × 3.6 μm2 footprint. The silicon OAM chip is formed by introducing a subwavelength surface structure (superposed holographic fork gratings) on top of a silicon waveguide, coupling the in-plane waveguide mode to the out-plane free-space OAM mode. We demonstrate in theory and experiment the broadband generation of polarization diversity OAM modes (x-/y-polarized OAM+1/OAM-1) from 1500 to 1630 nm with high purity and efficiency. The demonstrations of an ultra-compact broadband polarization diversity OAM generator may open up new perspectives for OAM-assisted N-dimensional optical multiplexing communications/interconnects and high-dimensional quantum communication systems.

Ultra-short and broadband polarization splitter based on PCF and metal surface plasmons resonance

An ultra-short and broadband polarization splitter based on PCF and metal surface plasmons resonance is proposed by designing a new structure and filling gold wire into certain air holes. The introducing of gold wire aims to couple with the basic modes. Then high birefringence is obtained to improve the splitter’s performances including length, bandwidth and maximal extinction ratio. The numerical results demonstrate that the splitter can be the shortest length of 100 μm (to our knowledge) and the broad bandwidth is 470 nm simultaneously. And when the splitter’s length is set as 104 μm, the widest bandwidth reaches the maximum 575 nm (to our knowledge) which covers almost all the E, S, C, L, and U communication band. In addition, the structure is relatively simple, so it is easy to fabricate using available methods. These properties will help designing splitter in the optical communication and sensing system.

Broadband Polarization Degeneracy of Guided Waves in Subwavelength Structured ZnO Pattern

Polarization degeneracy of electromagnetic plane waves in vacuum and in any bulk isotropic media is the keynote operational principle of many optical devices such as polarizers and interferometers. However, surface and guided waves spectra are typically either not degenerated at all or meet degeneracy only at very specific dispersion points. In this paper, we offer a design of a periodic photonic structure based on zinc oxide (ZnO) nanocylinders providing the broadband polarization degeneracy of the guided waves in the near-IR frequency range. We analyze the impact of the spatial dispersion and substrate on the degeneracy breaking. We offer the design based on ZnO nanocylinders with aluminum-doped zinc oxide substrate, achieving the degeneracy in the vicinity of a telecommunication wavelength, for the practical implementation. Finally, we propose and verify numerically a potentially important device–waveguide polarizer, which is the analogue to the $\lambda /4$ plate for the guided waves.

Design and fabrication of 3-D printed conductive polymer structures for THz polarization control.

In this paper, we numerically and experimentally demonstrate the inverse polarization effect in three-dimensional (3-D) printed polarizers for the frequency range of 0.5 - 2.7 THz. The polarizers simply consist of 3-D printed strip lines of conductive polylactic acid (CPLA, Proto-Pasta) and do not require a substrate or any further metallic deposition. The experimental and numerical results show that the proposed structure acts as a broadband polarizer between the range of 0.3 THz to 2.7 THz, in which the inverse polarization effect is clearly seen for frequencies above 0.5 THz. In the inverse polarization effect, the transmission of the transverse electric (TE) component exceeds that of the TM component, in contrast to the behavior of a typical wire-grid polarizer. We show how the performance of the polarizers depends on the spacing and thickness of the CPLA structure; extinction ratios higher than 20 dB are achieved. This is the first report using CPLA to fabricate THz polarizers, demonstrating the potential of using conductive polymers to design THz components efficiently and robustly.

Broadband Reflection in Polymer-Stabilized Cholesteric Liquid Crystals via Thiol-Acrylate Chemistry.

Thiols are prone to react with a multitude of various functional groups in high yields, which has been widely used for surface- and particle-patterning, bioorganic synthesis, polymer modification, imprint nanolithography, the fabrication of optical components, hydrogel synthesis, and the curing of hard protective coatings. In this work, a chiral thiol with a high helical twisting power was synthesized through a novel synthetic route with high selectivity, yield, and cost-effectiveness. It was then used to fabricate a liquid-crystal composite film with ultra-wide broadband reflection via thiol click chemistry. Cholesteric liquid-crystal materials with broadband reflection have many potential applications for broadband polarizers, polarizer-free displays, organic optical data storage media, smart switchable reflective windows, and continuous waveband laser protection.

Broadband Reflection in Polymer‐Stabilized Cholesteric Liquid Crystals via Thiol–Acrylate Chemistry

AbstractThiols are prone to react with a multitude of various functional groups in high yields, which has been widely used for surface‐ and particle‐patterning, bioorganic synthesis, polymer modification, imprint nanolithography, the fabrication of optical components, hydrogel synthesis, and the curing of hard protective coatings. In this work, a chiral thiol with a high helical twisting power was synthesized through a novel synthetic route with high selectivity, yield, and cost‐effectiveness. It was then used to fabricate a liquid‐crystal composite film with ultra‐wide broadband reflection via thiol click chemistry. Cholesteric liquid‐crystal materials with broadband reflection have many potential applications for broadband polarizers, polarizer‐free displays, organic optical data storage media, smart switchable reflective windows, and continuous waveband laser protection.

3D FSS polarizer for millimeter‐wave antenna applications

In this article, a novel wideband polarizer is presented, to convert the linear polarization of a directive antenna into circular polarization. This innovative polarizer is based on a 3D frequency selective surface (FSS) and consists of an array of 4×4 metal axial-mode helices. The proposed broadband polarizer operates in the 28 to 31 GHz frequency band. To demonstrate the principle, the 3D FSS is applied to a linearly polarized Yagi-Uda antenna operating in the same millimeter-wave frequency band. The measured and simulated results, by integrating the proposed 3D polarizer in front of the antenna, show that an axial ratio smaller than 3 dB is obtained over a wide bandwidth with gain enhancement.

Development of a confocal scanning microscope for fluorescence imaging and spectroscopy at variable temperatures.

We developed and tested a confocal scanning optical microscope that fits into a thermally controlled, commercial research cryostat designed for operation from ambient temperature down to below 4 K. The home-built microscope is a fiber-coupled, self-contained instrument based on readily available mechanical and optical components. Its sample module is sealed in a protective stainless steel tube that minimizes vibrations caused by the flow of cryogenic gas. A high numerical aperture microscope objective specifically designed for cryogenic and high-vacuum applications focuses the excitation light onto the sample, while the core of an optical fiber attached to an avalanche photodiode acts as the confocal detection pinhole. The sample is displaced using a piezotube scanner mounted on top of a three-axis, low-temperature nanopositioner assembly for coarse sample positioning. A broadband polarizing cube beam splitter in the emission path allows for polarization-resolved imaging and spectroscopy. Fluorescence excitation scans are acquired with custom-written software that correlates fluorescence photon counts with the output from a high precision wavelength meter, which is part of a narrow-band, tunable dye laser setup. The imaging and spectral data acquisition capabilities of the microscope were confirmed using a variety of samples and excitation wavelengths at temperatures ranging from 5 K to room temperature.

Dual Circular Polarization with Reduced Mutual Coupling Among Two Orthogonally Placed CPW-Fed Microstrip Antennas for Broadband Applications

In this communication, a design of Coplanar Waveguide-fed Dual Circularly Polarized Broadband Antenna (CPW-DCPBA) has been proposed. Here, a square slot consists of two orthogonal T-shaped asymmetric microstrip antennas protruded from a signal line of CPW feed has been presented. The microstrip antennas are isolated with an inverted-L type grounded strip embedded at the left corner, followed by a square strip. A modified meandered line (MML) is embedded with the square strip to achieve circular polarization at the lower frequency. An SRR is placed at the back of the antenna to enhance the 3-dB axial ratio bandwidth (ARBW). Measurements of the fabricated antenna show an impedance bandwidth of 88% and the 3-dB ARBW of 75%, in which return loss is better than 10 dB and the isolation among the antennas are better than 15 dB. A peak gain of 3.1–5.6 dBi is achieved within the axial ratio band. The antenna is fabricated on FR-4 substrate of 60 mm × 60 mm × 1.6 mm with an antenna area of 0.359λ 0 2 . The simulated results are in good agreement with the measured results which verified its usage for broadband applications.

Design and fabrication of copper-filled photonic crystal fiber based polarization filters.

This work demonstrates a broadband polarization filter based on copper-filled photonic crystal fiber (CFPCF). The proposed fiber is fabricated using the conventional stack-and-draw method. The polarization filter properties of the proposed CFPCF are investigated numerically by considering the cross-sectional scanning electron microscopy image of the fabricated CFPCF. It is observed that the magnitude of cross talk reached up to -206 dB over 0.8mm length with a broad bandwidth of 282nm at a central wavelength of 1790nm. In addition, the polarization characteristics of the CFPCF including cross talk, central wavelength, and bandwidth can be adjusted by varying the diameter of the copper wire. It is shown that the resonance wavelength of the proposed filter can be tuned over the wide range of wavelengths from 1390 to 1890nm. We have shown that by adjusting the copper wire diameter to 0.32Λ and 0.48Λ μm (Λ is pitch size), the proposed filter can operate at communication bands of 1310 and 1550nm, respectively. The results suggest high-potential of the proposed fiber for polarization filtering and other sensing applications.

Omnidirectional polarization beam splitter for white light.

As a key element in optical systems, a broadband and omnidirectional polarization beam splitter has been long desired. Here, based on anisotropic metamaterials, a perfect polarizing beam splitter is theoretically designed for the extremely broad frequency and angle bands without energy loss. When an electromagnetic wave is incident on the beam splitter, the transverse magnetic-polarized component suffers total reflection, while the transverse electric-polarized component is completely transmitted within the incident angle range [-90°, 90°] for the white light. This study provides a new approach to design an efficient polarizing beam splitter and may promote the development and applications of anisotropic metamaterials.

A different optical composition for a broadband linear polarization rotator

We suggest and experimentally demonstrate a broadband polarization rotator. Our device consist of a Fresnel’s rhomb retarder and a composite broadband half-wave plate with relative rotation α between their fast polarization axis. Such a optical device rotate the polarization plane of a linearly-polarized light at angle 2α over a wide range of wavelengths.

Adiabatic mode converters for silicon photonics: Power and polarization broadband manipulators

Abstract

Broadband Polarization Splitter-Rotator and the Application in WDM Receiver

Polarization sensitivity is a severe problem in photonic integration devices and chips based on the high-index contrast Si/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> system. In this paper, a broadband silicon polarization splitter-rotator (PSR) comprised of a bi-level taper and a counter-tapered coupler is proposed with a large fabrication tolerance. The measured insertion loss is lower than 0.7 dB/0.73 dB for the wavelength range from 1470 to 1580 nm with a crosstalk lower than -12.1 dB/-14.7 dB for TE and TM polarization input, respectively. By integrating the PSR with silicon arrayed waveguide gratings and germanium photodetectors (PDs) on a single chip, a polarization-insensitive wavelength-division-multiplexing (WDM) receiver is demonstrated with a polarization-dependent loss as low as 1.21 dB. High-speed operations are also achieved with clear eyes at 10 Gb/s.

Broadband TE Optical Isolators and Circulators in Silicon Photonics Through Ce:YIG Bonding

Optical isolators and circulators are fundamental building block in photonic integrated circuits to block undesired reflections and routing light according to a prescribed direction. In silicon photonics, heterogeneous integration of magneto-optic garnet bonded on a pre-patterned silicon layer has been demonstrated to be an effective solution for manufacturing optical isolators and circulators for TM polarized light. However, most integrated semiconductor lasers emit TE polarized light, which indicates the need to find a reliable solution for this polarization. In this paper, we demonstrated broadband optical isolators and circulators for TE polarized light based on heterogeneous bonding on the silicon photonics platform. To achieve this goal, an integrated adiabatic coupler and a broadband polarization rotator are designed and optimized. The nonreciprocal behavior is induced through an energy-efficient integrated electromagnet with a minimum power consumption of 3 mW. Two isolators/circulators are fabricated with small and large free spectral range, respectively. In the former case, an optical isolation ratio as large as 30 dB is measured at 1555 nm with an insertion loss of 18 dB, while for the broadband circulator, an optical isolation larger than 15 dB is guaranteed over more than 14 nm (1.75 THz) for all port combinations with an insertion loss between 14 and 18 dB at 1560 nm. Finally, it has been theoretically shown that the insertion loss can be reduced below 6 dB with design and fabrication improvements. To the best of the authors’ knowledge, the proposed integrated TE optical circulator is the first experimental demonstration of this device in silicon photonics.