Efficient Gratings Research Articles

Highly Efficient Polarization-Insensitive Grating Couplers on Thin-Film Lithium Niobate with an Integrated Gold Layer

The thin-film lithium niobate platform, which is emerging as a promising photonic integration platform, currently lacks a polarization-insensitive grating coupler (GC), a crucial component for polarization-independent fiber interfaces. This limitation restricts its use in many applications, such as polarization-insensitive modulation systems and polarization management. In this study, we propose a polarization-insensitive nonuniform GC, achieved by intersecting optimal TE- and TM-mode grating periods. Based on our simulation results, the proposed design delivers a coupling efficiency (CE) of 80% for TE and 78.5% for TM polarization, with a polarization-dependent loss of less than 0.14 dB at a wavelength of 1550 nm. The inserted gold layer, i.e., that inside the substrate layer, boosts the CEs of the optimal TE- and TM-mode GC by about 50%, resulting in a highly efficient, polarization-insensitive solution. This advancement enables on-chip polarization diversity applications on the thin-film lithium niobate platform. We also investigate the fabrication and alignment tolerances of the proposed design to ensure robust performance under practical conditions.

Method for designing highly efficient composite transmission gratings

We present and validate a new method for designing transmission gratings with high efficiency for the 1st diffraction order across the visible spectrum. The high efficiency is achieved by redirecting light to the 1st order via asymmetric composite elements, which scatter light in the same direction as the 1st diffraction order. By focusing on increasing the directional scattering of the grating elements, the design remains simple yet effective. As a result, the gratings are relatively easy to fabricate. Measurements of fabricated gratings show a relative increase of over 40% for a large part of the visible spectrum.

Dual-wavelength terahertz two-dimensional phase gratings based on all dielectric metasurfaces

Efficient and accurate phase gratings hold immense significance in the realization of large format heterodyne array receivers at terahertz frequencies. Metallic phase gratings have made substantial advancements in terms of operating wavelength and the number of diffraction beams. Like most other diffractive optical devices, metallic phase gratings are primarily optimized to operate at one specific wavelength. Metasurfaces compositing arrays of subwavelength nanostructures have been demonstrated with various optical functions, by freely modifying the polarization, phase, and amplitude of light. In this study, we present an approach to create a multi-wavelength phase grating compositing segments that incorporate multiple nanostructures. The resulting transmission phase grating not only exhibits uniform diffraction beams (2 × 2) but also achieves the same diffraction angles at both 1.31 and 2.7 THz. The measured total power efficiency of the diffraction beam pattern is 53.2% for 1.31 THz and 42.4% for 2.7 THz. These devices can be applied in terahertz astronomical observations and fluorescence microscopy applications, where multi-wavelength operation is necessary.

Experimental study and modeling of extreme ultraviolet 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers.

Multilayer coated diffraction gratings are crucial components for extreme ultraviolet (EUV) applications such as spectroscopy or spectro-imaging. However, for high groove density, the smoothening of the grating surface profile with multilayer deposition remains a limitation that requires further investigation. In this paper, we report on the design, characterization, and modeling of 4000 lines/mm diffraction gratings coated with periodic and aperiodic Al/Mo/SiC multilayers for EUV radiation. Two types of gratings with different groove depths are compared. Multilayer coatings were designed using a genetic algorithm to maximize the first-order diffraction efficiency in the 17-21 and 19-23nm wavelength ranges at normal incidence. Periodic and aperiodic multilayers with different numbers of layers were deposited by magnetron sputtering on the two types of fused silica gratings, and the grating groove profile evolution was measured by atomic force microscopy and cross-section transmission electron microscopy. The first-order diffraction efficiency was measured in the EUV at 5° incidence using monochromatic synchrotron radiation and modeled using the rigorous coupled-wave analysis method. The simulation models refined by using the Debye-Waller factor to account for the multilayer interfacial roughness show good agreement with experimental data. The results reported in this study will allow for designing efficient EUV multilayer gratings for high-resolution spectro-imaging instruments.

Interrogation Method with Temperature Compensation Using Ultra-Short Fiber Bragg Gratings in Silica and Polymer Optical Fibers as Edge Filters.

The use of simpler and less bulky equipment, with a reliable performance and at relative low cost is increasingly important when assembling sensing configurations for a wide variety of applications. Based on this concept, this paper proposes a simple, efficient and relative low-cost fiber Bragg grating (FBG) interrogation solution using ultra-short FBGs (USFBGs) as edge filters. USFBGs with different lengths and reflection bandwidths were produced in silica optical fiber and in poly(methyl methacrylate) (PMMA) microstructured polymer optical fiber (mPOF), and by adjusting specific inscription parameters and the diffraction pattern, these gratings can present self-apodization and unique spectral characteristics suitable for filtering operations. In addition to being a cost-effective edge filter solution, USFBGs and standard uniform FBGs in silica fiber have similar thermal sensitivities, which results in a straightforward operation without complex equipment or calculations. This FBG interrogation configuration is also quite promising for dynamic measurements, and due to its multiplexing capabilities multiple USFBGs can be inscribed in the same optical fiber, allowing to incorporate several filters with identical or different spectral characteristics at specific wavelength regions in the same fiber, thus showing great potential to create and develop new sensing configurations.

How Thin and Efficient Can a Metasurface Reflector Be? Universal Bounds on Reflection for Any Direction and Polarization

AbstractLight reflection plays a crucial role in a number of modern technologies. In this paper, analytical expressions for maximal reflected power in any direction and for any polarization are given for generic planar structures made of a single material represented by a complex scalar susceptibility. The problem of optimal light‐matter interactions to maximize reflection is formulated as the solution of an optimization problem in terms of the induced currents, subject to energy conservation and passivity, which admits a global upper bound by using Lagrangian duality. The derived upper bounds apply to a broad range of planar structures, including metasurfaces, gratings, homogenized films, photonic crystal slabs, and more generally, any inhomogeneous planar structure irrespective of its geometrical details. These bounds also set the limit on the minimum possible thickness, for a given lossy material, to achieve a desired reflectance. Moreover, the results allow the discovery of parameter regions where large improvements in the efficiency of a reflective structure are possible compared to existing designs. Examples are given of the implications of these findings for the design of superior and compact reflective components made of real, imperfect (i.e., lossy) materials, such as ultra‐thin and efficient gratings, polarization converters, and light‐weight mirrors for solar/laser sails.

Efficient diaphragm-based Fiber Bragg grating vacuum sensor

An efficient diaphragm-based fiber Bragg grating (FBG) vacuum sensor was designed, developed and experimentally demonstrated. The devised sensor was independently interrogated by spectral and intensity demodulation techniques. In spectral demodulation, a linear red-shift was observed in the Bragg spectrum with enhancement in the vacuum level with sensitivity and resolution of 18.5 pm/mbar and 6.644 mbar, respectively. In intensity demodulation, a chirped fiber Bragg grating (CFBG) was used as an optical filter for the Bragg spectrum of the FBG which linearly varied reflected light intensity from the FBG with enhancement in vacuum level inside the chamber. The intensity demodulation was able to detect the pressure difference in the chamber from 67 mbar to 0.2 μbar with sensitivity and resolution of 1.40 nW/mbar and 6.41 mbar, respectively. The sensor offered 2000 times faster response time than the reference commercial vacuum gauge for intensity demodulations. • We present an optical technique for direct measurement of vacuum level. • Fiber Bragg grating was used as transducer connected to a diaphragm. • The sensor was interrogated by spectral and intensity demodulation techniques. • The linear detection range of the sensor was 67 mbar to 0.2 μbar. • The sensor offered 2000 times faster response that the reference Pirani gauge.

Ultrafast laser inscription of efficient volume Bragg gratings deep in fused silica using active wavefront shaping

The maximum depth that photonic structures such as volume Bragg gratings (VBGs) can be precisely fabricated inside dielectric materials using ultrafast laser inscription (ULI) is limited by the aberration imparted on the laser beam by the air-substrate interface as it is focused into the substrate. Here, we use a computer-controlled spatial light modulator (SLM) to shape the wavefront of the ULI laser before it is focused into the substrate, such that the impact of this aberration on the manufacture of VBGs is minimized. We show that this technique allows us to inscribe efficient VBGs at depths in fused silica that would otherwise result in low efficiency VBGs. We find that an optimized “reference” grating fabricated at a mean depth of 200 µm without wavefront shaping exhibited a maximum relative first-order diffraction efficiency of 48%, whereas a grating fabricated at a mean depth of 900 µm using identical parameters exhibited an efficiency of 6.2% – both measured with 633 nm light polarized perpendicularly to the grating lines. Using the SLM to control the wavefront of the ULI laser beam, we were able to pre-compensate for the effect of the substrate surface aberration and fabricate gratings at a mean depth of 900 µm that increased the first-order relative diffraction efficiency to ∼42%. A further plasma study provided significant evidence to the effectiveness of Zernike polynomials for spherical aberration correction. Combing both plasma imaging and laser writing approaches, a set of polynomials for aberration correction at a range of depths was produced with scope for arbitrary depth correction.

Cr/C multilayer growth on a heavy metal layer for upgrading of high efficiency tender x-ray gratings.

To increase efficiency of single layer gratings used in the tender x-ray range, a high reflectance multilayer can be directly grown on single layer gratings. Multilayer growth quality was studied by depositing the Cr/C multilayer on a Pt single layer using flat substrates. Their structure quality and adhesion were characterized by atomic force microscopy (AFM), grazing incidence x-ray reflectivity (GIXRR), x-ray scattering (XRS), x-ray diffraction (XRD), and layer adhesion measurement. AFM results showed that the surface roughness was 0.218 nm for the multilayer without the Pt layer and 0.272 nm for the multilayer with the Pt layer. As GIXRR results showed, the average interface widths were 0.39 nm for the multilayer without the Pt layer and 0.42 nm for the multilayer with the Pt layer. XRS results indicated that the existence of a Pt layer enlarged slightly the roughness of the multilayer. Simulation results exhibited that these slight changes caused by the Pt layer had an insignificant effect on reflectivity. As XRD results displayed, the crystallization of the Pt layer had negligible effects on the crystallization of Cr in films. The layer adhesion measurement revealed that the critical loads to peel off the layer from the substrate were 84.64 mN for the multilayer without the Pt layer and 33.99 mN for the multilayer with the Pt layer. After 6 months, the latter layer structure is undamaged, demonstrating that the coating is not easily peeled off. This study proves the feasibility to upgrade a low efficiency single Pt layer grating to a highly efficient multilayer grating.

Deep learning for the design and characterization of high efficiency self-focusing grating

We demonstrate that the deep learning algorithm can considerably simplify the design and characterization of high efficient self-focusing varied line-spaced gratings. Our neural network is implemented with a recovery rate of up to 94% for the transmission function parameters. With numerical simulations, and optical experiments, we show that the self-focusing varied line-spaced gratings designed in such a way are endowed with enhanced functionalities, such as the intensity of first-order diffraction peak being enhanced with around a factor of 30 compared with the incident intensity, and a high ratio (about 60) between the peak intensity of the first order and the intensity of the zero-order. Our results allow the rapid design and characterization of self-focusing varied line-spaced gratings as well as optimal microstructures for targeted far-field diffraction patterns , which are playing key roles in spectroscopy and monochromatization applications. • A novel deep learning based self-focusing varied line spaced grating design method is proposed. • The grating parameters are retrieved from desired pattern with 94% recovery rate. • The experimental peak intensity ratio between first order and zero order of the binary 1D grating can be up to 86. • The high performance self-focusing grating is very easy to fabricate.

Visible-Wavelength-Tunable, Vortex-Beam Fiber Laser Based on a Long-Period Fiber Grating

In this letter, we report the first demonstration of a visible-wavelength-tunable, vortex-beam fiber laser based on a Sagnac-loop fiber filter and efficient long-period fiber grating. A 30 cm Pr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> /Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> -codoped ZBLAN fiber pumped by a 445 nm laser diode provides ~ 635 nm strong optical gain. A homemade dielectric mirror on the fiber end-facet and a Sagnac-loop fiber filter are used to build a wavelength-tunable fiber laser with a wavelength range of 633.89 to 636.02 nm. A self- fabricated long-period fiber grating acts as a mode converter in the cavity, and one-order vortex-beam including cylindrical vector beams (CVBs) and orbital angular momentum beams (OAMs) with a high mode purity are successfully obtained. Furthermore, we also measured the speckle contrast of the visible-wavelength fiber laser, and the speckle contrast is reduced by 17.13% through CVBs. This work could offer an effective approach to develop compact, wavelength-tunable vortex fiber lasers in the visible spectrum for applications to high-resolution imaging, laser display, visible light communications, and scientific research.

Design of efficient concave diffraction grating on 220 nm SOI platform for hybrid WDM–PDM​ (de)multiplexing

Abstract An efficient concave diffraction grating (CDG) for multi-wavelengths and dual polarizations is proposed and demonstrated on 220 nm SOI platform with systematic analyzation and simulation. The multilayer dielectric reflector is designed as an alternative to conventional deep-etched tooth to provide high reflectance with polarization insensitivity. Simulation results indicate that with proper selection of incident angles, the grating designed by this approach shows excellent performances in terms of diffraction efficiency, channel uniformity and next-channel crosstalk for both TE and TM polarizations. A comparative analysis between different grating tooth configurations and a detailed discussion about the fabrication tolerances are also presented. This design can be applied to wavelength and polarization hybrid multiplexing to further enhance the link capacity of optical transmissions and interconnects.

Erbium spectral-luminescent characteristics in bromide-fluoride photo-thermo-refractive glasses

Subject of Research. The paper presents the study of spectral-luminescent characteristics of erbium ions in bromide- fluoride photo-thermo-refractive glasses with 0.1 mol% of erbium oxide and 1-2 mol% of ytterbium oxide concentration. Such spectroscopic parameters as stimulated emission cross-section of erbium ions at a wavelength of 1.5 microns, the efficiency of energy transfer from ytterbium to erbium ions, and the quantum efficiency are calculated based on the measured absorption spectra of glasses under study. The gain/loss spectra of erbium ions in the infra-red range are calculated for various population inversion values. Methods. Bromide-fluoride photo-thermo-refractive glasses were synthesized in an electric furnace at the temperature of 1480 °C for 5 hours in the air. Judd-Ofelt theory was used for determination of spectroscopic intensity parameters. The amplification spectra were obtained using McCumber theory. Main Results. It was found that the stimulated emission cross-section of erbium ions at a wavelength of 1.5 microns is typical for erbium in silicate glasses. The quantum efficiency of radiation at the same wavelength reached 95 %, and the energy transfer efficiency was 86 %. The gain factor had a positive value when the population inversion parameter was higher than 0.5. Practical Relevance. Bromide-fluoride photo-thermo-refractive glasses activated with erbium and ytterbium ions have spectral-luminescent characteristics comparable with commercial laser silicate glasses. In addition, they allow for recording of highly efficient Bragg gratings. Thus, these glasses can be promising candidates for creating distributed feedback lasers with ultra-narrow spectral lines.

Adjoint Optimization of Efficient CMOS-Compatible Si-SiN Vertical Grating Couplers for DWDM Applications

Data communication in silicon photonic interconnects requires efficient and broadband on/off-chip coupling components. Recently, perfectly vertically-emitting grating couplers have been proposed to increase spatial I/O density of the optical link and potentially improve manufacturing costs and ease of optical beam characterization. In this article, adjoint optimization was leveraged in the design of low-loss single (silicon) and dual layer (silicon + silicon nitride) perfectly-vertical grating couplers that are compatible with a scalable silicon-on-insulator (SOI) platform for the 65 nm CMOS technology node. In simulation, the best design peaks at −0.52 dB insertion loss with a 1 dB-bandwidth of 24 nm at the 1310 nm datacom wavelength.

Theoretical analysis and optimization of highly efficient multilayer-coated blazed gratings with high fix-focus constant for the tender X-ray region.

The problem of X-ray diffraction from multilayer-coated blazed diffraction gratings is analyzed. Invalidity of the conventional condition of maximal diffraction efficiency observed in previous experiments is explained theoretically. This is attributed to two factors: contribution of anti-blaze facets to diffraction efficiency and effect of strongly asymmetric diffraction. We demonstrate that a proper choice of the multilayer d-spacing allows to design grating with the diffraction efficiency close to the maximal possible one throughout the tender X-ray range (E∼1-5 keV). An optimization procedure is suggested for the first time to choose the optimal grating parameters and the operation diffraction order to obtain a high fix-focus constant and high diffraction efficiency simultaneously in a wide spectral range.

Повышение эффективности полимерного нанокомпозита для формирования фотонных структур методом оптической записи

In this work, the conditions for optimizing an optical recording material based on photosensitive acrylate-urethane nanocomposites with silicon oxide nanoparticles, for efficient Bragg gratings recording by multibeam interference method is presented. It was shown that the choice of the most effective monomers and their concentrations in the nanocomposite which depends on their structure and polymerization rate, followed by determination of the optimal recording radiation intensity for the given composition, makes it possible to adjust the conditions for component diffusion and redistribution, and hence, the formation of regions with different refractive indices in the recording medium. These results can be useful for creating complex 2D and 3D photonic structures based on the targeted modification of photosensitive polymer nanocomposites with different properties (luminescent, nonlinear optical, etc.).

Polymer micro-lenses as an long-coupling-distance interfacing layer in low-cost optical coupling solution between optical fibers and photonic integrated waveguide circuits

We present a low-cost scheme for non-permanent optical signal coupling for prospective application in single use photonic integrated chips. The proposed scheme exploits the use of polymer kinoform microlenses. The feasibility of the proposed solution is demonstrated by the experimental investigation of the optical signal coupling from single mode optical fiber (SMF) to the test structure of SixNy integrated waveguide. Full Text: PDF ReferencesM. Smit et al., "An introduction to InP-based generic integration technology," Semiconductor Science and Technology, 29 (8), 083001, 2014 CrossRef R. Baets et al., "Silicon Photonics: silicon nitride versus silicon-on-insulator," in Optical Fiber Communication Conference, OSA Technical Digest (online) (Optical Society of America, 2016), paper Th3J.1. CrossRef K. Shiraishi et al., "A silicon-based spot-size converter between single-mode fibers and Si-wire waveguides using cascaded tapers," Appl. Phys. Lett. 91, 141120 (2007) CrossRef Y. Sobu et al., "GaInAsP/InP waveguide dual core spot size converter for optical fiber,"IEEE Photonic Society 24th Annual Meeting, 469-470, (2011). CrossRef F. Van Laere et al., "Compact and Highly Efficient Grating Couplers Between Optical Fiber and Nanophotonic Waveguides," Journal of Lightwave Technology, vol. 25, no. 1, pp. 151-156, Jan. 2007. CrossRef A. Kaźmierczak et al., "Light coupling and distribution or Si3N4/SiO2 integrated multichannel single mode sensing system," Opt. Eng. 48, 2009, pp. 014401 CrossRef M. Rossi et al., "Arrays of anamorphic phase-matched Fresnel elements for diode-to-fiber coupling," Appl. Opt. 34, 2483-2488 (1995) CrossRef M. Prasciolu et al, "Fabrication of Diffractive Optical Elements On-Fiber for Photonic Applications by Nanolitography," Japanese Journal of Applied Physics, Volume 42, (2003) CrossRef F.Schiappelli et al., "Efficient fiber-to-waveguide coupling by a lens on the end of the optical fiber fabricated by focused ion beam milling" Microelectronic Engineering Volumes 73-74, pp.397-404 (2004) CrossRef

Efficient long period fiber gratings inscribed with femtosecond pulses and an amplitude mask.

We present efficient long period fiber gratings written with femtosecond laser pulses at 800nm and an amplitude mask, to the best of our knowledge, for the first time. The measured transmission spectra depict strong resonances, while the total grating length and polarization-dependent loss could be significantly reduced compared to previous results. Two gratings are exemplarily shown-one in a standard single mode, and one in a large-mode-area fiber revealing a predictable spectrum without intermediate peaks due to the suppression of coupling to asymmetric higher-order cladding modes.

Optimized highly efficient multilayer-coated blazed gratings for the tender X-ray region.

The optimized design of multilayer-coated blazed gratings (MLBG) for high-flux tender X-ray monochromators was systematically studied by numerical simulations. The resulting correlation between the multilayer d-spacing and grating blaze angle significantly deviated from the one predicted by conventional equations. Three high line density gratings with different blaze angles were fabricated and coated by the same Cr/C multilayer. The MLBG with an optimal blaze angle of 1.0° showed a record efficiency reaching 60% at 3.1 keV and 4.1 keV. The measured efficiencies of all three gratings were consistent with calculated results proving the validity of the numerical simulation and indicating a more rigorous way to design the optimal MLBG structure.

Thermoplasmonic Activated Reverse-Mode Liquid Crystal Gratings

A new generation of reconfigurable optical components is conceived by bridging the photothermal properties of gold nanoparticles and the thermosensitivity of liquid crystalline materials. As such, gold nanorods (GNRs) heated using light are used to activate efficient hidden diffraction gratings realized in a blend made of a room temperature polymerizable liquid crystal (PLC) and nematic liquid crystal (NLC). Holographic liquid crystal polymer dispersed liquid crystal (HLCPDLC) gratings containing a small percentage of GNRs are fabricated with periodicity of 2.6 μm by means of a conventional UV holographic recording setup. The HLCPDLC structures are characterized using morphological, optical, and thermooptical techniques. Because of the initial refractive index match between the polymer-rich and LC-rich regions, the “grating” is hidden and the films appear transparent and nondiffractive. Illumination of these GNR-containing structures with a suitable light source (808 nm) induces a local heating due to the...