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Grating couplers are widely used to couple waveguide modes with the far field. Their usefulness is determined not only by energy efficiency but also by additional supported functionality. In this paper, a plasmonic is demonstrated grating on a silicon nitride slab waveguide that couples both transverse electric (TE) and transverse magnetic (TM) waveguide modes with circularly polarized light in the far field. Specifically, it is experimentally confirmed that circularly polarized light excites TE and TM modes propagating in opposite directions, and the direction is controlled by the handedness. The routing efficiency for normally incident light reaches up to 95%. The same structure operates in the outcoupling regime as well, demonstrating up to 97% degree of circular polarization, where the handedness is determined by the polarization and propagation direction of outcoupled modes. The results pave the way for the realization of polarization‐division multiplexers and demultiplexers, integrated circular polarization emitters, as well as detectors of the polarization state of the incident optical field. [ABSTRACT FROM AUTHOR]

Two-dimensional Ag/SiO 2 nanocomposite grating coupler with a periodicity of 600 nm was fabricated by etching an Ag-ion-implanted SiO 2 waveguide slab, which was evidenced to be able to convert a vertically incident beam in the wavelength range of 620–880 nm into four beams of guided waves without regard for the polarization of incident beam. Employing this coupler, the deflection of guided waves due to the diffraction distortion in the case of oblique incidence was clearly demonstrated. Moreover, to accurately predict the propagating direction of guided wave in the SiO 2 waveguide layer, a simple relation of the deflected angle of guided wave to the polar and azimuth angles of incident beam was deduced according to the oblique coupling of a one-dimensional Ag/SiO 2 nanocomposite grating coupler. The calculated results by using the deduced relation revealed that increasing the polar angle and/or azimuth angle of incident beam could lead to an increase in deflected angle of guided wave. Especially, for an azimuth angle of 90°, the calculated results agreed well with the experimental data, justifying the deduced relation. The above findings might be useful for the packaging and applications of grating couplers. [ABSTRACT FROM AUTHOR]

The silicon nitride (Si 3 N 4) waveguide platform becomes prominent in silicon photonics due to its ultralow propagation loss, negligible two-photon absorption, tolerance to phase error, and so on. Particularly, Si 3 N 4 photonic circuits provide nonlinear opportunities as thick waveguides enable dispersion engineering, which is fundamental for frequency comb and supercontinuum generation. However, grating couplers based on thick Si 3 N 4 waveguides, as the building block for any device, are prone to higher-order Bloch mode excitation, restricting the coupling efficiency dramatically, thus precluding practical applications that large bandwidth and are power restricted. We propose and demonstrate an 800 nm-thick Si 3 N 4 bi-layer fully etched grating coupler which is ultra-broadband and highly efficient. By applying the apodization technique, the bi-layer grating shows a conversion efficiency of −1.44 dB and a 1 dB bandwidth of 135 nm. Further integrated with a bottom reflector, a conversion efficiency of −1.01 dB with a 1-dB bandwidth of 117 nm is achieved. The proposed structures are highly fabrication tolerant and remain great performance regardless large structural parameter variations. It paves the way for a large-volume high-yield production towards fully-integrated frequency comb chip, optical beam forming network, Lidar, etc. [ABSTRACT FROM AUTHOR]

Despite the fact that the theoretical foundations of the sensitivity of waveguide grating based (bio)sensors are well-known, understood and their implications anticipated by the scientific community since several decades, to our knowledge, no prior publication has experimentally confirmed waveguide sensitivity for multiple film thicknesses, wavelengths and polarization of the propagating light. In this paper, the bulk refractive index sensitivity versus waveguide thickness of said refractometric sensors is experimentally determined and compared with predictions based on established theory. The effective refractive indices and the corresponding sensitivity were determined via the sensors' coupling angles at different cover refractive indices for transverse electric as well as transverse magnetic polarized illumination at various wavelengths in the visible and near-infrared. The theoretical sensitivity was calculated by solving the mode equation for a three layer waveguide. [ABSTRACT FROM AUTHOR]

Heterogeneous integration of III--V materials with silicon-on-insulator (SOI) waveguide circuitry by an adhesive die-to-wafer bonding process has been proposed as a solution to Si-based lasers and photodetectors. Here, we present the design and optimization of an InGaAs PIN photodetector vertically coupled with the underlying SOI waveguide, which could be readily fabricated using this bonding process. With the help of grating couplers, a thick bonding layer of 2.5 µm is applied, which inherently avoids the risk of low-bonding yield suffering in the evanescent coupling counterpart. An anti-reflection layer is also introduced between the bonding layer and the III--V layer stack to relieve the accuracy requirement for the bonding layer thickness. Besides, by optimizing the structure parameters, a high-absorption efficiency of 82% and a wide optical 1dB-bandwidth of 220nm are obtained. The analysis shows that the detection bandwidth of the present surface-illuminated photodetector is generally limited by transit-time in the i-InGaAs layer. The relationship of the detection bandwidth and the absorption efficiency versus the i-InGaAs layer thickness is presented for the ease of choosing proper structure parameters for specific applications. With the results presented here, the device can be readily fabricated. [ABSTRACT FROM AUTHOR]

Grating couplers are widely investigated as coupling interfaces between silicon-on-insulator waveguides and optical fibers. In this work, a high-efficiency and complementary metal-oxide-semiconductor (CMOS) process compatible grating coupler is proposed. The poly-Si layer used as a gate in the CMOS metal-oxide-semiconductor field effect transistor (MOSFET) is combined with a normal fully etched grating coupler, which greatly enhances its coupling efficiency. With optimal structure parameters, a coupling efficiency can reach as high as ∼70% at a wavelength of 1550 nm as indicated by simulation. From the angle of fabrication, all masks and etching steps are shared between MOSFETs and grating couplers, thereby making the high performance grating couplers easily integrated with CMOS circuits. Fabrication errors such as alignment shift are also simulated, showing that the device is quite tolerant in fabrication. [ABSTRACT FROM AUTHOR]

The use of grating couplers in high index contrast waveguides like silicon on insulator (SOI) offers several advantages over other coupling approaches, including better alignment tolerances and allowing for wafer-scale testing. The grating couplers were developed for nanometric SOI waveguides (Si-wires), and recently also for micrometric rib waveguides. In this paper we review our work in fiber-to-chip grating couplers for thick SOI rib waveguides, where a coupling efficiency of −2.2 dB was demonstrated experimentally. We also discuss the use of grating couplers to improve optical throughput ( étendue) of a planar waveguide Fourier-Transform (FT) spectrometer implemented in thick rib waveguides. [ABSTRACT FROM AUTHOR]

grating coupler system has been developed to measure refractive index gradients with high spatial (6.7 μm) and temporal (milliseconds) resolution. The system was applied to two-phase model systems consisting of water and non-aqueous pollution liquids. Refractive index gradients at the interfaces between the aqueous and organic phase of 1-butanol, hexane, and 1-heptanol were monitored under steady-state conditions. The temporal resolution was utilized in diffusion experiments with glycerol and sodium chloride in water, where the formation of a concentration gradient was studied. In a further application, the grating coupler system was modified to monitor low-level concentrations of aqueous pollution profiles as are caused by bacterial degradation in the aqueous phase. Toluene was selected as contaminant. The sensor sensitivity was improved by coating the sensor with the pre-concentrating polymers polydimethylsiloxane and Teflon® AF-2400. With the grating coupler setup, a multi-purpose instrument was created to measure high-resolution refractive index gradients with high temporal and spatial resolution in different fields of application. The new sensor system can be used to measure absolute refractive indices by covering parts of the sensing area with cover media of known refractive index. Coatings can be used for sensitivity improvement by pre-concentrating the sample, for selectivity by utilizing filtering properties of the coating, and as calibration standard for absolute refractive index measurements. [ABSTRACT FROM AUTHOR]

The numerical simulations of nanogratings integrated with gallium nitride (GaN) planar waveguides as well as the experimental in-coupling results are presented. A simulation tool based on the eigenmode expansion method and advanced boundary conditions provided a rigorous model of 400-nm-period grating couplers. A full-vectorial Maxwell solver allowed performing a number of simulations with varying grating parameters, where coupling efficiency, reflection and transmission characteristics of device were calculated. Gratings with different etch depths and arbitrary shapes were simulated using a staircase approximation, with an optimized number of steps per single slope. For the first time, an impact of dry etch processing on GaN coupler efficiency was evaluated, due to the inclusion of the sloped sidewalls, with regard to the technological constrains. Finally, the experimental results in the visible spectrum region (λ = 633 nm), for 400-nm-deep gratings etched in GaN waveguide, were presented together with theoretical data for binary and trapezoidal profiles of a grating, for different optical mode profiles ( $${{\rm TE}_{0}\div {\rm TE}_{3}\,{\rm modes}}$$). [ABSTRACT FROM AUTHOR]

Aiming at a handheld sensor to characterize chemical compositions, we propose a fully integrated absorption sensor based on infrared (IR) absorption in the evanescent field region of an integrated monomode waveguide utilizing thermally generated and detected IR radiation. For the coupling, two grating couplers are employed, which couple broadband thermal IR radiation into the waveguide and the attenuated IR radiation at the waveguide output toward a thermal IR detector, respectively. To achieve a spectroscopic measurement, we utilize the dispersive feature of the grating coupler, which couples the IR beams out of the waveguide at angles depending on the wavelength. Depending on the application, the spectral resolution might be more or less critical. In this paper, we report on the design of the grating couplers associated with the proposed system configuration influencing the spectral resolution. We also show measurements confirming the results obtained by numerical simulations aiming at the investigation of the dispersive feature of the grating couplers. [ABSTRACT FROM AUTHOR]

Abstract: We report the sol–gel chemistry reaction system and processes used to realize the first grating coupled monolithic solid state optical amplifier based on a Nd3+ doped sol–gel tapered rib waveguide. The sol–gel matrix composition is 50SiO2:50TiO2:10AlO1.5:1NdO1.5. The signal (1064nm) and pump (803nm) beams were coupled via grating couplers into and out of the rib waveguide. A photoluminescence lifetime of 138μs and a pump absorption cross-section of 1.79×10−24 m−2 were measured. A net internal gain of 3.75dB/cm was achieved. Theoretical simulations predict that this gain could be further improved. Analysis of the results suggests that the difference between achieved and predicted gain is due to low quantum efficiency caused by two-ion cross-relaxation and up-conversion, as manifestly evident by yellow light emitted from the device. The molecular-level processes during the doped sol–gel fabrication that could lead to these phenomena are discussed, and strategies are suggested to overcome these limitations in order to approach the full potential gain. [Copyright &y& Elsevier]

Silicon nitride photonics has enabled a broad class of low-loss integrated devices and chip-scale solutions that benefit from transparency over a wide wavelength range and compatibility with complementary metal-oxide-semiconductor process, allowing applications of tele/datacom, artificial intelligence, sensing and bio-photonics. As a complimentary platform to silicon-on-insulator photonics, low-loss silicon nitride waveguide technology was present to extend the capabilities of silicon photonics platform. Silicon nitride films prepared by low pressure chemical vapor deposition have low intrinsic loss and good repeatability, but silicon nitride film is prone to crack when the thickness is no less than 300 nm. In order to reduce high stress, the wafer-level silicon nitride waveguides with 400 nm thickness were fabricated by two-step deposition method on 200-mm complementary metal-oxide-semiconductor pilot-line. Due to the different linewidth change between single-stripe and multiple-stripe structure after the same layer etch process, the layout compensation is needed to correct this deviation. But the linewidth sensitivity for silicon nitride waveguides is relatively low, propagation losses in the C-band were slightly reduced from 0.66 dB/cm for 0.9 µm width down to 0.62 dB/cm for 1 µm width. [ABSTRACT FROM AUTHOR]

Abstract: Optical properties of silica–titania thin films prepared by the sol–gel technique are presented. The layers were deposited on glass slides or silica wafers by a dip coating process. Samples were prepared from different mixtures of silica and titania alkoxides. The stable, crack-free and transparent coatings were obtained after heating in the air at 60°C and 500°C. The layers coated on oxidized silica plates (on SiO2 optical substrate layers) can be used as planar waveguides. Coupling light into silica–titania thin films was possible by applying grating couplers. This functional device, fabricated by the holographic method, allows to couple laser light in and out of the planar waveguides by exciting various waveguide modes. The possibility of using silica–titania thin films with entrapped sensing molecules as an optical waveguide sensor is also discussed. [Copyright &y& Elsevier]

Abstract: In this work organic light emitting devices (OLEDs) were fabricated implementing gratings, in order to extract waveguided electroluminescence (EL). The gratings were recorded by exposing thin films of the molecular azo glass N, N′-bis (4-phenyl)-N, N′-bis [(4-phenylazo)-phenyl] benzidine (AZOPD) to holographic light patterns. The photopatterned AZOPD serves as hole transport material for devices with aluminum-tris(8-hydroxyquinoline) doped with 1% of 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran (Alq3:DCM) as emissive/electron transport layer. The corrugated devices showed enhanced emission in the forward direction. The emitted light is polarized preferably parallel to the grating lines. In addition, we have found a doubling in the total luminance with respect to the unstructured device. [Copyright &y& Elsevier]

By monolithic integration of a waveguide type grating with a quantum well laser, it is possible to construct various ultra-compact and high-performance devices. Since the graded refractive index separately-confined single-quantum-well DFB laser is superior in wavelength stability and low-loss passive waveguide formation is made possible by QW disordering, it is suitable for this type of integration. Examples of optical integrated circuits, fully functioning with one chip, are provided by a monolithic integrated-optic position/displacement sensor and a monolithic integrated-optic disk pickup. Their design, fabrication, and evaluation are carried out. In the former, a photocurrent variation with a period of 0.43 μm is obtained, so that a monolithic optical integrated sensor fully functioning with one chip was successfully attained. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(4): 10–18, 1998 [ABSTRACT FROM AUTHOR]

For high density integration of an optical integrated circuit, stacked waveguides and active interconnection between the waveguides must be attained. In this paper, two thin film slab waveguides stacked on a substrate were used to form a two-story structure. Grating couplers were formed in the waveguides to couple the two waveguides. The coupling equations were derived from mode coupling theory in order to clarify the co-directional and contra-directional coupling characteristics. In co-directional coupling, the wavelength selectivity is small, and high coupling efficiency cannot be expected (less than 25%) if a conventional uniform grating coupler is used. On the other hand, in contra-directional coupling, the wavelength selectivity is high, and high coupling efficiency can be expected if a long grating length and large coupling coefficient can be attained. A contra-directional coupling device with a coupling wavelength of 800 nm was designed and fabricated, and its wavelength dependence was evaluated by using the TE mode. The experimental half-value width was 0.3 nm, which is close to the calculated value of 0.2 nm, indicating that the coupler has a sharp wavelength selective coupling characteristic. © 1998 Scripta Technica, Electron Comm Jpn Pt 2, 81(4): 29–37, 1998 [ABSTRACT FROM AUTHOR]

The grating structure and its performance for coupling between fiber and silicon-based waveguide are studied and analyzed. The coupling mechanism is illustrated through the grating Bragg diffraction condition, and the effect of physical parameters and alignment parameters of the grating coupler on the coupling efficiency is analyzed by FDTD (Finite-difference time-domain). The optimum design values of the period, duty cycle, etching depth and alignment parameters of the grating coupler are obtained, and at the same time, the deviation analysis on the optimal size is carried out. We present high-efficiency grating couplers fabricated both in EBL (Electron Beam Lithography) and Deep Silicon Etching through process optimization on device dimension, which can be useful for grating coupler fabrication. [ABSTRACT FROM AUTHOR]