Grating-assisted Directional Coupler Research Articles

Highly Sensitive Refractive Index Sensor based on Silicon Nitride Strip Waveguide Directional Coupler

We propose a refractive index sensor, based on grating-assisted directional coupler, by comprising two coupled asymmetric strip waveguides with a top-loaded grating structure. We have studied variation of the sensitivity with the waveguide parameters to optimize the design of the sensor. The proposed sensor is an on-chip device with a high refractive index sensitivity of 104nm/RIU with $ \pm $ 140 nm fabrication tolerance, which is more than two times higher than the reported work, and the sensor can also achieve the ultrahigh sensitivity 105nm/RIU with the fabrication tolerance $ \pm $ 13nm. The sensor has large dynamic range from refractive index 1.0 to 1.55. We have also studied the propagation loss due to side wall roughness of the strip waveguide and found that it does not significantly affect sensor performance.

Electro-optic tunable grating-assisted optical waveguide directional coupler in lithium niobate

In this paper, a high-performance electro-optical tunable grating-assisted directional coupler (GADC) is proposed and demonstrated experimentally. Our proposed GADC consists of a two-mode waveguide (TMW) and a single-mode waveguide (SMW) formed with lithium niobate (LN). By introducing a long-period waveguide grating into the side-wall of the TMW to compensate for the phase mismatch of the fundamental modes of the SMW and TMW, the fundamental modes of the two waveguides can be coupled efficiently each other at a specific wavelength. Furthermore, push–pull electrode structure is introduced to achieve the electro-optic (EO) tuning function featuring high speed, low driving voltage, and large tuning range. The best LN GADC we fabricated on x-cut LN substrate using annealing proton exchange process shows a high isolation of ~ 34 dB at 1532.9 nm wavelength, quite large EO tuning efficiency of 1.195 nm/V (1526.4–1549.1 nm) or 1.736 nm/V (1576.1–1602.2 nm), and a thermo-optic tuning efficiency of 0.128 nm/°C (22–60 °C). Our proposed LN GADC can find applications in the fields of high-speed tunable wavelength filtering, mode filtering, and EO modulation.

Grating-Assisted Directional Coupler in Lithium Niobate for Tunable Mode Filtering

We propose a long-period grating assisted directional coupler in lithium niobate (LN) for the realization of electro-optic tunable mode filtering functionality. Our typical device, fabricated with the annealed proton-exchange process and designed for filtering out the fundamental mode in a two-mode LN waveguide, achieves a maximum mode extinction ratio of 29 dB at 1551.7 nm wavelength and a -20 dB bandwidth of ~2.5 nm with an electrical wavelength tuning sensitivity of 0.182 nm/V.

Grating-assisted non-linear directional coupler based optical switch with reduced critical power

We present a novel design of an optical switch based on grating-assisted non-linear directional coupler (GANDC) which consists of two non-identical waveguides, with a periodic refractive index perturbation in between the waveguides, along the direction of propagation. The proposed design of the optical switch reduces the critical power by a factor ~25, as compared to the conventional non-linear directional coupler-based switch, with potential applications in photonics.

Highly Sensitive Refractive Index Sensor Based on Silicon Nitride Strip Waveguide Directional Coupler

We propose a refractive index sensor, based on grating-assisted directional coupler, by comprising two coupled asymmetric strip waveguides with a top-loaded grating structure. We have studied variation of the sensitivity with the waveguide parameters to optimize the design of the sensor. The proposed sensor is an on-chip device with a high refractive index sensitivity of ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> nm/RIU with ±140 nm fabrication tolerance, which is more than two times higher than the reported work, and the sensor can also achieve the ultrahigh sensitivity ~10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> nm/RIU with the fabrication tolerance ±13 nm. The sensor has large dynamic range from refractive index 1.0 to 1.55. We have also studied the propagation loss due to side wall roughness of the strip waveguide and found that it does not significantly affect sensor performance.

301-nm wavelength tunable differentially driven all-polymer optical filter.

An optical filter based on all-polymer grating-assisted directional coupler is demonstrated. The wavelength tuning is differentially driven. The heater-electrodes created on the sidewalls of the polymer ridge can either blue- or red-shift the central wavelength. A total tuning range of 301 nm is achieved experimentally. The maximal local temperature gradient introduced across the coupler region is only 55°C-68°C. The filter is also insensitive to uniform ambient temperature change. Central wavelength jitter of ±3 nm is observed when the chip holder temperature cycles between 20°C and 80°C.

Design of a Grating-Assisted Lateral Directional Coupler by Impurity-Induced Quantum-Well Intermixing of InGaAs/GaAs

An asymmetric waveguide grating-assisted coupler, which is suitable for a coarse wavelength division multiplexing system, based on a quantum-well intermixing process in InGaAs/GaAs, has been designed. The proposed device processing is similar to that used in nominal microelectronics processing. The device has been modeled from the first principle of refractive- index change due to F-implantation and anneal. The coupler has been analyzed by quasi-static effective index as well as finite difference methods in conjunction with an orthonormal-mode beam propagation method. A power-coupling ratio of ~90% at a wavelength of 1.51 mum with a coupled wavelength bandwidth of ~22 nm has been obtained. The coupler lengths are within a few millimeters, and the device is suitable for multiwavelength integration.

Radiation loss of grating-assisted directional couplers using the Floquet-Bloch theory

The Floquet-Bloch theory (FBT) is used to determine the radiation loss of a grating-assisted directional coupler. Improper waves obtained by FBT represent leakage and produce radiation loss along the transverse direction. By summing the fields of all leaky harmonics, the power flow radiated into the superstrate and substrate can be calculated from Poynting's theorem. However, the locations at which these summations occur impact the accuracy. An analytic formula for radiation loss is also derived. Results from both the FBT and the analytic formula are compared with results obtained using the coupled-mode theory.

External cavity laser with high-sidemode suppression ratio using grating-assisted directional coupler

We propose and demonstrate, for the first time, a method to improve the sidemode suppression ratio (SMSR) of an external cavity laser (ECL) with a grating-assisted directional coupler as an external mirror. The ECL consisting of a Fabry-Pe/spl acute/rot laser diode and a grating-assisted coupler exhibited single-longitudinal-mode oscillation at Bragg wavelength with a high SMSR of 60 dB without any additional devices.

Ion beam etching of high resolution structures in Ta 2O 5 for grating-assisted directional coupler applications

An investigation on thin Ta 2O 5 films patterning using argon ion beam etching (IBE) is presented. The etch rates are characterised by varying the angle of incidence of the beam onto the substrate. Ta 2O 5 gratings with a period of 2.2 μm (1.1 μm linewidth) and 0.25 μm thickness are fabricated using an angle of incidence of 0°. The resulting Ta 2O 5 grating cross sectional profiles are analysed using AFM and SEM imaging. A fabrication method is thus demonstrated which could be used to implement wavelength selective gratings in applications such as grating-assisted directional couplers (GADCs).

Coupling to nanophotonic waveguides using a dual grating-assisted directional coupler

An analysis of a device we have called a dual grating-assisted directional coupler (DGADC) in silicon-on-insulator technology is presented. The device enables efficient coupling from optical fibres to small waveguides, typically <1 µm in cross-sectional dimensions. Coupling efficiency and device length are determined as functions of layer thicknesses and refractive indices, grating periods, depths and duty ratios, and finally wavelength. For our example calculation of coupling to a 250 nm silicon waveguide, this efficient and robust coupler can have coupling efficiency as high as 90%, while the wavelength range can have an FWHM of ~1.6 nm at resonance. Fabrication tolerances have also been analysed and they are up to two orders of magnitude larger than those of a GADC with only one grating.

Dual grating-assisted directional coupling between fibers and thin semiconductor waveguides

A novel dual grating-assisted directional coupler (DGADC) for coupling an optical fiber with a thin semiconductor waveguide is proposed. As an example, a DGADC in silicon-on-insulator is discussed. Maximum coupling efficiency in excess of 90% can be obtained using this coupler, while the highest coupling efficiency previously reported was only 40%.

Iterative method for the design of a nonperiodic grating-assisted directional coupler

We propose and investigate the optimal design of a nonperiodic grating-assisted directional coupler by iterative methods using the beam propagation method. Computer simulations were carried out at wavelengths of 0.8, 1.3, and 1.5 mum, which are often used in optical communications and networking. We found that the complete power coupling lengths can be reduced considerably in comparison with those in the case of the periodic grating-assisted waveguides with the same set of parameters.

Coupled-Mode Analysis of a Gating-Assisted Directional Coupler Using Singular Perturbation Technique - Abstract *

An accurate coupled-mode analysis of a grating-assisted directional coupler is presented using the singular perturbation technique. The optical fields in the coupler are represented by a linear combination of para-axial wave fields for each isolated waveguide. The para-axial wave fields are expanded using the multiple space-scales and solved so that they satisfy the para-axial wave equation for the coupled structure in the respective order of perturbation. This leads to the nonorthogonal coupled-mode equations which are accurate in the second order of perturbation. The proposed coupled-mode theory is applied to the analysis of two nonidentical coupled waveguides attached with a sinusoidal grating layer. The calculated wavelength response in the power transfer shows a close agreement with that obtained by a direct numerical analysis using the Fourier series expansion method.

Grating-assisted coupling of light between semiconductor and glass waveguides

Floquet-Bloch theory is used to calculate the electromagnetic fields in a leaky-mode grating-assisted directional coupler (LM-GADC) fabricated with semiconductor and glass materials. One waveguide is made from semiconductor materials (refractive index /spl ap/3.2) while the second is made from glass (refractive index /spl ap/1.45). The coupling of light between the two waveguides is assisted by a grating fabricated at the interface of the glass and semiconductor materials. Unlike typical GADC structures where power is exchanged between two waveguides using bound modes, this semiconductor/glass combination couples power between two waveguides using a bound mode (confined to the semiconductor) and a leaky mode (associated with the glass). The characteristics of the LM-GADC are discussed. Such LM-GADC couplers are expected to have numerous applications in areas such as laser-fiber coupling, photonic integrated circuits, and on-chip optical clock distribution. Analyses indicate that simple LM-GADC's can couple over 40% of the optical power from one waveguide to another in distances less than 1.25 mm.

Free space mode approximation of radiation modes for weakly guiding planar optical waveguides

The radiation modes of weakly guiding planar optical waveguides have been approximated and simplified as free space modes. The power spectrum of radiation modes has been numerically compared with that of free space modes. Except a very small angle (the angle between radiation direction and the waveguide walls), the approximation is accurate. Applying this approximation for radiation modes to the radiation loss calculation caused by random wall imperfections of a slab waveguide and a grating-assisted directional coupler, we have obtained very simple formulas and hence saved computation time. Our calculated results are close to those using exact radiation modes. Accuracy and limits by using this method have been discussed, Comparisons with other methods have also been made.

Coupled-mode analysis of wavelength filtering in a grating-assisted asymmetric three-waveguide directional coupler

The power transfer characteristics in a grating-assisted three-waveguide coupler are analyzed by using the nonorthogonal coupled-mode theory based on the isolated waveguide modes. The coupler consists of three asynchronous, parallel, planar waveguides in which the upper and lower surfaces of the intermediate waveguide are corrugated with different periods. When two phase-matching conditions and an additional constraint on the coupling constants are satisfied, the guided modes in the two outer waveguides couple selectively at a certain wavelength by means of the corrugated intermediate waveguide. It is shown that the wavelength selectivity in the power transfer between the two outer waveguides becomes noticeably sharper than that of the grating-assisted two-waveguide coupler and that the sidelobes in the wavelength response are almost completely suppressed.

Grating-assisted surface acoustic wave directional couplers

Physical properties of novel grating-assisted Y directional couplers are examined using the coupled-mode theory. A general formalism for the analysis of the lateral perturbed directional coupler properties is presented. Explicit expressions for waveguide key parameters such as coupling length, grating period, and other structural characterizations, are obtained. The influence of other physical properties such as time and frequency response or cutoff conditions are also analyzed. A plane grating-assisted directional coupler is presented and examined as a basic component in the integrated acoustic technology.

Coupled mode formulation for directional couplers with longitudinal perturbation

A grating-assisted directional coupler is investigated using an improved coupled-mode formulation for multi-waveguide systems with longitudinal perturbation. This approach is capable of handling three-dimensional structures as well as a complex index of refraction and nonisotropic media. The case of two coupled channel waveguides is closely examined, and a numerical analysis is carried out for several cases of slab structures. >

Radiation loss of grating-assisted directional coupler

The coupling between the guided modes and between guided and radiation modes of two parallel slab waveguides forming a directional coupler in order is computed to determine the radiation losses introduced by the coupling grating. The problem is solved in two stages. First, the guided modes for each waveguide are computed separately and the radiation modes are only determined for the more complicated of the two waveguides, the one that is nearer to the grating. Modifications caused by the presence of the opposite waveguide are then taken into account by computing first-order correction terms. For a practical example of slab waveguides defined in GaInAsP, the authors find that the radiation losses per power exchange length remain below 0.02 dB for a rectangular grating depth of 0.01 mu m.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>