Polarization-independent Waveguide Research Articles

Polarization-independent waveguide grating coupler using an optimized polysilicon overlay.

We propose and validate a new, to the best of our knowledge, approach to designing a polarization-independent waveguide grating coupler, using an optimized polysilicon overlay on a silicon grating structure. Simulations predicted coupling efficiencies of about -3.6 dB and -3.5 dB for TE and TM polarizations, respectively. The devices were fabricated using photolithography in a multi-project wafer fabrication service by a commercial foundry and have measured coupling losses of -3.96 dB for TE polarization and -3.93 dB for TM polarization.

Enhancement of magneto-optical properties in magnetic photonic crystal slab waveguide based on yttrium iron garnet

In this work, a polarization-independent waveguide based on magnetic photonic crystal (MPC) with a triangular lattice of air holes in Yttrium Iron Garnet (YIG) slab grown on alumina (Al2O3) substrate is proposed, where both TE-like and TM-like periodic band gaps overlap. YIG is well known for its attracting magneto-optical (MO) properties and used to produce a coupling between the TE and TM modes. Thus, a nonreciprocal effect can be obtained by applying an external magnetic field parallel to the direction of propagation. At 1550 nm, the complete photonic band gap is simulated and optimized using the three dimensional plane-wave expansion method. The aim of this study is to enhance Faraday rotation (FR) while maintaining a low modal birefringence. A numerical analysis in function of magnetic gyration (g) has been reported, using the BeamProp software. The results reveal a proportional relation between FR, Δn and g, such for g = 0.5, a large FR of 26.11×104 °/cm with Δn = 7×10−6. The results show a real improvement of this MPC structure based on YIG with larger FR, lower modal birefringence and minimal losses. The notable enhancement in the MO behaviour could improve the performance of optical isolators, and makes it suitable for nonreciprocal devices.

Polarization-independent waveguides based on the complete band gap of the two-dimensional photonic crystal slabs

Two-dimensional photonic crystal (PC) slabs exhibiting photonic band gaps (PBGs) in both TE and TM polarizations are of particular interest for a better control of light propagation and easy experimental fabrication. Through reducing the symmetry of the PC lattice, simultaneous realization of PBGs in both TE and TM polarizations within a certain frequency region can be achieved. Based on the air holes immersed in the silicon background, two kinds of two-dimensional PC slab structures are proposed. These are a honeycomb lattice of circular air holes with different sizes and larger circles, and smaller triangles alternately arranged, respectively. The light transmittances show that these two structures offer large complete band gaps. Different kinds of straight waveguides are designed, and their abilities of localizing the incident beam are studied by analysing the light transmittances and spatial field distributions, where the light energy is localized perfectly in the optimized waveguide structure. Our proposed PC slab waveguides are convenient for fabrication, and could be applied in future all-optical integrated circuits.

Demonstration of Polarization-Independent Surface Plasmon Resonance Polymer Waveguide for Refractive Index Sensing

The effect of the variation of the refractive index in a gold-coated ormocomp waveguide for sensing application is studied in this paper. The ormocomp waveguide is fabricated by using the nanoimprint method. A waveguide with width and height both equal to 10 μm is coated with 100 nm thick gold using the sputtering technique in order to introduce surface plasmon resonance at the vertical sides and also at the top of the structure. Here, polarization-independent waveguide is achieved by supporting both the plasmonic TE and TM modes and the light confinement in these modes are studied. Supermodes forming from coupling between the fundamental dielectric mode and the plasmonic supermode at the resonance peak are also investigated. This paper presents the numerical simulated results and also their experimental validations. For this structure, there are two dielectric-plasmon supermodes with two resonance peaks separated by 50 nm. A red shift is observed when the refractive index of the cladding material increases. The cladding material includes water (n = 1.333) and iso-propanol solutions with refractive indices of 1.344, 1.351, and 1.365. The gold-coated ormocomp waveguide has sensitivity of about 544.55 nm/RIU with a resolution of 5.3 × 10−3 RIU.

Sensitive magnetic field sensor using 2D magnetic photonic crystal slab waveguide based on BIG/GGG structure

A kind of magnetic field sensor (MFS) using a two-dimensional (2D) magnetic photonic crystal (MPC) slab waveguide as the sensing structure is proposed and investigated numerically. The slab structure is based on bismuth iron garnet (BIG), a well-known magnetic material with effective magnetooptical (MO) properties, sandwiched with gadolinium gallium garnet (GGG) as substrate. The complete photonic bandgap (PBG) of the 2D MPC is simulated and optimized for realization of polarization-independent waveguides. The simulation results show that the width and position of the complete PBG depend on the thickness of the BIG slab and the radius of the air holes used in the design. By reducing the lightwave propagation losses and enhancing the mode conversion ratio, increased sensitivity is obtained. Based on the Faraday effect, a good linear relationship is observed between the normalized output light intensity and the magnetic field strength as the gyrotropy parameter g is varied from 0.13 to 0.19, a g-range used as the sensor dynamic range. The remarkable enhancement in sensing performance due to the MO effect makes the designed device suitable for magnetic field sensing. The results are discussed to provide a basis for investigation of 2D MPC slab waveguides based on the same structure, which are of particular interest for development of highly sensitive MFSs.

Fabrication of polarization-independent waveguides deeply buried in lithium niobate crystal using aberration-corrected femtosecond laser direct writing

Writing optical waveguides with femtosecond laser pulses provides the capability of forming three-dimensional photonic circuits for manipulating light fields in both linear and nonlinear manners. To fully explore this potential, large depths of the buried waveguides in transparent substrates are often desirable to facilitate achieving vertical integration of waveguides in a multi-layer configuration, which, however, is hampered by rapidly degraded axial resolution caused by optical aberration. Here, we show that with the correction of the spherical aberration, polarization-independent waveguides can be inscribed in a nonlinear optical crystal lithium niobate (LN) at depths up to 1400 μm, which is more than one order of magnitude deeper than the waveguides written with aberration uncorrected femtosecond laser pulses. Our technique is beneficial for applications ranging from miniaturized nonlinear light sources to quantum information processing.

Polarization-independent waveguides in air holes photonic crystals and its slow light

A line-defect waveguide in a triangular lattice photonic crystal (PC) made of air holes in dielectric is demonstrated to support transverse magnetic (TM) as well as transverse electric (TE) guided modes simultaneously. A group of suitable geometric parameters were found to make the guided bands overlapped by means of Genetic Algorithm. The optimized waveguide realizes a polarization-independent single-mode transmission and wide operating bandwidth which reaches 0.012Δωa/(2πc). Moreover, the guided modes are shown to exhibit a wide-bandwidth slow light and an extremely low group velocity dispersion in most frequency range.

Reduced symmetry and analogy to chirality in periodic dielectric media

Much attention has been paid to photonic applications based on periodic media. Meanwhile, quasi-periodic and disordered media have extended the research domain and provided additional novelties for manipulating and controlling light propagation. This review article attempts to highlight the benefits of symmetry reduction in highly symmetric periodic photonic media, and applies the concept of chirality to all-dielectric materials arranged in special orders. Two-dimensional periodic structures known as photonic crystals (PCs) are highly symmetric in terms of structural patterns, due to the lattice types and shape of the elements occupying the PC unit-cell. We propose the idea of intentionally introducing reduced-symmetry, to search for anomalous optical characteristics so that these types of PCs can be used in the design of novel optical devices. Breaking either translational or rotational symmetries of PCs provides enhanced and additional optical characteristics such as creation of a complete photonic bandgap, wavelength demultiplexing, super-collimation, tilted self-collimation, and beam deflecting/routing properties. Utilizing these characteristics allows the design of several types of photonic devices such as polarization-independent waveguides, wavelength demultiplexers, beam deflectors, and routers. Moreover, reducing the symmetry in the PC unit-cell scale produces a novel feature in all-dielectric PCs that is known as chirality. On the basis of above considerations, it is expected that low-symmetric PCs can be considered as a potential structure in photonic device applications, due to the rich inherent optical properties, providing broadband operation, and being free of absorption losses.

Modelling of Polarization-Dependent Loss in Plasmonic Nanowire Waveguides

We have investigated the potential of using gold nanowires embedded in a dielectric cladding environment as polarization-independent long-range surface plasmon polariton waveguides at telecom wavelengths. We performed finite-element analysis on various symmetric and close-to-symmetric cross-sectional geometries and evaluated the effects of cladding thickness on the propagation and coupling loss. The calculations confirm that fabrication of polarization-independent waveguides with reasonable tolerances is feasible and that straight-waveguide insertion losses around 1.5 dB for short (0.5 mm) devices can be realized when coupling to and from conventional dielectric waveguide geometries.

Design and Application of Compact and Highly Tolerant Polarization-Independent Waveguides

In this paper, the design, fabrication, and application of a highly tolerant polarization-independent optical-waveguide structure suited for operation in the third communication window is presented. The waveguide structure has been optimized toward minimized sensitivity to technological tolerances and low fabrication complexity. The tolerance analysis has been based on the typical processing tolerances of the widely applied silicon-oxynitride technology, being plusmn3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> in refractive index, plusmn1% in thickness, and plusmn0.1 mum in channel width. The optimized waveguide design fulfills the criterion of a channel birefringence within 5times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup> , including processing tolerance. It also enables a fiber-to-chip coupling loss below 1 dB/facet and is suited for the realization of low-loss bends with a radius down to 600 mum. Based on this waveguide design, a passband-flattened optical wavelength filter with 50-GHz free spectral range has been realized and tested. The measured TE-TM shift of 0.03 nm confirms the polarization dependence of the optical waveguides being as low as 3times10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-5</sup>

Proposal for a polarization-independent integrated optical circulator

A polarization-independent waveguide circulator design is proposed which is well suited for monolithic integration. The circulator consists of multimode interferometers, half-wave plates, and Faraday rotators all in waveguide form. The length of the circulator is 712 mum. Simulations show the circulator to have 25 dB of isolation and 1.3 dB of insertion loss at a wavelength of 1.55 mum

Polarization-Independent Linear Waveguides in 3D Photonic Crystals

Using a symmetry-based approach, we have designed polarization-independent waveguides in a 3D photonic crystal. A comprehensive series of numerical experiments, involving the propagation of pulsed signals through long straight waveguide sections and sharp bends, quantitatively evaluates the bend-transmission coefficient over the entire bandwidth of the corresponding guided modes. High (approximately 95%) polarization-independent bend transmission is achieved within a certain frequency range.

Polarization-independent waveguide optical isolator based on nonreciprocal phase shift

A polarization-independent waveguide optical isolator based on nonreciprocal interference is proposed. The design uses simultaneous TE and TM nonreciprocal phase shifts obtained from geometric asymmetry in horizontal and vertical axes of waveguide cross section along with opposing transverse and vertical magnetic fields in interferometer arms. A design to achieve such an isolator is described. Both TE and TM nonreciprocal phase shifts comparable to the theoretical counterparts have also been experimentally observed from a single waveguide.

Birefringence compensation applying double-core waveguiding structures

For application in communication networks, polarization-independent planar optical waveguiding structures are essential, because the polarization state of the optical signal in an optical network is arbitrary. A new concept for polarization-independent waveguides simply adds a thin birefringence-compensating layer, which results in a double-core waveguiding structure. One of the major advantages of the double-core concept is its technological simplicity. The impact of the layer parameters is simulated and the compensating mechanisms are discussed. The feasibility of this concept is demonstrated on waveguides fabricated with silicon oxynitride technology, where the channel birefringence is tuned over a wide range(+1.6/spl times/10/sup -3/ to -1.1/spl times/10/sup -2/) by varying the thickness of the birefringence compensating layer.

Strained [formula omitted] MQW layers grown by CBE for optical components

We investigated the influence of strain on the optical birefringence in waveguides consisting of Ga x In 1 − x As InP multiquantum well layers. The range of growth parameters to achieve good quality material for the waveguide structures were determined. In a number of layer structures low-loss optical waveguides and PHASAR demultiplexers were fabricated and characterized. The polarization dependence of the demultiplexer can be reduced by using quantum wells strained in tension. Without any precaution, the tensile strain in the well is limited by relaxation at ϵ w ≈ −0.8%. The strain in the well can be further increased to ϵ w ≈ −1% by introducing a strain-balancing layer of a compressive strained InAs monolayer in the InP barrier. But polarization independence is not yet achieved. The results indicate, that there is no negative effect of the strain-balancing layer on the optical performance. Also a number of design criteria for polarization independent waveguides are determined.

Polarization-independent waveguide modulator using a novel quantum well with mass-dependent width

We propose a novel quantum well (QW) structure with two or four thin barriers inserted in which effective width of the well is mass-dependent. It is very useful for polarization-independent optical modulation. Incorporation of tensile strain is also discussed for further improvement of the polarization insensitivity. Fabrication of a modulator with lattice-matched QW's results in 6-V switching voltage with more than -10-dB extinction (--30 dB maximum) for both TE and TM modes over 29-nm wavelength range. Compared with the quasiparabolic QW, this new quantum well has better fabrication tolerance, wider wavelength range of operation, and higher yield.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thin-film garnet materials with zero linear birefringence for magneto-optic waveguide devices (invited)

The many advantages of thin-film waveguide magneto-optic devices have been difficult to realize because of the linear birefringence of these films, which makes it very difficult to achieve the required Faraday rotation. In epitaxial garnet films, this linear birefringence (the difference between the refractive indices of the TE and TM modes) can be reduced to small values by (1) growing single-mode multilayer films to minimize the shape effect, (2) growing the films in compression to control the photoelastic effect, and (3) annealing at high temperatures to eliminate the growth-induced effect. The remaining birefringence can be reduced to zero by growing the top active layer too thick so that the shape effect is smaller in magnitude than the stress effect, and then thinning it by chemical etching until the effects exactly cancel each other at a particular wavelength. An alternative method is to grow the top layer too thin and then to deposit a dielectric layer such as silicon nitride of the proper thickness to reduce the shape effect so that it exactly cancels the stress effect. Theoretical results on multilayer magneto-optic waveguides and successful etch-tuning experiments at 1.32 and 1.51 μm are presented. These tuning procedures can produce polarization-independent waveguide devices such as isolators or polarization rotators which may replace bulk or thick-film devices.