Y-branch spot-size converter for a buried silica waveguide with large index difference

Coupling loss occurs between a standard single-mode fiber and a silica waveguide when the difference between the refractive indices of the core and the cladding of the silica waveguide is high. We designed a Y-branch structure for use as a spot-size converter to reduce this coupling loss. The structure was tested with a three-dimensional beam-propagation method and was shown to exhibit a significantly reduced coupling loss, a low polarization-dependent loss, and a good tolerance of imprecision in fabrication. No additional fabrication steps are required for this proposed spot-size converter.     

Coupling-loss reduction of a vertically coupled microring resonator filter by spot-size-matched busline waveguides

To improve the input-output coupling loss of a vertically coupled microring resonator filter, we fabricated microring resonators on an antiresonant reflecting optical waveguide (ARROW) with a large spot size and on the rectangular busline waveguide with a spot-size transformer. The spot size and the tapered structure were optimally designed from the viewpoint of spot-size matching to single-mode fibers and the reduction of radiation loss. Clear dropping responses were demonstrated for the ARROW-based microring resonator filters, and the coupling loss was successfully reduced by 22 dB.     

Photonic devices based on preferential etching

We introduce a design concept of optical waveguides characterized by a practical and reproducible process based on preferential etching of crystalline silicon substrates. Low-loss waveguides, spot-size converters, and power dividers have been obtained with polymers. We have also aligned liquid crystals in the waveguides and demonstrated guided propagation. Therefore this technology is a suitable platform for soft-matter photonics and heterogeneous integration.     

Review of finite-element characterization of photonic devices

Abstract

We report the development and applications of finite-element-based numerical approaches for the characterization of a wide range of photonic devices. The full-vectorial finite-element method is considered for the modal solutions of uniform guided-wave sections and the least-squares boundary residual method is used to calculate the scattering coefficients for butt-coupled uniform waveguide sections. Several guided-wave components are discussed and their operating characteristics are illustrated. Particular emphasis is given to key photonic components such as the spot-size converters, Bragg gratings, polarization splitters, polarization rotators, multimode interference-based devices and modulators. Valuable results relating to the design and characterization of these important photonic components are presented.     

Variable spot-size waveguide for laser printing

A variable-spot-size Ti-LiNbO(3) waveguide that permits electro-optical longitudinal shifting of the focal point is described. A 70% spot-size increase is observed for the ±60-V power supply. This spot-size variation technique is used in a basic experiment on halftone laser printing.     

Highly efficient coupling semiconductor spot-size converter with an InP/InAlAs multiple-quantum-well core

A highly efficient spot-size converter (SSC) that uses a fiber-coupling tapered semiconductor waveguide is demonstrated. The waveguide core of this device consists of InGaAsP for semiconductor chip coupling and an InP/InAlAs multiple quantum well (MQW) for single-mode fiber (SMF) coupling. The equivalent refractive index of the MQW core is adjusted by controlling the well-to-barrier-layer thickness ratio to expand the chip spot size to that of SMF's. A high coupling efficiency of 1.4 dB can be obtained, and the lateral and axial misalignment tolerances for the SSC are 3 times better than those for conventional semiconductor waveguides. Moreover, this device has high reproducibility because of large fabrication tolerances.     

Micro-optically assisted high-index waveguide coupling

Adapting the concept of solid immersion lenses, we numerically study a micro-optical scheme for conventional high-index and photonic-crystal waveguide coupling by using a combination of different numerical methods such as ray tracing, angular-spectrum propagation, finite-difference time-domain simulations, and finite-element-method simulations. The numerical findings are discussed by means of impedance, group- or energy-velocity, spot-size, and phase-matching criteria. When fabrication constraints for high-index immersion lenses made of silicon are taken into account, a coupling efficiency of -80% can be reached for monomode silicon-on-insulator waveguides with a quadratic cross section of the core and rectangular cross sections of moderate aspect ratio. Similar coupling efficiencies of -80% can be obtained for silicon-on-insulator photonic-crystal waveguides. Tolerances that are due to misalignments and variations of the substrate thickness of the silicon lens are discussed.     

Optimization of compact lateral, vertical, and combined tapered spot-size converters by use of the beam-propagation method

A study of lateral, vertical, and combined spot-size converters is presented that employs full-vectorial numerical techniques such as modal solution and beam propagation based on the finite-element method. Spot-size expansion, coupling efficiency to an optical fiber, the mode-beating phenomenon, and transmission losses are demonstrated for all three spot-size-converter designs. Optimization of the device fabrication parameters is also reported. A significant improvement in the coupling efficiency and reduction of the device length are achieved when the length and the width are changed simultaneously.     

Precise recursive formula for calculating spot size in optical waveguides and accurate evaluation of splice loss

The spot size of a single-mode waveguide was defined from the viewpoint of a least-squares fit of the field profile to a Gaussian profile. The field profile was expanded in terms of Hermite-Gaussian functions, and a new precise recursive formula for calculating the spot size was derived. It was shown that our formula is equivalent to the best fitting of the offset coupling loss to that of a Gaussian profile and keeps its form against the Fourier transform that corresponds to the diffraction in the same manner as the Gaussian profile. The accuracy of conventional formulas and our new spot-size formula was compared with the exact value defined from the viewpoint of a least-squares fit to a Gaussian profile, and it was shown that our recursive formula is the most accurate of the approximate formulas. Next we proposed a new formula for calculating the splice loss between two waveguides and showed that our formula is more accurate than the conventional one, which involves only the spot size.     

The Stiles–Crawford Effect: spot-size ratio departure in retinitis pigmentosa

The Stiles–Crawford effect of the first kind is the retina’s compensative response to loss of luminance efficiency for oblique stimulation manifested as the spot-size ratio departure from the perfect power coupling for a normal human eye. In a retinitis pigmentosa eye (RP), the normal cone photoreceptor morphology is affected due to foveal cone loss and disrupted cone mosaic spatial arrangement with reduction in directional sensitivity. We show that the flattened Stiles–Crawford function (SCF) in a RP eye is due to a different spot-size ratio departure profile, that is, for the same loss of luminance efficiency, a RP eye has a smaller departure from perfect power coupling compared to a normal eye. Again, the difference in spot-size ratio departure increases from the centre towards the periphery, having zero value for axial entry and maximum value for maximum peripheral entry indicating dispersal of photoreceptor alignment which prevents the retina to go for a bigger compensative response as it lacks both in number and appropriate cone morphology to tackle the loss of luminance efficiency for oblique stimulation. The slope of departure profile also testifies to the flattened SCF for a RP eye. Moreover, the discrepancy in spot-size ratio departure between a normal and a RP eye is shown to have a direct bearing on the Stiles–Crawford diminution of visibility.     

Characterization of Modal Noise,Splice and Bending Loss in Single-mode Depressed Cladding Fibres

Abstract

Using a Gaussian function to approximate the modal field of a single-mode depressed cladding (or W-) fibre, we derive a simple expression for the modal spot-size, which depends only on the normalized frequency and two parameters that characterize the refractive-index profile. This spot-size can then be used to determine the expected splice and bend loss. The formalism for determining the spot-size can be generalized to approximate higher-order modes, which allows for a simple characterization of modal noise due to misaligned joints in single-mode fibres operating above cut-off.     

Waveguide analysis of heat-drawn and chemically etched probe tips for scanning near-field optical microscopy

We analyze two basic aspects of a scanning near-field optical microscope (SNOM) probe's operation: (i) spot-size evolution of the electric field along the probe with and without a metal layer, and (ii) a modal analysis of the SNOM probe, particularly in close proximity to the aperture. A slab waveguide model is utilized to minimize the analytical complexity, yet provides useful quantitative results--including losses associated with the metal coating--which can then be used as design rules.     

General formula for coupling-loss characterization of single-mode fiber collimators by use of gradient-index rod lenses

A general formula for determining the coupling loss between two single-mode fiber collimators with the simultaneous existence of separation, lateral offset and angular tilt misalignments, and spot-size mismatch is theoretically derived by use of the Gaussian field approximation. Based on this general formula, the formulas for coupling losses that are due to the misalignment of insert separation, lateral offset, and angular tilt are given. The formula for the coupling loss that is due to Gaussian spot-size mismatch of two single-mode collimators is also given. Good agreement between these formulas and experimental results is demonstrated with gradient-index rod lens-based fiber collimators operating in the 1300-nm band.     

Characterization of a compact silicon-based slot-to-strip waveguide crossing

A compact crossing scheme for a silicon-based slot-to-strip (or vice versa) waveguide is proposed and analyzed by using a finite-difference time-domain method, where a strip–multimode waveguide (SMW) crossing is used at the center and two logarithmically tapered slot-to-strip mode converters are incorporated into the ports with slot waveguides. For the input ports with slot waveguides and output ports with strip waveguides, the guided modes are efficiently transformed through the mode converter, and then enter into the SMW, where the fields converge at the center of the intersection due to the self-imaging effect. Hence, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to significant reductions of both crosstalk and radiation loss. The numerical results show that a slot-to-strip waveguide crossing operating at a wavelength of 1.55?μm can be achieved with insertion loss, crosstalk, and reflection of 0.134/0.182, ?36.18/?38.6, and ?35.8/?42.02?dB for input ports with slot/strip waveguides, respectively.     

Improved design of a polarization converter based on semiconductor optical waveguide bends

By using an efficient vector finite-element-based beam-propagation method, we present an improved design of a polarization converter. This design relies on the use of a single-section deeply etched bent semiconductor waveguide with slanted sidewalls. By careful adjustment of the bend radius, the waveguide width, and the sidewall angle we obtained a nearly 100% polarization conversion ratio with no appreciable radiation loss and a bending angle of less than 180 degrees .     

Optical waveguiding in epitaxial PbTiO(3) thin films

Epitaxial lead titanate (PbTiO(3)) thin films on SrTiO(3) (100) substrate were grown in situ by radio-frequency sputtering for optical waveguiding applications. The crystalline quality of the PbTiO(3) films deposited at 550 degrees C has been investigated through x-ray diffraction analysis. It indicates that thin films are completely c-axis oriented (rocking curve FWHM of 0.2 degrees for the 001 reflection). The transmission spectrum method has been used to measure the dispersion of the refractive index. At 632.8 nm, the PbTiO(3) film with an (001) orientation exhibits a refractive index of 2.61, which represents 98% of the bulk material. The prism-coupling technique has been also employed to determine the optical attenuation in the planar waveguide. In this study, we report a low propagation loss of 2.2 ? 0.2 dB/cm obtained in a PbTiO(3) optical waveguide.     

Whispering gallery modes of a curved antiresonant reflecting optical waveguide

The whispering gallery modes of a curved antiresonant reflecting optical waveguide are described and analyzed. We present a ray analysis method that can be used to derive an eigenmode equation and to analyze the loss characteristics of a waveguide in rectangular coordinates by using conformal transformation. With an optimized design of an antiresonant reflecting optical waveguide structure, the propagation loss of such a waveguide can be significantly reduced.     

Fabrication and analysis of a low-loss in-fiber active polymer waveguide

We present a method for fabricating an in-fiber electro-optic polymer waveguide within a D-shaped optical fiber. A combined process of selective chemical etching and spin coating creates a 2-cm in-fiber poly(methyl methacrylate)-DR1 dye polymer waveguide section with an overall insertion loss of micro 1.6 dB at 1550 nm. Numerical simulations show that, for in-fiber polymer waveguides to have low loss, the polymer layer's thickness must be kept below a certain value so that it will not support slab waveguide modes. Long transition regions between the unetched fiber and the polymer waveguide section also reduce loss. We analyze the efficiency of an in-fiber polymer waveguide by simulating its theoretical performance as an electro-optic modulator.     

Efficient Single-Mode GaAs/AlGaAs W Waveguide Phase Modulator with a Low Propagation Loss

We report a single-mode P-P-p-i-n-N-N GaAs/AlGaAs W waveguide phase modulator with a high phase modulation efficiency and a low propagation loss. The phase modulator with a W-shaped refractive-index profile utilizes a novel epilayer structure to reduce the propagation loss associated with doped layers and to obtain a phase modulation efficiency larger than those of P-i-N double heterostructure modulators. The phase shift and propagation loss were measured with a Fabry-Perot resonance method at 1.31-mum wavelength. A phase modulation efficiency as high as 34.6 degrees /V mm was measured for TE polarized light. Also propagation losses of less than 0.6 dB/cm were achieved. As a result, the W waveguide phase modulator that exhibits a high phase modulation efficiency and a low propagation loss have been experimentally realized for the first time as far as we know.     

Cut off in its prime: the influence of waveguides on microwavetechnology

A paper by Lord Rayleigh, published in February 1897, was seminal in establishing the waveguide as a possible transmission medium which was `out of the ordinary' for the time and, in the event, some years ahead of its practical application. The waveguide is now well established as an engineering tool, especially where there is a requirement for high power along with high sensitivity and low loss. The behaviour of waveguides in the below cut-off region, where the attenuation does not become infinite but reaches an asymptotic value dependent on the guide dimensions, is now the basis of a primary standard of attenuation used in standards laboratories throughout the world. Despite the high attenuation, `propagation' in a waveguide in the cut-off mode has attracted interest recently through claims that the group velocity may be of the order of four times the velocity of light