Single-mode Structures Research Articles

Large-scale optical switches by thermo-optic waveguide lens

Optical switches are desired in telecom and datacom as an upgrade to electrical ones for lower power consumption and expenses while improving bandwidth and network transparency. Compact, integrated optical switches are attractive thanks to their scalability, readiness for mass production, and robustness against mechanical disturbances. The basic unit relies mostly on a microring resonator or a Mach–Zehnder interferometer for binary “bar” and “cross” switching. Such single-mode structures are often wavelength / polarization dependent, sensitive to phase errors and loss-prone. Furthermore, when they are cascaded to a network, the number of control units grows quickly with the port count, causing high complexity in electronic wiring and drive circuit integration. Herein, we propose a new switching method by thermo-optic waveguide lens. Essentially, this multimode waveguide forms a square law medium by a pair of heater electrodes and focuses light within a chip by robust 1 × 1 imaging. A 1 × 24 basic switch is demonstrated with 32 electrodes and only two are biased at a time for a chosen output. By two-level cascading, the switch expands to 576 ports and only four electrodes are needed for one path. The chips are fabricated on wafer scale in a low-budget laboratory without resorting to foundries. Yet, the performance goes beyond state of the art for low insertion loss, low wavelength dependence and low polarization dependence. This work provides an original, alternative, and practical route to construct large-scale optical switches, enabling broad applications in telecom, datacom and photonic computing.

Contactless RF Probe Interconnect Technology Enabling Broadband Testing to the Terahertz Range

Radiofrequency (RF) probes based on 50- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\Omega$</tex-math></inline-formula> planar transmission lines play a key role in almost every stage of RF device development, establishing the physical contact between high-end instrumentation and the device. With the continuous downscaling of semiconductor technologies to reach into the millimeter-wave (30–300 GHz) and Terahertz (300 GHz to 3 THz) bands and devices exhibiting broader frequency response, current RF probe technology is the Achilles heel for precise and repeatable measurements. Here, we propose a novel RF probe technology based on the near-field coupling of single-mode dielectric waveguide structures, which according to our full-wave simulations provide an extremely broad frequency range covering from 0 Hz up to 340 GHz, the largest continuous bandwidth reported to date. A concept demonstrator using this approach shows contactless RF probing on test structures, which shows the path toward continuous measurements across the microwave, millimeter-wave, and Terahertz range.

Near-infrared single-mode optical waveguide fabrication in LiF crystal by carbon ion irradiation

In this study, a carbon-ion beam irradiation method with energy of 6.0 MeV was employed to prepare single-mode optical waveguide structures in LiF single crystal. The results of this study reveal that the visible light and near-infrared laser beams can be well-confined in the irradiated region that is influenced by the irradiation fluences. An end-face coupling system was used to estimate the propagation loss (1.3 dB cm−1) after annealing treatment. The surface damage of the irradiated regions in LiF was analysed based on the displacements per atom calculation using the stopping and range of ions in matter code.

Enhancement of transmission line model to analyze the matching section of a dielectric-loaded accelerator with sharp discontinuities

In this paper, a compact and single-mode X-band dielectric-loaded matching section with constant tilt angle, blended edges, and two discontinuities at both ends is analyzed by the transmission line model. This matching section is designed as a high-power injection coupler to a dielectric-loaded accelerating structure (DLA). Although the transmission line model is a powerful tool for single-mode structures, because of radial inhomogeneity and lack of single value for characteristic impedance in dielectric-loaded structures, this method is not applicable in non-uniform dielectric-loaded structures. To solve this problem, it is shown that a dielectric-loaded waveguide (DLW) can be modeled by a dielectric-filled waveguide (DFW) with the same parameters of the phase constant, power flow, and dielectric loss. The characteristic impedance of a dielectric-filled waveguide can be determined by three structural parameters of the real and imaginary parts of the dielectric material and inner radius. These structural parameters can be found by solving a non-linear equation system made by equality of the parameters of the dielectric-loaded and dielectric-filled waveguides. To analyze the matching section, it is stratified to low thickness and uniform DLWs in the propagation direction and every segment is substituted by its equal DFW model. However, because of sharp variation in dielectric region at two ends of the flared section, solving the non-linear equation system is difficult at these points due to the lack of a proper initial point for the answer. Therefore, two virtual sections are added at the discontinuities to smoothen the variation of the dielectric region. Then, the total transmission matrix and scattering matrix parameters can be obtained without considering the added virtual sections. Moreover, the matching section is simulated by a full-wave electromagnetic software and the results of the scattering matrix parameters are compared with the ones of the proposed method. It is shown that there is a good agreement between the total behavior of the simulation and analytical results and the proposed method has acceptable accuracy.

Effect of Dispersion-Enhanced Sensitivity in a Two-Mode Optical Waveguide with an Asymmetric Diffraction Grating.

Analysis of trends in the development of silicon photonics shows the high efficiency regarding the creation of optical sensors. The concept of bimodal sensors, which suggests moving away from the usual paradigm based only on single-mode waveguides and using the inter-mode interaction of guided optical waves in a two-mode optical waveguide, is developed in the present paper. In this case, the interaction occurs in the presence of an asymmetric periodic perturbation of the refractive index above the waveguide surface. Such a system has unique dispersion properties that lead to the implementation of collinear Bragg diffraction with the mode number transformation, in which there is an extremely high dependence of the Bragg wavelength on the change in the refractive index of the environment. This is called the “effect of dispersion-enhanced sensitivity”. In this paper, it is shown by numerical calculation methods that the effect can be used to create optical sensors with the homogeneous sensitivity higher than 3000 nm/RIU, which is many times better than that of sensors in single-mode waveguide structures.

Broadly tunable Tm/Ho-codoped fiber lasers based on temperature-sensitive single-mode-multimode-single-mode fiber structures.

We present all-fiber tunable lasers using single-mode-multimode-single-mode (SMS) structures involving a liquid cladding in the multimode section. We make use of large thermooptic coefficients of the refractive-index liquids to tune the laser oscillation wavelength. The oscillation wavelength is changed over a range of 100 nm from 1858 nm to 1958 nm by controlling the temperature of the SMS structures in the Tm/Ho-codoped fiber ring resonators.

Modeling multivariate dependencies for manufacturing single-mode I/O structures of integrated MMI-based splitters in glass sheets

For the realization of integrated optical interconnect systems, optical waveguide components are needed. A promising approach to manufacturing graded-index waveguide components is their embedding in thin glass sheets by ion-exchange processes. Conventionally, the modeling of integrated waveguide components manufactured by thermal diffusion processes involves the numerical solution of differential equations. In this work, a semi-analytical approach for the modeling of the concentration profile is extended by the Gauss–Newton optimization algorithm. This is done with the specific purpose of accurately modeling single-mode input and output waveguide structures of multimode-interference-based (MMI-based) optical splitters. The constraints imposed by the multivariate manufacturing process for the realization of these MMI-based components are elaborated. Corresponding correlations between these constraints and the according modeling of the devices are established.

Tailoring the response of single-mode–multimode–single-mode structures fabricated with a special low-index fiber to changes in the surrounding refractive index

An optical fiber refractive index (RI) sensor based on a single-mode–multimode–single mode (SMS) fiber structure fabricated with a coreless low-index multimode fiber (CLIMF) tapered to several diameters is presented. Both the response to changes in the external RI and its spectral dependence were strongly influenced by the length and diameter of the CLIMF. Different SMS sensors with diameters of 120, 100, 80, 60, 40 μm and the corresponding reimaging lengths at 1550 nm are immersed in RI of 1.32, 1.34, 1.36, 1.39, and 1.42 to study the response to changes in the external RI. The diameter of 80 μm yields the best sensitivity (540.7 nm / RIU) with a resolution of 6 × 10 − 5 in the RI range 1.32 to 1.39. By duplicating the reimaging length at 1550 nm for this diameter, the RI range increased to 1.42 and a maximum sensitivity of 1022 nm / RIU is achieved in the RI range from 1.39 to 1.42. Also the simulation results corroborate the wavelength positions of the peaks of the experimental spectrums. We show that the response range to changes in the external RI is controlled with the SMS geometry. This advantage makes the SMS sensor very suitable for biological applications.

Investigations on Integrated Optical Interference Couplers in PMMA for Visible and Infrared Range Fabricated by “Direct Writing” Excimer Laser Radiation

Abstract Optical waveguides were generated in Polymethylmethacrylate using “direct” laser and contact mask structuring. Excimer laser radiation (248 nm) produced a local refractive index increase thereby achieving light guiding in waveguides with widths from 2 to 15 $\mu$m. The near field output images were analyzed using visible and infrared laser light. The mode fields of single mode structures were evaluated to elaborate numerically the parameters of the index profile. An index increase up to 0.01 could be achieved using laser pulses at fluences below $0.15$ ${\textrm{J/cm}^{2}}$. Thus it was possible to design interference couplers of 5 $\mu$m waveguides width for the visible (635 nm to 670 nm) as well as for the infrared range at 1550 nm. The coupling properties as a function of the waveguide separation and the wavelength is investigated and found compatible with numerical simulations for the structure design. The presented approach allows for designing and fabricating single mode couplers in an efficient and simple manner.

Anisotropy-Induced Quantum Interference and Population Trapping between Orthogonal Quantum Dot Exciton States in Semiconductor Cavity Systems.

We describe how quantum dot semiconductor cavity systems can be engineered to realize anisotropy-induced dipole-dipole coupling between orthogonal dipole states in a single quantum dot. Quantum dots in single-mode cavity structures as well as photonic crystal waveguides coupled to spin states or linearly polarized excitons are considered. We demonstrate how the dipole-dipole coupling can control the radiative decay rate of excitons and form pure entangled states in the long time limit. We investigate both field-free entanglement evolution and coherently pumped exciton regimes, and show how a double-field pumping scenario can completely eliminate the decay of coherent Rabi oscillations and lead to population trapping. In the Mollow triplet regime, we explore the emitted spectra from the driven dipoles and show how a nonpumped dipole can take on the form of a spectral triplet, quintuplet, or a singlet, which has applications for producing subnatural linewidth single photons and more easily accessing regimes of high-field quantum optics and cavity-QED.

Photo-structurable polymer for interlayer single-mode waveguide fabrication by femtosecond laser writing

We present the fabrication and characterization of inter-layer single-mode light-guiding structures in a newly developed photo-structurable polymer via femtosecond laser writing. A host-guest polymer system utilizing external diffusion induces permanent and high refractive index modulation. Analyzing the complete curing and the impact of external diffusion on refractive index profile helps optimize the fabrication process. The fabrications of 4 × 2 array interconnect and 3-waveguide core fan-out device involve only a single layer, and need no stacking or alignment effort. The resulting pitch size and crosstalk between single-mode waveguides are 25 μm and −34 dB, respectively. Adding sensitizer into the photopolymer to increase its sensitivity in two-photon polymerization at a writing wavelength of 390 nm reduces the surface roughness of written structures by a factor of seven.

Laser direct writing of complex radially varying single-mode polymer waveguide structures

Increasing board-to-board and chip-to-chip computational data rates beyond 12.5 Gbs will require the use of single-mode polymer waveguides (WGs) that have high bandwidths and are able to be wavelength division multiplexed. Laser direct writing (LDW) of polymer WGs provides a scalable and reconfigurable maskless procedure compared to common photolithography fabrication. LDW of straights and radial curves are readily achieved using predefined drive commands of the two-axis direct drive linear stage system. Using the laser direct write process for advanced WG structures requires stage-drive programming techniques that account for specified polymer material exposure durations. Creating advanced structures such as WG S-bends into single-mode polymer WG builds provides designers with the ability to affect pitch control, optical coupling, and reduce footprint requirements. Fabrication of single-mode polymer WG segmented radial arcs is achieved through a smooth radial arc user-programmed defined mathematical algorithm. Cosine and raised-sine S-bends are realized through a segmentation method where the optimal incremental step length and bend dimensions are controlled to achieve minimal structure loss. Laser direct written S-bends are compared with previously published photolithographic S-bend results using theoretical bend loss models. Fabrication results show that LDW is a viable method in the fabrication of advanced polymer WG structures.

Microcavity morphology optimization

High spectral mode density of conventional optical cavities is detrimental to the generation of broad optical frequency combs and to other linear and nonlinear applications. In this work we optimize the morphology of high-$Q$ whispering gallery (WG) and Fabry-Perot (FP) cavities and find a set of parameters that allows treating them, essentially, as single-mode structures, thus removing limitations associated with a high density of cavity mode spectra. We show that both single-mode WGs and single-mode FP cavities have similar physical properties, in spite of their different loss mechanisms. The morphology optimization does not lead to a reduction of quality factors of modes belonging to the basic family. We study the parameter space numerically and find the region where the highest possible $Q$ factor of the cavity modes can be realized while just having a single bound state in the cavity. The value of the $Q$ factor is comparable with that achieved in conventional cavities. The proposed cavity structures will be beneficial for generation of octave spanning coherent frequency combs and will prevent undesirable effects of parametric instability in laser gravitational wave detectors.

Nanoporosity effect in optical loss of single-mode polymer waveguides

We fabricate single-mode polymer waveguide structures exhibiting polarization-independent ultra-low loss of 0.04 and 0.05 dB/cm at the 1310 and 1550 nm bands, respectively, with a Deltan of 1.6%. A porous structure that arises during the fabrication process is studied by considering its implications in the propagation loss based on the Rayleigh-Mie scattering loss mechanism. We demonstrate that the porous structure is to be reduced to the nanoscale (i.e., <10 nm) to realize waveguide structures with ultra-low propagation losses based on fabrications and measurements of morphologies with various degrees of porosity. Further, the bending loss and its respective polarization-dependent propagation loss behavior are analyzed to realize compact devices with ultra-low-loss and polarization-independent features, where a 4 mm bending radius is found to be adequate for such a performance over the 1550 nm band.

Strain and Temperature Sensing Characteristics of Single-Mode–Multimode–Single-Mode Structures

We present a comprehensive study of the strain and temperature-sensing characteristics of single-mode-multimode-single-mode (SMS) structures based on the modal interference of guided modes of graded index multimode fiber (MMF) section spliced in between two single-mode fibers. A detailed theoretical study of the structures in terms of the refractive index distribution, effect of dopant and their concentrations, and the variation of core diameter has been carried out. Our study shows that for the SMS structure with a GeO2-doped MMF there exists a critical wavelength on either side of which the spectrum shows opposite spectral shift with a change in temperature/strain, whereas for structures with a P2O5-doped MMF it shows monotonic red shift with increasing temperature/strain. It has been found that the critical wavelength shifts toward higher wavelengths with decreasing ldquoqrdquo value/doping concentration. Using different MMFs, both the red and blue spectral shifts have been observed experimentally. It has also been found that the SMS structure has higher sensitivity toward this critical wavelength. The study should find application in designing strain-insensitive high-sensitive temperature sensors or vice versa.

Two‐Way Molecular Switches with Large Nonlinear Optical Contrast

Molecular switches: Highly efficient acido- and photoswitchable frequency doublers (see scheme) based on the indolinooxazolidine core are studied by means of hyper-Rayleigh experiments and quantum-chemical calculations.To optimize the nonlinear optical (NLO) contrast, a series of indolinooxazolidine derivatives with electron-withdrawing substituents in the para position on the indolinic residue have been synthesized. Their linear and nonlinear optical properties have been characterized by UV-visible absorption and hyper-Rayleigh scattering measurements, as well as by ab initio calculations. The two-way photo- or pH-triggered switching mechanism has been demonstrated by comparing the absorption spectra of the zwitterionic and protonated open forms (POF). Hyper-Rayleigh measurements have revealed that the second-order NLO contrast between the closed indolinooxazolidine and the open pi-conjugated colored forms remain very large upon substitution. Theory and measurements show that for the POFs the amplitude of the first hyperpolarizability follows the Hammett parameters of the withdrawing groups. However, because the measurements are performed in resonance, to recover this behavior, elaborate procedures including homogeneous and inhomogeneous broadenings, as well as single-mode vibronic structures are necessary to extrapolate to the static limit.

Theoretical prediction of a dramatic Q-factor enhancement and degeneracy removal of whispering gallery modes in symmetrical photonic molecules

Coupled-microdisk photonic molecules with high-symmetry geometries that support nondegenerate whispering gallery modes with high quality factors are numerically designed, based on a two-dimensional model. Dramatic enhancement of calculated Q factors of nondegenerate modes as compared with single-microdisk whispering gallery modes of the same radial order is achieved by tuning the intercavity coupling distances. Potential applications of this work to the design of single-mode coupled-microdisk structures with large spontaneous emission factors and low threshold lasing operation and to random laser simulations are discussed.

Pathogen detection using single mode planar optical waveguides

We utilize an optical waveguide-based biosensor recently developed at Los Alamos National Laboratory for the sensitive and specific detection of protein markers. Our planar optical waveguides are based on single mode structures that provide high optical field intensity at the active surface while providing discrimination from sample background fluorescence through spatial filtering. We have incorporated sandwich immunoassays where the capture antibody is immobilized on a biocompatible film attached to the surface of an optical waveguide. The approach can be adapted to any marker protein and is faster (<10 min), more sensitive, and as specific as conventional enzyme-linked immunosorbent assays (ELISA, the current lab standard). Results are presented for the detection of several analytes of Bacillus anthracis including protective antigen, a virulence marker for B. anthracis, and the bacterial cell or cellular debris.

Simulating the modulation response of VCSELs: the effects of diffusion capacitance and spatial hole-burning

The comprehensive semiconductor laser simulator, Minilase, has been extended to simulate the dynamic response of vertical cavity surface emitting lasers (VCSELs). Unlike conventional rate equation approaches, Minilase is capable of correctly modelling nonlinear gain effects in the modulation response without the introduction of an empirical gain suppression factor. Various oxide-confined single-mode VCSEL structures are simulated with Minilase to examine the effects of real-space carrier transport on the modulation response in both vertical and lateral directions. It is demonstrated that a roll-off in the dynamic response is closely associated with the diffusion capacitance caused by vertical carrier leakage. By either grading the separate confinement regions or reducing the thickness of the SCH cavity, the vertical carrier leakage is shown to be greatly suppressed, and the modulation response is significantly improved. Simulations also reveal that the nonuniform HE11-like transverse optical intensity in the quantum well results in an overdamping of the relaxation oscillation, and this effect is greatly reduced by making the electrical aperture smaller than the optical aperture. Finally, a comparison with experimental results is presented and discussed.

Normally incident resonant grating reflection filters for efficient narrow-band spectral filtering of finite beams.

Resonant grating reflection filters operating at normal incidence are investigated. At normal incidence these structures are shown to have characteristics different from those of structures operated at oblique incidence. We show how higher-order diffraction within the structure laterally confines the incident energy about the point of incidence and results in a broadened angular selectivity. Multimode structures are shown to exhibit broader angular selectivities and narrower spectral linewidths than those of single-mode structures, achieving angular selectivities hundreds of times broader than what could be obtained at oblique incidence. When compared with that for oblique incidence, the increase in angular selectivity is shown to greatly improve the performance of these filters for spectrally filtering finite incident beams.