Unguided Modes Research Articles

Trapping and Amplification of Unguided Mode EMIC Waves in the Radiation Belt

AbstractElectromagnetic ion cyclotron (EMIC) waves can cause the scattering loss of the relativistic electrons in the radiation belt. They can be classified into the guided mode and the unguided mode, according to wave's propagation behavior. The guided mode waves have been widely investigated in the radiation belt, but the observation of the unguided mode waves have not been expected. Based on the observations of Van Allen Probes, we demonstrate for the first time the existence of the intense unguided L‐mode EMIC waves in the radiation belt according to the polarization characteristics. Growth rate analyses indicate that the hot protons with energies of a few hundred keV may provide the free energy for wave growth. The reflection interface formed by the spatial locations of local helium cutoff frequencies can be nearly parallel to the equatorial plane when the proton abundance ratio decreases sharply with ‐shell. This structure combined with hot protons may lead to the trapping and significant amplification of the unguided mode waves. These results may help to understand the nature of EMIC waves and their dynamics in the radiation belt.

Laser refrigeration of gas filled hollow-core fibres

We evaluate prospects, performance and temperature limits of a new approach to macroscopic scale laser refrigeration. The considered refrigeration device is based on exciplex-mediated frequency up-conversion inside hollow-core fibers pressurized with a dopant - buffer gas mixture. Exciplexes are excited molecular states formed by two atoms (dopant and buffer) which do not form a molecule in the ground state but exhibit bound states for electronically excited states. The cooling cycle consists of absorption of laser photons during atomic collisions inducing light assisted exciplex formation followed by blue-shifted spontaneous emission on the atomic line of the bare dopant atoms after molecular separation. This process, closely related to reversing the gain mechanism in excimer lasers, allows for a large fraction of collision energy to be extracted in each cycle. The hollow-core fiber plays a crucial role as it allows for strong light-matter interactions over a long distance, which maximizes the cooling rate per unit volume and the cooling efficiency per injected photon while limiting re-absorption of spontaneously emitted photons channeled into unguided radiation modes. Using quantum optical rate equations and refined dynamical simulations we derive general conditions for efficient cooling of both the gas and subsequently of the surrounding solid state environment. Our analytical approach is applicable to any specific exciplex system considered and reveals the shape of the exciplex potential landscapes as well as the density of the dopant as crucial tuning knobs. The derived scaling laws allow for the identification of optimal exciplex characteristics that help to choose suitable gas mixtures that maximize the refrigeration efficiency for specific applications.

Lateral Leakage in Silicon Photonics: Theory, Applications, and Future Directions

This paper presents an overview of lateral leakage in silicon photonics due to polarization coupling between the guided mode of a ridge waveguide and unguided slab mode in the etched cladding. We explain the physical origins and provide insight into how this effect can be suppressed or harnessed. We show how lateral leakage can manifest as new resonant behaviour and explore this effect in the context of bound states in the continuum. We review a number of applications for devices based on lateral leakage and present outlook for a new generation of polarization manipulators, antennas and filters.

The Light Source Metaphor Revisited—Bringing an Old Concept for Teaching Map Projections to the Modern Web

Map projections are one of the foundations of geographic information science and cartography. An understanding of the different projection variants and properties is critical when creating maps or carrying out geospatial analyses. The common way of teaching map projections in text books makes use of the light source (or light bulb) metaphor, which draws a comparison between the construction of a map projection and the way light rays travel from the light source to the projection surface. Although conceptually plausible, such explanations were created for the static instructions in textbooks. Modern web technologies may provide a more comprehensive learning experience by allowing the student to interactively explore (in guided or unguided mode) the way map projections can be constructed following the light source metaphor. The implementation of this approach, however, is not trivial as it requires detailed knowledge of map projections and computer graphics. Therefore, this paper describes the underlying computational methods and presents a prototype as an example of how this concept can be applied in practice. The prototype will be integrated into the Geographic Information Technology Training Alliance (GITTA) platform to complement the lesson on map projections.

Designing and Constructing the Strain Sensor Using Microbend Multimode Fiber

The microbend sensor is designed to experience a light loss when force is applied to the sensor. The periodic microbends cause propagating light to couple into higher order modes, the existing higher order modes become unguided modes. Three models of deform cells are fabricated at (3, 5, 8) mm pitchand tested by using MMF and laser source at 850 nm. The maximum output power of (8, 5, 3)mm model is (3, 2.7, 2.55)nW respectively at applied force 5N and the minimum value is (1.9, 1.65, 1.5)nW respectively at 60N.The strain is calculated at different microbend cells ,and the best sensitivity of this sensor for cell 8mm is equal to 0.6nW/N.

Designing and Constructing the Strain Sensor Using Microbend Multimode Fiber

The microbend sensor is designed to experience a light loss when force is applied to the sensor. The periodic microbends cause propagating light to couple into higher order modes, the existing higher order modes become unguided modes. Three models of deform cells are fabricated at (3, 5, 8) mm pitchand tested by using MMF and laser source at 850 nm. The maximum output power of (8, 5, 3)mm model is (3, 2.7, 2.55)nW respectively at applied force 5N and the minimum value is (1.9, 1.65, 1.5)nW respectively at 60N.The strain is calculated at different microbend cells ,and the best sensitivity of this sensor for cell 8mm is equal to 0.6nW/N.

Instructional guidance in microblogging-supported learning: insights from a multiple case study

Microblogging tools such as Twitter show potential to enrich classroom experience and benefit student learning. Research shows that instructional guidance is particularly necessary in computer-assisted learning environments, but no research has been done to study the effects of instructional guidance in microblogging-based learning. Using a multiple-case study design, the researcher examined student learning in terms of the amount of participation, ability to focus on task, and depth of thinking in guided, semi-guided, and unguided modes. The findings suggest that in guided environments, students achieved higher levels of learning, especially with respect to focusing on task and depth of thinking. Variations in depth of learning existed between the semi-guided and the guided mode. Students’ perceptions of the benefits and challenges of using microblogging across three cases were also analyzed. The study has implications for future research on using microblogging tools for educational purposes and pedagogical practice.

Coherent Coupling in High-Mesa Semiconductor Directional Coupler

The effect of coherent coupling on the performance of a directional coupler is investigated for the first time. Coherent coupling is the interference between a guided mode in a core layer and an unguided mode in a cladding layer (cladding mode) in a waveguide. It was experimentally confirmed that the cladding mode exists in an actual high-mesa semiconductor waveguide and it can be represented by a single propagation constant. The effect of coherent coupling on coupling loss and coupling efficiency in a directional coupler is analyzed using a new technique based on a matrix method. The analyzed results reveal that a small periodic oscillation of coupling loss of the transmitted light that depends on coupler length occurs owing to the coherent coupling in a strong-coupled directional coupler, and its effect should be considered when we minimize propagation loss in high-mesa semiconductor directional couplers. It is also shown that it is possible to design a directional coupler with both the desired coupling efficiency and low loss on the basis of the periodic oscillation of coupling loss.

Unguided plasmon-mode resonance in optically excited thin film: exact modal description of Kretschmann–Raether experiment

With the aim of studying electromagnetic surface wave resonance, we rigorously solve the homogeneous and inhomogeneous problem associated with an optically excited thin metallic film. We then demonstrate unambiguously that the excited eigenmode engendering plasmonic resonance in the so-called Kretschmann–Raether configuration is an unguided mode (i.e., with an anti-evanescent structure). This result, challenging the classical interpretation of the outgoing wave condition applied to surface waves, permits a quantitative interpretation of the attenuated total reflection curves.

Assembly simulations in virtual environments with optimized haptic path and sequence

Virtual assembly and disassembly simulations can be accomplished in intuitive and effective ways using haptic information in virtual environments (VEs). Potential problems in a given assembly scheme can be predicted by a user who may be able to suggest an alternative scheme in the VE. This paper describes an intelligent virtual assembly system in which an optimal assembly algorithm is used to allow haptic interactions during virtual assembly operations. This algorithm provides optimal paths for haptic guidance as well as an assembly sequence of the parts to be assembled. The performance of the given assembly schemes was simulated using a virtual assembly system. Experimental results showed that the haptic-path sequence-guidance (HSG) mode gave the best performance improvement in terms of accumulated assembly time (28.33%) and travel distance (15.05%) compared to the unguided mode, while the sequence-guidance (SG) mode alone increased performance by 15.33% for assembly time and 11.36% for travel distance. The experimental results were analyzed by the sub-tasks of gripper selection, inter-part movement, and part assembly. For the HSG mode, the greatest contributor to the time and distance reductions was the optimized haptic path, while for the SG mode, the reduced numbers of gripper exchanges and orientation change made the greatest contributions to reducing the assembly time and the travel distance. As a result, the optimized haptic path, as well as sequence guidance, enhanced the working performance of virtual assembly tasks.

Unified Approach for Coupling to Cladding and Radiation Modes in Fiber Bragg and Long-Period Gratings

Couplings of core mode to guided cladding and unguided radiation modes in fiber Bragg and long-period gratings are investigated by a unified approach based on complex mode matching method (CMMM). With the combination of the perfectly matched layer (PML) and the perfectly reflecting boundary (PRB), the continuous radiation modes can be well represented by a set of discrete complex modes so that simulation of coupling to radiation modes is greatly simplified and may be treated in the same fashion as guided modes. Numerical results of fiber Bragg and long-period gratings with refractive index of the outer cladding lower, equal, and higher than that of the inner cladding indicate that the unified approach is highly effective in the simulation of couplings to cladding and radiation modes.

Whistler wave excitation by a loop antenna in a bounded collisional magnetoplasma

Excitation of electromagnetic waves by spatially bounded given electric currents in the presence of an axially magnetized cylindrical plasma column surrounded by free space is studied. A rigorous solution for the total field comprising both the discrete and continuous parts of the spatial spectrum of the excited waves is found. Using the obtained field representation, an expression for the impedance of a loop antenna is derived and the contributions of guided and unguided modes of the column to the antenna impedance are determined for the whistler frequency range. Conditions have been revealed under which the real part of the antenna impedance is dominated by the contribution of guided waves. It is shown that when allowance for relatively small collisional losses in the plasma results in division of the guided whistler waves into weakly and strongly damped modes with significantly different field structures, appreciable redistribution of the antenna-launched power among the excited modes takes place. The results obtained are useful for clarification of the power-absorption mechanisms of whistler-range waves in a cylindrical magnetoplasma.

Synthesis of single- and multi-mode planar optical waveguides by a direct numerical solution of the Gel'fand–Levitan–Marchenko integral equation

The leapfrogging technique solving the Gel'fand–Levitan–Marchenko integral equation is a viable numerical tool for producing the refractive index profile associated with the design of single-mode graded index planar optical waveguides. This method is seen here to work also for the design of multi-mode waveguides of the same type, a case not examined previously in the literature. Truncated potentials are employed for modeling the behavior of guided and unguided modes of the waveguide. The accuracy of the method is checked against analytical expressions and new designs are produced for different waveguide material structures and optical frequencies.

Mode-discriminating electrooptic sampling for separating guided and unguided modes on coplanar waveguide

Mode-discriminating electrooptic sampling (MEOS) of coplanar waveguides was shown to discriminate between the symmetric quasi-TEM guided mode and asymmetric field distributions including unguided electromagnetic radiation. Radiation generated in a photoconductive switch and reflected from the back of the substrate was unambiguously identified. Ultrafast sampling of devices showed features in the transmitted pulse due to multiple substrate reflections. These features are removed using MEOS, leading to increased accuracy in determination of S-parameters.

Convective instabilities of electromagnetic ion cyclotron waves in the outer magnetosphere

The path‐integrated linear growth of electromagnetic ion cyclotron waves in the outer (L ≥ 7) magnetosphere is investigated using a realistic thermal plasma distribution with an additional anisotropic energetic ring current H+ to provide free energy for instability. The results provide a realistic simulation of the recent AMPTE observations. For conditions typical of the dayside magnetosphere, high plasma beta effects reduce the group velocity and significantly increase the spatial growth rates for left‐hand polarized instabilities just below the helium gyrofrequency ΩHe+, and on the unguided mode above Ω>He+ but below the cross over frequency ωcr. Relatively high densities, typical of the afternoon local time sector, favor these low group velocity effects for predominantly field‐aligned waves. Lower densities, typical of those found in the early morning local time sector, increase the group velocity but allow strong convective instabilities at high normalized frequencies well above ΩHe+. These waves are reflected in the magnetosphere and can exist for several equatorial transits without significant damping. They are left‐hand polarized only on the first equatorial crossing and become linearly polarized for the remainder of the ray path. Consequently, these waves should be observed with basically linear polarization at all frequencies and all latitudes in the early morning local time sector. Wave growth below ΩHe+ is severely limited owing to the narrow bandwidth for instability and the small resonant path lengths. In the afternoon sector, where plasma densities can exceed 107 m−3, intense convective amplification is possible both above and below ΩHe+. Waves below ΩHe+ are not subject to reflection when the O+ concentration is small and therefore should be observed with left‐hand polarization near the equator and essentially linear polarization at higher latitudes. Since the He+ concentration is usually large in the afternoon sector, guided mode waves above ΩHe+ reflect to form a background distribution with basically linear polarization. We suggest that the strong left‐hand polarized emissions observed by AMPTE in the afternoon sector near the equator are probably due to strongly growing low group velocity waves at frequencies just below ΩHe+, and on the unguided mode above ΩHe+.

Numerical modeling of coherent coupling and radiation fields in planar Y-branch interferometers

The beam-propagation method is used to study intermodal power transfer and radiation losses in slab waveguide splitters and combiners. Numerical computations demonstrate how modal power at an interferometer's recombination point exhibits an oscillatory dependence on the length of each distinct section of the structure. Recoupling of guided and unguided modes can be anticipated and adjusted so as to maximize output modal power and minimize sensitivity to transition lengths. Examples are given for systems formed from both angle bends and S bends.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Learner‐controlled Learning in the Affective Domain

This paper describes the use of interactive video to support the examination by young adults of their attitudes to Acquired Immune Deficiency Syndrome (AIDS), to sufferers and carriers of the disease, and to their own sexuality. Although there is cognitive content in the materials, the greatest part of the activity of learners using the materials is in terms of beliefs, feelings, perspectives, attitudes, relationships and so on. The package is being developed with substantial input from a wide range of counselling, educational, medical and technical bodies in Scotland, and represents an innovative use of the technology as well as a challenging learner‐driven environment for learners to explore their own feelings. That environment assumes that the materials are being mainly (but not exclusively) accessed by a small group of young people in the age range 16‐25, supported by an experienced human tutor acting as referee and providing support in the event that the activities of the learners become too psychologically uncomfortable and threatening. The package may also be used in unguided modes, by young people in groups or singly, and by the general public in an exhibition setting. The materials will be completed in mid‐1990.

Output coupling for closely confined Pb&amp;lt;inf&amp;gt;1-x&amp;lt;/inf&amp;gt;Sn&amp;lt;inf&amp;gt;x&amp;lt;/inf&amp;gt;Te double-heterostructure lasers

The output coupling of an idealized, symmetric model of a double-heterostructure (DH) laser is analyzed theoretically using parameters suitable to Pb 1-x Sn x Te. For the TE O mode incident at the laser mirror and for thin optical guiding regions such that only the TE O , TE 1 , TM O , and TM 1 modes may propagate, an exact formulation of the coupling problem is obtained including mode coupling at the mirror into the continuum of unguided radiation modes. Using this formulation, the power reflection and transmission coefficients, the fraction of incident power coupled into the radiation modes, the mirror illumination, and the far-field pattern are calculated for typical parameters. Significant mode coupling can occur, limiting the maximum external efficiency of such lasers. This and other potentially undesirable characteristics resulting from close optical confinement, such as large output beam divergence, must be considered in design criteria for DH structures in this alloy system.

Power Loss at Directional Change in Dielectric Waveguides

Wave propagation in a single-mode dielectric waveguide that changes in direction is treated theoretically. The directional change is represented as a sequence of straight waveguide sections joined by sharp corners. Power is exchanged between guided and unguided modes at the corners. Numerical results for slab waveguides illustrate that the loss for either a gradual curve or a sequence of two or more corner bends oscillates as a function of the distance over which the directional change takes place.