Order Waveguide Modes Research Articles

Photonic crystal surface emitting lasers with multiple-junction operating at high order waveguide mode

We propose a novel design for multi-junction photonic crystal surface emitting lasers (PCSELs) operating in higher-order waveguide modes to minimize internal losses. This paper details the design and simulation of a 2-junction PCSEL, including calculations of confinement factors, coupling coefficients, radiation loss, and threshold currents. We compare the performance of 2 J-PCSELs and 1 J-PCSELs, demonstrating the potential for highly efficient multi-junction PCSELs with improved power conversion efficiency and output power.

MeerKAT Holography Measurements in the UHF, L, and S Bands

Radio holographic measurements using the MeerKAT telescope are presented for each of its supported observing bands, namely UHF (544–1087 MHz), L (856–1711 MHz), and S (1750–3499 MHz). Because the UHF-band receiver design is a scaled version of that of the L band, the electromagnetic performance in these two bands are expectedly similar to one another. Despite also being linearly polarized, S-band receivers have an entirely different design and distinct performance characteristics from the lower two bands. As introduced in previous work for the L band, evidence of higher order waveguide mode activation also appears in S-band measurements but there are differences in its manifestation. Frequency-dependent pointing (beam squint), beamwidth, beam ellipticity, error beam, instrumental polarization, and cross-polarization power measurements are illustrated for each of MeerKAT’s observational bands in a side-by-side style to facilitate the comparison of features. The derivation of collimation errors and main reflector surface errors from measurements made at these relatively low observation frequencies is also discussed. Results include elevation and ambient temperature effects on collimation, as well as the signatures of collimation degrading over time. The accompanying data release includes a snapshot of full Jones matrix primary beam patterns for all bands and antennas with corresponding derived metrics.

Non-resonant recirculating light phase modulator

High efficiency and a compact footprint are desired properties for electro-optic modulators. In this paper, we propose, theoretically investigate, and experimentally demonstrate a recirculating phase modulator, which increases the modulation efficiency by modulating the optical field several times in a non-resonant waveguide structure. The “recycling” of light is achieved by looping the optical path that exits the phase modulator back and coupling it to a higher order waveguide mode, which then repeats its passage through the phase modulator. By looping the light back twice, we were able to demonstrate a recirculating phase modulator that requires nine times lower power to generate the same modulation index of a single pass phase modulator. This approach to modulation efficiency enhancement is promising for the design of advanced tunable electro-optical frequency comb generators and other electro-optical devices with defined operational frequency bandwidths.

Supercontinuum Generation in High Order Waveguide Mode with near-Visible Pumping Using Aluminum Nitride Waveguides

Optical sources emitting in the ultraviolet (UV) to near-infrared wavelength range are an enabling tools for a wide variety of applications. To achieve broadband coherent generation within visible ...

Nonparaxial Mode-Size Converter Using an Ultracompact Metamaterial Mikaelian Lens

Mode-division multiplexing (MDM) is a promising approach to increase data transmission capacity by using different modes as independent data channels. Intermodal crosstalk and coherent mode interference are detrimental to the performance of an MDM system and should be avoided in the mode converters and mode expanders for MDM. The Mikaelian lens is well known to be free from spherical aberrations and is therefore an excellent candidate for focusing high order waveguide modes which have main mode field distributions located off-axis. Here we proposed an ultracompact mode-independent mode-size converter using an integrated Mikaelian lens formed by refractive index engineering using a subwavelength grating. The Mikaelian lens is fabricated on the silicon-on-insulator (SOI) platform and can be used to convert the size of high order modes at the interface between the silicon waveguide and a multimode fiber in MDM systems. We present a proof-of-concept demonstration of a low loss spot size converter for three transverse electric (TE) modes between a 14 μm wide multimode waveguide grating coupler and a 1.56 μm wide multimode silicon waveguide. The measured insertion loss of the Mikaelian lens mode size converter is 0.21 dB for TE0 mode, 0.26 dB for TE1 mode and 0.28 dB for TE2 mode. 1-dB bandwidth of all the three modes are about 60 nm. The 15 μm long Mikaelian lens has comparable experimental performance as a 450 μm long adiabatic taper.

Low-Loss Transmission Line for a 3.4-kW, 93-GHz Gyro-Traveling-Wave Amplifier

In this article, a transmission line system for the propagation of millimeter-wave radiation is presented. The full system includes a TE11-to-TE01 mode converter, waveguide tapers, miter bends, and many straight sections. The design of each of these components is described, and the optimized simulation results are given. The mode converter shows a greater than 96% mode conversion efficiency that can be achieved over a 2% bandwidth at the ${W}$ -band. The miter bends demonstrate a transmission loss of 0.04 dB each over the same bandwidth when they are configured to introduce a mixture of higher order waveguide modes before the reflecting surface. An example transmission line system with a propagation length of 20 m, inclusive of four 90° bends with an oxygen-free high conductivity (OFHC) copper waveguide material, was studied over a 90–96-GHz frequency range and showed a 0.84-dB transmission loss at 93 GHz.

Systematic Evolution of Resonant Coupling Behavior Between Surface Plasmon Polaritons and Multi-waveguide Modes in Metal-Dielectric Multi-layers

A multi-layer structure consisting of metal and dielectric layers which allows coupling between surface plasmon polaritons and waveguide modes is studied by calculating reflectivity curves of incident light. A wide waveguide structure is employed to ensure SPP resonance with different order waveguide modes when varying coupling layer thicknesses. Multi-hybrid plasmonic waveguide modes (HPWG) are formed in the coupled system. The weight fractions of metal SPP deduced from losses of HPWG modes are calculated for each hybridized mode, which can be considered as a parameter of evaluating coupling strength. Three different evolution stages for hybrid modes are clearly distinguished based on the variation of loss with spacer layer thickness. The interaction between SPP and waveguide experiences a series of change from weak Fano resonance, via strong coupling, finally to weak coupling process. In case of strong coupling regime, the contribution of metal SPP for low order hybrid modes starts to increase and then decreases with decreasing spacer layer thicknesses. Furthermore, difference of resonant behavior for hybrid modes in both weak coupling stages is analyzed in detail.

Improvement to beam quality of optically pumped terahertz gas lasers with hole-coupling resonators

The hole-coupling method, which is commonly used in the optically pumped THz gas laser, will reduce the beam quality due to the hard-edge diffraction. It is shown that by optimizing the parameters of the resonator a diffraction-limited output beam can be obtained. A method is proposed for the selection of a higher order waveguide mode, whose first zero point along the radial direction coincides with the edge of the output hole coupler and the effect of hard edge diffraction is depressed. Measurements of beam quality are taken on an experimental prototype whose optical setup is designed by this method. A quasi-Gaussian beam output is obtained, and the beam quality M2 factor is about 1.15

Enhanced dielectric waveguide mode from the coupling of surface plasmon polaritons excited by a parallel electron beam

Abstract The coherent, power enhanced and mode selected light radiation from the coupling of surface plasmon polaritons (SPPs) and waveguide modes is presented here. The results of theoretical analysis show that this coupling occurs in the structure of a thin metal film with finite thickness dielectric medium loading excited by free electrons at certain beam energies, referred to as the coupling points. The research of particle-in-cell simulation is also carried out, and results agree well that of theoretical analyses. Because of the coupling, the power intensity of the coherent radiation is extremely enhanced up to 32 times compared with that transformed from SPPs by Cherenkov radiation directly in the infinite medium. And even at the non-coupling points, there is also a 5 times enhancement of the radiation power. The results also show that the main mode of the loading medium is changed from the zero order waveguide mode to the mode whose operating frequency is near to that of asymmetric SPPs mode. Accordingly, the results presented here is of great significance of developing miniature, integratable, high-power-density light radiation sources from visible light to ultra-violet rays at room temperature.

High Q reflection filter using a gradient-index membrane with a grating surface

High Q reflection filter using a gradient-index (GI) membrane with a grating surface is proposed. The thickness of GI membrane is very small comparing with the traditional multilayer reflection filter or the GI reflection filter, and the GI membrane can also break the restriction of the resonant excitation condition of the conventional guided-mode resonance (GMR) filter. High Q filtering features can be maintained even on the high-index substrate. The grating thickness of the GI membrane filter can be used to select the resonance wavelength with different quality factors (QFs), the reflection peak is blue-shifted, and the QF is decreased from 554.4 to 207.8 as the grating thickness is increased from 50 nm to 150 nm. The gradient coefficient of the GI membrane filter can be used to tailor the number of the reflection channels. The resonant excitations of high order waveguide modes confined in the GI membrane are responsible for the high Q filtering properties with multiple channels.

Hybrid multimode resonators based on grating-assisted counter-directional couplers.

Research thrusts in silicon photonics are developing control operations using higher order waveguide modes for next generation high-bandwidth communication systems. In this context, devices allowing optical processing of multiple waveguide modes can reduce architecture complexity and enable flexible on-chip networks. We propose and demonstrate a hybrid resonator dually resonant at the 1st and 2nd order modes of a silicon waveguide. We observe 8 dB extinction ratio and modal conversion range of 20 nm for the 1st order quasi-TE mode input.

Optimizing of the novel asymmetric plasmonic waveguide with two identical gratings to increase the SHG efficiency

In this paper a novel plasmonic waveguide is proposed to increase the second harmonic generation (SHG) efficiency by considering asymmetric plasmonic waveguide geometry and two identical grating separated with each other by spacing. The proposed structure consists of two different metals on both sides of lithium niobate. By using two different metals the nonlinear susceptibility of the waveguide would be increased noticeably causing to increase SHG process. On the other hand, it consists of two identical gratings on one side. By two identical gratings, the pump beam is coupled to two opposing SPP waves, which interfere with each other and results in SPP standing wave in the region between the two gratings. The needed phase matching condition is satisfied between the fundamental waveguide mode at the fundamental frequency and second order waveguide mode at the second harmonic frequency (SHF) by an appropriate design of the waveguide geometrical parameters. The details of structure including the metals of top and bottom, distance between two gratings, depth, and the duty cycle of gratings will be optimized to reach the highest SHG efficiency and the highest SHF optical power. It will be shown that by optimizing the geometry of proposed structure and using different metals, field enhancement in proposed waveguide can result in large enhancement of SHG. The SHG signal generated in proposed waveguide is more than four orders of magnitude higher than those previously reported. The device length is shorter than 3 µm and the normalized SHG conversion efficiency comes up to more than 9 × 107 W−1 cm−2.

Experimental realization of acoustic double zero index metamaterials

Acoustic double zero index metamaterial with simultaneous zero density and infinite bulk modulus induced by Dirac cone at the Brillouin zone center provide a practical solution for applications. The resulted finite impedance of this metamaterial can be designed to match with surrounding materials. However, such metamaterial consists of scatterers with lower sound speed than the matrix, which is fundamentally challenging for air acoustics because the sound speed in air is among the lowest in nature. Here, we experimentally realize double zero index metamaterial by periodically varying waveguide thickness, which modulates the speed sound of high order waveguide mode. With proper designed dimensions of the scatterers, a Dirac cone at 18.7 kHz formed by the degeneracy of one monopolar and two dipolar modes at the Brillouin zone center is obtain. The monopolar mode modulates the bulk modulus and the dipolar modes modulate the density, resulting in simultaneous zero density and infinite bulk modulus. In our exp...

Multipole surface plasmons in metallic nanohole arrays

The quasibound electromagnetic modes for the arrays of nanoholes perforated in thin gold film are analyzed both numerically by the rigorous coupled wave analysis (RCWA) method and semianalytically by the coupled mode method. It is shown that when the size of the nanohole occupies a large portion of the unit cell, the surface plasmon polaritons (SPPs) at both sides of the film are combined by the higher order waveguide modes of the holes to produce multipole surface plasmons: coupled surface plasmon modes with multipole texture on the elec-tric field distributions. Further, it is revealed that the multipole texture either enhances or suppresses the couplings between SPPs depending on their diffraction orders and also causes band inversion and reconstruction in the coupled SPP band structure. Due to the multipole nature of the quasibound modes, multiple dark modes coexist to produce a variety of Fano resonance structures on the transmission and reflection spectra.

Magnonic beam splitter: The building block of parallel magnonic circuitry

We demonstrate a magnonic beam splitter that works by inter-converting magnetostatic surface and backward-volume spin waves propagating in orthogonal sections of a T-shaped yttrium iron garnet structure. The inter-conversion is enabled by the overlap of the surface and volume spin wave bands. This overlap results from the demagnetising field induced along the transversely magnetised section(-s) of the structure and the quantization of the transverse wave number of the propagating spin waves (which are therefore better described as waveguide modes). In agreement with numerical micromagnetic simulations, our Brillouin light scattering imaging experiments reveal that, depending on the frequency, the incident fundamental waveguide magnonic modes may also be converted into higher order waveguide modes. The magnonic beam splitter demonstrated here is an important step towards the development of parallel logic circuitry of magnonics.

Transmission mode of a single layer graphene and its performance in the detection of the vibration spectrum of gas molecular

Since its successful preparation in 2004, graphene has attracted a great deal of attention, and the sensing application is an important research field. But nearly all the researches about graphene sensors focus on low frequency band, of which the mechanism is mainly dependent on the detection of charge carrier concentration and conductivity variation induced by the absorption of molecules. However, due to the fact that most of the molecules absorbed on the surface of graphene will induce the change of conductivity, this method is incapable of distinguishing different molecules. Transmission mode of a single molecular layer is studied based on Kubo formula and combined with a numerical method. The relation between transmission properties and effective mode index is analyzed, and the broadband localization capability of the waveguide mode is demonstrated. Meanwhile, the variation of the transmission intensity which is due to the interaction between the first order waveguide mode and the gas is adopted to retrieve the vibration spectrum of molecules. Taking the sensing of SO2, CO and C7H8 as examples, the effectiveness of this method is verified based on eigenmode analysis. Results show that the transmission spectrum is consistent with the variation spectrum of gas molecules; besides, in the transmission direction, the larger the interaction range, the greater the attenuation of mode transmission intensity will be. This study has provided a theoretical foundation for the realization of the detection and identification of gas moleculan fingerprints.

Efficient high order waveguide mode solvers based on boundary integral equations

For optical waveguides with high index contrast and sharp corners, high order full-vectorial mode solvers are difficult to develop, due to the field singularities at the corners. A recently developed method (the so-called BIE-NtD method) based on boundary integral equations (BIEs) and Neumann-to-Dirichlet (NtD) maps achieves high order of accuracy for dielectric waveguides. In this paper, we develop two new BIE mode solvers, including an improved version of the BIE-NtD method and a new BIE-DtN method based on Dirichlet-to-Neumann (DtN) maps. For homogeneous domains with sharp corners, we propose better BIEs to compute the DtN and NtD maps, and new kernel-splitting techniques to discretize hypersingular operators. Numerical results indicate that the new methods are more efficient and more accurate, and work very well for metallic waveguides and waveguides with extended mode profiles.

Use of mode subspace projections for depth discrimination with a horizontal line array: Theory and experimental results

A solution to the problem of acoustic source depth discrimination in a downward refracting, shallow-water ocean waveguide is presented for the case of a horizontal line array at endfire. The approach exploits the phenomenon of mode trapping, wherein a shallow acoustic source cannot excite the lowest order waveguide modes due to its evanescent amplitude dependence near the surface. The important implication of this "trapping" behavior is that, given sufficient spatial aperture, it provides a mechanism for differentiating a shallow acoustic noise source from one at depth. The method does not require array cant, or physical vertical aperture of any kind, but instead relies only on the sensitivity of a line array at endfire to differences in horizontal wave number to resolve low and high order mode subspace excitations. The only inputs to the algorithm are an approximate sound speed profile, water depth, and bottom type. The theoretical basis for the test statistic is first reviewed, followed by discussion of key requirements, and illustration of the concept using results from a RAM PE simulation for a downward refracting environment. Finally, the algorithm is experimentally demonstrated using data from a bottom-mounted HLA deployed in the moderately cluttered continental shelf environment of the Florida Straits.

Tropical daytime lower D‐region dependence on sunspot number

Observed phases and amplitudes of VLF radio signals propagating on (near) tropical all‐sea paths, both short, ∼300 km, and long, ∼10 Mm, are used to find daytime parameter changes for the lowest edge of the (D‐region of the) Earth's ionosphere as the solar cycle advanced from a very low sunspot number of ∼5 up to ∼60, in the period 2009–2011. The VLF phases, relative to GPS 1‐s pulses, and amplitudes were measured ∼100 km from the transmitter, where the direct ground wave is very dominant, ∼300 km from the transmitter, near where the ionospherically reflected waves form a (modal) minimum with the ground wave, and ∼10 Mm away where the lowest order waveguide mode is fully dominant. Most of the signals came from the 19.8 kHz, 1‐MW transmitter, NWC, North West Cape, Australia, propagating ENE, mainly over the sea, to the vicinity of Karratha and Dampier on the NW coast of Australia and then on to Kauai, Hawaii, ∼10.6 Mm from NWC. Observations from the 8.1‐Mm path NPM (21.4 kHz, Hawaii) to Dunedin, NZ, are also used. The sunspot number increase from ∼5 to ∼60 was found to coincide with a decrease in the height,H′, of the midday tropical ionosphere by 0.75 ± 0.25 km (from H′ ≈ 70.5 km to H′ ≈ 69.7 km) while the sharpness, β increased by 0.025 ± 0.01 km−1 (from β ≈ 0.47 km−1 to β ≈ 0.49 km−1) where H′ and βare the traditional height and sharpness parameters used by Wait and by the U.S. Navy in their Earth‐ionosphere VLF radio waveguide programs.

Multiple flare-angle horn feeds for sub-mm astronomy and cosmic microwave background experiments

Context. The use of large-format focal plane imaging arrays employing multiple feed horns is becoming increasingly important for the next generation of single dish sub-mm telescopes and cosmology experiments. Such receivers are being commissioned on both general purpose, common user telescopes and telescopes specifically designed for mapping intensity and polarisation anisotropies in the cosmic microwave background (CMB). Telescopes are currently being constructed to map the CMB polarisation that employ hundreds of feeds and the cost of manufacturing these feeds has become a significant fraction of the total cost of the telescope. Aims. We have developed and manufactured low-cost easy-to-machine smooth-walled horns that have a performance comparable to the more traditional corrugated feed horns that are often used in focal plane arrays. Our horns are much easier to fabricate than corrugated horns enabling the rapid construction of arrays with a large number of horns at a very low cost. Methods. Our smooth walled horns use multiple changes in flare angle to excite higher order waveguide modes. They are designed using a genetic algorithm to optimise the positions and magnitudes of these flare angle discontinuities. We have developed a fully parallelised software suite for the optimisation of these horns. We have manufactured prototype horns by traditional electroforming and also by a new direct drilling technique and we have measured their beam patterns using a far-field antenna test range at 230 GHz. Results. We present simulated and measured far-field beam patterns for one of our horn designs. They exhibit low sidelobe levels, good beam circularity and low cross-polarisation levels over a fractional bandwidth of 20%. These results offer experimental confirmation of our design technique, allowing us to proceed confidently in the optimisation of horns with a wider operational bandwidth. The results also show that the new manufacturing technique using drilling is successful, enabling the fabrication of large format arrays by repeatedly drilling into a single aluminium plate. This will enable the construction of focal plane arrays at a very low cost per horn. Conclusions. We have developed a new type of high performance feed horn that is fast and easy to fabricate. Having demonstrated the efficacy of our horn designs experimentally, we are building and testing a prototype focal plane array of 37 hexagonally close packed horns. This prototype array will be an important step towards building a complete CMB mapping receiver using these feed horns.