Mode Conversion Efficiency Research Articles

Experimental realization of an O-band compact polarization splitter and rotator

We experimentally realize a compact wideband polarization splitter and rotator (PSR) with CMOS compatibility. The fabricated PSR is then tested by utilizing a fabrication-tolerant TE-pass on-chip polarizer we propose to practically solve the issue of accurately aligning the polarizations in fiber and modes on chip. Both of these polarization handling devices take the advantage of bend structure that confines TE mode better than TM mode. The fabricated PSR has a high TM-TE and TE-TE mode conversion efficiency of -0.4 dB and -0.2 dB at 1310 nm, while the extinction ratio is better than 18 dB and the broad bandwidth exceeds 100 nm.

Generation of femtosecond optical vortex pulse in fiber based on an acoustically induced fiber grating.

We proposed a method for generation of a femtosecond optical vortex pulse in a two-mode fiber based on an acoustically induced fiber grating (AIFG) driven by a radio frequency source. Theoretical analysis and experimental results demonstrated that the left- and right-handed circular polarization fundamental modes of the femtosecond optical pulse could be converted to the linearly polarized ±1-order optical vortex modes through the AIFG with the mode conversion efficiency of ∼95%. The off-axial interference experiment and the polarization angle-dependent intensity examination were performed to verify the topological charge and the polarization state of the femtosecond optical vortex, respectively.

Compact circular waveguide TM 02 ‐TE 11 mode converter

Using the concept of a metallic cylinder and a linearly tapered metallic baffle loaded in a circular waveguide to convert TM02 circular waveguide mode into TE11 circular waveguide mode, a novel and compact circular waveguide TM02-TE11 mode converter is designed, fabricated, and measured. The detailed design methods, experimental setups and results are presented in this study. Simulation results show that the purity of output fundamental TE11 mode of the converter exceeds 99.6% for an operating frequency of 9.4 GHz and the power capability is as high as 1.66 GW, the mode conversion efficiency is more than 90% from 9.23 to 9.83 GHz band and the relative bandwidth is about 6.38%. A good agreement between measurements and simulations is observed. The circular waveguide TM02-TE11 mode converter has many advantages, such as the simple structure, coaxial input and output, high conversion efficiency, proper bandwidth and high power-handling capability. When used in the field of high power microwave transmission and radiation, the proposed converter could significantly enhance the compactness of all the radiation systems.

Shortcuts to adiabaticity in optical waveguides using fast quasiadiabatic dynamics.

We propose a fast quasiadiabatic approach to the design of optical waveguide devices. This approach distributes the system adiabaticity homogeneously over the device length, thus providing a shortcut to adiabaticity at a shorter device length. A mode sorting asymmetric Y junction is designed by redistributing the adiabaticity of a conventional linearly separating Y junction. Simple procedures for the design of fast quasiadiabatic devices are outlined, and the designed Y junction features large bandwidth at a shorter length than the conventional linearly separating Y junction. The proposed device is verified with beam propagation simulations. A mode conversion efficiency of larger than 99% is observed for the designed Y junction over a 220 nm range.

Design of a Broadband Quasi-Optical Mode Converter for $W$ -Band Gyro-TWTs

Mode converters for millimeter waves can reduce the transmission loss, generate the desired mode, and facilitate the application of high-power sources. A W-band broadband quasi-optical mode converter which converts a TE0,1 cylindrical waveguide mode into a linearly polarized Gaussian TEM00 mode has been realized and tested in this paper. The converter is composed of a wavefront transforming mirror, a polarization grating, and a phase correcting system. In order to obtain high conversion efficiency over a wide bandwidth and make the structure of the mode converter much simple and compact, the propagation path of the beam is optimized and a wavefront transforming mirror realized by an ellipsoidal mirror rather than a nonquadratic mirror is introduced. The conversion process in each section is displayed and the working principles are discussed in detail. The mode conversion characteristics are investigated by analyzing the measured radiation field pattern. Results of the measurement agree well with the numerical calculations. The proposed W-band mode converter has a conversion bandwidth of 21% when the mode conversion efficiency is over 95% and the cross polarization is below 1.7%. In addition to high conversion efficiency, low cross-polarization, and broad bandwidth, this converter features easy construction and compact structure.

An MMI-Based Mode (DE)MUX by Varying the Waveguide Thickness of the Phase Shifter

A two-mode (de)multiplexer [(DE)MUX] based on multimode interference couplers on InP substrate is proposed and experimentally demonstrated. A phase shifting section with a variation of the waveguide thickness is introduced to precisely control the phase shift to realize 100% mode conversion efficiency and multiplexing. Performance analysis of the (DE)MUX is carried out numerically, showing excellent properties with insertion loss <;1.2 dB and crosstalk <;-18.4 dB for both the TE 0 and TE 1 mode within the whole C-band. The proposed device is also fabricated and demonstrated on InP substrate.

Mode Conversion Behavior of Guided Wave in a Pipe Inspection System Based on a Long Waveguide.

To make clear the mode conversion behavior of S0-mode lamb wave and SH0-plate wave converting to the longitudinal mode guided wave and torsional mode guided wave in a pipe, respectively, the experiments were performed based on a previous built pipe inspection system. The pipe was wound with an L-shaped plate or a T-shaped plate as the waveguide, and the S0-wave and SH0-wave were excited separately in the waveguide. To carry out the objective, a meander-line coil electromagnetic acoustic transducer (EMAT) for S0-wave and a periodic permanent magnet (PPM) EMAT for SH0-wave were developed and optimized. Then, several comparison experiments were conducted to compare the efficiency of mode conversion. Experimental results showed that the T(0,1) mode, L(0,1) mode, and L(0,2) mode guided waves can be successfully detected when converted from the S0-wave or SH0-wave with different shaped waveguides. It can also be inferred that the S0-wave has a better ability to convert to the T(0,1) mode, while the SH0-wave is easier to convert to the L(0,1) mode and L(0,2) mode, and the L-shaped waveguide has a better efficiency than T-shaped waveguide.

Optical vortex generation with wavelength tunability based on an acoustically-induced fiber grating.

We presented a method to actualize the optical vortex generation with wavelength tunability via an acoustically-induced fiber grating (AIFG) driven by a radio frequency source. The circular polarization fundamental mode could be converted to the first-order optical vortex through the AIFG, and its topological charges were verified by the spiral pattern of coaxial interference between the first-order optical vortex and a Gaussian-reference beam. A spectral tuning range from 1540 nm to 1560 nm was demonstrated with a wavelength tunability slope of 4.65 nm/kHz. The mode conversion efficiency was 95% within the whole tuning spectral range.

Experimental demonstration of a two-mode (de)multiplexer based on a taper-etched directional coupler.

We experimentally demonstrate a compact, low-cross talk and fabrication-tolerant two-mode (de)multiplexer on the silicon-on-insulator platform. The device consists of a silicon wire waveguide coupled to a taper-etched waveguide. The partially etched taper structure is used to relax fabrication tolerance and thus to ensure high mode-conversion efficiency. The device is 68 μm in length, with a TE0-to-TE1 mode conversion loss of better than -0.8 dB demonstrated over the C-band wavelengths. In addition, the device demonstrates a low TE0-to-TE0 through waveguide insertion loss of better than -1.3 dB with modal cross talk lower than -26 dB, over a 65 nm wavelength range. Finally, we have experimentally demonstrated that the device is tolerant of fabrication errors of up to 20 nm. Better than -6 dB TE0-TE1 conversion loss with a cross talk lower than -23 dB over a 55 nm bandwidth has been obtained with a fabrication tolerance as large as 40 nm.

Is the bulk mode conversion important in high density helicon plasma?

In a high-density helicon plasma production process, a contribution of Trivelpiece-Gould (TG) wave for surface power deposition is widely accepted. The TG wave can be excited either due to an abrupt density gradient near the plasma edge (surface conversion) or due to linear mode conversion from the helicon wave in a density gradient in the bulk region (bulk mode conversion). By numerically solving the boundary value problem of linear coupling between the helicon and the TG waves in a background with density gradient, we show that the efficiency of the bulk mode conversion strongly depends on the dissipation included in the plasma, and the bulk mode conversion is important when the dissipation is small. Also, by performing FDTD simulation, we show the time evolution of energy flux associated with the helicon and the TG waves.

High-Order Mode Rotator on the SOI Integrated Platform

A high-order mode rotator based on a silicon-on-insulator strip waveguide with asymmetrical cross section is proposed and optimized to rotate the spatial distribution of the $E_{21}$ mode by 90° into that of the $E_{12}$ mode. The subscripts $i$ and $j$ of the mode $E_{ij}$ correspond to the number of lobes in the intensity distribution of the mode along the horizontal and vertical axes, respectively. The mode conversion efficiency and the corresponding length of the mode rotator device are presented as a function of design parameters for both the transverse magnetic (TM) and transverse electric (TE) polarizations. The fabrication tolerance of the structure is also analyzed. We further present simulation results of a two-mode multiplexing system based on $E_{21}$ and $E_{12}$ showing a mode extinction ratio exceeding 15 dB over a wide wavelength range of 100 nm.

On the propagation and mode conversion of auroral medium frequency bursts

AbstractAuroral medium frequency (MF) bursts are broadband, impulsive radio emissions associated with local substorm onsets. MF bursts consist of a characteristic fine structure whereby the higher frequencies arrive 10–100 ms before the lower frequencies. LaBelle (2011a) proposed that MF bursts originate as Langmuir/Z mode waves on the topside of the ionosphere that mode‐convert to LO mode waves and propagate to ground level, with the fine structure resulting by propagation delays due to the topside ionospheric density profile. We investigate three aspects of this mechanism. First, full‐wave calculations are used to simulate the MF burst fine structure using a realistic ionospheric density profile. The delay between the highest and lowest frequencies is 21 ms. This value is smaller than the experimentally determined delays of ∼100 ms presented in Bunch and LaBelle (2009), but differences between the topside electron number density profile used in the simulations and the number density profile during disturbed conditions make comparisons only approximate. Second, the Landau damping of Langmuir/Z mode waves on the topside ionosphere is calculated, assuming the electron distribution function consists of a cold background population (ne0) and a warm secondary population (nse). The Landau damping is small when nse/ne0 = 0.04% (consistent with Maggs and Lotko (1981)) but is significant when nse/ne0 &gt; 0.4%. Finally, full‐wave calculations are used to determine the mode conversion efficiency from Langmuir/Z mode waves to LO mode waves. These imply that waves would suffer an attenuation of wave energy density of approximately 5–10% if they are generated with their wave vectors in a narrow cone centered around the local magnetic field. Taken together, these calculations suggest that for small values of nse/ne0 &lt;0.4%, the mechanism proposed by LaBelle (2011a) is a plausible explanation for the origin of MF bursts.

Axicons for mode conversion in high peak power, higher-order mode, fiber amplifiers.

Higher-order mode fiber amplifiers have demonstrated effective areas as large as 6000 μm2, allowing for high pulse energy and peak power amplification. Long-period gratings are used to convert the fundamental mode to the higher-order mode at the entrance to the amplifier, and reconvert back to the fundamental at the exit, to achieve a diffraction limited beam. However, long period gratings are susceptible to nonlinearity at high peak power. In this work, we propose and demonstrate axicons for linear bulk-optic mode conversion at the output of higher order mode amplifiers. We achieve an M2 of less than 1.25 for 80% mode conversion efficiency. Experiments with pulsed amplifiers confirm that the mode conversion is free from nonlinearity. Furthermore, chirp pulse amplifier experiments confirm that HOM amplifiers plus axicon mode convertors provide energy scalability in femtosecond pulses, compared to smaller effective area, fundamental mode fiber amplifiers. We also propose and demonstrate a route towards fiber integration of the axicon mode convertor by fabricating axicons directly on the tip of the fiber amplifier end-cap.

A Universal Solution to &lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\rm TM}_{11}$&lt;/tex&gt;&lt;/formula&gt;-to-&lt;formula formulatype="inline"&gt;&lt;tex Notation="TeX"&gt;${\rm HE}_{11}$&lt;/tex&gt;&lt;/formula&gt; Circular Mode Converter Design

A universal solution to ${\rm TM}_{11}$ -to- ${\rm HE}_{11}$ mode converters in highly oversized circumferentially corrugated circular waveguides is proposed based on the nonuniform rational B-spline technique. A 35-GHz ${\rm TM}_{11}$ -to- ${\rm HE}_{11}$ mode converter of radius 16 mm is designed by the method and predicted to have a conversion bandwidth of 7.9% when the ${\rm HE}_{11}$ mode conversion efficiency is over 98.5%. The output mode content is extremely close to that of the ideal ${\rm HE}_{11}$ mode. The hot and cold tests show good property of the mode converter. The method offers universal solutions to the problems of the corrugated circular waveguide mode converters design at arbitrary radius and frequency.

Maximizing the Plasmonic Near-Field Transducer Efficiency to Its Limit for HAMR

Plasmonic near-field transducer (NFT) facilitates optical power transfer into a nano-spot in the recording media for heat-assisted magnetic recording. Typically, impedance matching is a condition for maximum power transfer between a given source and load. Even for waveguide-NFT-media stack system, we verify that matching the impedances of waveguide and media-stack at NFT resonance maximizes power transfer. However, it is not a sufficient condition. Since, NFT guides the light into the media-stack as symmetric and asymmetric optical modes, higher conversion of dielectric mode into a mode that exhibits maximum coupling into the media is the second condition for maximizing the power transfer into the media. In this study, operating wavelength range, material refractive indices, and NFT geometrical parameters are considered in the design space for maximizing the efficiency of an exemplary taper-based NFT structure. We first estimate the maximum efficiency and compare with the benchmark efficiency of “ideal” version of the considered NFT to confirm that the obtained efficiency is maximum. At maximum efficiency point, we show that the impedance matching between waveguide and high-index media stack is enhanced and mode-conversion efficiency into asymmetric plasmon mode is also enhanced because of high-index contrast feeding waveguide and suitable operating wavelength range.

Photonic-Quasi-TE-to-Hybrid-Plasmonic-TM Polarization Mode Converter

A novel ultracompact ( ${ $\mu$ m $^2$ ) polarization mode converter for converting quasi-TE $_{00}$ mode of a silicon strip waveguide (WG) to the fundamental quasi-TM mode of a hybrid-plasmonic WG (HPW) without lateral structures is described. The mode-evolution-based design consists of three-section asymmetric HPWs, each of which has a unique top silver structure. A newly defined asymmetry index whose minimum suggests the strongest decoupling of longitudinal field components in three-dimensional guiding structures is used to rapidly obtain a suboptimal design of top metallic structure without relying heavily on parameter sweeps using numerical computations. The optimized converter exhibits a mode conversion efficiency (MCE) of $88.79\%$ , an insertion loss (IL) of 0.5139 dB, and a polarization conversion efficiency (PCE) of $99.82\%$ at 1550 nm. The respective 1-dB optical bandwidths for the MCE and IL are about 104 and 108 nm, while the PCE is ${>}92\%$ over a 172 nm spectral range. The fabrication tolerances for top silver structures are also discussed.

Performance Analysis of a Broadband Second-Order Mode Converter Based on Multimode Interference Coupler and Phase Shifter

A novel mode converter with broad bandwidth, high mode conversion efficiency, and large fabrication tolerance is proposed and fabricated on a silicon-on-insulator (SOI) platform. By using a modified 3 $\times$ 3 multimode interference coupler and an enlarged butterfly-shaped phase shifter, mode conversion from the fundamental mode $(TE_0)$ to second-order mode $(TE_2) $ is achieved numerically and experimentally. Performance analysis of the mode converter is carried out numerically, and the results show high performance of the mode converter with mode conversion efficiency larger than 91% (i.e., insertion loss less than 0.40 dB) in the whole C-band. In addition, the mode converter has large fabrication tolerance. For example, the width and the length deviations of the multimode interference coupler can be as large as 50 and 250 nm, respectively, whereas the width and the length deviations of the phase shifter can be up to 25 and 400 nm, respectively.

Widely Wavelength-Tunable Mode Converter Based on Polymer Waveguide Grating

We design and fabricate a corrugated grating on a polymer waveguide for the realization of the conversion between the fundamental mode and the higher order mode. The transmission characteristics of the device are insensitive to the polarization state of the input light. Our typical fabricated device shows a mode conversion efficiency higher than 99% over a bandwidth of $\sim 4$ nm and a temperature sensitivity of 3.5 nm/°C, which allows the operation wavelength of the device to be tuned over the $C+L$ band with a temperature control <30°C. This mode converter could be integrated with other waveguide devices to realize more sophisticated functions for mode-division-multiplexing systems.

Dielectric loaded tm&lt;sub&gt;01&lt;/sub&gt; to te&lt;sub&gt;11&lt;/sub&gt; mode converter for S-band applications

In this paper, a compact, high efficiency TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> to TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode converter is presented. In the proposed structure, dielectric material is loaded below a tapered centre conductor, in lower section of a circular waveguide. The input TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode is initially transformed to TEM mode by the tapered centre conductor. The loaded dielectric material is designed to provide the desired phase shift for the mode conversion. Different dielectric materials are analyzed for loading to achieve high conversion efficiency and wide bandwidth. The proposed mode converter is fabricated with PTFE as dielectric for mode conversion and the results are verified through far field radiation pattern measurements. The mode conversion efficiency is more than 90% for 2.95-3.60 GHz band. Due to compactness and high conversion efficiency for wide bandwidth, it is suitable for S-band high power microwave (HPM) applications.

Influence of upper hybrid resonance localized oscillation on X-B mode conversion efficiency for high-β National Spherical Torus Experiment in nonlinear regime

Ever increasing needs and capabilities in high power radio frequency waves heating and current drive scenarios of present and future magnetic confined fusion plasmas motivate expansion of understanding for vast variety of ever upcoming nonlinearities in such levels of power. Among many motivating experiments, one of the most relevant and actively studied in the regime for electron Bernstein wave (EBW) heating is high-β National Spherical Torus Experiment. A very special type of large amplitude electron plasma oscillations known as localized upper hybrid (UH) mode is demonstrated. It is shown that the mutual synergetic interaction of EBW and the localized UH mode can significantly shift the resonance layer about △x∼0.9 mm compared to the prediction of linear theory and consequently can explain the considerable reduction of conversion value around 35% observed in our modelling. This reduction is due to scale up of density scale length, Ln, at the new UH resonance (UHR) location followed by the increase of Budden parameter, η, which varies from 0.18 predicted by linear aspect to 0.40 in new position of UHR layer obtained by our modelling. Moreover, the parametric instabilities in the form of ion decays and dispersion of localized UH mode, approximately 7 mm due to the finite electron temperature account, are also observed which have an important contribution in reduction of conversion efficiency.