Order Mode Coupling Research Articles

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital angular momentum (OAM) lasers have potential applications in large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus due to the advantages of simple structure, low cost and high peak power. At present, OAM mode femtosecond fiber lasers have made some breakthroughs in key parameters such as repetition frequency, pulse width, spectrum width, but it is difficult to achieve good overall performance. Besides, the repetition rate is tens of MHz at present. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. In coupler, the first order mode coupler with 3 dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3 dB polarization dependent loss is made by the technology of weak fused biconical taper. By combining the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber laser with over 100 MHz repetition rate is built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Comparing with traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indexes of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8 μm, which is the same as that of a single mode fiber. Comparing with DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required, which ensures a shorter overall cavity length and achieves high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101 nm 10 dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100 nm 10 dB bandwidth. Compared with the reported schemes, our scheme has good performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with excellent performance is expected to be widely used in OAM communication, particle manipulation and other research fields.

Orbital angular momentum mode femtosecond fiber laser with over 100 MHz repetition rate

Orbital Angular Momentum (OAM) lasers have potential demand in many applications such as large capacity communication systems, laser processing, particle manipulation and quantum optics. OAM mode femtosecond fiber laser has become the research focus with the advantages of simple structure, low cost and high peak power. The current OAM mode femtosecond fiber lasers have made breakthroughs in the repetition frequency, pulse width, spectrum width and other key parameters, but it is difficult to achieve good overall performance. Besides, the repetition rate is currently in tens of MHz. In this paper, a large-bandwidth mode coupler is made based on the mode phase matching principle. Among them, the first order mode coupler with 3dB polarization dependent loss is made by the technology of strong fused biconical taper, and the second order mode coupler with 0.3dB polarization dependent loss is made by the technology of weak fused biconical taper. Combined with the nonlinear polarization rotation mode-locking mechanism, OAM mode femtosecond fiber lasers with over 100 MHZ repetition rate are built. The achievement of the key parameters is attributed to the selection of dispersion shifted fibers that can accurately adjust intracavity dispersion. Compared to traditional dispersion compensation fibers (DCF), the group velocity dispersion is reduced by an order of magnitude, so it can better adjust intracavity dispersion to achieve the indicators of large spectral bandwidth and narrow pulse width. In addition, the diameter of the fiber is 8μm, which is the same as that of a single mode fiber. Compared to DCF, the fusion loss can be ignored, so only a shorter gain Erbium-doped fiber is required that ensure a shorter overall cavity length and achieve high repetition frequency. The experimental results show that the first order OAM mode fiber laser has 113.6 MHz repetition rate, 98 fs half-height full pulse width, and 101nm 10-dB bandwidth. Second-order OAM mode fiber laser has 114.9 MHz repetition rate, 60 fs half-height full pulse width, and 100nm 10-dB bandwidth. Compared with the reported schemes, our scheme has better performance in key parameters such as repetition rate, pulse width and spectral width. We believe that the OAM mode fiber laser with good over performance is expected to be more widely used in OAM communication, particle manipulation and other research fields.

Tunable Broadband Mode Converter Based on Long-Period Fiber Gratings at 2-μm Waveband

We demonstrate the fabrication of a broadband mode converter (MC) operating at 2 μm waveband based on long-period fiber gratings (LPFGs) written in the two-mode fiber (TMF) by CO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> laser. The proposed MC offers a 10-dB bandwidth of 58 nm, making it suitable for high-speed broad-band optical communication system at 2 μm waveband. The maximum conversion efficiency reaches 98.89% at wavelength of 2 μm. The torsion, strain, bending, and polarization characteristics of the broadband MC were measured experimentally. The torsion sensitivity of the LPFGs was measured to be 1.32 nm/(rad/m) at 2 μm waveband, which was almost three times higher than that of the grating with the same order mode coupling at 1.55 μm waveband. The proposed broadband MC can function as not only a promotion in fiber communication system but also a high sensitivity torsion sensor in fiber sensing system.

Coherent terahertz radiation with orbital angular momentum by helically microbunched electron beam

A scheme to generate coherent terahertz (THz) radiation carrying orbital angular momentum (OAM) through down-frequency conversion of optical lasers and higher order mode coupling is proposed and analyzed. Based on the laser–electron interaction in the undulator under a linear regime, the scheme utilizes dual-frequency lasers resonating at the harmonics of two consecutive undulators and a dispersion section to impose a THz helical microbunching in the electron beam. By choosing appropriate parameters of lasers and dispersion section, the density modulation can be down-converted to a THz spectral-domain and the central frequency can be extensively tuned together with the helical mode number. Then, the helically microbunched beam can be used as a source of THz OAM radiation in a downstream radiator. We expect that the intense vortex beam in the terahertz band will bring new opportunities for the research of quantum topological materials.

Measurement of rf voltage in long traveling wave accelerating structures with application to the CERN Super Proton Synchrotron

Since its inauguration in 1976, the Super Proton Synchrotron (SPS) at CERN relies on a traveling wave system for particle acceleration instead of making use of standing wave cavities that are more common in circular accelerators. These traveling wave structures (TWSs) are essentially periodically loaded waveguides operated at a fundamental frequency of about 200 MHz. In the frame of the LHC injector upgrade project, the SPS accelerating system is presently improved to better cope with the increased beam intensity required for the future High Luminosity LHC era. The available accelerating voltage provided by the TWSs by means of radio-frequency (rf) generators is a fundamental and valuable input for machine operation and particle tracking codes used to simulate beam phenomena in the SPS. Measurement of the voltage with well known nonresonant and on-axis perturbation methods is however infeasible due to the extreme mechanical length of the SPS structures. It is shown that a good estimate of the rf voltage can be obtained from off-axis measurement samples in each accelerating gap in addition to the in- and output reflection coefficients of the structures. The rf voltage for the LHC proton beam delivered by the system configuration that was installed during the first and second exploitation run of the LHC is measured by this method, not including the beam loading. The impact on the fundamental accelerating mode and the rf voltage of additional higher order mode (HOM) couplers, which are also required for the upgrade of the acceleration system, is assessed by the proposed method as well.

Purification of Selective Laser Melting Additive Manufactured Niobium for Superconducting RF-Applications Transactions on Applied Superconductivity

Superconducting radio frequency (SRF)-cavities in the field of particle accelerators are mostly manufactured from bulk or sheet niobium (Nb). Complex shapes of SRF-components like high order mode-couplers or cavities working at high fields are high in cost and lead-time. The fabrication of these parts via additive manufacturing promises a more effective production and higher flexibility in geometries. At CERN, additive manufacturing with pure Nb by selective laser melting (SLM) is under development for future particle accelerator components. In comparison with conventionally produced Nb SRF-components, made of highly pure Nb, the SLM-manufactured Nb material contains considerable impurities due to raw material processing and the SLM-process conditions itself. Contaminations with light elements, such as oxygen and nitrogen, introduced into the material's structure by the powder and the SLM process, contribute to an important lowering of the residual resistance ratio (RRR). In order to increase the RRR-values for conventionally manufactured SRF-cavities, titanium (Ti) gettering-heat treatments were investigated since the 1980s. This paper introduces the first representative Nb-material realized at CERN by additive manufacturing by SLM and the current studies on the post-treatment of sample material by vacuum firing and Ti-gettering-heat treatments. The samples are analyzed by mass spectroscopy and RRR-measurements in correlation with different process parameters. First trials with Ti-gettering heat treatments demonstrate an increase in the RRR from 7 to above 100, whereas the oxygen content could be reduced from 600 to 17 ppm. These results show that a purification post-treatment of SLM-manufactured Nb-components is feasible and pave the way for functioning SRF-components made by additive manufacturing.

Waveguide Slot Array Design With Compensation for Higher Order Mode Coupling Between Inclined Coupling Slots and Neighboring Radiating Slots

Existing design equations do not include higher order coupling effects between inclined coupling slots and longitudinal radiating slots of planar arrays. Several studies have shown that higher order mode coupling in the waveguide junctions causes errors in the excitation of radiating slots, which in turn result in antenna performance degradation. Efforts to compensate for these effects have to date been restricted to trial-and-error or applying full-wave analysis to an existing design and perturbing the slot dimensions to improve the array performance. In this paper, we present an efficient design approach that accounts for the higher order coupling in the waveguide junctions. Compensation is achieved with the addition of closed-form terms in the expression for the active admittance of the radiating slots straddling the coupling slot in each branch. In addition, we propose amendments to the method for calculation of coupling slot dimensions that accounts for phase differences between scattering parameters of coupling junctions. The effectiveness of the proposed procedure is demonstrated by means of two illustrative design examples.

Higher Order Mode Coupling Options for the eRHIC Crab Cavity

Higher Order Mode Coupling Options for the eRHIC Crab Cavity

Subpicosecond beam length measurement study based on theTM010mode

This article discusses a beam length measurement technique based on the ${\mathrm{TM}}_{010}$ modes of two cavities in frequency domain. Since the ${\mathrm{TM}}_{010}$ pickup monitor has a much better signal-to-noise ratio (SNR) than the other monitors, it can improve the measuring range and resolution limit of the beam length measurement in the frequency domain to achieve the real-time measurement of the pulse duration of the order of 1 ps or less with the resolution of about 10 fs. The root-mean-square length of a beam---which does not necessarily have a Gaussian distribution---can be obtained by choosing the working frequency of the cavity to be much less than the critical frequency of the beam. The modeling and analysis of the above theory were also made simultaneously. The resulting resolution was about 100 fs when the step size was chosen to be 200 fs in this simulation after the higher order mode coupling effect was eliminated and the beam offset was corrected by using the Bessel function. An efficient algorithm was also provided in case the measurement system would achieve the SNR of 112 dB when there could be some issue in signal processing. This algorithm can still provide the length result with the resolution of $3.5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ for a beam duration of 0.1 ps when the SNR of the difference signal is as low as 10 dB.

도파관 슬롯 커플러용 비대칭 복합 아이리스에 의해 급전되는 중심 종방향 션트-시리즈 결합 슬롯에 관한 연구

본 논문에서는 도파관 슬롯 배열 안테나 설계 시 분기 도파관 내에서 결합 슬롯과 복사 슬롯 간의 원하지 않는 고차 모드 결합을 줄일 수 있는 슬롯 커플러용 결합 요소를 제안하였다. 제안된 소자는 수직 교차하는 두 도파관이 공유하는 도파관 광벽에 위치한 중심 종방향 션트-시리즈 결합 슬롯과 이를 여기시키는 비대칭 복합아이리스로 구성된다. 먼저 HFSS를 사용하여 커플러 구조의 산란 계수를 구하고, 결합 슬롯의 T - 회로망 등가 회로의 파라미터를 추출하였다. 또한, 소자의 구조적인 치수를 변화시키면서 공진 길이 및 정규화 어드미턴스와같은 공진 특성을 분석하였다. 결합 슬롯으로부터 <TEX>${\lambda}_g/4$</TEX> 떨어진 단락 블록을 갖는 커플러를 설계 제작하여 측정한 실험 결과는 모의실험 결과와 잘 일치하였다. This paper proposes a new coupling element of microwave slot coupler for designing waveguide slot array which can reduce the effect of undesired higher order mode coupling between coupling and radiating slots in the branch waveguide. The proposed device is composed of a centered longitudinal shunt-series coupling slot at the center of broad wall shared by two crossed rectangular waveguides and an asymmetric compound iris that excites the coupling slot. We first have obtained scattering parameters for the proposed coupler by use of EM S/W tool HFSS and then extracted the parameters of T- network equivalent circuit for the coupling slot. We also have analyzed the resonant properties such as resonant length and normalized admittance by changing the geometrical dimensions. The measured results for the fabricated coupler with short-circuited block <TEX>${\lambda}_g/4$</TEX> away from the coupling slot are well agreed with the simulated ones.

Refractive Index Sensing by Using Mechanically Induced Long-Period Grating

In this paper, a refractive index sensor using a mechanically induced long-period grating is proposed and experimentally demonstrated. The sensing element is based on a composite optical waveguide, which is made of a single-mode fiber, a Teflon-cannula, and the medium pending to test. The research shows that the resonant wavelength of the high- order mode coupling is sensitive to the refractive index of the liquid. The pressure applied on the experimental setup should keep constant in the refractive index measurements for different liquids, where the mode coupling is in the undercoupling state. The sensitivity of the sensor for LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sub> mode resonance is about 2.78 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> refractive index unit in the refractive index range from 1.33 to 1.43.

Update on coaxial coupling scheme for International Linear Collider-type cavities

This paper reports on our efforts to develop a flangeable coaxial coupler for both higher order mode and fundamental coupling for nine-cell ILC-type cavities, which were designed in the early 1990's for pulsed operation with a duty factor less than 1%. The design of the coupler has been done in such a way that the rf magnetic flux $B$ at the flange connection was minimized and only a field of $&lt;5\text{ }\text{ }\mathrm{mT}$ would be present for an operation at an accelerating field ${E}_{\mathrm{acc}}\ensuremath{\sim}36\text{ }\text{ }\mathrm{MV}/\mathrm{m}$ ($B\ensuremath{\sim}150\text{ }\text{ }\mathrm{mT}$) in the cavity. Even though we achieved reasonably high $Q$ values at low field, the cavity/coupler combination was limited in the cw mode to only $\ensuremath{\sim}7\text{ }\text{ }\mathrm{MV}/\mathrm{m}$, where a thermally initiated degradation occurred. We believed that this limitation was caused by poor cooling of the shorting plate and inner tube in the coaxial coupler; therefore, we have improved the cooling conditions by initially drilling radial cooling channels every 30 degrees, then every 15 degrees into the shorting plate and eventually removing the ``bridges'' between the channels. This paper reports on our experiences with the modified coaxial coupler under cw and pulsed conditions.

An Optimization Procedure for Including the Higher Order Mode Coupling Between Coupling Slots in the Design of a Planar Slot Array

A simple iterative procedure to account for higher order mode coupling between adjacent coupling slots of a planar slot array is presented. The method converges rapidly and is easy to implement in conjunction with Elliott's design technique for planar arrays. The procedure is illustrated with an example and validated by a full-wave moment method analysis.

CSC - A procedure for coupled S-parameter calculations

Large rf-structures are sometimes too complex to be calculated entirely in single simulation runs. Usually, if the structure has open ports, the scattering properties - the so-called S-parameters - are of primary interest. As a matter of fact, they can be derived from scattering properties of parts of the entire structure, which are calculated in the procedure presented here in separate, less expensive simulations. A very compact representation of the underlying theory was found, which is presented in the paper. Furthermore, a Mathematica application called CSC based on this formulation is introduced. CSC calculates the scattering properties of an object, which are a combination of an arbitrary structure of segments with previously calculated S-parameters. To illustrate the use of CSC, three examples are shown: higher order mode (HOM) coupling properties of components of the TESLA Test Facility without and with accelerating cavities and the coupling of polarizations in chains of structures with slight deviations from circular cross sections.

Nonlinear saturation of Rayleigh-Taylor instability and generation of shear flow in equatorial spread-F plasma

Abstract. An analysis of low order mode coupling equations is used to describe the nonlinear behaviour of the Rayleigh-Taylor (RT) instability in the equatorial ionosphere. The nonlinear evolution of RT instability leads to the development of shear flow. It is found that there is an interplay between the nonlinearity and the shear flow which compete with each other and saturate the RT mode, both in the collisionless and collisional regime. However, the nonlinearly saturated state, normally known as vortices or bubbles, may not be stable. Under certain condition these bubbles are shown to be unstable to short scale secondary instabilities that are driven by the large gradients which develop within these structures. Some understanding of the role of collisional nonlinearity in the shear flow generations is also discussed.

3P060蛋白質分子内のエネルギー移動-常温の系でのモードカップリング

Mode coupling in a protein molecule was studied by a molecular dynamics simulation of the intramolecular vibrational energy transfer in myoglobin at near zero temperature. It was found that the vibrational energy is transferred from a given normal mode to a very few number of selective normal modes. These modes are selected by the relation between their frequencies, like Fermi resonance, governed by the third order mode coupling term. It was also confirmed that the coupling coefficients had high correlation with how much the coupled modes geometrically overlapped with each other.

Vibrational energy transfer in a protein molecule.

Mode coupling in a protein molecule was studied by a molecular dynamics simulation of the intramolecular vibrational energy transfer in myoglobin at near zero temperature. It was found that the vibrational energy is transferred from a given normal mode to a very few number of selective normal modes. These modes are selected by the relation between their frequencies, like Fermi resonance, governed by the third order mode coupling term. It was also confirmed that the coupling coefficients had high correlation with how much the coupled modes geometrically overlapped with each other.

Computation of the Electronic and Spectroscopic Properties of Carbohydrates Using Novel Density Functional and Vibrational Self-Consistent Field Methods

ABSTRACT A novel density functional method is presented for the calculation of electronic and thermodynamical properties of oligosaccharides. This method, termed K2-BVWN, offers two advantages; it scales as N 3, where N is the number of basis functions, and there are only two adjustable parameters. The current density functional method is tested in terms of reproducing high level gas phase ab initio calculations in eleven low energy conformers of D-glucopyranose including exo-anomeric and different hydroxymethyl orientations (G −, G +, and T). The K2-BVWN method is also tested in terms of reproducing the spectroscopic features of D-glucopyranose and D-mannopyranose (α/β) as compared with both a vibrational self-consistent field calculation (VSCF) as well as experimental infrared spectroscopy. The VSCF calculations offer the advantage that it is possible to include higher order mode coupling and anharmonic effects directly into the calculation of the vibrational frequencies. In general, the K2-BVWN method reproduces the ab initio energetic trends of the different conformers of D-glucose. While the absolute energies are not the same between the ab initio and the K2-BVWN method, both methods do predict a preference for the α-anomer in the gas phase (0.4 kcal/mol ab initio, 0.0 – 0.5 kcal/mol K2-BVWN). The K2-BVWN method was able to reproduce the experimental and VSCF calculated spectrum of both D-glucopyranose and D-mannopyranose in the frequency range between 1500 – 800 cm−1. Because the current density functional method is both relatively quick and accurate, it represents a significant advancement in the development of oligosaccharide force fields.

Design of layered ridge dielectric waveguide for millimeter and sub-millimeter wave circuits

Design rules for Layered Ridge Dielectric Waveguide (LRDW) are presented for the first time through simple figures and closed form equations. The Effective Dielectric Constant (EDC) method is used to develop the design rules that account for typical circuit specifications such as higher order mode suppression, dispersion, attenuation, and coupling between adjacent transmission lines. Comparisons between the design of LRDW, image guide, and millimeter-wave dielectric ridge guide are made.

Mode coupling theory for calculating the memory functions of flexible chain molecules: Influence on the long time dynamics of oligoglycines

A recent theory for the long time dynamics of flexible chain molecules is tested for the internal dynamics of triglycine and octaglycine, systems which are much more complicated than the previously studied alkanes. The theory extends the generalized Rouse (GR) theory used for the dynamics of polymers by providing a systematic procedure for including the contributions from internal friction and memory function matrices which are neglected in the GR theory. The mode-coupling method expresses the time correlation functions in terms of the eigenvalues and eigenfunctions of the diffusion operator and determines the eigenvalues by expanding the eigenfunctions in a suitable basis set. The greater complexity of the polyglycine interaction potential and the presence of cooperative local conformational transitions require including higher order mode coupling contributions than previously used. A major computational impediment induced by this requirement is the enormous growth in size of the basis set with the addition of the higher order mode coupling contributions that are needed to describe the influence of the memory functions. This impediment is alleviated by a new sorting procedure that includes in the basis set only the mode coupling functions with the slowest first order relaxation times. The theory is compared with Brownian dynamics (BD) simulations, so that both theory and simulation use identical, realistic potential functions and identical models for the solvent. The new method describes motions on time scales more than an order of magnitude longer than those accessible to molecular dynamics simulations. Inclusion of the memory functions greatly influences the dynamics, and the theory produces excellent agreement with the BD simulations for the long time motions. Individual BD trajectories exhibit the local and correlated conformational transitions.