Excitation Of Higher-order Modes Research Articles

Multichannel mode conversion and multiplexing based on a single spatial light modulator for optical communication

A method is proposed to achieve multichannel mode conversion and multiplexing by dividing a single spatial light modulator into several blocks with the mode conversion pattern and blazed grating loaded on each block. The conversion patterns realize the precise excitation of higher order modes using combined amplitude and phase modulation. The blazed gratings bring together incident beams, so these beams can be coupled into few-mode fiber (FMF). In the experiment, four higher order modes are precisely excited and converge with a tilt angle. Through the simulation method, these beams can be coupled into FMF with small tilt angles (0.0344 deg for LP 11 mode).

Modal content in hypocycloid Kagomé hollow core photonic crystal fibers.

The modal content of 7 and 19 cell Kagomé anti resonant hollow core fibers (K-ARF) with hypocycloid core surrounds is experimentally investigated through the spectral and spatial (S2) imaging technique. It is observed that the 7 and 19 cell K-ARF reported here, support 4 and 7 LP mode groups respectively, however the observation that K-ARF support few mode groups is likely to be ubiquitous to 7 and 19 cell K-ARFs. The transmission loss of the higher order modes (HOMs) was measured via S2 and a cutback method. In the 7 cell K-ARF it is found that the LP11 and LP21 modes have approximately 3.6 and 5.7 times the loss of the fundamental mode (FM), respectively. In the 19 cell it is found that the LP11 mode has approximately 2.57 times the loss of the FM, while the LP02 mode has approximately 2.62 times the loss of the FM. Additionally, bend loss in these fibers is studied for the first time using S2 to reveal the effect of bend on modal content. Our measurements demonstrate that K-ARFs support a few mode groups and indicate that the differential loss of the HOMs is not substantially higher than that of the FM, and that bending the fiber does not induce significant inter modal coupling. A study of three different input beam coupling configurations demonstrates increased HOM excitation at output and a non-Gaussian profile of the output beam if poor mode field matching is achieved.

Compact Corrugated Feedhorns With High Gaussian Coupling Efficiency and <inline-formula> <tex-math notation="LaTeX">$-60\;\text{dB}$</tex-math> </inline-formula> Sidelobes

We demonstrate that very high performance, extremely compact, scalar corrugated feedhorns can be designed and constructed by optimizing the excitation and phasing of the ${\rm HE}_{11}$ , ${\rm HE}_{12}$ , and ${\rm HE}_{13}$ modes near the throat of the horn while limiting excitation of higher order modes. We present the design and measurement of two families of dual-profiled horn, both with a directivity of 20 dBi that couple with very high efficiency to a fundamental Gaussian mode. The first was optimized for sidelobe performance and features sidelobes approaching $-$ 60 dB for a horn length of only 15.6 ${\lambda}$ . The second was designed to minimize horn length and to achieve sidelobe levels below $-35\;\text{dB}$ for a horn that is only 4.8 ${\lambda}$ long. The horns exhibit excellent coupling to the fundamental free-space Gaussian mode, with ${\rm LG}_{00}$ power coupling of 99.92% and 99.75%, respectively. We demonstrate excellent agreement between simulation and experiment at 94 GHz and simulate the performance over a 20% bandwidth. High-performance compact scalar horns are of interest because they reduce manufacturing risk at high frequencies, and reduce size and weight at lower frequencies, which can be important in horn arrays and space applications, where horn arrays often have serious weight and size restrictions.

Smooth and flat phase-locked Kerr frequency comb generation by higher order mode suppression.

High-Q microresonator is perceived as a promising platform for optical frequency comb generation, via dissipative soliton formation. In order to achieve a higher quality factor and obtain the necessary anomalous dispersion, multi-mode waveguides were previously implemented in Si3N4 microresonators. However, coupling between different transverse mode families in multi-mode waveguides results in periodic disruption of dispersion and quality factor, and consequently causes perturbation to dissipative soliton formation and amplitude modulation to the corresponding spectrum. Careful choice of pump wavelength to avoid the mode crossing region is thus critical in conventional Si3N4 microresonators. Here, we report a novel design of Si3N4 microresonator in which single-mode operation, high quality factor, and anomalous dispersion are attained simultaneously. The novel microresonator is consisted of uniform single-mode waveguides in the semi-circle region, to eliminate bending induced mode coupling, and adiabatically tapered waveguides in the straight region, to avoid excitation of higher order modes. The intrinsic quality factor of the microresonator reaches 1.36 × 106 while the group velocity dispersion remains to be anomalous at −50 fs2/mm. With this novel microresonator, we demonstrate that broadband phase-locked Kerr frequency combs with flat and smooth spectra can be generated by pumping at any resonances in the optical C-band.

Removing the Directional Degeneracy of LP11Mode in a Fused-Type Mode Selective Coupler

The removal of polarization and modal degeneracy in the excitation of higher order modes is realised in all-fiber mode-selective weakly-fused-couplers by controlling the coupler geometry. Experiments carried out with short couplers show strong polarization dependency, while long couplers with a slow varying cross-section have shown to remove polarization and spatial degeneracy in the first high order mode.

Higher Order Modes: A Solution for High Gain, Wide Band Patch Antennas for Different Vehicular Applications

Low-gain and narrow bandwidth problems are associated with patch antennas, and due to their inverse relation with each other, these problems cannot be improved simultaneously. It is well understood that the patch antenna excited at the higher order modes (HOMs) can overcome the problems of low gain and narrow bandwidth, provided that the modes are excited properly. For a given geometry, all the resonant frequencies are related in fixed ratios, providing no flexibility for the designer. Furthermore, high side lobe levels and undesirable radiation patterns are associated with the higher order modes. The excitation of higher order modes in given dimensions of the patch is achieved by surface modification and loading of slots on the patch. Two antennas based on a novel design procedure are discussed in the Ku and Ka bands to validate the proposed facts. The antenna in the Ka band has a peak gain of 8.5 dBi with 15.14% of bandwidth, while the one in the Ku band has a peak gain of 11.84 dBi with 17.6% of bandwidth. The design procedure is particularly important in designing antennas for applications like mobile satellite communication, wideband and multimedia applications, collision avoidance systems in vehicles, etc.

Ultrafast two-color all-optical transverse mode conversion in a graded-index fiber.

We report on experimental conversion of picosecond probe pulses from the fundamental mode (LP01) to the next higher-order mode (LP11) by optically induced transient long-period gratings in a graded-index fiber using subpicosecond control pulses. In contrast to former experiments, the temporally synchronized control (1030 nm) and probe pulses (1250 nm) were easily distinguishable by their wavelengths, allowing for the first direct observation of such an ultrafast transverse mode conversion. Despite a nonperfect pulse duration ratio between control and probe pulses as well as an unavoidable excitation of unwanted higher-order modes, a clear energy conversion of 18% between the LP01-mode and the LP11-mode was observed at 2.3 times lower control pulse energies compared to previous experiments.

Lateral force sensing arrangement based on an all fiber Fabry–Perot interferometer

In this work, lateral force sensing detection based on an all fiber intrinsic Fabry–Perot interferometer (IFPI) is presented. The IFPI generates several high-order reflected modes in conventional single-mode fiber (SMF), these modes were affected by lateral force distributed over a short section of SMF. Here, we analyze a fiber optic detection system based on intensity modulation reflection signal. The force is applied from 17 to 89lbsf, over a SMF section and excitation of high-order modes is analyzed. The system presents good sensitivity around 0.1dB/lbf and linear response in forward and backward direction. This system presents good mechanical strength, flexibility, and temperature-phase independent properties.

New Regime of Plasmonically Induced Transparency

Electromagnetically induced transparency is a phenomenon that drastically alters light-matter interactions such that an optically opaque medium becomes transparent. To date, it is widely accepted that broken symmetry is a necessary and sufficient condition for the realization of electromagnetically induced transparency (EIT) in nano-plasmonic structures. We have confirmed that even without broken symmetry, the plasmonically induced transparency (PIT) effect based on excitation of magnetic plasmon resonance can be created. We also demonstrated that in general, in the system allowing for higher-order mode excitation, broken symmetry may not be a sufficient condition for the realization of the EIT, in that the EIT may disappear even if the symmetry is broken.

Commissioning of the 28 GHz ECRH power transmission line for the TJ-II stellarator

The commissioning of the 28GHz power transmission line of the TJ-II stellarator, designed for the excitation of electron Bernstein waves (EBW) through the O-X-B mode conversion process, is presented in this paper. Based upon a comprehensive set of thermal measurements, its purpose is to go into details about the several problems that arouse during the whole process, namely higher order modes excitation because of the wider beam size and alignment mismatches at the waveguide mouth. All these drawbacks may have prevented the correct O-X mode conversion, thus providing a reasonable explanation for the unsuccessful EBW heating experiments.

Effect of spatial coherence on damage occurrence in multimode optical fibers.

We investigate the influence of spatial coherence on damage occurrence in highly multimode optical fibers using ultraviolet (UV) nanosecond pulses, with the aim of delivering high fluence in the UV. In some cases, the optical damage is initiated below the fiber facet damage threshold and takes place along the propagation path; such damage is believed to be caused by local constructive interference, creating "hot spots." In order to reduce the degree of spatial coherence, we used a large-diameter core (1.5mm) fiber as a mode scrambler. Different lengths of this large core fiber were used to deliver energy to a fiber core with a smaller diameter (0.6mm), in which the damage occurrence was observed. The experimental results indicate that there is a correlation between the degree of spatial coherence and the occurrence of optical damages, typically observed a few millimeters from the fiber facet. Numerical simulations, based on the beam-propagation method, support the degradation of spatial coherence, due to the excitation of high-order modes. Finally, by degrading the spatial coherence of the beam, we establish a new record by delivering more than 100mJ via a 1.5mm core diameter fiber in the UV, corresponding to ∼26 times the critical power for self-focusing. Our work sheds light on the ability to deliver high energies of nanosecond-pulsed UV laser radiation through multimode optical fibers.

Cumulative HOM Excitation and Transition Effects in LCLS-II

LCLS-II is a proposed upgrade of the most powerful X-ray laser in the world built at SLAC National Accelerator Laboratory. When complete, multiple free electron laser (FEL) sections will be powered by a continuous wave superconducting radio frequency (SRF) electron linac. The vital parameters for FEL radiation quality are beam emittance and beam transverse position stability. Excitation of high order modes (HOM) in SRF niobium cavities leads to additional beam power dissipation through incoherent and coherent losses. Energy stored in HOM may cause cumulative effects — transverse and longitudinal position displacement of the center of bunches, which leads to transverse and longitudinal emittance dilution. Cumulative effects due to dipole HOM excitation in LCLS-II are analyzed. Transition HOM effects are caused by sudden changes (periodic or single case) in the beam bunch structure. Bunch center position disturbance due to transition HOM effects is estimated.

Effect of multipole excitations in electron energy-loss spectroscopy of surface plasmon modes in silver nanowires

We have characterized the surface plasmon resonance (SPR) in silver nanowires using spatially resolved electron energy loss spectroscopy (EELS) in the scanning transmission electron microscope. Non-symmetric EELS spectra due to high-k SPR propagation along the nanowire and spectral shifts due to higher-order mode excitation are observed when the beam is positioned near the tip of the nanowire. When the beam is far from the tip region and on the side of nanowire, no spectral shifts are observed as the beam is scanned in the radial direction of the nanowire. The experimental spectra are compared with three different theoretical approaches: direct numerical calculation of the energy loss, analytical models for energy loss, and numerical simulations using an optical model. All three models reproduce the spectral shifts as the electron beam approaches the cap of the nanowire. The analytical model reveals the origin of the shifts in high-order plasmon mode excitation.

Optical trapping in the presence of higher order mode sources and interactions

The force fields produced by higher order mode (HOM) TMen1 sources are investigated analytically and numerically. Their application to optical manipulation and trapping is emphasized. It is found that the strong reactive fields excited by these advanced HOM nanoantennas significantly enhance both the gradient and near-field absorption-scattering forces, as well as increase the range of attraction of those absorption-scattering forces. Moreover, the impact on these force fields by the excitation of HOMs in a nanoparticle is addressed. These results are used to demonstrate the theoretical possibility of the generation of tractor beams by highly subwavelength sources on resonant dipolar nanoparticles. The analysis reveals that the associated attractive forces can be enhanced by several orders of magnitude before the HOMs induce any noticeable degradation of the radiation efficiency. The practical implementation of HOM TMen1 sources by means of active core-shell resonators, as well as their potential for the self-induced trapping of fluorescent molecules, is also addressed.

Broadband absorption enhancement in elliptical silicon nanowire arrays for photovoltaic applications.

Semiconducting nanowire arrays have emerged as a promising route toward achieving high efficiencies in solar cells. Here we propose a perpendicular elliptical silicon nanowire (PEE-SiNW) array for broadband light absorption in thin film silicon solar cells. Simulation results reveal that light absorption enhancement is originated from the split of the principal modes as well as the excitation of high order modes caused by the asymmetry of the elliptical nanowires and the enhanced mode coupling between adjacent elliptical nanowires attained by the appropriate arrangement of nanowires. An ultimate efficiency of 29.1% is achieved for the optimal PEE-SiNW array, which is 16.4% higher than that of the circular SiNW array with the same fill fraction.

Optical beam interactions with a periodic array of Fresnel zone plates

The interactions of first-order elegant Laguerre–Gaussian beams (ELG) with a two-dimensional periodic array are analysed theoretically and numerically. The structure consists of a periodic composition of two-zone Fresnel plates engraved in a silver film. The beam field is composed of periodic sequences of beams of circular or polar polarization incidence upon the structure. The beam axes coincide with the symmetry axes of every fourth Fresnel zone plate placed periodically along two orthogonal coordinates of a horizontal plane of the structure. It is shown that the beam-structure interaction results in substantial cross-polarization coupling, higher-order mode excitation, strong focussing and the extraordinary transmission of the optical field. An interpretation of the results is given per an analogy to the beam-structure interactions observed at planar, homogeneous and isotropic dielectric interfaces and layers.

Selective excitation of higher order modes in hollow-core PCF via prism-coupling.

Prism-coupling through the microstructured cladding is used to selectively excite individual higher order modes in hollow-core photonic crystal fibers (PCFs). Mode selection is achieved by varying the angle between the incoming beam and the fiber axis, in order to match the axial wavevector component to that of the desired mode. The technique allows accurate measurement of the effective indices and transmission losses of modes of arbitrary order, even those with highly complex transverse field distributions that would be extremely difficult to excite by conventional endfire coupling.

Higher order mode excitation for high gain microstrip patch antenna

Higher order mode excitation in the microstrip patch has been achieved to alleviate the low gain problem associated with this particular class of antennas. This higher order mode excitation is achieved by geometric modification of the radiating patch. The designed antenna structure is simple in nature, provides easy fabrication and manufacturing while exhibiting high gain characteristic. The proposed antenna has a measured gain of 10.21dBi at the operating frequency of 16GHz. The half power beam width is approximately 36° and 34° in the H- and E- planes respectively. The antenna can be used in the different applications related to the Ku band.

Inverted-relief cavity used in VCSELs to suppress higher-order transverse modes

One of the most acute VCSEL disadvantages is an excitation of higher-order transverse modes in devices emitting more intense radiation. The paper presents results of simulation of VCSEL performance with the aid of the inverted-relief cavity used to suppress those modes. This suppression method is very efficient and relatively simple from technological point of view, so it is worth to be recommended. Full Text: PDF References C. Jung, R. Jäger, M. Grabherr, P. Schnitzer, R. Michalzik, B. Weigl, S. Müller, K.J. Ebeling, "4.8 mW singlemode oxide confined top-surface emitting vertical-cavity laser diodes", Electron. Lett. 33, 1790 (1997). CrossRef W. Nakwaski, "VCSEL structures used to suppress higher-order transverse modes", Opto-Electr. Rev. 19, 119 (2011). CrossRef R. Sarzała, T. Czyszanowski, M. Wasiak, M. Dems, Ł. Piskorski, W. Nakwaski, K. Panajotov, "Numerical Self-Consistent Analysis of VCSELs", Adv. in Opt. Techn. (2012). CrossRef H. Martinsson, J.A. Vukušič, A. Larsson, "Single-mode power dependence on surface relief size for mode-stabilized oxide-confined vertical-cavity surface-emitting lasers", IEEE Photon. Technol. Lett. 12, 1129 (2000). CrossRef H.J. Unold, S.W.Z. Mahmoud, R. Jäger, M. Grabherr, R. Michalzik, K.J. Ebeling, "Large-area single-mode VCSELs and the self-aligned surface relief", IEEE J. Sel. Top. Quantum Electron. 7, 386 (2001). CrossRef D. Supper, J. Pfeiffer, M. Ilzhöfer, Ch. Giuliani, Ch. Degen, A.P. Lima, A. Ramakrishnan, G. Ebbinghaus, G. Steinle, "Long-wavelength VCSEL with integrated relief for control of singlemode emission", Opt. Comm. 267, 447 (2006). CrossRef

Mode conversion in nonlinear waveguides stimulated by the longitudinal bi-harmonic refractive index modulation

We study specific features of resonant mode conversion in nonlinear waveguides stimulated by the bi-harmonic longitudinal modulation of its parameters, which includes changes of the waveguide depth as well as its bending (in the one-dimensional case) or spiraling (in the two-dimensional case). We demonstrate the possibility of simultaneous excitation of higher-order modes of different parities and topologies with controllable energy weights. The output mode composition is highly sensitive to the variation in the input power and detuning from the resonant modulation frequency.