Mode Field Patterns Research Articles

Compact cyclic fiber three-mode converter based on mechanical fiber grating.

In this Letter, a compact cyclic mode converter (CMC) based on a mechanical fiber grating is proposed and fabricated to eliminate differential mode group delay and mode-dependent loss in the mode division multiplexing (MDM) transmission system. The proposed CMC can realize cyclical interchange of any input mode among the LP01/LP11a/LP11b modes, which requires only one mechanical grating. The mode conversion is evaluated by observing the mode field patterns of the fiber output. The experimental results prove that the introduction of CMC does not significantly degrade the transmission performance of the photonic lanterns back-to-back system. The insertion loss and the average cross talk of the whole system are lower than 5 dB and -11.3 dB, respectively. The proposed CMC provides a new method for reducing link damage in the MDM transmission system.

Optimizing two-dimensional polarization-diversity metagrating couplers for silicon photonics

Polarization-diversity couplers are promising industrially scalable optical devices that can couple optical signals with unknown polarization states into silicon photonic chips. Here we propose a novel, to the best of our knowledge, two-dimensional (2D) metagrating coupler (MGC) for the polarization-diversity system, which improves its coupling efficiency for both polarizations. Compared with the previously reported 2D grating couplers (GCs) with identical grating cells, the proposed apodized design gradually changes the aspect ratio and orientation angles of the elliptical patterns simultaneously. This 2D array of the varied grating cells modulates the diffracted mode field patterns and phases locally, and achieves a better overlap with the Gaussian fiber modes for both polarizations. The peak coupling loss (PCL) for the S-polarized and P-polarized light are − 1.6 d B and − 2.7 d B , respectively. The calculated polarization dependent loss is lower than 0.2 dB from 1522 nm to 1540 nm. The device is robust to the fabrication tolerance and fiber misalignment. The simulated fabrication tolerance analysis show that the etch depth error of ±20 nm results in less than 0.3 dB and 0.1 dB PCL drop, while the grating cell feature size error of ± 40 n m results in less than 0.6 dB and 0.2 dB PCL drop for both types of polarization light, respectively. The fiber misalignment should be within the range from − 2 µ m to + 2 µ m in two perpendicular directions, and the coupling angle deviation should be within the range from − 3 ∘ to + 3 ∘ , in order to assure 0.5 dB penalty loss for both polarizations. The performance of the design is insensitive to the mask misalignments. To assure 0.5 dB penalty loss for both polarizations, Δ θ is suggested to be within the range from − 3 ∘ to + 3 ∘ .

Circular-lattice microstructured optical fibers with near-zero ultra-flattened chromatic dispersion and high birefringence

In the present study, a dispersion flattened microstructured optical fiber (MOF) with extremely low dispersion value and high birefringence is proposed. The proposed MOF is composed by manipulating the number of air-holes, presents in different rings of a regular circular-lattice MOF. The dispersion and birefringence characteristics of the proposed MOF are numerically analyzed by applying finite difference field convergence approach. Moreover, the other light-guiding characteristics, e.g., fundamental mode-field pattern, effective refractive index, effective mode-field area and nonlinear characteristics of the fiber are explored as well. Numerical results show that flattened dispersion around 0 ± 1.3 ps/(nm.km) can be reached inside the spectral range 1120–2100 nm bounding a huge bandwidth of 980 nm. Numerical results also show that birefringence with the order of 10-4 can be obtained at 1550 nm.

Manipulation of lasing modes in a circular-side octagonal microcavity laser with a spatially distributed current injection.

We propose and demonstrate a circular-side octagonal microcavity (COM) semiconductor laser with a spatially distributed current injection for manipulating the lasing modes. There are two types of high-quality-factor whispering-gallery (WG) modes with distinct field patterns in a COM: the four-bounced quadrilateral modes and the eight-bounced octagonal modes. By designing two separated p-electrodes, the COM laser is divided into two regions that are pumped independently to select specific modes for lasing. The two types of WG modes lase simultaneously when the two regions are injected with equivalent currents. Degeneracy removal of the quadrilateral modes is observed in both simulation and experiment when the two regions are injected with inequivalent currents. The quadrilateral modes are suppressed when one of the two regions is un-injected or biased with a negative current, and single-octagonal-mode lasing is realized. The results show that the lasing modes can be efficiently manipulated with the spatially distributed current injection considering the distinct field patterns of different WG modes in the microcavities, which can promote the practical application of the microcavity lasers.

Spatial-light-modulator-based optical-fiber joint switch for few-mode multicore fibers.

To realize simplified cost-efficient optical networks with routing flexibility and scaling potential, a spatial-light-modulator-based optical-fiber joint switch for few-mode multicore fibers is proposed herein, which can route all spatial channels together as a unit. Numerical simulations and experiments were performed, and the results show that the signal paths for a 6-mode 19-core fiber can be simultaneously switched to the target output ports using the proposed method, and the mode-field patterns of the diffracted light can be maintained after joint switching. Further, the maximum port crosstalk can be reduced considerably from -11.6 to -25.1 dB by changing the position of the output port in the proposed method.

Mode control through anti-Hermitian coupling in regular-polygonal microcavities with non-uniform gain and loss.

We theoretically and numerically study optical modes in regular-polygonal microcavities with non-uniform gain and loss, where high quality (Q) whispering-gallery-like modes typically appear as superscar states. High Q superscar modes can be described by the propagating plane waves in an effective rectangle formed by unfolding the periodic orbits and exhibit regular and predictable spatial field distributions and transverse-mode spectra. With non-uniform gain and loss, anti-Hermitian coupling between the transverse modes with close frequencies occurs according to the mode coupling theory, which results in novel mode properties such as modified mode spectra and field patterns, and the appearance of exceptional points. Numerical simulation results are in good agreement with the theoretical analyses, and such analyses are also suitable for other kinds of high Q microcavities with non-uniform gain and loss. These results will be highly useful for studying non-Hermitian physics in optical microcavities and advancing the practical applications of microcavity devices.

Widely tunable single-mode lasers based on a hybrid square/rhombus-rectangular microcavity

Hybrid square/rhombus-rectangular lasers (HSRRLs) consisting of a Fabry–Perot (FP) cavity and a square/rhombus microcavity (SRM) are proposed and demonstrated for realizing single-mode lasing with a wide wavelength tuning range. The SRM is a deformed square microcavity with a vertex extended to the FP cavity to control the coupled mode field pattern in the FP cavity. Single-mode operation with a side-mode suppression ratio (SMSR) over 45.3 dB is realized, and a wide wavelength tuning range of 21 nm with SMSR >35 dB is further demonstrated by adjusting the injection currents of the SRM and the FP cavity simultaneously. Furthermore, a 3-dB modulation bandwidth of 14.1 GHz and an open-eye diagram at 35 Gb/s are demonstrated for the HSRRL.

Study of hole and slot type S-band pulsed power magnetron

This paper deals with the electromagnetic design of a high power pulsed magnetron capable of delivering 3.25 MW output power at operating frequency 2.998 ± 0.005 GHz using CST microwave studio. Eigen mode solver is used to find out the cold frequencies and RF field patterns of different modes of the magnetron. A capacitive type tuning mechanism is included in the magnetron simulation model to study the frequency tuning range. The cold test simulation of complete magnetron model with straps, output window, coupling loops and output coupler is carried out to compute the resonance spectrum, quality factor and circuit efficiency using frequency domain solver of CST microwave studio. Particle in cell simulation of the magnetron predicts output power, anode current and efficiency which are 3.25 MW, 117A and 56%, respectively, at operating voltage 50 kV and magnetic field of 1630 Gauss. A complete virtual prototype magnetron both in cold and hot conditions is demonstrated on computer.

Field stabilization of Alvarez-type cavities

Alvarez-type cavities are commonly used to reliably accelerate high quality hadron beams. Optimization of their longitudinal field homogeneity is usually accomplished by post-couplers, i.e. additional rods being integrated into the cavity. This paper instead proposes to use the stems that keep the drift tubes for that purpose. As their individual azimuthal orientations do not change the cavity's undisturbed operational mode, they comprise a set of free parameters that can be used to modify higher mode field patterns. The latter have significant impact on the robustness of the operational mode w.r.t. eventual perturbations. Several optimized stem configurations are presented and benchmarked against each other. The path to obtain these configurations is paved analytically and worked out in detail through simulations. It is shown that the method provides for flat field distributions and very low field tilt sensitivities without insertion of post-couplers.

Designing high-performance fiber laser based on ring structured PCF host

The tight modal confinement property with circularly symmetric fundamental mode-field pattern of a ring structured photonic crystal fiber (PCF) is utilized to improve the laser threshold and output characteristics of existing fiber lasers. We perform a detailed study of continuous-wave lasing characteristics in such erbium-doped fiber host by varying associated geometrical and optical parameters towards achieving a high performance and short length fiber laser. A finite difference mode-convergence analysis is implemented to determine the required modal properties of the guiding host, which is then used to solve standard propagation rate equations. The laser properties of the structure considered is compared with equivalent regular triangular-lattice photonic crystal fiber illustrating a significant improvement in the characteristics of fiber lasers reported so far.

Study on the similarity of photonic crystal ring resonator cavity modes and whispering-gallery-like modes in order to design more efficient optical power dividers

In this study, we discuss the wave pattern characteristics of confined cavity modes inside photonic crystal ring resonators (PCRRs). Despite different physical origins, these cavity modes are analogous to the whispering-gallery (WG-like) modes. Because of the absence of perfectly circular symmetry in our PC cavity, the WG-like modes are not degenerate, but they form a close doublet in which the field pattern of each of these doublets repeats itself by $$180^{\circ }/m$$180?/m where "m" is the azimuthal index number of WG-like modes. The cavity modes are named according to their symmetric field pattern, and WG-like ones are named by their azimuthal mode numbers "m." Based on the study of these similarities, we propose a $$1\times 2$$1×2 and a $$1\times 4$$1×4 T-junction- like power dividers. Through study of the similar properties of these cavity modes with WG-like modes, we have enhanced the output power of both dividers to 49.6 and 24.5 %, respectively. The permittivity of the hexagonal photonic crystal rod is $$\varepsilon _{h}= 12.04$$?h=12.04 surrounded by air as the background medium. For our PCRR, the dominant resonating mode is the cavity mode with hexapole field pattern or equivalently WG-like mode with azimuthal mode number of $$m\,=\,6$$m=6. The normalized frequencies for this mode and its doublet are $$a/\lambda = 0.348066$$a/?=0.348066 and 0.348301, respectively. The photonic band structure, PC waveguide guided mode and electric field patterns of the confined cavity modes inside the PCRR are calculated using the PWE method, and the transmission spectra are calculated by means of 2D-FDTD simulation method.

Observation of cladding modes spatio-spectral distribution in large mode area photonic crystal fiber

We report the observation of spatio-spectral distribution in cladding modes of a single-mode large mode area photonic crystal fiber. The cladding modes excitation was achieved without any external fiber exposure. The optical field patterns of the cladding modes within different pump wavelength are investigated. To the best of knowledge the spatio- spectral distribution in cladding modes of large mode photonic crystal fiber is demonstrated for the first time. The results are of immediate interest in applications demanding devices based on core and cladding mode coupling in photonic crystal fibers.

Matched feed design employing TE01 and TM11 modes in a smooth walled rectangular waveguide for cross-polar reduction in offset reflector antenna systems

Cross polar performance of an offset reflector antenna fed by a smooth walled rectangular matched feed employing TE01 and TM11 modes is presented. When studied with analytical mode field patterns, TE01+TM11 combination is found to give better cross-polar performance compared to combinations like TE01+TE11 and TE01+TE11+TM11. Physical optics (PO) has been used to compute secondary pattern. Mode coefficients and relative phases of different modes are evaluated using Particle Swarm Optimization (PSO) technique or contour plot. A shifted waveguide junction is used to generate the TM11 mode for realization of the matched feed. The desired mode coefficients are realized by proper choice of shifted waveguide dimensions and relative phase is maintained by proper choice of waveguide lengths. The performance evaluation of the complete antenna system is carried out using HFSS. A −30dB cross-polar bandwidth of 2.16% is achieved.

A Novel Matched Feed Structure for Achieving Wide Cross-polar Bandwidth for an Offset Parabolic Reflector Antenna System

This letter presents the details of a novel matched feed structure which promises improved cross-polar performance for an offset parabolic reflector antenna. Feed aperture formed by an intersection of circular and rectangular waveguides provides wide cross-polar bandwidth due to the cutoff wave numbers being close for the two modes used in conjugate matching. The mode coefficients and relative phase difference are estimated using particle swarm optimization (PSO) technique for the specified antenna system. Physical optics (PO) is used to evaluate the reflector pattern; feed radiation pattern is obtained using mode field patterns and cutoff wave numbers, which are computed using 2-D finite element method (FEM). The higher order mode which is combined with the main operating mode of the waveguide for conjugate matching is generated using two posts placed behind the aperture. The antenna system performance is evaluated using HFSS and CST Microwave Studio.

Dynamic characteristics of AlGaInAs/InP octagonal resonator microlaser

The static and dynamic characteristics are investigated for an AlGaInAs/InP octagonal resonator microlaser with a side length of 10.8 μm and a 2-μm-wide vertex output waveguide. The scattered light is observed by top-view infrared microscopic image and compared with simulated mode field pattern. Single-mode operation with the side-mode suppression ratio of 42 dB is achieved. Small-signal modulation responses with a 3-dB bandwidth of 15.7 GHz are reached at 287 K, and a clear eye diagram is demonstrated at 15 Gbit/s.

Eight channel transmit array volume coil using on-coil radiofrequency current sources.

At imaging frequencies associated with high-field MRI, the combined effects of increased load-coil interaction and shortened wavelength results in degradation of circular polarization and B1 field homogeneity in the imaging volume. Radio frequency (RF) shimming is known to mitigate the problem of B1 field inhomogeneity. Transmit arrays with well decoupled transmitting elements enable accurate B1 field pattern control using simple, non-iterative algorithms. An eight channel transmit array was constructed. Each channel consisted of a transmitting element driven by a dedicated on-coil RF current source. The coil current distributions of characteristic transverse electromagnetic (TEM) coil resonant modes were non-iteratively set up on each transmitting element and 3T MRI images of a mineral oil phantom were obtained. B1 field patterns of several linear and quadrature TEM coil resonant modes that typically occur at different resonant frequencies were replicated at 128 MHz without having to retune the transmit array. The generated B1 field patterns agreed well with simulation in most cases. Independent control of current amplitude and phase on each transmitting element was demonstrated. The transmit array with on-coil RF current sources enables B1 field shimming in a simple and predictable manner.

Investigations on high speed directly modulated microdisk lasers accounting for radial carrier hole burning

High-speed modulation characteristics are investigated for microdisk lasers theoretically and experimentally. In rate equation analysis, the microdisk resonator is radially divided into two regions under uniform carrier density approximation in each region. The injection current profile, carrier spatial hole burning, and diffusion are accounted for in the evaluation of small-signal modulation curves and the simulation of large-signal responses. The numerical results indicate that a wide mode field pattern in radial direction has merit for high-speed modulation, which is expected for coupled modes in the microdisk lasers connected with an output waveguide. For a 15-μm-radius microdisk laser connected with a 2-μm-wide output waveguide, the measured small-signal response curves with a low-frequency roll-off are well in agreement with the simulated result at a 2-μm radial width for the mode intensity distribution. The resonant frequencies of 7.2, 5.9, and 3.9 GHz are obtained at the temperatures of 287, 298, and 312 K from the small-signal response curves, and clear eye diagrams at 12.5 Gb/s with an extinction ratio of 6.1 dB are observed for the microdisk laser at the biasing current of 38 mA and 287 K.

Influences of carrier diffusion and radial mode field pattern on high speed characteristics for microring lasers

High-speed directly modulated microlasers are potential light sources for on-chip optical interconnection and photonic integrated circuits. In this Letter, dynamic characteristics are studied for microring lasers by rate equation analysis considering radial carrier hole burning and diffusion and experimentally. The coupled modes with a wide radial field pattern and the injection current focused in the edge area of microring resonator can greatly improve the high speed response curve due to the less carrier hole burning. The small-signal response curves of a microring laser connected with an output waveguide exhibit a larger 3 dB bandwidth and smaller roll-off at low frequency than that of the microdisk laser with the same radius of 15 μm, which accords with the simulation results.

Realisation of controllable transmission zeros by perturbation technique for designing dual‐mode filter using substrate integrated hexagonal cavity

Novel designs of dual-mode substrate integrated hexagonal cavity (SIHC) bandpass filters are proposed in this study. Different filters responses in terms of switching transmission zeros are realised using perturbation technique. With the variation of perturbed length, the change of modal resonant characteristics and field patterns of the degenerate modes are also studied. Two pole and four pole filters with transmission zeros are designed and fabricated. All the theoretical analysis with the help of coupling matrix are verified with experimental results, showing good agreement between them.

Modulation of Junction Defects Created by Crossing Photonic Crystal Waveguides

In optical circuits, functional elements are often placed on waveguide junctions. We show that a junction in photonic crystals (PCs) possesses localized modes very close to the bandpass of waveguides. Their mode resonances and field patterns can be controlled by modulating the PC waveguides. It is expected that functions such as switching and routing could be developed naturally by using junction modes without any special design of microcavities. Our calculation results can be explained by the coupled mode theory.