Suppression Of Higher Order Modes Research Articles

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.

Low-loss single-mode hollow-core fiber with anisotropic anti-resonant elements.

A hollow-core fiber using anisotropic anti-resonant tubes in the cladding is proposed for low loss and effectively single-mode guidance. We show that the loss performance and higher-order mode suppression is significantly improved by using symmetrically distributed anisotropic anti-resonant tubes in the cladding, elongated in the radial direction, when compared to using isotropic, i.e. circular, anti-resonant tubes. The effective single-mode guidance of the proposed fiber is achieved by enhancing the coupling between the cladding modes and higher-order-core modes by suitably engineering the anisotropic anti-resonant elements. With a silica-based fiber design aimed at 1.06 µm, we show that the loss extinction ratio between the higher-order core modes and the fundamental core mode can be more than 1000 in the range 1.0-1.65 µm, while the leakage loss of the fundamental core mode is below 15 dB/km in the same range.

Analysis of the Modal Content Into Large-Mode-Area Photonic Crystal Fibers Under Heat Load

Yb-doped double-cladding photonic crystal fibers are one of the key enabling factors for the development of high-power fiber lasers, due to their capability to provide a very large-mode-area together with the effective suppression of high-order modes, while allowing strong pump absorption and efficient conversion. Thermal effects are currently considered as the main bottleneck for future power scaling; since beyond a certain average power, they allow guidance of high order modes and energy transfer to them, causing a sudden degradation of the beam quality. In this paper, the effects of the heat load on the modes of double cladding fibers are thoroughly analyzed with a full-vector modal solver based on the finite-element method with integrated steady-state heat equation solver. Fibers with different inner cladding designs are compared to provide a deeper understanding of the mechanisms beyond the mode reconfinement and coupling. The influence of the fiber design on the robustness of the single-mode regime with respect to fiber heating has been demonstrated, providing a clear picture of the complex interaction between modes. On the basis of simulation results it has been possible to group fiber modes into three families characterized by peculiar reaction to heating.

Observation of Wakefield Suppression in a Photonic-Band-Gap Accelerator Structure.

We report experimental observation of higher order mode (HOM) wakefield suppression in a room-temperature traveling-wave photonic-band-gap (PBG) accelerating structure at 11.700GHz. It has been long recognized that PBG structures have the potential for reducing long-range wakefields in accelerators. The first ever demonstration of acceleration in a room-temperature PBG structure was conducted in 2005. Since then, the importance of PBG accelerator research has been recognized by many institutions. However, the full experimental characterization of the wakefield spectrum and demonstration of wakefield suppression when the accelerating structure is excited by an electron beam has not been performed to date. We conducted an experiment at the Argonne Wakefield Accelerator test facility and observed wakefields excited by a single high charge electron bunch when it passes through a PBG accelerator structure. Excellent HOM suppression properties of the PBG accelerator were demonstrated in the beam test.

Higher‐order mode absorption measurement of X-band choke-mode cavities in a radial line structure

An experiment is presented to study the higher-order mode (HOM) suppression of X-band choke-mode structures with a vector network analyzer (VNA). Specific radial line disks were built to test the reflection from the corresponding damping load and different choke geometries. The mismatch between the radial lines and the VNA was calibrated through a special multi-short-load calibration method. The measured reflections of different choke geometries showed good agreement with the theoretical calculations and verified the HOM absorption feature of each geometric design.

A Modified Bend-Resistant Multitrench Fiber With Two Gaps

We propose and investigate a modified multitrench fiber (MTF) design with two gaps numerically through the scalar FD-BPM algorithm. Excellent potential in mode area scaling, suppression of high-order modes (HOMs), and resistance to bending is demonstrated. Loss ratio between HOMs and the fundamental mode is more than 50 possibly over a wide range of high-index-ring thickness, gap width, bending radius, and bending orientation. Therefore, the required fabrication accuracy decreases. At 1050-nm wavelength and 20-cm bend radius, large effective mode area (920 μm2 ) is achievable with high loss ratio (>100). The fabricating technologies, carving grooves in the ordinary MTF and inserting rods in these grooves, can be possibly used to fabricate our modified two-gap-MTF.

Frequency multiplexing excitation network for ultra‐wideband coaxial waveguide feeds

A new technique for exciting the TE11 mode in a number of concentric coaxial waveguides is described. The excitation network is log-periodic and is designed to integrate a frequency multiplexing structure together with suppression of higher order modes (TE21 and TE31). The excitation network enables the implementation of very wide band nested coaxial feeds for reflector antennas where multiple waveguides are fed from a common input port. An excitation network providing continuous coverage over a 1–8 GHz frequency range was built and tested in a three-band coaxial waveguide feed.

Higher-order mode suppression in chalcogenide negative curvature fibers.

We find conditions for suppression of higher-order core modes in chalcogenide negative curvature fibers with an air core. An avoided crossing between the higher-order core modes and the fundamental modes in the tubes surrounding the core can be used to resonantly couple these modes, so that the higher-order core modes become lossy. In the parameter range of the avoided crossing, the higher-order core modes become hybrid modes that reside partly in the core and partly in the tubes. The loss ratio of the higher-order core modes to the fundamental core mode can be more than 50, while the leakage loss of the fundamental core mode is under 0.4 dB/m. We show that this loss ratio is almost unchanged when the core diameter changes and so will remain large in the presence of fluctuations that are due to the fiber drawing process.

Suppression of Higher Order Modes in an Array of Cavities Using Waveguides

Suppression of Higher Order Modes in an Array of Cavities Using Waveguides

Suppression of Higher Order Modes in an Array of Cavities Using Waveguides

An application of additional harmonic cavities operating at multiplies of the main RF system frequency of 400MHz is currently under discussionin the framework of the High Luminosity LHC upgrade program [1,2]. A structure consisting of two 800MHz single cell superconducting cavities with grooved beam pipes coupled by drift tubes has been suggested for implementation. However, it is desirable to increase the number of single cells installed in one cryomodule in order to decrease the number of transitions between “warm” and “cold” parts of the collider vacuum chamber. Unfortunately, it can lead to the appearance of higher order modes (HOM) trapped between the cavities. In order to solve this problem the methods of HOM damping with rectangular waveguides connected to the drift tubes were investigated and compared. We describe the results obtained for arrays of 2, 4 and 8 cavitiesin this paper.

Mode characteristics for unidirectional-emission microring resonator lasers

Mode characteristics are investigated numerically and experimentally for microring resonator lasers directly connected to an output waveguide. Optimized structural parameters are proposed for the microring resonators for realizing single-mode and unidirectional-emission operation. Finite-difference time-domain simulations indicate that the inner wall of the microring resonators introduces additional radiation loss and scattering loss, and so suppresses the higher-order transverse modes with higher field distributions in the internal region of the microrings. However, the mode Q factors can be enhanced several times for some modes with suitable inner-wall radii. AlGaInAs/InP microring lasers with inner-wall radii of 0, 5, and 7 μm and an outer-wall radius of 10 μm connected to a 2-μm-wide output waveguide are fabricated by planar techniques. Lasing spectra with longitudinal mode wavelength intervals of 11–12 nm and side-mode suppression ratios of 28–36 dB are observed, and suppression of higher-order transverse modes is achieved as the prediction of the simulation results.

Symmetry-Free Tm-Doped Photonic Crystal Fiber With Enhanced Mode Area

Thulium-doped fiber lasers have recently attracted a growing interest because of the possibility to combine emission around 2 μm with the high beam quality and compactness provided by the fiber medium. A key factor for power scaling of these devices, especially if operation in the short- and ultrashort-pulse regime is desired, is the availability of active fibers capable of joining large effective area and robust single-mode guiding. The strong thermo-optic effect originating from the large quantum defect of Tm ions hinders the possibility to easily find a design with such features, strongly improving the confinement of high-order modes and shrinking the mode area of the fundamental one. In this paper, a rod-type double-cladding photonic crystal fiber with inner cladding properly designed without any mirror symmetry in the cross-section is presented, and its guiding properties are thoroughly analyzed by means of numerical simulations, taking into account the effects of thermally-induced refractive index change. The results have shown that, with a careful choice of the structural parameters, the proposed fiber is capable of ensuring effective high-order mode suppression and effective area up to 3800 μm 2 when operating under heat load of about 300 W/m.

Comparative study of light propagation and single-mode operation in large-mode area fibers designed for 2-μm laser applications

Output performances of fiber-based optical systems, in particular fiber lasers and amplifiers, can be controlled using tailored fiber designs, gain profiles, and pump light overlap with the gain medium. Here, the performances of 2-μm light, propagating in three large-mode area fibers, a step-index fiber, a photonic crystal fiber (PCF), and a leakage channel fiber (LCF), designed to deliver a single-mode (SM) beam at this wavelength, were compared. Using the S 2 imaging technique, the transverse mode content has been decomposed, and propagation losses, SM purity, and mode-field area (MFA) were measured for various input mode overlap and coiling diameters. It was experimentally demonstrated that coiling the PCF and LCF to 40 and 20 cm in diameter, respectively, resulted in efficient higher-order mode suppression, pure SM beam delivery, moderate (∼1 dB) coil-induced losses in the fundamental mode, and nondistorted, large MFA (∼1600 μm 2 ) beam delivery.

Theory of filtered type-II parametric down-conversion in the continuous-variable domain: Quantifying the impacts of filtering

Parametric down-conversion (PDC) forms one of the basic building blocks for quantum optical experiments. However, the intrinsic multimode spectral-temporal structure of pulsed PDC often poses a severe hindrance for the direct implementation of the heralding of pure single-photon states or, for example, continuous-variable entanglement distillation experiments. To get rid of multimode effects narrowband frequency filtering is frequently applied to achieve a single-mode behavior. A rigorous theoretical description to accurately describe the effects of filtering on PDC, however, is still missing. To date, the theoretical models of filtered PDC are rooted in the discrete-variable domain and only account for filtering in the low gain regime, where only a few photon pairs are emitted at any single point in time. In this paper we extend these theoretical descriptions and put forward a simple model, which is able to accurately describe the effects of filtering on PDC in the continuous-variable domain. This developed straightforward theoretical framework enables us to accurately quantify the trade-off between suppression of higher-order modes, reduced purity and lowered Einstein-Podolsky-Rosen (EPR) entanglement, when narrowband filters are applied to multimode type-II PDC.

Novel bending-resistant design of two-layer low-index trench fiber with parabolic-profile core.

A novel design, two-layer low-index trench fiber with parabolic-profile core, is proposed and investigated numerically in this paper. Based on scalar FD-BPM algorithm, the excellent performance over other types of structures and great potential in mode area enlargement are demonstrated. The effective mode area of our design (D = 100μm) is approximately 890 μm2. Both the high order mode (HOM) suppression and bending resistance of our design are better than that of Multi-Trench Fiber (MTF). The mode loss ratio and effective mode area are independent on the bending radius. Due to the circular symmetry of our proposed configuration design, the bending property is not varied with the changing of bending directions.

Optimizing the configuration of a superconducting photonic band gap accelerator cavity to increase the maximum achievable gradients

We present a design of a superconducting rf photonic band gap (SRF PBG) accelerator cell with specially shaped rods in order to reduce peak surface magnetic fields and improve the effectiveness of the PBG structure for suppression of higher order modes (HOMs). The ability of PBG structures to suppress long-range wakefields is especially beneficial for superconducting electron accelerators for high power free-electron lasers (FELs), which are designed to provide high current continuous duty electron beams. Using PBG structures to reduce the prominent beam-breakup phenomena due to HOMs will allow significantly increased beam-breakup thresholds. As a result, there will be possibilities for increasing the operation frequency of SRF accelerators and for the development of novel compact high-current accelerator modules for the FELs.

Analysis of structurally sensitive loss in GaN-based VCSEL cavities and its effect on modal discrimination

Lateral loss causes optical energy to leave the laser cavity in the transverse, lateral, direction, and is sometimes neglected to simplify the numerical simulations. However, in contrast to outcoupling and absorption losses, we show that the lateral loss can change drastically with only nanometer-sized changes of the cavity structure, from being virtually zero to becoming the major source of cavity loss, since the cavity becomes antiguiding. This can be explained as the opening of a channel of efficient resonant lateral leakage of optical power at a certain oblique propagation angle. A number of different realizations of current apertures and top mirror designs in GaN-based VCSEL cavities, which have been suggested for realization of microcavity lasers emitting in the blue wavelength range, are simulated. Many of these are shown to lead to unintentional antiguiding, which can more than double the threshold gain for lasing. Notably, for strong enough antiguiding the resonant lateral leakage decreases so that the threshold gain values might again be tolerable. This regime has been suggested for robust single-mode operation since earlier predictions, building on analogies with slab waveguides, hinted at a very strong suppression of higher order modes. However, our simulations indicate that for the VCSEL cavities the derived formulas grossly overestimate the modal discrimination.

An investigation on optimum ridge width and exposed side strips width of terahertz quantum cascade lasers with metal-metal waveguides

The impacts of side exposed side strips (for high order modes suppression) and ridge width on terahertz (THz) quantum cascade laser (QCL) performance are investigated through numerical modeling and verified experimentally. Our results show that shrinking ridge width of THz QCLs with metal-metal waveguides leads to a substantial degradation of device performance due to higher optical loss resulting from the side-exposed strips in the highly-doped top contact layer. Nevertheless, the side-exposed strips facilitate single mode operation by strongly suppressing higher-order modes. An optimal width of the side exposed strips is obtained for achieving adequate higher-order mode suppression and maintaining sufficiently low fundamental mode loss.

Suppression of Higher-Order Lateral Modes in Broad-Area Diode Lasers by Resonant Anti-Guiding

For a maximum fiber-coupled power, high power broad-area diode lasers must operate with a small lateral far-field divergence at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with a low beam quality and wide emission angles. We present a new approach to suppress higher-order lateral modes based on an anti-guiding layer with a high refractive index inserted at the edges of the active stripe. Simulations of planar waveguides containing germanium layers reveal that strong mode-coupling effects occur. These are found to vary as a function of the thickness of the inserted layer. By embedding the germanium layer in the outer region of a ridge-waveguide, the mode coupling effects result in a reduction of the modal gain of higher-order lateral modes. The lateral far-field divergence of fabricated 90- μm stripe lasers emitting at 980 nm and containing such an anti-guiding layer is narrowed by 3° at a CW output power of 10 W.

High-Power Low-Pass Harmonic Filters With Higher-Order ${\rm TE}_{{\rm n}0}$ and Non-${\rm TE}_{{\rm n}0}$ Mode Suppression: Design Method and Multipactor Characterization

In this paper, a method to design high-power low-pass harmonic filters in rectangular waveguide technology is proposed. The new filters consist of a collection of smooth E-plane bandstop elements along the propagation direction and a smooth variation of the filter width. This yields to a broad rejected band for the fundamental TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> mode, together with higher-order ( TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n0</sub> and non- TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n0</sub> ) mode suppression. Two different examples with stringent requirements of the space industry are provided to demonstrate the capabilities of the new methodology. By means of high-power simulations and an extensive measurement campaign, it will be shown that the smoothness of the filter profile guarantees high-power operation even with small minimum mechanical gaps. Moreover, unlike classical techniques, our method is not restricted to filters with small gaps. Hence, filters with larger gaps (always fulfilling the demanding frequency specifications) are fabricated for even higher power-handling performance.