Mode Suppression Research Articles

4*10 Gbps WDM communication system based on a tunable V-cavity semiconductor laser.

This paper is about the V-cavity tunable semiconductor laser with a 1550 nm band used as a transmitter to build a wavelength division multiplexing (WDM) optical fiber communication link. In the experiment, a 20 km optical fiber communication link with a reasonable eye diagram and low bit error rate (BER) transmitted by 40 Gbps can be established. The experimental results show that a single laser can achieve a wavelength tuning range of 25 nm, reach 32 channels at a 100 GHz frequency interval, and the average side mode suppression ratio (SMSR) is above 39 dB. The advantages and application potential of V-cavity tunable semiconductor laser (VCL) for wavelength routing in optical communication networking are verified by experiments.

Searching for magnetic fields in pulsating A-type stars: the discovery of a strong field in the probable δ Sct star HD 340577 and a null result for the γ Dor star HR 8799

ABSTRACT Numerous δ Sct and γ Dor pulsators are identified in the region of the Hertzsprung–Russell diagram that is occupied by chemically peculiar magnetic Ap stars. The connection between δ Sct and γ Dor pulsations and the magnetic field in Ap stars is however not clear: theory suggests for magnetic Ap stars some critical field strengths for pulsation mode suppression by computing the magnetic damping effect for selected p and g modes. To test these theoretical considerations, we obtained PEPSI spectropolarimetric snapshots of the typical Ap star HD 340577, for which δ Sct-like pulsations were recently detected in Transiting Exoplanet Survey Satellite data, and the γ Dor pulsator HR 8799, which is a remarkable system with multiple planets and two debris discs. Our measurements reveal the presence of a magnetic field with a strength of several hundred Gauss in HD 340577. The measured mean longitudinal field would be the strongest field measured so far in a δ Sct star if the pulsational character of HD 340577 is confirmed spectroscopically. No magnetic field is detected in HR 8799.

A New Four-Step Commutation Method for Indirect Matrix Converter With Active-Voltage Vectors Being Applied

The existing two commutation methods of the rectifier stage for indirect matrix converter (IMC) are zero-current commutation and four-step commutation. The premise of zero-current commutation is that the inverter stage of IMC uses a zero-voltage vector, but when the inverter stage cannot use zero-voltage vectors (such as space overmodulation, common mode suppression modulation, etc.), the four-step commutation is adopted. The conventional four-step commutation method has a complex switching sequence and requires additional sensors. Based on the natural freewheeling characteristic of IMC and the switching sequence of zero-current commutation, a new four-step commutation method is proposed when active-voltage vectors are applied in the inverter stage, first turn off the two switches at 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> and 2 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">nd</sup> steps, and then turn on the two switches at 3 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">rd</sup> and 4 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sup> steps. The switching sequence of the new four-step commutation is not affected by the polarity of input voltage or output current direction, thus solving the complexity switching sequence problem of the conventional four-step commutation, and does not require additional voltage or current sensors, making the IMC commutation process safer and more reliable. Finally, this paper verifies the effectiveness of the new four-step commutation method of IMC through experiments.

Single-Mode-VCSEL With a Ring-Shaped Self-Aligned Recessed Metal Mode Filter

This article presents a single-mode vertical-cavity surface-emitting laser (VCSEL) using a novel ring-shaped self-aligned recessed metal (SARM) mode filter on the emitting window as a means of transverse mode control. The SARM structure acts as a spatial filter to suppress the higher-order transverse modes through selective recessed-etching of the top mirror and the self-aligned deposition of the p-type contact metals for cavity tuning and blocking the higher-order modes. The SARM mode filter could be a potentially high-yield alternative technique for the mass production of single-mode VCSELs, demonstrating a single-mode emission with 1.55-mW power and a 30.7-dB side mode suppression ratio.

High-Order Mode Suppression in Sapphire Fiber Bragg Gratings Based on Mode Field Matching Fusion Splicing for Single-Mode Demodulation Systems

High-Order Mode Suppression in Sapphire Fiber Bragg Gratings Based on Mode Field Matching Fusion Splicing for Single-Mode Demodulation Systems

All-Fiber 770 nm Laser Stabilized With Bichromatic Absorption Spectroscopy

A high-power, coherent, and frequency-stable 770 nm laser is vital for experiments in atom physics. This paper presents an all-fiber 770 nm laser based on second harmonic generation (SHG) and frequency-stabilized using the R2 absorption line of H <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> C <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">14</sup> N. This stabilization technique utilizes the acoustic effect to generate a dispersion-like error signal with a steep slope. Moreover, a cascaded gas cell configuration is utilized to double the frequency locking range. After frequency stabilization, the frequency offset has been effectively shifted to close zero. The laser provides an output power of 192 mW with a side mode suppression ratio (SMSR) larger than 35 dB, and a significant frequency noise suppression over 30 dB in the range of 10 to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Hz. The integral linewidth of the 770 nm laser is 28.1 kHz, which outperforms most semiconductor lasers. The proposed all-fiber configuration offers a new compact laser source potentially useful for quantum measurement.

Supersymmetry Laser Arrays with High‐Order Exceptional Point

Supersymmetry (SUSY) laser array with superpartner structure can suppress excess modes to achieve high‐intensity and high‐coherent radiation. Compared with complex superpartner, using parity time (PT) symmetry broken to manipulate SUSY laser arrays is a more flexible approach. Herein, based on ultrathin perovskite single crystal, a SUSY laser array is constructed without superpartner, but with auxiliary gain–loss structure. PT symmetry broken with high‐order exceptional point (EP) is realized in the visible spectral range. It intrinsically avoids superpartner field mismatch and improves side mode suppression ratio 8.8–12 dB for single‐mode lasing. Furthermore, variations in EP‐order enable the transition from single‐mode to dual‐mode or tri‐mode radiation and retain amplified radiation characteristics. In addition, these SUSY laser arrays with the high‐order EP have the potential to be small‐volume optical sensor devices. The new design combining SUSY and PT symmetry broken presents its potential in micro‐nanophotonic devices with the benefit of small size, flexibility in spectral control, expandability, simplicity, and multifunctionality feature.

Design of distributed feedback lasers grown on Si using time-domain traveling wave model combined with mode refractive index method

This paper proposes an optimization method combining the time-domain traveling wave model and the mode refractive index method, employed for characterizing both the transverse and longitudinal modes of quantum dot distributed feedback (DFB) lasers grown on Si. We use this method to optimize the overall performance of the Si-based DFB laser, and determine the material and chip structural parameters, including the ridge width, etching depth, grating thickness and grating position as optimization parameters. Here, the optimal DFB laser operating under fundamental transverse and single longitudinal mode is obtained. Its threshold current is as low as 5 mA, the slope efficiency is as high as 0.77 mW mA−1, and the side mode suppression ratio is up to 48 dB. When the injection current is 150 mA, the output power exceeds 100 mW. The corresponding ridge width, etching depth and grating thickness are 2 μm, 1.3 μm and 20 nm, respectively. The distance from the grating to the active region is 200 nm. Therefore, the novel method presented in this paper offers an effective scheme for the design of DFB lasers grown on Si with excellent performance.

Gyrokinetic simulation of short wavelength ion temperature gradient instabilities in the ADITYA-U tokamak

In this work, linear and nonlinear collisionless electrostatic simulation studies of the standard and short wavelength ion temperature gradient mode (SWITG) for experimental profiles and parameters of ADITYA-U tokamak are performed using the linear global eigenvalue gyrokinetic code GLOGYSTO and the nonlinear global gyrokinetic particle-in-cell code ORB5. All simulations are carried out with non-adiabatic ions and adiabatic electrons. The ADITYA-U tokamak which has recently been upgraded to divertor configuration, is small in size and well suited for investigation of micro-instabilities in the presence of density and temperature gradients. Due to steep density and temperature gradients, simulation shows that the SWITG mode naturally exists along with the standard ion temperature gradient (ITG) mode in ADITYA-U. In this work, the experimental shot# 33536 of the ADITYA-U tokamak is considered as a reference. There is good agreement in the growth rate and the real frequency values between GLOGYSTO and ORB5 with variations of less than . Two maxima of growth rate versus mode number are obtained, the first around is the standard ITG, the second around is the SWITG. Additionally, using linear stability analysis, it is observed that the SWITGs are suppressed for low values of i.e. only the standard ITG mode remains unstable. For the ADITYA-U tokamak, nonlinear global simulations with ORB5 are also carried out. Nonlinearly, SWITG dominating case results are compared with the conventional ITG case, where SWITG is relatively suppressed. The nonlinear contribution of the SWITG mode to the total thermal ion heat transport is found to be minimal due to an increased zonal flow shearing effect on the SWITG mode suppression, even though it may be linearly more unstable than the conventional long wavelength () ITG mode.

Research on Dissipation Dilution Mechanism and Boundary Dissipation Suppression Technique for High-Stress Graphene Nanoelectromechanical Resonator

The low-quality factor is a key bottleneck for the engineering and commercial application of graphene nanoelectromechanical resonators at room temperature. The hypothesis in dissipation dominated by the ohmic loss is difficult to cover this phenomenon. Mechanical loss may still be on the list of the main causes for the quality factor stress-modulation characteristics of graphene resonators. The dissipation dilution theory reveals the intrinsic energy and dissipation mechanism of the traditional high-stress silicon-based resonator, which may also be applied to two-dimensional (2D) materials if dominated by mechanical loss. Based on Zener’s model of anelasticity, combined with the edge-corrected mode shape, the stress dilution mechanism of the bending potential dissipation of the graphene resonator is revealed. On this basis, the resonator dissipation is decomposed into boundary dissipation and non-boundary dissipation parts, and the steep rise phenomenon of the bending dissipation density (curvature) in the boundary region is analyzed through theoretical calculation. The analysis reveals that boundary dissipation is dominant in bending dissipation. To effectively suppress the boundary dissipation, a novel design of a graphene resonator via soft-clamped phononic crystal (PnC) is proposed. The existence of localized mode (LM) and effective suppression of boundary dissipation are verified in the simulations of both triangular and honeycomb PnC lattices. The theoretical model developed in this paper provides a new window into the dissipation properties of graphene nanoelectromechanical resonators, and the design of graphene resonators via soft-clamped PnC is expected to provide a new route toward high-quality factors at room temperature.

A New Design of Filtering Patch Antennas With Enhanced Bandwidth and Harmonic Suppression

A new design method of filtering circular patch antenna with good harmonic suppression is proposed in this paper. According to the analysis of field distributions of the resonant modes on circular patch, we initially reveal the mechanism of high-order modes suppression by introducing a pair of common mode (CM) exciting signals with equal amplitude and same phase at specific locations on the circular patch antenna. Afterwards, to implement this design concept, the exciting signals realized by a T-shaped microstrip feed line with two pairs of extra parallel arms are arranged on both sides of the circular patch antenna. Owing to this excitation scheme, the influence of harmonics of the antenna has been reduced and two zeros on both sides of the passband are achieved by the cross-coupling form. Besides, an enhanced bandwidth is also realized attributing to an additional resonant mode introduced by the extra parallel arms. Finally, in order to verify the feasibility of the developed scheme, a prototype filtering antenna is fabricated and tested. Measured results show that the demonstrator exhibits an impedance bandwidth of 10.2% (2.42 GHz-2.68 GHz). Inside the passband, the radiation patterns keep basically consistent and the realized gain (at Φ = 0° and θ = 0°) is stable at 7.5 dBi. Compared with the traditional filtering antenna, the harmonic suppression bandwidth in this work ranges to 3.2 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> is the center frequency of the antenna) while maintaining a low profile.

High‐Power, Narrow‐Linewidth, and Low‐Noise Quantum Dot Distributed Feedback Lasers

AbstractSingle‐frequency semiconductor lasers represent a critical role in optical communications, light detection and ranging systems, photonics integrated circuits, etc. Here, combining atom‐like quantum dot (QD) materials and advanced lateral gratings, a high‐power, ultra‐low‐noise 1.3 µm InAs/GaAs QD distributed feedback laser is demonstrated. Stable single‐longitudinal‐mode output power of 100 mW from 25 ℃ to 85 ℃ is achieved with a maximum side mode suppression ratio of 62.6 dB, and the variations of threshold current and slope efficiency over the temperature range are slight, indicting a high temperature stability. A record‐narrow intrinsic linewidth of 1.62 kHz is achieved at 55 ℃ with a white noise level of merely 515 Hz2 Hz–1, and a minimum averaged relative intensity noise of only –166 dB/Hz between 0.1 GHz and 20 GHz is obtained at 25 ℃. Furthermore, a strong tolerance to external optical feedback (〉 –14 dB) is demonstrated in the range from 25 ℃ to 85 ℃, with a maximum value of –8 dB at 85 ℃. This high‐quality single‐frequency laser fabricated with simplified processes and compact size paves the way for its future large‐scale applications such as high‐capacity optical communication, high‐precision optical detection, high‐speed optical interconnections, etc.

Analytical model of direct-Bragg, exchange-Bragg, and evanescent coupling in a dual-waveguide structure and its application to single-mode lasing

Single-mode distributed-feedback lasing is explored for an asynchronous dual-waveguide uniform-grating structure that exhibits exchange-Bragg and evanescent coupling between optical modes of the two waveguides as well as direct-Bragg coupling between optical modes within each waveguide. Coupled-mode equations governing all coupling processes are presented and solved, yielding an analytical solution that is applicable to a variety of single-mode lasing mechanisms. We leverage this solution to elucidate the lasing mechanism in which the photonic bandgap (PBG) due to exchange-Bragg coupling is aligned to and suppresses a degenerate lasing mode associated with the direct-Bragg PBG of the active waveguide. PBG alignment and mode suppression are achieved for a range of values of Henry’s alpha, direct-Bragg coupling, exchange-Bragg coupling, and evanescent coupling that yield at-threshold gain margin and longitudinal power flatness superior to those of the benchmark λ/4-shifted DFB lasing structure. Such performance includes a gain margin ΔαL=0.88 for a minimum flatness F=0.0006, enabled by the power-transfer action of evanescent coupling. The dual-waveguide geometry and superior predicted performance make the structure compelling for both III-V and heterogeneous III-V-on-silicon platforms.

A Filter-Free Third Overtone Quartz Oscillator Based on Micromachining Technology.

This work investigates an AT-cut quartz crystal oscillator (XO) operating at its enhanced third overtone thickness shear (TS) mode while suppressing the fundamental TS mode by micromachining technology. A unique quartz plate structure has been proposed to alleviate the strong resonant signal from the first mode. A finite-element method (FEM) was used to characterize the first mode suppression while improving the performance of third overtone in terms of Q -factor and motional resistance ( Rm ) of the proposed device. In measurement, the unloaded Q -factor of the fundamental TS mode is from several tens of thousands (the reference design) to only several hundred (the proposed design). Meanwhile, the unloaded Q -factor of the third overtone response for the proposed design could reach Q ∼95 000 with a decent Rm . The Q -factor ratio attains 170-180 between the third overtone and fundamental modes. With the feature of low Q -factor on the fundamental mode, the oscillator system could successfully oscillate at a higher frequency without the help of bandpass filtering. Finally, the closed-loop oscillation measurement is performed for the phase noise (PN) characterization. The PN performance could reach -154.5 dBc/Hz at a 1-kHz offset at the carrier frequency of ∼ 60 MHz. This technique has proven its potential as a future solution of high-frequency quartz oscillator technology.

III-V-on-Si DFB Laser With Co-Integrated Power Amplifier Realized Using Micro-Transfer Printing

A C-band III-V-on-Si distributed feedback (DFB) laser with co-integrated power amplifier was realized using micro-transfer printing. The DFB laser exhibits single mode operation around 1540 nm at 20°C. By driving the DFB laser and co-integrated power amplifier simultaneously, up to 14 dBm waveguide-coupled output power with over 28 dB side mode suppression ratio was achieved at an overall bias current of 270 mA.

Sub-Kilohertz Narrow Linewidth Fiber Laser Based on Nanoparticles Doped Self-Injection Module

Fiber lasers with narrow linewidth are essential light sources for many prospects in advanced scientific fields. In this article, we proposed a scheme of self-injection locked single-frequency fiber laser based on a hybrid cavity configuration and a novel type of nanoparticles doped fiber. The newly proposed hybrid cavity was applied to achieve mode selection in the case of a long cavity which helped increase output power without sacrificing mode instability and single frequency performance. The nanoparticles doped self-injection module helped achieve a fiber laser with 3dB linewidth compressed from 16 kHz to 497 Hz and side mode suppression ratio of more than 60 dB. Furthermore, the sub-kilohertz fiber laser was amplified to more than 1 W with the polarization extinction ratio of ~30 dB. Throughout the work, experimental verification and analysis have been carried out to confirm the single frequency lasing performance, including center wavelength drift and slope efficiency variation. The demonstrated watt-level narrow linewidth fiber laser with low RIN and frequency noise has full potential for practical applications.

Numerical simulation of linewidth compression dynamics of external-cavity semiconductor laser

We theoretically simulate the linewidth compression dynamics of the semiconductor laser with a single external-cavity feedback. The evolution time from the periodic oscillation state to the equilibrium state is calculated by tracking the change of the intrinsic linewidth. The influences of the external-cavity length and feedback level are also explored. The linewidth evolution time of the external-cavity semiconductor laser can be extended with longer external cavity length and higher feedback level. Simultaneously, the intrinsic linewidth can be compressed while the side mode suppression ratio will be decreased. The simulation work show that the dynamic linewidth compression can be obtained by single external-cavity feedback, and nevertheless it is difficult to achieve the single-frequency output, which can instruct the design of the dynamical parameter tuning of the external-cavity semiconductor laser.

Effect of porous coatings on the nonlinear evolution of Mack modes in hypersonic boundary layers

The influence of a porous wall on the nonlinear evolution of Mack modes in a hypersonic boundary layer is studied by solving the nonlinear parabolized stability equations. The fundamental resonance of the second mode is particularly considered. It is found that the porous effect leads to (1) a much stronger mean-flow distortion in an indirect way, (2) a greater suppression of the saturated fundamental mode, and (3) slower amplification rates of the secondary instability modes, which eventually delays the transition onset. Detailed explanations of the three mechanisms are provided.

Ultrawideband De-Embedding Calibration for Magnetic Field Probes by Using Symmetric GCPW Calibrators

Microstrip line is commonly used as a calibrator for probe calibration. However, some issues need to be addressed for high frequency application, such as ripple and attenuation caused by the embedded part of the calibrator. In this paper, an ultra-wideband de-embedding calibration method (DCM) for magnetic field probe is proposed. Two symmetric non-soldering grounded coplanar waveguide (GCPW) calibrators, including Through and Line standards, are used to solve the de-embedded <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> -parameter matrix of a 3-port calibration system. Compared with the traditional calibration method (TCM), the proposed method can reduce the influence of the reflection and attenuation caused by the embedded part and preserve the intrinsic properties of the probe itself. By using the DCM with a symmetric and good matched GCPW calibrator, the ripples of the DCM decreases by 6.57 dB, which is 82 % of the TCM with a microstrip line, and the attenuation has been compensated by about 0.7 dB at 10 GHz. In addition, a new characterization method of common mode suppression ratio for H-field probe is also presented, and it is applicable to conducted emissions measurement in which a probe can detect signals without 90° rotation.

Suppression of Wall Modes in Rapidly Rotating Rayleigh-Bénard Convection by Narrow Horizontal Fins.

The heat transport by rapidly rotating Rayleigh-Bénard convection is of fundamental importance to many geophysical flows. Laboratory measurements are impeded by robust wall modes that develop along vertical walls, significantly perturbing the heat flux. We show that narrow horizontal fins along the vertical walls efficiently suppress wall modes ensuring that their contribution to the global heat flux is negligible compared with bulk convection in the geostrophic regime, thereby paving the way for new experimental studies of geophysically relevant regimes of rotating convection.