Search Results (12)

As one of the most precise timekeeping instruments ever developed, the optical clock will be used as the measuring equipment for the next generation of second definition. The demand for the miniaturization of optical clocks is progressively urgent. In this paper, a multi-channel radio frequency (RF) module with a 20% volume of the commercial module is designed and implemented for the transportable ⁴⁰Ca⁺ ion optical clock. Based on the double-crystal oscillator interlocking technique, a 1 GHz low-phase noise reference source is developed for direct digital synthesis. Through the simulation and optimization of the signal link design, the frequency range of the low phase-noise RF signal can reach 0–400 MHz with a 4 μHz resolution. Through two-stage power amplifying with different kinds of filters, it can achieve an output power of up to +33 dBm (2 W) at 100 MHz with a 25 dB phase noise lower than the commercial module at 1 Hz, and its third harmonic suppression ratio has been reduced by more than 20 dB at the frequency point of 300 MHz. This multi-channel RF module is used for the power stability and timing control test of a 729 nm clock laser to meet the requirements of the transportable ⁴⁰Ca⁺ optical clock. Additionally, this module can also be applied to other quantum systems such as the quantum absolute gravimeter, quantum gyroscopes, and quantum computers. Full article

In practical engineering applications, the vibration is often generated in various directions and can be harmful to the engineering equipment. Thus, it is necessary to develop vibration isolators that can reduce vibration in multiple directions. In this paper, we propose a planar two-dimensional vibration isolator based on compliant mechanisms. The proposed mechanism consists of two negative stiffness-compliant modules and two positive stiffness-compliant modules, which leads to the quasi-zero stiffness (QZS) property in the mechanism. The dynamic model is established by using the third-order Taylor expansion and the harmonic balance method. Based on the dynamic model, the influence of different parameters on the displacement transmissibility is discussed, including damping ratio, system stiffness, and excitation amplitude. Finally, we conducted the vibration isolation experiments and obtained the displacement transmissibility of the isolator. The results verify that the proposed isolator has good isolation performance for low-frequency vibration. Full article

High-harmonic generation from solid films is an attractive method for converting infrared laser pulses to ultraviolet and vacuum ultraviolet wavelengths and for examining the films using the generation process. In this work, AlN thin films grown on a sapphire substrate are studied. Below-band-gap third harmonics and above-band-gap fifth harmonics were generated using a Ti:sapphire oscillator running at 800 nm. A strong enhancement of the fifth-harmonic signal in the forward direction was observed from thicker 39 nm and 100 nm films compared to thinner 8 nm and 17 nm films. For the fifth harmonic generated in the backward direction, and also for the third harmonic in both the forward and backward directions, only a weak dependence of the harmonic signal on the film thickness was measured. Using both X-ray diffraction and dependence of the fifth harmonic on the laser polarization measurements, these behaviors are attributed to the crystallization and the grain size of the films, promising fifth-harmonic generation as a suitable tool to study AlN film properties. Full article

Low power brushless direct current (BLDC) motors are used in many consumer appliances. These motors have a relatively high winding resistance and therefore current control loop can be avoided in some cases, but fast and accurate speed regulation can be still needed. To minimize harmonics and generated sound noise, improved sinusoidal pulse width modulation (PWM) has been tested in the paper. As the most suitable commutation type, the sine wave with the third harmonic component has been selected. This type of communication reduced the torque ripple of the motor. This paper analyses the possibility to improve traditional proportional-integral-derivative (PID) speed regulator with Fuzzy logic block. A simulation model of BLDC motor, inverter, speed detection circuit and controller have been created. Simulation results showed that by applying the Fuzzy-based PID controller, the transient time can be reduced from 0.2 s to 0.05 s and overshoot can be avoided in comparison with traditional PID controller. Experimental results show a significant improvement in the motor dynamics—the overshoot and transient time were reduced twice. The difference with simulation results and experimental ones can be explained by delays introduced by the microcontroller. Full article

This paper investigates the possibilities of creating magnetic field sensors using the direct magnetoelectric (ME) effect in a monolithic heterostructure of amorphous ferromagnetic material/langatate. Layers of 1.5 μm-thick FeCoSiB amorphous ferromagnetic material were deposited on the surface of the langatate single crystal using magnetron sputtering. At the resonance frequency of the structure, 107 kHz, the ME coefficient of linear conversion of 76.6 V/(Oe∙cm) was obtained. Furthermore, the nonlinear ME effect of voltage harmonic generation was observed with an increasing excitation magnetic field. The efficiency of generating the second and third harmonics was about 6.3 V/(Oe²∙cm) and 1.8 V/(Oe³∙cm), respectively. A hysteresis dependence of ME voltage on a permanent magnetic field was observed due to the presence of α-Fe iron crystalline phases in the magnetic layer. At the resonance frequency, the monolithic heterostructure had a sensitivity to the AC magnetic field of 4.6 V/Oe, a minimum detectable magnetic field of ~70 pT, and a low level of magnetic noise of 0.36 pT/Hz^1/2, which allows it to be used in ME magnetic field sensors. Full article

This paper proposed angle measurement methods based on direct third harmonic generation (THG) in centrosymmetric crystals. The principles of the intensity-dependent and the wavelength-dependent angle measurement methods were illustrated. In this study, three prospective centrosymmetric crystals and two different phase-matching types were investigated in a wavelength range from 900 nm to 2500 nm. For the intensity-dependent method, a dispersion-less wavelength range was found from 1700 nm to 2000 nm for α-BBO and calcite. Compared with rutile, α-BBO and calcite had relatively better measurement performance based on the angle measurement sensitivity calculation. The wavelength-dependent method was considered in a dispersive range of around 1560 nm. The results suggested that α-BBO and calcite were also suitable for wavelength-dependent measurement. In addition, the effects of focusing parameters were considered in the simulation, and the optimized focal length (f = 100 mm) and the focused position (in the center of the crystal) were determined. Full article

This paper proposes a quasi-isotropic hybrid dielectric resonator antenna (DRA) and bow-tie patch with harmonics suppression. The suggested antenna consists of a DRA, a bow-tie patch, and a microstrip seventh-order Chebyshev low-pass filter. By loading a bow-tie patch on the designed DRA, a quasi-isotropic pattern is realized. The seventh-order Chebyshev low-pass filter was applied to the feed line, and harmonics were reduced in the section separate to the operating band to remove harmonics generated by the proposed antenna. The simulated $\mathrm{S}11$ that satisfies below −10 dB is 3.09–3.3 GHz (6.25%), and the measured S11 is 3.10–3.28 GHz (5.64%). The simulated gain difference considering all radiating regions ($0^{\circ }\le \phi \le {360}^{\circ }$ and $0^{\circ }\le \theta \le {180}^{\circ }$) is 7.211 dB. Compared with the antenna without a filter, the harmonic gain was reduced by 10.847 and 15.774 dB. The measured gain isolation of the operating band and the second and third harmonics are 10.10 and 18.94 dB, respectively. The proposed antenna is considered to be applicable to applications that require radio wave reception in all directions such as wireless point access points, internet of things and radio frequency identification, and is expected to contribute to reducing the size of the RF system. Full article

Traditional direct field-oriented control (DFOC) techniques with integral-proportional (PI) controllers have undesirable effects on the power quality and performance of variable speed contra-rotating wind power (CRWP) plants based on asynchronous generators (ASGs). In this work, a commanding technique based on the DFOC technique for ASG is presented on variable speed conditions to minimize the output power ripples and the total harmonic distortion (THD) of the grid current. A new DFOC strategy was designed based on third-order sliding mode (TOSM) control to minimize oscillations and the THD value of the current and active power of the ASG; the designed technique decreases the current THD from ASG and does not impose any additional undulations in different parts of ASG. The designed technique is simply implemented on traditional DFOC techniques in variable speed DRWP systems to ameliorate its effectiveness. Also, the results show that by using the designed TOSM controllers, in addition to regulating the active and reactive powers of the ASG-based variable speed CRWP system, the THD current and active power undulations of the traditional inverters can be minimized simultaneously, and the stator current became more like a sinusoidal form. Full article

In this work, direct irradiation by a Ti:Sapphire (100 fs) femtosecond laser beam at third harmonic (266 nm), with a moderate repetition rate (50 and 1000 Hz), was used to create regular periodic nanostructures upon polystyrene (PS) thin films. Typical Low Spatial Frequency LIPSSs (LSFLs) were obtained for 50 Hz, as well as for 1 kHz, in cases of one spot zone, and also using a line scanning irradiation. Laser beam fluence, repetition rate, number of pulses (or irradiation time), and scan velocity were optimized to lead to the formation of various periodic nanostructures. It was found that the surface morphology of PS strongly depends on the accumulation of a high number of pulses (10³ to 10⁷ pulses) at low energy (1 to 20 µJ/pulse). Additionally, heating the substrate from room temperature up to 97 °C during the laser irradiation modified the ripples’ morphology, particularly their amplitude enhancement from 12 nm (RT) to 20 nm. Scanning electron microscopy and atomic force microscopy were used to image the morphological features of the surface structures. Laser-beam scanning at a chosen speed allowed for the generation of well-resolved ripples on the polymer film and homogeneity over a large area. Full article

The modification of the optical properties of semiconductor quantum dots near plasmonic nanostructures has attracted significant attention in recent years due to the several potential applications of the coupled nanostructures in optoelectronics, biophotonics and quantum technologies, including sensors, light harvesting, quantum information processing and quantum communication, imaging, photocatalysis, solar cells and others. One of the methods for modifying the nonlinear optical susceptibilities in quantum dots near plasmonic nanostructures uses the change of the spontaneous decay rates of quantum emitters due to the Purcell effect in a tailored nanophotonic environment. In this work, using this idea, we study the modification of the third-order nonlinear optical susceptibilities and specifically the phenomena of degenerate four-wave mixing and third-harmonic generation in a quantum dot that is coupled to a spherical metallic nanoparticle. We find that the strong alteration of the quantum dot’s spontaneous decay rate near the metallic nanoparticle gives strong variation, either enhancement or suppression, of the phenomena of degenerate four-wave mixing and third-harmonic generation for different distances of the quantum dot from the surface of the metallic nanoparticle, depending on the electric dipole direction of the quantum dot. We also show that the degree of enhancement or suppression of the nonlinear optical susceptibilities differs for the studied phenomena and it is stronger for degenerate four-wave mixing than for third-harmonic generation. This work may have important potential applications in the creation of nanoscale photonic devices for various technological applications. Full article

Optical frequency combs are one of the most remarkable inventions in recent decades. Originally conceived as the spectral counterpart of the train of short pulses emitted by mode-locked lasers, frequency combs have also been subsequently generated in continuously pumped microresonators, through third-order parametric processes. Quite recently, direct generation of optical frequency combs has been demonstrated in continuous-wave laser-pumped optical resonators with a second-order nonlinear medium inside. Here, we present a concise introduction to such quadratic combs and the physical mechanism that underlies their formation. We mainly review our recent experimental and theoretical work on formation and dynamics of quadratic frequency combs. We experimentally demonstrated comb generation in two configurations: a cavity for second harmonic generation, where combs are generated both around the pump frequency and its second harmonic and a degenerate optical parametric oscillator, where combs are generated around the pump frequency and its subharmonic. The experiments have been supported by a thorough theoretical analysis, aimed at modelling the dynamics of quadratic combs, both in frequency and time domains, providing useful insights into the physics of this new class of optical frequency comb synthesizers. Quadratic combs establish a new class of efficient frequency comb synthesizers, with unique features, which could enable straightforward access to new spectral regions and stimulate novel applications. Full article

This study presents a self-testing platform with a foreground digital calibration technique for successive approximation register (SAR) analog-to-digital converters (ADCs). A high-accuracy digital-to-analog converter (DAC) with digital control is used for the proposed self-testing platform to generate the sinusoidal test signal. This signal is then implemented using an Arduino board, and the clock signal is generated to test the ADCs. In addition, fast Fourier transform and recursive discrete Fourier transform (RDFT) processors are adopted for dynamic performance evaluation and calibration of the ADCs. The third harmonic distortion caused by the non-linearity of the track-and-hold circuit, the mismatch of the DAC capacitor array, and the direct current (DC) offset of the comparator can be calculated using the processors to improve the ADC performance. The advantages of the proposed platform include its low cost, high integration, and no need for an extra analogy compensation circuit to deal with calibration. In this work a 12 bit SAR ADC and an RDFT processor are used in the Taiwan Semiconductor Manufacturing Co., Ltd. (TSMC) 0.18 μm standard complementary metal–oxide–semiconductor (CMOS) process with a sampling rate of 18.75 kS/s to validate the proposed method. The measurement results show that the signal-to-noise and distortion ratio is 55.07 dB before calibration and 61.35 dB after calibration. Full article