Showing papers on "Flattop published in 2019"

Silicon-on-insulator-based microwave photonic filter with widely adjustable bandwidth

Abstract: We demonstrate a silicon-based microwave photonic filter (MPF) with flattop passband and adjustable bandwidth. The proposed MPF is realized by using a 10th-order microring resonator (MRR) and a photodetector, both of which are integrated on a photonic chip. The full width at half-maximum (FWHM) bandwidth of the optical filter achieved at the drop port of the 10th-order MRR is 21.6 GHz. The ripple of the passband is less than 0.3 dB, while the rejection ratio is 32 dB. By adjusting the deviation of the optical carrier wavelength from the center wavelength of the optical bandpass filter, the bandwidth of the MPF can be greatly changed. In the experiment, the FWHM bandwidth of the proposed MPF is tuned from 5.3 to 19.5 GHz, and the rejection ratio is higher than 30 dB.

Utilization of the high spatial-frequency component in adaptive beam shaping by using a virtual diagonal phase grating

Abstract: A square flattop beam is a fundamental shape that is in high demand in various applications, such as ultra-high-power lasers, uniform surface processing and medical engineering. In this experiment, a new and simple scheme of the adaptive beam shaping system to generate a square flattop shape with high uniformity and edge steepness using virtual diagonal phase grating encoded on a spatial-light modulator and a 4f system is proposed. The grating vector kg is non-parallel to the normal vectors kx and ky of the objective beam profile to be extracted; thus, the residual and extracted components hit separately on the Fourier plane of the 4f system. Consequently, using a spatial-frequency filter passing components parallel to kx and ky, the residual components are blocked by the filter without loss of the high spatial-frequency domain of the extracted component. When the width of the filter was 1.0 mm, the edge of the shaped beam increased in height within 20 μm, which is less than 20% of that obtained with conventional vertical phase grating.

Generation of picosecond pulses with variable temporal profiles and linear polarization by coherent pulse stacking in a birefringent crystal shaper

Abstract: We report the study and demonstration of a new laser pulse shaping system capable of generating linearly polarized picosecond laser pulses with variable temporal profiles including symmetric intensity distributions, as well as non-symmetric distributions, which are highly desired by various applications. It is found that both high transmittance and high stability of the shaped pulse can be achieved simultaneously when crystals are set at a specific phase delay through fine control of the crystal temperature. Although multi-crystal pulse stacking with different configurations was reported before particularly for flattop pulse generation, this new configuration leads to new opportunities for many potential applications over a wide range of laser wavelengths, pulse repetition rate, time structures and power levels. A practical double-pass temporal shaping configuration that significantly reduces the number of crystals is also proposed in this paper as a result of present study.

Thermal-Hydraulic Analysis of the JT-60SA Central Solenoid Operation

Abstract: The minimum temperature margin of the JT-60SA CS coil in the standard (pulsed) operation scenario, with 5.5 MA plasma current and 75 s flattop, is computed using the state-of-the-art 4C code for different values of the coupling time constant n τ. The margin is >1 K, provided n τ < 130 ms.

Construction of a Pulsed Magnetic Field System for Regulating Strongly Dissipative Plasmas

Abstract: Pulsed magnetic field applied to controlling the dissipative plasma around spacecrafts may alleviate the blackout in communication. To validate it in the laboratory, a pulsed magnetic field system operated by a pulsed current derived from a novel power supply system to a magnetic coil with a special runway shape is constructed. The magnetic field with a flattop 12 ms and a minimum magnitude ~2 T during flattop is generated. The magnet is mounted inside of the cylindrical shell with a cone-shaped head to simulate the spacecraft. To improve the energy efficiency, an innovative power supply with multiple capacitors is proposed to generate a pulsed current with a flattop by discharging capacitors sequentially. Two thyristors are put in series as the main switches and a crowbar module is used to prevent current reversal. The target field was evaluated through a coupled transient analysis of the magnet and the power supply system. The thermal load and structural strength of the magnet were also studied through the coupled field analyses under both the normal operation and one extreme condition.

Generation of Nearly Flattop Ultrabroadband Response in a QPM Device Using Phase Shifter

Abstract: We have theoretically demonstrated the efficient generation of nearly flattop ultrabroadband response in a periodic quasiphase matching (QPM) device using a single phase shifter domain. The flat broadband response is generated using optical parametric generation (OPG) with a QPM device of grating period $\Lambda = \text{27}\;\mu$ m. The choice of grating period and pump source depends on the group velocities of signal and idler wavelengths to match close to the degeneracy point. We have identified the location of phase shifter domain, which generates a nearly flattop OPG gain bandwidth of $\Delta \lambda =\text{1.41}\;\mu$ m. The variations in broadband response are studied when the location and width of phase shifter domains are altered. When compared to a periodic device, the phase shifter domain included QPM structure exhibits higher tolerance to temperature in retaining the smoothness of the broadband response. Furthermore, we have generated the same ultrabroadband response by introducing multiple phase shifter domains at suitable locations in a periodic QPM device.

Demonstration of a cavity-based pulse compression system for pulse shape correction

Abstract: A compact X-band correction cavity chain (CC) has been successfully designed, fabricated and high power tested for the Compact Linear Collider (CLIC) project. The CC consists of eight spherical cavities and four polarizers, and each spherical cavity has a ${Q}_{0}$ of 45 000 and a diameter of 7 cm. The compact CC can correct a decaying pulse shape generated by a storage cavity without need of low efficiency pulse modulation or a solution based on long delay lines. A flattop output with small ripples can be obtained if the CC and the storage cavity are matched. Experiments of the CC with a barrel open cavity (BOC) were conducted on the XBOX2 test facility at CERN, and the high power results match the theoretical calculations. The appliance of the CC only reduces the pulse compression efficiency by 6% relatively if the transmission attenuation is ignored.

JACoW : Simulations of longitudinal beam stabilisation in the CERN SPS with BLonD

Abstract: The Super Proton Synchrotron (SPS) at CERN, the Large Hadron Collider (LHC) injector, will be pushed to its limits for the production of the High Luminosity LHC proton beam while beam quality and stability in the longitudinal plane are influenced by many effects. Particle simulation codes are an essential tool to study the beam instabilities. BLonD, developed at CERN, is a 2D particle-tracking simulation code, modelling the longitudinal phase space motion of single and multi-bunch beams in multi-harmonic RF systems. Computation of collective effects due to the machine impedance and space charge is done on a multi-turn basis. Various beam and cavity control loops of the RF system are implemented (phase, frequency and synchro-loops, and one-turn delay feedback) as well as RF phase noise injection used for controlled emittance blow-up. The longitudinal beam stability simulations during long SPS acceleration cycle (∼20 s) include a variety of effects (beam loading, particle losses, controlled blow-up, double RF system operation, low-level RF control, injected bunch distribution, etc.). Simulations for the large number of bunches in the nominal LHC batch (288) use the longitudinal SPS impedance model containing broad and narrow-band resonances between 50 MHz and 4 GHz. This paper presents a study of beam stabilisation in the double harmonic RF system of the SPS system with results substantiated, where possible, by beam measurements.

Ka-Band Linearizer Studies for a Compact Light Source

Abstract: The CompactLight project is currently developing the design of a next generation hard X-ray FEL facility, based on high-gradient X-band (12 GHz) structures, bright electron photo-injectors, and compact short period undulators. However, to improve the brightness limitations due to the nonlinear energy spread of the electron bunches, a Kaband (36 GHz) linearizer is being considered to provide a harmonic compensation during the bunch compression. In this paper, we analyze the feasibility of such linearizer.

100 MHz Reconfigurable Ultrafast Swept Source by Time Stretching of 100 nm Flat-top Spectrum

Abstract: The repetition rate of swept source determines the acquisition rate of optical coherence tomography and interrogation rate of optical sensing systems. The axial resolution of optical coherence tomography is limited by the swept range of the swept source. In this paper, we report an ultrafast time stretching swept source with up to 100 MHz repetition rate and a 100 nm flattop spectrum. The repetition rate can be reconfigured by incorporating with an optical modulator.

New Design of Vacuum Chambers for Radiation Shield Installation at Beam Injection Area of J-PARC RCS

Abstract: One of the issues in the J-PARC 3 GeV rapid cycling synchrotron is the high residual radiation dose around the beam injection point. A radiation shield is necessary to reduce radiation exposure of workers when maintenance is performed there. A space to install the radiation shield should be secured by newly designing a structure of the vacuum chamber at the injection point and the alumina ceramics beam pipes for the shift bump magnets. To make the space for the shield, the chamber is lengthened along the beam line and the cross-sectional shape is changed from circle to rectangle. The displacement and inner stress of the vacuum chamber due to atmospheric pressure were evaluated to be enough small by the calculation. For the ceramics beam pipe’s rf-shield, the damping resistor was effective to reduce the induced modulation voltages by the pulsed magnetic field. INTRODUCTION One of the issues in the J-PARC 3 GeV rapid cycling synchrotron (RCS) is the high residual radiation dose of the vacuum chambers and magnets in the beam injection area. The high radiation is caused by the nuclear scattering due to the interaction between the beam and a carbon foil for stripping the electrons of the injected H beam, which is called the 1 foil [1]. Fig. 1 shows a typical residual radiation distribution in the injection area. Residual dose in the downstream of the 1 foil is larger than that in the upstream due to the large scattering cross-section at the forward angle. The highest residual dose is estimated to be well over 15 mSv/h at the surface of the vacuum chamber at the beam injection point after the operation with an extracted beam power of 1 MW. The Effort to prevent workers radiation exposure during maintenance has been performed, such as a minimization of the foil hitting number [2], and control of the workers’ radiation dosage by recording the pocket dosimeters value every entering and leaving the tunnel. However, against predicted higher residual dose for the future higher beam power operation, radiation shields around the 1 foil should be considered. The problem against consideration of the radiation shield is the limited space around the vacuum chamber at the injection point. In this paper, the basic concept to make the space for the radiation shields and the related upgrade for the vacuum chamber and beam pipes are described. Firstly, the new configuration of the injection area under consideration is introduced. Next, the evaluation of the newly designed vacuum chamber at the injection point is described. Subsequently, the rf-shield on the alumina ceramics beam pipes, which is redesigned for the induced magnetic field damping, is mentioned. NEW SYSTEM CONFIGURATION UNDER DESIGNING Figure 2 a) shows the current configuration around the injection point. The 4 shift-bump magnets (SB1-4) create a bump orbit to match the injected H and circulating proton beam at the injection point. The beam orbit by the shiftbump magnets and the role of the 1 to 3 charge stripping foil are described in the reference [3]. The 2 foil strips the electron from the partially electron-stripped beam by the 1 foil, H. The current SB4 core is a split type to insert the 2 foil in the middle of the SB4. SB1-3 are designed to be the same as SB4 for mass production. Orbit calculation showed that the partially electron-stripped beam, H, cannot be transported to the injection beam dump if the position of the 2 foil is changed to upstream or downstream of the SB4. Therefore, as shown in Fig. 2 b), even in the new design, the SB4 core should be a split type. Only the SB2 and SB3 cores are changed to the combined Figure 1: Typical residual dose distribution at the beam injection area in the RCS about 5 hours after beam operation with 500 kW power. Figure 2: System configuration of the beam injection area. a) Current configuration. b) New configuration under designing for the radiation shield installation. a)

Stacked-actuators deformable mirror vs bimorph mirror for laser beam shaping

Abstract: We present our latest research results on intensity distribution transformation from Gaussian to a flattop and doughnut. The theoretical calculations and experimental results of the efficiency of different types of deformable mirrors are given. During the experiments the wavefront was measured with Shack-Hartmann sensor and then modified with bimorph deformable mirror to reach the desired intensity distribution in the far-field. Then the bimorph mirror was substituted with the stacked-actuators deformable mirror to confirm the simulations.

Shed frame test method in pilot tunnel digging technology

Abstract: The invention relates to the field of engineering construction, and provides a shed frame test method in a pilot tunnel digging technology. The method comprises the following steps that: test device building: a flattop shed frame is supported and fixed, a prefabricated bracket is supported and fixed below the flattop shed frame, and a pressure-bearing, height-regulation and force measurement device is arranged between the flattop shed frame and the prefabricated bracket, wherein the pressure-bearing, height-regulation and force measurement device has the functions of pressure bearing, height regulation and multidirectional force measurement; a loading process: the flattop shed frame loads preset pressure, meanwhile, the pressure-bearing, height-regulation and force measurement device provides supporting force, initial supporting force and pressure borne by the flattop shed frame are the same and have opposite directions; and unloading and regulation: supporting force provided by the pressure-bearing, height-regulation and force measurement device on a target position is regulated according to a surrounding rock load releasing rule. The invention provides the shed frame test methodin the pilot tunnel digging technology, the stress deformation state of the shed frame under surrounding rock pressure in different digging technologies and digging stages can be accurately obtained for guiding the structural design and the remediation repair of the shed frame, and construction cost is effectively reduced.

Antisymmetric distributed Bragg reflector as hybrid filter for compact integrated photonics

Abstract: Antisymmetric multi cavity resonator useful for flat top transmission is reported which is simple and compact than the similar conventional structures. The study of the output spectra is carried out in terms of layer thickness and change in angle of incidence. Electro-optic property of the EO polymer is utilized to convert flat-top output with full width half maximum (FWHM) bandwidth of 10 nm into narrowband output of 0.64 nm (FWHM bandwidth) by applying a maximum voltage of 500 V across the structure. It was shown here that by controlling the low index layer widths, angle of incidence of the light and applying a voltage of 300 V results in a flattop spectrum of FWHM bandwidth of 2.5 nm with tuning sensitivity 0.033 nm/V. The tuning sensitivity and bandwidth of the reported hybrid filter can also be customized comparatively at lower applied voltage than the previously reported electro-optically tunable filters.

Improving quantitative fluorescence imaging with flat field illumination

Abstract: We demonstrated flat-field illumination (FFI) for multi-color wide-field fluorescence microscopy using a refractionbased beam shaping system. The non-homogeneous illumination of a Gaussian intensity profile makes quantitative analysis in laser-assisted wide-field fluorescence microscopy very difficult. As contrasted with other approaches, our method is applicable to TIRF illumination, which effectively rejects background fluorescence. Our beam shaping device is extremely tolerant to variations in size of the incoming laser beam by accepting ± 10% variation, while being achromatic as well. This behavior originates from the well-balanced mapping of the incoming rays to the intended flattop beam profile in combination with a sophisticated material choice, which decreases the sensitivity to input beam diameter. The homogenous illumination profile of FFI will enable quantitative single-molecule analysis based on intensity information. This has powerful implications when combined with a pull-down assay, which can probe the oligomerization state of endogenous proteins. When combined with one-to-one fluorophore labeling, the stoichiometry of proteins related to neurodegenerative diseases could be readily determined by intensity distribution analysis, which is critical to not only diagnosing but also understanding the pathogenesis of these complex disorders that are particularly difficult to analyze. An additional application of FFI is high quality super-resolution imaging with a uniform spatial resolution over a large FOV, where the full power of the excitation beam could be utilized. A new optical design approach based on refractive freeform surfaces generating a square-shaped beam instead of a round one will be presented, which would yield greater illumination efficiency.

Flattop output extra-wide tuning range tunable laser design technique

Abstract: The laser oscillator design principle, which provides extended tunability together with a flattop output in the wide spectral range, is proposed and demonstrated on the examples of the dye laser. It is realized by using an high-reflecting (HR) moving intracavity mirror (scraper) with the sharp edge partially overlapping the laser radiation in the high-Q cavity and extracting it out. The technique provides the output laser beam with M 2 < 1.2 and can be applied to any type of lasers operating from the far-IR to the VUV spectral ranges. Besides, wide-band HR cavity mirrors are only used, and there are no special demands to the material of the mirror substrate.

Using over-damped resistor-inductor-capacitor circuits to synthesize an adjustable high voltage rectangular pulse.

Abstract: A novel method is proposed to synthesize an adjustable high voltage bipolar rectangular pulse by means of three over-damped resistor-inductor-capacitor nonsynchronous discharge circuits and the artificial current zero technology. The main advantage of the novel method is that the rise time and the flattop durations are adjustable independently. It is very suitable for the insulation test due to the output waveform being not sensitive to sample variety. A prototype was designed and tested. The results show that the prototype can output an adjustable unipolar rectangular pulse with 17 kV amplitude, 330 ns-5.45 µs flattop duration, and 110-350 ns rise time on an insulation sample.

Flat-top Supercontinuum Generation Based on Electro-optic Optical Frequency Comb

Abstract: We demonstrate an optical frequency comb-based supercontinuum (OFC-SC) with flattop spectra using pulse shaping technique and dispersion compensation. In addition, we achieve the short pulse width using the OFC-SC. © 2019 The Author(s)

Flat field illumination for improved fluorescence microscopy

Abstract: We demonstrated flat-field illumination (FFI) for multi-color wide-field fluorescence microscopy using a refraction-based beam shaping system. The non-homogeneous illumination of a Gaussian intensity profile makes quantitative analysis in laser-assisted wide-field fluorescence microscopy very difficult. As contrasted with other approaches, our method is applicable to TIRF illumination, which effectively rejects background fluorescence.Our beam shaping device is extremely tolerant to variations in size of the incoming laser beam by accepting ± 10% variation, while being achromatic as well. This behavior originates from the well-balanced mapping of the incoming rays to the intended flattop beam profile in combination with a sophisticated material choice, which decreases the sensitivity to input beam diameter. The homogenous illumination profile of FFI will enable quantitative single-molecule analysis based on intensity information. This has powerful implications when combined with a pull-down assay, which can probe the oligomerization state of endogenous proteins. When combined with one-to-one fluorophore labeling, the stoichiometry of proteins related to neurodegenerative diseases could be readily determined by intensity distribution analysis, which is critical to not only diagnosing but also understanding the pathogenesis of these complex disorders that are particularly difficult to analyze.An additional application of FFI is high quality super-resolution imaging with a uniform spatial resolution over a large FOV, where the full power of the excitation beam could be utilized. A new optical design approach based on refractive freeform surfaces generating a square-shaped beam instead of a round one will be presented, which would yield greater illumination efficiency.

High-power laser beam formation and focusing by means of adaptive optics : (Invited)

Abstract: The transformation of an intensity distribution from Gaussian to a flattop, doughnut, etc. is a very important task due to rapid development of laser technology and its applications. And the necessary result could be obtained with the use of flexible deformable mirrors that changes the phase of the beam and modifies the shape of the focal spot in the farfield zone. In this paper, we present the theoretical and experimental results of the flattop and doughnut beam formation with the use of a bimorph and stacked-actuator deformable mirrors as well as LC phase modulator.