Broadband mid-infrared pulse via intra-pulse difference frequency generation based on supercontinuum from multiple thin plates

We report on the generation of optical pulses with a nearly one octave-spanning spectrum ranging from 1300 nm to2500 nm at 1 kHz repetition rate, which are based on intra-pulse difference frequency generation(DFG) in β-barium borate crystal(β-BBO) and passively carrier-envelope-phase(CEP) stabilized. The DFG is induced by few-cycle pulses initiated from spectral broadening in multiple thin plates driven by a Ti: sapphire chirped-pulse amplifier. Furthermore, a numerical simulation is developed to estimate the conversion efficiency and output spectrum of the DFG. Our results show that the pulses from the DFG have the potential for seeding intense mid-infrared(MIR) laser generation and amplification to study strong-field physics and attosecond science.

Regional differences in rainfall frequency and amount over southwestern China

In this paper,the rainfall features in southwestern China are studied using daily rainfall station data.The rainfall features are distinct along the eastern and western edges of the Hengduan Mountains and over the mountains,especially in terms of rainfall frequency.The rainfall amounts and frequencies are much higher along the eastern and western edges than over the mountains,particularly during spring,which is partly contributed by the number and duration of rainfall events.The differences are more obvious in the nocturnal rainfall than in the daytime rainfall.The rainfall differences over the three regions could be affected by the large-scale environment.By analyzing reanalysis data,the large-scale circulations linked to the different rainfall features over southwestern China,and the interactions of these circulations with the topography are also discussed.

Velocity estimation of moving targets with stepped-frequency radar based on Doppler frequency difference

By analyzing the signal model of stepped-frequency waveform, a novel method for velocity measurement is proposed. The method is based on Doppler frequency difference which is achieved by using Hough transform. As the estimated velocity is inversely proportional to the frequency step size instead of the carrier frequency of the transmitted signal as the pulse-Doppler （PD） processing, the new algorithm can achieve much wider unambiguous velocity range. Furthermore, non-coherent integration of the sub-pulses with different carrier frequencies can be implemented by Hough trans- form to improve the anti-noise performance. Besides, field experimental results show that the high range resolution profile （HRRP） of a bullet with high speed can be reconstructed correctly without distortion.

Passive Localization Using Time Difference of Arrival and Frequency Difference of ArrivalWC

In order to improve the accuracy and engineering feasibility of four-Satellite localization system, the frequency difference measurement is introduced to the four-Satellite TDOA (Time Difference of Arrival) localization algorithm. The TDOA/FDOA (Frequency Difference of Arrival) localization algorithm is used to optimize the GDOP (geometric dilution of precision) of four-Satellite localization. The simulation results show that the absolute position measurement accuracy has little influence on TDOA/FDOA localization accuracy as compared with TDOA localization. Under the same conditions, TDOA/FDOA localization has better accuracy and its GDOP shows more uniform distribution in diamond configuration case. The localization accuracy of four-Satellite TDOA/FDOA is better than the localization accuracy of four-Satellite TDOA.

Difference-frequency ultrasound generation from microbubbles under dual-frequency excitation

The difference-frequency （DF） ultrasound generated by using parametric effect promises to improve detection depth owing to its low attenuation, which is beneficial for deep tissue imaging. With ultrasound contrast agents infusion, the harmonic components scattered from the microbubbles, including DF, can be generated due to the nonlinear vibration. A theoretical study on the DF generation from microbubbles under the dual-frequency excitation is proposed in formula based on the solution of the RPNNP equation. The optimisation of the DF generation is discussed associated with the applied acoustic pressure, frequency, and the microbubble size. Experiments are performed to validate the theoretical predictions by using a dual-frequency signal to excite microbubbles. Both the numerical and experimental results demonstrate that the optimised DF ultrasound can be achieved as the difference frequency is close to the resonance frequency of the microbubble and improve the contrast-to-tissue ratio in imaging.

Application of a characterized difference-frequency laser source to carbon monoxide trace detection

A tunable continuous wave（cw） mid-infrared（MIR） laser based on difference-frequency generation（DFG） in a 1.5-cm long AgGaS2 nonlinear crystal for trace gas detection is reported.Two visible and near-infrared diode lasers were used as pump and signal sources.The MIR-DFG laser was tunable in a wavelength range of 4.75 μm-4.88 μm.The phase-matching（PM） condition was non-critically achieved by adjusting the temperature of the crystal for fixed pairs of input pump and signal wavelengths.The required PM temperatures of the generated MIR-DFG wavelengths have been calculated by using three sets of recent Sellmeier equations and the temperature-dispersion equations of AgGaS2 given by Willer U,et al.（Willer U,Blanke T and Schade W 2001 Appl.Opt.40 5439）.Then the calculated PM temperatures are compared with the experimental values.The performance of the MIR-DFG laser is shown by the trace detection of the P（16） carbon monoxide（12C16O） absorption line in a laboratory-fabricated absorption cell.The enhanced sensitivity of about 0.6×10 4 was obtained through the long path absorption provided by consecutive reflections between coated cylindrical mirrors of a constructed cell.

Effects of Second-Order Sum-and Difference-Frequency Wave Forces on the Motion Response of a Tension-Leg Platform Considering the Set-down Motion

This paper presents a study on the motion response of a tension-leg platform(TLP) under first-and second-order wave forces, including the mean-drift force, difference and sum-frequency forces. The second-order wave force is calculated using the full-field quadratic transfer function(QTF). The coupled effect of the horizontal motions, such as surge, sway and yaw motions, and the set-down motion are taken into consideration by the nonlinear restoring matrix. The time-domain analysis with 50-yr random sea state is performed. A comparison of the results of different case studies is made to assess the influence of second-order wave force on the motions of the platform. The analysis shows that the second-order wave force has a major impact on motions of the TLP. The second-order difference-frequency wave force has an obvious influence on the low-frequency motions of surge and sway, and also will induce a large set-down motion which is an important part of heave motion. Besides, the second-order sum-frequency force will induce a set of high-frequency motions of roll and pitch. However, little influence of second-order wave force is found on the yaw motion.

Sensibility to Changes of Vibrational Modes of Excited Electron： Sum Frequency Signals Versus Difference Frequency Signals

In this paper, we investigate a two electronic level system with vibrational modes coupled to a Brownian oscillator bath. The difference frequency generation （DFG） signals and sum frequency generation （SFG） signals are calculated. It is shown that, for the same model, the SFG signals are more sensitive than the DFG signals to the changes of the vibrational modes of the electronic two-level system. Because the SFG conversion efficiency can be improved by using the time-delay method, the findings in this paper predict that the SFG spectrum may probe the changes of the microstructure more effectively.

Creation of multi-frequency terahertz waves by optimized cascaded difference frequency generation

A new scheme which generates multi-frequency terahertz(THz)waves from planar waveguide by the optimized cascaded difference frequency generation(OCDFG)is proposed.A THz wave with frequencyω_(T1)is generated by the OCDFG with two infrared pump waves,and simultaneously a series of cascaded optical waves with a frequency intervalω_(T1)is generated.The THz wave with a frequency of M-timesω_(T1)is generated by mixing the m-th-order and the(m+M)-th-order cascaded optical wave.The phase mismatch distributions of cascaded difference frequency generation(CDFG)are modulated by changing the thickness of planar waveguide step by step,thereby satisfying the phase-matching condition from first-order to high-order cascaded Stokes process step by step.As a result,the intensity of THz wave can be enhanced and modulated by controlling the cascading order of OCDFG.

Energy Scaling of Terahertz Pulses Produced through Difference Frequency Generation

We study the energy scaling of terahertz (THz) emission through difference frequency generation of near-infrared pulses, and demonstrate that Gigawatt few-cycle THz transients at the central frequency of 30 THz are produced from GaSe crystal pumped by two pulses at 1.65 and 1.95 micrometers, with the high quantum yield of 28%. Our analysis indicates that the high yield of DFG originates from the largely reduced group velocity mismatch as the long-wavelength pumping pulses are employed.

Theoretical research on terahertz wave generation from planar waveguide by optimized cascaded difference frequency generation

A novel scheme for high-efficiency terahertz(THz)wave generation based on optimized cascaded difference frequency generation(OCDFG)with planar waveguide is presented.The phase mismatches of each-order cascaded difference frequency generation(CDFG)are modulated by changing the thickness of the waveguide,resulting in a decrement of phase mismatches in cascaded Stokes processes and an increment of phase mismatches in cascaded anti-Stokes processes simultaneously.The modulated phase mismatches enhance the cascaded Stokes processes and suppress the cascaded anti-Stokes processes simultaneously,yielding energy conversion efficiencies over 25%from optical wave to THz wave at 100 K.

High-efficiency terahertz wave generation with multiple frequencies by optimized cascaded difference frequency generation

High-efficiency terahertz(THz) wave generation with multiple frequencies by optimized cascaded difference frequency generation(OCDFG) is investigated at 100 K using a nonlinear crystal consisting of a periodically poled lithium niobate(PPLN) part and an aperiodically poled lithium niobate(APPLN) part.Two infrared pump waves with a frequency difference ω_(T1) generate THz waves and a series of cascaded optical waves in the PPLN part by cascaded difference frequency generation(CDFG).The generated cascaded optical waves with frequency interval ω_(T1) then further interact in the APPLN part by OCDFG,yielding the following two advantages.First,OCDFG in the APPLN part is efficiently stimulated by inputting multi-order cascaded optical waves rather than the only two intense infrared pump waves,yielding unprecedented energy conversion efficiencies in excess of 37% at 1 THz at 100 K.Second,THz waves with M timesω_(T1) are generated by mixing the mth-order and the(m+M)th-order cascaded optical waves by designing poling period distributions of the APPLN part.

Mid-IR dual-wavelength difference frequency generation in uniform grating PPLN using index dispersion control

A novel widely tunable dual-wavelength mid-IR difference frequency generation （DFG） scheme with uniform grating periodically poled lithium niobate （PPLN） is presented in this paper. By using the temperature-dependent dispersion property of PPLN, the quasi-phase matching （QPM） peak for the pump may evolve into two separate ones and the wavelength spacing between them increases with the decrease of the crystal temperature. Such two pump QPM peaks may allow simultaneous dual-wavelength mid-IR laser radiations while properly setting the two fundamental pump wavelengths. With this scheme, mid-IR dual-wavelength laser radiations at around 3.228 and 3.548, 3.114 and 3.661, and 3.019 and 3.76 μm, are experimentally achieved for the crystal temperatures of 90, 65, and 30 ℃, respectively, based on the fiber laser fundamental lights.

Mid-Infrared Pulsed Laser Lithotripsy with a Tunable Laser Using Difference-Frequency Generation

A novel technique of lithotripsy was investigated with a mid-infrared tunable pulsed laser using difference-frequency generation (DFG). Human gallstone samples obtained from 24 patients were analyzed with their infrared absorption spectra. It was found that the principal components of the gallstones were different for the different patients and that the gallstone samples used in this research could be classified into four groups, i.e., mixed stones, calcium bilirubinate stones, cholesterol stones, and calcium carbonate stones. In addition, some gallstone samples had different compositions within the single stone. The mid-infrared laser tunable within a wavelength range of 5.5 - 10 μm was irradiated to the cholesterol stones at two different wavelengths of 6.83 and 6.03 μm, where the cholesterol stones had relatively strong and weak absorption peaks, respectively. As the result, the cholesterol stones were more efficiently ablated at the wavelength of 6.83 μm with the strong absorption peak. Therefore, it is suggested that the gallstones could be efficiently ablated by tuning the wavelength of the laser to the strong absorption peak of the gallstones. The higher efficiency of the ablation using the characteristic absorption peaks should lead to the safer treatment without damage to the surrounding normal tissues. In order to identify the composition of the gallstones in the patients, endoscopic and spectroscopic diagnosis using the DFG laser and an optical fiber probe made with two hollow optical fibers and a diamond attenuation total reflection prism should be useful. The absorption spectrum of the gallstones in the patients could be measured by measuring the energy of the DFG laser transmitted through the optical fiber probe and by scanning the wavelength of the DFG laser.

Dynamic Behaviors of A Marine Riser Under Two Different Frequency Parametric Excitations

The marine risers are often subjected to parametric excitations from the fluctuation top tension. The top tension on the riser may fluctuate with multiple frequencies caused by irregular waves. In this paper, the influence between different frequency components in the top tension on the riser system is theoretically simulated and analyzed. With the Euler-Bernoulli beam theory, a dynamic model for the vibrations of the riser is established. The top tension is set as fluctuating with time and it has two different frequencies. The influences from the fluctuation amplitudes, circular frequencies and phase angles of these frequency components on the riser system are analyzed in detail. When these two frequencies are fluctuating in the stable regions, the riser system may become unstable because ω1+ω2≈2Ωn. The fluctuation amplitudes of these frequencies have little effect on the components of the vibration frequencies of the riser. For different phase angles, the stability and dynamic behaviors of the riser would be different.

Compact mid-infrared dual-comb spectrometer over 3-4μm via intra-pulse difference frequency generation in LiNbO_(3) waveguides

The mid-infrared optical frequency comb is a powerful tool for gas sensing.In this study,we demonstrate a simple midinfrared dual-comb spectrometer covering 3±4μm in Li Nb O_(3)waveguides.Based on a low-power fiber laser system,the mid-infrared comb is achieved via intra-pulse difference frequency generation in the Li Nb O_(3)waveguide.We construct pre-chirp management before supercontinuum generation to control spatiotemporal alignment for pump and signal pulses.The supercontinuum is directly coupled into a chirped periodically poled Li Nb O_(3)waveguide for the 3±4μm idler generation.A mid-infrared dual-comb spectrometer based on this approach provides a 100 MHz resolution over25 THz coverage.To evaluate the applicability for spectroscopy,we measure the methane spectrum using the dualcomb spectrometer.The measured results are consistent with the HITRAN database,in which the root mean square of the residual is 3.2%.This proposed method is expected to develop integrated and robust mid-infrared dual-comb spectrometers on chip for sensing.

Different Frequency Synchronization Theory and Its Frequency Measurement Practice Teaching Innovation Based on Lissajous Figure Method

According to the requirement of multi-parameter time and frequency measurement without frequency normalization,a different frequency synchronization theory is proposed based on Lissajous figure method and the variation lawof Lissajous figure which are used in practice teaching of frequency measurement. The theory can achieve high-precision transmission and comparison of time and frequency and precise locking and tracking of phase and frequency,improve the level of scientific research on time and frequency for postgraduate,and promote practice teaching innovation of time frequency measurement for undergraduate. Utilizing the ratio of horizontal and vertical inflection point of the Lissajous figure,the nominal frequency of the measured signal is precisely calculated.The frequency deviation between the measured frequency and its nominal frequency can be obtained by combining the turning cycle of the Lissajous figure. By observing the phase relationship between the frequency standard signal and the measured signal,the accurate measurement of the frequency is implemented. Experimental results showthat the direct measurement and comparison better than the 10-11 order of magnitude with common frequency source can be finished between any signal frequencies.The frequency measurement method based on the theory has the advantage of simple operation,quick measurement speed,small error,lownoise and high measurement precision. It plays an important role in time synchronization,communications,metrology,scientific research,educational technology practice and equipment and other fields.

A Doppler Location Method Based on Virtual Path Difference

This paper presents a Doppler passive location method for moving targets with fixed single station using the Doppler frequency shift and time difference information.First,based on the relationship between frequency shift and path difference,the virtual path difference is calculated from the measured value of Doppler frequency shift by means of mean value correction.Then,under the assumption that the target is moving at a constant speed along a straight line,two coaxial virtual double base arrays are constructed by using the moving track of the moving target based on the method of fixed period time difference.On this basis,the moving distance of the moving target can be calculated by using the ratio relationship between the frequency difference and the radial distance between the two adjacent detection points in the middle of the array,and the linear solution of the two double base path difference positioning equations.At this point,the relative coordinate position of the moving target can be obtained by directly using the linear solution of the double base path difference positioning equation again.

Thermal camera based on frequency upconversion and its noise-equivalent temperature difference characterization

We present a scheme for estimating the noise-equivalent temperature difference(NETD)of frequency upconversion detectors(UCDs)that detect mid-infrared(MIR)light.In particular,we investigate the frequency upconversion of a periodically poled crystal based on lithium niobate,where an MIR conversion bandwidth of 220 nm can be achieved in a single-poled period by a special design.Experimentally,for an MIR radiating target at a temperature of 95℃,the NETD of the device was estimated to be 56 mK with an exposure time of 1 s.Meanwhile,a direct measurement of the NETD was performed utilizing conventional methods,which resulted in 48 mK.We also compared the NETD of our UCD with commercially available direct MIR detectors.We show that the limiting factor for further NETD reduction of our device is not primarily from the upconversion process and camera noise but from the limitations of the heat source and laser performance.Our detectors have good temperature measurement performance and can be used for a variety of applications involving temperature object identification and material structure detection.

High-frequency (50 Hz) electroacupuncture ameliorates cognitive impairment in rats with amyloid beta 1–42-induced Alzheimer＇s disease

Acupuncture has been shown to ameliorate cognitive impairment of Alzheimer’s disease.Acupoints and stimulation frequency influence the therapeutic effect of electroacupuncture.Rat models of Alzheimer’s disease were established by injecting amyloid beta 1–42（Aβ_（1–42））into the bilateral lateral ventricles.Electroacupuncture at 2,30,and 50 Hz was carried out at Baihui（GV20;15°obliquely to a depth of 2mm）and Shenshu（BL23;perpendicularly to 4–6 mm depth）,once a day for 20 minutes（each）,for 15 days,taking a break every 7 days.The Morris water maze test was conducted to assess the learning and memory.The expression levels of glycogen synthase kinase-3β（GSK-3β）,p Ser9-GSK-3β,p Tyr216-GSK-3β,amyloid precursor protein and Aβ_（1–40） in the hippocampus were determined by western blot assay.Results demonstrated that electroacupuncture treatment at different frequencies markedly improved learning and memory ability,increased synaptic curvatures,decreased the width of synaptic clefts,thickened postsynaptic densities,and downregulated the expression of GSK-3β,amyloid precursor protein,and Aβ_（1–40）.pSer9-GSK-3βexpression markedly decreased,while p Tyr216-GSK-3βexpression increased.High-frequency（50 Hz）electroacupuncture was more effective than low（2 Hz）or medium-frequency（30 Hz）electroacupuncture.In conclusion,electroacupuncture treatment exerts a protective effect against Aβ_（1–42）-induced learning and memory deficits and synapse-ultrastructure impairment via inhibition of GSK-3βactivity.Moreover,high-frequency electroacupuncture was the most effective therapy.