Qixian Peng

Analysis of photonic Doppler velocimetry data based on the continuous wavelet transform

The short time Fourier transform (STFT) cannot resolve rapid velocity changes in most photonic Doppler velocimetry (PDV) data. A practical analysis method based on the continuous wavelet transform (CWT) was presented to overcome this difficulty. The adaptability of the wavelet family predicates that the continuous wavelet transform uses an adaptive time window to estimate the instantaneous frequency of signals. The local frequencies of signal are accurately determined by finding the ridge in the spectrogram of the CWT and then are converted to target velocity according to the Doppler effects. A performance comparison between the CWT and STFT is demonstrated by a plate-impact experiment data. The results illustrate that the new method is automatic and adequate for analysis of PDV data.

Research on the magneto-optic current sensor for high-current pulses

A new magneto-optic current sensor with a dual-orthogonal configuration is proposed and demonstrated, which can resolve the problem which value of high-current pulses is ambiguous from the Faraday rotation in the conventional magneto-optic current sensor. In this dual-orthogonal configuration, the transmission axis of the polarizer makes two given angles, which are 0 degrees and 45 degrees , with the orientation of the s-polarized lights of two polarizing cube beam splitters. If the intrinsic linear birefringence of the sensing element is one-fourth less than the Faraday rotation generated from the high-current pulse, the measurement current influenced by it can be ignored and four outputs of the magneto-optic current sensor are in quadrature. The value of the current pulse can be calculated by an arctangent data reduction method, which can avoid the insensitive zone of the sine function, where the measurement current has a reduced accuracy. A measurement of the high-current pulse (up to 720 kA) of an electric gun, with good theoretical accuracy ( approximately 3%) and in agreement well with that of the calibrated Rogowski coil, is launched.

Research on the fiber Bragg grating sensor for the shock stress measurement

A fiber Bragg grating (FBG) sensor with an unbalanced Mach-Zehnder fiber interferometer for the shock stress measurement is proposed and demonstrated. An analysis relationship between the shock stress and the central reflection wavelength shift of the FBG is firstly derived. In this sensor, the optical path difference of the unbalanced Mach-Zehnder fiber interferometer is ~3.1 mm and the length of the FBG is 2 mm. An arctangent function reduction method, which can avoid sine functions insensitive zone where the shock stress measurement has a reduced accuracy, is presented. A shock stress measurement of water driven by one stage gun (up to 1.4 GPa), with good theoretical accuracy (~10%), is launched.

X-ray backlighting of imploding aluminium liners on PTS facility

The x-ray backlighting systems, including a 1.865 keV (Si Heα line) spherically bent crystal imaging system and an ∼8.3 keV (Cu Heα line) point-projection imaging system, newly fielded on the Primary Test Stand facility are introduced and its preliminary experimental results in radiography of the aluminium (Al) liners with seeded sinusoidal perturbations are presented. The x-ray backlighter source is created using a 1 TW, 1 kJ Nd: glass high power laser, kilo-joule laser system, recently constructed at China Academy of Engineering Physics. The ablation melt and instability of the imploding Al liner outer edge under the driving current of ∼7.5 MA are successfully observed using these two backlighting systems, respectively.

Time-resolved single-shot terahertz time-domain spectroscopy for ultrafast irreversible processes.

Pulsed terahertz spectroscopy is suitable for spectroscopic diagnostics of ultrafast events. However, the study of irreversible or single shot ultrafast events requires ability to record transient properties at multiple time delays, i.e., time resolved at single shot level, which is not available currently. Here by angular multiplexing use of femtosecond laser pulses, we developed and demonstrated a time resolved, transient terahertz time domain spectroscopy technique, where burst mode THz pulses were generated and then detected in a single shot measurement manner. The burst mode THz pulses contain 2 sub-THz pulses, and the time gap between them is adjustable up to 1 ns with picosecond accuracy, thus it can be used to probe the single shot event at two different time delays. The system can detect the sub-THz pulses at 0.1 THz-2.5 THz range with signal to noise ratio (SNR) of ∼400 and spectrum resolution of 0.05 THz. System design was described here, and optimizations of single shot measurement of THz pulses were discussed in detail. Methods to improve SNR were also discussed in detail. A system application was demonstrated where pulsed THz signals at different time delays of the ultrafast process were successfully acquired within single shot measurement. This time resolved transient terahertz time domain spectroscopy technique provides a new diagnostic tool for irreversible or single shot ultrafast events where dynamic information can be extracted at terahertz range within one-shot experiment.

All-optical framing photography based on hyperspectral imaging method

We propose and experimentally demonstrate a new all optical-framing photography that uses hyperspectral imaging methods to record a chirped pulse’s temporal-spatial information. This proposed method consists of three parts: (1) a chirped laser pulse encodes temporal phenomena onto wavelengths; (2) a lenslet array generates a series of integral pupil images;(3) a dispersive device disperses the integral images at void space of image sensor. Compared with Ultrafast All-Optical Framing Technology(Daniel Frayer,2013,2014) and Sequentially Time All-Optical Mapping Photography( Nakagawa 2014, 2015), our method is convenient to adjust the temporal resolution and to flexibly increase the numbers of frames. Theoretically, the temporal resolution of our scheme is limited by the amount of dispersion that is added to a Fourier transform limited femtosecond laser pulse. Correspondingly, the optimal number of frames is decided by the ratio of the observational time window to the temporal resolution, and the effective pixels of each frame are mostly limited by the dimensions M×N of the lenslet array. For example, if a 40fs Fourier transform limited femtosecond pulse is stretched to ~10ps, a CCD camera with 2048×3072 pixels can record ~15 framing images with temporal resolution of 650fs and image size of 100×100 pixels. As spectrometer structure, our recording part has another advantage that not only amplitude images but also frequency domain interferograms can be imaged. Therefore, it is comparatively easy to capture fast dynamics in the refractive index change of materials. A further dynamic experiment is being conducted.

High-peak-power terahertz sources pumped by high power laser and single-shot measurement of terahertz temporal waveform

Optical rectification of laser pulses in LiNbO3 crystal pumped by high power laser is one of the most powerful way to generate this high-peak-power terahertz pulses. It enhances the laser-terahertz transform efficiency by tilted-pulse-front pumping(TPFP) to fulfill phase match in the LiNbO3 crystal. However, comprehensive theoretical analysis is still lack. In this work, a detailed theoretical model to investigate the THz generation efficiency by using nonlinear susceptibility tensor of LiNbO3 crystal was presented. Based on femtosecond laser system, a setup to generate high-peak-power terahertz pulses and a time domain spectroscopy system are established. The property of generated terahertz pulses was analyzed by using terahertz camera and THz time domain system. We also realized the single-shot measurement of terahertz temporal waveform by using this terahertz source.

Application of focusing x-ray spectrograph with spatial resolution and uniform dispersion in Z-pinch plasmas measurement

Application of focusing x-ray spectrograph with spatial resolution and uniform dispersion in measurement of the imploding Al wire array z-pinch plasma is reported. Uniform dispersion (i.e., the linear dispersion is a constant, or in other words, the x-rays are dispersed on the detector with uniform spacing for every wavelength) is realized by bending the crystal of a spectrograph into a special shape. Since the spatial coordinate of the spectrum obtained by this spectrograph varies linearly with x-ray wavelength, it is very convenient for identification and processing of the experimental spectrum. The experimental results show that this spectrograph has high luminosity, high spectral and spatial resolution and is very suitable for the routine spectrum measurement on the Z-pinch facility or other high-energy-density-physics (HEDP) facilities.

Four-point bisensitivity velocity interferometer with a multireflection etalon

A four-point bisensitivity velocity interferometer system for any reflector (VISAR) with a renovative delay etalon is proposed and demonstrated. In this interferometer, we introduce a new film-coating strategy to accurately measure small velocity with relatively short and cheap etalon. Laser pointing to the etalon is split into two beams with different incident angles with each beam going through the etalon in different path. The beam with the smaller incident angle is reflected three times before it leaves the etalon, while the other beam with larger incident angle goes through the etalon to and forth only once. The delay time of the laser beam with smaller incident angle is almost three times longer than that of the beam with larger incident angle. In the example of the laser with a smaller incident angle, the velocity per fringe of this interferometer can be reduced by approximately three times. The etalon is optimized so that four laser beams can be penetrated in the vertical direction at the meantime. With an etalon of 200 mm in diameter and 150 mm in length, a four-point bisensitivity velocity interferometer can achieve the velocity per fringe of 100 and 350 m/s fringe. A measurement has been successfully undertaken for the steel flyer driven by the explosive where the developed interferometer applies.