A hybrid system for simultaneous velocity and distance measurements in dynamic experiments.

The accurate measurement of an object's motion requires knowledge of both its position and its velocity. The commonly used photonic Doppler velocimetry and broadband laser ranging techniques are ideal for conducting simultaneous dynamic experiments. Instead of employing two distinct measurement systems, we developed a hybrid system that can simultaneously measure velocity and distance in dynamic experiments. This reduces cost and complexity, particularly for multi-channel tests, by using only one probe, photodetector, and oscilloscope channel. By precisely synchronizing the velocity and distance data in time, the system can assist with interpreting the target's position and orientation.

Dual-channel Faraday rotation measurement for pulsed magnetic field.

The immunity and survivability in a strong electromagnetic environment make the Faraday Rotation (FR) method irreplaceable for measuring pulsed magnetic fields. We present a dual-channel FR method that achieves more stable and precise measurement and more applicable magnetic field types than the conventional FR method. By splitting the modulated light into two components, the ratio of the two components is transformed into a tangent function with two analyzers at a relative 45°. The use of terbium gallium garnet, which has a significant magneto-optical effect, as a magneto-optical medium enables higher sensitivity for measurement. We have successfully measured the variation in magnetic flux density of the capacitively driven pulsed magnetic field.

Influence of probe pressure on the pulsatile diffuse correlation spectroscopy blood flow signal on the forearm and forehead regions.

In a pilot study of 11 healthy adults (24 to 39 years, all male), we characterize the influence of external probe pressure on optical diffuse correlation spectroscopy (DCS) measurements of pulsatile blood flow obtained on the forearm and forehead. For external probe pressure control, a hand inflatable air balloon is inserted between the tissue and an elastic strap. The air balloon is sequentially inflated to achieve a wide range of external probe pressures between 20 and 250 mmHg on the forearm and forehead, which are measured with a flexible pressure sensor underneath the probe. At each probe pressure, the pulsatility index (PI) of arteriole blood flow on the forehead and forearm is measured with DCS (2.1-cm source-detector separation). We observe a strong correlation between probe pressure and PI on the forearm ( R = 0.66 , p < 0.001 ), but not on the forehead ( R = - 0.11 , p = 0.4 ). The forearm measurements demonstrate the sensitivity of the DCS PI to skeletal muscle tissue pressure, whereas the forehead measurements indicate that DCS PI measurements are not sensitive to scalp tissue pressure. Note, in contrast to pulsatility, the time-averaged DCS blood flow index on the forehead was significantly correlated with probe pressure ( R = - 0.55 , p < 0.001 ). This pilot data appears to support the initiation of more comprehensive clinical studies on DCS to detect trends in internal pressure in brain and skeletal muscle.

Multi-reference broadband laser ranging to increase the measuring range.

Broadband laser ranging (BLR) is an appropriate method to obtain absolute distance in dynamic experiments. In this article, we first analyze the performance limit for BLR and indicate that the measuring range can be hardly increased while keeping the distance or time resolution unchanged. Then, multi-reference BLR is proposed, which can break this limit and greatly increase the measuring range. Its validity is demonstrated by an experiment with an explosively driven aluminum surface flying over 100 mm. This method would improve the capability of BLR.

A multi-spot laser induced breakdown spectroscopy system based on diffraction beam splitter.

A quick simultaneous multispot laser induced breakdown spectroscopy (LIBS) system has been proposed. The basic idea is to combine the Diffraction Beam Splitter (DBS) with the linear-to-linear fiber bundle. The DBS divided the incident laser beam into five sub-beams, and then, a lens focused the incident sub-beam to produce a plasma array, where the distance between the neighbor subplasma was constant and the plasma emission was imaged on the fiber bundle. Each ablated spot on the sample generated the corresponding spectroscopy signals, which were collected by the defined fibers of the fiber bundle, propagated to the spectrograph slit, and then analyzed by the intensified charge-coupled device (ICCD) detector, where the two-dimensional capability of the charge-coupled device detectors was explored for the spectroscopy and position analyses. The five spectroscopy ribbon presented on the ICCD corresponded to the plasma radiation of the five ablated spots. The feasibility of a simultaneous multipoint spectroscopy detection at a single pulse ablation was confirmed by subjecting to the spectroscopy analyses of a copper plane, layered material, and magnesium-molybdenum mixture. The multispot LIBS system can be used in the ejecta research of detonation physics, fluid physics, and so on.

Ultrasensitive specific terahertz sensor based on tunable plasmon induced transparency of a graphene micro-ribbon array structure.

We proposed an ultrasensitive specific terahertz sensor consisting of two sets of graphene micro-ribbon with different widths. The interference between the plasmon resonances of the wide and narrow graphene micro-ribbons gives rise to the plasmon induced transparency (PIT) effect and enables ultrasensitive sensing in terahertz region. The performances of the PIT sensor have been analyzed in detail considering the thickness and refractive index sensing applications using full wave electromagnetic simulations. Taking advantage of the electrical tunability of graphene's Fermi level, we demonstrated the specific sensing of benzoic acid with detection limit smaller than 6.35 µg/cm2. The combination of specific identification and enhanced sensitivity of the PIT sensor opens exciting prospects for bio/chemical molecules sensing in the terahertz region.

Bi-fiber quasi-axis probe for photonic Doppler velocimetry for shock physics experiments.

Photonic Doppler velocimetry (PDV) has become a major domain velocity measurement technique for shock physics experiments. In this study, we propose and validate a bi-fiber quasi-axis probe for PDV that consists of a multimode source fiber and a single-mode detection fiber. This structure makes the received light by the probe totally reflected or scattered from the target, and, therefore, is free from the influence of unwanted return light. The light-receiving efficiency of the probe was calculated by the Monte Carlo method and was validated by static experiment. We validate that the feasibility of probe by dynamic experiments with measurement depth up to 6 cm. Above all, the bi-fiber quasi-axis probe is a promising supplementary probe for PDV for shock physics experiments.

Purified frequency modulation of a quantum cascade laser with an all-optical approach.

Purified frequency modulation (FM) is demonstrated in a standard middle-infrared quantum cascade laser by illuminating its front facet with two near-infrared (NIR) lasers. A 2 mW laser at 1550 nm is utilized to modulate the amplitude and frequency of a quantum cascade laser, and the associated amplitude modulation (AM) is suppressed by a 1.85 mW laser at 850 nm. Due to the hot carrier effect and the increment of electron temperature, the AM has been decreased. In addition, the free carrier concentration increases in the active region due to the two NIR illuminations, which enhance the FM. Purified FM is beneficial in improving the signal fidelity for free-space optical communication and high-speed FM spectroscopy.

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.

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 function's 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.

K-shell emission x-ray imaging of Z-pinch plasmas with a pinhole and a logarithmic spiral crystal.

An in-chamber, mini x-ray imaging instrument employs a pinhole and a logarithmic spiral crystal has been developed for obtaining K-shell line images of the imploding aluminum wire array on the "Yang" accelerator. The logarithmic spiral crystal acts as a monochromator and a non-dispersive mirror that reflects the pinhole image to a x-ray film detector with a very narrow photon energy bandwidth (<1 eV, mainly determined by the width of rocking curve of the crystal). Two imaging configurations with the use of Quartz (10 ̅10) crystal and Mica (002) crystal are designed, respectively, to image the Al Ly(α2) line (1727.7 eV) emission and Al He(α) intercombination line (1588.3 eV) emission. The primary experimental data corresponding to these two configurations are presented and discussed.

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.

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.