Pulsed Digital Holography Research Articles

Portable digital holographic particle analyzer (DHPA) for pneumatically conveyed fuel monitoring: Design and validation

Efficient, clean, and intelligent operation of coal-fired power plants plays a crucial role in the pursuit of energy conservation and carbon reduction. Online monitoring of pneumatically conveyed fuel is essential to furnish fundamental data for combustion optimization. This paper proposes a digital holographic particle analyzer (DHPA) with a portable design for monitoring pulverized fuel fineness. DHPA consists of a particle sampling and recycling probe, a digital inline holographic system, a controlling module, and software. The probe extracts representative fuel particles from the pipe, which are subsequently measured in a three-dimensional field by pulsed digital inline holography before being returned to the pipe. The performance tests of DHPA for coal fineness measurement encompass investigations into the dispersion of particles within the measurement volume, the minimum particle number necessary for reliable results, a comparison with the laser particle analyzer and sieving method, as well as sensitivity to mill parameter changes. It is recommended to capture at least 4 × 104 particles to ensure a credible outcome. DHPA exhibits comparability with the traditional sieving method, displaying an absolute error of 1.48% for fineness R90 measurement. Moreover, DHPA has been successfully employed for measuring and optimizing the coal fineness at full-scale power plants. On-site measurements have unequivocally confirmed the feasibility and effectiveness of DHPA for monitoring pulverized fuel.

Perforation induced ligament evolution in low-temperature swirl spray via off-axis holography

Ligaments generated from a perforated sheet are commonly observed in the primary breakup process and have a fundamental influence on spray characteristics. Ignition and combustion instabilities of aeroengines at high altitudes raise the demand for dynamic measurement of ligaments in a low-temperature spray. Picosecond pulsed digital off-axis holography (DOH) system with 65 kHz high-speed and 4.5μm equivalent pixel size high-resolution setups is established. Three-dimensional (3D) evolution of ligaments in a pressure swirl spray under fuel temperature of 245–250 K and fuel pressure of 0.2–0.8 MPa is characterized. The stringy object extraction method optimized by FMCs cross correlation algorithm successfully extracts a 3D arc-shaped ligament in the spray at 0.2 MPa. The transition of ligaments into droplets during two modes of sheet perforation as hole–edge and two-hole interactions at 0.5 MPa is quantified with time resolution up to 15.4μs. Both the nucleation and growth of perforations are found involved with surface waves, and their early diameter growth velocities remain stable in the short term, consistent with Taylor–Culick velocity characteristics. The 3D velocity of a pair of ligaments in the spray at 0.5 MPa is obtained based on the spatial displacement of their daughter droplets within double frames, indicating a downward helical movement trend after separation from the conical swirling sheet. This work demonstrates DOH a powerful tool for 3D morphology extraction and dynamic analysis of ligaments and perforations in the pressure swirl atomization scenario.

3D characterisation of debris clouds under hypervelocity impact with large-field pulsed digital in-line holography

To expand the field of view and obtain a full-scale image of a debris cloud, a large-field pulsed digital in-line holography (DIH) system is developed to study the three-dimensional (3D) positions and shapes of debris clouds generated by hypervelocity impact. A general model for the large-field pulsed DIH system is introduced. Derived from strict theoretical analysis, it suggests that the hologram recorded by a large-field pulsed DIH system can be described by an equivalent lensless system. At the Hypervelocity Impact Research Centre of the China Aerodynamics Research and Development Centre, based on hypervelocity impact equipment with a 7.6 mm bore, experiments on a 3 mm aluminium sphere impacting a 1 mm thick aluminium target plate with a velocity of 3.58 km/s were carried out. Ensuring the successful capture of the transient state of debris clouds, the large-field pulsed DIH system is synchronised with the impact event and the combination of the neutral density filter and the bandpass filter is proposed, to eliminate the plasma radiation and enhance the signal-to-noise ratio of the hologram. The experimental results show that the holographic fringes are clearly recorded and the detailed shapes and structures of both large and small aluminium fragments are observed after reconstruction. The structure of debris clouds can be divided into three parts: front, core and shell, agreeing well with results measured by laser shadowgraph. The study demonstrates the feasibility of a large-field pulsed DIH system for accurate measurements of ultrafast debris clouds and shows great potential in the diagnostics of hypervelocity impacts.

On-line measurement of pulverized coal fineness on a 300 MWe power plant with pulsed digital inline holography

Pulverized coal fineness (PCF) is a key parameter in coal-fired power plants, demanding accurate and on-line monitoring. This study presents on-line measurement of PCF on a 300 MWe coal-fired power plant with pulsed digital inline holography (DIH). A prototypal instrumentation system, comprised of sampling, measurement, collection, and output, is established and tested on the two mills of the plant. Fineness characterized by the size-area distribution (SAD) and various size-volume distributions (SVD) obtained by DIH are compared to the size-mass distribution (SMD) by the off-line sieving method. The absolute R90 deviations of SAD are 3.60% and 3.72% for the two mills, respectively, with the equivalent area circle diameter (Deac) to estimate the sieve size (Ds). A SVD for which Ds is estimated by the maximum inscribed circle diameter (Dmic) and the particle volume (VSeac) by the equivalent sphere with diameter of Deac yields the deviations of 2.18% and − 0.25%.DIH is applied to the on-line coal fineness monitoring on a 300 MWe power plant. An on-line holographic instrumentation system is established. Fineness results indicate the operation conditions of the tested mills. Different fineness characterizations are compared and two optimums are recommended.

Measurement of airblast atomization of low temperature kerosene with 25 kHz digital holography

Atomization of low temperature fuel is of great importance in a jet engine combustor, and its visualization and diagnostics are challenging, especially in the presence of shreds and ligaments. A 25 kHz picosecond pulsed digital inline holography system is applied to measure the spray produced by an airblast atomizer with low temperature kerosene. The breakup process of shreds and ligaments of -37∘C kerosene are visualized and analyzed, revealing the transition of liquid jet disintegrating into droplets under the atomizing air film. Three-dimensional positions and size distributions of droplets in spray are acquired. It is found that a decrease in the kerosene temperature increases shreds and ligaments in the spray, which eventually leads to reduction of droplet density and increase of droplet size, while increasing the air pressure can compensate the degradation. This work also demonstrates that high speed holographic imaging is a powerful tool in spray diagnostics and liquid breakup visualization.

Picosecond pulsed digital off-axis holography for near-nozzle droplet size and 3D distribution measurement of a swirl kerosene spray

Near-nozzle spray formation determines the spray structure of downstream dilute spray, and thus plays an important role in evaluating the performance of air blast atomizer. To capture the exact structure of the atomization field in near-nozzle region, the spray cones from the exit of a two-stage swirler to the 9 mm downstream area with the gas pressure of 0.5 kPa, 1.0 kPa, 3.0 kPa and the fuel pressure of 0.03 MPa and 0.71 MPa, are investigated using a picosecond pulsed laser digital off-axis holography (DOH) system. The spray structures and droplet characteristics in near-nozzle region are visualized with clear image of nonspherical droplets and then quantified. Semi-spray cone angles from the front view (x-y) projections of particle fields are obtained and results indicate that the angle increases with the increase of gas pressure within test range. From the overall view of spray field, the droplet peak diameter decreases with gas pressure, from about 80 μm at 0.54 kPa to about 30 μm at 3.05 kPa. In addition, cumulative volume distribution of droplet obtained in near-nozzle region agrees well with modified Rosin–Rammler distribution. From the local view, spatial distributions of the normalized droplet number density and Sauter mean diameters (SMD) ranging from 30 μm to 480 μm of the swirl spray field are statistically analyzed. Results indicate that as the gas pressure increases, the SMD distribution is more uniform. Under all gas pressure conditions, SMD distributions tend to be larger in the near-exit area and the region inside mist cone edge. The work also demonstrates that picosecond pulsed DOH is a powerful tool for 3D diagnostics of spray.

Three-dimensional characterization of debris clouds under hypervelocity impact with pulsed digital inline holography.

The increasing space debris poses a great threat to in-orbit spacecraft and satellites, because its hypervelocity impact can bring about fatal mechanical and electrical damage to them. This work applies pulsed digital inline holography (DIH) to measure three-dimensional (3D) positions and shapes of the debris clouds generated by the hypervelocity impact in the Whipple shield. Detailed operation procedures of synchronizing the pulse DIH system with the impact event and removing the strong plasma radiation are presented, ensuring the successful capture of the transient state of ultrafast ejecta. Experiments on a 2.25mm aluminum sphere impacting a 0.5mm thickness aluminum target plate with a velocity of 3.6 km/s are carried out at the Hypervelocity Impact Research Center of the China Aerodynamics Research and Development Center, and results show that the holographic fringes are clearly recorded and the debris fragments are reconstructed and located accurately, agreeing well with the results measured by laser shadowgraph. This work demonstrates the powerful capability and great potential of DIH in the diagnostics of hypervelocity impact.

Single-shot polarization digital holography for recording stimulated Raman scattering signal for time-resolved measurement of gaseous species

The stimulated Raman scattering (SRS) process is sensitive to the relation between the polarization direction of the two laser beams (the pump and the Stokes) that generate it. In this paper, we made use of the polarization sensitivity of the SRS process and used polarization-resolved pulsed digital holography to record the signal from one single-shot hologram. The pump beam polarization was kept vertical, while the Stokes beam polarization was 45 deg. The two polarization components of the Stokes beam were recorded in a single hologram by blending the Stokes beam with two reference beams with orthogonal polarization on the detector. The two components of the Stokes beam were separated in the Fourier domain, and the corresponding intensity maps were calculated. The vertically polarized component of the Stokes beam was amplified due to the SRS process, while the horizontal component experienced no gain. The difference between the vertically and horizontally polarized intensity maps, respectively, was calculated and Fourier transformed to separate the gain signal in the spatial frequency domain. The method was demonstrated on methane (CH4) gas at a pressure of 12 bars. Results show that SRS polarization holography is a promising technique for recording the SRS signal from one single-shot hologram for time-resolved monitoring of specific species.

Stimulated Raman scattering holography for time-resolved imaging of methane gas.

In this paper, pulsed digital holographic detection is coupled to the stimulated Raman scattering (SRS) process for imaging gases. A Q-switched Nd-YAG laser (532nm) has been used to pump methane gas (CH4) at pressures up to 12 bars. The frequency-tripled (355nm) beam from the same laser was used to pump an optical parametric oscillator (OPO). The Stokes beam (from the OPO) has been tuned to 629.93nm so that the frequency difference between the pump (532nm) and the Stokes beams fits a Raman active vibrational mode of the methane molecule (2922 cm-1). The pump beam has been spatially modulated with fringes produced in a Michelson interferometer. The pump and the Stokes beams were overlapped in time, space, and polarization on the gas molecules, resulting in a stimulated Raman gain of the Stokes beam and a corresponding loss of the pump beam through the SRS process. The resulting gain of the Stokes beam has been detected using pulsed digital holography by blending it with a reference beam on the detector. Two holograms of the Stokes beam, without and with the pump beam fringes present, were recorded. Intensity maps calculated from the recorded digital holograms showed amplification of the Stokes beam at the position of overlap with the pump beam fringes and the gas molecules. The gain of the Stokes beam has been separated from the background in the Fourier domain. A gain of about 4.5% at a pump beam average intensity of 4 MW/cm2 and a Stokes beam intensity of 0.16 MW/cm2 have been recorded at a gas pressure of 12 bars. The gain decreased linearly with decreasing gas pressure. The results show that SRS holography is a promising technique to pinpoint a specific species and record its spatial and temporal distribution.

Characteristic of femtosecond laser pulsed digital holography**Project supported by the National Natural Science Foundation of China (Grant No. 61275133).

Digital holography can be applied to ultrafast detection when a femtosecond laser pulse is used. In this paper, the interference process of two femtosecond laser pulses is studied and the recording process of the femtosecond laser pulsed digital hologram is simulated. Holograms at different recording angles are generated by integrating the instantaneous interference field. By analyzing the distribution of the reconstructed phase error, the characteristics of femtosecond laser pulsed digital holography are discussed.

Control of Material Transport Through Pulse Shape Manipulation—A Development Toward Designer Pulses

The variety of laser systems available to industrial laser users is growing and the choice of the correct laser for a material target application is often based on an empirical assessment. Industrial master oscillator power amplifier systems with tuneable temporal pulse shapes have now entered the market, providing enormous pulse parameter flexibility in an already crowded parameter space. In this paper, an approach is developed to design interaction parameters based on observations of material responses. Energy and material transport mechanisms are studied using pulsed digital holography, post process analysis techniques and finite-difference modelling to understand the key response mechanisms for a variety of temporal pulse envelopes incident on a silicon 〈1|1|1〉 substrate. The temporal envelope is shown to be the primary control parameter of the source term that determines the subsequent material response and the resulting surface morphology. A double peak energy-bridged temporal pulse shape designed through direct application of holographic imaging data is shown to substantially improve surface quality.

UV laser interaction with a fluorescent dye solution studied using pulsed digital holography

A frequency tripled Q-switched Nd-YAG laser (wavelength 355 nm, pulse duration 12 ns) has been used to pump Coumarin 153 dye solved in ethanol. Simultaneously, a frequency doubled pulse (532 nm) from the same laser is used to probe the solvent perpendicularly resulting in a gain through stimulated laser induced fluorescence (LIF) emission. The resulting gain of the probe beam is recorded using digital holography by blending it with a reference beam on the detector. Two digital holograms without and with the pump beam were recorded. Intensity maps were calculated from the recorded digital holograms and used to calculate the gain of the probe beam due to the stimulated LIF. In addition numerical data of the local temperature rise was calculated from the corresponding phase maps using Radon inversion. It was concluded that about 15% of the pump beam energy is transferred to the dye solution as heat while the rest is consumed in the radiative process. The results show that pulsed digital holography is a promising technique for quantitative study of fluorescent species.

Ultrafast nonlinear dynamics in thin GaN films studied by femtosecond digital holography

We studied the nonlinear propagation dynamics of intense 300 fs infrared pulses in thin GaN films by use of pulsed digital holography technique with 25 fs temporal resolution.

Holographic study of ultrafast optical excitation in GaN film induced by nonlinear propagation of light

The dynamics of pulse transformation and free-carrier generation associated with the nonlinear propagation of 1030 nm and 300 fs light pulses in Gallium nitride film has been studied by use of pulsed digital holography with 25 fs temporal resolution. Domination of the Kerr effect at the leading part and electron plasma at the trailing part of the pulse has been identified from the reconstructed spatiotemporal phase-contrast images. Experimental results have been supported by the simplified numerical model capable to reproduce the main features of a dynamic process.

Holographic measurement of thermal distortion during laser spot welding

Welding distortion is an important engineering topic for simulation and modeling, and there is a need for experimental verification of such models by experimental studies. High-speed pulsed digital holography is proposed as a measurement technique for out-of-plane welding distortion. To demonstrate the capability of this technique, measurements from a laser spot weld are presented. A complete two-dimensional deformation map with submicrometer accuracy was acquired at a rate of 1000 measurements per second. From this map, particular points of interest can be extracted for analysis of the temporal development of the final distortion geometry.

Single-shot diagnostic for the three-dimensional field distribution of a terahertz pulse based on pulsed digital holography

In this Letter, a pulsed digital holographic approach for detecting the three-dimensional (3D) field distribution of a freely propagating single terahertz (THz) pulse based on an electro-optic (E-O) sampling technique is proposed, by which the 3D field distribution of a single THz pulse sampled at different time points can be recorded in real-time on a series of subholograms and will be finally reconstructed as a series of two-dimensional spatial electric field distributions in a time series with a time resolution of femtosecond order. Simulation is carried out to demonstrate the process of the implementation, which confirmed the feasibility of the proposal.

Photoacoustic waves generated in blood studied using pulsed digital holography

We studied photoacoustic waves using pulsed digital holography. The acoustic waves were generated in a reindeer blood target by absorption of an IR laser pulse, lambda=1064 nm and pulse length=12 ns. The acoustic pressure waves were then imaged in water using a second collimated laser pulse at lambda=532 nm 2 micros after the first IR pulse. Quantitative information on acoustic wave properties such as three-dimensional shape and pressure distribution was calculated by applying the inverse Radon transform on the recorded projection. The pressure pulse had a flat and sharp front parallel with the blood surface, which indicates that the pressure was generated at the blood surface. The generated pressure was proportional to the laser fluence with the proportionality constant equal to 1.8+/-0.3 cm(-1). According to existing data, the proportionality constant should be 1.4 cm(-1) for oxygenated human blood, which made our calculations probable.

Depth-resolved measurement of phase gradients in a transient phase object field using pulsed digital holography

A technique to gain depth information from an image-plane digital holographic recording of a transient phase object positioned between a diffuser and an imaging system is demonstrated. The technique produces telecentric reconstructions of the complex amplitude throughout the phase volume using numerical lenses and the complex spectrum formulation of the diffraction integral. The in-plane speckle movements as well as the phase difference between the disturbed field and a reference field are calculated in a finite number of planes using a cross-correlation formulation. It is shown that depth information about in-plane phase gradients can be determined in two planes using reconstructed speckle fields from four different depths. In addition, the plane of optimum reconstruction for calculating the phase difference with maximum contrast is detected from the technique. The method is demonstrated on a measurement of a laser ablation process.

Angular multiplexing in pulsed digital holography for aperture synthesis

A new aperture-synthesis approach in a pulsed digital holography system for obtaining superresolution is proposed and experimentally demonstrated in which, with only a single laser pulse divided into two trains of subpulses, angular multiplexing can be applied to both the reference beam for different carrier frequency and the object beam for off-axis and on-axis illuminations. Furthermore, in the recording plane, multiple subholograms covering different spatial frequency ranges diffracted from the object can be recorded in a single frame of a CCD. Our analysis and results show that the resolution of the reconstructed image synthesized upon the reconstructed complex amplitudes of the subholograms can be improved effectively.

Finding the position of tumor inhomogeneities in a gel-like model of a human breast using 3-D pulsed digital holography

3-D pulsed digital holography is a noninvasive optical method used to measure the depth position of breast tumor tissue immersed in a semisolid gel model. A master gel without inhomogeneities is set to resonate at an 810 Hz frequency; then, an identically prepared gel with an inhomogeneity is interrogated with the same resonant frequency in the original setup. Comparatively, and using only an out-of-plane sensitive setup, gel surface displacement can be measured, evidencing an internal inhomogeneity. However, the depth position cannot be measured accurately, since the out-of-plane component has the contribution of in-plane surface displacements. With the information gathered, three sensitivity vectors can be obtained to separate contributions from x, y, and z vibration displacement components, individual displacement maps for the three orthogonal axes can be built, and the inhomogeneity's depth position can be accurately measured. Then, the displacement normal to the gel surface is used to find the depth profile and its cross section. Results from the optical data obtained are compared and correlated to the inhomogeneity's physically measured position. Depth position is found with an error smaller than 1%. The inhomogeneity and its position within the gel can be accurately found, making the method a promising noninvasive alternative to study mammary tumors.