Digital Holographic Particle Tracking Velocimetry for 3-D Transient Flow around an Obstacle in a Narrow Channel

Digital holographic particle tracking velocimetry (PTV) is developed by single high-speed camera and single double pulsed laser with high frequency pulses. This system can directly capture 1000 hologram fringe images for 1 second through a camera computer memory. The 3-D particle location is made of the reconstruction by using a computer hologram algorithm in a personal computer. This system can successfully be applied to instantaneous 3-D velocity measurement in the water flow with a square obstacle, and can obtain an average of 300 instantaneous velocity vectors.     

Quantitative visualization of micro-tube flow using micro-PIV

This paper describes PIV measurements ofthe flow field in a micro round tube with an internal diameter of 100 μm in order to examine micro-scale effects. Since the refractive index of the micro tube almost corresponds to that of water, the inner flow in the tube can be observed clearly. The micro PIV system has been developed using a microscope, a high sensitive CCD camera, a double pulsed Nd:YAG laser and optics. Applying the micro PIV technique to the flow, the velocity distributions with spatial resolution of 1.8 × 1.8 μm were measured even near the wall in the center plane of the round tube. It was found that the velocities near the tube wall were smaller than the theoretical values calculated by using Poiseuille’s law. It is believed that this disparity is due to micro-scale effects such as interference between particles and the wall, friction at the wall, surface tension and so on.     

Direct measurement of fluid velocity gradients at a wall by PIV image processing with stereo reconstruction

Velocity gradient is typically estimated in Particle Image Velocimetry (PIV) by differentiating a measured velocity field, which amplifies noise in the measured velocities. If gradients near a boundary are sought, such noise is usually greater than in bulk fluid, because of small tracer displacement, uncertainty in the effective positions of velocity vectors, intense deformation of tracer patterns, and laser reflection. We consider here a modified form of the Particle Image Distortion (PID) method todirectly calculate velocity gradients at a fixed wall, and refer it as “PIV/IG” (“Interface Gradiometry”). Results from synthetic 2D PIV images suggest our method achieves higher SNR and accuracy than velocity differentiation. Also, we have developed a procedure to reconstruct three-dimensional velocity gradient at a fixed wall the two non-zero components from PIV/IG data obtained in stereo views; these equations simplify considerably thanks to the no-slip condition. Experimental data from the bottom wall of turbulent open channel flow appear to suffer from a form of pixel locking. As with standard PIV, this underlines the importance of adequate tracer diameter in the images, sufficient seeding density, and of dynamic range of the camera sensor.     

Synchronously pumped femtosecond optical parametric oscillator based on AgGaSe2 tunable from 2 μm to 8 μm

The operation of a continuous-wave mode-locked silver gallium selenide (AgGaSe₂) optical parametric oscillator (OPO) is reported. The OPO was synchronously excited by 120-fs-long pulses of 1.55-μm radiation at a repetition rate of 82 MHz. The 1.55-μm radiation is generated by a noncritically phasematched cesium-titanyl-arsenate (CTA)-OPO pumped by a mode-locked Ti:sapphire laser. The AgGaSe₂-OPO generates signal and idler radiation in the range from 1.93 μm to 2.49 μm and from 4.1 μm to 7.9 μm, respectively. Up to 67 mW of signal wave output power has been obtained. The experimentally determined pulse duration and chirp parameters are in reasonable agreement with results from a numerical model taking into account group velocity mismatch, group velocity dispersion, self phase modulation, and chirp enhancement. Received: 6 August 1999 / Revised version: 4 October 1999 / Published online: 3 November 1999     

Performance evaluation of an optical flow technique applied to particle image velocimetry using the VSJ Standard Images

A Dynamic Programming based Optical Flow technique has been applied to the Particle Image Velocimetry (PIV) problem. It has been used for the extraction of dense velocity fields in a planar section of a fluid flow illuminated by a thin laser light sheet. Two (in-plane) components of the velocity vectors can be recovered using a single camera and all three components can be recovered using two or three cameras. Quantitative performance tests have been carried out on calibrated synthetic image sequences from the PIV Standard Project of the Visualization Society of Japan (VSJ). Results are presented for the 2D flow based sequences (STD01 to STD08 Standard Images) and the 3D flow based sequences (STD301, STD302, STD331 and STD337 Standard Images). The RMS error is within the 2–3% range and within the 4–8% range for recovery of the two-component and the three-component velocity vectors respectively.     

Multi-kHz mixture fraction imaging in turbulent jets using planar Rayleigh scattering

In this study, we describe the development of two-dimensional, high repetition-rate (10-kHz) Rayleigh scattering imaging as applied to turbulent flows. In particular, we report what we believe to be the first sets of high-speed 2D Rayleigh scattering images in turbulent non-reacting jets, yielding temporally correlated image sequences of the instantaneous mixture fraction field. Results are presented for turbulent jets of propane issuing into a low-speed co-flow of air at jet-exit Reynolds numbers of 10,000, 15,000, and 30,000 at various axial positions downstream of the jet exit. The quantitative high-speed mixture fraction measurements are facilitated by the use of a calibrated, un-intensified, high-resolution CMOS camera in conjunction with a unique high-energy, high-repetition rate pulse-burst laser system (PBLS) at Ohio State, which yields output energies of ∼200 mJ/pulse at 532 nm with 100-μs laser pulse spacing. The quality, accuracy, and resolution of the imaging system and the resulting image sets are assessed by (1) comparing the mean mixture fraction results to known scaling laws for turbulent jets, (2) comparing instantaneous images/mixture fraction profiles acquired simultaneously with the high-speed CMOS camera and a well-characterized, high-quantum efficiency CCD camera, and (3) comparing statistical quantities such as the probability density function of the mixture fraction results using the high-speed CMOS camera and the CCD camera. Results indicate accurate mixture fraction measurements and a high potential for accurately measuring mixture fraction gradients in both time and space.     

Three-dimensional dynamic optical manipulation by combining a diffractive optical element and a spatial light modulator

Since the introduction of computer-controlled spatial light modulators (SLMs), holographic optical tweezers have become an important tool for dynamic parallel optical manipulation. In this paper we clarify the usefulness of a new configuration for optical trapping that creates light patterns using the combination of a diffractive optical element (DOE) and an SLM. This configuration not only enables the use of the higher part of the SLM’s diffraction efficiency curve, because a simple hologram can be chosen for the SLM, but also achieves three-dimensional dynamic optical manipulation over a large spatial range. By switching blaze-like holograms displayed on the SLM, we demonstrated simultaneous transportation of three 6-μm-diameter polystyrene beads over a range of 90 μm in the vertical direction and 37.5 μm in the horizontal direction. Compared with the same manipulation executed using only the SLM, the range of this method is extended four-fold in the vertical direction and three-fold in the horizontal direction.     

Diode-pumped actively Q-switched Tm:YAG ring laser operation at room temperature

2-μm lasers with high pulse energy and long pulse width of hundreds of nanoseconds are needed urgently in the accurate wind velocity lidar systems. This paper presents the acoustic-optical Q-switched Tm:YAG laser performance in a pulsed-laser-diode end-pumping figure-eight ring resonator structure. Pulse energy and pulse width are investigated with the increasing of the incident pump energy at different repetition rate operation. Maximum energy of 4.6 mJ with the pulse width of 179.2 ns and 3.57 mJ with pulse width of 184.4 ns are obtained at the repetition rate of 20 and 100 Hz, respectively. Under free-running and Q-switched operation, the peak output wavelength is 2.014 μm at all time, and the beam quality factors are lower than 2 times diffraction-limited measured by a knife-edge traveling method.     

Femtosecond fiber laser based methane optical clock

An optical clock based on an Er³⁺ fiber femtosecond laser and a two-mode He–Ne/CH₄ optical frequency standard (λ=3.39 μm) is realized. Difference-frequency generation is used to down convert the 1.5-μm frequency comb of the Er³⁺ femtosecond laser to the 3.4-μm range. The generated infrared comb overlaps with the He–Ne/CH₄ laser wavelength and does not depend on the carrier–envelope offset frequency of the 1.5-μm comb. In this way a direct phase-coherent connection between the optical frequency of the He–Ne/CH₄ standard and the radio frequency pulse repetition rate of the fiber laser is established. The stability of the optical clock is measured against a commercial hydrogen maser. The measured relative instability is 1×10⁻¹² at 1 s and for averaging times less than 50 s it is determined by the microwave standard, while for longer times a drift of the He–Ne/CH₄ optical standard is dominant.     

X-ray emission from femtosecond laser micromachining

We characterize the spectral properties of X-rays generated from selected metal and semiconductor targets when 120-fs laser pulses are focused to intensities of∼10¹⁴–3×10¹⁵ W/cm² during laser micromachining in air. High fluxes of multi-keV-energy X-rays could be obtained with 280-μJ pulses at a 1 kHz repetition rate. The yield and spectral composition of the X-rays are found to depend sensitively on the processing conditions, and thus the X-ray emission is expected to be a novel indicator of optimal laser machining. Received: 17 July 2000 / Accepted: 27 October 2000 / Published online: 28 February 2001     

Laser wake field acceleration: the highly non-linear broken-wave regime

We use three-dimensional particle-in-cell simulations to study laser wake field acceleration (LWFA) at highly relativistic laser intensities. We observe ultra-short electron bunches emerging from laser wake fields driven above the wave-breaking threshold by few-cycle laser pulses shorter than the plasma wavelength. We find a new regime in which the laser wake takes the shape of a solitary plasma cavity. It traps background electrons continuously and accelerates them. We show that 12-J, 33-fs laser pulses may produce bunches of 3×10¹⁰ electrons with energy sharply peaked around 300 MeV. These electrons emerge as low-emittance beams from plasma layers just 700-μm thick. We also address a regime intermediate between direct laser acceleration and LWFA, when the laser-pulse duration is comparable with the plasma period. Received: 12 December 2001 / Published online: 14 March 2002     

Stereoscopic flow-tagging velocimetry

A laser-based method for measuring the three components of the velocity in a plane simultaneously and instantaneously without seed particles is presented. This is achieved by combining a laser flow-tagging technique with stereoscopic detection, in which the tagged flow is viewed from two different directions. A single CCD camera is employed for this purpose by using a new optical detection system. The flow tagging is performed by two consecutive laser pulses, i.e., “write” and “read” laser pulses. The write laser creates a grid of tracer molecules (NO) by inducing a photodissociation process. The three-dimensional motion of the tracer molecules is measured by a thick read laser sheet. Received: 22 July 1999 / Revised version: 5 August 1999 / Published online: 30 September 1999     

Generation of optical waveforms in 1.3-μm SOA-based fiber lasers

Passive optical waveform generation is obtained in fiber lasers using a 1.3-μm semiconductor optical amplifier (SOA) as the gain medium. Various waveforms, including square wave, staircase wave, triangular wave, pulse, and dark pulse are generated in SOA-based fiber lasers by adjusting intracavity polarization controllers. The passive waveform generation might be attributed to the SOA gain dynamics and the enhanced nonlinear interaction at the 1.3-μm zero dispersion wavelength of traditional single-mode fiber (SMF), as well as the interference effect between the two sub-cavities of fiber laser. With figure-8 cavity configuration, 1250th-order harmonic pulses have been successfully demonstrated. We have also obtained a free-running SOA-based fiber laser with 3-dB spectral width of 16 nm, and the center wavelength can be tuned over 45 nm range.     

Cross sections of excited-state absorption at 800 nm in erbium-doped ZBLAN fiber

4 I_13/2 and ⁴I_11/2 of erbium is measured in a fluorozirconate fiber in the wavelength range 780–840 nm. Using a pump- and probe-beam technique and choosing the pump wavelength such that the perturbation by pump ESA is minimized in the measurement, it is possible to determine the effective ESA cross sections, despite the fact that the excitation is distributed among two metastable levels. The derived ESA cross sections at 793 nm of 1.4×10^-21 cm² from the ⁴I_13/2 level and less than 0.1×10^-21 cm² from the ⁴I_11/2 level are in reasonable agreement with former results obtained from a rate-equation simulation of the erbium 3-μm laser. The corresponding ESA spectrum under 3-μm lasing conditions is derived. At the strongest ground-state absorption around 799 nm, decreasing ESA from the ⁴I_13/2 level is compensated by increasing ESA from the ⁴I_11/2 level, i.e., ESA losses cannot be avoided when pumping around 800 nm. This result is of relevance for possible high-power diode pumping of an erbium 3-μm double-clad fiber laser. Received: 20 January 1998     

Diode-pumped Q-switched Tm:LuAG ring laser operation at room temperature

2-μm lasers with high pulse energy and long pulse width of hundreds of nanoseconds are needed urgently in the accurate wind velocity lidar systems. This paper presented the acoustic-optical Q-switched Tm:LuAG laser performance in a pulsed-laser-diode end-pumping figure-eight ring resonator structure. Pulse energy and pulse width are investigated with the increasing of the incident pump energy at different repetition rate operation. Maximum energy of 3.3 mJ with the pulse width of 199 ns and 1.8 mJ with pulse width of 293 ns are obtained at the repetition rate of 20 and 50 Hz, respectively. Under Q-switched operation, the peak output wavelength is 2.022 μm at all time, and the beam quality factors are lower than 2 times diffraction-limited measured by a knife-edge traveling method.     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Difference frequency generation by optical rectification of femtosecond laser pulse in GaAs crystal with a periodic domain structure

We study the influence of spatial boundedness of a femtosecond laser pulse (FLP) on the efficiency of generation of difference frequency radiation (DFR) in a GaAs crystal with a periodic domain structure. It is shown that at propagation of DFR a spectral-angular filtration occurs in the transverse distribution of the beam. An expression is obtained for the frequency-angular spectrum of DFR. For a pump pulse with the duration of 100 fs, the beam radius of 24μm, the energy of 30 nJ, and the wavelength of 1.98μm propagating in 1.716-mm long GaAs crystal with the domain structure period of 74.6μm the efficiency of generation of DFR at the wavelength of 14μm is calculated to be close to 2×10⁻⁶.     

Parallel-plate RF resonator imaging of chemical shift resolved capillary flow

Magnetic resonance imaging has been introduced to study flow in microchannels using pure phase spatial encoding with a microfabricated parallel-plate nuclear magnetic resonance (NMR) probe. The NMR probe and pure phase spatial encoding enhance the sensitivity and resolution of the measurement. In this paper, ¹H NMR spectra and images were acquired at 100 MHz. The B₁ magnetic field is homogeneous and the signal-to-noise ratio of 30 μl doped water for a single scan is 8×10⁴. The high sensitivity of the probe enables velocity mapping of the fluids in the micro-channel with a spatial resolution of 13×13 μm. The parallel-plate probe with pure phase encoding permits the acquisition of NMR spectra; therefore, chemical shift resolved velocity mapping was also undertaken. Results are presented which show separate velocity maps for water and methanol flowing through a straight circular micro-channel. Finally, future performance of these techniques for the study of microfluidics is extrapolated and discussed.     

Effect of ZnO plasma plume dynamics on laser ablation

The dynamic behaviors and optical properties of a ZnO plasma plume produced by pulsed laser ablation using a Nd:YAG laser (wavelength: 532 nm, pulse width: 3 ns) were studied by fast photography using a commercial gated charge coupled device (CCD) camera linked with a delay circuit and by optical emission spectroscopy at various ambient oxygen pressures. Fast photography was conducted with a resolving power of 0.25 μs and the expansion behaviors of the laser ablation plume were observed. Plasma plume expansion velocity decreased with oxygen partial pressure. The flow of the plasma plume in the early stage of expansion of up to 3 ms agreed well with the drag model.     

A passively Q-switched Yb:YAG microchip laser

We present a diode-pumped passively Q-switched Yb:YAG microchip laser, using a semiconductor saturable absorber mirror. We obtained pulses with 1.1-μJ energy, 530-ps duration, 1.9-kW peak power, and a repetition rate of 12 kHz. The laser is oscillating in a single longitudinal mode. Received: 23 October 2000 / Published online: 7 February 2001