Optical Time-of-flight Research Articles

Plasma plume evolution of a capillary discharge based pulsed plasma thruster: An optical diagnosis study

This paper studied the plasma plume evolution process of a capillary discharge based pulsed plasma thruster. Time-resolved imaging and optical emission spectroscopy were applied to investigate the plume morphology and plasma species characteristics. It showed that ionized particles (mainly C II and F II) were accelerated early in the pulse and neutral particles (mainly C I and F I) later. An optical time-of-flight (OTOF) method was developed using a photodiode array combined with narrow bandpass filters. The equivalent streaming velocity of the plasma plume was evaluated with the OTOF technique. Measurements of individual species showed that both the ionized and the neutral species could be effectively accelerated by gasdynamic forces, and the ionized particles could reach a higher velocity. A Doppler shift measurement of the plasma plume was also performed to compare it with the findings from the OTOF method. The plasma plume streaming velocity of a thruster with a discharge energy of 5 J was measured and found to be (25.34 ± 0.17) km/s (OTOF) and (22.36 ± 4.02) km/s (Doppler shift). In addition, differences between the operation processes of the capillary discharge based pulsed plasma thruster and the electromagnetic pulsed plasma thruster were analyzed.

Consideration of optical time of flight measurement in laser induced plasmas

We considered some specifics of Optical Time Of Flight (OTOF) in Laser Induced Plasma if the OTOF signal is recorded at a distance of a few millimeters from the target. We demonstrated that the shape of OTOF signal is influenced by process of excitation transfer among the excited states. In case when the excitation transfer is dominant analogy with Time Of Flight (TOF) technique, developed for pulsed desorption flows into vacuum, is not meaningful. It is shown that at least two OTOF signals, for two different spectral lines, are necessary to record in order to estimate whether the excitation transfer is an issue or not. Assuming that expansion of the plasma is isotropic we derived a simple expression that takes into account geometric details concomitant for OTOF signal. It is shown that appropriate alignment of the spectrograph in respect to the laser beam position is essential for correct interpretation of experimental data and that asymptotic behavior of OTOF signal depends on a vertical opening of the slit and on the lens system. We demonstrated that proposed expression for OTOF signal provides consistent data for temperature T of heavy particles and center of mass velocity vc if OTOF signals are recorded at different distances from the target. For the same OTOF signals Shifted Maxwell-Boltzmann (SMB) function failed to correctly reproduce the both T and vc. We also discussed impact of inaccurate estimation of zero position in time and length scale on evaluated temperature of heavy particles, T, and center of mass velocity, vc. Importance of the geometric details is demonstrated considering two OTOF signals recorded for different vertical openings of the entrance slit while the rest of parameters were exactly the same. Temperature of the heavy particles T and the center of mass velocity vc, evaluated relying on the proposed expression, had a consistent values no matter if they were inferred from OTOF signal for large or small vertical slit opening. For the same OTOF signals Shifted Maxwell-Boltzmann function failed to correctly reproduce the both - center of mass velocity and temperature T of heavy particles. Also, T and vc estimated for OTOF signals recorded at different positions from the target, applying proposed expression, are consistent supporting assumption that expansion is dominantly isotropic and collisionless at chosen experimental conditions. We also discussed impact of inaccurate estimation of zero position in time and length scale on evaluated temperature of heavy particles, T, and center of mass velocity, vc.

Passive optical time-of-flight for non line-of-sight localization

Optical imaging through diffusive, visually-opaque barriers and around corners is an important challenge in many fields, ranging from defense to medical applications. Recently, novel techniques that combine time-of-flight (TOF) measurements with computational reconstruction have allowed breakthrough imaging and tracking of objects hidden from view. These light detection and ranging (LiDAR)-based approaches require active short-pulsed illumination and ultrafast time-resolved detection. Here, bringing notions from passive radio detection and ranging (RADAR) and passive geophysical mapping approaches, we present an optical TOF technique that allows passive localization of light sources and reflective objects through diffusive barriers and around corners. Our approach retrieves TOF information from temporal cross-correlations of scattered light, via interferometry, providing temporal resolution that surpasses state-of-the-art ultrafast detectors by three orders of magnitude. While our passive approach is limited by signal-to-noise to relatively sparse scenes, we demonstrate passive localization of multiple white-light sources and reflective objects hidden from view using a simple setup.

Influence of pulse width on the laser ablation of zinc in nitrogen ambient

Time-resolved spectroscopic measurements of expanding plasma plumes generated by irradiating a solid zinc target with laser pulses of 7 ns and 100 fs durations are carried out in the ambient pressure range of 0.05–200 Torr of nitrogen. At the relatively high input fluence of ~16 J/cm2, fast and slow atomic species are found to appear at different times in the optical time-of-flight (OTOF) spectra, the dynamics of which is primarily determined by the pulse duration of the excitation laser. In fs LPP, the average speed of fast species is unaffected by an increase in ambient pressure, while in ns LPP, the speed is found to reduce with pressure. The slow species shows a sharp peak in the OTOF spectra with a narrow velocity distribution for fs LPP, indicating a large number density and low electron temperature, which is consistent with optical emission spectroscopic (OES) studies. On the other hand, for ns LPP, the OTOF of slow species shows a more broadened profile which can be attributed to strong plume–laser interaction. The dynamics of slow species is heavily influenced by the presence of shock waves, which leads to the occurrence of much slower species at larger pressures.

Spatial characterization of laser induced Yb plasma in argon using optical emission spectroscopy: Pressure effect

• Laser induced Yb plasma at different pressure of Argon is spatially investigated. • O-TOF profiles and excitation temperature are used to characterize the plasma. • At 100 Pa of argon background gas, shock wave begins to be formed. • Drag force expansion describes the plasma propagation at pressures bigger than 1 Pa. • Two components of velocity distribution of the Yb atoms are estimated. Spatial and temporal behavior of laser induced Ytterbium plasma plume is studied using optical emission spectroscopy technique. A third harmonic Nd:YAG nanosecond laser was used to generate Yb plasma plume at different Argon background pressures (1, 10, 10 2 , 10 3 and 10 4 Pa). The plasma dynamics was investigated based on the spatial behavior of the excitation temperature coupled with optical time of flight (O-TOF) profiles of neutral Yb emission line (555.65 nm) along the propagation axe of the plasma plume. Drag force model was appropriate to describe the propagation dynamics at all pressures except of the lowest one (1 Pa) where free expansion model is dominant. The velocity distribution of Yb I atoms were extracted using two terms of Shifted Maxwell–Boltzmann (SMB) distribution. The correlation between the spatial comportment of both excitation temperature and O-TOF profiles is discussed.

Optical time of flight studies of lithium plasma in double pulse laser ablation: Evidence of inverse Bremsstrahlung absorption

The early stage of formation of lithium plasma in a collinear—double pulse laser ablation mode has been studied using optical time of flight (OTOF) spectroscopy as a function of inter-pulse delay time, the distance from the target surface and the fluence of the ablation lasers. The experimental TOF measurements were carried out for lithium neutral (670.8 nm and 610.3 nm), and ionic (548.4 nm and 478.8 nm) lines. These experimental observations have been compared with that for single pulse laser ablation mode. It is found that depending on the fluence and laser pulse shape of the first pre-ablation laser and the second main ablation laser, the plasma plume formation and its characteristic features can be described in terms of plume-plume or laser-plume interaction processes. Moreover, the enhancement in the intensity of Li neutral and ionic lines is observed when the laser-plume interaction is the dominant process. Here, we see the evidence of the role of inverse Bremsstrahlung absorption process in the initial stage of formation of lithium plasma in this case.

Robust Optical Time-of-Flight Range Imaging Based on Smart Pixel Structures

The reliable detection of the three-dimensional position of arbitrary objects in a scene is a key capability of most animals and one of the most important tasks in machine vision. Today's preferred technical solution is optical time-of-flight (TOF) range imaging, due to its simplicity, its distance resolution, its large and adaptable measurement range, as well as the absence of shadowing problems. In order significantly to extend the application areas of TOF 3-D cameras, in particular for outdoor use, we show how their performance can be improved in all relevant respects: background light suppression is improved by an order of magnitude by the minimum charge transfer method. Multicamera operation is achieved by a binary pseudo-noise modulation/demodulation technique. This method also avoids all practical ambiguity problems typically encountered with harmonic modulation. Higher temporal demodulation resolution becomes possible with a pixel structure employing lateral electric fields. We have realized such pixels with a commercially available CCD/CMOS process, and our measurement results confirm that gigahertz demodulation imaging is possible. The practicality of all theoretical concepts is demonstrated with a miniaturized TOF 3-D camera platform whose LED array light source is modulated at a typical rate of 20 MHz. Our work contributes, therefore, to opening up new application domains of the soaring optical TOF range imaging techniques.

Pseudonoise Optical Modulation for Real-Time 3-D Imaging With Minimum Interference

In optical time-of-flight (TOF) range imaging, harmonic intensity modulation of the illumination source is very common. By detecting the phase delay between emitted and reflected sinusoids, the distance can be measured accurately. However, this harmonic approach does not allow for the concurrent operation of several TOF range cameras because the arbitrary superposition of several differently parametrized sinusoids leads to a sinusoid with incorrect phase. To minimize inaccuracies by multi-camera interference (MCI), pseudonoise (PN) modulated intensity signals are employed for robust TOF range imaging. The time of flight is locally measured by correlating the incident light intensity with two time-shifted versions of the PN sequence, making use of smart demodulation pixels. We derive two fundamental expressions for the basic limitations of TOF measurements using PN sequences. Firstly, the precision of the distance measurement is limited by photon shot noise, and it essentially shows an inverse square root dependence of the number of detected photoelectrons. Secondly, MCI causes an inaccurate distance measurement given as the ratio of two sums. The denominator is the sum of two autocorrelation and two cross-correlation values; the nominator is the sum of one autocorrelation and one cross-correlation value. Due to the lack of a strict mathematical theory of correlation properties of m-sequences, an exhaustive numerical simulation was carried out to obtain expectation values of the distance measurement inaccuracy as a function of the sequence length and the number of interfering cameras. For experimental verification, an image sensor with 176 times 144 demodulation pixels was manufactured with a standard CMOS process offering a CCD option. Measurements taken with up to five concurrently operating sensors were in excellent agreement with our theoretical predictions concerning achievable distance accuracy. This confirms the aptness of PN techniques for multi-camera optical TOF range imaging.

Early stage emission spectroscopy study of metallic titanium plasma induced in air by femtosecond- and nanosecond-laser pulses

In this work the laser induced plasma obtained in air at atmospheric pressure by the interaction of a fs (femtosecond) or a ns (nanosecond) laser pulse with a metallic titanium target has been investigated by optical emission spectroscopy. The temporal evolution of plasma parameters such as electron number density and excitation temperature has been determined in order to highlight the processes involved when the emission spectra are acquired at short time delays from the ablating laser pulse. A survey of elementary processes implicated during plasma formation and expansion of ns- and fs-Laser Induced Plasma has been performed. Departures from equilibrium conditions are even discussed. The dynamic aspects corresponding to ns- and fs-LIP have been investigated by optical time of flight (TOF) and by fast emission imaging. The overall results have been used for clarifying the basic mechanisms occurring during plasma expansion due to either ns or fs laser source when experimental conditions usually used for laser-induced breakdown spectroscopy (LIBS) applications are employed.

A cold cesium atomic beam produced out of a pyramidal funnel

We have built an atomic funnel which produces a slow and cold cesium atomic beam. The atomic funnel is based on a pyramidal Magneto Optical Trap (MOT) with a small hole at its apex. Characterization of the funnel operation has been carried out by fluorescence emission and absorption spectroscopy, and optical time of flight (TOF) methods. The atomic beam has a longitudinal velocity in the range 8–12 m/s and a spread less than 1.5 m/s. The transverse temperature is close to the Doppler limit. Typically, an atom flux ∼4×10 9 atoms/ s is attained. These features, combined with the compactness and simplicity of the experimental arrangement, make this system an ideal source for experiments in atom lithography and atom optics.

Determination of Kinetic Energy Distribution in a Laser-Ablated Titanium Plume by Emission and Laser-Induced Fluorescence Spectroscopy

Pulsed laser deposition (PLD) of thin films has become a viable technique for a wide range of applications over the last few decades. As kinetic energy of the ablated plume is an important parameter in determining the quality of the film; consequently there is great interest in nonintrusive evaluation of the plume kinetic energy. Spectroscopic techniques such as optical time-of-flight (TOF) utilizes emission spectroscopy or laser-induced-fluorescence (LIF) and is an excellent method for this purpose since they offer temporal and spatial resolution as well as the capability of distinguishing different species. In this paper, the effects of laser fluence and background gas pressure on the kinetic energies of the ablated species are found by the optical time-of flight technique and by emission imaging. Laser-induced-fluorescence is employed for spectrally resolved imaging. The axial velocity of neutral titanium is found to be as high as 2×104 m/s. The distribution of species within the plume is also determined. [S0022-1481(00)01704-7]

Pulsed Laser Ablation of Boron Nitride

ABSTRACTThe probe beam deflection and optical time-of-flight method have been applied to study the dynamics of Nd:YAG laser (266 nm) ablation of pyrolytic boron nitride. Shock wave generation was observed and its velocity was derived by analysis of the acoustic transit time of the acoustic wave produced by laser ablation. The most probable velocities of B and B+ were obtained by optical time-of flight technique. The deposition of boron nitride on silicon substrate by laser ablation was attempted in nitrogen atmosphere. Cubic phase of boron nitride was observed as well as the hexagonal phase.

Interpretation of "optical time-of-flight" measurements in Si.

The optical time-of-flight (TOF) technique developed by Forchel, Hillmer, and co-workers for investigation of carrier transport in Si is examined. The TOF method detects the arrival of electron-hole pairs at the surface of a thin slab following their creation by pulsed laser excitation of the opposite surface. At high excitation levels, the optical TOF data were interpreted as evidence for rapid drift motion of electron-hole plasma at several times the sound velocity. At low excitation levels, the optical TOF results have been interpreted in terms of diffusive transport of excitons, surface recombination, and in some analyses, macroscopic drift. We show that both the high- and low-excitation cases are consistent with a very simple model that includes only classical diffusion and surface recombination. In particular, supersonic plasma drift is not required to explain the optical TOF results.