Laser-induced Breakdown Spark Research Articles

Multi-point line focused laser differential interferometer for high-speed flow fluctuation measurements.

A multi-point focused laser differential interferometer (FLDI) has been developed to measure density fluctuations at 16 points along a line. A pair of cylindrical lenses on the transmitter side of a conventional single-point FLDI instrument form two closely spaced (≤200µm), orthogonally polarized, parallel laser lines at the instrument's focus. On the receiver side of the instrument, the interference of the beams on a 16-element photodiode array results in a single line of measurements. The further addition of a Nomarski prism creates two separate measurement lines, and the addition of a second photodiode array to the instrument enables simultaneous measurements of density fluctuations along the two lines separated by several millimeters. These two lines of measurement can be conveniently oriented at any azimuthal angle relative to the instrument's optical axis on the measurement plane, coinciding with the instrument's focus. Two experiments were performed to demonstrate the capabilities of the instrument. In the first experiment, a laser-induced breakdown spark generated a traveling spherical shock wave, and measurements of the resulting density disturbance and wave velocity were obtained. These results were compared to high-speed schlieren images of the shock wave acquired at 400 kHz. In the second experiment, the multi-point FLDI instrument was used to measure density disturbances in the boundary layer of a flat plate in a Mach 6 freestream flow. The measurements were made along two lines, both approximately 6 mm in length, extending from the surface of the plate through the boundary layer. High-speed schlieren images were acquired at 100 kHz during separate wind tunnel runs at matching unit Reynolds numbers to visualize the unsteady boundary layer flow and compare to the FLDI measurements.

High-order wavefront aberrations due to a laser-induced breakdown spark in still air.

Experimental measurements of the wavefronts of the light from a laser-induced breakdown (LIB) spark in stationary air are presented for the full range of angles between the LIB laser and wavefront sensor up to 90°. The wavefront data are compared to measurements of the spark shape and position acquired from simultaneous photographic images of the spark. The results show that the aberrations from the LIB spark appear primarily as two modes determined from proper orthogonal decomposition, with the amplitude and importance of the modes depending on system parameters and viewing angle. Data are also presented demonstrating the link between the spark wavefront and the spark shape, and conclusions are drawn regarding how to minimize the spark aberrations for optical systems in which the spark is used as a light source to measure optical wavefronts.

Wave-Front Measurements of a Supersonic Boundary Layer Using Laser Induced Breakdown

The aero-optical effect of a flat-plate adiabatic boundary layer has been measured using the light generated by a laser-induced breakdown spark. The measurements were performed in a blowdown wind tunnel at freestream Mach numbers of 3 and 4.38. The tests showed that the aero-optical effect of boundary layers with rms optical path difference as low as could be accurately measured using the laser-induced breakdown spark, including their deflection-angle spatial spectra. The results demonstrate that, using the laser-induced breakdown spark as a source of illumination, it is possible to make accurate measurements of low-amplitude aero-optical effects in a manner that is self-contained, nonintrusive, and suitable for a flight-test environment.

Wavefront measurements of a laser-induced breakdown spark in still air

Experimental measurements of the wavefronts of the light from a laser-induced breakdown (LIB) spark in non-moving air are presented and compared to spark dimensional data acquired from photographic measurements of the spark. The data show that the variation in the spark emitted wavefront between ignitions can be directly related to the motion of the spark volumetric centroid. The dominant modal components of the emitted wavefront variations are presented, as well as quantitative results for the magnitude of the wavefront variations. The results are relevant to the use of LIB as a light source for the measurement of optical aberrations such as those caused by compressible (i.e., "aero-optic") flows around an aircraft in flight, and data are shown indicating that LIB could be successfully used to measure the aberrating effect of compressible shear layers and boundary layers at typical cruise Mach numbers.

Measurement of the properties of a CO 2 laser induced air-plasma by double floating probe and spectroscopic techniques

The laser-induced breakdown spark has recently been advanced as a method for real-time, in-situ spectrochemical analysis of gases. Many of these analyses take place in ambient air. To better characterize this source, we have measured the temporal variation of temperature and electron density in an air plasma induced by a CO 2 laser operating at 0.5 and 0.8 J/pulse. The electron temperature was measured by the double floating-probe technique (DFP). An excitation temperature for oxygen atoms was determined spectroscopically by Boltzmann plots. Electron density in the plasma was measured from the Stark broadening of the 715.6-nm line of 01. At 0.5 J/pulse, the DFP temperature ranged from 175000 K at 5 μs to less than 10000 K at 25 μs, while the 01 excitation temperature ranged from 19000 K at 1 μs to above 11 000 K at 25 μs. The excitation temperature and electron density agree with values calculated by others from local thermodynamic equilibrium models of an air plasma. While the electron temperature from the DFP method is much higher than the excitation temperature at 5 μs, at times greater than 25 μs the two have converged, implying thermodynamic equilibration between the species.

Laser photolysis and spectroscopy: a new technique for the study of rapid reactions in the nanosecond time range

We have developed a new, very rapid, spectroscopic recording technique with the aid of which photographic absorption spectra of transient intermediates with lifetimes in the nanosecond time range can be obtained. The technique is a hundred times faster than present flash spectrographic instrumentation and provides time-resolved absorption spectra over a wide spectral range in a single experiment. Frequency-doubling is used to obtain both a 347 nm laser pulse suitable for excitation and a 694 nm laser pulse from a single Q -switched ruby laser pulse. The 694 nm pulse is converted into a continuum for absorption spectroscopy by bringing it to a sharp focus in a suitable gas, so as to cause a laser-induced breakdown spark. The excitation pulse has a duration of 30 ns. The duration of the continuum pulse varies with the gas used. In 1 atm of oxygen it has a duration of 30 ns, is synchronized to within 10 ns with the excitation pulse and has an intensity adequate for single-shot flash spectroscopy. The laser-induced spark in 1 atm of xenon has a duration of several μs and provides an excellent background continuum, of essentially constant intensity, for kinetic spectroscopy over periods up to 1 μs, using an image-converter camera to provide time resolution. We have applied the laser photolysis technique to observing, for the first time, absorption spectra arising from the lowest excited singlet states of several aromatic hydrocarbons. The time-resolved spectra obtained show the decay of the new excited singlet absorption bands and the concomitant build-up of triplet-triplet absorption bands during the first microsecond following light absorption, thus depicting graphically the non-radiative process of intersystem crossing from the lowest excited singlet state to the triplet manifold. The new bands also serve to locate the energies of higher excited singlet levels which, in many cases, are inaccessible from the ground state. The new technique should find wide application to solid, liquid and gaseous systems and should contribute to the understanding of photochemical primary processes in the time range 10 -9 to 10 -6 s. Eventual extension of the technique to the 10 -12 s range appears possible by using mode-locked lasers.