We have used a nanosecond pulsed laser to study the dynamics of laser- induced breakdown with traditional optical detection on both nanosecond time scale and at later stages. Experiments on the induction of optical breakdown in the volume of liquid were performed using an Nd:YAG laser. It is shown that the optical breakdown in the liquid in the ultrasonic field is accompanied by an increase in the intensity of the spectral lines of potassium and oxygen with an increase in the amplitude and frequency of ultrasound. It was found that the effect of ultrasound on the intensity of the lines varies depending on the time of the breakdown evolution. Along with the optical spectra, the acoustic emission accompanying the pulsations of the cavitation bubble formed at the late stages of liquid breakdown source was studied. It was shown that the acoustic emission varies significantly with different ultrasound parameters. It is shown that an excited signal at its own switching frequency has a sufficiently high amplitude for its registration under typical experimental conditions. It is shown that the saturation effect is observed at frequencies above 200 kHz and at high ultrasound power, when the growth of the intensity of spectral lines slows down sharply. This effect indicates the possibility of using relatively small ultrasound powers for the implementation of the identified optoacoustic effects and spectroscopic properties in the laser breakdown in the liquid.
The method of laser induced breakdown spectroscopy (LIBS) was used to investigate intensive lines of sodium, calcium, potassium and magnesium in the aqueous solutions under the action of ultrasound at the frequencies of 10.7 kHz, 60 and 240 kHz. Acoustic radiation was generated within of cylinder transducers using a digital generator and powerful amplifier with maximum amplitude of 100 kPa. To excite optical breakdown in water we used a Nd:YAG laser with the wavelength of 532 and 1064nm. The changing of the delay of the control pulses allows to synchronize the time of maximum expansion of the liquid acoustic impulses and the start of the optical breakdown. The additional influence of ultrasonic radiation on the process of laser breakdown was studied. The increasing of the lines intensity under the influence of ultrasound is observed at all concentrations of the used solutions. It is shown that under the action of ultrasound on the dynamics of the cavity in a fluid the phase of the acoustic impact has a great significance. As a result it is shown that under the influence of ultrasound the strong strengthening of the intensity of spectral lines of elements dissolved in aqueous solutions is observed.
This article presents a method for automatic estimation of background intensities in the measurement of seawater fluorescence excited by pulsed laser radiation at wavelengths of 355 and 532 nm, and recorded by CCD matrix with an electron-optical converter (EOC). The problem to be solved is the variability of background intensities and background spatial distribution of CCD image caused mainly by EOC, including in the field of registration of useful signal of seawater fluorescence. This required the development of the numerical model that predicts the shape of the background of the entire image depending on the characteristics of the image areas in which the useful signal is not recorded.
The results of lidar sensing of the atmosphere in the continent-ocean transition zone obtained on three modifications of a femtosecond lidar: elastic scattering lidar, Raman lidar, white light lidar are discussed. In the mode of multi-frequency sensing (supercontinuum from the fundamental harmonic), the emission lines of the first positive system of the nitrogen molecule are registered. Comparisons of obtained data with the results of lidar sensing using of nanosecond laser pulses are presented.
The developed underwater laser induced breakdown spectrometer consists of two units: 1- remotely operated vehicle (ROV) with the next main characteristics: work deep – up to 150 meters, maximum speed of immersion 1 m/s, maximum cruise velocity - 2 m/s and 2 – spectrometer unit (SU) consist of a DPSS Nd: YAG laser excitation source (double pulse with 50 mJ energy for each pulse at wavelength 1064 nm, pulse width 12 ns and pulse repetition rate 1-15 Hz, DF251, SOL Instruments), a spectrum recording system (Maya HR4000 or 2000 Pro spectrometer, Ocean Optics) and microcomputer. These two units are connected by Ethernet network and registered spectral data are automatically processed in a MATLAB platform.
Research studies have been carried out the acoustic effects accompanying optical breakdown in water generated by focused laser and ultrasound radiation. Experiments were performed by using 532 nm pulses from a Brilliant B Nd:YAG laser. Acoustic radiation was produced by acoustic focusing systems in the form of hemispheres and rings at frequencies 10.7 kHz and 60 kHz. The experimental results show the sharply strengthened effects of acoustic emission from a breakdown zone by the joint influence of laser and ultrasonic irradiation. Various breakdown thresholds and character of acoustic emission in fresh and sea water are observed. The experimental result established that acoustic emission from optical breakdown of sea water in the presence and absence of ultrasound exceeds acoustic emission for the same experimental conditions in fresh water. Atomic lines of some chemical elements like sodium and magnesium were investigated for laser breakdown of water with the ultrasound field. The effect of magnification of these atomic lines resolution for salt water in ultrasound field was obtained. It is shown that the method of registration of acoustic emission from a breakdown zone allows to investigate thresholds and dynamics of laser breakdown which will be in accord with high-speed optical methods. The study revealed important practical applications of acoustic emission for breakdown and diagnostics of cavitation in opaque environments.
Excitation-emission matrix (EEM) technique is widely used to study the properties of organic matter in seawater. Additional variations of the exciting radiation parameters (in addition to the wavelengths), such as duration and intensity, allow to get more spectral information about the object of research, and allow to identify some individual components or to study the dynamics of the molecular changes in the sample of analyzed liquid.
One of the main limitations of the EEM is that usually necessary to carry out research in the laboratory on the specialized equipment. The paper describes the hardware system allowing carry out measurements in the field studies.
The hardware system includes a set of light-emitted diodes (LEDs) working in the range from 245 to 600 nm, which produce sequential excitation fluorescence of the sample liquid, which is detected using a 32-channel photomultiplier tube (PMT). Number of LEDs can vary from 7 to 16. Through the use of multi-channel photomultiplier reached the required measurement sensitivity and efficiency measurement is less than 1 minute for full cycle.
Minimum 4 channel in excitation range of 350-600 nm support the work of variable duration excitation pulses from microseconds to seconds. It makes possible to measure the fluorescence of the chlorophyll-a with closed or open reaction centers of phytoplankton cells. The use of multiple excitation radiation allows to activate various mechanisms of energy transfer in the photosynthetic apparatus of phytoplankton cells, improves measurement accuracy, reduces dependence on the variation in species composition and functional state of phytoplankton.
This paper presents the mobile complex to perform continuous measurements of chlorophyll-A concentrations and dissolved organic matter by the laser induced fluorescence method. The obtained data allow evaluating the state of the photosynthetic system of phytoplankton cells. In addition, the complex allows recording the CO2 gas analyzer data, the sea water saltiness sensors, pH, temperature, and solar illumination meter. The mobility this complex ensures a possibility working on different ships and under stationary conditions. The configuration of the complex can be promptly changed for solving the current problem by promptly adding or replacing various devices and sensors. The developed software integrates all devices into the complex. The enclosure of system allows use of conventional laboratory equipment, which is resistant to vibration, but not protected from the effects of the marine environment. Results of in situ measurements performed in Sea of Japan (Peter the Great Bay) by experimental setup are given.
Laser plasmas interaction and spectral characteristics of plasma were investigated at a laser breakdown in a
normal atmosphere with the purpose of improving laser-induced breakdown spectroscopy sensitivity. Colliding plasmas
interaction was investigated depending on mechanism of absorption wave of laser radiation and distance between foci.
Laser supported detonation wave, breakdown wave and fast wave of ionization are absorption wave observed in
experiment. It was shown that seed electrons for cascade breakdown in front of fast wave of ionization is occurred due
to oxygen molecules photoionization. Molecular emission and collapse of intensity of plasma continuum during the
initial moments of laser plasma expansion were registered. The line/continuum ratio was essentially increased in case of
laser plasmas interaction. Thus laser plasmas interaction improves sensitivity of LIBS.
Mechanisms of absorption wave of laser radiation and spectral characteristics of laser plasma were investigated at a laser breakdown in a normal atmosphere. Q-switched Nd:YAG laser operated at 1064 and 532 nm were used in experiment. Laser pulse consisted of prepulse and basic pulse for both laser wavelengths. Time interval between pulses was 15 ns, time duration (FWHM) was 4 ns for 532 nm wavelength, 5 ns for 1064 nm wavelength. Molecular emission and collapse of intensity of plasma continuum during the initial moments of laser plasma expansion were registered. It was carried out examination of colliding plasmas interaction which depended on absorption wave mechanism and distance between focal points of lens. The magnification of integrated intensity in case of plasmas interaction is registered. Decay time of continuous spectrum and lines emission was increased thus contrast of emission lines was also increased.
The results of atmosphere laser sounding carried out in frame of round the world expedition on the board of sail training ship "Nadezhda" are presented in this work. Nd:YAG laser was operated on wavelength λ=532 nm, energy per Q-switched pulse - 0.18 J, pulse duration - 4 ns. Laser repetition frquency was 10 Hz. Photomultiplies of backscattering signal receiving system operate in photon counting and analogue mode. Spatial resolution in photon counting mode - 240 m, in analogue mode - 6 m. The night dynamics and structure of boundary layers was investigated.
Influence of pulse duration and focussing mode the first and second harmoics radiation Nd:YAG lasers on shock waves characteristics arising at optical breakdown near to a surface of water was investigated in the present work.
The lasers with different parameters of radiation were applied to making the optical breakdwon. The first laser had pulse of duration 4 ns, energy 240 mJ, and wave length of radiation 1064 nm; the second laser had pulse of duration 18 ns, energy 400 mJ, and wave length of radiation 1064 nm; and the third laser had a uplse length 5 ns, energy 180 mJ, and wave length of radiation 532 nm. The laser pulse was focused by the lens (f=70mm) above a surface of a liquid, and then it was directed on surface and under surface of a liquid with the help of the microscrew. For each case, was carried out registration of optical breakdown and arising shock waves by the shadow camera. THe spectral complex "Flame Vision PRO System" with the time resolution about 3 ns was used to analyze the shadow images.
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