The paper presents a mathematical model of linear and nonlinear processes occurring due to propagation of femtosecond laser pulses in vitreous of the human eye. By methods of numerical simulation, we have solved a nonlinear spectral equation describing dynamics of two-dimensional TE-polarized radiation. The solution was performed in a homogeneous isotropic medium with instantaneous cubic nonlinearity without using slowly varying envelope approximation. For simulation we used medium with parameters close to the optical media of the eye. The model of femtosecond radiation takes into account the dynamics of the process of dispersion pulse broadening in time and occurrence of self-focusing of the retina when passing through vitreous body of the eye. The dependence between pulse duration on the retina and duration of the input pulse was found, as well as the values of power density at which self-focusing occurs. It was shown that the main mechanism of radiation damage caused while using Ti-sapphire laser is photoionization. The results coincide with the results obtained by other scientists. They can be used to create Russian laser safety standards for femtosecond laser systems.
The power consumption in the two-pulse bispectral primary source could be substantially decreased by replacing the SRS converters from 1.06 μm into 10.6 μm wavelength as the preamplifier cascades in СО2 laser channel at the same efficiency radiation of EUV source. The creation of high volume manufacturing lithography facilities with the technological standard of 10-20 nm is related to the implementation of resist exposure modes with pulse repetition rate of 100 kHz. Low power consumption of the proposed scheme makes it promising for the creation of LPP EUV sources.
New concept of EUV radiation power scaling in the intermediate focus of the illumination system is proposed. The multiplex source scheme based on combination of several sources with acceptable level power allows to concentrate EUV light on the total power level of 1kW and more have been developed. The experimental results showed that the power consumption in the double-pulse bi-spectral primary source for EUV lithography can be substantially decrease by replacing pre-amplifiers in power CO2 laser on the SRS converters wavelength 1.06 μm to 10.6 μm while maintaining efficiency of EUV radiation output of illuminated plasma.
There are already significant amounts of hazardous radioactive substances in the world. It, potentially, leads to a major damage and contamination of large areas. Laser sensing can serve as a highly effective method of searching and monitoring of radioactive contamination. We developed a laser system to detect accidental leakage of radioactive materials. Methods of fluorescence spectroscopy and Raman spectroscopy allow to detect a concentration of uranyl U235O2 and U238O2 at 500 ppb, and Sr90 and Cs137 at the level of 1 ppm at 100 m distance from the object.
Over the past 100 years, the rate of temperature in the Arctic increases almost twice higher than the average rate of warming of the planet. Identifying methane anomalies responsible for the temperature increase, by hiking trails in the Arctic requires great human labor. It is necessary to use lidar methods for search and identification of methane from permafrost. Necessary to create a Raman lidar for monitoring of emissions of methane hydrate from the permafrost. Hyperspectral resolution would resolve the isotope shifts in the Stokes spectra, thereby to determine the isotopic composition of methane ratio C14/C12 CH4 carbon emissions and identify the source for study (permafrost or oil deposits)
We have demonstrated airborne lidar possessing spectral resolution λ/Δλ ≥ 1000. Its ultraspectral resolution is provided by the dual polychromator based on large-sized stigmatic holographic gratings. The lidar was tested in a real-life flight conditions at the transcontinental gas pipeline. It has performed aero search of leaks and measurements of the leaked gas concentration.
The results of experimental and theoretical modeling of multistage Raman conversion in compressed hydrogen with a buffer gas under using the lasing of neodymium-crystalline mediums to obtain the seed radiation for CO2 power amplifier in the double-pulse and bispectral primary laser while maintaining efficiency of LPP EUV source are presented. This scheme allows as significantly decrease level of pumping consumption and tune a delay time up to 820 ns between a weak power (initiating a plasma cloud) and main power pulses irradiated the converter target. The lasing intensity in the focal spots was 0.2 GW/cm2 and 5TW/cm2 respectively for wavelengths of 1.064 μm and 9.2 μm.
It is shown that the power consumption in the double-pulse bi-spectral primary source for EUV lithography can
be substantially decrease by replacing pre-amplifiers in power CO2 laser with the SRS converters wavelength
1.06 μm to 10.6 μm while maintaining efficiency of EUV radiation output of illuminated plasma.
In this paper we consider using the terahertz (THz) time domain spectroscopy (TDS) for non destructive testing and determining the chemical composition of the vanes and rotor-blade spars. A versatile terahertz spectrometer for reflection and transmission has been used for experiments. We consider the features of measured terahertz signal in temporal and spectral domains during propagation through and reflecting from various defects in investigated objects, such as voids and foliation. We discuss requirements are applicable to the setup and are necessary to produce an image of these defects, such as signal-to-noise ratio and a method for registration THz radiation. Obtained results indicated the prospects of the THz TDS method for the inspection of defects and determination of the particularities of chemical composition of aircraft parts.
Remote laser spectroscopy availability for airborne search of radionuclides polution has been examined. Experiments were carried out under the CARS circuit. The method of remote detection a radionuclide in atmosphere from container burial places and in places of recycling the fuel waste of the atomic power station is elaborated. Preliminary results of investigation show the real possibility to register of leakage of a radionuclide with concentration at level of 1012÷1013 см-3 on a safe distance from the infected object.
We have developed airborn lidar system to detect pipeline leakage and explore oil and gas deposits. Test flights indicate that a sensitivity of 6 ppm for methane and 3 ppm for hydrogen sulfide has been reached for leakage detection. As estimations have shown the reliability of HHG detection can exceed 80% at the integration method of seismic prospecting and laser remote sensing.
We developed a Raman lidar with ultraspectral resolution for automatic airborne monitoring of pipeline leaks and for oil and gas exploration. Experiments were carried out under the CARS circuit. Minimal concentrations of 200 ppb of heavy hydrocarbon gas have been remotely measured in laboratory tests. Test flights indicate that a sensitivity of 6 ppm for methane and 2 ppm for hydrogen sulfide has been reached for leakage detection. As estimations have shown the reliability of heavy hydrocarbon gas detection by the integration method of seismic prospecting and remote laser sensing in CARS circuit can exceed 80%.
We developed a Raman lidar with ultraspectral resolution for automatic airborne monitoring of pipeline leaks and for oil and gas exploration. Test flights indicate that a sensitivity of 6 ppm for methane and 2 ppm for hydrogen sulfide has been reached for leakage detection.
Laser sensing can serve as a highly effective method of searching and monitoring of radioactive contamination. The first method is essence consists in definition the Sr90 and Сs137 concentration by excitation and registration of fluorescence at wavelength of λ = 0.347÷7.0 μm at laser sounding. The second method experiments were carried out under the Raman-scattering circuit. Preliminary results of investigation show the real possibility to register of leakage of a radionuclide with concentration at level of 108÷109 сm-3 on a safe distance from the infected object.
In this paper, we present review and latest results obtained in the scope of terahertz holographic and other
methods for phase retrieval in terahertz imaging. Not only accurate change of amplitude, but also rigorous
phase retrieval is essential for precise calculation of optical parameters of the samples in terahertz range. Pulse
terahertz holography introduced some years ago shows itself as perfect method for overall-object phase retrieval
technique, but in the same time it allows measurement with low signal to noise that leads to less precise derivation
of sample optical parameters. And certainly just point-by-point terahertz time-domain spectroscopy provides the
most precise information of sample phase, but it is rather time consuming and has low spatial resolution as well.
The other possible way assumes, in contrary to pulse terahertz holography and spectroscopy, using narrow-band
continuous terahertz source, which tunability might also make the measurement process easier. And diffraction
patterns registered with microbolometer array or any other terahertz intensity sensor placed at several different
distances from the object and/or taken for several different terahertz frequencies are used for phase retrieval in
this case. We present both numerical predictions and experimental results for the proposed methods, estimate
the achievable spatial and other limits of the techniques and compare them to the others used in different spectral
ranges.
Results of supercontinuum generation experiments in case of excitation by optical pulses with central wavelength
828 nm, duration 40-50 fs, energy up to 1,0-1,5 mJ are presented. Supercontinuum radiation was detected in crystal
DKDP and other crystals with χ(2)-nonlinearity at range from 400 nm to 1450 nm, as well as up to 2300 nm, where
radiation within blue range 400-480 nm and IR-range 900-2300 nm was caused by parametric processes. Obtained
radiation was analog of microstructured fibers supercontinuum radiation in the width of output spectrum.
In this paper, the results of using the method of angular spectrum of plane waves as a mathematical model
of the iterative algorithm for phase retrieval based on the intensity distributions formed by the radiation of
THz frequency range are presented. This mathematical model is designed to calculate the wave field propagation
in the near-field diffraction, and in comparison with the Fresnel transform is more effective when working
with the THz radiation. The criterion for selection of registration planes which provides the best quality of
wave field reconstruction was determined as a result of studying the characteristics of the longitudinal intensity
distributions.
Application of terahertz radiation for the creation of medical equipment and solving of biological problems has become
widely spread. From this point of view, the influence of THz radiation on the nerve fibers is of primary concern. In
addition, several studies indicated both stimulating and depressive effects on nerve cells. However, the mechanism of
this effect has not yet been studied, including the dose and exposure time. Our research was devoted to the impact of
broadband pulsed THz radiation in the frequency range of 0.05 to 2 THz on the neurite growth in the sensory ganglia of
10-12-day chicken embryos. Dependence of changes in functional responses of cells on the average output power has
been found. An increase in the stimulating effect was observed at the lowest power density used (0.5 μW/cm2). Through
non-destructive process and choosing the correct parameters of THz radiation, potential control of neural response
becomes possible, which can subsequently lead to new medical treatments.
Using the infrared matrix of pyroelectric or other photodetectors along with THz band pass filters with pulsed
or CW sources one can record the 2D intensity distribution of THz radiation with a high degree of monochromatization.
This allows one to use various approaches to solving the phase problem which were developed for
the visible frequencies. In this contribution we present the results of the numerical investigation of the wavefront
reconstruction using THz radiation at several wavelengths and taking the intensity distribution at various
distances.
An iterative wavefront retrieval method based on intensity measurements formed by several wavelengths is investigated
in the present contribution. This multiwavelength technique is extended to use the intensity distributions
recorded in various planes of the volume speckle field. The ability to retrieve the wavefront using speckle patterns
is demonstrated in experiment. Two different experimental techniques have been used. The first proposed
method allows one to record three different intensity distributions corresponding to the three CCD RGB channels
at single exposure. This gives the advantage in the analysis of fast processes, e.g. phase microscopy of moving
biological cell-like objects investigation. The second technique involves using a large number of wavelengths of
supercontinuum radiation formed by photonic-crystal fiber. This approach provides faster and more accurate
convergence of the proposed method, has simple and rugged recording scheme with fiber optic elements.
We present a wavefront retrieval method for radiation comprising several wavelengths. Both numerical models
and experimental results are presented. Numerical modeling implies iterative phase retrieval procedure for all
wavelengths in spectrum. For reconstruction we can use two different algorithms, one inherits from one proposed
by Osten, Pedrini and Almoro, the second one implies expansion by Hermite-Gauss or Laguerre-Gauss basis set
which allows to decrease calculation time consumption. In experiment, speckle patterns can be formed either
by spectral supercontinuum radiation from photonic-crystal fiber (PCF) or by Stokes components of stimulated
Raman scattering (SRS) from second harmonic of pulse Nd:YAG laser radiation in barium nitrate crystal.
In this paper, we present description of new methods developed for imaging with terahertz(THz) waves - methods of THz pulse holography. We present two methods - referenceless THz pulse holography and THz
pulse holography with reference beam. Modeling and theoretic results already presented earlier 1,2 are followed by setup for THz holography design and obtained experimental results on referenceless THz pulse holographic
object imaging.
In the present paper research results of broadband THz radiation influence in a range 0.1÷2 THz on some biological tissues
are presented. Theoretical modeling of THz radiation propagation through the fat sample is performed. Experimental
absorption spectra of samples of vegetable oil, nail tissue, skin tissue and blood are obtained. Spectra of these tissues differ
in a range of 0.1 ÷ 2 THz. Also they depend on water content. From these samples vegetable oil has the best transmission.
This paper is dedicated to THz pulse holography with reference beam, opposite to our last year report on THz pulse
holography without reference beam1.
With reference source, we can use not only wavefront inversion modelling for reconstruction, but also reconstruction
by shorter wavelength radiation that is much easier to be viewed. Using almost the same setup as in THz holography
with reference beam, after digital registration we can reproduce hologram at the desired THz wavelength scaled to the
desired reconstruction wavelength, then, we use standard scheme for reconstruction. Taking into account time needed for
computational reconstruction, the new approach looks promising especially for practical purposes.
A comparison between reconstruction techniques related to resolution and noise dependency is given.
The main idea of suggested THz holographic system is to measure amplitude-phase diffraction patterns of an object at
THz range and further digital processing of obtained data.
For check of the method verifying numerical calculations have been carried out and the holograms of amplitude and
phase arbitrary shaped transparencies were obtained and reconstructed numerically.
Possible advances in computational object reconstruction as well as reconstruction resolution are discussed and
implemented.
The method of optical signal generation with repetition rate frequency of about 30-50 THz based on interaction of two femtosecond laser pulses propagating collinearly in transparent nonlinear media with different group velocities is presented. It was shown that for high input pulses intensities their interaction may result in formation of quasi-discrete spectrum with temporal structure consisting of many ultrashort pulses of near rectangular shape. It was demonstrated that generated sequence could be used in information technologies.
We have presented a novel method of effective anti-Stokes SRS-generation in one-dimension photonic crystals by realization of SRS quasi-phase matching conditions. By numerical simulations we have studied crystals with different contrast in layers refraction indices. It was shown that the efficiency of anti-Stokes SRS generation may reach up to 30%.
From wave Maxwell equation we derive a system of differential equations described multiwave backward and forward stimulated Raman scattering with diffraction effects. The results of numerical simulations of this system are quantity and quality agreed with experimental results. The numerical simulations of quasi-phase matched forward and backward SRS showed that the influence of backward SRS on QPM structure realization results in the small difference between layers length of optimal QPM stmcture and small decreasing of resulting anti-Stokes conversion efficiency (~25%at
backward and forward SRS, ~3O% at forward SRS). The oscillations of optimal layers length are partially connected with backward SRS influence and with insufficient precision of layers length determination due to high computational complexity of this task.
The numerical model of stimulated Raman scattering (SRS) taking into account spatial dynamics of amplitude-phase characteristics and consisting of non-linear partial parabolic differential equations is developed. A finite-difference numerical method was used to solve these equations. Results of simulation are in good agreement with experimental data.
Numerical model describing transient stimulated Raman scattering and taking into account diffraction was developed and Stokes wave evolution in compressed hydrogen was simulated to study space-time dynamics of amplitude-phase SRS characteristics. Space-time intensity and phase dependencies as well as spectrum and spatial coherence function of pump and Stokes waves were obtained. Considerable difference in mentioned characteristics was found out for transient and quasi-stationary stimulated Raman scattering modes. More complicated space-time dependencies are typical for transient mode in comparison with quasi-stationary mode. However, under quasi-stationary conditions Stokes wave phase varies in wider limits, which results in spatial coherency lowering. Module of spatial coherency function value lowers to threshold and then becomes stable as conversion coefficient increases. Presence of Stokes beam focusing is shown at stimulated Raman scattering, which can be explained by competition of strong Raman amplification and diffraction. Results of simulations are in good agreement with experimental data.
We analyzed analytically and numerically the possibility of high intensity laser pulse compression at stimulated scattering from plasma noise and at the non-linear interaction of a short seed pulse with a contra-propagated main long pulse on the inhomogeneous plasma slab. Optimal conditions for highest compression ratio are obtained. Conversion efficiency in these conditions can reach a high level.
Faraday optical isolator is used in laser systems with amplifiers to protect master oscillator from reflected and backscattered amplified light. The main part of FOI is a magnet system with magneto-optical glass. For traditional magnet systems of two or several alternative toroidal magnets it was shown that with growth of inner radius length and mass increase. Besides that axial magnetic field of such systems does not exceed remanence of magnetic material. The goal of this paper was to solve this problem by constructing Faraday optical isolator on the base of multisector permanent magnets, the practical realization of magnets with non-uniform magnetization. On the base of those magnets, spherical and toroidal permanent magnet systems were constructed. For toroidal systems constructed on the base of multisector magnet with inner and outer radiuses r1 = 0.6 cm, r2 = 3.6 cm, length l = 1 cm and remanence of magnetic material B = 12 kOe axial magnetic field H = 14.4 kOe was calculated. For spherical systems constructed on the base of the same magnet axial magnetic field H = 19.6 kOe was calculated. The research of toroidal and spherical multisector magnet systems showed that they can be used for constructing of compact
Faraday optical isolators.
We took into account the generation of high-order Stokes and anti-Stokes forward and backward components in our method of anti-Stokes generation at stimulated Raman scattering in a medium exhibiting variations of the third-order nonlinearity along the direction of propagation. The dependence of anti-Stokes generation efficiency on pump and Stokes input intensity and pulse durations for hydrogen were studied. The results of our research can be used for creation of new effective nonlinear-optical frequency converters and Raman amplifiers.
The results of experimental investigation of dynamic holograms recording by 20 ns and 300 ps laser pulses in fullerene-containing solid-state matrices are presented in this paper. The spectral analysis in range 300 - 700 nm shows correlation between efficiency of dynamic holograms recording and presence of absorption band at nearly 330 nm. We use two types of solid-state matrices: porous glass and polymethylmethacrylate hosts, which were filled with fullerene C60. From the dynamic holograms recording experiments and from the spectral analysis we obtained data on sensitivity and stability of the media, which can be used later in practical applications like all-optical switching.
We consider the quasi-phase matching conditions taking into account high Stokes and anti-Stokes SRS generation. The influence of Raman gain dispersion at multiwave interaction results in increasing of the calculation accuracy. For transient SRS the maximum efficiency of quasi-phase matching anti-Stokes generation does not depend on pulse duration while the medium length greatly increases with transient increasing. The results of our research can be used for development of new effective nonlinear frequency converters.
The theoretical model of dynamic hologram recording in fullerene solutions based on the system of balanced equation and equation for absorption is constructed. The profiles and diffraction efficiencies of the amplitude and phase gratings are numerically calculated depending on input intensity. Due to nonlinear absorption the profile of amplitude grating for high energies becomes significantly non-sinusoidal and the amplitude of phase grating increases. Computation shows appearance of high diffraction orders and their number and intensity increase with increasing of input intensity. The comparison of theoretical results with results of experiment1 shows good adequacy of the model.
We report the theoretical and experimental results of dynamic holograms recording in nano-size porous glasses containing fullerine C60 and its use for optical limiting. To record a grating with spatial frequency 20 mm-1 we used laser pulses with 18 ns duration and 532 nm wavelength. Initial transmittance of the samples varied from 30% to 70%. We found that high diffraction orders appeared out of the medium during the experiment and investigated dependence self-diffraction on initial transmittance of the samples and in case when the samples were filled by toluene. It was found that the most effective self-diffraction is in situation when we use toluene and initial transmittance of the sample is nearly 70%. For explanation of our experimental results we made a numerical model of dynamic hologram in fullerene-containing media taking into consideration amplitude and phase grating forming during the pulse length. Our model showed good agreement with experimental results.
We took into account the generation of high-order Stokes and anti-Stokes components in our method of anti-Stokes generation at stimulated Raman scattering in a medium exhibiting variations of the third-order nonlinearity along the direction of propagation. The dependence of Raman gain dispersion on quasi-phase matching conditions was studied by numerical simulation. The possibility of effective periodical quasi-phase matching structure realization for hydrogen was determined. We received the multiwave models of media in which the efficiency of first anti-Stokes generation reached ~40%. The results of our research can be used for creation of new effective nonlinear-optical frequency converters and Raman amplifiers.
We present a new, to our knowledge, method of anti-Stokes generation at stimulated Raman scattering in media with variations of third order nonlinearity along longitudinal coordinate. By numerical simulation the quasi-phase matching condition in different media and achievement of high efficiency of transient anti-Stokes conversion are obtained. The dependence of energy conversion from pump into anti- Stokes wave on the relation of input intensity of pump and Stokes waves is computed. We received the models of media in which the efficiency of anti-Stokes generation exceeded 30 percent. The results of the study can be used for the development of new effective nonlinear-optical devices for laser frequency conversion.
The results of experimental and theoretical investigations of dynamic hologram recording and optical limiter improvement in nano-size porous glasses containing fullerene C60 and toluene solution of C60 are presented. TO record a grating with spatial frequency 20 mm-1 we used laser pulses with 18 ns duration and 532 nm wavelength. Initial transmittance of the samples varied from 32 percent to 73 percent. Experiment showed that high diffraction orders appeared during dynamic hologram recording, and if the input intensity increased the number of orders increased, too. Presence of high diffraction orders caused more effective optical limiting because of laser energy redistribution. Taking into consideration different processes including reverse saturable absorption a model of dynamic hologram recording in fullerene solution waw carried out. The result of experiments were analyzed by numerical modeling. Analysis of computations and experimental results showed adequacy of the used model and perspective of applying of dynamic hologram recording in media containing C60 toluene solution for improvement of optical limiting.
We investigate an extremely complex mechanism of spectral supercontinuum generation at the propagation of intensive femtosecond laser pulses in microstructure (photonic crystal) fibers beyond the slowly-varying envelope approximation. We discuss and compare theoretical approaches grounded on the considerations of field and spectral evolution of the pulses. We present new nonlinear spectral equation, first-order in propagation coordinate, applicable for the study of femtosecond supercontinuum generation in microstructure fiber with arbitrary linear dispersion. We demonstrate a very good agreement to independently published experimental data found in numerical simulations. We study formation and evolution of few-cycle solitary waves under the effects of Kerr and Raman nonlinearities.
We present a new, to our knowledge, method of Stokes-anti-Stokes Raman amplification in fiber, based on our idea of anti-Stokes generation at stimulated Raman scattering in media with variations of third order nonlinearity along longitudinal coordinate. The quasi-phase matching conditions at different pump wave intensity are obtained by numerical simulation. The dependence of layers lengths on pump wave intensity is computed. We received the models of media in which the Stokes-anti-Stokes amplification coefficient is higher than 10 dB. We propose the analytical model of quasi-phase matching stimulated Raman scattering in silica fiber which shows significant calculations rate improvement (~120 times) comparison with differential equations model. The results of our research can be used for creation of new effective optical fiber amplifiers.
Recently there has been considerable interest in the problems of optical imaging in turbid, strongly scattering media, such as tumours in biological tissues, objects in water, etc. To detect objects in the media the analysis of backscattering of picosecond signal can be used. In this paper we report about the influence of medium parameters and detector parameters on temporal profile of the reflected pulse and its intensity. Virtual experiments were carried out with the MONTE-CARLO method, and temporal profile of signal was obtained. The dependencies of the forepart and tail-part of the signal fronts, maximum position of the reflected signal and the reflection coefficient from the scattering particle density and cross section were obtained. These dependencies show that the tail-part of the signal is greatly decreased while the density is increased, compared to the forepart and maximum intensity position of the signal. These results can be used to analyze the scattering particle density and cross section in the turbid materials. Virtual experiments with the presence of various inhomogeneities were performed, which show that not only reflecting and absorbing solid objects, but also even density inhomogeneities can be detected.
We present a new, to our knowledge, method of Stokes-anti- Stokes Raman amplification in fiber, based on our idea of anti-Stokes generation at stimulated Raman scattering in media with variations of third order nonlinearity ((chi) (3)) along longitudinal coordinate. By numerical simulation the quasi-phase matching conditions at different pump wave intensity are obtained. The dependence of layers lengths on pump wave intensity is computed. We received the models of media in which the Stokes-anti-Stokes amplification coefficient is higher than 10 dB.
We present a new, to our knowledge, method of anti-Stokes generation at stimulated Raman scattering in media with variations of third order nonlinearity ((chi) (3)) along longitudinal coordinate. By numerical simulation the quasi- phase matching condition in different media and achievement of high efficiency of anti-Stokes conversion are obtained. The dependence of energy conversion from pump into anti-Stokes wave on the relation of input intensity of pump and Stokes waves is computed. We received the models of media in which the efficiency of anti-Stokes generation exceeded 30%. The results of the study can be used for the development of new effective nonlinear-optical devices for laser frequency conversion.
The temporal compression with amplification and generation of powerful femtosecond pulses using stimulated Raman scattering in gases is studied by numerical simulation. Optimal values of medium parameters (Raman gain coefficient, group velocities dispersion) for highest conversion efficiency are determined. The dependence of Stokes wave amplification on input pulse duration and intensity is analyzed. The results can be used for development of effective nonlinear conversion devices of ultrashort laser pulses.
Transient SRS in mixture of two Raman active media (doubled gas mixtures) with pumping by ultrashort (2 ps) pulses was theoretically analysed. The numerical simulations show that there is strong energy exchange between direct Stokes waves and combination Stokes line during ultrashort pulse propagation in gas mixture. It is necessary to achieve the same phase and temporal dependencies of direct Stokes pulses for effective energy conversion in combination Stokes line. One-peak pulse generation in combination Stokes line with temporal compression is possible at pump intensity from threshold value up to doubled level. The decrease of duration of pulse by 12 times at conversion efficiency of 6% was shown. Increase of input pump energy causes destruction of Stokes pulses form and formation of multipeak Stokes pulses. The results show that the optimum ratio of gas pressure for transient SRS generation of sum and difference combination Stokes lines is proportional to the relation of the g/T2 of mixture components or to the relation of their Raman cross-sections. Our calculation is in good agreement with experimental results1.
We report experimental and theoretical investigation of supercontinuum generation in broad spectral range from 500 nm to 2500 nm induced in water and bulk fused silica by 1mJ 150-fs pulses at 780 nm excitation wavelength. We find that experiments may be modeled theoretically by considering Raman- and Kerr-type nonlinearities such as transient stimulated Raman scattering, parametric four-photon mixing, self-phase modulation and cross-phase modulation.
The processes of transient SRS generation and amplification of continuous Stokes radiation were investigated by mathematical modeling. The detailed calculations of Stokes generation evolution in gaseous hydrogen, methane, crystal hydrogen and in barium nitride were carried out using high finesse resonators. It was shown, that the distinctive feature of resonator continuous SRS is the presence of relaxation oscillations at the initial moments of development of Stokes generation. The 100 times amplification fo the input pulsed Stokes signals with duration determined by the time characteristics of the resonator and SRS medium was shown. The results of numerical calculations correspond to the results of experiments.
We performed numerical experiments on dynamic hologram (grating) recording and relaxation in fullerene C60 solutions and carried out the comparison with experiment on self-diffraction (intensity up to 50 MW/cm2, pulse duration 10nm, wavelength 532 nm, angle between pump beams 0.16 rad). The grating recording was numerically simulated by the differential equations: (1) balanced equations system describing population in six-level system, (2) time-space equation for absorption from different levels, (3) equation for the hologram thermal relaxation. Using this method the time-space dependence of medium refraction index due to thermal absorption was determined. Based on phase grating recording the estimation of diffraction efficiency in different diffraction orders was determined. It was shown that during the recording the shape of phase thermal grating becomes nearly rectangular due to absorption on exited singlet and triplet sublevels. The long time of life of triplet sublevels of C60 influences phase grating relaxation. The calculated values of diffraction efficiency in 1-5 levels are almost equal to the values form the experiment. According to our results of numerical analysis we can make a conclusion that the model successfully describes recording and relaxation of thermal phase hologram in fullerene solution. Results of phase grating relaxation can be used for investigation of fullerene properties and life-times of triplet sublevels. Our model can be used for numerical simulation of phase and amplitude grating recording and relaxation in fullerene solution and fullerite with pumping by nano-pico and femtosecond pulses and CW radiation.
The concept for a new approach to generating and receiving optical system for sodium guide star excitation based on solid-state laser technology and the stimulated Raman scattering/amplification in compressed gases is presented. Requirements to laser system for laser guide star generation are analyzed. The most promising SRS media are chosen.
Inna Belousova, Vlidilen Belousov, Victor Bespalov, V. Grigorev, Oleg Danilov, A. Zevlakov, V. Zgonnick, Alexander Kalintsev, A. Kris'ko, N. Mironova, Eugene Sosnov, Alexander Ponomarev
We present the results of theoretical and experimental studies on creation of nonlinear optical systems on a base of fullerene-containing media: power radiation limiters, photorefractive media for dynamic hologram recording, and devices for controlling spatial and temporal parameters of radiation.
We present the review of femtosecond SRS generation and amplification in compressed gases. the aim of our work is to study SRS spectral and temporal structures in compressed gases with femtosecond light pulses and to optimize conditions of excitation in order to obtain pulses with the desired spectral, temporal, and energy properties. In what follows, we present the result of our studies of SRS amplification in compressed hydrogen pumped by femtosecond pluses of the second harmonic of radiation of a titanium- doped sapphire laser. Our aim was to estimate the feasibility of increasing efficiency of SRS conversion and the potentialities of using transient SRS for spectral-time selection and amplification of weak signals.
The equations describing the propagation dynamics of optical pulses consisting of several electrical field cycles and pulses with spectrum bandwidth comparable with the central frequency are presented. The numerical simulation of nonlinear optical phenomena observed in isotropic media with pumping by the pulses with the spectrum in area of the normal group dispersion is carried out. The spectrum ultrabroadening of a short pulse with the temporal broadening, the self-induced changes of the pulse polarization, the simultaneous generation of several Stokes, anti-Stokes components, the third harmonic and sum and difference frequencies generation with self-phase and cross- phase modulation are considered.
On the basis of the analysis of the wave equations for an electrical field of radiation and without use of slowly varying envelope approximation the self-action of femtosecond light pulses in transparent media is investigated. The results of numerical simulation of spectral supercontinuum evolution, accompanying temporary broadening of intensive pulses with a spectrum in the range of normal group dispersion of medium both with only electronic nonlinearity, and with simultaneous electronic and electronic-vibrational nonlinearities are presented. The opportunity of compression of pulses with supercontinuum spectrum in light formations consisting of one cycle of an electric field is predicted. It is shown that spectral superbroadening of the elliptically polarized radiation is accompanied by nonuniform rotation of a polarization ellipse.
The reflection of intense laser pulses consisting only of several light field cycles from nonlinear, isotropic and transparent dielectric media is theoretically investigated. It is shown, that the spectrum of the reflected pulse is enriched by odd harmonics and at close linear refraction indices of bordering media the duration of the reflected pulse can become less significant than the duration of incident pulse. It is shown, that the intense refracted wave has a strongly asymmetrically broadened spectrum in nonlinear medium.
Manifestations of quantum fluctuations in stimulated Raman scattering ( SRS ) have been intensively studied lately1. Quantum noise present in spontaneous scattering stimulates fluctuations both in phase and in amplitude characteristics of output Stokes emission. At present statistical properties of amplitude fluctuations are studied in detail and it is shown that in linear transient regime of SRS without pump depletion statistical scatter of Stokes energy Es has negative exponential distribution2. In saturation regime of SRS the amplitude fluctuations are almost suppressed simultaneously with pump depletion. The phase fluctuations manifest both in linear and in nonlinear SRS and produce beam pointing fluctuations3, a change in Stokes emission spectra from pulse to pulse4 and cause soliton appearance in Rayleigh component of SRS5. Phase noise is observed in saturation regime due to relation between phases of Stokes and phonon waves which is preserved in processes of stationary and quasistationary SRS amplification. Spectrum phase fluctuations manifest as fine structure of SRS spectrum4, fluctuations of average Stokes frequency and Stokes linewidth fluctuations. The evolution of phase fluctuations in time is determined by the nature of spontaneous scattering and polarization of Raman media and is of great interest as one can follow its development from vacuum modes level to saturation regime. However up to now this problem has not been studied yet.
We report efficient amplification of weak femtosecond signals by a stimulated Raman scattering process on vibrational and rotational components of pressurized H2 gas excited with 200-fs-duration frequency-doubled pulses from a regenerative-amplified Ti:sapphire laser. The amplification factor of 108 is obtained at the wavelength of 465 nm for weak seed pulses produced by white light continuum generation in glass.
The investigation showed step by step picture of formation and development of SRS spectra fine structure including processes of: separation of the central and side modes from broadband smooth spectrum; narrowing of the components linewidth; frequency shifting of the side components to the central component; generating modes competition leading to the smooth changing in their relative intensities; the narrowing of SRS spectrum envelope; and frequency shifting of all the lines towards long wave region. These processes are general and are observed practically in all the cases however each of them has its own peculiarities and characteristic times, determined by pump threshold exceeding and Stokes emission duration.
We present the results on stimulated Raman scattering in hydrogen excited by 200 fs-duration 1 kHz repetition rate and 0.6 mW average power Ti:sapphire laser (780 nm) and 0.2 W average power second harmonic (390 nm) pulses with linear and circuit polarization. We report on the spectral and temporal measurements of the various generated vibrational and rotational components at Stokes- and anti-Stokes frequencies in the wavelength range 289 - 1154 nm. At optimal conditions at 390 nm pump wavelength the SRS signal of 30 mW average power was obtained at the first (465 nm) and the second (577 nm) vibrational Stokes. The shortest measured duration of the vibrational Stokes components was by 3 times less than the duration of the pump pulses.
We present results on stimulated Raman scattering in H2, D2, HD and CH4 excited by 200 fs-duration 1 kHz repetition rate and 0.6 mW average power Ti:sapphire laser (780 nm) and 0.2 W average power second harmonic (390 nm) pulses with linear and circular polarization. Spectral and temporal measurements of the vibrational and rotational components at Stokes- and anti-Stokes frequencies in the wavelength range 289 - 1154 nm are reported. We observed 2.5 times shortening of the vibrational Stokes component pulses in H2 and D2 exited by 350 fs-duration second harmonic pulses.
In this paper recording possibilities of non-stationary gratings in active medium of photodissociative iodine laser and using four-wave mixing for phase conjugation in its own medium were experimentally investigated for the first time.
Scatter in data of bandwidths is investigated both theoretically and experimentally. The model of Stokes wave with statistical character of phase jumps originating in quantum fluctuations of vacuum field/polarization is offered. A good agreement between experimental result and theoretical curves is obtained.
Recently there has been an increase in researchers' attention to the holographic optical elements (HOE) making it possible to connect in free space a wide range of elements and channels of optical devices of computers or communication networks. One of the difficulties in use of such elements is spectral distinction between operation wavelength of HOE (800 - 1500 nm) and wavelengths of spectral sensitivity of high efficiency media for HOE recording such as dichromated gelatin, photopolymer and others. We report the results of investigation of HOE two-step recording previously used for laser wavefront correction.
In this paper we report results of experimental investigation of spectra-time structure of backward SRS in crystal calcite with nano- and sub-nanosecond pulse duration and phenomenological treatment of this structure.
The results of the investigation of a pulsed hololaser or recording of monochrome (sized 600x400 mm ) and two-color (size 300x400 mm ) holograms and topograms (included living beings) are presented. The holosystem is based on Nd phosphate glass laser with frequency doubling (A nm) and Raman conversion in compressed hydrogen (A nm). 1.
The results of the investigation of small-sized laser system on crystals KGd(W04)2 coherence are presented. This laser system generates pulses with durat i on of 3 ns or 0 . 2 ns . The i nvest i gat ion of the rec i pros i ty 1 aw and hologram diffraction efficiency of silver halide holographic materials on these durations are reported. With the help of this hololaser hologram recordings of diffuse objects are produced. 1 .
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