Q-switched Neodymium Laser Research Articles

The Role of Air in Laser-Induced Thermal Emission of Surface Layers of Porous Carbon Materials

The influence of the surrounding air on the amplitude and shape of thermal radiation pulses (at a wavelength of 430 nm) during the heating of the surface layer of a porous carbon material (to temperatures of the order of 2000–3000 K) by the radiation of a Q-switched neodymium laser is studied. When the pressure of the surrounding air is reduced to forevacuum conditions, the experiments showed a one-and-a-half-fold increase in the amplitude of pulsed signals of thermal radiation and an increase in the decay time of the glow by about a third. Numerical calculations of the dynamics of the temperature field in the surface layer of the material during the irradiation by nanosecond laser pulses are carried out. An improved model is used in the calculations, which accounts for (i) the porosity of the material and (ii) the temperature dependence of the coefficients of thermal conductivity and the heat capacities of carbon and air. To calculate the thermal conductivity of the porous material, a model of a cubic array of intersecting square rods is used. The satisfactory consistency of calculation results with experimental data is obtained. The above-mentioned improvements of the calculation model made it possible to reconcile the estimates of the thermal characteristics of surface layers of carbon, obtained from the emission decay data, with the reference data published in the literature.

Comparison of a single treatment with a Q-switched ruby laser and a Q-switched Nd:YAG neodymium laser for blue and black tattoo removal

BACKGROUND: Black and blue tattoo pigment are the most commonly used colors and can be injected at different depths. The effectiveness of tattoo removal is related to the selected wavelength, type of laser, energy parameters, number of procedures performed. Q-switched (QS) ruby and neodymium (Nd:YAG) lasers are most effective in removing blue and black tattoos due to their high peak power and ability to break the pigment into small particles that are absorbed by the macrophage system. In the literature available to us, it was not possible to find data on comparative efficacy and adverse events in their application.
 AIM: comparison of single treatment of blue and black tattoos with QS ruby laser (wavelength 694 nm) and neodymium Nd:YAG laser (wavelength 1064 nm).
 MATERIALS AND METHODS: 35 patients with blue and black tattoos were included in our study. The tattoos were divided into two parts or two adjacent tattoos were used on the same part of the body with the same density and color of the tattoo pigment; one side was treated with a ruby laser and the other with a neodymium laser. The comparison took into account the instantaneous reaction, the result of processing and side effects. Statistical significance was set at a power level 0.05.
 RESULTS: Edema and erythema were more common immediately after ruby laser treatment (power 0.05). The neodymium QS laser lightened tattoos better than the ruby QS laser after a single treatment (0.05 power). There was no difference in side effects between the two types of lasers.
 CONCLUSIONS: Neodymium Nd:YAG QS laser shows more efficiency and fewer procedures when removing blue and black tattoos. The instantaneous reaction after treatment with this type of laser is less pronounced than after the ruby laser.

Decay Kinetics of Thermal Radiation Emitted by Surface Layers of Carbon Materials under Pulsed Laser Excitation

The decay of thermal radiation emitted by surface layers of carbon materials excited by pulses of a Q-switched neodymium laser is investigated experimentally and theoretically. It is discovered that the decay curves can be approximated with satisfactory accuracy by a sum of two exponential components with decay times of about 10 and 100 ns. Changes in the decay curves under sample irradiation by a sequence of laser pulses can be interpreted as being the result of redistribution of intensities of these two components. Based on the results of computer simulation, the conclusion is drawn that the glow-decay time is determined by the ratio of the penetration depth of the laser radiation and the thermal-diffusion length, which creates an opportunity to determine the coefficient of temperature conductivity in a thin surface layer of the studied material at high temperatures (thousands of Kelvins).

Кинетика затухания теплового излучения поверхностных слоев углеродных материалов при импульсном лазерном возбуждении

For surface layers of carbon materials, under excitation by a Q-switched neodymium laser, the decay of laser-induced thermal emission is investigated both experimentally and theoretically. It is revealed that the decay curves can be fit with satisfactory accuracy by a sum of two exponential components with the decay times of the order of 10 and 100 ns. Under irradiation by a sequence of laser pulses, the observed transformations of the emission decay curves can be presented as redistribution of intensity of the above-mentioned two components. As a result of computer simulations, it is concluded that the emission decay time is determined by the ratio of the penetration depth of laser radiation and the thermal diffusion length. This fact opens a possibility for estimation of thermal diffusivity of an investigated material in a thin surface layer at high temperatures (thousands of Kelvins).

Initiation of PETN explosion by the second harmonic pulse of a neodymium laser

The explosion of PETN of density 1.73 g/cm3 was first initiated by the second-harmonic pulse of a Q-switched neodymium laser. It is shown that the primary process of energy absorption in this case is PETN molecule ionization involving two-photon absorption. The critical initiation energy density corresponding to a 50-% probability of explosion is 12.3 J/cm2.

Optical rectification effect in nanocarbon films

When a nanocarbon film obtained by plasmachemical deposition is fixed between two parallel electrodes and exposed to the pulsed radiation of a Q-switched neodymium laser, a pulsed electric voltage appears between the electrodes, with the pulse shape repeating the laser pulse envelope. It is shown that the amplitude and polarity of the pulsed voltage strongly depend on the angle of incidence and polarization of laser beam and on the spatial orientation of a carbon film with electrodes relative to the laser beam. The observed phenomenon exhibits all features characteristic of the optical rectification effect. For the optimum spatial orientation of a film, the factor of conversion of the laser pulse power into electric voltage amounted to 500 mV/MW, which is many times greater than the values observed in the case of optical rectification in well-known dielectric nonlinear optical crystals.

Pulsed mid-IR Cr2+:ZnS and Cr2+:ZnSe lasers pumped by Raman-shifted Q-switched neodymium lasers

The spectral and lasing properties of Cr2+:ZnS and Cr2+: ZnSe crystals pumped by Raman-shifted nanosecond neodymium lasers are studied. Tunable pulsed lasing is obtained in the mid-IR range between 2.05 and 2.40 μm. The minimum lasing threshold was 170 μJ at the maximum slope efficiency of 20 %.

Characteristics of Co2+:ZnS saturable absorber Q-switched neodymium lasers at 1.3 μm

Passively Q-switched neodymium lasers, Nd:YAG, Nd:YALO, and Nd:BEL, at 1.3 μm using the saturable absorber Co:ZnS were characterized. The best performance, about ∼0.35 mJ pulse energy and 300 ns pulse duration, was obtained using the Nd:BEL laser. In a Co:ZnS Q-switched Nd:YAG laser, sequential dual-wavelength Q-switched pulses at 1319 and 1338 nm were observed. The time separation between the output pulses was about 2–10 μs. Theoretical analysis is presented including analysis of the dual-mode Q-switched laser operation.

Efficient conversion into the near- and mid-infrared using a PPLN OPO

We report an efficient source of near- and mid-infrared radiation based on a periodically poled lithium niobate (PPLN) optical parametric oscillator (OPO). The pump source was a 15 ns pulse duration, 20 kHz repetition rate Q-switched diode-pumped neodymium laser. Resonated signals in the range 1465-1565 nm were generated, with corresponding idler wavelengths in the range 3323-3885 nm. The highest output power achieved was 1.5 W at 1547 nm, with 0.72 W at the corresponding idler wavelength of 3408 nm. This represents an optical-to-optical conversion efficiency of 42% from incident pump power to total OPO output. A minimum pulse energy threshold of 37.5 µJ was measured. The beam quality of both the signal and idler outputs degraded only slightly with respect to the pump beam.

Efficient wavelength conversion with high-power passively Q-switched diode-pumped neodymium lasers

Powerful CW diode-pumped Nd:YAG and Nd:YVO4 lasers Q-switched by Cr:YAG saturable absorbers demonstrate efficient (30%-60%) harmonic and parametric conversion, generating hundreds of milliwatts from ultraviolet to mid-infrared.

Experimental investigation of the coupling between fluctuations in the radiation energy at the fundamental and second-harmonic frequencies of a single-frequency passively Q-switched neodymium laser

An experimental investigation was made of the coupling between fluctuations in the fundamental and second-harmonic pulses generated by a single-frequency neodymium laser with passive Q switching of the resonator. An analysis of the experimental results led to the conclusion that fluctuations of the radiation energy at the fundamental frequency were accompanied by correlated fluctuations in the duration of the output pulses.

Laser flash effects on laser speckle shift visual evoked potential.

Steady-state visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally oscillating 10 degrees helium-neon laser speckle field (632.8 nm), moving vertically 2.5 degrees at 8 shifts per second. A 5-pulse flash train at the maximum permissible exposure (MPE) dose from a collimated Q-switched frequency-doubled neodymium laser (532 nm) was superimposed on the foveal stimulus and the subsequent disruption and recovery of the VEP measured. Minimal disruption of the response signal magnitude was demonstrated (0.1 greater than p greater than 0.05) which recovered within 300 ms of the termination of the pulse train. A small but significant (p less than 0.01) disruption of phase entrainment was also noted that recovered within the same period. This is contrasted with a second experiment with three monkeys in which an argon (514 nm) laser served both as the speckle stimulus source and as the shuttered flash. Exposure to collimated MPE argon radiation for 250 ms immediately depressed the VEP (97%, p less than 0.01) and showed recovery to 70% of the pre-flash baseline only after 3 s. Phase lock was also severely degraded for several seconds. These results imply that visual processing of nonacuity-limited medium contrast stimuli with broad spatial frequency content will probably not be materially affected by ultra-short pulsed laser exposure at these energy levels and frequencies. However, even safe levels of collimated continuous laser light may have severe effects on vision that could parallel flash effects seen with Xenon discharge flash lamps.

Flicker electroretinograms and visual evoked potentials in the evaluation of laser flash effects.

Electroretinograms (ERG's) and visual evoked potentials (VEP's) were recorded from four cynomolgus monkeys in response to a sinusoidally flickering argon laser beam (514 nm) producing a 50-micron spot on the fovea. Super-position of a 20-Hz train of six pulses of Q-switched (120 ns) frequency-doubled neodymium laser light (532 nm) at "safe" exposure energies in 250 ms had no significant effects on the ERG (p greater than 0.05). The VEP was disrupted significantly (p less than 0.001) but demonstrated recovery within 500 ms of the initial pulse. Therefore, flash effects of pulsed visible lasers at these doses on suprathreshold luminance processing are probably limited only to the exposure period.

Collision-induced I(52P1/2–52P3/2) radiative transition

An investigation was made of the dependences of the initial I(52P1/2)→I(52P3/2)+hν (λ = 1315nm) luminescence signal on the pressure and nature of the buffer gases. Atoms of I in the 52P1/2 state were formed, in initial concentrations of (0.5–3)×1016cm−3, as a result of photolysis of CF3I (0.01 atm) by the fourth harmonic of a Q-switched neodymium laser (E~0.6 J,τ ~2×10−3 sec). The I(52P1/2) luminescence was recorded by a germanium photodiode whose time constant was (3–4)×10−7 sec. When CF3I was diluted with Ar, Xe, and SF6 buffer gases, the intensity of the luminescence of the I(52P1/2) atoms increased because of an increase in the probability of the 52P1/2→52P3/2 transition as a result of collisions of the iodine atoms with the atoms or molecules of the buffer gases. The dependence of the probability of the investigated transition on the nature and pressure of the buffer gases was determined in the pressure range ~0–1 atm. Estimates were obtained of the oscillator strengths corresponding to the matrix elements of the induced electric dipole moment due to the deformation of the electron shells of the iodine atoms in collisions.

Pulse radiolysis and laser photolysis study of solvated electron solutions in hexamethylphosphortriamide

Laser photolysis ( Q-switched neodymium laser, 20 ns) and pulse radiolysis (2.2 μs electron pulses from a linear accelerator, dose per pulse ⋍2 krad) methods have been used in a study of the influence of light and ionizing radiation on electrochemically generated solvated electron solutions in hexamethylphosphortriamide (HMPA) at room temperature. The stoichiometry of association between e s - and metal cations (Na +) observed in the system is found to be (Na +, e 2 −2). It was found after a laser pulse that this complex with λ max = 750 nm is bleached and simultaneously a species absorbing in the i.r. (solvated electrons) is formed. The extinction coefficient of (Na +, e 2 -2) is 4 × 10 4 dm 3 mol −1 cm −1, the oscillator strength 0.7, and the quantum yield of photobleaching 0.075. The rate constant of (Na +, e 2 −2) formation K 1 = 1.2 × 10 6 dm 3 mol −1 s and the equilibrium constant between Na +, e s *minus; and the complex K ∗ = 2.3 × 10 4 dm 6 mol −2 .

Distribution of neodymium laser energy between modes

An experimental investigation was made of the spectral statistics of the radiation emitted by a Q-switched neodymium laser. The emission spectrum was recorded with a resolution equal to the model spacing. It was found that the density of the probability of the appearance of an axial mode of a given intensity was described by an exponential function and the mode intensities were not correlated with one another. The results were in good agreement with the hypothesis of the fluctuating nature of laser radiation. (AIP)

High-order multimode radiation statistics of aTEM 00 Q-switched neodymium laser

We interpret our recent experimental results concerning the dependence of the 11-photon Xe ionization rate on the axial-mode number of aQ-switched laser. If one takes the experimental procedure into account, the stochastical quantity of interest is the ratio between the instantaneous intensity and the mean intensity averaged over a round-trip duration. The enhancement factor normalized to the monomode case is the eleventh moment of that over-intensity. The measurements indicate that nearby longitudinal modes are FM-type phase correlated, while distant modes are phase independent. The radiation emitted by our Nd3+ glass laser have not the statistical properties of a pulsed Gaussian field, yet when the spectrum is sufficiently large (several Å) itsk-th-order over-intensity moment has the same valuek! as for a pulsed Gaussian field with a large spectrum.

Single-frequencyQ-switched neodymium laser

Reliable emission of one longitudinal and lowest transverse mode from a Q-switched neodymium laser was achieved using a specially designed resonant reflector as the exit mirror. This reflector acted as a longitudinal mode selector. The method used to design this selector is described and the characteristics of a single-frequency laser utilizing such a selector are reported.

Excitons in highly optically excited gallium phosphide

A study has been made of luminescence in weakly (10 15-10 16 cm -3) and heavily (10 18-10 19 cm -3) N-doped GaP crystals induced by 1.78, 2.34 and 3.56 eV photons from Q-switched ruby or neodymium lasers with a KDP crystal for second harmonic generation. The results which were obtained at excitations up to 10 20 cm -3 electron-hole pairs are interpreted as the transitions: single bound excitons, bound excitonic molecules, free excitons in weakly-doped GaP, as well as Auger processes and the formation of a new excitonic state similar to a solid metal with high density of single excitons and excitonic molecules bound to isoelectronic traps in heavily-doped GaP.

Optical pumping of neodymium glass by radiation emitted from a high-current discharge

A neodymium glass laser was pumped by the radiation emitted from a high-current pulse (τ = 70 μ sec) discharge. Such pumping .reduced the free-oscillation threshold by 40% compared with the pumping provided by standard lamps in an elliptic reflector. The use of a high-current discharge as the pumping source for Q-switched neodymium lasers increased considerably the laser efficiency.