International Journal of

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CSs have been theoretically predicted and recently experimentally demonstrated in broad area, vertical cavity driven semiconductor lasers (VCSELs) slightly below the lasing threshold. Above threshold, the simple adiabatic elimination of the polarization variable is not correct, leading to oscillatory instabilities with a spuriously high critical wave-number. To achieve real insight on the complete dynamical problem, we study here the complete system of equations and find regimes where a Hopf instability, typical of lasers above threshold, affects the lower intensity branch of the homogeneous steady state, while the higher intensity branch is unstable due to a Turing instability. Numerical results obtained by direct integration of the dynamical equations show that writable/erasable CSs are possible in this regime, sitting on unstable background

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Solid-state Raman lasers are known as important sources at normally difficult to access wavelengths, and our recent studies have shown that they also form the basis of a class of wavelength selectable lasers. This paper summarizes our recent studies in wavelength selectable Raman lasers in the visible (532-650 nm) and ultraviolet (266-321 nm). 

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Time resolved laser induced incandescence (LII) technique is used to measure size distribution of soot nanoparticles of candle's flame. Pulsed Nd:YAG laser is used to heat nanoparticles to incandescence temperature and the resulting signal is measured. Mass and energy balance equations are numerically solved to calculate temperature of soot particles in low fluence regime. Assuming Plank black body radiation and lognormal size distribution for soot particles, the intensity of LII signals are calculated. Using Levenberg-Marquart nonlinear regression algorithm and numerical and experimental LII signals, mean particle size and distribution width of soot nanoparticles are obtained.

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The composite cavity fiber laser (CCFL) is relatively simple in its fabrication, as it is essentially three wavelength matched Bragg gratings in a section of doped fiber. By using internal feedback with unequal sub-cavity lengths, unidirectional CCFLs with significantly asymmetric output power from its two outputs can be achieved. Preliminary results also show that it is possible for the lasing frequency of the two outputs to be different by a few GHz.

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In this work the effects of heat generation on the modes of Yb:Glass double clad fiber laser were investigated. The thermal dispersion and thermally-induced birefringence were considered when the gain medium becomes an anisotropic medium. The results showed considerable modifications of laser modes profiles, in particular for transfer magnetic (TM) and transfer electric (TE) modes which their polarization vectors possess radial and azimuthal components.

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This paper describes the passive Qswitch, based on polymer and organic dye BDN, of a pulsed Nd:YAG laser. Pulses of 27ns duration and peak power of 1.7MW for initial transmission of Q-switch 18.4% 0 T = have been obtained. We have obtained Q-switching efficiency of 77% for train of pulses of 31 and 76.2% 0 T = at the pump energy of 98J. We have also obtained the dependency of laser pulse characteristics on pump energy and initial transmission of Q-switch. We have obtained the damage threshold of this Q-switch at the pulse duration of 27ns to be13J cm2 .

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Electronic absorption, florescence excitation and AFM studies of selected homogeneous solutions and thin films of mesotetrakis (parahydroxyphenyl) porphyrin (THPP) (1), mesotetrakis (2, 3- dihydroxyphenyl) porphyrin 2, 3-OHPP) (2), mesotetrakis (3, 4-dihydroxyphenyl) porphyrin (3, 4-OHPP) (3) as well as Zn(II) derivative of 2, 3-OHPP (4) under UV lamp and green laser irradiation (532 nm) are acquired and analyzed.Our results indicate that in irradiation experiments, the products of the resulting porphyrins strongly depend on the careful choice of the light source. UV lamp irradiation produced J-type aggregated diacid porphyrins from free base porphyrins by simple photochemical technique. The stabilization of the aggregates structure may be achieved through hydrogen bonding between the protonated core and the peripheral hydroxyl groups, mediated by the chloride anions. The results of laser irradiation experiments shown the formation of J-aggregated porphyrin upon green laser (532 nm) irradiation.Green laser irradiation has been caused the hydrogen bonding interactions between neighboring porphyrin molecules through hydroxyl moieties to produce J-aggregated supramolecular porphyrins. The AFM topographic images and Dynamic Light Scattering (DLS) of THPP(1) under green laser irradiation have been indicated nano-structured porphyrin wires about 5 nm high approximately.

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The average velocity of water flow has been simultaneously measured with fluorescent and speckle imaging methods. The measured velocities with two methods are in good agreement with each other and it confirms that the speckle imaging method can be used as a confident method to measure the velocity of water flow in a dry leaf. Also the velocity of water flow through thick and thin xylems of a leaf can be measured with speckle imaging method and the ratio of thick xylem’s radius to thin xylem’s radius can be estimated with this method. Also the ratio was measured by monitoring the cross section of the leaf. The two measurements were in agreement.

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A capillary plasma z-pinch as an alternative active medium of soft X-Ray lasers was studied experimentally and theoretically. The theoretical analysis was based on the self consistent solution of the so called “snow plow” model. The dynamics of pinched plasma is determined by the capillary parameters and by the time dependence of electrical current passing through it. The current time dependence is strongly influenced by the electrical circuit connected to the capillary. In order to optimize the pinch dynamics from the point of view of laser pumping, the effects of the electrical circuit parameters on plasma evolution are studied in this paper

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We report the observations of dual wavelength amplified spontaneous emission from the solutions of a conjugated polymer poly [2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) in Tetrahydrofuran and 1, 2 Dichlorobenzene. We have prepared MEH-PPV using a modified procedure and purified several times in each step, the material offers low molecular weight, low polydispersity index and high thermal stability, which are some of the most important requirements for the fabrication of photo luminescent devices. The variation in the features of amplified spontaneous emission with increasing polymer concentration is presented. For intermediate polymer concentrations, narrow emissions were observed for the 0-0 and 0-1 vibronic peaks which were not reported in solutions to the best of our knowledge. The ASE characteristics sensitively depend on concentration as well as pump power. The gain studies show that MEH-PPV is a potential laser media. By varying the concentration of the solution and pump power we can use either of the two different wavelength bands for lasing applications.

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We report an experimental study on optical and magneto-optical properties of Cesubstituted yttrium iron garnet thin films incorporating gold nanoparticles. Au nanoparticles were formed by heating Au thin film on cubic quartz and garnet substrate in vacuum chamber and a Ce:YIG layer was deposited on them by the aid of Pulsed laser deposition method. A large enhancement of the longitudinal Kerr effect was obtained in sample with Au nanoparticles on quartz substrate and the effect of substrate material on improving optical and magneto-optical response of samples were investigated.

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In this article, we present a specific shape of disk laser which is side-pumped by four non-symmetric hollow- ducts. The use of non-symmetric hollow duct based on two goals of the uniformity of the pump light distribution profile and the homogeneity of pump light profile through the disk. First of all we simulated the pump light distribution in the disk by using Monte-Carlo ray tracing method. Then, by using finite element analysis (FEA) method, we calculated the absorbed pump light distribution through the disk for 12%, 14% and 20% concentration of Yb+3 ions. Finally, the results of calculation have been presented.

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We investigate theoretically the high-order harmonic spectrum extension and numerical generation of an intense isolated attosecond pulse from He+ ion irradiated by a two-color laser field. Our simulation results show that the chirp of the fundamental field can control HHG cutoff position. Also, these results show that the envelope forms of two fields are important factors for controlling the resultant attosecond pulses. Besides, the effects of relative intensity are investigated. As a result, by using the optimized conditions an intense isolated 126-as (attosecond) pulse can be observed

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We present a novel slanted faces of thin-disk composite Yb:YAG YAG laser which is side-pumped by four non-symmetric hollow- ducts. The pump light distribution in the disk is modeled by using Monte-Carlo ray tracing method. The temperature distribution inside the crystal is calculated by taking into account either the concentration of Yb+3 ion or the different transmission of laser output coupler. By using Finite Element Analysis (FEA) method, we calculated the absorption efficiency through the disk. The resonator is simulated by self consistently method. The resulting of optical efficiency and the output power of our laser have been modeled.

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The ABCD matrix method is used to simplifying the theoretical coupling efficiency calculation of Elegant Hermite-Cosh-Gaussian (EHChG) laser beams to a Single Mode Fiber (SMF) with a quadric lens formed on the tip. The integrals of coupling efficiency relation are calculated numerically by Boole method. Meanwhile, the structure parameters of surface-lensed fiber are optimized in numerical simulation to achieve maximum coupling efficiency. Results can give some guidance suggestions for designing suitable micro lenses in order to coupling the EHChG laser beams to the SMF.

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In this paper, transistor lasers (TLs) are used as an optical modulator for generation of ASK(Amplitude Shift Keying) and FSK (Frequency Shift Keying) optical signals. Our analysis is based on continuity equation, rate equations, and the theory of discontinuity of quasi-fermi level at the abrupt junction. Our simulation results indicate that, the specification of ASK and FSK optical signals, are affected by dynamical behavior of TL. Also our simulation results indicate that, the collector-emitter voltage amplitude should be small enough that the nonlinear properties of TLs do not destroy the modulated optical signals.

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In this research, tunability of a commercial diode laser has extended to about more than ± 11 nm using a V-shaped external-cavity fabricated around the laser. Although under normal condition it can be tuned up to about ± 4 nm just by changing its temperature and injection current. Such modified diode laser has then used in a difference-frequency generation (DFG) experimental setup as pump source in order to continuous tuning of the generated DFG spectrum up to about ± 100 nm from 4.76 m to 4.85 m in the mid-infrared region. An AgGaS2 crystal is used as nonlinear medium.

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Thanks to their unique optical and electromagnetic properties, noble-metal nanoparticles are proven very useful in many scientific fields, from nanotechnology to biology, including detection of cancer cells. Irradiated gold nanoparticles, as a nano-smolder could be widely used in biomedical contexts such as tumor therapy. Laser destruction of a cancerous tissue depends on thermal and physical properties of the tissue, therefore temperature quantification of an irradiated metallic nanoparticle in different materials could be followed by interesting applications. In this research we quantify the temperature of irradiated gold nanoparticles in paraffin which is the most commonly used material for embedding of biological tissues in pathology. We have shown that the temperature increase rate for irradiated gold nanoparticles with diameters of 78 nm, 97 nm, and 149 nm are 1.31, 1.40, and 2.28 ◦C/mW, respectively. Considering that the conductivity of a biological tissue is an important parameter on temperature raise and destruction, these results could yield a valuable insight into the cancer therapy.

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Micro size craters were created by interaction of nanosecond laser beam with titanium target in liquid media. The dimension of crater i.e. depth and width is important in some applications such as micromachining. When the interaction occurs in liquid environment, the ablated materials from the target expand into the liquid. The ablated material can affect the interaction process if the ablated material concentration increases. In this paper, we study the effect of ablated materials in liquids on the crater width. The crater dimension was characterized by using an optical microscope. The results show that not only the type of environment liquid is important in the final size of the created craters, but also the laser fluence and the liquid depth in which the interaction takes place is important in the crater size.

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In this paper, considering optical feedback as an optical injection, and taking in to account round-trip time role in the external cavity, a standard small signal analysis is applied on laser rate equations. By considering the relaxation oscillation (f2) and external cavity frequencies (f) ratio for semiconductor laser, field amplitude response gain, optical phase and carrier number for long external cavities (LEC) and short external cavities (SEC) are obtained. Laser output intensity and resonance peak dynamics have been shown by bifurcation diagrams. Furthermore, the effects of some control parameters, such as enhancement factor, pumping current and feedback strength, on response gain have been discussed in short and long external cavities. As a result, in optical injection, for SEC, compared to LEC, more varied dynamics are observed. Also, higher values of the response gain peak in SEC, in comparison with LEC, make SEC to be affected more by the injected beam. SEC provides greater bandwidth, and also better performance in the range of compared to LEC.