جستجوی مقالات مرتبط با کلیدواژه « تولید هماهنگ دوم » در نشریات گروه « فیزیک »

Laser beams carrying orbital angular momentum have found numerous applications in various branches of sciences especially in modern quantum communications. Frequency conversion of these beams using nonlinear crystals allows the desired spectral ranges to be achieved. In this work, first, the optimal value of the focusing parameter and the location of the focus of the pump beam with orbital angular momentum in second harmonic generation process are determined. Then, based on the dependence of the optimal value of the phase mismatch parameter on the Laguerre-Gaussian mode order contained in the pump beam, a novel method for varying of the orbital angular momentum of the generated second harmonic beam is presented. According to this method, for pump beams with fractional orbital angular momentum, by changing the focusing parameter, the angular momentum of the second harmonic generation beam can be continuously tuned.

Thermal effects strongly influence the process of second harmonic generation, especially at high fundamental powers. The generation of non-uniform temperature distribution in the crystal due to the absorption of fundamental and second harmonic waves leads to significant degradation of the efficiency of the second harmonic generation process. This absorption acts as a heat source in the crystal and prevents the crystal from remaining at the phase matching temperature, resulting in a phenomenon called thermal phase mismatch in the crystal. In this paper, first, a theoretical model to simulate the thermal effects in the second harmonic generation is introduced. Then, by considering a periodically poled crystal as a nonlinear crystal and adding a chirp to its periodicity, a solution to compensate for the thermal phase mismatch is presented. Simulation results show that the chirp of a periodically poled crystal compensates for the thermal effects on the nonlinear crystal and largely offsets the efficiency degradation in the second harmonic generation. For example, at the fundamental power of 20 W, the thermal effects reduce the second-harmonic conversion efficiency from 74% to 18% and using the chirped periodically poled crystal increases the efficiency to 50%.

Recently, polarization sensitive Second Harmonic Generation (pSHG) microscopy has become a powerful tool for the study of the noncenterosymmetric biological structures. This is due to the fact that pSHG has some intrinsically benefits such as high resolution and contrast, and it can also penetrate deeply inside the sample in a noninvasive manner. One drawback for the pSHG technique is that the imaging procedure is not fast enough to study in vivo samples or to monitor the dynamics of different tissues. This issue imposes some limitations on using the pSHG technique to study in vivo samples. Fortunately, recently Single Scan polarization sensitive Second Harmonic Generation (SS-pSHG) technique has been introduced as a fast alternative for the conventional pSHG technique. In this article, the results obtained from ex vivo biological samples of starch, human cornea and animal tendon have been compared in a pixel-to-pixel manner using pSHG and SS-pSHG techniques in forward direction. Even though the samples used here are ex vivo, the results of this study promise that the polarization sensitive SHG microscopy techniques have a great potential to study biological tissues in a noninvasive procedure. This issue is more important especially in the cases that samples are in vivo.

In this research, the nonlinear propagation equations of the fundamental and second harmonic waves coupled to the heat equation are solved for a focused Guassian beam inside two quasi-phase matched periodically poled MgO: PPLN and MgO: PPLT crystals, and the optimum focal length and radius of the continuous-wave laser beam with the wavelength of 1064 nm and with Gaussian profile, and also, the optimal phase-mismached parameter are calculated and the dependence of the second harmonic efficiency on these parameters is shown. Also, the effects of the thermal distribution inside the crystal on the second harmonic generation efficiency are investigated and it is shown how the thermal distribution reduces the second harmonic efficiency and, accordingly, the optimal crystal length for different incident powers is determined and shown that the lower crystal lengths should be selected for higher incident powers. Finally, a comparison between the second harmonic efficiencies in two crystals of MgO: PPLN and MgO: PPLT is performed considering the thermal distribution for different insident powers and it is shown that for the lower/ higher incident power than 21 watts, the crystal of MgO: PPLN / MgO: PPLT is more suitable and more efficient for second harmonic generation.

Second harmonic generation of Nd:YAG laser, as one of the most applicable and popular solid-state lasers, has been widely used in different fields of military applications and in diverse fields of medicine and industry etc., in last few decades. Non-linear crystals are mentioned as one of the most efficient and common methods for harmonic generating of lasers. In this research, we performed the simulation of the second and fourth harmonic generation of pulsed Nd:YAG laser with characteristics of 1064nm wavelength, 1joule/cm2 energy density, and 10ns pulse width, and considering the effects of temperature and divergence angle of the incident beam. The conversion efficiency of the second harmonic generation of this laser was investigated using KTP, BBO and LBO crystals. The simulation results suggested the KTP crystal as the most efficient crystal with 89.59% conversion efficiency. Formerly, achieving 80% efficiency using Nd: YAG laser with similar characteristics of the simulated specifications presented in this paper by using KTP crystal in an experimental configuration had been presented. In addition, the conversion efficiency of the fourth harmonic generation of this laser was investigated using BBO, CLBO and DKDP crystals. The simulation results suggested the BBO crystal as the most efficient crystal with 19.21% conversion efficiency. For the sake of comparison, 12% efficiency in the experimental layout with similar Nd:YAG laser specifications, presented in this article, had been presented in another article.

In this study، the efficiency of a self-doubler NYAB laser، which is simultaneously an active and a nonlinear crystal، in a double-pass cavity is investigated. To this end، first، a model based on the coupling of five differential equations describing the interaction of pump beam of 807 nm، laser beam of 1060 nm، and second harmonic beam of 530 nm is presented. In this model، the conversion of pump beam to laser beam through the fluorescence process is described by the laser rate-equations. The generation of second harmonic beam is performed through the nonlinear processes. The model consists the linear and nonlinear interactions of light as well as the optical absorption. The results give the laser and second harmonic generation efficiencies against the pump powers.

The poling process of polymers doped with chromophores is of great importance for the nonlinear properties of these materials. So, after having optimized the poling condition the second harmonic generation (SHG) was measured for PMMA polymer doped with Disperse Red 1. The dependence of SHG intensity to the poling voltage and poling temperature have been experimentally studied. The relaxation of dopant orientation in the polymer matrix was also measured.