We are introducing here conversion of eye hazardous laser to the eye safe region by Optical Parametric Oscillation (OPO) method, using nonlinear optical crystals. The practical optical parametric oscillator device consists of a nonlinear crystal enclosed in an optical resonant cavity pumped by Nd:YAG laser. Optical parametric devices generate broadly tunable coherent optical radiations by the phase-matching nonlinear optical interaction of an intense laser beam with this nonlinear crystal. In this process the high energy pump photon is converted into a pair of lower frequency signal and idler photons while conserving the total energy and momentum. Tunability of the signal-idler pair is usually achieved by changing the crystal birefringence of the angular dependence of the extraordinary index of the nonlinear crystal. The output energy is an eye safe radiation at either, or both the signal and idler wavelengths.
-This study concerns with the laser oscillator, threshold conditions, circulating power, and the Gaussian beam profile. Laser oscillator is a crucial device, which provides the feedback necessary to make a laser work. The regenerative laser oscillator is essentially a combination of two basic components: an optical amplifier, and an optical resonator, which serves to provide highly selective feedback. The optical resonator, comprised of two opposing plane-parallel or curved mirrors at right angles to the axis of active material, performs function of the feedback element, by coupling back in phase a portion of the signal emerging from the amplifing medium. The photons are reflected back and forth for many passes through the rod (amplifying medium), stimulating more and more emission on each pass. The mirrors can be gently curved so they tend to keep the light concentration inside the rod. One of the mirrors is 100 % reflective, but the other mirror transmits part of the light hitting it. This transmitted light is the output beam from the laser
Snow avalanches are formed in the mountainous snow regions mostly due to the presence of weak layers between the adjacent snow layers. Since the prevailing at mosphere in these regions is stable, weak snow layers are formed due to the presence of surface hoars, which is measured by surface layer heat flu xes. Surface hoar forms when relatively mo ist air over a cold snow surface beco mes over saturated with respect to the snow surface; causing a flu x of water vapor, which condenses on the surface, and creates feathery crystals (the icy layer equivalent to dew). Once buried, the resulting weak layer is a serious consideration for avalanche format ion. Surface hoar tends to form at night when the snow surface generally cools and the adjacent air becomes over saturated. The surface layer heat flu xes in the atmospheric boundary layer over Himalayan regions are calculated here by stability criteria using the bulk aerodynamic method.
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