كليدواژه :
دمش طولي پالسي , هدايت غيرفوريه , اجزاء محدود , نئوديوم: وانادات
چكيده فارسي :
براي برانگيختگي ترازهاي انرژي و توليد نور ليزر در محيط‌هاي جامد، از دمش نوري بهره گرفته مي‌شود. هنگام اعمال دمش به محيط كريستال، بخشي از توان دمشي تبديل به گرما شده و گراديان دمايي ايجاد مي‌شود. گراديان دمايي توليد شده مي‌تواند منجر به ظهور پديده‌هاي نامطلوب حرارتي و مكانيكي، نظير عدسي‌شدگي حرارتي، قطبيدگي خروجي، تنش‌هاي مكانيكي، كاهش بازده خروجي و حتي شكست بلور شود. به منظور بررسي و تحليل دقيق‌تر اين پديده‌هاي ترمومكانيكي، لازم است به جاي استفاده از نظريه‌ي انتقال حرارت هدايت فوريه، از نظريه‌ي انتقال حرارت هدايتي غيركلاسيك (داراي زمان آسايش) استفاده شود. در اين پژوهش، بر مبناي ارائه‌ي يك فرمولاسيون اجزاء محدود غيركلاسيك، توزيع دما و تنش در بلور صفحه‌يي نئوديوم: وانادات محاسبه مي‌شود. نتايج نشان مي‌دهد كه مقدار بيشينه دما و تنش محاسبه شده توسط مدل غيرفوريه، بزرگ‌تر از مدل فوريه بوده و نظريه‌ي غيرفوريه توان‌هاي شكست كوچك‌تري براي بلور پيش‌بيني مي‌كند.
چكيده لاتين :
In order to create a laser beam, one common method is to use beam pumping. In this method, beam is firstly emitted to the active medium of laser crystal and the electrons in the last shell of atoms are stimulated. With some internal interactions, laser beam will be created and some pumping energy is converted to heat in the crystal. In solid-state lasers, large amounts of pumping power may lead to heat generation in the active laser medium and a non-uniform temperature distribution in the crystal is generated. The temperature gradient causes thermal stress in the crystal structure and can be effective on thermomechanical behavior of the crystal and the quality of the laser beam. With increasing the pumping power, the heat generation within the active media is increased and consequently the thermal effects including the thermal lensing, polarization of output beam, mechanical stress, reducing the efficiency of output beam, are intensified. Even in this case, thermal stress causes failure. Hence to achieve a high quality beam laser, it is necessary to investigate the thermomechanical behavior of the laser crystal, through the designing of solid state lasers. In the past works, thermomechanical behavior of the crystal was investigated in framework of Fourier heat conduction theory (Heat wave diffusion speed is considered to be infinite), while this theory is not appropriate to study the heat conduction for medium under pulsed heat with appreciable relaxation time. Hence, in this research, by providing a finite element formulation based on the non-Fourier thermoelasticity (with considering relaxation time), distributions of temperature and stress in the Nd:YVO4 crystal are calculated. The results show that the maximum temperature and stress calculated by non-Fourier theory, are larger than those calculated by the Fourier theory and also the non-Fourier theory predicts lower failure power for crystal. In addition, if the relaxation time is lower than a certain value, the results of classical and nonclassical theories are closely equal to each other.
