الفهرس 
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Abstract
As they have the smallest size, compact design and wide range of emission laser wavelengths, laser diode and fiber laser are better than CW microchip laser. On the other hand, Q-switched solid state microchip laser is better than laser diode and fiber laser, because the energy storage capacity and laser induced threshold damage of laser diode are less than those of Q-switched microchip laser and long cavity of the fiber laser prevents achieving very short laser pulses. Yb:YAG was chosen as active medium due to its simple energy level scheme, long upper laser level lifetime (951 μs) which is suitable for storing energy, low quantum defects, large emission cross section, and large absorption cross-section for InGaAs laser emission. The combination of Cr4+:YAG and a doped-YAG gain medium, such as Yb:YAG, is particularly attractive from the point of view of an extremely robust device. Since both materials use the same host crystal YAG, they can be diffusion-bonded to each other in a way that blurs the distinction between a monolithic and a composite-cavity device. Both materials have the same thermal and mechanical properties and the same refractive index, and the bond between them can be sufficiently strong that the composite device acts in all ways as if it were a single crystal. Due to better mechanical and thermal properties compared to the glass and single crystal, transparent ceramics became high power end-pumping lasers candidate in numerous fields. Ceramic active medium can be heavily and homogeneously doped with laser-active ions. This work has been divided into two parts. In the first part, the performance of laser-diode end-pumped passively Q-switched Yb:YAG/Cr+4:YAG ceramic microchip laser system has been studied by numerical solution of modified coupled rate equations including the pumping rate and absorption of excited state of the saturable absorber. Fourth order Runge-Kutta method and MATLAB codes have been used in solving the modified coupled rate equations. As the thermal lens coefficient is very important in the stability of laser cavity, in the second part of this work, temperature distribution in Yb:YAG is studied via analytical solving of the heat equation, and the relation of the thermal focal length as a function of pulsed-pumping repetition rate is derived to have a technique of controlling thermal focal length by varying the pump repetition rate. The thermal focal length for ceramic and single crystal Yb:YAG is calculated and a comparison between the stability of ceramic and single crystal Yb:YAG/Cr:YAG is achieved by using ABCD matrix method. Finally the variation of the laser beam waist at the thermal focal point is calculated as a function of the pulsed-pump repetition rate.