Suppression of Laser Parasitic Oscillation Used Trivalent Samarium in Nd:YAG Ceramic Composite Rod

The laser parasitic oscillations in active media under high pumping condition limit the gain of a high power amplifier system. This problem occurs due to existence of closed optical paths by internal reflections of active medium or systems. This problem can be overcome by eliminating the internal reflections by making rough surface, grooves, or antireflection coating or using index matching fluid at the boundaries. These techniques have some disadvantages. Specifically, there is a limitation for suppressing parasitic oscillation at high pumping in rough surface, and there is deterioration in coating or index matching fluid due to exposure to light and coolant. However, index matching can be achieved even in solid, such as composite structure. Composite structure consists of two or more different media. It is inadequate for this purpose that there is refractive index difference between two media. Using same host and no adhesive may minimize the difference, but there are difficulties to achieve it in a crystal without optical quality problem and high cost. On the other hand, there is less such concern in the ceramic that the composite structure is fabricated by the sintering process. The ceramic material made by this process works optically as merely distribution difference of dopant ion. In the rod type material, since the parasitic oscillation occurs by reflection at its cylindrical surface, composite structure shall be concentric; using an analogy from fiber structure, the center portion is called a core part, and the outer portion is a clad part. It was reported that composite rod with non-doped YAG clad has better performance than normal rod in terms of slope efficiency [1] and extraction efficiency [2]. The parasitic oscillation may not be suppressed only using composite structure, because light reflects at the end surface of clad part. This concern will be solved by doping an absorber to clad part. The absorber must be transparent at excitation wavelength region and an opaque at lasing wavelength. In this study, we demonstrated the ceramic composite rod which is doped with neodymium in core part and samarium in clad part, to suppress parasitic oscillation. And also, we compared the thermal lens effect of this composite rod.