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Processing of Ceramics


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only a very small part of the white light can be converted into laser light. Therefore, the conversion efficiency was very low.

      In recent years, by using a semiconductor laser (for example, a GaAlAs type laser having a wavelength of 808 nm that can excite an Nd type laser) which can excite particularly the absorption wavelength band of the laser‐active element at pinpoint, light (excitation light)‐to‐light (Laser in gain media) conversion efficiency can be significantly improved. For example, the conversion efficiency in the case of Nd:YAG single‐crystal medium by conventional lamp excitation is about 3%, whereas by the semiconductor laser excitation system high efficiency up to 60% level can be achieved. It is a common knowledge to say that the solid laser gain medium is “optical grade single crystal with extremely low scattering.” Therefore, the suggestion of a sintered body (ceramics) to use in high‐tech optical technology was out of the question. Can you imagine how the ceramic including numerous scattering sources such as residual pores, secondary phases, and grain boundaries, etc. was able to use as a laser gain medium? It was a reckless challenge until it can contribute to laser science. Details are described in this chapter.

      2.2.1 Spontaneous Emission

      First, the fundamental part of the laser such as fluorescence generation and stimulated emission phenomenon will be explained. Transition metals such as Cr, Ti, Co, etc. or lanthanide rare earth elements such as Nd, Yb, etc. which participate in light emission are added to a general solid‐state laser gain medium. The light emission of Nd and Yb is described as a typical example relating to these laser emissions. Yb and Nd are known to be typical three and four‐level lasers, respectively.

Schematic illustration of the energy diagram of (a) three-level laser (Yb-doped) and (b) four level laser (Nd-doped).

      2.2.2 Stimulated Emission and Laser Generation