Growth of Optical Crystals by the Micro-Pulling-Down Method
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Crystals by the Micro-Pulling-Down Method Akira Yoshikawa and Valery Chani
Abstract The micro-pulling-down technique is a crystal growth method that has been mostly developed since 1992. The general scheme of the growth system is relatively simple: the melt (oxide, fluoride, metal) residing in a crucible is transported in downward through microcapillary channel(s) made in the bottom of the crucible. Two driving forces (capillary action and gravity) support the delivery of the melt to the liquid/solid growth interface formed under the crucible due to a properly established temperature gradient. Appropriate configuration of the crucible bottom allows for controlling of the crystal shape (fibers, rods, tubes, plates) and the dimensions of the crystals’ cross sections that range approximately from 0.1 to 10 mm. A great number of scientifically and industrially important optical crystal fibers have been successfully produced using this method.
Introduction Why Fiber-Shaped Crystals Are Desirable The solid-state laser industry is moving toward fabricating small light sources capable of high efficiency. Direct production of shape-ready miniature (0.1 to 10 mm) crystals with dimensions prescribed by the devices’ design would help in meeting these requirements. Considering the intended device integration and superior power scaling capability, the production, testing, and spectroscopic studies of new laser sources based on fiber-shaped crystals become additionally attractive. Production of such fibers may have an important impact on scientific developments and the consumer market. Both require the evolution of novel coherent microdevices, including those for industrial, medical, telecommunication, and high-power applications. In particular, the advantage of single crystalline fiber lasers as compared to commercially available double-clad amorphous fiber-based lasers is associated with the possibility of increasing the core dimension. This allows for achievement of large continuous wave powers or strong pulses under Q-switching operation (a laser with a non266
continuous pulsed output beam with extremely high peak power) at peak powers above a few tens of kW. Optical fiber crystals also have attracted the attention of the developers of medical imaging systems. The spatial resolution of positron emission tomography (PET) is currently around 3–4 mm, and its improvement is strongly required. A combination of the position sensitive avalanche photo diode (PSAPD) with a small diameter rod scintillator has been suggested to realize a higher spatial resolution. Recently, 1-mm resolution has been achieved with laboratory scale devices using 1.25 mm2 rod scintillators. However, the theoretical limit of PET resolution is expected to be as small as 100 µm. Therefore, the growth technology of 100µm diameter scintillator fiber crystals is also apt to become a topic of intense study.
How Fiber/Shaped Crystals Are Produced The two most common methods of growth of single crystalline fibers from the melt are laser-heated pedestal growth1,2
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