Strength enhancement of single crystal laser components
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The effect of chemical etching on the surface roughness and fracture strength of Nd:yttrium-aluminum-garnet single crystals was investigated. Etching the laser host material in a 1:1 mixture of concentrated sulfuric and phosphoric acid at elevated temperatures enabled material removal from the surface of laser rods cored from single-crystal boules. The roughness on the rod surface increased from the as-drilled condition after etching, but after 17 m of material was removed, the roughness remained approximately constant. The modulus of rupture increased from 133–530 MPa when 87 m of material was removed by the chemical etching procedure.
Solid-state lasers often use Nd-doped Y 3 Al 5 O 12 (Nd:YAG) single crystals as the lasing media. Single crystals of Nd:YAG are grown by the Czochralski process in which crystal growth is obtained by pulling from a melt at high temperatures,1 generating a single crystal boule larger than an individual laser component. These boules are then separated into rods or slabs for insertion into laser cavities. The cutting and grinding processes, which are required for fabricating these devices (both for flash lamp and diode-pumped applications), induce damage into the remaining structure. Damage on the surface of a brittle material will lead to stress concentrations, which in turn will be preferential sites for fracture due to thermal stresses that develop during pumping.2–5 This communication describes a method for increasing the fracture strength of a fabricated rod structure through removal of the as-processed surface of the rod. Other groups have measured the fracture toughness of singlecrystal Nd:YAG,6,7 but the actual devices used always have surface flaws present. The current study focused on the modulus of rupture measured for various processing conditions, as they can modify the roughness of the free surface and possibly remove subsurface microcracks when chemically etched. Nd:YAG rods 4 mm in diameter and approximately 55 mm in length were core drilled from single-crystal boules using a diamond-tipped coring bit, and were subjected to a four-point bend test at room temperature at 0.127 mm/s loading. The Nd concentration in the rods was 0.88% substitutional for Y. Five batches, each of several rods, were tested after different amounts of a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 11, Nov 2003
etching. The etching solution used consisted of a 1:1 mixture by volume of concentrated sulfuric acid and concentrated phosphoric acid, heated to temperature before the samples were placed in solution. The solution was heated using a hot plate, and was left stagnant during heating and etching. All of the rods in each batch were placed in a Teflon basket and were etched at the same time in the same solution. After etching, the rods were removed and air cooled prior to rinsing in water. The roughness of the rods was measured using a surface profilometer (SPN Technology, Inc., Goleta, CA). Four-point bend testing8 was carried out usin
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