Growth of Optical Quality Sapphire Single Crystal Fibers
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GROWTH OF OPTICAL QUALITY SAPPHIRE SINGLE CRYSTAL FIBERS Dieter H. Jundt, Martin. M. Fejer, and Robert. L. Byer Applied Physics Department, Stanford University,Stanford, California 94305
ABSTRACT Void-free sapphire single crystal fibers with diameters of 110 .tm and 60 .tm and lengths of over 2 m have been grown by the laser-heated pedestal growth method. The growth dynamics of the floating zone was studied and shows the features expected from a simple theoretical model. Optical losses have a minimum of 0.5 dB/m in the near infrared at 1064 nm. An absorption band centered at 400 nm results in losses of up to 20 dB/m in the visible. The fibers have potential applications in high temperature thermometry and in delivery systems for laser surgery. Absorption losses of 0.88 dB/m with a damage threshold higher than 1.2 kJ/cm 2 at 2936 nm made tissue ablation feasible with fibers several meters in length.
INTRODUCTION Sapphire ( oc-A1203 ) single-crystal fibers are well suited for sensor applications in high temperature or chemically hostile environments, 1 as well as for use in medical power-delivery systems operating at the 2936 nm Er:YAG wavelength 2 where silica glass fibers are highly absorbing. Sapphire fibers are stronger than other mid-infrared transmitting fibers3 and chemically inert.
GROWTH METHOD To grow the sapphire fibers, we used a laser-heated pedestal growth apparatus. 4 The tip of a rod of source material 5 was heated by a focused C02 laser, forming a molten droplet. By dipping an oriented seed crystal (in our case with the z-axis along the growth axis) into the droplet, a suspended molten zone was produced. The growth was then started, carefully controlling the ratio of the speeds at which the source rod is pushed into the zone and the fiber is pulled out. (See Fig. 1). For growth under equilibrium conditions, the reduction ratio of source to fiber diameter is given by rf/rm = 4
/vm f
(1)
where rm and rf are the source rod and fiber radii, and vm and vf denote the speeds of the source rod and the fiber, respectively. The fibers were typically grown in air at a speed of 4 mm per minute with a reduction ratio of source to fiber diameter of 3.5. A 6 cm long, 0.4 mm diameter source rod generates a 70 cm long 110 ý.tm diameter fiber. Fibers as long as 2.5 m were grown by using a two-step reduction from a 0.8 mm diameter source rod.
Mat. Res. Soc. Symp. Proc. Vol. 172. ©1990 Materials Research Society
274
Fig. 1: The laser-heated pedestal growth
interface
Molten zone
process and the parameters used to describe the molten zone shape.
melting interface source
rod
m4
Diameter variations, particularly those of high spatial frequency, are the major source of scatter loss in void-free single-crystal fibers. It is thus useful to characterize the influence of perturbations to the growth system, both as an aid in optimizing the growth parameters and for the design of closed-loop diameter-controllers. Following the analysis of meniscus-controlled growth in Ref. 6, one can show that if certain thermal e
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