REDUCTION OF THERMALLY INDUCED STRESS BIREFRINGENCE IN A SYNTHETIC CRYSTAL BY CONTROL OF GROWTH DIRECTION

摘要

1358064 Luminescent materials UNION CARBIDE CORP 1 Sept 1971 [26 Oct 1970] 40698/71 Heading C4S [Also in Division H1] Stress birefringence in a laser crystal is reduced by a suitable choice of the growth direction of the crystal in its manufacture. Fig. 1 shows a Nd doped YAG active medium 10 pumped by a helical flashlamp 12, the resulting thermal gradient in the medium causing stress variations and hence birefringence. When used with a polarizer 15 this results in considerable wastage of output as shown by arrow P L . For a conventional YAG crystal, which is grown in a [111] direction, the birefringence under such pumping is shown in Fig. 6 by the dotted line. This birefringence is not a function of the angle #, i.e. it is constant for all points at a given distance from the medium axis in a given transverse plane through the medium. For crystals grown in the [001] direction the birefringence is as shown by the dotted lines of Fig. 8; the birefringence is greater at some points than at corresponding points of Fig. 6, while in others it is less. If the plane of polarization set by the polarizer 15 in Fig. 1 is vertical in Fig. 8, i.e. coincident with the axis X in Fig. 8, the overall effect of birefringence is a minimum. If the plane is set to coincide with a 45 degree plane as shown by the dotted line marked Polarizer direction in Fig. 8 the overall birefringence is increased. The difference between the [111] birefringence and the maximum and minimum values for the [001] crystal vary with pumping energy (i.e. thermal gradient) as given in Fig. 9 (not shown).