Investigation of strain birefringence and wavefront distortion in 001 plates of KD2PO4

Abstract

Investigation of strain birefringence and wavefront distortion in 001 plates of KD2PO4J.J. De Yoreo and B.W. WoodsLawrence Livermore National LaboratoryLivermore, CA 94550ABSTRACTWhen 001 plates of KD2P04 (KD*P) are used in Pockels cells, strain induced refractive indexvariations result in beam depolarization and transmitted wavefront distortion. Thedepolarization is determined by the induced birefnngence while the wavefront distortion iscontrolled by the average index shift. Here we show that the birefringence is determined bythe shear stress in the xy-plane of the crystal while the average index shift depends only onthe normal stresses. Furthermore, for depolarization losses of 0.1 to 1 .0% and wavefrontdistortion of 0.1 to 1 .0k, the critical range of stress is 1 05 to 1 06 Pa. We also presentmeasured depolarization loss and wavefront distortion profiles for 5, 1 6 and 27cm, 95%deuterated, KD*P crystals. Using the analysis described above we show that the maximuminternal stresses in these crystals are within the critical range, but that the area averagedstresses are substantially lower. We find that crystals from different locations along the lengthof a boule have similar strain birefringence and wavefront distortion profiles indicating thatthe growth conditions which generate the internal strain persist throughout much of thegrowth history of the boule. Finally, we discuss potential sources of strain in KD*P.1. INTRODUCTIONA major upgrade of the Nova laser system at Lawrence Livermore Laboratory (LLNL) is beingproposed for construction in the late-i 990's. The current design for this upgradeincorporates a plasma-electrode Pockels cell with an aperture of approximately 1 000 cm2 inan optical switch and utilizes KDP or KD*P for the electro-optic medium. The baseline designrequires the Pockels cell medium itself to be over 99% efficient. This stringent requirementprecludes the use of KDP the Pockels cell medium because absorption losses alonewould greatly exceed this specification. In contrast, KD*P, with an absorption coefficient ofonly 5 x 1 cm1 is a good candidate for the Pockels cell medium1 . The maindisadvantage of using KD*P is that as grown crystals exhibit a high level of strain2 whichgenerates index variations and birefringence in the crystals through the stress-optic effect.The index variations lead to transmitted wavefront distortion and the birefringence results indepolarization of the beam. Both of these effects result in degradation of systemperformance. When the beam leaves the Pockels cell it passes through a polarizer and isrelayed to a set of frequency converting crystals. Beam depolarization generates directlosses in transmitted energy at the polarizer while wavefront distortion steers the beam awayfrom the phase matching direction and reduces conversion efficiency. To meet the NovaUpgrade performance goals, the overall depolarization losses and transmitted wavefront