Two-dimensional mapping of residual stress-induced birefringence in differently-grown semiconductors for optical communication applications

Semiconductor-based bulk optical modulators and switches demand material ho mogeneity and lack of residual birefringence, to guarantee low attenuation and noise performances. The fulfillment of such requirements strongly depen ds on crystal growth technique and machinery process. We have developed a p olarimetric two-dimensional mapping technique at lambda=1.5 mu m, that comp aratively characterizes the residual stress-induced birefringence of semico nductor modulator rods from a functional point of view, by evaluating the d istribution of effective birefringence and estimating the overall noise fig ure of merit (cross-talk) for optical switching applications. We have exami ned high-resistivity CdTe rods grown by the horizontal Bridgman technique, and compared results to data from GaAs single crystals grown by horizontal Bridgman, liquid encapsulated Czochralski and vapor pressure controlled Czo chralski. The birefringence has been measured with a resolution in the Delt a n approximate to 10(-7) range, spatial resolution can be adjusted, reachi ng 10 mu m. The performance of CdTe samples is strongly affected by the pre sence of dislocations and related localized stress fields, and acceptable p erformance is reached only for large diameter optical beams, which average birefringent effects on the cross-sectional area of the rod. High volume de nsity of dislocations is intrinsic to single crystals grown by the Bridgman method. Growing techniques from the vapor phase usually result in more hom ogeneous samples. Therefore, we think that the development of a suitable va por phase growth method is the key to obtain semi-insulating CdTe bulk crys tals to be used for high-performance optical modulation and switching. (C) 2000 Elsevier Science S.A. All rights reserved.