INVASIVENESS OF LITAO3 AND GAAS PROBES IN EXTERNAL E-O SAMPLING

Disturbances of signals on a coplanar waveguide (CPW) induced by the p resence of LiTaO3 and GaAs electrooptic probes in external electroopti c (EO) sampling has been simulated and compared quantitatively. The fi nite-difference time-domain (FD-TD) method is used to simulate the ful l wave field around a coplanar waveguide on a GaAs substrate in an ext ernal E-O sampling configuration. The results indicate that the induce d signal disturbance, or invasiveness, of a LiTaO3 probe is almost ten times that of a GaAs probe in terms of the magnitude of S11, but that LiTaO3 yields about two times the E-0 response for a given S11 and op tical probing wavelength. The transparency of LiTaO3 to shorter wavele ngths, however, allows an even higher sensitivity for this material re lative to GaAs. Our results suggest that these probes do not exhibit s ignificant invasiveness, magnitude of S11 smaller than -40 dB, if they are removed from contact by the distance of CPW's center conductor wi dth.