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

Disturbances of signals on a coplanar waveguide (CPW) induced by the presence of LiTaO3 and GaAs electrooptic probes in external electrooptic (EO) sampling has been simulated and compared quantitatively. The finite-difference time-domain (FD-TD) method is used to simulate the full wave field around a coplanar waveguide on a GaAs substrate in an external E-O sampling configuration. The results indicate that the induced 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 optical probing wavelength. The transparency of LiTaO3 to shorter wavelengths, however, allows an even higher sensitivity for this material relative to GaAs. Our results suggest that these probes do not exhibit significant invasiveness, magnitude of S11 smaller than -40 dB, if they are removed from contact by the distance of CPW's center conductor width.