Asymmetric coplanar-strip transmission lines for MMIC and integrated optic applications

Microwave transmission lines with coplanar electrodes on dielectric or semiconductor substrates are widely used in integrated optic travelling-wave modulators and optical control microwave attenuators/modulators. The reduction of the drive voltage in travelling-wave modulators and control power in optical control microwave attenuators is achieved by reducing the width of the gap between the coplanar strips. When reducing the gap width in these devices special care must be taken to keep the characteristic impedance, Z₀, high. Although symmetric coplanar and coplanar-strip waveguides, have been used, asymmetric coplanar-strip waveguides seem to be highly flexible in practical device design. Along with small sizes, the impedance, Z₀, effective dielectric constant, in _e, and microwave refractive index, n_m, can be adjusted by changing the width of one of the strips while keeping the width of the other strip and of the gap fixed. The basic parameters ( in _e,C,Z₀) of asymmetric coplanar-strip waveguides on finite thickness substrate are reported.