Design optimization of low-impedance high-speed optical modulators for digital performance

Limiting-eye patterns (LEPs) for various types of optical modulators are constructed from the inverse Fourier transform of the frequency-domain voltage-length integral (VLI). Various transmission-line circuits for low-characteristic-impedance (typically 23 /spl Omega/) modulators are optimized on the basis of the maximum opening of the LEP. The results show that localized pad capacitances, when used in combination with sections of a high-impedance (typically 75 /spl Omega/) transmission line, can be beneficial to the achievement of better modulator performance. Splitting the active waveguide into three segments can significantly increase the modulator bandwidth. However, passive-optical-waveguide sections several times longer than the active waveguides are required to achieve optimum performance. In the frequency domain, the simultaneous optimization of the bandwidth and the group-delay curve is shown to be crucial for achieving good digital performance.