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 Ω) 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 Ω) 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.