Low chirp electro-optic intensity modulation using planar microwave 180-degree hybrid directly prepared on LiNbO3 substrate

An electro-optic (EO) modulator integrated with the microwave planar circuit directly prepared onto a LiNbO₃ substrate for low chirp performance and compact configuration is introduced. A microwave rat-race (RR) circuit, which operates as the 180-degree hybrid, is designed on a z-cut LiNbO₃ substrate to create two modulation signals of the same amplitude in antiphase with each other from a single-input signal. Output ports of the RR were connected to the modulation electrodes on the substrate. The two signals drive two phase modulation parts of dual-electrode Mach-Zehnder modulator so that the symmetric interference are realized to obtain low-chirp intensity modulated light signals. The modulator was designed and fabricated for around 10 GHz modulation frequencies and 1550 nm light wavelength. The chirp parameters were measured to be less than 0.2 in the frequency range between 8 and 12 GHz. By compensating the unbalance of the light power splitting the chirp could be significantly reduced.