Traveling-Wave Mach-Zehnder Modulator Integrated With Electro-Optic Frequency-Domain Equalizer for Broadband Modulation

Broadband modulator is a key component of future high-capacity and high-baud-rate optical fiber communication. An electro-optic (EO) frequency-domain equalizer is proposed and investigated to increase the 3-dB bandwidth of optical modulators. The equalizer can be integrated with conventional traveling-wave modulators using a crossing waveguide, U-turn waveguide, or ferroelectric-domain-inversion. The 3-dB bandwidth of the modulator can be doubled by integrating the equalizer. As a theoretical consideration, we modeled several types of EO equalizers and investigated the function and parameter dependency of the equalizers in detail. For the experimental demonstration, a titanium-diffused lithium niobate optical waveguide modulator integrated with the EO equalizer was fabricated. The optical loss including fiber-coupling was as low as 5.4 dB, and the half-wave voltage was 3.7 V. The measured electro-optic response at 110 GHz is −0.4 dB, and that of the 3-dB bandwidth is larger than 110 GHz, which is the upper frequency limit of our measurement equipment. The 3-dB bandwidth of the modulator estimated using the fitting curve obtained from the measured electrical propagation loss of the modulator electrode was approximately 200 GHz.