Advanced Optical Modulators for Sub-THz-to-Optical Signal Conversion

The use of sub-terahertz (sub-THz) and/or THz bands is a method of achieving some attractive applications such as future large-capacity radio over fiber (RoF) networks. However, in the current scenario, the performance of devices operating in the THz band is considerably worse than that of devices operating in the microwave band. An optical modulator is a device that converts electrical signals such as microwave, sub-THz, and THz signals to optical signals, and their conversion efficiency decreases when they are operated at higher frequencies. In this article, we investigate two types of optical modulators. One is a long effective-length modulator to maximize its responsivity in the >100 GHz range. It has an advantage for band-limited applications such as RoF. The other is a broadband modulator integrated with an electro-optic (EO) frequency-domain equalizer. The fabricated modulator achieved an over 110-GHz 3-dB bandwidth by customizing the optical circuit diagram in a traveling-wave modulator, and in a numerical estimation, the 3-dB bandwidth reached sub-THz. We also investigated the modulation distortions of the modulator with the equalizer. Using the measurement results, the optical crosstalk in the EO equalizer of the fabricated modulator was estimated to be less than −30 dB, and the distortion attributable to the EO equalizer in the modulator was sufficiently small to be negligible. We also measured a third-order intermodulation distortion, and the results showed that integration of the equalizer does not cause a degradation of modulation linearity. The obtained spurious-free dynamic range was as high as 83.3 dB.