Tunable Subterahertz Wave Generation Based on Photonic Frequency Sextupling Using a Polarization Modulator and a Wavelength-Fixed Notch Filter

Optical frequency multiplication based on electrooptical modulation is an effective way to generate high-spectral-purity and frequency-tunable subterahertz waves. The previously demonstrated frequency-doubling and quadrupling techniques based on a Mach-Zehnder modulator have a low multiplication factor and suffer from bias drift problem and residual chirp. In this paper, a novel approach to achieving frequency sextupling using a polarization modulator and a wavelength-fixed optical notch filter is proposed and experimentally demonstrated. The method is free from bias drift problem and residual chirp, which can be used to generate high-spectral-purity subterahertz wave signals using relatively low-frequency electrical and optical devices. By using a narrow-bandwidth fiber Bragg grating as a wavelength-fixed optical notch filter, a high-spectral-purity microwave signal tunable from 18 to 27.6 GHz is generated when a microwave drive signal from 3 to 4.6 GHz is applied to the polarization modulator. The phase noise of the generated signal is measured as low as -107.57 dBc/Hz at a 10-kHz offset frequency. By replacing the narrow-bandwidth notch filter by an optical interleaver, a subterahertz wave tunable from 66 to 114 GHz is generated when the drive signal is tuned from 11 to 19 GHz. The distribution of the generated signal over optical fiber is investigated. The results show that the quality of the distributed subterahertz wave signal is maintained after transmission over a 40-km standard single-mode fiber.