High-Speed Optically Tunable RF Signal Generation with Low Power Threshold Based on On-Chip Germanium Photoelectric Effect

Radio frequency (RF) signal modulation and generation are crucial technologies for advanced radar systems and high-speed wireless communications. Compared with traditional electronic RF modulators, optically tunable RF modulators offer a wide bandwidth, rapid response, and dynamic tuning, among other features. These characteristics render these modulators viable solutions for satisfying the evolving demands of modern wireless communication systems. However, their practical applications are challenging due to their lack of scalability and compactness, as well as their high optical power requirements. This study demonstrates the generation of optically tunable microwave signals using an optically tunable silicon-based RF modulator. The optical tuning was achieved through the photoelectric effect of the germanium absorption region on a silicon photonic platform. When light is incident on the Ge absorption region, it generates charge carriers, thereby altering the equivalent circuit parameters of the device. Consequently, the frequency response of the RF modulators is altered. This effect was used for the generation of amplitude-frequency coded microwave signals at 1, 2.5, and 5 MHz. Both the optical power required for tuning and the response speed are orders of magnitude lower than those of free-space-illumination-based devices. Additionally, the generation of microwave signals was demonstrated by applying this modulator to an optically tunable RF oscillator, with a frequency-tuning range from 1 to 14.5 GHz.