Noise figure spectrum measurement of an optical fiber amplifier in a highly coherent microwave photonic link

Optical fiber amplifiers are widely used in high-speed laser communication, fiber optic sensing, time-frequency transfer, and other fields, serving as one of the core components in highly coherent microwave photonic links. The noise figure is a critical parameter for evaluating the noise level of optical fiber amplifiers. The frequency-resolved measurement method in the electrical domain offers accurate noise characterization introduced by the amplifier across different frequency bands. In this study, we precisely measured the relative intensity noise of the laser before and after amplification by the optical fiber amplifier and theoretically calculated the noise figure spectrum of the amplifier within the range of 1 kHz to 40 GHz. Then we construct a highly coherent microwave photonic link with an optical fiber amplifier and a narrow linewidth laser as the carrier. We compare the noise figure degradation of the microwave photonic link (MPL) when transmitting microwave signals at different frequencies, both with and without the addition of an amplifier. The results show that the level of the noise figure spectrum of the optical fiber amplifier is positively correlated with the degradation of the noise figure of the MPL in the corresponding frequency band. Additionally, factors such as the noise characteristics of the narrow linewidth laser source, the input optical power level to the amplifier, and the gain of the amplifier can all affect the noise figure spectrum.