Research on the linearity degradation caused by noise in microwave photonic temperature sensor with fiber ring structure

When the optical fiber ring is used as interferometer in microwave photonics (MWP) filter, researchers usually pay much attention to the sensing sensitivity, but are lack of experimental research on the deterioration of sensing linearity caused by the noise in this structure. Therefore, this paper studies the deterioration of the temperature sensing linearity in MWP filter, which is caused by the noise in optical fiber ring structure. In experiment, a sinusoidal microwave signal with the radio frequency(RF) amplitude of 4Vpp and the frequency range of 127MHz -147MHz are selected to modulate the broad-band light, and the bias voltage of the electro-optic modulator(EOM) is 3V. In the experiment, the 50m sensing fiber is heated, with the temperature range of 25°C-55°C. The filtered microwave signal is realized by the beating process in photodiode(PD), and then the signal is sent to the electrical spectrum analyzer (ESA). When the RF sweeping curves are recorded, the maximum peak detection(MPD) method is used to extract the resonant frequency of two resonant peaks at different temperatures respectively. Finally, the mean values of these resonant frequencies are calculated, and the first and multiple order polynomial is used to fit the above data. These results show that the resonant frequencies corresponding to different resonant peaks vary with the temperature with poor linearity. The first order fitting R-square of the resonant peak 1 and peak 2 are only 0.3855 and 0.2545, while the quartic fitting R-square can reach 0.8583 and 0.9589. It can be seen that there is a large noise in the optical fiber ring structure, which seriously affects the sensing performance of the MWP filter.