Microwave Photonic Interrogation of a High-Speed and High-Resolution Temperature Sensor Based on Cascaded Fiber-Optic Sagnac Loops

A high-speed and high-resolution temperature sensor based on two cascaded fiber-optic Sagnac loops (FSLs) interrogated using a microwave photonic (MWP) method is proposed and experimentally demonstrated. In the proposed system, an FSL is used as a sensor element. When it is experiencing temperate change, its spectrum is shifted, which leads to the shift in the spectrum of the cascaded FSLs. Instead of measuring the wavelength shift of the spectrum by using an optical spectrum analyzer (OSA), which has a low wavelength resolution and a slow scanning speed, we convert the optical spectrum to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping, and use a digital signal processor (DSP) to extract the sensing information. In our experiment, two cascaded FSLs with two different free spectrum ranges (FSRs) are employed. Due to the Vernier effect, multiple dips in the envelope of the optical spectrum are introduced. By passing a broadband frequency-chirped optical pulse generated by a frequency-swept laser source to the two cascaded FSLs, the sensing information is encoded in the optical spectrum and it is converted to the time domain due to SS-WTT mapping. By detecting the optical waveform at a photodetector (PD), a temporal microwave waveform with its shape identical to the optical spectrum is generated. By using a DSP, the time shift of a dip is measured and the sensing information is precisely demodulated. Experimental result shows that the proposed fiber-optic temperature sensor can provide a temperature resolution of 1.33 × 10 ⁻⁵ °C at a sensing speed of 23.497 kHz.