摘要： Since the free spectral range (FSR) of a fibre Fabry-Perot (f-FP) is the reciprocal of the time delay inside the interferometer, it can be used for the precise measurement of fibre properties (chromatic dispersion when measured as a function of wavelength, birefringence, etc.) or for the measurement of environmental changes (such as temperature, pressure, stress, etc.) which alter the fibre’s parameters. Here we propose a method to precisely measure the FSR of long-length f-FP based on an incoherent optical source and RF beat signal analysis. We built a 7-m long f-FP made of a single mode fibre (SMF) and investigated the use of several algorithms to evaluate the FSR. When measuring over time, we achieved an excellent accuracy down to 2.4 Hz (corresponding to a relative error of 1.5× 10??). To demonstrate its use in sensing application, we varied the f-FP temperature and monitored the associated FSR. We achieved a sensitivity of 345 Hz/K, leading to an excellent temperature resolution below 10 mK. Additionally, the achieved high accuracy in the measured FSR can also be used to measure fibre’s properties very precisely (e.g., zero chromatic dispersion wavelength) or to determine their temperature dependence. The method requires no alignment and requires only a few off-the-shelf components and instruments.