Design of Real-Time Free Spectral Range Measurement Based on HOPLL Technique

A real-time free spectral range (FSR) measurement system based on heterodyne optical phase-locked loop (HOPLL) is proposed for the first time to test the FSR of a fiber resonator. In this system, HOPLL is used to lock phase between the master and slave lasers, so that the frequency difference between them reach one FSR. In addition, PDH technique is used to establish a resonant tracking loop. By setting the loop control parameters, the master and slave lasers are respectively locked on two adjacent resonance peaks of the cavity. If the ambient temperature changes, the cavity length of the resonant cavity will change accordingly. Then the real-time change of FSR of the resonant cavity can be measured by detecting feedback change of the loop. The experimental results show that the linewidth of beat signal of the master and slave lasers after phase-locking is less than 10Hz, and also the frequency noise of the 10Hz bandwidth is 0.0154Hz which is 60dB lower than that before phase-locking. The researchers find that the FSR coefficient of thermal expansion is -210.35 ± 0.15Hz/°C, which is consistent with theoretical results. The relative measurement accuracy of 1.5 × 10
^-9
is achieved in this loop. Therefore, the experimental results effectively verify the actual application performance of the real-time FSR measurement based on HOPLL technique.