Dual Comb Shape Sensing Spectroscopy

Strain and torsion shape sensing are crucial components of Structural Health Monitoring (SHM), ensuring the safety of essential assets such as bridges, buildings, pipelines, aerospace structures, and wind turbines. This work demonstrates the experimental application of dual-comb FBG shape sensing spectroscopy, as shown in Fig. 1. The experimentally developed laser [1]–[3] is a ring-cavity laser incorporating in-cavity polarization-maintaining fiber (PMF) to achieve dual-comb generation using a polarization multiplexing technique. Due to variations in the refractive index within the PMF, ultrashort pulses propagate inside the laser cavity at different group velocities, forming dual combs with slightly different repetition frequencies. These frequencies can be tuned from a few hundred Hz to kHz by adjusting the pump power and in-cavity POC2. On account of the long cavity length (5.25 m) and the limited resolution of the OSA (20 pm), the dual-comb spectral lines are indistinguishable. However, the beating between these lines is temporally resolved through down-converting heterodyne detection, as shown in Fig. 1(b). This beating produces equally spaced multiple interferogram peaks with a separation of 1.66 ms. In the frequency domain, the interference between the two combs results in multi-heterodyne detection, where each line of the slow comb beats with the corresponding lines of the fast comb. The resulting beat notes, distributed in groups across the RF spectrum, produce a multi-peak structure, as shown in Fig. 1(c).