Mode-locked fiber laser with an embedded Sagnac interference filter for strain sensing

We demonstrate a saturable absorber-based mode-locked fiber laser with an embedded Sagnac interference filter for strain sensing. By selecting an appropriate birefringent fiber length in the loop, the interferometer exhibits two dips within the wavelength ranges of the amplified spontaneous emission spectrum, functioning as a bandpass filter for wavelength selection. Mode-locked pulses at the central wavelength of the bandpass filter can self-start by increasing the pump power. The laser-based axial strain sensing system retains the high sensitivity of a Sagnac interferometer while overcoming demodulation challenges caused by peak tracking of the interferometer spectrum. Strain sensing was achieved by monitoring the optical spectrum and radio frequency (RF) spectrum of the mode-locked pulses. Within a strain variation range of 0–1045 $\text{μ}ϵ$, the wavelength shift sensitivity reached 21 pm/$\text{μ}ϵ$. As strain increased, cavity length expansion resulted in a fundamental frequency RF spectrum shift sensitivity of $-$0.767 Hz/$\text{μ}ϵ$. The proposed system provides multiple types of strain demodulation options, and its compact structure holds the potential for structural health monitoring of bridges, pipelines, aviation materials, etc.