Strain measurement by using π-phase-shifted fiber Bragg grating with an ultra-narrow linewidth based on frequency-stabilized laser

The fiber Bragg grating has the characteristics of anti-electromagnetic interference, electrically passive operation, multi-point sensing, corrosion resistance, and compact size. An ultra-narrow linewidth transmission peak can be formed by introducing a π phase shift at the center of uniform fiber Bragg grating. But this PSFBG with an ultra-narrow linewidth is very sensitive to the input optical intensity. The photothermal effect generated by the input light inside the grating will cause the frequency shift, which will degrade the measurement precision of grating. At the same time, the frequency drift of the laser itself will also increase the measurement error. This article proposes a high-precision strain measurement method by use of the PSFBG with an ultra-narrow linewidth based on the frequency-stabilized laser. The incident laser is attenuated to the single-photon level for eliminating the photothermal effect in the PSFBG. The laser frequency is stabilized to the PSFBG with an ultra-narrow linewidth of 38 MHz by using the single-photon modulation technology. The influence of low-frequency flicker noise is eliminated with the 9 kHz high-frequency modulation. The filter bandwidth of lock-in amplifier is 312.5 Hz with the integration time and filter slope being 300 μs and 18 dB, respectively. The signal-to-noise ratio of error signal from the lock-in amplifier is 34. By tuning the resonant cavity length of the laser with the error signal, the output laser frequency is stabilized to the Bragg frequency of the PSFBG with an ultra-narrow linewidth of 38 MHz. The laser frequency fluctuation is bounded within 4 MHz over a period of 1000 s. The response sensitivity of Bragg wavelength to external strain in the range of 0 to 30 με is 1.2 pm/με with the standard error being 0.023 pm/με, and the linear fitting correlation coefficient is R²=0.997. Due to the random drift of Bragg wavelength caused by the environment temperature fluctuations, the corresponding strain measurement precision is 0.05 με. The high-precision strain measurement by using the PSFBG with an ultra-narrow linewidth based on the frequency-stabilized laser is achieved, which will play an important role in the field of aerospace engineering, civil engineering, transportation engineering, energy engineering, mechanical and equipment manufacturing, and so on.