Objective Fiber Bragg Grating (FBG) sensors, fabricated from quartz fibers, offer advantages such as repeatability, electromagnetic interference immunity, corrosion resistance, and radiation tolerance, enabling point or quasi-distributed measurements of strain, temperature, and pressure in harsh environments. To accurately capture transient physical signals in explosion-impact settings, FBG sensors require high-speed, precise wavelength demodulation. Traditional demodulation systems combining broadband sources with reflection wavelength detectors are limited by measurement speed. Advances in fiber optic sensing have increased interest in wavelength demodulation techniques, achieving MHz-level demodulation speeds. However, in FBG high-speed demodulation systems using tunable lasers, the transmission delay of transition fiber can cause significant wavelength demodulation errors, especially with longer fibers and higher demodulation speeds. Existing compensation methods, such as measuring fiber length or controlling laser scanning speed, have limitations in cost, precision, and applicability to multi-FBG configurations.

Methods This study proposes a new method using two standard hydrogen cyanide (HCN) gas absorption cells to calibrate and compensate for the time delay introduced by transition fiber. The absorption lines of HCN gas cells are stable and unaffected by temperature fluctuations. By selecting fixed wavelength absorption lines as reference points, the time delay caused by transition fiber is calculated through the peak collection delay difference of absorption lines in the two HCN gas cells. This delay is then integrated into the FBG wavelength calculation algorithm to compensate for the wavelength error caused by transition fiber in the tunable laser-based FBG demodulation system.

Results and Discussions Theoretical analysis indicates that the time delay originates from the transition fiber, and the time difference in collecting absorption lines from the two HCN gas cells accurately represents the time delay caused by the fiber. Experimental results show a linear relationship between transition fiber length and wavelength demodulation relative error, with a coefficient of determination R² of 0.999. Extensive experiments with different fiber lengths and FBG demodulation systems operating at different frequencies demonstrate that the wavelength error after compensation remains within ±10 pm. Additionally, the study successfully achieves time delay compensation for ultra-long transition fibers up to 1 kilometer, with a compensation rate exceeding 99.5%. The compensation efficiency for ultra-long fibers of 600 m and 1000 m in a 200 kHz demodulation system reaches 99.92% to 99.99%, while in a 100 kHz system, it ranges from 99.53% to 99.86%. This indicates that the wavelength demodulation delay error is primarily attributed to the transition fiber length rather than the FBG demodulation rate.

Conclusions This research addresses the critical challenge of delay error compensation in high-speed, long-distance FBG demodulation systems based on tunable lasers. By introducing two HCN gas absorption cells with different optical path lengths to generate a delay difference, and utilizing the absorption lines of HCN gas to measure the time difference and calculate the delay caused by transition fiber, the proposed method effectively compensates for the central wavelength demodulation error caused by fiber delay. Experimental results demonstrate that in a 200 kHz demodulation system, the maximum central wavelength demodulation error caused by transition fibers ranging from 0 to 400 m is approximately 8.8 nm, which is reduced to within 10 pm after compensation. Furthermore, the method has been successfully applied to ultra-long transition fibers of 600 m to 1000 m in FBG sensing systems with demodulation speeds of 100 kHz and 200 kHz, achieving a compensation efficiency of over 99.5%. This study significantly enhances the precision of high-speed FBG systems and provides a robust solution for wavelength demodulation error compensation in systems with varying demodulation frequencies, thereby expanding the application of wavelength-demodulated high-speed FBG sensors in measuring transient physical signals such as engine vibration and explosion impact.