•We propose a fast positioning algorithm to shorten the response time of ADMZI.•The principles of ADMZI and the fast positioning algorithm are described.•A positioning measurement experiment was carried out.•The processing speed is improved by 5 times.We propose a fast positioning algorithm for the asymmetric dual Mach–Zehnder interferometric infrared fiber vibration sensor. Using the approximately derivation method and the enveloping detection method, we successfully eliminate the asymmetry of the interference outputs and improve the processing speed. A positioning measurement experiment was carried out to verify the effectiveness of the proposed algorithm. At the sensing length of 85 km, the experimental results show that the mean positioning error is 18.9 m and the mean processing time is 116 ms. The processing speed is improved by 5 times compared to what can be achieved by using the traditional time-frequency analysis-based positioning method.

•We derive theanalytic expression for the external clock sampling error for long range OFDR system.•We analyze of impact factor of the external clock sampling error and experimentally demonstrate.•The spatial resolution can be improved by reducing external clock sampling error In OFDR.Utilizing an auxiliary interferometer to produce external clock signals as the data acquirement clock is widely used to compensate the nonlinearity of the tunable laser source (TLS) in optical frequency domain reflectometry (OFDR). However, this method is not always accurate because of the large optical length difference of both arms in the auxiliary interferometer. To investigate the deviation, we study the source and influence of the external clock sampling error in OFDR system. Based on the model, we find that the sampling error declines with the increase of the TLS's optical frequency tuning rate. The spatial resolution can be as high as 4.8 cm and the strain sensing location accuracy can be up to 0.15 m at the measurement length of 310 m under the minimum sampling error with the optical frequency tuning rate of 2500 GHz/s. Hence, the spatial resolution can be improved by reducing external clock sampling error in OFDR system.

We present a new method for multigas detection based on multiband fiber ring laser. Wavelength division multiplexing is used to realize multiband laser operation. Laser intracavity absorption spectroscopy combined with wavelength modulation technique is used to improve the measurement sensitivity. The second harmonic signal is extracted by the software-based lock-in algorithm for gas concentration retrieval. The relationship between the harmonic amplitudes and gas concentrations is investigated under both weak absorption and strong absorption to improve the accuracy of concentration retrieval. When the system is applied to detect the mixed gases of acetylene, carbon monoxide and carbon dioxide in C + L band, the minimum detectable concentrations are 0.6 ppm, 17.4 ppm and 19.2 ppm respectively.

We propose a novel method to separate overlapping absorption lines in gas detection based on fiber laser intracavity absorption spectroscopy. The method combines continuous wavelet transform, linear regression analysis and chaos particle swarm optimization together to retrieve the respective absorbance distributions of overlapping lines. The method’s principle is outlined and the rules for parameter selection are given. The performance of the method is demonstrated by both the simulated and experimental data. When the method is applied to the mixed gas concentration calibration of CO and CO2, high linearity is obtained with linear correlation coefficients of 0.9996 and 0.9997 respectively. The relative errors of CO and CO2 concentration retrieval are no more than 0.93% and 1.13% respectively.

The absorbance enhancement of multiple gas absorption lines is investigated by simulations and experiments in this paper. The relationship between cavity loss and gas absorbance of different absorption lines is simulated based on rate equation. For the purpose of using simple arithmetic mean instead of weighted arithmetic mean in calculating gas concentration by multiple absorption lines, the absorbance equality method is proposed and simulated. To verify the simulations, some experiments were carried out based on the intra-cavity fiber ring laser (ICFRL) gas sensing system. The experimental results of absorbance enhancement extremely match to the simulation results. More than 40 C2H2 absorption lines can be observed and a minimum detectable C2H2 concentration of 200 ppm is acquired according to the experiments.

An optical fiber amplifier loop (OFAL) for intra-cavity and ring-down cavity gas sensing is proposed in this paper. The action of the OFAL is investigated theoretically. A uniform model of the system is established using rate equations, from which the sensitivity enhancement factor can be obtained. Based on the model, the effects of pump power and system loss on the performance of the OFAL are deeply researched. At the same time, the expressions of the sensitivity for intra-cavity and ring-down cavity are deduced and discussed respectively. And the simulation of the sensitivities depending on the pump power and system loss is realized, which leads to the theoretical values of the system sensitivity limit.

The nonlinearity of a piezo-electrical transducer (PZT) driven tunable optical filter (TOF) is investigated and evaluated using FBG optical fiber sensing system including a broadband optical source and a tunable laser (TL) with a group of fiber Bragg gratings (FBGs). Polynomial fit is adopted to model the nonlinearity. Under three-order polynomial fit, the random error in wavelength measurement using this TOF and a reference FBG is minimized to be below 20 pm. At the same time, the absolute error in wavelength measurement is kept below 0.25 nm. Under four-order or higher polynomial fit, a sharp increase of the random measurement error is observed. This research reveals the possibility of using PZT driven TOFs in optical spectroscopy where high resolution, high-accuracy, and fast time-response is required.