An optical fiber displacement sensor based on the microwave photonics interferometric (MWPI) method is proposed and experimented, which provides an ideal solution for large range displacement measurement with high resolution. The sensor used a Michelson microwave photonics interferometer to sense the displacement with one sensing arm and a length-adjusted reference arm. The displacement variation would change the period of the microwave response function of the interferometer. According to the principle that the phase difference in one free spectral range (FSR) of the microwave response function is 360°, the displacement can be retrieved by the microwave response function by means of a vector network analyzer (VNA). A programmable path-switching true time delay line was used in the reference arm to decrease the microwave bandwidth. The measurement results show that the displacement sensing range is larger than 3 m and the measurement resolution is 31 μm. Finally, the measurement stability is tested, and the factors affecting the measurement resolution of this method and the main source of errors are investigated in detail.

A novel quartz enhanced photoacoustic spectroscopy (QEPAS) trace gas detection scheme is reported in this paper. A cylindrical lens was employed for near-infrared laser focusing. The laser beam was shaped as a planar line laser between the gap of the quartz tuning fork (QTF) prongs. Compared with a spherical lens-based QEPAS sensor, the cylindrical lens-based QEPAS sensor has the advantages of easier laser beam alignment and a reduction of stringent stability requirements. Therefore, the reported approach is useful in long-term and continuous sensor operation.

We first research the effects of femtosecond-laser-induced plasma (FLIP) on a laminar premixed methane/oxygen/nitrogen flame speed with a wide range of equivalence ratios ($0.8–2.0$) at atmospheric pressure. It is experimentally found that the flame speed increases by 30.8% at equivalence ratio 1.33, and the effects of the FLIP on the flame speed are more remarkable when the methane is rich. The self-emission spectra from the flame and the plasma are studied, and the presence of the oxygen atom is likely to be a key factor in enhancing flame speed.

Planar laser-induced fluorescence (PLIF) of toluene and acetone is performed for visualization of spatial distribution of oxygen in gas and oxygen mixing region in the nozzle flow field. The principle of the calibration methods of oxygen concentration distribution is expounded in this paper. As the two tracers had different situations on oxygen quenching effect, the ratio of the fluorescence images of the two tracers could be used to measure the oxygen concentration in the mixed area. In this way, the ratio of fluorescence signal intensity of toluene to acetone was found to be linearly related to oxygen concentration. By using this calibration relationship, we can measure the oxygen concentration distribution of the nozzle flow field with an accuracy of 12.6%. It indicates that this measurement method can be applied in a turbulent flow field. The variation trend of oxygen concentration distribution in the nozzle flow field can be analyzed by capturing multiple PLIF images in time sequence, which has the important reference value for the development of the nozzle burner flame.

The influence of femtosecond laser-induced plasma (FLIP) on the stability of a premixed ${\mathrm{CH}}_4/{\mathrm{O}}_2/{\mathrm{N}}_2$ flame is investigated at atmospheric pressure. The laser energy, laser repetition rate, the equivalence ratios, and the volume percentage of oxygen in ${\mathrm{O}}_2/{\mathrm{N}}_2$ blends are varied. Our findings indicate that the flame blow-off velocity is a function of these parameters. It has been experimentally found that the flame blow-off velocity increases by a factor of two with FLIP than without FLIP. A high-repetition-rate and a great energy laser-induced plasma flameholding, as a non-intrusive optical flameholding, may be a feasible alternative for any combustor.