Characterization of femtosecond laser-induced grating scattering of a continuous-wave laser light in air

Abstract

Nanosecond laser-induced grating scattering/spectroscopy (LIGS) technique has been widely applied for measuring thermodynamic parameters such as temperature and pressure in gaseous and liquid media. Recently, femtosecond (fs) laser was demonstrated to induce the grating and develop the fs-LIGS technique for gas thermometry. In this work, we systematically investigated the fs-LIGS signal generation using 35 fs, 800 nm laser pulses at 1 kHz repetition rate in ambient air by varying the pump laser energies, the probe laser powers and the temporal delays between two pump laser pulses. The stability of single-shot fs-LIGS signal was studied, from which we observed that the signal intensity exhibits a significant fluctuation while the oscillation frequency shows a much better stability. A 4.5% precision of the oscillation frequency was achieved over 100 single-shot signals. By using a previously-developed empirical model, the fs-LIGS signals were fitted using nonlinear least-squares fitting method, by which crucial time constants characterizing the signal decay process were extracted and their dependences on the pump laser energy were studied. From the measured results and theoretical analysis, we found that the appropriate range of the overall pump laser energy for reliable fs-LIGS measurements is approximately located within 80 ∼ 300 μJ. The limitations on the accuracy and precision of the fs-LIGS measurements, the origin of destructive influence of plasma generation on the signal generation as well as the electrostriction contribution were also discussed. Our investigations could contribute to a better understanding of the fs-LIGS process and further applications of the technique in single-shot gas thermometry and pressure measurements in various harsh conditions.

Deep learning assisted pressure measurements using femtosecond laser-induced grating scattering technique

“…The signal beam was reflected several times to filter out other light components, such as the 800 nm pump laser, and eventually detected with a photomultiplier tube (R943-02, Hamamatsu Photonics) and displayed on an oscilloscope (Tektronix DPO 4054, sampling rate 2.5 Gsps). More details regarding the experimental procedures can be found in our previous works [24,25].…”

“…In particular, the fs-LIGS signal is so strong that it can be directly seen by the naked eye, which makes the signal collection much easier; therefore, the practical application of the technique in harsh environments is more feasible. Characterization of the fs-LIGS signal generation, including the single-shot stability, the influence of pump laser energy and probe laser power, and the detrimental impact of plasma generation, was further performed systematically [25]. For applications, accurate temperature measurements up to 750 K with a deviation of approximately 10% in heated nitrogen gas flows have been demonstrated using this technique [24].…”