{"content":[{"text":"anti-stokes Raman scattering (CARS), are unable to detect. However, lasing techniques based \n on nanosecond pump laser pulses also possess some disadvantages. Firstly, generation of the \n lasing pulse along the pump laser beam as well as its long duration result in a very poor spatial \n resolution in the direction of the pump beam, allowing only measurements of vertical \n profiles [1–3,6]. Secondly, photochemical production of lasing species distorts the measured \n signals and therefore makes the measurements unreliable [2]. Thirdly, the lasing signal strength \n is not proportional to the number of excited atoms along its path until the saturation effect \n causes the signal to convert from exponential to linear growth. In other words, it requires \n sufficiently high laser power. \n In this letter, we demonstrate that the aforementioned difficulties of the lasing diagnostic \n technique can be overcome by using femtosecond laser pulses. With the use of a tunable deep- \n UV 125 femtosecond (fs) pump laser, a picosecond-time-scale 656 nm lasing pulse of atomic \n hydrogen was generated in a premixed flame in both the forward and backward directions. As \n shown in Fig. 1(a), the two-photon excitation transition occurs from the 1S ground state to the \n 3D excited state using a 205-nm pump laser. It is followed by relaxation from the 3D state to the \n 2P state (the Balmer-α line), releasing lasing emission at 656 nm wavelength (see the spectrum \n in Fig. 1(b)). An increased spectral bandwidth of the pump laser pulse, compared to nanosecond \n laser pulses, results in a lasing signal with a much broader detuning range of the pump laser \n wavelength. The characteristics of the backward 656 nm lasing pulse, including emission \n spectrum, spatial and temporal profiles, are analyzed. With the backward 656 nm lasing pulses \n generated from two welding flames, approximately 7 mm spatial resolution for atomic \n hydrogen detection was achieved, a significant progress towards single-ended diagnostics.","bbox":{"left":0.118928,"top":0.082055,"width":0.7622249999999999,"height":0.535405}},{"text":"Experimental setup \n The experimental setup is schematically illustrated in Fig. 1(c). A Chirped Pulsed Amplification \n (CPA) laser system (Coherent, Hidra-50) is used to deliver 125 fs, 800 nm laser pulse with \n maximum pulse energy of 30 mJ at 10 Hz repetition rate. This laser beam pumps an Optical \n Parametric Amplifier (OPA) followed by a frequency mixing apparatus (NirUVis unit), which \n can provide 205 nm laser pulses with a maximum pulse energy of approximately 50 µJ. The \n beam diameter is about 5 mm. The 205 nm pump laser propagates through a bulk CaF2 \n equilateral dispersive prism with an incident angle of 31.6º, in order to spatially separate the \n backward-propagating 656 nm radiation from the 205 nm pump laser beam. In addition, this \n configuration spectrally purifies the pump laser by dispersing residual frequencies. After the \n prism, the pulse energy of the pump beam has been reduced to approximately 20 µJ mainly due","bbox":{"left":0.118928,"top":0.648033,"width":0.762145,"height":0.26432500000000003}}],"images_bbox":[],"images_bbox_not_text_overlap":[]}