Amer Eynas, Gren Per, Edenharder Stefan, Sjödahl Mikael
Appl Opt. 2016 May 1;55(13):3429-34. doi: 10.1364/AO.55.003429.
In this paper, pulsed digital holographic detection is coupled to the stimulated Raman scattering (SRS) process for imaging gases. A Q-switched Nd-YAG laser (532 nm) has been used to pump methane gas (CH) at pressures up to 12 bars. The frequency-tripled (355 nm) beam from the same laser was used to pump an optical parametric oscillator (OPO). The Stokes beam (from the OPO) has been tuned to 629.93 nm so that the frequency difference between the pump (532 nm) and the Stokes beams fits a Raman active vibrational mode of the methane molecule (2922 cm). The pump beam has been spatially modulated with fringes produced in a Michelson interferometer. The pump and the Stokes beams were overlapped in time, space, and polarization on the gas molecules, resulting in a stimulated Raman gain of the Stokes beam and a corresponding loss of the pump beam through the SRS process. The resulting gain of the Stokes beam has been detected using pulsed digital holography by blending it with a reference beam on the detector. Two holograms of the Stokes beam, without and with the pump beam fringes present, were recorded. Intensity maps calculated from the recorded digital holograms showed amplification of the Stokes beam at the position of overlap with the pump beam fringes and the gas molecules. The gain of the Stokes beam has been separated from the background in the Fourier domain. A gain of about 4.5% at a pump beam average intensity of 4 MW/cm and a Stokes beam intensity of 0.16 MW/cm have been recorded at a gas pressure of 12 bars. The gain decreased linearly with decreasing gas pressure. The results show that SRS holography is a promising technique to pinpoint a specific species and record its spatial and temporal distribution.
在本文中，脉冲数字全息检测与受激拉曼散射（SRS）过程相结合用于气体成像。一台调Q Nd:YAG激光器（532纳米）被用于在高达12巴的压力下泵浦甲烷气体（CH₄）。来自同一激光器的三倍频（355纳米）光束被用于泵浦一个光学参量振荡器（OPO）。斯托克斯光束（来自OPO）被调谐到629.93纳米，使得泵浦光（532纳米）与斯托克斯光束之间的频率差符合甲烷分子的一个拉曼活性振动模式（2922厘米⁻¹）。泵浦光束通过迈克尔逊干涉仪产生的条纹进行空间调制。泵浦光束和斯托克斯光束在时间、空间和偏振上在气体分子处重叠，通过SRS过程导致斯托克斯光束的受激拉曼增益以及泵浦光束相应的损耗。斯托克斯光束产生的增益通过在探测器上与参考光束混合，利用脉冲数字全息术进行检测。记录了斯托克斯光束在有无泵浦光束条纹情况下的两张全息图。从记录的数字全息图计算出的强度图显示，在与泵浦光束条纹和气体分子重叠的位置，斯托克斯光束得到了放大。在傅里叶域中，斯托克斯光束的增益与背景分离。在12巴的气体压力下，泵浦光束平均强度为4兆瓦/厘米²且斯托克斯光束强度为0.16兆瓦/厘米²时，记录到约4.5%的增益。增益随气体压力降低而线性下降。结果表明，SRS全息术是一种用于精确识别特定物种并记录其空间和时间分布的有前景的技术。
