• Chinese Journal of Lasers
  • Vol. 48, Issue 21, 2101006 (2021)
Tiancheng Zheng1、3, Xianglong Cai1、2, Zhonghui Li1, Chencheng Shen1、3, Dong Liu1, Jingbo Liu1、*, and Jingwei Guo1、**
Author Affiliations
  • 1Key Laboratory of Chemical Lasers, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China
  • 2School of Science, Changchun University of Science and Technology, Changchun, Jilin 130022, China
  • 3University of Chinese Academy of Sciences, Beijing 100049, China
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    DOI: 10.3788/CJL202148.2101006 Cite this Article Set citation alerts
    Tiancheng Zheng, Xianglong Cai, Zhonghui Li, Chencheng Shen, Dong Liu, Jingbo Liu, Jingwei Guo. Visible Broadband Raman Lasers of H2∶CH4∶CO2 Gas Mixture Pumped by 532-nm Laser[J]. Chinese Journal of Lasers, 2021, 48(21): 2101006 Copy Citation Text show less

    Abstract

    Objective Mixtures of H2, CH4, and CO2 gases pumped using a 532-nm laser were studied as visible broadband Raman lasers to illuminate multispectral lasers. To conduct mixed-gas experiments, we must understand the Raman line-widths and gain coefficients of the three gases H2, CH4, and CO2, which are critical for adjusting the ratio of these three gases. The optimization process for mixing H2, CH4, and CO2 gases depends on two parameters: the input pump energy and the partial pressures of these gases. Notably, the most important step is to fully present the pressure- and pump energy-dependence of the Raman components of each order in H2, CH4, and CO2 gases. We hope to achieve simultaneous outputs of some spectral lines with nearly equal conversion efficiencies by controlling the pressure ratio between the three gases during the experiment of mixing the three Raman-active gases.

    Methods A schematic of the experiments used to study the multiwavelength Raman lasers generation using CH4, CO2, and H2 gases and their mixture in a single Raman cell is shown in Fig. 1. The radiation source was Nd∶YAG second harmonic at 532 nm, obtained from Beamtech Optronics Co., Ltd., with a spot diameter of ~8 mm, a divergence angle of less 1 mrad, and a pulse duration of 6 ns. All the experimental results were achieved at a fixed repetition frequency of 1 Hz. First, the pump beam was passed through a Pellin-Broca prism to avoid backward-shifted radiation. Then, it was focused using a lens L1 (f=1000 mm) at the center of the 1.80-m long Raman cell. Thereafter, the pump and Stokes beams were recollimated at the exit of the cell window using the same lens L2 (f=1000 mm) and dispersed via the same Pellin-Broca prism. Finally, the dichroic mirror was used to measure the backward scattering.

    Results and Discussions Fig. 3 presents the energy conversion efficiencies of the pump energy to various Raman components against the gas pressure fixed on the pump energy of 103 mJ for the H2, CH4, and CO2 gases, respectively. There are clear differences in energy distribution among the multiple orders of Stokes and anti-Stokes scattering in each of the three gases. We explain the favored BS1(the first Stokes in the backward direction) in CH4 for the large ratio between the forward and backward gain coefficients. Further, the strongest high-order Stokes in H2 can be observed owing to the biggest Raman gain. Four-wave mixing can play a considerable role in high pressure owing to the smallwave vector mismatch in CO2。Notably, various Stokes components among the three gases at the low pressure of 0.5 MPa have similar features (Fig.4): the conversion efficiencies for various Stokes components, especially for S1, will be stable at high pump energy. Fig.5(a) shows that the conversion efficiencies of different Raman components vary with the pump energy for a mixture of 0.45 MPa CH4, 0.4 MPa CO2, and 0.3 MPa H2. We can obtain 13 spectral lines (Table 2), in which the 574-, 630-, 683-, and 771-nm lasers own nearly equal conversion efficiencies (6.5%--8%) at the high energy region above 180 mJ, and the 532-, 853-, and 954-nm lasers reach 14%, 2.1%, and 1.9% efficiencies, respectively. Compared with the individual gas, we found that CH4 S1 (the first Stokes in the forward direction) yields the minimum threshold, followed by H2 and CO2. Furthermore, the S1 conversion efficiency of CH4 in the mixture is almost as good as that of the pure CH4 gas, while the S2 conversion efficiency (the second Stokes in the forward direction) is approximately two times than that of S1 owing to the additional H2 harming the BS1 in CH4. Compared with the unique H2 gas, the decrease of S1 (683 nm) in the mixture is due to a new pump laser that generates 853-nm laser by pumping CH4, which will compete with the formation of S2 (954 nm) in H2, leading to the decrease of the 954-nm laser. More attention should be given to investigate specific competitive mechanisms in the mixture.

    Conclusions In this paper, we have investigated the multispectral Raman system using H2, CH4, and CO2 gases as Raman media. The high-order Stokes light in H2 is the strongest among the three gases, especially S2. CH4 has the strongest BS1, causing the fierce competition with forward Raman light. The effect of S2 produced via four-wave mixing in CO2 gas is more obvious under high pressure. At low pressure, the conversion efficiency of each Stokes component (especially S1) of the three gases can reach saturation at high pump energy. We achieve the simultaneous outputs of 13 spectral lines, covering the wide visible spectral region by controlling the pressure ratio between different gases in the experiment of mixing the three Raman-active gases. A strong competition and interaction exist between the Raman-active gases in the mixture, especially between H2 and CH4gases. Thus, we conclude that combining multiple gases using a single laser and a single Raman cell can produce multispectral Raman laser outputs. Similarly, using 355-nm or other short-wavelength lasers, rich spectra can be generated, which will have paramount applications in the fields of laser color display, biomedicine, underwater communications, and atmospheric detection.

    Tiancheng Zheng, Xianglong Cai, Zhonghui Li, Chencheng Shen, Dong Liu, Jingbo Liu, Jingwei Guo. Visible Broadband Raman Lasers of H2∶CH4∶CO2 Gas Mixture Pumped by 532-nm Laser[J]. Chinese Journal of Lasers, 2021, 48(21): 2101006
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