| Vibrational stimulated Raman scattering (SRS) in gaseous O2, CH4 and H2 is studied by using a single longitudinal mode and broad bandwidth Nd:YAG laser as the pump source. The SRS conversion efficiencies in O2, H2 and CH4 gases are the same as or higher than the results in the references when the broad bandwidth laser is used as the pump source. However, when the single longitudinal mode laser is used as the pump source, some new experimental results are obtained as follows:1. Due to stimulated Brillouin scattering (SBS) suppression, the forward first-Stokes (FS1) conversion efficiency is only 3.8% in pure O2. In comparison, the accompanying SBS attains a reflectivity of 70%. However, by seeding an intense OPO beam into the Raman cavity, the FS1 quantum conversion efficiency now attains a peak value of 54%, while the SBS reflectivity reduces to 5%. On the other hand, the beam quality of the FS1 in an O2/He (41/59) mixture at total pressure 6.1Mpa excels that of the pump laser for a seeder energy of 5 mJ and pump energy of 50 mJ. However, at pump energies higher than 105 mJ, the thermal defocusing effect worsens the FS1 beam quality.2.Very intense backward first-Stokes (BS1) in H2/He mixture gas is generated by using a single longitudinal mode second-harmonic Nd: YAG laser as the pump source. The quantum conversion efficiency of BS1 is as high as 69% and the BS1 pulses are narrowed to 1.1ns. As a result, the BS1 peak power turns out to be 2.6 times that of the pump. The wave-front-reversed replica of the pump beam has been observed in H2/He mixture.3. Strong BS1 in H2 is observed when fundamental frequency Nd:YAG laser is used as the pump source. The peak quantum conversion efficiency of BS1 is obtained as high as 66%. Due to the opposite propagating directions, there iscompetition between FS1 and BS1, which leads to relaxation oscillation. As a result, the BS1 pulses are narrowed to about Ins brings that the BS1 peak power turns out twice that of the pump. Furthermore, the beam quality of BS1 is much better than that of both FS1 and pump.4. Very intense BS1 is generated in CH4. The peak quantum conversion efficiency of BS1 is as high as 73%. Due to the relaxation oscillation, the BS1 pulses are narrowed to about 1.2ns. As a result, the BS1 peak power turns out to be 2.7 times that of the pump. Furthermore, the beam quality of BS1 is much better than that of the pump and FS1. Moreover, the thermal defocusing greatly affects the SRS conversion efficiency and beam quality of FS1, without noticeably affecting that of BS1. This because that BS1 represents a wave-front-reversed replica of the pump beam, which can compensate the thermal distortions in Raman amplify process.
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