Researches On High Energy 2 μm Pulsed Fiber Laser And Cascaded Raman Fiber Laser Sources

In recent years, thulium-doped fiber lasers(TDFLs) have attracted people’s great attention with their operating wavelengths located in the eye-safety region. Thulium-doped fibers(TDFs) almost have a gain spectrum spanning from 1700 to 2100 nm, and they have been demonstrated to be highly effective media when used in obtaining high power and high pulse energy fiber laser source. In this thesis both theoretical and experimental studies on high pulse energy TDFLs are investigated. Further, studies on the cascaded Raman fiber laser source in the near-infrared and mid-infrared wavelength regions are presented. The primary contents are shown as follows:1. Theoretical models are brought to describe high pulse energy TDFL in this thesis, focusing on the study of the maximal extractable pulse energy in the thulium-doped fiber amplifier. Processes of energy storage and depletion in TDFs are also discussed in detail, with the conclusion of that there exists an optimal pulse repetition rate and an optimal gain fiber length when the pump power is fixed.2. In this thesis, temporal output characteristics of the gain-switched TDFL near threshold are also investigated. Particularly, in the unstable working state relatively stable output pulse trains with pulse repetition rate halving(PRR 1/2) with respect to the pump source is found. By using the resonantly-pumped gain-switched TDFL model, the physical insights of unstable working state and PRR 1/2 are revealed. And relatively stable working state with PRR 1/3 are forecasted through numerical modeling. Further, by reducing the environment-induced instabilities, we successfully demonstrate that PRR 1/3 can be achieved in the experiment.3. Experimental investigation of high pulse energy at 1980 nm are carried out. At first, a gain-switched TDFL working at 1980 nm is fabricated to be the seed pulse of a master oscillator power amplifier(MOPA) system. Then, energy scaling of seed pulse is achieved within the following two stages of thulium-doped fiber amplifiers. Maximal output pulse energy of 0.86 mJ is achieved in the MOPA system, and the corresponding output pulse peak power is calculated to be as high as 10 kW. It is demonstrated experimentally that seed pulses with lower repetition rates are much more beneficial for reaching the maximal extractable pulse energy if only there is no significant ASE generated in the amplifiers. Further, ways towards higher pulse energy output are discussed.4. Methods to obtain broadband cascaded Raman scattering source in short fibers are proposed. By pumping a 10 m length of highly germanium-doped fiber at 1064 nm, we demonstrate cascaded Raman scattering in the range of 1040-2100 nm. It is the first time to obtain such a broadband cascaded Raman scattering in a short step-index silica fiber. Further, it is proposed that mid-infrared cascaded Raman fiber laser source can be realized by pumping As2S3 fiber with TDFL at 2000 nm. And numerical investigations are conducted to validate that this method is feasible.